Increased platelet cytosolic calcium responses to low density lipoprotein in type II diabetes with and without hypertension

Candesartan, an angiotensin-II receptor blocker, ameliorates insulin resistance and hepatosteatosis by reducing intracellular calcium overload and lipid accumulation

Insulin resistance is a major contributor to the pathogenesis of several human diseases, including type 2 diabetes, hypertension, and hyperlipidemia. Notably, insulin resistance and hypertension share common abnormalities, including increased oxidative stress, inflammation, and organelle dysfunction. Recently, we showed that excess intracellular Ca²⁺, a known pathogenic factor in hypertension, acts as a critical negative regulator of insulin signaling by forming Ca²⁺-phosphoinositides that prevent the membrane localization of AKT, a key serine/threonine kinase signaling molecule. Whether preventing intracellular Ca²⁺ overload improves insulin sensitivity, however, has not yet been investigated. Here, we show that the antihypertensive agent candesartan, compared with other angiotensin-II receptor blockers, has previously unrecognized beneficial effects on attenuating insulin resistance. We found that candesartan markedly reduced palmitic acid (PA)-induced intracellular Ca²⁺ overload and lipid accumulation by normalizing dysregulated store-operated channel (SOC)-mediated Ca²⁺ entry into cells, which alleviated PA-induced insulin resistance by promoting insulin-stimulated AKT membrane localization and increased the phosphorylation of AKT and its downstream substrates. As pharmacological approaches to attenuate intracellular Ca²⁺ overload in vivo, administering candesartan to obese mice successfully decreased insulin resistance, hepatic steatosis, dyslipidemia, and tissue inflammation by inhibiting dysregulated SOC-mediated Ca²⁺ entry and ectopic lipid accumulation. The resulting alterations in the phosphorylation of key signaling molecules consequently alleviate impaired insulin signaling by increasing the postprandial membrane localization and phosphorylation of AKT. Thus, our findings provide robust evidence for the pleiotropic contribution of intracellular Ca²⁺ overload in the pathogenesis of insulin resistance and suggest that there are viable approved drugs that can be repurposed for the treatment of insulin resistance and hypertension.

Platelets Are at the Nexus of Vascular Diseases

Platelets are important actors of cardiovascular diseases (CVD). Current antiplatelet drugs that inhibit platelet aggregation have been shown to be effective in CVD treatment. However, the management of bleeding complications is still an issue in vascular diseases. While platelets can act individually, they interact with vascular cells and leukocytes at sites of vascular injury and inflammation. The main goal remains to better understand platelet mechanisms in thrombo-inflammatory diseases and provide new lines of safe treatments. Beyond their role in hemostasis and thrombosis, recent studies have reported the role of several aspects of platelet functions in CVD progression. In this review, we will provide a comprehensive overview of platelet mechanisms involved in several vascular diseases.

Increased intracellular Ca2+ concentrations prevent membrane localization of PH domains through the formation of Ca2+-phosphoinositides

Insulin resistance is a metabolic disorder in which target cells fail to respond to physiological levels of circulating insulin, leading to hyperinsulinemia and glucose intolerance. The molecular mechanism underlying insulin resistance is still largely unknown. Here, we found that intracellular Ca²⁺ overloading in obesity attenuates insulin-stimulated phosphorylation of protein kinase B and its downstream signaling by preventing membrane localization of various pleckstrin homology (PH) domains. When at high intracellular levels, Ca²⁺ binds tightly with phosphoinositides to yield Ca²⁺-phosphoinositides (PIPs), abrogating the membrane targeting of PH domains and disrupting insulin signaling. Thus, we identified a previously unknown physiological function of intracellular Ca²⁺ as a critical negative regulator of insulin signaling, especially through the formation of Ca²⁺-PIPs.

