Anabolic and Proliferative Potential of FST-344

FST-344 is a recombinant form of an endogenous glycoprotein that exists in two major isoforms generated by alternative mRNA splicing: FST-317 and FST-344, containing 288 and 315 amino acids after signal‐peptide cleavage, respectively.⁽¹⁾ Both isoforms share a core of 63 residues organized into three follistatin‐like domains (FSD1–3), each characterized by ten conserved cysteines that stabilize the tertiary structure. The FST-344 variant predominates in most tissues, accounting for over 95 % of follistatin transcripts, whereas FST-317 represents a minor fraction.

FST-344 was originally identified as an “activin‐binding protein” based on its potential to neutralize activin’s actions. Still, subsequent studies have suggested it may have a broader affinity across the transforming growth factor‑β (TGF‑β) superfamily. The transforming growth factor‑β (TGF‑β) superfamily is a large group of structurally related cytokines. This group is hypothesized to regulate cell growth, differentiation, and extracellular matrix production. Consequently, FST-344 is posited to regulate cytokines such as bone morphogenetic proteins (BMPs) and growth differentiation factors (GDFs) like GDF-8 (Growth Differentiation Factor 8), also known as myostatin. Research into FST-344 for its potential to inhibit myostatin and other anti-anabolic members of TGF‑β is ongoing.

 

Research

FST-344 and Myostatin

The FST‑344 is well known to engage with the TGF‑β superfamily ligands, specifically with myostatin, aka GDF-8, potentially mitigating myostatin’s potential negative actions in muscle cell hypertrophy. Specifically, myostatin is believed to control muscle cell proliferation and differentiation, potentially serving as an intrinsic brake against excessive muscular tissue fiber enlargement. Research by Rodino-Klapac et al. proposes that FST‑344 may bind to myostatin, potentially dampening its inhibitory actions.⁽²⁾ This may relieve part of myostatin’s constraint on muscle cell growth and specialization, possibly allowing for better support in muscular tissue mass in research settings. Specifically, the researchers commented, “[With] this approach, our translational studies show increased musc[ular tissue] size and strength.”

Mechanistically, FST-344 may act by occupying myostatin’s receptor‑binding surfaces. Consequently, FST-344 is posited to hinder association with the activin type IIB receptor–ALK4/5 complex, thereby dampening the Smad 2/3 phosphorylation cascade that normally represses Pax‑3, Myf‑5, and MyoD transcription. A reduction in this repression may permit continued myoblast proliferation and differentiation, ultimately favouring hypertrophy and some degree of muscular tissue hyperplasia, judging from indirect histology.

Research on research models suggests that muscular tissues may have enlarged by 1.5‑ to 2‑fold relative to controls, and muscular tissue fiber strength assays increased proportionally to the size. The researchers reported simultaneous falls in creatine kinase levels, which may suggest a potential protective action on sarcolemmal integrity, perhaps because fibres rendered larger by partial myostatin blockade are less susceptible to contraction‑induced damage. It is also important to note that long‑range off‑target binding of FST‑344 to other TGF‑β family ligands cannot be excluded, yet the additive muscle cell growth seen when follistatin over‑expression was combined with myostatin gene deletion hints at additional, myostatin‑independent actions via other TGF-β ligands.

For example, beyond myostatin, Rodino-Klapac et al. and Castonguay et al. also propose that FS‑344 may interact with other GDFs such as growth differentiation factor‑11 (GDF‑11), possibly to a lesser degree.⁽³⁾ GDF‑11 shares close structural homology with myostatin and signals through the same ActRIIB/ALK4/5 receptor complex; thus, FS‑315 may bind GDF‑11 dimers in the extracellular milieu, albeit with uncertain affinity. Such binding may hypothetically blunt GDF‑11’s impacts on cellular aging‑related tissue remodeling or muscle progenitor cell differentiation, potentially contributing to the robust hypertrophic response seen when FS‑315 is overexpressed alongside myostatin blockade.

FST-344 and Other TGF‑β Ligands

In addition to myostatin, FST‑344 seems to interact—albeit more weakly—with activin A and activin B. Activins are signaling proteins in the the TGF‑β superfamily that latch onto cell‑surface receptors like the activin type IIB receptor (ActRIIB), initiate Smad2/3 phosphorylation inside the cell, and drive gene programs that often tip the balance toward protein breakdown and tissue fibrosis.

