Angiotensin type 2 receptor actions contribute to angiotensin type 1 receptor blocker effects on kidney fibrosis

Effect of angiotensin type 2 receptor over-expression on the rat mesangial cell fibrotic phenotype: effect of gender

BACKGROUND AND AIM

The protective role of angiotensin type 2 receptors (AT2-Rs) is still controversial. As AT2-Rs are minimally expressed in adult tissues the aim of the current study was to over-express AT2-Rs in rat mesangial cells in order to ascertain their potential role in modulating renal scarring.

METHODS

Male and female mesangial cells were transiently transfected with AT2-R or control vector then 'injured' with macrophage-conditioned medium (MCM). Culture supernatants and extracted RNA were analysed for evidence of an anti-fibrotic phenotype.

RESULTS

Supernatant fibronectin levels in female mesangial cells treated with MCM were reduced in AT2-R transfected cells (p < 0.001) compared to controls. AT2-R transfected male cells showed a trend towards lower constitutive fibronectin levels. There was no effect of AT2-R transfection on TGF-β or TNF-α secretion; however, IL-1β levels were reduced in male cells treated with MCM. RT-PCR demonstrated that constitutive kallikrein mRNA levels were suppressed in both male and female AT2-R transfected cells. Bradykinin receptors (BkB2-R and BkB1-R) were unaffected in female cells although the BkB1-R was upregulated in male cells treated with MCM.

CONCLUSION

This data provides a case for AT2 receptors playing a protective role in rat mesangial cells independent of the effects of blood pressure control.

Obesity-related glomerulopathy and podocyte injury: a mini review

Obesity-related glomerulopathy (ORG) is morphologically defined as focal segmental glomerulosclerosis and glomerulomegaly. Podocyte hypertrophy and reduced density are related to proteinuria which in a portion of patients is in the nephrotic range and evolvs towards renal failure. This article reviews the pathogenetic mechanisms of podocyte injury or dysfunction and lists new possible antiproteinuric strategies based on pharmaceutical targeting of the reported pathogenetic mechanisms. The pathogenetic mechnisms discussed include: renin angiotensin system, plasminogen activation inhibitor-1 (PAI-1), lipid metabolism, adiponectin, macrophages and proinflammatory cytokines, oxidative stress. The proposed antiproteinuric strategies include: AT2 receptor blockers; adipokine complement C19 TNF-related protein-1 blocker; selective PAI-1 inhibitor; farnesoid x receptor activation; increase of circulating adiponectin; selective antiinflammatory drugs; more potent antioxidants (Heme oxigenase, NOX4 inhibitors). However, because ORG is a rare disease, the need for a long term pharmaceutical approach in obese proteinuric patients should be carefully evaluated and limited to the cases with progressive loss of renal function.

Obesity-related glomerulopathy and podocyte injury: a mini review

Obesity-related glomerulopathy (ORG) is morphologically defined as focal segmental glomerulosclerosis and glomerulomegaly. Podocyte hypertrophy and reduced density are related to proteinuria which in a portion of patients is in the nephrotic range and evolvs towards renal failure. This article reviews the pathogenetic mechanisms of podocyte injury or dysfunction and lists new possible antiproteinuric strategies based on pharmaceutical targeting of the reported pathogenetic mechanisms. The pathogenetic mechnisms discussed include: renin angiotensin system, plasminogen activation inhibitor-1 (PAI-1), lipid metabolism, adiponectin, macrophages and proinflammatory cytokines, oxidative stress. The proposed antiproteinuric strategies include: AT2 receptor blockers; adipokine complement C19 TNF-related protein-1 blocker; selective PAI-1 inhibitor; farnesoid x receptor activation; increase of circulating adiponectin; selective antiinflammatory drugs; more potent antioxidants (Heme oxigenase, NOX4 inhibitors). However, because ORG is a rare disease, the need for a long term pharmaceutical approach in obese proteinuric patients should be carefully evaluated and limited to the cases with progressive loss of renal function.