Insulin resistance, a key etiological factor in metabolic syndrome, is closely linked to ectopic lipid accumulation and increased intracellular Ca²⁺ concentrations in muscle and liver. However, the mechanism by which dysregulated intracellular Ca²⁺ homeostasis causes insulin resistance remains elusive. Here, we show that increased intracellular Ca²⁺ acts as a negative regulator of insulin signaling. Chronic intracellular Ca²⁺ overload in hepatocytes during obesity and hyperlipidemia attenuates the phosphorylation of protein kinase B (Akt) and its key downstream signaling molecules by inhibiting membrane localization of pleckstrin homology (PH) domains. Pharmacological approaches showed that elevated intracellular Ca²⁺ inhibits insulin-stimulated Akt phosphorylation and abrogates membrane localization of various PH domain proteins such as phospholipase Cδ and insulin receptor substrate 1, suggesting a common mechanism inhibiting the membrane targeting of PH domains. PH domain-lipid overlay assays confirmed that Ca²⁺ abolishes the binding of various PH domains to phosphoinositides (PIPs) with two adjacent phosphate groups, such as PI(3,4)P₂, PI(4,5)P₂, and PI(3,4,5)P₃. Finally, thermodynamic analysis of the binding interaction showed that Ca²⁺-mediated inhibition of targeting PH domains to the membrane resulted from the tight binding of Ca²⁺ rather than PH domains to PIPs forming Ca²⁺-PIPs. Thus, Ca²⁺-PIPs prevent the recognition of PIPs by PH domains, potentially due to electrostatic repulsion between positively charged side chains in PH domains and the Ca²⁺-PIPs. Our findings provide a mechanistic link between intracellular Ca²⁺ dysregulation and Akt inactivation in insulin resistance.

Diabetes mellitus, hypothalamic hypoestrogenemia, and coronary artery disease in premenopausal women (from the National Heart, Lung, and Blood Institute sponsored WISE study)

Diabetes mellitus (DM) portends a higher risk of coronary heart disease mortality in women compared with men. This relationship appears to be independent of traditional cardiac risk factors, and the role of reproductive hormones has been postulated. We assessed the relationship between DM, hypothalamic hypoestrogenemia (HHE), angiographic coronary artery disease (CAD), and major adverse cardiovascular events (MACE) during a median of 5.9 years in premenopausal women enrolled in the WISE Study. We evaluated 95 premenopausal women from WISE who underwent coronary angiography for suspected ischemia and were not using exogenous reproductive hormones. Results showed no difference in age between women with (n = 30) and without (n = 65) DM (43 +/- 6 years). DM was associated with hypertension, HHE, angiographic CAD, and coronary artery severity score (all p <0.05). Women with DM were twice as likely to have HHE (50% vs 26%; p = 0.02) compared with women without DM. The presence of both DM and HHE was associated with increased prevalence (40% vs 12% or 13%; p = 0.006) and severity of angiographic CAD (coronary artery severity score 19.9 +/- 19.2 vs 7.7 +/- 4.6 or 12.3 +/- 18.8; p = 0.008) compared with either HHE or DM alone, respectively. DM was moderately predictive of MACE. In conclusion, in premenopausal women undergoing coronary angiography for suspected myocardial ischemia, DM was associated with HHE. The presence of both DM and HHE predicted a greater burden of angiographic CAD. Prospective research is warranted to better understand causal relations between DM, endogenous hormones, and MACE in premenopausal women.

Diabetes-associated macrovasculopathy: pathophysiology and pathogenesis

The complications associated with diabetic vasculopathy are commonly grouped into two categories: microvascular and macrovascular complications. In diabetes, macrovascular disease is the commonest cause of mortality and morbidity and is responsible for high incidence of vascular diseases such as stroke, myocardial infarction and peripheral vascular diseases. Macrovascular diseases are traditionally thought of as due to underlying obstructive atherosclerotic diseases affecting major arteries. Pathological changes of major blood vessels leading to functional and structural abnormalities in diabetic vessels include endothelial dysfunction, reduced vascular compliance and atherosclerosis. Besides, advanced glycation end product formation interacts with specific receptors that lead to overexpression of a range of cytokines. Haemodynamic pathways are activated in diabetes and are possibly amplified by concomitant systemic hypertension. Apart from these, hyperglycaemia, non-enzymatic glycosylation, lipid modulation, alteration of vasculature and growth factors activation contribute to development of diabetic vasculopathy. This review focuses on pathophysiology and pathogenesis of diabetes-associated macrovasculopathy.

