DIFFERENT MOLECULAR AND MESSENGER-RIBONUCLEIC-ACID FORMS OF INSULIN-LIKE GROWTH FACTOR-BINDING PROTEIN-3 IN THE PREGNANT BABOON (PAPIO-ANUBIS)

The ratio of the serum concentrations of insulin-like growth factors ( IGF) to IGF-binding protein (IGFBP)-3 is highly correlated (Baxter & M artin 1986). During pregnancy in the baboon, this ratio is perturbed; serum IGFBP-3 concentrations increase 10-fold, yet IGF-I levels are un altered and IGF-II is increased only 2-fold (Giudice et al. 1993). The aims of this study were to determine the molecular distribution of IG FBP-3 and to identify the tissue source and form(s) of IGFBP-3 during pregnancy in the baboon. Serum of non-pregnant and pregnant baboons, a nd conditioned media of decidua and placental explant cultures were ch aracterized using neutral size-exclusion chromatography in combination with Western Ligand blot. Western immunoblot, an IGFBP-3 radioimmunoa ssay (RIA) and an IGFBP-3 protease assay. Localization of immunoreacti ve IGFBP-3 was determined by immunocytochemistry, and expression of IG FBP-3 mRNA in the placental and decidual explants was examined by Nort hern blot analysis. RIA confirmed that immunoreactive IGFBP-3 is incre ased 10-fold in pregnancy serum compared with non-pregnancy serum. Siz e-exclusion chromatography combined with an IGFBP-3 RIA revealed that, unlike non-pregnancy serum where 70% of the immunoreactive IGFBP-3 el utes in the 150 kDa ternary complex, equal amounts of immunoreactive I GFBP-3 were measured in pregnancy serum in the less than or equal to 1 50 and 60 kDa IGFBP regions. Western analysis revealed that non-pregna ncy serum contained predominantly a 45-40 kDa IGFBP-3 doublet and a 28 kDa immunoreactive form of IGFBP-3, while in pregnancy serum IGFBP-3 existed as 45-40 kDa doublet, as well as 26-28 kDa and 18 kDa immunore active forms. These alternative forms of IGFBP-3 were not attributable to detectable IGFBP-3 protease activity. To identify the source of th e increased serum levels of IGFBP-3 during pregnancy, we examined expl ant culture media of baboon decidua and placenta. Size-exclusion chrom atography combined with RIA and Western analysis revealed that: (1) mo re immunoreactive IGFBP-3 was produced by decidual cultures than by pl acental explants, but less 45-40 kDa IGFBP-3 was present in decidua; ( 2) the immunoreactive forms of IGFBP-3 detected in decidua were simila r to those found in maternal serum; (3) placental explants secreted on ly 45-40 kDa IGFBP-3 in culture. IGFBP-3 was immunohistochemically loc alized to the cells of placental villi, and to the perinuclear region of the decidual cells and staining for IGFBP-3 was more intense in the decidua than in the placenta. Northern analysis of the explant cultur es revealed two IGFBP-3 mRNA transcripts of 2.4 and 1.7 kb in both dec idua and placenta which may account for the different immunoreactive f orms of IGFBP-3 detected in the baboon. However analysis of non-pregna ncy liver also revealed two IGFBP-3 transcripts of 2.4 and 1.7 kb. The se data suggest that the two transcripts are not solely pregnancy-asso ciated and levels of protein may be the reason for detection of multip le immunoreactive IGFBP-3 fragments in pregnancy.