Dipeptidyl peptidase-4 inhibition ameliorates Western diet-induced hepatic steatosis and insulin resistance through hepatic lipid remodeling and modulation of hepatic mitochondrial function

Novel therapies are needed for treating the increasing prevalence of hepatic steatosis in Western populations. In this regard, dipeptidyl peptidase-4 (DPP-4) inhibitors have recently been reported to attenuate the development of hepatic steatosis, but the potential mechanisms remain poorly defined. In the current study, 4-week-old C57Bl/6 mice were fed a high-fat/high-fructose Western diet (WD) or a WD containing the DPP-4 inhibitor, MK0626, for 16 weeks. The DPP-4 inhibitor prevented WD-induced hepatic steatosis and reduced hepatic insulin resistance by enhancing insulin suppression of hepatic glucose output. WD-induced accumulation of hepatic triacylglycerol (TAG) and diacylglycerol (DAG) content was significantly attenuated with DPP-4 inhibitor treatment. In addition, MK0626 significantly reduced mitochondrial incomplete palmitate oxidation and increased indices of pyruvate dehydrogenase activity, TCA cycle flux, and hepatic TAG secretion. Furthermore, DPP-4 inhibition rescued WD-induced decreases in hepatic PGC-1α and CPT-1 mRNA expression and hepatic Sirt1 protein content. Moreover, plasma uric acid levels in mice fed the WD were decreased after MK0626 treatment. These studies suggest that DPP-4 inhibition ameliorates hepatic steatosis and insulin resistance by suppressing hepatic TAG and DAG accumulation through enhanced mitochondrial carbohydrate utilization and hepatic TAG secretion/export with a concomitant reduction of uric acid production.

Mineralocorticoid receptor blockade prevents Western diet-induced diastolic dysfunction in female mice

Overnutrition/obesity predisposes individuals, particularly women, to diastolic dysfunction (DD), an independent predictor of future cardiovascular disease. We examined whether low-dose spironolactone (Sp) prevents DD associated with consumption of a Western Diet (WD) high in fat, fructose, and sucrose. Female C57BL6J mice were fed a WD with or without Sp (1 mg·kg(-1)·day(-1)). After 4 mo on the WD, mice exhibited increased body weight and visceral fat, but similar blood pressures, compared with control diet-fed mice. Sp prevented the development of WD-induced DD, as indicated by decreased isovolumic relaxation time and an improvement in myocardial performance (<Tei index) and septal annular velocity (<E'-to-A' ratio), as assessed by echocardiography, as well as decreased diastolic relaxation time/increased diastolic initial filling rate, as assessed by MRI. The relationship between passive sarcomere length of cardiac myocytes and ventricular pressure was monitored using di-8-ANEPPS staining of the t-tubule network in hearts ex vivo. Sp administration led to longer sarcomere lengths at each pressure indicative of improved ventricular compliance in WD-fed mice. Sp also prevented left ventricular hypertrophy, interstitial fibrosis, and oxidative stress. Sp prevented the WD-induced increased expression of myocardial proinflammatory M1 macrophage markers monocyte chemoattractant protein-1 and CD11c and increased the expression of the anti-inflammatory M2 macrophage marker CD206. These findings demonstrate that WD-induced DD is associated with increased oxidant stress, fibrosis, and immune dysregulation. Mineralocorticoid receptor antagonism enhanced M2 macrophage polarization and ameliorated oxidant stress and fibrosis. This work supports a novel blood pressure-independent effect of MR antagonism as a strategy to prevent diet-induced DD in women. Mineralocorticoid antagonism; low-dose spironolactone; aldosterone;high-fat diet; high-fructose diet; oxidative stress; inflammation; cardiac hypertrophy; myocardial compliance.

DPP4 inhibition attenuates filtration barrier injury and oxidant stress in the zucker obese rat

OBJECTIVE

Obesity-related glomerulopathy is characterized initially by glomerular hyperfiltration with hypertrophy and then development of proteinuria. Putative mechanisms include endothelial dysfunction and filtration barrier injury due to oxidant stress and immune activation. There has been recent interest in targeting dipeptidyl peptidase 4 (DPP4) enzyme due to increasing role in non-enzymatic cellular processes.

METHODS

The Zucker obese (ZO) rat (aged 8 weeks) fed a normal chow or diet containing the DPP4 inhibitor linagliptin for 8 weeks (83 mg/kg rat chow) was utilized.

