C‐Jun, a signal‐transducing transcription factor of the AP‐1 family, normally implicated in cell cycle progression, differentiation and cell transformation, recently has also been linked to apoptosis. To explore further the functional roles of c‐Jun, a conditional allele was generated by fusion of c‐Jun with the hormone‐binding domain of the human estrogen receptor (ER). Here we demonstrate that increased c‐Jun activity is sufficient to trigger apoptotic cell death in NIH 3T3 fibroblasts. c‐Jun‐induced apoptosis is evident at high serum levels, but is enhanced further in factor‐deprived fibroblasts. Furthermore, apoptosis by c‐Jun is not accompanied by an increase in DNA synthesis. Constitutive overexpression of the apoptosis inhibitor protein Bcl‐2 delays the c‐Jun‐mediated cell death. The regions of c‐Jun necessary for apoptosis induction include the amino‐terminal transactivation and the carboxy‐terminal leucine zipper domain, suggesting that c‐Jun may activate cell death by acting as a transcriptional regulator. We further show that α‐fodrin, a substrate of the interleukin 1β‐converting enzyme (ICE) and CED‐3 family of cysteine proteases, becomes proteolytically cleaved in cells undergoing cell death by increased c‐Jun activity. Moreover, cell‐permeable irreversible peptide inhibitors of the ICE/CED‐3 family of cysteine proteases prevented the cell death.

The transcription factor AP‐1, composed of Jun and Fos proteins, is a major target of mitogen‐activated signal transduction pathways. However, little is known about AP‐1 function in normal cycling cells. Here we report that the quantity and the phosphorylation state of the c‐Jun and JunB proteins vary at the M–G1 transition. Phosphorylation of JunB by the p34cdc2–cyclin B kinase is associated with lower JunB protein levels in mitotic and early G1 cells. In contrast, c‐Jun levels remain constant while the protein undergoes N‐terminal phosphorylation, increasing its transactivation potential. Since JunB represses and c‐Jun activates the cyclin D1 promoter, these modifications of AP‐1 activity during the M–G1 transition could provide an impetus for G1 progression by a temporal increase in cyclin D1 transcription. These findings constitute a novel example of a reciprocal connection between transcription factors and the cell cycle machinery.