The proliferation of vascular smooth muscle cells (VSMCs) plays a critical role in the development of various vascular diseases such as atherosclerosis. In order for SMCs to proliferate, it is widely believed that cells undergo a phenotypic conversion from a differentiated contractile phenotype to a dedifferentiated synthetic phenotype. The transcription co-activator myocardin is well established to play a role in SMC differentiation and proliferation. Therefore, it is crucial to understand how myocardin expression and activity are regulated in VSMCs. We investigated the regulation of myocardin expression and activity in SMCs at two different levels. (1) Myocardin undergoes ubiquitylation and degradation via UPS, which leads to an increase in its transcriptional activity. However, the E3 ligase responsible for myocardin ubiquitylation has not been identified. Atrogin‐1 is a muscle-specific E3 ligase which down‐regulates myocardin protein during skeletal muscle differentiation. Therefore, it is possible that atrogin‐1 causes ubiquitylation, which leads to proteasome mediated degradation of myocardin protein in VSMCs, thereby increasing myocardin activity. Our results showed that in VSMCs, atrogin‐1 directly interacted with myocardin in the nuclei and caused ubiquitylation and subsequent proteasome mediated degradation of myocardin. UPS-mediated myocardin degradation increased myocardin transcriptional activity through an increase in RNA polymerase II recruitment to myocardin target gene promoters. Moreover, atrogin‐1 expression increased the contractility of VSMCs in vitro and increased the contractile response of mouse aortas to potassium chloride (KCl) and phenylephrine (PE) ex vivo. Atrogin-1 expression also decreased VSMC proliferation in vitro.
(2) The NF‐κB pathway is activated by various cytokines such as TNFα. Moreover, myocardin is known to inhibit VSMC proliferation by inhibiting the NF‐κB pathway. However, how the NF‐κB pathway activated by TNFα regulates myocardin remains unknown. We found that TNFα down‐regulated myocardin expression and activity in dedifferentiated cultured VSMCs by activating the NF‐κB pathway. TNFα-mediated myocardin downregulation decreased contractility and increased proliferation in cultured VSMCs. In addition, we found that in differentiated VSMCs, TNFα prevented myocardin mRNA degradation, which resulted in a further increase in myocardin expression and activity. TNFα-mediated myocardin mRNA stabilization resulted in increased contractility of VSMCs.