Recent Advances in Understanding the Molecular Pathophysiology of Angiotensin II Receptors: Lessons From Cell-Selective Receptor Deletion in Mice

The renin-angiotensin system (RAS) is an essential hormonal system involved in water and sodium reabsorption, renal blood flow regulation, and arterial constriction. Systemic stimulation of the RAS with infusion of the main peptide angiotensin II (Ang II) in animals as well as pathological elevation of renin (ie, renovascular hypertension) to increase circulatory Ang II in humans ultimately lead to hypertension and end organ damage. In addition to hypertension, accumulating evidence supports that the Ang II type 1 receptor exerts a critical role in cardiovascular and kidney diseases independent of blood pressure elevation. In the past 2 decades, the identification of an increased number of peptides and receptors has facilitated the concept that the RAS has detrimental and beneficial effects on the cardiovascular system depending on which RAS components are activated. For example, angiotensin 1-7 and Ang II type 2 receptors act as a counter-regulatory system against the classical RAS by mediating vasodilation. Although the RAS as an endocrine system for regulation of blood pressure is well established, there remain many unanswered questions and controversial findings regarding blood pressure regulation and pathophysiological regulation of cardiovascular diseases at the tissue level. This review article includes the latest knowledge gleaned from cell type-selective gene deleted mice regarding cell type-specific roles of Ang II receptors and their significance in health and diseases are discussed. In particular, we focus on the roles of these receptors expressed in vascular, cardiac, and kidney epithelial cells.

Chronic Kidney Disease Transdifferentiates Veins into a Specialized Immune-Endocrine Organ with Increased MYCN-AP1 Signaling

Most patients with end-stage renal disease (ESRD) and advanced chronic kidney disease (CKD) choose hemodialysis as their treatment of choice. Thus, upper-extremity veins provide a functioning arteriovenous access to reduce dependence on central venous catheters. However, it is unknown whether CKD reprograms the transcriptome of veins and primes them for arteriovenous fistula (AVF) failure. To examine this, we performed transcriptomic analyses of bulk RNA sequencing data of veins isolated from 48 CKD patients and 20 non-CKD controls and made the following findings: (1) CKD converts veins into immune organs by upregulating 13 cytokine and chemokine genes, and over 50 canonical and noncanonical secretome genes; (2) CKD increases innate immune responses by upregulating 12 innate immune response genes and 18 cell membrane protein genes for increased intercellular communication, such as CX3CR1 chemokine signaling; (3) CKD upregulates five endoplasmic reticulum protein-coding genes and three mitochondrial genes, impairing mitochondrial bioenergetics and inducing immunometabolic reprogramming; (4) CKD reprograms fibrogenic processes in veins by upregulating 20 fibroblast genes and 6 fibrogenic factors, priming the vein for AVF failure; (5) CKD reprograms numerous cell death and survival programs; (6) CKD reprograms protein kinase signal transduction pathways and upregulates SRPK3 and CHKB; and (7) CKD reprograms vein transcriptomes and upregulates MYCN, AP1, and 11 other transcription factors for embryonic organ development, positive regulation of developmental growth, and muscle structure development in veins. These results provide novel insights on the roles of veins as immune endocrine organs and the effect of CKD in upregulating secretomes and driving immune and vascular cell differentiation.

The Renin-Angiotensin-Aldosterone System in Metabolic Diseases and Other Pathologies

It has been our pleasure to have been able to develop two special issues within the International Journal of Molecular Sciences: (1) Renin-Angiotensin-Aldosterone System in Pathologies and (2) Renin-Angiotensin-Aldosterone System in Metabolism & Disease [...].

Abstract P111: Deletion Of Na + /H + Exchanger 3 Selectively In The Proximal Tubules Doesn’T Prevent The Development Of Doca-salt Hypertension In Pt- Nhe3 -/- Mice

We have previously shown that genetic deletion of Na + /H + exchanger 3 (NHE3) selectively in the proximal tubules lowered basal blood pressure and attenuated the development of angiotensin II (Ang II)-induced hypertension by promoting the pressure-natriuresis response. However, it is not known whether deletion of the NHE3 gene selectively in the proximal tubules attenuates salt-sensitive hypertension induced by DOCA-Salt. In the present study, we tested the hypothesis that the development of DOCA-Salt hypertension is attenuated by genetic deletion of NHE3 selectively in the proximal tubules of the kidney. To test the hypothesis, mice with proximal tubule-specific deletion of NHE3, i.e., PT- Nhe3 -/- , were generated using the iL- Sglt2-Cre / Nhe3 flox approach. Adult male PT- Nhe3 -/- mice were treated with or without DOCA (10 mg/10 g body wt., subcutaneously), supplemented with 2% high salt diet, for 4 weeks. Basal and weekly systolic (SBP), diastolic (DBP), and mean arterial pressure (MAP) were determined using the indirect tail-cuff method, whereas 24 hr. urinary sodium, potassium, and chloride excretion were determined using a metabolic cage. Compared with the time-control group (n=12), treatment with DOCA-salt moderately but significantly increased systolic SBP from 96 ± 3 mmHg to 126 ± 5 mmHg in PT- Nhe3 -/- mice (n=8; P <0.01). DBP and MAP increased to similar extents in response to DOCA-salt treatment ( P <0.05). As expected, DOCA-salt hypertension significantly increased the pressure-natriuresis response, i.e., 24 hr. urinary sodium excretion by 55% (Control: 152 ± 19 vs. DOCA-Salt: 236 ± 2 μmol/24 hr; P <0.01), potassium excretion by 52% (Control: 147 ± 16 vs. DOCA-Salt: 224 ± 18 μmol/24 hr; P <0.01), and chloride excretion by 71% (Control: 181 ± 27 vs. DOCA-Salt: 310 ± 31 μmol/24 hr; P <0.01), respectively. Taken together, these data do not support our hypothesis and suggest that deletion of NHE3 selectively in the proximal tubules doesn’t prevent the development of DOCA-Salt hypertension in PT- Nhe3 -/- mice.

Abstract 098: Double Deletion Of At 1a Receptors And Na + /h + Exchanger 3 In The Proximal Tubules Attenuates Angiotensin Ii-induced And 2k1c Goldblatt Hypertension In Proximal Tubule-specific Pt- Agtr1a -/- / Nhe3 -/- Mice

We have previously shown that genetic deletion of AT 1 (AT 1a ) receptors or Na + /H + exchanger 3 (NHE3) selectively in the proximal tubules attenuated the development of angiotensin II (Ang II)-induced hypertension in mice. The present study determined whether double deletion of AT 1a receptors and NHE3 selectively in the proximal tubules further attenuates the development of Ang II-induced or 2-kidney, 1-clip (2K1C) Goldblatt Hypertension. Proximal tubule-specific AT 1a receptor and NHE3 double knockout mice, PT- Agtr1a -/- / Nhe3 -/- , were generated using the iL- Sglt2-Cre / LoxP approach. Adult male wild-type (WT), PT- Agtr1a -/- , PT- Nhe3 -/- , and PT- Agtr1a -/- / Nhe3 -/- double knockout mice were treated with vehicle (or sham as time-controls), a slow pressor dose of Ang II infusion for 2 weeks (500 μg/kg body wt./day, i.p.), or induction of 2K1C Goldblatt hypertension by placing a silver clip (0.12 mm) on the left renal artery for 4 weeks, respectively. In WT mice, control systolic blood pressure was 118 ± 3 mmHg (n=9), which increased to ~143 ± 5 mmHg in response to Ang II infusion ( P <0.01, n=10) or to induction of 2K1C Goldblatt hypertension (n=12, P <0.01). By comparison, control systolic blood pressure was 15 ± 2 mmHg lower in age-matched PT- Agtr1a -/- mice ( P <0.01), or 13 ± 3 mmHg lower in PT- Nhe3 -/- mice than WT mice ( P <0.01). Double deletion of AT 1a receptors and NHE3 selectively in the proximal tubules further but only moderately decreased basal blood pressure to 98 ± 2 mmHg in PT- Agtr1a -/- / Nhe3 -/- double knockout mice ( P <0.05, n=10). In response to Ang II infusion, systolic blood pressure increased only to ~126 ± 3 mmHg in PT- Agtr1a -/- or PT- Nhe3 -/- mice ( P <0.01). Interestingly, 2K1C Goldblatt hypertension was markedly attenuated in PT- Agtr1a -/- (108 ± 3 mmHg, P <0.01, n=10), PT- Nhe3 -/- (110 ± 2 mmHg, P <0.01, n=10), or PT- Agtr1a -/- / Nhe3 -/- double knockout mice (103 ± 2 mmHg, P <0.01, n=8), respectively. Taken together, these data strongly support the hypothesis that the intratubular Ang II/AT 1a receptor/NHE3 signaling plays a key role in maintaining basal blood pressure homeostasis and the full development of Ang II-induced or 2K1C Goldblatt hypertension.

Kidney Angiotensin in Cardiovascular Disease: Formation and Drug Targeting

The concept of local formation of angiotensin II in the kidney has changed over the last 10-15 years. Local synthesis of angiotensinogen in the proximal tubule has been proposed, combined with prorenin synthesis in the collecting duct. Binding of prorenin via the so-called (pro)renin receptor has been introduced, as well as megalin-mediated uptake of filtered plasma-derived renin-angiotensin system (RAS) components. Moreover, angiotensin metabolites other than angiotensin II [notably angiotensin-(1-7)] exist, and angiotensins exert their effects via three different receptors, of which angiotensin II type 2 and Mas receptors are considered renoprotective, possibly in a sex-specific manner, whereas angiotensin II type 1 (AT1) receptors are believed to be deleterious. Additionally, internalized angiotensin II may stimulate intracellular receptors. Angiotensin-converting enzyme 2 (ACE2) not only generates angiotensin-(1-7) but also acts as coronavirus receptor. Multiple, if not all, cardiovascular diseases involve the kidney RAS, with renal AT1 receptors often being claimed to exert a crucial role. Urinary RAS component levels, depending on filtration, reabsorption, and local release, are believed to reflect renal RAS activity. Finally, both existing drugs (RAS inhibitors, cyclooxygenase inhibitors) and novel drugs (angiotensin receptor/neprilysin inhibitors, sodium-glucose cotransporter-2 inhibitors, soluble ACE2) affect renal angiotensin formation, thereby displaying cardiovascular efficacy. Particular in the case of the latter three, an important question is to what degree they induce renoprotection (e.g., in a renal RAS-dependent manner). This review provides a unifying view, explaining not only how kidney angiotensin formation occurs and how it is affected by drugs but also why drugs are renoprotective when altering the renal RAS. SIGNIFICANCE STATEMENT: Angiotensin formation in the kidney is widely accepted but little understood, and multiple, often contrasting concepts have been put forward over the last two decades. This paper offers a unifying view, simultaneously explaining how existing and novel drugs exert renoprotection by interfering with kidney angiotensin formation.

