PKCδ Regulates Mdm2 Independently of p53 in the Apoptotic Response to DNA Damage

Apoptosis is the key process in which cells with defective genome can be eliminated. Dys-regulation of apoptosis causes accumulation of irreparable mutation arisen from DNA damage and is the underlying cause of carcinogenesis. PKC delta is a multifunctional kinase involved in signal transduction of genotoxic-induced apoptosis. Previous studies have demonstrated that PKC delta transactivates p53 in response to DNA damage. These findings led us to determine if Mdm2, a nuclear phospho-protein and negative regulator of p53, could also be a PKC delta-modulated substrate. We discovered that inhibition of PKC delta down-regulates Mdm2 protein expression regardless of p53 status. Given that Mdm2 mRNA change was detected in p53-proficient, but not deficient cells, PKC delta affected Mdm2 on the post-translational level. Interestingly, treatment of MG132 restored Mdm2 expression to the steady-state level. Further investigation showed that PKC delta inhibited Mdm2 ubiquitination in p53-deficient cells and loss of PKC delta resulted in an increase in Mdm2 proteosomal degradation. Moreover, P300/CBP-associated factor (PCAF), an ubiquitin ligase 3 for Mdm2, was observed to participate in Mdm2 ubiquitination by PKC delta inhibition and knock-down of PCAF rescued Mdm2 diminution. Finally, as shown for PKC delta, Mdm2 was also required to exert pro-apoptotic response caused by genotoxic agents in p53-null cells. In addition, overexpression of Mdm2 restored inhibitory effect of apoptosis in cells silenced for PKC delta. Taken together, we conclude that PKC delta regulates Mdm2 expression distinctively of p53 pathway by affecting Mdm2 ubiquitination and maintenance of Mdm2 expression by PKC delta is important to ensure normal genotoxic cell death response in human cancer cells. (C) 2011 Wiley-Liss, Inc.