| Prohibitin is a 30kD protein with potent anti-proliferative functions. We have previously shown that prohibitin binds to the tumor suppressor protein Rb, represses E2F-mediated transcription and suppresses colony formation on plastic. Here we provide evidence that prohibitin is protective against apoptosis induced by the topoisomerase inhibitor camptothecin, suggesting that it has anti-apoptotic functions as well. Prohibitin levels were elevated in response to camptothecin, while Rb family members were degraded or inactivated. Camptothecin treatment induced E2F1-mediated transcriptional activity; this was repressed by prohibitin. In addition, we found that prohibitin is present mainly in the nucleus of two different breast carcinoma cell lines where it co-localized with E2F1 and p53. Prohibitin migrated to the mitochondria after apoptotic stimulation, as detected by co-localization with cytochrome c. Prohibitin still co-localized with p53 in the nuclear periphery after induction of apoptosis but the interaction with E2F1 was lost. Further, prohibitin bound to p53 in vitro and enhanced its transcriptional activity in transient transfection experiments. Prohibitin repressed growth of colonies in soft agar; E2F1 mediated growth in soft agar was repressed as well. An anti-sense prohibitin construct overcame most of the growth suppressive effects of p53, suggesting that growth suppressive properties of p53 might involve prohibitin function. Prohibitin also augmented p53 recruitment to an endogenous promoter, but repressed recruitment of E2F to a target site. This data suggests that prohibitin is capable of modulating apoptosis and cellular proliferation, probably by regulating the transcriptional activity of E2F1 and p53. |