Activation of renal angiotensin type 1 receptor contributes to the pathogenesis of progressive renal injury in a rat model of chronic cardiorenal syndrome

Although chronic cardiac dysfunction is known to progressively exacerbate renal injury, a condition known as type 2 cardiorenal syndrome (CRS), the mechanism responsible is largely unknown. The present study was undertaken to clarify the mechanism of renal injury in rats with both unilateral nephrectomy (NX) and surgically induced myocardial infarction (MI), corresponding to a model of type 2 CRS. Compared with a control group, rats with both MI and NX (MI+NX) exhibited progressive proteinuria during the experimental period (34 wk after MI surgery), whereas proteinuria was not observed in rats with MI alone and was moderate in rats with NX alone. The proteinuria in rats with MI+NX was associated with renal lesions such as glomerulosclerosis and infiltration of mononuclear cells and upregulation of the renal proinflammatory and -fibrotic cytokine and angiotensin II type 1a receptor (AT1aR) genes. In contrast, plasma renin activity was lowered in rats with MI+NX. Immunohistochemistry revealed that the increased AT1R protein was present mainly in renal interstitial mononuclear cells. Olmesartan medoxomil, an AT1R blocker, markedly reduced the proteinuria and infiltration of mononuclear cells, whereas spironolactone, a mineralocorticoid receptor blocker, did not. The present findings demonstrate the pathogenetic role of renal interstitial AT1R signaling in a model of type 2 CRS, providing evidence that AT1R blockade can be a useful therapeutic option for this syndrome.

Elevation of the antifibrotic peptide N-acetyl-seryl-aspartyl-lysyl-proline: a blood pressure-independent beneficial effect of angiotensin I-converting enzyme inhibitors

Blockade of the renin-angiotensin system (RAS) is well recognized as an essential therapy in hypertensive, heart, and kidney diseases. There are several classes of drugs that block the RAS; these drugs are known to exhibit antifibrotic action. An analysis of the molecular mechanisms of action for these drugs can reveal potential differences in their antifibrotic roles. In this review, we discuss the antifibrotic action of RAS blockade with an emphasis on the potential importance of angiotensin I-converting enzyme (ACE) inhibition associated with the antifibrotic peptide N-acetyl-seryl-aspartyl-lysyl-proline (AcSDKP).

Mesenchymal stem cell injection ameliorates chronic renal failure in a rat model

CKD (chronic kidney disease) has become a public health problem. The therapeutic approaches have been able to reduce proteinuria, but have not been successful in limiting disease progression. In this setting, cell therapies associated with regenerative effects are attracting increasing interest. We evaluated the effect of MSC (mesenchymal stem cells) on the progression of CKD and the expression of molecular biomarkers associated with regenerative effects. Adult male Sprague-Dawley rats subjected to 5/6 NPX (nephrectomy) received a single intravenous infusion of 0.5×106 MSC or culture medium. A sham group subjected to the same injection was used as the control. Rats were killed 5 weeks after MSC infusion. Dye tracking of MSC was followed by immunofluorescence analysis. Kidney function was evaluated using plasma creatinine. Structural damage was evaluated by H&E (haematoxylin and eosin) staining, ED-1 abundance (macrophages) and interstitial α-SMA (α-smooth muscle actin). Repairing processes were evaluated by functional and structural analyses and angiogenic/epitheliogenic protein expression. MSC could be detected in kidney tissues from NPX animals treated with intravenous cell infusion. This group presented a marked reduction in plasma creatinine levels and damage markers ED-1 and α-SMA (P<0.05). In addition, treated rats exhibited a significant induction in epitheliogenic [Pax-2, bFGF (basic fibroblast growth factor) and BMP-7 (bone morphogenetic protein-7)] and angiogenic [VEGF (vascular endothelial growth factor) and Tie-2] proteins. The expression of these biomarkers of regeneration was significantly related to the increase in renal function. Many aspects of the cell therapy in CKD remain to be investigated in more detail: for example, its safety, low cost and the possible need for repeated cell injections over time. Beyond the undeniable importance of these issues, what still needs to be clarified is whether MSC administration has a real effect on the treatment of this pathology. It is precisely to this point that the present study aims to contribute.

LOX-1 and angiotensin receptors, and their interplay

The renin-angiotensin system (RAS) plays an important role in regulating blood pressure, water-salt balance and the pathogenesis of cardiovascular diseases. Angiotensin II (Ang II) is the physiologically active mediator and mediates the main pathophysiological actions in RAS. Ang II exerts the effects by activating its receptors, primarily type 1 (AT1R) and type 2 (AT2R). Most of the known pathophysiological effects of Ang II are mediated by AT1R activation. The precise physiological function of AT2R is still not clear. Generally, AT2R is considered to oppose the effects of AT1R. Lectin-like oxidized low-density lipoprotein scavenger receptor-1 (LOX-1) is one of the major receptors responsible for binding, internalizing and degrading ox-LDL. The activation of LOX-1 has been known to be related to many pathophysiological events, including endothelial dysfunction and injury, fibroblast growth, and vascular smooth muscle cell hypertrophy. Many of these alterations are present in atherosclerosis, hypertension, and myocardial ischemia and remodeling. A growing body of evidence suggests the existence of a cross-talk between LOX-1 and Ang II receptors. Their interplays are embodied in the reciprocal regulation of their expression and activity. Their interplays are involved in a series of signals. Recent studies suggests that reactive oxygen species (ROS), nitric oxide (NO), protein kinase C (PKC) and mitogen activated protein kinases (MAPKs) are important signals responsible for their cross-talk. This paper reviews these aspects of dyslipidemia and RAS activation.

