Journal of Molecular and Cellular Cardiology 51 (2011) 141–143

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Journal of Molecular and Cellular Cardiology

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Editorial

Resisting resistin; it's good for the heart

1. Introduction

During the last two decades, the prevalence of obesity has reachedepidemic proportions worldwide, leading to the development of amultitude of comorbidities, including hypertension, diabetes mellitusand cardiovascular diseases [1,2]. Adipose tissue produces andsecretes a wide array of adipokines, which have key roles in situ(within the adipose tissue), and as circulating bioactive factors,suggesting that adipose tissue is an essential endocrine organ, inaddition to an energy storage depot that modulates fat mass andnutrient homeostasis [3,4]. Numerous hormones, growth factors andcytokines belong to the adipokine group, including tumor necrosisfactor-α (TNF-α), plasminogen activator inhibitor type 1 (PAI-1),leptin, adiponectin and the recently identified resistin [5]. Secretedadipokines signal to organs of metabolic importance including theliver, the skeletal and cardiac muscles and the immune systeminfluencing, directly or indirectly, fuel storage and mobilization, aswell as energy homeostasis, by regulating glucose, lipid and proteinmetabolism, cellular inflammation and atherosclerosis [3].

2. Resistin is an essential link between obesity, diabetesand inflammation

Resistin, the product of the RSTN gene, was discovered in 2001 bythe group of Mitchell Lazar as a target gene of the anti-diabetic drugthiozolidinedione (TZD), which was down-regulated in mouseadipocytes upon treatment [6]. It was “baptized” resistin because ofthe acquired insulin resistance that mice injected with resistindemonstrated [6–8]. Resistin is a ~12.5 kDa peptide hormone thatbelongs to the Resistin Like Molecules (RELM) family (also known asAdipose Tissue Specific Secretory Factor, ADSF, or Found in Inflam-matory Zone, FIZZ, family) of cystein-rich secreted proteins [9].Accordingly, the RELM/ADSF/FIZZ class of proteins is characterized bythe presence of a unique pattern of 11 cysteine residues and the factthat the proteins naturally form multimeric complexes. Human androdent resistins consist of 108 and 114 amino acids, as pre-peptides,however, their hydrophobic signal sequences are cleaved beforesecretion, yielding active peptides of 92 and 94 amino acids,respectively [10–12]. Resistin circulates in blood as a dimeric proteinconsisting of two active (cleaved) polypeptides, which are disulphidelinked [13]. In rodents, resistin is derived almost exclusively from fatcells [6,12,14], whereas in humans resistin is produced by inflamma-tory cells, primarily macrophages [15].

Several groups have studied the roles of resistin in the pathogenesisof obesity and diabetes. Early on, Steppan and colleagues demonstratedthat administration of recombinant resistin in normal mice impairedglucose tolerance, and prevented adipogenesis in murine 3T3-L1 cells,

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whereasblockingof resistinwith inhibitory antibodies improved insulinaction in diet-induced obese mice [6,8,9]. In concert with theseobservations, overexpression of resistin in healthy mice led to insulinresistance, causingdyslipidemia [16–18],whilemicedeficient in resistinwere protected fromobesity, due to reduced hepatic glucose production[19].Moreover, the levels of resistinwere found significantly elevated inthe blood plasma of genetic animal models of diabetes (db/db) andobesity (ob/ob) as well as diet-induced models of diabetic obesity[6,8,14,20]. Collectively, these lines of evidence suggested that resistinmight be an important link between obesity, glucose intolerance andinsulin resistance, and thus contribute to the development of diabetes.

