Synthetic insulin-like growth factor II

Chemical synthesis of peptides within the insulin superfamily

The synthesis of insulin has inspired fundamental advances in the art of peptide science while simultaneously revealing the structure-function relationship of this centrally important metabolic hormone. This review highlights milestones in the chemical synthesis of insulin that can be divided into two separate approaches: (i) disulfide bond formation driven by protein folding and (ii) chemical reactivity-directed sequential disulfide bond formation. Common to the two approaches are the persistent challenges presented by the hydrophobic nature of the individual A-chain and B-chain and the need for selective disulfide formation under mildly oxidative conditions. The extension and elaboration of these synthetic approaches have been ongoing within the broader insulin superfamily. These structurally similar peptides include the insulin-like growth factors and also the related peptides such as relaxin that signal through G-protein-coupled receptors. After a half-century of advances in insulin chemistry, we have reached a point where synthesis is no longer limiting structural and biological investigation within this family of peptide hormones. The future will increasingly focus on the refinement of structure to meet medicinal purposes that have long been pursued, such as the development of a glucose-sensitive insulin. Copyright © 2016 European Peptide Society and John Wiley & Sons, Ltd.

Fluorescent IGF-II analogues for FRET-based investigations into the binding of IGF-II to the IGF-1R

Convergent-based synthesis of native IGF-II and two IGF-II analogues, with coumarin fluorescent probes incorporated at residues 19 and 28, and their use in FRET-based identification of interactions with the type 1 insulin-like growth factor receptor (IGF-IR).

Presence of insulinlike growth factor receptors and lack of insulin receptors on fetal bovine smooth muscle cells

Previous investigations have demonstrated specific receptors and associated mitogenic actions for insulin and insulinlike growth factors I and II (IGF-I and II) in postnatal bovine aortic smooth muscle. Using fetal tissue we have observed different patterns of binding and action for these peptides. Smooth muscle cells isolated from near-term fetal bovine aortae were studied in early passage. Specific receptors for both IGF-I and IGF-II were identified. Specific binding averaged 5.7%/2.5 X 10(5) cells for IGF-I, and 16.2% for IGF-II, and 0.3% for insulin. High affinity Kd for both IGF receptors were nanomolar. IGF-II was fivefold less potent than IGF-I in displacing IGF-I binding. IGF-I showed no affinity for the IGF-II receptor. Insulin, at physiologic concentrations, was incapable of displacing either IGF-I or IGF-II binding. Cellular incorporation of [methyl-3H]thymidine was stimulated at the lowest dose of IGF-I tested, 0.5 ng/ml. IGF-II showed no effect up to 100 ng/ml, after which a sharp increase in incorporation was noted. Insulin had a similar effect only at concentrations greater than 0.5 micrograms/ml, with a maximal response noted at 5 to 10 micrograms/ml. Our results indicate that fetal bovine aortic smooth muscle cells have an abundance of IGF receptors but lack specific insulin receptors. In addition, IGF-II binding levels are three times higher than for IGF-I. These results are consistent with observations in other species, in which a predominance of IGF over insulin receptors has been demonstrated in fetal tissue, and provide further evidence for a role for the IGFs in embryonic cellular metabolism.

Insulinlike growth factor II expression and oval cell proliferation associated with hepatocarcinogenesis in woodchuck hepatitis virus carriers

