Renal and vascular injury induced by exogenous angiotensin II is AT1 receptor-dependent

(Pro)renin receptor decoy peptide PRO20 protects against adriamycin-induced nephropathy by targeting the intrarenal renin-angiotensin system

Adriamycin (ADR) administration in susceptible rodents such as the BALB/c mouse strain produces injury to the glomerulus mimicking human chronic kidney disease due to primary focal segmental glomerulosclerosis. The goal of the present study was to use this model to investigate antiproteinuric actions of the (pro)renin receptor decoy inhibitor PRO20. BALB/c mice were pretreated for 1 day with PRO20 at 500 μg·kg-1·day-1 via an osmotic minipump followed by a single injection of vehicle or ADR (10 mg/kg) via the tail vein. Albuminuria and renal function were analyzed at the fourth week post-ADR administration. ADR-treated mice exhibited severe proteinuria, hypoalbuminemia and hyperlipidemia, glomerulosclerosis, podocyte loss, tubulointerstitial fibrosis, and oxidative stress, accompanied by elevated urinary neutrophil gelatinase-associated lipocalin and kidney injury molecule-1, all of which were significantly attenuated by PRO20. Urinary and renal renin activity and angiotensin II were elevated by ADR and suppressed by PRO20. In parallel, urinary and renal H2O2 levels and renal NADPH oxidase 4 (Nox4) and transient receptor potential channel C6 (TRPC6) expression in response to ADR were all similarly suppressed. Taken together, the results of the present study provide the first evidence that PRO20 can protect against podocyte damage and interstitial fibrosis in ADR nephropathy by preventing activation of the intrarenal renin-angiotensin system and upregulation of Nox4 and TRPC6 expression. PRO20 may have a potential application in the treatment of ADR nephropathy.

Imbalance in Renal Vasoactive Enzymes Induced by Mild Hypoxia: Angiotensin-Converting Enzyme Increases While Neutral Endopeptidase Decreases

Chronic hypoxia has been postulated as one of the mechanisms involved in salt-sensitive hypertension and chronic kidney disease (CKD). Kidneys have a critical role in the regulation of arterial blood pressure through vasoactive systems, such as the renin-angiotensin and the kallikrein-kinin systems, with the angiotensin-converting enzyme (ACE) and kallikrein being two of the main enzymes that produce angiotensin II and bradykinin, respectively. Neutral endopeptidase 24.11 or neprilysin is another enzyme that among its functions degrade vasoactive peptides including angiotensin II and bradykinin, and generate angiotensin 1-7. On the other hand, the kidneys are vulnerable to hypoxic injury due to the active electrolyte transportation that requires a high oxygen consumption; however, the oxygen supply is limited in the medullary regions for anatomical reasons. With the hypothesis that the chronic reduction of oxygen under normobaric conditions would impact renal vasoactive enzyme components and, therefore; alter the normal balance of the vasoactive systems, we exposed male Sprague-Dawley rats to normobaric hypoxia (10% O₂) for 2 weeks. We then processed renal tissue to identify the expression and distribution of kallikrein, ACE and neutral endopeptidase 24.11 as well as markers of kidney damage. We found that chronic hypoxia produced focal damage in the kidney, mainly in the cortico-medullary region, and increased the expression of osteopontin. Moreover, we observed an increase of ACE protein in the brush border of proximal tubules at the outer medullary region, with increased mRNA levels. Kallikrein abundance did not change significantly with hypoxia, but a tendency toward reduction was observed at protein and mRNA levels. Neutral endopeptidase 24.11 was localized in proximal tubules, and was abundantly expressed under normoxic conditions, which markedly decreased both at protein and mRNA levels with chronic hypoxia. Taken together, our results suggest that chronic hypoxia produces focal kidney damage along with an imbalance of key components of the renal vasoactive system, which could be the initial steps for a long-term contribution to salt-sensitive hypertension and CKD.

(Pro)renin receptor mediates albumin-induced cellular responses: role of site-1 protease-derived soluble (pro)renin receptor in renal epithelial cells

Proteinuria is a characteristic of chronic kidney disease and also a causative factor that promotes the disease progression, in part, via activation of the intrarenal renin-angiotensin system (RAS). (Pro)renin receptor (PRR), a newly discovered component of the RAS, binds renin and (pro)renin to promote angiotensin I generation. The present study was performed to test the role of soluble PRR (sPRR) in albumin overload-induced responses in cultured human renal proximal tubular cell line human kidney 2 (HK-2) cells. Bovine serum albmuin (BSA) treatment for 24 h at 20 mg/ml induced renin activity and inflammation, both of which were attenuated by a PRR decoy inhibitor PRO20. BSA treatment induced a more than fivefold increase in medium sPRR due to enhanced cleavage of PRR. Surprisingly, this cleavage event was unaffected by inhibition of furin or a disintegrin and metalloproteinase 19. Screening for a novel cleavage enzyme led to the identification of site-1 protease (S1P). Inhibition of S1P with PF-429242 or siRNA remarkably suppressed BSA-induced sPRR production, renin activity, and inflammatory response. Administration of a recombinant sPRR, termed sPRR-His, reversed the effects of S1P inhibition. In HK-2 cells overexpressing PRR, mutagenesis of the S1P, but not furin cleavage site, reduced sPRR levels. Together, these results suggest that PRR mediates albumin-induced cellular responses through S1P-derived sPRR.

