Renal cell loss through cell suicide

Prevention of salt-induced renal injury by activation of NAD(P)H:quinone oxidoreductase 1, associated with NADPH oxidase

NADPH oxidase (NOX) is a predominant source of reactive oxygen species (ROS), and the activity of NOX, which uses NADPH as a common rate-limiting substrate, is upregulated by prolonged dietary salt intake. β-Lapachone (βL), a well-known substrate of NAD(P)H:quinone oxidoreductase 1 (NQO1), decreases the cellular NAD(P)H/NAD(P)(+) ratio via activation of NQO1. In this study, we evaluated whether NQO1 activation by βL modulates salt-induced renal injury associated with NOX-derived ROS regulation in an animal model. Dahl salt-sensitive (DS) rats fed a high-salt (HS) diet were used to investigate the renoprotective effect of NQO1 activation. βL treatment significantly lowered the cellular NAD(P)H:NAD(P)(+) ratio and dramatically reduced NOX activity in the kidneys of HS diet-fed DS rats. In accordance with this, total ROS production and expression of oxidative adducts also decreased in the βL-treated group. Furthermore, HS diet-induced proteinuria and glomerular damage were markedly suppressed, and inflammation, fibrosis, and apoptotic cell death were significantly diminished by βL treatment. This study is the first to demonstrate that activation of NQO1 has a renoprotective effect that is mediated by NOX activity via modulation of the cellular NAD(P)H:NAD(P)(+) ratio. These results provide strong evidence that NQO1 might be a new therapeutic target for the prevention of salt-induced renal injury.

Apoptosis and acute kidney injury

Improved mechanistic understanding of renal cell death in acute kidney injury (AKI) has generated new therapeutic targets. Clearly, the classic lesion of acute tubular necrosis is not adequate to describe the consequences of renal ischemia, nephrotoxin exposure, or sepsis on glomerular filtration rate. Experimental evidence supports a pathogenic role for apoptosis in AKI. Interestingly, proximal tubule epithelial cells are highly susceptible to apoptosis, and injury at this site contributes to organ failure. During apoptosis, well-orchestrated events converge at the mitochondrion, the organelle that integrates life and death signals generated by the BCL2 (B-cell lymphoma 2) protein family. Death requires the 'perfect storm' for outer mitochondrial membrane injury to release its cellular 'executioners'. The complexity of this process affords new targets for effective interventions, both before and after renal insults. Inhibiting apoptosis appears to be critical, because circulating factors released by the injured kidney induce apoptosis and inflammation in distant organs including the heart, lung, liver, and brain, potentially contributing to the high morbidity and mortality associated with AKI. Manipulation of known stress kinases upstream of mitochondrial injury, induction of endogenous, anti-apoptotic proteins, and improved understanding of the timing and consequences of renal cell apoptosis will inevitably improve the outcome of human AKI.

Regulation of cell survival by Na+/H+ exchanger-1

Na(+)/H(+) exchanger-1 (NHE1) is a ubiquitous plasma membrane Na(+)/H(+) exchanger typically associated with maintenance of intracellular volume and pH. In addition to the NHE1 role in electroneutral Na(+)/H(+) transport, in renal tubular epithelial cells in vitro the polybasic, juxtamembrane NHE1 cytosolic tail domain acts as a scaffold, by binding with ezrin/radixin/moesin (ERM) proteins and phosphatidylinositol 4,5-bisphosphate, which initiates formation of a signaling complex that culminates in Akt activation and opposition to initial apoptotic stress. With robust apoptotic stimuli renal tubular epithelial cell NHE1 is a caspase substrate, and proteolytic cleavage may permit progression to apoptotic cell death. In vivo, genetic or pharmacological NHE1 loss of function causes renal tubule epithelial cell apoptosis and renal dysfunction following streptozotocin-induced diabetes, ureteral obstruction, and adriamycin-induced podocyte toxicity. Taken together, substantial in vivo and in vitro data demonstrate that NHE1 regulates tubular epithelial cell survival. In contrast to connotations of NHE1 as an unimportant "housekeeping" protein, this review highlights that NHE1 activity is critical for countering tubular atrophy and chronic renal disease progression.

Gentamicin causes apoptosis at low concentrations in renal LLC-PK1 cells subjected to electroporation

