Interaction of somatomedin-C with an antibody directed against the synthetic C-peptide region of insulin-like growth factor-I

Aiming for the Insulin-like Growth Factor-1 system in breast cancer therapeutics

Despite the major discoveries occurred in oncology the recent years, breast malignancies remain one of the most common causes of cancer-related deaths for women in developed countries. Development of HER2-targeting drugs has been considered a breakthrough in anti-cancer approaches and alluded to the potential of targeting growth factors in breast cancer (BrCa) therapeutics. More than twenty-five years have passed since the Insulin-like Growth Factor-1 (IGF-1) system was initially recognized as a potential target candidate in BrCa therapy. To date, a growing body of studies have implicated the IGF-1 signaling with the BrCa biology. Despite the promising experimental evidence, the impression from clinical trials is rather disappointing. Several reasons may account for this and the last word regarding the efficacy of this system as a target candidate in BrCa therapeutics is probably not written yet. Herein, we provide the theoretical basis, as well as, a comprehensive overview of the current literature, regarding the different strategies targeting the various components of the IGF-1/IGF-1R axis in several pathophysiological aspects of BrCa, including the tumor micro-environment and cancer stemness. In addition, we review the rationale for targeting the IGF-1 system in the different BrCa molecular subtypes and in treatment resistant breast tumors with a focus on both the molecular mechanisms and on the clinical perspectives of such approaches in specific population subgroups. We also discuss the future challenges, as well as, the development of novel molecules and strategies targeting the system and suggest potential improvements in the field.

Insulin-like growth factor-1 signaling in cardiac aging

Cardiovascular disease (CVD) is the leading cause of death in most developed countries. Aging is associated with enhanced risk of CVD. Insulin-like growth factor-1 (IGF-1) binds to its cognate receptor, IGF-1 receptor (IGF-1R), and exerts pleiotropic effects on cell growth, differentiation, development, and tissue repair. Importantly, IGF-1/IGF-1R signaling is implicated in cardiac aging and longevity. Cardiac aging is an intrinsic process that results in cardiac dysfunction, accompanied by molecular and cellular changes. In this review, we summarize the current state of knowledge regarding the link between the IGF-1/IGF-1R system and cardiac aging. The biological effects of IGF-1R and insulin receptor will be discussed and compared. Furthermore, we describe data regarding how deletion of IGF-1R in cardiomyocytes of aged knockout mice may delay the development of senescence-associated myocardial pathologies. This article is part of a Special issue entitled Cardiac adaptations to obesity, diabetes and insulin resistance, edited by Professors Jan F.C. Glatz, Jason R.B. Dyck and Christine Des Rosiers.

Minireview: Were the IGF Signaling Inhibitors All Bad?

Preclinical studies in the 1980s defined a role for IGF signaling in the development and sustainability of the malignant process. Subsequently, antibody, tyrosine kinase, and ligand inhibitors of the IGF receptor were manufactured. In the past decade, numerous clinical trials have tested the efficacy of IGF receptor inhibitors in the treatment of advanced tumors. Early-phase trials in heavily pretreated populations showed promise with complete or partial responses in a few patients and stable disease in many more. Unfortunately, the results of the early-phase trials did not pan out to later-phase trials. The lack of use of biomarkers to define subsets of patients that may benefit from IGF receptor blockade and compensatory signaling via other growth factor receptors such as the insulin, GH, and epidermal growth factor receptors may have played a role in the lack of efficacy of IGF receptor inhibition in phase III trials. Although these trials failed to show benefit, the trials have revealed previously unknown knowledge regarding the complex nature of IGF signaling. The knowledge obtained from these trials will be useful in designing future trials studying inhibitors of growth factor signaling.

The relationship between aerobic exercise capacity and circulating IGF-1 levels in healthy men and women

OBJECTIVES

To determine whether aerobic capacity is associated independently with insulin-like growth factor-I (IGF-1) levels in healthy community-dwelling men and women.

