Fas-mediated apoptosis in Jurkat cells is suppressed in the pre-G2/M phase

The relationship between the cell cycle and Fas-mediated apoptosis was inve stigated using Jurkat cells. Analysis of the inducibility of apoptosis by a nti-Fas antibody during the cell cycle synchronized by the thymidine double -block method, showed that apoptosis was induced in only 50% of the G2/M ph ase cells, while most of cells in the other phases underwent apoptosis. The se observations indicate that G2/M phase cells are more resistant to Fas-me diated apoptosis than cells in other phases. Furthermore, a detailed analys is of G2/M phase found that only 20-30% of the cells underwent apoptosis 12 h after the removal of the second thymidine block (pre-G2/M phase). This s uggests that Fas-mediated apoptosis is potently suppressed during the pre-G 2/M phase. A possible explanation for the observation that cells in the pre -G2/M phase are less sensitive to anti-Fas antibody is lower expression lev el of Fas. To test this possibility, Fas expression levels on the cell surf ace during the cell cycle were examined. The content of Fas on the cell sur face, however, did not change appreciably during the cell cycle. Thus, the suppression of apoptosis in the pre-G2/M phase is determined downstream aft er the receipt of the apoptotic signal through Fas.