Binding characteristics of pro-insulin-like growth factor-II from cancer patients: binary and ternary complex formation with IGF binding proteins-1 to -6

Abstract

Many tumours secrete IGF-II in incompletely processed precursor forms. The ability of these pro-IGF-II forms to complex with the six IGF binding proteins (IGFBPs) is poorly understood. In this study, pro-IGF-II has been extracted from the serum and tumour tissue of two patients with non-islet cell tumour hypoglycaemia. These samples were used to study binary complex formation with IGFBPs-1 to -6 using competitive IGF-II binding assays and ternary complex formation with IGFBP-3 and IGFBP-5.In each case, IGFBPs-1 to -6 showed little difference in their ability to form binary complexes with recombinant IGF-II or tumour-derived pro-IGF-II forms, when the preparations were standardised according to IGF-II immunoreactivity. As previously described, ternary complex formation by acid-labile subunit (ALS) with IGFBP-3 and pro-IGF-II was greatly decreased compared with complex formation with mature IGF-II. In contrast, ALS bound similarly to IGFBP-5 in the presence of pro-IGF-II and mature IGF-II.These studies suggest that pro-IGF-II preferentially forms binary complexes with IGFBPs, and ternary complexes with IGFBP-5, rather than ternary complexes with IGFBP-3 as seen predominantly in normal serum. This may increase the tissue availability of serum pro-IGF-II, allowing its insulin-like potential to be realised.

Biochemical Characterization of Individual Human Glycosylated pro-Insulin-like Growth Factor (IGF)-II and big-IGF-II Isoforms Associated with Cancer

“…The findings presented here have also confirmed that IGFBP-2, IGFBP-3, and IGFBP-5 bind all IGF-II isoforms similarly to mature IGF-II, in agreement with Bond et al (16) but not with Elmlinger et al (17), who reported that pooled isolates of IGF-II isoforms from Ewing's sarcoma cell conditioned media appeared to retard binary complex formation with IGFBP-2 and IGFBP-3. Our approach has also confirmed early findings that IGFBP-3-based ternary complex formation by glycosylated IGF-II isoforms is severely compromised (16) and has refined these findings to show that big-IGF-II 1-87 and big-IGF-II 1-104 are the most potent at preventing its assembly.…”

“…Unglycosylated pro-IGF-II (1-156) and big-IGF-II (1-104), both produced in E. coli, were purchased from GroPep (Australia). Purified human IGFBP-2, IGFBP-3, and IGFBP-5 have been described elsewhere (16). The anti-IR mAb 83-7 and the anti-IGF-IR mAb 24-31 were produced in-house (23,24).…”

“…However, results showing that pooled IGF-II isoforms from NICTH serum or tumor tissue form binary complexes with IGFBP-1 to -6 (16) contrast with studies demonstrating pooled IGF-II isoforms from conditioned Ewing's sarcoma media not forming binary complexes with IGFBP-2 and IGFBP-3 (17). Similarly, evidence shows that glycosylated IGF-II isoforms from NICTH tumor tissue can escape ternary complex with IGFBP-3 and ALS by retarding ALS recruitment (16), but it is unknown what contribution the individual IGF-II isoforms make in this respect and what effect this has on bioavailability. Certainly, our studies in vivo have shown selective depletion of IGF-II isoforms, not mature IGF-II, from hepatocellular carcinoma tumors treated with DX-2647, an IGF-II neutralizing antibody (18).…”

Mitogenic activity of high molecular weight forms of insulin-like growth factor-II in amniotic fluid

“…Valenzano et al (1997) found no difference in the binding to IGFBP-1 and IGFBP-3 of the foetal bovine high M r weight forms of IGF-II. Binding characteristics of pro-IGF-II, extracted from tumours or serum from patients with cancer, to IGFBP-1 to -6 were established: precursor forms of IGF-II did not show any difference from mature IGF-II in their binding to IGFBPs (Bond et al 2000), except for the formation of a ternary complex with the acid labile subunit (ALS) and IGFBP-3. However, ALS is absent from AF.…”

Defective Expression of Prohormone Convertase 4 and Enhanced Expression of Insulin-like Growth Factor II by Pleural Solitary Fibrous Tumor Causing Hypoglycemia

“…In normal human serum, about 70-80% of the mature IGF-II form a ternary 150 kDa complex with IGF-BP3 and its acid-labile subunit. By contrast, the HMW form of IGF-II, which is not able to form the ternary complex, primarily forms a smaller binary 40 kDa complex with IGFBPs [10]. A binary complex can pass more readily through the capillary membrane due to its small molecular size to exert its insulin-like effects on target tissues via insulin receptors [9].…”