Activity and expression of the Na+/H+ exchanger in human endothelial cells cultured in high glucose

The Renoprotective Mechanisms of Sodium-Glucose Cotransporter-2 Inhibitors (SGLT2i)-A Narrative Review

Chronic kidney disease (CKD) is a noncommunicable condition that has become a major healthcare burden across the globe, often underdiagnosed and associated with low awareness. The main cause that leads to the development of renal impairment is diabetes mellitus and, in contrast to other chronic complications such as retinopathy or neuropathy, it has been suggested that intensive glycemic control is not sufficient in preventing the development of diabetic kidney disease. Nevertheless, a novel class of antidiabetic agents, the sodium-glucose cotransporter-2 inhibitors (SGLT2i), have shown multiple renoprotective properties that range from metabolic and hemodynamic to direct renal effects, with a major impact on reducing the risk of occurrence and progression of CKD. Thus, this review aims to summarize current knowledge regarding the renoprotective mechanisms of SGLT2i and to offer a new perspective on this innovative class of antihyperglycemic drugs with proven pleiotropic beneficial effects that, after decades of no significant progress in the prevention and in delaying the decline of renal function, start a new era in the management of patients with CKD.

Sodium-Glucose Cotransporter 2 Inhibition: Rationale and Mechanisms for Kidney and Cardiovascular Protection in People With and Without Diabetes

Large-scale randomized trials have demonstrated the remarkable capacity of sodium-glucose cotransporter 2 inhibitors to reduce the risk of cardiovascular outcomes and kidney disease progression, irrespective of the presence or absence of type 2 diabetes mellitus. Although the results of these trials have transformed clinical practice guidelines, the mechanisms underpinning the wide-ranging benefits of this class of agents remain incompletely understood and subject to ongoing investigation. Improvements in cardiometabolic risk factors such as glucose, blood pressure, body weight, and albuminuria likely contribute. However, other direct effects on physiological and cellular function, such as restoration of tubuloglomerular feedback, improvements in kidney and cardiac oxygenation and energy efficiency, as well as restoration of normal autophagy are also likely to be important. This review summarizes the rationale and potential mechanisms for cardiorenal protection with sodium-glucose cotransporter 2 inhibitors in people with and without diabetes, their relative importance, and the experimental and clinical lines of evidence supporting these hypotheses.

Exacerbated brain edema in a rat streptozotocin model of hyperglycemic ischemic stroke: Evidence for involvement of blood-brain barrier Na-K-Cl cotransport and Na/H exchange

Cerebral edema is exacerbated in diabetic ischemic stroke through poorly understood mechanisms. We showed previously that blood-brain barrier (BBB) Na-K-Cl cotransport (NKCC) and Na/H exchange (NHE) are major contributors to edema formation in normoglycemic ischemic stroke. Here, we investigated whether hyperglycemia-exacerbated edema involves changes in BBB NKCC and NHE expression and/or activity and whether inhibition of NKCC or NHE effectively reduces edema and injury in a type I diabetic model of hyperglycemic stroke. Cerebral microvascular endothelial cell (CMEC) NKCC and NHE abundances and activities were determined by Western blot, radioisotopic flux and microspectrofluorometric methods. Cerebral edema and Na in rats subjected to middle cerebral artery occlusion (MCAO) were assessed by nuclear magnetic resonance methods. Hyperglycemia exposures of 1-7d significantly increased CMEC NKCC and NHE abundance and activity. Subsequent exposure to ischemic factors caused more robust increases in NKCC and NHE activities than in normoglycemic CMEC. MCAO-induced edema and brain Na uptake were greater in hyperglycemic rats. Intravenous bumetanide and HOE-642 significantly attenuated edema, brain Na uptake and ischemic injury. Our findings provide evidence that BBB NKCC and NHE contribute to increased edema in hyperglycemic stroke, suggesting that these Na transporters are promising therapeutic targets for reducing damage in diabetic stroke.

Role of the sodium-hydrogen exchanger in mediating the renal effects of drugs commonly used in the treatment of type 2 diabetes

Diabetes is characterized by increased activity of the sodium-hydrogen exchanger (NHE) in the glomerulus and renal tubules, which contributes importantly to the development of nephropathy. Despite the established role played by the exchanger in experimental studies, it has not been specifically targeted by those seeking to develop novel pharmacological treatments for diabetes. This review demonstrates that many existing drugs that are commonly prescribed to patients with diabetes act on the NHE1 and NHE3 isoforms in the kidney. This action may explain their effects on sodium excretion, albuminuria and the progressive decline of glomerular function in clinical trials; these responses cannot be readily explained by the influence of these drugs on blood glucose. Agents that may affect the kidney in diabetes by virtue of an action on NHE include: (1) insulin and insulin sensitizers; (2) incretin-based agents; (3) sodium-glucose cotransporter 2 inhibitors; (4) antagonists of the renin-angiotensin system (angiotensin converting-enzyme inhibitors, angiotensin receptor blockers and angiotensin receptor neprilysin inhibitors); and (5) inhibitors of aldosterone action and cholesterol synthesis (spironolactone, amiloride and statins). The renal effects of each of these drug classes in patients with type 2 diabetes may be related to a single shared biological mechanism.

