Dual Cleavage Events Activate TGF-β

Abstract

Growth factors of the transforming growth factor-β (TGF-β) and bone morphogenetic protein (BMP) families are synthesized and maintained in inactive complexes, which are then cleaved into action by various proteinases. TGF-β interacts with the cleaved prodomain, called latency-associated peptide (LAP), which is disulfide linked to latent TGF-β-binding protein (LTBP), which then interacts with the extracellular matrix. The TGF-β-LAP-LTBP complex is called a large latent complex (LLC). Ge and Greenspan show that LTBP of the TGF-β1 LLC, but not LAP, is cleaved by the proteinase BMP1, which is a member of a family of proteinases, not BMP ligands, that are also known to process extracellular matrix proteins, chordin (a BMP antagonist), and the inhibitory prodomains of GDFs (growth and differentiation factors). By comparing mouse embryo fibroblasts (MEFs) from wild-type and bmp1 knockout mice, the authors showed that the bmp1 cells had increased deposition of LTBP (detected by immunofluorescence) but decreased processing of TGF-β to the active form (based on the TGF-β activity in cultured medium). Although in vitro LAP was not a substrate for BMP1, in vivo LAP cleavage was impaired in the bmp1 knockouts, suggesting that BMP1 cleavage of LTBP precedes LAP cleavage, which is required to release active TGF-β. Using pharmacological inhibitors and RNAi experiments, the authors demonstrated that matrix metalloproteinase 2 (MMP2) was required for LAP processing. In embryonic mice, the bmp1 knockouts displayed increased staining for fibrillar LTBP but decreased phosphorylation of Smad2 and 3, which are downstream of TGF-β, which is consistent with BMP1 contributing to activation of TGF-β. Thus, TGF-β activation appears to involve sequential cleavage events: First BMP1 cleavage of LTBP releases the LLC from the extracellular matrix, and then MMP2 cleavage of LAP releases TGF-β from the LLC. G, Ge, D. S. Greenspan, BMP1 controls TGFβ1 activation via cleavage of latent TGFβ-binding protein. J. Cell Biol. 175 , 111-120 (2006). [Abstract] [Full Text]