Angiotensin II AT(2) receptors inhibit growth responses in proximal tubule cells

Class A and C GPCR Dimers in Neurodegenerative Diseases

Neurodegenerative diseases affect over 30 million people worldwide with an ascending trend. Most individuals suffering from these irreversible brain damages belong to the elderly population, with onset between 50 and 60 years. Although the pathophysiology of such diseases is partially known, it remains unclear upon which point a disease turns degenerative. Moreover, current therapeutics can treat some of the symptoms but often have severe side effects and become less effective in long-term treatment. For many neurodegenerative diseases, the involvement of G proteincoupled receptors (GPCRs), which are key players of neuronal transmission and plasticity, has become clearer and holds great promise in elucidating their biological mechanism. With this review, we introduce and summarize class A and class C GPCRs, known to form heterodimers or oligomers to increase their signalling repertoire. Additionally, the examples discussed here were shown to display relevant alterations in brain signalling and had already been associated with the pathophysiology of certain neurodegenerative diseases. Lastly, we classified the heterodimers into two categories of crosstalk, positive or negative, for which there is known evidence.

Influence of Angiotensin II on cell viability and apoptosis in rat renal proximal tubular epithelial cells in in vitro studies

Introduction:

Angiotensin II (Ang II) is multifunctional peptide that plays an important role in blood pressure regulation and maintenance electrolyte homeostasis. It shows biological effects by activating two main receptors: AT₁ and AT₂. The aim of the present work was to investigate the effect of Ang II on NRK-52E cells in in vitro studies. Furthermore, an attempt was made to determine the effectiveness of the AT₁ and AT₂ receptor blocker activity (respectively, losartan and PD123319).

Methods:

The study was carried out using adherent NRK-52E cell line. Immunofluorescence and Western Blot method were used to confirm the presence of AT₁ and AT₂ receptors in the cells. The SRB and MTT tests showed decrease in the viability of NRK-52E cells incubated with Ang II in comparison to the control (without Ang II).

Results:

The blockade of the AT₁ receptor caused an increase in cell viability in comparison to cells incubated with Ang II only. The blockade of AT₂ receptor also triggered statistically significant increase in cell viability in comparison with cells only exposed to Ang II. Combined administration of blockers for both receptors (losartan and PD123319) decreased Ang II cytotoxicity against NRK-52E cell line. The apoptosis was only observed in cells incubated with Ang II in comparison with control cells. However, simultaneous use of both blockers caused statistically significant decrease in apoptosis.

Conclusions:

The result of our study indicates that Ang II causes damaging effect on NRK-52E cells by directing them to programmed cell death. It seems that not only does the AT₂ receptor itself play an important role in the induction of apoptosis, but also its interaction with AT₁ receptor does as well.

The involvement of angiotensin type 1 and type 2 receptors in estrogen-induced cell proliferation and vascular endothelial growth factor expression in the rat anterior pituitary

The aim of our study was to examine the involvement of renin-angiotensin system (RAS) in estrogen-induced lactotropes proliferation and vascular endothelial growth factor (VEGF) expression in rat pituitary. The study was performed on Fisher 344 rats underwent 8-day treatment with diethylstilboestrol (DES). The proliferation index (PCNA) and VEGF expression in pituitary sections were estimated using immunohistochemical methods. Treatment with DES increased the number of PCNA-positive cells, VEGF-positive cells, and VEGF-positive blood vessels in pituitary. Stimulatory effect of estrogen on cell proliferation and VEGF expression in blood vessels was attenuated by losartan, PD123319, and captopril. VEGF immunoreactivity in pituitary cells of DES-treated rats was decreased by AT1 antagonist and not changed by AT2 blocker and ACE inhibitor. Our findings suggest the involvement of RAS in DES-induced cell proliferation and VEGF expression in pituitary. Both the AT1 and AT2 receptors appear to mediate the estrogen-dependent mitogenic and proangiogenic effects in rat pituitary.

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.

Fluorofenidone inhibits Ang II-induced apoptosis of renal tubular cells through blockage of the Fas/FasL pathway

OBJECTIVES

The present study was designed to investigate the inhibitory effects of fluorofenidone on Ang II-induced apoptosis in renal tubular cells and the related signaling pathway.

