Use of site-specific antibodies to characterize the circulating form of big insulin-like growth factor II in patients with hepatitis C-associated osteosclerosis

Hepatitis C-associated osteosclerosis (HCAO) is a rare syndrome of adult-onset osteosclerosis. An understanding of the factor(s) leading to the stimulation of bone formation in these patients may provide novel anabolic approaches for the treatment of osteoporosis. We have demonstrated that HCAO patients have a specific increase in circulating big IGF-II (IGF-IIE) and IGF-binding protein-2 (IGFBP-2) levels, and that IGF-IIE and IGFBP-2 circulate together in a bioavailable, 50-kDa complex. Patients with nonislet cell tumor hypoglycemia (NICTH) also have increased circulating IGF-IIE and IGFBP-2 levels. However, HCAO patients do not exhibit hypoglycemia, nor do NICTH patients exhibit obvious osteosclerosis. Thus, to better understand the reason(s) for the differing clinical manifestations of the IGF-IIE excess in the two syndromes, we characterized IGF-IIE in HCAO and NICTH sera using recently developed antibodies (Ab) recognizing either the full-length IGF-IIE 89-amino acid C-terminal extension peptide (IIE_138-156 Ab) or specific cleavage forms of IGF-IIE (IIE_78-88 Ab and IIE_89-101 Ab). The predominant IGF-IIE form in HCAO serum migrated on SDS-PAGE as a single band at approximately 18 kDa that reacted with the IIE_89-101 Ab. On the other hand, the predominant form in NICTH serum migrated as a doublet of 14 and 16 kDa that reacted with the IIE_78-88 Ab. There results are consistent with differential processing of the IGF-IIE precursor at predicted cleavage sites producing IGF-IIE_1-104 and IGF-IIE_1-88 in HCAO and NICTH, respectively. As these two forms may have differing biological activities and/or targeting properties, our findings may explain at least in part the different manifestations of IGF-IIE overproduction in the two syndromes.