Interleukin-1β, but not IL-1α, mediates nerve growth factor secretion from rat astrocytes via type I IL-1 receptor

In astrocytes, nerve growth factor (NGF) synthesis and secretion is stimulated by the cytokine interleukin-1β (IL-1β). In the present study, the role of IL-1 receptor binding sites in the regulation of NGF release was evaluated by determining the pharmacological properties of astroglially localized IL-1 receptors, and, by comparing the effects of both the agonists (IL-1α and IL-1β) and the antagonist (IL-1ra)—members of the IL-1 family on NGF secretion from rat neonatal cortical astrocytes in primary culture. Using receptor-binding studies, binding of ¹²⁵I] IL-1β to cultured astrocytes was saturable and of high affinity. Mean values for the K_D and B_max were calculated to be 60.7±7.4 pM and 2.5±0.1 fmol mg^-1 protein, respectively. The binding was rapid and readily reversible. IL-1 receptor agonists IL-1α (K_i of 341.1 pM) and IL-1β (K_i 59.9 pM), as well as the antagonist IL-1ra (K_i 257.6 pM), displaced specific ¹²⁵I] IL-1β binding from cultured astrocytes in a monophasic manner. Anti-IL-1RI antibody completely blocked specific ¹²⁵I] IL-1β binding while anti-IL-1RII antibody had no inhibitory effect. Exposure of cultured astrocytes to IL-1α and IL-1β revealed the functional difference between the agonists in influencing NGF release. In contrast to IL-1β (10 U/ml), which caused a 3-fold increase in NGF secretion compared to control cells, IL-1α by itself had no stimulatory action on NGF release. The simultaneous application of IL-1α and IL-1β elicited no additive response. IL-1ra had no effect on basal NGF release but dose-dependently inhibited the stimulatory response induced by IL-1β. We concluded that IL-1β-induced NGF secretion from cultured rat cortical astrocytes is mediated by functional type I IL-1 receptors, whereas IL-1α and IL-1ra, in spite of their affinity for IL-1RI, have no effect on NGF secretion from these cells. Type II IL-1R is not present on rat neonatal cortical astrocytes.