Glucose-induced regulation of NHEs activity and SGLTs expression involves the PKA signaling pathway

Design, synthesis and biological evaluation of buthutin derivatives as cardioprotective agents

Natural products are the important sources in cardiovascular drug development. In this study, twenty-nine buthutin derivatives were designed, synthesized, and evaluated for their NHE-1 inhibition and protective effects on cardiomyocyte injury. The structure of the newly synthesized compounds had been confirmed by 1H-NMR, 13C-NMR, and HR-ESI-MS spectra. Among all target compounds at 1 μM, compounds 9d, 9f, 9k, 9m, and 9n, with a protection ratio exceeding 30%, exerted stronger protective effects on H9c2 cardiomyocyte than positive control dexrazoxane and buthutin A. Meanwhile, compounds 9k, 9m, and 9o showed the significant NHE-1 inhibitory activities on H9c2 cardiomyocyte, all with a dpHi/min value less than 0.23. What is more, compounds 9k, 9m, 9o and buthutin A all exhibited the specificity on NHE-1 inhibition. Molecular modelling studies suggested the ability of compounds 9m and 9o to establish interactions with three hydrogen bonds to Asp267 and Glu346 of NHE-1, but also the ability with much lower CDOCKER energies than positive control cariporide and buthutin A. The structure-activity relationship (SAR) studies suggested that the presences of amide group, four-carbon linker, and para hydroxyl benzene ring were advantageous pharmacophores for above two pharmacological actions. This research would open new avenues for developing amide-guanidine-based cardioprotective agents.

Development of a selective novel fluorescent substrate for sodium-dependent transporters

AIM

To synthesize, characterize, and validate 6FGA, a fluorescent glucose modified with a Cyanine5.5 at carbon-6 position, for probing the function of sodium-dependent glucose transporters, SGLT1 and SGLT2.

MAIN METHODS

The synthesis of fluorescent glucose analogue was achieved through "click chemistry" of Cyanine5.5-alkyne and 6-azido-6-deoxy-d-glucose. Cell system studies were conducted to characterize the in vivo transport properties.

KEY FINDINGS

Optical analyses revealed that 6FGA displayed similar spectral profiles to Cyanine5.5 in DMSO, allowing for concentration determination, thus supporting its utility in quantitative kinetic studies within biological assays. Uptake studies in cell system SGLT models, LLC-PK1 and HEK293 cells, exhibited concentration and time-dependent behavior, indicating saturation at specific concentrations and durations which are hallmarks of transported-mediated uptake. The results of cytotoxicity assays suggested cell viability at micromolar concentrations, enabling usage in assays for at least 1 h without significant toxicity. The dependence of 6FGA uptake on sodium, the co-transported cation, was demonstrated in LLC-PK1 and HEK293 cells. Fluorescence microscopy confirmed intracellular localization of 6FGA, particularly near the nucleus. Competition studies revealed that glucose tends to weakly reduce 6FGA uptake, although the effect did not achieve statistical significance. Assessments using standard SGLT and GLUT inhibitors highlighted 6FGA's sensitivity for probing SGLT-mediated transport.

SIGNIFICANCE

6FGA is a new fluorescent glucose analog offering advantages over existing probes due to its improved photophysical properties, greater sensitivity, enabling subcellular resolution and efficient tissue penetration in near-infrared imaging. 6FGA presents practicality and cost-effectiveness, making it a promising tool for nonradioactive, microplate-based assays at investigating SGLT-mediated glucose transport mechanisms.

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.

The anti-hypertensive effects of sodium-glucose cotransporter-2 inhibitors

INTRODUCTION

Hypertension is a well-established risk factor for cardiovascular (CV) events in patients with chronic kidney disease (CKD), heart failure, obesity, and diabetes. Despite the usual prescribed antihypertensive therapies, many patients fail to achieve the recommended blood pressure (BP) targets.

AREAS COVERED

This review summarizes the clinical BP-lowering data presented in major CV and kidney outcome trials for sodium-glucose cotransporter-2 (SGLT2) inhibitors, as well as smaller dedicated BP trials in high-risk individuals with and without diabetes. We have also highlighted potential mechanisms that may contribute to the antihypertensive effects of SGLT2 inhibitors, including natriuresis and hemodynamic changes, a loop diuretic-like effect, and alterations in vascular physiology.

EXPERT OPINION

The antihypertensive properties of SGLT2 inhibitors are generally modest but may be larger in certain patient populations. SGLT2 inhibitors may have an additional role as an adjunctive BP-lowering therapy in patients with hypertension at high risk of CV disease or kidney disease.

Acetazolamide in Acute Decompensated Heart Failure with Volume Overload

BACKGROUND

Whether acetazolamide, a carbonic anhydrase inhibitor that reduces proximal tubular sodium reabsorption, can improve the efficiency of loop diuretics, potentially leading to more and faster decongestion in patients with acute decompensated heart failure with volume overload, is unclear.

METHODS

In this multicenter, parallel-group, double-blind, randomized, placebo-controlled trial, we assigned patients with acute decompensated heart failure, clinical signs of volume overload (i.e., edema, pleural effusion, or ascites), and an N-terminal pro-B-type natriuretic peptide level of more than 1000 pg per milliliter or a B-type natriuretic peptide level of more than 250 pg per milliliter to receive either intravenous acetazolamide (500 mg once daily) or placebo added to standardized intravenous loop diuretics (at a dose equivalent to twice the oral maintenance dose). Randomization was stratified according to the left ventricular ejection fraction (≤40% or >40%). The primary end point was successful decongestion, defined as the absence of signs of volume overload, within 3 days after randomization and without an indication for escalation of decongestive therapy. Secondary end points included a composite of death from any cause or rehospitalization for heart failure during 3 months of follow-up. Safety was also assessed.

RESULTS

A total of 519 patients underwent randomization. Successful decongestion occurred in 108 of 256 patients (42.2%) in the acetazolamide group and in 79 of 259 (30.5%) in the placebo group (risk ratio, 1.46; 95% confidence interval [CI], 1.17 to 1.82; P<0.001). Death from any cause or rehospitalization for heart failure occurred in 76 of 256 patients (29.7%) in the acetazolamide group and in 72 of 259 patients (27.8%) in the placebo group (hazard ratio, 1.07; 95% CI, 0.78 to 1.48). Acetazolamide treatment was associated with higher cumulative urine output and natriuresis, findings consistent with better diuretic efficiency. The incidence of worsening kidney function, hypokalemia, hypotension, and adverse events was similar in the two groups.

CONCLUSIONS

The addition of acetazolamide to loop diuretic therapy in patients with acute decompensated heart failure resulted in a greater incidence of successful decongestion. (Funded by the Belgian Health Care Knowledge Center; ADVOR ClinicalTrials.gov number, NCT03505788.).

