Novel mechanism of reduced proliferation in ovarian clear cell carcinoma cells: Cytoplasmic sequestration of CDK2 by p27

ObjectiveOvarian clear cell carcinoma (CCC) carries a poor prognosis because of its insensitivity to chemotherapy. We previously found an association between reduced proliferation of CCC and chemoresistance; here we investigated the mechanism of the reduced proliferation.MethodsWe assessed cell cycle function by measuring the activity of cyclin-dependent kinases (CDKs) and the protein expression of cyclins, the CDK inhibitors, and p53 in 22 ovarian cancer cell lines and 60 human ovarian cancer specimens. We examined the cellular location of p27, p27 phosphorylated at threonine 157 (p27Thr157), and CDK2 protein by confocal microscopy and western blotting. We tested the effect of the inhibitor of phosphatidylinositol-3-kinase (PI3K) and small interfering RNA against p27 (si-p27) in two CCC cell lines (RMG-I, SMOV-2).ResultsCCC cells had lower CDK2 activity and higher p27 expression than serous adenocarcinoma (SA) cells. Low CDK2 activity correlated with high p27 protein expression. p27Thr157 sequestered CDK2 in the cytoplasm, but PI3K inhibitor or si-p27 maintained CDK2 in the nucleus and restored its activity. In human specimens, CDK2 was mostly in the cytoplasm and was spatially associated with p27; CDK2 activity was lower in the CCC than in the SA specimens. si-p27 enhanced the cytotoxic effect of cisplatin, doxorubicin, and gemcitabine in both RMG-I cells and SMOV-2 cells.ConclusionsReduced CDK2 activity via the cytoplasmic sequestration of CDK2 by p27Thr157 may contribute to suppression of CCC proliferation. A prospective study is needed to determine whether the cytoplasmic sequestration of CDK2 results in the chemoresistance of CCC.

Research highlights
► We explored the cell cycle regulation of clear cell carcinoma cells.
► High p27 expression directly suppressed cyclin-dependent kinase (CDK) 2 activity.
► Further, p27 sequestered CDK2 in the cytoplasm, contributing to inhibition of cell proliferation.