Physiology & Behavior 130 (2014) 157–169

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Physiology & Behavior

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Review

Advances in understanding the interrelations between leptin resistanceand obesity☆

Haitao Pan, Jiao Guo ⁎, Zhengquan Su ⁎⁎Key Research Center of Liver Regulation for Hyperlipidemia SATCM/Class III Laboratory ofMetabolism SATCM, Guangdong TCM Key Laboratory for Metabolic Diseases, Guangdong PharmaceuticalUniversity, Guangzhou, Guangdong, China

H I G H L I G H T S

• Leptin resistance is probably not only a significant symbol for obesity but is also an important risk factor for it.• The major advances that led to our current understanding of the possible mechanisms underlying leptin resistance• The role of leptin signals and leptin resistance in the regulation of the pathophysiological processes of obesity• Some effective treatments of leptin resistance are also discussed.

☆ Conflict of interest: There are no conflicts of interests⁎ Corresponding author. Tel./fax: +86 20 39352818.⁎⁎ Corresponding author. Tel.: +86 20 3935 2067; fax: +

E-mail addresses: wshxalb@163.com (J. Guo), suzhq@

http://dx.doi.org/10.1016/j.physbeh.2014.04.0030031-9384/© 2014 Elsevier Inc. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 9 October 2013Received in revised form 7 March 2014Accepted 2 April 2014Available online 12 April 2014

Keywords:ObesityLeptinLeptin resistanceInterrelations

Obesity, which has developed into a global epidemic, is a risk factor in most chronic diseases and some forms ofmalignancy. The discovery of leptin in 1994 has opened a new field in obesity research. Currently, we know thatleptin is the primary signal from energy stores and exerts negative feedback effects on energy intake. However,most individuals with diet-induced obesity (DIO) develop leptin resistance, which is characterized by elevatedcirculating leptin levels and decreased leptin sensitivity. To date, though various mechanisms have been pro-posed to explain leptin resistance, the exact mechanisms of leptin resistance in obesity are poorly understood.Consequently, it's an important issueworth discussing regardingwhat the exact interrelations between leptin re-sistance and obesity are. Here, we review the latest advancements in the molecular mechanisms of leptin resis-tance and the exact interrelations between leptin resistance, obesity, and obesity-related diseases, in order tosupply new ideas for the study of obesity.

© 2014 Elsevier Inc. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1582. Energy homeostasis and obesity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1583. Leptin and the factors influencing its secretion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1584. Leptin receptor signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1615. Leptin resistance and obesity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161

5.1. The mechanisms of leptin resistance in obesity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1615.1.1. Defective transport of leptin into the CNS across the BBB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1615.1.2. Attenuation of ObRb signaling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1625.1.3. Genetic variations of leptin and LEPR genes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1625.1.4. Endoplasmic reticulum (ER) stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1635.1.5. Others . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163

for any of the authors regarding the content described in the paper.

86 20 3935 2065.scnu.edu.cn (Z. Su).

158 H. Pan et al. / Physiology & Behavior 130 (2014) 157–169

5.2. The exact interrelations between leptin resistance, obesity and the obesity-related diseases . . . . . . . . . . . . . . . . . . . . . . . 1635.2.1. Leptin resistance, obesity and cardiovascular diseases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1645.2.2. Leptin resistance, obesity and neoplasmia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164

5.3. The treatments for leptin resistance-related obesity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1645.3.1. Caloric restriction and exercise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1645.3.2. Gene-targeted drug therapy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164

6. Discussions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166

1. Introduction

Obesity, the global epidemic, has attracted extensive attentionworldwide and has led to increasingly serious medical problems [1–3].Indeed, obesity is currently considered as a significant health threat be-cause nearly 35% of the adult populations in most developed countriesare clinically obese (WHO Statistics, 2013). Obesity and being over-weight are the fifth leading risk for global deaths and the risk factorsin various chronic diseases, including type 2 diabetes mellitus [4], car-diovascular diseases (CVDs) [5], immunity and inflammation [6,7], psy-chological deficits [8,9] and some malignancy conditions [10], such ascolorectal cancer [11] and breast cancer [12]. Thus, the prevalence ofobesity, the significance of obesity, and the economic costs of the dis-ease create an urgent need for better therapeutics and understandingof the physiological processes that balance energy intake and energy ex-penditure. A growing number of researchers have devoted themselvesto the study on the molecular mechanisms of obesity and obesity-related diseases [13–18], with the purpose of ascertaining the real rootof obesity for reasonable and effective therapies. There have been nu-merous theories to explain this type of complex physiological networkof obesity. However, the exactmechanismof obesity is not clear at pres-ent. Two central discoveries in the explanation of this network werethose of leptin, which is encoded by the obese gene (OB), and its longreceptor, leptin receptor b (ObRb) [12,19]. The hormone leptin, a 16-kDa polypeptide, is mainly synthesized and secreted by the white adi-pose tissue (WAT). Leptin acting on specific populations of neurons inthe brain, including hypothalamic, midbrain, and brainstem neurons,plays a central role in weight control by suppressing food intake and in-creasing energy expenditure [20–22]. Nevertheless, most individualswith DIO have high levels of circulating leptin [22,23]. More important-ly, the injection of additional leptin in obese individuals fails to counter-act obesity [20]. Consequently, the concept of leptin resistance emergedto explain the seemingly paradoxical elevated levels of leptin that occurin obesity.

However, a growing number of studies have shown that leptinresistance induced by obesity complicates efforts to distinguishthe mechanisms that predispose individuals to weight gain from themechanisms that result from obesity lately [21]. For this reason, leptinresistance is probably not only a significant symbol for obesity but isalso an important risk factor for it. In addition to its association withobesity, leptin is also involved in the regulation of other physiologicalprocesses, including reproduction [10,24], bone homeostasis [19] andimmune function [12], etc. Therefore, leptin resistance participates invarious pathological processes of other diseases, such as CVD [5], osteo-porosis [25], systemic inflammation [6,7] and depression [8,9]. In recentyears, a growing number of scientists have begun to explore the thera-peutic methods for obesity from the point of leptin resistance and alsothat of leptin resistance-related diseases. Here, we review themajor ad-vances that led to our current understanding of the possible mecha-nisms underlying leptin resistance, the exact interrelation betweenleptin resistance and obesity, and the role of leptin signals and leptin re-sistance in the regulation of the obese pathophysiological processes. Ad-ditionally, some effective treatments of leptin resistance are alsodiscussed.

2. Energy homeostasis and obesity

In recent years, there has been a growing emphasis on the correla-tion between nutrition and energy homeostasis [26,27], andmeanwhilethe studies on the effects of adipocytes on calorie homeostasis are inprocess [28]. Energy homeostasis is the balance between energy intake,energy spent and energy storage [29]. Inmost adults, although there ex-ists a large variation in daily food intake and energy expenditure, bodyweight is almost constant [30], which depends on the regulation ofthe balance between energy intake, in the form of food and drinks,and energy expenditure, in the forms of basal metabolism, physical ac-tivity and adaptive thermogenesis [18,28]. The adipocytes act as caloriestorage, which makes them well suited to the regulation of energy bal-ance. The adipose tissue also serves as a crucial integrator of energy ho-meostasis, because a host of regulating hormones for energy balancecan be secreted from the adipose tissue [28].

