Ovarian expression, cellular localization, and hormonal regulation of rat secretory phospholipase A₂: Increased expression by interleukin-1 and by gonadotropins

It has been suggested that ovulation may constitute a cyclic inflammatory-like process and that gonadotropin-inducible intraovarian interleukin (IL)-1, an established mediator of inflammation, may play a central role in this regard. In support of this hypothesis, our group has been able to document the ability of IL-1 to potently stimulate prostaglandin biosynthesis by cultured rat ovarian cells. Herein we explore the possibility that the prostaglandin-stimulating action of IL-1 is due, in part, to the enhanced expression of ovarian secretory phospholipase-A₂ (sPLA₂). A single sPLA₂ transcript of 1.4 kilobases was noted in all extraovarian tissues of immature rat origin subjected to Northern blot analysis. However, only a barely detectable signal was apparent in ovarian tissue. In contrast, the more sensitive RNase protection assay revealed the unequivocal presence of ovarian sPLA₂ transcripts. Cellular localization studies by way of in situ hybridization documented sPLA₂ transcripts primarily in the granulosa cell of the periovulatory ovary. Molecular probing of untreated cultured whole ovarian dispersates disclosed spontaneous elaboration of sPLA₂ transcripts as early as 20 h after the introduction of cells into culture. Treatment of cultured whole ovarian dispersates with IL-1β for 48 h produced a 1.7-fold increase (over the value in untreated controls) in the relative expression of sPLA₂ transcripts (p < 0.01) along with a 1.7-fold increase in media PLA₂ activity (p < 0.01). A more marked increase was documented for IL-1β- treated cultured isolated granulosa cells (12.5-fold increase, p < 0.001). Treatment of whole ovarian dispersates with an IL-1 receptor antagonist (ILl RA) produced a reduction in the basal expression of sPLA₂ transcripts (55% at the 5 μg/ml dose level; p < 0.01) and PLA₂ activity (40%; p < 0.01), thereby suggesting basal endogenous IL-1-like bioactivity. Treatment of cultured whole ovarian dispersates with either hCG or FSH led to 2.6-fold (p = 0.056) and 3-fold (p = 0.029) increases in the abundance of sPLA₂ transcripts, respectively, effects blocked by the concurrent presence of IL- 1RA. These observations 1) document the immature rat ovary as a site of sPLA₂ gene expression, 2) localize the relevant transcripts to the postovulatory granulosa cell, 3) confirm the presence of functional secreted ovarian PLA₂ activity, 4) reveal PLA₂ expression to be IL-1- and gonadotropin-dependent, and 5) suggest the existence of endogenous PLA₂- stimulating IL-1-like activity. These findings also suggest that the ability of hCG or FSH to up-regulate ovarian sPLA' transcripts may be due, in part, to the endogenous elaboration of IL-like activity.