NHE Activity Research Articles

Effects of Dapagliflozin in Combination with Hydrochlorothiazide on Cardiac Remodeling in Rats with Heart Failure After Myocardial Infarction.

In addition to its antihypertensive and diuretic effects, hydrochlorothiazide also demonstrates additional cardioprotective properties; however, the existence of a synergistic interaction between dapagliflozin and hydrochlorothiazide remains unclear.To establish a rat model of heart failure for investigating the effects and mechanisms of dapagliflozin in combination with hydrochlorothiazide during early intervention, H9c2 cells were cultured to validate their in vitro efficacy. The combination group exhibits a synergistic improvement in hemodynamics, ejection fraction, and a reduction in plasma B-type natriuretic peptide concentration. This combination effectively decreases collagen volume fraction and the expression of collagen I and III, p47phox, p67phox, NF-κB p65, Bax, and caspase-3. The combination group demonstrates a synergistic effect in enhancing cardiac function, attenuating oxidative stress and inflammation. The in vitro effects of the combination were demonstrated in H9c2 cardiomyocytes. In addition, the combination exhibits a more pronounced inhibitory effect on NHE1 expression. The expression of NHE1 in H9c2 cells is inhibited by hydrochlorothiazide, thereby alleviating the consequences of NHE1 overexpression. The results of molecular docking and kinetic simulations indicate a strong binding affinity (-6.1 kcal/mol) between hydrochlorothiazide and NHE1, resulting in the formation of a stable conformation. This may elucidate the underlying mechanism responsible for the synergistic effects of drugs.The combination of dapagliflozin and hydrochlorothiazide has synergistic effects on improving cardiac function, oxidative stress, and inflammation in rats with heart failure. Hydrochlorothiazide binds to and inhibits the expression of NHE1, thereby enhancing dapagliflozin's inhibitory effect on NHE activity. This mechanism potentially elucidates its enhanced cardioprotective effects.

Zinc-alpha2-glycoprotein modulates blood pressure by regulating renal lipid metabolism reprogramming - mediated urinary Na+ excretion in hypertension.

Organs modulating blood pressure are associated with a common cytokine known as adipokines. We chose Zinc-alpha2-glycoprotein (ZAG) due to its prioritized transcriptional level in the database. Previous studies showed that ZAG is involved in metabolic disorders. The aim of this study was to investigate its role in hypertension. Serum ZAG levels were assessed in hypertensive and healthy participants. Blood pressure was monitored in Azgp1-/- mice and other animal models by 24-hour ambulatory implanted telemetric transmitters and tail-cuff method. Multi-omics analysis of proteomics and metabolomics were performed to explore possible mechanisms. Serum ZAG levels were significantly decreased and associated with morning urine Na+ excretion in hypertensive participants in a cross-sectional study. This study firstly reported that Azgp1-/- mice exhibited increased blood pressure and impaired urinary Na+ excretion, which were restored by AAV9-mediated renal tubule Azgp1 rescue. Azgp1 knockout caused the reprogramming of renal lipid metabolism, and increased Na+/H+-exchanger (NHE) activity in the renal cortex. Administration with a NHE inhibitor EIPA reversed the impaired urinary Na+ excretion in Azgp1-/- mice. Moreover, the activity of carnitine palmitoyltransferase 1 (CPT1), a key enzyme of fatty acid β-oxidation, was decreased, and the levels of malonyl-CoA, an inhibitor of CPT1, were increased in renal cortex of Azgp1-/- mice. Renal Cpt1 rescue improved urinary Na+ excretion and blood pressure in Azgp1-/- mice, accompanied by decreased renal fatty acid levels and NHE activity. Finally, administration of recombinant ZAG protein improved blood pressure and urinary Na+ excretion in SHRs. Deficiency of Azgp1 increased the malonyl CoA-mediated inhibition of CPT1 activity, leading to renal lipid metabolism reprogramming, resulting in accumulated fatty acids and increased NHE activity, subsequently decreasing urinary Na+ excretion and causing hypertension. These findings may provide a potential kidney-targeted therapy in the prevention and treatment of hypertension.

Confinement controls the directional cell responses to fluid forces

Our understanding of how fluid forces influence cell migration in confining environments remains limited. By integrating microfluidics with live-cell imaging, we demonstrate that cells in tightly-but not moderately-confined spaces reverse direction and move upstream upon exposure to fluid forces. This fluid force-induced directional change occurs less frequently when cells display diminished mechanosensitivity, experience elevated hydraulic resistance, or sense a chemical gradient. Cell reversal requires actin polymerization to the new cell front, as shown mathematically and experimentally. Actin polymerization is necessary for the fluid force-induced activation of NHE1, which cooperates with calcium to induce upstream migration. Calcium levels increase downstream, mirroring the subcellular distribution of myosin IIA, whose activation enhances upstream migration. Reduced lamin A/C levels promote downstream migration of metastatic tumor cells by preventing cell polarity establishment and intracellular calcium rise. This mechanism could allow cancer cells to evade high-pressure environments, such as the primary tumor.

NHE1 regulation in NAFLD in vitro contributes to hepatocyte injury and HSC crosstalk.