The dysmetabolic syndrome, insulin resistance and increased cardiovascular (CV) morbidity and mortality in type 2 diabetes: aetiological factors in the development of CV complications

Insulin resistance often clusters with other cardiovascular risk factors, such as obesity, impaired glucose tolerance (IGT), hypertension, dyslipidaemia and impaired fibrinolysis. Collectively, these endocrine and metabolic disturbances are described as the dysmetabolic syndrome, which is also commonly called the "insulin resistance syndrome", the "metabolic syndrome", or "syndrome X". Insulin resistance, working in concert with the other components of the dysmetabolic syndrome, induces deleterious changes to the vascular endothelium and lipid profiles that directly and indirectly promote the progression of atherosclerosis. Insulin resistance in adipocytes, leading to decreased suppression of lipolysis by insulin, may be especially important in this regard. Reduced suppression of lipolysis by insulin in obese subjects is associated with increased levels of fatty acids that damage the arterial wall and promote atherosclerosis. The lipid profiles of insulin-resistant subjects are often characterised by the appearance of hypertriglyceridaemia and small, dense LDL-cholesterol, together with low HDL-cholesterol. In addition, adipocytes are highly active endocrine organs and secrete a range of substances that reduce insulin sensitivity further. The net result of these derangements is a vicious circle, wherein the development of insulin resistance is strongly associated with atherogenic lipid profiles and endothelial dysfunction which, in turn, exacerbates insulin resistance. The consequences for the individual with dysmetabolic syndrome include an increased risk of cardiovascular disease of up to 4-fold compared with subjects without the dysmetabolic syndrome.

Effects of statins on the vasculature: Implications for aggressive lipid management in the cardiovascular metabolic syndrome

The cardiovascular metabolic syndrome is a family of risk factors that predispose patients to develop diabetes and cardiovascular disease. Indeed, macrovascular, not microvascular, disease is the leading cause of death in these patients. The 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors (statins) exert both direct and indirect (cholesterol-lowering) effects on the vasculature. Clinical trials have shown that these agents reduce cardiovascular disease and cerebrovascular disease in persons with diabetes. However, their beneficial effects on diabetic dyslipidemia do not account for all of the observed risk reduction. Positive effects on nitric oxide metabolism, inflammation, coagulability, and adhesion of cells to the vascular endothelium likely contribute to the mechanism of action of these agents. These pleiotropic effects of statins on the vasculature will be discussed in this review.

Clinical review 145: Pleiotropic effects of statins: lipid reduction and beyond

There is accumulating evidence that statins have beneficial effects that are independent of their classical actions on lipoproteins. These effects include reductions in inflammation in the vasculature, kidney, and bone. Potential beneficial effects of these agents include enhancement of nitric oxide production in vasculature and the kidney. These agents appear to reduce bone fractures and may improve insulin sensitivity and reduce the likelihood of persons progressing from impaired glucose tolerance to type II diabetes. Potential beneficial pleiotropic effects of statins are covered in this review.

Diabetes mellitus and diabetes-associated vascular disease

Diabetes-related cardiovascular disease remains the leading cause of death in patients with type 2 diabetes. Hypertension is common among diabetics and has the same pathogenetic mechanisms as insulin resistance, in which the activated renin-angiotensin system contributes to the emerging high blood pressure and hyperglycemia. Hyperglycemia is one of the triggering factors for vascular dysfunction and clotting abnormalities and, therefore, for accelerated atherosclerosis in diabetes. Glycated hemoglobin levels, as a reflection of the degree of glycemia, are strongly associated with the risk of cardiovascular disease in diabetics and in the general population. Tight glycemic control, the treatment of dyslipidemia and raised blood pressure, in addition to the use of antiplatelet therapy, all powerfully reduce the risks associated with diabetes. Furthermore, angiotensin-converting enzyme inhibitors might offer additional cardioprotection to diabetics above that provided by blood pressure reduction.