RESULTS

Compared to lean controls, there were increases in plasma DPP4 activity along with proteinuria in ZO rats. ZO rats further displayed increases in glomerular size and podocyte foot process effacement. These findings occurred in parallel with decreased endothelial stromal-derived factor-1α (SDF-1α), increased oxidant markers, and tyrosine phosphorylation of nephrin and serine phosphorylation of the mammalian target of rapamycin (mTOR). DPP4 inhibition improved proteinuria along with filtration barrier remodeling, circulating and kidney tissue DPP4 activity, increased active glucagon like peptide-1 (GLP-1) as well as SDF-1α, and improved oxidant markers and the podocyte-specific protein nephrin.

CONCLUSIONS

These data support a role for DPP4 in glomerular filtration function and targeting DPP4 with inhibition improves oxidant stress-related glomerulopathy and associated proteinuria.

Abstract 200: Low Dose Spironolactone Treatment Prevents Cardiac Diastolic Dysfunction, Oxidant Stress and Fibrosis in a Female Model of Obesity in Concert with a Modulation of Inflammation

Obesity is a global epidemic with 1.5 billion overweight or obese people worldwide. The association of obesity and a high-fat/high-fructose Western diet (WD) markedly increases cardiovascular disease (CVD) particularly diastolic dysfunction in women. Few treatments exist for diastolic heart disease but, recent work implicates the mineralocorticoid receptor (MR) in inflammation, fibrosis and oxidant stress. We thus hypothesized that low-dose spironolactone (LDSp) could prevent diastolic dysfunction by reducing fibrosis, oxidant stress and inflammation. To test this hypothesis, we developed a female model of obesity induced diastolic dysfunction and examined preventative treatment with LDSp.

Four week-old C57BL6/J female mice were fed a WD with/without 1.0mg/kg/day of Sp (n=7 for each group). This dose of LDSp exerts no effect on blood pressure. After 16 weeks, we conducted detailed phenotypic analysis and assessed diastolic function by cardiac MRI. Immunohistochemistry was also done for cardiac oxidant stress, fibrosis, collagen content and insulin receptor (IRS-1) expression. Flow cytometry of heart tissue was utilized to examine inflammatory mechanisms.

We found WD feeding caused diastolic dysfunction that was prevented by LDSp (LV diastolic relaxation time 33.4 ± 1.2ms for WD, 20.6 ± 1.5ms for control and 24.3 ± 0.9ms for WD+LDSp, p <0.01). 3-nitrotyrosine staining showed significant cardiac oxidant stress with WD feeding that was ameliorated by LDSp. Cardiac fibrosis and the collagen 1:3 ratio were markedly increased with WD but prevented by LDSp. IRS-1 expression and phosphorylation were markedly reduced by WD feeding but not improved by LDSp. Flow cytometry showed evidence towards reduced M2 macrophage polarization with lower CD11b/CD301 double positive cells in WD fed hearts with trend toward improvement with LDSp.

These findings suggest WD induces cardiac diastolic dysfunction in females by increasing oxidant stress and fibrosis potentially mediated by deficiency in anti-inflammatory M2 macrophages. MR antagonism with spironolactone may prevent the decrease in M2 macrophage polarization reducing oxidant stress and fibrosis. This work supports a novel mechanism for spironolactone in treatment of WD induced heart disease.

Abstract 030: Dpp4 Inhibition Substantially Reverses Ang II-mediated Inflammation And Kidney Injury Independent Of BP Reduction

Objectives: DPP4 is thought to play an important role in co-stimulation of T-lymphocytes and activation of DPP4 is thought to play a role in hypertension both via degradation of glucagon like peptide-1 (GLP-1) and via direct mechanisms. However, the triggers for DPP4 activation in hypertension are largely unknown except for one study showing insulin-mediated release of DPP4 from adipocytes. Recently, our lab demonstrated increased DPP4 activity in the plasma and kidney tissue of a Western diet-fed C57Bl/6 mouse that had elevated BP, proteinuria and inflammation with M2 to M1 macrophage polarization. In the same model, we observed activation of kidney renin-angiotensin system (unpublished data). Additionally, we observed that Ang II (angiotensin II) is able to increase DPP4 activity in proximal tubule cells in vitro (PTCs, (unpublished data). Therefore, we hypothesized that Ang II activation of DPP4 in mice will lead to inflammation in the kidney and albuminuria.