Vertical Sleeve Gastrectomy Lowers SGLT2/Slc5a2 Expression in the Mouse Kidney

Bariatric surgery improves glucose homeostasis, but the underlying mechanisms are not fully elucidated. Here, we show that the expression of sodium-glucose cotransporter 2 (SGLT2/Slc5a2) is reduced in the kidney of lean and obese mice following vertical sleeve gastrectomy (VSG). Indicating an important contribution of altered cotransporter expression to the impact of surgery, inactivation of the SGLT2/Slc5a2 gene by clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 attenuated the effects of VSG, with glucose excursions following intraperitoneal injection lowered by ∼30% in wild-type mice but by ∼20% in SGLT2-null animals. The effects of the SGLT2 inhibitor dapaglifozin were similarly blunted by surgery. Unexpectedly, effects of dapaglifozin were still observed in SGLT2-null mice, consistent with the existence of metabolically beneficial off-target effects of SGLT2 inhibitors. Thus, we describe a new mechanism involved in mediating the glucose-lowering effects of bariatric surgery.

Sweet Taste Signaling: The Core Pathways and Regulatory Mechanisms

Sweet taste, a proxy for sugar-derived calories, is an important driver of food intake, and animals have evolved robust molecular and cellular machinery for sweet taste signaling. The overconsumption of sugar-derived calories is a major driver of obesity and other metabolic diseases. A fine-grained appreciation of the dynamic regulation of sweet taste signaling mechanisms will be required for designing novel noncaloric sweeteners with better hedonic and metabolic profiles and improved consumer acceptance. Sweet taste receptor cells express at least two signaling pathways, one mediated by a heterodimeric G-protein coupled receptor encoded by taste 1 receptor members 2 and 3 (TAS1R2 + TAS1R3) genes and another by glucose transporters and the ATP-gated potassium (K_ATP) channel. Despite these important discoveries, we do not fully understand the mechanisms regulating sweet taste signaling. We will introduce the core components of the above sweet taste signaling pathways and the rationale for having multiple pathways for detecting sweet tastants. We will then highlight the roles of key regulators of the sweet taste signaling pathways, including downstream signal transduction pathway components expressed in sweet taste receptor cells and hormones and other signaling molecules such as leptin and endocannabinoids.

The Pathophysiological Basis of Diabetic Kidney Protection by Inhibition of SGLT2 and SGLT1

SGLT2 inhibitors can protect the kidneys of patients with and without type 2 diabetes mellitus and slow the progression towards end-stage kidney disease. Blocking tubular SGLT2 and spilling glucose into the urine, which triggers a metabolic counter-regulation similar to fasting, provides unique benefits, not only as an anti-hyperglycemic strategy. These include a low hypoglycemia risk and a shift from carbohydrate to lipid utilization and mild ketogenesis, thereby reducing body weight and providing an additional energy source. SGLT2 inhibitors counteract hyperreabsorption in the early proximal tubule, which acutely lowers glomerular pressure and filtration and thereby reduces the physical stress on the filtration barrier, the filtration of tubule-toxic compounds, and the oxygen demand for tubular reabsorption. This improves cortical oxygenation, which, together with lesser tubular gluco-toxicity and improved mitochondrial function and autophagy, can reduce pro-inflammatory, pro-senescence, and pro-fibrotic signaling and preserve tubular function and GFR in the long-term. By shifting transport downstream, SGLT2 inhibitors more equally distribute the transport burden along the nephron and may mimic systemic hypoxia to stimulate erythropoiesis, which improves oxygen delivery to the kidney and other organs. SGLT1 inhibition improves glucose homeostasis by delaying intestinal glucose absorption and by increasing the release of gastrointestinal incretins. Combined SGLT1 and SGLT2 inhibition has additive effects on renal glucose excretion and blood glucose control. SGLT1 in the macula densa senses luminal glucose, which affects glomerular hemodynamics and has implications for blood pressure control. More studies are needed to better define the therapeutic potential of SGLT1 inhibition to protect the kidney, alone or in combination with SGLT2 inhibition.

Influence of carnosine and carnosinase-1 on diabetes-induced afferent arteriole vasodilation: implications for glomerular hemodynamics

BACKGROUND

Dysregulation in glomerular hemodynamics favors hyperfiltration in diabetic kidney disease (DKD). Although carnosine supplementation ameliorates features of DKD, its effect on glomerular vasoregulation is not known. We assessed the influence of carnosine and carnosinase-1 (CN1) on afferent glomerular arteriole vasodilation and its association with glomerular size, hypertrophy and nephrin expression in diabetic BTBR^ob/ob mice.

METHODS

Two cohorts of mice including appropriate controls were studied i.e., diabetic mice receiving oral carnosine supplementation (cohort 1) and human CN1 (hCN1) transgenic (TG) diabetic mice (cohort 2). Lumen area ratio (LAR) of the afferent arterioles and glomerular parameters were measured by conventional histology. Three-dimensional analysis using a tissue clearing strategy was also employed.

RESULTS

In both cohorts, LAR was significantly larger in diabetic BTBR^ob/ob vs non-diabetic BTBR^wt/ob mice (0.41±0.05 vs 0.26±0.07; p<0.0001) and (0.42±0.06 vs 0.29±0.04; p<0.0001), and associated with glomerular size (cohort 1: r =0.55, p=0.001; cohort 2: r=0.89, p<0.0001). LAR was partially normalized by oral carnosine supplementation (0.34±0.05 vs 0.41±0.05; p=0.004), but did not differ between hCN1 TG and wild type (WT) BTBR^ob/ob mice. In hCN1 TG mice, serum CN1 concentrations correlated with LAR (r=0.90; p=0.006). Diabetic mice displayed decreased nephrin expression and increased glomerular hypertrophy. This was not significantly different in hCN! TG BTBR^ob/ob mice (p=0,06 and p=0,08, respectively).

CONCLUSION

Carnosine and CN1 may affect intra-glomerular pressure in an opposing manner through regulation of afferent arteriolar tone. This study corroborates previous findings on the role of carnosine in the progression of DKD.

Cardiorenal Syndrome: The Role of Neural Connections Between the Heart and the Kidneys

The maintenance of cardiovascular homeostasis is highly dependent on tightly controlled interactions between the heart and the kidneys. Therefore, it is not surprising that a dysfunction in one organ affects the other. This interlinking relationship is aptly demonstrated in the cardiorenal syndrome. The characteristics of the cardiorenal syndrome state include alterations in neurohumoral drive, autonomic reflexes, and fluid balance. The evidence suggests that several factors contribute to these alterations. These may include peripheral and central nervous system abnormalities. However, accumulating evidence from animals with experimental models of congestive heart failure and renal dysfunction as well as humans with the cardiorenal syndrome suggests that alterations in neural pathways, from and to the kidneys and the heart, including the central nervous system are involved in regulating sympathetic outflow and may be critically important in the alterations in neurohumoral drive, autonomic reflexes, and fluid balance commonly observed in the cardiorenal syndrome. This review focuses on studies implicating neural pathways, particularly the afferent and efferent signals from the heart and the kidneys integrating at the level of the paraventricular nucleus in the hypothalamus to alter neurohumoral drive, autonomic pathways, and fluid balance. Further, it explores the potential mechanisms of action for the known beneficial use of various medications or potential novel therapeutic manipulations for the treatment of the cardiorenal syndrome. A comprehensive understanding of these mechanisms will enhance our ability to treat cardiorenal conditions and their cardiovascular complications more efficaciously and thoroughly.

