Human leukemia K562 cell mutant (K562/OA200) selected for resistance to okadaic acid (protein phosphatase inhibitor) lacks protein kinase C-epsilon, exhibits multidrug resistance phenotype, and expresses drug pump P-glycoprotein.

A human leukemia K562 cell mutant (K562/OA200) selected for resistance to okadaic acid (OA), an inhibitor of protein phosphatases 1 and 2A (PP1/PP2A), has been established. In wild type cells, the cytotoxicity of OA was associated with mitotic arrest and concentration- and time-dependent DNA fragmentation, a hallmark of apoptosis. The mutant was 100-fold more resistant to OA in terms of effects on these parameters. Although the synthesis of several proteins was altered, enzyme assay and immunoblot analysis indicated that the levels of PP1 and PP2A were unchanged in the mutant. Protein kinase C (PKC) assays and immunoblot analysis of calcium-dependent (cPKC) and calcium-independent (nPKC) isoforms revealed that nPKC-epsilon was strikingly absent in the mutant, which otherwise expressed in comparable amounts all other isotypes (cPKC-alpha, cPKC-beta, and nPKC-zeta) also present in the wild type. Northern blot analysis confirmed an absence of PKC-epsilon mRNA in the mutant cells. The OA200 cells were cross-resistant not only to another PP1/PP2A inhibitor, calyculin A, but also to structurally unrelated anticancer drugs (such as vinblastine and taxol) and furthermore, overexpressed the verapamil-sensitive drug pump P-glycoprotein at both the protein and mRNA levels. The mutant, however, was not cross-resistant to several PKC inhibitors tested including cardiotoxin, mastoparan, staurosporine, and an alkylphospholipid. Cardiotoxin, at a subtoxic concentration, enhanced by 6-fold vinblastine cytotoxicity in OA200 cells. These findings indicate that the multidrug resistance phenotype can be induced by cytotoxic agents other than conventional anticancer drugs, show that the development of multidrug resistance is not necessarily associated with increased cPKC activity, and identify certain PKC inhibitors that have potential as resistance modulators.