Diet induced obesity in rats reduces NHE3 and Na(+) /K(+) -ATPase expression in the kidney

The consumption of a high fat diet (HFD) is associated with proteinuria and altered sodium handling and excretion, which can lead to kidney disease. In the proximal tubule, the Na(+) /H(+) Exchanger 3 (NHE3) is responsible for normal protein reabsorption and the reabsorption of approximately 70% of the renal sodium load. It is the Na(+) /K(+) -ATPase that provides the driving force for the reabsorption of sodium and its exit across the basolateral membrane. This study investigates the effects that consumption of a HFD for 12 weeks has on NHE3 and Na(+) /K(+) -ATPase expression in the kidney. Western blot analysis identified a significant reduction in NHE3 and its modulator, phosphorylated protein kinase B, in renal lysate from obese rats. In the obese rats, a reduction in NHE3 expression in the proximal tubule may impact on the acidification of endosomes which are responsible for albumin uptake, suggesting a key role for the exchanger in protein endocytosis in obesity. Western blot analysis identified a reduction in Na(+) /K(+) -ATPase which could also potentially impact on albumin uptake and sodium reabsorption. This study demonstrates that consumption of a HFD for 12 weeks reduces renal NHE3 and Na(+) /K(+) -ATPase expression, an effect that may contribute to the albuminuria associated with obesity. Furthermore the reduction in these transporters is not likely to contribute to the reduced sodium excretion in obesity. These data highlight a potential link between NHE3 and Na(+) /K(+) -ATPase in the pathophysiological changes in renal protein handling observed in obesity.

Renal effects of chronic pharmacological manipulation of CB2 receptors in rats with diet-induced obesity

BACKGROUND AND PURPOSE

In diabetic nephropathy agonism of CB2 receptors reduces albuminuria and podocyte loss; however, the role of CB2 receptors in obesity-related nephropathy is unknown. The aim of this study was to determine the role of CB2 receptors in a model of diet-induced obesity (DIO) and characterize the hallmark signs of renal damage in response to agonism (AM1241) and antagonism (AM630) of CB2 receptors.

EXPERIMENTAL APPROACH

Male Sprague Dawley rats were fed a high-fat diet (HFD: 40% digestible energy from lipids) for 10 weeks. In another cohort, after 9 weeks on a HFD, rats were injected daily with either 3 mg·kg(-1) AM1241, 0.3 mg·kg(-1) AM630 or saline for 6 weeks.

KEY RESULTS

Ten weeks on a HFD significantly reduced renal expression of CB2 receptors and renal function. Treatment with AM1241 or AM630 did not reduce weight gain or food consumption in DIO. Despite this, AM1241 significantly reduced systolic BP, peri-renal adipose accumulation, plasma leptin, urinary protein, urinary albumin, urinary sodium excretion and the fibrotic markers TGF-β1, collagen IV and VEGF in kidney lysate. Treatment with AM630 of DIO rats significantly reduced creatinine clearance and increased glomerular area and kidney weight (gross and standardized for body weight). Diastolic BP, glucose tolerance, insulin sensitivity, plasma creatinine, plasma TGF-β1 and kidney expression of fibronectin and α-smooth muscle actin were not altered by either AM1241 or AM630 in DIO.

CONCLUSIONS

This study demonstrates that while agonism of CB2 receptors with AM1241 treatment for 6 weeks does not reduce weight gain in obese rats, it leads to improvements in obesity-related renal dysfunction.

LINKED ARTICLES

This article is part of a themed section on Endocannabinoids. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v173.7/issuetoc.

