ET(B) receptor protects the tubulointerstitium in experimental thrombotic microangiopathy

Effects of high- and low-dose aspirin on adaptive immunity and hypertension in the stroke-prone spontaneously hypertensive rat

Despite its well-known antithrombotic properties, the effect of aspirin on blood pressure (BP) and hypertension pathology is unclear. The hugely varying doses used clinically have contributed to this confusion, with high-dose aspirin still commonly used due to concerns about the efficacy of low-dose aspirin. Because prostaglandins have been shown to both promote and inhibit T-cell activation, we also explored the immunomodulatory properties of aspirin in hypertension. Although the common preclinical high dose of 100 mg/kg/d improved vascular dysfunction and cardiac hypertrophy, this effect was accompanied by indices of elevated adaptive immunity, renal T-cell infiltration, renal fibrosis, and BP elevation in stroke-prone spontaneously hypertensive rats and in angiotensin II-induced hypertensive mice. The cardioprotective effects of aspirin were conserved with a lower dose (10 mg/kg/d) while circumventing heightened adaptive immunity and elevated BP. We also show that low-dose aspirin improves renal fibrosis. Differential inhibition of the COX-2 isoform may underlie the disparate effects of the 2 doses. Our results demonstrate the efficacy of low-dose aspirin in treating a vast array of cardiovascular parameters and suggest modulation of adaptive immunity as a novel mechanism underlying adverse cardiovascular profiles associated with COX-2 inhibitors. Clinical studies should identify the dose of aspirin that achieves maximal cardioprotection with a new awareness that higher doses of aspirin could trigger undesired autoimmunity in hypertensive individuals. This work also warrants an evaluation of high-dose aspirin and COX-2 inhibitor therapy in sufferers of inflammatory conditions who are already at increased risk for cardiovascular disease.-Khan, S. I., Shihata, W. A., Andrews, K. L., Lee, M. K. S., Moore, X.-L., Jefferis, A.-M., Vinh, A., Gaspari, T., Dragoljevic, D., Jennings, G. L., Murphy, A. J., Chin-Dusting, J. P. F. Effects of high- and low-dose aspirin on adaptive immunity and hypertension in the stroke-prone spontaneously hypertensive rat.

Sodium butyrate suppresses angiotensin II-induced hypertension by inhibition of renal (pro)renin receptor and intrarenal renin-angiotensin system

OBJECTIVES

Butyrate, a short-chain fatty acid, is the end product of the fermentation of complex carbohydrates by the gut microbiota. Recently, sodium butyrate (NaBu) has been found to play a protective role in a number of chronic diseases. However, it is still unclear whether NaBu has a therapeutic potential in hypertension. The present study was aimed to investigate the role of NaBu in angiotensin II (Ang II)-induced hypertension and to further explore the underlying mechanism.

METHODS

Ang II was infused into uninephrectomized Sprague-Dawley rats with or without intramedullary infusion of NaBu for 14 days. Mean arterial blood pressure was recorded by the telemetry system. Renal tissues, serum samples, and 24-h urine samples were collected to examine renal injury and the regulation of the (pro)renin receptor (PRR) and renin.

RESULTS

Intramedullary infusion of NaBu in Sprague-Dawley rats lowered the Ang II-induced mean arterial pressure from 129 ± 6 mmHg to 108 ± 4 mmHg (P < 0.01). This corresponded with an improvement in Ang II-induced renal injury, including urinary albumin, glomerulosclerosis, and renal fibrosis, as well as the expression of inflammatory mediators tumor necrosis factor α, interleukin 6. The renal expression of PRR, angiotensinogen, angiotensin I-converting enzyme and the urinary excretion of soluble PRR, renin, and angiotensinogen were all increased by Ang II infusion but decreased by NaBu treatment. In cultured innermedullary collecting duct cells, NaBu treatment attenuated Ang II-induced expression of PRR and renin.

CONCLUSION

These results demonstrate that NaBu exerts an antihypertensive action, likely by suppressing the PRR-mediated intrarenal renin-angiotensin system.

