Acute renal failure. I. Relative importance of proximal vs. distal tubular injury

Kidney injury molecule-1: a tissue and urinary biomarker for nephrotoxicant-induced renal injury

Nephrotoxicity is a common side effect of therapeutic interventions, environmental insults, and exposure to toxicants in the workplace. Although biomarkers for nephrotoxicity are available, they often lack sensitivity and are not specific as indicators of epithelial cell injury. Kidney injury molecule-1 (Kim-1) is a type 1 membrane protein with extracellular immunoglobulin and mucin domains. The mRNA and protein for Kim-1 are expressed at very low levels in normal rodent kidney, but expression increases dramatically after injury in proximal tubule epithelial cells in postischemic rodent kidney and in humans during ischemic acute renal failure. To evaluate the utility of Kim-1 as a biomarker for other types of renal injury, we analyzed tissue and urinary expression in response to three different types of nephrotoxicants in the rat: S-(1,1,2,2-tetrafluoroethyl)-l-cysteine (TFEC), folic acid, and cisplatin. Marked increases in Kim-1 expression were confirmed by immunoblotting in all three models. The protein was shown to be localized to the proximal tubule epithelial cell by immunofluorescence. Furthermore, Kim-1 protein was detected in urine of toxicant-treated rats. The temporal pattern of expression in response to TFEC is similar to the Kim-1 expression pattern in the postischemic kidney. In folic acid-treated kidneys, Kim-1 is clearly localized to the apical brush border of the well-differentiated proximal tubular epithelial cells. After folic acid treatment, expression of Kim-1 is present in the urine despite no significant increase in serum creatinine. Cisplatin treatment results in early detection of urinary Kim-1 protein and diffuse Kim-1 expression in S3 cells of the proximal tubule. Kim-1 can be detected in the tissue and urine on days 1 and 2 after cisplatin administration, occurring before an increase in serum creatinine. The upregulation of expression of Kim-1 and its presence in the urine in response to exposure to various types of nephrotoxicants suggest that this protein may serve as a general biomarker for tubular injury and repair processes.

Molecular mechanisms of recovery from acute renal failure

OBJECTIVES

Despite technological advances in renal replacement therapy over the past few years, acute renal failure in the intensive care unit remains associated with high morbidity and mortality rates. In this article I review recent research aimed at elucidating mechanisms of renal recovery from acute injury.

DESIGN

Review of the literature.

CONCLUSIONS

A number of peptide growth hormones are reviewed, including epidermal growth factor, insulin-like growth factor-1, thyroxine, hepatocyte growth factor, and bone morphogenetic protein-7 promote renal regeneration in model systems. Unfortunately, despite promising studies in animal models of toxin and ischemia-induced acute tubular necrosis, human studies have not shown any clinical benefit. However, several of these molecules have not been studied in clinical trials. Existing pharmacologic strategies have a limited role in renal recovery. Finally, several recent studies have focused on the effects of renal replacement therapy on renal recovery, but additional studies are needed to confirm and extend these results.

Cell death induced by acute renal injury: a perspective on the contributions of apoptosis and necrosis

In humans and experimental models of renal ischemia, tubular cells in various nephron segments undergo necrotic and/or apoptotic cell death. Various factors, including nucleotide depletion, electrolyte imbalance, reactive oxygen species, endonucleases, disruption of mitochondrial integrity, and activation of various components of the apoptotic machinery, have been implicated in renal cell vulnerability. Several approaches to limit the injury and augment the regeneration process, including nucleotide repletion, administration of growth factors, reactive oxygen species scavengers, and inhibition of inducers and executioners of cell death, proved to be effective in animal models. Nevertheless, an effective approach to limit or prevent ischemic renal injury in humans remains elusive, primarily because of an incomplete understanding of the mechanisms of cellular injury. Elucidation of cell death pathways in animal models in the setting of renal injury and extrapolation of the findings to humans will aid in the design of potential therapeutic strategies. This review evaluates our understanding of the molecular signaling events in apoptotic and necrotic cell death and the contribution of various molecular components of these pathways to renal injury.

Chronic hyperosmolarity mediates constitutive expression of molecular chaperones and resistance to injury

Renal medullary cells are exposed to elevated and variable osmolarities and low oxygen tension. Despite the harsh environment, these cells are resistant to the effects of many harmful events. To test the hypothesis that this resistance is a consequence of these cells developing a stress tolerance phenotype to survive in this milieu, we created osmotically tolerant cells [hypertonic (HT) cells] by gradually adapting murine inner medullary collecting duct 3 cells to hyperosmotic medium containing NaCl and urea. HT cells have a reduced DNA synthesis rate, with the majority of cells arrested in the G(0)/G(1) phase of the cell cycle, and show constitutive expression of heat shock protein 70 that is proportional to the degree of hyperosmolarity. Unlike acute hyperosmolarity, chronic hyperosmolarity failed to activate MAPKs. Moreover, HT cells acquired protein translational tolerance to further stress treatment, suggesting that HT cells have an osmotolerant phenotype that is analogous to thermotolerance but is a permanent condition. In addition to osmotic shock, HT cells were more resistant to heat, H(2)O(2), cyclosporin, and apoptotic inducers, compared with isotonic murine inner medullary duct 3 cells, but less resistant to amphotericin B and cadmium. HT cells demonstrate that in renal medullary cells, hyperosmotic stress activates biological processes that confer cross-tolerance to other stressful conditions.