Traditionally, the adipocyte-derived regulating proteins (namelyadipocytokines) are categorized as pro-storage of energy includingresistin, tumor necrosis factor-α (TNF-α), interleukin-6 (IL-6) andretinol-binding protein-4 (RBP-4), and anti-storage of energy such asleptin, adiponectin, visfatin and omentin [2,31] (Fig. 1). The complicatedphysiological system for energy homeostasis consists of both afferentsignals and efferent effectors [32]. The afferent signals, which aremainlyin the form of hormones above, are integrated by the peripheral nervesand the central nervous system in the hypothalamus, brainstem, etc.The integrated signals regulate various types of neuropeptide in return,which modulate energy homeostasis and contribute to the stable stateof body weight [33]. Some regions in the hypothalamus, such as theventromedial hypothalamic nucleus (VMH) and the arcuate nucleus(ARC), have been thought to play central roles in the regulation of ener-gy homeostasis. VMHwas clearly associatedwith increased food intake,morbid obesity and insulin resistance, while damage tomore lateral hy-pothalamic structures was associated with anorexia and adipsia [34].ARC can produce α-melanocyte stimulating hormone (α-MSH), an an-orectic peptide formed from the cleavage of the proopiomelanocortin(POMC) protein [35]. This protein acts on melanocortin receptor types3 and 4 (MC3/4, respectively) that existed in various hypothalamic nu-clei to reduce food intake and energy expenditure in amanner similar toleptin [36]. Most peripheral signals for energy homeostasis, mainly theadipocyte-derived regulating proteins, accept the regulation ofmediobasal hypothalamic areas, particularly VMHandARC, constitutingthe hypothalamic homeostatic regulating circuits. The lesions of specificsections in the signal pathwayswill result in the energymetabolismdis-orders conducing to the development of obesity and related diseasessuch as diabetes. It is well worth mentioning that leptin has been con-sidered as one of the most important adipocytokines for energyhomeostasis.

3. Leptin and the factors influencing its secretion

The word “leptin” originates from the Greek root “leptos”, whichmeans “thin”, and cited by the medical community in 1994 [30]. Leptin,a 16 kDa polypeptide and the product of OB gene, is a cytokine-like cir-culating hormone produced and secreted predominantly by the

Fig. 1. Adipocyte-derived regulating proteins with varied effects on energy homeostasis. The pro-storaged proteins for energy including resistin, tumor necrosis factor-α (TNF-α),interleukin-6 (IL-6) and retinol-binding protein-4 (RBP-4); The anti-storaged proteins for energy including leptin, adiponectin, visfatin and omentin.

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adipocytes in theWAT, acting through their receptors (LEPRs) on specif-ic populations of neurons in the brain, including the hypothalamic,mid-brain, and brainstemneurons, andwhosemain function is to counteractobesity [22,37]. Mirroring the body's fat stores, leptin plays a crucial rolein the regulation of numerous neuroendocrine functions, from energyhomeostasis to a variety of additional processes such as reproduction[38,39], bone function [19], cardiovascular regulation [5] and immunefunction [12]. Leptin decreases body weight by both suppressing

Fig. 2. Schematic diagram for biological synthesis, secretion and actions of leptin as well as molrelation; ++: positive correlation;+: permissive role;−−: inhibition;−: Negative regulatioObRb: leptin receptor b.

appetite and promoting energy expenditure by directly targeting thehypothalamic neurons, including the increased expression of the an-orexigenic peptide α-MSH, which is derived from POMC cells in theARC, and the decreased expression of the orexigenic peptide neuropep-tide Y (NPY) neurons and agouti-related peptide (AgRP) neurons,which are also located in the ARC [40–42].

Various factors influence leptin secretion and expression (Fig. 2).The most important factors are the distribution of subcutaneous fat

ecular mechanisms and therapeutic principles of leptin resistance. Notes: +++: high cor-n;●: on study;▲: require further trials; FFAs: free fatty acids; ER: endoplasmic reticulum;

160 H. Pan et al. / Physiology & Behavior 130 (2014) 157–169

and the status of its energy stores because leptin is mainly expressed inthe adipose tissue, and circulating leptin concentrations in the fed stateare highly correlated with the degree of adiposity [20,43,44]. Leptin ex-pression also correlates with insulin level, as evidenced by the fact thatinsulin triggers leptin expression directly in isolated adipocytes [45] andenhances leptin levels when injected into rodents [46], as well as thediscovery that decreased leptin levels accompanied with low insulin re-sistance states and circulating leptin concentrations increased after in-sulin treatment [47]. Glucocorticoids directly induce hyperleptinemiaand stimulate leptin synthesis in cultured adipocytes [48], for example,leptin expression increases in response to the chronic elevation of corti-sol in humans [49]. In contrast, the secretion of leptin can also enhancethe concentration of circulating glucocorticoids [50]. Therefore, gluco-corticoids and leptin mutually promote each other. However, other re-lated studies have demonstrated that there is a negative correlationbetween the concentration of circulating leptin and glucocorticoidlevels [51]. Thus, the exact influence of glucocorticoids on leptin

Fig. 3. Schematic overview of ObRb and its role in leptin signaling. The ObRb and other five leconsisting of two cytokine receptor homology (CRH1 and CRH2) domains separated by an imIII) domains. The CRH2 domain and Ig-like domain are necessary and sufficient for leptin bind bbineswith the receptor via the recruitment of the box-1motif (less-well-conserved sequences)of JAK2. ObRb has three important conserved tyrosine residues in its cytoplasmic domain, indevated JAK2. The process that Y1138 recruits the transcription factor STAT3, togetherwith its phoregulates the expression and metabolism of POMC, SOCS3, NPY and AgRP. SOCS3 binding to Ycruitment of SHP2 to Y985, which is also a binding site for the negative modulator SOCS3homostasishomeostasis. PTP1B mediates the dephosphorylation of JAK2, limiting the extent oThe phosphorylated JAK2 protein also activates and phosphorylates IRS1 and IRS2 (insulin recepulation of the P13K pathway, and then regulating the balance between food intake and energy csuppresses the AMPK expression in hypothalamic acetyl-CoA carboxylase (ACC).

secretion should ultimately be confirmed based on the additional clini-cal trials in the future.