Methods: Pressor dose Ang II (1000ng/kg/min) was infused into C57Bl/6 mice for 3 wks and BP was measured in conscious mice by radiotelemetry. Kidney tissue was analyzed via DPP4 activity assay, FACS, Western Blot, light and electron microscopy.

Results: Ang II infused mice were characterized by elevated BP and albuminuria (4.5 fold) concomitant with mesangial widening and PTC injury. We also observed an increase in CD4+CD44 high CD62L low (1.5 fold), decrease in CD4+FoxP3 (1.5 fold) concomitant with increased expression of DPP4 on CD3+CD4+ T-cells. Saxagliptin (10mg/kg/day) administered in peanut butter starting at 1 wk after Ang II, decreased DPP4 activity (~50%) in the plasma and kidney tissue. Saxagliptin had no effect on BP and albuminuria although there was amelioration of mesangial widening and PTC injury. Saxagliptin also mitigated DPP4 expression on CD3+CD4+ T-cells and reversed the changes in memory T-cells and Tregs.

Conclusions: Ang II mediates BP increase and albuminuria via modulation of adaptive immunity which in turn may be regulated by DPP4. Saxagliptin reverses DPP4-mediated modulation of adaptive immunity and ameliorates kidney injury partially although it is unable to reverse established increases in BP and albuminuria.

Prevention of obesity-induced renal injury in male mice by DPP4 inhibition

Therapies to prevent renal injury in obese hypertensive individuals are being actively sought due to the obesity epidemic arising from the Western diet (WD), which is high in fructose and fat. Recently, activation of the immune system and hyperuricemia, observed with high fructose intake, have been linked to the pathophysiology of hypertension and renal injury. Because dipeptidyl peptidase 4 (DPP4) is a driver of maladaptive T-cell/macrophage responses, renal-protective benefits of DPP4 inhibition in the WD-fed mice were examined. Mice fed a WD for 16 weeks were given the DPP4 inhibitor MK0626 in their diet beginning at 4 weeks of age. WD-fed mice were obese, hypertensive, and insulin-resistant and manifested proteinuria and increased plasma DPP4 activity and uric acid levels. WD-fed mice also had elevated kidney DPP4 activity and monocyte chemoattractant protein-1 and IL-12 levels and suppressed IL-10 levels in the kidney, suggesting macrophage-driven inflammation, glomerular and tubulointerstitial injury. WD-induced increases in DPP4 activation in the plasma and kidney and proteinuria in WD mice were abrogated by MK0626, although blood pressure and systemic insulin sensitivity were not improved. Contemporaneously, MK0626 reduced serum uric acid levels, renal oxidative stress, and IL-12 levels and increased IL-10 levels, suggesting that suppression of DPP4 activity leads to suppression of renal immune/inflammatory injury responses to a WD. Taken together, these results demonstrate that DPP4 inhibition prevents high-fructose/high-fat diet-induced glomerular and tubular injury independent of blood pressure/insulin sensitivity and offers a potentially novel therapy for diabetic and obesity-related kidney disease.

Is there a role for the incretin system in blood pressure regulation?

Incretin-based therapies are now well established for diabetes management and are among the frontline agents for control of hyperglycemia. In addition to their antihyperglycemic effects, evidence is emerging on the role of these agents on blood pressure regulation, cardioprotective and renoprotective properties. Because of the pleiotropic nature of these affects, these agents could offer significant benefits with regards to the cardiorenal metabolic complications that are part of the diabetes and obesity epidemic in the United States and worldwide. We review the various known mechanisms or pathways by which incretin based therapy exerts its regulation of blood pressure with emphasis on novel mechanisms such as inflammation/immunomodulation and oxidative stress.

The role of tissue Renin-Angiotensin-aldosterone system in the development of endothelial dysfunction and arterial stiffness

