Insulin-like growth factor binding proteins in human endometrium: steroid-dependent messenger ribonucleic acid...

The insulin-like growth factor (IGF) autocrine/paracrine system is believed to play a role in endometrial differentiation and trophoblast growth. The IGFs bind with high affinity to a family of binding proteins (IGFBPs) that regulate the actions of the IGFs at their target cells. IGFBP-1, one of three well-characterized IGFBPs in humans, has been shown by numerous investigators to be present in secretory but not proliferative endometrium; however, no information is available with regard to two other human IGFBPs, IGFBP-2 and IGFBP-3, in normal human endometrium. We have examined expression of the messenger RNAs (mRNAs) encoding IGFBP-2 and IGFBP-3 in human proliferative endometrium [under the influence of estradiol (E2)] compared to secretory endometrium (under the influence of E2 and progesterone). Using a complementary DNA probe specific for IGFBP-2, by Northern analysis, expression of a 1.4 kilobase mRNA was found to be differentially expressed in secretory compared to proliferative phase endometrium. Using a complementary DNA probe encoding IGFBP-3, a 2.4 kilobase mRNA was found in endometrium and was also found to be differentially expressed in secretory compared to proliferative phase endometrium. Endometrial stromal cells were established in culture and revealed constitutive synthesis and secretion of IGFBP-2 and IGFBP-3 into the conditioned medium (CM), as detected by Western ligand blot analysis of the CM. Identification of the IGFBPs in the CM was made using IGFBP-2 and IGFBP-3 specific antiserum. In the presence of E2 and progesterone, there was an enhancement in the amount of IGFBP-3 and a marked enhancement of IGFBP-2 in the conditioned media, suggesting sex steroid-dependence of IGFBP-2 and, to a lesser extent, IGFBP-3 protein synthesis. Western ligand blot analysis of IGFBPs in serum throughout the menstrual cycle revealed no hormonal dependence in the serum IGFBPs, suggesting local action of the IGFBPs in endometrium. These data thus show that mRNAs encoding IGFBP-2 and IGFBP-3 are expressed in human endometrium, that their expression is differentially regulated in secretory compared to proliferative phase endometrium (different steroidogenic environments), and that the synthesis of both IGFBP-2 and IGFBP-3 proteins is regulated by steroid hormones.