β-Catenin-Dependent Signaling Pathway Contributes to Renal Fibrosis in Hypertensive Rats

The mechanism of hypertension-induced renal fibrosis is not well understood, although it is established that high levels of angiotensin II contribute to the effect. Since β-catenin signal transduction participates in fibrotic processes, we evaluated the contribution of β-catenin-dependent signaling pathway in hypertension-induced renal fibrosis. Two-kidney one-clip (2K1C) hypertensive rats were treated with lisinopril (10 mg/kg/day for four weeks) or with pyrvinium pamoate (Wnt signaling inhibitor, single dose of 60 ug/kg, every 3 days for 2 weeks). The treatment with lisinopril reduced the systolic blood pressure from 220 ± 4 in 2K1C rats to 112 ± 5 mmHg (P < 0.05), whereas the reduction in blood pressure with pyrvinium pamoate was not significant (212 ± 6 in 2K1C rats to 170 ± 3 mmHg, P > 0.05). The levels of collagen types I and III, osteopontin, and fibronectin decreased in the unclipped kidney in both treatments compared with 2K1C rats. The expressions of β-catenin, p-Ser9-GSK-3beta, and the β-catenin target genes cyclin D1, c-myc, and bcl-2 significantly decreased in unclipped kidney in both treatments (P < 0.05). In this study we provided evidence that β-catenin-dependent signaling pathway participates in the renal fibrosis induced in 2K1C rats.

Aggravated renal tubular damage and interstitial fibrosis in mice lacking guanylyl cyclase-A (GC-A), a receptor for atrial and B-type natriuretic peptides

BACKGROUND AND AIM

The infusion of chronic angiotensin II (Ang II) has been shown to promote renal interstitial fibrosis. To evaluate the pathophysiological significance of the natriuretic peptide-GC-A system, we infused Ang II (1.0 mg/kg/day) in GC-A-deficient mice (GC-A-KO).

METHODS

We used 5 groups (Wild-Saline n = 12, Wild-Ang II n = 14, GC-A-KO-Saline n = 11, GC-A-KO-Ang II n = 13, and GC-A-KO-Ang II-Hydralazine n = 10). Saline or Ang II was infused subcutaneously using an osmotic minipump for 3 weeks. Hydralazine was administered orally (0.05 g/L in drinking water).

RESULTS

Systolic blood pressure was significantly higher in the GC-A-KO-Saline group (130 ± 12 mmHg) than in the Wild-Saline group (105 ± 30 mmHg), and was similar to that in the Wild-Ang II (141 ± 17 mmHg) and GC-A-KO-Ang II-Hydralazine (140 ± 20 mmHg) groups. Systolic blood pressure was significantly higher in the GC-A-KO-Ang II group (159 ± 21 mmHg) than in the 4 other groups. Renal tubular atrophy and interstitial fibrosis were significantly more severe in the GC-A-KO-Ang II group (atrophy 13.4 %, fibrosis 12.0 %) than in the Wild-Saline (0, 2.0 %), Wild-Ang II (2.9, 4.4 %), and GC-A-KO-Saline (0, 2.6 %) groups. Hydralazine could not inhibit this aggravation (GC-A-KO-Ang II-Hydralazine 13.5, 11.3 %). The expression of monocyte chemotactic protein-1 in tubular cells, and F4/80 and alpha-smooth muscle actin in the interstitium was clearly detected in the Ang II-infused wild and GC-A-KO groups and was associated with renal tubular atrophy and interstitial fibrosis. The expression of E-cadherin in tubular cells was absent in the Ang II-infused wild and GC-A-KO groups and was associated with renal tubular atrophy.

CONCLUSIONS

The natriuretic peptide-GC-A system may play an inhibitory role in Ang II-induced renal tubular atrophy, interstitial fibrosis, and phenotypic transformation in renal tubular cells and fibroblasts.

Angiotensin receptor blocker protection against podocyte-induced sclerosis is podocyte angiotensin II type 1 receptor-independent

In the present study, we tested the hypothesis that the renoprotective effect of an angiotensin receptor blocker depends on the angiotensin II type 1 (AT(1)) receptor on podocytes. For this purpose, we generated podocyte-specific knockout mice for the AT(1) gene (Agtr1a) and crossed with NEP25, in which selective podocyte injury can be induced by immunotoxin, anti-Tac(Fv)-PE38. Four weeks after the addition of anti-Tac(Fv)-PE38, urinary albumin:creatinine ratio was not attenuated in Agtr1a knockout/NEP25 mice (n=18) compared with that in control NEP25 mice (n=13; 8.08+/-2.41 in knockout versus 4.84+/-0.73 in control). Both strains of mice showed similar degrees of sclerosis (0.66+/-0.17 versus 0.82+/-0.27 on a 0 to 4 scale) and downregulation of nephrin (5.78+/-0.45 versus 5.65+/-0.58 on a 0 to 8 scale). In contrast, AT(1) antagonist or an angiotensin I-converting enzyme inhibitor, but not hydralazine, remarkably attenuated proteinuria and sclerosis in NEP25 mice. Moreover, continuous angiotensin II infusion induced microalbuminuria similarly in both Agtr1a knockout and wild-type mice. Thus, angiotensin inhibition can protect podocytes and prevent the development of glomerulosclerosis independent of podocyte AT(1). Possible mechanisms include inhibitory effects on AT(1) of other cells or through mechanisms independent of AT(1). Our study further demonstrates that measures that directly affect only nonpodocyte cells can have beneficial effects even when sclerosis is triggered by podocyte-specific injury.

