Binding sites and binding properties of binary and ternary complexes of insulin-like growth factor-II (IGF-II), IGF-binding protein-3, and acid-labile subunit

We have examined regions of rat IGF-binding protein-3 (IGFBP-3) important for complex formations using two kinds of deletion mutants, three kinds of chimera molecules between rat IGFBP-3 and rat IGFBP-2, and a synthetic peptide (41 residues, Glu52-Ala92) derived from rat IGFBP-3. Solid-phase binding assays using 96-well microtiter plates were designed to quantitate the relative binding affinities. It was found that not only the IGFBP-3 derivatives with the amino-terminal, cysteine-rich domain (N domain) but also the synthetic peptide maintained affinity for IGF-II. Ternary complex formation was observed with full-length IGFBP-3 and chimera IGFBP, the carboxyl-terminal cysteine-rich domain (C domain) of which was derived from IGFBP-3, unlike the mutants lacking the C domain and the chimera IGFBPs, the C domain of which was derived from IGFBP-2. These results were confirmed by affinity cross-linking experiments. Furthermore, the IGFBP-3 derivatives that possessed the C domain of IGFBP-3 bound to the acid-labile subunit, even in the absence of IGFs. Finally, we observed sites in IGF-II important for the ternary complex formation using various IGF-II mutants. These IGF-II mutants, which contained a substitution of Tyr27 for Leu, had extremely reduced activity. These results strongly suggest that: 1) the N domain, containing at least Glu52-Ala92, of rat IGFBP-3 is important for binding to IGF-II; 2) the C domain of IGFBP-3 is essential for binding to the acid-labile subunit both in the presence and absence of IGF-II; and 3) Tyr27 of IGF-II is important for the ternary complex formation.

N-terminal deletion mutants of insulin-like growth factor-II (IGF-II) show Thr7 and Leu8 important for binding to insulin and IGF-I receptors and Leu8 critical for all IGF-II functions

To define the role of the N-terminal region of insulin-like growth factor-II (IGF-II) in its binding to insulin and IGF receptors, deletion mutants des-(1-5)-, des-(1-7)-, and des-(1-8)-recombinant (r) IGF-II, and the Gly8 for Leu substitution mutant of rIGF-II were prepared by site-directed mutagenesis, expressed in Escherichia coli, and purified. The binding affinity and mitogenic activity of these rIGF-II mutants as well as commercially available des-(1-6)-rIGF-II were analyzed. While the relative affinity of des-(1-5)- and des-(1-6)-rIGF-II for purified human insulin and IGF-I receptors remained at > or = 50% levels of that of rIGF-II, the affinity of des-(1-7)-rIGF-II decreased to approximately 10% and approximately 3%, respectively, of that of rIGF-II. When the octapeptide including Leu8 was removed prior to the Cys9-Cys47 intrachain bond, the relative affinity of this deletion mutant, des-(1-8)-rIGF-II, for these receptors dramatically decreased to < 1% of that of rIGF-II. Substituting Gly8 for Leu in rIGF-II decreased the affinity of this mutant for the IGF-I and insulin receptors to about the same extent. These results suggest that the side chains of Thr7 and Leu8 may play an important role in retaining all of the IGF-II functions. Decreases in the relative affinity for binding of the mutants to these receptors paralleled the decreases in their mitogenic potency for cultured Balb/c 3T3 cells. Although the relative affinity of des-(1-8)- or [Gly8]rIGF-II for rat IGF-II/CIM6-P (cation-independent mannose 6-phosphate) receptors was also < 1% of that of rIGF-II, the relative affinities of des-(1-5)-, des-(1-6)-, and des-(1-7)-rIGF-II for these receptors was significantly greater than that of rIGF-II. These results clearly demonstrate that Thr7 and Leu8 are important for binding to insulin and IGF-I receptors and Leu8 is critical for expression of all IGF-II functions.

Solution structure of human insulin-like growth factor II; recognition sites for receptors and binding proteins

The three-dimensional structure of human insulin-like growth factor II was determined at high resolution in aqueous solution by NMR and simulated annealing based calculations. The structure is quite similar to those of insulin and insulin-like growth factor I, which consists of an alpha-helix followed by a turn and a strand in the B-region and two antiparallel alpha-helices in the A-region. However, the regions of Ala1-Glu6, Pro31-Arg40 and Thr62-Glu67 are not well-defined for lack of distance constraints, possibly due to motional flexibility. Based on the resultant structure and the results of structure-activity relationships, we propose the interaction sites of insulin-like growth factor II with the type 2 insulin-like growth factor receptor and the insulin-like growth factor binding proteins. These sites partially overlap with each other at the opposite side of the putative binding surface to the insulin receptor and the type 1 insulin-like growth factor receptor. We also discuss the interaction modes of insulin-like growth factor II with the insulin receptor and the type 1 insulin-like growth factor receptor.

