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

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.     

A novel complex between the p65 subunit of NF-kappa B and c-Rel binds to a DNA element involved in the phorbol ester induction of the human urokinase gene.

The NF-kappa B subunits, p50 and p65, have extensive sequence homology with the c-rel proto-oncogene and the Drosophila morphogen dorsal. It has recently been shown that in vitro translated c-Rel can bind to DNA and form a complex with p50. However, the conditions for DNA binding of c-Rel in vivo and its DNA sequence specificity have not been established. Here we report the identification a novel heterodimeric complex that binds to a kappa B-like, phorbol ester (TPA) responsive DNA sequence, 5'-GGGAAAGTAC-3', in the 5' flanking region of the human urokinase (uPA) gene. This sequence was shown to bind two protein complexes, LC and UC. LC was indistinguishable from NF-kappa B as it reacted with antibodies recognizing the p50 subunit of NF-kappa B, and was shown by UV crosslinking to contain the p50 and p65 subunits of NF-kappa B. UC, on the other hand, strongly reacted with anti-v-Rel, but not with the anti-p50 antibodies, and was shown by crosslinking to contain 75 kDa and 85 kDa protein-DNA adducts. The 75 kDa and the 85 kDa adducts could be immunoprecipitated only by anti-p65 and anti-c-Rel antibodies, respectively, showing that c-Rel formed a heterodimer with p65. Both protein complexes were present in inactive forms in HeLa cell cytosol, and their nuclear translocation was induced by TPA. DNA binding of UC and LC could, furthermore, be inhibited by I kappa B-alpha.(ABSTRACT TRUNCATED AT 250 WORDS)