Renal proximal tubular cell fibronectin accumulation in response to glucose is polyol pathway dependent

Impact of Glucose in Peritoneal Dialysis: Saint or Sinner?

Peritoneal dialysis (PD) solutions using glucose as osmotic agent have been used for more than two decades as effective treatment for patients with end-stage renal disease. Although alternative osmotic agents such as amino acids and macromolecular solutions, including polypeptides and glucose polymers, are now available, glucose is still the most widely used osmotic agent in PD. It has been shown to be safe, effective, readily metabolized, and inexpensive. On the other hand, it is widely assumed that exposure of the peritoneal membrane to high glucose concentrations contributes to both structural and functional changes in the dialyzed peritoneal membrane. As in diabetes, glucose, either directly or indirectly through the generation of glucose degradation products or the formation of advanced glycation end products, may contribute to peritoneal membrane failure. Although efforts to reduce glucose toxicity have been made for years, only a few suggestions, such as dual-bag systems with bicarbonate as buffer system, have found broader acceptance. Recently, some interesting new approaches to the problem of glucose-related toxicity have been made, but further investigations will be necessary before they can be used clinically. This review will focus on adverse effects of glucose in PD solutions and summarize different aspects of glucotoxicity and potential therapeutic interventions.

A Pyruvate-Buffered Dialysis Fluid Induces Less Peritoneal Angiogenesis and Fibrosis than a Conventional Solution

Background

Conventional lactate-buffered peritoneal dialysis (PD) fluids containing glucose and glucose degradation products are believed to contribute to the development of fibrosis and angiogenesis in the dialyzed peritoneum. To reduce potential negative effects of lactate, pyruvate was substituted as a buffer and its effects on peritoneal pathological alterations were studied in a chronic peritoneal exposure model in the rat.

Methods

20 Wistar rats were infused intraperitoneally with pyruvate-buffered ( n = 9) or lactate-buffered PD fluid. After 20 weeks of daily infusion, peritoneal function was assessed. In omental peritoneal tissue, the number of blood vessels was analyzed following alpha-smooth muscle actin staining. The degree of fibrosis was quantitated in Picro Sirius Red-stained sections and by assessment of the hydroxyproline content. Plasma lactate/pyruvate and beta-hydroxybutyrate/acetoacetate (BBA/AA) ratios were determined. Plasma and dialysate vascular endothelial growth factor (VEGF) levels were quantitated by ELISA.

Results

The mass transfer area coefficient of creatinine was higher and the dialysate-to-plasma ratio of sodium was lower in pyruvate-treated animals compared to the lactatetreated group (0.11 vs 0.05 mL/min, p < 0.05, and 78% vs 89%, p < 0.05). The BBA/AA ratio tended to be lower in the pyruvate animals ( p = 0.07). The number of blood vessels was lower in pyruvate-treated animals (16 vs 37 per field, p < 0.001). Total surface area, luminal area, and wall/total area of the vessels were larger in the pyruvate group. The degree of fibrosis was lower in intersegmental and perivascular areas of pyruvate-exposed animals. Effluent VEGF was higher in the pyruvate group.

Conclusions

Replacement of lactate by pyruvate resulted in changes in peritoneal solute transport, accompanied by a reduction in both peritoneal membrane angiogenesis and fibrosis, suggesting potentially novel mechanisms to reduce glucose-driven alterations to the peritoneal membrane in PD patients.

Renal effects of a sodium-glucose cotransporter 2 inhibitor, tofogliflozin, in relation to sodium intake and glycaemic status

AIMS

Little is known about whether sodium intake is associated with the clinical effects of SGLT2 inhibitors (SGLT2is); however, SGLT2is may increase urinary sodium excretion. Thus, we investigated the impact of daily sodium intake on the estimated glomerular filtration rate (eGFR) via an SGLT2i, tofogliflozin (TOFO), in patients with type 2 diabetes (T2D).

METHODS

Individual-level data on 775 T2D patients in TOFO Phase 3 trials were analysed. Adjusted changes in variables during 52 weeks of TOFO therapy were compared according to basal daily salt intake (DSI), which was measured based on estimated daily urinary sodium excretion using the Tanaka formula. Multivariable analysis was used to investigate the impact of basal DSI on changes in eGFR at Weeks 4 and 52.

RESULTS

Sixty-six percent of participants were men; mean age, HbA1c, body mass index, eGFR_MDRD and median DSI were 58.5 years, 8.0%, 25.6 kg/m² , 83.9 mL/min/1.73 m² and 9.3 g/d, respectively. In all participants, eGFR_MDRD sharply dipped during Week 4, and gradually increased by Week 52, showing a significant increase overall from baseline to Week 52. Multivariable analysis showed that basal DSI and HbA1c levels were independently correlated with eGFR_MDRD changes at Weeks 4 and 52. Additionally, lower baseline HbA1c and DSI levels were significantly correlated with a greater increase in eGFR_MDRD at Week 52.

CONCLUSIONS

Dietary salt intake, in addition to glycaemic control, correlates with changed eGFR_MDRD via TOFO. Thus, an appropriate dietary approach to therapy should be considered before treatment of T2D patients with an SGLT2i.

Galectin-1 is a new fibrosis protein in type 1 and type 2 diabetes

Chronic exposure of tubular renal cells to high glucose contributes to tubulointerstitial changes in diabetic nephropathy. In the present study, we identified a new fibrosis gene called galectin-1 (Gal-1), which is highly expressed in tubular cells of kidneys of type 1 and type 2 diabetic mouse models. Gal-1 protein and mRNA expression showed significant increase in kidney cortex of heterozygous Akita^+/- and db/db mice compared with wild-type mice. Mouse proximal tubular cells exposed to high glucose showed significant increase in phosphorylation of Akt and Gal-1. We cloned Gal-1 promoter and identified the transcription factor AP4 as binding to the Gal-1 promoter to up-regulate its function. Transfection of cells with plasmid carrying mutations in the binding sites of AP4 to Gal-1 promoter resulted in decreased protein function of Gal-1. In addition, inhibition of Gal-1 by OTX-008 showed significant decrease in p-Akt/AP4 and protein-promoter activity of Gal-1 and fibronectin. Moreover, down-regulation of AP4 by small interfering RNA resulted in a significant decrease in protein expression and promoter activity of Gal-1. We found that kidney of Gal-1^-/- mice express very low levels of fibronectin protein. In summary, Gal-1 is highly expressed in kidneys of type 1 and 2 diabetic mice, and AP4 is a major transcription factor that activates Gal-1 under hyperglycemia. Inhibition of Gal-1 by OTX-008 blocks activation of Akt and prevents accumulation of Gal-1, suggesting a novel role of Gal-1 inhibitor as a possible therapeutic target to treat renal fibrosis in diabetes.-Al-Obaidi, N., Mohan, S., Liang, S., Zhao, Z., Nayak, B. K., Li, B., Sriramarao, P., Habib, S. L. Galectin-1 is a new fibrosis protein in type 1 and type 2 diabetes.

