INVOLVEMENT OF ARACHIDONIC-ACID CASCADE PATHWAYS IN INTERLEUKIN-6-STIMULATED CORTICOTROPIN-RELEASING FACTOR RELEASE INVITRO

We have demonstrated that centrally administered interleukin-6 (IL-6) stimulates adrenocorticotropin (ACTH) secretion by a direct effect on corticotropin-releasing factor (CRF) release from the hypothalamus. Since metabolites of the arachidonic acid cascade (AAC) have been implicated in mediating actions of cytokines in different tissues and some AAC inhibitors were able to block pyrogenic effects of cytokines and suppress IL-1-induced ACTH secretion, we decided to examine the mechanism of IL-6 action on CRF release in vitro. After a 60-min preincubation in Krebs-Ringer bicarbonate buffer, medial basal hypothalami (MBH) were preincubated for 30 min with dexamethasone (DEX), a phospholipase A2 (PLA2) inhibitor, to block arachidonic acid (AA) formation, or with inhibitors of AA metabolism: a cyclooxygenase inhibitor - indomethacin (IND); a lipoxygenase inhibitor - 5,8, 11-eicosatriynoic acid (ETI), and an epoxygenase inhibitor - clotrimazole (CLO). Then, the medium was discarded and MBH were incubated with medium or the above compounds and/or IL-6 for 30 min, and CRF release into the incubation medium was measured by radioimmunoassay. As reported previously, 10(-13) M IL-6 increased CRF release, which was significantly suppressed by DEX in a dose-dependent manner. The suppression was already highly significant at a concentration of 10(-11) M DEX and became maximal at 10(-7) M, at which concentration CRF release was no longer stimulated by IL-6. The response to IL-6 was completely blocked at the highest DEX concentration evaluated (10(-5) M). CLO also suppressed IL-6-induced CRF release with a minimal effective dose of 10(-9) M. Suppression was complete at 10(-7) and 10(-5) M. IL-6-stimulated CRF release was suppressed significantly only by higher than micromolar concentrations of IND and ETI. The results suggest that IL-6 stimulates CRF release by the activation of the AAC system and show that PLA2 and epoxygenase inhibitors are far more effective than cyclooxygenase and lipoxygenase inhibitors to block IL-6-stimulated CRF release in vitro. The results suggest an important role for the PLA2 pathway and the epoxygenase pathway in IL-6-induced CRF and ACTH release.