E Prostanoid-1 receptor regulates renal medullary alphaENaC in rats infused with angiotensin II

E Prostanoid (EP) receptors play an important role in urinary Na(+) excretion. In the kidney, the epithelial sodium channel (ENaC) is the rate-limiting-step for Na(+) reabsorption. We hypothesized that activation of EP1/EP3 regulates the expression of ENaC in the face of renin-angiotensin-aldosterone-system (RAAS) activation. In primary cultures of inner medullary collecting duct (IMCD) cells, sulprostone (EP1>EP3 agonist, 1 microM) and 17 Phenyl trinor (17 Pt, EP1 agonist, 10 microM) prevented the up-regulation of alphaENaC mRNA induced by aldosterone (10 nM). In Sprague-Dawley rats infused with angiotensin II (0.4 microg/kg/min), alphaENaC expression was up-regulated in renal cortex and medulla coincidently with high plasma aldosterone levels. Sulprostone and/or 17 Pt prevented this effect in renal medulla but not in cortex. Immunocytochemistry demonstrated that IMCD cells express EP1. Our results suggest that specific activation of EP1 receptor during RAAS activation antagonizes the action of aldosterone on alphaENaC expression in the renal medulla.

Human T and natural killer cells possess a functional renin-angiotensin system: further mechanisms of angiotensin II-induced inflammation

The renin-angiotensin system (RAS) plays an important role in the regulation of inflammation and in the progression of chronic kidney disease. Accumulation of inflammatory cells into the renal parenchyma has been a hallmark of chronic kidney disease; however, little is known concerning the presence and the function of RAS elements in T and natural killer (NK) cells. Here is reported a co-stimulatory effect of angiotensin II (AngII) by showing an augmentation of mitogen and anti-CD3-stimulated T and NK cell proliferation with AngII treatment. Angiotensinogen and AngI also generated the same effect, suggesting that NK and T cells have functional renin and angiotensin-converting enzyme activity. Indeed, they express renin, the renin receptor, angiotensinogen, and angiotensin-converting enzyme by mRNA analysis. Flow cytometric analysis and Western blot revealed angiotensin receptor 2 (AT(2)) expression in T and NK cells, whereas AT(1) expression was found in T and NK cells and monocytes by Western blot. These receptors were shown to be functional in calcium signaling, chemotaxis, and proliferation. However, AT(1) and AT(2) antagonists alone or in combination were unable to abrogate completely the effects of AngII, suggesting that another AngII receptor may also be functional in leukocytes. This is the first study to show that T and NK cells are fully equipped with RAS elements and are potentially capable of producing and delivering AngII to sites of inflammation. Because their chemotaxis is enhanced by AngII, this creates a potential inflammatory amplification system.

Rosiglitazone inhibits angiotensin II-induced CTGF expression in vascular smooth muscle cells - role of PPAR-gamma in vascular fibrosis

Angiotensin (Ang) II plays a pivotal role in vascular fibrosis, which leads to serious complications in hypertension and diabetes. Connective tissue growth factor (CTGF) is a potent profibrotic factor implicated in the Ang II-induced pathologic fibrosis process. PPAR-gamma activators thiazolidinediones have been recently reported to have beneficial vascular effects. However, their effects and related molecular mechanisms on extracellular matrix (ECM) turnover in vascular smooth muscle cells (VSMCs) are unknown. The present study evaluated the regulation of Ang II-induced CTGF, ECM production and cell growth by rosiglitazone in VSMCs. In aorta of Ang II-infused rats, CTGF expression was markedly increased, and type III collagen and fibronectin overexpression was observed. Cotreatment with rosiglitazone diminished these changes, whereas increased nuclear PPAR-gamma expression in VSMCs. In growth-arrested VSMCs, rosiglitazone attenuated the proliferation and apoptosis, increased PPAR-gamma production and activation, and reduced CTGF and ECM production in response to Ang II in a dose-dependent fashion. These inhibitory effects were attenuated by the pretreatment of cells with PPAR-gamma antagonist GW9662 or bisphenol A diglycidyl ether (BADGE). Furthermore, rosiglitazone inhibited Ang II-induced Smad2 production and phosphorylation but had no effect on transforming growth factor-beta(1) (TGF-beta(1)) expression. These results suggest that in Ang II-stimulated VSMCs, rosiglitazone caused an antiproliferative, antiapototic effect and reduces ECM production through mechanisms that include reducing CTGF expression, and a crosstalk between PPAR-gamma and Smad may be involved in the inhibitory effects of rosiglitazone. This novel finding suggests a role of PPAR-gamma activators in preventing Ang II-induced vascular fibrosis.

