title: The protein kinase C delta catalytic fragment targets Mcl-1 for degradation to trigger apoptosis.
abstract: Proteolytic cleavage and subsequent activation of protein kinase C (PKC) delta is required for apoptosis induced by a variety of genotoxic agent, including UV radiation. In addition, overexpression of the constitutively active PKCdelta catalytic fragment (PKCdelta-cat) is sufficient to trigger Bax activation, cytochrome c release, and apoptosis. While PKCdelta is a key apoptotic effector, the downstream target(s) responsible for the mitochondrial apoptotic cascade are not known. We found that expression of the active PKCdelta-cat in HaCaT cells triggers a reduction in the anti-apoptotic protein Mcl-1, similar to UV radiation. The down-regulation of Mcl-1 induced by PKCdelta-cat was not at the mRNA level but was due to decreased protein half-life. Overexpression of Mcl-1 protected HaCaT cells from both UV and PKCdelta-cat-induced apoptosis and blocked the release of cytochrome c from the mitochondria, indicating that Mcl-1 down-regulation was required for apoptosis signaling. Indeed, down-regulation of Mcl-1 with siRNA slightly increased the basal apoptotic rate of HaCaT cells and dramatically sensitized them to UV or PKCdelta-cat-induced apoptosis. HaCaT cells with down-regulated Mcl-1 had higher activated Bax protein, as measured by Bax cross-linking, indicating that Mcl-1 down-regulation is sufficient for Bax activation. Finally, recombinant PKCdelta could phosphorylate Mcl-1 in vitro, identifying Mcl-1 as a direct target for PKCdelta. Overall our results identify Mcl-1 as an important target for PKCdelta-cat that can mediate its pro-apoptotic effects on mitochondria to amplify the apoptotic signaling induced by a wide range of apoptotic stimuli.
authors.affiliation: Department of Pathology and the Oncology Institute, Skin Cancer Research Program, Loyola University Medical Center, Maywood, Illinois 60153, USA.
authors.forename: Leonid A
authors.initials: LA
authors.lastname: Sitailo
authors.forename: Shalini S
authors.initials: SS
authors.lastname: Tibudan
authors.forename: Mitchell F
authors.initials: MF
authors.lastname: Denning
mesh_terms: D017209:Apoptosis / Q000502:physiology*; D020134:Catalytic Domain / Q000502:physiology*; D002461:Cell Line, Transformed; D045304:Cytochromes c / Q000037:antagonists & inhibitors / Q000378:metabolism; D025461:Feedback, Physiological / Q000502:physiology; D006367:HeLa Cells; D006801:Humans; D008928:Mitochondria / Q000201:enzymology / Q000378:metabolism; D064549:Myeloid Cell Leukemia Sequence 1 Protein; D009363:Neoplasm Proteins / Q000037:antagonists & inhibitors / Q000096:biosynthesis / Q000378:metabolism*; D010446:Peptide Fragments / Q000502:physiology*; D051745:Protein Kinase C-delta / Q000096:biosynthesis / Q000235:genetics / Q000502:physiology*; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000037:antagonists & inhibitors / Q000096:biosynthesis / Q000378:metabolism*; D015398:Signal Transduction / Q000502:physiology; D014466:Ultraviolet Rays
chemical_list: D064549:Myeloid Cell Leukemia Sequence 1 Protein; D009363:Neoplasm Proteins; D010446:Peptide Fragments; D019253:Proto-Oncogene Proteins c-bcl-2; D045304:Cytochromes c; D051745:Protein Kinase C-delta