BRD4 Mediates Transforming Growth Factor-β-Induced Smooth Muscle Cell Differentiation from Mesenchymal Progenitor Cells.

Smooth muscle cell (SMC) differentiation plays a crucial role in angiogenesis and vasculogenesis during embryonic development. The underlying mechanisms controlling SMC differentiation, especially progenitor-specific regulation, however, remain largely unclear. In this study, we identified bromodomain-containing protein 4 (BRD4) as a novel regulator for SMC differentiation. Transforming growth factor-β (TGF-β) induces BRD4 expression in the initial phase of SMC differentiation of pluripotent murine 10T1/2 cells. BRD4 was found critical in mediating TGF-β-induced SMC differentiation. Knockdown of BRD4 with siRNA suppressed TGF-β-induced expression of SMC markers including α-SMA and SM22α. In addition, the BRD4 inhibitor JQ1 and degraders ARV-825 and dBET1 suppressed TGF-β-induced SMC marker gene expression. BRD4 regulates SMC differentiation by activating SMC marker gene transcription. BRD4 mediated SMC differentiation is independent of the phosphorylation of Smad2/3. Instead, BRD4 mediated TAZ expression induced by TGF-β. Consistent with the function of TAZ, the inhibition of BRD4 reduced nuclear retention of Smad3, thereby impairing Smad3 mediated SMC gene transcription. Myocardin is an important transcriptional modulator for SMC markers. Interestingly, the knockdown of BRD4 also attenuated the induction of myocardin due to TGF-β in 10T1/2 cells. Taken together, this study demonstrates that BRD4 is a novel modulator for SMC differentiation from mesenchymal progenitor cells through the regulation of TAZ and myocardin.