Protein kinase Cdelta stimulates proteasome-dependent degradation of C/EBPalpha 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/EBPalpha) 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/EBPalpha protein during apoptosis induction.


RESULTS
Upon onset of apoptosis induced by various kinds of inducers such as NSC606985, etoposide and others, C/EBPalpha 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 (PKCdelta), which was proteolytically activated during apoptosis induction tested, we showed that the active PKCdelta protein contributed to the increased degradation of C/EBPalpha protein. Three specific proteasome inhibitors antagonized C/EBPalpha degradation during apoptosis induction. More importantly, ectopic expression of PKCdelta-CF stimulated the ubiquitination of C/EBPalpha protein, while the chemical inhibition of PKCdelta action significantly inhibited the enhanced ubiquitination of C/EBPalpha protein under NSC606985 treatment. Additionally, silencing of C/EBPalpha expression by small interfering RNAs enhanced, while inducible expression of C/EBPalpha inhibited NSC606985/etoposide-induced apoptosis in leukemic cells.


CONCLUSIONS
These observations indicate that the activation of PKCdelta upon apoptosis results in the increased proteasome-dependent degradation of C/EBPalpha, which partially contributes to PKCdelta-mediated apoptosis.
Zhao
Meng
M
Institute of Health Sciences, Shanghai Institutes for Biological Sciences of Chinese Academy of Sciences and Shanghai Jiao Tong University School of Medicine, Shanghai, China.
Duan
Xu-Fang
XF
Zhao
Xu-Yun
XY
Zhang
Bo
B
Lu
Ying
Y
Liu
Wei
W
Cheng
Jin-Ke
JK
Chen
Guo-Qiang
GQ
D017209:Apoptosis / Q000187:drug effects; D001483:Base Sequence; D015153:Blotting, Western; D022763:CCAAT-Enhancer-Binding Protein-alpha / Q000235:genetics / Q000378:metabolism*; D053148:Caspase 3 / Q000378:metabolism; D045744:Cell Line, Tumor; D017931:DNA Primers; D015536:Down-Regulation; D005047:Etoposide / Q000494:pharmacology; D020868:Gene Silencing; D006801:Humans; D006868:Hydrolysis; D007938:Leukemia / Q000201:enzymology / Q000378:metabolism / Q000473:pathology*; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism*; D051745:Protein Kinase C-delta / Q000378:metabolism*; D012333:RNA, Messenger / Q000235:genetics; D034741:RNA, Small Interfering; D020133:Reverse Transcriptase Polymerase Chain Reaction; D014158:Transcription, Genetic; D054875:Ubiquitination
D022763:CCAAT-Enhancer-Binding Protein-alpha; D017931:DNA Primers; D012333:RNA, Messenger; D034741:RNA, Small Interfering; D005047:Etoposide; D051745:Protein Kinase C-delta; D053148:Caspase 3; D046988:Proteasome Endopeptidase Complex