Reactive oxygen species and molecular regulation of renal oxygenation

Hypoxia-inducible factors and tubular cell survival in isolated perfused kidneys

Adaptation to hypoxic environment is conferred through hypoxia-inducible transcription factors (HIFs). We have previously shown that the HIF system is transiently activated in vivo in radiocontrast-induced acute renal failure, associated with profound hypoxia in the renal medulla. Medullary thick ascending limbs (mTALs), the most affected nephron segments in this model, were virtually unable to mount an adaptive HIF response. Here, we study correlations between oxygenation, HIF activation, and cell viability in a related ex vivo model, the isolated perfused rat kidney (IPK). In IPKs perfused with cell-free oxygenated medium, severe medullary hypoxic damage developed, affecting 42+/-9% of mTALs in the mid-inner stripe. HIF-1alpha tubular immunostaining was noted with a zonal and tubular pattern largely similar to our findings in vivo: in 34+/-3% of collecting ducts (CDs) within the mid-inner stripe and extensively in the papillary tip, whereas mTALs were all HIF-negative. In IPKs supplemented with RBCs (improved oxygen supply), mTAL damage was totally prevented and CDs' HIF expression was attenuated (22+/-4%). By contrast, although measures designed to reduce medullary hypoxia by decreasing tubular reabsorptive activity (furosemide, ouabain, or high-albumin-non-filtering system) reduced mTAL damage, all paradoxically resulted in increased HIF expression in CDs (51+/-4%), and 17+/-3% of mTALs became immunostained as well. Our data confirm that CDs and mTALs have markedly different HIF responses, which correlate with their viability under hypoxic stress. mTALs transcriptional adaptation occurs within a narrow hypoxic range, and it appears that workload reduction can shift mTALs into this window of opportunity for HIF activation and survival.

Current understanding of HIF in renal disease

Hypoxia-inducible factors (HIF) are ubiquitous transcription factors regulated by oxygen-dependent proteolysis, and hence rapidly mount an adaptational response to hypoxia. The HIF system is apparently more complex than initially considered in the perspective of the increasing number of HIF target genes, and the inter-relationship with various additional regulatory pathways. Regional hypoxia is believed to play a major role in renal disease. Experimental data confirm a role for HIF in renal pathophysiology. The discovery of HIF prolyl-hydroxylases as key enzymes of oxygen sensing and HIF proteolysis offer new possibilities to therapeutically target HIF. Herein, we review basic concepts of HIF regulation, and existing data on HIF activation in renal disease.

Antioxidant activity in Spalax ehrenbergi: a possible adaptation to underground stress

The blind subterranean mole rat Spalax ehrenbergi superspecies has evolved adaptive strategies to cope with underground stress. Hypoxia is known to stimulate reactive oxygen species generation; however, mechanisms by which Spalax counteracts oxidative damage have not been investigated before. We studied in Spalax the oxidative status of the Harderian gland (HG), an organ which is particularly vulnerable to oxidative stress in many rodents. With regard to the sexual dimorphism found in this gland, differences between males and females were determined and compared to the surface-dwelling Syrian hamster. Our results show, for the first time, that Spalax exhibits remarkably low biomolecular damage, which implies the existence of physiological strategies to avoid oxidative damage under fluctuating O(2) and CO(2) levels existing in the mole rat's subterranean niche. Correspondingly, main antioxidant enzymes, such as superoxide dismutase (SOD), catalase, and glutathione reductase (GR), exhibited high activities in both genders; in particular, remarkably high levels were measured in SOD. SOD and GR activities showed statistically significant differences between sexes. Melatonin, an important circadian agent is also a very important antioxidant molecule and is synthesized in the Harderian glands (HGs) of Spalax. Therefore, the possible interaction between antioxidant enzymes and melatonin is suggested.

A biologic role of HIF-1 in the renal medulla

BACKGROUND

Activation of hypoxia-inducible factor-1 (HIF-1) is the primary defensive mechanism against hypoxia. HIF-1 activation generally occurs in pathologic disruption of tissue oxygenation. However, a biologic role of HIF-1 in the medulla of the kidney, which is considered perpetually hypoxic under physiologic conditions due to its unique circulation, remains to be elucidated.

METHODS

The expression of HIF-1alpha was detected by immunohistochemical analysis. Functional studies of HIF in medulla were carried out by gene transfer of various plasmids by retrograde injection via ureter.

RESULTS

Our immunohistochemical analysis detected HIF-1alpha in the inner stripe and the inner medulla of normal rats. Water deprivation increased the number of HIF-1alpha-positive cells, which may be mediated by an increase in medullar workload and a decrease in local blood flow. To perform functional studies, we performed gene transfer. Efficient expression of the transgene was confirmed using an enhanced green fluorescent protein (E-GFP) expressing vector. Our histologic and immunoblotting analysis detected the transgene product at the inner medulla and the inner stripe 48 hours after injection. Administration of negative-dominant HIF induced severe damage in the medulla of normal rats. In contrast, gene transfer of constitutively active HIF (HIF/VP16) induced expression of various HIF-regulated genes and protected the medulla against ischemic insults.

CONCLUSION

Our studies demonstrated a crucial role of HIF in the renal medulla under normal and hypoxic circumstances.

Heat Shock Protein 27 in Chronic Allograft Nephropathy: A Local Stress Response

BACKGROUND

Heat shock protein (HSP) 27 plays a cytoprotective role through its antioxidant, antiapoptotic, and actin-stabilizing properties during cell stress. The authors hypothesized that HSP27 is involved in chronic allograft nephropathy (CAN), a chronic state of inflammation and stress.

