Inhibitory effects of insulin-like growth factor (IGF)-binding proteins-1 and -3 on IGF-activated glucose consumption in mouse BALB/c 3T3 fibroblasts

High-fat diet reduces levels of type I tropocollagen and hyaluronan in rat skin

Although it is known that nutritional conditions affect the skin function, little information is available on the effect of a high-fat (HF) diet on skin. In this study, Sprague-Dawley rats were fed HF diets for 28 days, and we investigated the effect of this diet on type I tropocollagen and hyaluronan in rat skin. The HF diets reduced the levels of type I tropocollagen, COL1A1 mRNA, hyaluronan, and rat hyaluronan synthase (rhas)2 mRNA, which play a primary role in hyaluronan synthase in the dermis. However, rhas3 mRNA level in the skin was increased. The HF diets also decreased the skin mRNA expression of transforming growth factor (TGF)-beta1, which enhances the expression of COL1A1 and rhas2 mRNA and decreases rhas3 mRNA expression, and decreased the hepatic mRNA expression of insulin-like growth factor (IGF)-I, which enhances COL1A1, rhas2, and TGF-beta1 mRNA expression. The serum level of adiponectin, which promotes the syntheses of type I collagen and hyaluronan, was decreased in the HF diet groups. These findings suggest that an HF diet reduces the levels of type I tropocollagen and hyaluronan in the skin by suppressing the action of TGF-beta1, IGF-I and adiponectin, and these effects are deleterious for skin function.

Modulation of GH/IGF-1 axis: potential strategies to counteract sarcopenia in older adults

Aging is associated with progressive decline of skeletal muscle mass and function. This condition, termed sarcopenia, is associated with several adverse outcomes, including loss of autonomy and mortality. Due to the high prevalence of sarcopenia, a deeper understanding of its pathophysiology and possible remedies represents a high public health priority. Evidence suggests the existence of a relationship between declining growth hormone (GH) and insulin-like growth factor-1 (IGF-1) levels and age-related changes in body composition and physical function. Therefore, the age-dependent decline of GH and IGF-1 serum levels may promote frailty by contributing to the loss of muscle mass and strength. Preclinical studies showed that infusion of angiotensin II produced a marked reduction in body weight, accompanied by decreased serum and muscle levels of IGF-1. Conversely, overexpression of muscle-specific isoform of IGF-1 mitigates angiotensin II-induced muscle loss. Moreover, IGF-1 serum levels have been shown to increase following angiotensin converting enzyme inhibitors (ACEIs) treatment. Here we will review the most recent evidence regarding age-related changes of the GH/IGF-1 axis and its modulation by several interventions, including ACEIs which might represent a potential novel strategy to delay the onset and impede the progression of sarcopenia.

Insulin-like growth factor binding protein-3 induces insulin resistance in adipocytes in vitro and in rats in vivo

Insulin-like growth factor binding protein (IGFBP)-3 binds to IGF and modulates their actions and also possesses intrinsic activities. We investigated its effects on insulin action and found that when IGFBP-3 was added to fully differentiated 3T3-L1 adipocytes in culture, insulin-stimulated glucose transport was significantly inhibited to 60% of control in a time- and dose-dependent manner. Tumor necrosis factor (TNF)-alpha treatment also inhibited glucose transport to the same degree as IGFBP-3 and, in addition, increased IGFBP-3 levels 3-fold. Co-treatment with TNF-alpha and IGFBP-3 antisense partially prevented the inhibitory effect of TNF-alpha on glucose transport, indicating a role for IGFBP-3 in cytokine-induced insulin resistance. Insulin-stimulated phosphorylation of the insulin receptor was markedly decreased by IGFBP-3 treatment. IGFBP-3 treatment suppressed adiponectin expression in 3T3-L1 adipocytes. Infusion of IGFBP-3 to Sprague-Dawley rats for 3 h decreased peripheral glucose uptake by 15% compared with controls as well as inhibiting glycogen synthesis. Systemic administration of IGFBP-3 to rats for 7 d resulted in a dramatic 40% decrease in peripheral glucose utilization and glycogen synthesis. These in vitro and in vivo findings demonstrate that IGFBP-3 has potent insulin-antagonizing capability and suggest a role for IGFBP-3 in cytokine-induced insulin resistance and other mechanisms involved in the development of type-2 diabetes.