Insulinlike growth factor II (IGF-II) is a highly mitogenic fetal growth factor suspected of regulating the growth of a wide spectrum of tissues via an autocrine or paracrine mode of action or both. High steady-state levels of IGF-II RNA were detected in 45% of hepatocellular carcinomas (HCCs) arising from woodchuck livers with persistent woodchuck hepatitis virus (WHV) infection. Analysis of WHV RNA in the same HCCs revealed that HCCs with high levels of IGF-II RNA contained low or undetectable levels of WHV RNA and HCCs with low levels of IGF-II RNA contained high levels of WHV RNA. Integrated WHV DNA was present in HCCs from both groups, but viral DNA replicating forms were present, predominantly in HCCs with low levels of IGF-II. Several IGF-II RNAs, the most prominent of which were poly(A) species of approximately 3.75 and 1.1 to 1.3 kilobases, were detected only in precancerous nodules and HCCs. Levels of IGF-II were elevated two- to three-fold in the serum of woodchucks with chronic active hepatitis preceding the occurrence of HCC. Proliferation of a population of oval cells, which arise from portal tract regions in the liver, preceded the development of HCC and was a prominent feature of livers from which tumors with high levels of IGF-II occurred. The HCCs tended to have distinct histological features according to their growth factor status. Tumors with low levels of IGF-II were generally highly differentiated acinar-trabecular HCCs, whereas tumors with high levels of IGF-II were more anaplastic, with regions of fibrosis and fatty accumulation. A model to relate the pathology of WHV infection to oval cell proliferation and IGF-II expression in the development of these heterogeneous HCCs is presented.

Insulin-like growth factor II binding and action in human fetal fibroblasts

To investigate the role of insulin-like growth factor II (IGF-II) in human prenatal growth, IGF-II binding and biological action were studied in four lines of fetal and three lines of postnatal human fibroblasts. Specific binding of IGF-II was similar in both groups: 15.7% and 14.9% for fetal and postnatal fibroblasts, respectively. This was 5-10 times the amount of IGF-I binding found in these cells. IGF-I and IGF-II caused dose-dependent increases in [14C]aminoisobutyric acid (AIB) uptake. IGF-II was sevenfold less potent than IGF-I in stimulating this metabolic response in both fetal and postnatal fibroblasts. The maximal effect of IGF-II in stimulating [14C]AIB uptake approach that of IGF-I. Similar results were obtained when IGF-I and IGF-II stimulation of [3H]thymidine incorporation was compared in fetal and postnatal fibroblasts. Incubation in the presence of alpha IR-3, a monoclonal antibody to the type I IGF receptor, inhibited the ability of both IGF-I and IGF-II to stimulate [14C]AIB uptake and [3H]thymidine incorporation in fetal and postnatal cells. A monoclonal antibody to the insulin receptor did not affect IGF action. These data indicate that IGF-II is a potent metabolic and mitogenic stimulus for human fetal fibroblasts. However, despite the presence of abundant type II IGF receptors on both fetal and postnatal human fibroblasts, IGF-II stimulation of amino acid transport and DNA synthesis appears to be mediated through the type I rather than through its own type II IGF receptor.

Insulin-like growth factor-I augments gonadotropin-stimulated androgen biosynthesis by cultured rat testicular cells

In addition to the well-known growth stimulating effects of insulin-like growth factors (IGFs), recent studies suggest that these peptides may also modulate the differentiated functions of endocrine cells. Thus, in the present studies, we have investigated the actions of IGFs on androgen biosynthesis by cultured testicular cells. Treatment of cells obtained from adult hypophysectomized rats with LH (1 ng/ml) stimulated testosterone production 60-fold over basal levels. In contrast, treatment with either synthetic human IGF-I or IGF-II failed to stimulate androgen production. However, concomitant treatment of the LH-containing cultures with increasing doses of IGF-I (10-500 ng/ml) augmented testosterone production up to 70% over that seen with LH alone (ED50 = 67 ng/ml). Similar effects were obtained with IGF-II but this peptide was about 10-fold less potent than IGF-I. In addition, these peptides also stimulated the accumulation of pregnenolone and progesterone in the culture medium. Additional studies demonstrated specific binding of [125I]iodo-IGF-I to testicular cells. This binding was competed by IGF-related peptides with the potency order IGF-II = IGF-I greater than insulin whereas unrelated peptides did not compete. The cellular localization of these receptors was examined in testicular cells separated on a metrizamide density gradient. IGF-I receptors were evenly distributed between two cell peaks containing subpopulations of Leydig cells whereas much less binding was found in other testicular cell types. Coupled with recent findings indicating testicular production of IGF-I, the present results suggest that this peptide may act as a positive intratesticular modulator of Leydig cell differentiation.

