Transforming Growth Factor-β (TGF-β) Type I Receptor/ALK5-dependent Activation of the GADD45β Gene Mediates the Induction of Biglycan Expression by TGF-Î

We have recently shown that induction of biglycan (BGN) expression by transforming growth factor-β1 (TGF-β1) required sequential activation of both Smad and p38 mitogen-activated protein kinase signaling (Ungefroren, H., Lenschow, W., Chen, W.-B., and Kalthoff, H. (2003) J. Biol. Chem. 278, 11041–11049). Here, we have analyzed the receptors through which TGF-β1 controls expression of BGN and GADD45β, the latter of which is postulated to link early Smad signaling to delayed activation of p38. Ectopic expression of a dominant-negative mutant of the TGF-β type II receptor in PANC-1 cells abrogated TGF-β-induced BGN up-regulation. Similarly, inhibition of the TGF-β type I receptor/ALK5 with either SB431542 or by enforced stable expression of a kinasedead mutant greatly attenuated the TGF-β effect on both BGN and GADD45β expression in PANC-1 and MG-63 cells. The enhancing effect of ALK5 on TGF-β-mediated GADD45β and BGN expression and on GADD45β promoter activity was also dependent on its ability to activate Smad signaling, because an ALK5 mutant defective in Smad activation (TβRImL45) but with an otherwise functional kinase domain failed to mediate these responses. The TGF-β/ALK5 effect on p38 activation and BGN expression was mimicked by overexpression of GADD45β alone (in the absence of TGF-β stimulation) and suppressed upon antisense inhibition of GADD45β expression. These results show that TGF-β induces BGN expression through (the Smad-activating function of) ALK5 and GADD45β and suggest that the sensitivity of MyD118 to activation by TGF-β, which varies between tissues, ultimately determines the strength of the TGF-β effect on BGN.