Abundance of the Na-K-2Cl cotransporter NKCC2 is increased by high-fat feeding in Fischer 344 X Brown Norway (F1) rats

Renal NKCC2 Is Dual Regulated by the Synergy of 20-HETE and High-Fat Diet in CYP4F2 Transgenic Mice

INTRODUCTION

20-Hydroxyeicosatetraenoic acid (20-HETE) is the metabolite of cytochrome P450, which modulates blood pressure by inhibiting renal sodium transport. However, the molecular mechanisms underlying the role of 20-HETE in the development of obesity-related hypertension remain unclear, necessitating this study.

METHODS

Cytochrome P450 4F2 (CYP4F2) transgenic mice fed high-fat diet (HFD) were used as research animal models. The expression of renal ion transport molecules targeted by 20-HETE was evaluated by real-time PCR and Western blot (WB). The regulatory effect of 20-HETE and HFD on renal Na+-K+-2Cl- cotransporter, isoform 2 (NKCC2) was explored by immunoprecipitation, WB, and luciferase assay.

RESULTS

A 2-week HFD feeding dramatically decreased protein abundance but increased renal NKCC2 mRNA expression in CYP4F2 transgenic mice. The decrease in NKCC2 protein was demonstrated to be due to ubiquitination induced by the synergy between 20-HETE and HFD. The increased PPAR-γ protein in CYP4F2 transgenic mice fed HFD and the activation of rosiglitazone on the luciferase reporter construct of the NKCC2 promoter demonstrated that the increase in NKCC2 mRNA in CYP4F2 transgenic mice fed HFD was a consequence of elevated PPAR-γ protein induced by the synergy between 20-HETE and HFD.

CONCLUSIONS

Our data demonstrated that the synergy between 20-HETE and HFD could decrease NKCC2 protein via posttranslational ubiquitination, which was thought to be the main mechanism underlying the short-term effect in response to HFD and might be responsible for the adaptive modulation of renal NKCC2 to resist sodium retention. Moreover, the increased NKCC2 mRNA expression via PPAR-γ-induced transcriptional regulation was thought to be the main mechanism underlying the long-term effect in response to HFD and plays a pivotal role in the development of obesity-related hypertension.

Proximal tubular epithelial insulin receptor mediates high-fat diet-induced kidney injury

The role of insulin receptor (IR) activated by hyperinsulinemia in obesity-induced kidney injury is not well understood. We hypothesized that activation of kidney proximal tubule epithelial IR contributes to obesity-induced kidney injury. We administered normal-fat diet (NFD) or high-fat diet (HFD) to control and kidney proximal tubule IR–knockout (KPTIRKO) mice for 4 months. Renal cortical IR expression was decreased by 60% in male and female KPTIRKO mice. Baseline serum glucose, serum creatinine, and the ratio of urinary albumin to creatinine (ACR) were similar in KPTIRKO mice compared to those of controls. On HFD, weight gain and increase in serum cholesterol were similar in control and KPTIRKO mice; blood glucose did not change. HFD increased the following parameters in the male control mice: renal cortical contents of phosphorylated IR and Akt, matrix proteins, urinary ACR, urinary kidney injury molecule-1–to-creatinine ratio, and systolic blood pressure. Renal cortical generation of hydrogen sulfide was reduced in HFD-fed male control mice. All of these parameters were ameliorated in male KPTIRKO mice. Interestingly, female mice were resistant to HFD-induced kidney injury in both genotypes. We conclude that HFD-induced kidney injury requires renal proximal tubule IR activation in male mice.

Obesity, kidney dysfunction and hypertension: mechanistic links

Excessive adiposity raises blood pressure and accounts for 65-75% of primary hypertension, which is a major driver of cardiovascular and kidney diseases. In obesity, abnormal kidney function and associated increases in tubular sodium reabsorption initiate hypertension, which is often mild before the development of target organ injury. Factors that contribute to increased sodium reabsorption in obesity include kidney compression by visceral, perirenal and renal sinus fat; increased renal sympathetic nerve activity (RSNA); increased levels of anti-natriuretic hormones, such as angiotensin II and aldosterone; and adipokines, particularly leptin. The renal and neurohormonal pathways of obesity and hypertension are intertwined. For example, leptin increases RSNA by stimulating the central nervous system proopiomelanocortin-melanocortin 4 receptor pathway, and kidney compression and RSNA contribute to renin-angiotensin-aldosterone system activation. Glucocorticoids and/or oxidative stress may also contribute to mineralocorticoid receptor activation in obesity. Prolonged obesity and progressive renal injury often lead to the development of treatment-resistant hypertension. Patient management therefore often requires multiple antihypertensive drugs and concurrent treatment of dyslipidaemia, insulin resistance, diabetes and inflammation. If more effective strategies for the prevention and control of obesity are not developed, cardiorenal, metabolic and other obesity-associated diseases could overwhelm health-care systems in the future.

