Aldosterone and renal injury

Novel Pathophysiological Mechanisms in Hypertension

Hypertension is the most common disease affecting humans and imparts a significant cardiovascular and renal risk to patients. Extensive research over the past few decades has enhanced our understanding of the underlying mechanisms in hypertension. However, in most instances, the cause of hypertension in a given patient continues to remain elusive. Nevertheless, achieving aggressive blood pressure goals significantly reduces cardiovascular morbidity and mortality, as demonstrated in the recently concluded SPRINT trial. Since a large proportion of patients still fail to achieve blood pressure goals, knowledge of novel pathophysiologic mechanisms and mechanism based treatment strategies is crucial. The following chapter will review the novel pathophysiological mechanisms in hypertension, with a focus on role of immunity, inflammation and vascular endothelial homeostasis. The therapeutic implications of these mechanisms will be discussed where applicable.

Ginsenoside Rg1 reduces aldosterone-induced autophagy via the AMPK/mTOR pathway in NRK-52E cells

Aldosterone is a steroid hormone secreted from the adrenal cortex, which regulates blood pressure. Higher concentrations of aldosterone can cause several diseases, including hypertension, diabetic nephropathy and chronic kidney disease. Previous reports have demonstrated that aldosterone has a pathogenic role in renal injury via reactive oxygen species (ROS), which involves the regulation of autophagy. However, whether aldosterone can induce autophagy in renal tubular cells remains to be elucidated. In the present study, elevated autophagy was observed in rat renal tubular NRK-52E cells exposed to aldosterone, which was demonstrated by the increased number of autophagosomes, conversion of LC3-I to LC3-II and the expression of Beclin-1. The enhanced autophagy was accompanied by increased production of intracellular ROS, which was reversed by N-acetylcysteine, a specific inhibitor of ROS signaling. Furthermore, treatment with ginsenoside Rg1 reduced the aldosterone-induced autophagy and production of ROS, possibly through reducing the phosphorylation of AMPK and preserving mTOR activity. These findings demonstrated that aldosterone promoted ROS generation and increased autophagy in the NRK-52E cells. Ginsenoside Rg1 effectively relieved aldosterone-induced oxidative stress and abnormal autophagy, suggesting that Rg1 may be used as a potential therapeutic drug to inhibit the renal injury, which is induced by aldosterone.

Oxidative damages in tubular epithelial cells in IgA nephropathy: role of crosstalk between angiotensin II and aldosterone

Background

Inhibition of the renin-angiotensin-aldosterone system (RAAS) slows down the progression of chronic renal diseases (CKD) including IgA nephropathy (IgAN). Herein, we studied the pathogenetic roles of aldosterone (Aldo) in IgAN.

Methods

Human mesangial cells (HMC) was activated with polymeric IgA (pIgA) from IgAN patients and the effects on the expression of RAAS components and TGF-β synthesis examined. To study the roles of RAAS in the glomerulotubular communication, proximal tubular epithelial cells (PTEC) was cultured with conditioned medium from pIgA-activated HMC with eplerenone or PD123319, the associated apoptotic event was measured by the generation of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase and reactive oxygen species (ROS).

Results

Polymeric IgA up-regulated the Aldo synthesis and aldosterone synthase expression by HMC. The release of TGF-β by HMC was up-regulated synergistically by AngII and Aldo and this was inhibited by incubation of HMC with losartan plus eplerenone. Cultured PTEC express the mineralocorticoid receptor, but not synthesizing aldosterone. Apoptosis, demonstrated by cleaved PARP expression and caspase 3 activity, was induced in PTEC activated by conditioned medium prepared from HMC cultured with pIgA from IgAN patients. This apoptotic event was associated with increased generation of NADPH oxidase and ROS. Pre-incubation of PTEC with PD123319 and eplerenone achieved complete inhibition of PTEC apoptosis.

Conclusions

Our data suggest that AngII and Aldo, released by pIgA activated HMC, served as mediators for inducing apoptosis of PTEC in glomerulo-tubular communications. Crosstalk between AngII and Aldo could participate in determining the tubular pathology of IgAN.

Effects of mineralocorticoid receptor blockade on glucocorticoid-induced renal injury in adrenalectomized rats

OBJECTIVES

Aldosterone is well recognized as the selective physiological ligand for mineralocorticoid receptor in epithelia. However, in-vitro studies have demonstrated that the affinity of aldosterone and glucocorticoids for mineralocorticoid receptor is similar. We hypothesized that glucocorticoids are involved in the development of renal injury through an mineralocorticoid receptor-dependent mechanism.