Silencing of hypoxia-inducible factor-1α gene attenuates chronic ischemic renal injury in two-kidney, one-clip rats

Overactivation of hypoxia-inducible factor (HIF)-1α is implicated as a pathogenic factor in chronic kidney diseases (CKD). However, controversy exists regarding the roles of HIF-1α in CKD. Additionally, although hypoxia and HIF-1α activation are observed in various CKD and HIF-1α has been shown to stimulate fibrogenic factors, there is no direct evidence whether HIF-1α is an injurious or protective factor in chronic renal hypoxic injury. The present study determined whether knocking down the HIF-1α gene can attenuate or exaggerate kidney damage using a chronic renal ischemic model. Chronic renal ischemia was induced by unilaterally clamping the left renal artery for 3 wk in Sprague-Dawley rats. HIF-1α short hairpin (sh) RNA or control vectors were transfected into the left kidneys. Experimental groups were sham+control vector, clip+control vector, and clip+HIF-1α shRNA. Enalapril was used to normalize blood pressure 1 wk after clamping the renal artery. HIF-1α protein levels were remarkably increased in clipped kidneys, and this increase was blocked by shRNA. Morphological examination showed that HIF-1α shRNA significantly attenuated injury in clipped kidneys: glomerular injury indices were 0.71 ± 0.04, 2.50 ± 0.12, and 1.34 ± 0.11, and the percentage of globally damaged glomeruli was 0.02, 34.3 ± 5.0, and 6.3 ± 1.6 in sham, clip, and clip+shRNA groups, respectively. The protein levels of collagen and α-smooth muscle actin also dramatically increased in clipped kidneys, but this effect was blocked by HIF-1α shRNA. In conclusion, long-term overactivation of HIF-1α is a pathogenic factor in chronic renal injury associated with ischemia/hypoxia.

Silencing of hypoxia-inducible factor-1α gene attenuated angiotensin II-induced renal injury in Sprague-Dawley rats

Although it has been shown that upregulation of hypoxia-inducible factor (HIF)-1α is protective in acute ischemic renal injury, long-term overactivation of HIF-1α is implicated to be injurious in chronic kidney diseases. Angiotensin II (Ang II) is a well-known pathogenic factor producing chronic renal injury and has also been shown to increase HIF-1α. However, the contribution of HIF-1α to Ang II-induced renal injury has not been evidenced. The present study tested the hypothesis that HIF-1α mediates Ang II-induced renal injury in Sprague-Dawley rats. Chronic renal injury was induced by Ang II infusion (200 ng/kg per minute) for 2 weeks in uninephrectomized rats. Transfection of vectors expressing HIF-1α small hairpin RNA into the kidneys knocked down HIF-1α gene expression by 70%, blocked Ang II-induced HIF-1α activation, and significantly attenuated Ang II-induced albuminuria, which was accompanied by inhibition of Ang II-induced vascular endothelial growth factor, a known glomerular permeability factor, in glomeruli. HIF-1α small hairpin RNA also significantly improved the glomerular morphological damage induced by Ang II. Furthermore, HIF-1α small hairpin RNA blocked Ang II-induced upregulation of collagen and α-smooth muscle actin in tubulointerstitial region. There was no difference in creatinine clearance and Ang II-induced increase in blood pressure. HIF-1α small hairpin RNA had no effect on Ang II-induced reduction in renal blood flow and hypoxia in the kidneys. These data suggested that overactivation of HIF-1α-mediated gene regulation in the kidney is a pathogenic pathway mediating Ang II-induced chronic renal injuries, and normalization of overactivated HIF-1α may be used as a treatment strategy for chronic kidney damages associated with excessive Ang II.

Identification and developmental analysis of endothelin receptor type-A expressing cells in the mouse kidney

The endothelin (Edn) system plays pleiotropic roles in renal function and various disease processes through two distinct G protein-coupled receptors, Edn receptors type-A (Ednra) and type-B (Ednrb). However, difficulties in the accurate identification of receptor-expressing cells in situ have made it difficult to dissect their diverse action in renal (patho)physiology. We have recently established mouse lines in which lacZ and EGFP are 'knocked-in' to the Ednra locus to faithfully mark Ednra-expressing cells. Here we analyzed these mice for their expression in the kidney to characterize Ednra-expressing cells. Ednra expression was first observed in undifferentiated mesenchymal cells around the ureteric bud at E12.5. Thereafter, Ednra expression was widely observed in vascular smooth muscle cells, JG cells and mesenchymal cells in the interstitium. After growth, the expression became confined to vascular smooth muscle cells, pericytes and renin-producing JG cells. By contrast, most cells in the nephron and vascular endothelial cells did not express Ednra. These results indicate that Ednra expression may be linked with non-epithelial fate determination and differentiation of metanephric mesenchyme. Ednra-lacZ/EGFP knock-in mice may serve as a useful tool in studies on renal function and pathophysiology of various renal diseases.

