Vascular insulin-like growth factor-I resistance and diet-induced obesity

Impact of different hormones on the regulation of nitric oxide in diabetes

Polymetabolic syndrome achieved pandemic proportions and dramatically influenced public health systems functioning worldwide. Chronic vascular complications are the major contributors to increased morbidity, disability, and mortality rates in diabetes patients. Nitric oxide (NO) is among the most important vascular bed function regulators. However, NO homeostasis is significantly deranged in pathological conditions. Additionally, different hormones directly or indirectly affect NO production and activity and subsequently act on vascular physiology. In this paper, we summarize the recent literature data related to the effects of insulin, estradiol, insulin-like growth factor-1, ghrelin, angiotensin II and irisin on the NO regulation in physiological and diabetes circumstances.

Cixutumumab reveals a critical role for IGF-1 in adipose and hepatic tissue remodelling during the development of diet-induced obesity

High fat diet (HFD)-induced obesity leads to perturbation in the storage function of white adipose tissue (WAT) resulting in deposition of lipids in tissues ill-equipped to deal with this challenge. The role of insulin like growth factor-1 (IGF-1) in the systemic and organ-specific responses to HFD is unclear. Using cixutumumab, a monoclonal antibody that internalizes and degrades cell surface IGF-1 receptors (IGF-1 R), leaving insulin receptor expression unchanged we aimed to establish the role of IGF-1 R in the response to a HFD. Mice treated with cixutumumab fed standard chow developed mild hyperinsulinemia with no change in WAT. When challenged by HFD mice treated with cixutumumab had reduced weight gain, reduced WAT expansion, and reduced hepatic lipid vacuole formation. In HFD-fed mice, cixutumumab led to reduced levels of genes encoding proteins important in fatty acid metabolism in WAT and liver. Cixutumumab protected against blunting of insulin-stimulated phosphorylation of Akt in liver of HFD fed mice. These data reveal an important role for IGF-1 R in the WAT and hepatic response to short-term nutrient excess. IGF-1 R inhibition during HFD leads to a lipodystrophic phenotype with a failure of WAT lipid storage and protection from HFD-induced hepatic insulin resistance.

Concentrations of Insulin-like Growth Factors and Insulin-like Growth Factor-Binding Proteins and Respective Gene Expressions in Children before and after Hematopoietic Stem Cell Transplantation

Insulin-like growth factors (IGF-1 and IGF-2) and insulin-like growth factor-binding proteins (IGFBP-1 to -7) are involved in the regulation of cell proliferation and differentiation and may be associated with various metabolic parameters. The aim of our study was to compare levels of IGFs and IGFBPs and the expressions of their genes in children before and after hematopoietic stem cell transplantation (HSCT) to assess their potential as markers of late metabolic complications of HSCT. We also conducted additional comparisons with healthy controls and of correlations of IGF and IGFBP levels with anthropometric and biochemical parameters. We analyzed 19 children treated with HSCT and 21 healthy controls. We found no significant differences in the levels of IGFs and IGFBPs and expressions of their genes before and after HSCT, while IGF and IGFBP levels were significantly lower in children treated with HSCT compared with controls. We conclude that our results did not reveal significant differences between the levels of IGFs and IGFBPs before and after HSCT, which would make them obvious candidates for markers of late complications of the procedure in children. However, due to the very low number of patients this conclusion must be taken with caution and may be altered by further research.

Novel Paracrine Action of Endothelium Enhances Glucose Uptake in Muscle and Fat

[Figure: see text].

KLF4 (Kruppel-Like Factor 4)-Dependent Perivascular Plasticity Contributes to Adipose Tissue inflammation

OBJECTIVE

Smooth muscle cells and pericytes display remarkable plasticity during injury and disease progression. Here, we tested the hypothesis that perivascular cells give rise to Klf4-dependent macrophage-like cells that augment adipose tissue (AT) inflammation and metabolic dysfunction associated with diet-induced obesity (DIO). Approach and Results: Using Myh11-CreERT2 eYFP (enhanced yellow fluorescent protein) mice and flow cytometry of the stromovascular fraction of epididymal AT, we observed a large fraction of smooth muscle cells and pericytes lineage traced eYFP+ cells expressing macrophage markers. Subsequent single-cell RNA sequencing, however, showed that the majority of these cells had no detectable eYFP transcript. Further exploration revealed that intraperitoneal injection of tamoxifen in peanut oil, used for generating conditional knockout or reporter mice in thousands of previous studies, resulted in large increase in the autofluorescence and false identification of macrophages within epididymal AT as being eYFP+; and unintended proinflammatory consequences. Using newly generated Myh11-DreERT2tdTomato mice given oral tamoxifen, we virtually eliminated the problem with autofluorescence and identified 8 perivascular cell dominated clusters, half of which were altered upon DIO. Given that perivascular cell KLF4 (kruppel-like factor 4) can have beneficial or detrimental effects, we tested its role in obesity-associated AT inflammation. While smooth muscle cells and pericytes-specific Klf4 knockout (smooth muscle cells and pericytes Klf4Δ/Δ) mice were not protected from DIO, they displayed improved glucose tolerance upon DIO, and showed marked decreases in proinflammatory macrophages and increases in LYVE1+ lymphatic endothelial cells in the epididymal AT.

CONCLUSIONS

Perivascular cells within the AT microvasculature dynamically respond to DIO and modulate tissue inflammation and metabolism in a KLF4-dependent manner.

Acromegaly, inflammation and cardiovascular disease: a review

Acromegaly is characterized by Growth Hormone (GH) and Insulin-like Growth Factor 1 (IGF-1) excess. Uncontrolled acromegaly is associated with a strongly increased risk of cardiovascular disease (CVD), and numerous cardiovascular risk factors remain present after remission. GH and IGF-1 have numerous effects on the immune and cardiovascular system. Since endothelial damage and systemic inflammation are strongly linked to the development of CVD, and have been suggested to be present in both controlled as uncontrolled acromegaly, they may explain the presence of both micro- and macrovascular dysfunction in these patients. In addition, these changes seem to be only partially reversible after remission, as illustrated by the often reported presence of endothelial dysfunction and microvascular damage in controlled acromegaly. Previous studies suggest that insulin resistance, oxidative stress, and endothelial dysfunction are involved in the development of CVD in acromegaly. Not surprisingly, these processes are associated with systemic inflammation and respond to GH/IGF-1 normalizing treatment.

Effect of Anthocyanins Supplementation on Serum IGFBP-4 Fragments and Glycemic Control in Patients with Fasting Hyperglycemia: A Randomized Controlled Trial

BACKGROUND

Insulin-like growth factor binding protein-4 (IGFBP-4) fragments have been shown to be associated with cardiometabolic diseases. Anthocyanins as a subgroup of natural polyphenols could have benefits on treating cardiometabolic diseases. The aim of this study was to examine the effects of purified anthocyanins on serum IGFBP-4 fragments and glycemic control in patients with fasting hyperglycemia.

METHODS

A set of 121 participants with elevated fasting glucose (≥5.6 mmol/L), who were originally randomly assigned to anthocyanins (320 mg/day) or placebo groups, were included in this study. Serum IGFBP-4 fragments, fasting and postload glucose, insulin, and C-peptide after a three-hour oral glucose tolerance test (OGTT) were measured at baseline and at the end of 12 weeks.

