Striatal dopamine regulates systemic glucose metabolism in humans and mice.

The brain is emerging as an important regulator of systemic glucose metabolism. Accumulating data from animal and observational human studies suggest that striatal dopamine signaling plays a role in glucose regulation, but direct evidence in humans is currently lacking. We present a series of experiments supporting the regulation of peripheral glucose metabolism by striatal dopamine signaling. First, we present the case of a diabetes patient who displayed strongly reduced insulin requirements after treatment with bilateral deep brain stimulation (DBS) targeting the anterior limb of the internal capsule. Next, we show that DBS in this striatal area, which induced dopamine release, increased hepatic and peripheral insulin sensitivity in 14 nondiabetic patients with obsessive-compulsive disorder. Conversely, systemic dopamine depletion reduced peripheral insulin sensitivity in healthy subjects. Supporting these human data, we demonstrate that optogenetic activation of dopamine D1 receptor-expressing neurons in the nucleus accumbens increased glucose tolerance and insulin sensitivity in mice. Together, these findings support the hypothesis that striatal neuronal activity regulates systemic glucose metabolism.

Neuropeptide Y activity in the nucleus accumbens modulates feeding behavior and neuronal activity.

BACKGROUND: Neuropeptide Y (NPY) is a hypothalamic neuropeptide that plays a prominent role in feeding and energy homeostasis. Expression of the NPY Y1 receptor (Y1R) is highly concentrated in the nucleus accumbens (Acb), a region important in the regulation of palatable feeding. In this study, we performed a number of experiments to investigate the actions of NPY in the Acb. METHODS: First, we determined caloric intake and food choice after bilateral administration of NPY in the Acb in rats on a free-choice diet of saturated fat, 30% sucrose solution, and standard chow and whether this was mediated by the Y1R. Second, we measured the effect of intra-Acb NPY on neuronal activity using in vivo electrophysiology. Third, we examined co-localization of Y1R with enkephalin and dynorphin neurons and the effect of NPY on preproenkephalin messenger RNA levels in the striatum using fluorescent and radioactive in situ hybridization. Finally, using retrograde tracing, we examined whether NPY neurons in the arcuate nucleus projected to the Acb. RESULTS: In rats on the free-choice, high-fat, high-sugar diet, intra-Acb NPY increased intake of fat, but not sugar or chow, and this was mediated by the Y1R. Intra-Acb NPY reduced neuronal firing, as well as preproenkephalin messenger RNA expression in the striatum. Moreover, Acb enkephalin neurons expressed Y1R and arcuate nucleus NPY neurons projected to the Acb. CONCLUSIONS: NPY reduces neuronal firing in the Acb resulting in increased palatable food intake. Together, our neuroanatomical, pharmacologic, and neuronal activity data support a role and mechanism for intra-Acb NPY-induced fat intake.

Pten deletion in RIP-Cre neurons protects against type 2 diabetes by activating the anti-inflammatory reflex.

Inflammation has a critical role in the development of insulin resistance. Recent evidence points to a contribution by the central nervous system in the modulation of peripheral inflammation through the anti-inflammatory reflex. However, the importance of this phenomenon remains elusive in type 2 diabetes pathogenesis. Here we show that rat insulin-2 promoter (Rip)-mediated deletion of Pten, a gene encoding a negative regulator of PI3K signaling, led to activation of the cholinergic anti-inflammatory pathway that is mediated by M2 activated macrophages in peripheral tissues. As such, Rip-cre(+) Pten(flox/flox) mice showed lower systemic inflammation and greater insulin sensitivity under basal conditions compared to littermate controls, which were abolished when the mice were treated with an acetylcholine receptor antagonist or when macrophages were depleted. After feeding with a high-fat diet, the Pten-deleted mice remained markedly insulin sensitive, which correlated with massive subcutaneous fat expansion. They also exhibited more adipogenesis with M2 macrophage infiltration, both of which were abolished after disruption of the anti-inflammatory efferent pathway by left vagotomy. In summary, we show that Pten expression in Rip(+) neurons has a critical role in diabetes pathogenesis through mediating the anti-inflammatory reflex.

The islet estrogen receptor-α is induced by hyperglycemia and protects against oxidative stress-induced insulin-deficient diabetes.