Non-alcoholic fatty liver disease (NAFLD) is the fastest growing cause of liver-associated death globally. Whole-body knockout (KO) of Na+/H+ exchanger 1 (NHE1, SLC9A1) was previously proposed to protect against high fat diet-induced liver damage, however, mechanistic insight was lacking. The aim of the present work was to address this question in vitro to determine how NHE1 specifically in hepatocytes impacts lipid overload-induced inflammation, fibrosis, and hepatocyte- hepatic stellate cell (HSC) crosstalk. We induced palmitate (PA)-based steatosis in AML12 and HepG2 hepatocytes, manipulated NHE1 activity pharmacologically and by CRISPR-Cas knockout (KO) and -overexpression, and measured intracellular pH (pHi), steatosis-associated inflammatory and fibrotic mediators and cell death. PA treatment increased NHE1 mRNA levels but modestly reduced NHE1 protein expression and hepatocyte pHi. NHE1 KO in hepatocytes did not alter lipid droplet accumulation but reduced inflammatory signaling (p38 MAPK activity), lipotoxicity (4-HNE accumulation) and apoptosis (PARP cleavage). Conditioned medium from PA-treated hepatocytes increased expression of NHE1 and of the fibrosis regulator tissue inhibitor of matrix metalloproteases-2 (TIMP2) in LX-2 HSCs, in a manner abolished by NHE1 KO in hepatocytes. We conclude that NHE1 is regulated in NAFLD in vitro and contributes to the ensuing damage by aggravating hepatocyte injury and stimulating hepatocyte-HSC crosstalk.

Acidic preconditioning induced intracellular acid adaptation to protect renal injury via dynamic phosphorylation of focal adhesion kinase-dependent activation of sodium hydrogen exchanger 1

BackgroundDisruptions in intracellular pH (pHi) homeostasis, causing deviations from the physiological range, can damage renal epithelial cells. However, the existence of an adaptive mechanism to restore pHi to normalcy remains unclear. Early research identified H+ as a critical mediator of ischemic preconditioning (IPC), leading to the concept of acidic preconditioning (AP). This concept proposes that short-term, repetitive acidic stimulation can enhance a cell’s capacity to withstand subsequent adverse stress. While AP has demonstrated protective effects in various ischemia-reperfusion (I/R) injury models, its application in kidney injury remains largely unexplored.MethodsAn AP model was established in human kidney (HK2) cells by treating them with an acidic medium for 12 h, followed by a recovery period with a normal medium for 6 h. To induce hypoxia/reoxygenation (H/R) injury, HK2 cells were subjected to hypoxia for 24 h and reoxygenation for 1 h. In vivo, a mouse model of IPC was established by clamping the bilateral renal pedicles for 15 min, followed by reperfusion for 4 days. Conversely, the I/R model involved clamping the bilateral renal pedicles for 35 min and reperfusion for 24 h. Western blotting was employed to evaluate the expression levels of cleaved caspase 3, cleaved caspase 9, NHE1, KIM1, FAK, and NOX4. A pH-sensitive fluorescent probe was used to measure pHi, while a Hemin/CNF microelectrode monitored kidney tissue pH. Immunofluorescence staining was performed to visualize the localization of NHE1, NOX4, and FAK, along with the actin cytoskeleton structure in HK2 cells. Cell adhesion and scratch assays were conducted to assess cell motility.ResultsOur findings demonstrated that AP could effectively mitigate H/R injury in HK2 cells. This protective effect and the maintenance of pHi homeostasis by AP involved the upregulation of Na+/H+ exchanger 1 (NHE1) expression and activity. The activity of NHE1 was regulated by dynamic changes in pHi-dependent phosphorylation of Focal Adhesion Kinase (FAK) at Y397. This process was associated with NOX4-mediated reactive oxygen species (ROS) production. Furthermore, AP induced the co-localization of FAK, NOX4, and NHE1 in focal adhesions, promoting cytoskeletal remodeling and enhancing cell adhesion and migration capabilities.ConclusionsThis study provides compelling evidence that AP maintains pHi homeostasis and promotes cytoskeletal remodeling through FAK/NOX4/NHE1 signaling. This signaling pathway ultimately contributes to alleviated H/R injury in HK2 cells.

Na+/H+-exchange inhibition by cariporide is compensated via Na+,HCO3−-cotransport and has no net growth consequences for ErbB2-driven breast carcinomas

Defense against intracellular acidification of breast cancer tissue depends on net acid extrusion via Na+,HCO3−-cotransporter NBCn1/Slc4a7 and Na+/H+-exchanger NHE1/Slc9a1. NBCn1 is increasingly recognized as breast cancer susceptibility protein and promising therapeutic target, whereas evidence for targeting NHE1 is discordant. Currently, selective small molecule inhibitors exist against NHE1 but not NBCn1. Cellular assays—with some discrepancies—link NHE1 activity to proliferation, migration, and invasion; and disrupted NHE1 expression can reduce triple-negative breast cancer growth. Studies on human breast cancer tissue associate high NHE1 expression with reduced metastasis and—in some molecular subtypes—improved patient survival. Here, we evaluate Na+/H+-exchange and therapeutic potential of the NHE1 inhibitor cariporide/HOE-642 in murine ErbB2-driven breast cancer. Ex vivo, cariporide inhibits net acid extrusion in breast cancer tissue (IC50 = 0.18 μM) and causes small decreases in steady-state intracellular pH (pHi). In vivo, we deliver cariporide orally, by osmotic minipumps, and by intra- and peritumoral injections to address the low oral bioavailability and fast metabolism. Prolonged cariporide administration in vivo upregulates NBCn1 expression, shifts pHi regulation towards CO2/HCO3−-dependent mechanisms, and shows no net effect on the growth rate of ErbB2-driven primary breast carcinomas. Cariporide also does not influence proliferation markers in breast cancer tissue. Oral, but not parenteral, cariporide elevates serum glucose by ∼1.5 mM. In conclusion, acute administration of cariporide ex vivo powerfully inhibits net acid extrusion from breast cancer tissue but lowers steady-state pHi minimally. Prolonged cariporide administration in vivo is compensated via NBCn1 and we observe no discernible effect on growth of ErbB2-driven breast carcinomas.