Renal effects of guideline-directed medical therapies in heart failure: a consensus document from the Heart Failure Association of the European Society of Cardiology

Novel pharmacologic treatment options reduce mortality and morbidity in a cost-effective manner in patients with heart failure (HF). Undisputedly, the effective implementation of these agents is an essential element of good clinical practice, which is endorsed by the European Society of Cardiology (ESC) guidelines on acute and chronic HF. Yet, physicians struggle to implement these therapies as they have to balance the true and/or perceived risks versus their substantial benefits in clinical practice. Any worsening of biomarkers of renal function is often perceived as being disadvantageous and is in clinical practice one of the most common reasons for ineffective drug implementation. However, even in this context, they clearly reduce mortality and morbidity in HF with reduced ejection fraction (HFrEF) patients, even in patients with poor renal function. Furthermore these agents are also beneficial in HF with mildly reduced ejection fraction (HFmrEF) and sodium-glucose cotransporter 2 (SGLT2) inhibitors more recently demonstrated a beneficial effect in HF with preserved ejection fraction (HFpEF). The emerge of several new classes (angiotensin receptor-neprilysin inhibitor [ARNI], SGLT2 inhibitors, vericiguat, omecamtiv mecarbil) and the recommendation by the 2021 ESC guidelines for the diagnosis and treatment of acute and chronic HF of early initiation and titration of quadruple disease-modifying therapies (ARNI/angiotensin-converting enzyme inhibitor + beta-blocker + mineralocorticoid receptor antagonist and SGLT2 inhibitor) in HFrEF increases the likelihood of treatment-induced changes in renal function. This may be (incorrectly) perceived as deleterious, resulting in inertia of starting and uptitrating these lifesaving therapies. Therefore, the objective of this consensus document is to provide advice of the effect HF drugs on renal function.

Enhanced Expression and Function of Renal SGLT2 (Sodium-Glucose Cotransporter 2) in Heart Failure: Role of Renal Nerves

BACKGROUND

Recent clinical studies demonstrate that SGLT2 (sodium-glucose cotransporter 2) inhibitors ameliorate heart failure (HF). The present study was conducted to assess the expression and function of renal SGLT2 and the influence of enhanced renal sympathetic tone in HF.

METHODS

Four weeks after coronary artery ligation surgery to induce HF, surgical bilateral renal denervation (RDN) was performed in rats. Four groups of rats (Sham-operated control [Sham], Sham+RDN, HF and HF+RDN; n=6/group) were used. Immunohistochemistry and Western blot analysis were performed to evaluate the renal SGLT2 expression. One week after RDN (5 weeks after induction of HF), intravenous injection of SGLT2 inhibitor dapagliflozin were performed to assess renal excretory responses. In vitro, human embryonic kidney cells were used to investigate the fractionation of SGLT2 after norepinephrine treatment.

RESULTS

In rats with HF, (1) SGLT2 expression in the proximal tubule of the kidney was increased; (2) the response of increases in urine flow, sodium excretion, and glucose excretion to dapagliflozin were greater; and (3) RDN attenuated renal SGLT2 expression and normalized renal functional responses to dapagliflozin. In vitro, norepinephrine promoted translocation of SGLT2 to the cell membrane.

CONCLUSIONS

These results indicate that the enhanced tonic renal sympathetic nerve activation in HF increases the expression and functional activity of renal SGLT2. Potentiated trafficking of SGLT2 to cell surface in renal proximal tubules mediated by norepinephrine may contribute to this functional activation of SGLT2 in HF. These findings provide critical insight into the underlying mechanisms for the beneficial effects of SGLT2 inhibitors on HF reported in the clinical studies.

Pathophysiology of diabetic kidney disease: impact of SGLT2 inhibitors

Diabetic kidney disease is the leading cause of kidney failure worldwide; in the USA, it accounts for over 50% of individuals entering dialysis or transplant programmes. Unlike other complications of diabetes, the prevalence of diabetic kidney disease has failed to decline over the past 30 years. Hyperglycaemia is the primary aetiological factor responsible for the development of diabetic kidney disease. Once hyperglycaemia becomes established, multiple pathophysiological disturbances, including hypertension, altered tubuloglomerular feedback, renal hypoxia, lipotoxicity, podocyte injury, inflammation, mitochondrial dysfunction, impaired autophagy and increased activity of the sodium-hydrogen exchanger, contribute to progressive glomerular sclerosis and the decline in glomerular filtration rate. The quantitative contribution of each of these abnormalities to the progression of diabetic kidney disease, as well as their role in type 1 and type 2 diabetes mellitus, remains to be determined. Sodium-glucose co-transporter 2 (SGLT2) inhibitors have a beneficial impact on many of these pathophysiological abnormalities; however, as several pathophysiological disturbances contribute to the onset and progression of diabetic kidney disease, multiple agents used in combination will likely be required to slow the progression of disease effectively.

The pathophysiology and management of diuretic resistance in patients with heart failure

OBJECTIVES

The objectives of the study are to investigate the causes of diuretic resistance in patients with advanced congestive heart failure (CHF), since diuretics are the cornerstone of treatment of these patients. Several studies have shown that diuretic resistance in patients with advanced CHF is common, ranging from 25% to 50% in hospitalized patients.

METHODS

In order to get a current perspective as to the magnitude of diuretic resistance in such patients, a focused Medline search of the English language literature was conducted between 2015 and 2020 using the search terms, CHF, diuretics, treatment, resistance, frequency, and 30 papers with pertinent information were selected.

RESULTS

The analysis of data from the selected papers demonstrated that diuretic resistance is common in hospitalized patients with advanced CHF and frequently associated with renal failure, which is secondary to CHF.

CONCLUSIONS

Diuretic resistance appears to be common in patients with advanced CHF and it is mostly due to decreased cardiac output, low blood pressure, decreased glomerular filtration rate, decreased filtration of sodium, and increased tubular reabsorption of sodium. Diuretic resistance in such patients can be overcome with the combination of loop diuretics with thiazide and thiazide-like diuretics, aldosterone antagonists, as well as other agents. The data from these studies in combination with collateral literature will be discussed in this review.

