Mechanisms mediating the diuretic and natriuretic actions of the incretin hormone glucagon-like peptide-1

Gut hormones and gut microbiota: implications for kidney function and hypertension

Increased blood pressure (BP) and chronic kidney disease are two leading risk factors for cardiovascular disease. Increased sodium intake is one of the most important risk factors for development of hypertension. Recent data have shown that gut influences kidney function and BP by variety of mechanisms. Various hormones and peptides secreted from gut such as gastrin, glucocorticoids, Glucagon-like peptide-1 impact on kidney function and BP especially influencing sodium absorption from gut. These findings stimulate scientist to find new therapeutic options such as tenapanor for treatment of hypertension by blocking sodium absorption from gut. The gastrointestinal tract is also occupied by a huge community of microbes (microbiome) that under normal condition has a symbiotic relationship with the host. Alterations in the structure and function of the gut microbiota have been shown to play a key role in the pathogenesis and complications of numerous diseases including hypertension. Based on these data, in this review, we provide a summary of the available data on the role of gut and gut microbiota in regulation of BP and kidney function.

Novel developments in differentiating the role of renal and intestinal sodium hydrogen exchanger 3

The Na⁺/H⁺ exchanger isoform 3 (NHE3) facilitates Na⁺ absorption and H⁺ secretion and is expressed in the intestine, proximal tubule, and thick ascending limb of the kidney. While the function of NHE3 for Na⁺ and [Formula: see text](re)absorption has been defined using conventional NHE3 knockout mice (NHE3^-/-), the recent generation of conditional NHE3 knockout mice started to give critical new insight into the role of this protein by allowing for temporal and spatial control of NHE3 expression. For example, in contrast to NHE3^-/- mice, knockout of NHE3 in the S1 and S2 segments of the proximal tubule or along the entire tubule/collecting duct does not cause any lethality. Nonabsorbable NHE3 inhibitors have been developed, and preclinical as well as clinical trials indicate possible pharmacological use in fluid overload, hypertension, chronic kidney disease, hyperphosphatemia, and constipation. Some of the therapeutic considerations seem to be directly related to the pharmacodynamic properties of these drugs; however, little is known about the effects of these nonabsorbable NHE3 inhibitors on intestinal phosphate transport and the mechanisms so far remain elusive. This review focuses on novel findings of NHE3 in the intestine and the kidney as well as novel drug developments targeting NHE3.

Microvascular effects of the inhibition of dipeptidylpeptidase IV by linagliptin in nondiabetic hypertensive patients

BACKGROUND

Recent studies suggest vascular benefits of dipeptidylpeptidase IV (DPP-IV) inhibition in patients with diabetes mellitus. Only little is known about potential vascular effects of DPP-IV inhibitors in nondiabetic individuals. The aim of this study was to investigate the effect of DPP-IV inhibition in a nondiabetic hypertensive population.

METHOD

This was a double-blinded, randomized, placebo-controlled, mechanistic study, comparing microvascular effects of the DPP-IV inhibitor linagliptin with placebo in nondiabetic individuals with a history of arterial hypertension. Twenty-one patients received 5 mg linagliptin (5 women; age 67.6 ± 6.0 years; mean ± SD), whereas 22 patients were randomized to placebo (5 women; age 64.8 ± 7.1 years).

RESULTS

At baseline, after 6 and 12 weeks, retinal microcirculation and arterial blood pressure profiles were assessed. Moreover, blood samples were taken for the measurement of HbA1c, asymmetric dimethylarginine, C-reactive peptide, cyclic guanosinmonophosphate, transforming growth factor beta (TGF-ß1) and cystatin C. Retinal capillary perfusion increased by 23.7 ± 10.3% (mean ± SEM; P < 0.05), retinal arterial flow by 7.6 ± 0.6 (P < 0.05) and the retinal hyperemic response by 290 ± 263% (P < 0.05) during treatment with linagliptin. No change in retinal blood flow was found in the placebo group. Although blood pressure declined in both groups, a significant decline in TGF-ß1 by 9.3 ± 4.5% (P < 0.05) could only be observed in the linagliptin group. No significant change in other laboratory parameters could be observed in both groups.

CONCLUSION

Our study suggests microvascular and antifibrotic effects of linagliptin in a nondiabetic, hypertensive population.

The mechanism of glucagon-like peptide-1 participation in the osmotic homeostasis

We have found the physiological mechanism of intensification of the excessive fluid removal from the body under the action of glucagon-like peptide-1 and its analog exenatide. Under the water load in rats, exenatide significantly increased the clearance of lithium, reduced fluid reabsorption in the proximal tubule of the nephron and intensified reabsorption of sodium ions in the distal parts, which contributed to the formation of sodium-free water and faster recovery of osmotic homeostasis. Blocking this pathway with a selective antagonist of glucagon-like peptide-1 receptors slowed down the elimination of excessive water from the body.

Angiotensin II counteracts the effects of cAMP/PKA on NHE3 activity and phosphorylation in proximal tubule cells

Binding of angiotensin II (ANG II) to the AT₁ receptor (AT₁R) in the proximal tubule stimulates Na⁺/H⁺ exchanger isoform 3 (NHE3) activity through multiple signaling pathways. However, the effects of ANG II/AT₁R-induced inihibitory G protein (G_i) activation and subsequent decrease in cAMP accumulation on NHE3 regulation are not well established. We therefore tested the hypothesis that ANG II reduces cAMP/PKA-mediated phosphorylation of NHE3 on serine 552 and, in doing so, stimulates NHE3 activity. Under basal conditions, ANG II stimulated NHE3 activity but did not affect PKA-mediated NHE3 phosphorylation at serine 552 in opossum kidney (OKP) cells. However, in the presence of the cAMP-elevating agent forskolin (FSK), ANG II blocked FSK-induced NHE3 inhibition, reduced intracellular cAMP concentrations, lowered PKA activity, and prevented the FSK-mediated increase in NHE3 serine 552 phosphorylation. All effects of ANG II were blocked by pretreating OKP cells with the AT₁R antagonist losartan, highlighting the contribution of the AT₁R/G_i pathway in ANG II-mediated NHE3 upregulation under cAMP-elevating conditions. Accordingly, G_i inhibition by pertussis toxin treatment decreased NHE3 activity both in vitro and in vivo and, more importantly, prevented the stimulatory effect of ANG II on NHE3 activity in rat proximal tubules. Collectively, our results suggest that ANG II counteracts the effects of cAMP/PKA on NHE3 phosphorylation and inhibition by activating the AT₁R/G_i pathway. Moreover, these findings support the notion that NHE3 dephosphorylation at serine 552 may represent a key event in the regulation of renal proximal tubule sodium handling by ANG II in the presence of natriuretic hormones that promote cAMP accumulation and transporter phosphorylation.

Incretin-Based Therapies for Diabetic Complications: Basic Mechanisms and Clinical Evidence

An increase in the rates of morbidity and mortality associated with diabetic complications is a global concern. Glycemic control is important to prevent the development and progression of diabetic complications. Various classes of anti-diabetic agents are currently available, and their pleiotropic effects on diabetic complications have been investigated. Incretin-based therapies such as dipeptidyl peptidase (DPP)-4 inhibitors and glucagon-like peptide-1 receptor agonists (GLP-1RA) are now widely used in the treatment of patients with type 2 diabetes. A series of experimental studies showed that incretin-based therapies have beneficial effects on diabetic complications, independent of their glucose-lowering abilities, which are mediated by anti-inflammatory and anti-oxidative stress properties. Based on these findings, clinical studies to assess the effects of DPP-4 inhibitors and GLP-1RA on diabetic microvascular and macrovascular complications have been performed. Several but not all studies have provided evidence to support the beneficial effects of incretin-based therapies on diabetic complications in patients with type 2 diabetes. We herein discuss the experimental and clinical evidence of incretin-based therapy for diabetic complications.

ISN Forefronts Symposium 2015: Maintaining Balance Under Pressure-Hypertension and the Proximal Tubule

Renal control of effective circulating volume (ECV) is key for circulatory performance. When renal sodium excretion is inadequate, blood pressure rises and serves as a homeostatic signal to drive natriuresis to re-establish ECV. Recognizing that hypertension involves both renal and vascular dysfunction, this report concerns proximal tubule sodium hydrogen exchanger 3 (NHE3) regulation during acute and chronic hypertension. NHE3 is distributed in tall microvilli (MV) in the proximal tubule, where it reabsorbs a significant fraction of the filtered sodium. NHE3 redistributes, in the plane of the MV membrane, between the MV body, where NHE3 is active, and the MV base, where NHE3 is less active. A high-salt diet and acute hypertension both retract NHE3 to the base and reduce proximal tubule sodium reabsorption independent of a change in abundance. The renin angiotensin system provokes NHE3 redistribution independent of blood pressure: The angiotensin-converting enzyme (ACE) inhibitor captopril redistributes NHE3 to the base and subsequent angiotensin II (AngII) infusion returns NHE3 to the body of the MV and restores reabsorption. Chronic AngII infusion presents simultaneous AngII stimulation and hypertension; that is, NHE3 remains in the body of the MV, due to the high local AngII level and inflammation, and exhibits a compensatory decrease in abundance driven by the hypertension. Genetically modified mice with blunted hypertensive responses to chronic AngII infusion (due to lack of the proximal tubule AngII receptors interleukin-17A or interferon-γ expression) exhibit reduced local AngII accumulation and inflammation and larger decreases in NHE3 abundance, which improves the pressure natriuresis response and reduces the need for elevated blood pressure to facilitate circulating volume balance.