Essential role of Smad3 in angiotensin II-induced vascular fibrosis

Angiotensin II (Ang II) plays a pivotal role in vascular fibrosis, which leads to serious complications in hypertension and diabetes. However, the underlying signaling mechanisms are largely unclear. In hypertensive patients, we found that arteriosclerosis was associated with the activation of Smad2/3. This observation was further investigated in vitro by stimulating mouse primary aorta vascular smooth muscle cells (VSMCs) with Ang II. There were several novel findings. First, Ang II was able to activate an early Smad signaling pathway directly at 15 to 30 minutes. This was extracellular signal-regulated kinase 1/2 (ERK1/2) mitogen-activated protein kinase (MAPK) dependent but transforming growth factor-beta (TGF-beta) independent because Ang II-induced Smad signaling was blocked by addition of ERK1/2 inhibitor and by dominant-negative (DN) ERK1/2 but not by DN-TGF-beta receptor II (TbetaRII) or conditional deletion of TbetaRII. Second, Ang II was also able to activate the late Smad2/3 signaling pathway at 24 hours, which was TGF-beta dependent because it was blocked by the anti-TGF-beta antibody and DN-TbetaRII. Finally, activation of Smad3 but not Smad2 was a key and necessary mechanism of Ang II-induced vascular fibrosis because Ang II induced Smad3/4 promoter activities and collagen matrix expression was abolished in VSMCs null for Smad3 but not Smad2. Thus, we concluded that Ang II induces vascular fibrosis via both TGF-beta-dependent and ERK1/2 MAPK-dependent Smad signaling pathways. Activation of Smad3 but not Smad2 is a key mechanism by which Ang II mediates arteriosclerosis.

Origin of interstitial fibroblasts in an accelerated model of angiotensin II-induced renal fibrosis

To determine whether previous renal injury accelerates the progression of glomerulosclerosis and interstitial fibrosis, we examined the effect of treating rats with angiotensin II after Habu venom injury. After initiating disease, we examined the origin of interstitial myofibroblasts by locating alpha-smooth muscle actin (alpha-SMA)-positive and Na+,K+-ATPase-positive cells relative to interstitial space, tubular epithelial cells, the tubular basement membrane (TBM), and vascular structures. Tubular epithelial-mesenchymal transition was also assessed by examining TBM integrity and by using Texas Red (TR)-dextran in intravital tracking experiments. The staining of alpha-SMA-positive myofibroblasts dramatically increased in peritubular interstitial spaces 48 hours after Habu venom plus angiotensin II, particularly in and around perivascular and periglomerular regions, while tubular epithelial cells were alpha-SMA-negative. Na+,K+-ATPase-positive and TR-dextran-labeled cells were restricted to the tubular epithelium and excluded from the interstitium. By 7 and 14 days, expanded interstitial space contained only alpha-SMA-positive myofibroblasts without TR-dextran endocytic particles. Epithelium of atrophic tubules containing TR-dextran remained confined by surrounding interstitium and myofibroblasts. These studies indicate that early expansion of alpha-SMA-positive cells in the interstitium and loss of tubular area occur via encroachment of interstitial myofibroblasts from perivascular into atrophic tubular spaces rather than via epithelial-mesenchymal transition and migration of tubular cells through the TBM into the interstitium.

Activation of Gαq-Coupled Signaling Pathways in Glomerular Podocytes Promotes Renal Injury

The glomerular podocyte plays a key role in maintaining the integrity of the glomerular filtration barrier. This function may be regulated by activation of cell surface G protein-coupled receptors (GPCR). Studies suggest that podocytes express GPCR that are implicated in the pathogenesis of glomerular diseases. Common to these GPCR systems is activation of phospholipase C through the Gq alpha-subunit (Galpha q). For investigating the role of Galpha q-coupled signaling pathways in promoting renal injury in podocytes, a constitutively active Galpha q subunit (Galpha qQ > L) was expressed in glomerular podocytes using the mouse nephrin promoter. Transgenic (TG) mice demonstrated albuminuria as well as a decrease in both kidney mass and nephron number. By light microscopy, a portion of the TG mice had glomerular abnormalities, including focal to diffuse hypercellularity and segmental sclerosis. Consistent with injury-promoting effects of Galpha qQ > L, there was a significant reduction in podocalyxin mRNA as well as nephrin mRNA and protein levels in glomeruli from TG mice compared with non-TG controls. Expression of the transgene also seemed to increase susceptibility to glomerular injury, because treatment with puromycin aminonucleoside enhanced proteinuria in TG mice compared with non-TG littermate controls (4.2 +/- 1.0 [TG] versus 1.6 +/- 0.3 [non-TG] mg/24 h; P = 0.0161). Thus, activation of Galpha q in glomerular podocytes caused alterations in glomerular histomorphology, albuminuria, decreased nephron mass, and reduced glomerular expression of both nephrin and podocalyxin as well as enhanced susceptibility to glomerular damage induced by puromycin aminonucleoside. It is speculated that Galpha q-coupled signaling cascades may be important effector pathways mediating renal injury.

