Evidence that protein kinase C-α activation is a critical event in phorbol ester-induced multiple drug resistance in human colon cancer cells

We previously designed and characterized an in vitro model of the intrinsic drug resistance of human colon cancer. The human colonic epithelium is chronically exposed to endogenous protein kinase C (PKC) stimulatory factors, and our model demonstrated that activation of PKC induces resistance to multiple anticancer drugs in the metastatic human colon cancer cell line KM12L4a. PKC is an isozyme family with ten members, eight of which are phorbol ester-responsive. In this report, we show that thymeleatoxin (Tx), a daphnane tumor promoter that selectively activates the phorbol ester-responsive isozymes cPKC-α, -β₁, -β₂, and -γ, was just as effective in inducing drug resistance in KM12L4a cells as phorbol dibutyrate, a potent activator of all phorbol ester-responsive PKC isozymes. The induction of resistance by Tx was associated with a reduction in cytotoxic drug accumulation in KM12L4a cells. We demonstrated by immunoblot analysis and hydroxylapatite chromatography that KM12L4a cells express active cPKC-α but not cPKC-β₁, -β₂, or γ. Our results provide strong evidence that phorbol-ester activation of cPKC-α is sufficient for the induction of resistance observed in KM12L4a cells. The possibility that endogenous PKC activators may induce intrinsic drug resistance in clinical colon cancer by an analogous mechanism is strongly suggested by our detection of active cPKC-α in surgical specimens of human colon carcinomas.