Objective

Vascular endothelial growth factor (VEGF) stimulates pro-angiogenic signal transduction and cell function in part through activation of protein kinase C (PKC). Our aim was to examine how individual isoforms of PKC affect VEGF action.

Methods and Results

Transfection of bovine aortic endothelial cells with small interfering RNA (siRNA) targeting either PKCα, δ, or ε caused a reduction in the cognate PKC protein by 76–89% without changing expression of non-targeted isoforms. Downregulation of PKCε abrogated VEGF-stimulated phosphorylation of Akt at Ser473 and eNOS at Ser1179 and decreased VEGF-stimulated NO synthase activity in intact cells. In contrast, PKCα knockdown increased Akt and eNOS phosphorylation, while PKCδ knockdown had no significant effect. PKCε knockdown also decreased VEGF-stimulated Erk1/2 phosphorylation and abolished VEGF-stimulated DNA synthesis. Consistent with an effect on several pathways of VEGF signaling, VEGF receptor-2 (VEGFR2) tyrosine phosphorylation and expression of VEGFR2 protein and mRNA was decreased by 81, 90, and 84%, respectively, during knockdown of PKCε, but increased during PKCα knockdown.

Conclusions

By regulating VEGFR2 expression and activation, PKCε expression is critical for activation of Akt and eNOS by VEGF and contributes to VEGF-stimulated Erk activation, whereas PKCα has opposite effects.

Insulin contributes to skeletal muscle glucose uptake by increasing blood flow and recruiting perfused capillaries. In this issue, Kubota et al. (2011) show that deletion of IRS-2 in endothelial cells in mice causes impaired transcapillary insulin transport, decreased insulin-stimulated glucose uptake in muscle, and mild glucose intolerance.

Both cardio- and microvascular complications adversely affect the life quality of patients with diabetes and have been the leading cause of mortality and morbidity in this population. Cardiovascular pathologies of diabetes have an effect on microvenules, arteries, and myocardium. It is believed that hyperglycemia is one of the most important metabolic factors in the development of both micro- and macrovascular complications in diabetic patients. Several prominent hypotheses exist to explain the adverse effect of hyperglycemia. One of them is the chronic activation by hyperglycemia of protein kinase C (PKC), a family of enzymes that are involved in controlling the function of other proteins. PKC has been associated with vascular alterations such as increases in permeability, contractility, extracellular matrix synthesis, cell growth and apoptosis, angiogenesis, leukocyte adhesion, and cytokine activation and inhibition. These perturbations in vascular cell homeostasis caused by different PKC isoforms (PKC-α, -β1/2, and PKC-δ) are linked to the development of pathologies affecting large vessel (atherosclerosis, cardiomyopathy) and small vessel (retinopathy, nephropathy and neuropathy) complications. Clinical trials using a PKC-β isoform inhibitor have been conducted, with some positive results for diabetic nonproliferative retinopathy, nephropathy and endothelial dysfunction. This paper reviews current understanding of how PKC isoforms cause vascular dysfunctions and pathologies in diabetes.

Diabetic vascular complications are among the leading causes of morbidity and mortality in diabetic patients. In the past, many studies have focused on the mechanisms of hyperglycemia-induced chronic vascular complications via the formation of toxic metabolites such as oxidative stress, advanced glycosylated end products, persistent activation of protein kinase C, and increased sorbitol concentrations. However, vascular complications result from imbalances caused by increases in systemic toxic metabolites, such as those that occur under conditions of hyperglycemia and dyslipidemia, and by reductions in endogenous protective factors such as insulin, vascular endothelial growth factor, and platelet derived growth factor. This review outlines some of the evidence supporting the importance of enhancing endogenous regenerative factors.

