Interleukin-1β Regulates Phospholipase D-1 Expression in Rat Pancreatic β-Cells*

The cytokine interleukin (IL)-1beta induces a biphasic effect in rat pancreatic islets, with an early and transitory stimulation of insulin release followed by progressive functional suppression. To clarify the mechanisms involved in these effects, we have recently performed a differential display of messenger RNA (mRNA) by RT-PCR (DDRT-PCR) on rat beta-cells exposed for 6 or 24 h to IL-1beta. Among the different IL-1beta-induced genes, there was an early and transient increase in phospholipase D-1 (PLD1) expression. PLD1 can induce phosphatidic acid formation and subsequent activation of protein kinase C, a process which stimulates insulin release. In the present study, we characterized the regulation of PLD isoforms by IL-1beta in pancreatic beta-cells. By using different combinations of primers and RT-PCR, we observed that IL-1beta induces an early increase (2 and 6 h) in the expression of both alternatively spliced isoforms of PLD1 (PLD1alpha and 1b). Prolonged exposure to IL-1beta (12 and 24 h) caused a decrease of PLD1a mRNA expression compared with control beta-cells, and lead to a return of PLD1b mRNA to basal level. NG-methyl-L-arginine (LMA), a blocker of the inducible form of nitric oxide synthase (iNOS), prevented this late inhibitory effect of IL-1beta, suggesting that IL-1beta-induced decrease in PLD1a expression is NO-mediated. IL-1beta induced an early (2-6 h) and sustained (16-24 h) increase in PLD1a mRNA expression in insulin-producing RINm5F cells. This was paralleled by a cytokine-induced increase in PLD1 protein expression and enzyme activity. RINm5F cells, but not primary beta-cells, expressed PLD2, and the expression of this gene was not affected by IL-1beta. In conclusion, we have shown that the cytokine IL-1beta regulates PLD1 expression in primary and clonal beta-cells. The early induction of PLD1 probably contributes to the early stimulatory effects of IL-1beta on islet insulin release.