Thick Ascending Limb Sodium Transport in the Pathogenesis of Hypertension

The thick ascending limb plays a key role in maintaining water and electrolyte balance. The importance of this segment in regulating blood pressure is evidenced by the effect of loop diuretics or local genetic defects on this parameter. Hormones and factors produced by thick ascending limbs have both autocrine and paracrine effects, which can extend prohypertensive signaling to other structures of the nephron. In this review, we discuss the role of the thick ascending limb in the development of hypertension, not as a sole participant, but one that works within the rich biological context of the renal medulla. We first provide an overview of the basic physiology of the segment and the anatomical considerations necessary to understand its relationship with other renal structures. We explore the physiopathological changes in thick ascending limbs occurring in both genetic and induced animal models of hypertension. We then discuss the racial differences and genetic defects that affect blood pressure in humans through changes in thick ascending limb transport rates. Throughout the text, we scrutinize methodologies and discuss the limitations of research techniques that, when overlooked, can lead investigators to make erroneous conclusions. Thus, in addition to advancing an understanding of the basic mechanisms of physiology, the ultimate goal of this work is to understand our research tools, to make better use of them, and to contextualize research data. Future advances in renal hypertension research will require not only collection of new experimental data, but also integration of our current knowledge.

Renal tubule insulin receptor modestly promotes elevated blood pressure and markedly stimulates glucose reabsorption

Although the cause of hypertension among individuals with obesity and insulin resistance is unknown, increased plasma insulin, acting in the kidney to increase sodium reabsorption, has been proposed as a potential mechanism. Insulin may also stimulate glucose uptake, but the contributions of tubular insulin signaling to sodium or glucose transport in the setting of insulin resistance is unknown. To directly study the role of insulin signaling in the kidney, we generated inducible renal tubule-specific insulin receptor-KO mice and used high-fat feeding and mineralocorticoids to model obesity and insulin resistance. Insulin receptor deletion did not alter blood pressure or sodium excretion in mice on a high-fat diet alone, but it mildly attenuated the increase in blood pressure with mineralocorticoid supplementation. Under these conditions, KO mice developed profound glucosuria. Insulin receptor deletion significantly reduced SGLT2 expression and increased urinary glucose excretion and urine flow. These data demonstrate a direct role for insulin receptor-stimulated sodium and glucose transport and a functional interaction of insulin signaling with mineralocorticoids in vivo. These studies uncover a potential mechanistic link between preserved insulin sensitivity and renal glucose handling in obesity and insulin resistance.

High Fat Diets Sex-Specifically Affect the Renal Transcriptome and Program Obesity, Kidney Injury, and Hypertension in the Offspring

Obesity and related disorders have increased concurrently with an increased consumption of saturated fatty acids. We examined whether post-weaning high fat (HF) diet would exacerbate offspring vulnerability to maternal HF-induced programmed hypertension and kidney disease sex-specifically, with a focus on the kidney. Next, we aimed to elucidate the gene–diet interactions that contribute to maternal HF-induced renal programming using the next generation RNA sequencing (NGS) technology. Female Sprague-Dawley rats received either a normal diet (ND) or HF diet (D12331, Research Diets) for five weeks before the delivery. The offspring of both sexes were put on either the ND or HF diet from weaning to six months of age, resulting in four groups of each sex (maternal diet/post-weaning diet; n = 5–7/group): ND/ND, ND/HF, HF/ND, and HF/HF. Post-weaning HF diet increased bodyweights of both ND/HF and HF/HF animals from three to six months only in males. Post-weaning HF diet increased systolic blood pressure in male and female offspring, irrespective of whether they were exposed to maternal HF or not. Male HF/HF offspring showed greater degrees of glomerular and tubular injury compared to the ND/ND group. Our NGS data showed that maternal HF diet significantly altered renal transcriptome with female offspring being more HF-sensitive. HF diet induced hypertension and renal injury are associated with oxidative stress, activation of renin-angiotensin system, and dysregulated sodium transporters and circadian clock. Post-weaning HF diet sex-specifically exacerbates the development of obesity, kidney injury, but not hypertension programmed by maternal HF intake. Better understanding of the sex-dependent mechanisms that underlie HF-induced renal programming will help develop a novel personalized dietary intervention to prevent obesity and related disorders.

