Effects of pathophysiological concentrations of albumin on NHE3 activity and cell proliferation in primary cultures of human proximal tubule cells

Sodium-glucose Cotransporter Type 2 Inhibitors: A New Insight into the Molecular Mechanisms of Diabetic Nephropathy

Diabetic nephropathy is one of the chronic microvascular complications of diabetes and is a leading cause of end-stage renal disease. Fortunately, clinical trials have demonstrated that sodium-glucose cotransporter type 2 inhibitors could decrease proteinuria and improve renal endpoints and are promising agents for the treatment of diabetic nephropathy. The renoprotective effects of sodium-glucose cotransporter type 2 inhibitors cannot be simply attributed to their advantages in aspects of metabolic benefits, such as glycemic control, lowering blood pressure, and control of serum uric acid, or improving hemodynamics associated with decreased glomerular filtration pressure. Some preclinical evidence suggests that sodium-glucose cotransporter type 2 inhibitors exert their renoprotective effects by multiple mechanisms, including attenuation of oxidative and endoplasmic reticulum stresses, anti-fibrosis and anti-inflammation, protection of podocytes, suppression of megalin function, improvement of renal hypoxia, restored mitochondrial dysfunction and autophagy, as well as inhibition of sodium-hydrogen exchanger 3. In the present study, the detailed molecular mechanisms of sodium-glucose cotransporter type 2 inhibitors with the actions of diabetic nephropathy were reviewed, with the purpose of providing the basis for drug selection for the treatment of diabetic nephropathy.

Molecular Mechanisms of SGLT2 Inhibitor on Cardiorenal Protection

The development of sodium-glucose transporter 2 inhibitor (SGLT2i) broadens the therapeutic strategies in treating diabetes mellitus. By inhibiting sodium and glucose reabsorption from the proximal tubules, the improvement in insulin resistance and natriuresis improved the cardiovascular mortality in diabetes mellitus (DM) patients. It has been known that SGLT2i also provided renoprotection by lowering the intraglomerular hypertension by modulating the pre- and post- glomerular vascular tone. The application of SGLT2i also provided metabolic and hemodynamic benefits in molecular aspects. The recent DAPA-CKD trial and EMPEROR-Reduced trial provided clinical evidence of renal and cardiac protection, even in non-DM patients. Therefore, the aim of the review is to clarify the hemodynamic and metabolic modulation of SGLT2i from the molecular mechanism.

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.

Role of the sodium-hydrogen exchanger in mediating the renal effects of drugs commonly used in the treatment of type 2 diabetes

Diabetes is characterized by increased activity of the sodium-hydrogen exchanger (NHE) in the glomerulus and renal tubules, which contributes importantly to the development of nephropathy. Despite the established role played by the exchanger in experimental studies, it has not been specifically targeted by those seeking to develop novel pharmacological treatments for diabetes. This review demonstrates that many existing drugs that are commonly prescribed to patients with diabetes act on the NHE1 and NHE3 isoforms in the kidney. This action may explain their effects on sodium excretion, albuminuria and the progressive decline of glomerular function in clinical trials; these responses cannot be readily explained by the influence of these drugs on blood glucose. Agents that may affect the kidney in diabetes by virtue of an action on NHE include: (1) insulin and insulin sensitizers; (2) incretin-based agents; (3) sodium-glucose cotransporter 2 inhibitors; (4) antagonists of the renin-angiotensin system (angiotensin converting-enzyme inhibitors, angiotensin receptor blockers and angiotensin receptor neprilysin inhibitors); and (5) inhibitors of aldosterone action and cholesterol synthesis (spironolactone, amiloride and statins). The renal effects of each of these drug classes in patients with type 2 diabetes may be related to a single shared biological mechanism.

Albuminuria enhances NHE3 and NCC via stimulation of mitochondrial oxidative stress/angiotensin II axis

