Structure-function relationships associated with extracellular matrix alterations in diabetic glomerulopathy

Renal clearance of glycolaldehyde- and methylglyoxal-modified proteins in mice is mediated by mesangial cells through a class A scavenger receptor (SR-A)

AIMS/HYPOTHESIS

Glomerular mesangial expansion is a characteristic feature of diabetic nephropathy, and the accumulation of AGE in the mesangial lesion has been implicated as one of its potential causes. However, the route for the AGE accumulation in mesangial lesions in diabetic patients is poorly established.

METHODS

Glycolaldehyde-modified BSA (GA-BSA) and methylglyoxal-modified BSA (MG-BSA) were prepared as model AGE proteins, and their in vivo plasma clearance was examined in mice, and renal uptake by in vitro studies with isolated renal mesangial cells.

RESULTS

Both (111)In-GA-BSA and (111)In-MG-BSA were rapidly cleared from the circulation mainly by both the liver and kidney. Immunohistochemical studies with an anti-GA-BSA antibody demonstrated that intravenously injected GA-BSA accumulated in mesangial cells, suggesting that such cells play an important role in the renal clearance of circulating AGE proteins. Binding experiments at 4 degrees C using mesangial cells isolated from mice showed that (125)I-GA-BSA and (125)I-MG-BSA exhibited specific and saturable binding. Upon incubation at 37 degrees C, (125)I-GA-BSA and (125)I-MG-BSA underwent endocytic degradation by these cells. The binding of the ligands to these cells was inhibited by several ligands for scavenger receptors. The endocytic degradation of GA-BSA by mesangial cells from class A scavenger receptor (SR-A) knock-out mice was reduced by 80% when compared with that of wild-type cells. The glomerular accumulation of GA-BSA after its intravenous administration was attenuated in SR-A knock-out mice, as evidenced by immunohistochemical observations.

CONCLUSIONS/INTERPRETATION

These results raise the possibility that circulating AGE-modified proteins are subjected to renal clearance by mesangial cells, mainly via SR-A. This pathway may contribute to the pathogenesis of AGE-induced diabetic nephropathy.

New insights into the pathophysiology of diabetic nephropathy: from haemodynamics to molecular pathology

Although debated for many years whether haemodynamic or structural changes are more important in the development of diabetic nephropathy, it is now clear that these processes are interwoven and present two sides of one coin. On a molecular level, hyperglycaemia and proteins altered by high blood glucose such as Amadori products and advanced glycation end-products (AGEs) are key players in the development of diabetic nephropathy. Recent evidence suggests that an increase in reactive oxygen species (ROS) formation induced by high glucose-mediated activation of the mitochondrial electron-transport chain is an early event in the development of diabetic complications. A variety of growth factors and cytokines are then induced through complex signal transduction pathways involving protein kinase C, mitogen-activated protein kinases, and the transcription factor NF-kappaB. High glucose, AGEs, and ROS act in concert to induce growth factors and cytokines. Particularly, TGF-beta is important in the development of renal hypertrophy and accumulation of extracellular matrix components. Activation of the renin-angiotensin system by high glucose, mechanical stress, and proteinuria with an increase in local formation of angiotensin II (ANG II) causes many of the pathophysiological changes associated with diabetic nephropathy. In fact, it has been shown that angiotensin II is involved in almost every pathophysiological process implicated in the development of diabetic nephropathy (haemodynamic changes, hypertrophy, extracellular matrix accumulation, growth factor/cytokine induction, ROS formation, podocyte damage, proteinuria, interstitial inflammation). Consequently, blocking these deleterious effects of ANG II is an essential part of every therapeutic regiment to prevent and treat diabetic nephropathy. Recent evidence suggests that regression of diabetic nephropathy could be achieved under certain circumstances.

