Characterization of the three 125I-iodination isomers of human insulin-like growth factor I (IGF1)

A novel approach to identify two distinct receptor binding surfaces of insulin-like growth factor II

Very little is known about the residues important for the interaction of insulin-like growth factor II (IGF-II) with the type 1 IGF receptor (IGF-1R) and the insulin receptor (IR). Insulin, to which IGF-II is homologous, is proposed to cross-link opposite halves of the IR dimer through two receptor binding surfaces, site 1 and site 2. In the present study we have analyzed the contribution of IGF-II residues equivalent to insulin's two binding surfaces toward the interaction of IGF-II with the IGF-1R and IR. Four "site 1" and six "site 2" analogues were produced and analyzed in terms of IGF-1R and IR binding and activation. The results show that Val(43), Phe(28), and Val(14) (equivalent to site 1) are critical to IGF-1R and IR binding, whereas mutation to alanine of Gln(18) affects only IGF-1R and not IR binding. Alanine substitutions at Glu(12), Asp(15), Phe(19), Leu(53), and Glu(57) analogues resulted in significant (>2-fold) decreases in affinity for both the IGF-1R and IR. Furthermore, taking a novel approach using a monomeric, single-chain minimized IGF-1R we have defined a distinct second binding surface formed by Glu(12), Phe(19), Leu(53), and Glu(57) that potentially engages the IGF-1R at one or more of the FnIII domains.

Characterization of a second ligand binding site of the insulin receptor

Insulin binding to its receptor is characterized by high affinity, curvilinear Scatchard plots, and negative cooperativity. These properties may be the consequence of binding of insulin to two receptor binding sites. The N-terminal L1 domain and the C-terminus of the alpha subunit contain one binding site. To locate a second site, we examined the binding properties of chimeric receptors in which the L1 and L2 domains and the first Fibronectin Type III repeat of the insulin-like growth factor-I receptor were replaced by corresponding regions of the insulin receptor. Substitutions of the L2 domain and the first Fibronectin Type III repeat together with the L1 domain produced 80- and 300-fold increases in affinity for insulin. Fusion of these domains to human immunoglobulin Fc fragment produced a protein which bound insulin with a K(d) of 2.9 nM. These data strongly suggest that these domains contain an insulin binding site.

Relations between Plasma IGF-I Concentrations during Treatment with CIDR-based or Ovsynch Protocol for Timed AI and Conception in Early Postpartum Japanese Black Beef Cows

We examined the relations between plasma insulin-like growth factor (IGF) -I concentrations during treatment with CIDR-based or Ovsynch protocol for timed AI and conception and plasma steroid concentrations in early postpartum Japanese Black beef cows. Cows in the control group (Ovsynch; n = 21) underwent Ovsynch protocol (GnRH analogue on Day 0, PGF(2alpha) analogue on Day 7, and GnRH analogue on Day 9), with AI on Day 10, approximately 20 h after the second GnRH treatment. Cows in the Ovsynch+CIDR group (n = 22) received Ovsynch protocol plus a CIDR for 7 days (starting on Day 0). Cows in the further treatment group (EB+CIDR+GnRH; n = 22) received 2 mg of estradiol benzoate (EB) on Day 0 in lieu of the first GnRH treatment, followed by the same treatment as in the Ovsynch+CIDR protocol. Plasma IGF-I concentrations were determined on Days -7, 0, 7, 9 and 17. Conception rates were improved in the CIDR-combined groups (both CIDR-treated groups were combined) relative to Ovsynch group (P < 0.05) for cows with low IGF-I concentrations (<1,000 ng/ml) on Days -7, 0, and 7, but improved conception rate produced by the CIDR-based protocols did not occur in cows with a high IGF-I concentration (> or =1,000 ng/ml). Plasma estradiol-17beta concentrations increased from Day 0 to 7 (P < 0.05) and were unchanged from Day 7 to 9 in the Ovsynch group with low IGF-I concentrations on Day 0, while they were unchanged from Day 0 to 7 and increased from Day 7 to 9 (P < 0.05) in the Ovsynch group with high IGF-I concentrations on Day 0 and in the CIDR-combined group. Plasma progesterone concentrations in the Ovsynch group with low IGF-I concentrations on Day 0 were higher on Day 14 than in the Ovsynch group with high IGF-I concentrations on Day 0 and in the CIDR-combined group (P < 0.05). In conclusion, CIDR-based protocols may improve conception relative to Ovsynch in early postpartum beef cows with lower plasma IGF-I concentrations at the start of the protocols. This improvement is probably due to prevention of premature increases of estradiol-17beta and progesterone concentrations, which occurred in cows with low IGF-I concentrations treated with Ovsynch, by the CIDR treatment.