Angiotensin II and AT1a Receptors in the Proximal Tubules of the Kidney: New Roles in Blood Pressure Control and Hypertension

Contrary to public perception, hypertension remains one of the most important public health problems in the United States, affecting 46% of adults with increased risk for heart attack, stroke, and kidney diseases. The mechanisms underlying poorly controlled hypertension remain incompletely understood. Recent development in the Cre/LoxP approach to study gain or loss of function of a particular gene has significantly helped advance our new insights into the role of proximal tubule angiotensin II (Ang II) and its AT₁ (AT_1a) receptors in basal blood pressure control and the development of Ang II-induced hypertension. This novel approach has provided us and others with an important tool to generate novel mouse models with proximal tubule-specific loss (deletion) or gain of the function (overexpression). The objective of this invited review article is to review and discuss recent findings using novel genetically modifying proximal tubule-specific mouse models. These new studies have consistently demonstrated that deletion of AT₁ (AT_1a) receptors or its direct downstream target Na⁺/H⁺ exchanger 3 (NHE3) selectively in the proximal tubules of the kidney lowers basal blood pressure, increases the pressure-natriuresis response, and induces natriuretic responses, whereas overexpression of an intracellular Ang II fusion protein or AT₁ (AT_1a) receptors selectively in the proximal tubules increases proximal tubule Na⁺ reabsorption, impairs the pressure-natriuresis response, and elevates blood pressure. Furthermore, the development of Ang II-induced hypertension by systemic Ang II infusion or by proximal tubule-specific overexpression of an intracellular Ang II fusion protein was attenuated in mutant mice with proximal tubule-specific deletion of AT₁ (AT_1a) receptors or NHE3. Thus, these recent studies provide evidence for and new insights into the important roles of intratubular Ang II via AT₁ (AT_1a) receptors and NHE3 in the proximal tubules in maintaining basal blood pressure homeostasis and the development of Ang II-induced hypertension.

Abstract 17: Genetic Deletion Of Angiotensin Ii AT 1a Receptors Selectively In The Proximal Tubules Of The Kidney Attenuates Two-kidney, One-clip Goldblatt Hypertension In Pt- Agtr1a -/- Mice

The activation of the renin-angiotensin system (RAS) in the clipped kidney plays a critical role in the development of two-kidney, one-clip Goldblatt hypertension (2K1C), but the roles of angiotensin II (Ang II) and AT 1a receptors in the proximal tubules has not been determined previously. The present study tested the hypothesis that genetic deletion of AT 1a receptors selectively in the proximal tubules attenuates the development of 2K1C Goldblatt hypertension via AT 1a receptor-mediated, Na + /H + exchanger 3 (NHE3)-dependent mechanisms. To test the hypothesis, 2K1C Goldblatt hypertension was induced by placing a silver clip, 0.2 mm internal diameter, on the left renal artery for 4 weeks in adult male wild-type (WT), global AT 1a receptor knockout ( Agtr1a -/- ), proximal tubule (PT)-specific Agtr1a -/- (PT- Agtr1a -/- ), or PT- Nhe3 -/- mice, respectively. In WT mice, systolic blood pressure increased in a time-dependent manner reaching a maximal response by Week 3 (Basal: 112 ± 2 vs. 2K1C: 149 ± 4 mmHg, n=12, P <0.01). 2K1C Goldblatt hypertension in WT mice was associated with significant increases in renin mRNA expression in the clipped kidney (Control: 2366 ± 255 vs. Clipped: 3144 ± 569 copies/ng RNA, P <0.01) and decreases in renin mRNA expression in the nonclipped kidney (1738 ± 341 copies/ng RNA, P <0.05). Plasma Ang II levels were significantly increased in WT mice with 2K1C Goldblatt hypertension (Control: 50.2 ± 7.2 vs. 2K1C: 109.7 ± 17.2 pg/ml, P <0.05). Glomerular and tubulointerstitial fibrotic responses were also significantly increased in the clipped kidney ( P <0.01). In contrast to WT mice, the development of 2K1C hypertension was completely attenuated in Agtr1a -/- (Basal: 88 ± 4 vs. 2K1C: 92 ± 2 mmHg, n=9, n.s .), PT- Agtr1a -/- mice (Basal: 101 ± 2 vs. 2K1C: 104 ± 3 mmHg, n=12, n.s .) and PT- Nhe3 -/- mice (Basal: 103 ± 3 vs. 109 ± 5 mmHg, n=12, n.s .). Renin mRNA expression was not different in clipped and nonclipped kidney of Agtr1a -/- mice, but it was decreased in the nonclipped kidney of PT- Agtr1a -/- mice ( P <0.05). Taken together, these data suggest that genetic deletion of AT 1a receptors selectively in the proximal tubules attenuates the development of 2K1C Goldblatt hypertension via AT 1a receptor-mediated, Na + /H + exchanger 3 (NHE3)-dependent mechanisms.

Abstract P254: Sex Differences In Angiotensin Ii-induced Hypertension In Mice With Proximal Tubule-specific Deletion Of Mitochondrial Protein Sirtuin 3 In The Kidney

The development of Angiotensin II (Ang II)-induced hypertension is associated with mitochondrial dysfunction and kidney injury. Sirtuin 3 (SIRT3), a key mitochondrial protein, plays an important role in maintaining mitochondrial homeostasis. However, it remains unknown whether deletion of SIRT3 in the proximal tubules will alter the pressor and renal responses to Ang II in sex-different manners. In the present study, adult male, and female wild-type (WT) and mutant mice with proximal tubule-specific knockout of SIRT3, PT- Sirt3 -/- , were infused with or without a slow pressor dose of Ang II via an osmotic minipump (0.5 mg/kg/day, i.p.), supplemented with a 2% NaCI diet or losartan, 20 mg/kg/day, for 2 weeks. Systolic (SBP), diastolic (DBP), and mean arterial blood (MAP) pressure were determined using the tail-cuff method, whereas 24 hr. urinary sodium and potassium excretion were determined using a metabolic cage. Serum and urine creatinine were measured using colorimetric assays, whereas glomerular and tubulointerstitial injury was evaluated by Masson’s Trichrome staining. Basal SBP levels were lower in PT- Sirt3 -/- than in WT mice (SBP-WT: 112 ± 2 vs. SBP-PT- Sirt3 -/- : 93 ± 2 mmHg, P <0,01). The magnitude of Ang II-induced hypertension was similar between WT and PT- Sirt3 -/- mice with or without losartan treatment. Serum creatinine levels and urinary creatinine excretion were higher in PT- Sirt3 -/- mice than in WT mice ( P <0.05), but without significant sex differences in response to Ang II infusion or losartan treatment. Differences were found only in female WT and PT- Sirt3 -/- mice with lower 24 hr. urine and urinary creatinine excretion in response to Ang II infusion or losartan. Losartan significantly increased 24 hr. urinary potassium and chloride excretion in Ang II-infused male and female PT- Sirt3 -/- mice ( P <0.01). Finally, Ang II-infused PT- Sirt3 -/- mice showed significant renal cortical tubulointerstitial fibrotic responses ( P <0.05), but not glomerular fibrotic responses. We conclude that basal blood pressure is lower in male and female PT- Sirt3 -/- mice and that Ang II induces similar hypertensive and renal fibrotic responses in male and female PT- Sirt3 -/- mice without significant sex differences.

Sex differences in angiotensin II-induced hypertension and kidney injury: role of AT1a receptors in the proximal tubule of the kidney

In the present study, we tested the hypothesis that there are significant sex differences in angiotensin II (Ang II)-induced hypertension and kidney injury using male and female wildtype (WT) and proximal tubule-specific AT1a receptor knockout mice (PT-Agtr1a-/-). Twelve groups (n=8-12 per group) of adult male and female WT and PT-Agtr1a-/- mice were infused with a pressor dose of Ang II via osmotic minipump for 2 weeks (1.5 mg/kg/day, i.p.) and simultaneously treated with or without losartan (20 mg/kg/day, p.o.) to determine the respective roles of AT1a receptors in the proximal tubules versus systemic tissues. Basal systolic, diastolic, and mean arterial pressure were approximately 13 ± 3 mmHg lower (P<0.01), while basal 24-h urinary Na+, K+, and Cl- excretion were significantly higher in both male and female PT-Agtr1a-/- mice than WT controls (P<0.01) without significant sex differences between different strains. Both male and female WT and PT-Agtr1a-/- mice developed hypertension (P<0.01), and the magnitudes of the pressor responses to Ang II were similar between male and female WT and PT-Agtr1a-/- mice (n.s.). Likewise, Ang II-induced hypertension was significantly attenuated in both male and female PT-Agtr1a-/- mice (P<0.01). Furthermore, losartan attenuated the hypertensive responses to Ang II to similar extents in both male and female WT and PT-Agtr1a-/- mice. Finally, Ang II-induced kidney injury was attenuated in PT-Agtr1a-/- mice (P<0.01). In conclusion, the present study demonstrates that deletion of AT1a receptors in the proximal tubules of the kidney attenuates Ang II-induced hypertension and kidney injury without revealing significant sex differences.

