The
insulin-like growth factor II (
IGF-II) gene is overexpressed in many mesenchymal
tumors and can lead to non-
islet-cell tumor hypoglycemia (NICTH). ProIGF-II consists of the 67 aa of
IGF-II with a carboxyl 89-aa extension, the E domain. A derivative of proIGF-II containing only the first 21 aa of the E domain [proIGF-II-(E1-21)] has been isolated by others from normal serum and has O-linked glycosylation. We found that the "big
IGF-II" of normal serum, as detected by an RIA directed against residues 1-21 of the E domain of proIGF-II, was reduced in size by treatment with
neuraminidase and O-
glycosidase. The big
IGF-II, which is greatly increased in NICTH sera, was unaffected by
neuraminidase and O-
glycosidase treatment. We have also shown that big
IGF-II from normal serum is retained by
jacalin lectin columns and that big
IGF-II from NICTH serum was not retained, indicating that it lacked O-glycosylation. Normal O-linked glycosylation may be required for proper
peptidase processing of proIGF-II. The lack of normal O-linked glycosylation by
tumors may explain the predominance of big
IGF-II in NICTH sera. In normal serum, most of the
IGF-II is present in a 150-kDa ternary complex with
IGF-II binding protein (
IGFBP) 3 and alpha subunit. In NICTH serum, however, the complexes carrying big
IGF-II are < 50 kDa. We investigated whether big
IGF-II of NICTH was responsible for this abnormality.
Tumor big
IGF-II and
IGF-II were equally effective in forming the 150-kDa complex with purified
IGFBP-3 and 125I-labeled alpha subunit. Both 125I-labeled
IGF-II and 125I-labeled proIGF-II-(E1-21), when incubated with normal serum, formed the 150-kDa complex as detected by
Superose 12 exclusion chromatography. We conclude that the nonglycosylated big
IGF-II of NICTH serum can form normal complexes with serum IGFBPs. The defective binding in NICTH is attributable to defective
IGFBP-3 binding.