Salt-sensitive hypertension induced by decoy of transcription factor hypoxia-inducible factor-1alpha in the renal medulla

Renal Medullary Overexpression of Sphingosine-1-Phosphate Receptor 1 Transgene Attenuates Deoxycorticosterone Acetate (DOCA)-Salt Hypertension

BACKGROUND

Our previous studies showed that renal medullary sphingosine-1-phosphate receptor 1 (S1PR1) mediated sodium excretion, high salt intake increased S1PR1 level, deoxycorticosterone acetate (DOCA) blocked high salt-induced S1PR1 in the renal medulla, and that conditional knockout of S1PR1 in the collecting duct aggravated DOCA-salt hypertension. The present study tested the hypothesis that overexpression of S1PR1 transgene in the renal medulla attenuates the sodium retention and hypertension in DOCA-salt mouse model.

METHODS

Male C57BL/6J mice received renal medullary transfection of control or S1PR1-expressing plasmids and then DOCA-salt treatment. Renal sodium excretion and arterial pressure were compared between control and S1PR1-overexpressed mice in response to high salt loading or pressure natriuresis.

RESULTS

S1PR1-transfected mice showed significantly enhanced urinary sodium excretion in response to acute sodium loading (0.93 ± 0.27 in control vs. 4.72 ± 1.12 µmol/min/gKW in S1PR1-overexpressed mice, P < 0.05) and the pressure natriuresis (3.58 ± 1.77 vs. 9.52 ± 1.38, P < 0.05), less positive sodium balance in response to chronic high-salt intake (3.05 ± 0.39 vs. 1.65 ± 0.39 mmol/72 hr, P < 0.05), and consequently, the attenuation of DOCA-salt hypertension (134.2 ± 6.79 vs. 109.8 ± 3.54 mm Hg, P < 0.05). The αENaC protein amount in the renal medulla was not changed, however, the βENaC was significantly decreased and the γENaC was significantly increased in S1PR1-overexpressed mice. The immunostaining showed apical membrane translocation of γENaC, while no change of αENaC and βENaC in control mice, and that the apical membrane translocation of γENaC was blocked in S1PR1-treasffected mice.

CONCLUSIONS

These results suggested that activation of S1PR1 in the renal medulla attenuates DOCA-induced sodium retention and salt-sensitive hypertension associated with inhibition of ENaC.

No benefit of HIF prolyl hydroxylase inhibition for hypertensive renal damage in renovascular hypertensive rats

Introduction: We previously reported that malignant hypertension is associated with impaired capillary density of target organs. Here, we tested the hypothesis that stabilization of hypoxia-inducible factor (HIF) in a modified "preconditioning" approach prevents the development of malignant hypertension. To stabilize HIF, we employed pharmacological inhibition of HIF prolyl hydroxylases (PHD), that profoundly affect HIF metabolism. Methods: Two-kidney, one-clip renovascular hypertension (2K1C) was induced in rats; controls were sham operated. 2K1C rats received either intermittent injections of the PHD inhibitor ICA (2-(1-chloro-4-hydroxyisoquinoline-3-carboxamido) acetate) or placebo. Thirty-five days after clipping, the frequency of malignant hypertension was assessed (based on weight loss and the occurrence of characteristic vascular lesions). In addition, kidney injury was compared between all ICA treated versus all placebo treated 2K1C, regardless of the occurrence of malignant hypertension. HIF stabilization was evaluated by immunohistochemistry, and HIF target gene expression by RT-PCR. Results: Blood pressure was elevated to the same degree in ICA- and placebo-treated 2K1C compared to control rats. ICA treatment did not affect the frequency of malignant hypertension or the extent of kidney tissue fibrosis, inflammation, or capillary density. There was a trend towards higher mortality and worse kidney function in ICA-treated 2K1C rats. ICA increased the number of HIF-1α-positive renal tubular cell nuclei and induced several HIF-1 target genes. In contrast, expression of HIF-2α protein as well as HIF-2 target genes were markedly enhanced by 2K1C hypertension, irrespective of ICA treatment. Discussion: We conclude that intermittent PHD inhibition did not ameliorate severe renovascular hypertension in rats. We speculate that the unexpected strong renal accumulation of HIF-2α in renovascular hypertension, which could not be further augmented by ICA, may contribute to the lack of a benefit from PHD inhibition.

Natural compounds targeting mitochondrial dysfunction: emerging therapeutics for target organ damage in hypertension

Hypertension generally causes target organ damage (TOD) in the heart, brain, kidney, and blood vessels. This can result in atherosclerosis, plaque formation, cardiovascular and cerebrovascular events, and renal failure. Recent studies have indicated that mitochondrial dysfunction is crucial in hypertensive target organ damage. Consequently, mitochondria-targeted therapies attract increasing attention. Natural compounds are valuable resources for drug discovery and development. Many studies have demonstrated that natural compounds can ameliorate mitochondrial dysfunction in hypertensive target organ damage. This review examines the contribution of mitochondrial dysfunction to the development of target organ damage in hypertension. Moreover, it summarizes therapeutic strategies based on natural compounds that target mitochondrial dysfunction, which may be beneficial for preventing and treating hypertensive target organ damage.

The role of hypoxia-inducible factors in cardiovascular diseases

Cardiovascular diseases are the leading cause of death worldwide. During the development of cardiovascular diseases, hypoxia plays a crucial role. Hypoxia-inducible factors (HIFs) are the key transcription factors for adaptive hypoxic responses, which orchestrate the transcription of numerous genes involved in angiogenesis, erythropoiesis, glycolytic metabolism, inflammation, and so on. Recent studies have dissected the precise role of cell-specific HIFs in the pathogenesis of hypertension, atherosclerosis, aortic aneurysms, pulmonary arterial hypertension, and heart failure using tissue-specific HIF-knockout or -overexpressing animal models. More importantly, several compounds developed as HIF inhibitors or activators have been in clinical trials for the treatment of renal cancer or anemia; however, little is known on the therapeutic potential of these inhibitors for cardiovascular diseases. The purpose of this review is to summarize the recent advances on HIFs in the pathogenesis and pathophysiology of cardiovascular diseases and to provide evidence of potential clinical therapeutic targets.

Role of the renin-angiotensin system in NETosis in the coronavirus disease 2019 (COVID-19)

Myocardial infarction and stroke are the leading causes of death in the world. Numerous evidence has confirmed that hypertension promotes thrombosis and induces myocardial infarction and stroke. Recent findings reveal that neutrophil extracellular traps (NETs) are involved in the induction of myocardial infarction and stroke. Meanwhile, patients with severe COVID-19 suffer from complications such as myocardial infarction and stroke with pathological signs of NETs. Due to the extremely low amount of virus detected in the blood and remote organs (e.g., heart, brain and kidney) in a few cases, it is difficult to explain the mechanism by which the virus triggers NETosis, and there may be a different mechanism than in the lung. A large number of studies have found that the renin-angiotensin system regulates the NETosis at multiple levels in patients with COVID-19, such as endocytosis of SARS-COV-2, abnormal angiotensin II levels, neutrophil activation and procoagulant function at multiple levels, which may contribute to the formation of reticular structure and thrombosis. The treatment of angiotensin-converting enzyme inhibitors (ACEI), angiotensin II type 1 receptor blockers (ARBs) and neutrophil recruitment and active antagonists helps to regulate blood pressure and reduce the risk of net and thrombosis. The review will explore the possible role of the angiotensin system in the formation of NETs in severe COVID-19.

The hypoxia-inducible factor prolyl hydroxylase inhibitor FG4592 promotes natriuresis through upregulation of COX2 in the renal medulla

The renal medulla is a key site for the regulation of renal sodium excretion. However, the molecular mechanism remains unclear. Cyclooxygenase 2 (COX2) is specifically expressed in the renal medulla and contributes to the maintenance of the electrolyte/water balance in the body. Hypoxia-inducible factors (HIFs) have also been found to be expressed in the renal medulla, probably owing to the hypoxic conditions in the renal medulla. This study was designed to test the effects of HIF activation on renal sodium handling and renal medullary COX2 expression. Our data showed that HIF activation by the prolyl hydroxylase inhibitor (PHI) FG4592 enhanced natriuresis in mice challenged with a high-salt diet. In addition, FG4592 upregulated the expression of COX2 in the renal medulla. An in vitro study further supported the finding that HIF can induce the expression of COX2 and that this induction is mediated through direct binding to the promoter region of the Cox2 gene, facilitating its transcription. In addition, the COX2 inhibitor celecoxib diminished the natriuretic effect of FG4592. Together, these results suggest that HIF activation promotes sodium excretion through upregulation of COX2 in the renal medulla and therefore maintains sodium homeostasis in the body.