Cellular apoptosis induced by hyperglycemia occurs in many vascular cells and is critical to initiate diabetic pathologies. In the retina, pericyte apoptosis, the most specific vascular pathology attributed to hyperglycemia, is linked to the loss of PDGF actions due to unknown mechanisms. Our study demonstrated that hyperglycemia persistently activated PKCδ and p38α MAPK to increase the expression of a novel target, SHP-1, leading to PDGF receptor-β dephosphorylation and actions, and increased pericyte apoptosis, independent of NF-κB. These findings were also observed in diabetic mouse retinas, which were not reversed by achieving normoglycemia with insulin. Unlike diabetic controls, diabetic Prkcd−/− mice did not exhibit p38α MAPK/SHP-1 activation, PDGF resistance or acellular capillaries. Since PKCδ/p38α MAPK/SHP-1 activation are also induced in the brain pericytes and renal cortex by diabetes, these findings have elucidated a new pathway by which hyperglycemia can induce PDGF resistance and increase vascular cell apoptosis to cause diabetic vascular complications.

Renal hypertrophy and deposition of extracellular matrix proteins are consistent findings in diabetic nephropathy and these processes can be halted or reversed by euglycemic control. Using DNA microarray analysis of glomerular RNA from control and diabetic rats we found that the expression levels of insulin-like growth factor 1 receptor (IGF-1R) were increased while those of suppressor of cytokine signaling 2 (SOCS2) and STAT5 were decreased. All of these changes were normalized by islet cell transplantation. Overexpression of SOCS2 in rat mesangial cells inhibited IGF-1-induced activation of extracellular signal-regulated kinase, which subsequently reduced type IV collagen and DNA synthesis, an effect due to interaction of SOCS2 with IGF-1R. Inhibition of SOCS2 overexpression by small interfering RNA suppressed IGF-1R-mediated actions by preventing phosphorylation of tyrosine 317 in the p66Shc adaptor protein; however, overexpression of either SOCS1 or SOCS3 did not affect IGF-1R signaling. Insulin directly increased STAT5 and SOCS2 expression in mesangial cells. This study shows that insulin can inhibit the mitogenic action of IGF-1 in mesangial cells by regulating STAT5/SOCS2 expression. Insulin deficiency may contribute to the mesangial expansion found in diabetes through reduced STAT5/SOCS2 expression.

Diabetes can diminish the responsiveness to angiogenic factors (e.g., VEGF) important for wound healing and the treatment of ischemic diseases, and this study investigated the hypothesis that this effect can be reversed by altering Notch signaling. Aortic endothelial cells (ECs) isolated from diabetic mice demonstrated reduced sprouting capability in vitro, but adding a Notch inhibitor (DAPT) led to cell-density and VEGF-dose dependent enhancement of proliferation, migration and sprouting, in both 2-D and 3-D cultures, as compared to VEGF alone. The in vivo effects of VEGF and DAPT were tested in the ischemic hindlimbs of diabetic mice. Combining VEGF and DAPT delivery resulted in increased blood vessel density (~150%) and improved tissue perfusion (~160%), as compared to VEGF alone. To examine if DAPT would interfere with vessel maturation, DAPT was also delivered with a combination of VEGF and platelet derived growth factor (PDGF). DAPT and PDGF did not interfere with the effects of the other, and highly functional and mature networks of vessels could be formed with appropriate delivery. In summary, modulating Notch signaling enhances neovascularization and perfusion recovery in diabetic mice suffering from ischemia, suggesting this approach could have utility for human diabetics.

OBJECTIVE

To evaluate the extent of pancreatic β-cell function in a large number of insulin-dependent diabetic patients with a disease duration of 50 years or longer (Medalists).

RESEARCH DESIGN AND METHODS

Characterization of clinical and biochemical parameters and β-cell function of 411 Medalists with correlation with postmortem morphologic findings of 9 Medalists.