Epidemiological studies support the notion that arterial stiffness is an independent predictor of adverse cardiovascular events contributing significantly to systolic hypertension, impaired ventricular-arterial coupling and diastolic dysfunction, impairment in myocardial oxygen supply and demand, and progression of kidney disease. Although arterial stiffness is associated with aging, it is accelerated in the presence of obesity and diabetes. The prevalence of arterial stiffness parallels the increase of obesity that is occurring in epidemic proportions and is partly driven by a sedentary life style and consumption of a high fructose, high salt, and high fat western diet. Although the underlying mechanisms and mediators of arterial stiffness are not well understood, accumulating evidence supports the role of insulin resistance and endothelial dysfunction. The local tissue renin-angiotensin-aldosterone system (RAAS) in the vascular tissue and immune cells and perivascular adipose tissue is recognized as an important element involved in endothelial dysfunction which contributes significantly to arterial stiffness. Activation of vascular RAAS is seen in humans and animal models of obesity and diabetes, and associated with enhanced oxidative stress and inflammation in the vascular tissue. The cross talk between angiotensin and aldosterone underscores the importance of mineralocorticoid receptors in modulation of insulin resistance, decreased bioavailability of nitric oxide, endothelial dysfunction, and arterial stiffness. In addition, both innate and adaptive immunity are involved in this local tissue activation of RAAS. In this review we will attempt to present a unifying mechanism of how environmental and immunological factors are involved in this local tissue RAAS activation, and the role of this process in the development of endothelial dysfunction and arterial stiffness and targeting tissue RAAS activation.

Resistance to insulin and kidney disease in the cardiorenal metabolic syndrome; role for angiotensin II

The presence of insulin resistance is increasingly recognized as an important contributor to early stage kidney disease independent of the contribution of diabetes. Important in this relationship is the strong correlation between hyperinsulinemia and low levels of albuminuria (e.g. microalbuminuria). Recent work highlight mechanisms for glomerular/tubulointerstitial injury with excess insulin and emerging evidence identifies a unique role for insulin metabolic signaling and altered handling of salt reabsorption at the level of the proximal tubule. Evidence is also emerging for the role of insulin signaling in the glomerulus both epithelial and endothelial. Central to the mechanism of injury is inappropriate activation of the RAAS.

Abstract 391: Enhanced Coronary Vasoconstriction in Western Diet-Induced Obesity is Associated with Alterations in NHE1, SERCA2a and 3

Cardiovascular disease (CVD) remains a major risk factor and cause of mortality in type 2 diabetes but, the effect of western diet (WD) on coronary artery vasoconstriction has not been well characterized. Therefore, the purpose of this study was to identify potential calcium (Ca 2+ ) molecular mechanism(s) underlying alterations in coronary constriction in WD-induced obesity. C57Bl/6J mice were fed a WD or control diet for 16wks. Coronary vascular function was assessed with wire myography and human coronary vascular smooth muscle cells (VSMC) were cultured. WD enhanced coronary constriction to the thromboxane analog U46619. This was associated with increased coronary VSMC expression of sodium-hydrogen exchanger 1 (NHE1) and SERCA3 with decreased expression of SERCA2a. Oxidation of SERCA2a cysteine-674, a signal for SERCA2a degradation, was increased in WD coronary VSMC. Exposure of cultured human coronary VSMC to hyperinsulinemic conditions elicited similar changes in SERCA2a, 3 and NHE1. This study suggests that enhanced thromboxane-mediated coronary constriction in WD and insulin resistance/hyperinsulinemia may result from NHE1-dependent changes in intracellular pH and/or impaired Ca 2+ handling at the level of the sarcoplasmic reticulum.

Abstract 371: DPPIV Inhibitor MK0626 Prevents Western Diet Induced Renal Injury via Suppression of Kidney Tissue Ras

Objectives: Obesity is an independent risk factor for development and progression of renal injury. High fructose corn syrup consumption has coincided with the obesity epidemic in the United States. High fructose (60%) diets have been demonstrated to be associated with elevation in BP and worsening insulin resistance along with renal injury via increased hepatic production of uric acid. Recently, DPPIV inhibitors have been shown to improve diabetic changes and sodium excretion, effects that are beyond glycemic control. Therefore, the renal protective benefits of DPPIV inhibition in a clinically relevant Western diet fed mouse model were examined.

Methods: Mice fed a high fat/high fructose (WD) diet for 16 weeks and given a DPPIV inhibitor MK0626 in their diet were examined for metabolic parameters, inflammation, kidney renin-angiotensin system (RAS) and oxidative stress. Renal injury was assessed by biochemical, immunohistological and electron microscopy techniques. In vitro , angiotensin II (Ang II) effects on OKP-PTCs were assessed for mechanism.