Regulation of fibronectin levels by agmatine and spermine in mesangial cells under high-glucose conditions

Amines such as agmatine, putrescine, spermidine and spermine have been reported to be involved in a variety of physiological and biochemical phenomena. However, it is not known whether they are also involved in the homeostasis of intracellular fibronectin content via upregulation of protein kinase C (PKC), extracellular signal-regulated kinase (ERK), and transforming growth factor-beta1 (TGF-beta1). To determine this, we have studied the effect of multiple amines on fibronectin, TGF-beta1, ERK, and PKC levels in mesangial cells under high glucose conditions. All the amines tested (at 0.1-1 mM) affected neither the viability of mesangial cells for 42 h nor LDH release into the medium. Agmatine reduced TGF-beta1 and ERK levels but not PKC at concentrations of 0.1-1 mM. However, levels of fibronectin, TGF-beta1, ERK, and PKC were unaffected by either putrescine or spermidine. A decrease in fibronectin secretion was accompanied by decreases in TGF-beta1 and ERK. Such cumulative results lead us to hypothesize that agmatine reduces high glucose-induced fibronectin secretion via several pathways including ERK-TGF-beta1-fibronectin and spermine, via a decrease in TGF-beta1. Possible roles of enzymes involved in agmatine and polyamine biosynthesis are discussed in relation to secretion of ECM proteins.

Ramipril prevents microtubular changes in proximal tubules from streptozotocin diabetic rats

This study has investigated the microtubular cytoskeleton in rat glomerular and proximal tubule cells in experimental diabetes. The effect of treatment with ramipril on the relationship between microtubule organization and albuminuria in diabetes has also been examined. Diabetes was induced in male Sprague-Dawley rats by administration of streptozotocin (50 mg/kg, i.v.). Rats were treated with or without ramipril in their drinking water for 12 weeks. Diabetes was characterized by an increase in blood glucose level, glomerular filtration rate, and albumin excretion rate. Treatment of diabetic rats with ramipril did not affect glycaemic control, but reduced systolic blood pressure and prevented the rise in albuminuria and glomerular filtration rate. Immunohistochemistry was performed by using the ARK Peroxidase method with alpha-tubulin antibody. The regular, grainy staining pattern of the microtubules present in the renal proximal tubules from control kidneys was altered in diabetic animals, and appeared fragmented and striated. This was prevented by treatment with ramipril. Quantitative morphometric analysis revealed an increase in the percent proportional staining for alpha-tubulin in the proximal tubules of untreated diabetic rats (33.3 +/- 3.3%, n = 8, P < 0.05 vs control) compared with control rats (11.7 +/- 1.7%, n = 6), which was reduced by ramipril treatment (26.7 +/- 2.1%, n = 6, P < 0.05 vs untreated diabetic). Staining for alpha-tubulin in glomerular cells was unchanged in all groups. There was no significant difference in renal alpha-tubulin expression among all groups, as determined by real-time reverse transcription-polymerase chain reaction. These results raise the possibility that diabetes-induced changes in microtubules in the renal proximal tubules may contribute, in part, to the increase in albuminuria observed in diabetes.

Effect of high glucose on gene expression in mesangial cells: upregulation of the thiol pathway is an adaptational response

Pathological alterations in glomerular mesangial cells play a critical role in the development of diabetic nephropathy, the leading cause of end-stage renal disease. Molecular mechanisms mediating such alterations, however, remain to be fully understood. The present study first examined the effect of high glucose on the mRNA expression profile in rat mesangial cells using cDNA microarray. Based on variation-weighted criteria and with a false discovery rate of 4.3%, 459 of 17,664 cDNA elements examined were found to be upregulated and 151 downregulated by exposure to 25 mM d-glucose for 5 days. A large number of differentially expressed genes belonged to several functional categories, indicating high glucose had a profound effect on mesangial cell proliferation, protein synthesis, energy metabolism, and, somewhat unexpectedly, protein sorting and the cytoskeleton. Interestingly, several thiol antioxidative genes (glutathione peroxidase 1, peroxiredoxin 6, and thioredoxin 2) were found by microarray and confirmed by real-time PCR to be upregulated by high glucose. These changes suggested that the oxidative stress known to be induced in mesangial cells by high glucose might be buffered by upregulation of the thiol antioxidative pathway. Upregulation of thiol antioxidative genes also occurred in high-glucose-treated human mesangial cells and in glomeruli isolated from rats after 1 wk of streptozotocin-induced diabetes, but not in human proximal tubule cells. High glucose slightly increased lipid peroxidation and decreased the amount of reduced thiols in rat and human mesangial cells. Disruption of the thiol antioxidative pathway by two different thiol-oxidizing agents resulted in a three- to fivefold increase in high-glucose-induced lipid peroxidation. In summary, the present study provided a global view of the short-term effect of high glucose on mesangial cells at the level of mRNA expression and identified the upregulation of the thiol antioxidative pathway as an adaptational response of mesangial cells to high glucose.