RESULTS

Compared with placebo, anthocyanins increased serum IGFBP-4 fragments (net change 8.33 ng/mL, 95% CI [1.2, 15.47], p=0.023) and decreased fasting glucose (-0.4 mmol/L [-0.71, -0.1], p=0.01), 2-hour C-peptide (-1.02 ng/mL [-1.99, -0.04], p=0.041) and the 3-hour area under the curve (AUC) of C-peptide (-2.19 [-4.11, -0.27], p=0.026). No other significant difference in parameters for glycemic control and insulin resistance was observed.

CONCLUSION

Anthocyanins supplementation for 12 weeks improved serum IGFBP-4 fragments and decreased fasting glucose and postload C-peptide in patients with fasting hyperglycemia. Further studies are needed to confirm our findings and clarify the potential mechanism.

TRIAL REGISTRATION

ClinicalTrials.gov, NCT02689765. Registered on 6 February 2016, https://clinicaltrials.gov/ct2/show/NCT02689765.

IGFBP-1 in Cardiometabolic Pathophysiology-Insights From Loss-of-Function and Gain-of-Function Studies in Male Mice

We have previously reported that overexpression of human insulin-like growth factor binding protein (IGFBP)-1 in mice leads to vascular insulin sensitization, increased nitric oxide bioavailability, reduced atherosclerosis, and enhanced vascular repair, and in the setting of obesity improves glucose tolerance. Human studies suggest that low levels of IGFBP-1 are permissive for the development of diabetes and cardiovascular disease. Here we seek to determine whether loss of IGFBP-1 plays a causal role in the predisposition to cardiometabolic disease. Metabolic phenotyping was performed in transgenic mice with homozygous knockout of IGFBP-1. This included glucose, insulin, and insulin-like growth factor I tolerance testing under normal diet and high-fat feeding conditions. Vascular phenotyping was then performed in the same mice using vasomotor aortic ring studies, flow cytometry, vascular wire injury, and angiogenesis assays. These were complemented with vascular phenotyping of IGFBP-1 overexpressing mice. Metabolic phenotype was similar in IGFBP-1 knockout and wild-type mice subjected to obesity. Deletion of IGFBP-1 inhibited endothelial regeneration following injury, suggesting that IGFBP-1 is required for effective vascular repair. Developmental angiogenesis was unaltered by deletion or overexpression of IGFBP-1. Recovery of perfusion following hind limb ischemia was unchanged in mice lacking or overexpressing IGFBP-1; however, overexpression of IGFBP-1 stimulated hindlimb perfusion and angiogenesis in insulin-resistant mice. These findings provide new insights into the role of IGFBP-1 in metabolic and vascular pathophysiology. Irrespective of whether loss of IGFBP-1 plays a causal role in the development of cardiometabolic disorders, increasing IGFBP-1 levels appears effective in promoting neovascularization in response to ischemia.

Effects of IGF-1 on the Cardiovascular System

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Cardiovascular (CV) diseases are the most common health problems worldwide, with a permanent increase in incidence. Growing evidence underlines that insulin-like growth factor 1 (IGF-1) is a very important hormone responsible for normal CV system physiology. IGF-1 is an anabolic growth hormone, responsible for cell growth, differentiation, proliferation, and survival. Despite systemic effects, IGF-1 exerts a wide array of influences in the CV system affecting metabolic homeostasis, vasorelaxation, cardiac contractility and hypertrophy, autophagy, apoptosis, and antioxidative processes. The vasodilatory effect of IGF-1, is achieved through the regulation of the activity of endothelial nitric oxide synthase (eNOS) and, at least partly, through enhancing inducible NOS (iNOS) activity. Also, IGF-1 stimulates vascular relaxation through regulation of sodium/potassiumadenosine- triphosphatase. Numerous animal studies provided evidence of diverse influences of IGF-1 in the CV system such as vasorelaxation, anti-apoptotic and prosurvival effects. Human studies indicate that low serum levels of free or total IGF-1 contribute to an increased risk of CV and cerebrovascular disease. Large human trials aiming at finding clinical efficacy and outcome of IGF-1-related therapy are of great interest.

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We look forward to the development of new IGF 1 therapies with minor side effects. In this review, we discuss the latest literature data regarding the function of IGF-1 in the CV system in the physiological and pathophysiological conditions.

Insulin like growth factor 1 is linked to higher cardiovascular risk score in adults with type 2 diabetes mellitus and chronic kidney disease

INTRODUCTION

Insulin-like growth factor-1 (IGF-1) is a peptide that shares sequence homology with insulin and has endocrine, paracrine and autocrine functions, acts on endothelial cells, and stimulates angiogenesis. IGF1 also affects renal hemodynamics both directly and indirectly by interacting with the renin-angiotensin system.

OBJECTIVE

The study aimed at detecting a relation between age-adjusted IGF1 (AAIGF1) and cardiovascular risk score in adults with type 2 diabetes mellitus and chronic kidney disease.

DESIGN

Patients were 90 females and 42 males with different stages of CKD ranging from 0 to 4. After taking a consent, serum IGF1 was recorded and adjusted for the age of the patient using the IGF1 score equation: {[(log IGF-1 + 0.00625xage)-2.555]/0.104}. Both univariate and multivariate regression analyses of AAIGF1 to different metabolic parameters and microvascular complications of type 2 DM were done. A ROC curve for CV risk score was issued.

RESULTS

AAIGF1 showed a significant bidirectional change with the stage of CKD. Univariate analysis was done including cardiovascular parameters in relations to AAIGF1. A significant positive correlation was found between AAIGF1 and CV risk score (B = 0.036, p = 0.003), SBP (B = 0.030, P = 0.004) and DBP (B = 0.071, P = 0.000), with a reciprocal relation to EF (B = -0.050, P = 0.016). Multivariate regression showed a significant correlation between AAIGF1 and age, HOMAIR, HOMAB, Uric acid. A ROC curve with AUC of 0.675, P = 0.003, showed that AAIGF1 of approximately -1.7 is a cut off for intermediate CV risk (10 year risk score >7.5%).

CONCLUSION

AAIGF1 shows a bidirectional relation to the grade of chronic kidney disease in adults with type 2 DM. A cut off point for AAIGF1 was set to indicate intermediate CV risk score, which can encourage using AAIGF1 as a prognostic marker for higher CV risk score. Medications that modulate IGF1 level can affect CV risk.