Results: MK0626 ameliorated WD-induced increases in serum uric acid, oxidative stress and RAS. WD induced suppression of IL-10 was reversed by MK0626. There was a tendency to improve HOMA-IR by MK0626 but no effect on BP and body weights. Diet induced DPPIV activation in the plasma and kidney of WD mice was abrogated by MK0626 (~80%). WD mice were characterized by increased proteinuria (~3-fold), mesangial expansion and podocyte effacement and these changes were prevented by MK0626. In addition, the PTC endocytosis protein megalin and basilar canalicular network and mitochondrial ultrastructure abnormalities were reversed by MK0626. WD mice had decreased sodium excretion which was improved by MK0626. Ang II directly increased DPPIV activity and sodium hydrogen exchanger activity in PTCs and decreased megalin protein, which was effectively prevented by MK0626.

Conclusion: Thus, WD induced increases in DPPIV activity is associated with elevations in uric acid, renal RAS, inflammation and oxidative stress which may result in renal injury. These results suggest that DPPIV inhibitors prevent WD induced renal injury and offer a novel therapy for diabetic and obesity associated renal disease.

Dipeptidylpeptidase inhibition is associated with improvement in blood pressure and diastolic function in insulin-resistant male Zucker obese rats

Diastolic dysfunction is a prognosticator for future cardiovascular events that demonstrates a strong correlation with obesity. Pharmacological inhibition of dipeptidylpeptidase-4 (DPP-4) to increase the bioavailability of glucagon-like peptide-1 is an emerging therapy for control of glycemia in type 2 diabetes patients. Accumulating evidence suggests that glucagon-like peptide-1 has insulin-independent actions in cardiovascular tissue. However, it is not known whether DPP-4 inhibition improves obesity-related diastolic dysfunction. Eight-week-old Zucker obese (ZO) and Zucker lean rats were fed normal chow diet or diet containing the DPP-4 inhibitor, linagliptin (LGT), for 8 weeks. Plasma DPP-4 activity was 3.3-fold higher in ZO compared with Zucker lean rats and was reduced by 95% with LGT treatment. LGT improved echocardiographic and pressure volume-derived indices of diastolic function that were impaired in ZO control rats, without altering food intake or body weight gain during the study period. LGT also blunted elevated blood pressure progression in ZO rats involving improved skeletal muscle arteriolar function, without reducing left ventricular hypertrophy, fibrosis, or oxidative stress in ZO hearts. Expression of phosphorylated- endothelial nitric oxide synthase (eNOS)(Ser1177), total eNOS, and sarcoplasmic reticulum calcium ATPase 2a protein was elevated in the LGT-treated ZO heart, suggesting improved Ca(2+) handling. The ZO myocardium had an abnormal mitochondrial sarcomeric arrangement and cristae structure that were normalized by LGT. These studies suggest that LGT reduces blood pressure and improves intracellular Cai(2+) mishandling and cardiomyocyte ultrastructure, which collectively result in improvements in diastolic function in the absence of reductions in left ventricular hypertrophy, fibrosis, or oxidative stress in insulin-resistant ZO rats.

Angiotensin type 1 receptor resistance to blockade in the opossum proximal tubule cell due to variations in the binding pocket

Blockade of the angiotensin (ANG) II receptor type 1 (AT(1)R) with angiotensin receptor blockers (ARBs) is widely used in the treatment of hypertension. However, ARBs are variably effective in reducing blood pressure, likely due, in part, to polymorphisms in the ARB binding pocket of the AT(1)R. Therefore, we need a better understanding of variations/polymorphisms that alter binding of ARBs in heterogeneous patient populations. The opossum proximal tubule cell (OKP) line is commonly used in research to evaluate renal sodium handling and therefore blood pressure. Investigating this issue, we found natural sequence variations in the opossum AT(1)R paralleling those observed in the human AT(1)R. Therefore, we posited that these sequence variations may explain ARB resistance. We demonstrate that OKP cells express AT(1)R mRNA, bind (125)I-labeled ANG II, and exhibit ANG II-induced phosphorylation of Jak2. However, Jak2 phosphorylation is not inhibited by five different ARBs commonly used to treat hypertension. Additionally, nonradioactive ANG II competes (125)I-ANG II efficiently, whereas a 10-fold molar excess of olmesartan and the ANG II receptor type 2 blocker PD-123319 is unable to block (125)I-ANG II binding. In contrast, ANG II binding to OKP cells stably expressing rat AT(1A)Rs, which have a conserved AT(1)R-binding pocket with human AT(1)R, is efficiently inhibited by olmesartan. A novel observation was that resistance to ARB binding to opossum AT(1)Rs correlates with variations from the human receptor at positions 108, 163, 192, and 198 within the ARB-binding pocket. These observations highlight the potential utility of evaluating AT(1)R polymorphisms within the ARB-binding pocket in various hypertensive populations.