Sodium-hydrogen exchangers (NHE) in human cardiovascular diseases: interfering strategies and their therapeutic applications

Sodium-hydrogen exchangers (NHE) are among the main regulators of cell volume and intracellular concentration of hydrogen and sodium ions. By indirectly affecting sodium/calcium exchange across the plasma membrane, NHE can also influence the intracellular concentration of calcium. Excess activation of NHE or inappropriate sodium extrusion due to failure of ATP-dependent Na(+)/K(+) transport system can be deleterious during cardiac or peripheral organ ischemia. Besides being responsible for the regulation of intracellular pH and sodium-calcium inward currents, NHE isoform 1 (NHE-1), which is predominantly expressed in the cardiovascular system, influences the tone of the vessel wall in response to a variety of stimuli, including hypertonic stress. Because of the extensive involvement of NHE-1 in cardiac myocyte contracture and necrosis, stunning, reperfusion arrhythmias, as well as hypertension and myocardial diseases such as diabetic cardiomyopathy, efforts have been made in developing inhibitors of this transporter. We here review the biology and regulation of NHE, focusing on current knowledge of the role of NHE-1 as a potential target in the development of novel compounds that could play a role in cardiovascular homeostasis, both in physiological and pathological conditions.

Electrical plasticity and cardioprotection in myocardial ischemia--role of selective sodium channel blockers

The concept of electrical protection of the ischemic myocardium is in constant evolution and has recently been supported by experimental and clinical studies. Historically, antiplatelet agents, angiotensin-converting enzyme inhibitors, β-blockers, and statins have been all proposed as drugs conferring anti-ischemic cardioprotection. This was supported by the evidence consistently indicating that all these drugs were capable of reducing mortality and the risk of repeat myocardial infarction. The electrical plasticity paradigm is, however, a novel concept that depicts the benefits of improved sodium channel blockade with drugs such as ranolazine and cariporide. Although it has been hypothesized that the protective role of ranolazine depends on decreased fatty acid β-oxidation affecting preconditioning, we speculate against such a hypothesis, because inhibition of β-oxidation requires higher concentrations of the drug, above the therapeutic range. Rather, we discuss the key role of calcium overload reduction through inhibition of the late sodium current (I(Na)). Mechanisms driving cardioprotection involve the block of a cascade of complex ionic exchanges that can result in intracellular acidosis, excess cytosolic calcium, myocardial cellular dysfunction, and eventually cell injury and death. In this review we discuss the studies that demonstrate how electrical plasticity through sodium channel blockers can promote cardioprotection against ischemia in coronary heart disease.

NA+/H+ Exchanger 1-and Aquaporin-1-Dependent Hyperosmolarity Changes Decrease Nitric Oxide Production and Induce VCAM-1 Expression in Endothelial Cells Exposed to High Glucose

Since diabetic hyperglycaemia causes hyperosmolarity, we investigated the contribution of hyperosmolarity in the proinflammatory endothelial effects of hyperglycemia, and sought to unravel the mechanisms involved. Human aortic endothelial cells (HAEC) were incubated for short-term (1–3 days) or long-term (1–2 weeks) exposures to 5.5 mmol/L glucose (normoglycemia, basal), high glucose (25 and 45 mmol/L, HG), or a hyperosmolar control (mannitol 25 and 45 mmol/L, HM), in the presence or absence of the aquaporin-1 (AQP1) inihibitor dimethylsulfoxide (DMSO), the Na+/H+ exchanger 1 (NHE-1) inihibitor cariporide (CA), the protein kinase C (PKC) inihibitor calphostin C or the PKCβ isoform inhibitor LY379196 (LY). Both short- and long-term exposures to HG and HM decreased the expression of the active, phosphorylated form of endothelial nitric oxide synthase (Ser1146-eNOS) and, in parallel, increased vascular cell adhesion molecule(VCAM)-1 protein at immunoblotting. After 24 h incubation with HG/HM, we observed a significant similar and concentration-dependent enhancement of AQP1 expression. DMSO and CA inhibited hyperosmolarity-induced VCAM-1 expressions, while increasing nitrite levels and Ser1146-eNOS expression. Gene silencing by small interfering RNA reduced the expression of AQP1, and suppressed HG- and HM-stimulated VCAM-1 expression. Calphostin C and LY blunted hyperosmolarity-induced VCAM-1 expression, while increasing the expression of Ser1146-eNOS and nitrite production. Thus HG decreases eNOS activation and induces total VCAM-1 expression in HAEC through a hyperosmolar mechanism. These effects are mediated by activation of the water channels AQP1 and NHE-1, and a PKCβ-mediated intracellular signaling pathway. Targeting osmosignaling pathways may represent a novel strategy to reduce vascular effects of hyperglycemia.

