Endothelin-a receptor blockade improves renal microvascular architecture and function in experimental hypercholesterolemia

Vascular regression in the kidney: changes in 3D vessel structure with time post-irradiation

Though angiogenesis has been investigated in depth, vascular regression and rarefaction remain poorly understood. Regression of renal vasculature accompanies many pathological states such as diabetes, hypertension, atherosclerosis, and radiotherapy. Radiation decreases microvessel density in multiple organs, though the mechanism is not known. By using a whole animal (rat) model with a single dose of partial body irradiation to the kidney, changes in the volume of renal vasculature were recorded at two time points, 60 and 90 days after exposure. Next, a novel vascular and metabolic imaging (VMI) technique was used to computationally assess 3D vessel diameter, volume, branch depth, and density over multiple levels of branching down to 70 µm. Four groups of rats were studied, of which two groups received a single dose of 12.5 Gy X-rays. The kidneys were harvested after 60 or 90 days from one irradiated and one non-irradiated group at each time point. Measurements of the 3D vasculature showed that by day-90 post-radiation, when renal function is known to deteriorate, total vessel volume, vessel density, maximum branch depth, and the number of terminal points in the kidneys decreased by 55%, 57%, 28%, and 53%, respectively. Decreases in the same parameters were not statistically significant at 60 days post-irradiation. Smaller vessels with internal diameters of 70-450 µm as well as large vessels of diameter 451-850 µm, both decreased by 90 days post-radiation. Vascular regression in the lungs of the same strain of irradiated rats has been reported to occur before 60 days supporting the hypothesis that this process is regulated in an organ-specific manner and occurs by a concurrent decrease in luminal diameters of small as well as large blood vessels.

With a Little Help From My Friends: the Role of the Renal Collateral Circulation in Atherosclerotic Renovascular Disease

The collateral circulation can adapt to bypass major arteries with limited flow and serves a crucial protective role in coronary, cerebral, and peripheral arterial disease. Emerging evidence indicates that the renal collateral circulation can similarly adapt and thereby limit kidney ischemia in atherosclerotic renovascular disease. These adaptations predominantly include recruitment of preexisting microvessels for arteriogenesis, with de novo vessel formation playing a limited role. Yet, adaptations of the renal collateral circulation in renovascular disease are often insufficient to fully compensate for the limited flow within an obstructed renal artery and may be hampered by the severity of obstruction or patient comorbidities. Experimental strategies have attempted to circumvent limitations of collateral formation and improve the prognosis of patients with various ischemic vascular territories. These have included pharmacological approaches such as endothelial growth factors, renin-angiotensin-aldosterone system blockade, and If-channel-blockers, as well as interventions like preconditioning, exercise, enhanced external counter-pulsation, and low-energy shock-wave therapy. However, few of these strategies have been implemented in atherosclerotic renovascular disease. This review summarizes current understanding regarding the development of renal collateral circulation in atherosclerotic renovascular disease. Studies are needed to apply lessons learned in other vascular beds in the setting of atherosclerotic renovascular disease to develop new treatment regimens for this patient group.

Systemic biopolymer-delivered vascular endothelial growth factor promotes therapeutic angiogenesis in experimental renovascular disease

We recently developed a therapeutic biopolymer composed of an elastin-like polypeptide (ELP) fused to vascular endothelial growth factor (VEGF) and showed long-term renoprotective effects in experimental renovascular disease after a single intra-renal administration. Here, we sought to determine the specificity, safety, efficacy, and mechanisms of renoprotection of ELP-VEGF after systemic therapy in renovascular disease. We tested whether kidney selectivity of the ELP carrier would reduce off-target binding of VEGF in other organs. In vivo bio-distribution after systemic administration of ELP-VEGF in swine was determined in kidneys, liver, spleen, and heart. Stenotic-kidney renal blood flow and glomerular filtration rate were quantified in vivo using multi-detector computed tomography (CT) after six weeks of renovascular disease, then treated with a single intravenous dose of ELP-VEGF or placebo and observed for four weeks. CT studies were then repeated and the pigs euthanized. Ex vivo studies quantified renal microvascular density (micro-CT) and fibrosis. Kidneys, liver, spleen, and heart were excised to quantify the expression of angiogenic mediators and markers of progenitor cells. ELP-VEGF accumulated predominantly in the kidney and stimulated renal blood flow, glomerular filtration rate, improved cortical microvascular density, and renal fibrosis, and was accompanied by enhanced renal expression of VEGF, downstream mediators of VEGF signaling, and markers of progenitor cells compared to placebo. Expression of angiogenic factors in liver, spleen, and heart were not different compared to placebo-control. Thus, ELP efficiently directs VEGF to the kidney after systemic administration and induces long-term renoprotection without off-target effects, supporting the feasibility and safety of renal therapeutic angiogenesis via systemic administration of a novel kidney-specific bioengineered compound.

