Evaluation of cytotoxicity and genotoxicity of okadaic acid, a non-phorbol ester type tumour promoter, in V79 Chinese hamster lung cells.

Okadaic acid, a specific phosphatase inhibitor and non-phorbol ester type tumour promoter, was examined for cytotoxic and genotoxic properties in a hepatocyte-mediated assay with V79 Chinese hamster lung fibroblasts. Genetic endpoints measured were: mutation to 6-thioguanine resistance at the hgprt gene locus and induction of sister chromatid exchange (SCE). The cytotoxicity of okadaic acid to V79 cells was determined in the absence of hepatocytes at concentrations from 0 to 30 ng/ml medium. Okadaic acid decreased colony size at 20 ng/ml, reduced colony formation by 50% (LC(50)) at 24 ng/ml, and was lethal to 100% of the cells at concentrations above 30 ng/ml. Exposure of V79 cells to okadaic acid for as little as 24 hr without hepatocytes invoked morphological changes that were concentration related. By comparison, okadaic acid was less able to invoke changes in morphology of V79 cells when co-cultivated with rat hepatocytes, but did alter hepatocyte morphology. In genotoxicity tests, mutation frequencies were increased signficantly by okadaic acid only in the absence of hepatocytes and only at the highest tested concentration (20 ng/ml medium). In contrast, SCE frequencies were not affected when okadaic acid was tested alone or with hepatocyte activation at concentrations as high as 20 ng/ml. It was concluded that, within the limits of the test system used, okadaic acid was strongly cytotoxic and may function as a direct-acting mutagen, but was otherwise without cytogenetic activity towards V79 cells.