METHODS

Rat proximal tubular epithelial cells (NRK-52E) were used to examine the anti-apoptosis effects of fluorofenidone. Cell proliferation was assessed by methyl thiazolyl tetrazolium assay. Apoptosis was examined by AO/EB staining and TUNEL assay. The expression of Fas/FasL pathway members, including Fas, FasL, Bax, Bcl-2, Caspase-8, and Caspase-3 was detected by real-time RT-PCR and/or Western blot, respectively. The activity of Caspase-8 and Caspase-3 was detected by spectrophotometry.

RESULTS

Fluorofenidone didn't affect the proliferation of NRK-52E cells, but significantly inhibited the apoptosis of NRK-52E cells induced by Ang II. Fluorofenidone significantly reduced Ang II-induced increases in Fas, FasL, Bax, Caspase-8 and Caspase-3 at the mRNA level. Consistent with these observations, fluorofenidone also prevented Ang II-mediated up-regulation of FasL and Bax at the protein level. Additionally, Ang II-induced activation of Caspase-8 and Caspase-3 as well as Ang II-initiated downregulation of Bcl-2 at both mRNA and protein levels was all prevented by fluorofenidone.

CONCLUSIONS

Fluorofenidone can inhibit Ang II-induced apoptosis of renal tubular cells through blockage of the Fas/FasL pathway.

Angiotensin-(1-7) activates growth-stimulatory pathways in human mesangial cells

Angiotensin-(1-7) [Ang-(1-7)] is generated in part via ACE2-dependent degradation of angiotensin II (ANG II). In proximal tubular cells, Ang-(1-7) inhibits ANG II-stimulated phosphorylation of the mitogen-activated protein kinases (MAPKs) p38, extracellular signal-related kinase (ERK1/ERK2), and c-jun N-terminal kinase (JNK), suggesting that Ang-(1-7) protects against ANG II-mediated tubulointerstitial injury. We determined the effect of Ang-(1-7) on signaling and growth responses in cultured human mesangial cells. Ang-(1-7) increased phosphorylation of p38, ERK1/ERK2, and JNK MAPKs, which was blocked by the Ang-(1-7) antagonist A-779. Neither the AT(1) receptor antagonist losartan, nor the AT(2) antagonist PD123319 affected specific binding of [(125)I]Ang-(1-7) or Ang-(1-7)-stimulated p38 phosphorylation. Ang-(1-7) increased cell arachidonic acid release, an effect blocked by A-779. The p38 MAPK antagonist SB202190 completely prevented Ang-(1-7)-stimulated release of arachidonic acid, whereas inhibitors of ERK or JNK had no effect. Ang-(1-7) significantly enhanced DNA synthesis and increased production of transforming growth factor-beta1 (TGF-beta1), fibronectin, and collagen IV. Both A-779 and SB202190 blocked the Ang-(1-7)-stimulated increases in TGF-beta1, fibronectin, and collagen IV. These data indicate that Ang-(1-7) activates MAPK phosphorylation via binding to a specific receptor in human mesangial cells. Stimulation of p38 MAPK phosphorylation by Ang-(1-7) leads to release of arachidonic acid and production of TGF-beta1 and extracellular matrix proteins. We conclude that Ang-(1-7) exerts growth-stimulatory effects in human mesangial cells.

Specific receptor for angiotensinogen on human renal cells

BACKGROUND

We recently demonstrated the existence of an angiotensinogen (AGT) receptor on placental cells. It has been established that there is a tissue-specific renin-angiotensin system (RAS) in the human kidney. This study focused on whether human renal proximal tubule epithelial cells possessed an AGT receptor.

METHODS

Human renal proximal tubule epithelial cells were cultured in plastic wells. Binding assays were carried out by adding iodinated angiotensinogen ((125)I-AGT) to each culture well, with or without unlabeled AGT. The cells were washed, lysed, and the radioactivity was measured.

RESULTS

Human renal proximal tubule epithelial cells bound (125)I-AGT in a time-dependent manner. This binding was competitively and specifically inhibited by unlabeled AGT. Bound (125)I-AGT was competitively displaced by AGT. Acid washing removed 30% at 8 h, indicating that 70% bound AGT had been internalized. Scatchard plot binding analysis showed that the identified AGT receptor possessed a single class of high-affinity binding sites (K(d)=1.73 nmol, B(max)=23.39 pmol/10(6) cells).