Abstract P011: Novel Roles Of Global, Intestinal, And Kidney Proximal Tubule Sodium And Hydrogen Exchanger 3 In Angiotensin Ii-induced Hypertension

The present study used global ( Nhe3 -/- ), kidney-selective (tg Nhe3 -/- ), and proximal tubule-specific Na + /H + exchanger 3 (NHE3)-deficient mice (PT- Nhe3 -/- ) to test the hypothesis that NHE3 is required for the full development of angiotensin II (Ang II)-induced hypertension in mice. Four groups of adult male, age-matched wild-type (WT), global Nhe3 -/- , kidney-selective tg Nhe3 -/- and proximal tubule-specific Nhe3 -/- mice were infused with: a) saline; b) Ang II (10 pmol/min, i.v.); Ang II via an osmotic minipump for 2 weeks (1.5 mg/kg/day, i.p.); or treated with Ang II and losartan concurrently for 2 weeks (20 mg/kg/day, p.o.). Under basal conditions, global Nhe3 -/- , kidney-selective tg Nhe3 -/- and proximal tubule-specific Nhe3 -/- mice all showed significantly lower systolic, diastolic, and mean arterial pressure than wild-type mice (~15 ± 3 mmHg, P <0.01). The hypotensive phenotype in both global Nhe3 -/- and kidney-selective tg Nhe3 -/- mice was associated with abnormal intestinal structures, diarrhea, increased 24 h fecal Na + excretion, and salt wasting ( P <0.01). By contrast, there were no differences in intestinal structures and fecal Na + excretion between wild-type and PT- Nhe3 -/- mice. PT- Nhe3 -/- mice showed significant diuretic and natriuretic responses compared with wild-type mice ( P <0.01). Acute infusion of Ang II markedly increased arterial blood pressure in a time-dependent manner in wild-type mice, as expected ( P <0.01), but the pressure response was attenuated in global Nhe3 -/- , kidney-selective tg Nhe3 -/- , and PT- Nhe3 -/- mice ( P <0.01). Furthermore, the chronic pressor response to 2-week Ang II infusion was also significantly attenuated in Nhe3 -/- , tgNhe3 -/- , and PT- Nhe3 -/- mice, compared with wild-type mice ( P <0.01). Finally, concurrent treatment with losartan completely blocked the acute and chronic pressor responses to Ang II in wild-type, Nhe3 -/- , tg Nhe3 -/- , and PT- Nhe3 -/- mice (p<0.01). Taken together, these data support the proof of concept that NHE3 in the small intestines and the proximal tubules of the kidney is required for maintaining basal blood pressure homeostasis and for the development of Ang II-induced hypertension. Supported by NIH grants, 2R01DK102429-03A1, 1R56HL130988-01, and 2R01DK067299-10A1.

Abstract P033: PRESSOR, NATRIURETIC, AND RENAL RESPONSES IN ANGIOTENSIN II-INDUCED HYPERTENSION IN MALE AND FEMALE WILD-TYPE AND PROXIMAL TUBULE-SPECIFIC AT 1A RECEPTOR KNOCKOUT MICE

Dysregulation of intrarenal renin-angiotensin system is one of the key factors of human hypertension, but the mechanisms involved remain incompletely understood. To determine the roles of AT 1a receptors in the proximal tubules of the kidney, we infused angiotensin II (Ang II) for 2 weeks (40 ng / min, i.p.) in adult male and female wild-type C57BL/6J and mutant mice with deletion of AT 1a receptors in the proximal tubules (PT- Agtr1a -/- ), and treated with or without the AT 1 receptor blocker losartan (20 mg/kg/day, p.o.) (n=8 per group). The pressor response, 24 h urinary Na + excretion, glomerular and tubulointerstitial injury were compared between male and female wild-type and PT- Agtr1a -/- mice. Basal systolic, diastolic, and mean arterial blood pressure were about 13 ± 3 mmHg lower in male and female PT- Agtr1a -/- mice ( P <0.01), but no differences were observed between male and female wild-type or PT- Agtr1a -/- mice. In response to Ang II, both male and female wild-type and PT- Agtr1a -/- mice developed hypertension ( P <0.01), and the net pressor response were similar in male and female wild-type and PT- Agtr1a -/- mice (n.s.). However, absolute blood pressure was about 12 ± 3 mmHg lower in male and female PT- Agtr1a -/- mice ( P <0.01 vs. wild-type). Ang II-induced hypertension increased the natriuretic response in both male and female wild-type and PT- Agtr1a -/- mice ( P <0.01), but there were no significant differences between male and female wild-type and PT- Agtr1a -/- mice (n.s). Losartan did not increase the natriuretic responses further in all animals. Furthermore, Ang II-induced hypertension was associated with significant increases in glomerular and tubulointerstitial injury in male and female wild-type mice ( P <0.01), which were attenuated in male and female PT- Agtr1a -/- mice ( P <0.01). LOS treatment attenuated Ang II-induced hypertension and decreased Ang II-induced glomerular and tubulointerstitial injury in male and female wild-type and PT- Agtr1a -/- mice ( P <0.01). Taken together, we demonstrated that intratubular AT 1 (AT 1a ) receptors in the proximal tubules of the kidney plays a key role in maintaining basal blood pressure homeostasis and overall body salt and fluid balance, and the development of Ang II-induced hypertension and kidney injury.

Abstract P041: Proximal Tubule-specific Deletion Of Angiotensin Ii Type 1a Receptors In The Kidney Lowers Basal Blood Pressure And Attenuates Angiotensin Ii-induced Hypertension By Increasing Glomerular Filtration And The Pressure-natriuresis Response

The present study tested the hypothesis that intratubular angiotensin II (Ang II) and AT 1a receptors in the proximal tubules of the kidney plays an important role in basal blood pressure control and in the development of Ang II-induced hypertension. Mutant mice with proximal tubule-specific deletion of AT 1a receptors in the kidney, PT- Agtr1a -/- , were generated to test the hypothesis. Eight groups (n=7-12 per group) of adult male wild-type (WT) and PT- Agtr1a -/- mice were infused with or without Ang II for 2 weeks (1.5 mg/kg, i.p.). Basal systolic, diastolic, and mean arterial pressures were ~13 ± 3 mmHg lower in PT- Agtr1a -/- than WT mice ( P <0.01). Basal glomerular filtration rate (GFR), as measured using transdermal FITC-sinistrin, was significantly higher in PT- Agtr1a -/- mice (WT: 160.4 ± 7.0 μl/min vs. PT- Agtr1a -/- : 186.0 ± 6.0 μl/min, P <0.05). Basal 24 h urinary Na + excretion (U Na V) was significantly higher in PT- Agtr1a -/- than WT mice ( P <0.01). In response to Ang II infusion, both WT and PT- Agtr1a -/- mice developed hypertension, and the magnitude of the pressor response to Ang II was similar in WT (Δ43 ± 3 mmHg, P <0.01) and PT- Agtr1a -/- mice (Δ39 ± 5 mmHg, P <0.01). However, the absolute blood pressure level was still 16 ± 3 mmHg lower in PT- Agtr1a -/- mice ( P <0.01). Ang II significantly decreased GFR to 132.2 ± 7.0 μl/min in WT mice ( P <0.01), and to 129.4 ± 18.6 μl/min in PT- Agtr1a -/- mice ( P <0.01), respectively. In WT mice, U Na V increased from 139.3 ± 22.3 μmol/24 h in the control group to 196.4 ± 29.6 μmol/24 h in the Ang II-infused group ( P <0.01). In PT- Agtr1a -/- mice, U Na V increased from 172.0 ± 10.2 μmol/24 h in the control group to 264.7 ± 35.4 μmol/24 h in the Ang II-infused group ( P <0.01). The pressor response to Ang II was attenuated, while the natriuretic response was augmented by losartan in WT and PT- Agtr1a -/- mice ( P <0.01). Finally, proximal tubule-specific deletion of AT 1a receptors significantly augmented the pressure-natriuresis response and natriuretic responses to acute saline infusion ( P <0.01) or a 2% high salt diet ( P <0.01). We concluded that deletion of AT 1a receptors selectively in the proximal tubules lowers basal blood pressure and attenuates Ang II-induced hypertension by increasing GFR and promoting the natriuretic response in PT- Agtr1a -/- mice.

Angiotensin III/AT2 Receptor/NHE3 Signaling Pathway in the Proximal Tubules of the Kidney: A Novel Natriuretic and Antihypertensive Mechanism in Hypertension

See Article Kemp et al

Abstract P2028: Paracrine And Intracellular Angiotensin II Activate Protein Kinase C, Map Kinase Erk1/2, And Nuclear Factor-кB Signaling Responses In Murine Glomerular Mesangial Cells

The glomerular mesangial cells (MC) are a key target for the endocrine, paracrine, autocrine, and intracrine angiotensin II (ANG II). Increased local paracrine and intracrine ANG II formation in MCs may cause glomerular injury by promoting MC cell growth, hypertrophy and fibrosis. In the present study, we investigated the phospho-proteomic signaling and mitochondrial responses to paracrine or intracellular ANG II in murine MCs. MCs were grown to ~80% confluence in 6-well plates in DMEM and Ham’s F-12 medium, containing 5% fetal bovine serum and 1% penicillin-streptomycin, at 37°C under 95% O 2 /5% CO 2 . MCs were fasted in a serum-free medium for 24 h before being treated with ANG II (0.1 to 100 nM) for 5 to 60 min, or transfected with a mitochondria-targeting intracellular ANG II for 48 h (mito-ECFP/ANG II, 4 μg/well), with or without pretreatment with the AT 1 receptor blocker losartan (1 μM), AT 2 receptor blocker PD123319 (1 μM), or expression of mito-AT 2 receptors. Proteins were extracted for Western blot analysis of p-PKCα, p-ERK1/2, p38 MAPK, p65 and p50 subunits of NF-кB, or mitochondria protein complexes I to V, respectively. ANG II significantly increased p-ERK1/2 (Control: 0.20 ±0.07 vs. ANG II: 0.04 ± 0.07, p&lt0.05), p-PKCα (Control: 0.50 ± 0.10 vs. ANG II: 1.30 ±0.20, p&lt0.01), p38 MAPK (Control: 0.30 ±0.04 vs. ANG II: 0.60 ±0.06, p&lt0.01), and NF-кB responses (Control: 0.30 ±0.02 vs. ANG II: 0.55 ± 0.08, p&lt0.01) in dose- and time-dependent manners, with peak responses to ANG II occurred between 0.1 and 1.0 nM, 5-10 min ( p &lt0.05). These phopho-proteomic signaling responses were attenuated by losartan (p&lt0.01) or PD123319 (p&lt0.01), respectively. mito-ECFP/ANG II increased mitochondrial protein complex I &gt38%, Complex III &gt56%, Complex IV &gt89%, and Complex V &gt63%, respectively (p&lt0.01). The effects of mito-ECFP/ANG II on mitochondrial protein complexes were attenuated by losartan or the expression of mito-AT 2 receptors (p&lt0.01). These results suggest that paracrine and intracellular ANG II may activate AT 1 and AT 2 receptors to induce important MAP kinases ERK1/2, p-PKCα, p38 MAPK, NF-кB signaling, or mitochondria responses in MCs, which may contribute to ANG II-induced glomerular injury in hypertension and kidney diseases.