Protective effect of vascular endothelial growth factor against cardiopulmonary bypass-associated acute kidney injury in beagles

The present study aimed to examine the hypothesis that vascular endothelial growth factor (VEGF) has a protective effect against cardiopulmonary bypass (CPB)-associated acute kidney injury (AKI). Eighteen male beagles were randomly allocated to three groups (n=6 per group): Sham group, animals received sternotomy without going through CPB; CPB group, animals received CPB only; VEGF group, animals received CPB and VEGF. VEGF infusion was completed 1 h prior to the initiation of CPB. Renal microcirculation perfusion, serum creatinine (SCr) and blood urea nitrogen (BUN), histopathological injury score and apoptotic index were determined. Hypoxia inducible factor-1α, VEGF, phosphorylated (p)-Akt serine/threonine kinase (Akt), p-endothelial nitric oxide synthase (eNOS), cleaved caspase-3, B-cell lymphoma 2 (Bcl-2) and cluster of differentiation (CD)95 expression levels were assessed by western blot analysis, Enzyme-linked immunosorbent assay quantitative assays were used to evaluate tumor necrosis factor (TNF)-α, interleukin (IL)-6, superoxide dismutase and malondialdehyde levels. Renal microcirculation perfusion of the VEGF group was higher than that of the CPB group (P<0.05) and lower than that of the sham surgery group (P<0.05). SCr and BUN were significantly elevated after CPB in the CPB and VEGF groups, with significantly lower levels in group VEGF than group CPB. Renal pathology scores and apoptotic indices were significantly lower in the VEGF group than the CPB group. Levels of TNF-α, IL-6 in the VEGF group were significantly lower than in the CPB group. Levels of VEGF, p-Akt, p-eNOS and Bcl-2 expression in the VEGF group increased significantly in comparison with group CPB. Cleaved caspase-3 in the VEGF group was significantly lower than in the group CPB. CPB-associated reduction of renal microcirculation perfusion may predispose to AKI. VEGF appears to provide a protective effect on the kidneys through improvement in renal microperfusion.

Role of the Renal Microcirculation in Progression of Chronic Kidney Injury in Obesity

BACKGROUND

Obesity is largely responsible for the growing incidence and prevalence of diabetes, cardiovascular and renal diseases. Current strategies to prevent and treat obesity and its consequences have been insufficient to reverse the ongoing trends. Lifestyle modification or pharmacological therapies often produce modest weight loss which is not sustained and recurrence of obesity is frequently observed, leading to progression of target organ damage in many obese subjects. Therefore, research efforts have focused not only on the factors that regulate energy balance, but also on understanding mechanisms of target organ injury in obesity. Summary and Key Message: Microvascular (MV) disease plays a pivotal role in progressive kidney injury from different etiologies such as hypertension, diabetes, and atherosclerosis, which are all important consequences of chronic obesity. The MV networks are anatomical units that are closely adapted to specific functions of nutrition and removal of waste in every organ. Damage of the small vessels in several tissues and organs has been reported in obesity and may increase cardio-renal risk. However, the mechanisms by which obesity and its attendant cardiovascular and metabolic consequences interact to cause renal MV injury and chronic kidney disease are still unclear, although substantial progress has been made in recent years. This review addresses potential mechanisms and consequences of obesity-induced renal MV injury as well as current treatments that may provide protection of the renal microcirculation and slow progressive kidney injury in obesity.

Hypercholesterolemia Impairs Nonstenotic Kidney Outcomes After Reversal of Experimental Renovascular Hypertension

BACKGROUND

Revascularization of a stenotic renal artery improves kidney function only in select patients with renovascular hypertension (HT) secondary to atherosclerosis. However, the effects of reversal of renovascular HT (RRHT) on the nonstenotic kidney are unclear. We hypothesized that concurrent hypercholesterolemia (HC) attenuates nonstenotic kidney recovery.

METHODS

Female domestic pigs were randomized as Normal, renovascular HT, HT+RRHT, HTC (renovascular HT and HC), and HTC+RHT (n = 7 each). RRHT or sham was performed after 6 weeks of HT. Nonstenotic renal blood flow, glomerular filtration rate, and injurious pathways were studied 4 weeks later.

RESULTS

Mean arterial pressure increased similarly in HT and HTC and decreased after RRHT. Oxidative stress increased in HT and HTC kidneys, and decreased in HT+RRHT, but remained elevated in HTC+RRHT. Renal interstitial fibrosis, glomerulosclerosis, and tubular injury were all attenuated in HT+RRHT, but not HTC+RRHT. Endothelin-1 signaling and PGF2α isoprostane levels were elevated in both HTC and HTC+RRHT pigs.

CONCLUSIONS

RRHT reverses nonstenotic kidney injury in experimental renovascular HT, but concurrent HC blunts regression of kidney injury, possibly due to predominant vasoconstrictors and oxidative stress. These findings reinforce the contribution of the nonstenotic kidney and of prevailing cardiovascular risk factors to irreversibility of kidney dysfunction after revascularization.