Overexpression of MicroRNA-429 Transgene Into the Renal Medulla Attenuated Salt-Sensitive Hypertension in Dahl S Rats

BACKGROUND

We have previously shown that high salt stimulates the expression of miR-429 in the renal medulla, which induces mRNA decay of HIF prolyl-hydroxylase 2 (PHD2), an enzyme to promote the degradation of hypoxia-inducible factor (HIF)-1α, and increases the HIF-1α-mediated activation of antihypertensive genes in the renal medulla, consequently promoting extra sodium excretion. Our preliminary results showed that high salt-induced increase of miR-429 was not observed in Dahl S rats. This present study determined whether correction of this impairment in miR-429 would reduce PHD2 levels, increase antihypertensive gene expression in the renal medulla and attenuate salt-sensitive hypertension in Dahl S rats.

METHODS

Lentiviruses encoding rat miR-429 were transfected into the renal medulla in uninephrectomized Dahl S rats. Sodium excretion and blood pressure were then measured.

RESULTS

Transduction of lentiviruses expressing miR-429 into the renal medulla increased miR-429 levels, decreased PHD2 levels, and upregulated HIF-1α target gene NOS-2, which restored the adaptive mechanism to increase the antihypertensive gene after high-salt intake in Dahl S rats. Functionally, overexpression of miR-429 transgene in the renal medulla significantly improved pressure natriuretic response, enhanced urinary sodium excretion, and reduced sodium retention upon extra sodium loading, and consequently, attenuated the salt-sensitive hypertension in Dahl S rats.

CONCLUSIONS

Our results suggest that the impaired miR-429-mediated PHD2 inhibition in response to high salt in the renal medulla may represent a novel mechanism for salt-sensitive hypertension in Dahl S rats and that correction of this impairment in miR-429 pathway could be a therapeutic approach for salt-sensitive hypertension.

Collecting duct-specific knockout of sphingosine-1-phosphate receptor 1 aggravates DOCA-salt hypertension in mice

OBJECTIVE

We have previously reported that renal medullary sphingosine-1-phosphate (S1P) regulates sodium excretion via the S1P type-1 receptor (S1PR1). As S1PR1 is predominantly expressed in collecting ducts (CD), the present study tested the hypothesis that the CD-S1PR1 pathway plays a critical role in sodium excretion and contributes to salt-sensitive hypertension.

METHODS

CD-specific S1PR1 knockout mice were generated by crossing aquaporin-2-Cre mice with S1PR1-floxed mice. Renal sodium excretion and arterial pressure were compared between wild type and KO mice in response to high-salt challenges and treatment of deoxycorticosterone acetate (DOCA) salt.

RESULTS

Protein levels of renal medullary S1PR1 were increased by 100% after high-salt intake, whereas DOCA treatment with high-salt intake blocked the increase of S1PR1 levels. Urinary sodium excretions in knockout mice were decreased by 60% compared with wild type mice after acute intravenous sodium loading (0.84 ± 0.16 vs. 2.22 ± 0.62 μmole/min per g kwt). The pressure natriuresis was impaired in knockout mice compared with wild type mice (4.32 ± 1.04 vs. 8.73 ± 0.19 μmole/min per g kwt). The chronic high-salt intake-induced positive sodium balance was enhanced in knockout mice compared with wild type mice (5.27 ± 0.39 vs. 2.38 ± 1.04 mmol/100 g BW per 24 h). After 10-day DOCA-salt treatment, knockout mice developed more severe hypertension than wild type mice (SBP 142 ± 8 vs. 115 ± 4 mmHg).

CONCLUSION

The deletion of CD-S1PR1 reduced sodium excretion, promoted sodium retention, and accelerated DOCA-salt-induced salt-sensitive hypertension, suggesting that the CD-S1PR1 signaling is an important antihypertensive pathway by promoting sodium excretion and that impairment of renal medullary S1PR1 may represent a novel mechanism for salt-sensitive hypertension.

Effects of molidustat, a hypoxia-inducible factor prolyl hydroxylase inhibitor, on sodium dynamics in hypertensive subtotally nephrectomized rats

Hypoxia-inducible factor prolyl hydroxylase (HIF-PHD) inhibitors were developed for treatment of renal anemia. Patients applicable for HIF-PHD inhibitor treatment experience complications such as chronic kidney disease, whereby water and electrolyte homeostasis is disrupted. The effects of hypoxia-inducible factor stabilization on salt accumulation in the setting of reduced renal function remain unclear. In the present study, we investigated the effect of a HIF-PHD inhibitor, molidustat, on salt distribution and excretion in rats with subtotal nephrectomy-induced chronic kidney disease. Male Wistar rats were subjected to 5/6 nephrectomy. After confirming blood pressure elevation (>150 mmHg, at 4 weeks after surgery), rats were treated with molidustat. After 1 week of treatment, molidustat did not significantly improve blood cell volume or blood pressure. Distribution of sodium, potassium, and water in skin, carcass, and bone samples was not affected by molidustat. Furthermore, molidustat had no significant effect on urinary sodium excretion or concentration in response to acute oral salt loading (1 g/kg). In conclusion, molidustat did not affect distribution or excretion of salt in rats subjected to a model of nephron loss.

Polyprenol-Based Lipofecting Agents for In Vivo Delivery of Therapeutic DNA to Treat Hypertensive Rats

Development of efficient vectors for transfection is one of the major challenges in genetic engineering. Previous research demonstrated that cationic derivatives of polyisoprenoids (PTAI) may serve as carriers of nucleic acids. In the present study, the effectiveness of two PTAI-based formulations (PTAI-6–8 and 10–14) was investigated and compared to the commercial reagents. The purpose of applied gene therapy was to enhance the expression of vascular endothelial growth factor (VEGF-A) in the renal medulla of spontaneously hypertensive rats (SHR) and to test its potential as a novel antihypertensive intervention. In the first part of the study (in vitro), we confirmed that PTAI-based lipoplexes efficiently transfect XC rat sarcoma cells and are stable in 37 °C for 7 days. In the in vivo experiments, we administered selected lipoplexes directly to the kidneys of conscious SHR (via osmotic pumps). There were no blood pressure changes and VEGF-A level in renal medulla was significantly higher only for PTAI-10–14-based formulation. In conclusion, despite the promising results, we were not able to achieve VEGF-A expression level high enough to verify VEGF-A gene therapy usefulness in SHR. However, results of our study give important indications for the future development of PTAI-based DNA carriers and kidney-targeted gene delivery.

Effects of empagliflozin on nondiabetic salt-sensitive hypertension in uninephrectomized rats

Impaired pressure natriuresis (PN) underlies salt-sensitive hypertension, and renal inflammation and hypoxia-inducible factor-1 (HIF-1) have been implicated in the modulation of systemic hypertension. Although sodium-glucose cotransporter-2 (SGLT2) inhibitors were reported to lower blood pressure (BP) in type 2 diabetes mellitus, whether they have a role in nondiabetic hypertensive kidney diseases is unclear. The present study was undertaken to investigate whether nondiabetic salt-sensitive hypertension and accompanying renal inflammation are ameliorated by SGLT2 inhibition. Male Sprague-Dawley rats were randomly divided into three groups: sham controls (SCs), uninephrectomized controls (UCs), and empagliflozin-treated rats (ETs). All rats were fed a rodent diet with 8% NaCl throughout the study period. Empagliflozin was orally administered for 3 weeks after uninephrectomy. Systolic blood pressure was recorded weekly, and kidneys were harvested for immunoblotting, immunohistochemistry, and quantitative PCR analysis at the end of the animal experiment. Systolic BP was significantly decreased in ETs that were orally given empagliflozin for 3 weeks after uninephrectomy. Although ETs did not show any increase in weekly measured urine sodium, the right-shifted PN relationship in UCs was improved by empagliflozin treatment. The expression of HIF-1α was increased in the renal outer medulla of ETs. Consistent with this, HIF prolyl-hydroxylase-2 protein and mRNA were decreased in ETs. The abundance of CD3 and ED-1 immunostaining in UCs was reduced by empagliflozin treatment. The increased IL-1ß, gp91phox, and NOX4 mRNA levels in UCs were also reversed. Empagliflozin restored impaired PN in nondiabetic hypertensive kidney disease in association with increased renal medullary expression of HIF-1α and amelioration of renal inflammation.

Cellular cholesterol modifies flow-mediated gene expression

Downregulation of heme oxygenase-1 (HO-1), cyclooxygenase-2 (COX2), and nitric oxide synthase-2 (NOS2) in the kidneys of Dahl rodents causes salt sensitivity, while restoring their expression aids in Na⁺ excretion and blood pressure reduction. Loading cholesterol into collecting duct (CD) cells represses fluid shear stress (FSS)-mediated COX2 activity. Thus, we hypothesized that cholesterol represses flow-responsive genes necessary to effectuate Na⁺ excretion. To this end, CD cells were used to test whether FSS induces these genes and if cholesterol loading represses them. Mice fed either 0% or 1% cholesterol diet were injected with saline, urine volume and electrolytes were measured, and renal gene expression determined. FSS-exposed CD cells demonstrated increases in HO-1 mRNA by 350-fold, COX2 by 25-fold, and NOS2 by 8-fold in sheared cells compared with static cells (P < 0.01). Immunoblot analysis of sheared cells showed increases in HO-1, COX2, and NOS2 protein, whereas conditioned media contained more HO-1 and PGE₂ than static cells. Cholesterol loading repressed the sheared mediated protein abundance of HO-1 and NOS2 as well as HO-1 and PGE₂ concentrations in media. In cholesterol-fed mice, urine volume was less at 6 h after injection of isotonic saline (P < 0.05). Urinary Na⁺ concentration, urinary K⁺ concentration, and osmolality were greater, whereas Na⁺ excretion was less, at the 6-h urine collection time point in cholesterol-fed versus control mice (P < 0.05). Renal cortical and medullary HO-1 (P < 0.05) and NOS2 (P < 0.05) mRNA were repressed in cholesterol-fed compared with control mice. Cholesterol acts to repress flow induced natriuretic gene expression, and this effect, in vivo, may contribute to renal Na⁺ avidity.