CONCLUSION

The results of this study provide evidence for the presence of an AGT receptor on human renal proximal tubule epithelial cells. Our finding suggests that the AGT receptor may be an integral component of the renal RAS.

Role of reactive oxygen species in angiotensin II-mediated renal growth, differentiation, and apoptosis

Angiotensin II (ANG II) induces cell-cycle arrest of cultured proximal tubular cells, resulting in cellular hypertrophy. This ANG II-mediated hypertrophy is associated with the induction of p27(Kip1), an inhibitor of G1 phase cyclin-dependent kinase cyclin complexes. We have recently demonstrated that ANG II-mediated expression of p27(Kip1) and induction of cellular hypertrophy depend on the generation of reactive oxygen species (ROS). The effects of ROS are mediated by stimulation of mitogen-activated protein (MAP) kinases. p44/42 MAP kinase directly phosphorylates p27(Kip1) at serine-threonine residues and increases thereby its half-life time. AT2-receptor activation has been implicated in apoptosis and/or cell differentiation. Recent studies, however, revealed a more indirect role of hypoxia in the antiproliferative effects of ANG II transduced through AT2 receptors. We found that SM-20 is down-regulated in ANG II-stimulated PC12 cells that express only AT2 receptors. It turned out that SM20 is the rat homologue of a dioxygenase that regulates hypoxia-inducible factor 1 (HIF-1). ANG II induces HIF-1alpha by a posttranscriptional mechanism suggesting that SM20 down-regulation leads to stabilization of HIF-1. Thus, ANG II-induced ROS generation plays a pivotal role in several pathophysiological situations, leading to renal growth regulation and remodeling after injury.

Tight blood pressure control decreases apoptosis during renal damage

BACKGROUND

An excess rate of apoptosis could lead to the gradual loss of renal mass. In this study, we investigated the role of apoptosis in the renal damage secondary to hypertension.

METHODS

Spontaneously hypertensive rats with 5/6 renal mass reduction (subtotal nephrectomy) were distributed to receive no-treatment, 200 mg/L quinapril, 360 mg/L losartan, or triple therapy (200 mg/L hydralazine, 4 mg/L reserpine, and 100 mg/L hydrochlorothiazide) for 5 weeks. Sham-operated spontaneously hypertensive rats served as controls. Age-matched Wistar-Kyoto (WKY) rats, with or without subtotal nephrectomy, were also studied.

RESULTS

Nontreated spontaneously hypertensive rats + subtotal nephrectomy developed proteinuria, glomerular sclerosis, and tubulointerstitial lesions. In comparison to spontaneously hypertensive rats, an increment in the number of [proliferating cell nuclear antigen (PCNA)]-positive and apoptotic [terminal deoxynucleotidyl transferase (Tdt)-mediated deoxyuridine triphosphate biotin nick end labeling (TUNEL)]-positive tubular and glomerular cells was observed. By contrast, WKY + subtotal nephrectomy rats showed less severe morphologic lesions, and only the number of proliferating cells increased. By Western blot, an up-regulation of renal Bax (apoptosis inducer) was noted both in spontaneously hypertensive rats + subtotal nephrectomy and WKY + subtotal nephrectomy rats. By contrast, Bcl-xL (apoptosis protector) was up-regulated in WKY + subtotal nephrectomy rats but not in spontaneously hypertensive rats + subtotal nephrectomy. The administration of appropriate doses of quinapril, losartan, or triple therapy to spontaneously hypertensive rats + subtotal nephrectomy normalized systolic blood pressure, partially prevented proteinuria, renal lesions and apoptosis, and decreased Bax, but no changes were noted in Bcl-xL. The Bax/Bcl-xL index was significantly increased in spontaneously hypertensive rats + subtotal nephrectomy compared to sham-operated spontaneously hypertensive rats and decreased in treated groups.

CONCLUSION

The combination of renal mass reduction and hypertension caused severe renal lesions associated to an increment of apoptosis rate, mainly in tubular epithelial cells. Tight blood pressure control decreased the apoptosis rate and morphologic lesions. These studies suggest that changes in the expression of apoptosis-regulatory genes contribute to the progressive damage in hypertensive rats with renal mass reduction.