Luminal flow induces NADPH oxidase 4 translocation to the nuclei of thick ascending limbs

Superoxide (O₂⁻) exerts its physiological actions in part by causing changes in gene transcription. In thick ascending limbs flow‐induced O₂⁻ production is mediated by NADPH oxidase 4 (Nox4) and is dependent on protein kinase C (PKC). Polymerase delta interacting protein 2 (Poldip2) increases Nox4 activity, but it is not known whether Nox4 translocates to the nucleus and whether Poldip2 participates in this process. We hypothesized that luminal flow causes Nox4 translocation to the nuclei of thick ascending limbs in a PKC‐dependent process facilitated by Poldip2. To test our hypothesis, we studied the subcellular localization of Nox4 and Poldip2 using confocal microscopy and O₂⁻ production in the absence and presence of luminal flow. Luminal flow increased the ratio of nuclear to cytoplasmic intensity of Nox4 (N/C) from 0.3 ± 0.1 to 0.7 ± 0.1 (P < 0.01) and O₂⁻ production from 89 ± 15 to 231 ± 16 AU/s (P < 0.001). In the presence of flow PKC inhibition reduced N/C from 0.5 ± 0.1 to 0.2 ± 0.1 (P < 0.01). Flow‐induced O₂⁻ production was also blocked (flow: 142 ± 20 AU/s; flow plus PKC inhibition 26 ± 12 AU/s; P < 0.01). The cytoskeleton disruptor cytochalasin D (1 μmol/L) decreased flow‐induced Nox4 translocation by 0.3 ± 0.01 (P < 0.01); however, it did not reduce flow‐induced O₂⁻. Flow did not alter Poldip2 localization. We conclude that: (1) luminal flow elicits Nox4 translocation to the nucleus in a PKC‐ and cytoskeleton‐dependent process; (2) Nox4 activation occurs before translocation; and (3) Poldip2 is not involved in Nox4 nuclear translocation. Flow‐induced Nox4 translocation to the nucleus may play a role in O₂⁻‐dependent changes in thick ascending limbs.

Novel mechanisms of Na+ retention in obesity: phosphorylation of NKCC2 and regulation of SPAK/OSR1 by AMPK

Enhanced tubular reabsorption of salt is important in the pathogenesis of obesity-related hypertension, but the mechanisms remain poorly defined. To identify changes in the regulation of salt transporters in the kidney, C57BL/6 mice were fed a 40% fat diet [high-fat diet (HFD)] or a 12% fat diet (control diet) for 14 wk. Compared with control diet-fed mice, HFD-fed mice had significantly greater elevations in weight, blood pressure, and serum insulin and leptin levels. When we examined Na(+) transporter expression, Na(+)-K(+)-2Cl(-) cotransporter (NKCC2) was unchanged in whole kidney and reduced in the cortex, Na(+)-Cl(-) cotransporter (NCC) and α-epithelial Na(+) channel (ENaC) and γ-ENaC were unchanged, and β-ENaC was reduced. Phosphorylation of NCC was unaltered. Activating phosphorylation of NKCC2 at S126 was increased 2.5-fold. Activation of STE-20/SPS1-related proline-alanine-rich protein kinase (SPAK)/oxidative stress responsive 1 kinase (OSR1) was increased in kidneys from HFD-fed mice, and enhanced phosphorylation of NKCC2 at T96/T101 was evident in the cortex. Increased activity of NKCC2 in vivo was confirmed with diuretic experiments. HFD-fed mice had reduced activating phosphorylation of AMP-activated protein kinase (AMPK) in the renal cortex. In vitro, activation of AMPK led to a reduction in phospho-SPAK/phospho-OSR1 in AMPK(+/+) murine embryonic fibroblasts (MEFs), but no effect was seen in AMPK(-/-) MEFs, indicating an AMPK-mediated effect. Activation of the with no lysine kinase/SPAK/OSR1 pathway with low-NaCl solution invoked a greater elevation in phospho-SPAK/phospho-OSR1 in AMPK(-/-) MEFs than in AMPK(+/+) MEFs, consistent with a negative regulatory effect of AMPK on SPAK/OSR1 phosphorylation. In conclusion, this study identifies increased phosphorylation of NKCC2 on S126 as a hitherto-unrecognized mediator of enhanced Na(+) reabsorption in obesity and identifies a new role for AMPK in regulating the activity of SPAK/OSR1.