SETTING

The Baltimore Longitudinal Study on Aging (BLSA).

DESIGN

A cross-sectional analysis of data from the population-based cohort of the Baltimore Longitudinal Study of Aging (BLSA).

PARTICIPANTS

We studied 181 men and 92 women aged 20 to 93 years, volunteers in the Baltimore Longitudinal Study on Aging (BLSA). Subjects were free of endocrine, renal, hepatic, gastrointestinal, or cardiac diseases, and they were taking no medications known to interfere with the growth hormone-IGF-1 axis.

MEASUREMENTS

All subjects underwent a single measurement of serum IGF-1 in the fasting state, as well as peak VO2 determinations during maximal treadmill exercise testing performed within one visit of the IGF-1 determination. Dual energy X-ray absorptiometry (DEXA) scans were performed in a subset of 171 subjects (64 women and 107 men) for determination of fat free mass (FFM).

RESULTS

In the pooled group of women and men, univariate regression analysis revealed that age was correlated strongly with decreasing IGF-1 levels (r = -0.53, P < .001) and with peak VO2r = -0.56, P < .001). IGF-1 levels were also significantly correlated with peak VO2 (r = 0.29, P < .001). There were no significant gender-related differences in these relationships. On multivariate analysis, age (beta = -0.54, P < .001), but not peak VO2 (P = -0.01, P = .840), remained strongly associated with IGF-1 levels. After adjustment of peak VO2 for FFM in subjects with DEXA scans, results were similar.

CONCLUSIONS

These findings indicate that although both peak aerobic capacity and circulating IGF-1 levels decline with age, aerobic capacity is not independently related to circulating IGF-1 in healthy men and women across the adult life span.

Functional epitope mapping of insulin-like growth factor I (IGF-I) by anti-IGF-I monoclonal antibodies

Based on a collection of monoclonal antibodies (mAb) against insulin-like growth factor I (IGF-I), we have defined the IGF-I epitopes involved in the interaction with IGF-binding proteins (IGFBP) and IGF-I receptors. We have also characterized the ability of these antibodies to block IGF-I-induced survival of the IL-3-dependent Ba/F3 cell line. More than 140 hybridomas secreting IGF-I-specific mAb were characterized, of which 28 were studied in detail. They display apparent affinity constants ranging from less than 10(6) to 10(10) M-1 and varying crossreactivity with IGF-II, including 2 mAb with higher affinity for IGF-II than for IGF-I. None crossreact with insulin or any other growth factor tested. Using both enzyme immunoassays and real-time biospecific interaction analysis, we have identified 8 epitopic clusters related to the primary structure of IGF-I, according to mAb reactivity to synthetic peptides, proteolytic fragments of IGF-I, and various IGF-I mutants. The mAb panel also was used to map the IGF domains implicated in the interaction with IGFBP and IGF-I receptors. An IGF-I domain has been identified that remains exposed after IGF-I binding to IGFBP-1 or to IGFBP-3, which is recognized by 6 different mAb. The mAb in this group also bind IGF-I, when complexed to the type-1 IGF receptor on the murine pro-B cell line Ba/F3, and BALB/c 3T3 fibroblasts overexpressing the human receptor. Finally, IGF-I-promoted survival can be blocked with mAb specific for target epitopes, and their potential use in tumor cell growth control is discussed.

Effects of single nightly injections of growth hormone-releasing hormone (GHRH 1-29) in healthy elderly men