Treatment of hypertension in diabetes mellitus

In this article we emphasize the need for prompt intervention in diabetic patients with high blood pressure in order to protect the heart, brain, kidney, and the vascular tree against arteriosclerotic damage, which is the main cause of mortality in type 1, and particularly type 2 diabetes mellitus. Recent placebo-controlled, randomized trials indicate that compared with the nondiabetic population, a lower blood pressure threshold for intervention and a lower target blood pressure are adequate in terms of target organ protection. Although all major classes of antihypertensive drugs have demonstrated a potential benefit in treating diabetic hypertensive patients, blocking the renin-angiotensin system with angiotensin converting enzyme (ACE) inhibitors is especially useful in patients at high risk for myocardial infarction and/or renal damage. The new class of antihypertensive agents that block the angiotensin II receptor have renal effects very close to those observed with ACE inhibitors. The potential role of this new class in the treatment of hypertension in diabetes will depend on the results of ongoing trials.

Comorbidity of hypertension and diabetes: the fosinopril versus amlodipine cardiovascular events trial (FACET)

Macrovascular disease is the major cause of mortality in persons with type 2 diabetes mellitus, and hypertension is an important factor contributing to this high prevalence. High blood pressure is about twice as common in persons with diabetes mellitus as in those without. Up to 75% of diabetes-related cardiovascular complications are attributed to hypertension. These observations are part of the rationale for recommendations for more aggressive lowering of blood pressure (to < 130/85 mm Hg) in persons with coexistent diabetes and hypertension. This may require therapy with a combination of antihypertensive agents. The Fosinopril versus Amlodipine Cardiovascular Events Trial (FACET), discussed herein, supports the case for combination therapy with an angiotensin-converting enzyme (ACE) inhibitor and a calcium antagonist in diabetic patients with hypertension.

Effects of calcium antagonists on insulin sensitivity and other metabolic parameters

The antihypertensive efficacy of calcium antagonists appears to be comparable to that of oral diuretics when used as monotherapy. Peripheral vascular dilation appears to be the principal mechanism of the long-term blood pressure-lowering effects of calcium antagonists. The calcium antagonists appear to have beneficial effects with respect to maintenance of renal blood flow and glomerular filtration rate. Metabolic abnormalities associated with diuretic and beta-blocker antihypertensive therapy, such as hypokalemia, hypercalcemia, hyperuricemia, lipid changes, and hyperglycemia, are generally not observed with calcium antagonists.

Effects of insulin on calcium metabolism and platelet aggregation

The influence of insulin on platelets in vitro has not been exhaustively investigated. To clarify whether insulin affects Ca2+ metabolism in platelets directly or through alteration of other systems regulating intracellular Ca2+ homeostasis, we examined the effect of insulin both alone and in combination with prostaglandin E1 on platelet aggregation and Ca2+ metabolism. Incubation of rat platelets with insulin reduced thrombin-induced Ca2+ influx but did not change thrombin-evoked release of Ca2+ from internal stores or the size of internal Ca2+ stores. The interactive effects of insulin with prostaglandin E1 were only additive, and insulin did not augment the effects of prostaglandin E1 on platelet Ca2+ metabolism. In contrast, insulin did not inhibit thrombin-induced platelet aggregation but did augment inhibition of platelet aggregation by prostaglandin E1. Our results suggest that insulin inhibits platelet function by both prostaglandin E1-dependent and -independent mechanisms.

Hypertension in diabetes mellitus

Hypertension should be detected and treated early in diabetic patients. It has a marked contribution to the morbidity and mortality of diabetic individuals due to both atherosclerosis and microvascular disease. Antihypertensive treatment is an effective tool in slowing the progression of early and advanced diabetic nephropathy. Prospective studies addressing the effects of antihypertensive regimens on the incidence of CHF, stroke, and coronary artery disease in the diabetic population are not available. We assume that the beneficial effects of therapy apply to both diabetic and nondiabetic subjects. Glycemic control and the lipid profile are major concerns when selecting an antihypertensive drug. Because hyperinsulinemia and insulin resistance have been advocated as hypertensive and atherosclerotic risk factors, the effects of antihypertensive drugs on insulin action and plasma insulin levels may also become an important element in the selection of an antihypertensive agent. ACE inhibitors, calcium channel blockers, and alpha-adrenergic blockers probably offer the most favorable metabolic profile when compared with diuretics and beta-blockers and should be used as the initial drugs in most clinical settings.

