We have previously demonstrated that TGF-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. In the present study, we investigated the functional role of upstream TGF--Activated Kinase 1 (TAK1) and TAK1-Binding Protein 1 (TAB1) in the TGF-1 signaling cascade. Rapid activation of endogenous TAK1 activity by TGF-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-1 stimulation, whereas dominant-negative mutant of TAK1 (TAK1DN) suppressed TGF-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 p38 MAPK activity by ectopic expression of either TAB1 or wild-type p38 MAPK resulted in enhanced TGF-1-induced type I collagen expression. In contrast, constitutive expression of TAK1DN inhibited the collagen induction. Taken together, our data indicate that TAK1 and TAB1 play a pivotal role as upstream signal transducers activating MKK3-p38 MAPK signaling cascade that leads to the induction of type I collagen expression by TGF-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.