Proximal tubular cell sodium concentration in early diabetic nephropathy assessed by electron microprobe analysis

The interplay between sodium/glucose cotransporter type 2 and mitochondrial ionic environment

Mitochondrial volume is maintained through the permeability of the inner mitochondrial membrane by a specific aquaporin and the osmotic balance between the mitochondrial matrix and cellular cytoplasm. Various electrolytes, such as calcium and hydrogen ions, potassium, and sodium, as well as other osmotic substances, affect the swelling of mitochondria. Intracellular glucose levels may also affect mitochondrial swelling, although the relationship between mitochondrial ion homeostasis and intracellular glucose is poorly understood. This article reviews what is currently known about how the Sodium-Glucose transporter (SGLT) may impact mitochondrial sodium (Na+) homeostasis. SGLTs regulate intracellular glucose and sodium levels and, therefore, interfere with mitochondrial ion homeostasis because mitochondrial Na+ is closely linked to cytoplasmic calcium and sodium dynamics. Recently, a large amount of data has been available on the effects of SGLT2 inhibitors on mitochondria in different cell types, including renal proximal tubule cells, endothelial cells, mesangial cells, podocytes, neuronal cells, and cardiac cells. The current evidence suggests that SGLT inhibitors (SGLTi) may affect mitochondrial dynamics regarding intracellular Sodium and hydrogen ions. Although the regulation of mitochondrial ion channels by SGLTs is still in its infancy, the evidence accumulated thus far of the effect of SGLTi on mitochondrial functions certainly will foster further research in this direction.

Effect of renal tubule-specific knockdown of the Na+/H+ exchanger NHE3 in Akita diabetic mice

Na+/H+ exchanger isoform 3 (NHE3) contributes to Na+/bicarbonate reabsorption and ammonium secretion in early proximal tubules. To determine its role in the diabetic kidney, type 1 diabetic Akita mice with tubular NHE3 knockdown [Pax8-Cre; NHE3-knockout (KO) mice] were generated. NHE3-KO mice had higher urine pH, more bicarbonaturia, and compensating increases in renal mRNA expression for genes associated with generation of ammonium, bicarbonate, and glucose (phosphoenolpyruvate carboxykinase) in proximal tubules and H+ and ammonia secretion and glycolysis in distal tubules. This left blood pH and bicarbonate unaffected in nondiabetic and diabetic NHE3-KO versus wild-type mice but was associated with renal upregulation of proinflammatory markers. Higher renal phosphoenolpyruvate carboxykinase expression in NHE3-KO mice was associated with lower Na+-glucose cotransporter (SGLT)2 and higher SGLT1 expression, indicating a downward tubular shift in Na+ and glucose reabsorption. NHE3-KO was associated with lesser kidney weight and glomerular filtration rate (GFR) independent of diabetes and prevented diabetes-associated albuminuria. NHE3-KO, however, did not attenuate hyperglycemia or prevent diabetes from increasing kidney weight and GFR. Higher renal gluconeogenesis may explain similar hyperglycemia despite lower SGLT2 expression and higher glucosuria in diabetic NHE3-KO versus wild-type mice; stronger SGLT1 engagement could have affected kidney weight and GFR responses. Chronic kidney disease in humans is associated with reduced urinary excretion of metabolites of branched-chain amino acids and the tricarboxylic acid cycle, a pattern mimicked in diabetic wild-type mice. This pattern was reversed in nondiabetic NHE3-KO mice, possibly reflecting branched-chain amino acids use for ammoniagenesis and tricarboxylic acid cycle upregulation to support formation of ammonia, bicarbonate, and glucose in proximal tubule. NHE3-KO, however, did not prevent the diabetes-induced urinary downregulation in these metabolites.