Gentamicin accumulates in the lysosomes of kidney proximal tubular cells and causes apoptosis at clinically relevant doses. Gentamicin-induced apoptosis can be reproduced with cultured renal cells, but only at high extracellular concentrations (1 to 3 mM; 0.4 to 1.2 g/liter) because of its low level of uptake. We recently showed that gentamicin-induced apoptosis in LLC-PK1 cells involves a rapid (2-h) permeabilization of lysosomes and activation of the mitochondrial pathway of apoptosis (10 h). We now examine whether the delivery of gentamicin to the cytosol by electroporation would sensitize LLC-PK1 cells to apoptosis. Cells were subjected to eight pulses (1 ms) at 800 V/cm (square waves) in the presence of gentamicin (3 microM to 3 mM; 1.2 mg/liter to 1.2 g/liter); returned to gentamicin-free medium; and examined at 8 h for their Bax (a marker of mitochondrial pathway activation) contents by Western blotting and competitive reverse transcriptase PCR and at 24 h for apoptosis by 4',6'-diamidino-2'-phenylindole staining (confirmed by electron microscopy) and for necrosis (by determination of lactate dehydrogenase release). Nonelectroporated cells were incubated with gentamicin for 8 and 24 h. Significant increases in Bax levels (8 h) and apoptosis (24 h) were detected with 0.03 mM (13.2 mg/liter) gentamicin in electroporated cells compared with those achieved with 2 mM (928 mg/liter) in incubated cells. The increase in the Bax level was not associated with an increase in the level of its mRNA but was associated with the accumulation of ubiquitinated forms (probably as a result of impairment of its degradation by the proteasome). Assay of cell-associated gentamicin showed a marked, immediate, but transient accumulation in electroporated cells, whereas a slow, steady uptake was detected in incubated cells. The data indicate that cytosolic gentamicin triggers apoptosis. Sequestration of gentamicin in lysosomes would, to some extent, protect against apoptosis.

Carbon monoxide prevents apoptosis induced by uropathogenic Escherichia coli toxins

Urinary tract infections (UTIs) are often caused by Escherichia coli (E. coli). Previous studies have demonstrated that up-regulation of heme oxygenase-1 (HO-1) may trigger a survival mechanism against renal cell death induced by E. coli toxins. The present study analyses the role of carbon monoxide (CO), an end product of HO-1, in the survival mechanism. Moreover, we identified hemolysin as a putative pro-apoptotic toxin in the E. coli supernatant. Tubular cells were incubated with CO in the presence or absence of E. coli toxins. Uropathogenic or transformants of non-pathogenic strains expressing hemolysin were used. We found that the survival pathway during E. coli infection might be activated by HO-1-derived production of CO. The protection by CO was also associated with up-regulation of p21 protein expression. Furthermore, we found that in children with pyelonephritis, all the E. coli strains expressing hemolysin induced apoptosis. In E. coli strains not expressing hemolysin, only 45% of the strains could induce apoptosis. In conclusion, generation of CO elicited by HO-1 could promote survival signaling in renal cells. Hemolysin is one of the secreted toxins that are involved in inducing apoptosis during UTI.

Epidermal growth factor potentiates renal cell death in hydronephrotic neonatal mice, but cell survival in rats

BACKGROUND

Epidermal growth factor (EGF) markedly attenuates tubular apoptosis induced by unilateral ureteral obstruction (UUO) in the neonatal rat, and reduces apoptosis induced by mechanical stretch of cultured rat tubular cells.

METHODS

To investigate the role of EGF in modulating apoptosis resulting from UUO, neonatal wild type and mutant mice lacking EGF (knockout), or with diminished EGF receptor activity (waved-2 mutant) were compared to control mice for tubular apoptosis and atrophy. Rat and mouse kidneys were compared for localization of the EGF receptor. Apoptosis was also measured in cultured mouse tubular cells subjected to stretch and exposed to EGF.

RESULTS

UUO reduced endogenous renal EGF expression in wild-type mice. Unlike the rat, exogenous EGF did not decrease tubular apoptosis or atrophy in the obstructed kidney, and significantly increased stretch-induced apoptosis of cultured mouse tubular cells. Tubular apoptosis was 50% lower in the obstructed kidney of EGF knockout and waved-2 mice relative to wild type and heterozygous animals. Exogenous EGF increased tubular apoptosis and doubled atrophy in the obstructed kidney of waved-2 mice. Species differences in EGF receptor localization were detected in 3-day-old kidneys.

CONCLUSION

EGF acts as a survival factor in the neonatal rat, but potentiates tubular cell death in the neonatal mouse. Species differences are maintained in cultured cells, suggesting that differences in EGF receptor signaling underlie these opposing effects.

Long-term blood pressure control prevents oxidative renal injury

Arterial hypertension is a leading contributor to the progression of chronic renal disease. Short-term studies had addressed the role of oxidative stress in hypertensive nephropathy. We have now studied oxidative stress and caspase activation in a long-term model of hypertensive renal injury. Nontreated spontaneously hypertensive rats with uninephrectomy displayed severe arterial hypertension over a 36-week follow-up. Uncontrolled high blood pressure in the context of modest renal mass reduction resulted in significant histological renal injury. Blood pressure control by the angiotensin-converting enzyme (ACE) inhibitor, quinapril, or the AT1 receptor antagonist, losartan, decreased the degree of renal injury. Hypertensive renal injury was associated with evidence of activation of the apoptotic pathway (increased activation of caspase-3) and local renal (increased staining for 4-hydroxy-2-nonenal) and systemic [increased serum levels of 8-iso-prostaglandin F2alpha (8-iso-PGF2alpha)] lipid oxidation when compared with normotensive control rats. In addition, severe hypertension decreased the renal antioxidant defenses, as exemplified by decreased expression of Cu/Zn superoxide dismutase. Treatment with quinapril or losartan decreased caspase-3 activation, 4-hydroxy-2-nonenal staining, and 8-iso-PGF2alpha levels and increased Cu/Zn superoxide dismutase expression. These results suggest that hypertension-associated oxidative stress and its consequences may be decreased by either ACE inhibition or AT1 receptor antagonist, emphasizing the role of angiotensin II in hypertensive renal damage.