Dipeptidyl Peptidase IV Inhibition Exerts Renoprotective Effects in Rats with Established Heart Failure

Circulating dipeptidyl peptidase IV (DPPIV) activity is associated with worse cardiovascular outcomes in humans and experimental heart failure (HF) models, suggesting that DPPIV may play a role in the pathophysiology of this syndrome. Renal dysfunction is one of the key features of HF, but it remains to be determined whether DPPIV inhibitors are capable of improving cardiorenal function after the onset of HF. Therefore, the present study aimed to test the hypothesis that DPPIV inhibition by vildagliptin improves renal water and salt handling and exerts anti-proteinuric effects in rats with established HF. To this end, male Wistar rats were subjected to left ventricle (LV) radiofrequency ablation or sham operation. Six weeks after surgery, radiofrequency-ablated rats who developed HF were randomly divided into two groups and treated for 4 weeks with vildagliptin (120 mg/kg/day) or vehicle by oral gavage. Echocardiography was performed before (pretreatment) and at the end of treatment (post-treatment) to evaluate cardiac function. The fractional area change (FAC) increased (34 ± 5 vs. 45 ± 3%, p < 0.05), and the isovolumic relaxation time decreased (33 ± 2 vs. 27 ± 1 ms; p < 0.05) in HF rats treated with vildagliptin (post-treatment vs. pretreatment). On the other hand, cardiac dysfunction deteriorated further in vehicle-treated HF rats. Renal function was impaired in vehicle-treated HF rats as evidenced by fluid retention, low glomerular filtration rate (GFR) and high levels of urinary protein excretion. Vildagliptin treatment restored urinary flow, GFR, urinary sodium and urinary protein excretion to sham levels. Restoration of renal function in HF rats by DPPIV inhibition was associated with increased active glucagon-like peptide-1 (GLP-1) serum concentration, reduced DPPIV activity and increased activity of protein kinase A in the renal cortex. Furthermore, the anti-proteinuric effect of vildagliptin treatment in rats with established HF was associated with upregulation of the apical proximal tubule endocytic receptor megalin and of the podocyte main slit diaphragm proteins nephrin and podocin. Collectively, these findings demonstrate that DPPIV inhibition exerts renoprotective effects and ameliorates cardiorenal function in rats with established HF. Long-term studies with DPPIV inhibitors are needed to ascertain whether these effects ultimately translate into improved clinical outcomes.

Acute renal effects of the GLP-1 receptor agonist exenatide in overweight type 2 diabetes patients: a randomised, double-blind, placebo-controlled trial

AIMS/HYPOTHESIS

This study aimed to investigate the acute renal effects of the glucagon-like peptide-1 receptor agonist (GLP-1RA) exenatide in type 2 diabetes patients.

METHODS

We included overweight (BMI 25-40 kg/m(2)) men and postmenopausal women, aged 35-75 years with type 2 diabetes (HbA1c 48-75 mmol/mol; 6.5-9.0%) and estimated GFR ≥ 60 ml min(-1) 1.73 m(-2). Exenatide or placebo (NaCl solution, 154 mmol/l) was administrated intravenously in an acute, randomised, double-blind, placebo-controlled trial conducted at the Diabetes Center VU University Medical Center (VUMC). GFR (primary endpoint) and effective renal plasma flow (ERPF) were determined by inulin and para-aminohippurate clearance, respectively, based on timed urine sampling. Filtration fraction (FF) and effective renal vascular resistance (ERVR) were calculated, and glomerular hydrostatic pressure (PGLO) and vascular resistance of the afferent (RA) and efferent (RE) renal arteriole were estimated. Tubular function was assessed by absolute and fractional excretion of sodium (FENa), potassium (FEK) and urea (FEU), in addition to urine osmolality, pH and free water clearance. Renal damage markers, BP and plasma glucose were also determined.

RESULTS

Of the 57 patients randomised by computer, 52 were included in the final analyses. Exenatide (n = 24) did not affect GFR (mean difference +2 ± 3 ml min(-1) 1.73 m(-2), p = 0.489), ERPF, FF, ERVR or PGLO, compared with placebo (n = 28). Exenatide increased RA (p < 0.05), but did not change RE. Exenatide increased FENa, FEK, urine osmolality and pH, while FEU, urinary flow and free water clearance were decreased (all p < 0.05). Osmolar clearance and renal damage makers were not affected. Diastolic BP and mean arterial pressure increased by 3 ± 1 and 6 ± 2 mmHg, respectively, whereas plasma glucose decreased by 1.4 ± 0.1 mmol/l (all p < 0.05).

CONCLUSIONS/INTERPRETATION

Exenatide infusion does not acutely affect renal haemodynamics in overweight type 2 diabetes patients at normal filtration levels. Furthermore, acute GLP-1RA administration increases proximal sodium excretion in these patients.

TRIAL REGISTRATION

ClincialTrials.gov NCT01744236 FUNDING : The research leading to these results has been funded from: (1) the European Community's Seventh Framework Programme (FP7/2007-2013) under grant agreement number 282521 - the SAFEGUARD project; and (2) the Dutch Kidney Foundation, under grant agreement IP12.87.

Short-term effects of liraglutide on kidney function and vasoactive hormones in type 2 diabetes: a randomized clinical trial

AIMS

To investigate the effects of a single dose of 1.2 mg liraglutide, a once-daily glucagon-like peptide-1 (GLP-1) receptor agonist, on key renal variables in patients with type 2 diabetes.

METHODS

The study was a placebo-controlled, double-blind, crossover trial in 11 male patients with type 2 diabetes. Measurements included (51) Cr-EDTA plasma clearance estimated glomerular filtration rate (GFR) and MRI-based renal blood flow (RBF), tissue perfusion and oxygenation.

RESULTS

Liraglutide had no effect on GFR [95% confidence interval (CI) -6.8 to 3.6 ml/min/1.73 m(2) ] or on RBF (95% CI -39 to 30 ml/min) and did not change local renal blood perfusion or oxygenation. The fractional excretion of lithium increased by 14% (p = 0.01) and sodium clearance tended to increase (p = 0.06). Liraglutide increased diastolic and systolic blood pressure (3 and 6 mm Hg) and heart rate (2 beats per min; all p < 0.05). Angiotensin II (ANG II) concentration decreased by 21% (p = 0.02), but there were no effects on other renin-angiotensin system components, atrial natriuretic peptides (ANPs), methanephrines or excretion of catecholamines.

CONCLUSIONS

Short-term liraglutide treatment did not affect renal haemodynamics but decreased the proximal tubular sodium reabsorption. Blood pressure increased with short-term as opposed to long-term treatment. Catecholamine levels were unchanged and the results did not support a GLP-1-ANP axis. ANG II levels decreased, which may contribute to renal protection by GLP-1 receptor agonists.

New and old agents in the management of diabetic nephropathy

PURPOSE OF REVIEW

Diabetic nephropathy is a long-standing complication of diabetes mellitus and is responsible for more than 40% of end-stage renal disease cases in developed countries. Unfortunately, conventional renin-angiotensin-aldosterone system (RAAS) inhibitor medications only partially protect against the development and progression of diabetic nephropathy. Moreover, RAAS inhibitors have failed as primary prevention therapy in type 1 diabetes. Thus, agents targeting alternative pathogenic mechanisms leading to diabetic nephropathy have been intensively investigated, which is the topic of this review.

RECENT FINDINGS

Promising emerging agents have targeted neurohormonal activation (alternative components of the RAAS and neprilysin inhibition), tubuloglomerular feedback mechanisms (sodium glucose cotransporter 2 inhibition and incretin-based therapy) and renal inflammation/fibrosis.

SUMMARY

Evidence demonstrating the potential of these agents to protect and prevent progression of diabetic nephropathy is summarized in this review. There are dedicated clinical trials ongoing with these therapies, which have the potential to change the clinical practice.