Amiloride derivatives induce apoptosis by depleting ER Ca(2+) stores in vascular endothelial cells

BACKGROUND AND PURPOSE

Amiloride derivatives are blockers of the Na(+)/H(+) exchanger (NHE) and at micromolar concentrations have protective effects on cardiac and brain ischaemia/reperfusion injury but at higher concentrations also induce apoptosis. Here, we aimed to elucidate the mechanism related to this cytotoxic action.

EXPERIMENTAL APPROACH

We quantified the expression of genes associated with endoplasmic reticulum (ER) stress and measured changes in luminal ER Ca(2+) concentration ([Ca(2+)](ER)) with a 'cameleon' indicator, D1ER.

KEY RESULTS

Amiloride derivatives induced apoptosis in vascular endothelial cells, an effect that increased at alkaline extracellular pH. The potency order for cytotoxicity was 5-(N,N-hexamethylene)-amiloride (HMA) > 5-(N-methyl-N-isobutyl) amiloride > 5-(N-ethyl-N-isopropyl) amiloride (EIPA) >> amiloride. HMA dose-dependently increased the transcription of the ER stress genes GADD153 and GADD34 and rapidly depleted [Ca(2+)](ER), mimicking the effects of the sarco/endoplasmic reticulum ATPase (SERCA) inhibitor thapsigargin. The NHE1-specific inhibitor HOE 694 inhibited NHE activity by 87% but did not alter [Ca(2+)](ER). The decrease in [Ca(2+)](ER) evoked by amiloride derivatives was also observed in HeLa cells and was mirrored by an increase in cytosolic Ca(2+) concentration.

CONCLUSIONS AND IMPLICATIONS

Amiloride derivatives disrupt ER and cytosolic Ca(2+) homeostasis by a mechanism unrelated to NHE inhibition, most likely by interfering with the activity of SERCA. We propose that ER Ca(2+) depletion and subsequent ER stress provide a rationale framework for the apoptotic effects of amiloride derivatives.

Contributions of endothelin-1 and sodium hydrogen exchanger-1 in the diabetic myocardium

BACKGROUND

Endothelin-1 (ET-1) and sodium hydrogen exchanger-1 (NHE-1) are important mediators of several disease processes affecting the heart, especially relating to myocardial ischemia. There is evidence that their actions may be interrelated. Their contributions to diabetic heart disease have not been extensively documented. Accordingly, the aim of this study was to investigate the interactive roles of ET-1 and NHE-1 in the pathogenesis of diabetic cardiomyopathy, a significant cause of morbidity in diabetic patients.

METHODS

Streptozotocin-induced diabetic Sprague Dawley rats were treated with NHE-1 blocker cariporide or dual ET(A)/ET(B) blocker bosentan and were subsequently studied one, three and six months after induction of diabetes. These animals were compared with nondiabetic rats as well as with diabetic rats on poor blood glucose control.

RESULTS

Diabetes leads to hyperglycemia, reduced body weight gain and increased glycated hemoglobin levels. These animals exhibited focal myocardial fibrosis and increased ejection fraction, in association with a tendency to increased left ventricular wall thickness and heart weight, after six months of follow-up, both bosentan and cariporide prevented these responses. Diabetes also caused significant increases in ET-1 mRNA and protein expression in the heart at all time points, which was further augmented by cariporide treatment for three months. Diabetes did not affect either mRNA or protein expression of NHE-1, although these did decrease in hearts of diabetic animals treated with bosentan for six months.

CONCLUSIONS

These results indicate an important contribution of both ET-1 and NHE-1 in the pathogenesis of diabetic cardiomyopathy. These data suggest that NHE-1 may act as a downstream mediator in the production of ET-induced functional and structural changes in the myocardium in diabetes.

