c-Jun-Dependent CD95-L Expression Is a Rate-Limiting Step in the Induction of Apoptosis by Alkylating Agents

Abstract

Mouse 3T3 fibroblasts derived from fetuses lacking c-Jun were used to define an essential role of c-Jun, a main component of the transcription factor AP-1, in the cellular response to the alkylating agent methyl methanesulfonate (MMS). MMS represents the most potent and selective activator of the stress-induced kinases JNK/SAPK and p38, resulting in very efficient induction of c-Jun hyperphosphorylation and c-jun transcription. This agent induced apoptosis with high efficiency in wild-type cells but not in c-jun^−/− cells. Resistance to apoptosis was accompanied by impaired expression of CD95 ligand (CD95-L), a well-known inducer of apoptosis. The addition of recombinant CD95-L restored apoptosis sensitivity in c-jun^−/− fibroblasts. MMS-induced apoptosis in wild-type fibroblasts or human lymphocytes was strongly reduced by neutralizing CD95-L antibodies or transdominant negative FADD, confirming the importance of CD95 signalling in MMS-induced apoptosis. The loss-of-function approach in fibroblasts allowed the identification and dissection of c-Jun-dependent and -independent processes upstream or downstream of CD95 activation. We have found that c-Jun can act as a proapoptotic regulator in cells exposed to DNA damage via induction of CD95-L. Once activated, CD95-induced death signalling is not affected by the loss of c-Jun, demonstrating that only the initiation and not the execution of stress-induced apoptosis depends on c-Jun.

ACKNOWLEDGMENTS

We thank Jan-Paul Medema, Christian Behrens, and Klaus Hexel for help with cytometry; Melanie Sator-Schmidt and Sibylle Teurich for technical assistance; Dagmar Wilhelm and Andreas Dieckmann for sharing unpublished data; and Marina Schorpp-Kistner, Axel Behrens, and Kanaga Sabapathy for critical reading of the manuscript.

This work was supported by grants from the Deutsche Forschungsgemeinschaft (He-551/8-2) and the German-Israeli Cooperation in Cancer Research and the TMR and Biomedicine and Health programs (CT96-0044 and CT BMH4-98-3505) of the European Economic Community.