TGF-β-activated kinase 1 and TAK1-binding protein 1 cooperate to mediate TGF-β 1 -induced MKK3-p38 MAPK activation and stimulation of type I collagen

We have previously demonstrated that transforming growth factor-β 1 (TGF-β 1 ) rapidly activates the mitogen-activated protein kinase kinase 3 (MKK3)-p38 MAPK signaling cascade, leading to the induction of type I collagen synthesis in mouse glomerular mesangial cells (Wang L, Ma R, Flavell RA, Choi ME. J Biol Chem 277: 47257–47262, 2002). In the present study, we investigated the functional role of upstream TGF-β-activated kinase 1 (TAK1) and TAK1-binding protein 1 (TAB1) in the TGF-β 1 signaling cascade. Rapid activation of endogenous TAK1 activity by TGF-β 1 was observed in mouse mesangial cells. Transient overexpression of TAK1 with TAB1 enhanced the activation of MKK3 and p38 MAPK with or without TGF-β 1 stimulation, whereas a dominant-negative mutant of TAK1 (TAK1DN) suppressed TGF-β 1 -induced activation of MKK3 and p38 MAPK. Moreover, constitutive expression of TAK1DN reduced steady-state protein levels of MKK3 and p38 MAPK as well as MKK3 phosphorylation. Increased p38α MAPK activity by ectopic expression of either TAB1 or wild-type p38α MAPK resulted in enhanced TGF-β 1 -induced type I collagen expression. In contrast, constitutive expression of TAK1DN inhibited collagen induction. Taken together, our data indicate that TAK1 and TAB1 play a pivotal role as upstream signal transducers activating the MKK3-p38 MAPK signaling cascade that leads to the induction of type I collagen expression by TGF-β 1 . In addition, our findings also suggest that TAK1 has a novel function in regulation of the steady-state protein levels of MKK3 and p38 MAPK.