Inhibition of kidney proximal tubular glucose reabsorption does not prevent against diabetic nephropathy in type 1 diabetic eNOS knockout mice

BACKGROUND AND OBJECTIVE

Sodium glucose cotransporter 2 (SGLT2) is the main luminal glucose transporter in the kidney. SGLT2 inhibition results in glycosuria and improved glycaemic control. Drugs inhibiting this transporter have recently been approved for clinical use and have been suggested to have potential renoprotective benefits by limiting glycotoxicity in the proximal tubule. We aimed to determine the renoprotective benefits of empagliflozin, an SGLT2 inhibitor, independent of its glucose lowering effect.

RESEARCH DESIGN AND METHODS

We induced diabetes using a low dose streptozotocin protocol in 7-8 week old endothelial nitric oxide (eNOS) synthase knockout mice. We measured fasting blood glucose on a monthly basis, terminal urinary albumin/creatinine ratio. Renal histology was assessed for inflammatory and fibrotic changes. Renal cortical mRNA transcription of inflammatory and profibrotic cytokines, glucose transporters and protein expression of SGLT2 and GLUT1 were determined. Outcomes were compared to diabetic animals receiving the angiotensin receptor blocker telmisartan (current best practice).

RESULTS

Diabetic mice had high matched blood glucose levels. Empagliflozin did not attenuate diabetes-induced albuminuria, unlike telmisartan. Empagliflozin did not improve glomerulosclerosis, tubular atrophy, tubulointerstitial inflammation or fibrosis, while telmisartan attenuated these. Empagliflozin did not modify tubular toll-like receptor-2 expression in diabetic mice. Empagliflozin did not reduce the upregulation of macrophage chemoattractant protein-1 (MCP-1), transforming growth factor β1 and fibronectin mRNA observed in the diabetic animals, while telmisartan decreased transcription of MCP-1 and fibronectin. Empagliflozin increased GLUT1 mRNA expression and telmisartan increased SGLT2 mRNA expression in comparison to untreated diabetic mice. However no significant difference was found in protein expression of GLUT1 or SGLT2 among the different groups.

CONCLUSION

Hence SGLT2 inhibition does not have renoprotective benefits independent of glucose lowering.

Effects of SGLT2 inhibition in human kidney proximal tubular cells--renoprotection in diabetic nephropathy?

Sodium/glucose cotransporter 2 (SGLT2) inhibitors are oral hypoglycemic agents used to treat patients with diabetes mellitus. SGLT2 inhibitors block reabsorption of filtered glucose by inhibiting SGLT2, the primary glucose transporter in the proximal tubular cell (PTC), leading to glycosuria and lowering of serum glucose. We examined the renoprotective effects of the SGLT2 inhibitor empagliflozin to determine whether blocking glucose entry into the kidney PTCs reduced the inflammatory and fibrotic responses of the cell to high glucose. We used an in vitro model of human PTCs. HK2 cells (human kidney PTC line) were exposed to control 5 mM, high glucose (HG) 30 mM or the profibrotic cytokine transforming growth factor beta (TGFβ1; 0.5 ng/ml) in the presence and absence of empagliflozin for up to 72 h. SGLT1 and 2 expression and various inflammatory/fibrotic markers were assessed. A chromatin immunoprecipitation assay was used to determine the binding of phosphorylated smad3 to the promoter region of the SGLT2 gene. Our data showed that TGFβ1 but not HG increased SGLT2 expression and this occurred via phosphorylated smad3. HG induced expression of Toll-like receptor-4, increased nuclear deoxyribonucleic acid binding for nuclear factor kappa B (NF-κB) and activator protein 1, induced collagen IV expression as well as interleukin-6 secretion all of which were attenuated with empagliflozin. Empagliflozin did not reduce high mobility group box protein 1 induced NF-κB suggesting that its effect is specifically related to a reduction in glycotoxicity. SGLT1 and GLUT2 expression was not significantly altered with HG or empagliflozin. In conclusion, empagliflozin reduces HG induced inflammatory and fibrotic markers by blocking glucose transport and did not induce a compensatory increase in SGLT1/GLUT2 expression. Although HG itself does not regulate SGLT2 expression in our model, TGFβ increases SGLT2 expression through phosphorylated smad3.