Role of DNA De Novo (De)Methylation in the Kidney in Salt-Induced Hypertension

Numerous adult diseases involving tissues consisting primarily of nondividing cells are associated with changes in DNA methylation. It suggests a pathophysiological role for de novo methylation or demethylation of DNA, which is catalyzed by DNA methyltransferase 3 and ten-eleven translocases. However, the contribution of DNA de novo (de)methylation to these diseases remains almost completely unproven. Broad changes in DNA methylation occurred within days in the renal outer medulla of Dahl SS rats fed a high-salt diet, a classic model of hypertension. Intrarenal administration of anti-DNA methyltransferase 3a/ten-eleven translocase 3 GapmeRs attenuated high salt-induced hypertension in SS rats. The high-salt diet induced differential expression of 1712 genes in the renal outer medulla. Remarkably, the differential expression of 76% of these genes was prevented by anti-DNA methyltransferase 3a/ten-eleven translocase 3 GapmeRs. The genes differentially expressed in response to the GapmeRs were involved in the regulation of metabolism and inflammation and were significantly enriched for genes showing differential methylation in response to the GapmeRs. These data indicate a significant role of DNA de novo (de)methylation in the kidney in the development of hypertension in SS rats. The findings should help to shift the paradigm of DNA methylation research in diseases involving nondividing cells from correlative analysis to functional and mechanistic studies.

MicroRNA-214-3p in the Kidney Contributes to the Development of Hypertension

BACKGROUND

In spite of extensive study, the mechanisms for salt sensitivity of BP in humans and rodent models remain poorly understood. Several microRNAs (miRNAs) have been associated with hypertension, but few have been shown to contribute to its development.

METHODS

We examined miRNA expression profiles in human kidney biopsy samples and rat models using small RNA deep sequencing. To inhibit an miRNA specifically in the kidney in conscious, freely moving rats, we placed indwelling catheters to allow both renal interstitial administration of a specific anti-miR and measurement of BP. A rat with heterozygous disruption of the gene encoding endothelial nitric oxide synthase (eNOS) was developed. We used bioinformatic analysis to evaluate the relationship between 283 BP-associated human single-nucleotide polymorphisms (SNPs) and 1870 human miRNA precursors, as well as other molecular and cellular methods.

RESULTS

Compared with salt-insensitive SS.13BN26 rats, Dahl salt-sensitive (SS) rats showed an upregulation of miR-214-3p, encoded by a gene in the SS.13BN26 congenic region. Kidney-specific inhibition of miR-214-3p significantly attenuated salt-induced hypertension and albuminuria in SS rats. miR-214-3p directly targeted eNOS. The effect of miR-214-3p inhibition on hypertension and albuminuria was abrogated in SS rats with heterozygous loss of eNOS. Human kidney biopsy specimens from patients with hypertension or hypertensive nephrosclerosis showed upregulation of miR-214-3p; the gene encoding miR-214-3p was one of several differentially expressed miRNA genes located in proximity to human BP-associated SNPs.

CONCLUSIONS

Renal miR-214-3p plays a functional and potentially genetic role in the development of hypertension, which might be mediated in part by targeting eNOS.

High salt intake shifts the mechanisms of flow-induced dilation in the middle cerebral arteries of Sprague-Dawley rats

The goal of the present study was to examine the effect of 1 wk of high salt (HS) intake and the role of oxidative stress in changing the mechanisms of flow-induced dilation (FID) in isolated pressurized middle cerebral arteries of male Sprague-Dawley rats ( n = 15–16 rats/group). Reduced FID in the HS group was restored by intake of the superoxide scavenger tempol (HS + tempol in vivo group). The nitric oxide (NO) synthase inhibitor Nω-nitro-l-arginine methyl ester, cyclooxygenase inhibitor indomethacin, and selective inhibitor of microsomal cytochrome P-450 epoxidase activity N-(methylsulfonyl)-2-(2-propynyloxy)-benzenehexanamide significantly reduced FID in the low salt diet-fed group, whereas FID in the HS group was mediated by NO only. Cyclooxygenase-2 mRNA (but not protein) expression was decreased in the HS and HS + tempol in vivo groups. Hypoxia-inducible factor-1α and VEGF protein levels were increased in the HS group but decreased in the HS + tempol in vivo group. Assessment by direct fluorescence of middle cerebral arteries under flow revealed significantly reduced vascular NO levels and increased superoxide/reactive oxygen species levels in the HS group. These results suggest that HS intake impairs FID and changes FID mechanisms to entirely NO dependent, in contrast to the low-salt diet-fed group, where FID is NO, prostanoid, and epoxyeicosatrienoic acid dependent. These changes were accompanied by increased lipid peroxidation products in the plasma of HS diet-fed rats, increased vascular superoxide/reactive oxygen species levels, and decreased NO levels, together with increased expression of hypoxia-inducible factor-1α and VEGF.

NEW & NOTEWORTHY High-salt (HS) diet changes the mechanisms of flow-induced dilation in rat middle cerebral arteries from a combination of nitric oxide-, prostanoid-, and epoxyeicosatrienoic acid-dependent mechanisms to, albeit reduced, a solely nitric oxide-dependent dilation. In vivo reactive oxygen species scavenging restores flow-induced dilation in HS diet-fed rats and ameliorates HS-induced increases in the transcription factor hypoxia-inducible factor-1α and expression of its downstream target genes.

Regulation of hypoxia inducible factor/prolyl hydroxylase binding domain proteins 1 by PPARα and high salt diet

BACKGROUND

Hypoxia inducible factor (HIF)/prolyl hydroxylase domain (PHD)-containing proteins are involved in renal adaptive response to high salt (HS). Peroxisome proliferator activated receptor alpha (PPARα), a transcription factor involved in fatty acid oxidation is implicated in the regulation of renal function. As both HIF-1α/PHD and PPARα contribute to the adaptive changes to altered oxygen tension, this study tested the hypothesis that PHD-induced renal adaptive response to HS is PPARα-dependent.

METHODS

PPARα wild type (WT) and knock out (KO) mice were fed a low salt (LS) (0.03% NaCl) or a HS (8% NaCl) diet for 8 days and treated with hydralazine. PPARα and heme oxygenase (HO)-1 expression were evaluated in the kidney cortex and medulla. A 24-h urinary volume (UV), sodium excretion (UNaV), and nitrite excretion (UNOx V) were also determined.

RESULTS

PHD1 expression was greater in the medulla as compared to the cortex of PPARα WT mice (p<0.05) fed with a LS (0.03% NaCl) diet. The HS diet (8% NaCl) downregulated PHD1 expression in the medulla (p<0.05) but not the cortex of WT mice whereas expression was downregulated in the cortex (p<0.05) and medulla (p<0.05) of KO mice. These changes were accompanied by HS-induced diuresis (p<0.05) and natriuresis (p<0.05) that were greater in WT mice (p<0.05). Similarly, UNOx V, index of renal nitric oxide synthase (NOS) activity or availability and heme oxygenase (HO)-1 expression was greater in WT (p<0.05) but unchanged in KO mice on HS diet. Hydralazine, a PHD inhibitor, did not affect diuresis or natriuresis in LS diet-fed WT or KO mice but both were increased (p<0.05) in HS diet-fed WT mice. Hydralazine also increased UNOx V (p<0.05) with no change in diuresis, natriuresis, or HO-1 expression in KO mice on HS diet.

CONCLUSIONS

These data suggest that HS-induced PPARα-mediated downregulation of PHD1 is a novel pathway for PHD/HIF-1α transcriptional regulation for adaptive responses to promote renal function via downstream signaling involving NOS and HO.

Inhibition of microRNA-429 in the renal medulla increased salt sensitivity of blood pressure in Sprague Dawley rats

BACKGROUND

We have previously shown that high salt intake suppresses the expression of prolyl hydroxylase domain-containing protein 2 (PHD2), an enzyme promoting the degradation of hypoxia-inducible factor (HIF)-1α, and increases HIF-1α along with its target genes in the renal medulla, which promotes sodium excretion and regulates salt sensitivity of blood pressure. However, it remains unknown how high salt inhibits the expression of PHD2.

