Chronic hypoxia and tubulointerstitial injury: a final common pathway to end-stage renal failure

Involvement of asymmetric dimethylarginine (ADMA) in tubulointerstitial ischaemia in the early phase of diabetic nephropathy

BACKGROUND

Decreased peritubular capillary (PTC) flow due to impaired endothelial function elicits tubulointerstitial ischaemia, thereby enhancing renal damage in chronic kidney disease, including diabetic nephropathy. Since nitric oxide (NO) is a vasodilator and known to play an important role in the maintenance of PTC flow, it is conceivable that asymmetric dimethylarginine (ADMA), an endogenous inhibitor of NO synthase, may cause tubulointerstitial ischaemia, thus being involved in the progression of diabetic nephropathy. In this study, we investigated whether overexpression of dimethylarginine dimethylaminohydrolase (DDAH), an enzyme that degrades ADMA, could improve tubulointerstitial ischaemia in streptozotocin (STZ)-induced diabetic rats.

METHODS

Recombinant adenovirus vector encoding DDAH-I (Adv-DDAH) or control vector expressing bacterial beta-galactosidase (Adv-LZ) was intravenously administrated to diabetic rats. Three days after the treatment, effects of DDAH overexpression on plasma or urinary levels of ADMA or NO metabolites (NOx), tubulointerstitial ischaemia and renal expression of transforming growth factor-beta (TGF-beta) were evaluated.

RESULTS

Renal DDAH expression and activity were reduced in diabetic rats. Urinary levels of ADMA and TGF-beta were increased, while NOx levels were decreased in diabetic rats. Compared with control rats, pimonidazole-detected hypoxic areas were larger in the kidney of diabetic rats, although the number of capillaries in tubulointerstitial regions was not different between the two groups. In addition, renal expression levels of hypoxia-inducible factor-1alpha (HIF-1alpha) and TGF-beta were also increased in diabetic rats. DDAH overexpression significantly inhibited the increase of ADMA and the decrease of NOx and subsequently decreased urinary albumin excretion levels and ameliorated tubulointerstitial hypoxia and HIF-1alpha as well as TGF-beta expression in diabetic rats.

CONCLUSION

The present study demonstrated for the first time that the suppression of ADMA by DDAH overexpression could improve tubulointerstitial ischaemia and subsequent renal damage in experimental diabetic nephropathy. Substitution of DDAH protein or enhancement of its activity may become a novel therapeutic strategy for the treatment of early diabetic nephropathy.

Inhibition of advanced glycation end products (AGEs): an implicit goal in clinical medicine for the treatment of diabetic nephropathy?

Several factors are incriminated in the genesis of diabetic nephropathy (DN). To elucidate their interplays, we utilized a diabetic rat model with nephropathy (SHR/NDmcr-cp). This model is characterized by hypertension, obesity with the metabolic syndrome, diabetes with insulin resistance, and intrarenal AGE accumulation. Various therapeutic approaches were used to achieve renoprotection. Caloric restriction corrects metabolic abnormalities and protects the kidney without correcting hypertension. Anti-hypertensive agents, angiotensin II receptor blocker (ARB) and calcium channel blocker, lower blood pressure to the same extent, but only ARBs protect the kidney without changes in metabolic abnormalities. Glycemic control is better with insulin than with pioglitazone. The plasma insulin level is increased by insulin but decreased by pioglitazone which worsens the obesity. Nevertheless, pioglitazone provides renoprotection unlike insulin, perhaps as a result of the up-regulation of TGF-beta by hyperinsulinemia. Cobalt up-regulates the expression of a hypoxia-inducible factor (HIF) and its downstream genes (erythropoietin, VEGF, HO-1). It protects the kidney without correcting hypertension and metabolic abnormalities. Altogether, renoprotection is not necessarily associated with blood pressure or glycemic control. By contrast, it is almost always associated with a decreased AGE formation. AGE reduction may reflect a decreased oxidative stress as it is concomitant with a marked reduction of oxidative stress markers.

Chronic kidney disease at high altitude

With a prevalence of 10 to 11% in the general population, it is likely that many patients with chronic kidney disease will visit or reside in mountainous areas. Little is known, however, about whether short- or long-duration, high-altitude exposure poses a risk in this patient population. Given that many areas of the kidney are marginally oxygenated even at sea level and that kidney disease may result in further renal hypoxia and hypoxia-associated renal injury, there is concern that high altitude may accelerate the progression of chronic kidney disease. In this review, we address how chronic kidney disease and its management is affected at high altitude. We postulate that arterial hypoxemia at high altitude poses a risk of faster disease progression in those with preexisting kidney disease. In addition, we consider the risks of developing acute altitude illness in patients with chronic kidney disease and the appropriate use of medications for the prevention and treatment of these problems.

Metallothionein is upregulated by hypoxia and stabilizes hypoxia-inducible factor in the kidney

Recent studies underscore that chronic hypoxia in the tubulointerstitium is a final common pathway to progression to end-stage renal failure regardless of etiology. We used microarray analysis of rat kidneys made hypoxic by unilateral renal artery stenosis to measure transcriptomic events and clarify pathophysiological mechanisms of renal injury induced by chronic hypoxia. Many genes were upregulated in the kidney by chronic hypoxia, but we focused on metallothionein due to its antioxidative properties. Using tubular epithelial cells transfected with a reporter construct of luciferase, driven by the hypoxia-responsive elements (HRE), we found that addition of metallothionein to the culture media increased luciferase activity. This was associated with upregulation of the target genes of hypoxia-inducible factor (HIF), such as vascular endothelial growth factor and glucose transporter-1. Stimulation of the HIF-HRE pathway by metallothionein was confirmed by metallothionein overexpression. Hypoxia and exogenous metallothionein increased HIF-1alpha protein without changes in its mRNA levels, suggesting protein stabilization. Upregulation of the HIF-HRE system by metallothionein was associated with phosphorylation of ERK but not Akt. MEK inhibition and rapamycin decreased metallothionein-induced HIF activity. Our study shows that upregulation of metallothionein expression by hypoxia activates the HIF-HRE system through the ERK/mTOR pathway and may be a novel defense against hypoxia.