Imbalance of salt and water is a frequent and challenging complication of kidney disease, whose pathogenic mechanisms remain elusive. Employing an albumin overload mouse model, we discovered that albuminuria enhanced the expression of NHE3 and NCC but not other transporters in murine kidney in line with the stimulation of angiotensinogen (AGT)/angiotensin converting enzyme (ACE)/angiotensin (Ang) II cascade. In primary cultures of renal tubular cells, albumin directly stimulated AGT/ACE/Ang II and upregulated NHE3 and NCC expression. Blocking Ang II production with an ACE inhibitor normalized the upregulation of NHE3 and NCC in cells. Interestingly, albumin overload significantly reduced mitochondrial superoxide dismutase (SOD2), and administration of a SOD2 mimic (MnTBAP) normalized the expression of NHE3, NCC, and the components of AGT/ACE pathway affected by albuminuria, indicating a key role of mitochondria-derived oxidative stress in modulating renin-angiotensin system (RAS) and renal sodium transporters. In addition, the functional data showing the reduced urinary excretion of Na and Cl and enhanced response to specific NCC inhibitor further supported the regulatory results of sodium transporters following albumin overload. More importantly, the upregulation of NHE3 and NCC and activation of ACE/Ang II signaling pathway were also observed in albuminuric patient kidneys, suggesting that our animal model accurately replicates the human condition. Taken together, these novel findings demonstrated that albuminuria is of importance in resetting renal salt handling via mitochondrial oxidative stress-initiated stimulation of ACE/Ang II cascade. This may also offer novel, effective therapeutic targets for dealing with salt and water imbalance in proteinuric renal diseases.

Albuminuria Is an Appropriate Therapeutic Target in Patients with CKD: The Pro View

The presence of elevated levels of albuminuria is associated with an increased risk of progressive renal function loss over time. This association is found in various pathophysiological conditions, including diabetic nephropathy, hypertensive nephropathy, and various primary renal diseases, but also, the general, otherwise healthy population. Emerging data report that elevated albuminuria causes tubulointerstitial damage through activation of proinflammatory mediators, which ultimately leads to a progressive decline in renal function. Nowadays, various drugs are available that decrease the rate of GFR loss in patients with kidney disease. Well known are renin-angiotensin-aldosterone system inhibitors, but there are also other drugs and interventions, like intensive glucose control, anti-inflammatory agents (pentoxifylline), or a low-protein diet. These interventions have an additional effect beyond their original target, namely lowering albuminuria. Analyses from clinical trials show that the reduction in albuminuria observed during the first months of treatment with these drugs correlates with the degree of long-term renal protection: the larger the initial reduction in albuminuria, the lower the risk of ESRD during treatment. In addition, in treated patients, residual albuminuria is again the strongest risk marker for renal disease progression. These observations combined provide a strong argument that albuminuria is an appropriate therapeutic target in patients with CKD.

Chloride channel ClC-5 binds to aspartyl aminopeptidase to regulate renal albumin endocytosis

ClC-5 is a chloride/proton exchanger that plays an obligate role in albumin uptake by the renal proximal tubule. ClC-5 forms an endocytic complex with the albumin receptor megalin/cubilin. We have identified a novel ClC-5 binding partner, cytosolic aspartyl aminopeptidase (DNPEP; EC 3.4.11.21), that catalyzes the release of N-terminal aspartate/glutamate residues. The physiological role of DNPEP remains largely unresolved. Mass spectrometric analysis of proteins binding to the glutathione- S-transferase (GST)-ClC-5 C terminus identified DNPEP as an interacting partner. Coimmunoprecipitation confirmed that DNPEP and ClC-5 also associated in cells. Further experiments using purified GST-ClC-5 and His-DNPEP proteins demonstrated that the two proteins bound directly to each other. In opossum kidney (OK) cells, confocal immunofluorescence studies revealed that DNPEP colocalized with albumin-containing endocytic vesicles. Overexpression of wild-type DNPEP increased cell-surface levels of ClC-5 and albumin uptake. Analysis of DNPEP-immunoprecipitated products from rat kidney lysate identified β-actin and tubulin, suggesting a role for DNPEP in cytoskeletal maintenance. A DNase I inhibition assay showed a significant decrease in the amount of G actin when DNPEP was overexpressed in OK cells, suggesting a role for DNPEP in stabilizing the cytoskeleton. DNPEP was not present in the urine of healthy rats; however, it was readily detected in the urine in rat models of mild and heavy proteinuria (diabetic nephropathy and anti-glomerular basement membrane disease, respectively). Urinary levels of DNPEP were found to correlate with the severity of proteinuria. Therefore, we have identified another key molecular component of the albumin endocytic machinery in the renal proximal tubule and describe a new role for DNPEP in stabilizing the actin cytoskeleton.