Effects of sodium-glucose cotransporter 2 inhibitor, tofogliflozin, on the indices of renal tubular function in patients with type 2 diabetes

AIMS

Little is known of the effect of sodium-glucose cotransporter 2 (SGLT2) inhibitors on the renal tubules. We investigated the effect of the SGLT2 inhibitor, tofogliflozin (TOFO) on renal tubular indices, according to the degree of albuminuria, in type 2 diabetes mellitus (T2DM) patients with preserved renal function.

MATERIALS AND METHODS

A total of 988 patients, receiving TOFO, were enroled and divided into 3 groups, based on the urine albumin-to-creatinine ratio (UACR). The tubular indices (urinary N-acetyl-beta-d-glucosaminidase [NAG]-to-creatinine and urinary beta-2 microglobulin [beta2MG]-to-creatinine ratios) and UACR were log-transformed in the correlation analysis.

RESULTS

Treatment with TOFO led to similar reductions in glycated haemoglobin (HbA1c) levels, from baseline to week 24, across all groups. The NAG level increased in the normoalbuminuria group and decreased in the macroalbuminuria group significantly (P < .001, both), but did not change in the microalbuminuria group. Significant reductions in the UACR were observed in both microalbuminuria and macroalbuminuria groups (P < .001, both). Significant negative correlations between changes in the NAG and beta2MG levels and their corresponding baseline values were observed in all participants. The reduction in the UACR was negatively correlated with baseline levels. The changes in the tubular indices were positively correlated with reductions in the UACR across groups.

CONCLUSIONS

Logarithmic reductions in the renal tubular indices, via SGLT2 inhibition, were observed in patients with T2DM. TOFO may not only improve the degree of albuminuria but may also have protective effects on the tubules.

An evidence-based practice-oriented review focusing on canagliflozin in the management of type 2 diabetes

Caring for patients with type 2 diabetes mellitus (T2DM) has entered an era with many recent additions to the regimens used to clinically control their hyperglycemia. The most recent class of agents approved by the Food and Drug Administration (FDA) for T2DM is the sodium–glucose-linked transporter type 2 (SGLT2) inhibitors, which work principally in the proximal tubule of the kidney to block filtered glucose reabsorption. In the few years attending this new class arrival in the market, there has been a great deal of interest generated by the novel mechanism of action of SGLT2 inhibitors and by recent large outcome trials suggesting benefit on important clinical outcomes such as death, cardiovascular disease and kidney disease progression. In this review, we focus on canagliflozin, the first-in-class marketed SGLT2 inhibitor in the USA. In some cases, we included data from other SGLT2 inhibitors, such as outcomes in clinical trials, important insights on clinical features and benefits, and adverse effects. These agents represent a fundamentally different way of controlling blood glucose and for the first time in T2DM care to offer the opportunity to reduce glucose, blood pressure, and weight with effects sustained for at least 2 years. Important side effects include genital mycotic infections and the potential for orthostatic hypotension and rare instances of normoglycemic ketoacidosis. Active ongoing clinical trials promise to deepen our experience with the potential benefits, as well as the clinical risks attending the use of this new group of antidiabetic agents.

Sodium-glucose linked transporter-2 inhibitors in chronic kidney disease

SGLT2 inhibitors are new antihyperglycaemic agents whose ability to lower glucose is directly proportional to GFR. Therefore, in chronic kidney disease (CKD) the blood glucose lowering effect is reduced. Unlike many current therapies, the mechanism of action of SGLT2 inhibitors is independent of insulin action or beta-cell function. In addition, the mechanism of action of SGLT2 inhibitors is complementary and not alternative to other antidiabetic agents. SGLT2 inhibitors could be potentially effective in attenuating renal hyperfiltration and, consequently, the progression of CKD. Moreover, the reductions in intraglomerular pressure, systemic blood pressure, and uric acid levels induced by SGLT inhibition may potentially be of benefit in CKD subjects without diabetes. However, at present, only few clinical studies were designed to evaluate the effects of SGLT2 inhibitors in CKD. Consequently, safety and potential efficacy beyond blood glucose lowering should be better clarified in CKD. In this paper we provide an updated review of the use of SGLT2 inhibitors in clinical practice, with particular attention on subjects with CKD.

Pathophysiology of the diabetic kidney

Diabetes mellitus contributes greatly to morbidity, mortality, and overall health care costs. In major part, these outcomes derive from the high incidence of progressive kidney dysfunction in patients with diabetes making diabetic nephropathy a leading cause of end-stage renal disease. A better understanding of the molecular mechanism involved and of the early dysfunctions observed in the diabetic kidney may permit the development of new strategies to prevent diabetic nephropathy. Here we review the pathophysiological changes that occur in the kidney in response to hyperglycemia, including the cellular responses to high glucose and the responses in vascular, glomerular, podocyte, and tubular function. The molecular basis, characteristics, and consequences of the unique growth phenotypes observed in the diabetic kidney, including glomerular structures and tubular segments, are outlined. We delineate mechanisms of early diabetic glomerular hyperfiltration including primary vascular events as well as the primary role of tubular growth, hyperreabsorption, and tubuloglomerular communication as part of a "tubulocentric" concept of early diabetic kidney function. The latter also explains the "salt paradox" of the early diabetic kidney, that is, a unique and inverse relationship between glomerular filtration rate and dietary salt intake. The mechanisms and consequences of the intrarenal activation of the renin-angiotensin system and of diabetes-induced tubular glycogen accumulation are discussed. Moreover, we aim to link the changes that occur early in the diabetic kidney including the growth phenotype, oxidative stress, hypoxia, and formation of advanced glycation end products to mechanisms involved in progressive kidney disease.