Role of Vasopressin in the Regulation of Renal Sodium Excretion: Interaction with Glucagon-Like Peptide-1

The present study aimed to investigate the potential physiological role of vasopressin and the incretin hormone of the gastrointestinal tract (glucagon-like peptide-1; GLP-1) in the regulation of the water-salt balance in a hyperosmolar state as a result of sodium loadings. In rats, the administration of hypertonic NaCl solution resulted in a significant increase in natriuresis, which correlated with the vasopressin excretion rate. Natriuresis following an i.p. NaCl load (23.2 ± 1.4 μmol/min/kg) was enhanced by inhibition of V2 receptors (51.6 ± 3.7 μmol/min/kg, P < 0.05) and was reduced by a V1a antagonist injection (6.3 ± 1.1 μmol/min/kg, P < 0.05). Compared to i.p. salt administration, oral NaCl loading induced a significant increase in the plasma GLP-1 level within 5 min and resulted in more prominent natriuresis and a smaller increase in blood sodium concentration. It was hypothesised that the basis for the fast elimination of excess sodium following an oral NaCl load could be the involvement of GLP-1 in osmoregulation combined with vasopressin. It was demonstrated that GLP-1 mimetic exenatide (1.5 nmol/kg) produced a significant decrease in proximal reabsorption and an increase in fractional sodium excretion (from 0.15 ± 0.04% to 9 ± 1%). It was also shown that vasopressin at doses of 1-10 μg/kg and the selective V1a agonist (1 μg/kg) induced an increase in sodium fractional excretion to 10 ± 2% and 8 ± 2%, respectively. Combined administration of exenatide and V1a agonist revealed their cumulative natriuretic effect, and sodium fractional excretion increased by up to 18 ± 2%. These data suggest that GLP-1 combined with vasopressin could be involved in the regulation of sodium balance in the hyperosmolar state as a result of NaCl loading. Vasopressin regulates the reabsorption of a significant portion of filtered sodium in the distal segment of the nephron and modulates the natriuretic effect of GLP-1.

Anti-Inflammatory Effects of GLP-1-Based Therapies beyond Glucose Control

Glucagon-like peptide-1 (GLP-1) is an incretin hormone mainly secreted from intestinal L cells in response to nutrient ingestion. GLP-1 has beneficial effects for glucose homeostasis by stimulating insulin secretion from pancreatic beta-cells, delaying gastric emptying, decreasing plasma glucagon, reducing food intake, and stimulating glucose disposal. Therefore, GLP-1-based therapies such as GLP-1 receptor agonists and inhibitors of dipeptidyl peptidase-4, which is a GLP-1 inactivating enzyme, have been developed for treatment of type 2 diabetes. In addition to glucose-lowering effects, emerging data suggests that GLP-1-based therapies also show anti-inflammatory effects in chronic inflammatory diseases including type 1 and 2 diabetes, atherosclerosis, neurodegenerative disorders, nonalcoholic steatohepatitis, diabetic nephropathy, asthma, and psoriasis. This review outlines the anti-inflammatory actions of GLP-1-based therapies on diseases associated with chronic inflammation in vivo and in vitro, and their molecular mechanisms of anti-inflammatory action.

Mechanisms for the cardiovascular effects of glucagon-like peptide-1

Over the past three decades, at least 10 hormones secreted by the enteroendocrine cells have been discovered, which directly affect the cardiovascular system through their innate receptors expressed in the heart and blood vessels or through a neural mechanism. Glucagon-like peptide-1 (GLP-1), an important incretin, is perhaps best studied of these gut-derived hormones with important cardiovascular effects. In this review, I have discussed the mechanism of GLP-1 release from the enteroendocrine L-cells and its physiological effects on the cardiovascular system. Current evidence suggests that GLP-1 has positive inotropic and chronotropic effects on the heart and may be important in preserving left ventricular structure and function by direct and indirect mechanisms. The direct effects of GLP-1 in the heart may be mediated through GLP-1R expressed in atria as well as arteries and arterioles in the left ventricle and mainly involve in the activation of multiple pro-survival kinases and enhanced energy utilization. There is also good evidence to support the involvement of a second, yet to be identified, GLP-1 receptor. Further, GLP-1(9-36)amide, which was previously thought to be the inactive metabolite of the active GLP-1(7-36)amide, may also have direct cardioprotective effects. GLP-1's action on GLP-1R expressed in the central nervous system, kidney, vasculature and the pancreas may indirectly contribute to its cardioprotective effects.

Blood pressure and glucose independent renoprotective effects of dipeptidyl peptidase-4 inhibition in a mouse model of type-2 diabetic nephropathy

BACKGROUND

Despite the beneficial effects of type 4 dipeptidyl peptidase (DPP-4) inhibitors on glucose levels, its effects on diabetic nephropathy remain unclear.

METHOD

This study examined the long-term renoprotective effects of DPP-4 inhibitor linagliptin in db/db mice, a model of type 2 diabetes. Results were compared with the known beneficial effects of renin-angiotensin system blockade by enalapril. Ten-week-old male diabetic db/db mice were treated for 3 months with either vehicle (n = 10), 3 mg linagliptin/kg per day (n = 8), or 20 mg enalapril/kg per day (n = 10). Heterozygous db/m mice treated with vehicle served as healthy controls (n = 8).

RESULTS

Neither linagliptin nor enalapril had significant effects on the parameters of glucose metabolism or blood pressure in diabetic db/db mice. However, linagliptin treatment reduced albuminuria and attenuated kidney injury. In addition, expression of podocyte marker podocalyxin was normalized. We also analysed DPP-4 expression by immunofluorescence in human kidney biopsies and detected upregulation of DPP-4 in the glomeruli of patients with diabetic nephropathy, suggesting that our findings might be of relevance for human kidney disease as well.

CONCLUSION

Treatment with DPP-4 inhibitor linagliptin delays the progression of diabetic nephropathy damage in a glucose-independent and blood-pressure-independent manner. The observed effects may be because of the attenuation of podocyte injury and inhibition of myofibroblast transformation.

Acute renal haemodynamic effects of glucagon-like peptide-1 receptor agonist exenatide in healthy overweight men

AIMS

To determine the acute effect of glucagon-like peptide-1 (GLP-1) receptor agonist exenatide and the involvement of nitric oxide (NO) on renal haemodynamics and tubular function, in healthy overweight men.

METHODS

Renal haemodynamics and tubular electrolyte handling were measured in 10 healthy overweight men (aged 20-27 years; BMI 26-31 kg/m(2)) during intravenous administration of placebo (saline 0.9%), exenatide, and exenatide combined with the NO-synthase inhibitor L-N(G)-monomethyl arginine (L-NMMA). Glomerular filtration rate (GFR) and effective renal plasma flow (ERPF) were determined by inulin and para-aminohippurate clearance techniques, respectively, based on timed urine sampling. Glomerular hydrostatic pressure and vascular resistance of afferent and efferent renal arterioles were calculated using the Gomez formulae. Urinary electrolytes, osmolality and pH were also measured.

RESULTS

GFR increased by a mean of 18 ± 20 (+20%; p = 0.021) and ERPF increased by a median (interquartile range) of 68 (26; 197) ml/min/1.73 m(2) (+14%; p = 0.015) during exenatide infusion versus placebo. During L-NMMA infusion, exenatide increased GFR by mean 8 ± 12 ml/min/1.73 m(2) (+9%; p = 0.049). Exenatide increased estimated glomerular pressure by +6% (p = 0.015) and reduced afferent renal vascular resistance by -33% (p = 0.038), whereas these effects were blunted during L-NMMA infusion. Exenatide increased absolute and fractional sodium excretion, urinary osmolality and urinary pH. The tubular effects of exenatide were not altered by concomitant L-NMMA infusion.

CONCLUSIONS

Exenatide infusion in healthy overweight men acutely increases GFR, ERPF and glomerular pressure, probably by reducing afferent renal vascular resistance, and at least partially in an NO-dependent manner. As baseline renal haemodynamics in patients with type 2 diabetes differ from those in healthy individuals, clinical studies on the renal effects of GLP-1 receptor agonists are warranted.

Incretin-based therapy: renal effects

Glucagon like peptide-1 (GLP-1) analogues and dipeptidyl peptidase-4 (DPP-4) inhibitors are new classes of hypoglycemic agents with numerous pleiotropic effects. The review summarises data about the influence of GLP-1 analogues and DPP-4 inhibitors on structural and functional changes in diabetic kidneys. Growing evidence indicates that the kidney is one of the loci of the effects and degradation of GLP-1. The potency of the effects of GLP-1 in diabetic kidneys can be reduced by decrease in GLP-1 receptor expression or enhancement of GLP-1 degradation. In experimental models of diabetic nephropathy and non-diabetic renal injury, GLP-1 analogues and DPP-4 inhibitors slow the development of kidney fibrosis and prevent the decline of kidney function. The mechanisms of protective effect include hyperglycaemia reduction, enhancement of sodium excretion, suppression of inflammatory and fibrogenic signalling pathways, reduction of oxidative stress and apoptosis in the kidneys. In clinical studies, the urinary albumin excretion reduction rate while using the GLP-1 analogue and DPP-4 inhibitor treatment was demonstrated in patients with type 2 diabetes. Long-term impact of these agents on renal function in diabetes needs further investigations.