Mapping of the insulin-like growth factor II binding site of the Type I insulin-like growth factor receptor by alanine scanning mutagenesis

The Type I insulin-like growth factor receptor is a physiological receptor for insulin-like growth factor II (IGF-II). To characterize the molecular basis of the receptor's ligand binding properties, we have examined the effects of alanine mutations of residues in the ligand binding site of the receptor on its affinity for IGF-II. The functional epitope for IGF-II comprises residues in the N-terminal L1 domain and residues at the C-terminus of the alpha subunit. Cysteine rich domain residues do not appear to be critical for IGF-II binding.

Alanine scanning mutagenesis of a type 1 insulin-like growth factor receptor ligand binding site

The high resolution crystal structure of an N-terminal fragment of the IGF-I receptor, has been reported. While this fragment is itself devoid of ligand binding activity, mutational analysis has indicated that its N terminus (L1, amino acids 1-150) and the C terminus of its cysteine-rich domain (amino acids 190-300) contain ligand binding determinants. Mutational analysis also suggests that amino acids 692-702 from the C terminus of the alpha subunit are critical for ligand binding. A fusion protein, formed from these fragments, binds IGF-I with an affinity similar to that of the whole extracellular domain, suggesting that these are the minimal structural elements of the IGF-I binding site. To further characterize the binding site, we have performed structure directed and alanine-scanning mutagenesis of L1, the cysteine-rich domain and amino acids 692-702. Alanine mutants of residues in these regions were transiently expressed as secreted recombinant receptors and their affinity was determined. In L1 alanine mutants of Asp(8), Asn(11), Tyr(28), His(30), Leu(33), Leu(56), Phe(58), Arg(59), and Trp(79) produced a 2- to 10-fold decrease in affinity and alanine mutation of Phe(90) resulted in a 23-fold decrease in affinity. In the cysteine-rich domain, mutation of Arg(240), Phe(241), Glu(242), and Phe(251) produced a 2- to 10-fold decrease in affinity. In the region between amino acids 692 and 702, alanine mutation of Phe(701) produced a receptor devoid of binding activity and alanine mutations of Phe(693), Glu(693), Asn(694), Leu(696), His(697), Asn(698), and Ile(700) exhibited decreases in affinity ranging from 10- to 30-fold. With the exception of Trp(79), the disruptive mutants in L1 form a discrete epitope on the surface of the receptor. Those in the cysteine-rich domain essential for intact affinity also form a discrete epitope together with Trp(79).

The insulin-like growth factor (IGF) binding site of bovine insulin-like growth factor binding protein-2 (bIGFBP-2) probed by iodination

The insulin-like growth factor (IGF) binding site of bovine insulin-like growth factor binding protein 2 (bIGFBP-2) has been probed by chemical iodination. Tyrosyl residues of bIGFBP-2 were reacted by chloramine T-mediated iodination. The modification patterns of free bIGFBP-2 and bIGFBP-2 associated with insulin-like growth factor II (IGF-II) were compared by tryptic mapping using electrospray mass spectrometry and N-terminal sequencing. The presence of bound IGF-II resulted in protection of tyrosine at position 60 from iodination measured by the relative loss of tyrosine specific fluorescence and the incorporation of the radioisotope 125I. In addition, the pattern of iodine incorporation of bIGFBP-2 was not different whether IGF-I or IGF-II was the protective ligand. bIGFBP-2, when iodinated alone sustained a 8-fold loss of binding affinity for IGF-I and a 4-fold loss in binding affinity for IGF-II. In contrast, bIGFBP-2 iodinated while complexed with either IGF-I or IGF-II retained the same binding affinity for IGF-I or IGF-II as non-iodinated bIGFBP-2. We conclude that tyrosine 60 lies either in a region of bIGFBP-2 which directly interacts with both IGF-I and IGF-II or lies in a region of bIGFBP-2 which undergoes a conformational change that is important for IGF binding. Furthermore, iodination of tyrosine residues at positions 71, 98, 213, 226, and 269 has no detectable impact on binding of bIGFBP-2 to the IGFs.

Sustained signalling from the insulin receptor after stimulation with insulin analogues exhibiting increased mitogenic potency

The metabolic and mitogenic potencies of six different insulin analogues were determined by measuring glucose transport in primary adipocytes and DNA synthesis in CHO cells respectively. Three analogues showed a disproportionately high mitogenic potency compared with their metabolic potency, and were up to 7 times more mitogenically than metabolically potent when compared with human insulin. The mitogenic/metabolic potency ratio of the analogues was found to be inversely correlated with the insulin receptor dissociation rate constant (Kd) in an exponential fashion (r=0.99), with a disproportionately greater increase in mitogenic potential compared with metabolic potential for analogues with Kd values of less than 40% of that of human insulin. To investigate the molecular mechanisms behind the correlation between the increased half-life of the receptor-ligand complex (low Kd) and mitogenicity, 3 h time-course experiments were performed. Slow ligand dissociation from the insulin receptor induced a parallel sustained activation of the insulin receptor tyrosine kinase. A similar pattern was observed for insulin receptor autophosphorylation and Shc phosphorylation, whereas the duration of insulin receptor substrate-1 phosphorylation with low-Kd analogues and with insulin was similar Thus the increased half-life of the ligand-receptor complex induces sustained activation of the insulin receptor tyrosine kinase and sustained phosphorylation of Shc, which may be the cause of the disproportionately high mitogenic potency seen for some insulin analogues.