Deciphering the mechanisms of the Na+/H+ exchanger-3 regulation in organ dysfunction

The Na+/H+ exchanger-3 (NHE3) belongs to the mammalian NHE protein family and catalyzes the electro-neutral exchange of extracellular sodium for intracellular proton across cellular membranes. Its transport function is of essential importance for the maintenance of the body's salt and water homeostasis as well as acid-base balance. Indeed, NHE3 activity is finely regulated by a variety of stimuli, both acutely and chronically, and its transport function is fundamental for a multiplicity of severe and world-wide infection-pathological conditions. This review aims to provide a concise overview of NHE3 physiology and discusses the role of NHE3 in clinical conditions of prominent importance, specifically in hypertension, diabetic nephropathy, heart failure, acute kidney injury, and diarrhea. Study of NHE3 function in models of these diseases has contributed to the deciphering of mechanisms that control the delicate ion balance disrupted in these disorders. The majority of the findings indicate that NHE3 transport function is activated before the onset of hypertension and inhibited thereafter; NHE3 transport function is also upregulated in diabetic nephropathy and heart failure, while it is reported to be downregulated in acute kidney injury and in diarrhea. The molecular mechanisms activated during these pathological conditions to regulate NHE3 transport function are examined with the aim of linking NHE3 dysfunction to the analyzed clinical disorders.

Increased L-arginine transport via system b0,+ in human proximal tubular cells exposed to albumin

Albumin has complex effects on PTECs (proximal tubular epithelial cells) and is able to stimulate growth or injury depending on its bound moieties. Albumin itself is a mitogen, inducing proliferation through a number of pathways. In PTEC exposed to purified albumin, polyamines are required for entry into the cell cycle and are critical for proliferation. Polyamines are synthesized from L-ornithine (itself derived by the action of arginase on L-arginine), and the transport and availability of L-arginine may thus be important for subsequent polyamine-dependent proliferation. In the present study we investigated radiolabelled cationic amino-acid transport in cultured PTEC exposed to 20 mg/ml ultrapure recombinant human albumin, describing the specific kinetic characteristics of transport and the expression of transporters. L-[3H]Arginine transport capacity in human PTEC is increased after exposure for 24 h to human albumin, mediated by the broad-scope high-affinity system b0,+ and, to a lesser extent, system y+L (but not system y+) transport. Increased transport is associated with increased b0,+-associated transporter expression. Inhibition of phosphoinositide 3-kinase, a key regulator of albumin endocytosis and signalling, inhibited proliferation, but had no effect on the observed increase in transport. PTEC proliferated in response to albumin. L-Lysine, a competitive inhibitor of L-arginine transport, had no effect on albumin-induced proliferation; however, arginine deprivation effectively reversed the albumin-induced proliferation observed. In conclusion, in PTEC exposed to albumin, increased L-arginine transport is mediated by increased transcription and activity of the apical b0,+ transport system. This may make L-arginine available as a substrate for the downstream synthesis of polyamines, but is not critical for cell proliferation.

Magnetic resonance measurements of renal blood flow and disease progression in autosomal dominant polycystic kidney disease

Whether changes in renal blood flow (RBF) are associated with and possibly contribute to cystic disease progression in autosomal dominant polycystic kidney disease (ADPKD) has not been ascertained. The Consortium for Radiologic Imaging Studies of Polycystic Kidney Disease (CRISP) was created to develop imaging techniques and analyses to evaluate progression. A total of 131 participants with early ADPKD had measurements of RBF and total kidney (TKV) and cyst (TCV) volumes by magnetic resonance and of GFR by iothalamate clearance at baseline and 1, 2, and 3 yr. The effects of age, gender, body mass index, hypertension status, mean arterial pressure (MAP), TKV, TCV, RBF, renal vascular resistance (RVR), GFR, serum uric acid, HDL and LDL cholesterol, 24-h urine volume, sodium (UNaE) and albumin (UAE) excretions, and estimated protein intake were examined at baseline on TKV, TCV, and GFR slopes. TKV and TCV increased, RBF decreased, and GFR remained stable. TKV, TCV, RVR, serum uric acid, UAE, UNaE, age, body mass index, MAP, and estimated protein intake were positively and RBF and GFR negatively correlated with TKV and TCV slopes. TKV, RBF, UNaE, and UAE were independent predictors of TKV and TCV slopes (structural disease progression). TKV, TCV, RVR, and MAP were negatively and RBF positively correlated with GFR slopes. Regression to the mean confounded the analysis of GFR slopes. TKV and RBF were independent predictors of GFR decline (functional disease progression). In ADPKD, RBF reduction (1) parallels TKV increase, (2) precedes GFR decline, and (3) predicts structural and functional disease progression.

How does proteinuria cause progressive renal damage?