Dipeptidyl peptidase-4 inhibition by Saxagliptin prevents inflammation and renal injury by targeting the Nlrp3/ASC inflammasome

BACKGROUND

Glucagon-like peptide-1 (GLP-1) receptor activation delays the progression of diabetic nephropathy (DN) in rodents. The NOD-like receptor 3 (Nlrp3) inflammasome plays an important role in DN. Dipeptidyl peptidase-4 inhibitors (DPP4I) inhibit the degradation of endogenous GLP-1 and various other active substances. We assessed whether DPP4I attenuates diabetes-induced activation of the inflammasome and progression of DN in mice with type 2 diabetes mellitus (T2DM) and type 1 diabetes mellitus (T1DM).

METHODS

BTBR (T2DM), Akita (T1DM) and their matched non-diabetic control (wild-type (WT)) mice received 8-week treatment with Saxagliptin (Saxa) or vehicle.

RESULTS

Kidney weight and kidney/body weight ratio increased in the BTBR and Akita mice compared to their WT mice. Saxa attenuated these changes in the BTBR, but not in the Akita mice and had no effect in the WT mice. Serum blood urea nitrogen and creatinine significantly increased in the BTBR and Akita mice. Saxa attenuated the increase in the BTBR and Akita mice. Saxa improved glycemic control in the BTBR mice, but had no effect on glucose levels in the Akita and WT mice. Serum C reactive protein, tumor necrosis factor α (TNFα), interleukin (IL)-1β, IL-6 and IL-18 were significantly higher in the BTBR and Akita mice than in the WT mice. Saxa attenuated the increase in the BTBR and Akita mice. Kidney and adipose protein levels of apoptosis-associated speck-like protein 1, NLRP3, TNFα and Caspase-1 were higher in the BTBR and Akita mice than in the WT mice. Saxa reduced the levels in both types of diabetic mice.

CONCLUSIONS

Saxa attenuated diabetes-induced activation of the inflammasome and progression of DN. As Saxa did not affect glucose levels in the Akita mice, these effects are independent of glucose lowering.

One-year treatment with liraglutide improved renal function in patients with type 2 diabetes: a pilot prospective study

Unlike GLP-1, liraglutide is not cleared by the glomerulus and its pharmacokinetic is not altered in patients with mild renal impairment. The aim of our study was to analyze the effects of liraglutide on renal function in patients with type 2 diabetes. A twelve-month longitudinal prospective post-marketing study was performed. According to eGFR (estimated glomerular filtration rate) calculated with CKD-EPI equation, 84 consecutive patients were divided in Group A (eGFR > 90 ml/min) and Group B (eGFR < 90 ml/min). BMI, glucose, HbA1c, serum creatinine, microalbuminuria, and eGFR were evaluated at baseline and after 12 months of treatment. A reduction in fasting plasma glucose (p < 0.01), HbA1c (p < 0.003), BMI (p < 0.01), and systolic (p < 0.01) and diastolic blood pressure (p < 0.006) was recorded irrespective of eGFR category. Concerning renal function, creatinine levels had a trend to decrease in both groups. eGFR did not change in Group A, while it increased in Group B (p < 0.05) independently from the concomitant changes of other parameters. Moreover, seven out of 41 patients of Group B had increased eGFR levels which reached the normal values (>90 ml/min). At baseline, five patients had pathological microalbuminuria, but at 12 months three of them returned to normal albuminuria (p < 0.006). Total microalbuminuria levels improved in both groups (p < 0.02). According to preliminary data in animals, our study shows that liraglutide is effective in preserving eGFR in diabetic patients, increasing it in those with reduced renal function. This was associated with a decrease of frequency of patients positive to microalbuminuria. Further studies are needed to confirm these data.

Redistribution of Proximal and Distal Reabsorption of Water and Ions in Rat Kidney After Treatment with Glucagon-Like Peptide-1 Mimetic

Injection of a glucagon-like peptide-1 mimetic accelerated recovery of the initial status of water-salt balance in rats after water or saline load (2.5% NaCl). This effect is mediated by a decrease in proximal fluid reabsorption and change in ion and water transport in the distal part of renal tubules. We developed a new approach to calculation of additional fluid inflow from the proximal tubule to the distal tubule and distal sodium reabsorption under the influence of this mimetic in rats. The expected values corresponded to experimental results, which confirmed our hypothesis on the physiological mechanism for the involvement of the kidneys in the homeostatic effect of glucagon-like peptide-1 under variations in water-salt balance.

Advances in understanding relationship between GLP-1 based drugs and the kidney

The drugs based on glucagon-like peptide-1 (GLP-1) not only lower urinary protein, but also increase urine sodium excretion and improve the pathological changes of kidney disease. However, the mechanism is not very clear and may be associated with atrial natriuretic peptide, renin angiotensin axis, and oxidative stress. This review focuses on the progress in understanding the relationship between GLP-1 and the kidney.

Nephroprotection by Hypoglycemic Agents: Do We Have Supporting Data?

Current therapy directed at delaying the progression of diabetic nephropathy includes intensive glycemic and optimal blood pressure control, renin angiotensin-aldosterone system blockade and multifactorial intervention. However, the renal protection provided by these therapeutic modalities is incomplete. There is a scarcity of studies analysing the nephroprotective effect of antihyperglycaemic drugs beyond their glucose lowering effect and improved glycaemic control on the prevention and progression of diabetic nephropathy. This article analyzes the exisiting data about older and newer drugs as well as the mechanisms associated with hypoglycemic drugs, apart from their well known blood glucose lowering effect, in the prevention and progression of diabetic nephropathy. Most of them have been tested in humans, but with varying degrees of success. Although experimental data about most of antihyperglycemic drugs has shown a beneficial effect in kidney parameters, there is a lack of clinical trials that clearly prove these beneficial effects. The key question, however, is whether antihyperglycemic drugs are able to improve renal end-points beyond their antihyperglycemic effect. Existing experimental data are post hoc studies from clinical trials, and supportive of the potential renal-protective role of some of them, especially in the cases of dipeptidyl peptidase-4 inhibitors, glucagon-like peptide-1 receptor agonists and sodium-glucose cotransporter 2 inhibitors. Dedicated and adequately powered renal trials with renal outcomes are neccessary to assess the nephrotection of antihyperglycaemic drugs beyond the control of hyperglycaemia.

Proximal tubule NHE3 activity is inhibited by beta-arrestin-biased angiotensin II type 1 receptor signaling

Physiological concentrations of angiotensin II (ANG II) upregulate the activity of Na+/H+ exchanger isoform 3 (NHE3) in the renal proximal tubule through activation of the ANG II type I (AT1) receptor/G protein-coupled signaling. This effect is key for maintenance of extracellular fluid volume homeostasis and blood pressure. Recent findings have shown that selective activation of the beta-arrestin-biased AT1 receptor signaling pathway induces diuresis and natriuresis independent of G protein-mediated signaling. This study tested the hypothesis that activation of this AT1 receptor/beta-arrestin signaling inhibits NHE3 activity in proximal tubule. To this end, we determined the effects of the compound TRV120023, which binds to the AT1R, blocks G-protein coupling, and stimulates beta-arrestin signaling on NHE3 function in vivo and in vitro. NHE3 activity was measured in both native proximal tubules, by stationary microperfusion, and in opossum proximal tubule (OKP) cells, by Na+-dependent intracellular pH recovery. We found that 10−7 M TRV120023 remarkably inhibited proximal tubule NHE3 activity both in vivo and in vitro. Additionally, stimulation of NHE3 by ANG II was completely suppressed by TRV120023 both in vivo as well as in vitro. Inhibition of NHE3 activity by TRV120023 was associated with a decrease in NHE3 surface expression in OKP cells and with a redistribution from the body to the base of the microvilli in the rat proximal tubule. These findings indicate that biased signaling of the beta-arrestin pathway through the AT1 receptor inhibits NHE3 activity in the proximal tubule at least in part due to changes in NHE3 subcellular localization.

The physiological role of glucagon-like peptide-1 in the regulation of renal function

Glucagon like peptide-1 (GLP-1) is an incretin hormone constantly secreted from the intestine at low basal levels in the fasted state; plasma concentrations rise rapidly after nutrient ingestion. Upon release, GLP-1 exerts insulinotropic effects via a G protein-coupled receptor, stimulation of adenylyl cyclase, and cAMP generation. Although primarily involved in glucose homeostasis, GLP-1 can induce diuresis and natriuresis when administered in pharmacological doses in humans and rodents. However, whether endogenous GLP-1 plays a role in regulating renal function remains an open question. This study aimed to test the hypothesis that blockade of GLP-1 receptor (GLP-1R) signaling at baseline influences renal salt and water handling. To this end, the GLP-1R antagonist exendin-9 (100 μg·kg(-1)·min(-1)) or vehicle was administered intravenously to overnight-fasted male Wistar rats for 30 min. This treatment reduced urinary cAMP excretion and renal cortical PKA activity, demonstrating blockade of renal GLP-1R signaling. Exendin-9-infused-rats exhibited reduced glomerular filtration rate, lithium clearance, urinary volume flow, and sodium excretion compared with vehicle-infused controls. Exendin-9 infusion also reduced renal cortical Na(+)/H(+) exchanger isotope 3 (NHE3) phosphorylation at serine 552 (NHE3pS552), a PKA consensus site that correlates with reduced transport activity. Collectively, these results provide novel evidence that GLP-1 is a physiologically relevant natriuretic factor that contributes to sodium balance, in part via tonic modulation of NHE3 activity in the proximal tubule.