Hydrogen sulphide and tempol treatments improve the blood pressure and renal excretory responses in spontaneously hypertensive rats

Oxidative stress and suppressed H2S production lead to increased renal vascular resistance, disturbed glomerular hemodynamics, and abnormal renal sodium and water handling, contribute to the pathogenesis and maintenance of essential hypertension in man and the spontaneously hypertensive rat. This study investigated the impact of H2S and tempol alone and in combination on blood pressure and renal hemodynamics and excretory functions in the SHR. Groups of WKY rats or SHR (n=6) were treated for 4 weeks either as controls or received NaHS (SHR+NaHS), tempol (SHR+Tempol), or NaHS plus tempol (SHR+NaHS +Tempol). Metabolic studies were performed on days 0, 14, and 28, thereafter animals were anaesthetized to measure renal hemodynamics and plasma oxidative and antioxidant markers. SHR control rats had higher mean arterial blood pressure (140.0 ± 2 vs. 100.0 ± 3 mmHg), lower plasma and urinary H2S, creatinine clearance, urine flow rate and urinary sodium excretion, and oxidative stress compared to WKY (all p<0.05). Treatment either with NaHS or with tempol alone decreased blood pressure and oxidative stress and improved renal hemodynamic and excretory function compared to untreated SHR. Combined NaHS and tempol therapy in SHRs caused larger decreases in blood pressure (∼20-22% vs. ∼11-15% and ∼10-14%), increases in creatinine clearance, urinary sodium excretion and fractional sodium excretion and up-regulated the antioxidant status compared to each agent alone (all p<0.05). These findings demonstrated that H2S and tempol together resulted in greater reductions in blood pressure and normalization of kidney function compared with either compound alone.

Insulin increases the functional activity of the renal NaCl cotransporter

OBJECTIVES

Insulin is recognized to increase renal salt reabsorption in the distal nephron and hyperinsulinemic states have been shown to be associated with increased expression of the renal NaCl cotransporter (NCC). However, the effect of insulin on NCC functional activity has not been reported.

METHODS

Using a heterologous expression system of Xenopus laevis oocytes, a mouse distal convoluted cell line, mDCT15 cells, endogenously expressing NCC, and an ex-vivo kidney perfusion technique, we assessed the effect of insulin on the activity and phosphorylation of NCC. The signaling pathway involved was analyzed.

RESULTS

In Xenopus oocytes insulin increases the activity of NCC together with its phosphorylation at threonine residue 58. Activation of NCC by insulin was also observed in mDCT15 cells. Additionally, insulin increased the NCC phosphorylation in kidney under the ex-vivo perfusion technique. In oocytes and mDCT15 cells, insulin effect on NCC was prevented with inhibitors of phosphatidylinositol 3-kinase (PI3K), mTORC2, and AKT1 kinases, but not by inhibitors of MAP or mTORC1 kinases, suggesting that PI3K-mTORC2-AKT1 is the intracellular pathway required. Additionally, activation of NCC by insulin was not affected by wild-type or mutant versions of with no lysine kinase 1, with no lysine kinase 4, or serum glucocorticoid kinase 1, but it was no longer observed in the presence of wild-type or the dominant negative, catalytically inactive with no lysine kinase 3, implicating this kinase in the process.

CONCLUSION

Insulin induces activation and phosphorylation of NCC. This effect could play an important role in arterial hypertension associated with hyperinsulinemic states, such as obesity, metabolic syndrome, or type 2 diabetes mellitus.