Hepatic gene and protein expression of primary components of the IGF-I axis in long lived Snell dwarf mice

Recent evidence indicates that the GH/IGF-I axis plays a key role in the control of aging and longevity. To better understand this biological relationship we examined the mRNA and corresponding protein levels of primary IGF-I axis genes in the livers of young and aged long-lived Snell dwarf mice relative to their age-matched controls. We demonstrated that the level of IGF-I and ALS mRNAs is dramatically decreased in both young and aged dwarf livers, transcripts encoding IGF-IR and IGFBP-I are elevated in young dwarfs, but normalize to control levels in aged dwarf livers while transcripts encoding IGFBP-3 are elevated only in aged controls. Interestingly, regulation at the protein level of several IGF-I axis components in the Snell dwarf appears to involve both altered gene expression and post-translational regulation. In this study, we reveal both concordant and discordant relationships between mRNA and protein levels for particular components of the IGF-I axis, illustrating that some of these gene products are not solely regulated by transcriptional mechanisms. These results are consistent with a delay in the molecular maturation of the IGF-I axis in dwarf livers, suggesting the preservation of some neonatal characteristics in young adult and aged dwarf livers. Our studies provide gene expression and protein abundance profiles for components of IGF-I axis that are distinguishing characteristics of both young and aged dwarf mice, and suggest that delayed development of the IGF-I axis in the young adult Pit1(dw/dwJ) dwarf liver may play an important role in the endocrine regulation of mammalian longevity.

Use of mutagenesis to probe IGF-binding protein structure/function relationships

The IGF-binding proteins (IGFBPs) are multifunctional proteins that modulate IGF actions. To determine whether specific domains within these proteins account for specific functions, we and other laboratories have used in vitro mutagenesis. Prior experiments that used a variety of techniques had identified discrete regions within each protein that were proposed to account for specific functions. Alterations of these regions by substituting charged residues with neutral residues or hydrophobic residues with nonhydrophobic residues as well as domain swapping, i.e., substituting a domain from one specific form of IGFBP for the homologous domain in another form, has resulted in the elucidation of the functions of many of these specific sequences. Because the areas of protein sequence that are altered involve a limited number of amino acids, they generally do not alter the conformation of the entire protein; therefore, these specific substitutions can often be correlated with the functional changes that occur after mutagenesis. Mutants have been particularly useful for performing functional analyses in which the purified mutant protein is added to a biological test system. In some cases it has been possible to overexpress the mutagenized protein and determine whether the constitutively synthesized, mutant form of IGFBP has altered functional activity. These results have revealed that discrete regions of IGFBP sequence can mediate important and specific functional properties of these proteins.

Testes-specific transgene expression in insulin-like growth factor-I transgenic mice

Insulin-like growth factor-I (IGF-I) is a low molecular weight peptide that mediates the cell proliferating actions of growth hormone. Evidence exists indicating that IGF-I is produced by various cell types and this growth factor has been implicated in a variety of reproductive processes. To investigate the effect of IGF-I over-expression on reproductive systems, we generated three independent lines of transgenic mice harbouring a human IGF-I cDNA (hIGF-I) under the control of a Cytomegalovirus immediate early (CMV) promoter. The CMV promoter was used in an attempt to direct expression of IGF-I into a variety of tissues both reproductive and non-reproductive. Yet expression of the foreign hIGF-I gene, determined by Northern blot, was found to occur only in the testicular tissues of the male mice, apparently due to methylation of the transgene in all the tissues tested except the testes, which demonstrate transgene hypomethylation. Evaluation of the transgene expression during testicular development revealed that expression begins between 10 and 15 days of development, coinciding with the appearance of the zygotene and pachytene primary spermatocytes during early spermatogenesis, therefore indicating germ line expression of the transgene. Extensive study of the CMV-hIGF-I transgenic lines of mice has revealed that the effects of the transgene expression do not extend beyond the testicular tissues. No significant differences (P > 0.05) in the IGF-I serum levels, growth rates, or testicular histology have been observed between transgenic and non-transgenic male siblings. The ability of transgenic males to produce offspring also appears unaffected. Evaluation of the IGF binding protein (IGFBP) levels in the testicular tissues of CMV-hIGF-I transgenic mice by Western ligand blot revealed an increase in the concentration of testicular proteins with molecular weights corresponding to IGFBP-2 and IGFBP-3. These results suggest that the testicular over-expression of IGF-I induces increased IGFBP localization in this tissue. Inhibition of IGF activity by the IGFBPs would explain the lack of a dramatic physiological effect in the CMV-hIGF-I transgenic mice, despite the presence of elevated testicular IGF-I. The observation that testis specific IGF-I overexpression induces localization of IGFBPs in this tissue confirms the existence of a well regulated testicular IGF system and supports the convention that this growth factor plays an important role in testicular function.

