Previously, we showed that progesterone (P4) at physiologic concentrations (5nM-500nM) inhibits proliferation and migration of rat aortic smooth muscle cells (RASMCs). The P4-induced migration inhibition in RASMC was resulted from Rat sacroma homolog gene family, member A (RhoA) inactivation induced by activating the cSrc/AKT/ERK 2/p38 mitogen-activated protein kinase-mediated signaling pathway. We also demonstrated that up-regulation of cyclin-dependent kinase inhibitor 1B (p27kip1) is involved in the P4-induced migration inhibition in RASMC. Because P4 can increase formation of the p27kip1-RhoA complex in RASMC, this finding led us to hypothesize that the P4-induced inactivation in RhoA might be caused by up-regulation of p27kip1. Here, we showed that P4 increased phosphorylation of p27kip1 at Ser10 in the nucleus, which in turn caused p27kip1 translocation from the nucleus to the cytosol, subsequently increasing formation of the p27kip1-RhoA complex. These effects were blocked by knocking-down kinase-interacting stathmin (KIS) using KIS small interfering RNA. Knock-down of p27kip1 abolished the P4-induced decreases in the level of RhoA protein in RASMC. However, pretreatment of RASMC with the proteasome inhibitor, N-(benzyloxycarbonyl)leucinylleucinylleucinal (MG132), prevented the P4-induced degradation of p27kip1 and RhoA. Taken together, our investigation of P4-induced migration inhibition in RASMC showed a sequence of associated intracellular events that included 1) increase in formation of the KIS-p27kip1 complex in the nucleus; 2) phosphorylated nuclear p27kip1 at Ser10; 3) increased cytosolic translocation of p27kip1 and formation of the p27kip1-RhoA complex in the cytosol; and 4) degradation of p27kip1 and RhoA through the ubiquitin-proteasome pathway. These findings highlight the molecular mechanisms underlying P4-induced migration inhibition in RASMC.
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