Abstract We120: NHE1 Regulates Apoptosis Susceptibility in Sugen-Hypoxia Pulmonary Arterial Smooth Muscle Cells via Modulation of CHOP

Pulmonary hypertension (PH) is caused by lung vascular remodeling involving increased pulmonary arterial smooth muscle cell (PASMC) proliferation and apoptosis resistance. Na +/ H + exchanger (NHE) isoform 1 (NHE1) is a membrane protein regulating intracellular pH (pH i ) via movement of H + . Our lab found increased NHE activity is associated with hyperproliferation in PASMCs from the Sugen Hypoxia (SuHx) animal model of PH. In this study, we tested the hypothesis that increased NHE1 activity also causes apoptosis resistance in SuHx PASMCs via dysregulation of pH i . PASMCs isolated from distal pulmonary arteries of control and SuHx rats were treated with NHE inhibitor, [5-(N-ethyl-N-isopropyl)-amiloride; EIPA;1 uM], NHE1 inhibitor [cariporide;10 uM] or vehicle for 24 h before analyzing cell apoptosis via Hoechst staining under baseline and stimulated (250 uM H 2 O 2 ) conditions. Control PASMCs were infected with adenovirus containing green fluorescent protein (GFP), wild-type NHE1 (WT) or NHE1 lacking ion translocation activity (E262I). Protein expression and localization were assessed via immunoblotting. NHE activity was assessed from Na + -dependent recovery following an ammonium pulse. At baseline, apoptosis in SuHx PASMCs was similar to control cells but following H 2 O 2 stimulation , SuHx PASMCs were resistant to apoptosis (p<0.001). EIPA increased basal and stimulated apoptosis in SuHx cells. Surprisingly, treatment with cariporide did not alter SuHx apoptosis, suggesting that EIPA but not cariporide was able to sensitize SuHx cells to apoptosis, despite both drugs significantly inhibiting NHE activity, suggesting EIPA affects PASMC apoptosis through a mechanism other than pH i regulation. To test this possibility, control PASMCs were infected with GFP, WT or E262I. NHE activity was increased in cells with WT, but not E262I, when compared to GFP (p<0.001). Overexpression of either WT or E262I inhibited apoptosis following treatment with H2O2 (p<0.001). We next explored CAAT/enhancer binding protein homologous transcription factor (CHOP), known to be involved in induction of apoptosis. CHOP expression was increased by H2O2 in control cells but was decreased in SuHx PASMCs. EIPA increased CHOP levels in SuHx cells, whereas, expression of both WT and E262I constructs inhibited the H 2 O 2 -induced CHOP increase in control PASMCs. Our results indicate that NHE1 regulates apoptosis susceptibility in PASMCs during PH via modulation of CHOP rather than through pH i regulation.

Empagliflozin prevents heart failure through inhibition of the NHE1-NO pathway, independent of SGLT2

Sodium glucose cotransporter 2 inhibitors (SGLT2i) constitute the only medication class that consistently prevents or attenuates human heart failure (HF) independent of ejection fraction. We have suggested earlier that the protective mechanisms of the SGLT2i Empagliflozin (EMPA) are mediated through reductions in the sodium hydrogen exchanger 1 (NHE1)-nitric oxide (NO) pathway, independent of SGLT2. Here, we examined the role of SGLT2, NHE1 and NO in a murine TAC/DOCA model of HF. SGLT2 knockout mice only showed attenuated systolic dysfunction without having an effect on other signs of HF. EMPA protected against systolic and diastolic dysfunction, hypertrophy, fibrosis, increased Nppa/Nppb mRNA expression and lung/liver edema. In addition, EMPA prevented increases in oxidative stress, sodium calcium exchanger expression and calcium/calmodulin-dependent protein kinase II activation to an equal degree in WT and SGLT2 KO animals. In particular, while NHE1 activity was increased in isolated cardiomyocytes from untreated HF, EMPA treatment prevented this. Since SGLT2 is not required for the protective effects of EMPA, the pathway between NHE1 and NO was further explored in SGLT2 KO animals. In vivo treatment with the specific NHE1-inhibitor Cariporide mimicked the protection by EMPA, without additional protection by EMPA. On the other hand, in vivo inhibition of NOS with L-NAME deteriorated HF and prevented protection by EMPA. In conclusion, the data support that the beneficial effects of EMPA are mediated through the NHE1-NO pathway in TAC/DOCA-induced heart failure and not through SGLT2 inhibition.

SGLT2i Alleviates Atherosclerosis by Inhibiting NHE1 Activation to Protect against Macrophage Senescence Induced by Angiotensin II.

Sodium-dependent glucose transporter (SGLT2) inhibitors (SGLT2i) have been found to have anti-atherosclerotic effects in clinical treatment. The aim of this study was to explore whether angiotensin II (Ang II) induces changes in the expression of Na+/H+ exchanger of cytoplasmic membrane channel proteins (NHE1) and SGLT2 in macrophages and whether dapagliflozin (DAPA), an SGLT2i, protects against Ang II induced macrophage senescence by inhibiting NHE1 activation to alleviate Atherosclerosis (AS). After intervention with DAPA plus gavage or feeding them a high-fat diet, the mice's aortas were dissected, and oil red O staining was performed. Cell proliferation and toxicity detection, western blot, immunofluorescence, and β-galactosidase staining methods were adopted to detect cell activity, expressions of senescence-related genes, and number of senescent cells after different concentrations of Ang II or DAPA or plasmid NHE1 were treated with RAW264.7 cells. (1) The formation of AS plaques in ApoE -/- mice showed a downward trend under DAPA. (2) After the intervention of Ang II, the cell activity of RAW264.7 decreased, and the expression of senescent cells and related genes increased. (3) Under the Ang II condition, the expression of SGLT2 and NHE1 increased, and SGLT2, NHE1, and senescence-related genes decreased with the addition of DAPA. (4) The expression of NHE1, senescent cells and related genes decreased in RAW264.7 cells after DAPA treatment with plasmid NHE1 intervention. SGLT2i alleviates atherosclerosis by inhibiting NHE1 activation to protect against macrophage senescence induced by Ang II.