A role for tubular Na+/H+ exchanger NHE3 in the natriuretic effect of the SGLT2 inhibitor empagliflozin

Inhibitors of proximal tubular Na⁺-glucose cotransporter 2 (SGLT2) are natriuretic, and they lower blood pressure. There are reports that the activities of SGLT2 and Na⁺-H⁺ exchanger 3 (NHE3) are coordinated. If so, then part of the natriuretic response to an SGLT2 inhibitor is mediated by suppressing NHE3. To examine this further, we compared the effects of an SGLT2 inhibitor, empagliflozin, on urine composition and systolic blood pressure (SBP) in nondiabetic mice with tubule-specific NHE3 knockdown (NHE3-ko) and wild-type (WT) littermates. A single dose of empagliflozin, titrated to cause minimal glucosuria, increased urinary excretion of Na⁺ and bicarbonate and raised urine pH in WT mice but not in NHE3-ko mice. Chronic empagliflozin treatment tended to lower SBP despite higher renal renin mRNA expression and lowered the ratio of SBP to renin mRNA, indicating volume loss. This effect of empagliflozin depended on tubular NHE3. In diabetic Akita mice, chronic empagliflozin enhanced phosphorylation of NHE3 (S552/S605), changes previously linked to lesser NHE3-mediated reabsorption. Chronic empagliflozin also increased expression of genes involved with renal gluconeogenesis, bicarbonate regeneration, and ammonium formation. While this could reflect compensatory responses to acidification of proximal tubular cells resulting from reduced NHE3 activity, these effects were at least in part independent of tubular NHE3 and potentially indicated metabolic adaptations to urinary glucose loss. Moreover, empagliflozin increased luminal α-ketoglutarate, which may serve to stimulate compensatory distal NaCl reabsorption, while cogenerated and excreted ammonium balances urine losses of this "potential bicarbonate." The data implicate NHE3 as a determinant of the natriuretic effect of empagliflozin.

Segment-specific effects of resveratrol on porcine small intestinal dipeptide absorption depend on the mucosal pH and are due to different mechanisms: potential roles of different transport proteins and protein kinases

Numerous beneficial features of the polyphenol resveratrol (RSV) have been demonstrated in several tissues and cell culture models. There is also evidence, that RSV impairs intestinal nutrient transport but the underlying mechanisms are not understood. The aim of the present study was to evaluate whether RSV has also an impact on the H⁺-coupled transport of peptides via the peptide transporter 1 (PepT1) and to characterize RSV mediated changes in the apical abundance of nutrients transport proteins and protein kinases that may be involved. RSV decreased the H⁺-coupled transport of peptides in the porcine small intestines in a pH and location specific manner (jejunum vs ileum) as measured in Ussing chamber experiments. The comparison of the effects of RSV with the effects of the cAMP/PKA-activating agent forskolin indicates that different mechanisms may be responsible in the intestinal segments. Additionally, it seems that the transport of peptides and glucose in the jejunum are inhibited via the same mechanism while there might be two mechanisms involved in the ileum. Functional data and protein expression data indicate, that, besides PepT1, the activity of the Na⁺/H⁺-exchanger 3 (NHE3) may be involved. Protein kinase A (PKA) and AMP-activated kinase (AMPK) are both activated by RSV while the extracellular regulated kinase (ERK) and the serum and glucocorticoid induced kinase (SGK) are widely unaffected. Although PKA and AMPK are activated, AMPK seems not to be related to the effects of RSV. Additionally, both the functional data and the protein expression data reveal some interesting pH- and segment-specific differences.

Role of Deranged Energy Deprivation Signaling in the Pathogenesis of Cardiac and Renal Disease in States of Perceived Nutrient Overabundance

Sodium-glucose cotransporter 2 inhibitors reduce the risk of serious heart failure and adverse renal events, but the mechanisms that underlie this benefit are not understood. Treatment with SGLT2 inhibitors is distinguished by 2 intriguing features: ketogenesis and erythrocytosis. Both reflect the induction of a fasting-like and hypoxia-like transcriptional paradigm that is capable of restoring and maintaining cellular homeostasis and survival. In the face of perceived nutrient and oxygen deprivation, cells activate low-energy sensors, which include sirtuin-1 (SIRT1), AMP-activated protein kinase (AMPK), and hypoxia inducible factors (HIFs; especially HIF-2α); these enzymes and transcription factors are master regulators of hundreds of genes and proteins that maintain cellular homeostasis. The activation of SIRT1 (through its effects to promote gluconeogenesis and fatty acid oxidation) drives ketogenesis, and working in concert with AMPK, it can directly inhibit inflammasome activation and maintain mitochondrial capacity and stability. HIFs act to promote oxygen delivery (by stimulating erythropoietin and erythrocytosis) and decrease oxygen consumption. The activation of SIRT1, AMPK, and HIF-2α enhances autophagy, a lysosome-dependent degradative pathway that removes dangerous constituents, particularly damaged mitochondria and peroxisomes, which are major sources of oxidative stress and triggers of cellular dysfunction and death. SIRT1 and AMPK also act on sodium transport mechanisms to reduce intracellular sodium concentrations. It is interesting that type 2 diabetes mellitus, obesity, chronic heart failure, and chronic kidney failure are characterized by the accumulation of intracellular glucose and lipid intermediates that are perceived by cells as indicators of energy overabundance. The cells respond by downregulating SIRT1, AMPK, and HIF-2α, thus leading to an impairment of autophagic flux and acceleration of cardiomyopathy and nephropathy. SGLT2 inhibitors reverse this maladaptive signaling by triggering a state of fasting and hypoxia mimicry, which includes activation of SIRT1, AMPK, and HIF-2α, enhanced autophagic flux, reduced cellular stress, decreased sodium influx into cells, and restoration of mitochondrial homeostasis. This mechanistic framework clarifies the findings of large-scale randomized trials and the close association of ketogenesis and erythrocytosis with the cardioprotective and renoprotective benefits of these drugs.

Insulin enhances renal glucose excretion: relation to insulin sensitivity and sodium-glucose cotransport

INTRODUCTION

Insulin regulates renal glucose production and utilization; both these fluxes are increased in type 2 diabetes (T2D). Whether insulin also controls urinary glucose excretion is not known.

METHODS

We applied the pancreatic clamp technique in 12 healthy subjects and 13 T2D subjects. Each participant received a somatostatin infusion and a variable glucose infusion to achieve (within 1 hour) and maintain glycemia at 22 mmol/L for 3 hours; next, a constant insulin infusion (240 pmol/min/kg) was added for another 3 hours. Urine was collected separately in each period for glucose and creatinine determination.

RESULTS

During saline, glucose excretion was lower in T2D than controls in absolute terms (0.49 (0.32) vs 0.69 (0.18) mmol/min, median (IQR), p=0.01) and as a fraction of filtered glucose (16.2 (6.4) vs 19.9 (7.5)%, p<0.001). With insulin, whole-body glucose disposal rose more in controls than T2D (183 (48) vs 101 (48) µmol/kg_FFM/min, p<0.0003). Insulin stimulated absolute and fractional glucose excretion in controls (p<0.01) but not in T2D. Sodium excretion paralleled glucose excretion. In the pooled data, fractional glucose excretion was directly related to whole-body glucose disposal and to fractional sodium excretion (r=0.52 and 0.54, both p<0.01). In another group of healthy controls, empagliflozin was administered before starting the pancreatic clamp to block sodium-glucose cotransporter 2 (SGLT2). Under these conditions, insulin still enhanced both glucose and sodium excretion.

