Podocyte as the target for aldosterone: roles of oxidative stress and Sgk1

Paradoxical mineralocorticoid receptor activation and left ventricular diastolic dysfunction under high oxidative stress conditions

BACKGROUND

Salt status plays a pivotal role in angiotensin-II-induced organ damage by regulating reactive oxygen species status, and it is reported that reactive oxygen species activate mineralocorticoid receptors.

METHOD

To clarify the role of reactive oxygen species-related mineralocorticoid receptor activation in angiotensin-II-induced cardiac dysfunction, we examined the effect of the following: salt status; an MR antagonist, eplerenone; and an antioxidant, tempol in angiotensin-II-loaded Sprague-Dawley rats.

RESULTS

Angiotensin-II/salt-loading elevated blood pressure, and neither eplerenone nor tempol antagonized the rise in blood pressure significantly. Left ventricular diastolic function was monitored by measuring peak velocity of a mitral early inflow (E), the ratio of mitral early inflow to atrial contraction related flow (E/A), deceleration time of mitral early inflow and -dP/dt, the time constant (T), and filling pressure (left ventricular end-diastolic pressure) by echocardiography or cardiac catheterization. Despite the suppressed serum aldosterone, left ventricular diastolic function was deteriorated with angiotensin II/high salt, but not affected by angiotensin II/low salt. However, angiotensin-II/salt-induced cardiac dysfunction was restored by eplerenone and tempol. Nicotinamide adenine dinucleotide phosphateoxidase-derived superoxide formation was greater in the hearts of the angiotensin II/high-salt rats than of the angiotensin II/low-salt rats. The expression of the Na(+) -H(+) exchanger isoform 1, a target of mineralocorticoid receptor activation, was significantly increased in the angiotensin II/high-salt group. Both tempol and eplerenone inhibited the angiotensin-II/salt-induced upregulation of Na(+) -H(+) exchanger isoform 1.

CONCLUSION

These findings demonstrate that mineralocorticoid receptor activation by oxidative stress can cause left ventricular diastolic dysfunction in a rat model of mild hypertension.

Reversal of proteinuric renal disease and the emerging role of endothelin

Proteinuria is a major long-term clinical consequence of diabetes and hypertension, conditions that lead to progressive loss of functional renal tissue and, ultimately, end-stage renal disease. Proteinuria is also a strong predictor of cardiovascular events. Convincing preclinical and clinical evidence exists that proteinuria and the underlying glomerulosclerosis are reversible processes. This Review outlines the mechanisms involved in the development of glomerulosclerosis--particularly those responsible for podocyte injury--with an emphasis on the potential capacity of endothelin receptor blockade to reverse this process. There is strong evidence that endothelin-1, a peptide with growth-promoting and vasoconstricting properties, has a central role in the pathogenesis of proteinuria and glomerulosclerosis, which is mediated via activation of the ET(A) receptor. Several antiproteinuric drugs, including angiotensin-converting-enzyme inhibitors, angiotensin receptor antagonists, statins and certain calcium channel blockers, inhibit the formation of endothelin-1. Preclinical studies have demonstrated that endothelin receptor antagonists can reverse proteinuric renal disease and glomerulosclerosis, and preliminary studies in humans with renal disease have shown that these drugs have remarkable antiproteinuric effects that are additive to those of standard antiproteinuric therapy. Additional clinical studies are needed.

Regression of glomerulosclerosis in subtotally nephrectomized rats: effects of monotherapy with losartan, spironolactone, and their combination