METHODS AND RESULTS

Uninephrectomized (UNX) rats were treated with 1% NaCl and divided into three groups: vehicle, bilateral adrenalectomy (ADX) + hydrocortisone (HYDRO; 5 mg/kg/day, s.c.), ADX + HYDRO + eplerenone (0.125% in chow). HYDRO-treated UNX-ADX rats showed increased blood pressure and urinary albumin-to-creatinine ratio with an increase in the expression of the mineralocorticoid receptor target genes, serum and glucocorticoid-regulated kinases-1 and Na+/H+ exchanger isoform-1, in renal tissues. HYDRO treatment induced morphological changes in the kidney, including glomerulosclerosis and podocyte injury. Treatment with eplerenone markedly decreased the gene expression and reduced the albuminuria and renal morphological changes. In contrast, dexamethasone (0.2 mg/kg per day, s.c.) + UNX + ADX induced hypertension and albuminuria in different groups of rats. Eplerenone failed to ameliorate these changes.

CONCLUSIONS

Our findings indicate that chronic glucocorticoid excess could activate mineralocorticoid receptor and, in turn, induce the development of renal injury.

Aldosterone-induced mesangial cell proliferation is mediated by EGF receptor transactivation

Aldosterone (Aldo) stimulates glomerular mesangial cell (MC) proliferation, in part, through an ERK1/2-dependent pathway. In this study, we examined whether Aldo activation of ERK1/2 in MC is mediated through redox-dependent EGF receptor (EGFR) transactivation, as well as the involvement of other signaling mechanisms in Aldo-induced MC proliferation. Aldo increased human MC proliferation, as determined by [3H]thymidine incorporation and cell counts. This increase in proliferation was blocked by inhibition of the mineralocorticoid receptor (MR). Continuing our observations downstream in the signaling pathway, we examined the ability of Aldo to activate both the Ras/MAPK and the PI3K signaling pathways. Aldo increased Ki-RasA and Ki-RasA:GTP levels, and sequentially phosphorylated c-Raf, MAPK kinase (MEK1/2), and ERK1/2. Ki-RasA small interfering RNA (siRNA), the c-Raf inhibitor GW5074, and the MEK1/2 inhibitor PD98059 reduced Aldo-induced cell proliferation by ∼65%. Aldo also increased phosphorylation of PI3K, Akt, the mammalian target of rapamycin (mTOR), and the 70-kDa ribosomal S6 kinase (p70S6K1). Inhibition of the PI3K pathways by the selective PI3K inhibitor LY 294002, an Akt inhibitor, or the mTOR inhibitor rapamycin reduced cell proliferation by 51%. Combining LY 294002 and PD98059 completely blocked Aldo-induced MC proliferation. Next, we confirmed that Aldo exerts its effect on MAPK and PI3K activation, as well as on cell proliferation, by activating the EGFR. Pretreatment with the EGFR antagonist AG1478 inhibited MC proliferation, as well as the activation of Ras/MAPK and PI3K/Akt, suggesting that Ras/MAPK and PI3K/Akt activation occur downstream of EGFR activation. Finally, we examined the role of reactive oxygen species (ROS) in Aldo-induced transactivation of the EGFR. Aldo-induced ROS were predominantly generated by mitochondria. Pretreatment with the antioxidant N-acetyl-l-cysteine, catalase, SOD, mitochondrial respiratory chain complex I inhibitor rotenone (Rot), NADPH oxidase inhibitor apocynin, and DPI significantly inhibited Aldo-stimulated MC proliferation as well as EGFR transactivation. However, Rot reduced MC proliferation more potently than apocynin and DPI. In conclusion, Aldo stimulated cell proliferation through MR-mediated, redox-sensitive EGFR transactivation, which was dependent on the Ki-RasA/c-Raf/MEK/ERK and PI3K/Akt/mTOR/p70S6K1 signaling pathways in human MCs.

