Effect of a marine toxin on human peripheral blood monocytes

The effect of okadaic acid (OA), a non-TPA (12-0- tetradecanoylphorbol-13-acetate)-type tumor promoter, on interleukin 1 (IL-1) synthesis in human peripheral blood monocytes in vitro was examined using immunofluorescence and the mouse thymocyte assays. Stimulation of IL-1 was shown with 0.05 microgram OA/ml (the peak response), while concentrations of greater than 1.0 micrograms OA/ml showed significant inhibition of IL-1 synthesis by monocytes in the immunofluorescence analysis. Okadaic acid added directly to mouse thymocytes showed a peak response in IL-1 synthesis at 0.01 microgram OA/ml, while significant inhibition was shown for concentrations of OA greater than 0.1 microgram OA/ml. Supernatants of monocytes exposed to OA gave maximum stimulation at 0.05 microgram OA/ml for both 24 and 48 hr exposures. Significant inhibition of IL-1 synthesis in monocytes was shown by supernatants obtained from monocytes exposed 24 hr with 1.0 microgram OA/ml. Addition of various concentrations of OA to monocyte cultures in the presence of increasing concentrations of monoclonal antibody to OA showed significant reduction of the inhibition of IL-1 synthesis by OA at the 0.10 microgram level, but more significantly at the 1.0 microgram OA/ml level. The higher levels of OA associated with inhibition of IL-1 synthesis in monocytes in this study were comparable to the concentrations used in the tumor promotion studies by others. The reversal of the inhibitory effect of OA by the monoclonal antibody to OA is of interest and should be applicable to further studies on the mechanism of OA tumor promotion.