Effects of obesity on insulin: insulin-like growth factor 1 hybrid receptor expression and Akt phosphorylation in conduit and resistance arteries

Insulin and insulin-like growth factor-1 stimulate specific responses in arteries, which may be disrupted by diet-induced obesity. We examined (1) temporal effects of high-fat diet compared to low-fat diet in mice on insulin receptor, insulin-like growth factor-1 receptor, insulin receptor/insulin-like growth factor-1 receptor hybrid receptor expression and insulin/insulin-like growth factor-1-mediated Akt phosphorylation in aorta; and (2) effects of high-fat diet on insulin and insulin-like growth factor-1-mediated Akt phosphorylation and vascular tone in resistance arteries. Medium-term high-fat diet (5 weeks) decreased insulin-like growth factor-1 receptor expression and increased hybrid expression (~30%) only. After long-term (16 weeks) high-fat diet, insulin receptor expression was reduced by ~30%, insulin-like growth factor-1 receptor expression decreased a further ~40% and hybrid expression increased a further ~60%. Independent correlates of hybrid receptor expression were high-fat diet, duration of high-fat diet and plasma insulin-like growth factor-1 (all p < 0.05). In aorta, insulin was a more potent activator of Akt than insulin-like growth factor-1, whereas in resistance arteries, insulin-like growth factor-1 was more potent than insulin. High-fat diet blunted insulin-mediated vasorelaxation ( p < 0.01) but had no effect on insulin-like growth factor-1-mediated vasorelaxation in resistance arteries. Our findings support the possibility that hybrid receptor level is influenced by nutritional and metabolic cues. Moreover, vessel-dependent effects of insulin and insulin-like growth factor-1 on vascular tone and Akt activation may have implications in treating obesity-related vascular disease.

The insulin like growth factor and binding protein family: Novel therapeutic targets in obesity & diabetes

Background

Recent changes in nutrition and lifestyle have provoked an unprecedented increase in the prevalence of obesity and metabolic disorders. Recognition of the adverse effects on health has prompted intense efforts to understand the molecular determinants of insulin sensitivity and dysglycemia. In many respects, actions of insulin-like growth factors (IGFs) mirror those of insulin in metabolic regulation. Unlike insulin, however, the bioactivity of IGFs is regulated by a family of seven high-affinity binding proteins (IGFBPs) which confer temporospatial modulation with implications for metabolic homeostasis. In addition, evidence is accumulating that IGF-independent actions of certain of the IGFBPs can directly modulate insulin sensitivity.

Scope of review

In this review, we discuss the experimental data indicating a critical role for IGF/IGFBP axis in metabolic regulation. We highlight key discoveries through which IGFBPs have emerged as biomarkers or putative therapeutic targets in obesity and diabetes.

Major conclusions

Growing evidence suggests that several components of the IGF-IGFBP system could be explored for therapeutic potential in metabolic disorders. Both IGFBP-1 and IGFBP-2 have been favorably linked with insulin sensitivity in humans and preclinical data implicate direct involvement in the molecular regulation of insulin signaling and adiposity respectively. Further studies are warranted to evaluate clinical translation of these findings.

ATP sensitive K+ channel subunits (Kir6.1, Kir6.2) are the candidate mediators regulating ameliorating effects of pulsed magnetic field on aortic contractility in diabetic rats

Diabetes mellitus is a metabolic disease that causes increased morbidity and mortality in developed and developing countries. With recent advancements in technology, alternative treatment methods have begun to be investigated in the world. This study aims to evaluate the effect of pulsed magnetic field (PMF) on vascular complications and contractile activities of aortic rings along with Kir6.1 and Kir6.2 subunit expressions of ATP-sensitive potassium channels (K_ATP ) in aortas of controlled-diabetic and non-controlled diabetic rats. Controlled-diabetic and non-controlled diabetic adult male Wistar rats were exposed to PMF for a period of 6 weeks according to the PMF application protocol (1 h/day; intensity: 1.5 mT; consecutive frequency: 1, 10, 20, and 40 Hz). After PMF exposure, body weight and blood glucose levels were measured. Then, thoracic aorta tissue was extracted for relaxation-contraction and Kir6.1 and Kir6.2 expression experiments. Blood plasma glucose levels, body weight, and aortic ring contraction percentage decreased in controlled-diabetic rats but increased in non-controlled diabetic rats. PMF therapy repressed Kir6.1 mRNA expression in non-controlled diabetic rats but not in controlled diabetic rats. Conversely, Kir6.2 mRNA expressions were repressed both in controlled diabetic and non-controlled diabetic rats by PMF. Our findings suggest that the positive therapeutic effects of PMF may act through (K_ATP ) subunits and may frequently occur in insulin-free conditions. Bioelectromagnetics. 39:299-311, 2018. © 2018 Wiley Periodicals, Inc.

Insulinlike Growth Factor-Binding Protein-1 Improves Vascular Endothelial Repair in Male Mice in the Setting of Insulin Resistance

Insulin resistance is associated with impaired endothelial regeneration in response to mechanical injury. We recently demonstrated that insulinlike growth factor-binding protein-1 (IGFBP1) ameliorated insulin resistance and increased nitric oxide generation in the endothelium. In this study, we hypothesized that IGFBP1 would improve endothelial regeneration and restore endothelial reparative functions in the setting of insulin resistance. In male mice heterozygous for deletion of insulin receptors, endothelial regeneration after femoral artery wire injury was enhanced by transgenic expression of human IGFBP1 (hIGFBP1). This was not explained by altered abundance of circulating myeloid angiogenic cells. Incubation of human endothelial cells with hIGFBP1 increased integrin expression and enhanced their ability to adhere to and repopulate denuded human saphenous vein ex vivo. In vitro, induction of insulin resistance by tumor necrosis factor α (TNFα) significantly inhibited endothelial cell migration and proliferation. Coincubation with hIGFBP1 restored endothelial migratory and proliferative capacity. At the molecular level, hIGFBP1 induced phosphorylation of focal adhesion kinase, activated RhoA and modulated TNFα-induced actin fiber anisotropy. Collectively, the effects of hIGFBP1 on endothelial cell responses and acceleration of endothelial regeneration in mice indicate that manipulating IGFBP1 could be exploited as a putative strategy to improve endothelial repair in the setting of insulin resistance.

Serum Insulin-Like Growth Factor 1 and the Risk of Ischemic Stroke: The Framingham Study

BACKGROUND AND PURPOSE

Low insulin-like growth factor 1 (IGF-1) has been associated with increased risk of atherosclerosis and atrial fibrillation in cross-sectional studies. Yet, prospective data linking IGF-1 levels to the development of ischemic stroke remain inconclusive. We examined prospectively the association between serum IGF-1 levels and incident ischemic stroke.

METHODS

We measured serum IGF-1 levels in 757 elderly individuals (mean age 79±5, 62% women), free of prevalent stroke, from the Framingham original cohort participants at the 22nd examination cycle (1990-1994) and were followed up for the development of ischemic stroke. Cox models were used to relate IGF-1 levels to the risk for incident ischemic stroke, adjusted for potential confounders.

RESULTS

During a mean follow-up of 10.2 years, 99 individuals developed ischemic stroke. After adjustment for age, sex, and potential confounders, higher IGF-1 levels were associated with a lower risk of incident ischemic stroke, with subjects in the lowest quintile of IGF-1 levels having a 2.3-fold higher risk of incident ischemic stroke (95% confidence interval, 1.09-5.06; P=0.03) as compared with those in the top quintile. We observed an effect modification by diabetes mellitus and waist-hip ratio for the association between IGF-1 and ischemic stroke (P<0.1). In subgroup analyses, the effects were restricted to subjects with diabetics and those in top waist-hip ratio quartile, in whom each standard deviation increase in IGF-1 was associated with a 61% (hazard ratio, 0.39; 95% confidence interval, 0.20-0.78; P=0.007) and 41% (hazard ratio, 0.59; 95% confidence interval, 0.37-0.95; P=0.031) lower risk of incident ischemic stroke, respectively.