CONCLUSIONS

Acute exogenous insulin infusion jointly stimulates renal glucose and sodium excretion, indicating that the effect may be mediated by SGLTs. This action is resistant in patients with diabetes, accounting for their increased retention of glucose and sodium, and is not abolished by partial SGLT2 inhibition by empagliflozin.

Interplay of adenosine monophosphate-activated protein kinase/sirtuin-1 activation and sodium influx inhibition mediates the renal benefits of sodium-glucose co-transporter-2 inhibitors in type 2 diabetes: A novel conceptual framework

Long-term treatment with sodium-glucose co-transporter-2 (SGLT2) inhibitors slows the deterioration of renal function in patients with diabetes. This benefit cannot be ascribed to an action on blood glucose, ketone utilization, uric acid or systolic blood pressure. SGLT2 inhibitors produce a striking amelioration of glomerular hyperfiltration. Although initially ascribed to an action of these drugs to inhibit proximal tubular glucose reabsorption, SGLT2 inhibitors exert renoprotective effects, even in patients with meaningfully impaired levels of glomerular function that are sufficient to abolish their glycosuric actions. Instead, the reduction in intraglomerular pressures may be related to an action of SGLT2 inhibitors to interfere with the activity of sodium-hydrogen exchanger isoform 3, thereby inhibiting proximal tubular sodium reabsorption and promoting tubuloglomerular feedback. Yet, experimentally, such an effect may not be sufficient to prevent renal injury. It is therefore noteworthy that the diabetic kidney exhibits an important defect in adenosine monophosphate-activated protein kinase (AMPK) and sirtuin-1 (SIRT1) signalling, which may contribute to the development of nephropathy. These transcription factors exert direct effects to mute oxidative stress and inflammation, and they also stimulate autophagy, a lysosomally mediated degradative pathway that maintains cellular homeostasis in the kidney. SGLT2 inhibitors induce both AMPK and SIRT1, and they have been shown to stimulate autophagy, thereby ameliorating cellular stress and glomerular and tubular injury. Enhanced AMPK/SIRT1 signalling may also contribute to the action of SGLT2 inhibitors to interfere with sodium transport mechanisms. The dual effects of SGLT2 inhibitors on AMPK/SIRT1 activation and renal tubular sodium transport may explain the protective effects of these drugs on the kidney in type 2 diabetes.

Antihypertensive and Renal Mechanisms of SGLT2 (Sodium-Glucose Linked Transporter 2) Inhibitors

Empaglifolzin, canagliflozin, and dapagliflozin are SGLT2 (sodium-glucose linked transporter type 2) inhibitors for treatment of type 2 diabetes mellitus that also reduce blood pressure, mortality, and cardiovascular disease and slow the loss of glomerular filtration rate. SGLT2 inhibitors inhibit the coupled reabsorption of sodium and glucose from the proximal tubules, thereby increasing renal glucose and sodium excretion, but they have more widespread renal effects, including inhibition of the sodium:proton exchanger. They increase the delivery of sodium to the loop of Henle and can thereby activate the tubuloglomerular feedback response to correct glomerular hyperfiltration. There are multiple potential mechanisms whereby these drugs lower blood pressure and preserve kidney function that are the focus of this review.

Autophagy stimulation and intracellular sodium reduction as mediators of the cardioprotective effect of sodium-glucose cotransporter 2 inhibitors

In five large-scale trials involving >40 000 patients, sodium-glucose cotransporter 2 (SGLT2) inhibitors decreased the risk of serious heart failure events by 25-40%. This effect cannot be explained by control of hyperglycaemia, since it is not observed with antidiabetic drugs with greater glucose-lowering effects. It cannot be attributed to ketogenesis, since it is not causally linked to ketone body production, and the benefit is not enhanced in patients with diabetes. The effect cannot be ascribed to a natriuretic action, since SGLT2 inhibitors decrease natriuretic peptides only modestly, and they reduce cardiovascular death, a benefit that diuretics do not possess. Although SGLT2 inhibitors increase red blood cell mass, enhanced erythropoiesis does not favourably influence the course of heart failure. By contrast, experimental studies suggest that SGLT2 inhibitors may reduce intracellular sodium, thereby preventing oxidative stress and cardiomyocyte death. Additionally, SGLT2 inhibitors induce a transcriptional paradigm that mimics nutrient and oxygen deprivation, which includes activation of adenosine monophosphate-activated protein kinase, sirtuin-1, and/or hypoxia-inducible factors-1α/2α. The interplay of these mediators stimulates autophagy, a lysosomally-mediated degradative pathway that maintains cellular homeostasis. Autophagy-mediated clearance of damaged organelles reduces inflammasome activation, thus mitigating cardiomyocyte dysfunction and coronary microvascular injury. Interestingly, the action of hypoxia-inducible factors-1α/2α to both stimulate erythropoietin and induce autophagy may explain why erythrocytosis is strongly correlated with the reduction in heart failure events. Therefore, the benefits of SGLT2 inhibitors on heart failure may be mediated by a direct cardioprotective action related to modulation of pathways responsible for cardiomyocyte homeostasis.

Effect of Insulin on Proximal Tubules Handling of Glucose: A Systematic Review

Renal proximal tubules reabsorb glucose from the glomerular filtrate and release it back into the circulation. Modulation of glomerular filtration and renal glucose disposal are some of the insulin actions, but little is known about a possible insulin effect on tubular glucose reabsorption. This review is aimed at synthesizing the current knowledge about insulin action on glucose handling by proximal tubules. Method. A systematic article selection from Medline (PubMed) and Embase between 2008 and 2019. 180 selected articles were clustered into topics (renal insulin handling, proximal tubule glucose transport, renal gluconeogenesis, and renal insulin resistance). Summary of Results. Insulin upregulates its renal uptake and degradation, and there is probably a renal site-specific insulin action and resistance; studies in diabetic animal models suggest that insulin increases renal SGLT2 protein content; in vivo human studies on glucose transport are few, and results of glucose transporter protein and mRNA contents are conflicting in human kidney biopsies; maximum renal glucose reabsorptive capacity is higher in diabetic patients than in healthy subjects; glucose stimulates SGLT1, SGLT2, and GLUT2 in renal cell cultures while insulin raises SGLT2 protein availability and activity and seems to directly inhibit the SGLT1 activity despite it activating this transporter indirectly. Besides, insulin regulates SGLT2 inhibitor bioavailability, inhibits renal gluconeogenesis, and interferes with Na+K+ATPase activity impacting on glucose transport. Conclusion. Available data points to an important insulin participation in renal glucose handling, including tubular glucose transport, but human studies with reproducible and comparable method are still needed.

