Tubulointerstitial disease mediators of injury: the role of endothelin

Targeting the Endothelin A Receptor in IgA Nephropathy

Immunoglobulin A nephropathy (IgAN) is the most common primary glomerulonephritis worldwide and carries a substantial risk of kidney failure. New agency-approved therapies, either specifically for IgAN or for chronic kidney disease (CKD) in general, hold out hope for mitigating renal deterioration in patients with IgAN. The latest addition to this therapeutic armamentarium targets the endothelin-A receptor (ETAR). Activation of ETAR on multiple renal cell types elicits a host of pathophysiological effects, including vasoconstriction, cell proliferation, inflammation, apoptosis, and fibrosis. Blockade of ETAR is renoprotective in experimental models of IgAN and reduces proteinuria in patients with IgAN. This review discusses the evidence supporting the use of ETAR blockade in IgAN as well as addressing the potential role for this class of agents among the current and emerging therapies for treating this disorder.

Oxidized casein impairs antioxidant defense system and induces hepatic and renal injury in mice

SCOPE

Oxidized protein products (OPPs) can be easily found in meat and milk during processing and storage. Evidence supports that accumulation of endogenous OPPs plays a negative role in physiological metabolism. However, the impacts of dietary OPPs and the mechanisms have not been elucidated yet. The present study evaluated whether oral oxidized casein would destruct the antioxidant defense system and cause potential oxidized injury in mice liver and kidney.

METHODS AND RESULTS

We performed oxidized casein (modified respectively by H2O2-Cu and HClO) feeding experiments using KM mice (20-22 g). A 10-weeks feeding of oxidized casein as basal protein caused oxidative stress by increasing protein carbonylation (PC), advanced oxidation protein products (AOPPs), dityrosine (Dityr), lipid peroxidation and ROS levels in mice liver, kidney and blood (P<0.05). In mice liver and kidney, the mRNA expression of Nrf2, γ-GCS, HO-1, GPX-3, and GPX-4 up-regulated, the protein level of Nrf2 in nucleus increased. However, activities of anti-oxidant enzymes (CAT, SOD, and GPX) decreased (P<0.05). Moreover, histopathological examination displayed the formation of fibrous septa in mice liver and kidney after oxidized casein feeding.

CONCLUSION

Oxidized casein impairs antioxidant defense system and induces hepatic and renal fibrosis.

Ischemic injury underlies the pathogenesis of aristolochic acid-induced acute kidney injury

Aristolochic acid nephropathy (AAN) is a progressive tubulointerstitial renal disease caused by aristolochic acid intake. To determine the contribution of renal ischemia to the pathogenesis of AAN, we characterized changes in the expression of angiogenic factors and vasoactive substances, and then we evaluated the expression of a marker of hypoxia in an acute AAN rat model. Rats were orally administrated either a decoction of Aristolochiae manshuriensis that contained 20 mg/kg of aristolochic acid-I or an equal volume of distilled water (control group) once daily for 4 days or 7 days. Renal histology and serum creatinine were assessed. Expression of endothelin-1 (ET-1) and hypoxia inducible factor-1 alpha (HIF-1alpha) mRNA within renal cortex were determined by semiquantitative reverse-transcription polymerase chain reaction. Levels of ET-1, nitric oxide (NO), vascular endothelial growth factor (VEGF), and HIF-1alpha in kidneys were determined by radioimmunoassay, Griess method, Western blot, and immunohistochemistry, respectively. Tubular injury scores and ET-1 mRNA expression were increased in the AA-treated rats at both days 4 and 8, whereas serum creatinine level and ET-1 protein expression was increased only at day 4. In contrast, NO production in AA-treated rats was decreased at day 8 compared with the control group. Similarly, VEGF protein expression was reduced in the AA-treated rats at both days 4 and 8. A dramatic increase in nuclear staining for HIF-1alpha was observed mainly in the tubular cells of tubulointerstitial damage area in the AA-treated rats at day 8. The observed increase in HIF-1alpha protein expression, decrease in VEGF protein expression, and imbalance of vasoactive substances after induction of acute kidney injury by AA suggests that ischemic injury contributes to the pathogenesis of AAN.

Role of endothelin and endothelin receptor antagonists in renal disease

Endothelin (ET)-1 is a potent vasoconstrictor peptide with pro-inflammatory, mitogenic, and pro-fibrotic properties that is closely involved in both normal renal physiology and pathology. ET-1 exerts a wide variety of biological effects, including constriction of cortical and medullary vessels, mesangial cell contraction, stimulation of extracellular matrix production, and inhibition of sodium and water reabsorption along the collecting duct, effects that are primarily mediated in an autocrine/paracrine manner. Increasing evidence indicates that the ET system is involved in an array of renal disorders. These comprise chronic proteinuric states associated with progressive glomerular and tubulointerstitial fibrosis, including diabetic and hypertensive nephropathy, glomerulonephritis and others. In addition, ET-1 is causally linked to renal disorders characterized by increased renal vascular resistance, including acute ischaemic renal failure, calcineurin inhibitor toxicity, endotoxaemia, hepatorenal syndrome and others. Furthermore, derangement of the ET system may be involved in conditions associated with inappropriate sodium and water retention; for example, in congestive heart failure and hepatic cirrhosis. Both selective and non-selective ET receptor antagonist have been developed and tested in animal models with promising results. As key events in progressive renal injury like inflammation and fibrosis are mediated via both ET(A) and ET(B) receptors, while constrictor effects are primarily transduced by ET(A) receptors, dual ET receptor blockade may be superior over selective ET(A) antagonism. Several compounds have been developed with remarkable effects in several models of acute and progressive renal injury. Thus, clinical studies are required to assess whether these results can be confirmed in humans, hopefully leading to novel and effective therapeutic options with few side effects.

Why is proteinuria an ominous biomarker of progressive kidney disease?

Progressive tubule injury and interstitial fibrosis frequently accompany glomerulopathies associated with proteinuria. Clinical experience indicates that higher levels of proteinuria prior to, as well as after initiation of treatment predict more rapid decline in renal function and more pronounced tubulointerstitial injury. It has been proposed that filtration of potentially tubulotoxic plasma proteins is responsible for the observed correlations between proteinuria and progression (i.e., proteinuria is a cause and not only a consequence of progressive renal injury). Numerous attempts have been made to identify the species of putative tubulotoxic proteins in this progressive injury process, but much uncertainty persists. These uncertainties stem from nonphysiologic exposure of apical cell surfaces to proteins in vitro, the extremely high concentrations of various proteins tested in vitro, and the nonuniformity of end points measured. Furthermore, there is often a lack of correlation between in vitro and in vivo findings, and a lack of uniformity of results even for seemingly similar in vitro experiments. Less controversy is evident in the potential pathways whereby injured tubules evoke a tubulointerstitial inflammatory and fibrotic response, with many in vivo models serving to incriminate excessive cytokine and chemokine production, infiltration of various inflammatory cells, and the balance between apoptosis and cell proliferation. Despite many years of concerted efforts, we believe it is still unclear whether proteinuria is a cause (and if so, which species of protein), or only a consequence of progressive renal injury. Nevertheless, pending the resolution of these uncertainties by more decisive and unambiguous experimentation, the strongly predictive inverse relationship between level of proteinuria and long-term renal survival currently justifies aggressive antiproteinuric treatment strategies, with a goal of reducing protein excretion rate to the lowest level possible without the induction of symptoms or undue risk.