Zaprinast, an inhibitor of cGMP-selective phosphodiesterases, enhances the secretion of TNF-α and IL-1β and the expression of iNOS and MHC class II molecules in rat microglial cells

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 μg/ml)-induced secretion of TNF-α and IL-1β. Zaprinast also enhanced NO production induced by LPS or IFN-γ (100 U/ml), and in microglial cell cultures, but not in astrocyte cultures, zaprinast enhanced the basal and the IFN-γ-induced production of the cytokines, TNF-α and IL-1β, and of NO. This upregulation by zaprinast was partially inhibited by KT5823 (1.0 μM), an inhibitor of protein kinase G. The LPS-induced production of TNF-α, IL-1β, and NO was inhibited by ODQ (50 μM), an inhibitor of soluble guanylyl cyclase, and by KT5823. Immunohistochemical analysis of mixed glial cell cultures showed that LPS/IFN-γ-induced iNOS expression and the enhanced expression of iNOS by zaprinast were restricted to microglial cells. Zaprinast enhanced the IFN-γ (200 U/ml)-induced expression of MHC Class II molecules in astrocytes and microglial cells in mixed cultures, but did not enhance this IFN-γ-induced expression in pure astrocytes, which lacked paracrine TNF-α 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-α and IL-1β by activated microglial cells.