Modulation of the stress-induced synthesis of stress proteins by a phorbol ester and okadaic acid.

The expression of alphaB crystallin, hsp27, and hsp70 in C6 cells increased when they were exposed to arsenite (50 microM for 1 h) or heat (42 degrees C for 30 min), as detected by specific immunoassays, Western blot analysis, and Northern blot analysis. When cells were exposed to arsenite in the presence of 0.1 microM phorbol 12-myristate 13-acetate (PMA), an activator of protein kinase C, or 0.2 microM okadaic acid, an inhibitor of phosphoserine/phosphothreonine protein phosphatases, expression of alphaB crystallin was markedly enhanced. The induction of hsp27 and hosp70 expression was also stimulated to a considerable extent in the same cells. The stimulatory effect of PMA was further enhanced in the presence of okadaic acid, but it was strongly inhibited in the presence of 0.5 microM staurosporine, an inhibitor of protein kinase C. PMA and okadaic acid also stimulated the response to heat stress of the expression of alphaB crystallin, but they barely stimulated the response to heat stress of hsp27. The extent of stimulation of the arsenite-induced responses by PMA and okadaic acid was greater when the concentration of arsenite (i.e. the magnitude of the stress) was relatively low (25-50 microM). The arsenite-induced release of arachidonic acid from cells was also stimulated in the presence of PMA and/or akadaic acid, and the stimulatory effects of PMA and okadaic acid on the arsenite-induced accumulation of alphaB crystallin and hsp27 were strongly suppressed by quinacrine, an inhibitor of phospholipase A2. These results suggest that the stimulatory effects of PMA and okadaicacid on the stress responses are cuased, in part, by the increased metabolic activity of the arachidonic acid cascade, as a consequence of the activation of phospholipase A.