Loss of insulin action on the endothelium can cause endothelial
dysfunction and atherosclerosis. Hyperglycemia and elevated fatty acids
induced by diabetes can activate protein kinase C (PKC) β isoforms
and selectively inhibit insulin signaling via phosphatidylinositol 3-kinase
(PI3K)/Akt pathway to inhibit the activation of endothelial nitric oxide
synthase (eNOS) and metabolic actions.
To demonstrate that overexpressing PKCβ2 isoform in
endothelial cells can cause selective insulin resistance and exacerbate
atherosclerosis in the aorta.
Methods and Results
PKCβ2 isoform was overexpressed in endothelial cells using a
promoter of vascular endothelial cell-cadherin (VE-Cadherin). These mice
were cross-bred with ApoE-/- mice (Tg
(Prkcb)ApoE-/-). On a Western diet,
ApoE-/- mice did not differ in systemic
insulin sensitivity, glucose tolerance, plasma lipid or blood pressure.
Insulin action in endothelial cells and femoral artery from
Tg(Prkcb)ApoE-/- mice were impaired by
∼40% with respect to Akt/eNOS activation and
leukocyte-endothelial cell binding increased in cultured lung endothelial
cells from Tg(Prkcb)ApoE-/-mice compared to
ApoE-/- mice. Basal and angiotensin
stimulated big endothelin-1 (ET-1) levels were elevated in
Tg(Prkcb)ApoE-/- mice compared to
ApoE-/- mice. The severity of
atherosclerosis in the aorta from
Tg(Prkcb)ApoE-/- mice increased by
∼70% as measured by en face fat staining and plaque content
of the number of smooth muscle cells, macrophages and extracellular
Specific PKCβ2 activation in the endothelial cells caused
dysfunction and accelerated atherosclerosis due to loss of
insulin-stimulated Akt/eNOS activation and angiotensin induced increases in
Endothelium; endothelial dysfunction; protein kinase C; insulin resistance; atherosclerosis
Protein kinase C (PKC) activation, induced by hyperglycemia and angiotensin II (AngII), inhibited insulin-induced phosphorylation of Akt/endothelial nitric oxide (eNOS) by decreasing tyrosine phosphorylation of IRS2 (p-Tyr-IRS2) in endothelial cells. PKC activation by phorbol ester (phorbol myristate acetate [PMA]) reduced insulin-induced p-Tyr-IRS2 by 46% ± 13% and, similarly, phosphorylation of Akt/eNOS. Site-specific mutational analysis showed that PMA increased serine phosphorylation at three sites on IRS2 (positions 303, 343, and 675), which affected insulin-induced tyrosine phosphorylation of IRS2 at positions 653, 671, and 911 (p-Tyr-IRS2) and p-Akt/eNOS. Specific PKCβ2 activation decreased p-Tyr-IRS2 and increased the phosphorylation of two serines (Ser303 and Ser675) on IRS2 that were confirmed in cells overexpressing single point mutants of IRS2 (S303A or S675A) containing a PKCβ2-dominant negative or selective PKCβ inhibitor. AngII induced phosphorylation only on Ser303 of IRS2 and inhibited insulin-induced p-Tyr911 of IRS2 and p-Akt/eNOS, which were blocked by an antagonist of AngII receptor I, losartan, or overexpression of single mutant S303A of IRS2. Increases in p-Ser303 and p-Ser675 and decreases in p-Tyr911 of IRS2 were observed in vessels of insulin-resistant Zucker fatty rats versus lean rats. Thus, AngII or PKCβ activation can phosphorylate Ser303 and Ser675 in IRS2 to inhibit insulin-induced p-Tyr911 and its anti-atherogenic actions (p-Akt/eNOS) in endothelial cells.
