Interleukin-1β and catecholamines synergistically stimulate interleukin-6 release from rat C6 glioma cells in vitro:: a potential role for lysophosphatidylcholine

Interleukin-1 beta (IL-1 beta) and interleukin-6 (IL-6) are proinflammatory cytokines that affect the secretion of several neuroendocrine hormones. In addition, glial cells synthesize and release IL-6, suggesting a paracrine role for this cytokine in the brain. We have examined the regulation of IL-6 release from glial cells by cytokines and catecholamines. Forty ng/ml IL-1 beta induced a maximal 30-fold stimulation of IL-6 release (P < 0.01); higher and lower concentrations of IL-1 beta were less effective. In the presence of (Bu)(2)cAMP, IL-1 beta induced a strongly synergistic response with respect to IL-6 release; thus, the combination of these two agents resulted in a release of IL-6 that was much larger that the release attributed to either agent alone (i.e. 30-fold higher). Similarly, the combination of IL-1 beta and the diterpene forskolin (but not the inactive analog 1,9-dideoxyforskolin) or cholera toxin also resulted in a synergistic stimulation of C6 glioma IL-6 release. Thus. increases in intracellular cAMP concentrations act in a synergistic fashion with the IL-1 beta signaling pathway for IL-6 release. Because catecholamines increase intracellular cAMP levels, we investigated the effects of dopamine, epinephrine, and norepinephrine on IL-6 release. The combination of 1.0 to 100 mu M of each catecholamine with IL-1 beta resulted in the synergistic stimulation of IL-6 release. The coincubation of the beta-agonist isoproterenol and IL-1 beta resulted in a striking 25-fold synergistic induction of IL-6 release. The synergistic increases in IL-6 release caused by IL-1 beta and isoproterenol as well as IL-1 beta and norepinephrine were blocked by the pretreatment of C6 cells with the beta-receptor antagonist propranolol. Because lysophosphatidylcholine (LPC) may function as a second messenger for IL-1 beta, we also investigated the effects of LPC. Exogenous LPC (5 to 40 mu M) Stimulated IL-6 release from C6 glioma cells in a concentration-related manner (P < 0.01), The coincubation of LPC with norepinephrine provoked a synergistic release in IL-6 comparable with that obtained with IL-1 beta and norepinephrine. Exposure of [H-3] choline-labeled C6 cells to IL-1 beta resulted in an increase in the [H-3]LPC species as well as a decrease in [H-3] phosphatidylcholine. Finally, while TNF alpha was less efficacious than IL-1 beta for the stimulation of IL-6 release from C6 cells. the combination of IL-1 beta and TNF alpha resulted in a significant synergistic induction of IL-6 release. We have demonstrated that IL-1 beta stimulates IL-6 release from rat C6 glioma cells via a noncAMP-mediated mechanism that may involve LPC. The synergistic induction by cytokines and catecholamines of glial cell-derived IL-6 may subsequently affect inflammatory, neurodegenerative or neurotropic processes in the CNS.