METHOD AND RESULTS

The current study first revealed that high-salt-induced PHD2 inhibition was due to the enhanced decay of mRNA. We then found that high salt significantly increased the expression of miR-429, which was subsequently proven to target the 3'-untranslated region of PHD2 and reduce PHD2 levels, in the renal medulla. To define the functional role of renal medullary miR-429 in the regulation of PHD2/HIF-1α-mediated renal adaptation to high salt intake and salt sensitivity of blood pressure, we locally inhibited miR-429 in the renal medulla by locked nucleic acid anti-miR-429 in uninephrectomized rats. Our results demonstrated that inhibition of miR-429 remarkably increased the levels of PHD2, which disrupted PHD2-associated adaptive activation of HIF-1α-mediated gene expression in response to high salt in the renal medulla and consequently inhibited urinary sodium excretion, enhanced sodium retention in response to chronic sodium overloading, and as a result, produced a salt-sensitive hypertension.

CONCLUSION

It is concluded that miR-429 is an important upstream mediator in PHD2/HIF-1α-associated renal adaptation to high salt intake and that deficiency in miR-429-mediated PHD2 inhibition in response to high salt in the renal medulla may represent a pathogenic mechanism for salt-sensitive hypertension.

Intrarenal signaling mediated by CCK plays a role in salt intake-induced natriuresis

The natriuretic hormone CCK exhibits its gene transcripts in total kidney extracts. To test the possibility of CCK acting as an intrarenal mediator of sodium excretion, we examined mouse kidneys by 1) an in situ hybridization technique for CCK mRNA in animals fed a normal- or a high-sodium diet; 2) immuno-electron microscopy for the CCK peptide, 3) an in situ hybridization method and immunohistochemistry for the CCK-specific receptor CCKAR; 4) confocal image analysis of receptor-mediated Ca2+ responses in isolated renal tubules; and 5) metabolic cage experiments for the measurement of urinary sodium excretion in high-salt-fed mice either treated or untreated with the CCKAR antagonist lorglumide. Results showed the CCK gene to be expressed intensely in the inner medulla and moderately in the inner stripe of the outer medulla, with the expression in the latter being enhanced by high sodium intake. Immunoreactivity for the CCK peptide was localized to the rough endoplasmic reticulum of the medullary interstitial cells in corresponding renal regions, confirming it to be a secretory protein. Gene transcripts, protein products, and the functional activity for CCKAR were consistently localized to the late proximal tubule segments (S2 and S3) in the medullary rays, and the outer stripe of the outer medulla. Lorglumide significantly diminished natriuretic responses of mice to a dietary sodium load without altering the glomerular filtration rate. These findings suggest that the medullary interstitial cells respond to body fluid expansion by CCK release for feedback regulation of the late proximal tubular reabsorption.

Regulation and function of renal medullary cyclooxygenase-2 during high salt loading

Prostaglandins (PGs) are important autocrine/paracrine regulators that contribute to sodium balance and blood pressure control. Along the nephron, the highest amount of PGE₂ is found in the distal nephron, an important site for fine-tuning of urinary sodium and water excretion. Cylooxygenase-2 (COX-2) is abundantly expressed in the renal medulla and its expression along with urinary PGE₂ excretion is highly induced by chronic salt loading. Factors involved in high salt-induced COX-2 expression in the renal medulla include the hypertonicity, fluid shear stress (FSS), and hypoxia-inducible factor-1 alpha (HIF-1 alpha). Site-specific inhibition of COX-2 in the renal medulla of Sprague-Dawley rats causes sodium retention and salt-sensitive hypertension. Together, these results support the concept that renal medullary COX-2 functions an important natriuretic mediator that is activated by salt loading and its products promote sodium excretion and contribute to maintenance of sodium balance and blood pressure.

Characterization and Activation of NLRP3 Inflammasomes in the Renal Medulla in Mice

BACKGROUND/AIMS

Recent studies have indicated that local inflammatory mediators are importantly involved in the regulation of renal function. However, it remains unknown how such local inflammation is triggered intracellularly in the kidney. The present study was designed to characterize the inflammasome centered by Nlrp3 in the kidney and also test the effect of its activation in the renal medulla.

METHODS AND RESULTS

By immunohistochemistry analysis, we found that inflammasome components, Nlrp3, Asc and caspase-1, were ubiquitously distributed in different kidney areas. The caspase-1 activity and IL-1β production were particularly high in the renal outer medulla compared to other kidney regions. Further confocal microscopy and RT-PCR analysis showed that Nlrp3, Asc and caspase-1 were particularly enriched in the thick ascending limb of Henle's loop. In anesthetized mice, medullary infusion of Nlrp3 inflammasome activator, monosodium urate (MSU), induced significant decreases in sodium excretion and medullary blood flow without changes in mean arterial blood pressure and renal cortical blood flow. Caspase-1 inhibitor, Ac-YVAD-CMK and deletion of Nlrp3 or Asc gene abolished MSU-induced decreases in renal sodium excretion and MBF.

CONCLUSION

Our results indicate that renal medullary Nlrp3 inflammasomes represent a new regulatory mechanism of renal MBF and sodium excretion which may not depend on classical inflammatory response.

Renal Tumor Necrosis Factor α Contributes to Hypertension in Dahl Salt-Sensitive Rats

Tumor necrosis factor α (TNFα) is a major proinflammatory cytokine and its level is elevated in hypertensive states. Inflammation occurs in the kidneys during the development of hypertension. We hypothesized that TNFα specifically in the kidney contributes to the development of hypertension and renal injury in Dahl salt-sensitive (SS) rats, a widely used model of human salt-sensitive hypertension and renal injury. SS rats were chronically instrumented for renal interstitial infusion and blood pressure measurement in conscious, freely moving state. Gene expression was measured using real-time PCR and renal injury assessed with histological analysis. The abundance of TNFα in the renal medulla of SS rats, but not the salt-insensitive congenic SS.13^BN26 rats, was significantly increased when rats had been fed a high-salt diet for 7 days (n = 6 or 9, p < 0.01). The abundance of TNFα receptors in the renal medulla was significantly higher in SS rats than SS.13^BN26 rats. Renal interstitial administration of Etanercept, an inhibitor of TNFα, significantly attenuated the development of hypertension in SS rats on a high-salt diet (n = 7–8, p < 0.05). Glomerulosclerosis and interstitial fibrosis were also significantly ameliorated. These findings indicate intrarenal TNFα contributes to the development of hypertension and renal injury in SS rats.

Interactions between HIF-1α and AMPK in the regulation of cellular hypoxia adaptation in chronic kidney disease

Renal hypoxia contributes to chronic kidney disease (CKD) progression, as validated in experimental and human CKD. In the early stages, increased oxygen consumption causes oxygen demand/supply mismatch, leading to hypoxia. Hence, early targeting of the determinants and regulators of oxygen consumption in CKD may alter the disease course before permanent damage ensues. Here, we focus on hypoxia inducible factor-1α (HIF-1α) and AMP-activated protein kinase (AMPK) and on the mechanisms by which they may facilitate cellular hypoxia adaptation. We found that HIF-1α activation in the subtotal nephrectomy (STN) model of CKD limits protein synthesis, inhibits apoptosis, and activates autophagy, presumably for improved cell survival. AMPK activation was diminished in the STN kidney and was remarkably restored by HIF-1α activation, demonstrating a novel role for HIF-1α in the regulation of AMPK activity. We also investigated the independent and combined effects of HIF-1α and AMPK on cell survival and death pathways by utilizing pharmacological and knockdown approaches in cell culture models. We found that the effect of HIF-1α activation on autophagy is independent of AMPK, but on apoptosis it is partially AMPK dependent. The effects of HIF-1α and AMPK activation on inhibiting protein synthesis via the mTOR pathway appear to be additive. These various effects were also observed under hypoxic conditions. In conclusion, HIF-1α and AMPK appear to be linked at a molecular level and may act as components of a concerted cellular response to hypoxic stress in the pathophysiology of CKD.

Overexpression of hypoxia-inducible factor prolyl hydoxylase-2 attenuates hypoxia-induced vascular endothelial growth factor expression in luteal cells

Vascular endothelial growth factor (VEGF)-dependent angiogenesis has a crucial role in the corpus luteum formation and their functional maintenances in mammalian ovaries. A previous study by our group reported that activation of hypoxia‑inducible factor (HIF)‑1α signaling contributes to the regulation of VEGF expression in the luteal cells (LCs) in response to hypoxia and human chorionic gonadotropin. The present study was designed to test the hypothesis that HIF prolyl‑hydroxylases (PHDs) are expressed in LCs and overexpression of PHD2 attenuates the expression of VEGF induced by hypoxia in LCs. PHD2-overexpressing plasmid was transfected into LC2 cells, and successful plasmid transfection and expression was confirmed by reverse transcription quantitative polymerase chain reaction and western blot analysis. In addition, the present study investigated changes of HIF‑1α and VEGF expression after incubation under hypoxic conditions and PHD2 transfection. PHD2 expression was significantly higher expressed than the other two PHD isoforms, indicating its major role in LCs. Moreover, a significant increase of VEGF mRNA expression was identified after incubation under hypoxic conditions, which was, however, attenuated by PHD2 overexpression in LCs. Further analysis also indicated that this hypoxia‑induced increase in the mRNA expression of VEGF was consistent with increases in the protein levels of HIF‑1α, which is regulated by PHD-mediated degradation. In conclusion, the results of the present study indicated that PHD2 is the main PHD expressed in LCs and hypoxia‑induced VEGF expression can be attenuated by PHD2 overexpression through HIF‑1α‑mediated mechanisms in LCs. This PHD2-mediated transcriptional activation may be one of the mechanisms regulating VEGF expression in LCs during mammalian corpus luteum development.