Renal tubular epithelial cell self-injury through Fas/Fas ligand interaction promotes renal allograft injury

Tubular epithelial cells (TECs) coexpress Fas and Fas ligand (FasL), which could influence renal allograft injury. While TECs can resist apoptosis by Fas antibody, TEC apoptosis by contact with adjacent TECs has not been studied. Fas expression increased in TECs with cytokine treatment (IFN-gamma, TNF-alpha) while abundant FasL levels were not altered. Apoptosis (Annexin-V, DNA fragmentation) occurred in cytokine-treated TECs monolayers from C3H-HeJ mice by 24 h, but was absent in similarly treated TECs from Fas-deficient (lpr) or FasL-mutant (gld) mice, suggesting that 'self injury' occurred through Fas/FasL. Membrane labeling of TECs in cocultures confirmed that FasL-bearing TECs induced apoptosis when in contact with Fas-bearing TECs. Culturing TECs with allogeneic C57BL/6 (H-2b) splenocytes resulted in apoptosis and elimination of C3H-HeJ TECs by 48 h, with enhanced survival and reduced apoptosis using lpr or gld TECs. In a renal allograft model, survival of C57BL/6 recipients was greater (p < 0.05) and renal function improved (p < 0.001) using C3H-lpr or C3H-gld (H-2 k) donor kidneys compared with C3H-HeJ kidneys. These data demonstrate for the first time that cytokine-activated TECs can injure TECs through expression of functional FasL and Fas. We suggest that inhibition of TEC-TEC 'self injury' may be a novel strategy to augment renal allograft survival.

Insulin is a potent survival factor in mesangial cells: role of the PI3-kinase/Akt pathway

BACKGROUND

Elucidating the mechanisms of apoptosis is important for understanding the molecular mechanisms underlying glomerular disease. The phosphatidylinositol 3 kinase (PI3-kinase)/Akt pathway is essential for survival signaling in non-renal cells. However, little is known about the anti-apoptotic effect of insulin and the role of the PI3-kinase/Akt pathway in mesangial cells (MC) apoptosis.

METHODS

Apoptosis was induced in wild type, p27Kip1 (p27) -/- and p21Cip1/Waf1 (p21) -/- mouse MC by survival factor withdrawal, actinomycin D, ultraviolet (UV)-B irradiation and cycloheximide in the presence or absence of insulin (1 micromol/L) or insulin-like growth factor-I (IGF-I; 100 ng/mL). The activation and levels of Akt, extracellular signal regulated kinase (ERK) and specific cell cycle proteins were determined by Western blot analysis.

RESULTS

Insulin and IGF-I inhibited wild-type MC apoptosis induced by survival factor withdrawal, actinomycin D, ultraviolet-B irradiation and cycloheximide and in p27 -/- MC when apoptosis was induced by survival factor withdrawal. Akt was activated by insulin and IGF-I during apoptosis. Blocking PI3-kinase with LY294002 reduced Akt activation and abrogated the anti-apoptotic effect of insulin. ERK was activated during apoptosis and blocking ERK activation with U0126 or PD98059 partially rescued MC from apoptosis. Moreover, insulin also suppressed ERK activation during apoptosis. Our results also showed that the CDK-inhibitor p21 was increased by insulin and that p21 up-regulation was PI3-kinase/Akt pathway dependent. Furthermore, p21 -/- MC apoptosis induced by survival factor withdrawal was not rescued by insulin in contrast to the wild-type and p27 -/- MC. These data suggest that p21 may have a critical role in the anti-apoptotic effect of insulin.

CONCLUSIONS

Insulin is a potent survival factor for MC in response to a number of different apoptotic triggers, and this effect is mediated through the PI3-kinase/Akt pathway. Moreover, ERK and p21 may be involved in anti-apoptotic effect of insulin in MC.

Contribution of apoptotic cell death to renal injury

Cell number abnormalities are frequent in renal diseases, and range from the hypercellularity of postinfectious glomerulonephritis to the cell depletion of chronic renal atrophy. Recent research has shown that apoptosis and its regulatory mechanisms contribute to cell number regulation in the kidney. The role of apoptosis ranges from induction to repair and progression of renal injury. Death ligands and receptors, such as TNF and FasL, proapoptotic and antiapoptotic Bcl-2 family members and caspases have all been shown to participate in apoptosis regulation in the course of renal injury. These proteins represent potential therapeutic targets, which should be further explored.