Protein kinase C epsilon is overexpressed in primary human non-small cell lung cancers and functionally required for proliferation of non-small cell lung cancer cells in a p21/Cip1-dependent manner.

The protein kinase C (PKC) family of proteins plays important roles in growth regulation and is implicated in tumorigenesis. It has become clear that the role of PKC in tumorigenesis is cell context dependent and/or isoform specific. In this study, we showed for the first time by immunohistochemistry that overexpression of PKC epsilon was detected in the vast majority (>90%) of primary human non-small cell lung cancers (NSCLC) compared with normal lung epithelium. Inhibition of the PKC epsilon pathway using a kinase-inactive, dominant-negative PKC epsilon, PKC epsilon(KR), led to a significant inhibition of proliferation and anchorage-independent growth of human NSCLC cells in a p53-independent manner. This was accompanied by a specific induction of the cyclin-dependent kinase (cdk) inhibitor p21/Cip1 but not p27/Kip1. In response to serum stimulation, PKC epsilon(KR)-expressing cells showed a prolonged G(1)-S transition and delayed and reduced activation of cdk2 complexes, which was likely attributed to the increased binding of p21/Cip1 to cdk2. Furthermore, inhibition of PKC epsilon function either by expressing PKC epsilon(KR) or by small interfering RNA (siRNA)-mediated gene knockdown resulted in c-Myc down-regulation, which, in turn, regulated p21/Cip1 expression. Knockdown of PKC epsilon or c-Myc expression using siRNA led to induction of p21/Cip1 and attenuation of G(1)-S transition in NSCLC cells. Using p21(+/+) and p21(-/-) HCT116 isogenic cell lines, we further showed that growth inhibition by PKC epsilon(KR) required the function of p21/Cip1. Collectively, these results reveal an important role for PKC epsilon signaling in lung cancer and suggest that one potential mechanism by which PKC epsilon exerts its oncogenic activity is through deregulation of the cell cycle via a p21/Cip1-dependent mechanism.