DPP-4 Inhibitors as Therapeutic Modulators of Immune Cell Function and Associated Cardiovascular and Renal Insulin Resistance in Obesity and Diabetes

The prevalence of obesity and diabetes continues to rise in the United States and worldwide. These findings parallel the expansion of childhood obesity and diabetes. Obesity is a central component of the cardiorenal metabolic syndrome (CRS) which increases the risk for cardiovascular disease (CVD) and chronic kidney disease (CKD). The hallmark of obesity, CRS, and early type 2 diabetes is insulin resistance, a result of decreased insulin metabolic signaling due, in part, to enhanced serine phosphorylation and/or proteasome-mediated degradation of the insulin receptor substrate. Cardiovascular and renal insulin resistance significantly contributes to endothelial dysfunction, impaired cardiac diastolic and vascular relaxation, glomerular injury, and tubular dysfunction. In this context, multiple factors including oxidative stress, increased inflammation, and inappropriate activation of the renin-angiotensin-aldosterone and the sympathetic nervous system contribute to overweight- and obesity-induced systemic and tissue insulin resistance. One common link between obesity and the development of insulin resistance appears to be a low-grade inflammatory response resulting from dysfunctional innate and adaptive immunity. In this regard, there has been recent work on the role of dipeptidyl peptidase-4 (DPP-4) in modulating innate and adaptive immunity. The direct effects of DPP-4 on immune cells and the indirect effects through GLP-1-dependent and -independent pathways suggest effects of DPP-4 inhibition may have beneficial effects beyond glycemic control in improving CVD and renal outcomes. Accordingly, this review addresses new insights into the role of DPP-4 in immune modulation and the potential beneficial effects of DPP-4 inhibitors in insulin resistance and associated CVD and CKD prevention.

Abstract 606: DPP-4 Inhibition Improves Diastolic dysfunction in obese mice fed a Western Diet

Background and rationale: Obesity and diabetes are associated with insulin resistance, salt sensitive hypertension and cardiovascular dysfunction. Pharmacological inhibition of dipeptidylpeptidase-4 (DPP-4) to increase the bioavailability of glucagon like peptide-1 (GLP-1) is a contemporary therapy for control of glycemia in Type2 diabetes patients. Whether DPP-4 inhibition lowers blood pressures and improves pre-diabetic cardiomyopathy in a diet induced obesity (DIO) mouse model has not been examined in detail.

Methods: C57Bl/6 mice were fed high fat/high sucrose (Western diet, WD) for 16wks starting at ∼4 wks of age and treated with MK-0626 (10mg/kg/day, Merck).

Results: When compared to normal chow-fed mice, Western diet-fed (DIO) mice manifested increased MAP (15-20mmHg), heart (50%) and body weight (33%), increased blood sugars (FBS) and sodium retention (urine sodium 6.9μmol/day/gBW vs. 2.3μmol/day/gBW, respectively). In addition to correcting FBS, MK-0626 lowered MAP and increased sodium excretion and corrected cardiac diastolic dysfunction ( Table ). Indeed, echocardiographic indices of diastolic function, including tissue doppler E’/A’ ratio, estimates of LV filling pressure (E/E’), isovolemic relaxation time and the Tei index, all abnormal in mice fed a WD, were improved by MK-0626. Contemporaneous with these cardiac functional abnormalities, the mTOR/S6K and ERK1/2 growth signaling pathways were activated in DIO mice and improved by 16 wks of MK-0626 treatment.

Conclusion: These studies suggest that MK-0626 increases cardiac and renal bioavailability of GLP-1 and improves myocardial GLP-1R signaling, myocardial remodeling resulting in increased renal sodium excretion and improvements in cardiac diastolic function in pre-diabetic insulin resistant DIO mice.

Table: Summary of 2D M-Mode, pulse wave and tissue doppler Echo measurements on 20 wk old mice on control diet (CD), Western diet (WD), or Western Diet with MK-0626 (WD-MK). Values are mean ± SE. Numbers in parentheses are sample sizes. Paired t-tests: *p<0.05 CD vs WDC; † P<0.05 WDC vs WD-MKE, velocity of early mitral flow; E’, peak velocity of septal annulus; E/E’ index of LA filling pressure; Vp, flow propogation velocity; E/Vp, index of LV filling pressure; IVRT, isovolumic relaxation time; MPI, myocardial performance index.