Intercellular adhesion molecule-1-deficient mice are resistant against renal injury after induction of diabetes

Diabetic nephropathy is a leading cause of end-stage renal failure. Several mechanisms, including activation of protein kinase C, advanced glycation end products, and overexpression of transforming growth factor (TGF)-beta, are believed to be involved in the pathogenesis of diabetic nephropathy. However, the significance of inflammatory processes in the pathogenesis of diabetic microvascular complications is poorly understood. Accumulation of macrophages and overexpression of leukocyte adhesion molecules and chemokines are prominent in diabetic human kidney tissues. We previously demonstrated that intercellular adhesion molecule (ICAM)-1 mediates macrophage infiltration into the diabetic kidney. In the present study, to investigate the role of ICAM-1 in diabetic nephropathy, we induced diabetes in ICAM-1-deficient (ICAM-1(-/-)) mice and ICAM-1(+/+) mice with streptozotocin and examined the renal pathology over a period of 6 months. The infiltration of macrophages was markedly suppressed in diabetic ICAM-1(-/-) mice compared with that of ICAM-1(+/+) mice. Urinary albumin excretion, glomerular hypertrophy, and mesangial matrix expansion were significantly lower in diabetic ICAM-1(-/-) mice than in diabetic ICAM-1(+/+) mice. Moreover, expressions of TGF-beta and type IV collagen in glomeruli were also suppressed in diabetic ICAM-1(-/-) mice. These results suggest that ICAM-1 is critically involved in the pathogenesis of diabetic nephropathy.

Protective effects of chronic melatonin treatment against renal injury in streptozotocin-induced diabetic rats

The aim of this study was to investigate the effects of melatonin as an antioxidant, on prevention and treatment of streptozotocin (STZ)-induced diabetic renal injury in rats. Male Wistar rats were divided into four groups: (1) untreated, (2) melatonin-treated, (3) untreated diabetic (UD), (4) melatonin-treated diabetic (MD). Experimental diabetes was induced by single dose (60 mg/kg, i.p.) STZ injection. For 3 days prior to administration of STZ, melatonin was injected (200 microg/kg/day, i.p.); these injections were continued until the end of the study (4 weeks). Malondialdehyde (MDA) levels as a marker of lipid peroxidation were significantly increased in the renal homogenates of UD animals and decreased after melatonin administration. The activity of the antioxidative enzyme glutathione peroxidase (GSH-Px) was significantly reduced in UD rats. Melatonin treatment reversed STZ-induced reduction of GSH-Px activity without having an effect on blood glucose. Upon histopathological examination, it was observed that the melatonin treatment prevented the renal morphological damage caused by diabetes. Upon immunohistochemical investigation, glomerular anti-laminin beta1 staining decreased in MD rats. Additionally, no tubular anti-IGF-1 staining was observed in melatonin-treated rats. In conclusion, chronically administered melatonin reduced renal injury in STZ-induced diabetic rats and thus it may provide a useful therapeutic option in humans to reduce oxidative stress and the associated renal injury in patients with diabetes mellitus.

Expression of type-1 plasminogen activator inhibitor in the kidney of diabetic rat models

INTRODUCTION

Intrarenal coagulation and fibrinolysis are thought to be involved in the pathogenesis of diabetic nephropathy. However, gene expression of fibrinolytic factors in diabetic nephropathy has not been clearly defined. Therefore we determined the gene expression of fibrinolytic factors in the kidneys of diabetic rats.

