Insulin-like Growth Factor 1: “The Sulfation Factor”

Koray Kevin Celik

Endocrinology Seminar

Dr. David Champlin

Insulin-like growth factor 1 (IGF-1), also name Somatomedin C in the 1980’s, is a

protein hormone that in humans is encoded by the IGF1 gene. The growth factor was

initially identified in 1957 by Salmon and Daughaday, and was originally nominated as

the “Sulfation Factor” by the analog’s overall ability to stimulate 35-sulphate

incorporation into rat cartilage. IGF-1 is a hormone similar in molecular structure to

insulin. It plays an important role in childhood growth and continues to have

commanding anabolic effects in adults [3]. The growth factor and protein peptide consists

of 70 amino acids in a single chain with three intermolecular disulfide bridges, and has a

molecular weight of 7,649 Daltons.

Figure 1.0

Overalltherearethreemajorobservationsthatledtothediscoveryofthe

IGF/Somatomedinfamily:Thefirstofthethreemajorobservationsoccurredearly

in1957whenSalmonandDaughadayfirstwitnessedhowserumstimulatedthe

incorporationof35Sintoincubatedcartilage.Theserumofhypophysectomizedrats

lackedthesulfationactioninitsentirety.Furthermore,forsomereasonit’sactivity

couldnotbereconstitutedbytheadditionoffurtherquantitiesofgrowthhormone

(GH)tothematurationmedium,butratheritreappearedafterfurther

administrationofGHtohypophysectomizedrats[1].Theseannotationsarewhat

eventuallyinspiredthegreatDaughadaytohypothesizethattheuseof

unaccompaniedrecombinantgrowthhormonedoesnotactuallystimulategrowth

processes.TheuseofGHsatisfactorilyinducestheformationoffactorsthatmediate

themessageofgrowthhormonetoacertaindegree.Theseunfathomablefactors

ultimatelyleadtothesulfationofcartilageinvitro,whichreflectsgrowthinvivo.

Thesecondofthethreemajorobservationsthatledtothedetectionof

IGF/SomatodenCoriginatedfromhowthemajoritycellsrequireserumtogrowin

culture.Severalgrowthfactorsinserumareknowntobeaccountablefortheir

growth‐promotingproperties.In1972,PiersonandTeminextractedfactorsfrom

seruminwhich,theyentitledthemultiplication‐stimulatingactivity(MSA).These

factorshadmolecularweightsjustbelow10,000Dalton’sandweresubjectto

stimulatecellstoreplicatewhenaddedtoaculturemedium.Subsequently,they

hypothesizedandconfirmedthatculturedlivercellsdidindeedsecreteMSAintothe

culturemedium(s)[2].Thisdemonstrationwasthefirstofitskind,wherewe’dseen

thelikesofMSAbeingproducedprimarilyintheliverandthatMSAmayeventually

leadtoautocrinestimulationviathesamecellsbywhichitwasinitiallysecreted.

Thethirdmostimperativeobservationthatledtothediscoveryofinsulin‐

likegrowthfactorsstemmedfromavastlydifferentseriesofobservations.Itwas

notedingreatdetailthatserumemploysinsulin‐likeeffectsoninsulintargettissues

suchasmuscleandadiposetissue.Theeffectsofinsulinonseruminitiallywerenot

suppressedbytheadditionofanti‐insulinserum,and its effects were termed "non-

suppressible insulin-like activity" (NSILA) in the 1970s[3].Themoleculesfoundin

serumevidentlyareresponsiblefortheinsulin‐likeeffectsthatwereinvestigated

forandwerefinallyidentifiedasinsulin‐likegrowthfactorsIandII[4].

Additionally there is a synthetic analog of IGF-1, known as Mecasermin, IGF-1

LR3 (long recombinant receptor grade 3) that is sold legally under the trade name

Increlex. The LR3 is a long-term analog of human IGF-1, specifically designed and

manufactured for mammalian cell culture to support large-scale manufacturing of

recombinant biopharmaceuticals. Recombinant Human LR3 IGF-1 is a single, non-

glycosylated polypeptide chain containing 83 amino acids, which encompass the

complete human IGF-1 sequence with the substitution of an Arganine (R) for the

Glutamic Acid (E) at position three, consequently R3, and a 13 amino acid extension

peptide at the N terminus. This sequence alteration allows the recombinant hormone to

circumvent binding proteins as well as to increase its respective half-life within the blood.

R3 IGF-1 has been produced with the purpose of increasing biological activity and is

significantly more potent than human IGF-I in vitro. The growth factor is currently being

used for the treatment of growth failure and severe burns. It has a Chinese counterpart

that is often replicated and seen being sold and abused in great numbers within the

contexts of bodybuilding and professional sports.

Figure 2.0

Within the black market paradigm there are two types made commonly available,

Media and Receptor grades, respectively which ascertain predominantly to their overall

quality. The synthetic analogue is seen as a principal hormonal mediator of statural

growth. Under normal circumstances, after subcutaneous or intra-muscular injection,

growth hormone (GH) binds to its respective receptor in the liver, as well as in several

other tissues, and stimulates the synthesis and secretion of IGF-1. In target tissues, the

Type 1 IGF receptor, which is homologous to the insulin receptor, which is activated by

IGF-1, leading to intracellular signaling which agonizes multiple processes leading to

statural growth, hyperplasia and mitogenensis. The metabolic actions of IGF-1 are in

great detail directed at encouraging the overall uptake of glucose, fatty acids, and amino

acids so that metabolism supports growing tissues.