The possibility that proteinuria may accelerate kidney disease progression to end-stage renal failure has received support from the results of increasing numbers of experimental and clinical studies. Evidence indicating that this process occurs through multiple pathways, including induction of tubular chemokine expression and complement activation that lead to inflammatory cell infiltration in the interstitium and sustained fibrogenesis, is reviewed. Macrophages are prominent in the interstitial inflammatory infiltrate. This cell type mediates progression of renal injury to the extent that macrophage numbers in renal biopsy predict renal survival in patients with chronic renal disease. Chemoattractants and adhesive molecules for inflammatory cells are upregulated by excess ultrafiltered protein load of proximal tubular cells via activation of NF-kappaB-dependent and NF-kappaB-independent pathways. This mechanism is a potential target for therapeutic approaches, as shown by beneficial effects of manipulations with inhibitory molecules of NF-kappaB activation or of chemokine receptors in experimental studies. Targeting complement synthesis or activation in proximal tubule might offer novel therapeutic opportunities. Finally, proximal tubular cell receptors for uptake of plasma proteins that are under investigation may provide activation signals on excess tubular protein handling.

Albumin induces cellular fibrosis by upregulating transforming growth factor-beta ligand and its receptors in renal distal tubule cells

Albuminuria is indicative of nephropathy. However, little literature has focused on the role of albumin in renal distal tubule fibrosis. We used a well-defined distal tubule cell, Madin-Darby Canine Kidney (MDCK). Proliferation and cytotoxicity were examined. The conditioned supernatant was collected and subjected to ELISA assay for detection of fibronectin and TGF-beta1. Reverse transcription-PCR and Western blot assay were performed to evaluate the expression of mRNA and protein of two types of TGF-beta receptors (TbetaR). Flow cytometry assay and phosphotyrosine (pY)-specific antibodies were used to assay the phosphorylation status of TbetaR. We showed that albumin dose dependently (0, 0.1, 1, or 10 mg/ml) inhibited cellular growth in MDCK cells without inducing cellular cytotoxicity. In addition, albumin significantly upregulated the secretion of both fibronectin and TGF-beta1 at dose over 1 mg/ml. Moreover, 24 h pretreatment of albumin significantly enhanced exogenous TGF-beta1-induced secretion of fibronectin. These observations were reminiscent of the implications of TbetaR since TbetaR appears to correlate with the susceptibility of cellular fibrosis. We found that albumin significantly increased protein levels of type I TbetaR (TbetaRI) instead of type II receptors (TbetaRII). In addition, phosphorylation level of TbetaRII of both pY259 and pY424 was significantly enhanced instead of pY336. The novel observation indicates that extreme dose of albumin upregulates TGF-beta autocrine loop by upregulating TGF-beta1, TbetaRI, and the receptor kinase activity of TbetaRII by inducing tyrosine phosphorylation on key amino residue of TbetaRII in renal distal tubule cells. These combinational effects might contribute to the pathogenesis of renal fibrosis.

Drug Insight: thiazolidinediones and diabetic nephropathy—relevance to renoprotection

Up to a third of people with diabetes mellitus suffer end-stage renal failure due to diabetic nephropathy. Strategies to delay progression of diabetic nephropathy-including glycemic and blood pressure control, modification of the renin-angiotensin system and management of lipid levels with statins-have been effective, but development of new strategies is essential if the ever-increasing burden of this disease is to be minimized. Thiazolidinediones (TZDs) are a family of compounds used as oral hypoglycemic agents in patients with type 2 diabetes mellitus. The therapeutic effects of TZDs are largely a function of their activity as ligands of peroxisome proliferator-activated receptor gamma (PPARgamma), a transcription factor that has a central role in adipogenesis and insulin sensitization. In vitro animal and clinical studies have shown that TZDs ameliorate symptoms and pathogenic mechanisms of diabetic and nondiabetic nephropathy, including proteinuria, excessive deposition of glomerular matrix, cellular proliferation, inflammation and fibrosis. Many of these favorable effects occur under both normal and high-glucose conditions. The mechanisms responsible probably involve both PPARgamma-dependent and PPARgamma-independent pathways. So, TZDs and other agonists of PPARgamma offer promise for treatment of diabetic nephropathy; however, before their putative renoprotective effects can be translated into clinical practice, the complex mechanisms of PPARgamma activity and regulation will need to be investigated further.

