Protein kinase Cδ stimulates proteasome-dependent degradation of C/EBPα during apoptosis induction of leukemic cells

Background: The precise regulation and maintenance of balance between cell proliferation, differentiation and death in metazoan are critical for tissue homeostasis. CCAAT/enhancer-binding protein alpha (C/EBPa) has been implicated as a key regulator of differentiation and proliferation in various cell types. Here we investigated the potential dynamic change and role of C/EBPa protein during apoptosis induction. Methodology/Principal Findings: Upon onset of apoptosis induced by various kinds of inducers such as NSC606985, etoposide and others, C/EBPa expression presented a profound down-regulation in leukemic cell lines and primary cells via induction of protein degradation and inhibition of transcription, as assessed respectively by cycloheximide inhibition test, real-time quantitative RT-PCR and luciferase reporter assay. Applying chemical inhibition, forced expression of dominant negative mutant and catalytic fragment (CF) of protein kinase Cdelta (PKCd), which was proteolytically activated during apoptosis induction tested, we showed that the active PKCd protein contributed to the increased degradation of C/EBPa protein. Three specific proteasome inhibitors antagonized C/EBPa degradation during apoptosis induction. More importantly, ectopic expression of PKCd-CF stimulated the ubiquitination of C/EBPa protein, while the chemical inhibition of PKCd action significantly inhibited the enhanced ubiquitination of C/EBPa protein under NSC606985 treatment. Additionally, silencing of C/EBPa expression by small interfering RNAs enhanced, while inducible expression of C/EBPa inhibited NSC606985/etoposide-induced apoptosis in leukemic cells. Conclusions/Significance: These observations indicate that the activation of PKCd upon apoptosis results in the increased proteasome-dependent degradation of C/EBPa, which partially contributes to PKCd-mediated apoptosis.