Signaling pathways involved in renal oxidative injury: role of the vasoactive peptides and the renal dopaminergic system

The physiological hydroelectrolytic balance and the redox steady state in the kidney are accomplished by an intricate interaction between signals from extrarenal and intrarenal sources and between antinatriuretic and natriuretic factors. Angiotensin II, atrial natriuretic peptide and intrarenal dopamine play a pivotal role in this interactive network. The balance between endogenous antioxidant agents like the renal dopaminergic system and atrial natriuretic peptide, by one side, and the prooxidant effect of the renin angiotensin system, by the other side, contributes to ensuring the normal function of the kidney. Different pathological scenarios, as nephrotic syndrome and hypertension, where renal sodium excretion is altered, are associated with an impaired interaction between two natriuretic systems as the renal dopaminergic system and atrial natriuretic peptide that may be involved in the pathogenesis of renal diseases. The aim of this review is to update and comment the most recent evidences about the intracellular pathways involved in the relationship between endogenous antioxidant agents like the renal dopaminergic system and atrial natriuretic peptide and the prooxidant effect of the renin angiotensin system in the pathogenesis of renal inflammation.

Reactive oxygen species as important determinants of medullary flow, sodium excretion, and hypertension

The physiological evidence linking the production of superoxide, hydrogen peroxide, and nitric oxide in the renal medullary thick ascending limb of Henle (mTAL) to regulation of medullary blood flow, sodium homeostasis, and long-term control of blood pressure is summarized in this review. Data obtained largely from rats indicate that experimentally induced elevations of either superoxide or hydrogen peroxide in the renal medulla result in reduction of medullary blood flow, enhanced Na(+) reabsorption, and hypertension. A shift in the redox balance between nitric oxide and reactive oxygen species (ROS) is found to occur naturally in the Dahl salt-sensitive (SS) rat model, where selective reduction of ROS production in the renal medulla reduces salt-induced hypertension. Excess medullary production of ROS in SS rats emanates from the medullary thick ascending limbs of Henle [from both the mitochondria and membrane NAD(P)H oxidases] in response to increased delivery and reabsorption of excess sodium and water. There is evidence that ROS and perhaps other mediators such as ATP diffuse from the mTAL to surrounding vasa recta capillaries, resulting in medullary ischemia, which thereby contributes to hypertension.

Silencing of hypoxia-inducible factor-1α gene attenuates chronic ischemic renal injury in two-kidney, one-clip rats

Overactivation of hypoxia-inducible factor (HIF)-1α is implicated as a pathogenic factor in chronic kidney diseases (CKD). However, controversy exists regarding the roles of HIF-1α in CKD. Additionally, although hypoxia and HIF-1α activation are observed in various CKD and HIF-1α has been shown to stimulate fibrogenic factors, there is no direct evidence whether HIF-1α is an injurious or protective factor in chronic renal hypoxic injury. The present study determined whether knocking down the HIF-1α gene can attenuate or exaggerate kidney damage using a chronic renal ischemic model. Chronic renal ischemia was induced by unilaterally clamping the left renal artery for 3 wk in Sprague-Dawley rats. HIF-1α short hairpin (sh) RNA or control vectors were transfected into the left kidneys. Experimental groups were sham+control vector, clip+control vector, and clip+HIF-1α shRNA. Enalapril was used to normalize blood pressure 1 wk after clamping the renal artery. HIF-1α protein levels were remarkably increased in clipped kidneys, and this increase was blocked by shRNA. Morphological examination showed that HIF-1α shRNA significantly attenuated injury in clipped kidneys: glomerular injury indices were 0.71 ± 0.04, 2.50 ± 0.12, and 1.34 ± 0.11, and the percentage of globally damaged glomeruli was 0.02, 34.3 ± 5.0, and 6.3 ± 1.6 in sham, clip, and clip+shRNA groups, respectively. The protein levels of collagen and α-smooth muscle actin also dramatically increased in clipped kidneys, but this effect was blocked by HIF-1α shRNA. In conclusion, long-term overactivation of HIF-1α is a pathogenic factor in chronic renal injury associated with ischemia/hypoxia.

Renal overexpression of atrial natriuretic peptide and hypoxia inducible factor-1α as adaptive response to a high salt diet

In the kidney, a high salt intake favors oxidative stress and hypoxia and causes the development of fibrosis. Both atrial natriuretic peptide (ANP) and hypoxia inducible factor (HIF-1α) exert cytoprotective effects. We tested the hypothesis that renal expression of ANP and HIF-1α is involved in a mechanism responding to the oxidative stress produced in the kidneys of rats chronically fed a high sodium diet. Sprague-Dawley rats were fed with a normal salt (0.4% NaCl) (NS) or a high salt (8% NaCl) (HS) diet for 3 weeks, with or without the administration of tempol (T), an inhibitor of oxidative stress, in the drinking water. We measured the mean arterial pressure (MAP), glomerular filtration rate (GFR), and urinary sodium excretion (UV_Na). We evaluated the expression of ANP, HIF-1α, and transforming growth factor (TGF-β1) in renal tissues by western blot and immunohistochemistry. The animals fed a high salt diet showed increased MAP and UV_Na levels and enhanced renal immunostaining of ANP, HIF-1α, and TGF-β1. The administration of tempol together with the sodium overload increased the natriuresis further and prevented the elevation of blood pressure and the increased expression of ANP, TGF-β1, and HIF-1α compared to their control. These findings suggest that HIF-1α and ANP, synthesized by the kidney, are involved in an adaptive mechanism in response to a sodium overload to prevent or attenuate the deleterious effects of the oxidative stress and the hypoxia on the development of fibrosis.

Silencing of HIF prolyl-hydroxylase 2 gene in the renal medulla attenuates salt-sensitive hypertension in Dahl S rats

BACKGROUND

In response to high salt intake, transcription factor hypoxia-inducible factor (HIF) 1α activates many antihypertensive genes, such as heme oxygenase 1 (HO-1) 1 and cyclooxygenase 2 (COX-2) in the renal medulla, which is an important molecular adaptation to promote extra sodium excretion. We recently showed that high salt inhibited the expression of HIF prolyl-hydroxylase 2 (PHD2), an enzyme that promotes the degradation of HIF-1α, thereby upregulating HIF-1α, and that high salt-induced inhibition in PHD2 and subsequent activation of HIF-1α in the renal medulla was blunted in Dahl salt-sensitive hypertensive rats. This study tested the hypothesis that silencing the PHD2 gene to increase HIF-1α levels in the renal medulla attenuates salt-sensitive hypertension in Dahl S rats.

METHODS

PHD2 short hairpin RNA (shRNA) plasmids were transfected into the renal medulla in uninephrectomized Dahl S rats. Renal function and blood pressure were then measured.

RESULTS

PHD2 shRNA reduced PHD2 levels by >60% and significantly increased HIF-1α protein levels and the expression of HIF-1α target genes HO-1 and COX-2 by >3-fold in the renal medulla. Functionally, pressure natriuresis was remarkably enhanced, urinary sodium excretion was doubled after acute intravenous sodium loading, and chronic high salt-induced sodium retention was remarkably decreased, and as a result, salt-sensitive hypertension was significantly attenuated in PHD2 shRNA rats compared with control rats.

CONCLUSIONS

Impaired PHD2 response to high salt intake in the renal medulla may represent a novel mechanism for hypertension in Dahl S rats, and inhibition of PHD2 in the renal medulla could be a therapeutic approach for salt-sensitive hypertension.

Impaired pressure natriuresis is associated with interstitial inflammation in salt-sensitive hypertension

PURPOSE OF REVIEW

Impairment of the pressure natriuresis relationship is a central event in the pathogenesis of hypertension. Renal tubulointerstitial inflammation results in salt-sensitive hypertension and, until recently, the changes in pressure natriuresis induced by renal inflammation received little attention.

RECENT FINDINGS

Oxidative stress and increased intrarenal angiotensin II activity, in association with rarefaction and loss of peritubular vascular network, may be involved in the inflammation-induced blunting of the natriuresis resulting from increments in renal perfusion pressure.

SUMMARY

Here, we review the mechanisms for the impairment in pressure natriuresis resulting from renal tubulointerstitial inflammation in reference to the normal physiologic mechanisms involved in this response.

