Ovarian interleukin-1 receptor antagonist in rats: Gene expression, cellular localization, cyclic variation, and hormonal regulation of a potential determinant of interleukin-1 action

It is hypothesized that the intraovarian interleukin (IL)-1 system plays a prominent role in the regulation of follicular development and ovulation. A central component of the intraovarian IL-1 system is the IL-1 receptor ant agonist (IL-1RA), a protein acting as a pure IL-1 receptor antagonist and o ne for which intracellular (icIL-1RA) and secretory (sIL-1RA) varieties hav e been described. It was the objective of this study to explore rat ovarian IL-IRA gene expression, tea establish the identity and relative abundance of its alternative transcripts, to study its cellular localization, to dete rmine its cyclic variation, and to assess its hormonal regulation. Protecte d IL-1RA cDNA fragments corresponding to either sIL-1RA or icIL-1RA were ba rely detectable in untreated whole ovarian tissue of immature rat origin. H owever, sIL-1RA transcripts reached a maximal value (3.3-fold increase over untreated control values; p < 0.05) 12 h after hCG administration (time of projected ovulation). In situ hybridization localized IL-1RA to mural, ant ral, and cumulus granulosa cells, Modestly intense staining was also appare nt in oocytes, The basal pattern of sIL-1RA expression by cultured whole ov arian dispersates was characterized by a spontaneous increase to a peak val ue at 4 h. The early (4 h) sIL-1RA burst proved IL-1, nitric oxide-, and pr otein biosynthesis-independent. However, treatment with IL-1 beta led to a secondary sIL-1RA peak at 48 h, an effect that was substantially reversed b y IL-1RA. This stimulatory effect of IL-1 beta on IL-1RA expression proved relatively specific, and nitric oxide independent, but contingent upon de n ovo protein biosynthesis. The in vitro expression of icIL-1RA was barely de tectable, Taken together, these in vivo and in vitro observations 1) docume nt the rat ovary as a site of IL-1RA (sIL-1RA > cIL-1RA) expression, 2) loc alize the relevant transcripts to the granulosa cell, 3) disclose peak expr ession at the time of ovulation, and 4) establish IL-1 dependence.