T cell antigen receptor dependent signalling in human lymphocytes: cholera toxin inhibits interleukin-2 receptor expression but not interleukin-2 synthesis by preventing activation of a protein kinase C isotype, PKC-alpha.

Activation and translocation of protein kinases C is a key event in the regulation of T lymphocyte activation, proliferation and function. Stimulation of human peripheral blood lymphocytes with the monoclonal antibody BMA 031 raised against the T cell antigen receptor led to a bimodal activation of protein kinases C. The immediate activation and translocation of the protein kinase C isoform PKC-alpha was followed by activation and translocation of the protein kinase C-beta isoenzyme after 90 min of stimulation. Pretreatment of the cells with cholera toxin for 90 min completely abolished activation of protein kinase C-alpha. In sharp contrast, activation and translocation of protein kinase C-beta was not influenced by the bacterial toxin, suggesting that activation and translocation of different protein kinase C isoenzymes are regulated by distinct mechanisms of transmembrane signalling coupled to the T cell antigen receptor/CD3 complex. The expression of high affinity IL-2 receptors was completely inhibited by cholera toxin, while IL-2 synthesis and secretion were not influenced in BMA 031-stimulated human lymphocytes. Extensive control experiments have shown that the effects of cholera toxin were not mediated by its B subunit, and were independent of elevation of intracellular cAMP concentration, suggesting that cholera toxin interfered with a signalling pathway leading to activation of protein kinase C-alpha, which could be responsible for the inhibition of IL-2 receptor expression. This hypothesis was substantiated by the finding that upon introduction of antibodies against protein kinase C-alpha, IL-2 receptor gene expression was completely suppressed. The results suggest, that protein kinase C-alpha might be the major protein kinase C isoenzyme of a signal transduction cascade regulating IL-2 receptor expression in stimulated human lymphocytes.