Transcription factor activation during signal-induced apoptosis of immature CD4⁺CD8⁺ thymocytes: A protective role of c-Fos

Many signals that cause apoptotic cell death operate by inducing transcription and translation of other (presumably death effector) mediators, and it is well established that stimulus-induced apoptosis can often be blocked by inhibiting transcription and translation. Transcriptional regulation of apoptosis, however, is incompletely understood. To gain insight into nuclear events associated with signal-induced apoptosis during T cell development, we studied signal-induced apoptosis of ex vivo isolated immature CD8⁺4⁺ double-positive (DP) thymocytes. Stimuli utilizing the T cell receptor (TCR) signaling pathway or its parts (an αCD3/TCR monoclonal antibody, a Ca²⁺ ionophore, or a protein kinase C-activating phorbol ester) or a stimulus that antagonizes TCR signaling and apoptosis in T cell hybridoma (forskolin, a cyclic AMP-signaling activator) resulted in massive apoptosis of DP thymocytes. At the same time, these stimuli induced qualitatively similar but quantitatively unique patterns of inducible transcription factors (TFs) NF-κB/RelA-p50, AP-1 (Fos-Jun), and NUR-77. We focused our attention on the role of AP-1 (Fos-Jun) complex, which was strongly induced by all of the above stimuli and thus was a candidate for a proapoptotic TF. However, we found that AP-1/c-Fos induction was vital in prolonging DP thymocyte life, as judged by increased spontaneous and induced death of DP cells in Fos(-/-) mice. In direct support of this hypothesis, experiments with antisense oligonucleotides demonstrated that c-Fos plays an essential role in protecting normal DP thymocytes from Ca²⁺- and cAMP- induced apoptosis but not from TCR-mediated death. Together, these results demonstrate a physiological role for c-Fos in maintaining longevity of DP thymocytes.