Cooperation of NFκB and CREB to induce synergistic IL-6 expression in astrocytes

Astrocytes are critical players in the innate immune response of the central nervous system. Upon encountering proinflammatory stimuli, astrocytes produce a plethora of inflammatory mediators. Here, we have investigated how β2-adrenergic receptor activation modulates proinflammatory gene expression in astrocytes. We have observed that treatment of human 1321N1 astrocytes with the β-adrenergic agonist isoproterenol synergistically enhanced TNF-α-induced expression of the cytokine IL-6. The effect of isoproterenol was cAMP-dependent and mediated by the β2-adrenergic subtype. Using pharmacological inhibitors and siRNA we showed that protein kinase A (PKA) is an indispensable mediator of the synergy. Simultaneous induction with isoproterenol and TNF-α was moreover associated with combined recruitment of CREB and p65 to the native IL-6 promoter. The role of CREB and NFκB in promoting the synergy was corroborated using IL-6 promoter point mutants, as well as via siRNA-mediated silencing of CREB and NFκB. Interestingly, whereas CREB and NFκB usually compete for the limiting cofactor CREB binding protein (CBP), we detected enhanced recruitment of CBP at the IL-6 promoter in our system. The transcriptional synergy seems to be a gene specific process, occurring at the IL-6 and COX-2 gene, but not at other typical NFκB-dependent genes such as IL-8, ICAM-1 or VCAM-1. As astrocytic IL-6 overexpression has been associated with neuroinflammatory and neurodegenerative processes, our findings might have important physiological consequences.