Research by Schumann et al. suggests that in muscle cells or their supporting fibroblasts, such signals might encourage wasting and fibrosis of muscular tissue, and FST-344 may potentially inhibit that.⁽⁴⁾ Furthermore, Iskenderian et al. posit that FST-344 may limit the ability of activin A to activate its type IIB receptor on target cells, thereby potentially easing the pro‑inflammatory and pro‑fibrotic signaling cascades that activin A is thought to promote in dystrophic muscle cell models.⁽⁵⁾ The researchers also observed apparent “[better support for] muscle markers for necrosis, inflammation, and fibrosis.” Some muscle cell models also suggested a possible drop in CD68‑positive macrophage infiltration and a concurrent reduction in osteopontin and other fibrosis‑associated transcripts. These changes might reflect a shift from the macrophage phenotypes and myofibroblast activity fostered by activin A.

Hypertrophy in the test samples occurred in parallel. Yet, the disparity between FST‑344 and pure myostatin blockade hints that activin A neutralization might cooperate with—or possibly even augment—muscular fiber growth by tempering the inflammatory milieu that otherwise hampers regeneration. Furthermore, the previously mentioned research by Rodino-Klapac et al. suggests that FS‑344 may indirectly alter the availability of other TGF‑β modulators, such as FLRG or GASP‑1, by competing for overlapping ligand‑binding sites or by shifting the equilibrium among latent complexes.⁽²⁾ This indirect mechanism might further attenuate signals from multiple TGF‑β peptides, potentially broadening the anti‑fibrotic and pro‑hypertrophic potential of FS‑344 beyond myostatin blockade alone.

FST-344 and Insulin Synthesis

Binding to activin and myostatin reduces their engagement with TGF‑β receptors. FST-344 also appears to promote cell proliferation in pancreatic beta cells (the insulin-producing cells) and mediate other mechanisms, resulting in an overall increase in insulin synthesis. This has been observed in research by Zhao et al., which suggests that, more specifically, FST-344 dampened SMAD2/3 phosphorylation inside β‑cells.⁽⁶⁾ Once SMAD2/3 activity fell, the researchers reported a potential cascade favoring the insulin‑PI3K‑Akt pathway, increasing phosphorylated PI3K p85 and Akt. This may lead to an increase in insulin gene transcription and β‑cell proliferation. The net outcome appeared to be an expanded β‑cell mass and higher circulating insulin, without interacting with insulin resistance.

FST-344 and Cancer Cells

Research by Shi et al. suggests that FST-344 may have some impact on tumor cells mainly by sequestering activin and certain bone‑morphogenetic proteins, thereby reshaping the downstream SMAD pathways that govern proliferation, death, and motility.⁽⁷⁾ Because activin itself may either restrain or spur cell growth depending on lineage and context, the consequences of FST‑344 appear to vary widely amongst different cancer cell lines. Indeed, Shi et al. have observed that several cancer cell lines may proliferate faster under FST344 exposure.

Notably, LNCaP prostate‑carcinoma cells may grow faster when FST‑344 is added because the protein mops up extracellular activin, thus lifting a SMAD‑driven brake on DNA synthesis. DU145 prostate‑carcinoma cells behave similarly; once FST-344 neutralizes activin, their cell‑cycle machinery is no longer in check, and proliferation potentially resumes. Several melanoma cell lines also seem to expand in the presence of FST‑344, most likely because the molecule blocks the same activin‑dependent apoptotic signal that would otherwise curtail their survival.

Research by Zabkiewicz et al. has reported similar observations with MCF‑7 breast carcinoma cells, which may proliferate faster under FST-344 exposure due to a potential reduction of the apparent activin‑dependent brake on the G1‑to‑S transition.⁽⁸⁾ However, the researchers suggest that FST-344 may simultaneously dampen activin‑ or TGF‑β‑driven epithelial–mesenchymal transition pathways required for cancer cell migration. Thus, the current data positions FST‑344 as a molecule that may potentially uncouple proliferation from invasive dissemination and consequently reduce metastasis potential.

FST-344 and Retinal Cell Detachment

Research by Dağ et al. suggests that FST-344 may affect the normal function of retinal cells, leading to fluid accumulation below them and disrupting their normal function.⁽⁹⁾ Specifically, the researchers posit that FST‑344 may reach the eye through the fenestrated choriocapillaris and bind local members of the TGF‑β superfamily. Even though ocular myostatin transcripts are scarce, retinal pigment epithelium (RPE) and the inner layer of Bruch’s membrane express activin‑ and TGF‑β‑responsive receptors.