RESULTS

The Medalist cohort, with a mean ± SD disease duration and age of 56.2 ± 5.8 and 67.2 ± 7.5 years, respectively, has a clinical phenotype similar to type 1 diabetes (type 1 diabetes): mean ± SD onset at 11.0 ± 6.4 years, BMI at 26.0 ± 5.1 kg/m2, insulin dose of 0.46 ± 0.2 u/kg, ∼94% positive for DR3 and/or DR4, and 29.5% positive for either IA2 or glutamic acid decarboxylase (GAD) autoantibodies. Random serum C-peptide levels showed that more than 67.4% of the participants had levels in the minimal (0.03–0.2 nmol/l) or sustained range (≥0.2 nmol/l). Parameters associated with higher random C-peptide were lower hemoglobin A1C, older age of onset, higher frequency of HLA DR3 genotype, and responsiveness to a mixed-meal tolerance test (MMTT). Over half of the Medalists with fasting C-peptide >0.17 nmol/l responded in MMTT by a twofold or greater rise over the course of the test compared to fasting. Postmortem examination of pancreases from nine Medalists showed that all had insulin+ β-cells with some positive for TUNEL staining, indicating apoptosis.

CONCLUSIONS

Demonstration of persistence and function of insulin-producing pancreatic cells suggests the possibility of a steady state of turnover in which stimuli to enhance endogenous β cells could be a viable therapeutic approach in a significant number of patients with type 1 diabetes, even for those with chronic duration.

C57BL/6J and 129S6/Sv (B6 and 129) mice differ dramatically in their susceptibility to developing diabetes in response to diet- or genetically induced insulin resistance. A major locus contributing to this difference has been mapped to a region on mouse chromosome 14 that contains the gene encoding PKCδ. Here, we found that PKCδ expression in liver was 2-fold higher in B6 versus 129 mice from birth and was further increased in B6 but not 129 mice in response to a high-fat diet. PRKCD gene expression was also elevated in obese humans and was positively correlated with fasting glucose and circulating triglycerides. Mice with global or liver-specific inactivation of the Prkcd gene displayed increased hepatic insulin signaling and reduced expression of gluconeogenic and lipogenic enzymes. This resulted in increased insulin-induced suppression of hepatic gluconeogenesis, improved glucose tolerance, and reduced hepatosteatosis with aging. Conversely, mice with liver-specific overexpression of PKCδ developed hepatic insulin resistance characterized by decreased insulin signaling, enhanced lipogenic gene expression, and hepatosteatosis. Therefore, changes in the expression and regulation of PKCδ between strains of mice and in obese humans play an important role in the genetic risk of hepatic insulin resistance, glucose intolerance, and hepatosteatosis; and thus PKCδ may be a potential target in the treatment of metabolic syndrome.

Both insulin resistance and hyperinsulinemia have been reported to be independent risk factors for cardiovascular diseases. However, little is known regarding insulin signaling in the vascular tissues in insulin-resistant states. In this report, insulin signaling on the phosphatidylinositol 3-kinase (PI 3-kinase) and mitogen-activated protein (MAP) kinase pathways were compared in vascular tissues of lean and obese Zucker (fa/fa) rats in both ex vivo and in vivo studies. Ex vivo, insulin-stimulated tyrosine phosphorylation of insulin receptor β subunits (IRβ) in the aorta and microvessels of obese rats was significantly decreased compared with lean rats, although the protein levels of IRβ in the 2 groups were not different. Insulin-induced tyrosine phosphorylation of insulin receptor substrates 1 and 2 (IRS-1 and IRS-2) and their protein levels were decreased in the aorta of obese rats compared with lean rats. The association of p85 subunit to the IRS proteins and the IRS-associated PI 3-kinase activities stimulated by insulin in the aorta of obese rats were significantly decreased compared with the lean rats. In addition, insulin-stimulated serine phosphorylation of Akt, a downstream kinase of PI 3-kinase pathway, was also reduced significantly in isolated microvessels from obese rats compared with the lean rats. In euglycemic clamp studies, insulin infusion greatly increased tyrosine phosphorylation of IRβ- and IRS-2–associated PI 3-kinase activity in the aorta of lean rats, but only slight increases were observed in obese rats. In contrast, insulin stimulated tyrosine phosphorylation of MAP kinase (ERK-1/2) equally in isolated microvessels of lean and obese rats, although basal tyrosine phosphorylation of ERK-1/2 was higher in the obese rats. To our knowledge, these data provided the first direct measurements of insulin signaling in the vascular tissues, and documented a selective resistance to PI 3-kinase (but not to MAP kinase pathway) in the vascular tissues of obese Zucker rats.