Intermedin ameliorates vascular and renal injury by inhibition of oxidative stress

Intermedin (IMD) is a newly discovered peptide related to calcitonin gene-related peptide and adrenomedullin, and has been shown to reduce blood pressure and reactive oxygen species formation in vivo. In this study, we determined whether IMD exerts vascular and renal protection in DOCA-salt hypertensive rats by intravenous injection of adenovirus harboring the human IMD gene. Expression of human IMD was detected in the rat kidney via immunohistochemistry. IMD administration significantly lowered blood pressure, increased urine volume, and restored creatinine clearance. IMD also dramatically decreased superoxide formation and media thickness in the aorta. Vascular injury in the kidney was reduced by IMD gene delivery as evidenced by the prevention of glomerular and peritubular capillary loss. Moreover, IMD lessened morphological damage of the renal tubulointerstitium and reduced glomerular injury and hypertrophy. Attenuation of inflammatory cell accumulation in the kidney by IMD was accompanied by inhibition of p38MAPK activation and intercellular adhesion molecule 1 expression. In addition, IMD gene transfer resulted in a marked decline in myofibroblast and collagen accumulation in association with decreased transforming growth factor-beta1 levels. Furthermore, IMD increased nitric oxide excretion in the urine and lowered the amount of lipid peroxidation. These results demonstrate that IMD is a powerful renal protective agent with pleiotropic effects by preventing endothelial cell loss, kidney damage, inflammation, and fibrosis in hypertensive DOCA-salt rats via inhibition of oxidative stress and proinflammatory mediator pathways.

Regulation of oxygen utilization by angiotensin II in chronic kidney disease

Angiotensin II blockade delays progression of chronic kidney disease by modifying intrarenal hemodynamics, but the effects on metabolic adaptations are unknown. Using the remnant kidney model of chronic kidney disease in rats, we measured the effects of combined angiotensin II blockade with captopril and losartan on renal oxygen consumption (QO(2)) and factors influencing QO(2). Remnant kidneys had proteinuria and reductions in the glomerular filtration rate (GFR), renal blood flow (RBF) and nitric oxide synthase-1 protein expression while QO(2), factored by sodium reabsorption (QO(2)/TNa), was markedly increased. Combined blockade treatment normalized these parameters while increasing sodium reabsorption but, since QO(2) was unchanged, QO(2)/TNa also normalized. Triple antihypertensive therapy, to control blood pressure, and treatment with lysine, to increase GFR and RBF, did not normalize QO(2)/TNa, suggesting a specific effect of angiotensin II in elevating QO(2)/TNa. Inhibition of nitric oxide synthase increased QO(2) in the kidney of sham-operated rats but not in the remnant kidney of untreated rats. Our study shows that combined captopril and losartan treatment normalized QO(2)/TNa and functional nitric oxide activity in the remnant kidney independent of blood pressure and GFR effects, suggesting that other mechanisms in addition to hemodynamics underlie the benefits of angiotensin II blockade.

Urinary neutrophil gelatinase associated lipocalin (NGAL), a marker of tubular damage, is increased in patients with chronic heart failure

Renal impairment, as measured by reduced glomerular filtration rate (GFR) and increased urinary albumin excretion (UAE), is prevalent in patients with chronic heart failure (CHF) and is associated with reduced survival. The prevalence of structural tubular damage in CHF is unknown. We investigated 90 CHF patients and 20 age and sex matched healthy controls, and determined estimated GFR, UAE, N terminal-pro brain natriuretic peptide (NT-proBNP) and urinary neutrophil gelatinase associated lipocalin (NGAL) as a marker for tubular damage. CHF patients had significantly lower averaged estimated GFR (64+/-17 vs 90+/-12 mL/min/1.73 m(2), P<0.0001), but higher NT-proBNP and UAE levels (both P<0.0001). Median urinary NGAL levels were markedly increased in CHF patients compared to controls (175 (70-346) vs 37 (6-58) microg/gCr, P<0.0001). Both serum creatinine (r=0.26, P=0.006) and eGFR (r=-0.29, P=0.002) were significantly associated with urinary NGAL levels as were NT-proBNP and UAE but to a lesser extent. In conclusion, renal impairment in CHF patients is not only characterised by decreased eGFR and increased UAE, but also by the presence of tubular damage, as measured by increased urinary NGAL concentrations.

Nicotinic acetylcholine receptor expression and regulation in the rat kidney after ischemia-reperfusion injury

The cholinergic anti-inflammatory pathway is a mechanism whereby local inflammation is modulated by the brain via the vagus nerve and nicotinic acetylcholine receptors (nAChRs). The nAChR family are ligand-gated ion channels that consist of many different subtypes formed by the specific assembly of five polypeptide subunits including alpha1-10, beta1-4, gamma, delta, and epsilon. The alpha7 receptor (alpha7nAChR) mediates the anti-inflammatory effects of cholinergic stimulation. We recently demonstrated that cholinergic agonists attenuate renal ischemia-reperfusion (I/R) injury in rats. We also showed that tubular epithelial cells express functional nAChRs in vitro. The current studies report the expression, localization, and regulation of the alpha7nAChR in the rat kidney after I/R injury. We also examined, in this model, potential interactions between cholinergic stimulation and the STAT3 pathway, a key signaling cascade that has been linked to alpha7nAChR activation. RT-PCR and immunohistochemistry showed constitutive expression of many nAChR subunits. Immunohistochemistry localized basal alpha7nAChR expression to the endothelium of cortical peritubular capillaries, and its distribution was upregulated after I/R injury. Western blotting also showed an increase in alpha7nAChR subunit protein after renal I/R injury. Interestingly, pretreatment with nicotine, which improves the outcome after renal I/R injury, reduced the alpha7nAChR protein after I/R injury. Finally, we found that I/R injury stimulated the STAT3 pathway, whereas pretreatment with nicotine downregulated its activation. These results suggest that the alpha7nAChR plays an important role in the pathophysiology of renal I/R injury.