Sodium-glucose linked transporter-2 inhibitors: potential for renoprotection beyond blood glucose lowering?

The proximal tubule's sodium-glucose linked transporter-2 (SGLT2) accounts for the vast majority of glucose reabsorption by the kidney. Its selective inhibition, accordingly, leads to substantial glycosuria, lowering blood glucose, and facilitating weight loss in individuals with diabetes. During the past year, two SGLT2 inhibitors, canagliflozin and dapagliflozin, have been approved for the treatment of type 2 diabetes. Beyond their anti-hyperglycemic properties, however, this new class of drugs has several other attributes that provide a theoretical basis for kidney protection. Like agents that block the renin-angiotensin system, SGLT2 inhibitors also reduce single-nephron glomerular filtration rate (SNGFR) in the chronically diseased kidney, though by quite different mechanisms. Additional potentially beneficial effects of SGLT2 inhibition include modest reductions in blood pressure and plasma uric acid. Finally, cell culture studies indicate that glucose uptake from the tubular lumen, as well as from the basolateral compartment, can contribute to proximal tubular production of extracellular matrix proteins. Whether such attributes will translate into reducing the progression of chronic kidney disease will require the undertaking of long-term, dedicated studies.

Tuberin inhibits production of the matrix protein fibronectin in diabetes

Exposure of proximal tubular epithelial cells to high glucose contributes to the accumulation of tubulointerstitial and matrix proteins in diabetic nephropathy, but how this occurs is not well understood. We investigated the effect of the signaling molecule tuberin, which modulates the mammalian target of rapamycin pathway, on renal hypertrophy and fibronectin expression. We found that the kidney mass was significantly greater in partially tuberin-deficient (TSC2(+/-) ) diabetic rats than wild-type diabetic rats. Furthermore, TSC2(+/-) rats exhibited significant increases in the basal levels of phospho-tuberin and fibronectin expression in the kidney cortex. Increased levels of phosphorylated tuberin associated with an increase in fibronectin expression in both wild-type and TSC2(+/-) diabetic rats. Treatment with insulin abrogated the diabetes-induced increase in fibronectin expression. In vitro, high glucose enhanced fibronectin expression in TSC2(+/-) primary proximal tubular epithelial cells; both inhibition of Akt and inhibition of the mammalian target of rapamycin could prevent this effect of glucose. In addition, forced expression of tuberin in tuberin-null cells abolished the expression of fibronectin protein. Taken together, these data suggest that tuberin plays a central role in the development of renal hypertrophy and in modulating the production of the matrix protein fibronectin in diabetes.

Implication of dysregulation of the canonical wingless-type MMTV integration site (WNT) pathway in diabetic nephropathy

AIMS/HYPOTHESIS

The wingless-type MMTV integration site (WNT) pathway mediates multiple physiological and pathological processes, such as inflammation, angiogenesis and fibrosis. The aim of this study was to investigate whether canonical WNT signalling plays a role in the pathogenesis of diabetic nephropathy.

METHODS

Expression of WNT ligands and frizzled receptors in the canonical WNT pathway in the kidney was compared at the mRNA level using real-time RT-PCR between Akita mice, streptozotocin-induced diabetic rats and db/db mice and their respective non-diabetic controls. Renal function was evaluated by measuring the urine albumin excretion. Human renal proximal tubular epithelial cells were treated with high-glucose medium and 4-hydroxynonenal (HNE). Levels of β-catenin, connective tissue growth factor and fibronectin were determined by western blot analysis.

RESULTS

Some of the WNT ligands and frizzled receptors showed increased mRNA levels in the kidneys of Akita mice, streptozotocin-induced diabetic rats and db/db mice compared with their non-diabetic controls. Renal levels of β-catenin and WNT proteins were upregulated in these diabetic models. Lowering the blood glucose levels by insulin attenuated the activation of WNT signalling in the kidneys of Akita mice. In cultured human renal proximal tubular epithelial cells, both high glucose and HNE activated WNT signalling. Inhibition of WNT signalling with a monoclonal antibody blocking LDL-receptor-related protein 6 ameliorated renal inflammation and fibrosis and reduced proteinuria in Akita mice.

CONCLUSIONS/INTERPRETATION

The WNT pathway is activated in the kidneys of models of both type 1 and 2 diabetes. Dysregulation of the WNT pathway in diabetes represents a new pathogenic mechanism of diabetic nephropathy and renders a new therapeutic target.

Ethyl pyruvate ameliorates albuminuria and glomerular injury in the animal model of diabetic nephropathy

Pyruvate is an endogenous antioxidant and anti-inflammatory substance. The present study was implemented to investigate the protective effect of ethyl pyruvate (EP) against the development and progression of diabetic nephropathy in an in vivo and in vitro model. Diabetic rats were prepared by injecting streptozotocin (65 mg/kg). Those that developed diabetes after 72 h were treated with EP (40 mg/kg) intraperitoneally. Diabetic rats without pyruvate treatment and nondiabetic rats were used for control. As an in vitro experiment, rat mesangial cells cultured primarily from Sprague-Dawley rats were treated in high-glucose (HG; 50 mM) or normal-glucose (NG; 5 mM) conditions and with or without pyruvate. Pyruvate-treated diabetic rats exhibited decreased albuminuria and attenuated NADPH-dependent reactive oxygen species generation. Immunohistochemistry showed reduced laminin, type IV collagen, and fibronectin deposition in the glomeruli compared with nontreated diabetic rats. Parallel changes were shown in tissue mRNA and protein expression levels of monocyte chemoattractant protein-1, transforming growth factor-β1, laminin, fibronectin, and type IV collagen in the kidney. Concordantly, protective effects were also exhibited in the mesangial cell culture system. These findings suggest that pyruvate protects against kidney injury via NADPH oxidase inhibition. The present study established that activation of NADPH oxidase plays a crucial role in diabetes-induced oxidative stress, glomerular hypertrophy, and ECM molecule expression. Pyruvate exhibited a renoprotective effect in the progression of experimental diabetic nephropathy. Future research is warranted to investigate the protective mechanism of pyruvate more specifically in relation to NADPH oxidase in diabetic nephropathy.