The effect of tezosentan after cold ischemia and renal artery clamping as a model of reperfusion injury in newborn piglets

BACKGROUND

Cold ischemia and clamping of the renal artery contribute to acute tubular necrosis and renal dysfunction of transplant grafts. The mechanism of ischemic injury is not fully understood, but endothelin (ET)-1 and -2 have been found to participate in reperfusion injury. ET receptor blockade has been shown to have renoprotective effects in both warm and cold reperfusion injury.

OBJECTIVE

We sought to assess the effect of tezosentan, a competitive ET antagonist, on piglet renal function during cold ischemia and renal artery clamping.

DESIGN/METHODS

Sixteen piglets (7 to 10 days old) were prepped and assigned to three experimental groups: piglets with kidneys clamped (KCLAMP), with kidneys wrapped in ice (KICE), and piglets treated with tezosentan injected after 45 minutes of clamping and ice (KTEZO). Preexperiment parameters including vital signs, urine volume, glomerular filtration rate (GFR), paraaminohippuric acid clearance (CPAH), fractional excretion of sodium and potassium (FeNa, FeK), and renal blood flow (RBF) were measured at baseline, then at 1- and 2-hour intervals.

RESULTS

The decrease in urine volume was comparable in both KCLAMP and KICE groups, but no UV decrease was observed in KTEZO group. RBF and GFR were similar (26% to 52% decrease) in all three groups. FeNa decreased by >50% in KICE, whereas it increased by 60% in KTEZO when compared with baseline. A similar increase in FeK was observed in all three groups.

CONCLUSIONS

Cold ischemia and clamping have deleterious effects on RBF, GFR, and FeNa. ET blockade did not have a renoprotective effect except on urine volume when given soon after the injury.

Pathophysiological Mechanisms of Salt-Dependent Hypertension

Changes in salt intake are associated in general with corresponding changes in arterial blood pressure. An exaggerated increment in blood pressure driven by a salt load is characteristic of salt-sensitive hypertension, a condition affecting more than two thirds of individuals with essential hypertension who are older than 60 years. In the last decade, significant insight was gained about the role of the kidney in the increment in blood pressure induced by sodium retention. The present review focuses on the pathophysiological characteristics of the blood pressure increase driven by expansion of extracellular fluid and the increment in plasma sodium concentration. In addition, we discuss systemic and renal conditions that result in decreased urinary sodium excretion and were implicated in the development of salt-sensitive hypertension.

Pathogenesis and prognosis of thrombotic microangiopathy

Thrombotic microangiopathy (TMA) is a clinicopathological syndrome characterized by thrombosis formation in the microvasculature of various organs. Included in the broad category of TMA are the hemolytic uremic syndrome (HUS) and thrombotic thrombocytopenic purpura (TTP). Typical HUS is caused by Escherichia coli O157:H7, which produces the Shiga-like toxins; Stx-1 and Stx-2. In addition to damaging endothelial cells via the inhibition of protein synthesis, Shiga-like toxins also activate endothelial cells to produce inflammatory mediators, amplifying the prothrombogenic state. Although most patients with typical HUS recover renal functions, recent analysis has shown that typical HUS is not a benign disease in the long term. Genetic abnormalities of complement regulatory proteins predispose patients to atypical HUS. Mutations in factor H, membrane cofactor protein, and factor I are known to be associated with atypical HUS. Atypical HUS forms have a poor outcome and show recurrent and progressive courses. Autoimmune IgG inhibitors of a disintegrin and metalloprotease, with thrombospodin-1-like domains (ADAMTS) 13 and mutations of the ADAMTS13 gene lead to the development of TTP. Without treatment, TTP is associated with a very high mortality rate. As it is for atypical HUS, plasma exchange is currently the most feasible treatment for TTP. Etiological diagnosis at the bedside and the development of disease-specific therapeutic modalities will enable us to optimize the management of patients with TMA and improve their prognosis in the future.