High glucose transactivates the EGF receptor and up-regulates serum glucocorticoid kinase in the proximal tubule

BACKGROUND

Serum glucocorticoid regulated kinase (SGK-1) is induced in the kidney in diabetes mellitus. However, its role in the proximal tubule is unclear. This study determined the expression and functional role of SGK-1 in PTCs in high glucose conditions. As the epidermal growth factor (EGF) receptor is activated by both EGF and other factors implicated in diabetic nephropathy, the relationship of SGK-1 with EGFR activity was assessed.

METHODS

mRNA and protein expression of SGK-1 and mRNA expression of the sodium hydrogen exchanger NHE3 were measured in human PTCs exposed to 5 mmol/L (control) and 25 mmol/L (high) glucose. The effects of SGK-1 on cell growth, apoptosis, and progression through the cell cycle and NHE3 mRNA were examined following overexpression of SGK-1 in PTCs. The role of EGFR activation in observed changes was assessed by phospho-EGFR expression, and response to the EGFR blocker PKI166. SGK-1 expression was then assessed in vivo in a model of streptozotocin-induced diabetes mellitus type 2.

RESULTS

A total of 25 mmol/L glucose and EGF (10 ng/mL) increased SGK-1 mRNA (P < 0.005 and P < 0.002, respectively) and protein (both P < 0.02) expression. High glucose and overexpression of SGK-1 increased NHE3 mRNA (P < 0.05) and EGFR phosphorylation (P < 0.01), which were reversed by PKI166. SGK-1 overexpression increased PTC growth (P < 0.0001), progression through the cell cycle (P < 0.001), and increased NHE3 mRNA (P < 0.01), which were all reversed with PKI166. Overexpression of SGK-1 also protected against apoptosis induced in the PTCs (P < 0.0001). Up-regulation of tubular SGK-1 mRNA in diabetes mellitus was confirmed in vivo. Oral treatment with PKI166 attenuated this increase by 51%. No EGF protein was detectable in PTCs, suggestive of phosphorylation of the EGFR by high glucose and downstream induction of SGK-1.

CONCLUSION

The effects of high glucose on PTC proliferation, reduced apoptosis and increased NHE3 mRNA levels are mediated by EGFR-dependent up-regulation of SGK-1.

Albumin stimulates cell growth, L-arginine transport, and metabolism to polyamines in human proximal tubular cells

BACKGROUND

Pure albumin stimulates proximal tubular epithelial cell (PTEC) proliferation, and may have a role in homeostasis in health, as well as in disrupted PTEC turnover in proteinuric nephropathies. We investigated a role for arginine and its metabolites, the polyamines, in this process, given the ability of polyamines to trigger proliferation in other mammalian cells.

METHODS

[(3)H]-L-arginine uptake was examined after incubation with 20 mg/mL recombinant human serum albumin (rHSA) in HK-2 PTEC monolayers. Nitric oxide synthase (NOS) and arginase activity was measured; NOS, arginase, and ornithine decarboxylase (ODC) expression was identified by semiquantitative reverse transcription-polymerase chain reaction (RT-PCR). Polyamine synthesis and intracellular amino acid concentrations were compared using high-performance liquid chromatography, and cell growth measured by [(3)H]-thymidine incorporation.

RESULTS

In HK-2 PTEC exposed to 20 mg/mL rHSA for 24 hours, cell proliferation as determined by [(3)H]-thymidine incorporation was increased. In parallel, L-arginine transport capacity was increased in a dose- and time-dependent manner. This effect was specific to rHSA, and was not seen with transferrin or immunoglobulin G. The intracellular concentration of L-arginine remained unchanged, although L-ornithine was increased with rHSA incubation. rHSA up-regulated type II arginase mRNA, and increased arginase activity, although no difference in nitric oxide synthase expression or activity was seen. ODC mRNA was increased, as were intracellular polyamine concentrations. alpha-Difluoromethylornithine (DFMO), an ODC inhibitor, reduced intracellular polyamine concentrations and rHSA-induced cell proliferation to control levels.

CONCLUSION

The arginine-ornithine-polyamine pathway appears enhanced in PTEC incubated with rHSA and is involved in cellular proliferation; this may offer novel approaches to understanding progressive proteinuric nephropathies.

