Signal transduction pathways mediating astrocyte IL-6 induction by IL-1 beta and tumor necrosis factor-alpha.

One immune function of astrocytes is IL-6 production. Synthesis of IL-6 within the central nervous system (CNS) can produce several different responses, acting on glia, neurons, and lymphocytes infiltrating brain tissue, and some of these effects are associated with CNS autoimmune disease. IL-6 gene expression in astrocytes is regulated by cytokines, infectious agents, neuropeptides, and neurotransmitters, and most of these stimuli interact synergistically. To examine the integration of these diverse factors in the control of IL-6 production, we have studied the involvement of underlying signal transduction processes using neonatal rat astrocytes. We have focused on signal transduction related to the stimulation of IL-6 gene expression by IL-1 beta and TNF-alpha. Our results indicate that stimuli related to protein kinase C (PKC), such as PMA and calcium ionophore A23187, increase IL-6 expression, whereas pharmacologic inhibitors of PKC inhibit IL-6 induction by IL-1 beta and TNF-alpha. Furthermore, both IL-1 beta and TNF-alpha stimulate PKC activity in astrocytes. Stimulators of the cAMP pathway, such as cholera toxin, forskolin, and dibutyryl cAMP, also induced astrocyte IL-6 gene expression. However, inhibition of the cAMP pathway effector, protein kinase A, did not reduce the induction of astrocyte IL-6 gene expression in response to IL-1 beta or TNF-alpha, and an ELISA for cAMP detected only very small increases in cAMP synthesis in response to these cytokines. These data suggest that although cAMP does activate astrocyte IL-6 gene expression, it is the PKC pathway that plays a primary role in the stimulation of astrocyte IL-6 gene expression by IL-1 beta and TNF-alpha.