Dual inhibition of renin-angiotensin-aldosterone system and endothelin-1 in treatment of chronic kidney disease

Inhibition of the renin-angiotensin-aldosterone system (RAAS) plays a pivotal role in treatment of chronic kidney diseases (CKD). However, reversal of the course of CKD or at least long-term stabilization of renal function are often difficult to achieve, and many patients still progress to end-stage renal disease. New treatments are needed to enhance protective actions of RAAS inhibitors (RAASis), such as angiotensin-converting enzyme (ACE) inhibitors (ACEIs) or angiotensin receptor blockers (ARBs), and improve prognosis in CKD patients. Inhibition of endothelin (ET) system in combination with established RAASis may represent such an approach. There are complex interactions between both systems and similarities in their renal physiological and pathophysiological actions that provide theoretical rationale for combined inhibition. This view is supported by some experimental studies in models of both diabetic and nondiabetic CKD showing that a combination of RAASis with ET receptor antagonists (ERAs) ameliorate proteinuria, renal structural changes, and molecular markers of glomerulosclerosis, renal fibrosis, or inflammation more effectively than RAASis or ERAs alone. Practically all clinical studies exploring the effects of RAASis and ERAs combination in nephroprotection have thus far applied add-on designs, in which an ERA is added to baseline treatment with ACEIs or ARBs. These studies, conducted mostly in patients with diabetic nephropathy, have shown that ERAs effectively reduce residual proteinuria in patients with baseline RAASis treatment. Long-term studies are currently being conducted to determine whether promising antiproteinuric effects of the dual blockade will be translated in long-term nephroprotection with acceptable safety profile.

Vascular Endothelial Growth Factor and Podocyte Protection in Chronic Hypoxia: Effects of Endothelin-A Receptor Antagonism

BACKGROUND

Podocytes are major components of the filtration barrier and a renal source of vascular endothelial growth factor (VEGF). Chronic renovascular disease (RVD) progressively degrades the renal function, accompanied by podocyte damage and a progressive reduction in VEGF. We showed that the endothelin (ET) pathway contributes to this pathological process and ET-A (but not ET-B) receptor antagonism protects the kidney in RVD. We hypothesize that ET-A-induced renoprotection is largely driven by the protection of podocyte integrity and function.

METHODS

To mimic the renal environment of chronic RVD, human podocytes were incubated under chronic hypoxia for 96 h and divided in untreated or treated with an ET-A or ET-B receptor antagonist. Cells were quantified after 96 h. Cell homogenates and media were obtained after 1, 24 and 96 h to quantify production of VEGF, anti-VEGF soluble receptor s-Flt1, and the expression of apoptotic mediators. A separate set of similar experiments was performed after addition of a VEGF-neutralizing antibody (VEGF-NA).

RESULTS

Hypoxia decreased podocyte number, which was exacerbated by ET-B but improved after ET-A antagonism. Production of VEGF was preserved by ET-A antagonism, whereas s-Flt1 increased in hypoxic cells after ET-B antagonism only, accompanied by a greater expression of pro-apoptotic mediators. On the other hand, treatment with VEGF-NA diminished ET-A-induced protection of podocytes.

CONCLUSION

ET-A antagonism preserves podocyte viability and integrity under chronic hypoxia, whereas ET-B antagonism exacerbates podocyte dysfunction and death. Enhanced bioavailability of VEGF after ET-A antagonism could be a pivotal mechanism of podocyte protection that significantly contributes to ET-A receptor blockade-induced renal recovery in chronic RVD.

Et and diabetic nephropathy: preclinical and clinical studies

The incidence of progressive kidney disease associated with diabetes continues to increase worldwide. Only partial renoprotection is achieved by current standard therapy with angiotensin-converting enzyme inhibitors and/or angiotensin-receptor blockers, increasing the need for novel therapeutic approaches. Experimental studies have provided evidence of a pathogenic role for endothelin-1 (ET-1) and its cognate receptors in the development and progression of diabetic nephropathy. ET-1, mainly through the activation of ETA receptor, contributes to renal cell injury, inflammation, and fibrosis. In animal models of type 1 and type 2 diabetes, ETA-selective antagonists have been shown to provide renoprotective effects, supplying the rationale for clinical trials in patients with diabetic nephropathy with ETA-receptor antagonists administered in addition to renin-angiotensin system blockade.

Endothelin A receptor antagonist, atrasentan, attenuates renal and cardiac dysfunction in Dahl salt-hypertensive rats in a blood pressure independent manner

Proteinuria is a hallmark of chronic kidney disease (CKD) and cardiovascular disease (CVD), and a good predictor of clinical outcome. Selective endothelin A (ET_A) receptor antagonist used with renin-angiotensin system (RAS) inhibitors prevents development of proteinuria in CKD. However, whether the improvement in proteinuria would have beneficial effects on CVD, independent of RAS inhibition, is not well understood. In this study, we investigated whether atrasentan, an ET_A receptor antagonist, has renal and cardiovascular effects independent of RAS inhibition. Male Dahl salt sensitive (DSS) rats, at six weeks of age, received water with or without different doses of atrasentan and/or enalapril under high salt (HS) diet or normal diet (ND) for 6 weeks. At the end of 12th week, atrasentan at a moderate dose significantly attenuated proteinuria and serum creatinine without reducing mean arterial pressure (MAP), thereby preventing cardiac hypertrophy and improving cardiac function. ACE inhibitor enalapril at a dose that did not significantly lowered BP, attenuated cardiac hypertrophy while moderately improving cardiac function without reducing proteinuria and serum creatinine level. Nonetheless, combined therapy of atrasentan and enalapril that does not altering BP exerted additional cardioprotective effect. Based on these findings, we conclude that BP independent monotherapy of ET_A receptor antagonist attenuates the progression of CKD and significantly mitigates CVD independent of RAS inhibition.