Renal Benefits of SGLT 2 Inhibitors and GLP-1 Receptor Agonists: Evidence Supporting a Paradigm Shift in the Medical Management of Type 2 Diabetes

Diabetic nephropathy (DN) is one of the most perilous side effects of diabetes mellitus type 1 and type 2 (T1DM and T2DM).). It is known that sodium/glucose cotransporter 2 inhibitors (SGLT 2i) and glucagone like peptide-1 receptor agonists (GLP-1 RAs) have renoprotective effects, but the molecular mechanisms are still unknown. In clinical trials GLP-1 analogs exerted important impact on renal composite outcomes, primarily on macroalbuminuria, possibly through suppression of inflammation-related pathways, however enhancement of natriuresis and diuresis is also one of possible mechanisms of nephroprotection. Dapagliflozin, canagliflozin, and empagliflozin are SGLT2i drugs, useful in reducing hyperglycemia and in their potential renoprotective mechanisms, which include blood pressure control, body weight loss, intraglomerular pressure reduction, and a decrease in urinary proximal tubular injury biomarkers. In this review we have discussed the potential synergistic and/or additive effects of GLP 1 RA and SGLT2 inhibitors on the primary onset and progression of kidney disease, and the potential implications on current guidelines of diabetes type 2 management.

Dapagliflozin Attenuates Na+/H+ Exchanger-1 in Cardiofibroblasts via AMPK Activation

PURPOSE

We assessed whether the SGLT-2 inhibitor dapagliflozin (Dapa) attenuates the upregulation of the cardiac Na⁺/H⁺ exchanger (NHE-1) in vitro in mouse cardiofibroblasts stimulated with lipopolysaccharides (LPS) and whether this effect is dependent on adenosine monophosphate kinase (AMPK) activation.

METHODS

Mouse cardiofibroblasts were exposed for 16 h to Dapa (0.4 μM), AMPK activator (A769662 (10 μM)), AMPK inhibitor (compound C (CC) (10 μM)), an SGLT-1 and SGLT-2 inhibitor (phlorizin (PZ) (100 μM)), Dapa+CC, or Dapa+PZ, and then stimulated with LPS (10 ng/ml) for 3 h. NHE-1 mRNA levels were assessed by rt-PCR and total AMPK, phosphorylated-AMPK (P-AMPK), NHE-1, and heat shock protein-70 (Hsp70) protein levels in the whole cell lysate by immunoblotting. In addition, NHE-1 protein levels attached to Hsp70 were assessed by immunoprecipitation.

RESULTS

Exposure to LPS significantly reduced P-AMPK levels in the cardiofibroblasts. A769662 and Dapa equally increased P-AMPK. The effect was blocked by CC. Phlorizin had no effect on P-AMPK. LPS exposure significantly increased NHE-1 mRNA levels. Both Dapa and A769662 equally attenuated this increase. The effect of Dapa was blocked with CC. Interestingly, none of the compounds significantly affected NHE-1 and Hsp70 protein levels in the whole cell lysate. However, LPS significantly increased the concentration of NHE-1 attached to Hsp70. Both Dapa and A69662 attenuated this association and CC blocked the effect of Dapa. Again, phlorizin had no effect and did not alter the effect of Dapa.

CONCLUSIONS

Dapa increases P-AMPK in cardiofibroblasts exposed to LPS. Dapa attenuated the increase in NHE-1 mRNA and the association between NHE-1 and Hsp70. This effect was dependent on AMPK.

Role of the sympathetic nervous system in regulation of the sodium glucose cotransporter 2

BACKGROUND

The sympathetic nervous system (SNS) regulates glucose metabolism in various organs including the kidneys. The sodium glucose cotransporter 2 (SGLT2) mediates glucose reabsorption in renal proximal tubules and its inhibition has been shown to improve glucose control, cardiovascular and renal outcomes. We hypothesized that SNS-induced alterations of glucose metabolism may be mediated via regulation of SGLT2.

METHOD

We used human renal proximal tubule cells to investigate the effects of noradrenaline on SGLT2 regulation. Mice fed a high-fat diet were oral gavaged with dapagliflozin and the expression of noradrenaline and tyrosine hydroxylase was measured in the kidney and heart.

RESULTS

Noradrenaline treatment resulted in a pronounced increase in SGLT2 and interleukin (IL)-6 expression in HK2 cells and promoted translocation of SGLT2 to the cell surface. In vivo, dapagliflozin treatment resulted in marked glucosuria in high-fat diet-fed mice. SGLT2 inhibition significantly reduced high-fat diet-induced elevations of tyrosine hydroxylase and noradrenaline in the kidney and heart. We also aimed to assess the levels of hypertension-related cytokines in the kidneys of our mice treated with and without dapagliflozin. Excitingly, we demonstrate that SGLT2 inhibition with dapagliflozin promoted a trend towards reduced tumour necrosis factor-alpha and elevated IL-1β protein levels in the kidney.

CONCLUSION

Our in-vitro and in-vivo studies provide first evidence for an important cross-talk between the SNS and SGLT2 regulation that may not only account for SNS-induced alterations of glucose metabolism but potentially contribute to cardiovascular and renal protection observed with SGLT2 inhibitors.

Empagliflozin: a new strategy for nephroprotection in diabetes

Empagliflozin, an inhibitor of sodiumglucose symporter type 2 (SGLT2), is a new class of antidiabetic agents with numerous pleiotropic effects. The review summarises data on the influence of empagliflozin on the structural and functional changes in the kidneys of the models of diabetes mellitus (DM) and of patients with DM. A literature search was conducted using the databases of Medline/PubMed, Scopus, Web of Science, ClinicalTrials.gov and eLibrary. The experimental results showed a decrease in the blood glucose level, blood pressure, glomerular hyperfiltration and overexpression of proinflammatory and fibrogenic factors in the kidneys under the influence of empagliflozin. Most clinical studies have demonstrated the albuminuria-lowering effect of empagliflozin in patients with type 2 DM. The EMPA-REG OUTCOME study has demonstrated slowing of the chronic kidney disease progression, decrease in the incidence of end-stage renal failure and death from renal causes in patients with type 2 DM undergoing the empagliflozin treatment compared with those receiving placebo. The mechanisms of the nephroprotective effect of empagliflozin included systemic and renal effects. The decrease in hyperglycaemia, blood pressure and body weight; reduction in glomerular hyperfiltration; enhancement of sodium excretion and suppression of inflammatory and fibrogenic signalling pathways in the kidneys may help slow the development of diabetic kidney damage under the influence of empagliflozin. The possibility of extrapolating the confirmed properties of empagliflozin to other SGLT2 inhibitors needs further investigation.

Is it time to think about the sodium glucose co-transporter 2 sympathetically?

Disturbances in glucose homeostasis are a key feature of the metabolic syndrome and type 2 diabetes. Renal glucose reabsorption is an important factor in glycaemic control. Glucose reabsorption in the proximal tubules is mediated by the sodium glucose co-transporter 2. The capacity for glucose reabsorption is increased in type 2 diabetes and contributes significantly to hyperglycaemia and impaired glucose control. Understanding the mechanisms underpinning the regulation of the sodium glucose co-transporter 2 is therefore of high clinical relevance. However, despite recent advances in the field and the availability of pharmacological inhibitors of this glucose transporter for the treatment of type 2 diabetes, the mechanisms that regulate sodium glucose co-transporter 2 expression are not fully understood. The sympathetic nervous system is an important modulator of glucose homeostasis, and sympathetic hyperactivity is a characteristic feature of obesity, the metabolic syndrome and type 2 diabetes. Sympathetic inhibition either achieved pharmacologically or by renal sympathetic denervation has been associated with improved glucose control. Importantly, sympathetic nerves innervate the proximal tubules of the kidney where they have been shown to regulate the expression of other transporters such as the sodium hydrogen exchanger 3. This review aims to explore the evidence for the regulation of sodium glucose co-transporter 2-mediated glucose reabsorption by the sympathetic nervous system.