By sequestering myostatin and activin, FST‑344 may tilt the local Smad‑signalling balance toward a state that favors matrix remodelling enzymes over structural collagens. This may make the subretinal environment more permissive to fluid collection, leading to its detachment when collected behind the retinal cell. Future measurements of matrix metalloproteinase activity in RPE cultures after FST‑344 exposure would help clarify this pathway.

You can find Follistatin-344 for sale with 99% purity, on our website (available for research use only).

NOTE: These products are intended for laboratory research use only. This peptide is not intended for personal use. Please review and adhere to our Terms and Conditions before ordering.

 

References:

1. Shi L, Resaul J, Owen S, Ye L, Jiang WG. Clinical and Therapeutic Implications of Follistatin in Solid Tumours. Cancer Genomics Proteomics. 2016 11-12;13(6):425-435. doi: 10.21873/cgp.20005. PMID: 27807065; PMCID: PMC5219916.
2. Rodino-Klapac LR, Haidet AM, Kota J, Handy C, Kaspar BK, Mendell JR. Inhibition of myostatin with emphasis on follistatin as a therapy for muscle disease. Muscle Nerve. 2009 Mar;39(3):283-96. doi: 10.1002/mus.21244. PMID: 19208403; PMCID: PMC2717722.
3. Castonguay R, Lachey J, Wallner S, Strand J, Liharska K, Watanabe AE, Cannell M, Davies MV, Sako D, Troy ME, Krishnan L, Mulivor AW, Li H, Keates S, Alexander MJ, Pearsall RS, Kumar R. Follistatin-288-Fc Fusion Protein Promotes Localized Growth of Skeletal Muscle. J Pharmacol Exp Ther. 2019 Mar;368(3):435-445. doi: 10.1124/jpet.118.252304. Epub 2018 Dec 18. PMID: 30563942.
4. Schumann C, Nguyen DX, Norgard M, Bortnyak Y, Korzun T, Chan S, St Lorenz A, Moses AS, Albarqi HA, Wong L, Michaelis K. Increasing lean muscle mass in mice via nanoparticle-mediated hepatic delivery of follistatin mRNA. Theranostics. 2018 Oct 22;8(19):5276.
5. Iskenderian A, Liu N, Deng Q, Huang Y, Shen C, Palmieri K, Crooker R, Lundberg D, Kastrapeli N, Pescatore B, Romashko A, Dumas J, Comeau R, Norton A, Pan J, Rong H, Derakhshan K, Ehmann DE. Myostatin and activin blockade by engineered follistatin(FST-344) results in hypertrophy and improves dystrophic pathology in the mdx mouse more than myostatin blockade alone in skeletal Muscle. 2018 Oct 27;8(1):34. doi: 10.1186/s13395-018-0180-z. PMID: 30368252; PMCID: PMC6204036.
6. Zhao C, Qiao C, Tang RH, Jiang J, Li J, Martin CB, Bulaklak K, Li J, Wang DW, Xiao X. Overcoming Insulin Insufficiency by Forced Follistatin Expression in β-cells of db/db Mice. Mol Ther. 2015 May;23(5):866-874. doi: 10.1038/mt.2015.29. Epub 2015 Feb 13. PMID: 25676679; PMCID: PMC4427879.
7. Shi L, Resaul J, Owen S, Ye L, Jiang WG. Clinical and Therapeutic Implications of Follistatin in Solid Tumours. Cancer Genomics Proteomics. 2016 11-12;13(6):425-435. doi: 10.21873/cgp.20005. PMID: 27807065; PMCID: PMC5219916.
8. Zabkiewicz C, Resaul J, Hargest R, Jiang WG, Ye L. Increased Expression of Follistatin in Breast Cancer Reduces Invasiveness and Clinically Correlates with Better Survival. Cancer Genomics Proteomics. 2017 Jul-Aug;14(4):241-251. doi: 10.21873/cgp.20035. PMID: 28647698; PMCID: PMC5572302.
9. Dağ U, Çağlayan M, Öncül H, Alakuş MF. Central serous chorioretinopathy associated with high-dose follistatin-344(FST-344): a retrospective case series. Int Ophthalmol. 2020 Nov;40(11):3155-3161. doi: 10.1007/s10792-020-01501-6. Epub 2020 Jul 15. PMID: 32671599.