Insulin-like growth factor (IGF) binding proteins modulate the glucocorticoid-dependent biological effects of IGF-II in cultured fetal rat hepatocytes

The role of insulin-like growth factor binding proteins (IGFBPs) in regulation by IGF-II of glycogenesis and DNA synthesis was investigated in hepatocytes isolated from fetal rat livers at days 15 and 18 of gestation and grown in the presence or absence of cortisol. IGFBP-1 was clearly revealed by Western ligand blot and immunoblot analysis of IGFBPs secreted into conditioned media. Its production and cellular messenger RNA (mRNA) were positively regulated by cortisol and increased in older cells. In the absence of IGFBP (fresh medium), glycogenesis, and DNA synthesis were stimulated by IGF-II and insulin. In each case, cortisol enhanced this stimulation. In the presence of IGFBPs (cell-conditioned media), IGF-II stimulation of DNA synthesis and to a lesser extent glycogenesis was inhibited. The degree of inhibition was directly related to IGFBP-1 production. IGFBPs had no effect on stimulation of glycogenesis and DNA synthesis by des(1-6)IGF-II, a structural analog of IGF-II that does not bind to IGFBPs. Insulin, whose biological effects were not modified by conditioned media, inhibited IGFBP-1 production. Comparison of the dose dependence of the two bioactivities showed that DNA synthesis was more sensitive to IGF-II than glycogenesis. Our results suggest that in the case of DNA synthesis the effects of IGF-II are mediated via the IGF-I receptor and those of insulin via the insulin receptor, whereas in the case of glycogenesis both are mediated via the insulin receptor. In conclusion, IGF-II and insulin stimulation of glycogenesis and DNA synthesis in cultured fetal hepatocytes depends on the presence of glucocorticoid and the stage of development. IGF-II action is negatively regulated by IGFBP-1 whose synthesis increases in the presence of glucocorticoids.

Insulin-Like Growth Factor Binding Protein-1 Is Elevated in Patients With Polycythemia Vera and Stimulates Erythroid Burst Formation In Vitro

