Proinflammatory cytokines produced by activated glial cells may in turn augment the immune/inflammatory reactions of glial cells through autocrine and paracrine routes. The NO/cGMP signaling represents one of the reactions of activated glial cells. We investigated whether the production of proinflammatory cytokines by glial cells is affected by NO-dependent downstream cGMP signaling. In primary cultures of mixed astrocytes and microglial cells, zaprinast (0.1 mM), an inhibitor of cGMP-selective phosphodiesterases, enhanced the basal and LPS (1.0 mug/ml)-induced secretion of TNF-alpha and IL-1beta. Zaprinast also enhanced NO production induced by LPS or IFN-gamma (100 U/ml), and in microglial cell cultures, but not in astrocyte cultures, zaprinast enhanced the basal and the IFN-gamma-induced production of the cytokines, TNF-alpha, and IL-1beta, and of NO. This upregulation by zaprinast was partially inhibited by KT5823 (1.0 muM), an inhibitor of protein kinase G. The LPS-induced production of TNF-alpha, IL-1beta, and NO was inhibited by ODQ (50 muM), an inhibitor of soluble guanylyl cyclase, and by KT5823. Immunohistochemical analysis of mixed glial cell cultures showed that LPS/IFN-gamma-induced NOS expression and the enhanced expression of NOS by zaprinast were restricted to microglial cells. Zaprinast enhanced the IFN-gamma (200 U/ml)-induced expression of MHC Class II molecules in astrocytes and microglial cells in mixed cultures, but did not enhance this IFN-gamma-induced expression in pure astrocytes, which lacked paracrine TNF-alpha from microglial cells. Summarizing, zaprinast, which is associated with cGMP/protein kinase G signaling, may augment central immune/inflammatory reactions, possibly via the increased production of TNF-alpha and IL-1beta by activated microglial cells. (C) 2002 Wiley-Liss, Inc.
