Ovarian expression, cellular localization, and hormonal regulation of rat secretory phospholipase A(2): 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-A(2) (sPLA(2)). A single sPLA(2) 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 sPLA, transcripts. Cellular localization studies by way of in situ hybridization documented sPLA(2) transcripts primarily in the granulosa cell of the periovulatory ovary. Molecular probing of untreated cultured whole ovarian dispersates disclosed spontaneous elaboration of sPLA(2) transcripts as early as 20 h after the introduction of cells into culture. Treatment of cultured whole ovarian dispersates with IL-1 beta for 48 h produced a 1.7-fold increase (over the value in untreated controls) in the relative expression of sPLA(2) transcripts (p < 0.01) along with a 1.7-fold increase in media PLA(2) activity (p < 0.01). A more marked increase was documented for IL-1 beta-treated cultured isolated granulosa cells (12.5-fold increase, p < 0.001). Treatment of whole ovarian dispersates with an IL-1 receptor antagonist (IL-1RA) produced a reduction in the basal expression of sPLA(2) transcripts (55% at the 5 mu g/ml dose level; p < 0.01) and PLA(2) 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 sPLA(2) transcripts, respectively, effects blocked by the concurrent presence of IL-1RA. These observations 1) document the immature rat ovary as a site of sPLA(2) gene expression, 2) localize the relevant transcripts to the postovulatory granulosa cell, 3) confirm the presence of functional secreted ovarian PLA(2) activity, 4) reveal PLA(2) expression to be IL-1- and gonadotropin-dependent, and 5) suggest the existence of endogenous PLA(2)-stimulating IL-1-like activity. These findings also suggest that the ability of hCG or FSH to up-regulate ovarian sPLA(2) transcripts may be due, in part, to the endogenous elaboration of IL-like activity.