Cyclosporine nephropathy: morphometric analysis of the medullary thick ascending limb

Role of Hypoxia in Renal Failure Caused by Nephrotoxins and Hypertonic Solutions

Hypoxia plays a role in the pathogenesis of acute kidney injury under diverse clinical settings, including nephrotoxicity. Although some nephrotoxins exert direct renal parenchymal injury, likely with consequent altered oxygenation, others primarily reduce renal parenchymal oxygenation, leading to hypoxic tubular damage. As outlined in this review, nephrotoxin-related renal hypoxia may result from an altered renal oxygen supply (cyclosporine), enhanced oxygen consumption for tubular transport (agents inducing osmotic diuresis), or their combination (nonsteroidal anti-inflammatory drugs, radiocontrast agents, and others). Most agents causing hypoxic renal injury further supress physiologic low medullary Po₂, in which a limited regional blood supply barely matches the intense regional tubular transport and oxygen consumption. The medullary tubular transport and blood supply are finely matched, securing oxygen sufficiency. Predisposition to hypoxia-mediated nephrotoxicity by medical conditions, such as chronic kidney disease or diabetes, may be explained by malfunctioning of control systems that normally maintain medullary oxygenation. However, this propensity may be diminished by hypoxia-mediated adaptive responses governed by hypoxia-inducible factors. Recent reports have suggested that inhibitors of sodium-glucose cotransporters and the administration of hypertonic saline may be added to the growing list of common therapeutic interventions that intensify medullary hypoxia, and potentially could lead to hypoxic acute kidney injury.

Cyclosporine A induces endothelin-converting enzyme-1: Studies in vivo and in vitro

AIM

Cyclosporine A (CsA) induces renal vasoconstriction and hypoxia and enhances the expression of endothelin-1 (ET-1) pro-hormone (pre-pro-ET-1), plausibly leading to a feed-forward loop of renal vasoconstriction, hypoxia and enhanced synthesis of the potent vasoconstrictor ET-1. Endothelin-converting enzyme (ECE)-1 cleaves big endothelin to generate endothelin (ET)-1 and is upregulated by hypoxia via hypoxia-inducible factor (HIF). We hypothesized that in addition to the direct induction of ET-1 synthesis, CsA might also intensify renal ECE-1 expression, thus contributing to enhanced ET-1 synthesis following CsA.

METHODS

CsA was administered to Sprague Dawley rats (120 mg/kg/SC) for 4 days, and renal HIF and ECE-1 expression were assessed with Western blots and immunostaining. Human umbilical vein endothelial cells (HUVEC) and proximal tubular cell line (HK-2) were subjected to CsA, and ECE-1 induction was evaluated using real-time mRNA PCR and Western blots.

RESULTS

Cyclosporine A intensified renal parenchymal ECE-1 expression in the rat kidney, particularly in distal nephron segments, along with renal hypoxia (detected by pimonidazole adducts) and HIF expression, in line with our recent observations showing episodic hypoxia in mice subjected to CsA. Furthermore, in cultured normoxic HUVEC and HK-2 cells, CsA dose-dependently induced both pre-pro-ET-1 and ECE-1 mRNA and protein expression, with enhanced ET-1 generation.

CONCLUSION

CsA induces ECE-1 via both hypoxic and non-hypoxic pathways. ECE-1 may contribute to increased renal ET-1 generation following CsA, participating in a feed-forward loop of renal parenchymal hypoxia and ET synthesis.

RENAL PARENCHYMAL OXYGENATION AND HYPOXIA ADAPTATION IN ACUTE KIDNEY INJURY

The pathogenesis of acute kidney injury (AKI), formally termed acute tubular necrosis, is complex and, phenotypically, may range from functional dysregulation without overt morphological features to literal tubular destruction. Hypoxia results from imbalanced oxygen supply and consumption. Increasing evidence supports the view that regional renal hypoxia occurs in AKI irrespective of the underlying condition, even under circumstances basically believed to reflect 'direct' tubulotoxicity. However, at present, it is remains unclear whether hypoxia per se or, rather, re-oxygenation (possibly through reactive oxygen species) causes AKI. Data regarding renal hypoxia in the clinical situation of AKI are lacking and our current concepts regarding renal oxygenation during acute renal failure are presumptive and largely derived from experimental studies. There is robust experimental evidence that AKI is often associated with altered intrarenal microcirculation and oxygenation. Furthermore, renal parenchymal oxygen deprivation seems to participate in the pathogenesis of experimental AKI, induced by exogenous nephrotoxins (such as contrast media, non-steroidal anti-inflammatory drugs or amphotericin), sepsis, pigment and obstructive nephropathies. Sub-lethal cellular hypoxia engenders adaptational responses through hypoxia-inducible factors (HIF). Forthcoming technologies to modulate the HIF system form a novel potential therapeutic approach for AKI.