Impact of anemia after renal transplantation on patient and graft survival and on rate of acute rejection

BACKGROUND AND OBJECTIVES

The impact of posttransplantation anemia on patient survival, renal allograft survival, and rate of acute rejection is not known.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

A total of 1023 patients who underwent kidney transplantation at one center from January 1992 through June 2003 were retrospectively analyzed. Posttransplantation anemia was defined as mean hemoglobin <11 g/dl after 3 mo after transplantation. Data on demographics, pretransplantation dialysis, previous transplant history, pretransplantation hemoglobin, degree of HLA mismatch, and donor characteristics were collected. Some of the posttransplantation data that were collected in addition to the hemoglobin included delayed graft function; diabetes; hypertension; induction and maintenance of immunosuppressive regimen; posttransplantation infections; and use of angiotensin-converting enzyme inhibitor/angiotensin receptor blocker, statins, aspirin, and beta blockers. Cox regression models were used to assess the effects of posttransplantation anemia on each outcome: Mortality, graft survival, and rate of acute rejection. Median follow-up time was 4 yr.

RESULTS

During the entire follow-up period, there were 89 (9%) deaths, 143 (14%) acute rejection episodes, and 235 (23%) kidney losses. In multivariate Cox regression models, being anemic after transplantation, after the first 90 d, was associated with increased overall mortality and increased renal allograft loss. Posttransplantation anemia was also associated with increased acute rejection rates.

CONCLUSIONS

This study shows that posttransplantation anemia is associated with worse patient and graft survival and higher rates of acute rejection when compared with nonanemic renal transplant recipients.

Ischemic injury underlies the pathogenesis of aristolochic acid-induced acute kidney injury

Aristolochic acid nephropathy (AAN) is a progressive tubulointerstitial renal disease caused by aristolochic acid intake. To determine the contribution of renal ischemia to the pathogenesis of AAN, we characterized changes in the expression of angiogenic factors and vasoactive substances, and then we evaluated the expression of a marker of hypoxia in an acute AAN rat model. Rats were orally administrated either a decoction of Aristolochiae manshuriensis that contained 20 mg/kg of aristolochic acid-I or an equal volume of distilled water (control group) once daily for 4 days or 7 days. Renal histology and serum creatinine were assessed. Expression of endothelin-1 (ET-1) and hypoxia inducible factor-1 alpha (HIF-1alpha) mRNA within renal cortex were determined by semiquantitative reverse-transcription polymerase chain reaction. Levels of ET-1, nitric oxide (NO), vascular endothelial growth factor (VEGF), and HIF-1alpha in kidneys were determined by radioimmunoassay, Griess method, Western blot, and immunohistochemistry, respectively. Tubular injury scores and ET-1 mRNA expression were increased in the AA-treated rats at both days 4 and 8, whereas serum creatinine level and ET-1 protein expression was increased only at day 4. In contrast, NO production in AA-treated rats was decreased at day 8 compared with the control group. Similarly, VEGF protein expression was reduced in the AA-treated rats at both days 4 and 8. A dramatic increase in nuclear staining for HIF-1alpha was observed mainly in the tubular cells of tubulointerstitial damage area in the AA-treated rats at day 8. The observed increase in HIF-1alpha protein expression, decrease in VEGF protein expression, and imbalance of vasoactive substances after induction of acute kidney injury by AA suggests that ischemic injury contributes to the pathogenesis of AAN.

The role of capillary density, macrophage infiltration and interstitial scarring in the pathogenesis of human chronic kidney disease

To assess the relationship between interstitial capillary density and interstitial macrophages we prospectively measured these factors in situ in 110 patients with chronic kidney disease. Macrophage numbers and urinary MCP-1/CCL2 levels significantly correlated inversely with capillary density which itself significantly correlated inversely with chronic damage and predicted disease progression. In 54 patients with less than 20% chronic damage, there was a significant correlation between the urinary albumin to creatinine ratio and MCP-1/CCL2, and MCP-1/CCL2 and macrophages but not between MCP-1/CCL2 and capillary density. Conversely, in 56 patients with over 20% chronic damage there was no correlation between MCP-1/CCL2 and macrophages but there were significant inverse correlations between capillary density and both macrophages and chronic damage. The expression of VEGF mRNA significantly correlated with macrophage infiltration, capillary density and chronic scarring. In an ischemic-hypertensive subgroup there was upregulation of the hypoxia marker carbonic anhydrase IX and with over 20% chronic damage an increased macrophage to CCR2 ratio. Our study shows that proteinuria and MCP-1/CCL2 are important for macrophage recruitment in early disease. As renal scarring evolves, alternative pathways relating to progressive tissue ischemia secondary to obliteration of the interstitial capillary bed predominate.

High Prevalence of chronic kidney disease among patients with sleep related breathing disorder (SRBD)

Sleep apnea syndrome, a sleep-related breathing disorder (SRBD) of which obstructive sleep apnea syndrome (OSAS) is representative, is often associated with obesity, and therefore patients with SRBD might have a high prevalence of chronic kidney disease (CKD). However, the relationship between obesity and the prevalence of CKD has not yet been investigated in a large cohort of patients with SRBD. The Okinawa Nakamura Clinic Sleep Apnea Syndrome (ONSLEEP) registry contains records for all patients evaluated by full-scale polysomnography (PSG) from September 1990 to the end of 2003 (n=5,651). We studied the total of 4,056 (71.8%) of these patients who had an apnea hypopnea index (AHI) of more than 5 events per hour. The glomerular filtration rate (GFR) was estimated using the abbreviated Modification of Diet in Renal Disease equation in the 1,624 patients for whom serum creatinine data was obtained at the time of the PSG. We defined CKD as a GFR of less than 60 mL/min/1.73 m2. The mean age was 49.9+/-13.5 (mean+/-SD) years; the mean body mass index (BMI) was 28.4+/-5.0 (mean+/-SD) kg/m2. We compared the findings with those from participants in the 1993 general screening registry in Okinawa (n=94,267). From among the total 94,267 screening participants, we selected 7,454 subjects who were age- and sex-matched to the experimental group with SRBD; the ratio of cases to controls was thus approximately 1:4. CKD was detected in 496 (30.5%) patients, with SRBD a higher incidence than that in the screened population (9.1%); the adjusted odds ratio (95% confidence interval) was 4.542 (3.922-5.260, p<0.0001). In contrast to the screened population, the prevalence of CKD decreased as BMI increased (it was 35.7% in SRBD patients with a BMI<25.0 kg/m2, 31.4% in those with a BMI 25.0 to 29.9 kg/m2, and 25.2% in those with a BMI > or =30.0 kg/m2); in the controls the values were 8.1%, 10.5%, and 10.6%, respectively. Taken together, these results suggest that surveillance of CKD is warranted among SRBD patients, particularly those who are not obese.

