Mechanisms of protein kinase C epsilon down-regulation by transforming growth factor-beta in lung cancer cells
Protein kinase C epsilon (PKC{varepsilon}), a diacylglycerol (DAG)/phorbol ester-regulated PKC isoform, has been widely linked to oncogenesis and metastasis. PKC{varepsilon} plays important roles in the regulation of motility and invasiveness in non-small cell lung cancer (NSCLC). We previously reported that this kinase becomes prominently down-regulated upon TGF-{beta}-induced epithelial-to-mesenchymal transition (EMT), which leads to prominent phenotypic changes. While the phorbol ester PMA causes down-regulation of PKC, {delta} and {varepsilon} within hours, TGF-{beta} requires at least 4 days to reduce the expression levels of PKC{varepsilon} without affecting the expression of other PKCs, an effect that parallels the acquisition of a mesenchymal phenotype. Despite the prominent transcriptional component involved in EMT, we found that PKC{varepsilon} down-regulation does not involve changes in PKC{varepsilon} mRNA levels and was entirely independent of transcriptional activation of the PRKCE gene. Further mechanistic analysis revealed that the reduction in PKC{varepsilon} expression is dependent on proteasomal and endolysosomal pathways, but independent of autophagy processing mechanisms. Site-directed mutagenesis of Lys312 and Lys321 in PKC{varepsilon} prevented its down-regulation in response to either TGF-{beta} or the phorbol ester PMA. The shift in PKC{varepsilon} isozyme levels depending on cell plasticity underscores relevant functional consequences by modulating the expression of this oncogenic/metastatic kinase and highlights key roles of protein stability mechanisms in the control of PKC{varepsilon} phenotypic outcomes.