Interferon‐γ and lipopolysaccharide reduce cAMP responses in cultured glial cells: Reversal by a type IV phosphodiesterase inhibitor

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Abstract
The aim of the present study was to determine whether two classical macrophage activators, bacterial lipopolysaccharide (LPS) and interferon-γ (IFN-γ) could affect the accumulation of the second messenger cAMP in cultured rat microglia and astrocytes. Purified microglia and astrocyte secondary cultures obtained from the neonatal rat were grown for 3 days in basal medium Eagle (BME) + 10% fetal calf serum (FCS). Exposure of microglia to LPS resulted into a dose- and time-dependent decrease in the accumulation of cAMP induced by receptor-mediated (isoproterenol or prostaglandin E2) or direct (forskolin) activation of adenylate cyclase. The inhibitory effect of LPS was rapid (a 10 min preincubation was sufficient to approach a maximal effect), occurred at low doses (IC50 = 1.2 ng/ml), and was not abrogated by pertussis toxin. A selective inhibitor of type IV phosphodiesterase (rolipram, 100 nM) prevented the effect of LPS on cAMP accumulation, while inhibitors of other forms of phosphodiesterase were unable to do so. IFN-γ (100 u/ml) also caused a depression of the evoked cAMP accumulation in microglia after a 10 min preincubation, and its effect was prevented by rolipram, as in the case of LPS. Astrocytes differed from microglia in that LPS (1–100 ng/ml) did not inhibit the accumulation of cAMP induced by either isoproterenol or forskolin; on the other hand, IFN-γ did have an inhibitory effect (though less pronounced than in microglia) that could be prevented by rolipram. Our observations indicate that two potent activators of microglia acting at different receptors, LPS and IFN-γ, can diminish the accumulation of cAMP through a common mechanism, the stimulation of a specific form of cAMP phosphodiesterase. The fact that IFN-γ, but not LPS, was effective in astrocytes suggests that LPS receptors are scarcely, if at all, expressed in these cells, or that they are differently coupled to second messengers. Selective inhibitors of type IV phosphodiesterase might prevent some of the obnoxious actions of LPS or IFN-γ in the living organism.