Involvement of protein kinase Cε (PKCε) in thyroid cell death -: A truncated chimeric PKCε cloned from a thyroid cancer cell line protects thyroid cells from apoptosis

The protein kinase C (PKC) family has been implicated in the regulation of apoptosis. However, the contribution of individual PKC isozymes to this process is not well understood. We reported amplification of the chromosome 2p21 locus in 28% of thyroid neoplasms, and in the WRO thyroid carcinoma cell line. By positional cloning we identified a rearrangement and amplification of the PKC epsilon gene, that maps to 2p21, in WRO cells. This resulted in the overexpression of a chimeric/ truncated PKC epsilon (Tr-PKC epsilon) mRNA, coding for N-terminal amino acids 1-116 of the isozyme fused to an unrelated sequence. Expression of the Tr-PKC epsilon protein in PCCL3 cells inhibited activation-induced translocation of endogenous PKC epsilon, but its kinase activity was unaffected, consistent with a dominant negative effect of the mutant protein on activation-induced translocation of wild-type PKC epsilon and/or displacement of the isozyme to an aberrant subcellular location. Cell lines expressing Tr-PKC epsilon grew to a higher saturation density than controls. Moreover, cells expressing Tr-PKC epsilon were resistant to apoptosis, which was associated with higher Bcl-2 levels, a marked impairment in p53 stabilization, and dampened expression of Bax. These findings point to a role for PKC epsilon in apoptosis-signaling pathways in thyroid cells, and indicate that a naturally occurring PKC epsilon mutant that functions as a dominant negative can block cell death triggered by a variety of stimuli.