Protective effect of COMP-angiopoietin-1 on cyclosporine-induced renal injury in mice

BACKGROUND

Peritubular capillary injury induces chronic hypoxia in the renal tubulointerstitium, and renal peritubular capillary dysfunction is an early event that contributes to tubulointerstitial fibrosis. Cyclosporine A (CsA) is a potent immunosuppressant and improves survival of renal allografts. However, the limitation of CsA use is chronic nephrotoxicity. A soluble, stable and potent angiopoietin-1 (Ang1) variant, cartilage oligomeric matrix protein (COMP)-Ang1 has been developed. We investigated whether COMP-Ang1 ameliorates CsA-induced renal injury.

METHODS

CsA-treated mice were injected with recombinant adenovirus expressing either COMP-Ang1 or LacZ. Histology, inflammatory, haemodynamic and fibrotic parameters, and signalling pathway were evaluated.

RESULTS

Histologic examination showed that COMP-Ang1 significantly decreased CsA-induced tubular damage and tubulointerstitial fibrosis. CsA-induced increases in macrophage infiltration and expression of MCP-1 and ICAM-1 after CsA treatment were significantly reduced by COMP-Ang1. Treatment with COMP-Ang1 also decreased the CsA-induced increases in TGF-beta1 and Smad 2/3 levels while increasing Smad 7 levels. Laser-Doppler sonographic findings and endothelial factor VIII staining revealed that COMP-Ang1 preserved the integrity of peritubular vasculature and intrarenal haemodynamics from the CsA-induced renal injury. COMP-Ang1 inhibited tubular cell apoptosis while increasing tubular cell proliferation in CsA-induced renal injury.

CONCLUSIONS

These results indicate that COMP-Ang1 exhibited a protective effect on damaged peritubular capillaries, haemodynamic alteration and inflammation in CsA-induced renal injury. Thus, COMP-Ang1 may be useful as a therapeutic and prophylactic agent for specific protection against endothelial dysfunction and inflammation.

Cellular oxygen sensing in health and disease

To avoid localised problems resulting from excess or inadequate oxygen, all cells and tissues have the ability to sense and respond to changes in oxygen levels. Despite their rich blood supply, the kidneys have unique properties with respect to oxygen that enable them to act as specialised organs, sensing oxygen delivery as well as rendering them prone to hypoxic injury. Essential to normal growth and development, as well as the control of energy metabolism, angiogenesis and erythropoiesis, cellular oxygen homoeostasis is central to the pathophysiology of anaemia, ischaemia, inflammation and cancer, both within the kidney and more generally. A major transcriptional pathway, predominantly regulated by hypoxia-inducible factor (HIF), controls many hundreds of genes, either directly or indirectly, that serve to modulate both the supply and consumption of oxygen. Recent advances have illuminated the mechanisms underlying the regulation of HIF by oxygen and have defined novel therapeutic targets. The challenge now is for us to understand the complexities generated by multiple isoforms of the various components of oxygen sensing, the identification of additional levels of control, and the tissue specific responses to activation of the HIF pathway.

Hemoglobin is expressed by mesangial cells and reduces oxidant stress

Hemoglobin (Hb) serves as the main oxygen transporter in erythrocytes, but it is also expressed in nonhematopoietic organs, where it serves an unknown function. In this study, microarray and proteomic analyses demonstrated Hb expression in the kidney. Rat kidneys were perfused extensively with saline, and glomeruli were isolated by several techniques (sieving, manual dissection, and laser capture-microdissection). Reverse transcriptase-PCR revealed glomerular alpha- and beta-globin expression, and immunoblotting demonstrated expression of the protein. In situ hybridization studies showed expression of the globin subunits in the mesangium, and immunostaining confirmed this localization of Hb. Furthermore, globin mRNA expression was detected in primary cultures of rat mesangial cells but not in cultured glomerular endothelial or epithelial cells. For investigation of Hb function in mesangial cells, the SV40-MES13 murine mesangial cell line was transfected with a vector expressing alpha- and beta-globins; this overexpression reduced production of hydrogen peroxide-induced intracellular radical oxygen species and enhanced cell viability against oxidative stress. In summary, Hb is expressed by rat mesangial cells, and its potential functions may include antioxidative defense.

Proteinuria in diabetic kidney disease: a mechanistic viewpoint

Proteinuria is the hallmark of diabetic kidney disease (DKD) and is an independent risk factor for both renal disease progression, and cardiovascular disease. Although the characteristic pathological changes in DKD include thickening of the glomerular basement membrane and mesangial expansion, these changes per se do not readily explain how patients develop proteinuria. Recent advances in podocyte and glomerular endothelial cell biology have shifted our focus to also include these cells of the glomerular filtration barrier in the development of proteinuria in DKD. This review describes the pathophysiological mechanisms at a cellular level which explain why patients with DKD develop proteinuria.