The cannabinoid receptor 1 and its role in influencing peripheral metabolism

Evidence from in vitro and in vivo studies has demonstrated the deleterious pathological effects of a dysregulated endocannabinoid system. Increased stimulation of the cannabinoid receptor 1 (CB1 ) and subsequent downstream cellular signalling are both causative in the deleterious pathological effects observed in a number of diseases. When the CB1 cell signalling cascade is blocked, this results in whole body weight-loss, leading to a reduction in obesity and associated co-morbidities. In the central nervous system; however, CB1 antagonism results in adverse psychological side effects. Blockade of CB1 via peripheral acting compounds that do not cross the blood-brain barrier have been determined to have beneficial effects in metabolic tissues such as the liver and skeletal muscle. These results support the notion that peripheral blockade of CB1 using pharmacological antagonists is a viable target for the treatment of the current epidemic of obesity and its associated co-morbidities.

GPR119 regulates genetic markers of fatty acid oxidation in cultured skeletal muscle myotubes

Gene knockout and agonist studies indicate that activation of the G protein-coupled receptor, GPR119, protects against diet-induced obesity and insulin resistance. It is not known if GPR119 activation in skeletal muscle mediates these effects. To address this uncertainty, we measured GPR119 expression in skeletal muscle and determined the effects of PSN632408, a GPR119 agonist, on the expression of genes and proteins required for fatty acid and glucose oxidation in cultured myotubes. GPR119 expression was readily detected in rat skeletal muscle and mRNAs were induced by 12 weeks of high-fat feeding. Treatment of cultured mouse C₂C₁₂ myotubes with 5 μM PSN632408 or 0.5 mM palmitate reduced expression of mRNAs encoding fatty acid oxidation genes to similar extents. More so, treatment with PSN632408 decreased AMPKα (Thr172 phosphorylation) activity in the absence of palmitate and ACC (Ser79 phosphorylation) activity in the presence of palmitate. In human primary myotubes PSN632408 decreased expression of PDK4 and AMPKα2 mRNA in myotubes derived from obese donors. These data suggest GPR119 activation in skeletal muscle may impair fatty acid and glucose oxidation.

The interaction between megalin and ClC-5 is scaffolded by the Na⁺-H⁺ exchanger regulatory factor 2 (NHERF2) in proximal tubule cells

Albumin endocytosis in the proximal tubule is mediated by a number of proteins, including the scavenger receptor megalin/cubilin and the PSD-95/Dlg/ZO-1 (PDZ) scaffolds NHERF1 and NHERF2. In addition, in a number of in vitro and in vivo models, the loss of ClC-5 results in a decreased cell surface expression and whole cell level of megalin, suggesting an interaction between these two proteins in vivo. We investigated if ClC-5 and megalin interact directly, and as ClC-5 binds to NHERF2, we investigated if this PDZ scaffold was required for a megalin/ClC-5 complex. GST-pulldown and immunoprecipitation experiments using rat kidney lysate demonstrated an interaction between ClC-5 and megalin, which was mediated by their C-termini. As this interaction may be controlled by a scaffold protein, we characterised any interaction between megalin and NHERF2. Immunoprecipitation experiments indicated that megalin interacts with NHERF2 in vivo, and that this interaction was via an internal NHERF binding domain in the C-terminus of megalin and PDZ2 and the C-terminus of NHERF2. Silencing NHERF2 had no effect on megalin protein levels in the whole cell or plasma membrane. Using siRNA against NHERF2, we demonstrated that NHERF2 was required to facilitate the interaction between megalin and ClC-5. Using fusion proteins, we characterised a protein complex containing ClC-5 and megalin, which is scaffolded by NHERF2, in the absence of any other proteins. Importantly, these observations are the first to describe an interaction between megalin and ClC-5, which is scaffolded by NHERF2 in proximal tubule cells.