The female steroid, 17ß-estradiol (E2), is important for pancreatic ß-cell function and acts via at least three estrogen receptors (ER), ERα, ERß, and the G-protein coupled ER (GPER). Using a pancreas-specific ERα knockout mouse generated using the Cre-lox-P system and a Pdx1-Cre transgenic line (PERαKO ⁻/⁻), we previously reported that islet ERα suppresses islet glucolipotoxicity and prevents ß-cell dysfunction induced by high fat feeding. We also showed that E2 acts via ERα to prevent ß-cell apoptosis in vivo. However, the contribution of the islet ERα to ß-cell survival in vivo, without the contribution of ERα in other tissues is still unclear. Using the PERαKO ⁻/⁻ mouse, we show that ERα mRNA expression is only decreased by 20% in the arcuate nucleus of the hypothalamus, without a parallel decrease in the VMH, making it a reliable model of pancreas-specific ERα elimination. Following exposure to alloxan-induced oxidative stress in vivo, female and male PERαKO ⁻/⁻ mice exhibited a predisposition to ß-cell destruction and insulin deficient diabetes. In male PERαKO ⁻/⁻ mice, exposure to E2 partially prevented alloxan-induced ß-cell destruction and diabetes. ERα mRNA expression was induced by hyperglycemia in vivo in islets from young mice as well as in cultured rat islets. The induction of ERα mRNA by hyperglycemia was retained in insulin receptor-deficient ß-cells, demonstrating independence from direct insulin regulation. These findings suggest that induction of ERα expression acts to naturally protect ß-cells against oxidative injury.

Loss of neurotensin receptor-1 disrupts the control of the mesolimbic dopamine system by leptin and promotes hedonic feeding and obesity.

Neurons of the lateral hypothalamic area (LHA) control motivated behaviors such as feeding and ambulatory activity, in part by modulating mesolimbic dopamine (DA) circuits. The hormone, leptin, acts via the long form of the leptin receptor (LepRb) in the brain to signal the repletion of body energy stores, thereby decreasing feeding and promoting activity. LHA LepRb neurons, most of which contain neurotensin (Nts; LepRb(Nts) neurons) link leptin action to the control of mesolimbic DA function and energy balance. To understand potential roles for Nts in these processes, we examined mice null for Nts receptor 1 (NtsR1KO). While NtsR1KO mice consume less food than controls on a chow diet, they eat more and become obese when fed a high-fat, high-sucrose palatable diet; NtsR1KO mice also exhibit augmented sucrose preference, consistent with increased hedonic feeding in these animals. We thus sought to understand potential roles for NtsR1 in the control of the mesolimbic DA system and LHA leptin action. LHA Nts cells project to DA-containing midbrain areas, including the ventral tegmental area (VTA) and the substantia nigra (SN), where many DA neurons express NtsR1. Furthermore, in contrast to wild-type mice, intra-LHA leptin treatment increased feeding and decreased VTA Th expression in NtsR1KO mice, consistent with a role for NtsR1 signaling from LHA LepRb neurons in the suppression of food intake and control of mesolimbic DA function. Additionally, these data suggest that other leptin-regulated LHA neurotransmitters normally oppose aspects of Nts action to promote balanced responses to leptin.

Importance of ß-Catenin in glucose and energy homeostasis.

In settings of increased insulin demand, failure to expand pancreatic ß-cells mass leads to diabetes. Genome-wide scans of diabetic populations have uncovered several genes associated with susceptibility to type 2 diabetes and a number of them are part of the Wnt signaling. ß-Catenin, a Wnt downstream effector participates in pancreatic development, however, little is known about its action in mature ß-cells. Deletion of ß-Catenin in Pdx1 pancreatic progenitors leads to a decreased ß-cell mass and impaired glucose tolerance. Surprisingly, loss of ß-catenin made these mice resistant to high fat diet because of their increased energy expenditure and insulin sensitivity due to hyperactivity. The complexity of this phenotype was also explained in part by ectopic expression of Cre recombinase in the hypothalamus. Our data implicates ß-Catenin in the regulation of metabolism and energy homeostasis and suggest that Wnt signaling modulates the susceptibility to diabetes by acting on different tissues.

Leptin action via neurotensin neurons controls orexin, the mesolimbic dopamine system and energy balance.

