Porcine ovarian cells express messenger ribonucleic acids for the acid-labile subunit and insulin-like growth factor binding protein-3 during follicular and luteal phases of the estrous cycle.

Abstract

It has recently been shown that, in follicular fluid, as in the circulation, insulin-like growth factors (IGFs)-I and -II exist in a ternary complex with IGF binding protein-3 (IGFBP-3) and the acid-labile subunit (ALS). The current study was designed to determine whether ovarian follicular and luteal cells could synthesize IGFBP-3 and ALS. Ovaries were collected, during the follicular and early luteal phases, from mature pigs whose cycles were synchronized with PGF2alpha. We studied IGFBP-3 and ALS messenger RNA (mRNA) by in situ hybridization. These transcripts were colocalized with aromatase mRNA, a marker of healthy granulosa cells. IGFBP-3 mRNA was equally expressed in granulosa cells of all growing follicles. In contrast, granulosa cell ALS mRNA levels were higher (P < 0.05) in preantral and small antral follicles than in large antral follicles. In thecal cells, expression of mRNA for IGFBP-3, ALS and cyclin D1 (a marker of cell proliferation) was restricted to healthy (aromatase-expressing) follicles. In those follicles, thecal expression of IGFBP-3 mRNA was low or absent in preantral follicles but increased (P < 0.05) in antral follicles. Thecal cell ALS transcripts peaked in small antral follicles (P < 0.05) and then declined. In granulosa cells of atretic follicles, transcripts for aromatase were greatly reduced, whereas IGFBP-3 mRNA levels remained high. In contrast, ALS transcript levels were greatly reduced in both granulosa (P < 0.05) and thecal cells (P < 0.001) of atretic follicles. After ovulation, IGFBP-3 mRNA was moderately expressed in granulosa luteins but strongly detected in a few theca-derived cells and in vascular endothelial cells. This study demonstrates that follicular fluid IGFBP-3 and ALS, like the IGFs, originate (at least in part) from the ovary. The ability of follicular cells to synthesize, assemble, and store all components of the ternary complex may be critical in determining the bioavailability of follicular IGF-I and -II.