Insulin-like growth factor I affects the intraovarian interleukin-1 system: Evidence for suppression of type I interleukin-1 receptor expression and enhancement of secretory phospholipase A₂ expression and activity

This study assessed the possibility that the intraovarian insulin-like growth factor I (IGF-I) system interacts with the intraovarian interleukin-1 (IL-1) system, the central role of which has been the subject of increasing attention. To this end, whole ovarian dispersates from immature rats were cultured for 48 h in the absence or presence of IGF-I or IGF-binding protein-3 (IGFBP-3), with or without IL-1β. Cellular RNA content was subjected to a solution hybridization, RNase protection assay with gel-purified [³²P]-UTP-labelled antisense riboprobes for rat IL-1β, type I IL-1 receptor (IL-1R) and secretory phospholipase A₂ (sPLA₂). PLA₂ activity in conditioned media was assayed by measuring the release of [³H]-labelled palmitic acid from the sn-2 position of [³H]-labelled phosphatidylcholine dipalmitoyl (PCDP) substrate. Treatment with IGF-I resulted in a significant (P< 0.01) decrease in type I IL-1R transcripts (an effect which was reversed by co-treatment with IL-1β), was without effect on IL-1β transcripts, and significantly (P < 0.05) increased sPLA2 gene expression (an effect which was further enhanced by co-treatment with IL-1β). Treatment with IGF-I resulted in a significant increase in extracellular PLA₂ activity over untreated control. These observations suggest that IGF-I may down-regulate ovarian IL-1 action by decreasing type I IL-1R gene expression, while up-regulating sPLA₂ gene expression and activity. These findings are consistent with a role for IGF-I in suppressing IL-1 actions while promoting the generation of prostaglandins. It is tempting to speculate that IGF-I, an intraovarian regulator concerned with promoting folliculogenesis, may be also entwined with priming the prostaglandin-producing potential in anticipation of subsequent ovulation.