Hypokalemic nephropathy is associated with impaired angiogenesis

Hypokalemic nephropathy is associated with alterations in intrarenal vasoactive substances, leading to vasoconstriction, salt-sensitivity, and progression of interstitial fibrosis. In this study, we investigated whether hypokalemic nephropathy might also involve impaired renal angiogenesis. Sprague-Dawley rats that were fed low-potassium diets developed peritubular capillary loss that began in the inner stripe of the outer medulla (week 2) and progressed to the outer stripe of the outer medulla (week 4) and cortex (week 12). These changes were associated with increased macrophage infiltration, increased expression of both monocyte chemoattractant protein-1 and TNF-alpha, and a loss of vascular endothelial growth factor and endothelial nitric oxide synthase. Renal thiobarbituric acid-reactive substances, markers of oxidative stress, were increased late in disease. In conclusion, hypokalemic nephropathy is associated with impaired renal angiogenesis, evidenced by progressive capillary loss, reduced endothelial cell proliferation, and loss of VEGF expression.

Mechanisms of disease: the hypoxic tubular hypothesis of diabetic nephropathy

Diabetic nephropathy is traditionally considered to be a primarily glomerular disease, although this contention has recently been challenged. Early tubular injury has been reported in patients with diabetes mellitus whose glomerular function is intact. Chronic hypoxia of the tubulointerstitium has been recognized as a mechanism of progression that is common to many renal diseases. The hypoxic milieu in early-stage diabetic nephropathy is aggravated by manifestations of chronic hyperglycemia-abnormalities of red blood cells, oxidative stress, sympathetic denervation of the kidney due to autonomic neuropathy, and diabetes-mellitus-induced tubular apoptosis; as such, tubulointerstitial hypoxia in diabetes mellitus might be an important early event. Chronic hypoxia could have a dominant pathogenic role in diabetic nephropathy, not only in promoting progression but also during initiation of the condition. Early loss of tubular and peritubular cells reduces production of 1,25-dihydroxyvitamin D3 and erythropoietin, which, together with dysfunction of their receptors caused by the diabetic state, diminishes the local trophic effects of the hormones. This diminution could further compromise the functional and structural integrity of the parenchyma and contribute to the gradual decline of renal function.

Oxidative stress, plasminogen activator inhibitor 1, and lung fibrosis

Fibrosis is characterized by excessive accumulation of extracellular matrix (ECM) in basement membranes and interstitial tissues, resulting from increased synthesis or decreased degradation of ECM or both. The plasminogen activator/plasmin system plays an important role in ECM degradation, whereas the plasminogen activator inhibitor 1 (PAI-1) is a physiologic inhibitor of plasminogen activators. PAI-1 expression is increased in the lung fibrotic diseases and in experimental fibrosis models. The deletion of the PAI-1 gene reduces, whereas the overexpression of PAI-1 enhances, the susceptibility of animals to lung fibrosis induced by different stimuli, indicating an important role of PAI-1 in the development of lung fibrosis. Many growth factors, including transforming growth factor beta (TGF-beta) and tumor necrosis factor alpha (TNF-alpha), as well as other chemicals/agents, induce PAI-1 expression in cultured cells and in vivo. Reactive oxygen and nitrogen species (ROS/RNS) have been shown to mediate the induction of PAI-1 by many of these stimuli. This review summarizes some recent findings that help us to understand the role of PAI-1 in the development of lung fibrosis and ROS/RNS in the regulation of PAI-1 expression during fibrogenesis.

Heart failure and anemia: mechanisms and pathophysiology

Anemia is a common comorbidity in patients with heart failure and affects up to 50% of patients, depending on the definition of anemia used and on the population studied. Presence of anemia and lower hemoglobin (Hgb) concentrations are powerful independent predictors of adverse outcomes in heart failure. Even small reductions in Hgb are associated with worse outcomes. Correction of anemia may be useful in improving heart failure outcomes. However, the causes of anemia in heart failure are not entirely clear. Specific causes of anemia such as hematinic abnormalities are seen only in a minority of subjects. Renal dysfunction and neurohormonal and proinflammatory cytokine activation appear to contribute to anemia of chronic disease in the majority of the patients, resulting in inappropriate erythropoietin production and defective iron utilization. Under normal conditions, reduced tissue oxygenation due to chronic anemia results in non-hemodynamic and hemodynamic compensatory responses to enhance oxygen carrying capacity. Erythropoiesis is the predominant non-hemodynamic response to hypoxia, but because erythropoiesis is defective in heart failure, hemodynamic mechanisms predominate. Hemodynamic responses are complex and involve a vasodilation-mediated high-output state with neurohormonal activation. The high-output state initially helps to increase oxygen transport. However, the hemodynamic and neurohormonal alterations could potentially have deleterious long-term consequences and could contribute to anemia's role as an independent risk factor for adverse outcomes.

Biliverdin reductase mediates hypoxia-induced EMT via PI3-kinase and Akt

Chronic hypoxia in the renal parenchyma is thought to induce epithelial-to-mesenchymal transition (EMT), leading to fibrogenesis and ultimately end-stage renal failure. Biliverdin reductase, recently identified as a serine/threonine/tyrosine kinase that may activate phosphatidylinositol 3-kinase (PI3K) and Akt, is upregulated in response to reactive oxygen species that may accompany hypoxia. We investigated this potential role of biliverdin reductase in hypoxia-induced renal tubular EMT. Expression of biliverdin reductase was upregulated in a human proximal tubule cell line (HK-2) cultured in hypoxic conditions (1% O2), and this was accompanied by reduced expression of E-cadherin and increased expression of the mesenchymal marker vimentin. Inhibiting PI3K reversed these changes, consistent with EMT. In normoxic conditions, overexpression of biliverdin reductase promoted similar characteristics of EMT, which were also reversed by inhibiting PI3K. Furthermore, using small interfering RNA (siRNA) to knockdown biliverdin reductase, we demonstrated that the enzyme associates with phosphorylated Akt and mediates the hypoxia-induced EMT phenotype. In vivo, expression of biliverdin reductase increased in the tubular epithelia of 5/6-nephrectomized rats, and immunohistochemistry of serial sections demonstrated similar localization of phosphorylated Akt and biliverdin reductase. In conclusion, biliverdin reductase mediates hypoxia-induced EMT through a PI3K/Akt-dependent pathway.