Angiotensin II accelerates and renin-angiotensin system blockade halts progression; blockade with high doses even reverses established glomerulosclerosis. Aldosterone also accelerates progression of glomerulosclerosis, partially independently of angiotensin II. The purpose of this study was to assess the relative ability of an angiotensin receptor type 1 (AT1) blocker, a mineralocorticoid receptor blocker, and their combination to reverse glomerulosclerosis. Sprague-Dawley rats were subjected to subtotal renal ablation (SNX) or sham operation. Eight weeks after surgery, they were either euthanized or allocated to treatment with vehicle, losartan, spironolactone, their combination, or unspecific antihypertensive treatment (dihydralazine) for 4 wk. Renal morphology was evaluated by stereology in tissues obtained using pressure-controlled perfusion fixation. Systolic blood pressure was significantly higher in SNX compared with sham-operated animals and decreased in all treatment groups. Compared with wk 8 after SNX, the glomerulosclerosis index (GSI) had increased further by week 12 in the vehicle- and dihydralazine-treated groups but was significantly lowered in the SNX+losartan as well as in the SNX+losartan+spironolactone groups and had not progressed further in the SNX+spironolactone group. The study confirms the partial regression of established glomerulosclerosis in subtotally nephrectomized rats after high-dose AT1 receptor blockade. Nonhyperkalemic doses of spironolactone prevented the increase but failed to decrease the GSI below the 8-wk level and preserved podocyte numbers. Combining the AT1 blocker with mineralocorticoid receptor blockade failed to further increase the regression of glomerulosclerosis.

Mineralocorticoid receptor blockade ameliorates peritoneal fibrosis in new rat peritonitis model

Peritoneal fibrosis (PF) is an important complication of long-term peritoneal dialysis. Although mineralocorticoid and mineralocorticoid receptor (MR) have attracted increasing attention in the field of vascular injury, including the heart, kidney, and vessels, little is known about the role of mineralocorticoid in PF. This work was designed to explore the effects of MR blockade on PF. We developed a new model of PF in rats based on mechanical scraping of the peritoneum. This model is characterized by acute-phase inflammation (neutrophil and macrophage infiltration on days 0–3) and late-phase PF (α-smooth muscle actin-positive fibroblast infiltration, type III collagen accumulation, and neoangiogenesis on days 7–14). Peritoneal thickening peaked on day 14. MR was expressed in rat peritoneum and a rat fibroblast cell line. Expression of its effector kinase [serum- and glucocorticoid-induced kinase-1 (Sgk1)], transforming growth factor-β (TGF-β), plasminogen activator inhibitor-1 (PAI-1), and CD31-positive vessels increased during the course of PF. Rats were treated with spironolactone, angiotensin receptor blockade (ARB), or angiotensin-converting enzyme inhibitor (ACEI)-ARB-spironolactone starting at 6 h after peritoneal scraping. All parameters, including peritoneal thickening, number of macrophages and CD31-positive vessels, and expression of monocyte chemoattractant protein-1, TGF-β, PAI-1, and Sgk1, were significantly suppressed by spironolactone (10 mg·kg−1·day−1). The effects of spironolactone (10 and 20 mg·kg−1·day−1) were very similar to those of triple blockade. ARB, but not ACEI, significantly reduced peritoneal thickening. Furthermore, peritoneal function assessed by peritoneal equilibration test was significantly improved by spironolactone. Our results suggest that MR is a potential target to prevent inflammation-induced PF in patients on peritoneal dialysis.

Aldosterone in salt-sensitive hypertension and metabolic syndrome

Metabolic syndrome, which is caused by obesity, is now a global pandemic. Metabolic syndrome is an aggregation of hypertension, diabetes and dyslipidaemia. Insulin resistance is a key factor in the development of these components of metabolic syndrome. Concerning the mechanism for the development of hypertension in metabolic syndrome, the lack of insulin resistance in the kidney increases sodium reabsorption by hyperinsulinaemia, leading to sodium retention in the body, and resultant salt-sensitive hypertension. Moreover, hyperaldosteronism, which is caused by adipocyte-derived aldosterone-releasing factors, induces not only salt-sensitive hypertension, but also proteinuria in obese hypertensive rats. Salt loading markedly aggravates proteinuria and induces cardiac diastolic dysfunction in obese hypertensive rats, suggesting that salt and aldosterone exert unfavourable synergistic actions on the cardiovascular system, possibly through the overproduction of oxidative stress. In turn, reactive oxygen species (ROS), which are induced by adipokines such as tumour necrosis factor-alpha, non-esterified fatty acids, angiotensinogen etc., can activate the mineralocorticoid (MR) receptor, in an aldosterone-independent fashion. Therefore, aldosterone/MR activation plays a key role not only in the development of salt-sensitive hypertension, but also in cardiovascular injury in metabolic syndrome, possibly through its function as a feed-forward system.

