Constitutive presence of a catalytic fragment of protein kinase C epsilon in a small cell lung carcinoma cell line.

Protein kinase C (PKC) has been implicated in a variety of cellular responses such as proliferation, differentiation, and secretion. We assessed the role of PKC in the mitogenic effects of gastrin-releasing peptide (in a small cell lung cancer (SCLC) cell line. Using antisera that specifically recognize the PKC isoforms alpha, beta, gamma, delta, and epsilon, we determined that PKC epsilon is the major isoform in the SCLC cell line NCI-N417, followed by PKC alpha and delta. In addition to the 90-kDa PKC epsilon, our anti-PKC epsilon antiserum specifically detected a 40-kDa immunoreactive protein. Treatment of the cells with either 20 nM phorbol myristate acetate or 50 nM GRP enhanced significantly the level of the 40-kDa protein in a time-dependent (1-8 h), cycloheximide-sensitive fashion. Subcellular fractionation revealed that 90% of PKC epsilon was in particulate form, while the 40-kDa immunoreactive protein was cytosolic. To test the hypothesis that the 40-kDa soluble protein represented a catalytically independent PKC epsilon fragment, cytosolic extracts were assayed for kinase activity. 45-50% of the activity was apparent in the absence of the PKC activators phosphatidylserine and diacylglycerol. This effector-independent kinase activity was further purified by affinity chromatography using a synthetic peptide corresponding to the pseudosubstrate region of PKC epsilon (ERMRPRKRQGAVRRRV) coupled to Sepharose. The partially purified protein, recognized by the anti-PKC epsilon antiserum, exhibited histone kinase activity with kinetics similar to those of the tryptically generated catalytic fragment of brain PKC epsilon. This activity was inhibited by staurosporine (IC50 = 1 x 10(-8) M) and by the pseudosubstrate inhibitor peptide (IC50 = 7.7 x 10(-8) M). The SCLC kinase and the brain PKC epsilon catalytic fragment were similar as indicated by the relative sizes of the PKC epsilon immunoreactive peptides generated with protease V8 from Staphylococcus aureus (Mr approximately 37,000, 34,000, 28,000, 26,000, and 25,000). Taken together, we conclude that a variant SCLC cell line expresses a constitutively active catalytic fragment of PKC epsilon. Regulation by 12-O-tetradecanoyl-13-acetate or GRP via de novo protein synthesis suggests a novel mechanism of control of PKC diversity with implications for small cell lung cancer and possibly other malignancies.