The proximal tubule in the pathophysiology of the diabetic kidney

Diabetic nephropathy is a leading cause of end-stage renal disease. A better understanding of the molecular mechanism involved in the early changes of the diabetic kidney may permit the development of new strategies to prevent diabetic nephropathy. This review focuses on the proximal tubule in the early diabetic kidney, particularly on its exposure and response to high glucose levels, albuminuria, and other factors in the diabetic glomerular filtrate, the hyperreabsorption of glucose, the unique molecular signature of the tubular growth phenotype, including aspects of senescence, and the resulting cellular and functional consequences. The latter includes the local release of proinflammatory chemokines and changes in proximal tubular salt and fluid reabsorption, which form the basis for the strong tubular control of glomerular filtration in the early diabetic kidney, including glomerular hyperfiltration and odd responses like the salt paradox. Importantly, these early proximal tubular changes can set the stage for oxidative stress, inflammation, hypoxia, and tubulointerstitial fibrosis, and thereby for the progression of diabetic renal disease.

Angiotensin-(1-7) activates a tyrosine phosphatase and inhibits glucose-induced signalling in proximal tubular cells

Background. In the diabetic kidney, stimulation of mitogen-activated protein kinases (MAPKs) leads to extracellular matrix protein synthesis. In the proximal tubule, angiotensin-(1–7) [Ang-(1–7)] blocks activation of MAPKs by angiotensin II. We studied the effect of Ang-(1–7) on signalling responses in LLC-PK₁ cells in normal (5 mM) or high (25 mM) glucose.

Methods. The p38 MAPK was assayed by immunoblot, Src homology 2-containing protein-tyrosine phosphatase-1 (SHP-1) activity was measured after immunoprecipitation, cell protein synthesis was determined by [³H]-leucine incorporation and transforming growth factor-β1 (TGF-β1), fibronectin and collagen IV were assayed by immunoblots and/or ELISA.

Results. High glucose stimulated p38 MAPK. This response was inhibited by Ang-(1–7) in a concentration-dependent fashion, an effect reversed by the receptor Mas antagonist A-779. Ang-(1–7) increased SHP-1 activity, via the receptor Mas. An inhibitor of tyrosine phosphatase, phenylarsine oxide, reversed the inhibitory effect of Ang-(1–7) on high glucose-stimulated p38 MAPK. Ang-(1–7) inhibited high glucose-stimulated protein synthesis, and blocked the stimulatory effect of glucose on TGF-β1. Conversely, Ang-(1–7) had no effect on glucose-stimulated synthesis of fibronectin or collagen IV.

Conclusions. These data indicate that in proximal tubular cells, binding of Ang-(1–7) to the receptor Mas stimulates SHP-1, associated with the inhibition of glucose-stimulated p38 MAPK. Ang-(1–7) selectively inhibits glucose-stimulated protein synthesis and TGF-β1. In diabetic nephropathy, Ang-(1–7) may partly counteract the profibrotic effects of high glucose.

From hyperglycemia to diabetic kidney disease: the role of metabolic, hemodynamic, intracellular factors and growth factors/cytokines

At present, diabetic kidney disease affects about 15-25% of type 1 and 30-40% of type 2 diabetic patients. Several decades of extensive research has elucidated various pathways to be implicated in the development of diabetic kidney disease. This review focuses on the metabolic factors beyond blood glucose that are involved in the pathogenesis of diabetic kidney disease, i.e., advanced glycation end-products and the aldose reductase system. Furthermore, the contribution of hemodynamic factors, the renin-angiotensin system, the endothelin system, and the nitric oxide system, as well as the prominent role of the intracellular signaling molecule protein kinase C are discussed. Finally, the respective roles of TGF-beta, GH and IGFs, vascular endothelial growth factor, and platelet-derived growth factor are covered. The complex interplay between these different pathways will be highlighted. A brief introduction to each system and description of its expression in the normal kidney is followed by in vitro, experimental, and clinical evidence addressing the role of the system in diabetic kidney disease. Finally, well-known and potential therapeutic strategies targeting each system are discussed, ending with an overall conclusion.

Hyaluronan attenuates transforming growth factor-beta1-mediated signaling in renal proximal tubular epithelial cells

Increased expression of hyaluronan (HA) has been associated with both acute renal injury and progressive renal disease, although the functional significance of this remains unclear. There is overwhelming evidence that transforming growth factor (TGF)-beta1 is critical to the development of progressive renal disease. Recent studies suggest an interaction between HA and TGF-beta signaling in cancer cell biology. The aim of this study was to examine the potential role of HA as a modulator of TGF-beta1 function in renal proximal tubular epithelial cells (PTC). Under resting conditions, co-localization of the principal receptor for HA, CD44, and both the TGF-beta type I and type II receptors was demonstrated by immunoprecipitation and western analysis and further confirmed by immunocytochemistry and confocal microscopy. Stimulation of PTC with TGF-beta1 led to increased synthesis of both type III and type IV collagen assessed by Western analysis. Addition of HA did not alter collagen synthesis, but abrogated TGF-beta1-mediated increase in type III and type IV collagen. This effect was blocked by the addition of a blocking antibody to CD44 and also by inhibition of MAP kinase kinase (MEK) activity. Furthermore HA decreased TGF-beta1 activation of a luciferase-SMAD responsive construct, and decreased translocation of SMAD4 into the cell nucleus. We have previously demonstrated an anti-migratory effect of TGF-beta1 in a scratch wounding model. As with HA antagonism of TGF-beta1 extracellular matrix generation, HA reduced the anti-migratory effect of TGF-beta1 in a CD44-dependent manner. In contrast to the effect of TGF-beta1 on collagen synthesis, which is SMAD-dependent, the anti-migratory effect of TGF-beta1 in this model is known to be dependent of activation of RhoA. In the presence of HA, TGF-beta1-mediated activation of RhoA was also abrogated in a CD44-dependent manner. The results suggest that co-localization of CD44 and TGF-beta receptors facilitate modulation of both SMAD and non-SMAD-dependent TGF-beta1-mediated events by HA. Our results therefore suggest that alteration of HA synthesis may represent an endogenous mechanism to limit renal injury.