MATERIALS AND METHODS

As a model of type1 diabetes male Sprague-Dawley rats were used. They were divided into three groups: control, streptozotocin (STZ)-induced diabetic, and insulin-treated diabetic. Otsuka Long-Evans Tokushima Fatty (OLETF) rats were used as a model of type 2 diabetes; and Long-Evans Tokushima Otsuka (LETO) rats, as the control. Renal gene expressions of type-1 plasminogen activator inhibitor (PAI-1), tissue-type PA (tPA), and urokinase-type PA (uPA) were examined by real-time PCR. Localization of PAI-1 mRNA was investigated by in situ hybridization.

RESULTS

Renal PAI-1 mRNA levels (versus control) were increased by 60-80% in STZ-induced diabetic rats (10 days or 3 weeks post STZ injection); and insulin treatment reduced this increased expression to the control level. In OLETF rats (38 weeks old), the renal PAI-1 mRNA level was 2.5-fold higher than that in age-matched LETO rats. Both tPA and uPA mRNA levels were significantly lower than those in LETO rats. PAI-1 mRNA was observed in intraglomerular cells and tubular epithelial cells of both models.

CONCLUSIONS

Renal PAI-1 gene expression is up-regulated in both type 1 and type 2 diabetic rats, and changes in gene expressions of fibrinolytic factors may play important roles in the development and pathogenesis of diabetic nephropathy.

Troglitazone suppresses the secretion of type I collagen by mesangial cells in vitro

BACKGROUND

Our laboratory has shown that troglitazone, a thiazolidinedione and peroxisomal proliferator activated receptor gamma (PPAR-gamma) agonist, prevents mesangial expansion and glomerulosclerosis in diabetic rats. We investigated and compared the action of two PPAR agonists at the level of the mesangial cell.

METHODS

Rat mesangial cells were grown in medium containing 5 mmol/L glucose, 30 mmol/L glucose, or 5 mmol/L glucose plus 25 mmol/L mannitol. The cultures were either left untreated, treated with 10 micromol/L troglitazone (PPAR-gamma), or 100 micromol/L clofibrate (PPAR-alpha). The following parameters were used to assess mesangial cell responses: detection of PPAR-gamma and -alpha mRNA, the degree of PPAR-gamma and -alpha activation, spread cell area, total protein production, and laminin and type I collagen production.

RESULTS

Reverse transcription-polymerase chain reaction (RT-PCR) showed the presence of PPAR-gamma and -alpha mRNA in rat mesangial cells. PPAR-gamma and -alpha proteins are active in mesangial cells and the extent of activation is affected by different glycemic conditions. Troglitazone and clofibrate treatment corrected in part the increase in spread cell area seen in mesangial cells in hyperglycemic conditions. However, neither agonist corrected the increase in total protein production induced by hyperglycemia. Treatment with troglitazone resulted in a significant, specific decrease in type I collagen along with a slight decrease in laminin production in both medium conditions. Clofibrate had no effect on laminin synthesis in either medium condition but did decrease type I collagen synthesis in cells grown in hyperglycemic conditions.

CONCLUSION

PPAR-alpha and -gamma mRNA signaling pathways are in place and active in mesangial cells. Both agonists affect the phenotypic behavior of mesangial cells and ameliorate changes resulting from hyperglycemia. The data indicate that the correction of mesangial cell phenotype by troglitazone may influence production/deposition of a pathological fibrotic connective tissue matrix (that is, type I collagen) by these cells.