Brown adipocyte progenitor population is modified in obese and diabetic skeletal muscle

Brown adipose tissue mitochondria express the unique thermogenic uncoupling protein-1. Recently, brown adipocyte progenitors have been identified in the CD34+ cell population of human skeletal muscle. The aims of this study were firstly to determine if obesity and diabetes have altered amounts of muscle brown adipocyte progenitors and, secondly, to establish if the latter are correlated with clinical parameters of obesity and diabetes. Body mass index (BMI), plasma glucose, insulin, cholesterol and triglycerides as well as homeostasis model assessment were measured in lean (n=10), obese (n=18) and obese-diabetic (n=15) subjects and muscle biopsies were taken from the rectus abdominus. CD34 being also expressed on endothelial cells, we measured CD31, another endothelial marker, and expressed the brown adipocyte progenitors, as the CD34/CD31 mRNA ratio. The latter was significantly reduced in the obese vs lean subjects suggesting a smaller pool of brown adipocyte progenitors. More strikingly, for lean and obese subjects negative correlations were observed between the CD34/CD31 mRNA ratios and BMI, fasting insulin levels and homeostasis model assessment. These correlations highlight the potential physiological relevance of the muscle CD34/CD31 mRNA ratio.

Adiponectin decreases pyruvate dehydrogenase kinase 4 gene expression in obese- and diabetic-derived myotubes

AIM

To investigate the effects of globular adiponectin (gAd) on gene expression and whether these effects are mediated through 3',5'-cyclic monophosphate-activated protein kinase in skeletal muscle myotubes obtained from lean, obese and obese diabetic individuals.

METHODS

Rectus abdominus muscle biopsies were obtained from surgical patients to establish primary skeletal muscle cell cultures. Three distinct primary cell culture groups were established (lean, obese and obese diabetic; n = 7 in each group). Once differentiated, these cultures were then exposed to gAd or 5-aminoimidazole-4-carboxamide-1-beta-d-ribofuranoside (AICAR) for 6 h.

RESULTS

Stimulation with gAd decreased pyruvate dehydrogenase kinase 4 (PDK4) gene expression in the obese and diabetic samples (p < or = 0.05) and increased cytochrome c oxidase (COX) subunit 4 (COXIV) gene expression in the myotubes derived from lean individuals only (p < 0.05). AICAR treatment also decreased PDK4 gene expression in the obese- and diabetic-derived myotubes (p < or = 0.05) and increased the gene expression of the mitochondrial gene, COXIII, in the lean-derived samples only (p < 0.05).

CONCLUSIONS

This study demonstrated distinct disparity between myotubes derived from lean compared with obese and obese diabetic individuals following gAd and AICAR treatment. Further understanding of the regulation of PDK4 in obese and diabetic skeletal muscle and its interaction with adiponectin signalling is required as this appears to be an important early molecular event in these disease states that may improve blood glucose control and metabolic flux.

Effects of cannabinoid receptors on skeletal muscle oxidative pathways

The endocannabinoids, a recently discovered endogenous, lipid derived, signaling system regulating energy metabolism, have effects on central and peripheral energy metabolism predominantly via the cannabinoid receptor type 1 (CB1). CB1 is expressed centrally in the hypothalamus and nucleus accumbens and peripherally in adipocytes and skeletal muscle. This study determined the effect of endocannabinoids on the expression of genes regulating energy metabolism in human skeletal muscle. Primary cultures of myotubes (lean and obese; n=3/group) were treated with the cannabinoid receptor agonist, anandamide (AEA) (0.2 and 5microM) and the CB1 specific antagonist AM251 (0.2 and 5microM) separately and in combination for 24h. The expression of mRNA for AMP-activated protein kinase (AMPK) alpha 1 (alpha1) and alpha 2 (alpha2), pyruvate dehydrogenase kinase 4 (PDK4) and peroxisome proliferator-activated receptor-gamma co-activator-1alpha (PGC-1alpha) were determined using 'Real Time' RT-PCR. AMPKalpha1 mRNA increased in lean and obese myotubes in response to AM251 (P<0.05). AEA inhibited the effect of AM251 on AMPKalpha1 mRNA levels in myotubes from lean and obese subjects (P<0.05); the dose-response curve was shifted to the left in the obese. In response to AM251, irrespective of the presence of AEA, PDK4 expression was decreased in lean and obese myotubes (P<0.05). Taken together these data suggest that endocannabinoids regulate pathways affecting skeletal muscle oxidation, effects particularly evident in myotubes from obese individuals.