Oxidized low density lipoprotein inhibits platelet plasma membrane Ca(2+)-ATPase

Oxidized low density lipoprotein (LDL) has been shown to enhance platelet activation. Since platelet activation is accompanied by an increase in cytosolic calcium, the effects of oxidized LDL on plasma membrane Ca(2+)-ATPase, plasma membrane fluidity and cytoplasmic calcium were studied in human platelets and purified platelet plasma membranes. Our results demonstrate that oxidized LDL, but not native LDL, inhibits the activity of Ca(2+)-ATPase in purified platelet plasma membranes (P < 0.01). Addition of the free radical scavenger alpha-tocopherol had no effect on the ability of oxidized LDL to inhibit the Ca(2+)-ATPase. An increased cytoplasmic calcium level in whole platelets was induced by oxidized LDL (P < 0.01), indicating that the plasma membrane Ca(2+)-extrusion pump may also be inhibited in vivo by oxidized LDL, although other mechanisms for the increase in cytoplasmic calcium are possible. Since no change in membrane fluidity was observed in platelet plasma membranes exposed to oxidized or native LDL as estimated by steady state trimethylammonium diphenylhexatriene (TMA-DPH) anisotropy, oxidized LDL does not affect the Ca(2+)-ATPase by grossly changing the membrane environment. The present results suggest that exposure of platelets to oxidized LDL causes inhibition of the plasma membrane Ca(2+)-ATPase which contributes to the observed increase in cytoplasmic calcium and increased sensitivity to agonists.

Serotonin-induced increases in platelet cytosolic calcium concentration in depressed, schizophrenic, and substance abuse patients

Exaggerated intracellular calcium responses to challenges with serotonin (5-HT) have been reported in depression. In our studies, consistent with previous reports, patients with depression exhibited an exaggerated increase in 5-HT-stimulated intracellular calcium concentration ([Ca++]i). Basal cytosolic calcium was elevated in both calcium-free and 1 mM calcium media in depressed patients. the increased responsiveness to 5-HT was seen in both conditions. Patients with schizophrenia and substance abuse did not differ from normal controls. The 5-HT response was correlated with diastolic blood pressure (r = 0.33, p = 0.02): however, this association did not fully account for the exaggerated [Ca++]i responses in the depressed group. These findings suggest that exaggerated increases in [Ca++]i in response to serotonin are a characteristic of depressed patients not shared with schizophrenic and substance abuse patients. The relationship of depression to hypertension, two conditions that share abnormalities of calcium homeostasis, warrants further study.

Normalization of abnormal coronary vasomotion by calcium antagonists in patients with hypertension

BACKGROUND

Endothelial dysfunction with a loss of endothelium-dependent vasodilation has been reported in patients with arterial hypertension. The purpose of the present study was to evaluate coronary vasomotor response to dynamic exercise in patients with coronary artery disease with and without arterial hypertension and to determine the effect of calcium antagonists on coronary vasomotion.

METHODS AND RESULTS

Cross-sectional areas of a normal and a stenotic coronary vessel segment were examined in 79 patients with coronary artery disease at rest and during supine bicycle exercise (Ex). Change in luminal area after acute administration of a calcium antagonist (diltiazem or nicardipine), during exercise, and after sublingual nitroglycerin (percent change compared with rest = 100%) was assessed by biplane quantitative coronary arteriography. Patients were divided into two groups: Group 1 (control) consisted of 48 patients without (normotensive subjects, n = 30; hypertensive subjects, n = 18) and group 2 of 31 patients with (normotensive subjects, n = 15; hypertensive subjects, n = 16) pretreatment with a calcium antagonist immediately before exercise. The groups did not differ with regard to clinical characteristics or hemodynamic data measured during exercise. Mean aortic pressure at rest, however, was significantly increased in hypertensive patients compared with normotensive subjects in group 1 (103 mm Hg versus 92 mm Hg, P < .01) and group 2 (110 mm Hg versus 98 mm Hg, P < .025). In group 1, exercise-induced vasomotor response was significantly different between normotensive and hypertensive patients in normal (+20% versus +1%, P < .003) and stenotic vessels (-5% versus -20%, P < .025). However, in group 2 there was coronary vasodilation in normotensive and hypertensive patients for both normal (delta Ex +23% versus +21%, P = NS) and stenotic vessel segments (+24% versus +26%, P = NS).

