RelA/p65 is a molecular target for the immunosuppressive action of protein kinase A.

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Abstract
Stimulation of the protein kinase A (PKA) signalling pathway exerts an inhibitory effect on the proliferation of numerous cells, including T lymphocytes. In CD4+ T helper cells, stimulation of PKA leads to suppression of interleukin 2 (IL‐2) induction, while induction of the genes coding for the lymphokines IL‐4 and IL‐5 is enhanced. We show that the differential effect of PKA activity on induction of the IL‐2 and IL‐4 genes is mediated through their promoters. One major target of the suppressive effect of PKA is the kappa B site in the IL‐2 promoter. A kappa B site is missing in the IL‐4 promoter. Mutations preventing factor binding to the IL‐2 kappa B site result in a loss of PKA‐mediated suppression of IL‐2 promoter activity. Furthermore, activation of the PKA signalling pathway impairs the inducible activity of multiple kappa B sites of the IL‐2 promoter, but not of other factor binding sites. The reduction in activity of kappa B sites in activated and PKA‐stimulated T cells is accompanied by changes in the concentration and DNA binding of Rel/NF‐kappa B factors. Stimulation of the PKA pathway in Jurkat T cells with the PKA activator forskolin leads to an increase in synthesis of c‐Rel and p105/p50, while synthesis of p65/RelA remains unchanged. However, nuclear translocation and DNA binding of p65 is distinctly impaired, probably due to a retarded degradation of I kappa B‐alpha. In a similar way, stimulation of the PKA signalling pathway inhibits nuclear translocation of p65 and generation of nuclear kappa B complexes in peripheral T lymphocytes from murine lymph nodes. These results indicate that PKA‐mediated suppression of NF‐kappa B activity plays an important role in the control of activation of peripheral T lymphocytes.