Can urinary excretion rate of malondialdehyde, uric acid and protein predict the severity and impending death in perinatal asphyxia?

BACKGROUND

Perinatal asphyxia (PA) associated with multi-organ damage is a leading cause of neonatal mortality and morbidity. We evaluated if urinary malondialdehyde:creatinine (UMDA:Cr), uric acid:creatinine (UUA:Cr) and protein:creatinine (UP:Cr) vary with the severity of PA and if these parameters can predict the impending death in PA.

METHODS

Study included 20 asphyxiated and 20 healthy newborn males. Hypoxic-ischemic encephalopathy (HIE) staging, APGAR (activity, pulse, grimace, appearance and respiration) score and urinary protein, uric acid, creatinine and MDA were evaluated.

RESULTS

UMDA:Cr, UUA:Cr and UP:Cr were significantly higher and correlated with APGAR and HIE in PA. By regression analysis also, urinary parameters were found to have significant association with HIE stage and APGAR in PA. Receiver operating characteristics (ROC) curve of UP:Cr, UUA:Cr and UMDA:Cr showed area under curve of 0.896 (p=0.003), 0.859 (p=0.008) and 0.849 (p=0.010) with cut-off value of 9.04 mg, 2.34 mg and 3.49 microg/mg of creatinine respectively that can optimally predict the impending death in PA. SDS-PAGE of unconcentrated urine detected both high (73 kDa and 68 kDa) and low molecular weight proteins (52 kDa, 47 kDa, 25 kDa and 20 kDa) in PA but not in controls.

CONCLUSION

Urinary excretion rate of uric acid, MDA and proteins is higher and has potential to act as biochemical markers for severity evaluation and death prediction in PA.

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.

The mineralocorticoid receptor: insights into its molecular and (patho)physiological biology

The last decade has witnessed tremendous progress in the understanding of the mineralocorticoid receptor (MR), its molecular mechanism of action, and its implications for physiology and pathophysiology. After the initial cloning of MR, and identification of its gene structure and promoters, it now appears as a major actor in protein-protein interaction networks. The role of transcriptional coregulators and the determinants of mineralocorticoid selectivity have been elucidated. Targeted oncogenesis and transgenic mouse models have identified unexpected sites of MR expression and novel roles for MR in non-epithelial tissues. These experimental approaches have contributed to the generation of new cell lines for the characterization of aldosterone signaling pathways, and have also facilitated a better understanding of MR physiology in the heart, vasculature, brain and adipose tissues. This review describes the structure, molecular mechanism of action and transcriptional regulation mediated by MR, emphasizing the most recent developments at the cellular and molecular level. Finally, through insights obtained from mouse models and human disease, its role in physiology and pathophysiology will be reviewed. Future investigations of MR biology should lead to new therapeutic strategies, modulating cell-specific actions in the management of cardiovascular disease, neuroprotection, mineralocorticoid resistance, and metabolic disorders.

Salt-induced nephropathy in obese spontaneously hypertensive rats via paradoxical activation of the mineralocorticoid receptor: role of oxidative stress