Expression of receptors for epidermal growth factor and insulin-like growth factor I by ZR-75-1 human breast cancer cell variants is inversely related: the effect of steroid hormones on insulin-like growth factor I receptor expression

We have investigated the expression of insulin-like growth factor I receptors (IGFR) by the ZR-75-1 human breast cancer cell line and tamoxifen-resistant (ZR-75-9a1) and oestrogen-independent (ZR-PR-LT) variants. ZR-75-1 cells expressed 6633+/-953 receptors per cell,(K(d) 0.24+/-0.06 nM). IGFR expression was reduced in ZR-75-9a1 cells (1180+/-614 receptors per cell, K(d) 0.13+/-0.05) and increased in the ZR-PR-LT cell line (18 430+/-3210 receptors per cell, K(d) 0.24+/-17). A comparison of these data with previously published findings for epidermal growth factor receptor (EGFR) expression by these cell lines revealed that IGFR and EGFR expression are inversely related in the variant lines whereas ZR-75-1 cells express similar numbers of both receptors. Since the changes in IGFR expression observed are associated with changes in steroid hormone receptor status, we also investigated the effects of oestradiol, the synthetic progestin ORG 2058 and dexamethasone on IGFR expression. Oestradiol increased IGFR expression only in the ZR-75-1 cell line. Low concentrations of ORG 2058 increased IGFR levels in the two cell lines positive for progesterone receptor (ZR-75-1 and ZR-PR-LT). High concentrations of ORG 2058 increased IGFR expression in all cell lines, as did dexamethasone. These data suggest that EGFR and IGFR expression may be linked in breast cancer, and that EGFR/IGFR ratios in breast cancer may be a more sensitive prognostic indicator than EGFR expression alone. Regardless of basal IGFR expression by the cell studied, ORG 2058 increased IGFR expression, possibly via both the progesterone and glucocorticoid receptors.

A single-chain insulin-like growth factor I/insulin hybrid binds with high affinity to the insulin receptor

1. To investigate the structure/function relationship of the interaction between ligand and receptor in the insulin-like growth factor I (IGF-I) and insulin receptor systems we have prepared and characterized a single-chain insulin/IGF-I hybrid. The single-chain hybrid consists of the insulin molecule combined with the C domain of IGF-I. The single-chain hybrid was found to bind with high affinity to both truncated soluble insulin receptors and membrane-bound holoreceptors. The affinity for interacting with the soluble truncated insulin receptors was 55-94% relative to insulin, and affinity for membrane-bound insulin receptors was 113% of that of insulin. Furthermore we found that the affinity of the single-chain hybrid molecule for IGF-I receptors was 19-28% relative to IGF-I. 2. The affinity of the single-chain hybrid for chimeric insulin/IGF-I receptors exceeded that of either natural ligand. This indicates that coordinately changing domains of the receptors and the ligands can induce higher affinity of ligand for receptor, supporting the idea that these receptors have a common ligand-binding site [Kjeldsen, Andersen, Wiberg, Rasmussen, Schäffer, Balschmidt, Møller and Møller (1991) Proc. Natl. Acad. Sci. U.S.A. 88, 4404-4408]. 3. In contrast with what was generally assumed about the ligand structure required for binding to the insulin receptor we demonstrate the first single-chain insulin analogue that can bind with high affinity to the insulin receptor.

A model for insulin binding to the insulin receptor

The affinities of a number of insulin analogues for the human insulin receptor, a truncated soluble form of the insulin receptor, and the human insulin-like growth factor 1 receptor were determined. Insulin analogues with substitutions in the A13 or B17 positions were shown to have anomalous binding properties. This suggests that these positions, which are located in the hexamer-forming surface on the opposite side of the molecule from the classical binding site, constitute a second domain of the molecule important for receptor binding. In the present work, a model is proposed where each of the two alpha subunits of the insulin receptor contributes with a different binding region to the formation of the high-affinity binding site. Subsequently, a second molecule of insulin is able to bind to a low-affinity site involving only one of the alpha subunits, thus accounting for the curvilinear Scatchard plot. The affinity of the low-affinity site could be estimated using a high-affinity insulin analogue as the tracer. The model also provides the framework for a molecular explanation of the negative cooperativity phenomenon.