Age-related reductions in growth hormone (GH) and insulin-like growth factor-I (IGF-I) may contribute to decreased muscle mass and strength in older persons. The relationship of this phenomenon to skeletal muscle bioenergetics has not been reported. We sought to determine whether administration of GH-releasing hormone (GHRH) would sustain increases in GH and IGF-I and improve skeletal muscle function and selected measures of body composition and metabolism. We measured GH secretion, muscle strength, muscle histology, and muscle energy metabolism by phosphorus nuclear magnetic resonance spectroscopy (31P-NMRS), body composition, and endocrine-metabolic functions before and after 6 weeks of treatment. Eleven healthy, ambulatory, non-obese men aged 64 to 76 years with low baseline IGF-I levels were treated at home as outpatients by nightly subcutaneous self-injections of 2 mg GHRH for 6 weeks. We measured GH levels in blood samples obtained every 20 minutes from 8:00 PM to 8:00 AM; AM serum levels of IGF-I, IGF binding protein-3 (IGFBP-3), and GH binding protein (GHBP); muscle strength; muscle histology; the normalized phosphocreatine abundance, PCr/[PCr + Pi], and intracellular pH in forearm muscle by NMRS during both sustained and ramped exercise; body composition by dual-energy x-ray absorptiometry (DEXA); lipid levels; and glucose, insulin, and GH levels during an oral glucose tolerance test (OGTT). GHRH treatment increased mean nocturnal GH release (P < .02), the area under the GH peak ([AUPGH] P < .006), and GH peak amplitude (P < .05), with no change in GH pulse frequency or in levels of IGF-I, IGFBP-3, or GHBP Two of six measures of muscle strength, upright row (P < .02) and shoulder press (P < .04), and a test of muscle endurance, abdominal crunch (P < .03), improved. GHRH treatment did not alter exercise-mediated changes in PCr/[PCr + Pi] or intracellular pH, but decreased or abolished significant relationships between changes in PCr/[PCr + Pi] or pH and indices of muscle strength. GHRH treatment did not change weight, body mass index, waist to hip ratio, DEXA measures of muscle and fat, muscle histology, glucose, insulin, or GH responses to OGTT, or lipids. No significant adverse effects were observed. These data suggest that single nightly doses of GHRH are less effective than multiple daily doses of GHRH in eliciting GH- and/or IGF-I-mediated effects. GHRH treatment may increase muscle strength, and it alters baseline relationships between muscle strength and muscle bioenergetics in a manner consistent with a reduced need for anaerobic metabolism during exercise. Thus, an optimized regimen of GHRH administration might attenuate some of the effects of aging on skeletal muscle function in older persons.

Chemical synthesis of insulin-like growth factor II

Human insulin-like growth factor II (IGF-II) with 67 amino acids and three disulfide bridges has been synthesized by the solid-phase method. The synthetic hormone is shown to be homogeneous in high performance liquid chromatography (HPLC), high performance partition chromatography (HPPC), and chromatofocusing. It is indistinguishable from natural hormone in HPLC, peptide map of thermolysin digests, amino acid composition and radioreceptor binding assay. Thus, synthetic and natural IGF-II are identical.

Synthetic insulin-like growth factor II

Human insulin-like growth factor II with 67 amino acid residues and three disulfide bridges has been synthesized by the solid-phase method. Homogeneity of the synthetic product is ascertained by chromatofocusing, high performance liquid chromatography and amino acid analysis. In both radioimmunoassay and radioreceptor assay, the synthetic product is indistinguishable from the natural hormone.

Insulin-like growth factor characteristics of an acidic non-suppressible insulin-like activity