Other factors influencing the circulating concentration of leptinwere reported in several researches in the field. Glucose and/or its me-tabolites play permissive roles in the secretion and expression of leptin[52] and leptin signaling [53], and glucose dose-dependently enhancesleptin signaling and leptin sensitivity, at least in part, by attenuatingthe ability of AMPK to inhibit leptin signaling. Circulating free fattyacids (FFAs) serve as suppressors of leptin secretion, that may be asso-ciated with the hyposensitivity to food caused by FFAs in circulation[54]. The administration of thyroid hormone decreases leptin levels inrodents, but that is not a major determinant of plasma leptin levels[55]. Meanwhile, there is an interaction effect between leptin, sex hor-mones and growth hormones, which is likely to result in metabolic dis-orders involving sex hormones and growth retardation in obeseadolescents [56]. Additionally, infections, endotoxins and some cyto-kines stimulate leptin synthesis and secretion reported by several

ptin receptors receptor isoforms all share the same ligand-binding extracellular domain,munoglobulin-like (Ig-like) and followed by two membrane proximal fibronectin III (FNinding and the receptor activation. After the activation of ObRb, Janus kinase 2 (JAK2) com-adjoining themembrane domain of cell, which leads to the phosphorylation and activationntifying with positions Y985, Y1077 and Y1138, which can be phosphorylated by the acti-sphorylation by JAK2, dimerisation dimerization and translocation into the arcuate nucleus,985 attenuates ObRb signaling, completing the negative feedback regulatory loop. The re-, positively engages the ERK pathway, which improves the effect of leptin on energyf leptin action in cultured cells, and also increases the expression of SOCS3 via ER stress.torsubstrate substrates 1, 2) after leptin binding to it’s recruitment,which lead to the stim-onsumption by increasing the expression of POMC in the ARC. The phosphorylation of JAK2

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studies [57]. To sum up, there have been so many influencing factorscontributing to the level of expression and secretion of leptin that pro-vides a wide field for the further studies on leptin.

4. Leptin receptor signals

Plasma leptin level was found to be highly correlated with theamount of body fat [20] and has a central effect on the regulation offeeding behavior and energy expenditure by activating the hypothalam-ic leptin receptors (LEPRs) [58–60]. The LEPR, the product of the LEPRgene, is a member of the class I cytokine receptor family, and has atleast six splice variants, ObRa–ObRf [61]. Notably, the long receptorObRb mediates essentially all known physiological effects of leptin inenergy homeostasis [62–64] because genetic deficiency of ObRb resultsin pronounced hyperorexia and morbid obesity in animals [65,66]. Lep-tin binding to the domain of extracellular of ObRb triggers the domain ofactivated box-1 and amino acid at 31–36 chain onObRb, and the recruit-ment of the tyrosine kinase Janus kinase-2 (JAK2), resulting in the phos-phorylation and activation of JAK2 [4,67,68] (Fig. 3). Activated JAK2phosphorylates three tyrosine residues in the cytoplasmic domain ofObRb, which includes Tyr985, Tyr1077 and Tyr1138 [69]. Of these resi-dues, phosphorylated Tyr1138 (pY1138) recruits signal transducer andactivator of transcription 3 (a transcription factor, STAT3), which be-comes phosphorylated by JAK2 [70,71]. Phosphorylated STAT3(pSTAT3) then homodimerizes and translocates to the arcuate nucleusin the hypothalamus, where it increases the expression and neuronalexcitability of POMC and inhibits that of AgRP and NPY [40,72,73]. Onthe one hand, POMC is subsequently processed into α-MSH, whichcan suppress appetite, promote energy expenditure and decreasebody weight. On the other hand, the activation of STAT3 inhibits AgRPexpression, which can suppress the activity of NPY and subsequentlyblock NPY inhibition against reproduction and the growth axis, whichfinally cause the inhibition of appetite and increase energy expenditure[74].

This complicated physiological process is regulated by both positiveand negative regulators. The negative regulators act as feedback inhibi-tors of leptin signaling by binding to pY985 and preventing the activa-tion of the JAK2/STAT3 pathway [75], including suppressor of cytokinesignaling protein-3 (SOCS3) and protein tyrosine phosphatase 1B(PTP1B). The positive regulator Src-homology 2 domain 1 (SH2B1)markedly enhances JAK2 activity, thereby promoting the activation ofthe leptin-signaling pathways downstream of JAK2 [76,77]. Additional-ly, endoplasmic reticulum (ER) stress can also prevent the activation ofthe JAK2/STAT3 pathway and conduces to the hyposensitivity of leptinand the development of obesity by means of combining with PTP1B toeach other.

In addition to the JAK2/STAT3 signaling pathway, the activation ofObRb also results from the activations of extracellular signal-regulatedkinase (ERK) and phosphoinositide-3-kinase (PI3K) pathways [78].The involvement of the ERK pathway is supported by the fact that theinhibition of ERK blocked the effect of leptin on food intake and bodyweight. SHP2 has been shown to promote signaling through the ERKpathway, though other data indicated that SHP2 might suppress JAK2/STAT3 signaling by decreasing JAK2 phosphorylation [12,79]. Leptin-stimulated activation of hypothalamic phosphatidylinositol-3-kinase(PI3K) is impaired in DIO [80], and the pharmacological inhibition ofPI3K activity blocks the anorectic effect of leptin [81,82], while chronicactivation of the hypothalamic PI3K pathway by ObR neuron-targeteddeletion of the PIP3 phosphatase PTEN increases leptin sensitivity anddecreases adiposity [83,84], suggesting that the PI3K pathway in non-POMC hypothalamic neurons appears tomediate the long-term anorec-tic effects of leptin. The effect of PI3K promoting leptin actions is medi-ated by the inhibition of FoxO1, which can antagonize leptin actionin vivo [12]. The activation of both pathways has been shown to be re-quired for proper regulation of energy and glucose homeostasis [84].

Corresponding with the abovementioned regulating pathways,including ERK and P13K, the AMPK and mTOR pathways are also in-volved in the regulation of leptin signals. Leptin stimulates hypo-thalamic acetyl-CoA carboxylase (ACC), a key enzyme in fatty acidbiosynthesis, via the inhibition of AMPK [85,86]. Dominant negativeAMPK expression in the hypothalamus is sufficient to reduce foodintake and body weight; in contrast, the fundamental activation ofhypothalamic AMPK attenuates leptin's anorexigenic response [87,88]. It has been reported that glucose and its metabolites playpermissive roles in leptin signaling and that glucose enhances lep-tin sensitivity, at least in part, by attenuating the ability of AMPKto inhibit leptin signaling [54]. Leptin increases the mammalianhypothalamic target of rapamycin (mTOR) activity, and the inhibi-tion of mTOR signaling by rapamycin attenuates leptin's anorecticeffect [89]. Exposure to a high-fat diet decreases mTOR signalingwithin the hypothalamus, suggesting that reduced hypothalamicmTOR activity contributes to the development of hyperphagia andweight gain during DIO [90].

5. Leptin resistance and obesity

Noticeably, the reduced abilities of peripheral leptin and centralObRb to collaborate in the suppression of appetite and the promotionof energy expenditure can be taken for the crucial risk factors for the de-velopment of obesity. Therefore, elevated peripheral leptin level is likelyto control or reverse obesity. However, the hope that leptin might be a“miracle cure” in the treatment of obesity was remote. Rather thanbeing leptin deficient, most obese humans and animals have high levelsof circulating leptin [20,91,92], and increased leptin fails to resist the de-velopment of obesity [93–95]. Meanwhile, treatment with leptin alonein obese individuals fails to counteract common obesity [94–96]. Thistransparent leptin ineffectiveness and hyposensitivity is identified asleptin resistance [19,39,97–100]. A large number of studies demonstrat-ed that most individuals with DIO manifest leptin resistance character-ized by increased leptin levels in the blood and decreased leptinsensitivity, which is in proportion to the individual's adipose mass andBMI [92,98–103].

