Modulation of cytokine expression in PB-3c mastocytes by IBMX and PMA.

Stimulation of mast cells, either via the IgE receptor or with calcium ionophore triggers the production of several cytokines, such as interleukins-3, -4, -5, and -6, and GM-CSF. In PB-3c mastocytes, ionophore-induced IL-3 and GM-CSF expression is primarily the result of mRNA stabilization, and is enhanced by oncogenic ras. Apart from mobilizing calcium, the IgE receptor activation leads to production of DAG and elevation of cAMP levels, thereby activating protein kinases C and A, respectively. The influence of these two secondary messengers on cytokine production was examined using the cAMP elevating agent IBMX, the phorbol ester PMA, and the staurosporine derivative CGP 41251, which preferentially inactivates PKC. IBMX was determined to be a potent coinducer of IL-3 expression, whereas elevation of IL-6 and GM-CSF was more pronounced in PMA-treated cells. Both PMA and IBMX were shown to act posttranscriptionally on IL-3, by extending the half-life of the mRNA. Ionophore-induced cytokine expression appears to require serine/threonine kinase activity, as it could be abolished by treatment with the drug CGP 41251. Our results therefore suggest that the factors regulating cytokine expression and mRNA stability are subject to regulation by serine/threonine phosphorylation.