Evolutionary origins of eukaryotic sodium/proton exchangers

More than 200 genes annotated as Na+/H+ hydrogen exchangers (NHEs) currently reside in bioinformation databases such as GenBank and Pfam. We performed detailed phylogenetic analyses of these NHEs in an effort to better understand their specific functions and physiological roles. This analysis initially required examining the entire monovalent cation proton antiporter (CPA) superfamily that includes the CPA1, CPA2, and NaT-DC families of transporters, each of which has a unique set of bacterial ancestors. We have concluded that there are nine human NHE (or SLC9A) paralogs as well as two previously unknown human CPA2 genes, which we have named HsNHA1 and HsNHA2. The eukaryotic NHE family is composed of five phylogenetically distinct clades that differ in subcellular location, drug sensitivity, cation selectivity, and sequence length. The major subgroups are plasma membrane (recycling and resident) and intracellular (endosomal/TGN, NHE8-like, and plant vacuolar). HsNHE1, the first cloned eukaryotic NHE gene, belongs to the resident plasma membrane clade. The latter is the most recent to emerge, being found exclusively in vertebrates. In contrast, the intracellular clades are ubiquitously distributed and are likely precursors to the plasma membrane NHE. Yeast endosomal ScNHX1 was the first intracellular NHE to be described and is closely related to HsNHE6, HsNHE7, and HsNHE9 in humans. Our results link the appearance of NHE on the plasma membrane of animal cells to the use of the Na+/K(+)-ATPase to generate the membrane potential. These novel observations have allowed us to use comparative biology to predict physiological roles for the nine human NHE paralogs and to propose appropriate model organisms in which to study the unique properties of each NHE subclass.

Renal tubule albumin transport

Albumin is the most abundant protein in serum and contributes to the maintenance of oncotic pressure as well as to transport of hydrophobic molecules. Although albumin is a large anionic protein, it is not completely retained by the glomerular filtration barrier. In order to prevent proteinuria, albumin is reabsorbed along the proximal tubules by receptor-mediated endocytosis, which involves the binding proteins megalin and cubilin. Endocytosis depends on proper vesicle acidification. Disturbance of endosomal acidification or loss of the binding proteins leads to tubular proteinuria. Furthermore, endocytosis is subject to modulation by different signaling systems, such as protein kinase A (PKA), protein kinase C (PKC), phosphatidylinositol 3-kinase (PI3-K) and transforming growth factor beta (TGF-beta). In addition to being reabsorbed in the proximal tubule, albumin can also act as a profibrotic and proinflammatory stimulus, thereby initiating or promoting tubulo-interstitial diseases.

Expression of the Na+/H+ exchanger regulatory protein family in genetically hypertensive rats

OBJECTIVE

To examine a possible involvement of a regulatory protein of Na+/H+ exchanger (NHE) in the increased renal NHE activity in spontaneously hypertensive rats (SHR), we investigated mRNA expression of inhibitory members of the NHE regulatory protein family, NHERF1 and NHERF2, in the kidney.

DESIGN

Prehypertensive 4-week-old and hypertensive 11-week-old SHR and age-matched Wistar-Kyoto (WKY) rats were used to determine the changes in NHE activity and NHERF family expression in the kidney. Dahl salt sensitive (DS) and resistant rats were also used to examine whether these changes are specific for SHR.

METHODS

mRNA expression in the kidney was quantified by RNase protection assay. The NHE activity in primary cultured proximal tubular cells was measured as Na-dependent pHi recovery rate by the NH4Cl prepulse technique with 2'7'-bis-(2-carboxyethyl)-5.6-carboxyfluorescein (BCECF).

RESULTS

NHERF1 mRNA expression was significantly decreased in both prehypertensive and hypertensive SHR in comparison with age-matched WKY rats, whereas NHERF2 mRNA expression was significantly increased in SHR only in the hypertensive period. Antihypertensive treatment did not abolish these changes seen in control SHR. On the other hand, hypertensive DS rats fed a high-salt diet showed significant decreases in NHE activity and NHE3 mRNA expression compared with normotensive DS rats fed a low-salt diet, without significant changes in NHERF1 and NHERF2 mRNA expression.

CONCLUSION

These results suggest that decreased expression of NHERF1 may be related to the enhanced NHE activity in SHR and that these changes are likely to be genetically determined, whereas the increased NHERF2 expression may be induced as a compensatory mechanism.