Neutrophil-independent mechanisms of caspase-1- and IL-18-mediated ischemic acute tubular necrosis in mice

Having recently described the injurious role of caspase-1-mediated production of the proinflammatory cytokine IL-18 in ischemic acute renal failure (ARF), we report here on the effect of the newly developed caspase inhibitor Quinoline-Val-Asp(Ome)-CH(2)-OPH (OPH-001) on caspase-1, IL-18, neutrophil infiltration, and renal function in ischemic ARF. C57BL/6 mice with ischemic ARF treated with OPH-001 had a marked (100%) reduction in blood urea nitrogen (BUN) and serum creatinine and a highly significant reduction in morphological acute tubular necrosis (ATN) score compared with vehicle-treated mice. OPH-001 significantly reduced the increase in caspase-1 activity and IL-18 and prevented neutrophil infiltration in the kidney during ischemic ARF. To evaluate whether this lack of neutrophil infiltration was contributing to the protection against ischemic ARF, a model of neutrophil depletion was developed. Neutrophil-depleted mice had a small (18%) reduction in serum creatinine during ischemic ARF but no reduction in ATN score despite a lack of neutrophil infiltration in the kidney. Remarkably, caspase-1 activity and IL-18 were significantly increased in the kidney in neutrophil-depleted mice with ARF. In addition, IL-18 antiserum-treated neutrophil-depleted mice with ischemic ARF had a significant (75%) reduction in serum creatinine and a significant reduction in ATN score compared with vehicle-treated neutrophil-depleted mice. These results suggest a novel neutrophil-independent mechanism of IL-18-mediated ischemic ARF.

Neutrophil-independent mechanisms of caspase-1- and IL-18-mediated ischemic acute tubular necrosis in mice

Having recently described the injurious role of caspase-1-mediated production of the proinflammatory cytokine IL-18 in ischemic acute renal failure (ARF), we report here on the effect of the newly developed caspase inhibitor Quinoline-Val-Asp(Ome)-CH(2)-OPH (OPH-001) on caspase-1, IL-18, neutrophil infiltration, and renal function in ischemic ARF. C57BL/6 mice with ischemic ARF treated with OPH-001 had a marked (100%) reduction in blood urea nitrogen (BUN) and serum creatinine and a highly significant reduction in morphological acute tubular necrosis (ATN) score compared with vehicle-treated mice. OPH-001 significantly reduced the increase in caspase-1 activity and IL-18 and prevented neutrophil infiltration in the kidney during ischemic ARF. To evaluate whether this lack of neutrophil infiltration was contributing to the protection against ischemic ARF, a model of neutrophil depletion was developed. Neutrophil-depleted mice had a small (18%) reduction in serum creatinine during ischemic ARF but no reduction in ATN score despite a lack of neutrophil infiltration in the kidney. Remarkably, caspase-1 activity and IL-18 were significantly increased in the kidney in neutrophil-depleted mice with ARF. In addition, IL-18 antiserum-treated neutrophil-depleted mice with ischemic ARF had a significant (75%) reduction in serum creatinine and a significant reduction in ATN score compared with vehicle-treated neutrophil-depleted mice. These results suggest a novel neutrophil-independent mechanism of IL-18-mediated ischemic ARF.

Oxygen transport across vasa recta in the renal medulla

In this model of oxygen transport in the renal medullary microcirculation, we predicted that the net amount of oxygen reabsorbed from vasa recta into the interstitium is on the order of 10(-6) mmol/s, i.e., significantly lower than estimated medullary oxygen requirements based on active sodium reabsorption. Our simulations confirmed a number of experimental findings. Low medullary PO(2) results from the countercurrent arrangement of vessels and an elevated vasa recta permeability to oxygen, as well as high metabolic needs. Diffusional shunting of oxygen between descending vasa recta (DVR) and ascending vasa recta also explains why a 20-mmHg decrease in initial PO(2) at the corticomedullary junction only leads to a small drop in papillary tip PO(2) (<2 mmHg with baseline parameter values). Conversely, small changes in the consumption rate of DVR-supplied oxygen, in blood flow rate, in hematocrit, or in capillary permeability to oxygen, beyond certain values sharply reduce interstitial PO(2). Without erythrocytes, papillary tip PO(2) cannot be maintained above 10 mmHg, even when oxygen consumption is zero.

