Similar effects of cyclosporine and verapamil on lymphokine, interleukin 2 receptor, and proto-oncogene expression

Since calcium channel blocking agents and CsA exert an antiproliferative effect upon T cell mitogenesis, we have compared and characterized their immunosuppressive properties at the level of gene activation. Verapamil (greater than or equal to 30 microM), which blocks T cell mitogenesis and a rise in cytosolic calcium, was added to cultures of peripheral blood mononuclear cells stimulated with phytohemagglutinin (5 micrograms/ml) and phorbol myristate acetate (5 ng/ml). Northern blot analysis was performed using cDNA probes for the p55 interleukin 2 receptor (Tac; IL-2R), interleukin 2 and c-myc at 20 hr of culture. Accumulation of IL-2 encoding mRNA within the cytoplasm was completely abrogated by verapamil. However, IL-2R and c-myc encoding mRNA were clearly detectable in verapamil-treated cell cultures. Surface expression of the Tac protein in mitogen-activated T cells was also not blocked by verapamil as shown by FACS analysis. In companion experiments with CsA, verapamil only partially inhibits the intracellular processes leading to T cell activation. A calcium-independent pathway may exist for the expression of IL-2R and c-myc, while an increase of intracellular Ca2+ may provide the additional signal for IL-2 gene expression. Although the in vitro concentrations of verapamil used in these experiments are in excess of common clinically therapeutic levels, the results help clarify the mode of CsA action and may provide a new tool to dissect the early events of T cell activation.