Aldosterone is implicated in the pathogenesis of proteinuria and chronic kidney disease. We previously demonstrated the contribution of elevated serum aldosterone in the early nephropathy of SHR/NDmcr-cp (SHR/cp), a rat model of metabolic syndrome. In the present study, we investigated the effect of salt loading on renal damage in SHR/cps and explored the underlying mechanisms. SHR/cps fed a high-sodium diet for 4 weeks developed severe hypertension, massive proteinuria, and advanced renal lesions. High salt also worsened glomerular podocyte impairment. Surprisingly, selective mineralocorticoid receptor (MR) antagonist eplerenone dramatically ameliorated the salt-induced proteinuria and renal injury in SHR/cps. Although salt loading reduced circulating aldosterone, it increased nuclear MR and expression of aldosterone effector kinase Sgk1 in the kidney. Gene expressions of transforming growth factor-beta1 and plasminogen activator inhibitor-1 were also enhanced in the kidneys of salt-loaded SHR/cps, and eplerenone completely inhibited these injury markers. To clarify the discrepancy between decreased aldosterone and enhanced MR signaling by salt, we further investigated the role of oxidative stress, a putative key factor mediating salt-induced tissue damage. Interestingly, antioxidant Tempol attenuated the salt-evoked MR upregulation and Sgk1 induction and alleviated proteinuria and renal histological abnormalities, suggesting the involvement of oxidative stress in salt-induced MR activation. MR activation by salt was not attributed to increased serum corticosterone or reduced 11beta-hydroxysteroid dehydrogenase type 2 activity. In conclusion, sodium loading exacerbated proteinuria and renal injury in metabolic syndrome rats. Salt reduced circulating aldosterone but caused renal MR activation at least partially via induction of oxidative stress, and eplerenone effectively improved the nephropathy.

Aldosterone and end-organ damage

Aldosterone concentrations are inappropriately high in many patients with hypertension, as well as in an increasing number of individuals with metabolic syndrome and sleep apnoea. A growing body of evidence suggests that aldosterone and/or activation of the MR (mineralocorticoid receptor) contributes to cardiovascular remodelling and renal injury in these conditions. In addition to causing sodium retention and increased blood pressure, MR activation induces oxidative stress, endothelial dysfunction, inflammation and subsequent fibrosis. The MR may be activated by aldosterone and cortisol or via transactivation by the AT(1) (angiotenin II type 1) receptor through a mechanism involving the EGFR (epidermal growth factor receptor) and MAPK (mitogen-activated protein kinase) pathway. In addition, aldosterone can generate rapid non-genomic effects in the heart and vasculature. MR antagonism reduces mortality in patients with CHF (congestive heart failure) and following myocardial infarction. MR antagonism improves endothelial function in patients with CHF, reduces circulating biomarkers of cardiac fibrosis in CHF or following myocardial infarction, reduces blood pressure in resistant hypertension and decreases albuminuria in hypertensive and diabetic patients. In contrast, whereas adrenalectomy improves glucose homoeostasis in hyperaldosteronism, MR antagonism may worsen glucose homoeostasis and impairs endothelial function in diabetes, suggesting a possible detrimental effect of aldosterone via non-genomic pathways.

Suppressing renal NADPH oxidase to treat diabetic nephropathy

Renal nicotinamide adenine dinucleotide phosphate reduced form (NADPH) oxidase is an important source of oxidative stress and its expression is enhanced in the glomerulus and distal tubules of diabetic nephropathy. High glucose-induced protein kinase C signalling or renal angiotensin II signalling increases the membrane translocation of cytosolic component p47phox. NADPH oxidase-derived reactive oxygen species (ROS) in the podocytes damage the glomerular basement membrane and the slit diaphragm causing proteinuria, and mesangial and glomerular endothelial NADPH oxidase increase TGF-beta and cause collagen and fibronectin accumulation. Tubular NADPH oxidase stimulated by angiotensin II or aldosterone contributes to sodium retention and to tubulointerstitial damage. Thus, inhibition of the renal renin-angiotensin II-aldosterone system with angiotensin-converting enzyme inhibitor, angiotensin II type 1 receptor blocker or selective aldosterone inhibitor indirectly suppresses NADPH oxidase reducing renal ROS, proteinuria and glomerulosclerosis. Statins are also effective in blocking the membrane translocation of Rac, especially in diabetes with hypercholesterolemia where ROS is produced by the intrinsic NADPH oxidase and by the activated macrophages. A medical herb, picrorhiza, inhibits the membrane translocation of p47phox, is a specific inhibitor of NADPH oxidase and, more so than superoxide dismutase mimetics, may be a promising strategy for the treatment of diabetic nephropathy.