Sex Differences in Ammoniagenesis in Obese, Diabetic, ZSF1 Rats

Diabetic ketoacidosis develops when insulin is insuffcient to clear blood sugar into cells for energy, then the liver breaks down fat for fuel with production of ketoacids. Lower pH activates proximal tubule PEPCK expression to replenish bicarbonate and excrete acid via NHE3 as ammonia, known as, “ammoniagenesis.” Baseline female (F) rodents exhibit greater ammoniagenesis and less NHE3 activity than males (M). We examined ammoniagenesis in male (M) and female (F) ZSF1 rats, a model of obesity and diabetic kidney disease. ZSF1 rats are born lean (L) or obese (O). OM display hyperglycemia and glucosuria at 10 wk, not yet evident in OF at 18 wk. We tested three hypotheses in ZSF1 rats (n=4-6/group): 1) ammoniagenesis rate is greater in OM and OF than LM and LF, 2) LF exhibit a higher rate of ammoniagenesis than OF, and 3) the blood glucose lowering SGLT2 inhibitor empagliflozin (SGLT2i) reduces ammoniagenesis in OM. Ammoniagenesis was measured as ammonia/24 hr/tibia length because OM and OF weigh 50% more than LM and LF. Abundance of PEPCK and NHE3 was assessed by semiquantitative immunoblots. Titratable acidity (TA) was measured as classically defined. In OM, ammoniagenesis was 6X greater (P< 0.0001), and TA elevated (P=0.0003) versus LM. Ammoniagenesis was not different in OF vs LF (P=0.99). Ammoniagenesis was only 2X greater in OM vs OF (P=0.001) indicative of higher baseline ammoniagenesis in F vs M. PEPCK abundance was 2.8X greater in both OM and OF vs LM and LF (P< 0.001), and NHE3 abundance was lower by 35 and 20%, respectively, in OM and OF vs LM and LF (P< 0.003). Oral SGLT2i treatment (10 mg/d, n=5-6/group, 6 wks starting at 20 wk old) vs vehicle lowered plasma glucose 35%, doubled urine glucose excretion, but did not significantly alter ammoniagenesis, PEPCK or NHE3 abundance (but tended to reduce urine TA). These findings demonstrate that at 16 weeks, OM exhibit highly elevated ammoniagenesis and TA along with hyperglycemia. In contrast, OF do not exhibit hyperglycemia nor elevated ammoniagenesis, illustrating a resistance to diabetic ketoacidosis at 16 wk, despite obesity, higher TA (P= 0.003), higher PEPCK and lower NHE3. In 20 wk old OM ZSF1, SGLT2i treatment does not reverse ammoniagenesis despite lowering plasma glucose for 6 weeks. R01 DK083785, R25DK113659, NIH Step Up U24 DK115255,. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Sglt2-independent effects of gliflozins on proximal tubule function

Background: Beyond glycemic control, Sglt2 inhibitors (Sglt2i) have protective effects on renal function. Sglt2i-mediated reductions in Na+ reabsorption have been suggested to protect PT cells from damage. The mechanisms responsible for renoprotection are unknown and have been suggested to involve inhibition of NHE3 leading to reduced ATP-dependent tubular workload and mitochondrial oxygen consumption. NHE3 activity is also important for regulation of endosomal pH, and the effects of Sglt2i on endocytic function are unknown. Aim of this study: to dissect the direct and immediate effects of SGLT2i on PT cell functions using opossum kidney (OK) PT cells, which represents a powerful physiologically-relevant system model. Methods: We tested the direct effects of Sglt2i on Na+-dependent fluid transport and endocytic uptake in a highly differentiated cell culture model that recapitulates the morphology, metabolism, and high endocytic and transport capacity of PT cells in vivo. The effect of gliflozins on albumin uptake and on fluid transport was quantified in OK PT cells. The effect of canagliflozin vs the NHE3 inhibitor S3226 on early endosome pH in OK cells was measured using fluorescence ratio imaging. Effects of gliflozins, NHE3 inhibitors, and AMPK pathway perturbants were determined by immunoblotting. 10 week-old male C57BL/6 mice were given canagliflozin or empagliflozin by oral gavage daily following vehicle gavage. Urine was collected via metabolic cages at baseline and after 24 h and 48h. Creatinine and albumin were measured by ELISA. Results: Canagliflozin but not empagliflozin reduced fluid transport across cell monolayers and inhibited endocytic uptake of albumin. These effects were independent of glucose and occurred at clinically relevant concentrations of drug. Canagliflozin did not affect endosomal pH, and albumin uptake was further reduced when added together with the selective NHE3 inhibitor S3226. Additionally, canagliflozin did not alter NHE3 phosphorylation when added alone, but prevented the reduction in phosphorylation caused by the selective NHE3 inhibitor S3226. Mice given a single dose of canagliflozin excreted twice as much urine over 24 h compared with empagliflozin-treated animals, suggesting that off-target effects of canagliflozin contribute significantly to reduced Na+-dependent fluid transport in vivo. Conclusions: Canagliflozin reduces fluid transport and albumin uptake in PT cells via effects on mitochondrial function upstream of the AMPK/mTOR axis. Sglt2i-mediated reductions in Na+-dependent fluid transport in the PT are secondary to reduced mitochondrial function and are not NHE3-specific. ASN Pre-doctoral fellowship Award, National Institutes of Health: R01 DK118726, R01 DK125049, S10 OD021627, Dialysis Clinic Inc. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Physical and functional interaction between NHE3 and SGLT2 in the renal proximal tubule of male and female rats