Role of impaired peritubular capillary and hypoxia in progressive interstitial fibrosis after 56 subtotal nephrectomy of rats

AIM

To investigate the potential role of peritubular capillary (PTC) loss and subsequent hypoxia as a pathogenic factor in interstitial fibrosis after renal ablation in rats.

METHODS

PTC loss and tubulointerstitial hypoxia in remnant kidney rats (SNTx group), sham-operated rats (sham group) and normal animals (normal group) were assessed by peritubular CD141-positive staining lumina and tubulointerstitial hypoxia-inducible factor alpha subunit 1 (HIF-1alpha) expression, respectively, at the time points of week 3, week 6 and week 12. The related cardinal factors contributing to interstitial fibrosis, such as transforming growth factor-beta1 (TGF-beta1), alpha-smooth muscle actin (alpha-SMA) were also evaluated and analysed in the context of progressive PTC loss. Expression of TGF-beta1 mRNA in cultured renal tubular epithelial cells (MDCK cells) exposed to hypoxia was also investigated.

RESULTS

PTC loss and tubulointerstitial hypoxia were noted even in the early stage (week 3) when the interstitial fibrosis was mild, and were persistent in the process of developing interstitial fibrosis. An in vitro study showed that hypoxia enhanced TGF-beta1 mRNA expression in the MDCK cells.

CONCLUSION

PTC loss or hypoxic milieu in the tubulointerstitium is a pathological event, which may contribute to the developing interstitial fibrosis mediated by direct hypoxic effects and via hypoxia-induced TGF-beta1 expression.

Transdifferentiation of cultured tubular cells induced by hypoxia

BACKGROUND

Tubulointerstitial fibrosis leads to progressive kidney disease and, ultimately, may result in end-stage renal disease (ESRD). Myofibroblasts, which express alpha-smooth muscle actin (alpha-SMA) in their cytoplasm, regulate renal fibrogenesis. Recent studies suggest that certain interstitial myofibroblasts derive from renal tubular cells that have undergone epithelial-mesenchymal transformation (EMT) (transdifferentiation). However, the role(s) of hypoxia, which is involved in progressive kidney disease, on tubular EMT remains unclear.

METHODS

Immortalized rat proximal tubular cells (IRPTC) were cultured in normobaric hypoxia (1% O2) for 3, 6, or 15 days, with match control in normoxic conditions. alpha-SMA, vimentin, and desmin chosen as markers of EMT were measured by immunocytochemistry and immunoblots collagen I production and cell motility were chosen as functional assays. Various concentrations of cobaltous chloride (CoCl2) were used as hypoxic mimickers. In vivo studies were carried out in a chronic ischemic kidney model.

RESULTS

Immunohistochemical studies revealed increased expression of alpha-SMA. Striking morphologic changes were detected after 6 days of hypoxia for alpha-SMA-positive fibroblast-like cells (SMA + fib) and after 15 days for alpha-SMA-positive myofibroblast-like cells (SMA + myo). Immunoblots confirmed these findings. Collagen I production increased in a time-dependent manner parallel to alpha-SMA expression. Cell motility assays demonstrated that transformed cells had higher migratory capacity than normal tubular cells. Cobaltous salt also induced alpha-SMA and collagen I synthesis. Chronic ischemic kidney revealed in vivo tubular EMT at day 7.

CONCLUSION

Hypoxia can induce tubular EMT. This process may play an important role in progression of kidney disease.