Chemistry and antihypertensive effects of tempol and other nitroxides

Nitroxides can undergo one- or two-electron reduction reactions to hydroxylamines or oxammonium cations, respectively, which themselves are interconvertible, thereby providing redox metabolic actions. 4-Hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl (tempol) is the most extensively studied nitroxide. It is a cell membrane-permeable amphilite that dismutates superoxide catalytically, facilitates hydrogen peroxide metabolism by catalase-like actions, and limits formation of toxic hydroxyl radicals produced by Fenton reactions. It is broadly effective in detoxifying these reactive oxygen species in cell and animal studies. When administered intravenously to hypertensive rodent models, tempol caused rapid and reversible dose-dependent reductions in blood pressure in 22 of 26 studies. This was accompanied by vasodilation, increased nitric oxide activity, reduced sympathetic nervous system activity at central and peripheral sites, and enhanced potassium channel conductance in blood vessels and neurons. When administered orally or by infusion over days or weeks to hypertensive rodent models, it reduced blood pressure in 59 of 68 studies. This was accompanied by correction of salt sensitivity and endothelial dysfunction and reduced agonist-evoked oxidative stress and contractility of blood vessels, reduced renal vascular resistance, and increased renal tissue oxygen tension. Thus, tempol is broadly effective in reducing blood pressure, whether given by acute intravenous injection or by prolonged administration, in a wide range of rodent models of hypertension.

Aldosterone induces mesangial cell apoptosis both in vivo and in vitro

Both clinical and experimental reports indicate that aldosterone contributes to the progression of renal failure independent of its hemodynamic effects. In the present study, we evaluated effect of aldosterone on human mesangial cell (MC) growth. Aldosterone induced apoptotic and mitogenic effects on MCs. Aldosterone promoted MC apoptosis in a dose- and time-dependent manner. Spironolactone, a mineralocorticoid receptor antagonist, inhibited aldosterone-induced MC apoptosis. Similarly, antioxidants and free radical scavengers partially attenuated proapoaptotic effects of aldosterone. Aldosterone also enhanced dephosphorylation of phospho-Bad and accumulation of cytosolic cytochrome c in MCs. In in vivo studies, rats were randomly assigned to receive normal saline, aldosterone, or eplerenone + aldosterone for 28 days. Systolic blood pressure, urinary albumin excretion rate, serum creatinine, and aldosterone were measured. Aldosterone-infused rats developed elevated systolic blood pressure and albuminuria when compared with control rats. Aldosterone-treated rats also showed greater numbers of apoptosed MCs. This proapoptotic effect of aldosterone was inhibited by eplerenone, a selective aldosterone antagonist. These findings suggest that aldosterone, besides its hemodynamic effects, may also directly contribute to the occurrence of MC apoptosis.

Eplerenone potentiates the antiproteinuric effects of enalapril in experimental nephrotic syndrome

Nephrotic syndrome (NS) is a clinical state characterized by massive proteinuria and edema. It is believed that nephrin and podocin are involved in the development of proteinuria. The proteinuria and effects of eplerenone alone or combined with enalapril on nephrin/podocin abundance in rats with NS have not yet been studied. Therefore, the present study was designed to examine the early (beginning 2 days before NS induction) and late (beginning 2 wk after NS induction) effects of eplerenone and enalapril, alone or combined, on proteinuria and nephrin/podocin abundance in rats with adriamycin-induced NS. Adriamycin caused a significant increase in daily protein excretion (UprV; from 26.96 ± 3.43 to 958.57 ± 56.7 mg/day, P < 0.001) and cumulative proteinuria [from 900.33 ± 135.5 to 22,490.62 ± 931.26 mg ( P < 0.001)] during 6 wk. Early treatment with enalapril significantly decreased UprV from 958.6 ± 56.7 to 600.31 ± 65.13 mg/day ( P < 0.001) and cumulative proteinuria to 12,842.37 ± 1,798.17 mg/6 wk ( P < 0.001). Similarly, early treatment with eplerenone produced a profound antiproteinuric effect: UprV decreased from 958.57 ± 56.7 to 593.38 ± 21.83 mg/day, P < 0.001, and cumulative proteinuria to 16,601.84 ± 1,334.31 mg/6 wk; P < 0.001. An additive effect was obtained when enalapril and eplerenone were combined: UprV decreased from 958.57 ± 56.69 to 424.17 ± 38.54 mg/day, P < 0.001, and cumulative protein excretion declined to 10,252.88 ± 1,011.3 mg/6 wk, P < 0.001. These antiproteinuric effects were associated with substantial preservation of glomerular nephrin and podocin. In contrast, late treatment with either enalapril or eplerenone alone or combined mildly decreased UprV and cumulative proteinuria. Thus pretreatment with eplerenone or enalapril is effective in reducing daily and cumulative protein excretion and preservation of nephrin/podocin. More profound antiproteinuric effects were obtained when enalapril and eplerenone were combined.

