TGF-β1 regulates the expression and transcriptional activity of TAZ protein via a Smad3-independent, myocardin-related transcription factor-mediated mechanism

Hippo pathway transcriptional coactivators TAZ and YAP and the TGF-beta 1 (TGF beta) effector Smad3 regulate a common set of genes, can physically interact, and exhibit multilevel crosstalk regulating cell fate-determining and fibrogenic pathways. However, a key aspect of this cross-talk, TGF beta-mediated regulation of TAZ or YAP expression, remains uncharacterized. Here, we show that TGF beta induces robust TAZ but not YAP protein expression in both mesenchymal and epithelial cells. TAZ levels, and to a lesser extent YAP levels, also increased during experimental kidney fibrosis. Pharmacological or genetic inhibition of Smad3 did not prevent the TGF beta-induced TAZ up-regulation, indicating that this canonical pathway is dispensable. In contrast, inhibition of p38 MAPK, its downstream effector MK2 (e.g. by the clinically approved antifibrotic pirferidone), or Akt suppressed the TGF beta-inducedTAZexpression. Moreover, TGF beta elevated TAZmRNAin a p38-dependent manner. Myocardin-related transcription factor (MRTF) was a central mediator of this effect, as MRTF silencing/inhibition abolished the TGF beta-induced TAZ expression. MRTF overexpression drove the TAZ promoter in a CC(A/T-rich) 6GG (CArG) box-dependent manner and induced TAZ protein expression. TGF beta did not act by promoting nuclear MRTF translocation; instead, it triggered p38-and MK2-mediated, Nox4-promoted MRTF phosphorylation and activation. Functionally, higher TAZ levels increased TAZ/TEAD-dependent transcription and primed cells for enhancedTAZactivity upon a second stimulus (i. e. sphingosine 1-phosphate) that induced nuclear TAZ translocation. In conclusion, our results uncover an important aspect of the cross-talk between TGF beta and Hippo signaling, showing that TGF beta induces TAZ via a Smad3-independent, p38-and MRTF-mediated and yet MRTF translocation-independent mechanism.