Estrogen upregulates renal angiotensin II AT1 and AT2 receptors in the rat

We studied renal AT1 and AT2 receptors in male, female, ovariectomized and ovariectomized-estrogen-treated Wistar-Hanover and Wistar-Kyoto rats. AT1 receptors and AT1A receptor mRNA predominated, with no significant differences between males and females. AT2 receptor expression was restricted in female rats to the capsule, the transition zone between outer and inner medulla, the endothelium lining the papilla, and arcuate arteries and veins. There were no AT2 receptors in male rats, while male mice express substantial numbers of estrogen-dependent AT2 receptors. Arcuate arteries and veins expressed AT1B mRNA in males and females, and AT2 mRNA in females only. AT1 receptor and AT2 receptor expression were estrogen-dependent, with increases in AT1 and AT2 receptor expression after estrogen treatment in ovariectomized rats. Estrogen treatment increased prostaglandin E2 (PGE2) and cGMP concentrations in the renal medulla, and eNOS expression in cortical arteries. In rodents, expression of renal Angiotensin II receptor types is estrogen-dependent, with significant species, strain and area differences. Our results support an important role for AT2 receptors in the regulation of renal function and in the protective effects of estrogen in the kidney.

Oxidative stress in hypertension and chronic kidney disease: role of angiotensin II

Angiotensin II, via the type 1 (AT1) receptor, stimulates oxidative stress. The vasculature, interstitium, juxtaglomerular apparatus, and the distal nephron in the kidney express nicotinamide adenine dinucleotide phosphate (NADPH) oxidase that generates superoxide anion, which is an important component of angiotensin II-induced oxidative stress. The angiotensinogen gene is stimulated by NF-kappaB activation, which is sensitive to the redox ratio, providing a positive feedback loop that can upregulate angiotensin II production. Oxidative stress can accompany hypertension in many models, including the spontaneously hypertensive rat (SHR), angiotensin II-infused rats, renovascular hypertension, and the deoxycorticosterone acetate (DOCA) salt model of hypertension. AT1 receptor antagonists can abrogate the effects of angiotensin II on oxidative stress, thus providing an important mechanistic insight onto the renal protective effects of these agents in conditions associated with angiotensin II excess.

Local induction of angiotensin-converting enzyme in the kidney as a mechanism of progressive renal diseases

Angiotensin Converting Enzyme (ACE) or Kininase II has a pivotal role determining the local activity of the renin angiotensin and kallikrein kinin systems. Angiotensin II (Ang II), a main hormone of the renin system, has a well established participation as a renal injury agent in models of renal disease with tubulointerstitial fibrosis. Although, since its discovery, ACE has been known to convert Ang I to Ang II, and to inactivate bradykinin (BK), only recently has been emerged evidence for a role of BK with renal protective and antifibrotic effects opposing Ang II. Pertinent to the tubulointerstitial injury, where infiltration and proliferation of macrophages and fibroblast occur, ACE also regulates the levels of the natural hemoregulatory peptide, N-acetyl-seryl-aspartyl-lysyl-proline (Ac-SDKP). Owing the importance of tissue ACE, its distribution was studied in several models of renal injury. The results showed increased ACE in proximal tubules and ACE induction in the cell infiltrated tubulointerstitium (macrophages and myofibroblasts) of injured kidneys from hypokalemic, Goldblatt hypertensive, Ang II and phenylefrine infused rats, and in both human diabetic and membranous nephropathies. ACE, in addition to Ang II generation, may play a pathogenic role through the hydrolysis of BK and Ac-SDKP. Thus, local increase in ACE can be a novel mechanism involved in tubulointerstitial renal injury, providing, from an anatomical ground, a pathological basis for the putative deleterious effect of ACE in the diseased kidneys, and the beneficial effect of ACE inhibition.

Renal angiotensin II up-regulation and myofibroblast activation in human membranous nephropathy

BACKGROUND

The molecular mechanisms of renal injury and fibrosis in proteinuric nephropathies are not completely elucidated but the renin-angiotensin system (RAS) is involved. Idiopathic membranous nephropathy (MN), a proteinuric disease, may progress to renal failure. Our aim was to investigate the localization of RAS components in MN and their correlation with profibrotic parameters and renal injury.

METHODS

Renal biopsies from 20 patients with MN (11 with progressive disease) were studied for the expression of RAS components [angiotensin-converting enzyme (ACE) and angiotensin II (Ang II)] by immunohistochemistry. Transforming growth factor-beta (TGF-beta) and platelet-derived growth factor (PDGF)-BB were studied by by in situ hybridization, and myofibroblast transdifferentiation by alpha-smooth muscle actin (alpha-SMA) staining.