Chronic blockade of endothelin A and B receptors using macitentan in experimental renovascular disease

BACKGROUND

Emerging research has identified the endothelin (ET)-1 pathway as a potential target for novel renoprotective therapies. We recently showed that selective ET-A receptor antagonism in chronic renovascular disease (RVD) improves renal function and reduces renal injury. Although ET-A and -B have opposing roles, in some clinical situations they may induce similar effects. Thus, we hypothesized that simultaneous blockade of the ET-A and -B receptors would protect the kidney during RVD.

METHODS

Unilateral RVD was induced in pigs. After 6 weeks, single-kidney function was quantified in vivo using multi-detector computer tomography. Pigs were subsequently divided into untreated (RVD, n = 7) or daily-treated with the dual ET-A/B receptor antagonist macitentan (RVD + macitentan, n = 6) for 4 weeks. At 10 weeks, in vivo studies were repeated, then pigs were euthanized and ex vivo studies performed in the stenotic kidney to quantify inflammation, fibrosis, microvascular density and remodeling.

RESULTS

Four weeks of macitentan therapy modestly improved renal blood flow (29%, P = 0.06 versus pre-treatment) and showed protective effects on the renal parenchyma by attenuating inflammation and glomerulosclerosis, reducing apoptosis and tubular casts and improving albuminuria and cortical microvessel density. No overt adverse effects were observed.

CONCLUSION

Possibly by inducing a pro-survival renal microenvironment, macitentan increased renal microvascular density, promoted cell survival and decreased injury, which in turn improved stenotic kidney hemodynamics in our model. Our results further support the safety of using macitentan in patients with concomitant chronic renal disease and supported the feasibility of a new strategy that may preserve the stenotic kidney in RVD.

Endothelin-a receptor antagonism after renal angioplasty enhances renal recovery in renovascular disease

Percutaneous transluminal renal angioplasty/stenting (PTRAS) is frequently used to treat renal artery stenosis and renovascular disease (RVD); however, renal function is restored in less than one half of the cases. This study was designed to test a novel intervention that could refine PTRAS and enhance renal recovery in RVD. Renal function was quantified in pigs after 6 weeks of chronic RVD (induced by unilateral renal artery stenosis), established renal damage, and hypertension. Pigs with RVD then underwent PTRAS and were randomized into three groups: placebo (RVD+PTRAS), chronic endothelin-A receptor (ET-A) blockade (RVD+PTRAS+ET-A), and chronic dual ET-A/B blockade (RVD+PTRAS+ET-A/B) for 4 weeks. Renal function was again evaluated after treatments, and then, ex vivo studies were performed on the stented kidney. PTRAS resolved renal stenosis, attenuated hypertension, and improved renal function but did not resolve renal microvascular rarefaction, remodeling, or renal fibrosis. ET-A blocker therapy after PTRAS significantly improved hypertension, microvascular rarefaction, and renal injury and led to greater recovery of renal function. Conversely, combined ET-A/B blockade therapy blunted the therapeutic effects of PTRAS alone or PTRAS followed by ET-A blockade. These data suggest that ET-A receptor blockade therapy could serve as a coadjuvant intervention to enhance the outcomes of PTRAS in RVD. These results also suggest that ET-B receptors are important for renal function in RVD and may contribute to recovery after PTRAS. Using clinically available compounds and techniques, our results could contribute to both refinement and design of new therapeutic strategies in chronic RVD.

Drugs to foster kidney regeneration in experimental animals and humans

BACKGROUND

The incidence of kidney diseases is increasing worldwide and they are emerging as a major public health problem. Once mostly considered inexorable, renal disease progression can now be halted and lesions can even regress with drugs such as angiotensin-converting enzyme inhibitors (ACEi) and angiotensin II type I receptor blockers, indicating the possibility of kidney repair.

SUMMARY

The discovery of renal progenitor cells lining the Bowman capsule of adult rat and human kidneys has shed light on the mechanism of repair by ACEi. Parietal progenitors are a reservoir of cells that contribute to podocyte turnover in physiological conditions. In the early phases of renal disease these progenitors migrate chaotically and subsequently proliferate, accumulating in Bowman's space. The abnormal behavior of parietal progenitors is sustained by the activation of CXCR4 receptors in response to an increased production of the chemokine SDF-1 by podocytes activated by the inflammatory environment. Ang II, via the AT1 receptor, also contributes to progenitor cell proliferation. The CXCR4/SDF-1 and Ang II/AT1 receptor pathogenic pathways both pave the way for lesion formation and subsequent sclerosis. ACEi normalize the CXCR4 and AT1 receptor expression on progenitors, limiting their proliferation, concomitant with the regression of hyperplastic lesions in animals, and in a patient with crescentic glomerulopathy.

KEY MESSAGE

Understanding the molecular and cellular determinants of regeneration triggered by renoprotective drugs will reveal novel pathways that might be challenged or targeted by pharmacological therapy.

