1. ¹Department of Molecular Biology and Immunology, University of North Texas Health Science Center, Fort Worth, TX, USA
2. ²Institute for Cancer Research, University of North Texas Health Science Center, Fort Worth, TX, USA

Correspondence: Professor A Basu, Department of Molecular Biology and Immunology, University of North Texas Health Science Center, 3500 Camp Bowie Boulevard, Fort Worth, TX 76107, USA. E-mail: abasu@hsc.unt.edu

³These authors contributed equally to this work.

Received 30 August 2007; Revised 3 January 2008; Accepted 3 January 2008; Published online 3 March 2008.

Abstract

Protein kinase C (PKC) acts as an antiapoptotic protein and inhibits tumor necrosis factor- (TNF)-related apoptosis-inducing ligand (TRAIL)-induced apoptosis in MCF-7 breast cancer cells. Members of the TNF receptor superfamily trigger apoptosis independent of the tumor suppressor protein p53, which primarily affects DNA damage-induced apoptosis. We have previously shown that PKC acts upstream of Akt to inhibit receptor-initiated cell death. Since Akt can regulate p53, we have examined the involvement of p53 in PKC-mediated TRAIL resistance. Overexpression of PKC in MCF-7 cells (MCF-7/PKC) caused a decrease in p53 and an increase in human homolog of murine double minute 2 (Hdm2) and phospho-Hdm2. Depletion of p53 by siRNA attenuated, whereas depletion of Hdm2 enhanced TRAIL-mediated apoptosis. Knockdown of Akt decreased Hdm2 phosphorylation, increased p53 level and potentiated TRAIL-induced cell death. Depletion of PKC from MCF-7 cells caused an increase in p53, whereas knockdown of p53 caused a decrease in Bid mRNA. Depletion of Akt from MCF-7/PKC cells resulted in an increase in p53 and Bid. These results suggest that PKC mediates TRAIL resistance by Akt-mediated phosphorylation of Hdm2 resulting in suppression of p53 expression and downregulation of Bid in MCF-7 breast cancer cells.