5.1. The mechanisms of leptin resistance in obesity

In view of the great influence of leptin resistance on the develop-ment of obesity, it's an important subject for treating obesity to eluci-date the mechanisms of “leptin resistance”. Currently, severalmechanisms have been proposed to explain leptin resistance, includingdefective leptin transport across the blood–brain barrier (BBB) [104], at-tenuation of leptin signaling [105,106], the deficiency and variations ofLEP and LEPR genes [3,19,21,107], ER stress [67,68,108], inflammation[6], excessive bioavailability of heavy metals such as Zn [109,110], andothers (Fig. 2).

5.1.1. Defective transport of leptin into the CNS across the BBBThe results of a large number of studies displayed that obese individ-

uals exhibit high peripheral leptin levels but relatively lower cerebro-spinal fluid (CSF) concentrations, suggesting that defective leptintransport into the central nervous system (CNS) across the BBB con-duces to leptin resistance [104,111–115]. Indeed, leptinmediates the in-hibition of appetite and increased energy expenditure only under theconditions that nomadic leptin is transported across the BBB and ulti-mately binds to its receptors initiating the signals for energy homeosta-sis in some hypothalamus populations [104]. By binding to themegalin,a kind of binding protein at the choroid plexus epithelium, leptin trig-gers its transportation into the CNS in the study of Dietrich et al. [116].Triglycerides act as the regulators for theprocess because decreasing tri-glycerides can strengthen the anorectic effect of leptin by enhancingleptin transport across the BBB [117]. Acute phase C-reactive protein(CRP) is also likely to contribute to leptin resistance by preventing leptin

162 H. Pan et al. / Physiology & Behavior 130 (2014) 157–169

from crossing the BBB [118]. The process of leptin transport balance isregulated by two LEPRs, ObRa andObRe. ObRamediates leptin transportacross the BBB [119,120] and ObRe inhibits leptin transport bycounteracting the function of ObRa [121]. Under normal conditions,the actions and number of these two receptors are in balance. Thus, sub-sequent studies are needed to clarify the equilibrium correlation be-tween ObRa and ObRe under normal and obese states.

5.1.2. Attenuation of ObRb signalingThe attenuation of ObRb signaling is mainly due to two parallel mo-

lecular mechanisms including the up-regulation of suppressor of cyto-kine signaling (SOCS3) in the cytoplasm and that of protein tyrosinephosphatase-1b (PTP1B) in the endoplasmic reticulum. Both of themare all involved in the regulation of ObRb signaling pathway, particular-ly JAK2/STAT3.

SOCS3 is a critical protein that inhibits the signal transduction pro-cess of various cytokines in the body, including leptin [105]. It's themost crucial procedure for the inhibition of appetite and increased ener-gy expenditure that leptin mediates STAT3 phosphorylation, and thenincreases POMC expression as well as inhibits the activities of NPY andAgRP. By binding to Tyr985 of ObRb and JAK2, SOCS3 inhibits theleptin-induced phosphorylation of STAT3 signaling through a feedbacknegative mechanism [121]. Mice with deletions of SOCS3 in the wholebrain or POMC are resistant to DIO [122,123]. The incidence of DIOand leptin resistance is decreased in rats with hypothalamic SOCS3 si-lencing by RNAi, which also confirms the conclusion above [124]. Fur-ther studies have indicated that JAK2 and SOCS3 experience leptin-dependent immunoprecipitation [75,105]. This result supports therole of SOCS3 in developing leptin resistance effectively.

PTP1B is a class 1 non-receptor protein tyrosine phosphatase (PTP),which is attached to the cytoplasmic face of the endoplasmic reticulumin the hypothalamic regionswhere leptin-responsive neurons are locat-ed [125]. A high-fat diet is accompanied by elevated PTP1B expression,suggesting that PTP1B may play a crucial role in the etiology of leptinresistance [126]. Both systemic, neuron-specific and POMC neuron-specific deletions of PTP1B improve leptin sensitivity and protect indi-viduals from DIO [127,128]. However, PTP1B in POMC neurons regu-lates energy expenditure but not food intake, and other neuronalpopulations are responsible for the impact of PTP1B on both food intakeand energy expenditure [30]. PTP1B binds to and dephosphorylatesJAK2, thereby inhibiting leptin signaling [125,129]. PTP1B expressionis increased by high-fat feeding and inflammation [130] and can be reg-ulated by oxidation, phosphorylation, sumoylation and proteolysis [30].The exact correlation between the expression and activity of PTP1B andleptin resistance remains unclear; however, this particular correlation isthe focus of gene therapy for leptin resistance in the future.

Other mechanisms of leptin resistance focusing on the attenuation ofObRb signaling are explored by some biologists. TC-PTP, a non-receptor,classical PTP in the N1 hypotype, also acts as the critical negativeregulator of hypothalamic ObRb signaling because mice that lack TCPTPin neuronal cells have enhanced leptin sensitivity, and are resistant tohigh-fat-diet-induced weight gain and the development of leptin resis-tance [46]. The insulin receptor-signaling pathway can interfere withleptin signaling at the level of JAK2 through SHP-2-dependent pathways,which can decrease the level of JAK2 phosphorylation, as the result ofhyperinsulinemia contributing to the pathogenesis of leptin resistance[48]. The attenuation of ObRb signaling has been considered as one ofthe leading factors, and also the primary effect on the central leptin resis-tance in the hypothalamus. In addition to JAK2/STAT3, other signalingpathways, such as the phosphatidylinositol 3-kinase/Akt (P13K/Akt),AMPK and mTOR pathways, participate jointly in the regulation ofObRb signal transduction [88,131,132]. Consequently, the studies onthe expression level of regulators in ObRb signaling pathways above,and simultaneously explorations on the actions and regulations ofother five LEPR signaling pathways canmake better sense of leptin resis-tance in multiple perspectives and also provide the effective academic

supports for the treatments on leptin resistance. There are mainlysomekinds of regulators in the other signaling pathways of ObRb, includ-ing phosphatase and tensin homolog deleted on chromosome 10 (PTEN)[30,39], a type of negative regulator of the PI3K pathway serving as theinhibitor of P13K phosphatase; ACC [133], a key enzyme in fatty acid bio-synthesis and an important intermediary in the attenuation of AMPK forinhibiting food intake; and Akt [12,134,135], the reinforcer for hypotha-lamic mTOR anorexigenic activity by phosphorylating and inactivatingTSC2 for the combination of TSC1/TSC2 complex, etc.