Characterization of new monoclonal antibodies to human insulin-like growth factor-II and their application in western immunoblot analysis

Four immunoglobulin G1 class monoclonal antibodies (mAbs; 1D5, 1D9, 2B11, and 2H11) were produced against recombinant human insulin-like growth factor-II (rhIGF-II). Enzyme-linked immunosorbent assay established that these four mAbs specifically recognized rhIGF-II and hIGF-II, but not rhIGF-I. mAbs 1D5, 1D9, and 2H11 did not cross-react with mouse rIGF-II, although there are only six amino acid differences between mouse IGF-II and human IGF-II. The epitope for each mAb was partially defined by enzyme-linked immunosorbent assay using mouse-human chimera IGF-II mutants and other IGF-II mutants that were prepared by site-directed mutagenic procedures. These results indicated that the epitopes of mAbs 1D5, 1D9, and 2H11 are in the C-domain of hIGF-II around Ala32 to Ser36, and that of 2B11 is in the carboxyl-terminal end of the B- and A-domains of hIGF-II. A specific and sensitive RIA was developed using mAb 2H11. In this RIA, IGF-II variant ([RLPG/S29]IGF-II) and rhIGF-II competed equally with [125I]IGF-II for binding to mAb 2H11. Similar results were produced when mAbs 1D5, 1D9, and 2B11 substituted for 2H11. The potential usefulness of mAb 2H11 in an immunoblot procedure to characterize the heterogeneity of IGF-II in the sera and tumor tissues of patients with nonislet cell tumor hypoglycemia was evaluated. A procedure that combined acid-ethanol extraction of serum or tumor tissues and immunoaffinity concentration of the extracted IGF-II with mAb 2H11-immobilized resin was found to be an effective way to prepare the samples. In Western immunoblots, a quantity of rhIGF-II as low as 3 ng could be identified, whereas 200 ng rhIGF-I or rat IGF-II were not recognized. The levels of IGF-II in the sera of 12 patients with nonislet cell tumor hypoglycemia varied from normal to about twice normal. The mol wt (M(r)) of this IGF-II was between 10-17K. There was little of the processed 7.5K M(r) IGF-II in the sera of these patients. Finally, the source of the high M(r) forms of IGF-II was the tumor, because the ratios of high M(r) forms of IGF-II to 7.5K IGF-II changed dramatically from 99:1 and 91:9 to 4:96 and 32:68 in two patients after successful excision of their tumors.(ABSTRACT TRUNCATED AT 250 WORDS)

The design, expression, and characterization of human insulin-like growth factor II (IGF-II) mutants specific for either the IGF-II/cation-independent mannose 6-phosphate receptor or IGF-I receptor

Five mutants of recombinant insulin-like growth factor-II (rIGF-II) that bound with high affinity to either the IGF-II/cation-independent mannose 6-phosphate (IGF-II/CIM6-P) or the IGF-I receptor were prepared by site-directed mutagenic procedures, expressed as fusion proteins in the larva of Bombyx mori or Escherichia coli, purified to homogeneity, renatured, and characterized in terms of their receptor binding affinities and specificities as well as their biological activities. Class I mutants in which Phe26, Tyr27, and Val43 were substituted with Ser, Leu, and Leu, respectively, bound to enriched preparations of rat placental IGF-II/CIM6-P receptors with apparent equilibrium dissociation constants (Kd(app)) that were only slightly greater, i.e. 0.10, 0.05, and 0.06 nM, than that of rIGF-II (0.04 nM) or hIGF-II (0.03 nM). In contrast, replacing Phe26 with Ser resulted in 5- and 20-fold decreases in the affinities of this mutant for highly purified human placental IGF-I and insulin receptors, respectively. The affinities of the two other Class I mutants, [Leu27]- and [Leu43]rIGF-IIs, for these two receptors were reduced 80- to 220-fold. The affinities of Class II mutants, i.e. [Thr48,Ser49,Ile50]- and [Arg54,Arg55] rIGF-IIs, for IGF-I receptors were as potent as rIGF-II; however, they bound very poorly or not at all to the IGF-II/CIM6-P receptor. In the binding study of those mutant rIGF-IIs, IGF-II was observed to have an unexpectedly high affinity for pure human placental insulin receptor preparations. For example, the affinities of hIGF-II, rIGF-II, and two Class II rIGF-II mutants for the insulin receptor were only 3-, 9-, and 5-fold less, respectively, than that of porcine insulin. In two biological assay systems, i.e. the stimulation of DNA synthesis in Balb/c 3T3 cells and glycogen synthesis in HepG2 cells, the Kd(app) of the rIGF-II mutants for the IGF-I receptor but not the IGF-II/CIM6-P receptor correlated with their abilities to produce biological responses.

High-level expression and characterization of hepatitis B virus surface antigen in silkworm using a baculovirus vector

The Bombyx mori nuclear polyhedrosis virus (BmNPV) in the silkworm was used successfully for mass production of biologically active foreign genes under the control of the polyhedrin promoter. This system was adapted for the production of large amounts of hepatitis B virus surface antigens (HBsAg). The DNA fragments coded for the middle protein, which is composed of the S protein with the pre-S2 region, were cloned, the signal protein gene of beta-IFN was added, and both were inserted into a cloning vector. After co-transfection with wild-type BmNPV, stable recombinant viruses were isolated by the limiting dilution method. Infected silkworm larvae with the recombinants expressed HBsAg at high levels (400-600 micrograms/ml). These products, consisting of two polypeptides with molecular weights of approximately 25,000 (p25) and glycosylated P25 (GP28), were purified as assembled 22-nm particles. We demonstrated that HBsAg from silkworms consists of S protein with 7 amino acids of Pre-S2.