Abstract 077: Proximal Tubule-Specific Deletion of the NHE3 (na + /h + Exchanger 3) in the Kidney Attenuates Angiotensin II-Induced Hypertension in Mice

The Na + /H + exchanger 3 (NHE3), a ~85 kDa protein encoded by the SLC9A3 gene, is the most important Na + transporter in the proximal tubules of the kidney. Angiotensin II (ANG II) is well-recognized to increase proximal tubule Na + reabsorption by stimulating NHE3 expression and activity in the proximal tubules. However, a direct cause and effect relationship between ANG II and NHE3 in the proximal tubules in ANG II-induced hypertension has not been determined previously. The present study directly tested the hypothesis that NHE3 in the proximal tubules of the kidney is required for the development of ANG II-induced hypertension using proximal tubule-specific NHE3 knockout mice (PT- Nhe3 -/- ). Specifically, PT- Nhe3 -/- mice were generated using the SGLT2-Cre / Nhe3 loxlox approach, whereas ANG II-induced hypertension was studied in 12 groups (n=5-12 per group) of adult male and female wild-type (WT) and PT- Nhe3 -/- mice. Under basal conditions, systolic (SBP), diastolic (DBP), and mean arterial blood pressure (MAP) were significantly lower in male and female PT- Nhe3 -/- than WT mice ( P <0.01). A high pressor, 1.5 mg/kg/day, i.p., or a slow pressor dose of ANG II, 0.5 mg/kg/day, i.p., for 2 weeks significantly increased SBP, DBP, and MAP in male and female WT mice ( P <0.01), but the hypertensive response to ANG II was markedly attenuated in male and female PT- Nhe3 -/- mice ( P <0.01). ANG II impaired the pressure-natriuresis response in WT mice, whereas proximal tubule-specific deletion of NHE3 improved the pressure-natriuresis response in ANG II-infused PT- Nhe3 -/- mice ( P <0.01). AT 1 receptor blocker losartan completely blocked ANG II-induced hypertension in both WT and PT- Nhe3 -/- mice ( P <0.01). However, inhibition of nitric oxide synthase with L-NAME had no effect on ANG II-induced hypertension in WT or PT- Nhe3 -/- mice (n.s.). Furthermore, ANG II-induced hypertension was significantly attenuated by an orally absorbable NHE3 inhibitor AVE0657. In conclusion, NHE3 in the proximal tubules of the kidney is required for the full development of ANG II-induced hypertension. Our results suggest that NHE3 in the proximal tubules may be therapeutically targeted to treat hypertension induced by ANG II or associated with increased NHE3 expression in the proximal tubules.

Abstract P2016: Proximal Tubule-Specific Knockout of Angiotensin II Type 1a Receptors in the Kidney Augments the Natriuretic Response to Angiotensin III in Mice

Angiotensin III (ANG III), a major metabolite of the potent vasopressor hormone ANG II, reportedly binds and activates AT 2 receptors to oppose the AT 1 receptor-mediated antinatriuretic effect of ANG II in the proximal tubules of the kidney. The present study tested the hypothesis that global ( Agtr1a -/- ) or proximal tubule-specific deletion of AT 1a receptors in the kidney (PT- Agtr1a -/- ) attenuates the blood pressure, but augments renal Na + excretory responses to ANG III in mice. Proximal tubule AT 1a receptor knockout mice (PT- Agtr1a -/- ) were generated using the Sglt2-Cre/ Agtr1a -floxed approach. Adult male wild-type littlemates (WT), global ( Agtr1a -/- ), and PT- Agtr1a -/- mice (n =7-10 for each group) were anesthetized and infused with saline or increasing doses of ANG III, at 10, 50, and 100 ng/min, i.v ., respectively, for 60 min each. An additional group of PT- Agtr1a -/- mice (n=7) was infused with ANG III at 1.5 mg/kg/day, i.p ., via osmotic minipump for 2 weeks. Basal systolic, diastolic, and mean arterial blood pressure were significantly lower ( p <0.01), whereas 24 h urinary Na + excretion was significantly higher in Agtr1a -/- and PT- Agtr1a -/- than WT mice ( p <0.01). ANG III significantly increased systolic blood pressure from 82 ± 5 mmHg to 116 ± 8 mmHg in anesthetized WT mice (n=8, p <0.01 vs. baseline), and from 75 ± 3 mmHg to 109 ± 6 mmHg in anesthetized PT- Agtr1a -/- mice (n=10, p <0.01 vs. baseline). There was no significant difference in the net pressor response to ANG III between WT and PT- Agtr1a -/- mice. A similar pressor effect to ANG III was confirmed in conscious PT- Agtr1a -/- mice (Basal: 97 ± 2 mmHg vs. ANG III: 111 ± 5 mmHg, n=7, p <0.05). Global deletion of AT 1a receptors completely blocked the pressor effect of ANG III in Agtr1a -/- mice (n=7, p <0.05 vs. WT or PT- Agtr1a -/- mice). In WT mice, ANG III induced small, but significant, antinatriuretic responses at 10 and 50 ng/min ( p <0.05), respectively. Conversely, ANG III induced small, but significant, natriuretic responses at 50 and 100 ng/min in PT- Agtr1a -/- mice ( p <0.05). These results suggest that the pressor effect of ANG III is mediated by systemic AT 1a receptors, whereas the natriuretic effect of ANG III in PT- Agtr1a -/- mice may involve AT 2 receptors in the proximal tubules of the kidney.

Abstract 102: Genetic Deletion of Sirtuin 3 (SIRT3) in the Mitochondria of the Proximal Tubules Aggravates Angiotensin II-Induced Hypertension by Impairing the Pressure-Natriuresis Response in Proximal Tubule-Specific SIRT3 Knockout Mice

Sirtuin 3 (SIRT3), a key mitochondrial protein in the sirtuin family, plays a key role in maintaining normal mitochondrial function via its anti-oxidative, anti-aging, and anti-inflammatory effects. Conversely, angiotensin II (ANG II), a key peptide of the renin-angiotensin system, plays a key role in the development of cardiovascular and hypertensive injury through its vasoconstrictive, pro-oxidative, pro-growth, and proinflammatory actions, in part by suppressing mitochondrial SIRT3 expression and inducing mitochondrial dysfunction. The present study tested the hypothesis that genetic deletion of SIRT3 selectively in the proximal tubules of the kidney aggravates ANG II-induced hypertension in proximal tubule (PT)-specific SIRT3 knockout mice (PT-SIRT3-KO). PT-SIRT3-KO mice were generated using the SGLT2-Cre/SIRT3-loxP approach. ANG II-dependent hypertension was induced by infusing a slow pressor dose of ANG II, 0.5 mg/kg/day, i.p., and a 2% Na + diet for 2 weeks in adult male wildtype (WT) and PT-SIRT3-KO mice (n=7-12 per group). PT-SIRT3-KO mice developed normally, and showed no abnormal cardiac and kidney structures. Under basal conditions, systolic, diastolic and mean arterial pressure were significantly lower (WT: 119 ± 3 mmHg vs. PT-SIRT3-KO: 111 ± 3 mmHg, P <0.05), whereas urinary Na + excretion was significantly higher in PT-SIRT3-KO than WT mice (WT: 167.9 ± 5.8 μmol/24 h vs. PT-SIRT3-KO: 217.9 ± 7.9 μmol/24 h, P <0.05), without altering urinary K + excretion. ANG II infusion significantly increased systolic blood pressure by 24 ± 3 mmHg in WT mice ( P <0.01), whereas it increased systolic blood pressure by 38 ± 3 mmHg in PT-SIRT3-KO mice ( P <0.01). Conversely, ANG II-induced hypertension was associated with a marked natriuretic response in WT mice (547.7 ± 19.9 μmol/24 h), which was attenuated in PT-SIRT3-KO mice (312.1 ± 11.1 μmol/24 h, P <0.01). Proximal tubule-specific SIRT3 deletion slightly increased the kidney to body wt. ratio (n.s.), but significantly attenuated the heart to body wt. ratio in PT-SIRT3-KO mice ( P <0.01). Our results suggest that genetic deletion of mitochondrial SIRT3 in the proximal tubules of the kidney aggravates ANG II-induced hypertension by impairing the pressure-natriuretic response in PT-SIRT3-KO mice.

