Search

Over the last 20 years, there has been an increased appreciation of the long-term sequelae of acute kidney injury (AKI) and the potential development of chronic kidney disease (CKD). Several pathophysiological features have been proposed to mediate the AKI to CKD progression including maladaptive alterations in tubular, interstitial, inflammatory and vascular cells. These alterations likely interact to culminate in the progression to CKD. In this article we focus primarily on evidence of vascular rarefaction secondary to AKI, and the potential mechanisms by which rarefaction occurs in relation to other alterations in tubular and interstitial compartments. We further focus on the potential that rarefaction contributes to renal hypoxia. Consideration the role of hypoxia in the AKI to CKD transition focuses on experimental evidence of persistent renal hypoxia following AKI and experimental maneuvers to evaluate the influence of hypoxia, per se, in progressive disease. Finally, consideration of methods to evaluate hypoxia in patients is provided with the suggestion that non-invasive measurement of renal hypoxia may provide insight into progression in post AKI patients.

Ku70 plays an important role in DNA damage repair and prevention of cell death. Previously, we reported that apoptosis caused a decrease in cellular Ku70 levels. In this study, we analyzed the mechanism of how Ku70 levels decrease during drug-induced apoptosis. In HeLa cells, staurosporin (STS) caused a decrease in Ku70 levels without significantly affecting Ku70 mRNA levels. We found that Ku70 protein was highly ubiquitinated in various cell types, such as HeLa, HEK293T, Dami (a megakaryocytic cell line), endothelial, and rat kidney cells. An increase in ubiquitinated Ku70 protein was observed in apoptotic cells, and proteasome inhibitors attenuated the decrease in Ku70 levels in apoptotic cells. These results suggest that the ubiquitin-proteasome proteolytic pathway plays a role in decreasing Ku70 levels in apoptotic cells. Ku70 forms a heterodimer with Ku80, which is required for the DNA repair activity of Ku proteins. We also found that Ku80 levels decreased in apoptotic cells and that Ku80 is a target of ubiquitin. Ubiquitinated Ku70 was not found in the Ku70-Ku80 heterodimer, suggesting that modification by ubiquitin inhibits Ku heterodimer formation. We propose that the ubiquitin-dependent modification of Ku70 plays an important role in the control of cellular levels of Ku70.

Resistance to ischemic acute renal failure in the Brown Norway rat: A new model to study cytoprotection. Background An in vivo model of intrinsic resistance to ischemia could be invaluable to define how specific pathways to injury or putative protectors from injury affect the severity of acute renal failure (ARF). The purpose of this study was to determine whether separate rat strains had differential sensitivity to renal ischemia, characterize the extent of protection, and begin to define differences in gene expression that might impact on the severity of ARF. Methods The sensitivity to 45 minutes of renal ischemia in Sprague-Dawley rat (SD) was compared with 2 lines of Brown-Norway rats (BN/Mcw, BN/Hsd). Constitutive and inducible stress protein expression was compared between strains. Results At 24hours' reperfusion, SD rats had higher creatinine (3.4mg/dL), elevated Na and water excretion, and proximal tubule necrosis. Both strains of BN rats were resistant to loss of renal function (Scr = 0.9mg/dL at 24hours' reflow) and had preserved renal morphology. BN rats had no redistribution of Na,K-ATPase into detergent-soluble cortical extracts found early (15 minutes) after ischemia in SD rats. Hsc73 expression did not differ between strains and was not induced by ischemia. Compared with SD, induction of Hsp25 and 72 by renal ischemia was blunted in both BN strains. Constitutive Hsp25 was higher in both BN-Mcw and BN-Hsd compared with SD rat kidney. Constitutive Hsp72 was significantly higher only in BN-Mcw kidneys. Immunohistochemistry showed baseline Hsp72 and 25 expression was increased in proximal tubules of BN-Mcw versus SD. Conclusion BN rat kidney is resistant to ischemic injury and provides a new model for studying cytoprotective mechanisms. Initial study of strain-specific gene expression suggests particular stress proteins are among the potential mechanisms contributing to protection against ARF.