5.1.3. Genetic variations of leptin and LEPR genes

5.1.3.1. Genetic deficiency of LEPR gene. The LEPRs, the product of LEPRgene, mediate the effect of leptin on the regulation of feeding behaviorand energy expenditure by accepting the combination of leptin on cyto-membrane [136]. A famous study designed by Halaas et al. demonstrat-ed that the phenotype of db/db mice knocked out the ObRb gene wassimilar to that of leptin-deficient ob/ob mice, including morbid obesity,hyperlipidemia, decreased insulin sensitivity and infertility [59,62,66],suggesting that the genetic deficiency of LEPR can result in leptin resis-tance directly. Other related studies also demonstrated that the geneticLEPR deficiency especially lacking of the transmembrane and intracellu-lar domains reduces the binding level of leptin to LEPR, which in turnimpairs the LEPR signaling transduction pathway within the CNS, suchas JAK2/STAT3, leading to the development of severe obesity and leptinresistance [77,137]. However, an in vivo study in ELKOmice designed byPan et al. [138] showed that the mutation or deficiency of LEPRexpressed in some peripheral regions, such as the endothelium ofblood vessels, can provide partial positive resistance to DIO, which isprobably the opposite effect compared with neural LEPR. The mecha-nisms of that have been unclear now, with the possible reason thatthe endothelial LEPRmaymerely function as a negative regulator of DIO.

5.1.3.2. Genetic variations of leptin and LEPR genes. Leptin resistance isstrongly associated with obesity and simultaneously appears to be aheritable trait characterized by the genetic variations at LEP or LEPRgene between people in different regions [139,140]. The cause of the ge-netic variation at LEP or LEPR gene is that LEP or LEPR gene contains anumber of single nucleotide polymorphisms respectively, which canlead to the impaired LEPR expression, the failure of cellular leptin action,and the development of obesity [37]. Mutations in the polymorphismsabove are reported to result in severe obesity in both animal andhuman models [141,142], and calculating the allelic frequencies ofthese polymorphisms between different human subjects can effectivelyshow the ethnic variations suffering from obesity and leptin resistance.

5.1.3.2.1. Genetic variation of LEPR gene. Genetic variation at LEPRgene may play a crucial role in the pathophysiology of human obesity,and leptin-resistant state [139]. Numerous studies indicated that theLEPR gene contains a number of single nucleotide polymorphisms,therein, the Lys109Arg, Gln223Arg, and Lys656Asn are the three mostcommon polymorphisms, the association of which with obesity hasbeen evaluated in different populations all over the world [143]. How-ever, the results of previous studies have shown conflicting results. Anearlier study reported a lack of relationship between the genetic varia-tion in LEPR gene and obesity in a white British male population [144].Subsequently, Park et al. [145] identified 35 sequence variants in LEPRgene and some of them associate with BMI in a Korean population, in-cluding the LEPR gene Lys109Arg, Gln223Arg and Lys656Asnpolymorphisms.

A famous study designed byMasuo et al. [146] explored the correla-tions between the three polymorphisms above, sympathetic neutral ac-tivity, blood pressure, and obesity in 129 healthy male individuals. Thestudy showed a significantly higher prevalence of Asn656 allele andArg223 allele in overweight/obese subjects by measuring the fat massand plasma leptin levels, and no difference of Lys109Arg frequency be-tween these subjects. However, the study performed by Yiannakouris

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et al. [147] reported that only the Arg223 allele variation of LEPR, ratherthan LEPR Lys109Arg and Lys656Asn polymorphisms, associates withobesity and leptin resistance in a Greek population of young subjects.The conclusions were well verified by the study of Guizar-Mendozaet al. [148] that evaluated the potential role of genetic variation at theLEPR gene in obesity and obesity-related traits in Mexican adolescents.Consequently, it's pronounced that the specific genetic variation ofGln223Arg in LEPR gene acts as the primary factor for the genetic varia-tion of obesity and leptin-resistant state to a large extent, andwhich hasalso been confirmed by a growing number of studies concerned.

The Heritage Family Study showed a significant association betweenLEPRGln223Arg andBMI, skin thickness, fatmass, and leanmusclemassin a Quebec family [149]. The Genetic Variation Family Study reported aconsistent relationship between LEPR Gln223Arg and BMI in Caucasianmen [150]. An association was noted between LEPR Gln223Arg and ab-dominal fat mass in postmenopausal Dutchwomen [151]. A study fromBrazil with a multiethnic adult group suggested a strong correlation be-tween obesity and LEPR Gln223Arg [152]. Another study by Matteviet al. [153] in a Brazilian population reported that the association be-tween LEPR Gln223Arg and BMI was stronger in non-smokers than inthe general population, which also indicated that smoking serves asone of the risk factors for obesity. Meanwhile, the Gln223Arg variationat LEPR gene is possibly associated with the development of obesity,leptin resistant-states and obesity-related diseases such as hyperten-sion, type 2 diabetes mellitus, and hyperlipidemia in Chinese in someprovinces of China [154-157].

Nonetheless, a study of 92 obese and 99 lean Turkish children per-formed by Komsu-Ornek et al. [158] showed that there is no significantcorrelation between LEPR gene Gln223Arg and circulating leptin levels,which was also reported by the study of Paracchini et al. [159]. In viewof this inconsistency between the results of numerous studies above,authors consider that the primary cause of the inconsistency may bedue to the complex pathogenesis of obesity, especially leptin-resistantobesity, which involves a large number of both genetic and environ-mental factors. However, the genetic variations of LEPR gene exert cru-cial influences on the susceptivity and morbidity of obesity in terms ofthe environmental factors.

5.1.3.2.2. Genetic variation of leptin gene. A pedigree-based cross-sectional study by Livshits et al. [160] in an apparently healthy popula-tion of European origin strongly suggested that the genetic variations inbodymass index, skinfolds, and even body circumferences, as well as ofsystolic blood pressure are due to genetic variation of circulating leptinlevels. They confirmed that this type of genetic variation is also causedby the genetic mutation of leptin gene. LEP gene polymorphisms are as-sociated with sweet preference and obesity by affecting the molecularmechanisms of leptin activation in the hypothalamic neurons [161]. Nu-merous studies indicated that the G-2548A polymorphism is the mostnotable variation in LEP gene recently.

Certainly, LEP gene G-2548A polymorphism contributes to thedevelopments of obesity and obesity-related diseases to varyingdegrees in people all over the world, with the different allelic frequen-cies of G/A in Han nationality children in China [162,163], Aryan inFrance [164], and the women in Brazil [165]. In most Chinese, the LEP−2548 G/G plays a genetic recessive role in the development ofextreme obesity, which is similar to that of the women in Brazil, but isopposite to that of Aryan in France. However, Zhang et al. reportedthat LEP gene G-2548A polymorphism is not associatedwith abdominalobesity, but is significantly correlated with abdominal obesity cerebralinfarction in Han nationality children in China [166]. Meanwhile, thesame result was showed by the study of Xiang et al. [167] in Tujiachildren in China. The differences between people in various areasmay be mainly caused by race, regional or age differences.