Reversibility of glucose-induced changes in mesangial cell extracellular matrix depends on the genetic background

Adequate glycemic control protects most patients with diabetes from nephropathy, but a substantial fraction of patients develop progressive disease despite lowering glycemia. We isolated mesangial cells (MC) from the glomeruli of mouse strains that model these two outcomes in patients with diabetes, namely those that have the propensity (ROP) or resistance (B6) to develop progressive diabetic nephropathy. We determined the nature and reversibility of changes in selected extracellular matrix-related molecules after chronic exposure to elevated glucose concentration. MC were exposed to 25 mmol/l glucose for 5 weeks followed by 6 mmol/l glucose and 19 mmol/l mannitol for an additional 5 weeks. Matrix metalloproteinase-2 (MMP-2) and transforming growth factor-beta(1) (TGF-beta(1)) levels increased in B6 MC exposed to 25 mmol/l glucose but returned to baseline levels when the glucose concentration was reduced to 6 mmol/l. MMP-2 and TGF-beta(1) were higher in ROP MC at baseline and increased in response to 25 mmol/l glucose, but remained elevated when glucose concentration was reduced. Type I collagen expression and accumulation increased in a reversible manner in B6 MC exposed to 25 mmol/l glucose. However, type I collagen expression was higher in ROP MC at baseline and remained unaffected by changes in glucose concentration. Thus, 25 mmol/l glucose induced reversible changes in MMP-2, TGF-beta(1), and type I collagen in MC of sclerosis-resistant mice but not in MC from sclerosis-prone mice. Therefore, progressive diabetic nephropathy may be secondary to stable alterations in the phenotype of MC as a result of the interplay between the genetic background and elevated glucose concentrations.

Alterations of vitronectin and its receptor alpha(v) integrin in the rat renal glomerular wall during diabetes

Vitronectin, a multifunctional glycoprotein present in blood and extracellular matrix, is not only a member of the cell adhesion molecules, but also a regulator of proteolytic enzyme cascades, thereby providing a unique regulatory factor for proteolytic degradation of extracellular matrix and tissue remodeling. Vitronectin interacts with the cell surface through integrins of the alpha(v)-related system. Because vitronectin and its receptor may have a role in various renal physiological and pathological processes, we evaluated their expression in renal tissues of streptozotocin-induced short- and long-term hyperglycemic rats by applying quantitative immunoelectron microscopy and Western blot analysis. Vitronectin was shown over the glomerular basement membrane (GBM) and mesangial matrix (MM), whereas alpha(v) was located along the plasma membrane of endothelial, epithelial, and mesangial cells. Although distribution patterns of vitronectin and alpha(v) integrin labeling in renal tissues from short- and long-term hyperglycemic rats, as well age-matched normoglycemic rats, were similar, increases in their immunoreactive sites were detected in hyperglycemic conditions. Changes also were present in old compared with young normoglycemic animals. The diabetes-related increase in vitronectin was more significant in the GBM than MM, whereas the increase in alpha(v) integrin was as significant in podocytes as mesangial cells. Western blot analysis, performed on isolated glomerular material from normoglycemic and hyperglycemic animals, confirmed those changes. Our results suggest that vitronectin and its receptor, alpha(v) integrin, must have defined roles in molecular mechanisms involved in the pathogenesis of both diabetic and aging nephropathy.

Vascular endothelial growth factor is essential for hyperglycemia-induced structural and functional alterations of the peritoneal membrane

Long-term peritoneal dialysis is associated with the development of functional and structural alterations of the peritoneal membrane. Long-term exposure to the high glucose concentrations in conventional peritoneal dialysate has been implicated in the pathogenesis of peritoneal hyperpermeability and neoangiogenesis. Vascular endothelial growth factor (VEGF) is an endothelial-specific growth factor that potently stimulates microvascular permeability and proliferation. High glucose exposure upregulates VEGF expression in various cell types and tissues. This study investigated whether VEGF plays a pathogenetic role in hyperglycemia-induced microvascular dysfunction in the peritoneal membrane. The peritoneal microcirculation of streptozotocin-induced diabetic rats and age-matched controls was studied in vivo with a combination of functional and morphologic techniques. The diabetic microcirculation was characterized by an elevated transport of small solutes, indicating the presence of an increased effective vascular surface area. The leakage of FITC-albumin was more rapid in diabetic vessels, suggesting hyperpermeability for macromolecules. Structurally, an increased vascular density with focal areas of irregular capillary budding was found in the diabetic peritoneum. The hyperglycemia-induced structural and functional microvascular alterations were prevented by long-term treatment with neutralizing anti-VEGF monoclonal antibodies, whereas treatment with isotype-matched control antibodies had no effect. VEGF blockade did not influence microvascular density or macromolecular leakage in control rats, demonstrating specificity for the hyperglycemia-induced alterations. The present results thus support an causative link among high glucose exposure, upregulation of VEGF, and peritoneal microvascular dysfunction.