RESULTS

ACE immunostaining was elevated in tubular cells and appeared in interstitial cells colocalized in alpha-actin-positive cells in progressive disease. Elevated levels of Ang II were observed in tubules and infiltrating interstitial cells. TGF-beta and PDGF mRNAs were up-regulated mainly in cortical tubular epithelial cells in progressive disease (P < 0.01) and correlated with the myofibroblast transdifferentiation (r = 0.8, P < 0.01 for TGF-beta; r = 0.6, P < 0.01 for PDGF). Moreover, in serial sections of progressive cases, the ACE and Ang II over-expression was associated with the tubular expression of these pro-fibrogenic factors, and with the interstitial infiltration and myofibroblast activation.

CONCLUSION

Intrarenal RAS is selectively activated in progressive MN. De novo expression of ACE at sites of tubulointerstitial injury suggests that the in situ Ang II generation could participate in tubular TGF-beta up-regulation, epithelial-myofibroblast transdifferentiation, and disease progression. These results suggest a novel role of Ang II in human tubulointerstitial injury.

Renin-angiotensin system activation and interstitial inflammation in human diabetic nephropathy

BACKGROUND

The molecular mechanisms of renal injury in diabetic nephropathy (DN) are not completely understood, although inflammatory cells play a key role. The renin-angiotensin system (RAS) is involved in kidney damage; however, few studies have examined the localization of RAS components in human DN. Our aim was to investigate in renal biopsies the expression of RAS and their correlation with proinflammatory parameters and renal injury.

METHODS

The biopsies from 10 patients with type 2 diabetes mellitus and overt nephropathy were studied for the expression of RAS components by immunohistochemistry (IH). In addition, by Southwestern histochemistry we studied the in situ detection of the activated nuclear factor kappa B (NFkappaB), and by IH and/or in situ hybridization (ISH), the expression of monocyte chemoattractant protein-1 (MCP-1) and regulated upon activation, normal T cell expressed and secreted (RANTES), whose genes are regulated by NFkappaB.

RESULTS

Angiotensin-converting enzyme (ACE) immunostaining was elevated in tubular cells and appeared in interstitial cells. Elevated levels of angiotensin II (Ang II) immunostaining were observed in tubular and infiltrating interstitial cells. There was also a down-regulation of AT1 and up-regulation of AT2 receptors. An activation of NFkappaB and a marked up-regulation of NFkappaB-dependent chemokines mainly in tubular cells was observed. Elevated levels of NFkappaB, chemokines, and Ang II in tubules were correlated with proteinuria and interstitial cell infiltration.

CONCLUSIONS

Our results show that in human DN, RAS components are modified in renal compartments, showing elevated local Ang II production, activation of tubular cells, and induction of proinflammatory parameters. These data suggest that Ang II contributes to the renal inflammatory process, and may explain the molecular mechanisms of the beneficial effect of RAS blockade.

Prolonged infusion of angiotensin II into normal rats induces stellate cell activation and proinflammatory events in liver

Recent evidence indicates that angiotensin II (ANG II) plays an important role in liver fibrogenesis. However, the underlying mechanisms are largely unknown. In advanced chronic liver diseases, circulating levels of ANG II are frequently elevated. We investigated the hepatic effects of prolonged systemic infusion of ANG II in normal rats. Saline or ANG II at subpressor and pressor doses (15 and 50 ng.kg-1.min-1, respectively) were infused to normal rats for 4 wk through a subcutaneous osmotic pump. Infusion of ANG II resulted in liver injury, as assessed by elevated serum liver enzymes. Livers from ANG II-perfused rats showed activation of JNK and ERK as well as increased NF-kappaB and activating protein-1 DNA-binding activity. Moreover, ANG II perfusion induced oxidative stress, increased concentration of proinflammatory cytokines, and upregulated the inflammatory proteins inducible nitric oxide synthase and cyclooxygenase-2. Histological examination of the livers from ANG II-infused rats showed mild portal inflammation as well as thickening and thrombosis of small hepatic vessels. ANG II-treated livers showed accumulation of CD43-positive inflammatory cells and activated hepatic stellate cells (HSCs) at the pericentral areas. A slight increase in collagen synthesis was observed, as assessed by Sirius red staining and hepatic hydroxyproline. All of these effects were observed when ANG II was perfused at subpressor and pressor doses. ANG II also accelerated the activation of primary cultured rat HSCs. In conclusion, increased systemic ANG II can induce liver injury by promoting proinflammatory events and vascular damage. ANG II-induced hepatic effects are not dependent on increase in arterial pressure.