Sodium-glucose cotransporter type 2 (SGLT2) and sodium/hydrogen exchanger-3 (NHE3) serve as the primary renal proximal tubule transporters responsible for the apical uptake of filtered glucose and NaHCO3/NaCl, respectively. Our previous studies demonstrated that chronic treatment with the selective SGLT2 inhibitor empagliflozin reduces NHE3 activity in the proximal tubule. Furthermore, we observed that the non-specific SGLT1/SGLT2 inhibitor, phlorizin, acutely inhibits NHE3 in the absence of luminal glucose, suggesting that the impact of SGLT inhibition on NHE3 may not be attributed to high tubular glucose concentrations. Additionally, our investigations revealed that SGLT2, but not SGLT1, colocalizes with NHE3 in the renal proximal tubule. This study aimed to gain insights into the functional and physical association of NHE3 and SGLT2 in the renal proximal tubule. In vivo microperfusion was conducted in male and female Wistar rats using glucose-free perfusates containing vehicle or empagliflozin (1 μM). Empagliflozin treatment significantly decreased the flow rate of bicarbonate reabsorption (JHCO3−) in both male (2.109±0.091 vs. 1.119±0.096 nmol/cm2∙s) and female rats (1.750±0.037 vs. 1.128±0.062 nmol/cm2∙s), indicating that SGLT2 inhibitors inhibit NHE3 activity through glucose-independent mechanisms. By employing nondenaturant detergents, we utilized blue native polyacrylamide gel electrophoresis (BN-PAGE) to preserve native protein-protein interactions. Double staining with antibodies against specific partner candidates revealed that bands attributed to NHE3, MAP17, PDZK1, and SGLT2 migrate similarly at ~480 kDa in samples from rat renal cortex, as supported by independent experiments. Rigorous controls were implemented to address concerns related to autofluorescence, antibody cross-reactivity, and signal specificity, confirming the validity of our results. Furthermore, complexes involving SGLT2 and NHE3 at an apparent ~480 kDa molecular weight were observed in male and female rats. Unlike SGLT2, SGLT1 bands in BN-PAGE did not overlap with the NHE3 signal, which is consistent with our earlier immunofluorescence experiments. Collectively, our data suggest that NHE3 and SGLT2 are functionally and physically associated with the renal proximal tubule. Future experiments will be undertaken to further characterize the physical association between NHE3 and SGLT2 and to determine whether the inhibition of NHE3 by SGLT2 inhibitors is mediated through the disruption of the NHE3-PDZK1-MAP17-SGLT2 multiprotein complex. Funding: São Paulo Research Foundation (FAPESP). This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Regulation of renal acid-base balance by Epac isoforms in proximal tubule and collecting duct

Proper regulation of acid-base homeostasis has utmost significance for well-being and survival of the living organisms. Up to 50% of emergency room patients have notable disturbances in pH and associated morbific manifestations, which impedes effective treatment. Kidneys play a central role in regulation of acid-base balance by reabsorbing HCO3− via NHE-3 in the proximal tubule (PT) and secreting H+ via V-ATPase pump by the intercalated cells of collecting duct (CD). Exchange proteins directly activated by cAMP (Epac) are most prominently expressed in the proximal tubule and collecting ducts, the sites critical for acid-base transport in the renal tubule. However, the significance and physiological relevance of Epac isoforms (Epac1 and Epac2) for controlling HCO3− reabsorption and H+ secretion and by extension, the systemic pH regulation is not known. Here, we used transgenic mouse models lacking both or individual Epac1 and Epac2 isoforms to examine the systemic adaptation to acid-base insults and to inquire on the perturbed molecular signaling mechanisms and end-effectors in freshly isolated split-opened PTs and CDs using biochemical and fluorescent imaging tools. Deletion of Epac1&2 caused a dramatic decrease in NHE-3 protein levels in an additive manner. Moreover, we detected a marked redistribution of NHE-3 to the base of brush border in proximal tubule cells rendering its inactivation. Consistently, NHE-3 activity was more than 70% decreased upon Epac1&2 deletion, as was assessed by a rate of recovery after intracellular acidification in freshly isolated split-opened PTs. Furthermore, V-ATPase activity was similarly decreased in A-type of intercalated cells in freshly isolated CDs despite compensatory accumulation on the apical side, as monitored with confocal microscopy in cortical and medullary kidney sections of the knockout. In unstressed conditions, Epac1&2 deletion did not affect arterial pH and HCO3− levels, while urinary pH was significantly reduced. Dietary acid load (280 mM NH4Cl in drinking water for 3 days) unmasked severe hyperchloremic metabolic acidosis with pH 7.17±0.01 and HCO3− 14.5±0.6 mM and impaired urinary acidification. Deletion of Epac2 alone recapitulated the phenotype of double knockout, whereas Epac1−/− mice did not demonstrate notable deficiencies in response to dietary acid load. Overall, we concluded that Epac signaling plays a major role in urinary acidification with the dominant contribution of Epac2 isoform by regulating acid-base transport in both proximal tubule (via NHE-3) and the collecting duct (via V-ATPase). Epac1&2 and Epac2 deletion of impairs renal adaptation to the acid load resulting in severe metabolic acidosis. This research was supported by NIH-NIDDK DK117865, DK119170, AHA EIA35260097 (to O. Pochynyuk) and AHA-23POST1020372 (to K. Pyrshev). This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Na+/H+ exchanger isoform 1 (NHE1) confers resistance to apoptosis in pulmonary arterial smooth muscle cells (PASMCs)

