Decreased ternary complex formation and predominance of a 29 kDa IGFBP-3 fragment in human fetal serum

Insulin-like growth factor-I (IGF-I) has been proposed to be important in the endocrine control of fetal growth in humans, although serum IGF-I concentrations are 10-fold lower than during rapid pubertal growth. However, the bioavailability of IGF-I in fetal serum may be increased by changes in the specific IGF binding proteins (IGFBPs). We have recently suggested that the bioavailability of circulating IGF-I is increased in the human fetus due to the molar excess of IGF-I plus IGF-II relative to IGFBP-3 as well as the increased concentrations of IGFBP-2, which does not form a long-lived ternary complex. We have presently studied ternary complex formation between IGF, IGFBP-3, and acid labile subunit (ALS) to further assess if IGF-I bioavailability is increased in human fetal serum. In 19-35 week gestation fetal sera, a markedly decreased formation of the ternary complex was demonstrated by the general absence of IGFBP-3 (detected by Western immunoblotting) in the ~ 130-150 kDa ternary complex after neutral size chromatography. The predominant form of IGFBP-3 in fetal serum was a 29 kDa fragment, which, following deglycosylation by Endoglycosidase-F, was demonstrated to consist of a ~ 20 kDa protein core. Despite the predominance of the 29 kDa IGFBP-3 fragment, we have previously demonstrated that the IGFBP-3 protease activity is not increased in fetal serum, in contrast to pregnancy or non-insulin dependent diabetes mellitus (NIDDM) sera. The 29 kDa IGFBP-3 and the less prominent intact 38-40 kDa IGFBP-3 in fetal sera were both present in the binary complex (~ 30-50 kDa) and in the almost absent ternary complex (~ 130-150 kDa). The apparently similar IGFBP-3 fragments found in pregnancy and normal adult sera do participate in ternary complex formation. Therefore, the predominance of IGFBP-3 fragments may contribute, but do not fully explain the decreased ternary complex formation in fetal serum. The ALS levels in fetal serum were assessed by the ability of endogenous ALS to shift exogenously added ~ 30-50 kDa binary complex consisting of [¹²⁵I]-IGFBP-3, preincubated with IGF-II, to the ~ 130-150 kDa ternary complex. ALS levels were generally low, although an increase with increasing gestational age was observed. Low fetal serum ALS expression may additionally contribute to the decrease in ternary complex formation. We suggest that attenuation of ternary complex formation may be one mechanism by which the fetus increases the bioavailability of circulating IGF-I, thereby permitting endocrine regulation of fetal growth.