Lysophosphatidylcholine stimulates interleukin-6 release from rat anterior pituitary cells in vitro

Interleukin-6 (IL-6) is a B-cell differentiation-inducing cytokine that affects the secretion of several neuroendocrine hormones. Normal rat anterior pituitary (AP) cells synthesize and release IL-6, suggesting a paracrine role for the stimulation of AP hormone release by this cytokine. We have previously reported that IL-1 beta enhances IL-6 release and phospholipase A2 (PLA2)-mediated hydrolysis of phosphatidylcholine (PC) in AP cells. Because lysophosphatidylcholine (LPC) may function as a second messenger for IL-1 beta, we have investigated the effects of exogenous LPC on IL-6 release from AP cells in vitro. AP cells from male Long-Evans rats were dispersed and cultured for 5-6 days in 96-well (100,000 cells/well) culture plates. Cells were rinsed and incubated in the absence or presence of 1.25-40 microM LPC 18:0 (stearoyl) for 6 h, and IL-6 concentrations determined using the 7-TD1 cell bioassay. LPC 18:0 significantly (P < 0.01) stimulated IL-6 release up to 10-fold in a concentration-related manner. In contrast, LPC 18:0 did not affect PRL release. LPC species substituted with progressively shorter saturated 1-acyl chains (16:0-10:0) were less effective for IL-6 induction. Examination of structurally related glycerophospholipid species revealed the specificity of the LPC stimulation of IL-6 release. Thus, 1.25-40 microM lysophosphatidylethanolamine (LPE; 18:0) and lysophosphatidic acid (LPA; 18:0) were without significant effect on AP IL-6 release, demonstrating the specific functional requirement for the phosphorylcholine headgroup. Hydrolysis of the structurally related choline-linked phospholipid sphingomyelin (SM) has been implicated in IL-1 beta action in certain cell types. Similarly, 1.25-20 microM lysosphingomyelin (sphingosylphosphorylcholine; SPC) also significantly (P < or = 0.01) stimulated IL-6 release from AP cells, although SPC exhibited discernibly lower potency and efficacy than LPC. An acyl analog of platelet-activation factor (PAF), i.e. 18:0-2:0 PC (1-stearoyl-2-acetoyl-sn-glycero-3-phosphorylcholine), differs from LPC by an acetyl group in the sn-2 position; PAF was at least as effective as LPC for the stimulation of IL-6 release from AP cells in vitro. Stimulation of IL-6 release by LPC 18:0 was completely suppressed by pharmacological inhibitors of protein kinase C such as H7 (20 microM) and chelerythrine (5 microM). In addition, H7 (20 microM) abolished the stimulation of IL-6 release by IL-1 beta (0.16-100 ng/mL). These findings demonstrate that LPC, acyl PAF, or SPC (but not other lysophospholipids) stimulate IL-6 release from AP cells in vitro. We conclude that LPC-mediated activation of protein kinase C is involved in the stimulatory actions of IL-1 beta in AP cells.