Imbalance of T-cell subsets in angiotensin II-infused hypertensive rats with kidney injury

Blockade of angiotensin (Ang) II is efficient in various renal diseases. Although interest has focused on the hemodynamic changes and reduction of proteinuria, recent studies emphasize the nonhemodynamic effects of Ang II on kidney injury. The aim of this study was to clarify the mechanisms of Ang II on the immune system that alter the balance of helper T-cell (Th) subsets. We used a continuous, Ang II infusion model of rats that develop hypertension, proteinuria, and tubulointerstitial damage, including de novo expression of alpha-smooth muscle actin and loss of endothelial cells. We isolated T cells from the spleen and measured cytokine levels by ELISA systems. Ang II-infused rats showed an increase in the Th1 cytokine gamma-interferon and a decrease in the Th2 cytokine interleukin-4. The same change in cytokine mRNA expression in the spleen and kidney was confirmed by quantitative polymerase chain reaction analysis. Our ELISPOT assay showed an increase in the number of gamma-interferon-secreting T cells by Ang II. To investigate whether these changes were specific effects of Ang II, we treated the model rats with the Ang II receptor blocker (ARB) olmesartan or the nonspecific vessel dilator hydralazine. Administration of the ARB ameliorated disease manifestations and the imbalance in Th subsets, whereas hydralazine did not, despite comparable effects on blood pressure. These results demonstrate a direct role of Ang II in the modification of Th balance. The imbalance of Th subsets was associated with hypertensive kidney injury induced by Ang II. Some of the beneficial effects of ARBs might be explained by their immunomodulatory reactions.

Molecular mechanisms of angiotensin II-induced vascular injury

Blockers of the renin-angiotensin system are used in the treatment of several cardiovascular and renal diseases, including hypertension, atherosclerosis, and cardiac failure. Angiotensin II plays an essential role in the pathogenesis of these diseases through the regulation of cell growth, inflammation, and fibrosis. There are two main angiotensin II receptors, AT(1) and AT(2). The AT(1) receptor is responsible for most of the pathophysiologic actions of angiotensin II, including cell proliferation, production of growth factors and cytokines, and fibrosis. AT(2) causes antiproliferation and counteracts the cell growth induced by AT(1) activation. We review the mechanisms whereby AT(1) and AT(2) receptors elicit their respective actions. We discuss the current understanding of the signaling mechanisms involved in angiotensin II-induced vascular damage, describing the mediators (growth factors and cytokines) and intracellular signals (activation of protein kinases, transcription factors, and redox pathways) implicated in these processes, with special emphasis on novel information and open questions.

Estrogen upregulates renal angiotensin II AT(2) receptors

AT(2) receptors may act in opposition to and in balance with AT(1) receptors, their stimulation having beneficial effects. We found renal AT(2) receptor expression in female mice higher than in male mice. We asked the question of whether such expression might be estrogen dependent. In male, female, ovariectomized, and estrogen-treated ovariectomized mice, we studied renal AT(1) and AT(2) receptors by immunocytochemistry and autoradiography, AT(2) receptor mRNA by RT-PCR, and cAMP, cGMP, and PGE(2) by RIA. AT(1) receptors predominated. AT(2) receptors were present in glomeruli, medullary rays, and inner medulla, and in female kidney capsule. AT(1) and AT(2) receptors colocalized in glomeruli. Female mice expressed fewer glomerular AT(1) receptors. Ovariectomy decreased AT(1) receptors in medullary rays and capsular AT(2) receptors. Estrogen administration normalized AT(1) receptors in medullary rays and increased AT(2) receptors predominantly in capsule and inner medulla, and also in glomeruli, medullary rays, and inner stripe of outer medulla. In medullas of estrogen-treated ovariectomized mice there was higher AT(2) receptor mRNA, decreased cGMP, and increased PGE(2) content. We propose that the protective effects of estrogen may be partially mediated through enhancement of AT(2) receptor stimulation.

Interstitial inflammation, sodium retention, and the pathogenesis of nephrotic edema: a unifying hypothesis

BACKGROUND

The pathophysiology of edema in the nephrotic syndrome is controversial. Some investigators believe that sodium retention may result from a primary renal defect that causes an "overfilled" blood volume. In contrast, other authors believe that fluid escapes the vascular compartment due a low oncotic pressure, and sodium retention is a compensatory physiological response to an "underfilled" blood volume. The patients that best fit the "underfilled" hypothesis are children with minimal-change nephrotic syndrome (MCNS).

METHODS

We analyzed critically the available evidence for and against each proposed pathogenic mechanism in the light of recent evidence indicating that the inflammatory infiltrate may play a role in primary renal sodium retention.

RESULTS

Inflammatory infiltrate in the kidney is a constant characteristic in nephrotic syndrome associated with primary sodium retention and it is absent in most cases of MCNS in children

CONCLUSIONS

We propose that primary sodium retention in the nephrotic syndrome depends on the existence and the intensity of renal inflammatory infiltrate, conspicuously absent in most cases of MCNS in children and present in other conditions associated with massive proteinuria. The tubulointerstitial inflammatory infiltrate is associated with increased vasoconstrictive mediators that result in increased tubular sodium reabsorption and with glomerular hemodynamic changes that reduce filtered sodium load.

