The mitochondrial permeability transition (MPT)- A NewTarget for human papillomavirus (HPV) 16 E6-enhanced apoptosis

HPV 16 E6 protein targets p53 for rapid proteolysis, which is believed to result in inhibition of p53-mediated apoptosis. Recent data indicate that factors responsible for triggering apoptosis reside in the cytoplasm of cells and not in the nucleus. The findings that mitochondria are required in certain cell-free models for induction of apoptosis and that Bcl-2 is localized to mitochondria, have focused attention on the role of the MPT in apoptosis. Here we present data that HPV 16 E6 expression sensitizes cells to MPT-induced apoptosis; this sensitization occurs only in the presence of wildtype (wt) p53 expression. The extent of apoptosis induced by atractyloside (an inducer of the MPT) was determined in normal, temperature-sensitive (ts) p53, and HPV 16 E6 transfected J2-3T3 cells, and in HPV expressing Siha, Hela and CaSki cells. C33a cells, which express mutant (mt) p53, were also exposed to atractyloside in the presence or absence of HPV 36 E6 expression. The sensitivity of cells to atractyloside-induced apoptosis was found to be: E6-J2-3T3 > CaSki > normal-J2 3T3 > ts p53-J2-3T3 > vector-J2-3T3 > Hela > SiHa > C33a > C33a 16 E6. Cyclosporin A, an inhibitor of the MPT, and ICE-1 protease inhibitor proteced against atractyloside-induced apoptosis in all cells tested. These findings indicate that: 1) HPV 16 E6 protein expression sensitizes cells to atractyloside-induced apoptosis; 2) HPV 36 E6 sensitization requires p53 experssion; and 3) HPV 16 E6 targets the mitochondria through a protease sensitive mechanism.