Overexpression of integrin β1 inhibits proliferation of hepatocellular carcinoma cell SMMC-7721 through preventing Skp2-dependent degradation of p27 via PI3K pathway

Integrins may play important roles in many cellular events, such as cell proliferation, differentiation, and apoptosis. We showed previously that overexpression of integrin beta 1 inhibits cell proliferation in SMMC-7721 cells. Here we reported that one of the cyclin-dependent kinase (CDK) inhibitors, P27(Kip1) was involved in proliferation-inhibition induced by overexpression of integrin beta 1. Overexpression of integrin beta 1 upregulated p27(Kip1) at the protein level, but not mRNA level. The knock-down of p27(KiP1) expression restored cell growth in integrin beta 1-overexpressing cells. Cycloheximide (Chx) treatment and pulse-chase experiments revealed that overexpression of integrin beta 1 prolonged the half-life of p27(KiP1) by inhibiting its degradation. Proteasome inhibitor (MG132) treatment of the cells indicated that proteasome mediated degradation of p27, and Skp2-dependent degradation might be prevented. Overexpression of integrin beta 1 decreased Skp2 at mRNA level, which was regulated by cell adhesion and the subsequent adhesion-dependent signaling. Overexpression of integrin beta 1 reduced cell adhesion, accordingly, inactivated the phosphoinositide 3-kinase (PI3K) signaling. PI3K inhibitor LY294002 upregulated p27(Kip1) at post-translational level and downregulate Skp2 at mRNA level, which could mimic the effects of integrin beta 1 overexpression on p27(Kip1) and Skp2. Together, these results suggested that overexpression of integrin beta 1 inhibited cell proliferation by preventing the Skp2-dependent degradation of p27(Kip1) via PI3K pathway.