Effect of albumin on 14C-alpha-Methyl-D-Glucopyranoside uptake in primary cultured renal proximal tubule cells: involvement of PLC, MAPK, and NF-kappaB

A growing body of evidence implicates albumin has an important regulatory function in renal proximal tubule cells (PTCs). In present study, the effect of bovine serum albumin (BSA) on 14C-alpha-methyl-D-glucopyranoside (alpha-MG) uptake and its related signal molecules were examined in the primary cultured rabbit renal PTCs. BSA significantly increased uptake of alpha-MG, a distinctive proximal tubule marker, as well as expression level of Na+/glucose cotransporters (SGLT1 and SGLT2) proteins. The BSA-induced increase of alpha-MG uptake was completely blocked by actinomycin D and cycloheximide. Neomycin or U 73122 (PLC inhibitors), BAPTA/AM or TMB-8 (intracellular Ca2+ mobilization inhibitors) completely abolished BSA-induced increase of alpha-MG uptake. BSA significantly increased IPs accumulation, but did not affect Ca2+ uptake. Effect of BSA on alpha-MG uptake was blocked by PD 98059, but did not SB 203580. BSA increased phosphorylation of p44/42 mitogen activated protein kinase (MAPK) in a time-dependent manner. NAC or catalase (antioxidants) significantly blocked BSA-induced increase of H2O2 formation and alpha-MG uptake. BSA activated NF-kappaB translocation into nucleus. PDTC, SN50, and TLCK (NF-kappaB inhibitors) also completely blocked BSA-induced increase of alpha-MG uptake, NF-kappaB p65 and phospho IkappaB-alpha activation. In conclusion, BSA stimulates alpha-MG uptake and its action is partially correlated with PLC, MAPK, or NF-kappaB signal molecules in primary cultured renal PTCs.

PKC-alpha-mediated remodeling of the actin cytoskeleton is involved in constitutive albumin uptake by proximal tubule cells

One key role of the renal proximal tubule is the reabsorption of proteins from the glomerular filtrate by constitutive receptor-mediated endocytosis. In the opossum kidney (OK) renal proximal tubule cell line, inhibition of protein kinase C (PKC) reduces albumin uptake, although the isoforms involved and mechanisms by which this occurs have not been identified. We used pharmacological and molecular approaches to investigate the role of PKC-alpha in albumin endocytosis. We found that albumin uptake in OK cells was inhibited by the pan-PKC blocker bisindolylmaleimide-1 and the isoform-specific PKC blockers Go-6976 and 2',3,3',4,4'-hexahydroxy-1,1'-biphenyl-6,6'-dimethanol dimethyl ether, indicating a role for PKC-alpha. Overexpression of a kinase deficient PKC-alpha(K368R) but not wild-type PKC-alpha significantly reduced albumin endocytosis. Western blot analysis of fractionated cells showed an increased association of PKC-alpha-green fluorescent protein with the membrane fraction within 10-20 min of exposure to albumin. We used phalloidin to demonstrate that albumin induces the formation of clusters of actin at the apical surface of OK cells and that these clusters correspond to the location of albumin uptake. These clusters were not present in cells grown in the absence of albumin. In cells treated either with PKC inhibitors or overexpressing kinase-deficient PKC-alpha(K368R) this actin cluster formation was significantly reduced. This study identifies a role for PKC-alpha in constitutive albumin uptake in OK cells by mediating assembly of actin microfilaments at the apical membrane.

Pioglitazone increases renal tubular cell albumin uptake but limits proinflammatory and fibrotic responses

BACKGROUND

Peroxisome proliferator-activated receptor gamma (PPARgamma) agonists, which are known to be critical factors in lipid metabolism, have also been reported to reduce proteinuria. The mechanism and its relevance to progressive nephropathy have not been determined. The aims of this study were to assess the direct effects of a PPARgamma agonist on tubular cell albumin uptake, proinflammatory and profibrotic markers of renal pathology, using an opossum kidney model of proximal tubular cells.

METHODS

Cells were exposed to pioglitazone (10 micromol/L) in the presence and absence of low-density lipoprotein (LDL) 100 microg/mL +/- exposure to albumin 1 mg/mL. Results were expressed relative to control (5 mmol/L glucose) conditions.

RESULTS

Pioglitazone caused a dose-dependent increase in tubular cell albumin uptake (P < 0.0001). Despite the increase in albumin reabsorption, no concurrent increase in inflammatory or profibrotic markers were observed. Exposure to LDL increased monocyte chemoattractant protein-1 (MCP-1) (P < 0.05) and transforming growth factor-beta1 (TGF-beta1) (P < 0.05) production, which were reversed in the presence of pioglitazone. LDL induced increases in MCP-1 and TGF-beta1 were independent of nuclear factor-kappaB (NF-kappaB) transcriptional activity. In contrast, tubular exposure to albumin increased tubular protein uptake, in parallel with an increase in MCP-1 (P= 0.05), TGF-beta1 (P < 0.02) and NF-kappaB transcriptional activity (P < 0.05), which were unaffected by concurrent exposure to pioglitazone.

