Differential down-regulation of protein kinase C selectively affects IgE-dependent exocytosis and inositol trisphosphate formation

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Abstract
Short-term treatment of rat basophilic leukaemia (RBL-2H3) cells with the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA) activates protein kinase C (PKC) and results in the inhibition of the IgE-dependent formation of inositol phosphates, but in the potentiation of serotonin secretion. Long-term treatment with TPA, which depletes the cells of their endogenous PKC, eliminates both Ca2(+)-ionophore- and TPA- as well as IgE-dependent secretion, but it potentiates by 1.7-fold IgE-induced inositol phosphate formation. Taken together, these observations strongly suggest that the dual actions of TPA on IgE-dependent responses are both mediated by PKC. The opposing effects of TPA are differentially down-regulated. Following TPA treatment, the rate by which the cells lose their ability to undergo exocytosis is faster than the rate at which inhibition of inositol phosphates formation is relieved and their production potentiated. In addition, both processes show different sensitivities to inhibitors of PKC action. Whereas IgE-dependent secretion is completely blocked by the PKC inhibitors K252a, H-7 and sphingosine [concns. causing 50% inhibition (IC50 values) = 25 ng/ml 80 microns and 30 microns respectively], these inhibitors do not relieve inhibition of inositol phosphate formation by TPA, nor do they potentiate this response. These results may imply that the bidirectional control exerted by PKC on IgE-dependent responses is mediated by its different isoenzymes.

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