In patients with diabetes, atherosclerosis is the main reason for impaired life expectancy, and diabetic nephropathy and retinopathy are the largest contributors to end-stage renal disease and blindness, respectively. An improved therapeutic approach to combat diabetic vascular complications might include blocking mechanisms of injury as well as promoting protective or regenerating factors, for example by enhancing the action of insulin-regulated genes in endothelial cells, promoting gene programs leading to induction of antioxidant or anti-inflammatory factors, or improving the sensitivity to vascular cell survival factors. Such strategies could help prevent complications despite suboptimal metabolic control.
To characterize glucagon-like peptide (GLP)-1 signaling and its effect on renal endothelial dysfunction and glomerulopathy. We studied the expression and signaling of GLP-1 receptor (GLP-1R) on glomerular endothelial cells and the novel finding of protein kinase A–dependent phosphorylation of c-Raf at Ser259 and its inhibition of angiotensin II (Ang II) phospho–c-Raf(Ser338) and Erk1/2 phosphorylation. Mice overexpressing protein kinase C (PKC)β2 in endothelial cells (EC-PKCβ2Tg) were established. Ang II and GLP-1 actions in glomerular endothelial cells were analyzed with small interfering RNA of GLP-1R. PKCβ isoform activation induced by diabetes decreased GLP-1R expression and protective action on the renal endothelium by increasing its degradation via ubiquitination and enhancing phospho–c-Raf(Ser338) and Ang II activation of phospho-Erk1/2. EC-PKCβ2Tg mice exhibited decreased GLP-1R expression and increased phospho–c-Raf(Ser338), leading to enhanced effects of Ang II. Diabetic EC-PKCβ2Tg mice exhibited greater loss of endothelial GLP-1R expression and exendin-4–protective actions and exhibited more albuminuria and mesangial expansion than diabetic controls. These results showed that the renal protective effects of GLP-1 were mediated via the inhibition of Ang II actions on cRaf(Ser259) and diminished by diabetes because of PKCβ activation and the increased degradation of GLP-1R in the glomerular endothelial cells.
Diabetes results in vascular changes and dysfunction, and vascular complications are the leading cause of morbidity and mortality in diabetic patients. There has been a continual increase in the number of diabetic nephropathy patients and epidemic increases in the number of patients progressing to end-stage renal diseases. To identify targets for therapeutic intervention, most studies have focused on understanding how abnormal levels of glucose metabolites cause diabetic nephropathy, which is of paramount importance in devising strategies to combat the development and progression of diabetic nephropathy. However, less studied than the systemic toxic mechanisms, hyperglycemia and dyslipidemia might inhibit the endogenous vascular protective factors such as insulin, vascular endothelial growth factor, and platelet-derived growth factor. In this review, we highlight the importance of enhancing endogenous protective factors to prevent or delay diabetic nephropathy.
Hyperglycemia; protein kinase C (PKC)β; advanced glycation end products (AGEs); platelet-derived growth factor (PDGF); vascular endothelial growth factor (VEGF)
To correlate changes between VEGF expression with systemic and retinal oxidative stress and inflammation in rodent models of obesity induced insulin resistance and diabetes.
Retinal VEGF mRNA and protein levels were assessed by RT-PCR and VEGF ELISA, respectively. Urinary 8-hydroxydeoxyguanosine (8-OHdG), blood levels of C-reactive protein (CRP), malondialdehyde (MDA), and CD11b/c positive cell ratio were used as systemic inflammatory markers. Retinal expression of Nox2, Nox4, and p47phox mRNA levels were measured as oxidative stress markers. TNF-α, inter-cellular adhesion molecule-1 (ICAM-1), IL1β, and activation of nuclear factor κB (NF-κB) were used as retinal inflammatory markers.