The role of Sgk-1 in the upregulation of transport proteins by PPAR-{gamma} agonists in human proximal tubule cells

BACKGROUND

Cellular sodium and water transport are dysregulated in diabetes mellitus. Synthetic peroxisome proliferator-activated receptor gamma (PPAR-gamma) agonists are currently used in the treatment of type 2 diabetes, but their use is limited by fluid retention. Recent data suggest that PPAR-gamma agonists stimulate distal tubular epithelial Na transport, potentially through the serine glucocorticoid kinase-1 (Sgk-1)-dependent regulation of the epithelial Na channel. We have recently demonstrated that Sgk-1 additionally regulates sodium reabsorption through the proximal tubular sodium hydrogen exchanger-3 (NHE3). However, the effects of PPAR-gamma agonists on Sgk-1, the water channel proteins aquaporins and on sodium transport in human proximal tubule cells (PTCs) have not previously been studied.

METHODS

PTCs were exposed to the PPAR-gamma agonists, pioglitazone and the more selective PPAR-gamma agonist L-805645 with and without the Sgk inhibitor (GSK650394A). PPAR-gamma, Sgk-1, NHE3, AQP 1 and 7 mRNA and protein expression were determined by semi-quantitative PCR and western blot. The Sgk-1-specific effect was determined using Sgk-1 siRNA.

RESULTS

Exposure of PTCs to 10 muM pioglitazone and 8 microM L-805645 increased the mRNA and protein expression of PPAR-gamma (P < 0.005), NHE3 and Sgk-1 (both P < 0.05). The expression of AQPs 1 and 7 was increased by pioglitazone and L-805645 (both P < 0.05). The increases in NHE3 and AQPs 1 and 7 were significantly reduced by pharmacological inhibition of Sgk and when cultures were exposed to Sgk-1-specific siRNA.

CONCLUSIONS

PPAR-gamma agonists enhanced the expression of NHE3, AQP 1 and 7 channels in human proximal tubule cells through Sgk-1-dependent pathways.

The role of renal proximal tubular cells in diabetic nephropathy

We now know that the rate of progression of diabetic nephropathy, like all progressive renal disease, correlates with the degree of corticointerstitial fibrosis. Therefore, much interest has focused on the contribution of the resident cells in the renal cortex to this process. This article reviews the evidence that the epithelial cells of the proximal tubule are major players in orchestrating events in the corticointerstitium in diabetic nephropathy. More specifically, it addresses their role in extracellular matrix turnover, generation of cytokines, and recruitment of inflammatory cells, as well as examining the concept that they are the source of the interstitial myofibroblasts, which are the principal mediators of the fibrotic process.

Renal tubular Na(+)-K(+)-ATPase in diabetes mellitus: relationship to metabolic abnormality

This study was undertaken to evaluate the effect of long-term diabetes on Na(+)-K(+)-ATPase in isolated nephron segments in five groups of rats: 1) controls of 7 wk duration (7 WD), 2) diabetes mellitus (DM) of 7 WD, 3) DM of 7 WD treated with insulin replacement, 4) DM rats of 25 WD, and 5) control rats of 25 WD. The blood glucose (BG) values in the first three groups were 123 +/- 9, 450 +/- 25, and 302 +/- 30 mg/dl; the glomerular filtration rate (GFR) was 1.34 +/- 0.08, 1.80 +/- 0.10, and 1.77 +/- 0.08 ml/min; and urinary sodium excretion was 0.94 +/- 0.05, 1.76 +/- 0.10, and 1.40 +/- 0.07 mu eq/min. Na(+)-K(+)-ATPase in group 2 increased in all segments studied (P < 0.001, group 1 vs. 2 for all). In group 3, Na(+)-K(+)-ATPase normalized in proximal convoluted (PC), proximal straight (PS), and distal convoluted (DC) tubules (P < 0.001, group 2 vs. group 3 for all), whereas in the outer medullary thick ascending limb (OMTAL) the correction was partial and in the CTAL and CCD there was no correction. In group 4 BG was 420 +/- 20 mg/100 ml compared with 123 +/- 9 in group 5 (P < 0.001), and GFR was 1.19 +/- 0.11 ml/min vs. 1.15 +/- 0.11 in group 5 (P = not significant).(ABSTRACT TRUNCATED AT 250 WORDS)