As one of the important molecular mechanisms of obesity, the ge-netic variations in LEPR and/or LEP genes have been taken seriously inrecent years. However, because of the differences of statistical methods,regions, nationalities, ages, etc., there exist multiple results of studies

aiming at the association between a kind of genetic variation in specificgene and obesity. Moreover, the pathogenesis of obesity involves notonly the genetic factors but also the environmental influences. Furtherresearches should focus on the roles of both gene–gene and gene–envi-ronment interaction in the developments of obesity and leptin resis-tance by means of genomic and proteomic methods.

5.1.4. Endoplasmic reticulum (ER) stressIncreasing evidence suggests that stress signals can impair endoplas-

mic reticulum (ER) function, leading to an accumulation of unfoldedproteins that result in ER stress [168,169]. The accumulation of unfoldedproteins that is toxic to cells markedly inhibits leptin-induced STAT3phosphorylation in in vitro studies performed by Hosoi et al. [170], pre-vents the activation of the JAK2/STAT3 pathway, and consequently con-tributes to leptin resistance [171]. Increasing evidence showed that ERstress has been discovered in multiple tissues in leptin-resistant andobese animals, including the hypothalamus, and inhibits leptin signal-ing. There is a regulatory mechanism of SOCS3 in dysfunction of leptinsignaling via activation of IKKβ and ER stress [12]. By binding to the en-doplasmic reticulum, PTP1B dephosphorylates JAK2 and activatesSOCS3 in the ARC via the activation of IKKβ/NF-κB, and sequentially in-terrupts central LEPR signaling and leptin action in the hypothalamusleading to leptin resistance. The deactivation of IKKβ significantly con-duces to the remission of obesity and leptin resistance [172], indicatingthat IKKβ acts as one of the most important regulatory factors in leptinresistance associated with the endoplasmic reticulum.

5.1.5. OthersActivation of inflammatory signaling has also been detected in the

hypothalamus of obese rats [43].WAT is awell known active participantin regulating the physiological processes, and plays a primary role in thedevelopment of a triad of hormonal imbalance (leptin resistance, adren-aline resistance, insulin resistance). Therefore, the dysfunction or intox-ication of the WAT might result in leptin resistance and obesity [45].Moreover, toll-like receptor 4 (TLR4) signaling by fatty acids can also in-hibit leptin-induced STAT3 activation [44], which is considered themain mechanism of leptin resistance by inflammatory reactions.

Taneja et al. [110] showed that although the Zn treated ratsdisplayed a higher circulating leptin level, these rats were unable to re-duce food intake and gained in body weight compared with the controlgroup, which suggested that excessive bioavailability of Zn induces lep-tin resistance through the increased uptake of nutrients at the intestinallevel, leading to the growth of adipocytes that aggravate leptin synthesisand its release into the blood stream. However, another relevant studyindicated that Zn-induced leptin resistance could be attenuated throughrestoring the ionic balance of Zn, Cu andMg through the inclusion of anantioxidant-enriched modified poultry egg [50]. Although further clini-cal studies are required before these eggs are put to use for human con-sumption, the prospective of the treatments for Zn-induced leptinresistance gives researchers an idea of what to expect.

5.2. The exact interrelations between leptin resistance, obesity and theobesity-related diseases

As discussed above for the definition and molecular mechanisms ofleptin resistance in obesity, it's vitally important to elaborate the exactinterrelations between leptin resistance and obesity for the preventionsand treatments of obesity. Firstly, leptin informs the brain of the fatstored levels to regulate the accumulations and content of body fatpads from the following aspects: 1) by binding to and activating ObRbin thehypothalamus, leptin decreases appetite and food intakewhile in-creasing energy expenditure for energy homeostasis; and 2) leptin di-rectly suppresses the processes of body lipid biosynthesis by theactivation of JAK2/STAT3 [173,174]. This regulating process of leptinfor the levels of stored body fat and energy homeostasis has beenaccounted as the positive regulation, and individuals suffer from the

164 H. Pan et al. / Physiology & Behavior 130 (2014) 157–169

morbid obesity in case that the positive regulation is damaged or inter-fered by various internal and external factors such as leptin resistanceprimarily.

Implicit in the definition of leptin resistance is the ideas that a path-ological state in which elevated peripheral (adipocyte and circulating)leptin levels coincident with ongoing hyperphagia and morbid obesity,a failure of endogenous hyperleptinemia to reduce appetite or to in-crease energy expenditure, a decreased responsiveness to the exoge-nous leptin administration significantly, a failure of leptin to activatekey signaling molecules, STAT3 for instance, within the specific area inthe hypothalamus from amolecular standpoint, and lastly the processesthat promote and/or result from obesity impair leptin action [23,26,175]. Individuals that presented reduced leptin sensitivity may predis-pose to obesity than the normal ones, because these obese strains ap-pear to have blunted leptin sensitivity even prior to the developmentof obesity, which also suggests that leptin resistance serves as a predis-posing factor for obesity [176,177]. However, leptin resistance is proba-bly not only an important risk factor but also a significant symbol forobesity. A growingnumber of biologists have reached extensive consen-sus on the formulation that leptin resistance is a hallmark of obesity, es-pecially DIO [91], indicating that drugs aimed at ameliorating leptinresistancemight prevent the development of DIO. Consequently, under-standing themechanisms thatmay underlie “leptin resistance” is crucialfor both determining the causes of obesity and identifying the potentialmechanisms that may be targeted for therapy.

Leptin resistance resulting in the failure of elevated leptin levels tocontrol or reverse obesity enhances the susceptibility to obesity, andobesity can also aggravate leptin resistance, which further sustains obe-sity, leading to a vicious cycle of escalatingmetabolic derangement [20,99,178,179]. Studies in the complicated interrelations between obesityand leptin resistance will open a new field for obesity; meanwhile,other potential mechanisms for leptin resistance-related obesity andthe studies focusing on obesity-related diseases may provide the basisfor a novel treatment of obesity and understanding the role of leptin re-sistance in these diseases across-the-board.