Disparate effects of single endothelin-A and -B receptor blocker therapy on the progression of renal injury in advanced renovascular disease

We hypothesized that chronic specific endothelin (ET)-A receptor blockade therapy would reverse renal dysfunction and injury in advanced experimental renovascular disease. To test this, unilateral renovascular disease was induced in 19 pigs and after 6 weeks, single-kidney hemodynamics and function was quantified in vivo using computed-tomography. All pigs with renovascular disease were divided such that 7 were untreated, 7 were treated with ET-A blockers, and 5 were treated with ET-B blockers. Four weeks later, all pigs were re-studied in vivo, then euthanized and ex vivo studies performed on the stenotic kidney to quantify microvascular density, remodeling, renal oxidative stress, inflammation, and fibrosis. RBF, GFR, and redox status were significantly improved in the stenotic kidney after ET-A but not ET-B blockade. Furthermore, only ET-A blockade therapy reversed renal microvascular rarefaction and diminished remodeling, which was accompanied by a marked decreased in renal inflammatory and fibrogenic activity. Thus, ET-A but not ET-B blockade ameliorated renal injury in pigs with advanced renovascular disease by stimulating microvascular proliferation and decreasing the progression of microvascular remodeling, renal inflammation and fibrosis in the stenotic kidney. These effects were functionally consequential since ET-A blockade improved single kidney microvascular endothelial function, RBF, and GFR, and decreased albuminuria.

Renal vascular structure and rarefaction

An intact microcirculation is vital for diffusion of oxygen and nutrients and for removal of toxins of every organ and system in the human body. The functional and/or anatomical loss of microvessels is known as rarefaction, which can compromise the normal organ function and have been suggested as a possible starting point of several diseases. The purpose of this overview is to discuss the potential underlying mechanisms leading to renal microvascular rarefaction, and the potential consequences on renal function and on the progression of renal damage. Although the kidney is a special organ that receives much more blood than its metabolic needs, experimental and clinical evidence indicates that renal microvascular rarefaction is associated to prevalent cardiovascular diseases such as diabetes, hypertension, and atherosclerosis, either as cause or consequence. On the other hand, emerging experimental evidence using progenitor cells or angiogenic cytokines supports the feasibility of therapeutic interventions capable of modifying the progressive nature of microvascular rarefaction in the kidney. This overview will also attempt to discuss the potential renoprotective mechanisms of the therapeutic targeting of the renal microcirculation.

Early superoxide scavenging accelerates renal microvascular rarefaction and damage in the stenotic kidney

Renal artery stenosis (RAS), the main cause of chronic renovascular disease (RVD), is associated with significant oxidative stress. Chronic RVD induces renal injury partly by promoting renal microvascular (MV) damage and blunting MV repair in the stenotic kidney. We tested the hypothesis that superoxide anion plays a pivotal role in MV dysfunction, reduction of MV density, and progression of renal injury in the stenotic kidney. RAS was induced in 14 domestic pigs and observed for 6 wk. Seven RAS pigs were chronically treated with the superoxide dismutase mimetic tempol (RAS+T) to reduce oxidative stress. Single-kidney hemodynamics and function were quantified in vivo using multidetector computer tomography (CT) and renal MV density was quantified ex vivo using micro-CT. Expression of angiogenic, inflammatory, and apoptotic factors was measured in renal tissue, and renal apoptosis and fibrosis were quantified in tissue sections. The degree of RAS and blood pressure were similarly increased in RAS and RAS+T. Renal blood flow (RBF) and glomerular filtration rate (GFR) were reduced in the stenotic kidney (280.1 ± 36.8 and 34.2 ± 3.1 ml/min, P < 0.05 vs. control). RAS+T kidneys showed preserved GFR (58.5 ± 6.3 ml/min, P = not significant vs. control) but a similar decreases in RBF (293.6 ± 85.2 ml/min) and further decreases in MV density compared with RAS. These changes were accompanied by blunted angiogenic signaling and increased apoptosis and fibrosis in the stenotic kidney of RAS+T compared with RAS. The current study shows that tempol administration provided limited protection to the stenotic kidney. Despite preserved GFR, renal perfusion was not improved by tempol, and MV density was further reduced compared with untreated RAS, associated with increased renal apoptosis and fibrosis. These results suggest that a tight balance of the renal redox status is necessary for a normal MV repair response to injury, at least at the early stage of RVD, and raise caution regarding antioxidant strategies in RAS.

Therapeutic modalities in diabetic nephropathy: standard and emerging approaches

Diabetes mellitus is the leading cause of end stage renal disease and is responsible for more than 40% of all cases in the United States. Current therapy directed at delaying the progression of diabetic nephropathy includes intensive glycemic and optimal blood pressure control, proteinuria/albuminuria reduction, interruption of the renin-angiotensin-aldosterone system through the use of angiotensin converting enzyme inhibitors and angiotensin type-1 receptor blockers, along with dietary modification and cholesterol lowering agents. However, the renal protection provided by these therapeutic modalities is incomplete. More effective approaches are urgently needed. This review highlights the available standard therapeutic approaches to manage progressive diabetic nephropathy, including markers for early diagnosis of diabetic nephropathy. Furthermore, we will discuss emerging strategies such as PPAR-gamma agonists, Endothelin blockers, vitamin D activation and inflammation modulation. Finally, we will summarize the recommendations of these interventions for the primary care practitioner.