Impaired pressure natriuresis resulting in salt-sensitive hypertension is caused by tubulointerstitial immune cell infiltration in the kidney

Immune cell infiltration of the kidney is a constant feature in salt-sensitive hypertension (SSHTN). We evaluated the relationship between the renal inflammation and pressure natriuresis in the model of SSHTN that results from transient oral administration of N(ω)-nitro-L-arginine methyl ester (L-NAME). Pressure natriuresis was determined in Wistar rats that received 4 wk of a high-salt (4% NaCl) diet, starting 1 wk after stopping L-NAME, which was administered alone (SSHTN group, n = 17) or in association with mycophenolate mofetil (MMF; MMF group, n = 15). The administration of MMF in association with L-NAME is known to prevent the subsequent development of SSHTN. Control groups received a high (n = 12)- and normal (0.4%)-salt diet (n = 20). Rats with SSHTN had increased expression of inflammatory cytokines and oxidative stress. The severity of hypertension correlated directly (P < 0.0001) with the number of tubulointerstitial immune cells and angiotensin II-expressing cells. Pressure natriuresis was studied at renal arterial pressures (RAPs) of 90, 110, 130, and 150 mmHg. Glomerular filtration rate was similar and stable in all groups, and renal blood flow was decreased in the SSHTN group. Significantly decreased natriuresis (P < 0.05) was found in the SSHTN group at RAPs of 130 and 150 mmHg, and there was an inverse correlation (P < 0.01) between the urinary sodium excretion and the number of tubulointerstitial inflammatory cells (lymphocytes and macrophages) and cells expressing angiotensin II. We conclude that tubulointerstitial inflammation plays a key role in the impairment of pressure natriuresis that results in salt-dependent hypertension in this experimental model.

Implication of CD38 gene in podocyte epithelial-to-mesenchymal transition and glomerular sclerosis

CD38 is a multifunctional protein involving in a number of signalling pathways. Given that the lack of CD38 is considered as a dedifferentiation marker of lymphocytes and other cells, we hypothesized that CD38 and its signalling pathway may participate in the epithelial-to-mesenchymal transition (EMT) process of podocytes and thereby regulates the integrity of glomerular structure and function. Western blot analysis and RT-PCR demonstrated that renal tissue CD38 expression was lacking in CD38(-/-) mice or substantially reduced in renal CD38 shRNA-transfected WT (CD38-shRNA) mice compared to CD38(+/+) littermates. Confocal fluorescent microscopy demonstrated the reduced expression of epithelial markers (P-Cadherin, ZO-1 and podocin) and increased expression of mesenchymal markers (FSP-1, α-SMA and desmin) in the glomeruli of CD38(-/-) and CD38-shRNA mice compared to CD38(+/+) mice. Morphological examinations showed profound injury in the glomeruli of CD38(-/-) or CD38-shRNA mice compared to CD38(+/+) mice. This enhanced glomerular injury in CD38(-/-) or CD38-shRNA mice was accompanied by increased albuminuria and proteinuria. DOCA/high salt treatment further decreased the expression of epithelial markers and increased the abundance of mesenchymal markers, which were accompanied by more increased glomerular damage index and mean arterial pressure in CD38(-/-) and CD38-shRNA mice than CD38(+/+) mice. In vitro studies showed that inhibition of CD38 enhances the EMT in podocytes. In conclusion, our observations reveal that the normal expression of CD38 importantly contributes to the differentiation and function of podocytes and the defect of this gene expression may be a critical mechanism inducing EMT and consequently resulting in glomerular injury and sclerosis.

Overexpression of HIF prolyl-hydoxylase-2 transgene in the renal medulla induced a salt sensitive hypertension

Renal medullary hypoxia-inducible factor (HIF)-1α and its target genes, such as haem oxygenase and nitric oxide synthase, have been indicated to play an important role in the regulation of sodium excretion and blood pressure. HIF prolyl hydroxylase domain-containing proteins (PHDs) are major enzymes to promote the degradation of HIF-1α. We recently reported that high salt intake suppressed the renal medullary PHD2 expression and thereby activated HIF-1α-mediated gene regulation in the renal medulla in response to high salt. To further define the functional role of renal medullary PHD2 in the regulation of renal adaptation to high salt intake and the longer term control of blood pressure, we transfected PHD2 expression plasmids into the renal medulla in uninephrectomized rats and determined its effects on pressure natriuresis, sodium excretion after salt overloading and the long-term control of arterial pressure after high salt challenge. It was shown that overexpression of PHD2 transgene increased PHD2 levels and decreased HIF-1α levels in the renal medulla, which blunted pressure natriuresis, attenuated sodium excretion, promoted sodium retention and produced salt sensitive hypertension after high salt challenge compared with rats treated with control plasmids. There was no blood pressure change in PHD2-treated rats that were maintained in low salt diet. These results suggested that renal medullary PHD2 is an important regulator in renal adaptation to high salt intake and a deficiency in PHD2-mediated molecular adaptation in response to high salt intake in the renal medulla may represent a pathogenic mechanism producing salt sensitive hypertension.

Hypoxia inducible factor-1α-mediated gene activation in the regulation of renal medullary function and salt sensitivity of blood pressure

Many enzymes that produce natriuretic factors such as nitric oxide synthase (NOS), hemeoxygenase-1 (HO-1) and cyclooxygenase-2 (COX-2) are highly expressed in the renal medulla. These enzymes in the renal medulla are up-regulated in response to high salt intake. Inhibition of these enzymes within the renal medulla reduces sodium excretion and increases salt sensitivity of arterial blood pressure, indicating that these enzymes play important roles in kidney salt handling and renal adaptation to high salt challenge. However, it remains a question what mechanisms mediate the activation of these enzymes in response to high salt challenge in the renal medulla. Interestingly, these enzymes are oxygen sensitive genes and regulated by transcription factor hypoxia-inducible factor (HIF)-1α. Our recent serial studies have demonstrated that: 1) High salt intake stimulates HIF-1α-mediated gene expression, such as NOS, HO-1 and COX-2, in the renal medulla, which may augment the production of different antihypertensive factors in the renal medulla, mediating renal adaptation to high salt intake and regulating salt sensitivity of arterial blood pressure. 2) HIF prolyl-hydroxylase 2 (PHD2), an enzyme that promotes the degradation of HIF-1α, is highly expressed in renal medulla. High salt intake suppresses the expression of PHD2 in the renal medulla, which increases HIF-1α-mediated gene expressions in the renal medulla, thereby participates in the control of salt sensitivity of blood pressure. 3) The high salt-induced inhibition in PHD2 and the consequent activation of HIF-1α in the renal medulla is not observed in Dahl salt sensitive hypertensive (Dahl/ss) rats. Correction of these defects in PHD2/HIF-1α-associated molecular adaptation in the renal medulla improves sodium excretion, reduces sodium retention and attenuates saltsensitive hypertension in Dahl/ss rats. In conclusion, PHD2 regulation of HIF-1α-mediated gene activation in the renal medulla is an important molecular adaptation to high salt intake; impaired PHD2 regulation of HIF-1α-mediated gene activation in the renal medulla may be responsible for the salt-sensitive hypertension in Dahl/ss rats; correction of these defects may be used to as therapeutic strategies for the treatment of salt-sensitive hypertension.

Delayed ischemic preconditioning contributes to renal protection by upregulation of miR-21

Delayed ischemic preconditioning effectively protects kidneys from ischemia-reperfusion injury but the mechanism underlying renal protection remains poorly understood. Here we examined the in vivo role of microRNA miR-21 in the renal protection conferred by delayed ischemic preconditioning in mice. A 15 minute renal ischemic preconditioning significantly increased the expression of miR-21 by 4 hours and substantially attenuated ischemia-reperfusion injury induced 4 days later. A locked nucleic acid-modified anti-miR-21 given at the time of ischemic preconditioning knocked down miR-21 and significantly exacerbated subsequent ischemia-reperfusion injury in the mouse kidney. Knockdown of miR-21 resulted in significant upregulation of programmed cell death protein 4, a pro-apoptotic target gene of miR-21, and substantially increased tubular cell apoptosis. Hypoxia inducible factor-1α in the kidney was activated after ischemic preconditioning and blockade of its activity with a decoy abolished the up-regulation of miR-21 in cultured human renal epithelial cells treated with the inducer cobalt chloride. In the absence of ischemic preconditioning, knockdown of miR-21 alone did not significantly affect ischemia-reperfusion injury in the mouse kidney. Thus, upregulation of miR-21 contributes to the protective effect of delayed ischemic preconditioning against subsequent renal ischemia-reperfusion injury.