Proximal Tubule-Specific Deletion of the NHE3 (Na+/H+ Exchanger 3) in the Kidney Attenuates Ang II (Angiotensin II)-Induced Hypertension in Mice

The present study directly tested the hypothesis that the NHE3 (Na⁺/H⁺ exchanger 3) in the proximal tubules of the kidney is required for the development of Ang II (angiotensin II)-induced hypertension using PT-Nhe3^-/- (proximal tubule-specific NHE3 knockout) mice. Specifically, PT-Nhe3^-/- mice were generated using the SGLT2-Cre/Nhe3^loxlox approach, whereas Ang II-induced hypertension was studied in 12 groups (n=5-12 per group) of adult male and female wild-type (WT) and PT-Nhe3^-/- mice. Under basal conditions, systolic blood pressure, diastolic blood pressure, and mean arterial blood pressure were significantly lower in male and female PT-Nhe3^-/- than WT mice (P<0.01). A high pressor, 1.5 mg/kg per day, intraperitoneal or a slow pressor dose of Ang II, 0.5 mg/kg per day, intraperitoneal for 2 weeks significantly increased systolic blood pressure, diastolic blood pressure, and mean arterial blood pressure in male and female WT mice (P<0.01), but the hypertensive response to Ang II was markedly attenuated in male and female PT-Nhe3^-/- mice (P<0.01). Ang II impaired the pressure-natriuresis response in WT mice, whereas proximal tubule-specific deletion of NHE3 improved the pressure-natriuresis response in Ang II-infused PT-Nhe3^-/- mice (P<0.01). AT₁ receptor blocker losartan completely blocked Ang II-induced hypertension in both WT and PT-Nhe3^-/- mice (P<0.01). However, inhibition of nitric oxide synthase with L-N^G-Nitroarginine methyl ester had no effect on Ang II-induced hypertension in WT or PT-Nhe3^-/- mice (not significant). Furthermore, Ang II-induced hypertension was significantly attenuated by an orally absorbable NHE3 inhibitor AVE0657. In conclusion, NHE3 in the proximal tubules of the kidney may be a therapeutical target in hypertension induced by Ang II or with increased NHE3 expression in the proximal tubules.

Proximal Tubule-Specific Deletion of the NHE3 (Na+/H+ Exchanger 3) Promotes the Pressure-Natriuresis Response and Lowers Blood Pressure in Mice

The present study directly tested the hypothesis that deletion of the NHE3 (Na⁺/H⁺ exchanger 3) selectively in the proximal tubules of the kidney lowers basal blood pressure by increasing the pressure-natriuresis response in mice. Adult male and female, age-matched wild-type (WT) littermates and proximal tubule-specific NHE3 knockout mice (PT- Nhe3^-/-; n=6-16 per group) were studied for (1) basal phenotypes of electrolytes and pH, blood pressure, and kidney function; (2) the pressure-natriuresis response using the mesenteric, celiac, and abdominal arterial occlusion technique; and (3) the natriuretic responses to acute saline expansion (0.9% NaCl, 10% body weight, intraperitoneal) or 2-week of 2% NaCl diet. Under basal conditions, PT- Nhe3^-/- mice showed significantly lower systolic, diastolic, and mean arterial blood pressure ( P<0.01) than WT mice ( P<0.01). PT- Nhe3^-/- mice also exhibited significantly greater diuretic ( P<0.01) and natriuretic responses than WT mice ( P<0.01), without altering 24-hour fecal Na⁺ excretion, plasma pH, Na⁺, and bicarbonate levels. In response to increased renal perfusion pressure by 30 mm Hg, the pressure-natriuresis response increased 5-fold in WT mice ( P<0.01), but it increased 8-fold in PT- Nhe3^-/- mice ( P<0.01). In response to 10% acute saline expansion or 2-week 2% NaCl diet, more pronounced natriuretic responses were demonstrated in PT- Nhe3^-/- than WT mice ( P<0.01). Our results support the scientific premise and physiological relevance that NHE3 in the proximal tubules plays an essential role in maintaining basal blood pressure homeostasis, and genetic deletion of NHE3 selectively in the proximal tubules of the kidney lowers blood pressure by increasing the pressure natriuretic response.

Genetic and genomic evidence for an important role of the Na+/H+ exchanger 3 in blood pressure regulation and angiotensin II-induced hypertension

The sodium (Na⁺)/hydrogen (H⁺) exchanger 3 (NHE3) and sodium-potassium adenosine triphosphatase (Na⁺/K⁺-ATPase) are two of the most important Na⁺ transporters in the proximal tubules of the kidney. On the apical membrane side, NHE3 primarily mediates the entry of Na⁺ into and the exit of H⁺ from the proximal tubules, directly and indirectly being responsible for reabsorbing ~50% of filtered Na⁺ in the proximal tubules of the kidney. On the basolateral membrane side, Na⁺/K⁺-ATPase serves as a powerful engine driving Na⁺ out of, while pumping K⁺ into the proximal tubules against their concentration gradients. While the roles of NHE3 and Na⁺/K⁺-ATPase in proximal tubular Na⁺ transport under in vitro conditions are well recognized, their respective contributions to the basal blood pressure regulation and angiotensin II (ANG II)-induced hypertension remain poorly understood. Recently, we have been fortunate to be able to use genetically modified mouse models with global, kidney- or proximal tubule-specific deletion of NHE3 to directly determine the cause and effect relationship between NHE3, basal blood pressure homeostasis, and ANG II-induced hypertension at the whole body, kidney and/or proximal tubule levels. The purpose of this article is to review the genetic and genomic evidence for an important role of NHE3 with a focus in the regulation of basal blood pressure and ANG II-induced hypertension, as we learned from studies using global, kidney- or proximal tubule-specific NHE3 knockout mice. We hypothesize that NHE3 in the proximal tubules is necessary for maintaining basal blood pressure homeostasis and the development of ANG II-induced hypertension.

Abstract P248: Overexpression of an Intracellular Angiotensin II Fusion Protein Selectively in the Mitochondria of Mouse Proximal Tubule Cells Induces Mitochondrial Stress and Glycolytic Responses: Roles of Nitric Oxide and Superoxide

Endocrine and paracrine angiotensin II ( ANG II) activates cell surface AT 1 receptors to induce mitochondrial superoxide (O 2 - ) production and uncoupling of eNOS, leading to ANG II-induced renal and hypertensive injury. Little is known, however, whether ANG II is internalized to the mitochondria to activate AT 1 or AT 2 receptors. We tested the hypothesis that ANG II in the mitochondria exerts important dual roles on mitochondrial stress and glycolytic responses via the AT 1a /NADPH oxidase/O 2 .- and AT 2 /eNOS/NO signaling pathways. To test the hypothesis, a mitochondria-targeting ANG II protein (mito-ECFP/ANG II) was expressed selectively in the mitochondria of mouse proximal tubule cells, and treated with or without AT 1 receptor blocker losartan (10 μM), AT 2 receptor blocker PD123319 (10 μM), a non-selective NOS inhibitor L-NAME (10 mM), or a scavenger of mitochondrial superoxide, mito-TEMPO (10 μM) for 48 h, respectively. The mitochondrial stress and glycolytic responses were then determined using Seahorse XF e 24 Extracellular Flux Analyzer. Expression of mito-ECFP/ANG II alone significantly increased oxygen consumption rate (OCR) (Control: 259.8 ± 52.2 vs. mito-ECFP/ANG II: 512.5 ± 75.6 pmol/min; p <0.01, n=8) and extracellular acidification rate (ECAR) (Control: 13.5 ± 2.2 vs. mito-ECFP/ANG II: 19.3 ± 2.4 mpH/min; p <0.01, n=8), respectively. These mito-stress responses were blocked by losartan ( p <0.01, n=8), but not by PD123139 ( n.s. ). Furthermore, the mito-stress response of mito-ECFP/ANG II was significantly attenuated by mito-TEMPO (288.1 ± 10.3 pmol/min; p <0.01, n=8), and augmented by L-NAME (623.5 ± 8.7 pmol/min, p <0.05, n=8). Finally, the mito-stress response to mito-ECFP/ANG II expression was associated with significant increases in mitochondrial redox carries, Complex I (NADH coenzyme Q reductase), Complex II (succinate dehydrogenase), Complex III (cytochrome bc 1 complex) and Complex IV proteins (cytochrome c oxidase) ( p <0.01). We concluded that ANG II in the mitochondrial induces important mitochondrial stress and glycolytic responses primarily via the AT 1a /NADPH oxidase/O 2 .- signaling pathways in mouse proximal tubule cells.

Abstract P381: Deletion of Na + /H + Exchanger 3 Selectively in the Proximal Tubule of the Kidney Augments Acute and Chronic Natriuretic Responses to Saline Volume Expansion in Mice

The Na + /H + exchanger 3 (NHE3), a ~85 kDa protein encoded by the SLC9A3 gene, plays a key role in mediating Na + reabsorption in the proximal tubule of the kidney and in maintaining blood pressure homeostasis. In the present study, we tested the hypothesis that deletion of NHE3 selectively in the proximal tubule of the kidney, but not globally, augments acute and chronic natriuretic responses to saline volume expansion in mice. Adult male wildtype ( Nhe3 +/+ ), global NHE3 knockout ( Nhe3 -/- ) and proximal tubule-specific NHE3 knockout mice (PT- Nhe3 -/- ) (n=8-13/group) were subject to acute saline administration of 10% body wt., i.p., or a 2% high salt diet for 2 weeks to determine and compare acute or chronic natriuretic responses. In response to 10% acute saline volume expansion, urinary Na + excretion increased 6.5 fold from basal 18.9 ± 5.0 μmol/3h to 122.2 ± 7.7 μmol/3h in wildtype mice ( p <0.01). By comparison, urinary Na + excretion increased 8.4 fold from basal 26.2 ± 3.9 μmol/3h to 206.5 ± 5.7 μmol/3h in PT- Nhe3 -/- mice ( p <0.01), whereas the natriuretic response increased 4.8 fold in Nhe3 -/- mice ( p <0.01). The differences in the natriuretic responses to acute saline expansion between Nhe3 +/+ , Nhe3 -/- and PT- Nhe3 -/- mice were statistically significant ( p <0.01). However, the diuretic response to acute water volume expansion (10% of body wt., i.p.) was not different between Nhe3 +/+ and PT- Nhe3 -/- mice ( n.s. ). In response to 2% NaCl diet for 2 weeks, 24 h urinary Na + excretion increased 19% in Nhe3 +/+ mice ( n.s. ), whereas the natriuretic response increased 43% in PT- Nhe3 -/- mice ( p <0.01). Interestingly, the natriuretic responses to 2% high salt diet were greater in adult female wildtype (Δ66%, p <0.01) and PT- Nhe3 -/- mice (Δ62%, p <0.01), respectively. By contrast, the natriuretic response to 2% high salt diet was significantly attenuated in global Nhe3 -/- mice ( p <0.01). These data suggest that the deletion of NHE3 selectively in the proximal tubule of the kidney, but not globally, augments the natriuretic responses to acute or chronic saline volume expansion in PT- Nhe3 -/- mice.