Leptin acts on leptin receptor (LepRb)-expressing neurons throughout the brain, but the roles for many populations of LepRb neurons in modulating energy balance and behavior remain unclear. We found that the majority of LepRb neurons in the lateral hypothalamic area (LHA) contain neurotensin (Nts). To investigate the physiologic role for leptin action via these LepRb(Nts) neurons, we generated mice null for LepRb specifically in Nts neurons (Nts-LepRbKO mice). Nts-LepRbKO mice demonstrate early-onset obesity, modestly increased feeding, and decreased locomotor activity. Furthermore, consistent with the connection of LepRb(Nts) neurons with local orexin (OX) neurons and the ventral tegmental area (VTA), Nts-LepRbKO mice exhibit altered regulation of OX neurons and the mesolimbic DA system. Thus, LHA LepRb(Nts) neurons mediate physiologic leptin action on OX neurons and the mesolimbic DA system, and contribute importantly to the control of energy balance.

Conditional gene targeting in mouse pancreatic ß-Cells: analysis of ectopic Cre transgene expression in the brain.

OBJECTIVE: Conditional gene targeting has been extensively used for in vivo analysis of gene function in ß-cell biology. The objective of this study was to examine whether mouse transgenic Cre lines, used to mediate ß-cell- or pancreas-specific recombination, also drive Cre expression in the brain. RESEARCH DESIGN AND METHODS: Transgenic Cre lines driven by Ins1, Ins2, and Pdx1 promoters were bred to R26R reporter strains. Cre activity was assessed by ß-galactosidase or yellow fluorescent protein expression in the pancreas and the brain. Endogenous Pdx1 gene expression was monitored using Pdx1(tm1Cvw) lacZ knock-in mice. Cre expression in ß-cells and co-localization of Cre activity with orexin-expressing and leptin-responsive neurons within the brain was assessed by immunohistochemistry. RESULTS: All transgenic Cre lines examined that used the Ins2 promoter to drive Cre expression showed widespread Cre activity in the brain, whereas Cre lines that used Pdx1 promoter fragments showed more restricted Cre activity primarily within the hypothalamus. Immunohistochemical analysis of the hypothalamus from Tg(Pdx1-cre)(89.1Dam) mice revealed Cre activity in neurons expressing orexin and in neurons activated by leptin. Tg(Ins1-Cre/ERT)(1Lphi) mice were the only line that lacked Cre activity in the brain. CONCLUSIONS: Cre-mediated gene manipulation using transgenic lines that express Cre under the control of the Ins2 and Pdx1 promoters are likely to alter gene expression in nutrient-sensing neurons. Therefore, data arising from the use of these transgenic Cre lines must be interpreted carefully to assess whether the resultant phenotype is solely attributable to alterations in the islet ß-cells.

Carcinoembryonic antigen-related cell adhesion molecule 2 controls energy balance and peripheral insulin action in mice.

BACKGROUND & AIMS: The carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1) is a transmembrane glycoprotein with pleotropic functions, including clearance of hepatic insulin. We investigated the functions of the related protein CEACAM2, which has tissue-specific distribution (kidney, uterus, and crypt epithelia of intestinal tissues), in genetically modified mice. METHODS: Ceacam2-null mice (Cc2-/-) were generated from a 129/SvxC57BL/6J background. Female mice were assessed by hyperinsulinemic-euglycemic clamp analysis and indirect calorimetry and body fat composition was measured. Cc2-/- mice and controls were fed as pairs, given insulin tolerance tests, and phenotypically characterized. RESULTS: Female, but not male Cc2-/- mice exhibited obesity that resulted from hyperphagia and reduced energy expenditure. Pair feeding experiments showed that hyperphagia led to peripheral insulin resistance. Insulin action was normal in liver but compromised in skeletal muscle of female Cc2-/- mice; the mice had incomplete fatty acid oxidation and impaired glucose uptake and disposal. The mechanism of hyperphagia in Cc2-/- mice is not clear, but appears to result partly from increased hyperinsulinemia-induced hypothalamic fatty acid synthase levels and activity. Hyperinsulinemia was caused by increased insulin secretion. CONCLUSIONS: In mice, CEACAM2 is expressed by the hypothalamus. Cc2-/- mice develop obesity from hyperphagia and reduced energy expenditure, indicating its role in regulating energy balance and insulin sensitivity.

Modulation of the mesolimbic dopamine system by leptin.