Heterogeneity of insulin-like growth factor-I affinity for the insulin-like growth factor-II receptor: comparison of natural, synthetic and recombinant DNA-derived insulin-like growth factor-I

Although insulin-like growth factors (IGF) I and II bind with high affinity to structurally discrete receptors, they bind with a lesser affinity to each other's receptor. We have evaluated the affinity of five different IGF-I preparations (three natural IGF-I preparations, one synthetic preparation, and one recombinant DNA-derived) for the IGF-II receptor in rat placental membranes, 18-54,SF cells and BRL-3A cells. In all tissues tested, the natural IGF-I preparations demonstrated an affinity for the IGF-II receptor which was 10-20% that of IGF-II. However, the recombinant and synthetic IGF-I preparations exhibited substantially lower affinities than natural IGF-I for this receptor, with only 10-25% reduction in (125-I)iodo IGF-II binding at peptide concentrations up to 400 ng/ml. Radioimmunoassay of the natural IGF-I preparations with an antibody directed against the unique C-peptide region of IGF-II demonstrated that contamination of IGF-I preparations with immunoreactive IGF-II could not exceed 5%. These results demonstrate that IGF-I purified from human plasma has a different affinity for the IGF-II receptor than does synthetic or recombinant IGF-I. Furthermore, these data are consistent with the hypothesis that IGF-I, itself, may be heterogeneous, and that subforms may vary in their affinities for the IGF receptors. Alternatively, IGF-I preparations which have been considered to be pure may be contaminated with small amounts of IGF-II, resulting in overestimation of the affinity of IGF-I for the type II IGF receptor.

Identification of insulin-like growth factor-II in human seminal and follicular fluids

Antisera raised in rabbits against synthetic insulin-like growth factor-II (IGF-II) were used to develop a specific radioimmunoassay (RIA) for IGF-II. Affinity purified antibodies showed 6% cross-reactivity with IGF-I but failed to recognize insulin even at 10 micrograms/tube. Utilizing this RIA system, immunoreactive IGF-II was identified in the pooled samples of human follicular fluid and seminal plasma. The acid-ethanol precipitates of human seminal and follicular fluids were chromatographed on Sephadex G-50 column and the IGF-II immunoreactive fractions were subjected to reversed-phase high performance liquid chromatography. It was found that immunoactive IGF-II was eluted in the same location as that of synthetic IGF-II. The data indicate for the first time that human seminal plasma and follicular fluid contain significant amounts of IGF-II.

Effect of an anti-insulin-like growth factor I receptor antibody on insulin-like growth factor II stimulation of DNA synthesis in human fibroblasts

To investigate the role of insulin-like growth factor II in the control of DNA synthesis in human fibroblasts, dose-response curves for insulin-like growth factor I and II stimulation of [3H]thymidine incorporation were compared in the absence and presence of alpha IR-3, a highly specific monoclonal antibody directed against the type I insulin-like growth factor receptor. Specific binding of [125I]insulin-like growth factor I to human fibroblast monolayer cultures was inhibited 60-70% in the presence of alpha IR-3. alpha IR-3 had no effect on [125I]insulin-like growth factor II binding to human fibroblasts. However, alpha IR-3 inhibited both insulin-like growth factor I and II stimulated [3H]thymidine incorporation. These data indicate that the type II insulin-like growth factor receptor does not function as a transducer of insulin-like growth factor II's mitogenic effect in human fibroblasts.

Identification of receptors for insulin-like growth factor II in two insulin-like growth factor II producing cell lines

Specific high affinity membrane receptor(s) for insulin-like growth factor II have been characterized in two cell lines which produce this hormone and have the ability to proliferate in serum-free media. These receptor(s) have no affinity for either insulin or biosynthetic insulin-like growth factor I. Affinity cross-linking and sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed an apparent Mr of 250K which does not change with disulfide bond reduction. Our findings are consistent with an autocrine function for insulin-like growth factor II and indicate that these continuous cell lines may provide unique systems for further investigations of this hormone and its receptor.