Recent Updates on the Proximal Tubule Renin-Angiotensin System in Angiotensin II-Dependent Hypertension

It is well recognized that the renin-angiotensin system (RAS) exists not only as circulating, paracrine (cell to cell), but also intracrine (intracellular) system. In the kidney, however, it is difficult to dissect the respective contributions of circulating RAS versus intrarenal RAS to the physiological regulation of proximal tubular Na(+) reabsorption and hypertension. Here, we review recent studies to provide an update in this research field with a focus on the proximal tubular RAS in angiotensin II (ANG II)-induced hypertension. Careful analysis of available evidence supports the hypothesis that both local synthesis or formation and AT1 (AT1a) receptor- and/or megalin-mediated uptake of angiotensinogen (AGT), ANG I and ANG II contribute to high levels of ANG II in the proximal tubules of the kidney. Under physiological conditions, nearly all major components of the RAS including AGT, prorenin, renin, ANG I, and ANG II would be filtered by the glomerulus and taken up by the proximal tubules. In ANG II-dependent hypertension, the expression of AGT, prorenin, and (pro)renin receptors, and angiotensin-converting enzyme (ACE) is upregulated rather than downregulated in the kidney. Furthermore, hypertension damages the glomerular filtration barrier, which augments the filtration of circulating AGT, prorenin, renin, ANG I, and ANG II and their uptake in the proximal tubules. Together, increased local ANG II formation and augmented uptake of circulating ANG II in the proximal tubules, via activation of AT1 (AT1a) receptors and Na(+)/H(+) exchanger 3, may provide a powerful feedforward mechanism for promoting Na(+) retention and the development of ANG II-induced hypertension.

The vasoprotective axes of the renin-angiotensin system: Physiological relevance and therapeutic implications in cardiovascular, hypertensive and kidney diseases

The renin-angiotensin system (RAS) is undisputedly one of the most prominent endocrine (tissue-to-tissue), paracrine (cell-to-cell) and intracrine (intracellular/nuclear) vasoactive systems in the physiological regulation of neural, cardiovascular, blood pressure, and kidney function. The importance of the RAS in the development and pathogenesis of cardiovascular, hypertensive and kidney diseases has now been firmly established in clinical trials and practice using renin inhibitors, angiotensin-converting enzyme (ACE) inhibitors, type 1 (AT₁) angiotensin II (ANG II) receptor blockers (ARBs), or aldosterone receptor antagonists as major therapeutic drugs. The major mechanisms of actions for these RAS inhibitors or receptor blockers are mediated primarily by blocking the detrimental effects of the classic angiotensinogen/renin/ACE/ANG II/AT₁/aldosterone axis. However, the RAS has expanded from this classic axis to include several other complex biochemical and physiological axes, which are derived from the metabolism of this classic axis. Currently, at least five axes of the RAS have been described, with each having its key substrate, enzyme, effector peptide, receptor, and/or downstream signaling pathways. These include the classic angiotensinogen/renin/ACE/ANG II/AT₁ receptor, the ANG II/APA/ANG III/AT₂/NO/cGMP, the ANG I/ANG II/ACE2/ANG (1-7)/Mas receptor, the prorenin/renin/prorenin receptor (PRR or Atp6ap2)/MAP kinases ERK1/2/V-ATPase, and the ANG III/APN/ANG IV/IRAP/AT₄ receptor axes. Since the roles and therapeutic implications of the classic angiotensinogen/renin/ACE/ANG II/AT₁ receptor axis have been extensively reviewed, this article will focus primarily on reviewing the roles and therapeutic implications of the vasoprotective axes of the RAS in cardiovascular, hypertensive and kidney diseases.

Luminal ANG II is internalized as a complex with AT1R/AT2R heterodimers to target endoplasmic reticulum in LLC-PK1 cells

ANG II has many biological effects in renal physiology, particularly in Ca²⁺ handling in the regulation of fluid and solute reabsorption. It involves the systemic endocrine renin-angiotensin system (RAS), but tissue and intracrine ANG II are also known. We have shown that ANG II induces heterodimerization of its AT₁ and AT₂ receptors (AT₁R and AT₂R) to stimulate sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA) activity. Thus, we investigated whether ANG II-AT₁R/AT₂R complex is formed and internalized, and also examined the intracellular localization of this complex to determine how its effect might be exerted on renal intracrine RAS. Living cell imaging of LLC-PK₁ cells, quantification of extracellular ANG II, and use of the receptor antagonists, losartan and PD123319, showed that ANG II is internalized with AT₁R/AT₂R heterodimers as a complex in a microtubule-dependent and clathrin-independent manner, since colchicine-but not Pitstop2-blocked this process. This result was confirmed by an increase of β-arrestin phosphorylation after ANG II treatment, clathrin-mediated endocytosis being dependent on dephosphorylation of β-arrestin. Internalized ANG II colocalized with an endoplasmic reticulum (ER) marker and increased levels of AT₁R, AT₂R, and PKCα in ER-enriched membrane fractions. This novel evidence suggests the internalization of an ANG II-AT₁/AT₂ complex to target ER, where it might trigger intracellular Ca²⁺ responses.

Abstract 065: Overexpression of an Intracellular Angiotensin II Fusion Protein Selectively in the Mitochondria of the Proximal Tubules Elevates Blood Pressure in Mice via AT 1a Receptor-mediated Mitochondrial Respiratory and Glycolysis Stress

An intracrine mitochondrial renin-angiotensin system (RAS) has recently been identified in various animal and human tissues, but whether the mitochondrial RAS plays a physiological role in the regulation of blood pressure remains unknown. The present study tested whether overexpression of an intracellular angiotensin II fusion protein, ECFP/ANG II, selectively in the mitochondria of the proximal tubules alters blood pressure, and whether the effects may involve AT 1a receptors and the Na + /H + exchanger 3 (NHE3). An adenoviral vector encoding ECFP/ANG II, a mitochondria targeting sequence, and the sglt2 promoter, Ad-sglt2-mito-ECFP/ANG II, was constructed for proximal tubule- and mitochondria-specific overexpression for 2 weeks. In adult male C57BL/6J mice, overexpression of mito-ECFP/ANG II in the mitochondria of the proximal tubules increased systolic blood pressure (SBP) significantly (Control: 116 ± 3 vs. mito-ECFP/ANG II: 128 ± 3 mmHg; p <0.01, n=15). The blood pressure-increasing effect of Ad-sglt2-mito-ECFP/ANG II was blocked in proximal tubule-specific AT 1a -KO mice (Control: 105 ± 2 vs. mito-ECFP/ANG II: 104 ± 4 mmHg; n.s ., n=7), or in proximal tubule-specific NHE3-KO mice (Control: 108 ± 3 vs. mito-ECFP/ANG II: 107 ± 3 mmHg; n.s ., n=13), respectively. In further experiments, mouse proximal tubule cells were transfected with Ad-sglt2-mito-ECFP/ANG II for 48 h and treated with the AT 1 blocker losartan (10 μM) or the AT 2 blocker PD123319 (10 μM) to measure mitochondrial respiratory and glycolytic function using Seahorse XF Cell Mito and XF Glycolysis Stress Tests. The mito-ECFP/ANG II expression was robust and colocalized with MitoTracker® Red FM. Overexpression of mito-ECFP/ANG II markedly increased oxygen consumption rate (OCR) (Control: 139.4 ± 9.2 vs. mito-ECFP/ANG II: 236.3 ± 12.6 pmol/min; p <0.01, n=12) and extracellular acidification rate (ECAR) (Control: 8.8 ± 0.6 vs. mito-ECFP/ANG II: 11.8 ± 1.2 mpH/min; p <0.01, n=12), respectively. Losartan blocked the effects of mito-ECFP/ANG II on OCR and ECAR, whereas PD123319 had no effect. We conclude that intracellular ANG II may activate AT 1 receptors in the mitochondria of the proximal tubules to alter mitochondrial respiratory and glycolytic function and arterial blood pressure.

Abstract 011: An Orally Absorbable Potent NHE3 Inhibitor Attenuates Angiotensin II-induced Hypertension Primarily by Inhibiting NHE3 in the Proximal Tubule of the Kidney

We have recently shown that angiotensin (ANG II)-induced hypertension was attenuated in mice with global ( Nhe3 -/- ) and Nhe3 -/- mice with transgenic rescue of the NHE3 gene selectively in small intestines (tg Nhe3 -/- ), suggesting an important role of NHE3 in the development of ANG II-dependent hypertension. In this study, we specifically tested whether the pharmacological inhibition of NHE3 mainly in the proximal tubules of the kidney attenuates ANG II-dependent hypertension induced by a low and slow pressor dose of ANG II supplemented with a high salt diet. Overall, 9 groups (n=5-12) of adult male C57BL/6J mice were infused with or without ANG II (500 μg/kg/day, i.p. via minipump) and supplemented with or without a 2% NaCl diet to slowly and moderately increase systolic blood pressure (SBP) in 2 weeks. ANG II alone increased SBP from 116 ± 2 mmHg to 140 ± 2 mmHg ( p <0.01), and supplement of ANG II with a 2% NaCl diet further increased SBP to 147 ± 4 mmHg ( p <0.05). Concurrent treatment with an orally active, absorbable NHE3 inhibitor AVE0657 (Sanofi-Aventis; 20 mg/kg/day, p.o.) significantly decreased SBP to 125 ± 4 mmHg in ANG II-infused mice ( p <0.01), and to 134 ± 6 mmHg in ANG II-infused mice supplemented with 2% NaCl ( p <0.01), respectively. Further treatment with AVE0657 and losartan, an AT 1 receptor blocker (20 mg/kg/day, p.o.), completely normalize SBP in mice treated with ANG II and 2% NaCl to control (115 ± 5 mmHg, p <0.01). In the kidney, AVE0657 significantly increased 24h urinary Na + excretion from 157.1 ± 6.7 to 207.7 ± 8.1 μmol/24h ( p <0.01) without altering 24h urine excretion or SBP. Furthermore, AVE0657 did not significantly alter 24 h fecal Na + excretion in non ANG II-infused (4.99 ± 0.37 μmol/24h, n.s.) or ANG II-infused mice (4.19 ± 0.67 μmol/24h, n.s.), compared with control (4.02 ± 0.20 μmol/24h, n.s. ) or global Nhe3 -/- mice (50.8 ± 0.8 μmol/24h, p <0.01). Since small intestines in the gut and the proximal tubules of the kidney express the vast majority of NHE3 in the body, these results provide preclinical evidence and perspectives that orally absorbable NHE3 inhibitors may be pharmacologically beneficial to prevent and treat hypertension induced by ANG II and a high salt, mainly by inhibiting NHE3 in the proximal tubule of the kidney.