p38 MAPK and MAPK kinase 3/6 mRNA and activities are increased in early diabetic glomeruli

BACKGROUND

The p38 mitogen-activated protein kinase (MAPK) pathway is activated by several stress factors, potentially leading to cellular apoptosis and growth. Little is known about the pattern of glomerular p38 MAPK pathway activation during the course of diabetic nephropathy (DN). We examined the activity and expression of the p38 MAPK pathway members, p38 MAPK, MKK3/6, cAMP-responsive element binding protein (CREB), and MAPK phosphatase-1 (MKP-1), in experimental DN in rats over the course of four months.

METHODS

Control (C; N = 16) and diabetic (DM; N = 16) rats were studied. Four rats from each group were sacrificed monthly, and competitive reverse transcription-polymerase chain reaction and Western blot were performed with microdissected and sieved glomeruli, respectively.

RESULTS

Glomerular p38 MAPK mRNA expression was significantly higher in DM than C (P < 0.01) throughout the four-month period. Western blot revealed an average 3.1-fold increase in p38 MAPK protein throughout the study period (P < 0.05). However, p38 MAPK activity was significantly increased only in one- and two-month diabetic glomeruli. Glomerular MKK3/6 and CREB mRNA as well as activity were significantly increased only in one- and two-month DM compared with C. MKP-1 mRNA showed a similar pattern.

CONCLUSIONS

Glomerular p38 MAPK activity was increased in early DN. Parallel to this, we also showed, to our knowledge for the first time, that there were increased MKK3/6 and CREB activities and mRNA expression. This activated p38 MAPK pathway in diabetic glomeruli may, in part, play a role in the pathogenesis of early hypertrophy and extracellular matrix accumulation.

Delayed branching of endothelial capillary-like cords in glycated collagen I is mediated by early induction of PAI-1

Development of micro- and macrovascular disease in diabetes mellitus (DM) warrants a thorough investigation into the repertoire of endothelial cell (EC) responses to diabetic environmental cues. Using human umbilical vein EC (HUVEC) cultured in three-dimensional (3-D) native collagen I (NC) or glycated collagen I (GC), we observed capillary cord formation that showed a significant reduction in branching when cells were cultured in GC. To gain insight into the molecular determinants of this phenomenon, HUVEC subjected to GC vs. NC were studied using a PCR-selected subtraction approach. Nine different genes were identified as up- or downregulated in response to GC; among those, plasminogen activator inhibitor-1 (PAI-1) mRNA was found to be upregulated by GC. Western blot analysis of HUVEC cultured on GC showed an increase in PAI-1 expression. The addition of a neutralizing anti-PAI-1 antibody to HUVEC cultured in GC restored the branching pattern of formed capillary cords. In contrast, supplementation of culture medium with the constitutively active PAI-1 reproduced defective branching patterns in HUVEC cultured in NC. Ex vivo capillary sprouting in GC was unaffected in PAI-1 knockout mice but was inhibited in wild-type mice. This difference persisted in diabetic mice. In conclusion, the PCR-selected subtraction technique identified PAI-1 as one of the genes characterizing an early response of HUVEC to the diabetic-like interstitial environment modeled by GC and responsible for the defective branching of endothelial cells. We propose that an upregulation of PAI-1 is causatively linked to the defective formation of capillary networks during wound healing and eventual vascular dropout characteristic of diabetic nephropathy.