Retinal VEGF mRNA and protein expression increased in Zucker diabetic fatty (ZDFfa/fa) rats and streptozotosin (STZ) induced diabetic Sprague-Dawley rats, after two months of disease, but not in Zucker fatty (ZF) rats. Systemic markers of oxidative stress and inflammation were elevated in insulin resistant and diabetic rats. Some oxidative stress and inflammatory markers (TNF-α, IL-6, ICAM-1, and IL1-β) were upregulated in the retina of ZDFfa/fa and STZ diabetic rats after 4 months of disease. In contrast, activation of NF-κB in the retina was observed in high fat fed nondiabetic and diabetic cis-NF-κBEGFP mice, ZF, ZDFfa/fa, and STZ-induced diabetic rats.
Only persistent hyperglycemia and diabetes increased retinal VEGF expression. Some markers of inflammation and oxidative stress were elevated in the retina and systemic circulation of obese and insulin resistant rodents with and without diabetes. Induction of VEGF and its associated retinal pathologies by diabetes requires chronic hyperglycemia and factors in addition to inflammation and oxidative stress.
Only chronic diabetes induced late markers of inflammation and oxidative stress in the retina correlated with increased VEGF expression, but insulin resistance alone caused systemic and retinal inflammation and no increase in VEGF elevation.
Insulin resistance has been associated with the progression of chronic kidney disease in both diabetes and obesity. This study characterizes insulin signaling in renal tubules and glomeruli in insulin resistant and diabetic states.
Insulin-induced phosphorylation of insulin receptor substrate-1 (IRS1), Akt, endothelial nitric oxide (eNOS), and glycogen synthase kinase 3α (GSK3α) were selectively inhibited in the glomeruli but not in the renal tubules of both streptozotocin (STZ)-diabetic and Zucker fatty, insulin resistant rats compared to non-diabetic and Zucker lean rats. Protein levels, but not the mRNA expression, of IRS1 were decreased only in the glomeruli of STZ-diabetic rats and increased its association with ubiquitination. Protein kinase C (PKC) β isoform inhibitor, ruboxistaurin (RBX), treatment enhanced insulin actions and elevated IRS1 expression. In glomerular endothelial cells, high glucose inhibited phosphorylation of Akt, eNOS and GSK3α, decreased IRS1 protein expression and increased association with ubiquitination. Overexpression of IRS1 or the addition of RBX reversed the inhibitory effects of high glucose.
Selective inhibition of the IRS1/PI3K/Akt pathway and insulin activation of eNOS and GSK3α in the glomeruli in diabetes and insulin resistance is partly due to increased IRS1 degradation and PKCβ activation. The loss of insulin's effect on endothelial eNOS and GSK3α activation may contribute to the glomeropathy observed in diabetes and obesity.
diabetic nephropathy; insulin resistance; obesity; insulin receptor substrate-1; protein kinase C β
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.
Insulin resistance plays a central role in the development of the metabolic syndrome, but how it relates to cardiovascular disease remains controversial. Liver insulin receptor knockout (LIRKO) mice have pure hepatic insulin resistance. On a chow diet, LIRKO mice have a proatherogenic lipoprotein profile with reduced HDL cholesterol and VLDL particles that are markedly enriched in cholesterol. This is due to increased secretion and decreased clearance of apoB-containing lipoproteins, coupled with decreased triglyceride secretion secondary to increased expression of PGC-1β, which promotes VLDL secretion, but decreased expression of SREBP-1c, SREBP-2 and their targets, the lipogenic enzymes and the LDL receptor. Within twelve weeks on an atherogenic diet, LIRKO mice show marked hypercholesterolemia, and 100% of LIRKO mice, but 0% of controls, develop severe atherosclerosis. Thus, insulin resistance at the level of the liver is sufficient to produce the dyslipidemia and increased risk of atherosclerosis associated with the metabolic syndrome.
