The ovarian expression of the antigonadotropic insulin-like growth factor binding protein-2 is theca-interstitial cell-selective: evidence for hormonal regulation

Abstract

To begin the process of identification and charactization of rat ovarian insulin-like growth factor binding proteins, we have undertaken to explore the ovarian expression, cellular localization, and hormonal regulation of the insulin-like growth factor binding protein-2 (IGFBP-2) gene for which an antigonadotropic potential has recently been demonstrated. To this end, a solution hybridization/RNase protection assay was employed wherein total ovarian RNA (20 micrograms) from immature (21-23 days old) female rats was hybridized with a [32P]-labeled IGFBP-2 riboprobe. As in liver, a single protected fragment (550 bases long) corresponding to IGFBP-2 transcripts was identified in whole ovarian material. Cellular localization studies revealed the IGFBP-2 gene to be exclusively expressed in the theca-interstitial rather than the granulosa cell compartment. To confirm the cellular distribution of the IGFBP-2 protein, media conditioned by cultured granulosa or theca-interstitial cells were subjected to immunoprecipitation using two IGFBP-2-directed polyclonal antisera. Expectedly, both antibodies (but not non-immune rabbit serum) readily immunoprecipitated the 28 kDa rat IGFBP-2 species generated by hepatic BRL-3A cells. Similarly, both antibodies effectively immunoprecipitated an IGFBP the size of rat IGFBP-2 elaborated by theca-interstitial cells. In contrast, neither antibody immunoprecipitated the 28-29 kDa IGFBP species elaborated by granulosa cells otherwise readily apparent in conventional Western ligand blots. Hypophysectomy resulted in a 3-fold decrease (P less than 0.05) in the relative (densitometrically-quantified) abundance of ovarian IGFBP-2 transcripts, a diametrically opposed effect (P less than 0.05) being noted at the level of the liver. In contrast, treatment of immature hypophysectomized rats with a diethylstilbestrol-containing subcutaneous silastic implant for a total of 5 days resulted in a concordant 3-fold increase (P less than 0.05) in the relative abundance of IGFBP-2 transcripts in both ovary and liver when compared with untreated hypophysectomized controls. Taken together, these findings document rat ovarian IGFPB-2 gene expression to be theca-interstitial (rather than granulosa) cell-selective, and subject to upregulatory control by pituitary principle(s) and/or by estrogens. Although equally estrogen-dependent, hepatic IGFBP-2 gene expression proved constitutive in nature and subject to (diametrically opposed) inhibitory control by (potentially distinct) pituitary principle(s).