Augmentation of angiotensinogen expression in the proximal tubule by intracellular angiotensin II via AT1a/MAPK/NF-кB signaling pathways

Long-term angiotensin II (ANG II) infusion significantly increases ANG II levels in the kidney through two major mechanisms: AT1 receptor-mediated augmentation of angiotensinogen (AGT) expression and uptake of circulating ANG II by the proximal tubules. However, it is not known whether intracellular ANG II stimulates AGT expression in the proximal tubule. In the present study, we overexpressed an intracellular cyan fluorescent ANG II fusion protein (Ad-sglt2-ECFP/ANG II) selectively in the proximal tubule of rats and mice using the sodium and glucose cotransporter 2 (sglt2) promoter. AGT mRNA and protein expression in the renal cortex and 24-h urinary AGT excretion were determined 4 wk following overexpression of ECFP/ANG II in the proximal tubule. Systolic blood pressure was significantly increased with a small antinatriuretic effect in rats and mice with proximal tubule-selective expression of ECFP/ANG II (P < 0.01). AGT mRNA and protein expression in the cortex were increased by >1.5-fold and 61 ± 16% (P < 0.05), whereas urinary AGT excretion was increased from 48.7 ± 5.7 (n = 13) to 102 ± 13.5 (n = 13) ng/24 h (P < 0.05). However, plasma AGT, renin activity, and ANG II levels remained unaltered by ECFP/ANG II. The increased AGT mRNA and protein expressions in the cortex by ECFP/ANG II were blocked in AT1a-knockout (KO) mice. Studies in cultured mouse proximal tubule cells demonstrated involvement of AT1a receptor/MAP kinases/NF-кB signaling pathways. These results indicate that intracellular ANG II stimulates AGT expression in the proximal tubules, leading to increased AGT formation and secretion into the tubular fluid, which contributes to ANG II-dependent hypertension.

Role of the Na+/H+ exchanger 3 in angiotensin II-induced hypertension in NHE3-deficient mice with transgenic rescue of NHE3 in small intestines

The role of Na(+/)H(+) exchanger 3 (NHE3) in the kidney in angiotensin II (ANG II)-induced hypertension remains unknown. The present study used global NHE3-deficient mice with transgenic rescue of the Nhe3 gene in small intestines (tgNhe3(-/-)) to test the hypothesis that genetic deletion of NHE3 selectively in the kidney attenuates ANG II-induced hypertension. Six groups of wild-type (tgNhe3(+/+)) and tgNhe3(-/-) mice were infused with either vehicle or ANG II (1.5 mg/kg/day, i.p., 2 weeks, or 10 nmol/min, i.v., 30 min), treated with or without losartan (20 mg/kg/day, p.o.) for 2 weeks. Basal systolic blood pressure (SBP) and mean intra-arterial blood pressure (MAP) were significantly lower in tgNhe3(-/-) mice (P < 0.01). Basal glomerular filtration rate, 24 h urine excretion, urinary Na(+) excretion, urinary K(+) excretion, and urinary Cl(-) excretion were significantly lower in tgNhe3(-/-) mice (P < 0.01). These responses were associated with significantly elevated plasma ANG II and aldosterone levels, and marked upregulation in aquaporin 1, the Na(+)/HCO3 cotransporter, the α1 subunit isoform of Na(+)/K(+)-ATPase, protein kinase Cα, MAP kinases ERK1/2, and glycogen synthase kinase 3 α/β in the renal cortex of tgNhe3(-/-) mice (P < 0.01). ANG II infusion markedly increased SBP and MAP and renal cortical transporter and signaling proteins in tgNhe3(+/+), as expected, but all of these responses to ANG II were attenuated in tgNhe3(-/-) mice (P < 0.01). These results suggest that NHE3 in the kidney is necessary for maintaining normal blood pressure and fully developing ANG II-dependent hypertension.

Abstract P600: Conditional Knockout of AT1a Receptors Selectively in The Proximal Tubules Attenuates Renal Ischemia-Reperfusion Injury in Mice

The activation of the intrarenal renin-angiotensin system plays an important role in the pathogenesis of renal ischemia-reperfusion injury, but the underlying cellular and signaling mechanisms remain incompletely understood. In the present study, we tested the hypothesis that conditional knockout of AT1a receptors selectively in the proximal tubules attenuates renal ischemia-reperfusion injury in mice. Three groups (n=5-8/per group) of adult male C57BL/6J (WT), global AT1a receptor-deficient (AT1a-KO), and conditional proximal tubule-specific AT1a-KO mice (PT-AT1a-KO) were subjected to sham surgery or 30 min unilateral left kidney ischemia, followed by reperfusion for 24 h or 7 days, respectively. Under basal conditions, systolic blood pressure was 25 ± 3 mmHg lower in global AT1a-KO (p<0.01) or 13 ± 3 mmHg lower in PT-AT1a-KO mice (p<0.05), respectively. Systolic blood pressure, 24 h urine and urinary sodium excretion were not significantly altered in all strains 24 h or 7 days after renal ischemia-reperfusion. Kidney wt. to body wt. ratio, but not heart wt. to body wt. ratio, was significantly increased in both AT1a-KO and PT-AT1a-KO mice (p<0.05). Renal ischemia-reperfusion injury was assessed by Masson Trichrome staining and compared between WT, AT1a-KO and PT-AT1a-KO mice. No significant glomerular, tubulointerstital, and peri-vascular fibrotic responses were observed in sham controls of all strains. However, significant fibrotic responses were observed in the glomeruli, cortical tubulointerstitial and peri-vascular tissues in WT mice 24 h or 7 days after renal ischemia-reperfusion (p<0.01). Surprisingly, however, glomerular, tubulointerstital, and peri-vascular fibrotic responses were significantly worsen, rather than improved, in global AT1a-KO mice (p<0.01). By comparison, conditional deletion of AT1a receptors selectively in the proximal tubules markedly attenuated glomerular, tubulointerstital, and peri-vascular fibrotic responses in PT-AT1a-KO mice 24 h or 7 days after renal ischemia-reperfusion (p<0.01). Our results suggest that AT1a receptors in the proximal tubule play an important role in the pathogenesis of renal ischemia-reperfusion injury, and thus may represent a therapeutic target in the future.

Abstract 077: In Vivo Evidence of AT1a Receptor-Mediated Uptake of Angiotensin II by the Proximal Tubule Visualized by Intravital Multiphoton Imaging

The development of all forms of angiotensin II (ANG II)-dependent hypertension is associated with higher levels of intrarenal ANG II, which are greater than can be explained on the basis of circulating ANG II and suppressed cortical renin expression. In the present study, we used intravital multiphoton imaging to test the hypothesis that AT1 (AT1a) receptor-mediated uptake of ANG II by the proximal tubules of the kidney plays a major role in the underlying mechanisms. Adult male Munich-Wistar rats were anesthetized with Inactin and continuously infused with a pressor dose of Alexa 488-conjugated ANG II (50 ng/min, i.v.) for 2 hr. Time-dependent proximal tubular uptake responses of Alexa 488-ANG II were studied with mean arterial blood pressure maintained at ~150 mmHg throughout the experiment. After 30 min infusion, very low levels of Alexa 488-ANG II were visualized within proximal tubules and cortical collecting ducts (CCDs) (p<0.05). However, high levels of Alexa 488-ANG II were accumulated in the lumen of CCDs, but not that of proximal tubules. After 1 hr infusion, moderate levels of Alexa 488-ANG II were visualized in the proximal tubules, but not in the glomeruli and CCDs. The most striking uptake responses were visualized in all segments of proximal tubules after 2 hr infusion. Internalized Alexa 488-ANG II was predominantly localized to the basolateral side, where it was colocalized with tetramethyl rhodamine methyl ester (TMRM), a mitochondrial membrane potential-dependent dye. TMRM is a lipophilic cationic dye that is primarily accumulated in the mitochondria of proximal tubules. Some internalized Alexa 488-ANG II was visualized around and over the nuclei. Furthermore, the uptake of Alexa 488-ANG II by proximal tubules was significantly attenuated in caveolin 1-knockout mice (p<0.01), and blocked in AT1a receptor-knockout mice (p<0.01). Our results provide strong intravital multiphoton microscopic evidence that circulating ANG II is primarily taken up by the proximal tubules of the kidney via an AT1a receptor-mediated mechanism, and that internalized ANG II may be transported to the mitochondria and the nucleus, where it may alter mitochondrial and nuclear function in the proximal tubules of the kidney.