Nutritional status modulates many forms of reward-seeking behavior, with caloric restriction increasing the drive for drugs of abuse as well as for food. Understanding the interactions between the mesolimbic dopamine (DA) system (which mediates the incentive salience of natural and artificial rewards) and the neural and hormonal systems that sense and regulate energy balance is thus of significant importance. Leptin, which is produced by adipocytes in proportion to fat content as a hormonal signal of long-term energy stores, acts via its receptor (LepRb) on multiple populations of central nervous system neurons to modulate neural circuits in response to body energy stores. Leptin suppresses feeding and plays a central role in the control of energy balance. In addition to demonstrating that leptin modulates hypothalamic and brainstem circuits to promote satiety, recent work has begun to explore the mechanisms by which leptin influences the mesolimbic DA system and related behaviors. Indeed, leptin diminishes several measures of drug and food reward, and promotes a complex set of changes in the mesolimbic DA system. While many of the details remain to be worked out, several lines of evidence suggest that leptin regulates the mesolimbic DA system via multiple neural pathways and processes, and that distinct sets of LepRb neurons each modulate unique aspects of the mesolimbic DA system and behavior in response to leptin.

Ventral tegmental area leptin receptor neurons specifically project to and regulate cocaine- and amphetamine-regulated transcript neurons of the extended central amygdala.

Leptin acts via its receptor (LepRb) to regulate neural circuits in concert with body energy stores. In addition to acting on a number of hypothalamic structures, leptin modulates the mesolimbic dopamine (DA) system. To determine the sites at which LepRb neurons might directly influence the mesolimbic DA system, we examined the distribution of LepRb neurons and their projections within mesolimbic brain regions. Although the ventral tegmental area (VTA) contains DA LepRb neurons, LepRb neurons are absent from the amygdala and striatum. Also, LepRb-EGFPf mice (which label projections from LepRb neurons throughout the brain) reveal that few LepRb neurons project to the nucleus accumbens (NAc). In contrast, the central amygdala (CeA) and its rostral extension receive copious projections from LepRb neurons. Indeed, LepRb-specific anterograde tracing demonstrates (and retrograde tracing confirms) that VTA LepRb neurons project to the extended CeA (extCeA) but not the NAc. Consistently, leptin promotes cAMP response element-binding protein phosphorylation in the extCeA, but not NAc, of leptin-deficient animals. Furthermore, transgenic mice expressing the trans-synaptic tracer wheat germ agglutinin in LepRb neurons reveal the innervation of CeA cocaine- and amphetamine-regulated transcript (CART) neurons by LepRb neurons, and leptin suppresses the increased CeA CART expression of leptin-deficient animals. Thus, LepRb VTA neurons represent a subclass of VTA DA neurons that specifically innervates and controls the extCeA; we hypothesize that these neurons primarily modulate CeA-directed behaviors.

Leptin acts via leptin receptor-expressing lateral hypothalamic neurons to modulate the mesolimbic dopamine system and suppress feeding.

The lateral hypothalamic area (LHA) acts in concert with the ventral tegmental area (VTA) and other components of the mesolimbic dopamine (DA) system to control motivation, including the incentive to feed. The anorexigenic hormone leptin modulates the mesolimbic DA system, although the mechanisms underlying this control have remained incompletely understood. We show that leptin directly regulates a population of leptin receptor (LepRb)-expressing inhibitory neurons in the LHA and that leptin action via these LHA LepRb neurons decreases feeding and body weight. Furthermore, these LHA LepRb neurons innervate the VTA, and leptin action on these neurons restores VTA expression of the rate-limiting enzyme in DA production along with mesolimbic DA content in leptin-deficient animals. Thus, these findings reveal that LHA LepRb neurons link anorexic leptin action to the mesolimbic DA system.

Critical roles for the TSC-mTOR pathway in ß-cell function.

