Abnormal proliferation of vascular smooth muscle cells (VSMCs) contributes to various vascular diseases, but the factors that maintain VSMCs in a quiescent state remain poor understood. Phosphatidylinositol 3 kinases (PI3Ks) are important protein kinases that regulate vascular cell proliferation, but the biological and pathological functions of p55γ, a regulatory subunit of PI3K, and its regulation in the cardiovascular system are completely unknown. We aimed to determine the relationship between p55γ and vascular proliferation and neointimal formation.
In the present study, we have demonstrated that p55γ expression is markedly downregulated in primary cultured VSMCs in response to mitogenic stimulation and in carotid arteries after balloon injury, and that overexpression p55γ profoundly inhibits mitogenic stimuli and injury induced VSMC proliferation as well as neointimal formation. p55γ overexpression inhibited, whereas knockdown of p55γ promoted PDGF-BB- and serum-induced VSMC proliferation. Importantly, in vivo adenoviral gene transfer of p55γ into carotid arteries attenuated, while knockdown of p55γenhanced balloon injury-induced neointimal formation. Furthermore, p55γ sequentially upregulated p53 and p21, resulting in cell-cycle arrest in S phase; knockdown of either p53 or p21 blocked p55γ-induced VSMC growth arrest. Mechanistically, p55γ interacted with and stabilized p53 protein by blocking MDM2-mediated p53 ubiquitination and degradation, subsequently activating its target gene p21. Concurrently, p55γ upregulated Bcl-xl expression, which counterbalanced p53-mediated apoptosis. These findings mark p55γ as a novel upstream regulator of the p53-p21 signaling pathway which negatively regulates VSMC proliferation, suggesting that malfunction of p55γ may trigger vascular proliferative disorders.
* Correspondence to Chun-Mei Cao (firstname.lastname@example.org) or Rui-Ping Xiao (email@example.com).