mTORC1 activation in podocytes is a critical step in the development of diabetic nephropathy in mice

Diabetic nephropathy (DN) is among the most lethal complications that occur in type 1 and type 2 diabetics. Podocyte dysfunction is postulated to be a critical event associated with proteinuria and glomerulosclerosis in glomerular diseases including DN. However, molecular mechanisms of podocyte dysfunction in the development of DN are not well understood. Here we have shown that activity of mTOR complex 1 (mTORC1), a kinase that senses nutrient availability, was enhanced in the podocytes of diabetic animals. Further, podocyte-specific mTORC1 activation induced by ablation of an upstream negative regulator (PcKOTsc1) recapitulated many DN features, including podocyte loss, glomerular basement membrane thickening, mesangial expansion, and proteinuria in nondiabetic young and adult mice. Abnormal mTORC1 activation caused mislocalization of slit diaphragm proteins and induced an epithelial-mesenchymal transition-like phenotypic switch with enhanced ER stress in podocytes. Conversely, reduction of ER stress with a chemical chaperone significantly protected against both the podocyte phenotypic switch and podocyte loss in PcKOTsc1 mice. Finally, genetic reduction of podocyte-specific mTORC1 in diabetic animals suppressed the development of DN. These results indicate that mTORC1 activation in podocytes is a critical event in inducing DN and suggest that reduction of podocyte mTORC1 activity is a potential therapeutic strategy to prevent DN.

Changes in angiotensin receptors expression play a pivotal role in the renal damage observed in spontaneously hypertensive rats

The renal renin-angiotensin system plays a central role in the development of hypertension. The aim of this work was to verify the expression of angiotensin II receptors AT1R and AT2R in the microsomal fraction of renal cortex and correlate this with the development of hypertension and renal damage in spontaneously hypertensive rats (SHR) using Wistar-Kyoto rats (WKY) as controls. AT1R expression increased (126%) and AT2R expression decreased (66%) in 4-wk-old SHR; AT2 expression decreased in 14-wk-old SHR (61%) compared with respective age-matched WKY. These modifications were correlated to the increase in protein kinase C activity and decrease in protein kinase A activity. Four-week-old SHR showed large accumulations of macrophages in kidney glomerulus and the tubulointerstitial area, dense cortical collagen deposition, and arterial proliferative changes in the walls of arterioles and medium-sized vessels. Similar modifications were also observed in 14-wk-old SHR. Four-week-old SHR treated with losartan (30 mg·kg−1·day−1) or hydralazine (15 and 30 mg·kg−1·day−1) by gavage for 10 wk did not develop hypertension. The decrease in AT2R expression and renal damage observed in SHR remained even after treatment with hydralazine. On the other hand, losartan treatment prevented the modifications observed in 14-wk-old SHR, indicating that renal injuries are caused specifically by AT1 rather than an increase in blood pressure. Our results indicate that the imbalance in AT1R and AT2R expression is associated with an inflammatory process that contributes to renal injury in adult SHR and to the development of hypertension.

Angiotensin AT₂ receptor stimulation inhibits early renal inflammation in renovascular hypertension

Angiotensin II type 2 receptor (AT₂R) counteracts most effects of angiotensin II type 1 receptor (AT(1)R). We hypothesized that direct AT₂R stimulation reduces renal production of the inflammatory cytokines tumor necrosis factor-α (TNF-α), interleukin-6 (IL-6), and transforming growth factor-β1 (TGF-β1) and enhances the production of nitric oxide (NO) and cyclic guanosine 3',5'-monophosphate (cGMP) in the clipped kidney of 2-kidney, 1-clip (2K1C) hypertension rat model. We used Sprague-Dawley rats to evaluate changes in renal interstitial fluid recovery levels of TNF-α, IL-6, NO, and cGMP; renal expression of AT₁R, AT₂R, TGF-β1, TNF-α, and IL-6 in sham and 2K1C rats treated for 4 days with vehicle, AT₂R agonist compound 21 (C21), or AT₂R antagonist PD123319 (PD), alone and combined (n=6, each group). Systolic blood pressure increased significantly in 2K1C and was not influenced by any treatment. Clipped kidneys showed significant increases in renal expression of AT₁R, AT₂R, TNF-α, IL-6, TGF-β1 and decreases in NO and cGMP levels. These factors were not influenced by PD treatment. In contrast, C21 caused significant decrease in renal TNF-α, IL-6, TGF-β1 and an increase in NO and cGMP levels. Combined C21 and PD treatment partially reversed the observed C21 effects. Compared to sham, there were no significant changes in TNF-α, IL-6, TGF-β1, NO, or cGMP in the nonclipped kidneys of 2K1C animals. We conclude that direct AT₂R stimulation reduces early renal inflammatory responses and improves production of NO and cGMP in renovascular hypertension independent of blood pressure reduction.