TSC1 is a tumor suppressor that associates with TSC2 to inactivate Rheb, thereby inhibiting signaling by the mammalian target of rapamycin (mTOR) complex 1 (mTORC1). mTORC1 stimulates cell growth by promoting anabolic cellular processes, such as translation, in response to growth factors and nutrient signals. To test roles for TSC1 and mTORC1 in ß-cell function, we utilized Rip2/Cre to generate mice lacking Tsc1 in pancreatic ß-cells (Rip-Tsc1cKO mice). Although obesity developed due to hypothalamic Tsc1 excision in older Rip-Tsc1cKO animals, young animals displayed a prominent gain-of-function ß-cell phenotype prior to the onset of obesity. The young Rip-Tsc1cKO animals displayed improved glycemic control due to mTOR-mediated enhancement of ß-cell size, mass, and insulin production but not determinants of ß-cell number (proliferation and apoptosis), consistent with an important anabolic role for mTOR in ß-cell function. Furthermore, mTOR mediated these effects in the face of impaired Akt signaling in ß-cells. Thus, mTOR promulgates a dominant signal to promote ß-cell/islet size and insulin production, and this pathway is crucial for ß-cell function and glycemic control.

Critical role for hypothalamic mTOR activity in energy balance.

The mammalian target of rapamycin (mTOR) promotes anabolic cellular processes in response to growth factors and metabolic cues. The TSC1 and TSC2 tumor suppressors are major upstream inhibitory regulators of mTOR signaling. Mice with Rip2/Cre-mediated deletion of Tsc1 (Rip-Tsc1cKO mice) developed hyperphagia and obesity, suggesting that hypothalamic disruption (for which Rip2/Cre is well known) of Tsc1 may dysregulate feeding circuits via mTOR activation. Indeed, Rip-Tsc1cKO mice displayed increased mTOR signaling and enlarged neuron cell size in a number of hypothalamic populations, including Pomc neurons. Furthermore, Tsc1 deletion with Pomc/Cre (Pomc-Tsc1cKO mice) resulted in dysregulation of Pomc neurons and hyperphagic obesity. Treatment with the mTOR inhibitor, rapamycin, ameliorated the hyperphagia, obesity, and the altered Pomc neuronal morphology in developing or adult Pomc-Tsc1cKO mice, and cessation of treatment reinstated these phenotypes. Thus, ongoing mTOR activation in Pomc neurons blocks the catabolic function of these neurons to promote nutrient intake and increased adiposity.

Regulation of vascular endothelial growth factor expression and vascularization in the myocardium by insulin receptor and PI3K/Akt pathways in insulin resistance and ischemia.

OBJECTIVE: This study characterized the role of insulin receptors and resistance on vascular endothelial growth factor (VEGF) expression and myocardial vascularization in physiological conditions and after ischemia. METHODS AND RESULTS: Cardiac microvascular density was reduced by 30% in insulin-resistant Zucker fatty rats versus lean controls. This was associated with a parallel 40% inhibition of insulin-stimulated activation of both Akt and VEGF expression in the myocardium and cardiomyocytes. In contrast, the activation of Erk1/2 by insulin remained unchanged. In cultured cardiomyocytes, insulin or insulin-like growth factor (IGF)-1 increased VEGF mRNA and protein expression by 2-fold. Inhibition of PI3K/Akt, especially Akt2-mediated cascades but not the Ras/MEK/Erk pathway, using chemical inhibitors, dominant negative adenoviral constructs, or siRNA approaches suppressed VEGF mRNA expression by insulin. Ventricular tissues from muscle insulin receptor knockout (MIRKO) mice, which lack insulin receptors in the myocardium, have significant reductions in insulin but not IGF-1 signaling, VEGF expression, and vascular density before and after ischemia versus controls. CONCLUSIONS: Insulin regulates VEGF gene expression and vascularization in the myocardium specifically via insulin receptors and the activation of PI3K/Akt pathway. Selective inhibition of this pathway may lead to the decreases in VEGF expression and capillary density in the myocardium of patients with insulin resistance.

Differential regulation of angiotensin II-induced expression of connective tissue growth factor by protein kinase C isoforms in the myocardium.