Incretins and selective renal sodium-glucose co-transporter 2 inhibitors in hypertension and coronary heart disease

Hyperglycemia is associated with an increased risk of cardiovascular disease, and the consequences of intensive therapy may depend on the mechanism of the anti-diabetic agent(s) used to achieve a tight control. In animal models, stable analogues of glucagon-like peptide-1 (GLP-1) were able to reduce body weight and blood pressure and also had favorable effects on ischemia following coronary reperfusion. In a similar way, dipeptidyl peptidase IV (DPP-IV) showed to have favorable effects in animal models of ischemia/reperfusion. This could be due to the fact that DPP-IV inhibitors were able to prevent the breakdown of GLP-1 and glucose-dependent insulinotropic polypeptide, but they also decreased the degradation of several vasoactive peptides. Preclinical data for GLP-1, its derivatives and inhibitors of the DPP-IV enzyme degradation suggests that these agents may be able to, besides controlling glycaemia, induce cardio-protective and vasodilator effects. Notwithstanding the many favorable cardiovascular effects of GLP-1/incretins reported in different studies, many questions remain unanswered due the limited number of studies in human beings that aim to examine the effects of GLP-1 on cardiovascular endpoints. For this reason, long-term trials searching for positive cardiovascular effects are now in process, such as the CAROLINA and CARMELINA trials, which are supported by small pilot studies performed in humans (and many more animal studies) with incretin-based therapies. On the other hand, selective renal sodium-glucose co-transporter 2 inhibitors were also evaluated in the prevention of cardiovascular outcomes in type 2 diabetes. However, it is quite early to draw conclusions, since data on cardiovascular outcomes and cardiovascular death are limited and long-term studies are still ongoing. In this review, we will analyze the mechanisms underlying the cardiovascular effects of incretins and, at the same time, we will present a critical position about the real value of these compounds in the cardiovascular system and its protection.

Dipeptidyl peptidase-4 inhibitors in progressive kidney disease

PURPOSE OF REVIEW

Dipeptidyl peptidase-4 (DPP-4) inhibitors are incretin-based drugs approved for the treatment of type 2 diabetes. The main action of DPP-4 inhibitors is to increase the level of incretin hormones such as glucagon-like peptide-1 (GLP-1), thereby stimulating insulin secretion from pancreatic β cells. Recently emerging evidence suggests the pleiotropic extrapancreatic function of GLP-1 or DPP-4 inhibitors, including kidney and cardiovascular protection. Here, we review the effects of DPP-4 inhibitors on progressive kidney disease such as diabetic nephropathy from a therapeutic point of view.

RECENT FINDINGS

A growing number of studies in animal models and human diseases have shown that DPP-4 inhibition ameliorates kidney disease by a process independent of glucose lowering. Possible mechanisms underlying such protective properties include the facilitation of natriuresis and reduction of blood pressure, and also local effects of the reduction of oxidative stress, inflammation and improvement of endothelial function in the kidney. DPP-4 inhibitors may also restore other DPP-4 substrates which have proven renal effects.

SUMMARY

Treatment of diabetes with DPP-4 inhibitors is likely to involve a variety of extrapancreatic effects including renal protection. Such pleiotropic action of DPP-4 inhibitors might occur by both incretin-dependent and incretin-independent mechanisms. Conclusive evidence is needed to translate the favorable results from animal models to humans.

Pleiotropic effects of the dipeptidylpeptidase-4 inhibitors on the cardiovascular system

Dipeptidylpeptidase-4 (DPP-4) is a ubiquitously expressed transmembrane protein that removes NH2-terminal dipeptides from various substrate hormones, chemokines, neuropeptides, and growth factors. Two known substrates of DPP-4 include the incretin hormones glucagon-like peptide-1 (GLP-1) and gastric inhibitory peptide, which are secreted by enteroendocrine cells in response to postprandial hyperglycemia and account for 60–70% of postprandial insulin secretion. DPP-4 inhibitors (DPP-4i) block degradation of GLP-1 and gastric inhibitory peptide, extend their insulinotropic effect, and improve glycemia. Since 2006, several DPP-4i have become available for treatment of type 2 diabetes mellitus. Clinical trials confirm that DPP-4i raises GLP-1 levels in plasma and improves glycemia with very low risk for hypoglycemia and other side effects. Recent studies also suggest that DPP-4i confers cardiovascular and kidney protection, beyond glycemic control, which may reduce the risk for further development of the multiple comorbidities associated with obesity/type 2 diabetes mellitus, including hypertension and cardiovascular disease (CVD) and kidney disease. The notion that DPP-4i may improve CVD outcomes by mechanisms beyond glycemic control is due to both GLP-1-dependent and GLP-1-independent effects. The CVD protective effects by DPP-4i result from multiple factors including insulin resistance, oxidative stress, dyslipidemia, adipose tissue dysfunction, dysfunctional immunity, and antiapoptotic properties of these agents in the heart and vasculature. This review focuses on cellular and molecular mechanisms mediating the CVD protective effects of DPP-4i beyond favorable effects on glycemic control.

Glucagon-like peptide 1 receptor agonist (GLP-1 RA): long-term effect on kidney function in patients with type 2 diabetes

AIMS

In a short-term study including 31 patients with type 2 diabetes, glucagon-like peptide 1 receptor agonist (GLP-1 RA) treatment was associated with a significant reversible decline in GFR. Twenty-three patients re-initiated GLP-1 RA treatment after the primary study, and the aim was to investigate the long-term effect on kidney function.

METHODS

We included 30 patients in a one-year extension study, all initially treated with liraglutide for seven weeks. During follow-up 23 were treated with liraglutide and seven untreated. Primary outcome was change in GFR ((51)Cr-EDTA plasma clearance).

RESULTS

Patients were 61.5 (10.0) years and HbA(1c) 60.1 (13.8) mmol/mol. Baseline GFR was 100.6 (24.9) mL/min/1.73 m(2) and was reduced by 11 (95% CI: 6.6-15.7, p < 0.001) mL/min/1.73 m(2), independent of change in 24-h systolic blood pressure (SBP), weight, UAER or HbA(1c) (p≥0.33). Geometric mean (IQR) of UAER was 25.5 (9.9-50.9) mg/d and was reduced by 27 (95% CI: 5-44; p = 0.020)%, and 24-h SBP was reduced by 8.2 (p = 0.048) mmHg. No changes occurred in untreated patients.

CONCLUSIONS

Long-term treatment with liraglutide was associated with a reduction in measured GFR similar to the effect during short-term treatment, suggesting a metabolic or haemodynamic reversible effect and not structural changes. Moreover, UAER and 24-h SBP were reduced.

TRIAL REGISTRATION

ClinicalTrials.gov identifier: NCT01499108.

Comparison of effects of sitagliptin and voglibose on left ventricular diastolic dysfunction in patients with type 2 diabetes: results of the 3D trial

Background

Left ventricular (LV) diastolic dysfunction is frequently observed in patients with type 2 diabetes. Dipeptidyl peptidase-4 inhibitor (DPP-4i) attenuates postprandial hyperglycemia (PPH) and may have cardio-protective effects. It remains unclear whether DPP-4i improves LV diastolic function in patients with type 2 diabetes, and, if so, it is attributable to the attenuation of PPH or to a direct cardiac effect of DPP-4i. We compared the effects of the DPP-4i, sitagliptin, and the alpha-glucosidase inhibitor, voglibose, on LV diastolic function in patients with type 2 diabetes.

Methods

We conducted a prospective, randomized, open-label, multicenter study of 100 diabetic patients with LV diastolic dysfunction. Patients received sitagliptin (50 mg/day) or voglibose (0.6 mg/day). The primary endpoints were changes in the e’ velocity and E/e’ ratio from baseline to 24 weeks later. The secondary efficacy measures included HbA1c, GLP-1, lipid profiles, oxidative stress markers and inflammatory markers.