Abstract 478: Is Ang II-mediated mTOR/S6K1 Activation in Proximal Tubule Cells mTORC2-dependent?

Introduction: Activation of the mTOR nutrient sensing pathway in early diabetes has been implicated in glomerulomegaly, hyperfiltration and hypertrophy of the nephrons. Recent studies show that excess/deletion of mTOR/raptor (mTORC1) in the podocytes results in diabetic changes. Interestingly, mTOR exists in another complex called mTORC2 (mTOR/rictor), which has been shown to phosphorylate Akt at S473 and is proposed to regulate cell motility and actin cytoskeleton. Recently, growth factors such as angiotensin II (Ang II) have been identified as activators of mTORC1 in cardiomyocytes, intestinal epithelial cells and embryonic cell. However, the precise mechanism of mTORC1 activation in the proximal tubule cells (PTC) is largely unknown. We hypothesized that Ang II activation of mTORC1 is dependent on mTORC2 activation.

Methods: Mice were subjected to Ang II (200ng/kg/min) via osmotic minipump and whole kidney lysates analyzed for mTORC1 activation. In parallel, acute Ang II (10 -7 M, 10min) treatment of opossum PTCs stably expressing rat AT1 B R was carried out on 24 hour starved cultures preceded by 1 hour pretreatment with various inhibitors.

Results: Ang II activated mTORC1/S6 kinase in kidney lysates (1.5-fold) and cultured PTCs (3-5 fold) as evidenced by increased phosphorylation of T389-S6K1, S240/244-RPS6 and T36/45-4EBP1. Ang II also activated mTORC2 (2-fold) as evidenced by p-S473-Akt. Upstream, Ang II caused phosphorylation of EGF receptor and inhibition of EGFR activation resulted in 30% inhibition of Akt activation and partial inhibition of mTOR/S6 kinase. Inhibition of PKC also resulted in partial inhibition of mTOR/S6 activation but the magnitude (50%) was higher than inhibition of EGFR signaling. Rapamycin (10nM) inhibited mTORC1 activation and not mTORC2 but PP242 (ATP site inhibitor, 8nM) completely abolished both mTORC1 and mTORC2 activation.

Conclusion: Ang II activation of mTOR/S6K follows a distinct pattern in PTC when compared to other cell types. Ang II induced activation of mTOR/S6 kinase is mediated by both PKC and EGFR activation. mTORC2 appears to play a central role in regulating Ang II induced activation of mTOR/S6 kinase.

Anemia and chronic kidney disease: making sense of the recent trials

Anemia is a very common complication of chronic kidney disease (CKD). Anemia confers significant risk of cardiovascular disease and contributes to decreased quality of life. Anemia in CKD patients can be multi-factorial, including but not invariably due to the underlying renal insufficiency. Identifying the type of anemia is important in this group of patients and can often be challenging. Diagnosing anemia of renal disease due to erythropoietin (EPO) deficiency is a diagnosis of exclusion. Erythropoiesis stimulating agents (ESA) are the mainstay for the treatment of anemia secondary to CKD. However, over the last four years the use of ESA in the treatment of anemia in CKD patients has undergone a severe interrogation as several trials have reported adverse outcomes with targeting higher hemoglobin (Hb) levels with these agents. Thereby, this review describes the pathophysiology of anemia in CKD patients, diagnosis and the current role of ESA's as it relates to anemia of CKD as well as safety and efficacy of ESA's.

Gestational Diabetes and the Offspring: Implications in the Development of the Cardiorenal Metabolic Syndrome in Offspring

The risk of developing type 2 diabetes and cardiovascular disease in women who had previously been diagnosed with gestational diabetes (GDM) is well established. There is increasing evidence that the offspring of women with GDM are at increased risk for the development of all components of the cardiorenal metabolic syndrome. Overall, it appears that these offspring have an increased risk for overweight/obesity, insulin resistance, higher blood pressure, renal disease, and type 2 diabetes. However, distinct differences in regional populations, lack of routine screening and treatment of GDM worldwide, and long follow-up periods for offspring represent a challenge in assessing the risk for development of these abnormalities in the offspring of women who have had GDM.