Carbamylated Erythropoietin Improves Angiogenesis and Protects the Kidneys from Ischemia-Reperfusion Injury

Several studies have shown that erythropoietin (EPO) can protect the kidneys from ischemia-reperfusion injury and can raise the hemoglobin (Hb) concentration. Recently, the EPO molecule modified by carbamylation (CEPO) has been identified and was demonstrated to be able to protect several organs without increasing the Hb concentration. We hypothesized that treatment with CEPO would protect the kidneys, partly due to the increased peritubular capillaries. The therapeutic effect of CEPO was evaluated using an endothelial tube formation assay in vitro, and a rat ischemia-reperfusion injury model in vivo. EPO treatment showed the tendency of increased tube formation, while CEPO treatment induced more capillary-like formation than EPO. Ischemia-reperfusion-induced kidneys exhibited characteristic nuclei of apoptosis in tubular epithelial cells with decreased peritubular capillaries, while EPO treatment inhibited tubular apoptosis with preserved endothelial cells. Moreover, CEPO-treated kidneys showed minimal tubular apoptosis with increased peritubular capillary endothelial cells. In conclusion, we identified a new therapeutic approach using CEPO to protect kidneys from ischemia-reperfusion injury by promoting angiogenesis.

The effect of losartan on hemoglobin concentration and renal outcome in diabetic nephropathy of type 2 diabetes

Angiotensin-converting enzyme inhibitors and angiotensin receptor blockers can decrease hemoglobin, causing anemia and this may be an independent risk factor for chronic kidney disease progression. We studied the relationship between a decline in hemoglobin and outcome in 1513 patients with type 2 diabetes and kidney disease by a post hoc analysis of the RENAAL Study (Reduction of Endpoints in NIDDM with the Angiotensin II Antagonist Losartan) with an average follow-up of 3.4 years. The relationship between baseline and year-1 hemoglobin and treatment on end-stage renal disease (ESRD) and ESRD or death was evaluated using multivariate Cox models (covariates: baseline hemoglobin, proteinuria, serum albumin, serum creatinine, and year-1 hemoglobin). Compared with placebo, losartan treatment was associated with a significant decrease of hemoglobin, with the largest between-group difference at 1 year. After adjustment, there were significant relative risk reductions for losartan compared with placebo for ESRD and for ESRD or death regardless of the baseline hemoglobin even in those patients with a baseline hemoglobin below 120 g l(-1). Hence, the renoprotective properties of losartan were maintained despite a significant lowering of the hemoglobin concentration.

Uric acid excretion and dopamine-induced glomerular filtration response in patients with IgA glomerulonephritis

BACKGROUND/AIM

It is unknown to what extent uric acid (UA) may affect vessel function and participate in tubulointerstitial damage. We examined the relationship between intrarenal vessel function and serum UA and its excretion in association with urinary N-acetyl-beta-D-glucosaminidase (NAG).

METHODS

In 50 IgA patients (mean age 34.7 +/- 9.3 years) and 15 controls (mean age 33.5 +/- 6.9 years) with a creatinine clearance of 99.4 +/- 21.6 and 118.1 +/- 17.2 ml/min, respectively, the renal vascular function was estimated based on the dopamine-induced glomerular filtration response (DIR; see text). The DIR was measured using two 120-min creatinine clearance values (before and after intravenous administration of 2 g/kg/min dopamine). Serum UA, triglycerides and cholesterol and urinary NAG (24 h) and protein and UA excretion were measured.

RESULTS

Patients with IgA nephropathy versus controls: DIR 8.80 +/- 6.6 vs. 12.83% (p < 0.01), NAG 7.25 +/- 3.30 vs. 4.69 +/- 1.12 U/g creatinine (p < 0.01), and fractional UA excretion 7.80 +/- 2.20 versus 6.29 +/- 1.80% (p < 0.01). A negative correlation between DIR and NAG was found; regression analysis showed a more prominent relationship in the patients (NAG = 9.99 - 0.29x DIR) than in the controls (NAG = 5.50 - 0.06x DIR). UA and urate excretion and NAG in the patients correlated with DIR (r = -0.39, p < 0.02; r = -0.29, p < 0.04, and r = 0.59, p < 0.001, respectively). Multivariate analysis showed an association of DIR (R(2) = 0.39) with NAG but not with proteinuria and UA and UA excretion; the NAG excretion (R(2) = 0.56) correlated significantly with UA and DIR.

CONCLUSION

It is suggested that UA plays a role, associated with tubular dysfunction, in the regulation of intrarenal vessel function.

Anemia as a risk factor for chronic kidney disease

Chronic kidney disease (CKD) is an important and leading cause of end-stage renal disease (ESRD) and moreover, plays a role in the morbidity and mortality due to cardiovascular disease, infection, and cancer. Anemia develops during the early stages of CKD and is common in patients with ESRD. Anemia is an important cause of left ventricular hypertrophy and congestive heart failure. Correction of anemia by erthyropoiesis-stimulating agent (ESA) has been shown to improve survival in patients with congestive heart failure. Anemia is counted as one of the non-conventional risk factors associated with CKD. Hypoxia is one of the common mechanisms of CKD progression. Treatment by ESA is expected to improve quality of life, survival, and prevent the CKD progression. Several clinical studies have shown the beneficial effects of anemia correction on renal outcomes. However, recent prospective trials both in ESRD and in CKD stages 3 and 4 failed to confirm the beneficial effects of correcting anemia on survival. Similarly, treatment of other risk factors such as hyperlipidemia by statin showed no improvement in the survival of dialysis patients. Given the high prevalence of anemia in ESRD and untoward effects of anemia in CKD stages 3 and 4, appropriate and timely intervention on renal anemia using ESA is required for practicing nephrologists and others involved in the care of high-risk population. Lessons from the recent studies are to correct renal anemia (hemoglobin <10 g/dl not hemoglobin > or =13 g/dl). Early intervention for renal anemia is a part of the treatment option in the prevention clinic. In this study, clinical significance of anemia management in patients with CKD is discussed.