J. Clin. Invest. 104:447–457 (1999).

Hyperglycemia can cause vascular dysfunctions by multiple factors including hyperosmolarity, oxidant formation, and protein kinase C (PKC) activation. We have characterized the effect of hyperglycemia on p38 mitogen-activated protein (p38) kinase activation, which can be induced by oxidants, hyperosmolarity, and proinflammatory cytokines, leading to apoptosis, cell growth, and gene regulation. Glucose at 16.5 mM increased p38 kinase activity in a time-dependent manner compared with 5.5 mM in rat aortic smooth muscle cells (SMC). Mannitol activated p38 kinase only at or greater than 22 mM. High glucose levels and a PKC agonist activated p38 kinase, and a PKC inhibitor, GF109203X, prevented its activation. However, p38 kinase activation by mannitol or tumor necrosis factor-α was not inhibited by GF109203X. Changes in PKC isoform distribution after exposure to 16.5 mM glucose in SMC suggested that both PKC-β2 and PKC-δ isoforms were increased. Activities of p38 kinase in PKC-δ– but not PKC-β1–overexpressed SMC were increased compared with control cells. Activation of p38 kinase was also observed and characterized in various vascular cells in culture and aorta from diabetic rats. Thus, moderate hyperglycemia can activate p38 kinase by a PKC-δ isoform–dependent pathway, but glucose at extremely elevated levels can also activate p38 kinase by hyperosmolarity via a PKC-independent pathway.

Although excessive fructose intake is epidemiologically linked with dyslipidemia, obesity, and diabetes, the mechanisms regulating plasma fructose are not well known. Cells transfected with sodium/glucose cotransporter 5 (SGLT5), which is expressed exclusively in the kidney, transport fructose in vitro; however, the physiological role of this transporter in fructose metabolism remains unclear. To determine whether SGLT5 functions as a fructose transporter in vivo, we established a line of mice lacking the gene encoding SGLT5. Sodium-dependent fructose uptake disappeared in renal brush border membrane vesicles from SGLT5-deficient mice, and the increased urinary fructose in SGLT5-deficient mice indicated that SGLT5 was the major fructose reabsorption transporter in the kidney. From this, we hypothesized that urinary fructose excretion induced by SGLT5 deficiency would ameliorate fructose-induced hepatic steatosis. To test this hypothesis we compared SGLT5-deficient mice with wild-type mice under conditions of long-term fructose consumption. Paradoxically, however, fructose-induced hepatic steatosis was exacerbated in the SGLT5-deficient mice, and the massive urinary fructose excretion was accompanied by reduced levels of plasma triglycerides and epididymal fat but fasting hyperinsulinemia compared with fructose-fed wild-type mice. There was no difference in food consumption, water intake, or plasma fructose between the two types of mice. No compensatory effect by other transporters reportedly involved in fructose uptake in the liver and kidney were indicated at the mRNA level. These surprising findings indicated a previously unrecognized link through SGLT5 between renal fructose reabsorption and hepatic lipid metabolism.

Both insulin and IGF-1 have been implicated in control of retinal endothelial cell growth, neovascularization, and diabetic retinopathy. To precisely define the role of insulin and IGF-1 signaling in endothelium in these processes, we have used the oxygen-induced retinopathy model to study mice with a vascular endothelial cell–specific knockout of the insulin receptor (VENIRKO) or IGF-1 receptor (VENIFARKO). Following relative hypoxia, VENIRKO mice show a 57% decrease in retinal neovascularization as compared with controls. This is associated with a blunted rise in VEGF, eNOS, and endothelin-1. By contrast, VENIFARKO mice show only a 34% reduction in neovascularization and a very modest reduction in mediator generation. These data indicate that both insulin and IGF-1 signaling in endothelium play a role in retinal neovascularization through the expression of vascular mediators, with the effect of insulin being most important in this process.