CONCLUSIONS

IGF-1 levels were inversely associated with ischemic stroke, especially among persons with insulin resistance.

Attenuated IGF-1 predicts all-cause and cardiovascular mortality in a Black population: A five-year prospective study

BACKGROUND

Inconsistent findings are reported on whether insulin-like growth factor-1 (IGF-1) is protective or harmful in predicting hypertension, carotid wall thickness and mortality. We determined the five-year prognostic value of IGF-1 for these outcomes in a large Black population prone to hypertension and cardiovascular disease.

DESIGN

A longitudinal study as part of the PURE (Prospective Urban and Rural Epidemiology) study, North West Province, South Africa.

METHODS

We measured IGF-1 and IGF binding protein-3 (IGFBP-3) in 1038 HIV-uninfected participants (age range 32-94 years) and assessed blood pressure, carotid intima-media thickness and mortality.

RESULTS

Over five years 116 deaths occurred. Baseline IGF-1 was similar in survivors and non-survivors (p = 0.50), but tended to be higher in survivors upon adjustment for IGFBP-3 and covariates (p = 0.061). Normotensives and hypertensives (p = 0.072), and those with carotid intima-media thickness < 0.9 mm and ≥ 0.9 mm also displayed similar baseline IGF-1 (p = 0.55). Multivariable-adjusted Cox-regression indicated high IGF-1 predicting lower risk for all-cause mortality (hazard ratio 0.45; 0.23-0.88) and cardiovascular mortality (hazard ratio 0.26; 0.08-0.83) when also adjusting for IGFBP-3. When including normo- and hypertensives at baseline, high IGF-1 was related to normotension at follow-up (hazard ratio 0.68; 0.49-0.95). We found no association with carotid intima-media thickness (hazard ratio 0.59; 0.31-1.14).

CONCLUSION

In a Black South African population with low socio-economic status and harmful health behaviours, we found a protective independent association between IGF-1 and hypertension, cardiovascular and all-cause mortality, with no association with carotid wall thickness.

Insulin-Like Growth Factor-1 Receptor Deficiency in Macrophages Accelerates Atherosclerosis and Induces an Unstable Plaque Phenotype in Apolipoprotein E-Deficient Mice

BACKGROUND

We have previously shown that systemic infusion of insulin-like growth factor-1 (IGF-1) exerts anti-inflammatory and antioxidant effects and reduces atherosclerotic burden in apolipoprotein E (Apoe)-deficient mice. Monocytes/macrophages express high levels of IGF-1 receptor (IGF1R) and play a pivotal role in atherogenesis, but the potential effects of IGF-1 on their function are unknown.

METHODS AND RESULTS

To determine mechanisms whereby IGF-1 reduces atherosclerosis and to explore the potential involvement of monocytes/macrophages, we created monocyte/macrophage-specific IGF1R knockout (MΦ-IGF1R-KO) mice on an Apoe(-/-) background. We assessed atherosclerotic burden, plaque features of stability, and monocyte recruitment to atherosclerotic lesions. Phenotypic changes of IGF1R-deficient macrophages were investigated in culture. MΦ-IGF1R-KO significantly increased atherosclerotic lesion formation, as assessed by Oil Red O staining of en face aortas and aortic root cross-sections, and changed plaque composition to a less stable phenotype, characterized by increased macrophage and decreased α-smooth muscle actin-positive cell population, fibrous cap thinning, and decreased collagen content. Brachiocephalic artery lesions of MΦ-IGF1R-KO mice had histological features implying plaque vulnerability. Macrophages isolated from MΦ-IGF1R-KO mice showed enhanced proinflammatory responses on stimulation by interferon-γ and oxidized low-density lipoprotein and elevated antioxidant gene expression levels. Moreover, IGF1R-deficient macrophages had decreased expression of ABCA1 and ABCG1 and reduced lipid efflux.

CONCLUSIONS

Our data indicate that macrophage IGF1R signaling suppresses macrophage and foam cell accumulation in lesions and reduces plaque vulnerability, providing a novel mechanism whereby IGF-1 exerts antiatherogenic effects.

Endothelial IGF-1 Receptor Signalling in Diabetes and Insulin Resistance

Despite contemporary medical therapy, people with diabetes and insulin resistance experience substantially increased risk of cardiovascular events caused by atherosclerosis. Dysfunction of the endothelium is a key phase in early atherogenesis and represents a promising therapeutic target. The evolutionarily related insulin and insulin-like growth factor-1 (IGF-1) axes are implicated in the development of vascular disease. In this review, we summarise recent developments in our understanding of how modulating the IGF-1 axis influences vascular disease in the setting of insulin resistance. By contrasting data from models of altered insulin and/or IGF-1 signalling, we emphasise the complex spatiotemporal interplay of these systems in health and disease. We then discuss therapeutic opportunities, before detailing important gaps in our knowledge relevant to therapeutic translation.

Endothelial cells and the IGF system

Endothelial cells line blood vessels and modulate vascular tone, thrombosis, inflammatory responses and new vessel formation. They are implicated in many disease processes including atherosclerosis and cancer. IGFs play a significant role in the physiology of endothelial cells by promoting migration, tube formation and production of the vasodilator nitric oxide. These actions are mediated by the IGF1 and IGF2/mannose 6-phosphate receptors and are modulated by a family of high-affinity IGF binding proteins. IGFs also increase the number and function of endothelial progenitor cells, which may contribute to protection from atherosclerosis. IGFs promote angiogenesis, and dysregulation of the IGF system may contribute to this process in cancer and eye diseases including retinopathy of prematurity and diabetic retinopathy. In some situations, IGF deficiency appears to contribute to endothelial dysfunction, whereas IGF may be deleterious in others. These differences may be due to tissue-specific endothelial cell phenotypes or IGFs having distinct roles in different phases of vascular disease. Further studies are therefore required to delineate the therapeutic potential of IGF system modulation in pathogenic processes.

Haploinsufficiency of the Insulin-Like Growth Factor-1 Receptor Enhances Endothelial Repair and Favorably Modifies Angiogenic Progenitor Cell Phenotype

Objectives—

Defective endothelial regeneration predisposes to adverse arterial remodeling and is thought to contribute to cardiovascular disease in type 2 diabetes mellitus. We recently demonstrated that the type 1 insulin-like growth factor receptor (IGF1R) is a negative regulator of insulin sensitivity and nitric oxide bioavailability. In this report, we examined partial deletion of the IGF1R as a potential strategy to enhance endothelial repair.

Approach and Results—

We assessed endothelial regeneration after wire injury in mice and abundance and function of angiogenic progenitor cells in mice with haploinsufficiency of the IGF1R (IGF1R +/− ). Endothelial regeneration after arterial injury was accelerated in IGF1R +/− mice. Although the yield of angiogenic progenitor cells was lower in IGF1R +/− mice, these angiogenic progenitor cells displayed enhanced adhesion, increased secretion of insulin-like growth factor-1, and enhanced angiogenic capacity. To examine the relevance of IGF1R manipulation to cell-based therapy, we transfused IGF1R +/− bone marrow–derived CD117 + cells into wild-type mice. IGF1R +/− cells accelerated endothelial regeneration after arterial injury compared with wild-type cells and did not alter atherosclerotic lesion formation.

