Albumin and Glucose Effects On Cell Growth Parameters, Albumin Uptake and Na⁺/H⁺-Exchanger Isoform 3 in OK Cells

Abstract

Background: The degree of albuminuria, the presence of sodium-dependent hypertension, and histological evidence of both tubular and interstitial pathology correlate with the progression of diabetic nephropathy. The sodium-hydrogen exchanger NHE-3 plays an integral role in both sodium reabsorption and receptor-mediated albumin endocytosis in proximal tubular cells (PTCs). The aim of this study was to investigate the direct effects of hyperglycemia and albumin on cell growth parameters, NHE-3 protein expression and albumin uptake in an in vitro model of PTCs. Methods: Opossum kidney (OK) cells were exposed to 5 mmol/l glucose (control) or 25 mmol/l (high) glucose in the presence or absence of either 0.1 or 1.0 g/l bovine serum albumin (BSA) for up to 72 hrs prior to study. 20 mmol/l mannitol + 5 mmol/l glucose was used as a control for hyperosmolality. The cell number, the degree of cell swelling, cell protein content and NHE-3 protein expression were assessed. Cellular albumin uptake and the role of NHE in both control and high glucose conditions were determined by FITC-BSA ± NHE-inhibitor ethyl isopropyl amiloride (EIPA). Results: High glucose and the hyperosmolar control induced cellular hypertrophy, which was not modified in the presence of albumin. Cell volume was initially increased by 1.0 g/l BSA, +/-high glucose, which normalized over 48-72 hrs. All experimental conditions induced an early and sustained increase in NHE-3 protein expression. High glucose increased albumin uptake, independent of changes in osmolality. EIPA reduced the albumin uptake in PTCs with kinetics supporting the role of NHE-3 in this process. Conclusion: These results suggest that exposure of PTCs to high glucose concentrations promotes osmolality mediated cell hypertrophy and increased tubular albumin reabsorption linked to an increase in NHE-3 expression. It is postulated that this increase in albumin uptake due to high glucose exposure may lead to proinflammatory protein overload of PTCs, ultimately impairing the compensatory increase in tubular albumin reabsorption..

How Does Proteinuria Cause Progressive Renal Damage?

“…Other studies confirmed the pathway (26,27) and revealed reactive oxygen as a second messenger (18,28). Protein overload elicited rapid generation of hydrogen peroxide in human proximal tubular cells, an effect that, together with NF-B activation, was prevented by antioxidants (28).…”

Renoprotective Effect of Pioglitazone by the Prevention of Glomerular Hyperfiltration through the Possible Restoration of Altered Macula Densa Signaling in Rats with Type 2 Diabetic Nephropathy

“…The activation of Na-K-ATPase due to an excess of filtered glucose in the lumen has been discussed in relation to its influence on the tubular absorption in diabetic nephropathy for more than a decade [50,51]. Increased activity of tubular salt transporters, such as Na-H exchanger 3, has also been reported as a particular feature of diabetic nephropathy [47,52], which might also contribute to an increase in the tubular reabsorption. In addition, some recent works have shown the possible involvement of megalin, insulin receptor substrate [53] and serum glucocorticoid-regulated kinase 1 [54] in the enhanced tubular reabsorption in diabetic nephropathy.…”

Nedd4-2 Functionally Interacts with ClC-5

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