CONCLUSION

These findings suggest that dyslipidemia potentiates renal pathology through mechanisms that may be modified by PPARgamma activation independent of NF-kappaB transcriptional activity. In contrast, tubular exposure to protein induces renal damage through NF-kappaB-dependent mechanisms that are unaffected by PPARgamma activation.

MOLECULAR CHANGES IN PROXIMAL TUBULE FUNCTION IN DIABETES MELLITUS

Diabetic kidney disease is initially associated with hypertension and increased urinary albumin excretion. The hypertension is mediated by enhanced volume expansion due to enhanced salt and water retention by the kidney. The increased urinary albumin is not only due to increased glomerular leak, but also to a decrease in albumin reabsorption by the proximal tubule. The precise molecular mechanisms underlying these two phenomena and whether there is any link between the increase in Na(+) retention and proteinuria remain unresolved. There is significant evidence to suggest that increased Na(+) retention by the proximal tubule Na(+)/H(+) exchanger isoform 3 (NHE3) can play a role in some forms of hypertension. Increased NHE3 activity in models of diabetes mellitus may explain, in part, the enhanced salt retention observed in patients with diabetic kidney disease. The NHE3 also plays a role in receptor-mediated albumin uptake in the proximal tubule. The uptake of albumin requires the assembly of a macromolecular complex that is thought to include the megalin/cubulin receptor, NHE3, the vacuolar type H(+)-ATPase (v-H(+)-ATPase), the Cl(-) channel ClC-5 and interactions with the actin cytoskeleton. The NHE3 seems to exist in two functionally distinct membrane domains, one involved with Na(+) reabsorption and the other involved in albumin uptake. The present review focuses on the evidence derived from in vivo studies, as well as complementary studies in cell culture models, for a dual role of NHE3 in both Na(+) retention and albumin uptake. We suggest a possible mechanism by which disruption of the proximal tubule albumin uptake mechanism in diabetes mellitus may lead to both increased Na(+) retention and proteinuria.

The effect of high glucose and PPAR-gamma agonists on PPAR-gamma expression and function in HK-2 cells

Peroxisome proliferator-activated receptor-gamma (PPAR-gamma) are ligand-activated transcription factors that regulate cell growth, inflammation, lipid metabolism, and insulin sensitivity. PPAR-gamma in the human kidney has been described. However, the role of PPAR-gamma in proximal tubular cells with respect to cell growth and inflammation in diabetic nephropathy is largely unknown. We evaluated the effect of high (30 mM) D-glucose, thiazolidinedione pioglitazone (10 microM), and the selective PPAR-gamma agonist L-805645 (8 microM) on PPAR-gamma expression, growth, and inflammatory parameters in the proximal tubular model of HK-2 cells. PPAR-gamma was present in HK-2 cells and upregulated with 30 mM D-glucose to 177 +/- 31.2% of control (P < 0.05). PPAR-gamma activation was induced by pioglitazone to a similar level to that observed by exposure to high glucose but maximally induced by the selective agonist L-805645. However, L-805645 reduced cell viability in both 5 and 30 mM d-glucose to 73.8 +/- 3.1 and 77.6 +/- 1.4% of control (both P < 0.0001). In parallel, thymidine incorporation was reduced with L-805645 in both 5 and 30 mM D-glucose to 33.3 +/- 3.4 and 37.9 +/- 2.2%, respectively (both P < 0.0001). Flow cytometry demonstrated increased apoptosis and G(1) phase arrest in association with an increase in p21(cip1/waf1) in cells exposed to L-805645. Exposure to 30 mM D-glucose did not significantly change AP-1 promoter activity (89.0 +/- 5.5% of control); however, the addition of L-805645 significantly reduced it to 62.2 +/- 2.7% of control (P < 0.0001). Thirty nanomolar D-glucose induced transforming growth factor-beta(1) to 137.7 +/- 16.9% of control (P < 0.05), and L-805645 was able to suppress this to 68.7 +/- 5.7% of control (P < 0.01 vs. d-glucose). Exposure to 30 mM D-glucose reduced monocyte chemoattractant protein 1 levels to 78.6 +/- 7.1% (P < 0.05) of control, with the reduction more marked in the presence of either pioglitazone (P < 0.01) or L-805645 (P < 0.01). In summary, high glucose upregulates PPAR-gamma and when significantly induced demonstrates anti-proliferative and anti-inflammatory effects.