Workshop: endothelial cell dysfunction leading to diabetic nephropathy : focus on nitric oxide

Clinical manifestations of diabetic nephropathy are an expression of diabetic microangiopathy. This review revisits the previously proposed Steno hypothesis and advances our hypothesis that development of endothelial cell dysfunction represents a common pathophysiological pathway of diabetic complications. Specifically, the ability of glucose to scavenge nitric oxide is proposed as the initiation phase of endothelial dysfunction. Gradual accumulation of advanced glycated end products and induction of plasminogen activator inhibitor-1, resulting in the decreased expression of endothelial nitric oxide synthase and reduced generation of nitric oxide, are proposed to be pathophysiologically critical for the maintenance phase of endothelial dysfunction. The proposed conceptual shift toward the role of endothelial dysfunction in diabetic complications may provide new strategies for their prevention.

Gelatinase B (MMP-9) is not essential in the normal kidney and does not influence progression of renal disease in a mouse model of Alport syndrome

Matrix metalloproteinases are matrix degrading enzymes implicated in many biological processes, including development and inflammation. Gelatinase B (gelB; also known as MMP-9) is expressed in the kidney and is hypothesized to be involved in basement membrane remodeling and in preventing pathogenic accumulation of extracellular matrix in the kidney. Inhibition of gelB activity in metanephric organ culture disrupts branching morphogenesis of the ureteric bud, suggesting that gelB plays a role in kidney development in vivo. We studied kidneys of gelB-deficient mice to search for developmental, histological, molecular, ultrastructural, and functional defects. Surprisingly, no differences between gelB-/- and control kidneys were detected, and renal function was normal in gelB mutants. In addition, gelB-/- embryonic kidneys developed normally in organ culture. Gelatinase B-deficient mice were bred with Col4a3-/- mice, a model for Alport syndrome, to determine whether gelB influences the progression of glomerulonephritis. This is an important question, as it has been hypothesized that proteases are involved in damaging Alport glomerular basement membrane. However, the presence or absence of gelB did not affect the rate of progression of renal disease. Thus, gelB does not have a discernible role in the normal kidney and gelB is not involved in the progression of glomerulonephritis in a mouse model of Alport syndrome.

Localization of olfactomedin-related glycoprotein isoform (BMZ) in the golgi apparatus of glomerular podocytes in rat kidneys

Agene encoding olfactomedin-related glycoprotein was isolated from rat glomerulus despite its prior identification as a neuron-specific gene. The mRNA expression was remarkably intense in renal glomerulus and brain and faint in the lung and eye among rat systemic organs. Although the brain contained four mRNA variants (AMY, AMZ, BMY, and BMZ) transcribed from a single gene, the glomerulus, lung, and eye expressed only two variants (BMZ and BMY). The glycoprotein was intensely immunolocalized in glomerular podocytes and neurons by using an antibody against synthetic peptide of the M region, but weak in endothelial cells of the kidney and lung. Bronchiolar epithelial cells in the lung, and ciliary, corneal, and iris epithelial cells in the eye were also stained. Immunogold electron microscopy revealed selective localization of olfactomedin-related glycoprotein at the Golgi apparatus in podocytes. In glomerular culture, the staining was also intense at a juxtanuclear region in synaptopodin-positive epithelial cells of irregular shape (phenotypic feature of podocytes), whereas it was weak in synaptopodin-negative ones of cobblestone-like appearance (phenotypic feature of parietal epithelial cells of Bowman's capsule). Interestingly, Western blot analysis identified an intense band corresponding to BMZ isoform and another faint band corresponding to BMY isoform in the glomerulus, whereas the intensity of these two bands were nearly equal in the lung and eye. In the brain, four bands corresponding to four isoforms were observed apparently. Computer sequence analysis predicted coiled-coil structures in the secondary structure of the glycoprotein similar to those in Golgi autoantigens, suggesting significant roles in the unique functions of the Golgi apparatus in rat podocytes and neurons.