Endothelin-A receptor blockade slows the progression of renal injury in experimental renovascular disease

Endothelin (ET)-1, a potent renal vasoconstrictor with mitogenic properties, is upregulated by ischemia and has been shown to induce renal injury via the ET-A receptor. The potential role of ET-A blockade in chronic renovascular disease (RVD) has not, to our knowledge, been previously reported. We hypothesized that chronic ET-A receptor blockade would preserve renal hemodynamics and slow the progression of injury of the stenotic kidney in experimental RVD. Renal artery stenosis, a major cause of chronic RVD, was induced in 14 pigs and observed for 6 wk. In half of the pigs, chronic ET-A blockade was initiated (RVD+ET-A, 0.75 mg·kg(-1)·day(-1)) at the onset of RVD. Single-kidney renal blood flow, glomerular filtration rate, and perfusion were quantified in vivo after 6 wk using multidetector computer tomography. Renal microvascular density was quantified ex vivo using three-dimensional microcomputer tomography, and growth factors, inflammation, apoptosis, and fibrosis were determined in renal tissue. The degree of stenosis and increase in blood pressure were similar in RVD and RVD+ET-A pigs. Renal hemodynamics, function, and microvascular density were decreased in the stenotic kidney but preserved by ET-A blockade, accompanied by increased renal expression of vascular endothelial growth factor, hepatocyte growth factor, and downstream mediators such as phosphorilated-Akt, angiopoietins, and endothelial nitric oxide synthase. ET-A blockade also reduced renal apoptosis, inflammation, and glomerulosclerosis. This study shows that ET-A blockade slows the progression of renal injury in experimental RVD and preserves renal hemodynamics, function, and microvascular density in the stenotic kidney. These results support a role for ET-1/ET-A as a potential therapeutic target in chronic RVD.

Renovascular disease, microcirculation, and the progression of renal injury: role of angiogenesis

Emerging evidence supports the pivotal role of renal microvascular disease as a determinant of tubulo-interstitial and glomerular fibrosis in chronic kidney disease. An intact microcirculation is vital to restore blood flow to the injured tissues, which is a crucial step to achieve a successful repair response. The purpose of this review is to discuss the impact and mechanisms of the functional and structural changes of the renal microvascular network, as well as the role of these changes in the progression and irreversibility of renal injury. Damage of the renal microcirculation and deterioration of the angiogenic response may constitute early steps in the complex pathways involved in progressive renal injury. There is limited but provocative evidence that stimulation of vascular proliferation and repair may stabilize renal function and slow the progression of renal disease. The feasibility of novel potential therapeutic interventions for stabilizing the renal microvasculature is also discussed. Targeted interventions to enhance endogenous renoprotective mechanisms focused on the microcirculation, such as cell-based therapy or the use of angiogenic cytokines have shown promising results in some experimental and clinical settings.

Role of renal microcirculation in experimental renovascular disease

BACKGROUND

Renal artery stenosis (RAS) causes renal injury partly via microvascular (MV) endothelial dysfunction and damage. Vascular endothelial growth factor (VEGF) is crucial for preservation of microvasculature and promotes vascular proliferation and endothelial repair. We have previously shown that MV rarefaction is associated with decreased VEGF in the kidney exposed to chronic RAS, accompanied by deteriorated renal function and fibrosis. We hypothesized that preserving the renal microcirculation in the stenotic kidney will halt the progression of renal damage.

METHODS

Unilateral RAS was induced in 16 pigs. In eight, VEGF (0.05 micrograms/kg) was infused intra-renally at the onset of RAS. After 6 weeks, single-kidney haemodynamics and function were assessed using in vivo multi-detector computed tomography (CT). Renal microvessels, angiogenic pathways and morphology were investigated ex vivo using micro-CT, real-time PCR and histology.

RESULTS

Blood pressure and degree of RAS was similar in RAS and RAS + VEGF pigs. Single-kidney renal blood flow (RBF) and glomerular filtration rate (GFR) were reduced in RAS compared to Normal (221.1 +/- 46.5 and 29.9 +/- 3.8 vs. 522.5 +/- 60.9 and 49.3 +/- 3.4 mL/min, respectively, P < 0.05), accompanied by decreased cortical MV density and increased renal fibrosis. Pre-emptive administration of VEGF preserved MV architecture, attenuated fibrosis and normalized RBF and GFR (510.8 +/- 50.9 and 39.9.1 +/- 4.1 mL/min, P = not significant vs. Normal).

CONCLUSIONS

This study underscores the importance of the renal microcirculation in renovascular disease. Intra-renal administration of VEGF preserved renal MV architecture and function of the stenotic kidney, which in turn preserved renal haemodynamics and function and decreased renal fibrosis. These observations suggest that preventing renal MV loss may be a potential target for therapeutic approaches for patients with chronic renovascular disease.