Pulmonary hypertension (PH) is a condition caused by remodeling of the lung vasculature due to abnormal cell turnover involving increased cell proliferation and resistance to apoptosis. The mechanisms controlling abnormal cell growth and survival are incompletely understood. We previously found that Na+/H+ exchanger (NHE) activity is increased in PASMCs from PH patients and the Sugen Hypoxia (SuHx) animal model of PH. NHE isoform 1 (NHE1) is a membrane protein that regulates intracellular pH (pHi) and cell volume via export of H+ and import of Na+. This protein also has a cytosolic tail region shown to be involved in non-canonical functions such as cytoskeletal interactions and cell signaling via protein-protein interactions. NHE1 has been associated with increased PASMC proliferation; however, it is unclear if increased NHE activity, dysregulation of pHi or cytoskeletal interactions are involved in resistance to apoptosis. In this study, we tested the hypothesis that increased expression of NHE1 leads to apoptosis resistance in PASMCs via dysregulation of pHi. We first used the rat SuHx model of PH in which male Wistar rats receive a single subcutaneous injection of the vascular endothelial growth factor receptor inhibitor, SU5416, followed by 3 wks of exposure to hypoxia (10% O2) and 2 wks of normoxia. PASMCs were isolated from distal pulmonary arteries of control and SuHx Wistar rats and treated with the NHE inhibitor, 5-(N-ethyl-N-isopropyl)-amiloride (EIPA; 1 uM), or vehicle for 24 h before analyzing cell apoptosis via Hoechst staining under baseline and stimulated conditions (250 uM H2O2). Endoplasmic reticulum stress was measured using the fluorescent stain, Thioflavin T (ThT). In some experiments, cells from control rats were infected with adenovirus containing green fluorescent protein (AdGFP; control for infection), wild-type NHE1 (AdNHE1-WT) or NHE1 mutants: AdNHE1-E262I lacking ion translocation activity and AdNHE1-EM lacking the ezrin binding site which prevents cytoskeletal interactions. NHE1 protein expression and localization were assessed via immunoblotting (total and surface proteins). Effcacy of Na+/H+ translocation was assessed from Na+-dependent recovery following an ammonium pulse. At baseline, despite increased ThT levels in SuHx cells, apoptosis was not different compared to control cells, indicating SuHx cells have suppressed apoptotic response to increased ER stress. Following treatment with EIPA, apoptosis was increased in SuHx cells, possibly indicating they became sensitized to the high levels of ER stress and were able to proceed into the apoptotic pathway. Similarly, following treatment with H2O2, SuHx PASMCs showed resistance to apoptosis compared to control cells, which was alleviated with pre-treatment with EIPA. In control cells, wild-type NHE1 and NHE1 mutants were expressed in similar amounts which led to an expected increase in NHE activity for WT and NHE1-EM, but not for NHE1-E262I when compared to GFP. We found that enhanced expression of NHE1-WT was suffcient to inhibit apoptosis following treatment with H2O2. Interestingly, both NHE1 mutants also significantly decreased PASMC apoptosis following H2O2 stimulation. Our results indicate NHE1 is necessary for apoptosis resistance in PASMCs from animals with PH and that increased expression is suffcient to confer resistance to apoptosis in control PASMCs. Additionally, these results suggest neither pHi regulation nor interaction between NHE1 and ezrin are essential for NHE1-induced apoptosis resistance, indicating another mechanism through which NHE1 confers resistance to apoptosis in PH. Based on these findings, we speculate that development of NHE1-related therapeutic targets could potentially restore cell apoptosis and reverse disease. AHA 23PRE1022720, HL159906, HL073859. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Deletion of dipeptidyl peptidase 4 attenuates angiotensin II-induced blood pressure rise and enhances NHE3 phosphorylation at serine 552 in the kidneys of male and female mice

Dipeptidyl peptidase 4 (DPP4) physically associates with the Na+/H+ exchanger isoform3 (NHE3), and DPP4 inhibitors reduce NHE3-dependent sodium reabsorption in the renal proximal tubule. Angiotensin II (Ang II) is a key mediator of sodium and water retention by the kidneys, partly through the activation of proximal tubule NHE3. Recent studies from our group reveled that Ang II activates DPP4 by, both in vivo and in vitro, and that inhibition of several canonical and non-canonical pathways can suppress this activation. However, the impact of this interaction on renal sodium handling and blood pressure remains unclear. This study aimed to test the hypothesis that DPP4 deletion mitigates the blood pressure elevation induced by Ang II in male and female mice, at least partly due to the downregulation of NHE3 in the renal proximal tubule. Male and female mice with global DPP4 deletion or wild-type controls were acutely administered either a pressor dose of Ang II (1000 ng/kg/minute) or saline. Blood pressure was measured before and one hour after the administration of Ang II or saline by tail-cuff plethysmography. Kidneys were subsequently collected for protein homogenate preparation. Increased DPP4 activity was observed in response to acute Ang II stimulation in both male and female wild-type mice, with female mice exhibiting higher basal DPP4 activity. As expected, DPP4 knockout mice exhibited minimal DPP4 activity. Baseline blood pressure did not differ significantly between male wild-type and DPP4 knockout mice or between females. In line with our hypothesis, both male and female DPP4 knockout mice displayed a reduced increase in blood pressure in response to acute Ang II administration compared to their wild-type counterparts. Furthermore, the phosphorylation of NHE3 at serine 552 (pS552-NHE3), a surrogate marker of NHE3 inhibition, was higher in wild-type females than males. Interestingly, pS552-NHE3 was fivefold higher in male DPP4 knockout mice and threefold higher in female DPP4 knockout mice compared to their wild-type counterparts. Additionally, the increase in pS552-NHE3 one hour after Ang II administration was higher in males (1208 ± 119 vs. 544 ± 54 %, P < 0.01) and females (385 ± 18 vs. 271 ± 27%, P < 0.01) DPP4 knockout mice compared to wild-type animals. Moreover, ERK, a well-known effector of the AT1R signaling cascade, was phosphorylated in a similar manner as NHE3 in response to Ang II in males but not in females. In conclusion, our data demonstrate that the absence of DPP4 reduces the rise in blood pressure induced by acute Ang II stimulation, possibly through Ang II/AT1R signaling pathways influencing phosphorylation of downstream effectors such as NHE3. Furthermore, we observe significant sex differences in Ang II effects on DPP4 activity, and AT1R signaling cascades. FAPESP 2021/14524-3 NIDDKD - K08DK115886. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Renal upregulation of NCC counteracts empagliflozin-mediated NHE3 inhibition in normotensive but not in hypertensive rats