5.2.1. Leptin resistance, obesity and cardiovascular diseasesObesity, which is mainly characterized by leptin resistance, is con-

sidered to be one of the leading risk factors for cardiovascular diseases(CVDs) such as hypertension, atherosclerosis, and hyperlipidemia[180]. Simultaneously it has been suggested that leptin resistancecould be an important link between obesity and the development ofCVD [181]. A study by evaluating the associations between congestiveheart failure (CHF) and CVD in 818 elderly suggested that higher circu-lating leptin levels are consistent with a greater risk of CHF and CVD[182]. In the past 3 to 5 years, increasing evidence indicated that leptinresistance, presenting hyperleptinemia and blunted leptin sensitivity,appears to enhance cardio-renal functions by generalizing the sympa-thetic nervous system (SNS) potentially leading to obesity-related hy-pertension [148,183-185], and stimulate vascular inflammation [186],oxidative stress and hypertrophy [5], which in turn contributes to thepathogenesis of hypertension, atherosclerosis, and left ventricular hy-pertrophy, etc. One of the deep-seated mechanisms of that is possiblythat central leptin resistance disorders hypothalamic regulation of ener-gy homeostasis, which results in obesity and increased lipid production.This kind of morbid obesity and lipid synthesis may lead to ectopic lipiddeposition and lipotoxicity in peripheral organs such as cardiomyocyteand vascular wall [187], which in turn conduces to the development ofCVD. Simultaneously, the reduced leptin sensitivity causes the de-creased production of endothelial NO synthase, which plays an impor-tant role in vasodilatation [188], aggravating the progressions ofvascular diseases including hypertension. Another pathogenesis ofCVD in leptin-resistant obesity is that leptin resistance would result ina reduction of leptin-induced inhibition of the sterol regulatoryelement-binding protein 1c (SREBP1c), which regulates lipid accumula-tion in tissues. The abnormal deposition of lipids induced by

overexpression of SREBP1c in nonadipose organs, including the liverand cardiac and skeletal muscles, contributes significantly to the emer-gence of CVD [189,190].

5.2.2. Leptin resistance, obesity and neoplasmiaObesity increases not only the risk of CVD but also that of various

types of neoplasia. The correlation between weight gain and deathresulting from cancer was confirmed more than 30 years ago. Sincethe discovery of ob gene and leptin, a growing number of biologistshave devoted themselves to exploring the exact associations betweenleptin resistance and a variety of neoplasms, hoping to provide thenew preventive and therapeutic measures for these tumors.

Leptin resistance may be involved in the development of varioustypes of cancer, such as colorectal cancer and endometrial cancer,which has been testified in recent years. Firstly, a high consistency of el-evated circulating leptin level with these types of cancer has been re-ported in numerous studies [191-195]. It is well known that fathomeostasis is regulated by leptin and that resistance to leptin resultsin lipotoxicity and cellular lipoapoptosis. The impaired leptin activityleads to the deposition of excess fat in non-adipose tissues where it un-dergoes oxidative stress and the cells in the target tissues are thus ex-posed to large amounts of lipid metabolites such as ceramides, nitricoxide andperoxides [196]which can lead to oncogenesis or lipoapotosis[197]. Secondly, experimental studies have shown that the elevated cir-culating leptin level has the anti-apoptotic and/or mitogenic effects incertain cancer cell lines, such as the breast, esophagus, colon, and pros-tate, and that the anti-apoptotic and mitogenic effects can be inhibitedby the inhibition of MAPK and P13K pathways, suggesting that the acti-vation of MAPK and P13K pathways induces the effects above for thepathological basis of leptin resistance-related neoplasia [198-200]. Last-ly, some kinds of neoplasms, including colorectal, breast, endometrial,prostate, and gastric cancers, overexpress LEPR,with a possible explana-tion that leptin may promote cell proliferation, invasion, andmetastasisthrough JAK2/STAT3 signaling pathway [197,201-206]. Consequently,though the precise mechanisms of neoplasia above in leptin-resistantobesity have not been confirmed absolutely, it is meaningful for provid-ing the preventive and therapeutic strategies for neoplasms to under-stand these mechanisms.

5.3. The treatments for leptin resistance-related obesity

5.3.1. Caloric restriction and exerciseIt is well-known that caloric restriction plays an important role in

the treatment of obesity, which is similar to the role in overcoming lep-tin resistance [207]. A study by Berriel et al. [208] in DIOmice has shownthat three days after abrogating a high-caloric diet, the mice displayingleptin resistance and DIO produced a high sensitivity to leptin again.Some related studies reported that regular and long-term exercise notonly decreases the plasma leptin concentration and the expression ofObRb mRNA in the arcuate nucleus in the hypothalamus but also acti-vates STAT3 and AMPK signaling pathways that are suppressed by ahigh dose of dexamethasone in rats [48,209]. Zhou [210] demonstratedthat free aerobic exercise reduces SOCS3mRNA expression in the hypo-thalamus of rats that were fed a high fat diet, and ultimately reduces theinhibition of the JAK2/STAT3 signaling pathway, suggesting that exer-cise can prevent leptin-resistant obesity. Meanwhile, in vivo study byTan et al. [211] showed that the effect of an exercise-dietary interven-tion is superior to that of exercise or diet intervention alone. Therefore,the fundamental and effective therapy for leptin resistance is the combi-nation of caloric restriction and exercise. However, it's necessary for themajority of severe obesity to explore the gene-targeted drug therapies.

5.3.2. Gene-targeted drug therapyThe primarymanifestations of leptin resistance are increased circulat-

ing leptin levels and a decreased sensitivity to leptin in the regulation ofbody weight and energy balance. However, the underlying molecular

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mechanisms of leptin resistance are various, which requires the re-searchers to develop specific therapies targeted at different molecularmechanisms, segments and proteins in LEPR signaling pathway. At pres-ent, the gene-targeted drug therapy has become the leading treatmentfor leptin resistance with the development of gene detection technologyand the in-depth study on themolecularmechanisms of leptin resistance.

5.3.2.1. Down-regulation of SOCS3 and/or PTP1B. The central leptin resis-tance is characterized by diminished hypothalamic LEPRs and impairedleptin signal transduction. The down-regulated expression of LEPRgene-related negative regulating proteins, such as SOCS3 and PTP1B,can increase the signal transduction of ObRb and prevent the develop-ment of DIO. Rats with hypothalamic SOCS3 silencing by RNAi are resis-tant to DIO, leptin resistance and metabolic disorders, suggesting thatafter the down-regulation of the SOCS3 gene by RNAi, the restorationof JAK2/STAT3 recovers the natural physiological action of leptin andnormal body weight [124]. Meanwhile, Liu et al. [212] showed thatobese rats with leptin resistance that were injected with SOCS3 smallhair RNA (shRNA) lentivirus presented the convalescent actions of lep-tin. The recombinant adenoviral vector that harbors clock genes (RAV-HCG) in a different study performed by Xie et al. [213] was showed toreduce the obese indexes of DIOmice, increase leptin sensitivity and de-crease accumulated SOCS3 in the arcuate nucleus, which suggests notonly that RAV-HCG can improve leptin action, but also that the inhibi-tion of clock genes in the central nervous system feeding centers playsan important role in leptin resistance. Consequently, increasing leptinsensitivity through a signal transduction pathway involved SOCS3 is ex-pected to be able to restore the biological function of leptin in regulatingthe energy balance in obese patients.

Leptin resistance is associatedwith increased activity and expressionof PTP1B, which appears to be a promising therapeutic target for thetreatment of leptin resistance [214,215]. Inhibition of PTP1B by a selec-tive PTP1B inhibitor, the leptin-induced STAT3 activationwas enhancedin cells [216]. Recently, though the inhibiting drugs of PTP1B have notbeen found, a number of studies have indicated that PTP1B is well ac-cepted as drug target and is now thought as the focus of inhibitor dis-covery all over the world [217]. Consequently, it's necessary tounderstand the structural details of PTP1B molecule relevant to the in-teractions with inhibitors, and the progress towards compounds withenhanced membrane permeability, affinity, specificity, and potency onintracellular PTP1B [216,218].