Unlike each drug alone, lisinopril if combined with avosentan promotes regression of renal lesions in experimental diabetes

In the present study, we evaluated the effect of simultaneously blocking angiotensin II synthesis and endothelin (ET)-1 activity as a multimodal intervention to implement renoprotection in overt diabetic nephropathy. Mechanisms underlying combined therapy effectiveness were addressed by investigating podocyte structure and function and glomerular barrier size-selective properties. Uninephrectomized rats made diabetic by streptozotocin received orally placebo, lisinopril (12.5 mg/l), the ET(A) receptor antagonist avosentan (30 mg/kg), or their combination from 4 (when animals had proteinuria) to 8 mo. Proteinuria, renal damage, podocyte number, nephrin expression, and glomerular size selectivity by graded-size Ficoll molecule fractional clearance were assessed. Combined therapy normalized proteinuria, provided complete protection from tubulointerstitial damage, and induced regression of glomerular lesions, while only a partial renoprotection was achieved by each drug alone. Lisinopril plus avosentan restored to normal values the number of podocytes. Single therapies only limited podocyte depletion. Defective nephrin expression of diabetes was prevented by each drug. Altered glomerular size selectivity to large macromolecules of diabetic rats was remarkably improved by lisinopril and the combined treatment. Avosentan ameliorated peritubular capillary architecture and reduced interstitial inflammation and fibrosis. The ACE inhibitor and ET(A) receptor antagonist induced regression of glomerular lesions in overt diabetic nephropathy. Regression of renal disease was conceivably the result of the synergistic effect of the ACE inhibitor of preserving glomerular permselective properties and the ET(A) antagonist in improving tubulointerstitial changes. These findings provide mechanistic insights to explain the antiproteinuric effect of this combined therapy in diabetes.

Angiogenesis in the kidney: a new therapeutic target?

PURPOSE OF REVIEW

The prevalence of chronic kidney disease has been growing consistently for the past decades. Renal failure is often associated with defective angiogenesis, and recognition of the contribution of the renal microcirculation to the progression of chronic renal disease may aid in the development of therapeutic interventions.

RECENT FINDINGS

Intra-renal proliferation, remodeling, and/or rarefaction of microvessels in response to injury can all aggravate nephron damage, and experimental evidence suggests that they may constitute the early steps in the complex pathways involved in progressive renal injury. Recent studies showed the benefits of targeted interventions deemed to promote neovascularization (e.g. progenitor cells, growth factors) on the ischemic myocardium and brain and in a few models of renal disease.

SUMMARY

Evidence of aberrant renal microvascular architecture in various forms of renal disease provides the impetus to attempt modulating the renal microcirculation to interfere with the disease process. Targeted interventions to preserve the renal microcirculation may not only decrease the evolving injury in renal vascular disease but also potentially constitute a coadjuvant intervention to become part of a comprehensive management plan to improve the success of parallel strategies to preserve renal function, such as revascularization.

Endothelin receptor selectivity in chronic kidney disease: rationale and review of recent evidence

Endothelin (ET) is a potent vasoconstrictory peptide with proinflammatory and profibrotic properties that exerts its biological effects through two pharmacologically distinct receptor subtypes, namely ET(A) and ET(B). In addition to its substantial contribution to normal renal function, a large body of evidence suggests that derangement of the renal ET system is involved in the initiation and progression of chronic kidney disease (CKD) in diabetes, hypertension and glomerulonephritis. Thus, the use of ET receptor antagonists (ERAs) may offer potential novel treatment strategies in CKD. Recent literature on the role of the renal ET system in the healthy kidney was reviewed. In addition, an unbiased PubMed search was performed for studies published during the last 5 years that addressed the effects of ERAs in CKD. A particular objective was to extract information regarding whether selective or nonselective ERAs may have therapeutic potential in humans. ET-1 acts primarily as an autocrine or paracrine factor in the kidney. In normal physiology, ET-1 promotes diuresis and natriuresis by local production and action through ET(B) receptors in the renal medulla. In pathology, ET-1 mediates vasoconstriction, mesangial-cell proliferation, extracellular matrix production and inflammation, effects that are primarily conveyed by ET(A) receptors. Results obtained in animal models and in humans with the use of ERAs in CKD are encouraging; nevertheless, it is still under debate which receptor subtype should be targeted. According to most studies, selective inhibition of ET(A) receptors appears superior compared with nonselective ERAs because this approach does not interfere with the natriuretic, antihypertensive and ET clearance effects of ET(B) receptors. Although preliminary data in humans are promising, the potential role of ERAs in patients with CKD and the question of which receptor subtype should be targeted can only be clarified in randomized clinical trials.

The importance of endothelin-1 for microvascular dysfunction in diabetes

Most of the late diabetic complications such as retinopathy, nephropathy, and neuropathy, have their basis in disturbed microvascular function. Structural and functional changes in the micro-circulation are present in diabetes mellitus irrespective of the organ studied, and the pathogenesis is complex. Endothelial dysfunction, characterized by an imbalance between endothelium-derived vasodilator and vasoconstrictor substances, plays an important role in the pathogenesis of diabetic microangiopathy. Increased circulating levels of endothelin-1 (ET-1), a potent vasoconstrictor peptide, has been found in patients with diabetes, and a positive correlation between plasma ET-1 levels and microangiopathy in patients with type 2 diabetes has been demonstrated. In addition to its direct vasoconstrictor effects, enhanced levels of ET-1 may contribute to endothelial dysfunction through inhibitory effects on nitric oxide (NO) production. Vascular endothelial dysfunction may precede insulin resistance, although the feature of insulin resistance syndrome includes factors that have negative effects on endothelial function. Furthermore, ET-1 induces a reduction in insulin sensitivity and may take part in the development of the metabolic syndrome. In the following, the mechanisms by which ET-1 contributes to the development of diabetic microangiopathy and the potentially beneficial effect of selective ET_A receptor antagonists are discussed.