Sodium-glucose cotransporter-2 inhibitors (SGLT2i) reduce blood pressure (BP) in hypertensive patients. However, the underlying molecular mechanisms remain largely unknown. Proximal tubule (PT) NHE3-mediated sodium reabsorption is inhibited by SGLT2i in normotensive rodents, yet no BP-lowering effect is observed under this scenario. This study examined the effect of empagliflozin (EMPA) on renal tubular sodium transport in normotensive and spontaneously hypertensive rats (SHRs). It also tested the hypothesis that EMPA-mediated PT NHE3 inhibition in normotensive rats is associated with upregulation of distal nephron apical sodium transporters. Administration of EMPA for 14 days reduced BP in 12-week-old SHRs but not in age-matched Wistar rats. PT NHE3 was inhibited by EMPA treatment in both Wistar and SHRs. EMPA increased NCC expression and phosphorylation levels in Wistar but not SHRs. SHRs exhibited higher cortical and medullary NKCC2 and β-ENaC subunit mRNA and higher medullary α-ENaC expression levels than Wistar rats, none of which were affected by EMPA. Another set of male Wistar rats was treated with EMPA, the NCC inhibitor hydrochlorothiazide (HCTZ), EMPA combined with HCTZ or vehicle for 14 days. In these rats, BP was reduced in response to EMPA and HCTZ combined but not by EMPA or HCTZ alone. Altogether, these data suggest that upregulation of NCC counteracts EMPA-mediated inhibition of PT NHE3 in male normotensive rats, thereby maintaining their baseline BP. Moreover, reducing NHE3 activity without further upregulation of the main apical sodium transporters beyond the PT may contribute to the BP-lowering effect of SGLT2i in experimental models and patients with hypertension. FAPESP 2016/22140-7 and 2021/14534-3. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

SGLT2-independent effects of canagliflozin on NHE3 and mitochondrial complex I activity inhibit proximal tubule fluid transport and albumin uptake.

Beyond glycemic control, SGLT2 inhibitors (SGLT2is) have protective effects on cardiorenal function. Renoprotection has been suggested to involve inhibition of NHE3 leading to reduced ATP-dependent tubular workload and mitochondrial oxygen consumption. NHE3 activity is also important for regulation of endosomal pH, but the effects of SGLT2i on endocytosis are unknown. We used a highly differentiated cell culture model of proximal tubule (PT) cells to determine the direct effects of SGLT2i on Na+-dependent fluid transport and endocytic uptake in this nephron segment. Strikingly, canagliflozin but not empagliflozin reduced fluid transport across cell monolayers and dramatically inhibited endocytic uptake of albumin. These effects were independent of glucose and occurred at clinically relevant concentrations of drug. Canagliflozin acutely inhibited surface NHE3 activity, consistent with a direct effect, but did not affect endosomal pH or NHE3 phosphorylation. In addition, canagliflozin rapidly and selectively inhibited mitochondrial complex I activity. Inhibition of mitochondrial complex I by metformin recapitulated the effects of canagliflozin on endocytosis and fluid transport, whereas modulation of downstream effectors AMPK and mTOR did not. Mice given a single dose of canagliflozin excreted twice as much urine over 24 h compared with empagliflozin-treated mice despite similar water intake. We conclude that canagliflozin selectively suppresses Na+-dependent fluid transport and albumin uptake in PT cells via direct inhibition of NHE3 and of mitochondrial function upstream of the AMPK/mTOR axis. These additional targets of canagliflozin contribute significantly to reduced PT Na+-dependent fluid transport in vivo.NEW & NOTEWORTHY Reduced NHE3-mediated Na+ transport has been suggested to underlie the cardiorenal protection provided by SGLT2 inhibitors. We found that canagliflozin, but not empagliflozin, reduced NHE3-dependent fluid transport and endocytic uptake in cultured proximal tubule cells. These effects were independent of SGLT2 activity and resulted from inhibition of mitochondrial complex I and NHE3. Studies in mice are consistent with greater effects of canagliflozin versus empagliflozin on fluid transport. Our data suggest that these selective effects of canagliflozin contribute to reduced Na+-dependent transport in proximal tubule cells.

Renal upregulation of NCC counteracts empagliflozin-mediated NHE3 inhibition in normotensive but not in hypertensive male rat.