CONCLUSIONS

Abnormal coronary vasomotion during exercise can be observed in hypertensive patients with reduced vasodilator response in normal arteries and enhanced vasoconstrictor response in stenotic arteries. Calcium antagonists prevent the abnormal response of normal and stenotic coronary arteries to exercise in hypertensive patients and thus may compensate for endothelial dysfunction with reduced vasodilator response to exercise.

Diabetes mellitus and associated hypertension, vascular disease, and nephropathy. An update

Because considerable important information has been published since our previous review, this update concentrates on new findings with regard to cardiovascular and renal risk factors contributing to the striking morbidity and mortality of these coexisting diseases. For example, a large body of investigative data has recently emerged suggesting or delineating a pathogenic role for hyperglycemic-related glycosylation and oxidation of lipoproteins and vascular and renal tissues. Great strides have recently been made in the understanding of platelet, coagulation, lipoprotein, and endothelial abnormalities in the pathogenesis of cardiovascular and renal disease associated with diabetes mellitus and hypertension. Major progress has been made in clarifying the pathophysiology of glomerulosclerosis and other processes involved in the progression of diabetic nephropathy. Furthermore, accumulating data surveyed in this review address new and promising pharmacological interventions that specifically address these pathophysiological mechanisms.

Cytosolic Ca profile of resting and thrombin-stimulated platelets from black women with NIDDM

In this study, human platelets were used as a cellular model for exploring cytosolic free Ca (Cai) regulation in non-insulin-dependent diabetes mellitus (NIDDM). Cai levels were monitored in resting and thrombin-stimulated platelets from obese females with NIDDM; obese, nondiabetic women, and nonobese, nondiabetic women. All subjects were black. Significant and marked elevation of basal Cai levels was observed in platelets from the diabetic subjects when no aspirin was used during platelet isolation. However, no significant differences were observed in Cai between aspirin-treated platelets from women with NIDDM and platelets from nondiabetic women. The rate of the Cai return to basal level after thrombin stimulation was significantly lower in platelets from the diabetic subjects, suggesting an abnormality in platelet Ca extrusion or sequestration in NIDDM. Platelet Cai levels positively correlated with low-density lipoprotein cholesterol/high-density lipoprotein cholesterol ratio (LDL/HDL) and fasting blood glucose. These findings suggest abnormalities in platelet Cai homeostasis in NIDDM that are influenced by the serum lipid profile and perhaps glucose.

Increased calcium stores in platelets from African Americans

Differences in cation transport have been observed between African Americans and whites. These differences may underlie the increased predisposition of African Americans to essential hypertension. To further explore these racial differences, we used platelets as a cellular model for calcium regulation. We measured 45Ca fluxes in platelets from 21 African American and 25 white men. Additionally, using fura 2, we measured cytosolic free calcium levels in resting platelets and platelets treated with ouabain and thrombin. Platelet 45Ca uptake was described by two exchangeable pools: a small, rapidly exchangeable pool and a larger, slowly exchangeable pool. Both pools were larger in platelets from African Americans than from whites (263 versus 185 pmol per 1 x 10(8) platelets for the rapidly exchangeable pool, P < .05; 744 versus 532 pmol per 1 x 10(8) platelets for the slowly exchangeable pool, P < .01). 45Ca washout was described by a rapidly exchangeable pool and a static pool. The former was also higher in platelets from African Americans than from whites (246 versus 202 pmol per 1 x 10(8) platelets, P < .01). The cytosolic free calcium concentrations in resting platelets were lower in African Americans than in whites. After treatment with ouabain and thrombin, the sustained posttransient levels of cytosolic free calcium increased to a greater extent in platelets from African Americans (46.7 nmol/L) than from whites (34.5 nmol/L, P = .033). Platelets from African Americans demonstrate higher intracellular calcium stores than platelets from whites. This racial difference could explain the sensitivity of African Americans to vasoactive agents acting through calcium mobilization from intracellular stores and cytosolic calcium.