Cutting Out Growth Factor Signaling

Science's STKE  02 Mar 2004:
Vol. 2004, Issue 222, pp. tw78-TW78
DOI: 10.1126/stke.2222004TW78

HtrA1, a serine protease that functions as a tumor suppressor, contains a signal sequence for secretion and an N-terminal domain that resembles follastatin, a protein that binds activin [a member of the transforming growth factor-β (TGF-β) family]. Oka et al. investigated the possibility that HtrA1 could bind to and thereby antagonize the effects of proteins in this family of growth and differentiation factors and discovered that, although HtrA1 inhibited signaling from TGF-β family proteins, it did so through an unexpected mechanism. The authors used in situ hybridization and immunostaining to show that, in mice, HtrA1 mRNA was expressed at times and places during development associated with TGF-β family protein activity. HtrA1 bound to TGF-β1, TGF-β2, bone morphogenetic protein 4 (BMP4), growth differentiation factor 5 (GDF5), and activin in vitro and inhibited BMP4- and TGF-β1-dependent signaling in cotransfection assays in C2C12 myoblasts. Signaling from a constitutively active BMP4 receptor was not affected, which indicates that inhibition took place prior to the activation of intracellular pathways. Chick embryos injected with 293T cells expressing HtrA1 showed localized developmental deficits consistent with inhibition of BMP4 signaling. Unexpectedly, deletion analysis indicated that the HtrA1 protease domain and an adjacent linker region--but not the domain that resembles follastatin--were required for binding to BMP4 and GDF5 and inhibition of BMP4 signaling. Mutational analysis confirmed that protease activity was required for inhibition of BMP4 signaling. Thus, HtrA1 does antagonize the effects of TGF-β proteins, but this antagonism is independent of the follastatin-like domain and depends on its protease activity.

C. Oka, R. Tsujimoto, M. Kajikawa, K. Koshiba-Takeuchi, J. Ina, M. Yano, A. Tsuchiya, Y. Ueta, A. Soma, H. Kanda, M. Matsumoto, M. Kawaichi, HtrA1 serine protease inhibits signaling mediated by TGF-β family proteins. Development 131, 1041-1053 (2004). [Abstract] [Full Text]