New concepts in endothelin control of sodium balance

1. Endothelin (ET)-1, which was originally found to be secreted by the vascular endothelium, is highly expressed in the kidney, particularly in the renal medulla. 2. Recent studies using genetic models have provided significant breakthroughs in the role of ET-1 in the kidney. For example, ET-1 in the medullary collecting duct physiologically regulates water and salt reabsorption, thereby controlling blood pressure. Surprisingly, to explain the blood pressure regulation both ET(A) and ET(B) receptors are necessary in collecting duct. In fact, we recently revealed that ET(A) receptor stimulation in the renal medulla was natriuretic and diuretic. 3. The expression and secretion of ET-1 in the renal medulla are regulated by multiple mechanisms, such as changes in osmolality, exaggerated renin-angiotensin system activity and hypoxia. The changes in the renal medullary ET system are likely to work as compensatory 'protective' natriuretic factors in response to high sodium exposure in the kidney. 4. In the present review, we focus on recent publications that describe our current knowledge of the functional role of renal medullary ET-1, including the recently characterized actions of ET(A) receptors, the second messenger systems, mechanisms of stimulating ET-1 production and how the ET system is involved in the development of hypertension.

Lysosome fusion to the cell membrane is mediated by the dysferlin C2A domain in coronary arterial endothelial cells

Dysferlin has recently been reported to participate in cell membrane repair in muscle and other cells through lysosome fusion. Given that lysosome fusion is a crucial mechanism that leads to membrane raft clustering, the present study attempted to determine whether dysferlin is involved in this process and its related signalling, and explores the mechanism underlying dysferlin-mediated lysosome fusion in bovine coronary arterial endothelial cells (CAECs). We found that dysferlin is clustered in membrane raft macrodomains after Fas Ligand (FasL) stimulation as detected by confocal microscopy and membrane fraction flotation. Small-interfering RNA targeted to dysferlin prevented membrane raft clustering. Furthermore, the translocation of acid sphingomyelinase (ASMase) to membrane raft clusters, whereby local ASMase activation and ceramide production--an important step that mediates membrane raft clustering--was attenuated. Functionally, silencing of the dysferlin gene reversed FasL-induced impairment of endothelium-dependent vasodilation in isolated small coronary arteries. By monitoring fluorescence quenching or dequenching, silencing of the dysferlin gene was found to almost completely block lysosome fusion to plasma membrane upon FasL stimulation. Further studies to block C2A binding and silencing of AHNAK (a dysferlin C2A domain binding partner), showed that the dysferlin C2A domain is required for FasL-induced lysosome fusion to the cell membrane, ASMase translocation and membrane raft clustering. We conclude that dysferlin determines lysosome fusion to the plasma membrane through its C2A domain and it is therefore implicated in membrane-raft-mediated signaling and regulation of endothelial function in coronary circulation.

Overexpression of HIF-1α transgene in the renal medulla attenuated salt sensitive hypertension in Dahl S rats

Hypoxia inducible factor (HIF)-1α-mediated gene activation in the renal medulla in response to high salt intake plays an important role in the control of salt sensitivity of blood pressure. High salt-induced activation of HIF-1α in the renal medulla is blunted in Dahl S rats. The present study determined whether the impairment of the renal medullary HIF-1α pathway was responsible for salt sensitive hypertension in Dahl S rats. Renal medullary HIF-1α levels were induced by either transfection of HIF-1α expression plasmid or chronic infusion of CoCl₂ into the renal medulla, which was accompanied by increased expressions of anti-hypertensive genes, cyclooxygenase-2 and heme oxygenase-1. Overexpression of HIF-1α transgenes in the renal medulla enhanced the pressure natriuresis, promoted the sodium excretion and reduced sodium retention after salt overload. As a result, hypertension induced by 2-week high salt was significantly attenuated in rats treated with HIF-1α plasmid or CoCl₂. These results suggest that an abnormal HIF-1α in the renal medulla may represent a novel mechanism mediating salt-sensitive hypertension in Dahl S rats and that induction of HIF-1α levels in the renal medulla could be a therapeutic approach for the treatment of salt-sensitive hypertension.

NAD(P)H oxidase-dependent intracellular and extracellular O2•- production in coronary arterial myocytes from CD38 knockout mice

Activation of NAD(P)H oxidase has been reported to produce superoxide (O(2)(•-)) extracellularly as an autocrine/paracrine regulator or intracellularly as a signaling messenger in a variety of mammalian cells. However, it remains unknown how the activity of NAD(P)H oxidase is regulated in arterial myocytes. Recently, CD38-associated ADP-ribosylcyclase has been reported to use an NAD(P)H oxidase product, NAD(+) or NADP(+), to produce cyclic ADP-ribose (cADPR) or nicotinic acid adenine dinucleotide phosphate, which mediates intracellular Ca(2+) signaling. This study was designed to test a hypothesis that the CD38/cADPR pathway as a downstream event exerts feedback regulatory action on the NAD(P)H oxidase activity in production of extra- or intracellular O(2)(•-) in mouse coronary arterial myocytes (CAMs). By fluorescence microscopic imaging, we simultaneously monitored extra- and intracellular O(2)(•-) production in wild-type (CD38(+/+)) and CD38 knockout (CD38(-/-)) CAMs in response to oxotremorine (OXO), a muscarinic type 1 receptor agonist. It was found that CD38 deficiency prevented OXO-induced intracellular but not extracellular O(2)(•-) production in CAMs. Consistently, the OXO-induced intracellular O(2)(•-) production was markedly inhibited by CD38 shRNA or the CD38 inhibitor nicotinamide in CD38(+/+) CAMs. Further, Nox4 siRNA inhibited OXO-induced intracellular but not extracellular O(2)(•-) production, whereas Nox1 siRNA attenuated both intracellular and extracellular O(2)(•-) production in CD38(+/+) CAMs. Direct delivery of exogenous cADPR into CAMs markedly elevated intracellular Ca(2+) and O(2)(•-) production in CD38(-/-) CAMs. Functionally, CD38 deficiency or Nox1 siRNA and Nox4 siRNA prevented OXO-induced contraction in isolated perfused coronary arteries in CD38 WT mice. These results provide direct evidence that the CD38/cADPR pathway is an important controller of Nox4-mediated intracellular O(2)(•-) production and that CD38-dependent intracellular O(2)(•-) production is augmented in an autocrine manner by CD38-independent Nox1-derived extracellular O(2)(•-) production in CAMs.

Renal tubulointerstitial hypoxia: cause and consequence of kidney dysfunction

1. Intrarenal oxygen availability is the balance between supply, mainly dependent on renal blood flow, and demand, determined by the basal metabolic demand and the energy-requiring tubular electrolyte transport. Renal blood flow is maintained within close limits in order to sustain stable glomerular filtration, so increased intrarenal oxygen consumption is likely to cause tissue hypoxia. 2. The increased oxygen consumption is closely linked to increased oxidative stress, which increases mitochondrial oxygen usage and reduces tubular electrolyte transport efficiency, with both contributing to increased total oxygen consumption. 3. Tubulointerstitial hypoxia stimulates the production of collagen I and α-smooth muscle actin, indicators of increased fibrogenesis. Furthermore, the hypoxic environment induces epithelial-mesenchymal transdifferentiation and aggravates fibrosis, which results in reduced peritubular blood perfusion and oxygen delivery due to capillary rarefaction. 4. Increased oxygen consumption, capillary rarefaction and increased diffusion distance due to the increased fibrosis per se further aggravate the interstitial hypoxia. 5. Recently, it has been demonstrated that hypoxia simulates the infiltration and maturation of immune cells, which provides an explanation for the general inflammation commonly associated with the progression of chronic kidney disease. 6. Therapies targeting interstitial hypoxia could potentially reduce the progression of chronic renal failure in millions of patients who are otherwise likely to eventually present with fully developed end-stage renal disease.

SNARE-mediated rapid lysosome fusion in membrane raft clustering and dysfunction of bovine coronary arterial endothelium

The present study attempted to evaluate whether soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) mediate lysosome fusion in response to death receptor activation and contribute to membrane raft (MR) clustering and consequent endothelial dysfunction in coronary arterial endothelial cells. By immunohistochemical analysis, vesicle-associated membrane proteins 2 (VAMP-2, vesicle-SNAREs) were found to be abundantly expressed in the endothelium of bovine coronary arteries. Direct lysosome fusion monitoring by N-(3-triethylammoniumpropyl)-4-[4-(dibutylamino)styryl]pyridinium dibromide (FM1-43) quenching demonstrated that the inhibition of VAMP-2 with tetanus toxin or specific small interfering ribonucleic acid (siRNA) almost completely blocked lysosome fusion to plasma membrane induced by Fas ligand (FasL), a well-known MR clustering stimulator. The involvement of SNAREs was further confirmed by an increased interaction of VAMP-2 with a target-SNARE protein syntaxin-4 after FasL stimulation in coimmunoprecipitation analysis. Also, the inhibition of VAMP-2 with tetanus toxin or VAMP-2 siRNA abolished FasL-induced MR clustering, its colocalization with a NADPH oxidase unit gp91(phox), and increased superoxide production. Finally, FasL-induced impairment of endothelium-dependent vasodilation was reversed by the treatment of bovine coronary arteries with tetanus toxin or VAMP-2 siRNA. VAMP-2 is critical to lysosome fusion in MR clustering, and this VAMP-2-mediated lysosome-MR signalosomes contribute to redox regulation of coronary endothelial function.