Persistent proteinuria up-regulates angiotensin II type 2 receptor and induces apoptosis in proximal tubular cells

Apoptosis is implicated in the progressive cell loss and fibrosis both at glomerular and tubulointerstitial level. In this study, we examined the potential mechanisms by which persistent proteinuria (protein-overload model) could induce apoptosis. After uninephrectomy (UNX), Wistar rats received daily injections of 0.5 g of bovine serum albumin (BSA)/100 g body weight or saline. Both at day 8 and day 28, rats receiving BSA had proteinuria and renal lesions characterized by tubular atrophy and/or dilation and mononuclear cell infiltration. In relation to control-UNX rats, renal cortex of nephritic rats showed an increment in AT2 mRNA (reverse transcriptase-polymerase chain reaction) and protein (Western blot) expression. In both groups, AT2 receptor immunostaining was mainly localized in proximal tubular cells. Rats with persistent proteinuria showed a significantly increased number of terminal dUTP nick-end labeling positive apoptotic cells compared with UNX-controls, both in glomeruli and tubulointerstitium. Double staining for apoptosis and AT2 receptor showed that most terminal dUTP nick-end labeling positive cells were found in tubules expressing AT2 receptor. Using an antibody that recognizes the active form caspase-3, we observed an increment in caspase-3 activation in rats receiving BSA with respect to those receiving saline. Rats with persistent proteinuria showed a diminution in the phosphorylation of Bcl-2 with respect to UNX-controls both at day 8 and day 28. By contrast, no changes were observed either in the Bax or in the Bcl-2 protein levels. The administration of BSA to UNX rats induced a diminution in the phosphorylation of ERK with respect to UNX-control at all times studied. The changes observed in ERK activities took place without alterations of ERK1/2 protein levels. In summary, our data suggest that persistent proteinuria causes apoptosis in tubular cells through the activation of AT2 receptor, which can, in turn, inhibit MAP kinase (ERK1/2) activation and Bcl-2 phosphorylation.

Angiotensin II mediates Tyr-dephosphorylation in rat fetal kidney membranes

Angiotensin II (Ang II) elicits a variety of physiological effects through specific Ang II receptors in numerous tissues. In addition, Ang II is a modulator of cellular growth and exerts a positive or negative effect on cell growth depending on which receptor subtype is activated. Expression of the intrarenal AT2 receptors occurs at its highest levels in the fetal kidney, with a rapid decline after birth. In the present paper, we performed a study on the signaling mechanism of Ang II receptors in rat fetal (E20) kidney, a rich source of AT2 receptors, where both Ang II receptor subtypes are present. Ang II induces Tyr-dephosphorylation of proteins in rat fetal kidney membranes. The response is dose-dependent, with a reduction of 20% with respect to the control (100%), signal that is completely reversed by Ang IIAT2 competitor PD123319. Orthovanadate, the inhibitor of phospho-Tyr-phosphatases (PTPase), reverts Ang II effect, suggesting the involvement of a protein tyrosine phosphatase. The peptide analog of Ang II, CGP42112, exhibits an agonist effect, which is dose-dependent. Thus, in rat fetal (E20) kidney, the Ang-induced protein Tyr-dephosphorylation of several proteins is mediated by AT2 receptors, mechanism that involves an orthovanadate sensitive PTPase.

Role of apoptosis in the pathogenesis of acute renal failure

Renal tubular cells die by apoptosis as well as necrosis in experimental models of ischemic and toxic acute renal failure as well as in humans with acute tubular necrosis. It is not yet possible, however, to determine the relative contribution of these two forms of cell death to loss of renal tubular cells in acute tubular necrosis. The beneficial effect of administering growth factors to animals with acute tubular necrosis is probably related to the potent antiapoptotic (survival) effects of growth factors as well as to their proliferative effects. Rapamycin inhibits both of these effects of growth factors and delays the recovery of renal function after acute tubular necrosis by inhibiting renal tubular cell regeneration and by increasing renal tubular cell loss by apoptosis. The administration of caspase inhibitors ameliorates ischemia-reperfusion injury in multiple organs including the kidney. However, the extent to which this protective effect of caspase inhibition is caused by reduced intrarenal inflammation, or by amelioration of renal tubular cell loss due to apoptosis, remains uncertain. In addition to caspase inhibition, the apoptotic pathway offers many potential targets for therapeutic interventions to prevent renal tubular cell apoptosis.