Conclusions—

Haploinsufficiency of the IGF1R is associated with accelerated endothelial regeneration in vivo and enhanced tube forming and adhesive potential of angiogenic progenitor cells in vitro. Partial deletion of IGF1R in transfused bone marrow–derived CD117 + cells enhanced their capacity to promote endothelial regeneration without altering atherosclerosis. Our data suggest that manipulation of the IGF1R could be exploited as novel therapeutic approach to enhance repair of the arterial wall after injury.

Interaction between insulin-like growth factor-1 and atherosclerosis and vascular aging

The process of vascular aging encompasses alterations in the function of endothelial (ECs) and vascular smooth muscle cells (VSMCs) via oxidation, inflammation, cell senescence and epigenetic modifications, increasing the probability of atherosclerosis. Aged vessels exhibit decreased endothelial antithrombogenic properties, increased reactive oxygen species generation, inflammatory signaling and migration of VSMCs to the subintimal space, impaired angiogenesis and increased elastin degradation. The key initiating step in atherogenesis is subendothelial accumulation of apolipoprotein B-containing low-density lipoproteins resulting in activation of ECs and recruitment of monocytes. Activated ECs secrete 'chemokines' that interact with cognate chemokine receptors on monocytes and promote directional migration. Recruitment of immune cells establishes a proinflammatory status, further causing elevated oxidative stress, which in turn triggers a series of events including apoptotic or necrotic death of vascular and nonvascular cells. Increased oxidative stress is also considered to be a key factor in mechanisms of aging-associated changes in tissue integrity and function. Experimental evidence indicates that insulin-like growth factor-1 exerts antioxidant, anti-inflammatory and pro-survival effects on the vasculature, reducing atherosclerotic plaque burden and promoting features of atherosclerotic plaque stability.

Revisiting the relationship between blood pressure and insulin-like growth factor-1

Conflicting evidence exists on the relationship between blood pressure (BP) and insulin-like growth factor-1 (IGF-1). We reviewed available articles and pooled extrapolated regression coefficients for the association between BP and total IGF-1 as reported in the literature and included additional data from 912 individuals from the general population. We identified 20 studies including 11 704 subjects. We also measured total IGF-1, insulin-like binding protein-3, and BP in 912 black and white men and women from South Africa (aged 20-70 years). When plotting positive and negative weighed regression coefficients (29 data points) against IGF-1, we found a significant positive relationship (r=0.31; P<0.001; n=11 704) intercepting the 0 point at 191 ng/mL IGF-1, suggesting an inverse BP/IGF-1 relationship in low IGF-1 conditions, and a positive relationship in overtly high IGF-1 conditions. In conclusion, our findings suggest that the relationship between BP and IGF-1 is dependent on, or related to, IGF-1 concentrations, as an expression of direct or reverse causality. Low IGF-1 bioavailability (associated with aging and vascular deterioration), resistance to IGF-1, and the complex interplay between IGF-1 and other vasoactive hormones could mask the vasoprotective functions of IGF-1 in cross-sectional studies or could modify their functions in prospective studies.

Leptin receptor deficient diabetic (db/db) mice are compromised in postnatal bone regeneration

Increased fragility fracture risk with improper healing is a frequent and severe complication of insulin resistance (IR). The mechanisms impairing bone health in IR are still not fully appreciated, which gives importance to studies on bone pathologies in animal models of diabetes. Mice deficient in leptin signaling are widely used models of IR and its comorbidities. Leptin was first recognized as a hormone, regulating appetite and energy balance; however, recent studies have expanded its role showing that leptin is a link between insulin-dependent metabolism and bone homeostasis. In the light of these findings, it is intriguing to consider the role of leptin resistance in bone regeneration. In this study, we show that obese diabetic mice lacking leptin receptor (db/db) are deficient in postnatal regenerative osteogenesis. We apply an ectopic osteogenesis and a fracture healing model, both showing that db/db mice display compromised bone acquisition and regeneration capacity. The underlying mechanisms include delayed periosteal mesenchymatic osteogenesis, premature apoptosis of the cartilage callus and impaired microvascular invasion of the healing tissue. Our study supports the use of the db/db mouse as a model of IR associated bone-healing deficits and can aid further studies of mesenchymatic cell homing and differentiation, microvascular invasion, cartilage to bone transition and callus remodeling in diabetic fracture healing.

Importance of insulin resistance to vascular repair and regeneration

Metabolic insulin resistance is apparent across a spectrum of clinical disorders, including obesity and diabetes, and is characterized by an adverse clustering of cardiovascular risk factors related to abnormal cellular responses to insulin. These disorders are becoming increasingly prevalent and represent a major global public health concern because of their association with significant increases in atherosclerosis-related mortality. Endogenous repair mechanisms are thought to retard the development of vascular disease, and a growing evidence base supports the adverse impact of the insulin-resistant phenotype upon indices of vascular repair. Beyond the impact of systemic metabolic changes, emerging data from murine studies also provide support for abnormal insulin signaling at the level of vascular cells in retarding vascular repair. Interrelated pathophysiological factors, including reduced nitric oxide bioavailability, oxidative stress, altered growth factor activity, and abnormal intracellular signaling, are likely to act in conjunction to impede vascular repair while also driving vascular damage. Understanding of these processes is shaping novel therapeutic paradigms that aim to promote vascular repair and regeneration, either by recruiting endogenous mechanisms or by the administration of cell-based therapies.

Endothelium-specific insulin resistance leads to accelerated atherosclerosis in areas with disturbed flow patterns: a role for reactive oxygen species

OBJECTIVE

Systemic insulin resistance is associated with a portfolio of risk factors for atherosclerosis development. We sought to determine whether insulin resistance specifically at the level of the endothelium promotes atherosclerosis and to examine the potential involvement of reactive oxygen species.

METHODS

We cross-bred mice expressing a dominant negative mutant human insulin receptor specifically in the endothelium (ESMIRO) with ApoE(-/-) mice to examine the effect of endothelium-specific insulin resistance on atherosclerosis.

RESULTS

ApoE(-/-)/ESMIRO mice had similar blood pressure, plasma lipids and whole-body glucose tolerance, but blunted endothelial insulin signalling, in comparison to ApoE(-/-) mice. Atherosclerosis was significantly increased in ApoE(-/-)/ESMIRO mice at the aortic sinus (226 ± 16 versus 149 ± 24 × 10(3) μm(2), P = 0.01) and lesser curvature of the aortic arch (12.4 ± 1.2% versus 9.4 ± 0.9%, P = 0.035). Relaxation to acetylcholine was blunted in aorta from ApoE(-/-)/ESMIRO mice (Emax 65 ± 41% versus 103 ± 6%, P = 0.02) and was restored by the superoxide dismutase mimetic MnTMPyP (Emax 112 ± 15% versus 65 ± 41%, P = 0.048). Basal generation of superoxide was increased 1.55 fold (P = 0.01) in endothelial cells from ApoE(-/-)/ESMIRO mice and was inhibited by the NADPH oxidase inhibitor gp91ds-tat (-12 ± 0.04%, P = 0.04), the NO synthase inhibitor L-NMMA (-8 ± 0.02%, P = 0.001) and the mitochondrial specific inhibitor rotenone (-23 ± 0.04%, P = 0.006).