Hyaluronan and proximal tubular cell migration

BACKGROUND

The ubiquitous polysaccharide hyaluronan has been associated with both acute renal injury and progressive renal disease. The aim of this study was to examine the effect of hyaluronan on proximal tubular cell migration.

METHODS

The proximal tubular cell line, HK-2 cells, were grown in monolayer culture, and cell migration following addition of hyaluronan characterized in an in vitro model of injury that we have previously developed and characterized.

RESULTS

Addition of well-defined preparations of exogenous hyaluronan increased cell migration; however, optimum enhancement of migration was seen with hyaluronan of high molecular weight. Activation of the mitogen-activated protein kinase (MAPK) signaling cascade, as assessed by increased expression of the dually phosphorylated active form of MAPK, could be demonstrated following addition of hyaluronan. This was blocked by the addition of a specific antibody to the hyaluronan receptor, CD44. Hyaluronan-dependent enhanced migration was also abrogated by addition the CD44 blocking antibody, and by inhibition of MAPK kinase (MEK) activity. Generation of a denuded area also led to increased synthesis of endogenous hyaluronan and activation of MAPK, and blockage of either CD44 or MAPK activation inhibited proximal tubule cell (PTC) migration and re-epithelialization under nonstimulated conditions.

CONCLUSION

We have demonstrated that hyaluronan activation of the MAPK pathway through binding to its receptor CD44, enhances proximal tubule cell (PTC) migration. In addition, the results suggest that mechanical injury of PTC stimulated hyaluronan generation. These observations may have implications for both recovery from acute tubular injury and progressive renal fibrosis.

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.

Regulation of proximal tubular epithelial cell CD44-mediated binding and internalisation of hyaluronan

BACKGROUND

Increased expression of the connective tissue polysaccharide hyaluronan (HA) in the renal corticointerstitium is associated with progressive renal fibrosis. Numerous studies have demonstrated involvement proximal tubular epithelial cells in the fibrotic process and in the current study we have characterised their expression of the HA receptor, CD44, and examined changes in CD44 expression and function in response to either IL-1beta or glucose.

METHODS

Characterisation of CD44 splice variant expression was carried out in primary cultures of human proximal tubular cells (PTC) and HK2 cells. Binding and internalisation HA was examined by addition of exogenous of fluorescein-HA (fl-HA), and expression of CD44 examined by immunoblot analysis and flow cytometry. Alteration in "functional" CD44 was determined by immunoprecipitation of CD44 following stimulation in the presence of fl-HA.

RESULTS

PTC, both primary culture and the PTC cell line, HK2, express at least 5 CD44 splice variants, the expression of which are not altered by addition of either IL-1beta or 25mM D-glucose. Addition of either stimulus increased cell surface binding and internalisation of fl-HA and increased expression of functionally active CD44. Increased binding and internalisation of fl-HA, was blocked by anti-CD44 antibody, and by the inhibition of O-glycosylation.

CONCLUSIONS

The data demonstrate that stimuli inducing PTC HA synthesis also regulate PTC-HA interactions. Furthermore increased HA binding and internalisation is the result of post-translational modification of CD44 by O-glycosylation, rather than by alteration in expression of CD44 at the cell surface, or by alternate use of CD44 splice variants.

Reduced tubular cation transport in diabetes: prevented by ACE inhibition

BACKGROUND

The renal clearance of organic cations is important for the homeostasis of a number of exogenous and endogenous compounds. The organic cation transporters (OCTs) situated on the basolateral surface of proximal tubular cells mediate active cation excretion. Alterations of cation transport may occur in diabetes, although the role of the OCTs has not been previously assessed.

METHODS

Experimental diabetes was induced in rats with streptozotocin (55 mg/kg) and animals were randomly assigned to receive ramipril (3 mg/mL) in drinking water for 24 weeks. In a second protocol, rats were infused with angiotensin II (Ang II) at a dose of 58.3 ng/kg/min for 2 weeks via an implanted osmotic pump. Expression of the OCTs and renal clearance of the endogenous cation N-methyl-nicotinamide (NMN) was assessed.

RESULTS

Diabetes was associated with a reduction in gene and protein expression of both OCT-1 and OCT-2 and a reduction in NMN clearance. These effects were prevented by ramipril, associated with the prevention of albuminuria and tubular injury as demonstrated by the expression of osteopontin and glutathione peroxidase 3 (GPX-3). An infusion of Ang II also reduced NMN clearance but without altering the renal expression of OCTs.

CONCLUSION

We hypothesize that reduced expression of OCTs in diabetes may be a marker of tubular injury. However, Ang II may also directly augment renal cation clearance independent of changes in transporter expression. Together these effects may provide additional mechanism to explain treatment-related improvements in creatinine clearance and renoprotection in diabetes following blockade of the renin-angiotensin system (RAS).

Glucose-mediated induction of TGF-beta 1 and MCP-1 in mesothelial cells in vitro is osmolality and polyol pathway dependent

BACKGROUND

Glucose is converted to sorbitol and then to fructose via the polyol pathway that has been implicated in the pathogenesis of organ damage. The contribution of the polyol pathway to mesothelial cell activation has, however, not been fully determined.

METHODS

The effect of increasing glucose concentrations on transforming growth factor-beta 1 (TGF-beta 1) and monocyte chemoattractant protein-1 (MCP-1) secretion by human peritoneal mesothelial cells (HPMC) was examined. The importance of the polyol pathway was identified by its specific inhibition with an aldose reductase inhibitor.

RESULTS

Incubation of HPMC with 5 to 100 mmol/L glucose resulted in an induction of aldose reductase mRNA and intracellular sorbitol accumulation accompanied by the induction of TGF-beta 1 and MCP-1 mRNA expression and protein secretion. Mannitol at the same concentrations also induced aldose reductase, TGF-beta 1 and MCP-1 mRNA and protein expression but at a lower level than glucose. Sorbinil dose-dependently reduced both intracellular sorbitol levels (79.8% reduction of 60 mmol/L D-glucose induced intracellular sorbitol with 100 micromol/L sorbinil (N = 3, P < 0.01) and glucose-induced TGF-beta 1 and MCP-1 secretion. Mannitol induced TGF-beta 1 and MCP-1 secretion was not reduced by sorbinil. The addition of 15 to 40 mmol/L sodium lactate, either alone or in the presence of D-glucose enhanced TGF-beta 1 and MCP-1 secretion, which was inhibited by sorbinil. In contrast, sodium pyruvate appeared to antagonize D-glucose-induced TGF-beta 1 and MCP-1 secretion.

