Multidrug-resistant human KB carcinoma cells are highly resistant to the protein phosphatase inhibitors okadaic acid and calyculin A. Analysis of potential mechanisms involved in toxin resistance.

In this study we show that multidrug-resistant (MDR) human KB-V1 cells are highly resistant to the cytotoxicity of okadaic acid and calyculin A, 2 toxins from marine sponges that are potent inhibitors of type-1 and type-2A protein phosphatases (PP1 and PP2A). Cytotoxicity and colony-forming assays indicated that, relative to parental drug-sensitive KB-3 cells, KB-V1 cells are 35-fold more resistant to okadaic acid and 70-fold more resistant to calyculin A. Cytotoxicity of the toxins was associated with mitotic arrest characterized by chromosome scattering and over-condensation, with KB-3 cells being more sensitive than KB-V1 cells and calyculin A being more potent than okadaic acid. The resistance of KB-V1 cells to both okadaic acid and calyculin A was completely reversed by verapamil, suggesting that the toxins may be transported by P-glycoprotein (P-gp). To further assess the possibility of an interaction with P-gp, the toxins were employed as potential modulators of the photoaffinity labeling of P-gp by [3H]azidopine. Relative to vinblastine, which effectively competed with [3H]azidopine for P-gp photolabeling, calyculin A was 100-fold less potent and okadaic acid did not inhibit photolabeling at concentrations up to 50 microM. To determine whether the resistance mechanism involved differences in toxin-sensitive phosphatase activity, the activity was assayed in extracts from both cell lines and found to be slightly higher (1.6-fold) in KB-V1 than in KB-3 cells. Our results demonstrate a novel, marked resistance of MDR KB-V1 cells to these phosphatase inhibitors and suggest that a major mechanism of resistance may involve toxin transport by P-gp at sites apparently different from those which bind azidopine.