Cyclosporine A inhibits the adaptive responses to hypertonicity: a potential mechanism of nephrotoxicity

Cell survival in the hypertonic environment of the renal medulla is dependent on the intracellular accumulation of protective organic solutes through the induction of genes whose transcriptional regulation is mediated in part by interaction between osmotic response elements and the transcription nuclear factor of activated T lymphocyte 5. It is shown that cyclosporine A (CsA) prevents the nuclear translocation of the transcription nuclear factor of activated T lymphocyte 5 and inhibits osmotic response element-mediated reporter gene expression. The expression of mRNA for hypertonicity-induced genes (aldose reductase, betaine/gamma-amino-n-butyric acid transporter 1, and heat shock protein 70) is also decreased in the medulla of CsA-treated rats. CsA inhibits the increase of betaine/gamma-amino-n-butyric acid transporter 1 and heat shock protein 70 mRNA in osmotically stressed MDCK cells, blocks cell proliferation under isotonic conditions, and augments hypertonicity-induced apoptosis. Histologic examination of the kidneys of CsA-treated rats shows a marked increase in apoptosis in the renal medulla where hypertonicity normally prevails. The data are consistent with calcineurin-mediated induction of hypertonic stress-response genes, and they suggest that CsA nephrotoxicity may in part result from inhibition of the adaptive responses to hypertonicity occurring during the urinary concentrating mechanism.

Compensated heart failure predisposes to outer medullary tubular injury: studies in rats

BACKGROUND

Heart failure (HF) is considered a putative factor predisposing to acute renal failure (ARF). Since outer medullary hypoxic injury may play an important role in the pathogenesis of acute tubular necrosis, we explored the impact of experimental HF on the propensity to develop ARF with hypoxic medullary injury following the inhibition of prostaglandin and nitric oxide synthesis.

METHODS

Compensated, high-output HF was induced in Sprague-Dawley rats by aorto-caval fistula. At the eighth to ninth postoperative day, the rats were injected with indomethacin and N(omega) nitro-L-arginine methyl ester (L-NAME; ARF protocol) and were sacrificed 24 hours later for morphologic evaluation.

RESULTS

Kidney function comparably declined in HF-ARF rats and in control sham operated animals (CTR-ARF). Nevertheless, outer medullary hypoxic damage with medullary thick ascending limb (mTAL) necrosis occurred almost exclusively in the HF-ARF group (11 +/- 4% vs. 0.2 +/- 0.2% of tubules in CTR-ARF, P < 0.03). In a third group of HF animals subjected to vehicles only (HF-Nil), kidney function was preserved and renal morphology remained intact. Papillary-tip necrosis was consistently found in all animals subjected to indomethacin and L-NAME, irrespective of preconditioning. Morphometric evaluation disclosed that HF was not associated with mTAL hypertrophy.

CONCLUSIONS

Incipient HF predisposes to hypoxic outer medullary injury, probably reflecting the impact of regional vasoconstrictive stimuli rather than tubular hypertrophy when protective local vasodilating mechanisms are hampered. The presence and extent of outer medullary hypoxic damage cannot be predicted from the functional derangement, which in the experimental settings may also represent prerenal azotemia or papillary damage.

Smooth muscle-specific actin levels in the urine of renal transplant recipients: correlation with cyclosporine or tacrolimus nephrotoxicity

