INDUCTION OF APOPTOSIS BY C-FOS PROTEIN

The role of c-Fos in apoptosis was examined in two Syrian hamster embr yo cell lines (sup(+)I and sup(-)II) and a human colorectal carcinoma cell line (RKO), using the chimeric Fos-estrogen receptor fusion prote in c-FosER. As previously reported, contrasting responses were observe d when these two cell lines were placed under growth factor deprivatio n conditions; sup(+)I cells were highly susceptible to apoptosis, wher eas sup(-)II cells were resistant. In this report, we show that the ac tivated c-FosER protein induces apoptosis in sup(-)II preneoplastic ce lls in serum-free medium, indicating that c-Fos protein can induce apo ptotic cell death in these cells. c-Fos-induced apoptosis was not bloc ked by the protein synthesis inhibitor cycloheximide, suggesting that the c-Fos transcriptional activation activity is not involved. This co nclusion was further supported by the observation that overexpression of v-Fos, which is highly proficient in transcriptional activation but deficient in the transcriptional repression activity associated with c-Fos, did not induce apoptosis. Constitutively expressed Bcl-2 delaye d the onset of low-serum-induced apoptosis in sup(+)I cells and enhanc ed survival in sup(-)II cells. Further, coexpression of Bcl-2 and c-Fo sER in sup(+)I or sup(-)II cells protected the cells from c-FosER-indu ced apoptosis. The possibility that c-FosER-induced apoptosis requires a p53 function was examined. Colorectal carcinoma RKO(p53+/+) cells, which do not normally undergo apoptosis in serum-free medium, showed a poptotic DNA fragmentation upon expression and activation of c-FosER. Further, when the wild-type p53 protein was diminished in the RKO cell s by infection with the papillomavirus E6 gene, subsequent c-FosER-ind uced apoptosis was blocked. The data suggest that c-Fos protein plays a causal role in the activation of apoptosis in a p53-dependent manner . This activity does not require new protein synthesis and is blocked by overexpression of Bcl-2 protein.