Renal handling of endogenous lithium in experimental diabetes mellitus in the rat

1. The usefulness of determining the renal handling of endogenous lithium as a marker of proximal tubular sodium reabsorption was assessed in streptozotocin induced diabetes mellitus in the Sprague-Dawley rat. 2. The clearance and fractional excretion of lithium were determined before and following the development of diabetes mellitus, and compared with measurements of proximal tubular reabsorption made directly using micropuncture techniques. Endogenous lithium was measured in order to avoid the toxic tubular effects of exogenously administered lithium salts. 3. Although a trend existed for a reduction in the fractional excretion of lithium in diabetic animals (1.8 +/- 0.3 vs 2.4 +/- 0.5%; P greater than 0.20), this did not reach statistical significance and did not accurately reflect the change in directly measured tubular Na reabsorption. 4. The decrease in proximal tubular Na reabsorption demonstrated in diabetic animals treated with phlorizin was not significantly reflected in the fractional lithium excretion, although again a corresponding trend was evident (1.9 +/- 0.8 vs 0.6 +/- 0.2%; P greater than 0.10. 5. In summary, the significant alterations in tubular Na handling in diabetes mellitus, previously demonstrated directly using micropuncture techniques, are not reflected in the renal handling of endogenous lithium. This indirect method is inadequate to assess proximal tubular Na transport in experimental diabetes mellitus.

Tubular sodium handling and tubuloglomerular feedback in compensatory renal hypertrophy

Tubular sodium handling and tubuloglomerular feedback (TGF) activity were assessed in established compensatory renal hypertrophy in Sprague Dawley rats. Hyperfiltration at the level of the single nephron was confirmed 4-6 weeks following a reduction in renal mass. TGF activity, determined as the difference between late proximal and early distal measurements of single-nephron glomerular filtration rate (SNGFR), was significantly increased in compensatory renal hypertrophy, being 7.8 +/- 1.0 vs 23.3 +/- 1.9 vs 25.5 +/- 2.6 nl/min (P for analysis of variance less than 0.05) following sham operation, unilateral nephrectomy, and 1 1/3 nephrectomy, respectively. Enhanced net tubular Na transport was also observed, with total Na reabsorption up to the late proximal site being 1.8 +/- 0.2 vs 2.7 +/- 0.1 vs 3.1 +/- 0.3 nmol/min (P less than 0.05), and to the early distal site being 3.4 +/- 0.5 vs 5.8 +/- 0.6 vs 7.9 +/- 0.8 nmol/min (P less than 0.05) in the three animal groups respectively. Comparison of proximal tubular length demonstrated a 71.9 +/- 8.1% increase in uninephrectomised vs sham-operated animals. This increase was proportionately greater than the increase in proximal Na reabsorption (50.0 +/- 4.0%) observed in the corresponding animal groups. Concurrent electron microprobe experiments in uninephrectomised and sham-operated animals demonstrated that the proximal tubular intracellular Na concentration was significantly lower following uninephrectomy (16.8 +/- 0.6 vs 18.9 +/- 0.5 mmol/kg wet weight, P less than 0.01), in association with evidence of reduced basolateral Na/K-ATPase activity.(ABSTRACT TRUNCATED AT 250 WORDS)