Results

The study was completed with 40 patients in the sitagliptin group and 40 patients in the voglibose group. There were no significant changes in the e’ velocity and E/e’ ratio from baseline to 24 weeks later in both groups. However, analysis of covariance demonstrated that pioglitazone use is an independent factor associated with changes in the e’ and E/e’ ratio. Among patients not using pioglitazone, e’ increased and the E/e’ ratio decreased in both the sitagliptin and voglibose groups. GLP-1 level increased from baseline to 24 weeks later only in the sitagliptin group (4.8 ± 4.7 vs. 7.3 ± 5.5 pmol/L, p < 0.05). The reductions in HbA1c and body weight were significantly greater in the sitagliptin group than in the voglibose group (−0.7 ± 0.6 % vs. −0.3 ± 0.4, p < 0.005; −1.3 ± 3.2 kg vs. 0.4 ± 2.8 kg, p < 0.05, respectively). There were no changes in lipid profiles and inflammatory markers in both groups.

Conclusions

Our trial showed that sitagliptin reduces HbA1c levels more greatly than voglibose does, but that neither was associated with improvement in the echocardiographic parameters of LV diastolic function in patients with diabetes.

Trial registration

Registered at http://www.umin.ac.jp under UMIN000003784

Blood pressure-lowering effects of incretin-based diabetes therapies

Glucagon-like peptide-1 receptor (GLP-1) agonists and dipeptidyl-peptidase-4 (DPP-4) inhibitors are therapies that are used to treat hyperglycemia in patients with type 2 diabetes mellitus. Although both of these medication types primarily lower prandial and fasting blood glucose levels by enhanced GLP-1 receptor signalling, they have distinct mechanisms of action. Whereas DPP-4 inhibitors boost patient levels of endogenously produced GLP-1 (and glucose-dependent insulinotropic peptide) by preventing its metabolism by DPP-4 enzymatic activity, GLP-1 receptor agonists are either synthetic analogues of human GLP-1 or exendin-4 based molecules. They are tailored to resist hydrolysis by DPP-4 activity and to provide longer durability in the circulation compared with native GLP-1. Several roles for incretin-based diabetes therapies beyond the endocrine pancreas and their glycemic-lowering properties have now been described, including attenuation of cardiac myocyte injury and reduction in post-ischemic infarction size after cardiovascular insult. Favourable outcomes have also been observed on systolic blood pressure reduction, postprandial intestinal lipoprotein metabolism, endothelial cell function, modulation of innate immune-mediated inflammation and surrogate markers of renal function. As hypertension is an independent risk factor for premature death in patients with type 2 diabetes, potential favourable extrapancreatic actions, particularly within the heart, blood vessels and kidney, for this drug class are of considerable clinical interest. Herein, we highlight and provide critical appraisal of the clinical data supporting the antihypertensive effects of GLP-1 receptor agonists and DPP-4 inhibitors and link possible mechanisms of action to clinical outcomes reported for this drug class.

GLP-1 receptor agonists: Nonglycemic clinical effects in weight loss and beyond

OBJECTIVE

Glucagon-like peptide-1 (GLP-1) receptor agonists are indicated for treatment of type 2 diabetes since they mimic the actions of native GLP-1 on pancreatic islet cells, stimulating insulin release, while inhibiting glucagon release, in a glucose-dependent manner. The observation of weight loss has led to exploration of their potential as antiobesity agents, with liraglutide 3.0 mg day(-1) approved for weight management in the US on December 23, 2014, and in the EU on March 23, 2015. This review examines the potential nonglycemic effects of GLP-1 receptor agonists.

METHODS

A literature search was conducted to identify preclinical and clinical evidence on nonglycemic effects of GLP-1 receptor agonists.

RESULTS

GLP-1 receptors are distributed widely in a number of tissues in humans, and their effects are not limited to the well-recognized effects on glycemia. Nonglycemic effects include weight loss, which is perhaps the most widely recognized nonglycemic effect. In addition, effects on the cardiovascular, neurologic, and renal systems and on taste perception may occur independently of weight loss.

CONCLUSIONS

GLP-1 receptor agonists may provide other nonglycemic clinical effects besides weight loss. Understanding these effects is important for prescribers in using GLP-1 receptor agonists for diabetic patients, but also if approved for chronic weight management.

Acute hemodynamic and renal effects of glucagon-like peptide 1 analog and dipeptidyl peptidase-4 inhibitor in rats

BACKGROUND

Glucagon-like peptide 1 (GLP-1) analogs and dipeptidyl peptidase-4 (DPP4) inhibitors are a newer class of antidiabetics named as incretin-based therapy. In addition to the homeostatic control of glucose, the incretin-based therapy has shown beneficial effects on the cardiovascular system in preclinical and clinical studies. However, there is limited information on their renal effects. To this end, we assessed the acute hemodynamic and renal effects of a GLP-1 analog, Liraglutide, and a DPP4 inhibitor, MK-0626.

METHODS

Experiments were performed in anesthetized male Sprague-Dawley rats. Three ascending doses of Liraglutide (3, 9, and 27 nmol/kg/h) or MK-0626 (1 mg/kg) with or without GLP-1 peptide (2.4, 4.8, or 9.6 pmol/kg/min) were administered. Blood pressure (BP) and heart rate (HR) were recorded from an indwelling catheter. Glomerular filtration rate (GFR) and renal blood flow (RBF) were assessed by inulin and para-aminohippurate clearance, respectively. Renal excretory function was assessed in metabolic studies.

RESULTS

Both Liraglutide and MK-0626 plus GLP-1 evoked significant diuretic and natriuretic responses and increased GFR. MK-0626 alone increased RBF. Liraglutide at 27 nmol//kg/h and MK-0626 plus GLP-1 at 9.6 pmol/kg/min also increased HR, whereas BP was not affected.

CONCLUSION

The results of the present study demonstrated that a GLP-1 analog and a DPP4 inhibitor may have beneficial effects on renal sodium and water handling. Additionally, the DPP4 inhibitor, MK-0626, favorably affects renal hemodynamics by increasing RBF. However, exceedingly high levels of GLP-1 receptor agonists may adversely affect the cardiovascular system in acute setting, as demonstrated by an acute increase in HR.

Potential role of dipeptidyl peptidase IV in the pathophysiology of heart failure

Dipeptidyl peptidase IV (DPPIV) is a widely expressed multifunctional serine peptidase that exists as a membrane-anchored cell surface protein or in a soluble form in the plasma and other body fluids. Numerous substrates are cleaved at the penultimate amino acid by DPPIV, including glucagon-like peptide-1 (GLP-1), brain natriuretic peptide (BNP) and stromal cell-derived factor-1 (SDF-α), all of which play important roles in the cardiovascular system. In this regard, recent reports have documented that circulating DPPIV activity correlates with poorer cardiovascular outcomes in human and experimental heart failure (HF). Moreover, emerging evidence indicates that DPPIV inhibitors exert cardioprotective and renoprotective actions in a variety of experimental models of cardiac dysfunction. On the other hand, conflicting results have been found when translating these promising findings from preclinical animal models to clinical therapy. In this review, we discuss how DPPIV might be involved in the cardio-renal axis in HF. In addition, the potential role for DPPIV inhibitors in ameliorating heart disease is revised, focusing on the effects of the main DPPIV substrates on cardiac remodeling and renal handling of salt and water.

Activation of GLP-1 receptors on vascular smooth muscle cells reduces the autoregulatory response in afferent arterioles and increases renal blood flow

Glucagon-like peptide (GLP)-1 has a range of extrapancreatic effects, including renal effects. The mechanisms are poorly understood, but GLP-1 receptors have been identified in the kidney. However, the exact cellular localization of the renal receptors is poorly described. The aim of the present study was to localize renal GLP-1 receptors and describe GLP-1-mediated effects on the renal vasculature. We hypothesized that renal GLP-1 receptors are located in the renal microcirculation and that activation of these affects renal autoregulation and increases renal blood flow. In vivo autoradiography using (125)I-labeled GLP-1, (125)I-labeled exendin-4 (GLP-1 analog), and (125)I-labeled exendin 9-39 (GLP-1 receptor antagonist) was performed in rodents to localize specific GLP-1 receptor binding. GLP-1-mediated effects on blood pressure, renal blood flow (RBF), heart rate, renin secretion, urinary flow rate, and Na(+) and K(+) excretion were investigated in anesthetized rats. Effects of GLP-1 on afferent arterioles were investigated in isolated mouse kidneys. Specific binding of (125)I-labeled GLP-1, (125)I-labeled exendin-4, and (125)I-labeled exendin 9-39 was observed in the renal vasculature, including afferent arterioles. Infusion of GLP-1 increased blood pressure, RBF, and urinary flow rate significantly in rats. Heart rate and plasma renin concentrations were unchanged. Exendin 9-39 inhibited the increase in RBF. In isolated murine kidneys, GLP-1 and exendin-4 significantly reduced the autoregulatory response of afferent arterioles in response to stepwise increases in pressure. We conclude that GLP-1 receptors are located in the renal vasculature, including afferent arterioles. Activation of these receptors reduces the autoregulatory response of afferent arterioles to acute pressure increases and increases RBF in normotensive rats.