Protein kinase C (PKC) and angiotensin II (AngII) can regulate cardiac function in pathological conditions such as in diabetes or ischemic heart disease. We have reported that expression of connective tissue growth factor (CTGF) is increased in the myocardium of diabetic mice. Now we showed that the increase in CTGF expression in cardiac tissues of streptozotocin-induced diabetic rats was reversed by captopril and islet cell transplantation. Infusion of AngII in rats increased CTGF mRNA expression by 15-fold, which was completely inhibited by co-infusion with AT1 receptor antagonist, candesartan. Similarly, incubation of cultured cardiomyocytes with AngII increased CTGF mRNA expression by 2-fold, which was blocked by candesartan and a general PKC inhibitor, GF109203X. The role of PKC isoform-dependent action was further studied using adenoviral vector-mediated gene transfer of dominant negative (dn) PKC or wild type PKC isoforms. Expression of dnPKCalpha, -epsilon, and -zeta isoforms suppressed AngII-induced CTGF expression in cardiomyocytes. In contrast, expression of dominant negative PKCdelta significantly increased AngII-induced CTGF expression, whereas expression of wild type PKCdelta inhibited this induction. This inhibitory effect was further confirmed in the myocardium of transgenic mice with cardiomyocyte-specific overexpression of PKCdelta (deltaTg mice). Thus, AngII can regulate CTGF expression in cardiomyocytes through a PKC activation-mediated pathway in an isoform-selective manner both in physiological and diabetic states and may contribute to the development of cardiac fibrosis in diabetic cardiomyopathy.

Characterization of multiple signaling pathways of insulin in the regulation of vascular endothelial growth factor expression in vascular cells and angiogenesis.

The effects of insulin on vascular endothelial growth factor (VEGF) expression in cultured vascular cells and in angiogenesis were characterized. Insulin increased VEGF mRNA levels in mouse aortic smooth muscle cells from 10(-9) to 10(-7) m with an initial peak of 3.7-fold increases at 1 h and a second peak of 2.8-fold after 12 h. The first peak of VEGF expression was inhibited by LY294002, an inhibitor of phosphatidylinositol (PI) 3-kinase, and by the overexpression of dominant negative forms of p85 subunit of PI 3-kinase or Akt. Inhibitors of MEK kinase, PD98059, or overexpression of dominant negative forms of Ras was ineffective. In contrast, the chronic effect of insulin on VEGF expression was partially inhibited by both LY294002 or PD98059 as well as by the overexpression of dominant negatives of PI 3-kinase or Ras. The importance of PI 3-kinase-Akt pathway on VEGF expression was confirmed in mouse aortic smooth muscle cells isolated from insulin receptor substrate -1 knockout (IRS-1-/-) mice that showed parallel reductions of 46-49% in insulin-stimulated VEGF expression and PI 3-kinase-Akt activation. Insulin-induced activation of PI 3-kinase-Akt on hypoxia-induced VEGF expression and neovascularization was reduced by 40% in the retina of neonatal hypoxia model using IRS-1-/- mice. Thus, unlike other cells, insulin can regulate VEGF expression by both IRS-1/PI 3-kinase-Akt cascade and Ras-MAPK pathways in aortic smooth muscle cells. The in vivo results provide direct evidence that insulin can modulate hypoxia-induced angiogenesis via reduction in VEGF expression in vivo.

Decreased cardiac expression of vascular endothelial growth factor and its receptors in insulin-resistant and diabetic States: a possible explanation for impaired collateral formation in cardiac tissue.

BACKGROUND: Inadequate angiogenic response to ischemia in the myocardium of diabetic patients could result in poor collateral formation. Yet, excessive neovascularization in the retina causes proliferative diabetic retinopathy. Since vascular endothelial growth factor (VEGF) is the major angiogenic factor expressed in response to hypoxia, we have characterized expression of VEGF and its receptors in retina, renal glomeruli, aorta, and myocardium in insulin-resistant and diabetic states. Methods and Results- The expression of mRNA and protein for VEGF and its receptors, VEGF-R1 and VEGF-R2, in the myocardium was decreased significantly by 40% to 70% in both diabetic and insulin-resistant nondiabetic rats. Twofold reductions in VEGF and VEGF-R2 were observed in ventricles from diabetic patients compared with nondiabetic donors. In contrast, expression of VEGF and its receptors were increased 2-fold in retina and glomeruli from diabetic or insulin-resistant rats. Insulin treatment of diabetic rats normalized changes in both cardiac and microvascular tissues. Insulin increased VEGF mRNA expression in cultured rat neonatal cardiac myocytes. CONCLUSIONS: The results documented for the first time that differential regulation of VEGF and its receptors exist between microvascular and cardiac tissues, which can be regulated by insulin. These results provide a potential explanation for concomitant capillary leakage and neovascularization in the retina and inadequate collateral formation in the myocardium of insulin-resistant and diabetic patients.