Atypical protein kinase c ζ as a target for chemosensitization of tumor cells

Exposure of tumor cells to cytotoxic agents simultaneously activates a variety of intracellular signaling pathways. Some of these pathways involve enzymes from the protein kinase C (PKC) family of serine/threonine kinases. This family includes isoenzymes that negatively influence cell death, whereas other demonstrate an opposite effect. The present study analyzes the role of the ζ atypical PKC isoform in tumor cell response to cytotoxic agents. Using a histone H1 phosphorylation assay, we showed that both tumor necrosis factor α and etoposide activate PKCζ in U937 human leukemic cells. Stable transfection of a kinase-dead, dominantnegative PKCζ mutant in U937 cells decreases Bcl-2 expression while increasing the expression of Bax and several procaspases. This transfection also prevents etoposide-induced nuclear factor-κB nuclear translocation and accumulation of X-linked inhibitor of apoptosis protein. PKCζ inhibition accelerates the occurrence of apoptosis in leukemic cells exposed to etoposide and tumor necrosis factor α. This sensitization was confirmed in vitro by use of a clonogenic assay. In addition, PKCζ inhibition sensitized tumor cells grown in nude mice to etoposide. These results indicate that PKCζ isoform is a protective signals that is activated in tumor cells exposed to a cytotoxic agent. This inducible resistance factor thus appears an attractive target for chemosensitization of tumor cells.