Erythropoiesis-stimulating agents: past and future

Renal anemia is a well-recognized complication of chronic kidney disease (CKD), and the deficiency of erythropoietin (EPO) is the primary cause. Observational population-based studies continue to demonstrate the association of low hemoglobin with adverse outcomes, and renal failure, cardiac failure, and anemia all may interact to cause or worsen each other, the so-called cardio-renal anemia syndrome. Treatment of anemia can be successfully achieved with the use of erythropoiesis-stimulating agents (ESAs). From a mechanistic point of view, however, the therapeutic benefits of ESA could be far beyond the correction of anemia. ESA modulates a broad array of cellular processes that include progenitor stem cell development, cellular integrity, and angiogenesis. A pleiotropic effect of EPO has been shown in the central nervous system, the cardiovascular system, and the kidney. While recent results of randomized controlled trials have established that there is little support for normalizing hemoglobin in CKD patients, the results of these studies do not negate renoprotective effects of EPO. A large number of patients with CKD will benefit from early recognition and appropriate correction of anemia with ESA.

Stratégies pour faire régresser les lésions de fibrose rénale

The deterioration of renal function in chronic kidney disease is related to the progression of renal fibrosis, which was long considered unavoidable. Today, the reversibility of renal fibrotic lesions is a reality, although still clinically rare. Because angiotensin II is highly profibrotic, blocking its action effectively protects the kidney, as numerous clinical trials have shown. The development of interstitial fibrosis is secondary to the epithelial-to-mesenchymal transition induced by transforming growth factor (TGF)-beta. Bone morphogenic protein-7 (BMP-7) and hepatocyte growth factor (HGF) induce the reverse transition and thus open up perspectives for treatment. Degradation of the extracellular matrix by matrix metalloproteinases or other enzymes is another therapeutic pathway. Renal regeneration may be promoted by modulation of hypoxia-inducible factor-1 (HIF-1) and vascular endothelial growth factor (VEGF).

Hypoxia and the HIF system in kidney disease

The kidney is sensitive to changes in oxygen delivery. This sensitivity has the merit of facilitating the kidneys in their adjustment of erythropoietin (EPO) production to changes in oxygen supply. The main determinant of EPO synthesis is the transcriptional activity of its gene in kidneys, which is related to local oxygen tensions. Regulation of EPO production is mediated by hypoxia-inducible factor (HIF). When local oxygen tension decreases, accumulated HIF binds to the key sequence of the EPO gene, the hypoxia-responsive element (HRE), and activates transcription of EPO. HIF consists of a constitutive beta-subunit and one of alternative oxygen-regulated HIF alpha-subunits (HIF-1alpha, HIF-2alpha, and HIF-3alpha), and HIF-2alpha is responsible for erythropoietin production. However, the high sensitivity to changes in oxygen tension also makes the kidney prone to hypoxic injury. Severe energy depletion and subsequent activation of a number of critical alterations in metabolism occurs under hypoxic conditions. Hypoxia is also a profibrogenic stimulus. In addition to ischemic acute renal failure, hypoxia can also play a crucial role in the development of nephrotoxic acute kidney injury, radiocontrast nephropathy, and acute glomerulonephritis. Furthermore, accumulating evidence suggests that chronic hypoxia is a final common pathway to end-stage kidney failure in chronic kidney disease. Given that renal hypoxia has pivotal roles on the development and progression of both acute and chronic kidney disease, hypoxia can be a valid therapeutic target for chronic kidney disease. Activation of HIF leads to expression of a variety of adaptive genes in a coordinated manner. Studies utilizing HIF-stimulating agents proved efficacy in various kidney disease models, suggesting that HIF activation is an ideal target of future therapeutic approaches.

Chronic hypoxia aggravates renal injury via suppression of Cu/Zn-SOD: a proteomic analysis

Accumulating evidence suggests a pathogenic role of chronic hypoxia in various kidney diseases. Chronic hypoxia in the kidney was induced by unilateral renal artery stenosis, followed 7 days later by observation of tubulointerstitial injury. Proteomic analysis of the hypoxic kidney found various altered proteins. Increased proteins included lipocortin-5, calgizzarin, ezrin, and transferrin, whereas the decreased proteins were alpha(2u)-globulin PGCL1, eukaryotic translation elongation factor 1alpha(2), and Cu/Zn superoxide dismutase (SOD1). Among these proteins, we focused on Cu/Zn-SOD, a crucial antioxidant. Western blot analysis and real-time quantitative PCR analysis confirmed the downregulation of Cu/Zn-SOD in the chronic hypoxic kidney. Furthermore, our laser capture microdissection system showed that the expression of Cu/Zn-SOD was predominant in the tubulointerstitium and was decreased by chronic hypoxia. The tubulointerstitial injury estimated by histology and immunohistochemical markers was ameliorated by tempol, a SOD mimetic. This amelioration was associated with a decrease in levels of the oxidative stress markers 4-hydroxyl-2-nonenal and nitrotyrosine. Our in vitro studies utilizing cultured tubular cells revealed a role of TNF-alpha in downregulation of Cu/Zn-SOD. Since the administration of anti-TNF-alpha antibody ameliorated Cu/Zn-SOD suppression, TNF-alpha seems to be one of the suppressants of Cu/Zn-SOD. In conclusion, our proteomic analysis revealed a decrease in Cu/Zn-SOD, at least partly by TNF-alpha, in the chronic hypoxic kidney. This study, for the first time, uncovered maladaptive suppression of Cu/Zn-SOD as a mediator of a vicious cycle of oxidative stress and subsequent renal injury induced by chronic hypoxia.