CONCLUSION

These data suggest that the polyol pathway and osmolality contribute to the regulation of HPMC function by glucose. Control of polyol pathway activation might reduce glucose-mediated damage to the peritoneal membrane and promote its long-term survival.

Fibronectin expression in proximal tubules from ischemic rat kidneys without reperfusion

The expression of fibronectin (FN), one of the extracellular matrix proteins, was studied in isolated renal proximal tubules in a in vivo rat model of unilateral renal ischemia without reperfusion. FN is involved in cell-extracellular matrix interactions and defective cell-extracellular matrix interactions have been hypothesized to contribute to ischemic renal failure. The expression of FN was investigated by reverse transcription-polymerase chain reaction (RT-PCR), Elisa and Western blot. Isolated proximal tubules from control and post-ischemic rat kidneys were used. ATP, intracellular calcium content, and alkaline phosphatase were also measured to describe the effects associated to 40 min of ischemia. Control tubules expressed FN. Forty minutes of ischemia promoted diminished ATP levels and phosphatase alkaline activity, and increased intracellular calcium in isolated proximal tubules. An increased abundance of FN was observed by ischemic tubules as compared with control tubules. To determine quantitatively the value of FN content, ELISA method was performed. The ischemic tubules also expressed higher amount of FN mRNA. Three amplification products were obtained from both ischemic and control proximal tubular cDNA. The relative amounts of each of the obtained products were the same, strongly suggesting that the augmentation of the FN gene transcription during ischemia is not associated to a modification in the splicing pattern. Moreover, this expression is increased after 40 min of ischemia, not followed by reperfusion.

Renal connective tissue growth factor induction in experimental diabetes is prevented by aminoguanidine

The aim of this study was to determine whether aminoguanidine (AG), an inhibitor of advanced glycation, prevents expression of the profibrotic cytokine, connective tissue growth factor (CTGF), as well as accumulation of the previously reported CTGF-dependent matrix protein, fibronectin, in a model of experimental diabetic nephropathy. Diabetic animals were randomly allocated into groups receiving 32 wk of AG or vehicle. Diabetic rats showed increases in CTGF mRNA and protein expression as well as in advanced glycation end-product (AGE) and fibronectin immunostaining, compared with nondiabetic rats. In the diabetic kidney, the increase in CTGF gene and protein expression as well as expression of the extracellular matrix protein fibronectin were prevented by AG. To further explore the relationship between AGEs and mesangial CTGF and fibronectin production, cultured human mesangial cells were exposed in vitro to soluble AGE-BSA and carboxymethyl lysine-BSA, and this led to induction of both CTGF and fibronectin. On the basis of our in vitro findings in mesangial cells linking AGEs to CTGF expression, the known prosclerotic effects of CTGF, and the ability of AG to attenuate mesangial expansion, it is postulated that the antifibrotic effects of AG in this animal model may be partially mediated by CTGF.

Interaction between the transforming growth factor-beta type II receptor/Smad pathway and beta-catenin during transforming growth factor-beta1-mediated adherens junction disassembly

The aim of the current study was to examine the influence of transforming growth factor (TGF)-beta 1 on proximal tubular epithelial cell-cell interaction, with particular emphasis on the regulation of adherens junction complex formation. Stimulation of the proximal tubular cell line HK-2 cells by TGF-beta 1 led to loss of cell-cell contact and disassembly of both adherens and tight junctional complexes. Adherens junction disassembly was associated with reduction of both Triton-soluble and Triton-insoluble E-cadherin, and an increase in detergent-soluble beta-catenin. Under these conditions, immunoprecipitation and Western analysis demonstrated decreased association of beta-catenin, both with E-cadherin, alpha-catenin, and the cell cytoskeleton. Confocal microscopy after immunostaining, showed decreased intensity of peripheral E-cadherin staining, and redistribution of beta-catenin expression to a perinuclear location. Tight junction disassembly was manifest by a reduction in the expression of Triton-soluble occludin and ZO-1 by Western analysis and their disassociation manifested by immunostaining and confocal microscopy. Loss of cell-cell contact and disassembly of adherens junctions were seen after addition of TGF-beta 1 to the basolateral aspect of the cells. Immunoprecipitation experiments demonstrated co-localization of E-cadherin, beta-catenin, and TGF-beta 1 RII in unstimulated cells. After TGF-beta 1 stimulation, the TGF-beta 1 RII no longer associated with either E-cadherin or beta-catenin. Dissociation of the adherens junction protein from the TGF-beta 1 receptor was associated with increased beta-catenin tyrosine phosphorylation and decreased threonine phosphorylation. Furthermore after receptor ligand binding, beta-catenin became associated with the TGF-beta 1-signaling molecules Smad3 and Smad4.

Goto-Kakizaki rat is protected from proteinuria after induction of anti-Thy1 nephritis

Hyperglycemia, although necessary, alone is insufficient for the development of progressive diabetic nephropathy. Two factors implicated in its pathogenesis are mesangial cell activation and/or proliferation and monocyte/macrophage influx. We have shown that prolonged hyperglycemia in the Goto-Kakizaki (GK) rat is associated with renal structural changes similar to those in patients with diabetes before the onset of progressive nephropathy. The aim of the current study is to examine the role of mesangial cell injury and macrophage influx on renal structure and function. After induction of nephritis in either hyperglycemic GK rats or normoglycemic Wistar rats by the administration of Ox-7 antibody, the degree of mesangiolysis and subsequent mesangial proliferation was no different between GK and Wistar rats. Similarly, macrophage influx and mesangial cell activation (assessed by alpha-smooth actin expression) was no different between the two groups. Wistar rats developed marked albuminuria; conversely, no significant proteinuria or albuminuria was seen in GK rats. Analysis of glomerular proteoglycans (PGs) showed an increase in (35)S incorporation into heparan sulfate PGs of GK compared with Wistar rats, with no alteration in glycosaminoglycan chain size or charge density. These changes were kidney specific and not seen in spleen, lung, or heart tissue. Western blot analysis showed increased agrin core protein expression in whole-kidney homogenates of untreated GK rats. Induction of Thy1.1 nephritis was associated with reduced expression of agrin in both GK and Wistar rats. However, agrin expression was greater in GK rats at all times. In summary, acute mesangial cell injury associated with a macrophage influx did not initiate progressive diabetic nephropathy in GK rats. Despite a similar magnitude of glomerular/mesangial injury, GK rats, in contrast to normoglycemic Wistar rats, did not develop proteinuria after the administration of anti-Thy1 antibody. We postulate that altered expression of agrin in this model accounts for the lack of proteinuria and thus may protect against progressive nephropathy.

