TNFα-mediated cell death is independent of Cdc25A

Tumor necrosis factor (TNFs) have been shown to be synthesized by ovarian carcinomas, and may therefore affect tumor cells in an autocrine manner. Therefore, we investigated the effects of recombinant TNFs on ovarian carcinoma cells N.1 and examined expression of the proto-oncogenes c-myc and cdc25A which are known to play a prominent role in apoptosis. TNFα elicited apoptosis in N.1 cells within 72 h which was shown by typical morphological changes, DNA fragmentation and signature type cleavage of poly(ADP-ribose) polymerase into a 89 kDa proteolytic peptide. TNFα-induced apoptosis was accompanied by constitutive c-Myc expression, although the mRNA level of phosphatase cdc25A was suppressed within 24 h of TNFα treatment and the protein level decreased after 48 h. Cdc25A tyrosine phosphatase is an activator of the cdk2-cyclin E complex which allows for cell cycle progression. As expected, we found TNFα-mediated Cdc25A down-regulation to inhibit Cdk2 activity. Cdc25A suppression was related to TNFα-induced apoptosis but not to a TNFα-induced G0 arrest because cyclin D1 expression was unaffected and the gene gas6 (growth arrest specific 6) was not induced. Arresting cells by treatment with genistein prevented TNFα-triggered apoptosis and inhibited c-myc expression. TNFα-induced apoptosis is not accompanied by cell cycle arrest which may be due to constitutive c-Myc expression, although Cdc25A and Cdk2 activity is also down-regulated. High c-Myc and low Cdc25A activity might present conflicting signals to the cell cycle machinery which are incompatible with cell survival.