5.3.2.2. Activation of POMC directly. The most important leptin-sensitivetarget regions mediating directly leptin's effects on energy balance arePOMC neurons. The central POMC gene therapy, activating POMC, canovercome central leptin resistance in that α-MSH, the product ofPOMC, can effectively suppress appetite, promote energy expenditureand decrease body weight, which makes POMC neurons the importantand useful drug target regions [106,219]. For this reason, the exogenousdrugs that activate POMC, such as MT-II reported by Pierroz et al. [220]can effectually bypass central leptin resistance. However, few re-searches demonstrated that direct leptin action on POMC neuronsdoes not reduce food intake, but is sufficient to stabilize the level of glu-cose in mouse lacking LEPRs [221]. Further studies should be concen-trated on increasing the activity of POMC in regulating ObRb signalsand simultaneously exploring the deep interrelations between POMCin the central and the biological actions of LEPRs.

5.3.2.3. Improvement of leptin transport into the CNS across the BBB. TheBBB presents a tremendous challenge for the delivery of drugs tothe central nervous system (CNS), especially the hypothalamic re-gions. A leptin agonist modified by the addition of pluronic block co-polymers (P85-leptin) guided by the strategic drug delivery to brain(SDDB) [222] can retain the biological functions of leptin and simul-taneously can transport leptin across the BBB independent of somekinds of transporters such as ObRa [223]. The decreasing level of

triglycerides may also attenuate leptin resistance generated by theimpaired leptin transportation across the BBB in the study by Bankset al. [117]. Though the acute phase CRP is likely to contribute to lep-tin resistance by preventing leptin from crossing the BBB, a lateststudy by Hsuchou et al. [118] demonstrated that CRP at a high doseincreases the paracellular permeability of the CNS for leptin's entry.Meanwhile, the improvement of the binding affinity between circu-lating leptin and its binding proteins like megalin can effectively in-crease the level of central leptin concentration, which has beenconfirmed in the study performed by Dietrich et al. [116]. Thus, in-creasing leptin transport into the CNS across the BBB, which is con-ducive to LEPR signaling transduction, is a useful strategy tocombat the central leptin resistance.

5.3.2.4. Attenuation of ER stress. ER stress has been identified as oneof thecentral mechanisms resulting in leptin resistance, which provides newfields for the treatment of leptin resistance. Hosoi et al. [108] showedpreviously that fluvoxamine, a selective serotonin reuptake inhibitorused for depression, can effectively attenuate the endoplasmic reticu-lum stress-induced leptin resistance by reversing the ER-stress-induced impairment of leptin-induced STAT3 phosphorylation. Zhenget al. [224] demonstrated that carbon monoxide (CO), a gaseous mole-cule, could ameliorate leptin resistance during ER stress induced bythapsigargin or tunicamycin in human cells. Zhou et al. [225] reportedthat a type of sweet tea (ST) leaf extract can effectively attenuate leptinresistance and produce anti-hyperlipidemic activity by ameliorating thestate of oxidative stress of the ER. Meanwhile, Ropelle et al. [226]showed that the recombinant pro-inflammatory cytokine IL-6 and theanti-inflammatory protein IL-10 combined with physical exercise cansuppress leptin resistance by inhibiting IKKβ/NFκB and ER stress. In asubsequent study, Jeong et al. [227] reported that AMPK activation bylowmolecular weight fucoidan (LMWF), a drug for treatments of meta-bolic disorder widely, could potentially prevent obesity by amelioratingER stress-related leptin resistance. Consequently, the further explora-tions on the correlations between leptin resistance and ER stress willprovide a novel therapeutic strategy for leptin resistance.

Additionally, other therapies for leptin resistance have been devel-oped in some specific studies. Aubert et al. [228] reported thatmetforminpresents anorectic effects in vivo after its administration was modulatedby the hypothalamic ObRb in gene level, which sequentially increasethe central sensitivity to leptin. Gong et al. [229] confirmed thatelectroacupuncture treatment could effectively reduce body weightand noticeably increase the leptin sensitivity of DIO rats through regulat-ing the expression of leptin andObRb possibly.Moreover, Yan et al. [230]demonstrated that catgut implantation at acupoint could also appropri-ately adjust central and peripheral leptin levels and promote hypotha-lamic OB-R gene expression in leptin-resistant rats. Tam et al. [231]showed that JD5037, a type of peripherally restricted cannabinoid-1 re-ceptor (CB1R) inverse agonist, has anti-obesity effects by reversing leptinresistance, which can effectively decrease appetite and reduce weightmediated by resensitizing DIO mice to endogenous leptin, and sequen-tially reverse hyperleptinemia by reducing leptin expression and secre-tion in adipocytes and improving leptin clearance via the kidney. Tosum up, increasing treatments, mainly the gene drug therapies, havebeen developed according to the different leptin-resistant mechanismsand other solutions should also be explored with the discoveries ofnew targets in LEPR signaling pathways.

6. Discussions

Obesity is a complicated metabolic syndrome that is caused by vari-ous factors and molecular mechanisms. Leptin resistance, the centralmechanism of obesity, has been a major concern for 20 years. Duringthe two decades at the turn of the century, thousands of researchershave devoted themselves to the studies of leptin resistance and count-less research achievements have been published all over the world.

166 H. Pan et al. / Physiology & Behavior 130 (2014) 157–169

Our hope is not on the quantity of increasing papers in different journalsblindly but to bestow the gospels upon obese patients with further re-search on leptin resistance and obesity. Consequently, the future explo-rations of leptin resistance should be focused on the following points:

(1) The in-depth researches ought to be the regulation of obese geneexpression and its individual difference, the exact associationsbetween the leptin gene and/or LEPR gene polymorphisms andthe pathogenesis of obesity.

(2) The relationships between leptin resistance and the external en-vironment in addition to the genetics, such as diet, gender, age,and work. Meanwhile, the peripheral role of leptin should notbe neglected, particularly in the context of enhanced immune re-sponses in autoimmune diseases.

(3) The causal and conditional relationships between leptin resis-tance and obesity.

(4) Last but not the least, the exploration of gene-targeted drugs andcombined drugs based on leptin signaling pathways should becarried out reasonably for promoting the relief of obesity.

To sum, additional studies in the pathogenesis of leptin resistance,and finding the new therapeutic targets of LEPR signal pathways by so-phisticated genetic engineering technology, aswell as the exploration ofthe novel gene-targeted drugsmight be the new direction of obesity re-search and therapy. Meanwhile, the close interrelations between obesi-ty, leptin resistance, and obesity-related diseases, such as CVD andneoplasia, should be lucubrated carefully in the future.

Acknowledgments

This project was financially supported by the National Natural ScienceFoundation of China (no. 81173107), the Science and Technology Plan-ning Project of Guangzhou, China (no. 11A52130094), and the Scienceand Technology Planning Project of Zhongshan, China (no. 2009H017).

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