The role of p27 phosphorylation in mediating atRA sensitivity of ovarian carcinoma cells

Abstract

All trans retinoic acid (atRA) has been shown to inhibit the growth of CAOV3 ovarian carcinoma cells. This results from arrest of the cell cycle during the G1 phase. G1 checkpoint is regulated by a multitude of molecules, including the retinoblastoma family of proteins, cyclin dependent kinases (Cdks), and cyclin dependent kinase inhibitors. P27, a cyclin dependent kinase inhibitor regulating G1 checkpoint has been shown to have elevated levels in response to atRA treatment. In these studies we investigated the role of p27 phosphorylation in mediating atRA induced growth inhibition. Our results show that atRA treatment of atRA sensitive CAOV3 cells leads to an increase in the levels of S10 phosphorylation of p27 in both nuclear and cytoplasmic cell compartments. This increase was accompanied by a decrease in the levels of skp2 protein, protein that plays a critical role in the degradation of p27. Similar results were not observed in SKOV3 cells which are not growth inhibited by atRA treatment. Finally, we demonstrated that overexpression of a mutant of p27 that cannot be phosphorylated on S10 induces a dominant negative effect on the endogenous p27 activity. This dominant negative effect reverses the atRA effect on p27 binding to CDKs, on inhibition of CDK activity, on the expression of S phase genes and ultimately on the inhibition of growth of ovarian carcinoma cells. These results suggest that hypophosphorylation of the serine10 locus of p27 might be one of the mechanisms that lead to atRA resistance of ovarian carcinoma cell. It is possible that atRA resistant ovarian tumors constitute an environment that hinders S10 phosphorylation and that by modulating the activity of the kinase(s) responsible for this event the atRA resistance can be overcome.