Hyperosmotic stress induces nuclear factor-kappaB activation and interleukin-8 production in human intestinal epithelial cells

Inflammatory bowel disease of the colon is associated with a high osmolarity of colonic contents. We hypothesized that this hyperosmolarity may contribute to colonic inflammation by stimulating the proinflammatory activity of intestinal epithelial cells (IECs). The human IEC lines HT-29 and Caco-2 were used to study the effect of hyperosmolarity on the IEC inflammatory response. Exposure of IECs to hyperosmolarity triggered expression of the proinflammatory chemokine interleukin (IL)-8 both at the secreted protein and mRNA levels. In addition, hyperosmotic stimulation induced the release of another chemokine, GRO-alpha. These effects were because of activation of the transcription factor, nuclear factor (NF)-kappaB, because hyperosmolarity stimulated both NF-kappaB DNA binding and NF-kappaB-dependent transcriptional activity. Hyperosmolarity activated both p38 and p42/44 mitogen-activated protein kinases, which effect contributed to hyperosmolarity-stimulated IL-8 production, because p38 and p42/44 inhibition prevented the hyperosmolarity-induced increase in IL-8 production. In addition, the proinflammatory effects of hyperosmolarity were, in a large part, mediated by activation of Na(+)/H(+) exchangers, because selective blockade of Na(+)/H(+) exchangers prevented the hyperosmolarity-induced IEC inflammatory response. In summary, hyperosmolarity stimulates IEC IL-8 production, which effect may contribute to the maintenance of inflammation in inflammatory bowel disease.

NHE blockade inhibits chemokine production and NF-kappaB activation in immunostimulated endothelial cells

Na(+)/H(+) exchanger (NHE) activation has been documented to contribute to endothelial cell injury caused by inflammatory states. However, the role of NHEs in regulation of the endothelial cell inflammatory response has not been investigated. The present study tested the hypothesis that NHEs contribute to endothelial cell inflammation induced by endotoxin or interleukin (IL)-1beta. NHE inhibition using amiloride, 5-(N-ethyl-N-isopropyl)-amiloride, and 5-(N-methyl-N-isobutyl)amiloride as well as the non-amiloride NHE inhibitors cimetidine, clonidine, and harmaline suppressed endotoxin-induced IL-8 and monocyte chemoattractant protein (MCP)-1 production by human umbilical endothelial vein cells (HUVECs). The suppressive effect of amiloride on endotoxin-induced IL-8 production was associated with a decreased accumulation of IL-8 mRNA. NHE inhibitors suppressed both inhibitory (I)kappaB degradation and nuclear factor (NF)-kappaB DNA binding, suggesting that a decrease in activation of the IkappaB-NF-kappaB system contributed to the suppression of HUVEC inflammatory response by NHE blockade. NHE inhibition decreased also the IL-1beta-induced HUVEC inflammatory response, because amiloride suppressed IL-1beta-induced E-selectin expression on HUVECs. These results demonstrate that maximal activation of the HUVEC inflammatory response requires a functional NHE.

Cardioprotective effect of chronic hyperglycemia: effect on hypoxia-induced apoptosis and necrosis

It is generally accepted that mild forms of diabetes render the heart resistant to an ischemic insult. Because myocytes incubated chronically in medium containing high concentrations of glucose (25 mM) develop into a diabetes-like phenotype, we tested the hypothesis that high-glucose treatment diminishes hypoxia-induced injury. In support of this hypothesis, we found that cardiomyocytes incubated for 3 days with medium containing 25 mM glucose showed less hypoxia-induced apoptosis and necrosis than cells exposed to medium containing 5 mM glucose (control). Indeed, whereas 27% of control cells became necrotic after 1 h of chemical hypoxia with 10 mM deoxyglucose and 5 mM amobarbital (Amytal), only 11% of the glucose-treated cells became necrotic. Similarly, glucose treatment reduced the extent of apoptosis from 32% to 12%. This beneficial effect of glucose treatment was associated with a 40% reduction in the Ca(2+) content of the hypoxic cell. Glucose treatment also mediated an upregulation of the cardioprotective factor Bcl-2 but did not affect the cellular content of the proapoptotic factors Bax and Bad. Nonetheless, the phosphorylation state of Bad was shifted in favor of its inactive, phosphorylated form after high-glucose treatment. These data suggest that glucose treatment renders the cardiomyocyte resistant to hypoxia-induced apoptosis and necrosis by preventing the accumulation of Ca(2+) during hypoxia, promoting the upregulation of Bcl-2, and enhancing the inactivation of Bad.

Polymorphism of the 5' untranslated region of NHE1 gene associated with type-I diabetes

The ubiquitous form of the sodium-hydrogen exchanger, NHE1, is devoted to the regulation of intracellular pH and cell volume. In addition, NHE1 activity is stimulated by growth factors and increased NHE rates are found in both circulating and immortalized cells during diabetes or diabetic nephropathy. In this context, we searched for polymorphisms of the 5'-flanking regulatory region of NHE1 gene in subjects with type-I diabetes. We identified a C/T transition 696 bases upstream the translation initiation start site which disrupts a repeated palindromic GC sequence. The TT genotype was significantly more frequent in type-1 diabetics and may have functional importance. Genetic linkage between NHE1 and diabetes has been previously described in NOD mice strains with consequences on NHE rates. Hence, the polymorphism described hereby may act as a predisposition factor to type-I diabetes or to diabetic complications, and may be useful to investigate the genetic involvement of NHE1 in human pathophysiology.