Adaptation to hypoxia in the diabetic rat kidney

Hypoxia of the kidney in diabetes could predispose it to develop acute and chronic renal failure. To examine the relationship between renal hypoxia and renal failure, we measured hypoxia (as a pimonidazole adducts), hypoxia-inducible factors (HIFs), and a hypoxia target gene heme oxygenase-1. The studies were performed in rats with streptozotocin (STZ)-induced diabetes, Cohen diabetes sensitive rats, and during short-term artificial hyperglycemia in rats induced by intravenous glucose and octreotide. STZ-treated rats received insulin, the superoxide dismutase mimetic tempol, or contrast medium. Radiocontrast media causes hypoxia and HIF induction. Hypoxia, HIFs, and heme oxygenase were undetectable in controls, but transiently activated in STZ-treated and the Cohen diabetes sensitive rats. Different patterns of HIFs and pimonidazole were observed between the three models. Insulin abolished pimonidazole and HIF induction, whereas tempol lead to increased HIFs and heme oxygenase induction at similar levels of pimonidazole. When compared with control rats, STZ-treated rats exhibited more intense and protracted renal pimonidazole, with augmented hypoxia inducible factor production and reduced GFR following contrast media. Our data suggest that both regional hypoxia and hypoxia adaptation transiently occur in early stages of experimental diabetes, largely dependent on hyperglycemia or after contrast media. Tempol may augment the HIF response in diabetes.

Heme oxygenase-1 modulates the expression of the anti-angiogenic chemokine CXCL-10 in renal tubular epithelial cells

The turnover and repair of peritubular capillaries is essential for the maintenance of normal renal tubular structure and function. Following injury, ineffective capillary repair/angiogenesis may result in chronic disease, whereas effective repair attenuates the injury process. Thus the process of healing in the kidney is likely dependent on an intricate balance between angiogenic and anti-angiogenic factors to maintain the renal microvasculature. We investigated the role of cytoprotective heme oxygenase-1 (HO-1) in the regulation of chemokines in human renal proximal tubular epithelial cells (RPTEC). Transfection of RPTEC with a HO-1 overexpression plasmid promoted a marked induction in the mRNA expression of the anti-angiogenic chemokine CXCL-10, along with angiogenic chemokines CXCL-8 and CCL-2. Utilizing a CXCL-10 promoter luciferase construct, we observed that HO-1-induced CXCL-10 expression is regulated at the transcriptional level. However, with increases in concentrations and time intervals of HO-1 induction, there was a marked decrease in CXCL-10 expression. Using pharmacological inhibitors, we found that HO-1-induced early robust CXCL-10 transcription is mediated through the PKC signaling pathway. To evaluate the functional significance of HO-1-induced CXCL-10 release, we cultured human vascular endothelial cells in the absence and presence of culture supernatants of the HO-1 plasmid-transfected RPTEC. We found that early (24 h) supernatants of the HO-1 plasmid-transfected cells (RPTEC) inhibited endothelial cell proliferation, and this effect was blocked by addition of a CXCL-10 neutralizing antibody. Thus HO-1 can regulate the expression of the anti-angiogenic CXCL-10 and may alter a critical balance between angiogenic vs. anti-angiogenic factors that are important to maintain renal microvasculature during injury.

The effect of renal transplantation on adiponectin and its isoforms and receptors

Insulin resistance (IR) and other proatherogenic risk factors associated with end-stage kidney disease (ESKD) are improved by renal transplantation. Adiponectin is a protein with insulin-sensitizing, anti-inflammatory, and antiatherogenic properties. It exists in several isoforms, but the high molecular weight (HMW) isoform correlates best with insulin sensitivity. Paradoxically, the levels of this protein and its HMW isoform are increased in ESKD. We measured the homeostasis model assessment for insulin resistance (HOMA-IR), plasma adiponectin and its isoforms, and messenger RNA for adiponectin receptors (AdipoR1 and AdipoR2) on peripheral blood mononuclear cells in 54 stable transplant recipients, 50 patients established on hemodialysis, and 52 controls; groups were matched for body mass index and sex. HOMA-IR values were significantly higher in patients with ESKD compared with controls (P < .0005) and transplant patients (P < .05) but there was no difference between the latter 2 groups. Adiponectin levels were also higher in patients with ESKD compared with controls (P < .0005), and although levels were lower in the transplant group, they remained higher than in controls (P < .0001). However, although the absolute amount of HMW isoform in transplant patients remained higher than in controls (P < .0001), the proportion was similar, and less than in patients with ESKD (P < .005). The absolute amount of the HMW isoform correlated with superior metabolic indices in all 3 cohorts. AdipoR1 and AdipoR2 messenger RNA levels after transplantation were significantly lower than those of ESKD subjects (P < .0001, P < .01), but transplant patients had less AdipoR1 than controls, although their AdipoR2 levels were higher. AdipoR1 correlated with AdipoR2 in all 3 cohorts. We conclude that HOMA-IR was lower in the transplant group compared with the group on hemodialysis and this coincided with lower total adiponectin levels and absolute amount of the HMW isoform and AdipoR on peripheral blood mononuclear cells. Lower AdipoR after transplantation may be secondary to immunosuppression and/or an improvement in glomerular filtration rate and the uremic milieu.

Potential role of fractalkine receptor expression in human renal fibrogenesis

The inhibition of several chemokine/chemokine receptors has been shown to reduce progressive renal interstitial fibrosis. In this study, we examined the expression of the CX(3)C receptor in human renal biopsies with interstitial fibrosis and from normal kidneys by real-time polymerase chain reaction (PCR) and immunohistochemistry. The CX(3)C receptor was not only detected in mononuclear, tubular epithelial, and dendritic cells but also in alpha-smooth muscle actin and vimentin-positive interstitial myofibroblasts in fibrotic kidneys. Real-time PCR indicated a significant upregulation of CX(3)C receptor mRNA in fibrotic kidneys compared with non-fibrotic nephropathies or donor biopsies. In renal fibroblasts in vitro, hydrogen peroxide increased the expression of the CX(3)C receptor, an increase that was inhibited by N-acetylcysteine and catalase. However, neither proinflammatory nor profibrotic cytokines resulted in this upregulation. Stimulation of fibroblasts by CX(3)C ligand led to a significant enhancement of migration, which was abrogated by pre-incubation with a blocking anti-CX(3)C receptor antibody. Our studies indicate that renal fibrosis is associated with the expression of CX(3)C receptors on human renal fibroblasts. The expression is induced by reactive oxygen species suggesting a role of oxidative stress.