Insulin-like growth factor binding proteins in sera of pregnant nonhuman primates

Abstract

Insulin-like growth factors (IGFs) are mitogenic peptides that are important for fetal and maternal tissue growth during pregnancy. They circulate complexed primarily with a serum binding protein, IGFBP-3, which regulates the availability of the IGFs to their target tissues. We previously reported that in pregnant women, serum IGFBP-3 levels, assessed by Western ligand blotting, decline markedly beginning at 6 weeks gestation due to a circulating protease that cleaves IGFBP-3 into a 29-kilodalton (kDa) protein and lower mol wt (M(r)) fragments. In the current study, we compared IGFBP profiles, IGFBP-3 and IGFBP-1 levels, and IGFBP protease activities in sera from pregnant and nonpregnant women, baboons, and rhesus monkeys, using Western ligand blotting, IGFBP-specific immunoassays, IGFBP-3 protease assay, and zymographic gel electrophoresis. Serum IGFBP profiles in nonpregnant human and nonhuman primates were similar and were not cycle-dependent. IGFBP-3 (37-43 kDa), IGFBP-2 (31 kDa), and IGFBP-1 (28 kDa) were identified in all three species using IGFBP-specific human antisera. A 24-kDa IGFBP was also present and is believed to be IGFBP-4. Serum IGFBP-1 levels increased throughout gestation in human and nonhuman primates. Serum IGFBP-2 and putative IGFBP-4 were barely detectable in all three species from midgestation to term, but increased several days postpartum. In contrast, serum IGFBP-3 profiles differed markedly between species during gestation. Rather than the decrease seen in human pregnancy serum, there was an increase in circulating IGFBP-3 levels in nonhuman primates. Furthermore, for both baboon and rhesus monkey, the M(r) of serum IGFBP-3 was about 2 kDa greater in pregnant than in nonpregnant animals, and deglycosylation studies suggested that the higher M(r) forms may be alternatively glycosylated or may have a unique primary structure. As in nonpregnant women, serum IGFBP-3 protease activity was absent in nonpregnant and pregnant baboons. However, rhesus monkey serum contained a calcium-dependent protease that cleaved recombinant human IGFBP-3 into unique fragments, compared to the human pregnancy enzyme. Unlike human pregnancy serum, which proteolyzes IGFBP-3, in human nonpregnancy serum, rhesus serum incubated under similar conditions did not result in proteolysis of rhesus IGFBP-3, suggesting that the IGFBP-3 protease in human pregnancy serum is not present in the circulation of the rhesus monkey. To assess proteolytic activity in these sera, zymographic polyacrylamide gel analysis, using gelatin as a substrate, was performed. A minor band of proteolytic activity (72 kDa) was observed in all three species throughout gestation.(ABSTRACT TRUNCATED AT 400 WORDS)

Proteolytic Activity Degrading Insulin-like Growth Factor-Binding Protein−2, −3, −4, and −5 in Healthy Growing and Atretic Follicles in the Pig Ovary1

“…These gelatinases were also present in ovine follicular fluid (not shown) and human pregnant serum (Fig. 6, lane h), as previously described [22,31]. Two minor gelatinases were also observed at about 70 and 72 kDa in porcine follicular fluid, their identity being unknown.…”

Preeclampsia Is Associated with Low Circulating Levels of Insulin-Like Growth Factor I and 1,25-Dihydroxyvitamin D in Maternal and Umbilical Cord Compartments*

“…The increase in serum IGF-I levels during pregnancy (3,46) may not be associated with an elevation in pituitary GH production, as circulating levels of this hormone do not increase during this physiological stage (47). During pregnancy, serum IGFBP-3 concentrations decreased markedly, and this decrease has been attributed to an endogenous pregnancy-related serum IGFBP-3 proteolytic activity (33,42,46,49). In addition, the finding in this study of similar concentrations of IGFBP-3 in both NT and PE groups may indicate the low bioavailability of IGF-I during preeclampsia.…”

Maternal‐placental insulin‐like growth factor (IGF) signaling and its importance to normal embryo‐fetal development

“…There are some reported differences in the regulation and proteolysis of IGFBPs (particularly IGFBP3) between humans and non-human primates (Giudice et al, 1993), but in general the function of IGF signaling between humans and non-human primates appears similar. Interestingly, PAPP-A has been localized to the syncytiotrophoblasts (Bonno et al, 1994), which is consistent with PAPP-A-induced proteolysis of IGFBP releasing bound IGFs at the placental barrier to bind IGF-1R for subsequent signaling.…”