Cyclic AMP potentiation of cytokine-induced nitric oxide synthase activity in a murine astrocyte cell line

Astrocytes in culture have been previously shown to express inducible nitric oxide synthase (iNOS) following treatment with cytokines such as interleukin-1 beta (IL-1 beta) and interferon-gamma (IFN-gamma). We report here on the effects of the cyclic nucleotide analogues 8-bromo-cyclic AMP and 8-bromo-cyclic GMP on cytokine-stimulated iNOS gene expression in a cultured murine astrocyte cell line. In these cells, neither 8-bromo-cyclic AMP nor S-bromo-cyclic GMP alone was able to stimulate iNOS activity. Similarly, neither IL-1 beta nor IFN-gamma was capable of independently stimulating iNOS expression. Co-stimulation with both cytokines, however, resulted in measurable increases in iNOS activity, and correlated to increases in iNOS mRNA levels. The addition of 8-bromo-cyclic AMP, but not 8-brorno-cyclic GMP, was found to further enhance the expression of iNOS activity induced by IL-1 beta and IFN-gamma co-stimulation. This potentiation effect of 8-bromo-cyclic AMP correlated to a further elevation in iNOS mRNA levels over that produced by cytokine co-stimulation alone. However, 8-bromo-cyclic AMP co-treatment with either cytokine alone did not stimulate iNOS activity, indicating that the signal transduction pathway(s) involved in the potentiation effect of 8-bromo-cyclic AMP is functional only in the presence of both cytokines. These results indicate that cyclic AMP-mediated processes can participate in modulating the expression of astrocyte iNOS when the appropriate combinations of stimulatory cytokines are present. (C) 1997 Elsevier Science Ltd.