Mechanisms of tubulointerstitial injury in the kidney: final common pathways to end-stage renal failure

There are many different glomerular disorders, including glomerulonephritis, diabetic nephropathy, and hypertensive nephrosclerosis. However, once glomerular damage reaches a certain threshold, the progression of renal disease is consistent and irreversible. Recent studies emphasized the crucial role of tubulointerstitial injury as a mediator of progression of kidney disease. One common mechanism that leads to renal failure via tubulointerstitial injury is massive proteinuria. Accumulating evidence suggests critical effects of filtered macromolecules on tubular cells, including lysosomal rupture, energy depletion, and tubular injury directly induced by specific components such as complement components. Another common mechanism is chronic hypoxia in the tubulointerstitium. Tubulointerstitial damage results in the loss of peritubular capillaries, impairing blood flow delivery. Interstitial fibrosis also impairs oxygen diffusion and supply to tubular cells. This induces chronic hypoxia in this compartment, rendering a vicious cycle. Development of novel therapeutic approaches against these final common pathways will enable us to target any types of renal disease.

Tubulointerstitial disease: role of ischemia and microvascular disease

PURPOSE OF REVIEW

Tubulointerstitial injury is characteristic of aging-associated renal injury and progressive renal disease. Salt-sensitive hypertension is also associated with tubulointerstitial inflammation, especially when accompanied by microvascular disease. Here we summarize recent studies on the pathogenesis and consequences of tubulointerstitial disease, emphasizing the role of ischemia and the microvasculature.

RECENT FINDINGS

Tubulointerstitial injury occurs via several mechanisms of which one of the most important is chronic ischemia. Recent studies suggest that chronic vasoconstriction may contribute to the renal injury associated with angiotensin II, catecholamines, nitric oxide inhibition, hypokalemia, hyperuricemia, and cyclosporine nephropathy. Salt-sensitivity may result as a consequence of the tubulointerstitial inflammatory response to these conditions, and this appears to be perpetuated by the development of preglomerular vascular disease. With progression of tubulointerstitial disease there is also a loss of peritubular capillaries, and stimulating microvascular growth with angiogenic factors can stabilize renal function in these models.

SUMMARY

Ischemia secondary to vasoconstriction or to structural changes of the renal vasculature may have important consequences both in terms of mediating salt-sensitive hypertension and renal progression. Angiogenic factors may have potential benefit in preventing or treating these conditions.

Endothelin a receptor blockade and endothelin B receptor blockade improve hypokalemic nephropathy by different mechanisms

Hypokalemia causes renal tubulointerstitial injury with an elevation in renal endothelin-1 (ET-1). It was hypothesized that hypokalemic tubulointerstitial injury is ameliorated by the blockade of ET-A receptors (ETA), whereas ET-B receptor (ETB) antagonism may exacerbate the injury, because ETB is thought to mediate vasodilation. Rats were fed a K(+)-deficient diet alone (LC) or with an ETA-selective antagonist ABT-627 (LA) or an ETB-selective antagonist A-192621 (LB) for 8 wk. Control rats were on a normal K(+) diet alone or with the ETA-selective or ETB-selective antagonists. The severity of hypokalemia was not significantly different among LA, LB, and LC. LC developed tubulointerstitial injury with an elevation of renal preproET-1 mRNA level. There was an increase in tubular osteopontin expression, macrophage infiltration, collagen accumulation, and tubular cell hyperplasia. ETA blockade significantly ameliorated all parameters for renal injury in the cortex without suppressing local ET-1 and ETA expression. By contrast, ETB blockade significantly reduced local ET-1 and ETA expression and improved the injury to a similar extent in the cortex. In the medulla, ETA or ETB blockade only partially blocked renal injury. ETA blockade did not affect BP in normokalemic or hypokalemic rats. ETB blockade induced a BP elevation with a decrease in urinary Na(+) excretion in normokalemic but not in hypokalemic rats. These results indicate that ET-1 can mediate hypokalemic renal injury in two different ways: by directly stimulating ETA and by locally promoting endogenous ET-1 production via ETB. Thus, ETA as well as ETB blockade may be renoprotective in hypokalemic nephropathy.

New therapeutics that antagonize endothelin: promises and frustrations

The discovery of endothelin--a highly potent endogenous vasoconstrictor - in 1988 has led to considerable efforts to develop antagonists of endothelin receptors that could have therapeutic potential in disorders including hypertension, heart failure and renal diseases. However, in general, the results of trials in humans have not mirrored the highly promising effects in animal disease models. Here, we discuss preclinical and clinical results with endothelin antagonists, and consider possible approaches to fully realizing the potential of endothelin antagonism.