The pathophysiological roles of resistin have been investigated inhumans, too, although there has been considerable controversy. Somestudies reported increased levels of resistin in the plasma of patientsthat had suffered myocardial infarction as well as in obese and type 2diabetic patients, compared to lean healthy control subjects [21,22].Consistent with this, TZD treatment resulted in decreased levels ofplasma resistin in type 2 diabetic patients [23,24]. Others, however,failed to show any differences in resistin expression between normaland diabetic samples or an association between the levels of resistinand TZD treatment [25–28]. While the effects of resistin in glucosemetabolism are still unclear, an alternate hypothesis postulated thatin humans resistin might play a role in inflammatory processes, giventhat it is primarily derived frommacrophages. In agreement with this,increased plasma resistin levels were associated with the develop-ment and extent of severity of inflammatory and metabolic diseases,including sepsis and septic shock, inflammatory bowel syndrome,coronary artery disease and rheumatoid arthritis [29–32]. Given thedirect or indirect association of resistin with these diverse patholog-ical conditions, it has been suggested that resistin might represent anessential link between insulin resistance, metabolic syndromes,inflammation and atherosclerosis [3].

3. The effects of resistin in the cardiovascular system—observationsfrom animal models and cultures of isolated cells

Recently, several groups have directed their efforts towards theelucidation of the roles of resistin in the development and potentialexacerbation of cardiovascular diseases. Rothwell and colleagueswere the first to report that pre-conditioning of an isolated rat heartpreparation with human recombinant resistin, prior to ischemia,significantly impaired reperfusion recovery and contractile activity.This effect was mediated by an NFκB stimulated cascade, whichpromoted the release of the pro-inflammatory cytokine TNFα, and thesecretion of hypertrophic cardiac markers [33]. In line with thismodel, Wang and colleagues demonstrated that resistin expressionwas markedly up-regulated in a cardiac volume overload murine

142 Editorial

model subjected to aorta-caval shunt [34]. In contrast, however, Gaoand colleagues reported that pre-treatment of mouse hearts withresistin hada cardioprotective effect against either ischemia/reperfusioninjury or left anterior descending (LAD) coronary artery ligation, bydramatically reducing cardiocyte apoptosis and infarct size, throughactivation of PI3K/Akt/PKCε/KATP-dependent pathways [35].

Conversely, neonatal rat ventricular myocytes (NRVM) subjected tocyclic stretch exhibited significantly increased expression of resistin atboth the mRNA and protein levels, mediated by TNFα, ERK kinase andNFκB-regulated pathways, while treatment of NRVMwith recombinantor secreted resistin impairedglucose transport [34]. Consistentwith this,adenoviral overexpression of resistin in cultures of NRVM resulted inenhanced protein synthesis and cell size, induced the expression of fetalgenes and activated known hypertrophic signal responses [36].Moreover, adenoviral overexpression of resistin in cultures of adult ratventricularmyocytes (ARVM)depressed contractility by impairingCa2+

homeostasis and altering myofilament activation [36].Although the aforementioned studies provided important insights

about the effects of acute manipulation of the expression of resistin inthe myocardium, it was only recently that Chemaly and colleaguesexamined the consequences of long-term resistin overexpression inthe cardiovascular system in vivo[37]. To achieve long-term expres-sion of resistin in the heart, Chemaly et al used adeno associated virus(AAV) serotype 9, which was introduced to rats through tail veininjection. AAV mediated gene transfer is a promising new method forgene therapy, as AAV exhibits high infection capability of non-dividing and dividing cells, lacks pathogenicity and stably integratesinto the host cell's genome at a specific site [38,39]. Importantly, selectserotypes, with AAV9 being the most potent, have been shown totransduce cardiac (and skeletal) muscles with high efficiency [40–42].Consistent with this, AAV9 has been used to deliver cytoskeletal,membrane and signaling proteins, and more recently to down-regulate Ca2+ cycling proteins in animal models of heart failure,resulting in considerable improvement of cardiac function [43,44].