The biological activities of an acidic form of non-suppressible insulin-like activity (ILA pI 4.8) have been studied. ILA pI 4.8 was isolated from Cohn fraction IV-1 of human serum by pH 5.5 ion-exchange chromatography on SP-Sephadex. Carrier-bound ILA was eluted at pH 9.7 and then sequentially gel chromatographed in 1% formic acid on Sephadex G-75 and Bio-Gel P-30. The low-Mr (7000) active material was subjected to flat bed isoelectric focusing. Overall recovery was 87 munit of insulin equivalents/100 g of Cohn fraction IV-1, with a specific activity in the range 4-10 munit/mg of protein, representing a purity of 1-6%. This material has been tested in a variety of insulin-like growth factor (IGF)/somatomedin assay systems. It stimulated, in a dose-related manner, [14C]glucose conversion into lipid by isolated rat adipocytes, 35SO4(2-) incorporation into weanling rat costal cartilage and [3H]thymidine incorporation into DNA of cultured human fibroblasts. Like IGF-I and -II, ILA pI 4.8 was able to inhibit degradation of 125I-insulin by crude homogenates of rat liver. In addition, the biological activity of ILA pI 4.8 was completely suppressible by a recently described inhibitor of IGF-I and IGF-II. ILA pI 4.8 was able to compete, in a parallel manner, with 125I-IGF-I and 125I-IGF-II and, at higher doses, with 125I-insulin in a placental radioreceptor assay. No cross-reactivity was seen in a radioimmunoassay for IGF-I and -II C-peptides, but at higher concentrations parallel displacement was observed in a somatomedin C/IGF-I radioimmunoassay using two different antisera. These data indicate that ILA pI 4.8 does possess many of the biological activities previously reported for the IGFs. Since ILA pI 4.8 does occur naturally in serum, it would appear reasonable to tentatively include it as one of the IGF/somatomedin family.

Insulin-like growth factors. Studies in diabetics with and without retinopathy

To determine whether two insulin-like growth factors (IGF I and IGF II) influence the course of diabetic retinopathy, we measured the concentrations of these factors in 80 adult patients with diabetes and in 62 control subjects. In seven patients with Type I diabetes and rapidly deteriorating vision as a result of proliferative and exudative retinopathy, the serum concentration of IGF I was 722 +/- 41 ng per milliliter (mean +/- S.E.M.), as compared with 381 +/- 48 ng per milliliter in 26 patients who had Type I diabetes without retinopathy or with less severe forms of it, and 302 +/- 15 ng per milliliter in the controls (P less than 0.001 for both comparisons). Serum concentrations of IGF II were normal in subjects with Type I diabetes but were somewhat depressed in those with Type II disease. Whether elevated serum concentrations of IGF I cause the accelerated development of retinopathy in some patients remains to be determined. Such levels do appear to identify patients at high risk for rapid deterioration of vision, and hence may be useful in selecting patients for more intensive or alternative forms of therapy.

Insulin-like growth factor I/somatomedin-C (IGF-I/SM-C) and glucocorticoids synergistically regulate mitosis in competent human fibroblasts

In serum-free medium, insulin-like growth factor-I/somatomedin-C (IGF-I/SM-C) was weakly mitogenic for adult human fibroblasts in culture. However, in the presence of 0.5% human hypopituitary serum (HHS), which by itself had little effect, there was a marked dose-dependent response to IGF-I/SM-C with a 10- to 20-fold increase in [3H]thymidine incorporation at 25 ng/ml IFG-I/SM-C. With the further addition of dexamethasone or hydrocortisone to the combination of IGF-I/SM-C + 0.5% HHS, there was a dramatic synergistic effect resulting in a 60- to 70-fold increase in [3H]thymidine incorporation. This stimulation was two times greater than that seen with 20% FCS. In contrast, glucocorticoids had no effect in serum-free medium or with HHS alone. These [3H]thymidine incorporation results were clearly supported by cell replication studies. Dose-response curves for 125I IGF-I/SM-C binding and IGF-I/SM-C stimulation of [3H]thymidine incorporation were similar with 1/2 maximal effects for both at 5 ng/ml. However, the striking synergism seen with glucocorticoids occurred in the absence of any glucocorticoid-induced change in IGF-I/SM-C binding, indicating that the interaction of IGF-I/SM-C and glucocorticoids occurs at a postreceptor level. These data demonstrate that in the presence of a low concentration of HHS, IGF-I/SM-C and glucocorticoids stimulate complete cell cycle traverse and replication of human fibroblasts.

Total synthesis of insulin-like growth factor I (somatomedin C)

Human insulin-like growth factor I (somatomedin C) with 70 amino acid residues and three disulfide bridges has been synthesized by the solid-phase method. The synthetic product behaves as a pure polypeptide in paper electrophoresis, isoelectric focusing, and high-performance liquid chromatography, Its elution behavior in Sephadex G-50, isoelectric point, amino acid composition, and growth-stimulating activity in bovine adrenal cortical cells or granulosa cells are comparable to those reported for the natural product. In radioimmunoassay, the synthetic product is indistinguishable from the natural hormone when either synthetic or natural product is used as the labeled ligand.