Willow bark extract (WBE) is listed in the European Pharmacopoeia and has been traditionally used for treating fever, pain, and inflammation. Recent studies have demonstrated its clinical usefulness. This study investigated the antioxidative effects of WBE in human umbilical vein endothelial cells (HUVECs) and Caenorhabditis elegans. WBE prevented oxidative-stress-induced cytotoxicity of HUVECs and death of C. elegans. WBE dose-dependently increased mRNA and protein expression levels of the nuclear factor erythroid 2-related factor 2 (Nrf2) target genes heme oxygenase-1, γ-glutamylcysteine ligase modifier and catalytic subunits, and p62 and intracellular glutathione (GSH) in HUVECs. In the nematode C. elegans, WBE increased the expression of the gcs-1::green fluorescent protein reporter, a well-characterized target of the Nrf2 ortholog SKN-1, in a manner that was SKN-1-dependent. WBE increased intranuclear expression and DNA binding of Nrf2 and the activity of an antioxidant response element (ARE) reporter plasmid in HUVECs. WBE-induced expression of Nrf2-regulated genes and increased GSH levels in HUVECs were reduced by Nrf2 and p38 small interfering (si) RNAs and by the p38-specific inhibitor SB203580. Nrf2 siRNA reduced the cytoprotective effect of WBE against oxidative stress in HUVECs. Salicin, a major anti-inflammatory ingredient of WBE, failed to activate ARE–luciferase activity, whereas a salicin-free WBE fraction showed intensive activity. WBE induced antioxidant enzymes and prevented oxidative stress through activation of Nrf2 independent of salicin, providing a new potential explanation for the clinical usefulness of WBE.
Willow bark extract; Nrf2; Antioxidant enzyme; HUVEC; Caenorhabditis elegans; Oxidative stress; Free radicals
Vascular dysfunction is a major contributor to diabetes complications. It is also the primary physiologic cause of erectile dysfunction and considered an independent predictor of cardiovascular disease (CVD) in males over age 40 years. A cohort of individuals with 50 or more years of type 1 diabetes, Joslin Medalists, have low rates of small but not large vessel complications. This study aims to identify the prevalence and longitudinal association of sexual dysfunction (SD) with CVD in Joslin Medalists.
RESEARCH DESIGN AND METHODS
Description and association of self-assessment of SD in males of the Medalist cohort by self-reported sexual problems with CVD. SD is validated through the use of the abbreviated International Index of Erectile Dysfunction (IIEF).
Of 301 males in the Medalist Study, 69.8% reported a history of SD. Unadjusted risk factors included elevated glycated hemoglobin (HbA1c) (P = 0.02), elevated BMI (P = 0.03), higher total cholesterol (P = 0.02), lower HDL (P < 0.01), and increased levels of interleukin-6 (P = 0.03). SD was independently associated with CVD (age-, HbA1c-, and BMI-adjusted OR 1.9 [95% CI 1.0–3.5]). In adjusted analyses, retinal, neural, and renal complications were not associated (P > 0.05) with SD. Current report of SD (IIEF score ≤17) in a subset of Medalists was significantly correlated with self-reported longitudinal SD.
SD in those with extreme-duration type 1 diabetes is independently associated with CVD, representing a large-vessel pattern. The findings suggest that SD may predict CVD in those with type 1 diabetes of long duration. These individuals have also been found to be relatively free of microvascular complications.
The purpose of this study is to examine the effectiveness of a culturally specific pilot clinic for Asian Americans (AA) in reaching glycemic target and to characterize factors affecting the attainment of glycemic control in comparison with white counterparts.
This electronic health record review included all new AA patients with type 2 diabetes (n = 109) in a culturally specific program and a randomly selected sample of new white patients with type 2 diabetes (n = 218) in the adult clinic within the same time period and diabetes center.
AA and whites had a comparable proportion of patients with A1C ≤7% (32.1%, 34.9%; P = .621) at baseline and after 12 months of care (48.6%, 56.0%; P = .210), with a similar A1C decline (−0.9% ± 1.6%, −0.8% ± 1.7%, P = .710) by 12 months. Factors associated with the lack of success in reaching target in AA but not in whites included older age, lower educational attainment, less likelihood of having health insurance, and a need for more educational visits. The percentage of AA reaching A1C ≤7%, as compared to whites, worsened among those with highest initial A1C when stratified by ascending quartiles (96.7% vs 85.2%, P = .101; 61.9% vs 58.9%, P = .813; 24.0% vs 37.7%, P = .230; 15.2% vs 35.4%, P = .044).
