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 d ependent DNA fragmentation, a hallmark of apoptosis. The mutant was 10 0-fold more resistant to OA in terms of effects on these parameters. A lthough the synthesis of several proteins was altered, enzyme assay an d immunoblot analysis indicated that the levels of PP1 and PP2A were u nchanged in the mutant. Protein kinase C (PKC) assays and immunoblot a nalysis of calcium-dependent (cPKC) and calcium-independent (nPKC) iso forms revealed that nPKC-epsilon was strikingly absent in the mutant, which otherwise expressed in comparable amounts all other isotypes (cP KC-alpha, cPKC-beta, and nPKC-zeta) also present in the wild type. Nor thern blot analysis confirmed an absence of PKC-epsilon mRNA in the mu tant cells. The OA200 cells were cross-resistant not only to another P P1/PP2A inhibitor, calyculin A, but also to structurally unrelated ant icancer drugs (such as vinblastine and taxol) and furthermore, overexp ressed the verapamil-sensitive drug pump P-glycoprotein at both the pr otein and mRNA levels. The mutant, however, was not cross-resistant to several PKC inhibitors tested including cardiotoxin, mastoparan, stau rosporine, and an alkylphospholipid. Cardiotoxin, at a subtoxic concen tration, enhanced by 6-fold vinblastine cytotoxicity in OA200 cells. T hese 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 a ssociated with increased cPKC activity, and identify certain PKC inhib itors that have potential as resistance modulators.