CONCLUSIONS

Insulin resistance specifically at the level of the endothelium leads to acceleration of atherosclerosis in areas with disturbed flow patterns such as the aortic sinus and the lesser curvature of the aorta. We have identified a potential role for increased generation of reactive oxygen species from multiple enzymatic sources in promoting atherosclerosis in this setting.

Diurnal variation in vascular and metabolic function in diet-induced obesity: divergence of insulin resistance and loss of clock rhythm

Circadian rhythms are integral to the normal functioning of numerous physiological processes. Evidence from human and mouse studies suggests that loss of rhythm occurs in obesity and cardiovascular disease and may be a neglected contributor to pathophysiology. Obesity has been shown to impair the circadian clock mechanism in liver and adipose tissue but its effect on cardiovascular tissues is unknown. We investigated the effect of diet-induced obesity in C57BL6J mice upon rhythmic transcription of clock genes and diurnal variation in vascular and metabolic systems. In obesity, clock gene function and physiological rhythms were preserved in the vasculature but clock gene transcription in metabolic tissues and rhythms of glucose tolerance and insulin sensitivity were blunted. The most pronounced attenuation of clock rhythm occurred in adipose tissue, where there was also impairment of clock-controlled master metabolic genes and both AMPK mRNA and protein. Across tissues, clock gene disruption was associated with local inflammation but diverged from impairment of insulin signaling. We conclude that vascular tissues are less sensitive to pathological disruption of diurnal rhythms during obesity than metabolic tissues and suggest that cellular disruption of clock gene rhythmicity may occur by mechanisms shared with inflammation but distinct from those leading to insulin resistance.

Cell-specific insulin resistance: implications for atherosclerosis

Insulin resistance is increasingly acknowledged as an independent risk factor for cardiovascular disease. Despite this, our understanding of the cellular and molecular mechanisms that might account for this relationship remain incompletely understood. A key challenge has been in distinguishing between a 'whole-body' milieu of inflammation and oxidative stress from the ramifications of cell-specific resistance to insulin. Transgenic models have now begun to explore the cellular influences of insulin resistance on vascular biology, with novel implications for atherosclerosis across a range of cells including endothelial cells, endothelial progenitor cells, vascular smooth muscle cells, macrophages and fibroblasts. Emerging data from these models have also begun to challenge conventional dogma. In particular, the findings across various cell types are disparate with some even implying a protective influence on vascular biology. We now review these data, highlighting recent advances in our understanding of cellular resistance to insulin as well as those areas where there remains a paucity of data.

A combinatorial in silico and cellular approach to identify a new class of compounds that target VEGFR2 receptor tyrosine kinase activity and angiogenesis

BACKGROUND AND PURPOSE

Vascular endothelial growth factor receptor 2 (VEGFR2) is an attractive therapeutic target for the treatment of diseases such as cancer. Small-molecule VEGFR2 inhibitors of a variety of chemical classes are currently under development or in clinical use. In this study, we describe the de novo design of a new generation pyrazole-based molecule (JK-P3) that targets VEGFR2 kinase activity and angiogenesis.

EXPERIMENTAL APPROACH

JK-P compound series were designed using de novo structure-based identification methods. Compounds were tested in an in vitro VEGFR2 kinase assay. Using primary endothelial cells, JK-P compounds were assessed for their ability to inhibit VEGF-A-stimulated VEGFR2 activation and intracellular signalling. We tested these compounds in cell migration, proliferation and angiogenesis assays.

KEY RESULTS

JK-P3 and JK-P5 were predicted to bind the VEGFR2 kinase domain with high affinity, and both compounds showed pronounced inhibition of endogenous VEGFR2 kinase activity in primary human endothelial cells. Only JK-P3 inhibited VEGF-A-stimulated VEGFR2 activation and intracellular signalling. Interestingly, JK-P3 inhibited endothelial monolayer wound closure and angiogenesis but not endothelial cell proliferation. Both compounds inhibited fibroblast growth factor receptor kinase activity in vitro, but not basic fibroblast growth factor-mediated signalling in endothelial cells.

CONCLUSIONS AND IMPLICATIONS

This is the first report that describes an anti-angiogenic inhibitor based on such a pyrazole core. Using a de novo structure-based identification approach is an attractive method to aid such drug discovery. These results thus provide an important basis for the development of multi-tyrosine kinase inhibitors for clinical use in the near future.

Novel role of the IGF-1 receptor in endothelial function and repair: studies in endothelium-targeted IGF-1 receptor transgenic mice

We recently demonstrated that reducing IGF-1 receptor (IGF-1R) numbers in the endothelium enhances nitric oxide (NO) bioavailability and endothelial cell insulin sensitivity. In the present report, we aimed to examine the effect of increasing IGF-1R on endothelial cell function and repair. To examine the effect of increasing IGF-1R in the endothelium, we generated mice overexpressing human IGF-1R in the endothelium (human IGF-1R endothelium-overexpressing mice [hIGFREO]) under direction of the Tie2 promoter enhancer. hIGFREO aorta had reduced basal NO bioavailability (percent constriction to N(G)-monomethyl-l-arginine [mean (SEM) wild type 106% (30%); hIGFREO 48% (10%)]; P < 0.05). Endothelial cells from hIGFREO had reduced insulin-stimulated endothelial NO synthase activation (mean [SEM] wild type 170% [25%], hIGFREO 58% [3%]; P = 0.04) and insulin-stimulated NO release (mean [SEM] wild type 4,500 AU [1,000], hIGFREO 1,500 AU [700]; P < 0.05). hIGFREO mice had enhanced endothelium regeneration after denuding arterial injury (mean [SEM] percent recovered area, wild type 57% [2%], hIGFREO 47% [5%]; P < 0.05) and enhanced endothelial cell migration in vitro. The IGF-1R, although reducing NO bioavailability, enhances in situ endothelium regeneration. Manipulating IGF-1R in the endothelium may be a useful strategy to treat disorders of vascular growth and repair.

Aging, atherosclerosis, and IGF-1

Insulin-like growth factor 1 (IGF-1) is an endocrine and autocrine/paracrine growth factor that circulates at high levels in the plasma and is expressed in most cell types. IGF-1 has major effects on development, cell growth and differentiation, and tissue repair. Recent evidence indicates that IGF-1 reduces atherosclerosis burden and improves features of atherosclerotic plaque stability in animal models. Potential mechanisms for this atheroprotective effect include IGF-1-induced reduction in oxidative stress, cell apoptosis, proinflammatory signaling, and endothelial dysfunction. Aging is associated with increased vascular oxidative stress and vascular disease, suggesting that IGF-1 may exert salutary effects on vascular aging processes. In this review, we will provide a comprehensive update on IGF-1's ability to modulate vascular oxidative stress and to limit atherogenesis and the vascular complications of aging.