Influence of maternal low protein diet during pregnancy on hepatic gene expression signature in juvenile female porcine offspring

SCOPE

Epidemiological and experimental evidence indicates that maternal nutrition status contributes to long-term changes in the metabolic phenotype of the offspring, a process known as fetal programming.

METHODS AND RESULTS

We have used a swine model (Sus scrofa) to analyze consequences of a maternal low protein diet (about 50% of control) during pregnancy on hepatic lipid metabolism and genome-wide hepatic gene expression profile of juvenile female offspring (mean age 85 days). We found 318 S. scrofa genes to be differentially expressed in the liver at age 85 days. In the low protein offspring group key genes of fatty acid de novo synthesis were downregulated whereas several genes of lipolysis and phospholipid biosynthesis were upregulated. qRT-PCR analysis of selected genes verified microarray data and revealed linear correlations between gene expression levels and slaughter weight. Hepatic cholesterol 7α hydroxylase protein expression tended to be lower in the low protein group. Total lipid and triglyceride content and fatty acid composition of total lipids were not different between groups.

CONCLUSION

A maternal low protein diet during pregnancy induces a distinct hepatic gene expression signature in juvenile female pigs which was not translated into phenotypical changes of liver lipid metabolism.

The role of nitric oxide in the dysregulation of the urine concentration mechanism in diabetes mellitus

Uncontrolled diabetes mellitus results in osmotic diuresis. Diabetic patients have lowered nitric oxide (NO) which may exacerbate polyuria. We examined how lack of NO affects the transporters involved in urine concentration in diabetic animals. Diabetes was induced in rats by streptozotocin. Control and diabetic rats were given L-NAME for 3 weeks. Urine osmolality, urine output, and expression of urea and water transporters and the Na-K-2Cl cotransporter were examined. Predictably, diabetic rats presented with polyuria (increased urine volume and decreased urine osmolality). Although metabolic parameters of control rats were unaffected by L-NAME, treated diabetic rats produced 30% less urine and osmolality was restored. UT-A1 and UT-A3 were significantly increased in diabetic rat inner medulla. While L-NAME treatment alone did not alter UT-A1 or UT-A3 abundance, absence of NO prevented the upregulation of both transporters in diabetic rats. Similarly, AQP2 and NKCC2 abundance was increased in diabetic animals however, expression of these transporters were unchanged by L-NAME treatment of diabetes. Increased expression of the concentrating transporters observed in diabetic rats provides a compensatory mechanism to decrease solute loss despite persistent glycosuria. Our studies found that although diabetic-induced glycosylation remained increased, total protein expression was decreased to control levels in diabetic rats treated with L-NAME. While the role of NO in urine concentration remains unclear, lowered NO associated with diabetes may be deleterious to the transporters’ response to the subsequent osmotic diuresis.

Cellular distribution of NKCC2 in the gastric mucosa and its response to short-term osmotic shock

The Na(+)-K(+)-2Cl(-) cotransporter-2 (NKCC2) has long been recognized as a "kidney-specific" transporter and is important in salt reabsorption. NKCC2 has been found in the gastric mucosa; however, its cellular distribution and function remain obscure. The present study characterized the distribution pattern of NKCC2 in mammalian gastric mucosa and investigated its response to osmotic challenge. Reverse transcription with the polymerase chain reaction, Western blot and immunofluorescence were used to determine NKCC2 expression and localization. The effect of osmotic shock on NKCC2 expression was studied in isolated gastric mucosa with variable osmolarity treatment. Results from all of the above studies were compared with those of NKCC1. Our data indicated that NKCC1 and NKCC2 were expressed in the gastric mucosa of rat, mouse and human. The mRNA transcripts and proteins for NKCC1 and NKCC2 were broadly expressed in the rat gastric mucosa. In rat and mouse, NKCC1 was largely confined to the lower part of the oxyntic and pyloric gland areas, whereas NKCC2 extended throughout the gastric glands. NKCC1 immunoreactivity was strongly expressed in the parietal and chief cells but was weaker in the mucous cells. NKCC2 was abundantly located in the parietal and mucous cells but faintly distributed in the chief cells. Hypertonic treatment increased the protein level of NKCC1 and caused evident membrane translocation. In contrast, NKCC2 was significantly downregulated and no obvious membrane translocation was observed. Thus, NKCC2 displayed a more ubiquitous distribution in the gastric mucosa and might work coordinately with NKCC1 to maintain cell volume homeostasis under hypertonic conditions.