Thiazide-induced subtle renal injury not observed in states of equivalent hypokalemia

Hydrochlorothiazide (HCTZ) is used to manage hypertension and heart failure; however, its side effects include mild hypokalemia, metabolic abnormalities, and volume depletion, which might have deleterious effects on renal and endothelial function. We studied whether HCTZ cause renal injury and/or altered vasoreactivity and if these changes are hypokalemia-dependent. Rats were given a normal diet or a diet moderately low in potassium K+ with or without HCTZ. Animals fed either a low K+ diet alone or HCTZ developed mild hypokalemia. There was no significant difference in systolic blood pressure in the different treatment groups. All three groups with hypokalemia had mild proteinuria; low K(+)-HCTZ rats had reduced creatinine clearance. HCTZ-treated rats displayed hypomagnesemia, hypertriglyceridemia, hyperglycemia, insulin resistance, and hyperaldosteronism. No renal injury was observed in the groups without HCTZ; however, increased kidney weight, glomerular ischemia, medullary injury, and cortical oxidative stress were seen with HCTZ treatment. Endothelium-dependent vasorelaxation was reduced in all hypokalemic groups and correlated with reduced serum K+, serum, and urine nitric oxide. Our results show that HCTZ is associated with greater renal injury for the same degree of hypokalemia as the low K+ diet, suggesting that factors such as chronic ischemia and hyperaldosteronism due to volume depletion may be responsible agents. We also found impaired endothelium-dependent vasorelaxation was linked to mild hypokalemia.

Aldosterone promotes proximal tubular cell apoptosis: role of oxidative stress

Aldosterone has attracted significant consideration for its role in the progression of renal injury. Since apoptotic cell loss contributes to the deterioration of renal function, we examined the effect of aldosterone on tubular cell apoptosis. To determine dose and time course effect, human renal proximal tubular (HK2) cells were treated with aldosterone at different doses and for variable time periods followed by evaluation for apoptosis. To determine the role of mineralocorticoid receptors (MR) and oxidative stress, HK2 cells were treated with either vehicle or aldosterone in the presence or absence of spironolactone/antioxidants/free radical scavengers (FRS) followed by evaluation for apoptosis. The presence of MR was evaluated using RT-PCR. Reactive oxygen species (ROS) generation was evaluated using redox-sensitive dyes. Effect of aldosterone was evaluated on dephosphorylation of phospho-Bad and accumulation of cytosolic cytochrome c. Human tubular cells express MR. Aldosterone promotes tubular cell apoptosis in a dose- and time-dependent manner. This effect of aldosterone is mediated through MR and associated with generation of ROS. Antioxidants and FRS partially attenuated the proapoaptotic effect of aldosterone. Aldosterone enhanced dephosphorylation of phospho-Bad and accumulation of cytosolic cytochrome c. We conclude that aldosterone-induced tubular cell apoptosis is mediated through the activation of the mitochondrial pathway and generation of ROS.

Aldosterone up-regulates production of plasminogen activator inhibitor-1 by renal mesangial cells

In vivo studies have demonstrated that aldosterone is an independent contributor to glomerulosclerosis. In the present study, we have investigated whether aldosterone itself mediated glomerulosclerosis, as angiotensin II (Ang II) did, by inducing cultured renal mesangial cells to produce plasminogen activator inhibitor-1 (PAI-1), and whether these effects were mediated by aldosterone-induced increase in transforming growth factor beta(1) (TGF-beta(1)) expression and cellular reactive oxygen species (ROS) activity. Quiescent rat mesangial cells were treated by aldosterone alone or by combination of aldosterone and spironolactone, Ang II, neutralizing antibody to TGF-beta(1) or antioxidant Nacetylcysteme (NAC). This study indicate that aldosterone can increase PAI-1 mRNA and protein expression by cultured mesangial cells alone, which is independent of aldosterone-induced increases in TGF-beta(1) expression and cellular ROS. The effects on PAI-1, TGF-beta(1) and ROS generation were markedly attenuated by spironolactone, a mineralocorticoid receptor antagonist, which demonstrate that mineralocorticoid receptor (MR) may play a role in mediating these effects of aldosterone.