Aldosterone induces vascular insulin resistance by increasing insulin-like growth factor-1 receptor and hybrid receptor

OBJECTIVE

We previously showed that aldosterone induces insulin resistance in rat vascular smooth muscle cells (VSMCs). Because insulin-like growth factor-1 receptor (IGF1R) affects insulin signaling, we hypothesized that aldosterone induces vascular insulin resistance and remodeling via upregulation of IGF1R and its hybrid insulin/insulin-like growth factor-1 receptor.

METHODS AND RESULTS

Hybrid receptor expression was measured by immunoprecipitation. Hypertrophy of VSMCs was evaluated by (3)H-labeled leucine incorporation. Aldosterone (10 nmol/L) significantly increased protein and mRNA expression of IGF1R and hybrid receptor in VSMCs but did not affect insulin receptor expression. Mineralocorticoid receptor blockade with eplerenone inhibited aldosterone-induced increases in IGF1R and hybrid receptor. Aldosterone augmented insulin (100 nmol/L)-induced extracellular signal-regulated kinase 1/2 phosphorylation. Insulin-induced leucine incorporation and α-smooth muscle actin expression were also augmented by aldosterone in VSMCs. These aldosterone-induced changes were significantly attenuated by eplerenone or picropodophyllin, an IGF1R inhibitor. Chronic infusion of aldosterone (0.75 μg/hour) increased blood pressure and aggravated glucose metabolism in rats. Expression of hybrid receptor, azan-positive area, and oxidative stress in aorta was increased in aldosterone-infused rats. Spironolactone and tempol prevented these aldosterone-induced changes.

CONCLUSIONS

Aldosterone induces vascular remodeling through IGF1R- and hybrid receptor-dependent vascular insulin resistance. Mineralocorticoid receptor blockade may attenuate angiopathy in hypertensive patients with hyperinsulinemia.

The insulin-like growth factor-1 receptor is a negative regulator of nitric oxide bioavailability and insulin sensitivity in the endothelium

OBJECTIVE

In mice, haploinsufficiency of the IGF-1 receptor (IGF-1R(+/-)), at a whole-body level, increases resistance to inflammation and oxidative stress, but the underlying mechanisms are unclear. We hypothesized that by forming insulin-resistant heterodimers composed of one IGF-1Rαβ and one insulin receptor (IR), IRαβ complex in endothelial cells (ECs), IGF-1R reduces free IR, which reduces EC insulin sensitivity and generation of the antioxidant/anti-inflammatory signaling radical nitric oxide (NO).

RESEARCH DESIGN AND METHODS

Using a number of complementary gene-modified mice with reduced IGF-1R at a whole-body level and specifically in EC, and complementary studies in EC in vitro, we examined the effect of changing IGF-1R/IR stoichiometry on EC insulin sensitivity and NO bioavailability.

RESULTS

IGF-1R(+/-) mice had enhanced insulin-mediated glucose lowering. Aortas from these mice were hypocontractile to phenylephrine (PE) and had increased basal NO generation and augmented insulin-mediated NO release from EC. To dissect EC from whole-body effects we generated mice with EC-specific knockdown of IGF-1R. Aortas from these mice were also hypocontractile to PE and had increased basal NO generation. Whole-body and EC deletion of IGF-1R reduced hybrid receptor formation. By reducing IGF-1R in IR-haploinsufficient mice we reduced hybrid formation, restored insulin-mediated vasorelaxation in aorta, and insulin stimulated NO release in EC. Complementary studies in human umbilical vein EC in which IGF-1R was reduced using siRNA confirmed that reducing IGF-1R has favorable effects on NO bioavailability and EC insulin sensitivity.

CONCLUSIONS

These data demonstrate that IGF-1R is a critical negative regulator of insulin sensitivity and NO bioavailability in the endothelium.

The role of IGF-1 resistance in obesity and type 2 diabetes-mellitus-related insulin resistance and vascular disease

IMPORTANCE OF THE FIELD

The insulin-resistant conditions of type 2 diabetes mellitus (T2DM) and obesity are a major cause of cardiovascular disease on a global scale. These disorders are not only a cause of ill health but are a huge financial drain on healthcare systems. T2DM leads to an increased risk of cardiovascular mortality equivalent to over 10 years of ageing while obesity independent of T2DM also leads to a substantially increased risk of acute myocardial infarction. Recent trials of therapeutic agents and approaches to preventing the cardiovascular complications of type 2 diabetes have been disappointing.

AREAS COVERED IN THIS REVIEW

The role of insulin resistance in the endothelium in the regulation of the anti-atherosclerotic signalling molecule NO and a potential important role for IGF-1 in vascular NO production.

WHAT THE READER WILL GAIN

A comprehensive understanding of how insulin and IGF-1 regulate vascular function and the effect of type 2 diabetes on these pathways.

TAKE HOME MESSAGE

The roles of insulin and IGF-1 in vascular function are complex and intimately related. Nevertheless IGF-1 signalling in the arterial wall has the potential to be manipulated to protect the vasculature against the development of atherosclerosis and its devastating complications.

IGF-1 and atherothrombosis: relevance to pathophysiology and therapy

IGF-1 (insulin-like growth factor-1) plays a unique role in the cell protection of multiple systems, where its fine-tuned signal transduction helps to preserve tissues from hypoxia, ischaemia and oxidative stress, thus mediating functional homoeostatic adjustments. In contrast, its deprivation results in apoptosis and dysfunction. Many prospective epidemiological surveys have associated low IGF-1 levels with late mortality, MI (myocardial infarction), HF (heart failure) and diabetes. Interventional studies suggest that IGF-1 has anti-atherogenic actions, owing to its multifaceted impact on cardiovascular risk factors and diseases. The metabolic ability of IGF-1 in coupling vasodilation with improved function plays a key role in these actions. The endothelial-protective, anti-platelet and anti-thrombotic activities of IGF-1 exert critical effects in preventing both vascular damage and mechanisms that lead to unstable coronary plaques and syndromes. The pro-survival and anti-inflammatory short-term properties of IGF-1 appear to reduce infarct size and improve LV (left ventricular) remodelling after MI. An immune-modulatory ability, which is able to suppress 'friendly fire' and autoreactivity, is a proposed important additional mechanism explaining the anti-thrombotic and anti-remodelling activities of IGF-1. The concern of cancer risk raised by long-term therapy with IGF-1, however, deserves further study. In the present review, we discuss the large body of published evidence and review data on rhIGF-1 (recombinant human IGF-1) administration in cardiovascular disease and diabetes, with a focus on dosage and safety issues. Perhaps the time has come for the regenerative properties of IGF-1 to be assessed as a new pharmacological tool in cardiovascular medicine.