Clinical effectiveness of telmisartan alone or in combination therapy for controlling blood pressure and vascular risk in the elderly

Elderly patients (age ≥ 65 years) with hypertension are at high risk for vascular complications, especially when diabetes is present. Antihypertensive drugs that inhibit the renin-angiotensin system have been shown to be effective for controlling blood pressure in adult and elderly patients. Importantly, renin-angiotensin system inhibitors were shown to have benefits beyond their classic cardioprotective and vasculoprotective effects, including reducing the risk of new-onset diabetes and associated cardiovascular effects. The discovery that the renin-angiotensin system inhibitor and angiotensin II type 1 (AT(1)) receptor blocker (ARB), telmisartan, can selectively activate the peroxisome proliferator-activated receptor-γ (PPARγ, an established antidiabetic drug target) provides the unique opportunity to prevent and treat cardiovascular complications in high-risk elderly patients with hypertension and new-onset diabetes. Two large clinical trials, ONTARGET (Ongoing Telmisartan Alone in combination with Ramipril Global Endpoint Trial) and TRANSCEND (Telmisartan Randomized AssessmeNt Study in ACE-I iNtolerant subjects with cardiovascular disease) have assessed the cardioprotective and antidiabetic effects of telmisartan. The collective data suggest that telmisartan is a promising drug for controlling hypertension and reducing vascular risk in high-risk elderly patients with new-onset diabetes.

Acute hyperglycemia rapidly stimulates VEGF mRNA translation in the kidney. Role of angiotensin type 2 receptor (AT2)

Angiotensin II (Ang II) and vascular endothelial growth factor (VEGF) are important mediators of kidney injury in diabetes. Acute hyperglycemia increased synthesis of intrarenal Ang I and Ang II and resulted in activation of both Ang II receptors, AT1 and AT2, in the kidney. Losartan (specific AT1 antagonist) or PD123319 (specific AT2 antagonist) did not affect hyperglycemia but prevented activation of renal AT1 and AT2, respectively. In murine renal cortex, acute hyperglycemia increased VEGF protein but not mRNA content after 24 h, which suggested translational regulation. Blockade of AT2, but not AT1, prevented increase in VEGF synthesis by inhibiting translation of VEGF mRNA in renal cortex. Acute hyperglycemia increased VEGF expression in wild type but not in AT2 knockout mice. Binding of heterogeneous nuclear ribonucleoprotein K to VEGF mRNA, which stimulates its translation, was prevented by blockade of AT2, but not AT1. The Akt-mTOR-p70(S6K) signaling pathway, involved in the activation of mRNA translation, was activated in hyperglycemic kidneys and was blocked by the AT2 antagonist. Elongation phase is an important step of mRNA translation that is controlled by elongation factor 1A (eEF1A) and 2 (eEF2). Expression of eEF1A and activity of eEF2 was higher in kidney cortex from hyperglycemic mice and only the AT2 antagonist prevented these changes. To assess selectivity of translational control of VEGF expression, we measured expression of fibronectin (FN) and laminin β1 (lamβ1): acute hyperglycemia increased FN expression at both protein and mRNA levels, indicating transcriptional control, and did not affect the expression of lamβ1. To confirm results obtained with PD123319, we induced hyperglycemia in AT2 knockout mice and found that in the absence of AT2, translational control of VEGF expression by hyperglycemia was abolished. Our data show that acute hyperglycemia stimulates Ang II synthesis in murine kidney cortex, this leads to AT2 activation and stimulation of VEGF mRNA translation, via the Akt-mTOR-p70(S6K) signaling pathway. Our data show that exclusive translational control of protein expression in the kidney by acute hyperglycemia is not a general phenomenon, but do not prove that it is restricted to VEGF.