Acute kidney dysfunction after ischemia-reperfusion injury may be a consequence of persistent intrarenal vasoconstriction. Regulators of G protein Signaling (RGS) proteins are GTPase activating proteins for heterotrimeric G proteins that can regulate vascular tone. RGS4 is expressed in vascular smooth muscle cells in the kidney. However, RGS4 protein levels are low in many tissues as a consequence of N-end rule-mediated polyubiquination and proteasomal degradation. In this work, the role of RGS4 in the renal response to ischemia/reperfusion injury (IRI) was investigated. A murine model of IRI was employed and RGS4-null mice (R4KO) were highly sensitized to the development of renal dysfunction after IRI. Furthermore, R4KO mice exhibited reduced renal blood flow after IRI. Kidneys were studied for intrinsic RGS4 function by ex vivo isolation. R4KO kidneys exhibited increased renal vasoconstriction in response to endothelin-1 infusion. The intrinsic renal activity of RGS4 was examined in an in vivo model of syngeneic renal transplantation. Transplanted R4KO kidneys exhibited significantly reduced reperfusion blood flow and increased renal cell death. To increase RGS4 activity, wild type mice were administered the proteasomal inhibitor MG-132 and this resulted in increased renal RGS4 protein. Furthermore, MG-132 treatment inhibited the development of renal dysfunction after IRI in wild type - but not R4KO - mice. These results demonstrate that RGS4 antagonizes the development of renal dysfunction in response to IRI.

DIABETES IS NOW the principal cause of end-stage renal disease (ESRD). Greater than 40% of approximately 350,000 patients with ESRD in the United States have diabetes as their primary disease (1997 data).1 The rate of incidence for diabetic ESRD is greater than twice that of nondiabetic ESRD (1992-1996).1 Based on these data, one can predict an epidemic of ESRD comprising mostly diabetic patients in the near future. Because of graft limitations for renal transplant, dialysis remains the primary treatment regimen. Research efforts have been directed toward attenuating the progression of chronic renal failure by targeting the selfperpetuating fibrotic process in the kidney. Diabetic nephropathy with renal insufficiency is characterized by scarring in the glomeruli and the tubulointerstitium. It generally is well accepted that there are common and convergent pathways that are activated in the glomerular, tubular, and interstitial cells in diabetes that promote fibrosis. Transforming growth factor (TGF)-b represents perhaps the most central player in the fibrogenic process because of its activity in increasing extracellular matrix production and inhibiting its degradation.2 Interest in TGF-b as a mediator of kidney disease spawns primarily from the groundbreaking work of Border et al,3,4 which showed that blockade of TGF-b inhibited fibrotic deposition in the glomeruli of rats treated with anti-Thy 1.1 antibodies. Since the publication of those studies, TGF-b activity, expression, or both have been evaluated in nearly all renal disease models characterized by scarring in the glomerulus or tubulointerstitium.2 Primary among these are models of diabetic nephropathy. Ziyadeh et al, whose article is the subject of this Journal Club,5 have been at the forefront of many major discoveries regarding the potential pathologic role of TGF-b in the setting of diabetic nephropathy. They and others have shown that animal models of type 1 and 2 diabetes are associated with elevated renal expression of TGFb.6,7 These investigators have measured the increased production of TGF-b by the kidneys of patients with type 2 diabetes.8 Mechanistically, they have shown that elevated glucose concentration stimulates the production of TGF-b in cultured renal mesangial and tubular cells in vitro.9-11 Despite complicated renal anatomy and a diversity of renal responses to hyperglycemia, a relatively straightforward hypothesis can be proposed: Elevated plasma glucose or glycated proteins elicit renal fibrosis by stimulating the local production of TGF-b. The exact mechanism by which TGF-b activity is enhanced in the diabetic state is not yet clear. Work from this group and others suggests, however, that increased TGF-b activity results in part from increased transcriptional activity.12,13 Multiple signaling pathways, including DAG/ PKC-b, inositol triphosphate, cyclooxygenase, and PLA2, have been implicated.2,14 A putative

Acute Kidney Injury (AKI) is a significant contributor to morbidity and mortality, affecting 35% of critical care patients and 7% of hospital admissions nationwide. Furthermore, the 60 day mortality rate for AKI patients requiring renal replacement therapy was as high as 53.6%. Understanding the molecular underpinnings responsible for the variation in response to injury is an important step to addressing AKI. Previous work on AKI resistant Brown Norway s rats indicated that several proteins active in metabolism were overexpressed post-injury. Among these was Isocitrate Dehydrogenase 2 (IDH2), a mitochondria-enzyme that has a crucial role in metabolism.. In order to assess the contribution of heightened expression of IDH2 to AKI resistance, the protein was overexpressed in a highly oxygen-dependent cell line derived from mouse proximal tubule (S3 cells). Overexpression was confirmed via western blotting. The membrane potential of the mitochondria was assessed by use of the cationic dye JC-1. Mitochondrial polarization, as measured by the red-shift of JC-1 dye, was increased by 11% (p

Acute kidney injury following folate administration is characterized by a vascular remodeling that is initially proliferative but subsequently results in vascular endothelial loss. Interventions directed toward promoting endothelial growth may preserve vascular structure and therefore renal function. However, angiopoietin-1 therapy in the setting of folate-induced acute kidney injury resulted in an expanded fibrotic response despite apparent preservation of the vasculature, indicating that renal repair responses are complex and vascular-directed therapies should be approached with caution.