While a culturally specific diabetes program in a specialty setting achieved a similar glycemic outcome for AA compared with whites, reasons for not reaching glycemic target differed. The findings suggest that the elimination of diabetes disparities requires not only culturally and linguistically specific programs, but must also identify and address the socio-environmental differences unique to each population.
The prevalence of diabetes is rising dramatically among Asians, with increased consumption of the typical Western diet as one possible cause. We explored the metabolic responses in East Asian Americans (AA) and Caucasian Americans (CA) when transitioning from a traditional Asian diet (TAD) to a typical Western diet (TWD), which has not been reported before. This 16-week randomized control pilot feasibility study, included 28AA and 22CA who were at risk of developing type 2 diabetes. Eight weeks of TAD were provided to all participants, followed by 8 weeks of isoenergy TWD (intervention) or TAD (control). Anthropometric measures, lipid profile, insulin resistance and inflammatory markers were assessed. While on TAD, both AA and CA improved in insulin AUC (−960.2 µU/mL×h, P = 0.001) and reduced in weight (−1.6 kg; P<0.001), body fat (−1.7%, P<0.001) and trunk fat (−2.2%, P<0.001). Comparing changes from TAD to TWD, AA had a smaller weight gain (−1.8 to 0.3 kg, P<0.001) than CA (−1.4 to 0.9 kg, P = 0.001), but a greater increase in insulin AUC (AA: −1402.4 to 606.2 µU/mL×h, P = 0.015 vs CA: −466.0 to 223.5 µU/mL×h, P = 0.034) and homeostatic static model assessment-insulin resistance (HOMA-IR) (AA: −0.3 to 0.2, P = 0.042 vs CA: −0.1 to 0.0, P = 0.221). Despite efforts to maintain isoenergy state and consumption of similar energy, TAD induced weight loss and improved insulin sensitivity in both groups, while TWD worsened the metabolic profile.
Trial Registration: ClinicalTrials.gov NCT00379548
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.
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.
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.
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.
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.
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.
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.
Transforming growth factor-beta1 (TGF-B1) is a highly pleiotropic cytokine whose functions include a central role in the induction of fibrosis.
To investigate the hypothesis that elevated plasma levels of TGF-B1 are positively associated with incident heart failure (HF).
Participants and Methods
The hypotheses were tested using a two-phase case-control study design, ancillary to the Cardiovascular Health Study – a longitudinal, population-based cohort study. Cases were defined as having an incident HF event after their 1992-93 exam and controls were free of HF at follow-up. TGF-B1 was measured using plasma collected in 1992-93 and data from 89 cases and 128 controls were used for analysis. The association between TGF-B1 and risk of HF was evaluated using the weighted likelihood method, and odds ratios (OR) for risk of HF were calculated for TGF-B1 as a continuous linear variable and across quartiles of TGF-B1.
The OR for HF was 1.88 (95% confidence intervals [CI] 1.26 to 2.81) for each nanogram increase in TGF-B1, and the OR for the highest quartile (compared to the lowest) of TGF-B1 was 5.79 (95% CI 1.65 – 20.34), after adjustment for age, sex, C-reactive protein, platelet count and digoxin use. Further adjustment with other covariates did not change the results.
Higher levels of plasma TGF-B1 were associated with an increased risk of incident heart failure among older adults. However, further study is needed in larger samples to confirm these findings.
transforming growth factor-beta; heart failure; fibrosis; growth factors; cardiac remodeling
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.
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.
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.
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.
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.
Protein Kinase C; Vascular cell biology; Diabetes; Cardiovascular diseases