Regulation of TAZ Expression by Cyclic Stretch

Abstract

It is increasingly recognized that fibrosis‐related phenotype transitions (e.g. EMT and fibrobrolast‐myofibroblast transition) are brought about by the interplay between chemical (e.g. TGFβ) and mechanical (e.g. stretch) stimuli, which, in part, act through cytoskeleton‐regulated transcription factors. Among these, the Hippo effector TAZ and the Rho/actin‐regulated myocardin‐related transcription factor (MRTF) are TGFβ‐ and mechano‐sensitive transcriptional co‐activators, which play key roles in the pathogenesis of organ fibrosis. Previously we have shown a multilevel crosstalk between TAZ and MRTF. Moreover, we found that TGFβ is a strong inducer of TAZ protein and mRNA expression, an effect mediated by p38‐dependent MRTF activation, independent of a net nuclear translocation of MRTF or TAZ. These results revealed that, in addition to TAZ nuclear translocation, the regulation of TAZ expression (a hitherto poorly characterized process) is also a critical feature of fibrogenesis. Since mechanical stimuli are prominent inducers of fibrosis, we sought to determine if cyclic stretch impacts TAZ expression and to investigate the potential role of the RhoGTPase/MRTF signaling pathway in this process. Cyclic stretch (10%, 1 Hz) increased MRTF‐transcriptional activity, measured by a CArG box‐ (cis‐element for MRTF) containing luciferase reporter construct, between 3 and 6 hr in C3H10T1/2 cells. Overexpression of Rho family small GTPases (RhoA, Rac1 and Cdc42) known to be activated by stretch and to induce MRTF translocation, were sufficient to enhance the activity of a TAZ promoter luciferase reporter (TAZ‐Luc). Consistently, cyclic stretch increased TAZ protein and mRNA expression, and activated TAZ‐Luc. TAZ‐Luc activation was abolished by MRTF inhibition or the mutation of the CArG box within the reporter (mtTAZ‐Luc). The stretch‐induced activation of TAZ expression was a universal phenomenon, since similar effects were observed in Interstitial Rat Mesenchyme Cells (IRMC) and Human Pulmonary Microvascular Endothelial Cells (HPMEC) as well. Finally, since the redox‐regulator pirin was recently suggested to be an upstream regulator of MRTF, we checked its potential involvement in the regulation of TAZ expression. While CCT‐251236, a pirin‐binding compound abolished the TGFβ‐induced TAZ expression, pirin silencing failed to recapitulate this effect. This suggests that the drug has alternative targets and MRTF can be activated in a pirin‐independent manner. Taken together, our studies show that critical chemical and mechanical fibrogenic stimuli increase TAZ expression and MRTF is a key mediator of this effect, both via classic (Rho‐mediated, translocation‐dependent) and by “non‐canonical” (phosphorylation‐induced activation) mechanisms. We propose that the ensuing rise in TAZ expression potently accelerates and exacerbates fibrogenic phenotype transitions.Support or Funding InformationKidney Foundation of Canada and Natural Sciences and Engineering Research Council of Canada (AK)