Effects of forskolin on Kupffer cell production of interleukin-10 and tumor necrosis factor alpha differ from those of endogenous adenylyl cyclase activators: Possible role for adenylyl cyclase 9

Proinflammatory cytokines like tumor necrosis factor alpha (TNF-alpha) that are released from Kupffer cells may trigger liver inflammation and damage. Hence, endogenous mechanisms for limiting TNF-alpha expression are crucial for avoiding the development of sepsis. Such mechanisms include the anti-inflammatory actions of interleukin-10 (IL-10) as well as signaling induced by the intracellular second messenger cyclic AMP (cAMP). Kupffer cells express several receptors that activate cAMP synthesis, including E-prostanoid receptors and P-adrenergic receptors. The expression and role of specific adenylyl cyclases in the inhibition of Kupffer cell activation have so far not been subject to study. Pretreatment of rat Kupffer cell cultures with cAMP analogues [8-(4-chlorophenyl)-thio-cAMP], adenylyl cyclase activator (forskolin), or ligands for G-coupled receptors (isoproterenol or prostaglandin E-2) 30 min before the addition of lipopolysaccharide (LPS) (1 mu g/ml) caused attenuated TNF-a levels in culture medium (forskolin/isoproterenol, P <= 0.05; prostaglandin E-2, P <= 0.01). Forskolin also reduced IL-10 mRNA and protein (P :5 0.05), which was not observed with the other cAMP-inducing agents. Furthermore, we found that rat Kupffer cells express high levels of the forskolin-insensitive adenylyl cyclase 9 compared to whole liver and that this expression is down-regulated by LPS (P <= 0.05). We conclude that regulation of TNF-alpha and IL-10 in Kupffer cells depends on the mechanism by which cAMP is elevated. Forskolin and prostaglandin E-2 differ in their effects, which suggests a possible role of forskolin-insensitive adenylyl cyclases like adenylyl cyclase 9.