Insulin receptors (IRs) on vascular endothelial cells have been suggested to participate in insulin-regulated glucose homeostasis. To directly address the role of insulin action in endothelial function, we have generated a vascular endothelial cell IR knockout (VENIRKO) mouse using the Cre-loxP system. Cultured endothelium of VENIRKO mice exhibited complete rearrangement of the IR gene and a more than 95% decrease in IR mRNA. VENIRKO mice were born at the expected Mendelian ratio, grew normally, were fertile, and exhibited normal patterns of vasculature in the retina and other tissues. Glucose homeostasis under basal condition was comparable in VENIRKO mice. Both eNOS and endothelin-1 mRNA levels, however, were reduced by approximately 30–60% in endothelial cells, aorta, and heart, while vascular EGF expression was maintained at normal levels. Arterial pressure tended to be lower in VENIRKO mice on both low- and high-salt diets, and on a low-salt diet VENIRKO mice showed insulin resistance. Thus, inactivation of the IR on endothelial cell has no major consequences on vascular development or glucose homeostasis under basal conditions, but alters expression of vasoactive mediators and may play a role in maintaining vascular tone and regulation of insulin sensitivity to dietary salt intake.

Insulin and such insulinlike growth factors as multiplication stimulating activity (MSA) are related polypeptides that have common biological activities. Both insulin and MSA produce acute metabolic responses (stimulation of glucose oxidation in isolated fat cells) as well as growth effects (stimulation of [3H]thymidine incorporation into DNA in cultured fibroblasts). In addition, most cells have separate receptors for insulin and insulinlike growth factors, and both peptides have weaker affinity for each other's specific receptors than for their own. To determine, therefore, whether these effects are mediated by receptors for insulin, insulinlike growth factors, or both, we have selectively blocked insulin receptors with a specific antagonist, namely Fab fragments derived from naturally occurring antibodies to the insulin receptor.

In rat adipocytes, 10 μg/ml of antireceptor Fab inhibited insulin binding by 90%, whereas it inhibited MSA binding <5%. The anti-insulin receptor Fab is without intrinsic biological activity, but acts as a competitive inhibitor of insulin receptors. Blockade of insulin receptors with Fab fragments produced a 30-fold rightward shift in the dose response for stimulation of glucose oxidation by both insulin and MSA. The dose-response curves for stimulation of oxidation by vitamin K5 and spermine, agents that stimulate glucose oxidation through noninsulin receptor pathways, were not affected by the blockade of insulin receptors with Fab antibody fragments. These data suggest that this acute metabolic effect of both insulin and MSA is mediated via the insulin receptor.

In cultured human fibroblasts, 10 μg/ml of Fab inhibited insulin binding by 90% and MSA binding by 15%. In fibroblasts, however, blockade of the insulin receptor did not alter the dose response for stimulation of thymidine incorporation into DNA by either insulin or MSA. Furthermore, intact antireceptor antibody immunoglobulin (Ig)G, which produces multiple other insulinlike effects, and Fab fragments of antireceptor antibody did not stimulate thymidine incorporation. These data demonstrate directly that the insulin receptor mediates the metabolic effects of insulin and MSA, whereas the growth-promoting action of both peptides is mediated by the MSA receptor or other growth factors.

OBJECTIVE

To assess complication prevalence and identify protective factors in patients with diabetes duration of ≥50 years. Characterization of a complication-free subgroup in this cohort would suggest that some individuals are protected from diabetes complications and allow identification of endogenous protective factors.