Previously, we found that, in the myeloproliferative disorder polycythemia vera (PV), circulating erythroid progenitor cells were hypersensitive to insulin-like growth factor I (IGF-I), an effect shown to occur through the IGF-I receptor. Also, in cells of PV patients, the IGF-I receptor was hyperphosphorylated on tyrosine residues under basal conditions, and its tyrosine phosphorylation in response to exogenous IGF-I was strongly augmented. Thus, because IGF-I appeared to play a role in the pathogenesis of PV, we wished to assess its level in the circulation of these patients. Normally, most of the circulating IGF-I is bound to specific high-affinity IGF binding proteins that can regulate its activity. We determined the circulating levels of IGF-I and two of its key binding proteins, IGFBP-1 and IGFBP-3. In two separate experiments, plasma samples from a total of 23 PV patients age- and sex-matched with 41 normal individuals were compared by radioimmunoassay. The levels of IGFBP-1 in patients with PV (37.80 ± 4.33 μg/L) were more than fourfold higher than in normals (9.34 ± 1.34 μg/L) or patients with secondary erythrocytosis (9.47 ± 1.96 μg/L), whereas the plasma concentrations of IGFBP-3 and IGF-I in these patients were similar to those of normal subjects. Because circulating IGFBP-1 levels may be influenced by insulin, we measured the concentrations of insulin in the same samples. Our data showed that the elevation of circulating IGFBP-1 in PV could not be attributed to low levels of insulin in these patients. The substantial increase in concentration of IGFBP-1 was confirmed on ligand blots performed with 125I–IGF-I. IGFBP-1 can be either inhibitory or stimulatory to the action of IGF-I under different conditions. We reasoned that if IGFBP-1 were stimulatory for erythropoiesis, an elevated IGFBP-1 level could help to explain the increased sensitivity to IGF-I observed in PV. If IGFBP-1 were inhibitory, it might suggest a compensatory mechanism in which a hyperphosphorylated IGF-I receptor in PV might induce a negative modulator of IGF-I action, in this case IGFBP-1. To distinguish between these two hypotheses, we titrated the effect of IGFBP-1 in the presence of IGF-I with respect to erythroid burst formation and found that IGFBP-1 was strikingly stimulatory. The elevated level of IGFBP-1 coupled with its ability to stimulate erythroid burst formation provide an attractive mechanism to account for the increased sensitivity of erythroid progenitor cells to IGF-I and the consequent overproduction of red blood cells characteristic of PV.

Insulin-like growth factor binding proteins and their role in controlling IGF actions

The insulin-like growth factor binding proteins (IGFBPs) are a family of six proteins that bind to insulin-like growth factor-I and -II with very high affinity. Because their affinity constants are between two- and 50-fold greater than the IGF-I receptor, they control the distribution of the IGFs among soluble IGFBPs in interstitial fluids, IGFBPs bound to cell surfaces or extracellular matrix (ECM) and cell surface receptors. Although there are six forms of insulin-like growth factor binding proteins, most interstitial fluids contain only three or four forms, and usually only one or two predominate. The proteins differ significantly in their biochemical characteristics, and this accounts for many of the differences that have been observed in their biological actions. Several different types of protease cleave these binding proteins. Proteolytic cleavage generally inactivates the binding proteins or reduces their ability to bind to IGF-I or -II substantially. Several cell types have been shown to secrete these proteases; therefore, the factors that regulate protease activity can control binding protein actions indirectly. Other post-translational modifications, such as glycosylation and phosphorylation, have been shown to alter IGF binding protein activity. While binding protein actions have been studied extensively in vitro, many of the in vivo activities of these proteins remain to be defined.

Components of the IGF system mediate the opposing effects of tamoxifen on endometrial and breast cancer cell growth

The involvement of the IGF system in the growth regulation of hormone-dependent (e.g. endometrial and breast) cancer cells was studied. We chose two opposing effects of tamoxifen: the paradoxical stimulation of Ishikawa endometrial cancer cells growth and its inhibitory effect on MCF-7 mammary cancer cells. The results clearly confirm our working hypothesis that the IGF system is involved in growth regulation of these cancer cells irrespective of the direction of the drug effect. The following parameters of the IGFs system were studied: IGF-I receptors, IGF-I stimulated protein tyrosine phosphorylation, and membrane-associated and secreted IGF-binding proteins (IGFBPs). In Ishikawa cells, tamoxifen, similar to estradiol, increased IGF-I stimulated tyrosine phosphorylation of cellular substrates in accordance with its effect on cell growth. This effect of tamoxifen was inverted in MCF-7 cells. Tamoxifen did not affect the number or affinity of IGF-I receptors in both Ishikawa and MCF-7 cells, however, it caused a three-fold decrease in membrane-associated IGFBPs in the endometrial cells but an increase in these proteins in breast cancer cells. Similar but much less pronounced changes in soluble IGFBPs were observed. Our results indicate that the opposing growth effects of tamoxifen an endometrial and mammary cancer cells are associated with modulation of the IGF system components, mainly with reciprocal changes in membrane-associated IGFBPs.