Jatrophone and 12-O-tetradecanoyl phorbol-13-acetate antagonism of stimulation of natural killer activity and lymphocyte proliferation.

We have recently reported that the diterpene jatrophone antagonizes the effects of phorbol ester in pharmacogical studies. In order to investigate further whether this action is associated with an inhibition of protein kinase C activity, we examined the effect of jatrophone on the stimulation of lymphocyte activities which are dependent on the protein kinase C pathway. Jatrophone (0.02-0.32 microM) caused concentration-dependent and equipotent inhibition of human lymphocyte proliferation induced by 5 micrograms/ml of phytohemagglutinin or by a combination of 100 ng/ml of 12-O-tetradecanoyl phorbol-13-acetate (TPA) plus 0.15 microM ionomicyn, with IC50 values (and their 95% confidence limits) of 53.4 (42.6-65.3) nM and 48.4 (39.4-59.8) nM, respectively. Jatrophone also blocked, in a concentration-dependent fashion, the murine lymphocyte proliferation stimulated by 5 micrograms/ml of concanavalin A, with an IC50 value of 63.5 (51.2-76.5) nM. The inhibition was not due to a toxic effect as the pre-incubation of lymphocytes for 48 h with 0.32 microM jatrophone did not impair the proliferation after removal of the diterpene from the culture medium. Human lymphocytes when pre-treated with 10 ng/ml TPA had a 3 times higher spontaneous natural killer activity against K562 cells and an increased expression of CD69. In addition, jatrophone inhibited both spontaneous and TPA-stimulated natural killer activity and the expression of CD69. Jatrophone concentrations that inhibited 75% of lymphocyte proliferation did not impair the intracellular increase in Ca2+ flux in lymphocytes stimulated by phytohemagglutinin. These results indicate that jatrophone is a potent inhibitor of activation of lymphocytes, probably through inhibition of the protein kinase C pathway.