Gene expression and identification of gene therapy targets in diabetic nephropathy

A number of novel genes that are up-regulated in diabetic kidneys have been identified. Recently, transforming growth factor-beta (TGF-beta)--driven secreted proteins, i.e., connective tissue growth factor (CTGF) and gremlin, were identified. They are up-regulated in kidneys of diabetic animals and modulate the biology of mesangial cells. CTGF mediates TGF-beta--induced matrix overproduction by the mesangial cells. Gremlin is a putative antagonist of bone morphogenetic protein-2 that blocks mesangial cell proliferation. Thus, gremlin may modulate the biology of mesangium by stimulating mesangial cell proliferation and in turn production of matrix. In addition, transcriptionally regulated kinases, serum glucocorticoid-regulated kinase and munc-13 have been identified. The former stimulates renal tubular Na+ transport and is involved in hyperfiltraion of diabetic kidneys by a Na+ transport feedback mechanism. Munc-13 has been shown to induce apoptosis in hyperglycemic state via diacylglycerol-activated, PKC-independent signaling pathway. Another pathway relevant to diabetic nephropathy is polyol pathway, where glucose is reduced to sorbitol by aldose reductase. Recently, a renal-specific reductase of the aldo-keto reductase family was isolated. It is up-regulated in diabetic mice, and this could serve as a suitable target for gene therapy in renal complications of diabetes. Several mitochondrial genome-encoded genes, such as, cytochrome oxidase and NADH dehydrogenase, are up-regulated in diabetic kidneys. A novel nuclear-encoded mitochondrial gene, i.e., translocase inner mitochondrial membrane 44 (Tim44), is up-regulated in diabetic kidneys, and it may also serve as another target for molecular therapeutic intervention at the core storage energy sites, i.e., mitochondria. In this review, these novel differentially regulated genes that respond to hyperglycemic stress are described, and they may serve as possible targets for gene therapy in the treatment of diabetic nephropathy.

In-vitro studies of aluminium-induced toxicity on kidney proximal tubular cells

The toxicity of aluminium (Al) to various cells is well described. However, little is known about its effect on kidney cells, which can be exposed to relatively high concentrations. In this study, the effect of aluminium as the citrate complex in concentrations up to 100 microM/l was investigated using a monolayer cell culture of kidney proximal tubular cells (PTC). Aluminium was found to be slightly toxic; at 100 microM/l the PTCs lost viability by 15, 20 and 24% after incubation for 24, 48 and 72 h, respectively. Viability was significantly reduced (P<0.001) after 48 h incubation with aluminium concentrations of 25, 50, 75 and 100 microM/l compared with controls. Lactate dehydrogenase (LDH) release was significantly increased (P<0.001) with 100 microM/l Al to 44.67+/-1.76 and 50.33+/-0.88 compared with controls 24+/-1.00 and 28.33 2.34 U/l after 24 and 48 h incubation, respectively, indicating damage to the plasma membrane. However, N-acetyl-beta-D-glucosaminidase (NAG) release in the medium of cells exposed to aluminium showed no difference from control values (P>0.1). Glucose consumption in aluminium-exposed cells at 100 microM/l was slightly, but not significantly (P=0.14), increased during 48 h incubation. Electron micrographs of cells exposed to aluminium at 100 microM/l showed a slight reduction in microvilli density and the cell tight junctions were not as clear compared with the control cells. Pretreatment with protective agents glutathione and tiopronin partly restored the viability of kidney proximal tubular cells to control values, whereas vitamin C and/or cysteine showed no effect. This study indicates that aluminium may show toxicity to kidney cells in culture. Several sites on the cell, i.e. microvilli, membrane and the cell junction, seem to be affected, however the mechanism(s) of damage remain unclear.

Regulation of renal proximal tubular epithelial cell hyaluronan generation: implications for diabetic nephropathy

BACKGROUND

Proximal tubular cells (PTCs) contribute to pathological changes in the renal interstitium by the generation of cytokines and alterations in the composition of the extracellular matrix. Hyaluronan (HA) is a ubiquitous connective tissue polysaccharide that regulates cell function and tissue remodeling. In the current study, we investigated the regulation of HA generation by PTCs.

METHODS

Primary cultures of human PTCs were grown to confluence and stimulated under serum-free conditions with either interleukin-1 (IL-1) or 25 mmol/L D-glucose. Alterations in HA generation were detected by enzyme-linked immunosorbent assay, and alterations in HA synthase gene expression were examined by reverse transcription-polymerase chain reaction. Subsequently, the mechanisms of IL-1 beta and glucose-induced alterations in HA were examined utilizing HK-2 cells.

RESULTS

Stimulation of human PTCs (HPTCs) with either IL-1 beta or 25 mmol/L D-glucose led to a significant increase in the HA concentration in the culture supernatant. In contrast, stimulation of HPTCs with transforming growth factor-beta1, basic fibroblast growth factor, or platelet-derived growth factor-AB did not stimulate HA production. The addition of IL-1 beta or 25 mmol/L D-glucose also increased HA generation in HK-2 cells and was associated with the induction of HAS2 mRNA. HAS3 mRNA was constitutively expressed and was not influenced by the addition of either stimulus. HAS1 mRNA expression was not detected in either unstimulated or stimulated cells. Inhibition of gene transcription or protein synthesis abolished HA production in response to either IL-1 beta or glucose. Inhibition of nuclear factor-kappa B (NF-kappa B) activation either by sulindac or by the proteosome inhibitor (PSI) abrogated both IL-1 beta and glucose-mediated alteration in HA synthesis.