Hypoxia-inducible transcription factors stabilization in the thick ascending limb protects against ischemic acute kidney injury

Hypoxia-inducible transcription factors (HIF) protect cells against oxygen deprivation, and HIF stabilization before ischemia mitigates tissue injury. Because ischemic acute kidney injury (AKI) often involves the thick ascending limb (TAL), modulation of HIF in this segment may be protective. Here, we generated mice with targeted TAL deletion of the von Hippel-Lindau protein (Vhl), which mediates HIF degradation under normoxia, using Tamm-Horsfall protein (Thp)-driven Cre expression. These mice showed strong expression of HIF-1α in TALs but no changes in kidney morphology or function under control conditions. Deficiency of Vhl in the TAL markedly attenuated proximal tubular injury and preserved TAL function following ischemia-reperfusion, which may be partially a result of enhanced expression of glycolytic enzymes and lactate metabolism. These results highlight the importance of the thick ascending limb in the pathogenesis of AKI and suggest that pharmacologically targeting the HIF system may have potential to prevent and mitigate AKI.

Modulation of pressure-natriuresis by renal medullary reactive oxygen species and nitric oxide

The renal pressure-natriuresis mechanism is the dominant controller of body fluid balance and long-term arterial pressure. In recent years, it has become clear that the balance of reactive oxygen and nitrogen species within the renal medullary region is a key determinant of the set point of the renal pressure-natriuresis curve. The development of renal medullary oxidative stress causes dysfunction of the pressure-natriuresis mechanism and contributes to the development of hypertension in numerous disease models. The purpose of this review is to point out the known mechanisms within the renal medulla through which reactive oxygen and nitrogen species modulate the pressure-natriuresis response and to update the reader on recent advances in this field.

NAD(P)H oxidase and renal epithelial ion transport

A fundamental requirement for cellular vitality is the maintenance of plasma ion concentration within strict ranges. It is the function of the kidney to match urinary excretion of ions with daily ion intake and nonrenal losses to maintain a stable ionic milieu. NADPH oxidase is a source of reactive oxygen species (ROS) within many cell types, including the transporting renal epithelia. The focus of this review is to describe the role of NADPH oxidase-derived ROS toward local renal tubular ion transport in each nephron segment and to discuss how NADPH oxidase-derived ROS signaling within the nephron may mediate ion homeostasis. In each case, we will attempt to identify the various subunits of NADPH oxidase and reactive oxygen species involved and the ion transporters, which these affect. We will first review the role of NADPH oxidase on renal Na(+) and K(+) transport. Finally, we will review the relationship between tubular H(+) efflux and NADPH oxidase activity.

The effect of chronic candesartan therapy on the metabolic profile and renal tissue cytokine levels in the obese Zucker rat

The effect of candesartan, an angiotensin-II type-1 receptor antagonist, on the metabolic profile and renal inflammation is unclear. We evaluated this relationship by feeding male lean (LZ) and obese (OZ) Zucker rats chow or chow with candesartan (23.5 mg/kg · diet) for 14 weeks (n = 6–8/treatment/body type). Candesartan reduced serum triglycerides, plasma creatinine, urine albumin, and renal cortical collagen and glycogen deposition in the OZ. An ELISA-based cytokine array revealed that candesartan normalized elevated renal interleukin (IL) 1-β and monocyte chemoattractant protein-1 (MCP-1) levels in OZ. Nonetheless, candesartan impaired glucose tolerance, and did not lower blood insulin or glucose levels. Moreover, renal IL-1α, -2, -4, -6 and -10 tumor necrosis factor-α, interferon-γ, were significantly reduced in OZ relative to LZ, and increased by candesartan. Furthermore, candesartan increased growth-regulated oncogene, transforming growth factor-β1 and IL-18 in OZ kidneys to a level higher than LZ or untreated OZ. Candesartan did not affect renal cytokine levels in LZ. Overall, candesartan attenuated renal disease and improved renal function in OZ, despite mixed effects on metabolic factors and cytokines. Reduced plasma triglycerides and/or renal MCP-1 and IL-1β may have had a role in this protection. However, these effects were clearly independent of any improvement in glucose tolerance.