Efficacy and safety of atrasentan in patients with cardiovascular risk and early atherosclerosis

Endothelin plays an important role in the pathogenesis of atherosclerosis. The aim of the study was to evaluate the safety and hemodynamic and metabolic responses to 6 months treatment with atrasentan, the selective endothelin-A receptor antagonist. Seventy-two patients with multiple cardiovascular risk factors and nonobstructive coronary artery disease on coronary angiogram were randomly assigned in a double-blind manner to atrasentan or placebo. Mean aortic blood pressure decreased from 92+/-10 to 80+/-10 mm Hg (P<0.001) in the atrasentan group and did not change in the placebo group (93+/-10 and 92+/-11 mm Hg; P=0.84). The difference between the groups was significant (P<0.001). No effect on heart rate was observed. In a subgroup of patients not treated with angiotensin-converting enzyme inhibitor, creatinine level decreased in the atrasentan versus the placebo group (P=0.011). Fasting glucose (P=0.026), glycosylated hemoglobin level (P=0.041), triglyceride l (P=0.013), lipoprotein-A (P=0.046), and uric acid levels (P=0.048) decreased significantly in the atrasentan group compared with the placebo group. No progression of angiographic coronary disease was observed. The most common adverse effects with atrasentan were nasal stuffiness, headache, and edema. In conclusion, 6 months of treatment with atrasentan results in a reduction of blood pressure and improvement in glucose and lipid metabolism. These findings suggest the beneficial role of atrasentan in the treatment of hypertension and metabolic syndrome.

Microcirculation and oxidative stress

The microcirculation is a complex and integrated system, transporting oxygen and nutrients to the cells. The key component of this system is the endothelium, contributing to the local balance between pro and anti-inflammatory mediators, hemostatic balance, as well as vascular permeability and cell proliferation. A constant shear stress maintains vascular endothelium homeostasis while perturbed shear stress leads to changes in secretion of vasodilator and vasoconstrictor agents. Increased oxidative stress is a major pathogenetic mechanism of endothelial dysfunction by decreasing NO bioavailability, promoting inflammation and participating in activation of intracellular signals cascade, so influencing ion channels activation, signal transduction pathways, cytoskeleton remodelling, intercellular communication and ultimately gene expression. Targeting the microvascular inflammation and oxidative stress is a fascinating approach for novel therapies in order to decrease morbidity and mortality of chronic and acute diseases.

Effects of statins on renal function

Patients with chronic kidney disease (CKD) are much more likely to die of cardiovascular disease than end-stage renal disease. Dyslipidemia is highly prevalent in patients with CKD and may contribute to the elevated cardiovascular risk as well as CKD progression. Statins are lipid-lowering drugs that appear to protect the kidneys via cholesterol reduction as well as noncholesterol-mediated mechanisms. Subgroup analyses of major clinical studies and meta-analyses of smaller trials indicate that statin therapy slows the decline of the glomerular filtration rate. Additionally, statins appear to reduce proteinuria in patients with CKD. Statins are well recognized to reduce cardiovascular morbidity and mortality in patients with and without documented cardiovascular disease and in certain high-risk populations, such as persons with diabetes mellitus. However, conclusive evidence for improved cardiovascular outcomes with statin therapy for CKD is not yet available. Several ongoing studies are evaluating the effect of statins on cardiovascular end points in patients with CKD and may provide data needed to support adjunctive use of these agents in this high-risk population.

Role of renal cortical neovascularization in experimental hypercholesterolemia

Hypercholesterolemia induces renal inflammation and neovascularization, associated with renal endothelial dysfunction and injury. Neovascularization might conceivably represent a defense mechanism to sustain renal perfusion. Therefore, the present study was designed to test the hypothesis that preventing neovascularization using thalidomide, a potent anti-inflammatory and antiangiogenic agent, would impair basal renal hemodynamics in experimental hypercholesterolemia. Single-kidney function and hemodynamic responses to endothelium-dependent challenge were assessed in pigs after 12 weeks of hypercholesterolemia, hypercholesterolemia chronically supplemented with thalidomide (4 mg/kg per day), and normal controls. Renal microvascular architecture was then studied ex vivo using 3D microcomputed tomography imaging and inflammation, angiogenesis, and oxidative stress explored in renal tissue. The density of larger microvessels (200 to 500 microm) was selectively decreased in hypercholesterolemia plus thalidomide and accompanied by a decreased fraction of angiogenic, integrin beta(3)-positive microvessels (9.9%+/-0.9% versus 25.5%+/-1.7%; P<0.05 versus hypercholesterolemia), implying decreased angiogenic activity. Furthermore, thalidomide increased renal expression of endothelial NO synthase and decreased tumor necrosis factor-alpha and renal inflammation but did not decrease oxidative stress. Thalidomide also decreased basal renal blood flow and glomerular filtration rate but normalized the blunted renal hemodynamic responses in hypercholesterolemia. Attenuated inflammation and pathological angiogenesis achieved in hypercholesterolemia by thalidomide are accompanied by restoration of renovascular endothelial function but decreased basal renal hemodynamics. This study, therefore, suggests that neovascularization in the hypercholesterolemic kidney is a compensatory mechanism that sustains basal renal vascular function.