Exendin-4 attenuates renal tubular injury by decreasing oxidative stress and inflammation in streptozotocin-induced diabetic mice

In this study, we aimed to research the restorative effects of exendin-4, a GLP-1 analog, on renal tubular injury in streptozotocin-induced diabetes model. BALB/c male mice were divided into four groups: non-diabetic, non-diabetic + exendin-4 (3 μg/kg), diabetic and diabetic + exendin-4. In our diabetic model, we observed renal injury mainly in tubular area rather than glomeruli and exendin-4 decreased tubular injury with its glucose lowering effect. Besides, PCNA positive tubular cells, activities of LDH and Na(+)-K(+)-ATPase were also significantly declined by the administration of exendin-4. Furthermore, exendin-4 attenuated the levels of ROS, MDA, 8-OHdG, proinflammatory cytokines (TNF-α, IL-1β), chemokine MCP-1, ICAM-1, and fibrosis-related molecules (transforming growth factor β1 and fibronectin). In consistent with reducing tubular injury, macrophage infiltration and both MCP-1 and ICAM-1 production in tubular cells were decreased. These results indicate that exendin-4 may decrease renal tubular injury seen in the beginning of diabetic nephropathy by decreasing ROS production and inflammation.

Glucagon-like peptide-1 and blood pressure in young and healthy adults from the general population

Hypertension and diabetes mellitus are highly correlated, but the underlying mechanisms are only partly understood. Therefore, the aim of our study was to investigate the relationships between plasma levels of glucagon-like peptide-1, a key factor in the regulation of glucose homeostasis, and various blood pressure indices. Healthy adults aged 25 to 41 years were enrolled in a population-based study. Established cardiovascular disease, diabetes mellitus, or a body mass index >35 kg/m(2) were exclusion criteria. Fasting plasma glucagon-like peptide-1 levels as determined with a novel high-sensitive assay and ambulatory blood pressure data were available in 1479 participants not using antihypertensive treatment. Median age of our population was 38 years. Mean systolic and diastolic blood pressure across increasing glucagon-like peptide-1 quartiles were 120.6, 122.8, 123.2, and 124.9 mm Hg and 77.1, 78.7, 78.9, and 79.9 mm Hg, respectively. We found a linear relationship of glucagon-like peptide-1 with 24-hour ambulatory blood pressure after multivariable adjustment (β per 1 log-unit increase 2.01; 95% confidence interval, 1.02-3.00; P<0.0001 for systolic and 1.22; 0.47-1.97; P=0.002 for diastolic blood pressure). In separate analyses, glucagon-like peptide-1 was significantly related to both awake (β per 1 log-unit increase 2.05; 1.02-3.09; P=0.0001 for systolic and 1.15; 0.35-1.96; P=0.005 for diastolic blood pressure) and asleep blood pressure (β per 1 log-unit increase 1.34; 0.26-2.42; P=0.01 for systolic and 1.05; 0.26-1.84; P=0.009 for diastolic blood pressure). In conclusion, plasma levels of glucagon-like peptide-1 are significantly associated with both systolic and diastolic blood pressure levels.

Endogenous Glucagon-Like Peptide-1 as a Potential Mediator of the Resolution of Diabetic Kidney Disease following Roux en Y Gastric Bypass: Evidence and Perspectives

Diabetic kidney disease in patients with type 2 diabetes strongly correlates with the incidence of major cardiovascular events and all-cause mortality. Pharmacological and lifestyle based management focusing on glycaemic, lipid, and blood pressure control is the mainstay of treatment but efficacy remains limited. Roux en Y gastric bypass is an efficacious intervention in diabetes. Emerging evidence also supports a role for bypass as an intervention for early diabetic kidney disease. This paper firstly presents level 1 evidence of the effects of bypass on hyperglycaemia and hypertension and then summarises emerging data on its effects on diabetic kidney disease. Glucagon-like peptide-1 is implicated as a central mediator of diabetes resolution following bypass through the incretin effect. It has been ascribed vasodilatory, pronatriuretic, and antioxidant properties and its exogenous administration or optimisation of its endogenous levels via dipeptidyl peptidase IV inhibition results in antioxidant and antiproteinuric effects in preclinical models of DKD. Some evidence is emerging of translation of coherent effects in the clinical setting. These findings raise the question of whether pharmacotherapy targeted at optimising circulating hormone levels may be capable of recapitulating some of the effects of bypass surgery on renal injury.

Effects of GLP-1 in the kidney

The incretin hormone, glucagon-like peptide-1 (GLP-1), stimulates insulin secretion and forms the basis of a new drug class for diabetes treatment. GLP-1 has several extra-pancreatic properties which include effects on kidney function. Although renal GLP-1 receptors have been identified, their exact localization and physiological role are incompletely understood. GLP-1 increases natriuresis through inhibition of the sodium-hydrogen ion exchanger isoform 3 in the proximal tubule. This may in part explain why GLP-1 receptor agonists have antihypertensive effects. Glomerular filtration rate is regulated by GLP-1, but the mechanisms are complex and may depend on e.g. glycaemic conditions. Atrial natriuretic peptide or the renin-angiotensin system may be involved in the signalling of GLP-1-mediated renal actions. Several studies in rodents have shown that GLP-1 therapy is renoprotective beyond metabolic improvements in models of diabetic nephropathy and acute kidney injury. Inhibition of renal inflammation and oxidative stress probably mediate this protection. Clinical studies supporting GLP-1-mediated renal protection exist, but they are few and with limitations. However, acute and chronic kidney diseases are major global health concerns and measures improving renal outcome are highly needed. Therefore, the renoprotective potential of GLP-1 therapy need to be thoroughly investigated in humans.

Improvements in the metabolic milieu following Roux-en-Y gastric bypass and the arrest of diabetic kidney disease

Roux-en-Y gastric bypass (RYGB) is an efficacious intervention for morbid obesity and has a diabetes-remitting effect in patients with obesity and type 2 diabetes mellitus, which occurs prior to significant weight loss. Roux-en-Y gastric bypass is also associated with early and sustained reductions in the risk factor profile for the progression of diabetic complications. Attention is therefore now being placed on RYGB as a metabolic intervention with the capacity to yield therapeutic benefit in relation to the progression of diabetic complications, such as diabetic kidney disease. As alterations in gut anatomy following RYGB coincide with attendant shifts in downstream enteroendocrine signals with direct and indirect resolutionary effects on the kidney, the concept of an endocrine gut-kidney axis post-RYGB is growing. With the model of a gut-kidney axis in mind, this article summarizes emerging data on the effects of RYGB on risk factors for diabetic kidney disease (hyperglycaemia, dyslipidaemia and hypertension), highlighting a potential role for glucagon-like peptide 1 in risk factor reduction.

Intestinal regulation of urinary sodium excretion and the pathophysiology of diabetic kidney disease: a focus on glucagon-like peptide 1 and dipeptidyl peptidase 4

The tubular hypothesis of glomerular filtration and nephropathy in diabetes is a pathophysiological concept that assigns a critical role to the tubular system, including proximal tubular hyper-reabsorption and growth, which is relevant for early glomerular hyperfiltration and later chronic kidney disease. Here we focus on how harnessing the bioactivity of hormones released from the gut may ameliorate the early effects of diabetes on the kidney in part by attenuating proximal tubular hyper-reabsorption and growth. The endogenous tone of the glucagon-like peptide 1 (GLP-1)/GLP-1 receptor (GLP-1R) system and its pharmacological activation are nephroprotective in diabetes independent of changes in blood glucose. This is associated with suppression of increases in kidney weight and glomerular hyperfiltration, which may reflect, at least in part, its inhibitory effects on tubular hyper-reabsorption and growth. Inhibition of dipeptidyl peptidase 4 (DPP-4) is also nephroprotective independent of changes in blood glucose and involves GLP-1/GLP-1R-dependent and -independent mechanisms. The GLP-1R agonist exendin-4 induces natriuresis via activation of the GLP-1R. In contrast, DPP4 inhibition increases circulating GLP-1, but drives a GLP-1R-independent natriuretic response, implying a role for other DPP-4 substrates. The extent to which the intrarenal DPP-4/GLP-1 receptor system contributes to all these changes remains to be established, as does the direct impact of the system on renal inflammation.

Physiology and pathophysiology of incretins in the kidney

PURPOSE OF REVIEW

Incretin-based therapy with glucagon-like peptide-1 receptor (GLP-1R) agonists and dipeptidyl peptidase-4 (DPP-4) inhibitors is considered a promising therapeutic option for type 2 diabetes mellitus. Cumulative evidence, mainly from preclinical animal studies, reveals that incretin-based therapies also may elicit beneficial effects on kidney function. This review gives an overview of the physiology, pathophysiology, and pharmacology of the renal incretin system.