Combination of direct renin inhibition with angiotensin type 1 receptor blockade improves aldosterone but does not improve kidney injury in the transgenic Ren2 rat

Enhanced renin-angiotensin-aldosterone system (RAAS) activation contributes to proteinuria and chronic kidney disease by increasing glomerular and tubulointerstitial oxidative stress, promotion of fibrosis. Renin activation is the rate limiting step in angiotensin (Ang II) and aldosterone generation, and recent work suggests direct renin inhibition improves proteinuria comparable to that seen with Ang type 1 receptor (AT(1)R) blockade. This is important as, even with contemporary use of AT(1)R blockade, the burden of kidney disease remains high. Thereby, we sought to determine if combination of direct renin inhibition with AT(1)R blockade in vivo, via greater attenuation of kidney oxidative stress, would attenuate glomerular and proximal tubule injury to a greater extent than either intervention alone. We utilized the transgenic Ren2 rat with increased tissue RAS activity and higher serum levels of aldosterone, which manifests hypertension and proteinuria. Ren2 rats were treated with renin inhibition (aliskiren), AT(1)R blockade (valsartan), the combination (aliskiren+valsartan), or vehicle for 21days. Compared to Sprague-Dawley controls, Ren2 rats displayed increased systolic pressure (SBP), circulating aldosterone, proteinuria and greater urine levels of the proximal tubule protein excretory marker beta-N-acetylglucosaminidase (β-NAG). These functional and biochemical alterations were accompanied by increases in kidney tissue NADPH oxidase subunit Rac1 and 3-nitrotyrosine (3-NT) content as well as fibronectin and collagen type III. These findings occurred in conjunction with reductions in the podocyte-specific protein podocin as well as the proximal tubule-specific megalin. Further, in transgenic animals there was increased tubulointerstitial fibrosis on light microscopy as well as ultrastructural findings of glomerular podocyte foot-process effacement and reduced tubular apical endosomal/lysosomal activity. Combination therapy led to greater reductions in SBP and serum aldosterone, but did not result in greater improvement in markers of glomerular and tubular injury (i.e. β-NAG) compared to either intervention alone. Further, combination therapy did not improve markers of oxidative stress and podocyte and proximal tubule integrity in this transgenic model of RAAS-mediated kidney damage despite greater reductions in serum aldosterone and BP levels.

Mineralocorticoid receptor-dependent proximal tubule injury is mediated by a redox-sensitive mTOR/S6K1 pathway

BACKGROUND/AIMS

The mammalian target of rapamycin (mTOR) is a serine kinase that regulates phosphorylation (p) of its target ribosomal S6 kinase (S6K1), whose activation can lead to glomerular and proximal tubular cell (PTC) injury and associated proteinuria. Increased mTOR/S6K1 signaling regulates signaling pathways that target fibrosis through adherens junctions. Recent data indicate aldosterone signaling through the mineralocorticoid receptor (MR) can activate the mTOR pathway. Further, antagonism of the MR has beneficial effects on proteinuria that occur independent of hemodynamics.

METHODS

Accordingly, hypertensive transgenic TG(mRen2)27 (Ren2) rats, with elevated serum aldosterone and proteinuria, and age-matched Sprague-Dawley rats were treated with either a low dose (1 mg/kg/day) or a conventional dose (30 mg/kg/day) of spironolactone (MR antagonist) or placebo for 3 weeks.

RESULTS

Ren2 rats displayed increases in urine levels of the PTC brush border lysosomal enzyme N-acetyl-β-aminoglycosidase (β-NAG) in conjunction with reductions in PTC megalin, the apical membrane adherens protein T-cadherin and basolateral α-(E)-catenin, and fibrosis. In concert with these abnormalities, Ren2 renal cortical tissue also displayed increased Ser2448 (p)/activation of mTOR and Thr389 (p)-S6K1 and increased 3-nitrotyrosine (3-NT) content, a marker for peroxynitrite. Low-dose spironolactone had no effect on blood pressure but decreased proteinuria and β-NAG comparable to a conventional dose of this MR antagonist. Both doses of spironolactone attenuated ultrastructural maladaptive alterations and led to comparable reductions in (p)-mTOR/(p)-S6K1, 3-NT, fibrosis, and increased expression of α-(E)-catenin, T- and N-cadherin.

CONCLUSIONS

Thereby, MR antagonism improves proximal tubule integrity by targeting mTOR/S6K1 signaling and redox status independent of changes in blood pressure.