New Generation Calcium Channel Blockers in Hypertensive Treatment

During a couple of decades, a number of antihypertensive drugs have been developed, and the choice of hypertension treatment has been expanded. Among antihypertensive drugs, calcium channel blockers, which inhibit L-type voltage-gated calcium channels, are potent vasodilators, and have been used as a first- or second-line drug. Dihydropyridine-class calcium channel blockers are categorized into three generations according to the length of activity, and long-acting calcium channel blockers cause less activation of sympathetic nervous system, and are reported to offer beneficial action compared with short-action agents. Furthermore, novel types of calcium channel blockers have been developed that possess the blocking action on other calcium channel subtypes (T- and N-type), and exert agent-specific action apart from their class effects, such as the effects on heart rate and renin/aldosterone release. These additional benefits conferred by T/N-type calcium channel blockade are anticipated to provide organ protective actions in the treatment of hypertension, in addition to the blood pressure-lowering effect of L-type calcium channel blockade. In conclusion, novel calcium channel blockers with sustained activity and T/N-type calcium channel blocking action could provide more beneficial effects than classical blockers, and may expand the clinical utility of these agents.

Molecular Mechanisms and Therapeutic Strategies of Chronic Renal Injury: Renoprotective Effects of Aldosterone Blockade

Recent clinical and pre-clinical studies have indicated the utility of mineralocorticoid receptor (MR) antagonists in renal injury. We have demonstrated in rats that chronic treatment with aldosterone results in severe proteinuria and renal injury, characterized by glomerular changes, tubulointerstitial fibrosis, and collagen accumulation. We also observed increased reactive oxygen species (ROS) generation and mitogen-activated protein kinases (MAPKs) activity in renal cortical tissues. Treatment with a selective MR antagonist, eplerenone, prevented elevation of ROS levels and MAPK activity, as well as ameliorating renal injury. In vitro studies revealed that MRs are highly expressed in rat glomerular mesangial cells (RMC) and rat renal fibroblasts. In RMC, aldosterone induces cellular injuries through NADPH oxidase-dependent ROS production and/or MAPK activation. Aldosterone-induced renal cellular injuries were markedly attenuated by treatment with eplerenone. These data suggest that aldosterone induces renal injury through activation of MRs and support the notion that MR blockade has beneficial effects on aldosterone-dependent renal injury through mechanisms that cannot be simply explained by hemodynamic changes. In this review, we summarized our recent findings pertaining to the roles of aldosterone and MRs in the pathogenesis of renal injury. Potential molecular mechanisms responsible for aldosterone/MR-induced renal injury were also discussed.

Growth of embryonic renal parenchyme at the interphase of a polyester artificial interstitium

The construction of an artificial kidney module by tissue engineering or the application of cell-based therapies for the treatment of renal failure requires exact information regarding the cellbiological mechanisms of parenchyme development in combination with different kinds of biomaterials. To learn more about these processes tissue cultures are frequently used experimental tools. However, apart from experiments with early kidney anlagen there is a lack of suitable in-vitro models regarding the generation and long-term maintenance of renal tubules. In the present paper we like to demonstrate an advanced culture technique, which allows to generate tubular elements derived from renal stem cells. For the growth of tubules it is essential to fine-tune the interface between the embryonic tissue and the dead fluid space within a perfusion culture container by offering a polyester artificial interstitium. Culture was performed in IMDM supplemented with hormones and growth factors but using serum-free conditions over 14 days. Formation of tissue was then analysed by immunohistochemistry and two-dimensional (2D) electrophoresis. Culture in pure IMDM leads to a complete loss of tissue formation. In contrast, application of aldosterone (A) induces the development of numerous polarised tubules. Surprisingly, addition of epidermal growth factor (EGF), a cocktail of insulin, transferrin and selenium (ITS), retinoic acid (RA), cholecalciferol (VitD3) or bovine pituitary extract (BPT) does not further improve development of tubules, but leads to intensive cell clustering and a decrease of tubule formation. 2D Western blots of developing tissue probed with soybean agglutinin (SBA) reveal a unique pattern of newly detected proteins. It is found that growth factors do not support but abolish protein spots upregulated by aldosterone. It remains to be investigated, which cellbiological effect stimulates the embryonic cells to develop tubules in competition to cell clusters at the interphase of an artificial interstitium.