Adenosine attenuates oxidant injury in human proximal tubular cells via A(1) and A(2a) adenosine receptors

We have recently demonstrated protection against renal ischemic-reperfusion injury in vivo by A(1)- and A(2a)-adenosine receptor (AR) modulations. To further elucidate the signaling cascades of AR-induced cytoprotection against reperfusion/oxidant-mediated injury, immortalized human proximal tubule (HK-2) cells were treated with H(2)O(2). H(2)O(2) caused dose- and time-dependent HK-2 cell death that was measured by lactate dehydrogenase release and trypan blue dye uptake. Adenosine protected against H(2)O(2)-induced HK-2 cell death by means of A(1)- and A(2a)-AR activation. A(1)-AR-mediated protection involves pertussis toxin-sensitive G proteins and protein kinase C, whereas A(2a)-AR-mediated protection involves protein kinase A activation by means of cAMP and activation of the cAMP response element binding protein. Moreover, protein kinase A activators (forskolin and Sp-isomer cAMP) also protected HK-2 cells against H(2)O(2) injury. De novo gene transcription and protein synthesis are required for both A(1)- and A(2a)-AR-mediated cytoprotection as actinomycin D and cycloheximide, respectively, blocked cytoprotection. Chronic treatments with a nonselective AR agonist abolished the protection by adenosine. Moreover, chronic treatments with a nonselective AR antagonist increased the endogenous tolerance of HK-2 cells against H(2)O(2). We concluded that A(1)- and A(2a)-AR activation protects HK-2 cells against H(2)O(2)-induced injury by means of distinct signaling pathways that require new gene transcription and new protein synthesis.

Single injection of naked plasmid encoding hepatocyte growth factor prevents cell death and ameliorates acute renal failure in mice

Hepatocyte growth factor (HGF) is a pleiotrophic factor that plays an important role in tissue repair and regeneration after injury. The expression of both HGF and its c-met receptor genes is rapidly upregulated after acute renal injury induced by folic acid. In this study, the role of exogenous HGF in preventing acute renal failure by systemic administration of naked plasmid containing human HGF cDNA driven under the cytomegalovirus promoter (pCMV-HGF) was examined in mice. Intravenous injection of pCMV-HGF plasmid produced substantial levels of human HGF protein in mouse kidneys. Simultaneous injection of HGF plasmid DNA significantly ameliorated renal dysfunctions and accelerated recovery from the acute injury induced by folic acid. Of interest, preadministration of HGF plasmid 24 h before folic acid injection dramatically protected renal epithelial cells from both apoptotic and necrotic death and preserved the structural and functional integrity of renal tubules. Expression of HGF transgene activated protein kinase B/Akt kinase and preserved prosurvival Bcl-xL protein expression in vivo. These results indicate that a single, intravenous injection of naked plasmid containing HGF gene not only promotes renal regeneration after injury but also protects tubular epithelial cells from the initial injury and cell death in the first place. These data suggest that HGF gene therapy may provide a new avenue for exploring a novel therapeutic strategy for clinical acute renal failure.

Cell proliferation in the loop of henle in the developing rat kidney

In the developing rat kidney, there is no separation of the medulla into an outer and inner zone. At the time of birth, ascending limbs with immature distal tubule epithelium are present throughout the renal medulla, all loops of Henle resemble the short loop of adult animals, and there are no ascending thin limbs. It was demonstrated previously that immature thick ascending limbs in the renal papilla are transformed into ascending thin limbs by apoptotic deletion of cells and transformation of the remaining cells into a thin squamous epithelium. However, it is not known whether this is the only source of ascending thin limb cells or whether cell proliferation occurs in the segment undergoing transformation. This study was designed to address these questions and to identify sites of cell proliferation in the loop of Henle. Rat pups, 1, 3, 5, 7, and 14 d old, received a single injection of 5-bromo-2'-deoxyuridine (BrdU) 18 h before preservation of kidneys for immunohistochemistry. Thick ascending and descending limbs were identified by labeling with antibodies against the serotonin receptor, 5-HT(1A), and aquaporin-1, respectively. Proliferating cells were identified with an antibody against BrdU. BrdU-positive cells in descending and ascending limbs of the loop of Henle were counted and expressed as percentages of the total number of aquaporin-1-positive and 5-HT(1A)-positive cells in the different segments. In the developing kidney, numerous BrdU-positive nuclei were observed in the nephrogenic zone. Outside of this location, BrdU-positive tubule cells were most prevalent in medullary rays in the inner cortex and in the outer medulla. BrdU-labeled cells were rare in the papillary portion of the loop of Henle and were not observed in the lower half of the papilla after 3 d of age. BrdU-labeled nuclei were not observed in segments undergoing transformation or in newly formed ascending thin limb epithelium. It was concluded that the growth zone for the loop of Henle is located around the corticomedullary junction, and the ascending thin limb is mainly, if not exclusively, derived from cells of the thick ascending limb.