Abstract P167: Conditional Deletion of the Na+/H+ Exchanger 3 in the Proximal Tubule of the Kidney Promotes Pressure Natriuresis in Mice

The development and progression of most, if not all, forms of hypertension appear to converge on a final common pathway or mechanism, i.e., increased renal salt retention due to significantly impaired pressure natriuresis responses. Yet the factors responsible for resetting the pressure natriuresis response in hypertension remain to be determined. The present study tested the hypothesis that the upregulation of the sodium and hydrogen exchanger 3 (NHE3) impairs while selective knockout of this transporter in the proximal tubule promotes pressure natriuresis and lowers blood pressure in mice. Proximal tubule-specific NHE3 knockout mice (PT-NHE3-KO) were generated by the Cre/Lox approach, and confirmed by complementary PCR, Western blot, and immunofluorescent imaging of NHE3 expression in the proximal tubule of the kidney, respectively. No abnormal histological phenotypes were observed in small intestines and the kidney of PT-NHE3-KO mice. Compared with wild-type mice (WT, n=12), PT-NHE3-KO mice (n=10) had significantly lower basal systolic blood pressure (WT: 116 ± 3 mmHg versus PT-NHE3-KO: 103 ± 3 mmHg, p<0.01). Mean intra-arterial pressure was also significantly lower in anesthetized PT-NHE3-KO mice (WT: 91 ± 3 mmHg versus PT-NHE3-KO: 77 ± 5 mmHg, p<0.01) (n=8 each). The lower basal blood pressure in PT-NHE3-KO mice was associated with higher basal urine flow (WT: 0.81 ± 0.09 ml/day versus PT-NHE3-KO: 1.12 ± 0.06 ml/day, p<0.05), urinary sodium (WT: 120.1 ± 17.5 μmol/day versus PT-NHE3-KO: 197.1 ± 24.5 μmol/day, p<0.01), potassium (WT: 171.7 ± 26.0 μmol/day versus PT-NHE3-KO: 310.3 ± 24.3 μmol/day, p<0.01), and chloride excretion (WT: 125.7 ± 22.5 μmol/day versus PT-NHE3-KO: 222 ± 29.2 μmol/day, p<0.05) (n=8-12 for each group). In response to ~25 mmHg increase in renal perfusion pressure in both strains, urinary sodium excretion was increased by 4-fold in WT mice (n=12, p<0.01), whereas it increased by 7-fold in PT-NHE3-KO mice (n=10, p<0.01). We concluded that NHE3 in the proximal tubule of the kidney plays an important role in the regulation of proximal tubular sodium reabsorption and blood pressure homeostasis, and that selective deletion of NHE3 in the proximal tubule promotes the pressure natriurestic response and lowers blood pressure in mice.

Role of the Na+/H+ exchanger 3 in angiotensin II-induced hypertension

The renal mechanisms responsible for angiotensin II (ANG II)-induced hypertension remain incompletely understood. The present study tested the hypothesis that the Na(+)/H(+) exchanger 3 (NHE3) is required for ANG II-induced hypertension in mice. Five groups of wild-type (Nhe3(+/+)) and Nhe3(-/-) mice were treated with vehicle or high pressor doses of ANG II (1.5 mg/kg/day ip, via minipump for 2 wk, or 10 pmol/min iv for 30 min). Under basal conditions, Nhe3(-/-) mice had significantly lower systolic blood pressure (SBP) and mean intra-arterial pressure (MAP) (P < 0.01), 24 h urine (P < 0.05), urinary Na(+) (P < 0.01) and urinary K(+) excretion (P < 0.01). In response to ANG II, SBP and MAP markedly increased in Nhe3(+/+) mice in a time-dependent manner, as expected (P < 0.01). However, these acute and chronic pressor responses to ANG II were significantly attenuated in Nhe3(-/-) mice (P < 0.01). Losartan blocked ANG II-induced hypertension in Nhe3(+/+) mice but induced marked mortality in Nhe3(-/-) mice. The attenuated pressor responses to ANG II in Nhe3(-/-) mice were associated with marked compensatory humoral and renal responses to genetic loss of intestinal and renal NHE3. These include elevated basal plasma ANG II and aldosterone and kidney ANG II levels, salt wasting from the intestines, increased renal AQP1, Na(+)/HCO3 (-), and Na(+)/K(+)-ATPase expression, and increased PKCα, mitogen-activated protein kinases ERK1/2, and glycogen synthase kinase 3αβ signaling proteins in the proximal tubules (P < 0.01). We concluded that NHE3 in proximal tubules of the kidney, along with NHE3 in intestines, is required for maintaining basal blood pressure as well as the full development of ANG II-induced hypertension.

Abstract 405: Novel Signaling Mechanisms by Which Intracellular Angiotensin II Induces Na + /HCO 3 - CotransporterExpression In The Proximal Tubule of The Kidney

Previous studies have shown that endocrine and/or paracrine angiotensin II (ANG II) plays an important role in the regulation of sodium and bicarbonate reabsorption in the proximal tubule of the kidney. However, it is not known whether intracellular (or intracrine) ANG II also plays a role in these responses in the proximal tubule. The present study tested the hypothesis that overexpression of an intracellular cyan fluorescent fusion protein of ANG II (ECFP/ANG II) in the proximal tubule of the kidney induces the expression of the Na + /HCO 3 - cotransporter via MAPK- and NF-kB signaling pathways. To test the hypothesis, transport-competent mPCTs from wild-type and type 1a ANG II receptor-deficient mice (AT 1a -KO) were transfected with ECFP/ANG II, and treated with the AT 1 receptor blocker losartan, the MEK1/MEK2 inhibitor U0126, or the NF-κB inhibitor RO 106-9920. In wild-type mPCT cells, the expression of ECFP/ANG II more than doubled total and/or phosphorylated NHE3 antiporter and Na + /HCO 3 - cotransporter proteins (p<0.01). These response were accompanied by more than threefold increases in phospho-ERK 1/2, p65 subunit of NF-κB, and phospho-IKKα/β (Ser 176/180) proteins (p<0.01). Pretreatment of mPCT cells with losartan, U0126, or RO 106-9920 significantly blocked the effects of ECFP/ANG II (p<0.01). Furthermore, the effects of ECFP/ANG II were significantly attenuated in mPCT cells of AT 1a -KO mice (p<0.01),. In wild-type C57BL/6J mice, adenovirus-mediated overexpression of ECFP/ANG II selectively in the proximal tubule of the kidney, driven by the sodium and glucose cotransporter 2 (sglt2) promoter, significantly increased blood pressure, total and/or phosphorylated NHE3 and Na + /HCO 3 - proteins, and proximal tubular lithium reabsorption (p<0.01). These responses to ECFP/ANG II as observed in C57BL/6J mice were also attenuated in AT 1a -KO mice (p<0.01). Our results strongly suggest that intracellular ANG II may induce NHE3 and Na + /HCO 3 - expression, and increase proximal tubular sodium and bicarbonate reabsorption via AT 1a receptor-mediated activation of MAP kinases ERK 1/2 and NF-κB signaling pathways.

Role of caveolin 1 in AT1a receptor-mediated uptake of angiotensin II in the proximal tubule of the kidney

Caveolin 1 (CAV-1) functions not only as a constitutive scaffolding protein of caveolae but also as a vesicular transporter and signaling regulator. In the present study, we tested the hypothesis that CAV-1 knockout (CAV-1 KO) inhibits ANG II type 1 [AT1 (AT1a)] receptor-mediated uptake of ANG II in the proximal tubule and attenuates blood pressure responses in ANG II-induced hypertension. To determine the role of CAV-1 in mediating the uptake of FITC-labeled ANG II, wild-type (WT) mouse proximal convoluted tubule cells were transfected with CAV-1 small interfering (si)RNA for 48 h before AT1 receptor-mediated uptake of FITC-labeled ANG II was studied. CAV-1 siRNA knocked down CAV-1 expression by >90% (P < 0.01) and inhibited FITC-labeled ANG II uptake by >50% (P < 0.01). Moreover, CAV-1 siRNA attenuated ANG II-induced activation of MAPK ERK1/2 and Na(+)/H(+) exchanger 3 expression, respectively (P < 0.01). To determine whether CAV-1 regulates ANG II uptake in the proximal tubule, Alexa 488-labeled ANG II was infused into anesthetized WT and CAV-1 KO mice for 60 min (20 ng/min iv). Imaging analysis revealed that Alexa 488-labeled ANG II uptake was decreased by >50% in CAV-1 KO mice (P < 0.01). Furthermore, Val(5)-ANG II was infused into WT and CAV-1 KO mice for 2 wk (1.5 mg·kg(-1)·day(-1) ip). Basal systolic pressure was higher, whereas blood pressure and renal excretory and signaling responses to ANG II were attenuated, in CAV-1 KO mice (P < 0.01). We concluded that CAV-1 plays an important role in AT1 receptor-mediated uptake of ANG II in the proximal tubule and modulates blood pressure and renal responses to ANG II.

Mechanisms of AT1a receptor-mediated uptake of angiotensin II by proximal tubule cells: a novel role of the multiligand endocytic receptor megalin

The present study tested the hypothesis that the multiligand endocytic receptor megalin is partially involved in the uptake of ANG II and downstream signaling responses in mouse proximal tubule cells (mPCT) by interacting with AT1a receptors. mPCT cells of wild-type (WT) and AT1a receptor-deficient (AT1a-KO) mice were treated with vehicle, the AT1 receptor blocker losartan (10 μM), or a selective megalin small interfering (si) RNA for 48 h. The uptake of fluorescein (FITC)-labeled ANG II (10 nM, 37°C) and downstream signaling responses were analyzed by fluorescence imaging and Western blotting. AT1a receptors and megalin were abundantly expressed in mPCT cells, whereas AT1a receptors were absent in AT1a-KO mPCT cells (P < 0.01). In WT mPCT cells, FITC-ANG II uptake was visualized at 30 min in the cytoplasm and in the nuclei 1 h after exposure. Losartan alone completely blocked the uptake of FITC-ANG II, whereas megalin siRNA inhibited only 30% of the response (P < 0.01). The remaining FITC-ANG II uptake in the presence of megalin siRNA was completely abolished by losartan. ANG II induced threefold increases in phosphorylated MAP kinases ERK1/2 and a onefold increase in phosphorylated sodium and hydrogen exchanger 3 (NHE3) proteins, which were also blocked by losartan and megalin-siRNA. By contrast, losartan and megalin siRNA had no effects on these signaling proteins in AT1a-KO mPCT cells. We conclude that the uptake of ANG II and downstream MAP kinases ERK1/2 and NHE3 signaling responses in mPCT cells are mediated primarily by AT1a receptors. However, megalin may also play a partial role in these responses to ANG II.