Silencing of hypoxia-inducible factor-1α gene attenuated angiotensin II-induced renal injury in Sprague-Dawley rats

Although it has been shown that upregulation of hypoxia-inducible factor (HIF)-1α is protective in acute ischemic renal injury, long-term overactivation of HIF-1α is implicated to be injurious in chronic kidney diseases. Angiotensin II (Ang II) is a well-known pathogenic factor producing chronic renal injury and has also been shown to increase HIF-1α. However, the contribution of HIF-1α to Ang II-induced renal injury has not been evidenced. The present study tested the hypothesis that HIF-1α mediates Ang II-induced renal injury in Sprague-Dawley rats. Chronic renal injury was induced by Ang II infusion (200 ng/kg per minute) for 2 weeks in uninephrectomized rats. Transfection of vectors expressing HIF-1α small hairpin RNA into the kidneys knocked down HIF-1α gene expression by 70%, blocked Ang II-induced HIF-1α activation, and significantly attenuated Ang II-induced albuminuria, which was accompanied by inhibition of Ang II-induced vascular endothelial growth factor, a known glomerular permeability factor, in glomeruli. HIF-1α small hairpin RNA also significantly improved the glomerular morphological damage induced by Ang II. Furthermore, HIF-1α small hairpin RNA blocked Ang II-induced upregulation of collagen and α-smooth muscle actin in tubulointerstitial region. There was no difference in creatinine clearance and Ang II-induced increase in blood pressure. HIF-1α small hairpin RNA had no effect on Ang II-induced reduction in renal blood flow and hypoxia in the kidneys. These data suggested that overactivation of HIF-1α-mediated gene regulation in the kidney is a pathogenic pathway mediating Ang II-induced chronic renal injuries, and normalization of overactivated HIF-1α may be used as a treatment strategy for chronic kidney damages associated with excessive Ang II.

Acid sphingomyelinase gene deficiency ameliorates the hyperhomocysteinemia-induced glomerular injury in mice

Hyperhomocysteinemia (hHcys) enhances ceramide production, leading to the activation of NADPH oxidase and consequent glomerular oxidative stress and sclerosis. The present study was performed to determine whether acid sphingomyelinase (Asm), a ceramide-producing enzyme, is implicated in the development of hHcys-induced glomerular oxidative stress and injury. Uninephrectomized Asm-knockout (Asm(-/-)) and wild-type (Asm(+/+)) mice, with or without Asm short hairpin RNA (shRNA) transfection, were fed a folate-free (FF) diet for 8 weeks, which significantly elevated the plasma Hcys level compared with mice fed normal chow. By using in vivo molecular imaging, we found that transfected shRNAs were expressed in the renal cortex starting on day 3 and continued for 24 days. The FF diet significantly increased renal ceramide production, Asm mRNA and activity, urinary total protein and albumin excretion, glomerular damage index, and NADPH-dependent superoxide production in the renal cortex from Asm(+/+) mice compared with that from Asm(-/-) or Asm shRNA-transfected wild-type mice. Immunofluorescence analysis showed that the FF diet decreased the expression of podocin but increased desmin and ceramide levels in glomeruli from Asm(+/+) mice but not in those from Asm(-/-) and Asm shRNA-transfected wild-type mice. In conclusion, our observations reveal that Asm plays a pivotal role in mediating podocyte injury and glomerular sclerosis associated with NADPH oxidase-associated local oxidative stress during hHcys.

Renal oxidative stress, oxygenation, and hypertension

Hypertension is closely associated with progressive kidney dysfunction, manifested as glomerulosclerosis, interstitial fibrosis, proteinuria, and eventually declining glomerular filtration. The postulated mechanism for development of glomerulosclerosis is barotrauma caused by increased capillary pressure, but the reason for development of interstitial fibrosis and the subsequently reduced kidney function is less clear. However, it has been hypothesized that tissue hypoxia induces fibrogenesis and progressive renal failure. This is very interesting, since recent reports highlight several different mechanisms resulting in altered oxygen handling and availability in the hypertensive kidney. Such mechanisms include decreased renal blood flow due to increased vascular tone induced by ANG II that limits oxygen delivery and increases oxidative stress, resulting in increased mitochondrial oxygen usage, increased oxygen usage for tubular electrolyte transport, and shunting of oxygen from arterial to venous blood in preglomerular vessels. It has been shown in several studies that interventions to prevent oxidative stress and to restore kidney tissue oxygenation prevent progression of kidney dysfunction. Furthermore, inhibition of ANG II activity, by either blocking ANG II type 1 receptors or angiotensin-converting enzyme, or by preventing oxidative stress by administration of antioxidants also results in improved blood pressure control. Therefore, it seems likely that tissue hypoxia in the hypertensive kidney contributes to progression of kidney damage, and perhaps also persistence the high blood pressure.

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.

Neovascular age-related macular degeneration: opportunities for development of first-in-class biopharmaceuticals

Age-related macular degeneration (AMD) is a condition that may cause blindness. The prevalence of the disease in the Western world is estimated at 1-2% of the population. Over the past decade, treatment of neovascular AMD has been shifting from destruction of newly formed blood vessels towards inhibitors that silence the vascular endothelial growth factor (VEGF) pathway. Such agents are often first-in-class biopharmaceuticals that benefit from the fact that they can be locally administered in an immune-privileged environment with slow clearance. These new VEGF pathway inhibitors have improved therapeutic effects over conventional treatment and have promoted the identification of novel targets for inhibition of AMD angiogenesis. This review describes the rationale behind the shift from conventional to current treatment options and discusses investigational, most notably biopharmaceutical, drugs that are in clinical trials. It also provides possible points for improvement of these treatments, specifically regarding their delivery.

NMDA receptor-mediated activation of NADPH oxidase and glomerulosclerosis in hyperhomocysteinemic rats

This study investigated the role of NMDA receptor in hyperhomocyteinemia (hHcys)-induced NADPH oxidase (Nox) activation and glomerulosclerosis. Sprague-Dawley rats were fed a folate-free (FF) diet to produce hHcys, and a NMDA receptor antagonist, MK-801, was administrated. Rats fed the FF diet exhibited significantly increased plasma homocysteine levels, upregulated NMDA receptor expression, enhanced Nox activity and Nox-dependent O(2)(.-) production in the glomeruli, which were accompanied by remarkable glomerulosclerosis. MK-801 treatment significantly inhibited Nox-dependent O(2)(.-) production induced by hHcys and reduced glomerular damage index as compared with vehicle-treated hHcys rats. Correspondingly, glomerular deposition of extracellular matrix components in hHcys rats was ameliorated by the administration of MK-801. Additionally, hHcys induced an increase in tissue inhibitor of metalloproteinase-1 (TIMP-1) expression and a decrease in matrix metalloproteinase (MMP)-1 and MMP-9 activities, all of which were abolished by MK-801 treatment. In vitro studies showed that homocysteine increased Nox-dependent O(2)(.-) generation in rat mesangial cells, which was blocked by MK-801. Pretreatment with MK-801 also reversed homocysteine-induced decrease in MMP-1 activity and increase in TIMP-1 expression. These results support the view that the NMDA receptor may mediate Nox activation in the kidney during hHcys and thereby play a critical role in the development of hHcys-induced glomerulosclerosis.

Role of sphingolipid mediator ceramide in obesity and renal injury in mice fed a high-fat diet

The present study tested a hypothesis that excess accumulation of sphingolipid, ceramide, its metabolites, or a combination contributes to the development of obesity and associated kidney damage. Liquid chromatography/mass spectrometry analysis demonstrated that C57BL/6J mice on the high-fat diet (HFD) had significantly increased plasma total ceramide levels compared with animals fed a low-fat diet (LFD). Treatment of mice with the acid sphingomyelinase (ASMase) inhibitor amitriptyline significantly attenuated the HFD-induced plasma ceramide levels. Corresponding to increase in plasma ceramide, the HFD significantly increased the body weight gain, plasma leptin concentration, urinary total protein and albumin excretion, glomerular damage index, and adipose tissue ASMase activity compared with the LFD-fed mice. These HFD-induced changes were also significantly attenuated by treatment of mice with amitriptyline. In addition, the decline of plasma glucose concentration after an intraperitoneal injection of insulin (0.15 U/kg b.wt.) was more sustained in mice on the HFD with amitriptyline than on the HFD alone. Intraperitoneal injection of glucose (3 g/kg b.wt.) resulted in a slow increase followed by a rapid decrease in the plasma glucose concentration in LFD and HFD plus amitriptyline-treated mice, but such blood glucose response was not observed in HFD-fed mice. Immunofluorescence analysis demonstrated a decrease in the podocin and an increase in the desmin in the glomeruli of HFD-fed mice compared with the LFD and HFD plus amitriptyline-treated mice. In conclusion, our results reveal a pivotal role for ceramide biosynthesis in obesity, metabolic syndrome, and associated kidney damage.