Sodium-glucose cotransporter-2 inhibitors (SGLT2i) reduce blood pressure (BP) in patients with hypertension, yet the precise molecular mechanisms remain elusive. SGLT2i inhibits proximal tubule (PT) NHE3-mediated sodium reabsorption in normotensive rodents, yet no hypotensive effect is observed under this scenario. This study examined the effect of empagliflozin (EMPA) on renal tubular sodium transport in normotensive and spontaneously hypertensive rats (SHRs). It also tested the hypothesis that EMPA-mediated PT NHE3 inhibition in normotensive rats is associated with upregulation of distal nephron apical sodium transporters. EMPA administration for 14 days reduced BP in 12-wk-old SHRs but not in age-matched Wistar rats. PT NHE3 activity was inhibited by EMPA treatment in both Wistar and SHRs. In Wistar rats, EMPA increased NCC activity, mRNA expression, protein abundance, and phosphorylation levels, but not in SHRs. SHRs showed higher NKCC2 activity and an abundance of cleaved ENaC α and γ subunits compared with Wistar rats, none of which were affected by EMPA. Another set of male Wistar rats was treated with EMPA, the NCC inhibitor hydrochlorothiazide (HCTZ), and EMPA combined with HCTZ or vehicle for 14 days. In these rats, BP reduction was observed only with combined EMPA and HCTZ treatment, not with either drug alone. These findings suggest that NCC upregulation counteracts EMPA-mediated inhibition of PT NHE3 in male normotensive rats, maintaining their baseline BP. Moreover, the reduction of NHE3 activity without further upregulation of major apical sodium transporters beyond the PT may contribute to the BP-lowering effect of SGLT2i in experimental models and patients with hypertension.NEW & NOTEWORTHY This study suggests that reduced NHE3-mediated sodium reabsorption in the renal proximal tubule may account, at least in part, for the BP-lowering effect of SGLT2 inhibitors in the setting of hypertension. It also demonstrates that chronic treatment with SGLT2 inhibitors upregulates NCC activity, phosphorylation, and expression in the distal tubule of normotensive but not hypertensive rats. SGLT2 inhibitor-mediated upregulation of NCC seems crucial to counteract proximal tubule natriuresis in subjects with normal BP.

Empagliflozin mitigates cardiac hypertrophy through cardiac RSK/NHE-1 inhibition

BackgroundSGLT2i reduce cardiac hypertrophy, but underlying mechanisms remain unknown. Here we explore a role for serine/threonine kinases (STK) and sodium hydrogen exchanger 1(NHE1) activities in SGLT2i effects on cardiac hypertrophy. MethodsIsolated hearts from db/db mice were perfused with 1 µM EMPA, and STK phosphorylation sites were examined using unbiased multiplex analysis to detect the most affected STKs by EMPA. Subsequently, hypertrophy was induced in H9c2 cells with 50 µM phenylephrine (PE), and the role of the most affected STK (p90 ribosomal S6 kinase (RSK)) and NHE1 activity in hypertrophy and the protection by EMPA was evaluated. ResultsIn db/db mice hearts, EMPA most markedly reduced STK phosphorylation sites regulated by RSKL1, a member of the RSK family, and by Aurora A and B kinases. GO and KEGG analysis suggested that EMPA inhibits hypertrophy, cell cycle, cell senescence and FOXO pathways, illustrating inhibition of growth pathways. EMPA prevented PE-induced hypertrophy as evaluated by BNP and cell surface area in H9c2 cells. EMPA blocked PE-induced activation of NHE1. The specific NHE1 inhibitor Cariporide also prevented PE-induced hypertrophy without added effect of EMPA. EMPA blocked PE-induced RSK phosphorylation. The RSK inhibitor BIX02565 also suppressed PE-induced hypertrophy without added effect of EMPA. Cariporide mimicked EMPA’s effects on PE-treated RSK phosphorylation. BIX02565 decreased PE-induced NHE1 activity, with no further decrease by EMPA. ConclusionsRSK inhibition by EMPA appears as a novel direct cardiac target of SGLT2i. Direct cardiac effects of EMPA exert their anti-hypertrophic effect through NHE-inhibition and subsequent RSK pathway inhibition.

Predicting sex differences in the effects of diuretics in renal epithelial transport during angiotensin II-induced hypertension.

Chronic angiotensin II (ANG II) infusion is an experimental model that induces hypertension in rodents. The natriuresis, diuresis, and blood pressure responses differ between males and females. This is perhaps not unexpected, given the rodent kidney, which plays a key role in blood pressure regulation, exhibits marked sex differences. Under normotensive conditions, compared with males, the female rat nephron exhibits lower Na+/H+ exchanger 3 (NHE3) activity along the proximal tubule but higher Na+ transporter activities along the distal segments. ANG II infusion-induced hypertension induces a pressure natriuretic response that reduces NHE3 activity and shifts Na+ transport capacity downstream. The goals of this study were to apply a computational model of epithelial transport along a rat nephron 1) to understand how a 14-day ANG II infusion impacts segmental electrolyte transport in male and female rat nephrons and 2) to identify and explain any sex differences in the effects of loop diuretics, thiazide diuretics, and K+-sparing diuretics. Model simulations suggest that the NHE3 downregulation in the proximal tubule is a major contributor to natriuresis and diuresis in hypertension, with the effects stronger in males. All three diuretics are predicted to induce stronger natriuretic and diuretic effects under hypertension compared with normotension, with relative increases in sodium excretion higher in hypertensive females than in males. The stronger natriuretic responses can be explained by the downstream shift of Na+ transport load in hypertension and by the larger distal transport load in females, both of which limit the ability of the distal segments to further elevate their Na+ transport.NEW & NOTEWORTHY Sex differences in the prevalence of hypertension are found in human and animal models. The kidney, which regulates blood pressure, exhibits sex differences in morphology, hemodynamics, and membrane transporter distributions. This computational modeling study provides insights into how the sexually dimorphic responses to a 14-day angiotensin II infusion differentially impact segmental electrolyte transport in rats. Simulations of diuretic administration explain how the natriuretic and diuretic effects differ between normotension and hypertension and between the sexes.