Recent advances in the biochemistry and physiology of the insulin-like growth factor/somatomedin family

The insulin-like growth factors (IGF) or somatomedins (Sm) are a family of low molecular weight circulating growth factors which have a major, but not absolute, dependence on GH, and have been shown to stimulate body growth and skeletal metabolism in vivo. They are thus considered to mediate the effects of GH on skeletal growth. In humans, the family consists of two well-characterized forms--IGF-I or SmC (a basic peptide) and IGF-II (a "neutral" peptide)--as well as perhaps two less well characterized forms--SmA (a neutral peptide) and an acidic insulin-like activity (ILA pI 4.8). Similar IGF/Sm species have been found and well-characterized in rat serum. Some higher mol wt forms also exist in tissues and body fluids and may represent possible precursor forms. On the basis of in vitro, clinical and in vivo evidence it has been postulated that IGF-I is the primary growth factor in the adult, whilst IGF-II is probably a major foetal growth factor. In vitro the IGF/Sms have a variety of effects including (1) acute insulin-like metabolic actions, which are observed primarily in insulin target tissues and are initiated largely at insulin receptors, and (2) longer term effects, associated with cell growth and skeletal tissue metabolism, and which occur in traditionally non-insulin target tissues, primarily via IGF/Sm receptors. These observations, together with the circumstantial clinical evidence favouring a close association between IGF levels and growth status, clearly indicate a role for IGF/Sms in growth regulation. This concept is now fully supported by the demonstration that IGF-I infused into hypophysectomized (GH-deficient) rats results in increased growth and skeletal metabolism. The physiological regulation of the expression of net IGF activity in vivo is complex and is controlled by the following three determinants: the levels of IGFs, the levels of the specific carrier-proteins and the levels of IGF inhibitors. Both IGFs and their carrier-proteins are influenced by the GH status of the animal as well as by other hormones, nutritional status and chronic illness. Little is known yet about the control of the various IGF inhibitors that have been described. Of importance, however, is the general concept that normal growth is dependent on an adequate balance between all three determinants and that some regard must be had for the contribution of each in determining the overall potential for growth under given circumstances.

Radioimmunological determination of insulinlike growth factors I and II in normal subjects and in patients with growth disorders and extrapancreatic tumor hypoglycemia

Serum levels of immunoreactive insulinlike growth factors (IGF) I and II were determined by a modified IGF I and a new IGF II radioimmunoassay in normal children and adults, and in patients with acromegaly, isolated growth hormone deficiency, and extrapancreatic tumor hypoglycemia. Serum samples were gel filtered by a simple routine procedure at acidic pH to dissociate and separate IGF from the IGF carrier protein. Mean immunoreactive IGF I levels (+/- SD; corrected for crossreactivity of IGF II) were 193 +/- 58 ng/ml in normal adult subjects, 712 +/- 245 ng/ml in acromegalic patients and 24 +/- 14 ng/ml in patients with isolated growth hormone deficiency. The lack of growth hormone alone, irrespective of an otherwise normal hormonal status, appears to be responsible for the drastic decrease of IGF I levels. Oversecretion of growth hormone does not increase the levels of immunoreactive IGF II: mean levels (+/- SD; corrected for crossreactivity of IGF I) in normal and acromegalic subjects are virtually identical (647 +/- 126 and 641 +/- 189 ng/ml, respectively). Apparently, normal growth hormone levels stimulate IGF II production already maximally. However in growth hormone deficiency immunoreactive IGF II is significantly decreased (252 +/- 99 ng/ml). Thus, IGF II, like IGF I, is growth hormone dependent. But in contrast to IGF I, the growth hormone dependence of IGF II seems to become apparent only at subnormal growth hormone levels. In normal children IGF I is age dependent: it is low in newborn cord sera (51 +/- 20 ng/ml) and gradually rises into the adult range with increasing age. At the onset of and during puberty mean IGF I levels lie above prepubertal values. In contrast, IGF II levels in normal children are independent of age and pubertal stage beyond the first year of life, whereas newborns have significantly lower IGF II values. Hypoglycemia resulting from extrapancreatic tumors is not associated with increased immunoreactive IGF I or II levels. IGF I is decreased in most of the sera (mean level +/- SD:56 +/- 39 ng/ml) whereas IGF II lies in the normal range (556 +/- 195 ng/ml).