CONCLUSION

This study demonstrates, to our knowledge for the first time, that increased HA synthesis in response to either IL-1 beta or elevated 25 mmol/L D-glucose is associated with NF-kappa B-activated transcription of HAS2.

Identification of a renal-specific oxido-reductase in newborn diabetic mice

Aldose reductase (ALR2), a NADPH-dependent aldo-keto reductase (AKR), is widely distributed in mammalian tissues and has been implicated in complications of diabetes, including diabetic nephropathy. To identify a renal-specific reductase belonging to the AKR family, representational difference analyses of cDNA from diabetic mouse kidney were performed. A full-length cDNA with an ORF of 855 nt and yielding a approximately 1.5-kb mRNA transcript was isolated from a mouse kidney library. Human and rat homologues also were isolated, and they had approximately 91% and approximately 97% amino acid identity with mouse protein. In vitro translation of the cDNA yielded a protein product of approximately 33 kDa. Northern and Western blot analyses, using the cDNA and antirecombinant protein antibody, revealed its expression exclusively confined to the kidney. Like ALR2, the expression was up-regulated in diabetic kidneys. Its mRNA and protein expression was restricted to renal proximal tubules. The gene neither codistributed with Tamm-Horsfall protein nor aquaporin-2. The deduced protein sequence revealed an AKR-3 motif located near the N terminus, unlike the other AKR family members where it is confined to the C terminus. Fluorescence quenching and reactive blue agarose chromatography studies revealed that it binds to NADPH with high affinity (K(dNADPH) = 66.9 +/- 2.3 nM). This binding domain is a tetrapeptide (Met-Ala-Lys-Ser) located within the AKR-3 motif that is similar to the other AKR members. The identified protein is designated as RSOR because it is renal-specific with properties of an oxido-reductase, and like ALR2 it may be relevant in the renal complications of diabetes mellitus.

The effect of sorbinil, an aldose reductase inhibitor, on aortic function in control and streptozotocin-induced diabetic rats

1. The present study investigates the effect of treatment of 14-day streptozotocin-diabetic rats with the aldose reductase inhibitor, sorbinil, on changes ex vivo in aortic vasoconstriction and vasodilation. 2. Maximum contractile responses and aortic sensitivity to phenylephrine were significantly enhanced in aortae from 14-day diabetic rats, in accordance with our previous data. 3. Endothelium-dependent relaxations to carbachol were, in contrast, depressed, although endothelium-independent relaxations to forskolin and sodium nitroprusside were unaltered. 4. Sorbinil treatment of diabetic animals failed to prevent any of these diabetes-induced alterations in aortic function, and indeed exacerbated some of these alterations. In addition, sorbinil treatment caused altered aortic responses in control animals, which sometimes mirrored those found in diabetic animals. 5. It can be concluded that sorbinil may have actions in addition to, and independent of, polyol pathway inhibition. Thus, sorbinil may not be an effective tool for the investigation of aldose reductase inhibition within the vascular system of the rat.

Aldose reductase inhibitors: therapeutic implications for diabetic complications

The 'late complications' of diabetes mellitus, i.e., nephropathy, neuropathy and retinopathy are firmly rooted in inadequate control of blood glucose: hyperglycaemia. Hyperglycaemia causes elevated cytosolic glucose and/or rates of glucose metabolism, i.e., 'hyperglysolia,' within cells of vulnerable tissues. Although the molecular basis for the pathogenic effects of hyperglysolia remains to be proven, substantial evidence points to a key role for increased glucose metabolism through a cytosolic enzyme, aldose reductase (AR). Recent human genetic and biochemical data link polymorphisms of the AR gene (technically called the AR2 gene) and elevated tissue levels of AR with strongly altered risks for diabetic complications. Despite several genetic reports failing to confirm such an association, there are now ten concordant reports from five continents that certain polymorphisms of the AR gene are associated with an ~ 3- to 20-fold higher risk for diabetic complications. Moreover, in US and European diabetic study populations the principle allele of the AR gene associated with elevated disease risk, the Z-2 allele, correlates with an ~ 2- to 3-fold increase in AR expression. These results, together with recent clinical, experimental and pharmacological data, provide powerful new support for the rationale for research and development of aldose reductase inhibitors (ARIs) targeted at slowing the progression of diabetic complications. Although past clinical trials of ARIs have been disappointing, this has stemmed from overly optimistic expectations, inadequate trial designs and lack of pharmacological robustness and/or acceptable systemic toleration of the agents tested. However, a more realistic and encouraging perspective for therapeutic expectations for ARIs has arisen from recent data revealing that the seemingly modest short-term effects of intensified glycaemic control and of pancreatic transplantation are followed by substantial long-term benefits on diabetic complications. In addition, robust inhibition of AR in human nerve has recently yielded dose-dependent efficacy on nerve structure and function. Thus, the quest for well-tolerated, potent ARIs continues to be a worthy and more urgent objective than ever before.

The tubulointerstitium in progressive diabetic kidney disease: more than an aftermath of glomerular injury?

Although the glomerulus, particularly the mesangium, has been the focus of intense investigation in diabetes, tubulointerstitial injury is also a major feature of diabetic nephropathy and an important predictor of renal dysfunction. The renal tubule in diabetes is subject to both direct and indirect pathogenetic influences as a consequence of its position in the nephron and its resorptive function. On exposure to glucose, proximal tubular cells elaborate vasoactive hormones, including angiotensin II and injurious cytokines such as transforming growth factor-beta (TGF-beta), as well as extracellular matrix proteins. In turn, angiotensin II may further increase TGF-beta expression in both proximal tubular and interstitial cells, thus amplifying the stimulus to fibrogenesis in the renal tubulointerstitium. In addition to these mostly direct influences, the renal tubule, particularly its proximal segment, is exposed to glomerular effluent. In the diabetic state, this includes large quantities of advanced glycation end products and glucose and, at later stages in the evolution of diabetic nephropathy, protein, all of which are factors that may induce TGF-beta expression and fibrosis. Diabetic nephropathy should therefore be viewed as a disease affecting the entire nephron. Continued exploration into tubulointerstitial disease in addition to glomerular injury in diabetes may help provide further insights into the pathogenesis of diabetic nephropathy and additional targets for therapeutic intervention.