Figure 3.0

As seen in the image above, the diagram depicts circulating IGF-1 levels in the

blood and in tissue. The majority of circulating IGF-1 is credited being produced in the

liver. Hepatic IGF-1 production is subject to complex regulation by both hormonal and

nutritional factors. Various IGF-binding proteins (IGFBP’s) are also produced in the

liver. In IGF-responsive tissues, the ligands IGF1 and IGF2 as well as IGFBPs can be

delivered through the circulation from the liver, but IGFs and IGFBPs can also be

produced locally through autocrine or paracrine mechanisms [5]. These mechanisms

habitually comprise interactions between stromal- and epithelial-cell subpopulations.

Furthermore, Growth hormone is predominantly produced in the pituitary gland under the

mechanism of the hypothalamic factors growth-hormone-releasing hormone (GHRH)

with Somatostatin (SMS) acting as the major stimulator of IGF-1 production [6].

Figure 4.0

(Above) IGF-1 and its receptor IGF-1R come together to undertake a resilient

proliferative signaling system. This system when agonized correctly stimulates the likes

various forms of mitogenensis and halts the cell signal known as apoptosis (cell suicide).

In real life situations IGF-1 acts as a precursor for countless bodily growth hormone

responses, many are still being discovered today. One of the main components of the

IGF-1 mitogenic signaling factor is the association of the receptor tyrosine kinase with

Shc, Grb2, and Sos-1 to activate ras and the Map kinase cascade (raf, Mek, Erk) [6]. The

culmination of the signaling pathways is seen at the end of the Map kinase pathway,

which, is a modification of specific transcription factor activity, such as activation of

ELK transcription factors. Serum response factor (SRF) and AP-1 contribute to mitogenic

signaling by many factors. Phosphorylation of IRS-1 and PI3 kinase activation are also

involved in IGF-1 signaling, similar to insulin signaling.

Figure 5.0

As previously discussed the IGF-1 receptor is a tyrosine kinase cell-surface

receptor that binds to either IGF1 or IGF2. Above we see the respective receptor(s) being

triggered by IGF-1 both at the target site(s) as well as downstream.

IGF-binding proteins and IGFBP proteases have significant roles in regulating

ligand bioavailability. Ligands are delivered either from remote sites of production

through the circulation or are locally produced. There is strong evidence that certain

IGFBPs also have direct growth-regulatory actions that can stimulate beneficial

mitogenensis as well as stimulate the growth of proto-oncogenes. The local

bioavailability of ligands is subject to complex physiological regulation and is

abnormally high in most cancers. IGF-binding proteins chef responsibilities include

prolonging the half-life of the IGFs, as well as having a strong affinity for IGFs

comparable to IGF1R. Moreover, it is noted that there is fierce competition between

IGFBPs and IGF1R for the available ligands in tissue and in blood [5]. This potential for

competition leads to the latter effect of increasing IGF1R activation. Further, The IGF2R

binds to IGF2, but has no tyrosine kinase domain and appears to act as a negative

influence on proliferation by reducing the amount of IGF2 available for binding to

IGF1R. Certain IGFBP proteases, which are more often than not produced by neoplastic

cells, cleave IGFBPs and can discharge free ligand and thereby increase IGF1R

activation [6]. Subsequently, ligand binding to IGF1R, its tyrosine kinase action is

activated, which further agonizes the signaling pathway through intracellular networks

that control cell proliferation and cell survival.

Key downstream networks include the PI3K-AKT-TOR system and the RAF-

MAPK systems. Activation of these pathways stimulates proliferation and inhibits

apoptosis, and stimulates the TOR, target of rapamycin. Such mutations have been shown

to inhibit key molecules involved in insulin signaling (IIS) and the nutrient signaling

pathway Target of Rapamycin (TOR). These mutations in TOR (in this case RSKS-1)

result in a 30 percent lifespan extension, while mutations in IIS (Daf-2) often result in a

doubling of lifespan in the worms [7].

References

[1] Binoux, M., R Hossenlopp, C. Lassarre and D. Seurin. 1980. Somatomedin production by rat liver in organ culture. I. Validity of the technique. Influence of the released material on cartilage sulphation. Effects of growth hormone and insulin. Acta Endocrinol. 93:73.

[2] Dulak, N. and H. J. Temin. 1973. A partially purified polypeptide fraction from rat liver cell conditioned medium with multiplication stimulating activity for embryo fibroblasts. J. Cell. Physiol. 81:153.

[3] Froesch, E. R., MD, Ch Scmid, PHD, I. Zangger, MD, and E. Eigenmann. "Journal of Clinical Investigation." Editorial. EFFECTS OF IGF/SOMATOMEDINS ON GROWTH AND DIFFERENTIATION OF MUSCLE AND BONE Jan. 1986: 57-75. Print.

[4] Rinderknecht, E. and R. E. Humbel. 1978a. The amino acid sequence of human insulin-like growth factor I and its structural homology with proinsulin. J. Biol. Chem. 253:2769.

[5] Perrini,, Sebastio, Luigi Laviola, Marcos C. Carreira, Angello Cignarelli, Annalisa Natalicchio, and Francesco Giorgino. "The GH/IGF1 Axis and Signaling Pathways in the Muscle and Bone: Mechanisms Underlying Age-related Skeletal Muscle Wasting and Osteoporosis." Journal of Endocrinology -review (n.d.): 201-10. 2011. Web. 04 Mar. 2014

[6] "IGF-1 Technical Mode of Action on Proliferation and Neoplasia- Everything You

Need to Know." Peptide Resource All About Peptides. Purepeptide.com,

2011. Web. 04 Mar. 2014.

[7] Pankaj Kapahi, PhD et al. Germline Signaling Mediates the Synergistically

Prolonged Longevity by Double Mutations in daf-2 and rsks-1 in C. elegans. Cell

Reports, December 2013