The serum insulin-like growth factor-II/mannose-6-phosphate receptor in normal and diabetic pregnancy

Abstract

The extracellular domain of the insulin-like growth factor-II/mannose-6-phosphate (IGF-II/Man-6-P) receptor is present in the circulation of several species including man. The purpose of the present study was to establish whether this truncated receptor is present in higher concentrations in fetal sera compared with adult sera and whether the metabolic status of the individual alters serum concentrations of this protein. Nondiabetic and diabetic pregnant women were studied throughout gestation, and at term fetal cord sera were obtained. Levels of IGF-I increased throughout pregnancy in normal and diabetic women. IGF-II levels significantly increased during the third trimester in both groups and levels of IGF-I and IGF-II were significantly elevated in fetal cord samples from diabetic women only. Serum samples were gel-filtered on Sephadex G-200, and column fractions were assayed for binding of radiolabeled IGF-II and IGF-I. There was specific binding (SB) of IGF-II in the void volume fractions in all samplex examined. Normal women had 3% ± 0.5% SB, whereas in cord sera SB was 5% ± 0.7% and in pregnant sera 10% ± 2%. There was no difference in SB in fetal cord or pregnant samples from normal and diabetic women. In addition, there was a peak of binding activity of both IGF-I and -II in γ-globulin and postalbumin fractions of the columns in pregnant and nonpregnant women, but only in postalbumin fractions in fetal cord samples. Western blotting using a specific antiserum (no. 3637) against rat IGF-II/Man-6-P receptor was performed on aliquots of Sephadex G-200 void volume pools from all groups; a band of approximately 210 kd (without dithiothreitol) was seen. The band appeared to be more intense in pregnant women and to be increased in both the fetal cord samples and maternal samples from diabetic women. Based on binding and Western blotting, fetal cord samples had more receptor than nonpregnant normal women. When fetal cord, maternal, and adult nonpregnant sera Sephadex G-200 pools were assayed for IGF-II content by radioreceptor assay (RRA) after gel filtration in acid, only 1% to 2% of circulating IGF-II was associated with the truncated IGF-II/Man-6-P receptor in all cases. These data demonstrate that the extracellular domain (ecto-domain) of the IGF-II/Man-6-P receptor circulates in fetal cord serum and is elevated compared with that in normal adult women. Maternal concentration of the receptor is also increased in pregnancy. The ecto-domain of the IGF-II/Man-6-P receptor is not a significant carrier of the peptide and may reflect the increased levels of the peptide during pregnancy.