On this pageFour Growth-Factor Signaling Pathways at a Glance
Several families of growth-factor signaling peptides are used in research to probe how cells sense and respond to growth signals. This overview covers four of them at the receptor and pathway level: IGF-1 LR3 and the IGF-1 receptor, GHRH analogs at the GHRH receptor, GHSR agonists at the ghrelin receptor, and the follistatin/myostatin axis of the TGF-β superfamily. It describes molecular structure, receptors, signaling cascades and laboratory characterization only; it makes no statements about effects in people or animals.
Four Growth-Factor Signaling Pathways at a Glance
| Peptide family | Example research compounds | Receptor | Receptor class | Main intracellular cascade |
|---|---|---|---|---|
| IGF-1 analogs | IGF-1 LR3 | IGF-1R | Receptor tyrosine kinase | IRS–PI3K–Akt–mTOR; Ras–MAPK |
| GHRH analogs | Sermorelin, CJC-1295, Tesamorelin | GHRHR | Class B GPCR | Gs–adenylyl cyclase–cAMP–PKA |
| GHSR agonists | Ipamorelin, GHRP-2, GHRP-6, Hexarelin | GHSR-1a | Class A GPCR | Gq/11–PLC–IP3–Ca²⁺ |
| Activin/myostatin regulators | Follistatin (binds myostatin) | ActRIIB / ALK4/5 (for myostatin) | Serine/threonine kinase receptors | SMAD2/3–SMAD4 |
IGF-1 LR3 and IGF-1 Receptor Signaling
IGF-1 LR3 (Long R3 IGF-1) is an 83-amino-acid analog of insulin-like growth factor 1, approximately 9.1 kDa. It consists of the 70-residue human IGF-1 sequence with arginine in place of glutamate at position 3 (the "R3") and a 13-residue N-terminal extension (the "Long"). Both changes greatly reduce binding to the six IGF-binding proteins (IGFBP-1 to -6), which otherwise sequester native IGF-1 in culture media and biological fluids.
IGF-1 LR3 activates the IGF-1 receptor (IGF-1R), a disulfide-linked α2β2 heterotetramer with intrinsic tyrosine kinase activity. Ligand binding triggers autophosphorylation of the β-subunits, followed by two main branches:
- IRS-1/2 → PI3K → Akt → mTORC1, with downstream readouts such as phosphorylation of p70S6K and 4E-BP1;
- Shc → Grb2/SOS → Ras → Raf → MEK → ERK1/2, the MAPK branch.
Because it is largely unaffected by IGFBPs, IGF-1 LR3 is widely used in cell-culture media formulations and as a tool ligand for IGF-1R phosphorylation assays. See IGF-1 LR3 research peptide.
GHRH Analogs and GHRH Receptor Signaling
Growth hormone-releasing hormone (GHRH) is a 44-residue hypothalamic peptide; its N-terminal 29 residues retain full activity at the GHRH receptor (GHRHR), a class B G protein-coupled receptor expressed on pituitary somatotroph cells. GHRHR couples to Gs, activating adenylyl cyclase and raising cAMP, which activates protein kinase A and the transcription factor CREB; the pituitary-specific factor Pit-1 is also part of this transcriptional program.
Research GHRH analogs form a structure–activity series:
- Sermorelin: GHRH(1-29)-NH2, the unmodified active fragment (Sermorelin).
- CJC-1295: GHRH(1-29) with D-Ala2, Gln8, Ala15 and Leu27 substitutions that resist dipeptidyl peptidase-4 cleavage; available without DAC or with a C-terminal maleimidopropionyl-lysine (DAC) that binds albumin (CJC-1295 no DAC / Ipamorelin blend).
- Tesamorelin: full-length GHRH(1-44) with an N-terminal trans-3-hexenoyl group (Tesamorelin).
GHSR Agonists and Ghrelin Receptor Signaling
The growth hormone secretagogue receptor type 1a (GHSR-1a) is a class A GPCR whose endogenous ligand is ghrelin, a 28-residue peptide that requires octanoylation of Ser3 by ghrelin O-acyltransferase (GOAT) for receptor activity. GHSR-1a signals mainly through Gq/11, phospholipase C, IP3 and intracellular calcium release, and it is notable for unusually high constitutive (ligand-independent) activity.
Synthetic GHSR agonists used in receptor pharmacology include:
- Ipamorelin: Aib-His-D-2-Nal-D-Phe-Lys-NH2, described in early pharmacology as a selective GHSR agonist (Ipamorelin);
- GHRP-6: His-D-Trp-Ala-Trp-D-Phe-Lys-NH2, one of the first synthetic growth hormone-releasing peptides;
- GHRP-2: D-Ala-D-2-Nal-Ala-Trp-D-Phe-Lys-NH2;
- Hexarelin: a GHRP-6 analog containing 2-methyl-D-tryptophan.
Typical in vitro readouts are calcium flux, IP1 accumulation, β-arrestin recruitment and radioligand or fluorescent-ligand binding in GHSR-1a-expressing cell lines.
Follistatin and the Myostatin Pathway
Myostatin, also called growth differentiation factor 8 (GDF-8), is a member of the TGF-β superfamily. It is synthesized as a precursor that is cleaved by furin-type proprotein convertases, leaving the mature dimer non-covalently bound to its prodomain in a latent complex. Cleavage of the prodomain by BMP-1/tolloid metalloproteinases releases active ligand.
Active myostatin binds the type II activin receptors ActRIIB (and ActRIIA), which recruit and phosphorylate the type I receptors ALK4 or ALK5. These serine/threonine kinases phosphorylate SMAD2 and SMAD3, which partner with SMAD4 and translocate to the nucleus to regulate transcription. GDF-11 is a close homolog that signals through the same receptors.
Follistatin is a secreted glycoprotein that binds activins, myostatin and some BMPs with high affinity and blocks their access to receptors. Alternative splicing produces precursors of 344 and 317 residues, which yield the mature FST315 and FST288 isoforms after removal of the signal peptide; "Follistatin 344" in research catalogues refers to the longer precursor form. Follistatin-like 3 (FSTL3) is a related binding protein. In the laboratory, follistatin and myostatin are typically studied with SMAD2/3 phosphorylation assays, SMAD-responsive luciferase reporters (for example CAGA-luc) and binding assays.
Characterizing Growth-Factor Signaling Peptides
The analytical approach depends on the size and origin of the molecule:
- Synthetic peptides (Sermorelin, CJC-1295, Tesamorelin, Ipamorelin and the GHRPs): identity by mass spectrometry and purity by reverse-phase high-performance liquid chromatography, with appearance, fill amount and lot number.
- Recombinant proteins (IGF-1 LR3, follistatin, myostatin): identity by mass spectrometry or N-terminal sequencing, purity by SDS-PAGE and/or HPLC, and often a cell-based bioactivity measure and endotoxin result.
All are stored lyophilized, sealed and protected from light at −20 °C or below, and dissolved material is aliquoted to avoid repeated freezing and thawing. Small proteins such as IGF-1 LR3 are often reconstituted in dilute acid and diluted into carrier-protein-containing buffer to limit adsorption to plastic. For individual compound profiles, see our top 10 research peptides overview.
Purity and identity data for each lot are published on our Certificates of Analysis page, and our research-only disclaimer sets out terms of sale. NuRev supplies these growth-factor signaling peptides for laboratory research use only; it is not for human or veterinary use.