Inhibition of poly(ADP-ribose) polymerase attenuates ischemic renal injury in rats

The enzyme, poly(ADP-ribose) polymerase (PARP), effects repair of DNA after ischemia-reperfusion (I/R) injury to cells in nerve and muscle tissue. However, its activation in severely damaged cells can lead to ATP depletion and death. We show that PARP expression is enhanced in damaged renal proximal tubules beginning at 6-12 h after I/R injury. Intraperitoneal administration of PARP inhibitors, benzamide or 3-amino benzamide, after I/R injury accelerates the recovery of normal renal function, as assessed by monitoring the levels of plasma creatinine and blood urea nitrogen during 6 days postischemia. PARP inhibition leads to increased cell proliferation at 1 day postinjury as assessed by proliferating cell nuclear antigen and improves the histopathological appearance of kidneys examined at 7 days postinjury. Furthermore, inhibition of PARP increases levels of ATP measured at 24 h postischemia compared with those in vehicle-treated animals. Our data indicate that PARP activation is a part of the cascade of molecular events that occurs after I/R injury in the kidney. Although caution is advised, transient inhibition of PARP postischemia may constitute a novel therapy for acute renal failure.

Identification of a novel kidney-specific gene downregulated in acute ischemic renal failure

To gain further insights into the molecular mechanisms involved in acute renal failure, we have isolated a new gene from rat and human, named KSP32 (kidney-specific protein with a molecular mass of 32 kDa). KSP32 encodes a novel gene that shows little homology to other mammalian proteins. It, however, shares extensive homology with several proteins found in the nematode Caenorhabditis elegans and plants. The expression of KSP32 mRNA is highly restricted to kidney. In situ hybidization analysis revealed that the expression of KSP32 mRNA was prominent in the boundary of kidney cortex and outer medulla, exhibiting a raylike formation extending from the medulla into the cortex. Finally, KSP32 mRNA was dramatically downregulated in rat following induction of acute ischemic renal failure. Rapid loss of KSP32 mRNA expression was observed beginning at approximately 5 h following renal injury and mRNA levels remained depressed for at least 96 h. Both KSP32 mRNA levels as well as renal function recovered 14 days after injury. Administration of an endothelin receptor antagonist (SB-209670), known to restore renal function, significantly increased KSP32 expression.

Acute renal failure. III. The role of growth factors in the process of renal regeneration and repair

This review, which is the final installment in a series devoted to controversial issues in acute renal failure (ARF) (3, 47), will examine available information regarding the role of growth factors in ARF. In general, studies in this area have fallen into two broad categories: 1) those that have examined the renal expression of genes encoding growth factors or transcriptional factors associated with the growth response that is induced after ARF, and 2) those that have examined the efficacy of exogenously administered growth factors in accelerating recovery of renal function in experimental models of ARF. Despite the vast amount of information that has accumulated in these two areas of investigation, our understanding of the mechanisms involved in the process of regeneration and repair after ARF, and the role of growth factors in this response, remains rudimentary. This overview, contributed to by a number of experts in the field, is designed to summarize present knowledge and to highlight potentially fertile areas for future research in this area.

Heme oxygenase-1 gene ablation or expression modulates cisplatin-induced renal tubular apoptosis

Heme oxygenase-1 (HO-1) is a 32-kDa microsomal enzyme that catalyzes the conversion of heme to biliverdin, releasing iron and carbon monoxide. Induction of HO-1 occurs as a protective response in cells/tissues exposed to a wide variety of oxidant stimuli. The chemotherapeutic effects of cis-diamminedichloroplatinum(II) (cisplatin), a commonly used anticancer drug, are limited by significant nephrotoxicity, which is characterized by varying degrees of renal tubular apoptosis and necrosis. The purpose of this study was to evaluate the functional significance of HO-1 expression in cisplatin-induced renal injury. Our studies demonstrate that transgenic mice deficient in HO-1 (-/-), develop more severe renal failure and have significantly greater renal injury compared with wild-type (+/+) mice treated with cisplatin. In vitro studies in human renal proximal tubule cells demonstrate that hemin, an inducer of HO-1, significantly attenuated cisplatin-induced apoptosis and necrosis, whereas inhibition of HO-1 enzyme activity reversed the cytoprotective effect. Overexpression of HO-1 resulted in a significant reduction in cisplatin-induced cytotoxicity. These studies provide a basis for future studies using targeted gene expression of HO-1 as a therapeutic and preventive modality in high-risk settings of acute renal failure.