Short- and long-term regulation of intestinal Na+/H+ exchange by Toll-like receptors TLR4 and TLR5

Inappropriate activation of pattern recognition receptors has been described as a potential trigger in the development of inflammatory bowel disease (IBD). In this study, we evaluated the activity and expression of Na(+)/H(+) exchanger (NHE) subtypes in T84 intestinal epithelial cells during Toll-like receptor 4 (TLR4) activation by monophosphoryl lipid A and TLR5 by flagellin. NHE activity and intracellular pH were evaluated by spectrofluorescence. Additionally, kinase activities were evaluated by ELISA, and siRNA was used to specifically inhibit adenylyl cyclase (AC). Monophosphoryl lipid A (MPLA) (0.01-50.00 μg/ml) and flagellin (10-500 ng/ml) inhibited NHE1 activity in a concentration-dependent manner (MPLA short term -25.2 ± 5.0%, long term -31.9 ± 4.0%; flagellin short term -14.9 ± 2.0%, long term -19.1 ± 2.0%). Both ligands triggered AC3, PKA, PLC, and PKC signal molecules. Long-term exposure to flagellin and MPLA induced opposite changes on NHE3 activity; flagellin increased NHE3 activity (∼10%) with overexpression of membrane protein, whereas MPLA decreased NHE3 activity (-17.3 ± 3.0%). MPLA and flagellin simultaneously had synergistic effects on NHE activity. MPLA and flagellin impaired pHi recovery after intracellular acidification. The simultaneous exposure to MPLA and flagellin induced a substantial pHi reduction (-0.55 ± 0.03 pH units). Activation of TLR4 and TLR5 exerts marked inhibition of NHE1 activity in intestinal epithelial cells. Transduction mechanisms set into motion during TLR4-mediated and long-term TLR5-mediated inhibition of NHE1 activity involve AC3, PKA, PLC, and PKC. However, short- and long-term TLR4 activation and TLR5 activation might use different signaling pathways. The physiological alterations on intestinal epithelial cells described here may be useful in the development of better IBD therapeutics.

SGLT inhibitors attenuate NO-dependent vascular relaxation in the pulmonary artery but not in the coronary artery

Inhibitors of sodium-glucose cotransporter (SGLT)2 are a new class of oral drugs for type 2 diabetic patients that reduce plasma glucose levels by inhibiting renal glucose reabsorption. There is increasing evidence showing the beneficial effect of SGLT2 inhibitors on glucose control; however, less information is available regarding the impact of SGLT2 inhibitors on cardiovascular outcomes. The present study was designed to determine whether SGLT inhibitors regulate vascular relaxation in mouse pulmonary and coronary arteries. Phlorizin (a nonspecific SGLT inhibitor) and canagliflozin (a SGLT2-specific inhibitor) relaxed pulmonary arteries in a dose-dependent manner, but they had little or no effect on coronary arteries. Pretreatment with phlorizin or canagliflozin significantly inhibited sodium nitroprusside (SNP; a nitric oxide donor)-induced vascular relaxation in pulmonary arteries but not in coronary arteries. Phlorizin had no effect on cGMP-dependent relaxation in pulmonary arteries. SNP induced membrane hyperpolarization in human pulmonary artery smooth muscle cells, and pretreatment of cells with phlorizin and canagliflozin attenuated SNP-induced membrane hyperpolarization by decreasing K(+) activities induced by SNP. Contrary to the result observed in ex vivo experiments with SGLT inhibitors, SNP-dependent relaxation in pulmonary arteries was not altered by chronic administration of canagliflozin. On the other hand, canagliflozin administration significantly enhanced SNP-dependent relaxation in coronary arteries in diabetic mice. These data suggest that SGLT inhibitors differentially regulate vascular relaxation depending on the type of arteries, duration of the treatment, and health condition, such as diabetes.

Probing SGLT2 as a therapeutic target for diabetes: basic physiology and consequences

Traditional treatments for type 1 and type 2 diabetes are often associated with side effects, including weight gain and hypoglycaemia that may offset the benefits of blood glucose lowering. The kidneys filter and reabsorb large amounts of glucose, and urine is almost free of glucose in normoglycaemia. The sodium-dependent glucose transporter (SGLT)-2 in the early proximal tubule reabsorbs the majority of filtered glucose. Remaining glucose is reabsorbed by SGLT1 in the late proximal tubule. Diabetes enhances renal glucose reabsorption by increasing the tubular glucose load and the expression of SGLT2 (as shown in mice), which maintains hyperglycaemia. Inhibitors of SGLT2 enhance urinary glucose excretion and thereby lower blood glucose levels in type 1 and type 2 diabetes. The load-dependent increase in SGLT1-mediated glucose reabsorption explains why SGLT2 inhibitors in normoglycaemic conditions enhance urinary glucose excretion to only ~50% of the filtered glucose. The role of SGLT1 in both renal and intestinal glucose reabsorption provides a rationale for the development of dual SGLT1/2 inhibitors. SGLT2 inhibitors lower blood glucose levels independent of insulin and induce pleiotropic actions that may be relevant in the context of lowering cardiovascular risk. Ongoing long-term clinical studies will determine whether SGLT2 inhibitors have a safety profile and exert cardiovascular benefits that are superior to traditional agents.

Angiotensin receptor blocker telmisartan suppresses renal gluconeogenesis during starvation

The kidney plays an important role in gluconeogenesis during starvation. To clarify the anti-diabetic action of angiotensin receptor blockers, we examined the effects of telmisartan on the sodium-glucose co-transporters (SGLT) and the pathways of renal gluconeogenesis in streptozotocin-induced diabetes mellitus (DM) rats. At 4 weeks, the DM rats treated with/without telmisartan for 2 weeks and normal control rats were used for the study after a 24-hour fast. SGLT2 expressed on the brush border membrane of the proximal convoluted tubules increased in the DM rats, but decreased in the rats treated with telmisartan. The expression of restriction enzymes of gluconeogenesis, glucose-6-phosphatase, and phosphoenolpyruvate carboxykinase increased in the proximal tubules in the DM rats, whereas these enzymes decreased in the kidneys of the rats treated with telmisartan. The elevated cytoplasmic glucose-6-phosphate and glucose levels in the kidney of DM rats significantly decreased in those treated with telmisartan, whereas those levels in the liver did not show significant change. Meanwhile, the high plasma glucose levels in the DM rats during the intravenous insulin tolerance tests were ameliorated by telmisartan. The increased fasting plasma glucose levels after 24 hours of starvation in the DM rats thus returned to the control levels by telmisartan treatment. In conclusion, the increased renal SGLT2 expression, elevated renal gluconeogenesis enzymes and extent of insulin-resistance in the DM rats were ameliorated by telmisartan therapy, thus resulting in decreased plasma glucose levels after 24 hours of fasting.