Dwarfism in the pygmy. An isolated deficiency of insulin-like growth factor I

Insulin-like growth factors (IGFs) I and II, which are present in normal human beings, were measured in serum samples from 11 pygmies from the Central African Republic, 31 controls, and 12 patients with growth hormone deficiency. The mean serum concentration of IGF-I (+/- S.E.M.) was 68.6 +/- 8 ng per milliliter in pygmies, as compared with 193 +/- 10 ng per milliliter in controls (P less than 0.001) and 24 +/- 4 ng per milliliter in patients with growth hormone deficiency (P less than 0.05). Mean serum concentrations of IGF-II in controls, pygmies, and growth hormone-deficient patients were 647 +/- 22, 503 +/- 37, and 252 +/- 29 ng per milliliter, respectively. The serum IGF-I concentration was within the normal range in only one pygmy, whereas IGF-II values were within the normal range in 10 of 11. Pygmies appear to have a major defect in the production of IGF-I.

Somatomedin receptor of human placenta: solubilization, photolabeling, partial purification, and comparison with insulin receptor

Using a recently isolated human basic somatomedin (basic SM) similar to insulin-like growth factor I (IGF-I), we studied both the photoaffinity-labeled and unlabeled basic-SM receptor solubilized from human placental cell membranes. Unlike the result with the insulin receptor, high yields of soluble basic-SM-binding activity are obtained with Triton X-100. The soluble basic-SM receptor retains high-affinity (Kd approximately 0.3 nM) peptide-specific binding of basic SM, similar to the binding present in particulate placenta membranes; the receptor exhibits a comparatively low affinity for insulin (Kd approximately 3 microM). On Sepharose 6B, like the crude soluble insulin receptor, the basic-SM receptor migrates as a species with an apparent Stokes radius of 7.2 nm; unlike the insulin receptor, the basic-SM receptor does not, under similar conditions, yield a smaller binding species (apparent Stokes radius 3.8 nm). Upon photoaffinity labeling with 125I-labeled basic SM, one principal specifically labeled constituent is detected. Upon gel electrophoresis in the presence of 2-mercaptoethanol, the photolabeled constituent, like the insulin receptor, migrates as a species with an apparent molecular weight of about 140,000; in the absence of reducing agent, a molecular weight greater than 240,000 is observed. Lectin-agarose affinity chromatography yields a 30-fold purification both of the basic-SM-binding activity and the photolabeled constituent. Anti-insulin receptor antibody does not appear to precipitate the basic-SM receptor. We conclude that the basic-SM receptor of human placenta is a glycoprotein, remarkably similar to (an isoreceptor) but distinct from the insulin receptor previously characterized in this tissue.

Somatomedin-C mediates growth hormone negative feedback by effects on both the hypothalamus and the pituitary

Somatomedin-C stimulates somatostatin release to a maximum of 390 percent of basal release during short-term (20-minute) incubation of rat hypothalamus. It has no effect on basal or stimulated growth hormone release from primary cultures of rat adenohypophyseal cells during a 4-hour incubation, but inhibits stimulated release by more that 90 percent after 24 hours. These findings suggest that somatomedin-C participates in the growth hormone negative feedback loop with an immediate effect on hypothalamic somatostatin and a delayed effect on the anterior pituitary.