Ten weeks following overexpression of resistin, animals displayedincreased ratio of left ventricular (LV) weight/body weight andincreased end systolic LV volume, indicative of developing cardiachypertrophy. These changes were accompanied by dramatic increasesin the expression levels of at least twomolecular markers of stress andhypertrophy, atrial natriuretic factor (ANF) and β-myosin heavy chain(β-MHC). In vivo hemodynamic analysis of the treated animalsdemonstrated impaired contractile activity, mediated at least in partby increased levels of phospholamban (PLN), the sarco endoplasmicreticulum Ca2+ ATPase 2a (SERCA2a) inhibitor. Histological evalua-tion further revealed that the resistin overexpressing myocardiumappeared largely fibrotic, displaying elevated expression of essentialprofibrotic targets, including collagen 1 and 3. The fibrotic myocar-dium exhibited increased levels of apoptosis, too. Thus, the amountsof the anti-apoptotic Bcl-2 gene were considerably diminished,whereas the levels of the pro-apoptotic Bax and death caspase-3were notably increased, indicating a clear shift towards the release ofpro-apoptotic molecules, promoting cell death. Most notably, resistinoverexpressing hearts displayed higher levels of intracellular reactiveoxygen species (ROS). This increase was attributed to enhancedexpression and activation of NADPH oxidase, a superoxide-producingenzyme. A number of inflammatory cytokines, including TNFα and itscognate receptor TNFα receptor 1, were also upregulated, leading tothe activation of downstream signaling cascades, including NFκBactivation, which is a critical event in cytokine-induced inflammation.Taken together, these findings strongly suggested that long-termexpression of resistin in vivo significantly impaired LV structure andcontractility by eliciting apoptotic and inflammatory responses.

Elevated levels of circulating resistin (hyperresistinemia) havebeen correlated with myocardial dysfunction in humans and inrodents [3]. The study by Chemaly and colleagues is the first oneto investigate the effects of long-term resistin expression in the

cardiovascular system in vivo[37]. Cardiac remodeling associated withan overt hypertrophic phenotype, abnormal Ca2+ handling, wide-spread fibrosis, increased apoptosis and prominent inflammationwere the manifestations of resistin's overexpression, faithfullyrecapitulating the major features of diabetic cardiomyopathy. Conse-quently, Chemaly and colleagues proposed that resistin might be “amolecular determinant of diabetic cardiomyopathy” [37]. Undoubt-edly, their findings are convincing and exciting, as they point to anovel therapeutic target with manifold and interlinked activities invarious disease conditions. Since diabetic cardiomyopathy is amultifactorial disease, a number of additional parameters need to beconsidered, however. Does the hypertrophic phenotype developed bythe treated animals progressively worsen (i.e. beyond the 10-weektime point)? Does age or gender affect the onset and severity ofmyocardial remodeling? Do nutritional and exercise regimes factor inthe observed cardiac dysfunction? Conversely, does long-term down-regulation of resistin improve the cardiac pathology of diabetic andobese animals?What major signaling cascades does resistin regulate?More importantly, does human and rodent resistin share the samepathophysiological properties and activities? How safe is it totranslate knowledge gained from in vitro culture settings and animalmodels to human subjects, especially given the relatively lowsequence homology between rodent and human resistin (~59%),and the fact that they are produced by different tissue sources [45]?Notwithstanding the numerous questions that still need to beaddressed regarding the biology of resistin, using a series of eleganttechnologies Chemaly and colleagues were able to show that long-term expression of resistin in rat hearts induced cardiac remodelingand dysfunction.

Sources of funding

The author is supported by grants from the National Institutes ofHealth (R01 AR52768 and R21HL106197), the American HeartAssociation (GRNT3780035) and a Pilot Grant from Johns HopkinsPhysical Sciences-Oncology Center (U54CA143868), The Johns HopkinsInstitute for NanoBioTechnology.

Disclosures

None.

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Aikaterini Kontrogianni-KonstantopoulosUniversity of Maryland, School of Medicine,

Department of Biochemistry and Molecular Biology,Baltimore, MD 21201, USA

Tel.: +1 410 706 5788; fax: +1 410 706 8297.E-mail address: akons001@umaryland.edu.

11 May 2011