Lack of dietary carbohydrates induces hepatic growth hormone (GH) resistance in rats

GH is a well established regulator of growth, lipid, and glucose metabolism and therefore important for fuel utilization. However, little is known about the effects of macronutrients on the GH/IGF system. We used low-carbohydrate/high-fat diets (LC-HFD) as a model to study the impact of fat, protein, and carbohydrates on the GH/IGF-axis; 12-wk-old Wistar rats were fed either regular chow, a moderate, protein-matched LC-HFD, or a ketogenic LC-HFD (percentage of fat/protein/carbohydrates: chow, 16.7/19/64.3; LC-HF-1, 78.7/19.1/2.2; LC-HF-2, 92.8/5.5/1.7). After 4 wk, body and tibia length, lean body mass, and fat pad weights were measured. Furthermore, we investigated the effects of LC-HFD on 1) secretion of GH and GH-dependent factors, 2) expression and signaling of components of the GH/IGF system in liver and muscle, and 3) hypothalamic and pituitary regulation of GH release. Serum concentrations of IGF-I, IGF binding protein-1, and IGF binding protein-3 were lower with LC-HF-1 and LC-HF-2 (P < 0.01). Both LC-HFD-reduced hepatic GH receptor mRNA and protein expression, decreased basal levels of total and phosphorylated Janus kinase/signal transducers and activators of transcription signaling proteins and reduced hepatic IGF-I gene expression. Hypothalamic somatostatin expression was reduced only with LC-HF-1, leading to increased pituitary GH secretion, higher IGF-I gene expression, and activation of IGF-dependent signaling pathways in skeletal muscle. In contrast, despite severely reduced IGF-I concentrations, GH secretion did not increase with LC-HF-2 diet. In conclusion, lack of carbohydrates in LC-HFD induces hepatic GH resistance. Furthermore, central feedback mechanisms of the GH/IGF system are impaired with extreme, ketogenic LC-HFD.

Insulin resistance impairs circulating angiogenic progenitor cell function and delays endothelial regeneration

OBJECTIVE

Circulating angiogenic progenitor cells (APCs) participate in endothelial repair after arterial injury. Type 2 diabetes is associated with fewer circulating APCs, APC dysfunction, and impaired endothelial repair. We set out to determine whether insulin resistance adversely affects APCs and endothelial regeneration.

RESEARCH DESIGN AND METHODS

We quantified APCs and assessed APC mobilization and function in mice hemizygous for knockout of the insulin receptor (IRKO) and wild-type (WT) littermate controls. Endothelial regeneration after femoral artery wire injury was also quantified after APC transfusion.

RESULTS

IRKO mice, although glucose tolerant, had fewer circulating Sca-1(+)/Flk-1(+) APCs than WT mice. Culture of mononuclear cells demonstrated that IRKO mice had fewer APCs in peripheral blood, but not in bone marrow or spleen, suggestive of a mobilization defect. Defective vascular endothelial growth factor-stimulated APC mobilization was confirmed in IRKO mice, consistent with reduced endothelial nitric oxide synthase (eNOS) expression in bone marrow and impaired vascular eNOS activity. Paracrine angiogenic activity of APCs from IRKO mice was impaired compared with those from WT animals. Endothelial regeneration of the femoral artery after denuding wire injury was delayed in IRKO mice compared with WT. Transfusion of mononuclear cells from WT mice normalized the impaired endothelial regeneration in IRKO mice. Transfusion of c-kit(+) bone marrow cells from WT mice also restored endothelial regeneration in IRKO mice. However, transfusion of c-kit(+) cells from IRKO mice was less effective at improving endothelial repair.

CONCLUSIONS

Insulin resistance impairs APC function and delays endothelial regeneration after arterial injury. These findings support the hypothesis that insulin resistance per se is sufficient to jeopardize endogenous vascular repair. Defective endothelial repair may be normalized by transfusion of APCs from insulin-sensitive animals but not from insulin-resistant animals.

Impaired hypothalamic regulation of endocrine function and delayed counterregulatory response to hypoglycemia in Magel2-null mice

Hypothalamic dysfunction may underlie endocrine abnormalities in Prader-Willi syndrome (PWS), a genetic disorder that features GH deficiency, obesity, and infertility. One of the genes typically inactivated in PWS, MAGEL2, is highly expressed in the hypothalamus. Mice deficient for Magel2 are obese with increased fat mass and decreased lean mass and have blunted circadian rhythm. Here, we demonstrate that Magel2-null mice have abnormalities of hypothalamic endocrine axes that recapitulate phenotypes in PWS. Magel2-null mice had elevated basal corticosterone levels, and although male Magel2-null mice had an intact corticosterone response to restraint and to insulin-induced hypoglycemia, female Magel2-null mice failed to respond to hypoglycemia with increased corticosterone. After insulin-induced hypoglycemia, Magel2-null mice of both sexes became more profoundly hypoglycemic, and female mice were slower to recover euglycemia, suggesting an impaired hypothalamic counterregulatory response. GH insufficiency can produce abnormal body composition, such as that seen in PWS and in Magel2-null mice. Male Magel2-null mice had Igf-I levels similar to control littermates. Female Magel2-null mice had low Igf-I levels and reduced GH release in response to stimulation with ghrelin. Female Magel2-null mice did respond to GHRH, suggesting that their GH deficiency has a hypothalamic rather than pituitary origin. Female Magel2-null mice also had higher serum adiponectin than expected, considering their increased fat mass, and thyroid (T(4)) levels were low. Together, these findings strongly suggest that loss of MAGEL2 contributes to endocrine dysfunction of hypothalamic origin in individuals with PWS.

Metabolic impact of adult-onset, isolated, growth hormone deficiency (AOiGHD) due to destruction of pituitary somatotropes

Growth hormone (GH) inhibits fat accumulation and promotes protein accretion, therefore the fall in GH observed with weight gain and normal aging may contribute to metabolic dysfunction. To directly test this hypothesis a novel mouse model of adult onset-isolated GH deficiency (AOiGHD) was generated by cross breeding rat GH promoter-driven Cre recombinase mice (Cre) with inducible diphtheria toxin receptor mice (iDTR) and treating adult Cre^+/−,iDTR^+/− offspring with DT to selectively destroy the somatotrope population of the anterior pituitary gland, leading to a reduction in circulating GH and IGF-I levels. DT-treated Cre^−/−,iDTR^+/− mice were used as GH-intact controls. AOiGHD improved whole body insulin sensitivity in both low-fat and high-fat fed mice. Consistent with improved insulin sensitivity, indirect calorimetry revealed AOiGHD mice preferentially utilized carbohydrates for energy metabolism, as compared to GH-intact controls. In high-fat, but not low-fat fed AOiGHD mice, fat mass increased, hepatic lipids decreased and glucose clearance and insulin output were impaired. These results suggest the age-related decline in GH helps to preserve systemic insulin sensitivity, and in the context of moderate caloric intake, prevents the deterioration in metabolic function. However, in the context of excess caloric intake, low GH leads to impaired insulin output, and thereby could contribute to the development of diabetes.

IGF-1, oxidative stress and atheroprotection

Atherosclerosis is a chronic inflammatory disease in which early endothelial dysfunction and subintimal modified lipoprotein deposition progress to complex, advanced lesions that are predisposed to erosion, rupture and thrombosis. Oxidative stress plays a crucial role not only in initial lesion formation but also in lesion progression and destabilization. Although most growth factors are thought to promote vascular smooth muscle cell proliferation and migration, thereby increasing neointima, recent animal studies indicate that insulin-like growth factor (IGF)-1 exerts both pleiotropic anti-oxidant effects and anti-inflammatory effects, which together reduce atherosclerotic burden. This review discusses the effects of IGF-1 in models of vascular injury and atherosclerosis, emphasizing the relationship between oxidative stress and potential atheroprotective actions of IGF-1.