Cyclosporine A (CSA) and tacrolimus (FK506) are powerful immunosuppressive agents that have proven useful for antirejection therapy in patients with solid organ transplants, including kidney. However, both drugs are nephrotoxic, each producing similar histological patterns of injury to renal tubules and preglomerular arterioles, and this toxicity is a major cause of renal allograft dysfunction. A renal transplant biopsy presently represents the most reliable means of diagnosing nephrotoxicity caused by CSA or tacrolimus and distinguishing it from acute rejection. Because CSA and tacrolimus nephrotoxicity often involve arteriolar smooth muscle, whereas vascular smooth muscle is rarely involved in acute rejection, we investigated if the appearance of a smooth muscle-specific isoform of alpha-actin (SMA) in the urine of renal transplant recipients about to undergo a biopsy for graft dysfunction correlated with biopsy evidence of CSA or tacrolimus toxicity. Eighty-nine urine samples from 61 patients, plus 6 samples from healthy control subjects, were analyzed in a blinded manner by enzyme-linked immunosorbent assay using a specific anti-SMA monoclonal antibody. For the patient samples, the results of these assays were then correlated with the biopsy findings. Those 40 cases in which the biopsy showed evidence of CSA or tacrolimus nephrotoxicity had a significantly (P < 0.01) greater SMA level in the corresponding urine samples (0.089 +/- 0.126 microgram/mL; mean +/- SD) than the 49 cases without toxicity (0.018 +/- 0.027 microgram/mL) or 6 control subjects (0.003 +/- 0.007 microgram/mL), although there was considerable overlap of SMA values among these groups. The greatest SMA levels were seen in patients with CSA or tacrolimus nephrotoxicity that was likely to be relatively acute, namely those with thrombotic microangiopathy and those without previous biopsy evidence of toxicity. SMA levels correlated significantly with the estimated severity of arteriolopathy on biopsy. In patients with tubular but not arteriolar lesions of CSA or tacrolimus toxicity, the mean SMA level was not significantly greater than that in patients without toxicity. Urine SMA levels in patients with a biopsy specimen showing acute rejection were not significantly different from those in patients without rejection, and there was no correlation between urine SMA level and severity of rejection. Whereas the degree of overlap of SMA levels in patients with and without nephrotoxicity was far too great to consider this assay as a potential alternative to renal transplant biopsy for the diagnosis of nephrotoxicity, the assay may have potential as a marker for active arteriolar injury in renal transplant recipients and other patients receiving CSA or tacrolimus therapy.

Localization of cyclophilin A and cyclophilin C mRNA in murine kidney using RT-PCR

Cyclosporin A (CsA), which is widely used as an immunosuppressant, has a nephrotoxic side effect. The mechanism of this nephrotoxicity is not well understood; however, recent studies suggest that cyclophilin (cyp) is responsible for mediating the immunosuppressive action of CsA through the interaction with the Ca(2+)- and calmodulin-dependent phosphatase, calcineurin. While cyp A mRNA is expressed ubiquitously, cyp C mRNA has been shown to be topically expressed, including in the kidney. We examined: (1) distribution of cyp A and cyp C mRNA in microdissected murine nephron segments, using a combination of reverse transcription and polymerase chain reaction (RT-PCR) techniques, and (2) the effect of CsA administration on cyp C mRNA expression in proximal convoluted tubule. Among the nephron segments examined, large signals for cyp C PCR product were detected in proximal convoluted tubule and proximal straight tubule. Our data showed that the distribution of cyp C mRNA was uneven, and it mainly existed in segments that are relatively sensitive to CsA toxicity. In contrast, cyp A mRNA was found to be distributed almost equally along the nephron segments examined. By CsA administration, the signal for cyp C mRNA PCR product was increased. These results suggest that cyp C may play some role in the renal tubular disorder observed in CsA nephrotoxicity.

Urinary enzymes as biomarkers of renal injury in experimental nephrotoxicity of immunosuppressive drugs

Urinary excretion of N-acetyl-beta-D-glucosaminidase (NAG) and of alanine-aminopeptidase (AAP) was studied after administration of cyclosporine A (CSA A), FK 506, or the corresponding vehicles to salt-depleted rats. On days 7, 14, and 28 after treatment for CSA and day 14 after treatment for FK 506, measurements of the urinary enzymes, serum creatinine (SCr), creatinine clearance (ClCr), and blinded renal histology were done. After 1 week on CSA there was a dramatic increase of 489% in the urinary excretion of AAP (162.6 IU/g Cr, CSA vs. 27.6 IU/g Cr control, p < .03), a significant decrease of 32% in ClCr, a significant increase of 41% in SCr, and mild proximal tubular atrophy and vacuolization. After 2 or 4 weeks of CSA treatment there were no more differences in the urinary AAP between CSA and control rats, but the urinary excretion of NAG was increased: 29.6 IU/g Cr, CSA vs. 20.9 IU/g Cr, control, p < .03 on day 14 and 26.9 IU/g Cr, CSA vs. 21.5 IU/g Cr, control, p < .008 on day 28. At the same time there was a progressive decline of the ClCr, a progressive increase in the SCr, and an increase in the severity of the histological lesion. After 14 days of treatment with FK 506 we observed a striking elevation in urinary AAP (62.6 IU/g Cr, FK 506 vs. 36.0 IU/g Cr, control, p < .01) consistent with a significant decrease in ClCr, a significant increase in SCr, and a moderate proximal tubular vacuolization and atrophy.(ABSTRACT TRUNCATED AT 250 WORDS)