Atypical protein kinase C zeta 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 zeta atypical PKC isoform in tumor cell response to cytotoxic agents. Using a histone H1 phosphorylation assay, we showed that both tumor necrosis factor alpha and etoposide activate PKCzeta in U937 human leukemic cells. Stable transfection of a kinase-dead, dominant-negative PKCzeta 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-kappaB nuclear translocation and accumulation of X-linked inhibitor of apoptosis protein. PKCzeta inhibition accelerates the occurrence of apoptosis in leukemic cells exposed to etoposide and tumor necrosis factor alpha. This sensitization was confirmed in vitro by use of a clonogenic assay. In addition, PKCzeta inhibition sensitized tumor cells grown in nude mice to etoposide. These results indicate that PKCzeta 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.