RESEARCH DESIGN AND METHODS

Cross-sectional, observational study of 351 U.S. residents who have survived with type 1 diabetes for ≥50 years (Medalists). Retinopathy, nephropathy, neuropathy, and cardiovascular disease were assessed in relation to HbA1c, lipids, and advanced glycation end products (AGEs). Retrospective chart review provided longitudinal ophthalmic data for a subgroup.

RESULTS

A high proportion of Medalists remain free from proliferative diabetic retinopathy (PDR) (42.6%), nephropathy (86.9%), neuropathy (39.4%), or cardiovascular disease (51.5%). Current and longitudinal (the past 15 years) glycemic control were unrelated to complications. Subjects with high plasma carboxyethyl-lysine and pentosidine were 7.2-fold more likely to have any complication. Of Medalists without PDR, 96% with no retinopathy progression over the first 17 years of follow-up did not experience retinopathy worsening thereafter.

CONCLUSIONS

The Medalist population is likely enriched for protective factors against complications. These factors might prove useful to the general population with diabetes if they can be used to induce protection against long-term complications. Specific AGE combinations were strongly associated with complications, indicating a link between AGE formation or processing with development of diabetic vasculopathy.

Objective

Classic features of type 1 and type 2 diabetes may not apply in Asian Americans, due to shared absence of common HLA DR-DQ genotype, low prevalence of positive anti-islet antibodies and low BMI in both types of diabetes. Our objective was to characterize diabetic phenotypes in Asian Americans by clamp and clinical features.

Materials/Methods

This was a cross-sectional study conducted in a referral center. Thirty East young Asian American adult volunteers (27.6±5.5 years) with type 1, type 2 diabetes or controls underwent hyperinsulinemic euglycemic clamp to assess insulin resistance and DEXA to assess adiposity.

Results

Gender, BMI, waist/hip ratio, leptin, LDL, anti-GAD, anti-IA2 antibodies and C-reactive protein were similar among three groups. Serum C-peptide, adiponectin, free fatty acid, HDL concentrations and truncal fat by DEXA, were different between diabetic groups. Glucose disposal rate by clamp was lowest in type 2 diabetes, followed by type 1 diabetes and controls (5.43±2.70, 7.62±2.59, 8.61±2.37 mg/min/kg, respectively, p = 0.001). Free fatty acid concentration universally plummeted during steady state of the clamp procedure regardless of diabetes types in all three groups. Adipocyte fatty acid binding protein in the entire cohort (r = −0.625, p = 0.04) and controls (r = −0.869, p = 0.046) correlated best with insulin resistance, independent of BMI.

Conclusions

Type 2 diabetes in Asian Americans was associated with insulin resistance despite having low BMI as type 1 diabetes, suggesting a potential role for targeting insulin resistance apart from weight loss. Adipocyte fatty acid binding protein, strongly associated with insulin resistance, independent of adiposity in the young Asian American population, may potentially serve as a biomarker to identify at-risk individuals. Larger studies are needed to confirm this finding.

Summary

To determine whether insulin action on endothelial cells promotes or protects against atherosclerosis, we generated apolipoprotein E null mice in which the insulin receptor gene was intact or conditionally deleted in vascular endothelial cells. Insulin sensitivity, glucose tolerance, plasma lipids, and blood pressure were not different between the two groups, but atherosclerotic lesion size was more than 2-fold higher in mice lacking endothelial insulin signaling. Endothelium-dependent vasodilation was impaired and endothelial cell VCAM-1 expression was increased in these animals. Adhesion of mononuclear cells to endothelium in vivo was increased 4-fold compared with controls, but reduced to below control values by a VCAM-1 blocking antibody. These results provide definitive evidence that loss of insulin signaling in endothelium, in the absence of competing systemic risk factors, accelerates atherosclerosis. Therefore, improving insulin sensitivity in the endothelium of patients with insulin resistance or type 2 diabetes may prevent cardiovascular complications.