METHODS

The authors used the Fisher 344-to-Lewis model of CAN. Transplants were performed in 3-month-old recipient rats. HSP27 mRNA and protein levels were determined using semiquantitative polymerase chain reaction, microarray (stress-toxicity, GEArray) analyses, gene sequencing, immunoblotting, and immunohistochemical analyses at 10 days and 6 months posttransplant. P38 mitogen-activated protein kinase (MAPK), manganese (Mn) superoxide dismutase (SOD), copper-zinc (CuZn) SOD, FasL, Bax, hypoxia-inducible factor (HIF)-1alpha, and CD3 lymphocytes were studied in parallel as selective biomarkers of oxidative stress (OS), apoptosis, hypoxia, and graft-infiltrating immune cells.

RESULTS

Six months after transplantation, kidney allografts displayed histologic and functional features of CAN, including tubular atrophy, interstitial fibrosis, glomerulosclerosis, and increased proteinuria and serum creatinine levels. HSP27 mRNA and protein levels in CAN were reduced by 50% and 85%, respectively (P=0.04). Immunohistochemical analyses revealed a "shift" in HSP27 from the medulla to the cortex in allografts with CAN. Bax, phosphorylated p38-MAPK, HIF-1alpha, and MnSOD followed a parallel relocation pattern. CD3 lymphocyte density and tubular FasL expression were also greater in the cortex of allografts with CAN. Time-course analyses revealed that most of these changes were present as early as 10 days posttransplant.

CONCLUSIONS

The shift of HSP27 from the medulla to the cortex, combined with greater CD3, p38-MAPK, Bax, FasL, HIF-1alpha, and MnSOD immunoreactivity in this area of the kidney, likely represents an allograft-level response to CAN-related OS-hypoxia.

Kidney Aging: From Phenotype to Genetics

Aging is a physiological process that causes structural and functional changes in human body systems, sometimes leading to various organ failure. As far as the kidney is concerned, both genetic factors and environmental agents may influence the tissues damage in elderly people and the related loss of function. On the other hand, functional adaptations to structural changes appear to be compromised by co-morbid conditions that are frequently found in elderly people, such as atherosclerosis and hypertension. It is not yet known whether physiological aging is inevitably accompanied by a decline in renal function or how rapidly it might happen. The discovery of molecular mechanisms responsible for tissue damage in aging could offer new perspectives on interventions. The role of nitric oxide, oxidative stress, the renin-angiotensin system, changes in length of telomeres, and klotho gene expression are important subjects for further in-depth studies about aging. A better understanding of physiological renal aging could improve the clinical approach to this process and widen the therapeutic possibilities offered by transplantation.

Two cases of renovascular hypertension and ischemic renal dysfunction: reliable choice of examinations and treatments

We experienced two aged patients with atherosclerotic renovascular stenosis associated with hypertension and ischemic nephropathy. Both patients exhibited sudden rise in blood pressure (BP) and progressive aggravation of renal dysfunction. In these patients, the use of contrast medium to screen for renal artery stenosis (RAS) ran the risk of further deterioration of renal function. We therefore used magnetic resonance angiography (MRA), which is less conducive to renal damage, to screen for RAS. One-sided RAS was treated by percutaneous transluminal angioplasty of the renal artery (PTRA) and stenting. As a result, BP decreased in both patients. Serum creatinine (Cr) decreased slightly in one patient, whereas, in the other, serum Cr increased transiently and then decreased and stabilized to pre-treatment levels. Thus, although it is unclear whether the combination of PTRA and stenting is among the best treatments for patients with RAS and moderate-to-severe renal dysfunction, PTRA and stenting are clearly of benefit in selected patients. In addition, recent progress in characterizing the pathophysiology of ischemic nephropathy associated with renovascular hypertension has created interest in the therapeutic potential of angiotensin II receptor antagonists, sympatholytic agents, and antioxidants. Therefore, we discuss the therapeutic utility of PTRA and stenting and the above-mentioned medications in patients with RAS and renal dysfunction.

Cortical microvascular remodeling in the stenotic kidney: role of increased oxidative stress

OBJECTIVE

Mechanisms of renal injury distal to renal artery stenosis (RAS) remain unclear. We tested the hypothesis that it involves microvascular remodeling consequent to increased oxidative stress.

METHODS AND RESULTS

Three groups of pigs (n=6 each) were studied after 12 weeks of RAS, RAS+antioxidant supplementation (100 IU/kg vitamin E and 1 g vitamin C daily), or controls. The spatial density and tortuousity of renal microvessels (<500 microm) were tomographically determined by 3D microcomputed tomography. The in situ production of superoxide anion and the expression of vascular endothelial growth factor (VEGF), its receptor VEGFR-2, hypoxia-inducible-factor (HIF)-1alpha, von Hippel-Lindau (VHL) protein, and NAD(P)H oxidase (p47phox and p67phox subunits) were determined in cortical tissue. RAS and RAS+antioxidant groups had similar degrees of stenosis and hypertension. The RAS group showed a decrease in spatial density of cortical microvessels, which was normalized in the RAS+antioxidant group, as was arteriolar tortuousity. RAS kidneys also showed tissue fibrosis (by trichrome and Sirius red staining), increased superoxide anion abundance, NAD(P)H oxidase, VHL protein, and HIF-1alpha mRNA expression. In contrast, expression of HIF-1alpha, VEGF, and VEGFR-2 protein was downregulated. These were all significantly improved by antioxidant intervention.

CONCLUSIONS

Increased oxidative stress in the stenotic kidney alters growth factor activity and plays an important role in renal microvascular remodeling, which can be prevented by chronic antioxidant intervention.