Serum "big insulin-like growth factor II" from patients with tumor hypoglycemia lacks normal E-domain O-linked glycosylation, a possible determinant of normal propeptide processing.

Abstract

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 0-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-H, was reduced in size by treatment with neuraminidase and 0-glycosidase. The big IGF-II, which is greatly increased in NICTH sera, was unaffected by neuraminidase and 0-glycosidase treatment. We have also shown that big IGF-H from normal serum is retained byjacalin lectin columns and that big IGF-H from NICTH serum was not retained, indicating that it lacked 0-glycosylation. Normal O-linked glycosylation may be required for proper peptidase processing of proIGF-H. The lack of normal 0-linked glycosylation by tumors may explain the predominance of big IGF-ll in NICTH sera. In normal serum, most of the IGF-ll is present in a 150-kDa ternary complex with IGF-U binding protein (IGFBP) 3 and a subunit. In NICTH serum, however, the complexes carrying big IGF-II are <50 kDa. We investigated whether big IGF-ll of NICTH was responsible for this abnormality. Tumor big IGF-H and IGF-HI were equally effective in forming the 150-kDa complex with purified IGFBP-3 and 12SI-labeled a subunit. Both 12SI-labeled IGF-ll and 12SI-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.Insulin-like growth factor (IGF) II is synthesized as proIGF-II, which consists of the 67 aa of IGF-II and an 89-aa carboxyl-terminal extension, the E domain (1). Processing of proIGF-II normally occurs in a stepwise fashion, and cleavage after the single lysine at position 21 ofthe E domain yields a 10.5-kDa peptide [proIGF-II-(E1-21)] that has been isolated from serum (2). A larger variant of proIGF-II-(El-21), with 0-linked glycosylation on threonine at position 8 of the E domain, has also been isolated from serum (3).Many mesenchymal and some renal, adrenal, and hepatic cell tumors have increased expression of the IGF-II gene and synthesize and secrete IGF-II peptides (4). In some patients with large tumors, the secreted IGF-II peptides can produce hypoglycemia. However, measurement of these peptides with the IGF-II RIA has given inconsistent results (5, 6). Two recent findings provided an explanation for these inconsisThe publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.tencies. The predomina...

IGF2 and cancer

“…Post-translational processing begins with cleavage of the signal peptide to yield pro-IGF2 (1-156). This is followed by O-linked glycosylation of the 89-residue E-domain that may promote further processing (Daughaday et al 1993). Pro-IGF2 then undergoes sequential proteolysis to mature IGF2 (1-67) that lacks the E-domain.…”

“…The reason for impaired proteolytic processing of pro-IGF2 is unclear but recent studies have suggested mechanisms. First, processing may fail because of the absence of glycosylation (Daughaday et al 1993). Secondly, the quantity of pro-IGF2 produced may overwhelm the proteolytic capacity of the tumour cells (Zapf 1993).…”

IGF-binding proteins are multifunctional and act via IGF-dependent and -independent mechanisms

“…Such an event does not occur for two reasons: 1) formation of 150 kDa (ALS-IGFBP-3/IGFBP-5-IGF) complexes that cannot cross the vascular endothelial cell barrier allow high concentrations of IGFs to accumulate in the blood without risk of hypoglycemia; 2) much of the IGF in the remaining 40-50 kDa complex is bound to inhibitory IGFBPs, and the IGFs in this complex are not bioavailable unless this complex is broken down (Rajaram et al 1997). The importance of IGFBPs in preventing the insulin-like effects of IGFs is evident from studies involving non-islet cell tumor hypoglycemia (NICTH), in which hypoglycemia is seen in the absence of detectable insulin because of the overproduction of partially processed biologically active IGF-II by mesenchymal tumors (Daughaday et al 1993, Zapf 1994. The state of hypoglycemia found in patients with NICTH is associated with an increase in the serum concentration of pro IGF-II, a decrease in the circulating concentration of the 150 kDa complex, and a corresponding increase in the circulating concentration of 50 kDa complex (Daughaday et al 1993, Zapf 1994, Rajaram et al 1997.…”

Role of pro-IGF-II processing by proprotein convertase 4 in human placental development

“…In addition to mature IGF-II, two ''big'' variants, IGF-II (1-87) and IGF-II (1-104), have been identified in human and bovine serum (10)(11)(12)(13). They are 11-17 kDa in size, contain 87 or 104 amino acids, respectively, and differ in glycosylation (10)(11)(12)(13)(14). It has been suggested that big IGF-II is biologically active but may have different activity in tumor cells when compared with mature IGF-II (15).…”