The mechanisms and therapeutic potential of SGLT2 inhibitors in diabetes mellitus

The kidneys in normoglycemic humans filter 160-180 g of glucose per day (∼30% of daily calorie intake), which is reabsorbed and returned to the systemic circulation by the proximal tubule. Hyperglycemia increases the filtered and reabsorbed glucose up to two- to three-fold. The sodium glucose cotransporter SGLT2 in the early proximal tubule is the major pathway for renal glucose reabsorption. Inhibition of SGLT2 increases urinary glucose and calorie excretion, thereby reducing plasma glucose levels and body weight. The first SGLT2 inhibitors have been approved as a new class of antidiabetic drugs in type 2 diabetes mellitus, and studies are under way to investigate their use in type 1 diabetes mellitus. These compounds work independent of insulin, improve glycemic control in all stages of diabetes mellitus in the absence of clinically relevant hypoglycemia, and can be combined with other antidiabetic agents. By lowering blood pressure and diabetic glomerular hyperfiltration, SGLT2 inhibitors may induce protective effects on the kidney and cardiovascular system beyond blood glucose control.

Sodium glucose cotransporter 2 and the diabetic kidney

PURPOSE OF REVIEW

Reabsorption of glucose in the proximal tubule occurs predominantly via the sodium glucose cotransporter 2 (SGLT2). There has been intense interest in this transporter as a number of SGLT2 inhibitors have entered clinical development. SGLT2 inhibitors act to lower plasma glucose by promoting glycosuria and this review aims to outline the effect on the diabetic kidney of this hypoglycaemic agent.

RECENT FINDINGS

This review provides an overview of recent findings in this area: the transcriptional control of SGLT2 expression in human proximal tubular cells implicates a number of cytokines in the alteration of SGLT2 expression; experimental data show that SGLT2 inhibition may correct early detrimental effects of diabetes by reducing proximal tubular sodium and glucose transport, suggesting a possible renoprotective effect independent of the glucose lowering effects of these agents; and the nonglycaemic effects of SGLT2 inhibitors may have an impact on renal outcomes.

SUMMARY

The available clinical evidence shows consistent reduction in glycaemic parameters and some evidence suggests additional effects including weight loss and mild blood pressure reduction. There are some side effects that warrant further investigation and establishing whether SGLT2 inhibition provides a renal benefit relies on future long-term studies with specific renal end-points.

Na+/H+-exchanger-1 inhibition counteracts diabetic cataract formation and retinal oxidative-nitrative stress and apoptosis

The Na⁺-H⁺-exchanger-1 (NHE-1) controls intracellular pH and glycolytic enzyme activities, and its expression and activity are increased by diabetes and high glucose. NHE-1-dependent upregulation of the upper part of glycolysis, under conditions of inhibition (lens) or insufficient activation (retina) of glyceraldehyde 3-phosphate dehydrogenase, underlies diversion of the excessive glycolytic flux towards several pathways contributing to oxidative stress, a causative factor in diabetic cataractogenesis and retinopathy. This study evaluated the role for NHE-1 in diabetic cataract formation and retinal oxidative stress and apoptosis. Control and streptozotocin-diabetic rats were maintained with or without treatment with the NHE-1 inhibitor cariporide (Sanofi-Aventis, 10 mgkg-1d-1) for 3.5 months. In in vitro studies, bovine retinal pericytes and endothelial cells were cultured in 5 or 30 mM glucose, with or without 10 µM cariporide, for 7 days. A several-fold increase of the by-product of glycolysis, α-glycerophosphate, indicative of activation of the upper part of glycolysis, was present in both rat lens and retina at an early (1-month) stage of streptozotocin-diabetes. Cariporide did not affect diabetic hyperglycemia and counteracted lens oxidative-nitrative stress and p38 MAPK activation, without affecting glucose or sorbitol pathway intermediate accumulation. Cataract formation (indirect ophthalmoscopy and slit-lamp examination) was delayed, but not prevented. The number of TUNEL-positive cells per flat-mounted retina was increased 4.4-fold in diabetic rats (101 ± 17 vs. 23 ± 8 in controls , P<0.01), and this increase was attenuated by cariporide (45 ± 12, P<0.01). Nitrotyrosine and poly(ADP-ribose) fluorescence and percentage of TUNEL-positive cells were increased in pericytes and endothelial cells cultured in 30 mM glucose, and these changes were at least partially prevented by cariporide. In conclusion, NHE-1 contributes to diabetic cataract formation, and retinal oxidative-nitrative stress and apoptosis. The findings identify a new therapeutic target for diabetic ocular complications.

Functional role of sodium glucose transporter in high glucose-mediated angiotensin type 1 receptor downregulation in human proximal tubule cells

Previously, we have demonstrated human angiotensin type 1 receptor (hAT(1)R) promoter architecture with regard to the effect of high glucose (25 mM)-mediated transcriptional repression in human proximal tubule epithelial cells (hPTEC; Thomas BE, Thekkumkara TJ. Mol Biol Cell 15: 4347-4355, 2004). In the present study, we investigated the role of glucose transporters in high glucose-mediated hAT(1)R repression in primary hPTEC. Cells were exposed to normal glucose (5.5 mM) and high glucose (25 mM), followed by determination of hyperglycemia-mediated changes in receptor expression and glucose transporter activity. Exposure of cells to high glucose resulted in downregulation of ANG II binding (4,034 ± 163.3 to 1,360 ± 154.3 dpm/mg protein) and hAT(1)R mRNA expression (reduced 60.6 ± 4.643%) at 48 h. Under similar conditions, we observed a significant increase in glucose uptake (influx) in cells exposed to hyperglycemia. Our data indicated that the magnitude of glucose influx is concentration and time dependent. In euglycemic cells, inhibiting sodium-glucose cotransporters (SGLTs) with phlorizin and facilitative glucose transporters (GLUTs) with phloretin decreased glucose influx by 28.57 ± 0.9123 and 54.33 ± 1.202%, respectively. However, inhibiting SGLTs in cells under hyperglycemic conditions decreased glucose influx by 53.67 ± 2.906%, while GLUT-mediated glucose uptake remained unaltered (57.67 ± 3.180%). Furthermore, pretreating cells with an SGLT inhibitor reversed high glucose-mediated downregulation of the hAT(1)R, suggesting an involvement of SGLT in high glucose-mediated hAT(1)R repression. Our results suggest that in hPTEC, hyperglycemia-induced hAT(1)R downregulation is largely mediated through SGLT-dependent glucose influx. As ANG II is an important modulator of hPTEC transcellular sodium reabsorption and function, glucose-mediated changes in hAT(1)R gene expression may participate in the pathogenesis of diabetic renal disease.