Protein phosphatase 2A is a negative regulator of transforming growth factor-beta1-induced TAK1 activation in mesangial cells

TAK1 (transforming growth factor (TGF)-beta-activated kinase 1) is a serine/threonine kinase that is rapidly activated by TGF-beta1 and plays a vital function in its signal transduction. Once TAK1 is activated, efficient down-regulation of TAK1 activity is important to prevent excessive TGF-beta1 responses. The regulatory mechanism of TAK1 inactivation following TGF-beta1 stimulation has not been elucidated. Here we demonstrate that protein phosphatase 2A (PP2A) plays a pivotal role as a negative regulator of TAK1 activation in response to TGF-beta1 in mesangial cells. Treatment with okadaic acid (OA) induces autophosphorylation of Thr-187 in the activation loop of TAK1. In vitro dephosphorylation assay suggests that Thr-187 in TAK1 is a major dephosphorylation target of PP2A. TGF-beta1 stimulation rapidly activates TAK1 in a biphasic manner, indicating that TGF-beta1-induced TAK1 activation is tightly regulated. The association of PP2A(C) with TAK1 is enhanced in response to TGF-beta1 stimulation and closely parallels TGF-beta1-induced TAK1 activity. Attenuation of PP2A activity by OA treatment or targeted knockdown of PP2A(C) with small interfering RNA enhances TGF-beta1-induced phosphorylation of TAK1 at Thr-187 and MKK3 (MAPK kinase 3). Endogenous TAK1 co-precipitates with PP2A(C) but not PP6(C), another OA-sensitive protein phosphatase, and knockdown of PP6(C) by small interfering RNA does not affect TGF-beta1-induced phosphorylation of TAK1 at Thr-187 and MKK3. Moreover, ectopic expression of phosphatase-deficient PP2A(C) enhances TAK1-mediated MKK3 phosphorylation by TGF-beta1 stimulation, whereas the expression of wild-type PP2A(C) suppresses the MKK3 phosphorylation. Taken together, our data indicate that PP2A functions as a negative regulator in TGF-beta1-induced TAK1 activation.