Regulation and function of a hemicleaved Activin-A processing intermediate in melanoma

Activin-A is a Transforming Growth Factor-B (TGFB)-related cytokine that regulates various biological processes including cell proliferation and differentiation through autocrine, paracrine or endocrine signaling. Activin-A expression is upregulated in multiple cancer types, including melanoma, where it is thought to promote primary and metastatic tumor growth by facilitating immune evasion. In lung and colorectal cancers, high Activin-A plasma concentrations also correlate with systemic weight loss (cachexia) and poor prognosis. Mature Activin-A derives from a 91 kDa precursor dimer of two INHBA chains that is cleaved by proprotein convertases (PCs) to displace an inhibitory prodomain. Once cleaved, Activin-A can bind to type I and type II Activin receptors leading to activation of Smad2/3 signaling pathway. However, the regulation of Activin-A maturation and its role in vivo are unknown. Unexpectedly, our laboratory found that cleavage of a single INHBA chain independently of any known PCs gives rise to a 'hemicleaved' processing intermediate that can already stimulate Smad signaling, albeit at reduced level compared to the fully mature 25 kDa form. The aim of this project is to identify the protease responsible for hemicleaved Activin-A synthesis and its influence on PC-mediated cleavage of the second INHBA chain, and to evaluate its potential role in regulating tumor-host interactions. To detect this PC-like protease (PCLP), I initiated an siRNA screen using a murine protease siRNA library.