Renal cortical vasoconstriction contributes to development of salt-sensitive hypertension after angiotensin II exposure

Rats that are administered angiotensin II (AngII) for 2 wk develop persistent salt-sensitive hypertension, which can be prevented by the immunosuppressor mycophenolate mofetil (MMF) given during the AngII infusion. This study examined the contribution of glomerular hemodynamics (GFR dynamics) in the post-AngII hypertensive response to a high-salt diet (HSD) and the effect of MMF treatment. During AngII administration, rats developed severe hypertension (systolic BP [SBP], 185 +/- 3.9 mmHg), proteinuria, afferent and efferent vasoconstriction, and glomerular hypertension. Rats that received AngII+MMF showed similar responses to AngII; however, they developed lower proteinuria (P < 0.05). At 2 wk, AngII was withdrawn and SBP returned toward normal. Rats were then placed on an HSD (4% NaCl), resulting in a progressive increase in SBP (155 +/- 8.2 mmHg at week 1 and 163 +/- 4.5 mmHg at week 5). GFR dynamic alterations persisted after AngII was stopped, i.e., afferent and efferent vasoconstriction, decreased glomerular plasma flow and single-nephron GFR, and lower ultrafiltration coefficient. These changes correlated with the thickening of the afferent arteriole and with focal tubulointerstitial injury. In the AngII+MMF group, SBP remained unchanged throughout the HSD period (146 +/- 2.3 mmHg at week 1 and 148 +/- 4.4 mmHg at week 5) in association with less afferent arteriolar thickening and tubulointerstitial injury. Single-nephron GFR, glomerular plasma flow, efferent resistance, and ultrafiltration coefficient returned to normal with a significant reduction in afferent resistance. These results suggest a critical role of cortical vasoconstriction in salt-sensitive hypertension. The MMF-induced prevention of these changes suggests that immune mechanisms are involved in the vasoconstrictive response.

Hypokalemia induces renal injury and alterations in vasoactive mediators that favor salt sensitivity

We investigated the hypothesis that hypokalemia might induce renal injury via a mechanism that involves subtle renal injury and alterations in local vasoactive mediators that would favor sodium retention. To test this hypothesis, we conducted studies in rats with diet-induced K+ deficiency. We also determined whether rats with hypokalemic nephropathy show salt sensitivity. Twelve weeks of hypokalemia resulted in a decrease in creatinine clearance, tubulointerstitial injury with macrophage infiltration, interstitial collagen type III deposition, and an increase in osteopontin expression (a tubular marker of injury). The renal injury was greatest in the outer medulla with radiation into the cortex, suggestive of an ischemic etiology. Consistent with this hypothesis, we found an increased uptake of a hypoxia marker, pimonidazole, in the cortex. The intrarenal injury was associated with increased cortical angiontensin-converting enzyme (ACE) expression and continued cortical angiotensin II generation despite systemic suppression of the renin-angiotensin system, an increase in renal endothelin-1, a decrease in renal kallikrein, and a decrease in urinary nitrite/nitrates and prostaglandin E(2) excretion. At 12 wk, hypokalemic rats were placed on a normal-K+ diet with either high (4%)- or low (0.01%)-NaCl content. Despite correction of hypokalemia and normalization of renal function, previously hypokalemic rats showed an elevated blood pressure in response to a high-salt diet compared with normokalemic controls. Hypokalemia is associated with alterations in vasoactive mediators that favor intrarenal vasoconstriction and an ischemic pattern of renal injury. These alterations may predispose the animals to salt-sensitive hypertension that manifests despite normalization of the serum K+.

Mycophenolate mofetil prevents salt-sensitive hypertension resulting from angiotensin II exposure

BACKGROUND

Interstitial mononuclear cell infiltration is a feature of experimental models of salt-sensitive hypertension (SSHTN). Since several products of these cells are capable of modifying local vascular reactivity and sodium reabsorption, we investigated whether mycophenolate mofetil (MMF), a drug known to inhibit infiltration and proliferation of immune cells, would modify the SSHTN induced by angiotensin II (Ang II) infusion.

METHODS

Sprague-Dawley rats received Ang II for two weeks using subcutaneous minipumps. A high-sodium (4% NaCl) diet was started on the third week and was maintained until the eighth week. MMF (30 mg/kg, N = 15), an immunosuppressive drug, or vehicle (N = 15) was given daily by gastric gavage during the initial three weeks. Sham-operated rats (N = 9) were used as controls. Body weight, blood pressure (tail-cuff plethysmography), and serum creatinine were determined weekly. Urinary malondialdehyde (MDA) excretion, renal histology, and immunohistology, including the presence of Ang II and superoxide-producing cells, were analyzed at the end of Ang II infusion and at eight weeks.

RESULTS

MMF treatment did not modify hypertension induced during exogenous Ang II infusion, but prevented the subsequent SSHTN. Tubulointerstitial injury resulting from Ang II infusion was significantly reduced by MMF treatment, as were proliferative activity, T-cell infiltration and activation (interleukin-2 receptor expression), superoxide-producing cells, and urinary MDA excretion. Ang II-producing cells were present in the renal tubulointerstitium of rats with SSHTN (60 +/- 30 Ang II-positive cells/mm(2) at 8 weeks) and were reduced by two thirds in the MMF-treated group. Forty percent of lymphocytes infiltrating the tubulointerstitium stained positive for Ang II. The expression of Ang II receptors in the kidney was unmodified.

CONCLUSIONS

SSHTN resulting from Ang II infusion is associated with infiltration and activation of immune cells that produce Ang II. MMF treatment reduces these features and prevents the development of SSHTN.