RECENT FINDINGS

Activation of GLP-1R in the kidney leads to diuretic and natriuretic effects, possibly through direct actions on renal tubular cells and sodium transporters. Moreover, there is evidence that incretin-based therapy reduces albuminuria, glomerulosclerosis, oxidative stress, and fibrosis in the kidney, partially through GLP-1R-independent pathways. Molecular mechanisms by which incretins exert their renal effects are understood incompletely, thus further studies are needed.

SUMMARY

The GLP-1R and DPP-4 are expressed in the kidney in various species. The kidney plays an important role in the excretion of incretin metabolites and most GLP-1R agonists and DPP-4 inhibitors, thus special attention is required when applying incretin-based therapy in renal impairment. Preclinical observations suggest direct renoprotective effects of incretin-based therapies in the setting of hypertension and other disorders of sodium retention, as well as in diabetic and nondiabetic nephropathy. Clinical studies are needed in order to confirm translational relevance from preclinical findings for treatment options of renal diseases.

The potential for renoprotection with incretin-based drugs

Incretin-based drugs, i.e., glucagon-like peptide-1 (GLP-1) receptor agonists and dipeptidyl peptidase-4 (DPP-4) inhibitors, are widely used for the treatment of type 2 diabetes. In addition to the primary role of incretins in stimulating insulin secretion from pancreatic β-cells, they have extra pancreatic functions beyond glycemic control. Indeed, recent studies highlight the potential beneficial effects of incretin-based therapy in diabetic kidney disease (DKD). Experimental studies using various diabetic models suggest that incretins protect the vascular endothelium from injury by binding to GLP-1 receptors, thereby ameliorating oxidative stress and the local inflammatory response, which reduces albuminuria and inhibits glomerular sclerosis. In addition, there is some evidence that GLP-1 receptor agonists and DPP-4 inhibitors mediate sodium excretion and diuresis to lower blood pressure. The pleiotropic actions of DPP-4 inhibitors are ascribed primarily to their effects on GLP-1 signaling, but other substrates of DPP-4, such as brain natriuretic peptide and stromal-derived factor-1α, may have roles. In this review, we summarize recent studies of the roles of incretin-based therapy in ameliorating DKD and its complications.

Effects of the once-weekly glucagon-like peptide-1 receptor agonist dulaglutide on ambulatory blood pressure and heart rate in patients with type 2 diabetes mellitus

Glucagon-like peptide-1 receptor agonists, used to treat type 2 diabetes mellitus, are associated with small reductions in systolic blood pressure (SBP) and increases in heart rate. However, findings based on clinic measurements do not adequately assess a drug's 24-hour pharmacodynamic profile. The effects of dulaglutide, a once-weekly glucagon-like peptide-1 receptor agonist, on BP and heart rate were investigated using ambulatory BP monitoring. Patients (n=755; 56±10 years; 81% white; 48% women), with type 2 diabetes mellitus, taking ≥1 oral antihyperglycemic medication, with a clinic BP between 90/60 and 140/90 mm Hg were randomized to dulaglutide (1.5 or 0.75 mg) or placebo subcutaneously for 26 weeks. Ambulatory BP monitoring was performed at baseline and at 4, 16, and 26 weeks. The primary end point was change from baseline to week 16 in mean 24-hour SBP, a tree gatekeeping strategy compared the effects of dulaglutide to placebo. Both doses of dulaglutide were noninferior to placebo for changes in 24-hour SBP and diastolic blood pressure, and dulaglutide 1.5 mg significantly reduced SBP (least squares mean difference [95% confidence interval]), -2.8 mm Hg [-4.6, -1.0]; P≤0.001). Dulaglutide 0.75 mg was noninferior to placebo (1.6 bpm; [0.3, 2.9]; P≤0.02) for 24-hour heart rate (least squares mean difference [95% confidence interval]), but dulaglutide 1.5 mg was not (2.8 bpm [1.5, 4.2]). Dulaglutide 1.5 mg was associated with a reduction in 24-hour SBP and an increase in 24-hour heart rate. The mechanisms responsible for the observed effects remain to be clarified.

Changes in the activity and expression of protein phosphatase-1 accompany the differential regulation of NHE3 before and after the onset of hypertension in spontaneously hypertensive rats

AIM

The Na(+) /H(+) exchanger NHE3 activity decreases in the proximal tubule of spontaneously hypertensive rats (SHRs) as blood pressure increases, and this reduction is correlated with higher NHE3 phosphorylation levels at the PKA consensus site serine 552. This study tested the hypothesis that this lowered NHE3 activity is associated with an increase in PKA activity and expression, and/or a decrease in protein phosphatase-1 (PP1) activity and expression.

METHODS

Proximal tubule NHE3 activity was measured as the rate of bicarbonate reabsorption by stationary microperfusion. NHE3 phosphorylation and protein expression were determined by immunoblotting. PKA and PP1 activities were determined using specific substrates under optimal enzymatic conditions.

RESULTS

The PKA activator, 6-MB-cAMP, increased the phosphorylation levels of NHE3 at serine 552 in the renal cortex; this increase happens to a much greater extent in young pre-hypertensive SHRs (Y-SHRs) compared to adult SHRs with established hypertension (A-SHRs). Likewise, the inhibitory effect of 6-MB-cAMP on NHE3 transport activity was much more pronounced in the proximal tubules of Y-SHRs than in those of A-SHRs. Renal cortical activity of PKA was not significantly different between Y-SHRs and A-SHRs. On the other hand, Y-SHRs exhibited higher protein phosphatase 1 (PP1) activity, and their expression of the PP1 catalytic subunit PP1α in the renal cortex was also higher than in A-SHRs.

CONCLUSION

Collectively, these results support the idea that the lower NHE3 transport activity and higher phosphorylation occurring after the development of hypertension in SHRs are due, at least in part, to reduced PP1-mediated dephosphorylation of NHE3 at serine 552.

GLP-1 receptor localization in monkey and human tissue: novel distribution revealed with extensively validated monoclonal antibody

Glucagon-like peptide 1 (GLP-1) analogs are increasingly being used in the treatment of type 2 diabetes. It is clear that these drugs lower blood glucose through an increase in insulin secretion and a lowering of glucagon secretion; in addition, they lower body weight and systolic blood pressure and increase heart rate. Using a new monoclonal antibody for immunohistochemistry, we detected GLP-1 receptor (GLP-1R) in important target organs in humans and monkeys. In the pancreas, GLP-1R was predominantly localized in β-cells with a markedly weaker expression in acinar cells. Pancreatic ductal epithelial cells did not express GLP-1R. In the kidney and lung, GLP-1R was exclusively expressed in smooth muscle cells in the walls of arteries and arterioles. In the heart, GLP-1R was localized in myocytes of the sinoatrial node. In the gastrointestinal tract, the highest GLP-1R expression was seen in the Brunner's gland in the duodenum, with lower level expression in parietal cells and smooth muscle cells in the muscularis externa in the stomach and in myenteric plexus neurons throughout the gut. No GLP-1R was seen in primate liver and thyroid. GLP-1R expression seen with immunohistochemistry was confirmed by functional expression using in situ ligand binding with (125)I-GLP-1. In conclusion, these results give important new insight into the molecular mode of action of GLP-1 analogs by identifying the exact cellular localization of GLP-1R.

Functional role of glucose metabolism, osmotic stress, and sodium-glucose cotransporter isoform-mediated transport on Na+/H+ exchanger isoform 3 activity in the renal proximal tubule

Na(+)-glucose cotransporter 1 (SGLT1)-mediated glucose uptake leads to activation of Na(+)-H(+) exchanger 3 (NHE3) in the intestine by a process that is not dependent on glucose metabolism. This coactivation may be important for postprandial nutrient uptake. However, it remains to be determined whether SGLT-mediated glucose uptake regulates NHE3-mediated NaHCO3 reabsorption in the renal proximal tubule. Considering that this nephron segment also expresses SGLT2 and that the kidneys and intestine show significant variations in daily glucose availability, the goal of this study was to determine the effect of SGLT-mediated glucose uptake on NHE3 activity in the renal proximal tubule. Stationary in vivo microperfusion experiments showed that luminal perfusion with 5 mM glucose stimulates NHE3-mediated bicarbonate reabsorption. This stimulatory effect was mediated by glycolytic metabolism but not through ATP production. Conversely, luminal perfusion with 40 mM glucose inhibited NHE3 because of cell swelling. Notably, pharmacologic inhibition of SGLT activity by Phlorizin produced a marked inhibition of NHE3, even in the absence of glucose. Furthermore, immunofluorescence experiments showed that NHE3 colocalizes with SGLT2 but not SGLT1 in the rat renal proximal tubule. Collectively, these findings show that glucose exerts a bimodal effect on NHE3. The physiologic metabolism of glucose stimulates NHE3 transport activity, whereas, supraphysiologic glucose concentrations inhibit this exchanger. Additionally, Phlorizin-sensitive SGLT transporters and NHE3 interact functionally in the proximal tubule.