Abrogation of p53-induced G1 arrest by the HPV 16 E7 protein does not inhibit p53-induced apoptosis

Wild type (wt) p53 expressed from a temperature-sensitive construct (ts p53) can induce both G1 cell cycle arrest and apoptosis in the p53-negative J3D mouse T lymphoma line (Wang et al, 1995). The human papillomavirus (HPV) 16 E7 protein has been shown to prevent p53-induced G1 cell cycle arrest following DNA damage. We asked whether inhibition of p53-induced G1 arrest by overexpression of the HPV16 E7 protein in the ts p53-transfected J3D cells would interfere with p53-induced apoptosis. Whereas a majority of the ts p53-expressing J3D cells were arrested in the G1 phase 22 h after induction of wt p53 by temperature shift to 32 degrees C, the E7/ts p53-expressing cells showed only a minor increase in the number of cells in G1 at this time point. In addition, the E7/ts p53-expressing cells showed a much less dramatic reduction in number of cells in S phase than the ts p53-expressing cells. This demonstrates that E7 at least partially rescues the cells from p53-induced G1 arrest. In contrast, overexpression of HPV16 E7 did not have any effect on the kinetics nor the frequency of p53-triggered apoptotic death, as shown by FACS analysis, trypan blue exclusion, and DNA fragmentation analysis. These findings support the notion that p53-induced G1 arrest and p53-induced apoptosis are two separate independent pathways.