Overexpression of Hp95 induces G1 phase arrest in confluent HeLa cells

Xp95, a protein recently identified in Xenopus laevis, is potentially invol ved in progesterone-induced Xenopus oocyte maturation. In this study, we cl oned a human homologue of Xp95, designated Hp95, and examined the effect of its overexpression on the growth properties of human malignant HeLa cells which have lost the contact inhibition of cell proliferation. We observed t hat although HeLa cells did not undergo GI phase arrest at any stage after confluence, they were able to downregulate their GI phase CDK activities in response to confluence. When Hp95 was overexpressed in HeLa cells by trans fection with a constitutive or an inducible expression vector containing a full-length Hp95 transgene, HeLa cells became able to undergo Gl phase arre st and form a monolayer culture after confluence. However, the GI phase CDK activities in these Hp95 overexpressing cells were not inhibited further a s compared to control cells after confluence. These results indicate that t he defects in HeLa cells that cause the loss of contact inhibition of cell proliferation are in components downstream of the GI phase CDKs and that ov erexpression of Hp95 counteracts some of these defects.