Amelioration of accelerated diabetic mesangial expansion by treatment with a PKC beta inhibitor in diabetic db/db mice, a rodent model for type 2 diabetes

Activation of protein kinase C (PKC) is implicated as an important mechanism by which diabetes causes vascular complications. We have recently shown that a PKC beta inhibitor ameliorates not only early diabetes-induced glomerular dysfunction such as glomerular hyperfiltration and albuminuria, but also overexpression of glomerular mRNA for transforming growth factor beta1 (TGF-beta1) and extracellular matrix (ECM) proteins in streptozotocin-induced diabetic rats, a model for type 1 diabetes. In this study, we examined the long-term effects of a PKC beta inhibitor on glomerular histology as well as on biochemical and functional abnormalities in glomeruli of db/db mice, a model for type 2 diabetes. Administration of a PKC beta inhibitor reduced urinary albumin excretion rates and inhibited glomerular PKC activation in diabetic db/db mice. Administration of a PKC beta inhibitor also prevented the mesangial expansion observed in diabetic db/db mice, possibly through attenuation of glomerular expression of TGF-beta and ECM proteins such as fibronectin and type IV collagen. These findings provide the first in vivo evidence that the long-term inhibition of PKC activation in the renal glomeruli can ameliorate glomerular pathologies in diabetic state, and thus suggest that a PKC beta inhibitor might be an useful therapeutic strategy for the treatment of diabetic nephropathy.

Renal basement membrane components

Renal basement membrane components. Basement membranes are specialized extracellular matrices found throughout the body. They surround all epithelia, endothelia, peripheral nerves, muscle cells, and fat cells. They play particularly important roles in the kidney, as demonstrated by the fact that defects in renal basement membranes are associated with kidney malfunction. The major components of all basement membranes are laminin, collagen IV, entactin/nidogen, and sulfated proteoglycans. Each of these describes a family of related proteins that assemble with each other in the extracellular space to form the basement membrane. Over the last few years, new basement membrane components that are expressed in the kidney have been discovered. Here, the major components and their localization in mature and developing renal basement membranes are described. In addition, the phenotypes of basement membrane component gene mutations, both naturally occurring and experimental, are discussed, as is the aberrant deposition of basement membrane proteins in the extracellular matrix in several renal diseases.

Molecular mechanisms of renal hypertrophy: role of p27Kip1

There are two fundamentally different growth responses for cells comprising the nephron: hyperplasia or hypertrophy. Cells that progress through the normal cell cycle double their DNA content and eventually divide during mitosis. Those cells that hypertrophy stop the growth process in the G1-phase of the cell cycle; while they increase in size, protein and RNA content, they cannot duplicate their set of chromosomes because they never pass through the S-phase of the cell cycle. Hypertrophy may be an early compensatory mechanism to initially replace the loss of functioning tissue, however, this maladaptive process eventually fosters progressive loss of renal function. Since progression of the cell through the G1 to S-phases is regulated by cyclins D, E and A, which in turn bind and activate cyclin dependent kinases (CDKs), evidence has been accumulating on a particular CDK-inhibitor protein, p27Kip1, which is speculated to be a key to the complex process of the G1/S cell cycle transition. This article examines the mechanisms of the proliferative growth response following acute tubular necrosis, and compensatory hypertrophy of glomerular and tubule cells, with a particular focus on the protein p27Kip1.

Mechanism of action of angiotensin-receptor blocking agents

Angiotensin II is the most active hormone of the renin-angiotensin system. In humans, two angiotensin receptors have been identified: AT(1) and AT(2). In adults, most of the effects of angiotensin II are mediated by the AT(1) receptor; the function of the AT(2) receptor is not yet well established. Angiotensin II has both systemic and local paracrine effects. Increased activity of angiotensin II and stimulation of the AT(1) receptor have been linked to the development of several cardiovascular and renal diseases, including hypertension, heart failure, left ventricular hypertrophy, and diabetic nephropathy. Over the past two decades, angiotensin-converting enzymes have been used to manage these diseases. However, the side effects and less-than- maximum therapeutic effects of angiotensin-converting enzyme inhibitors, particularly in the decrease of mortality associated with congestive heart failure, have led to the development of AT(1)-receptor blockers.

Multiple promoter elements are required for the stimulatory effect of insulin on human collagenase-1 gene transcription. Selective effects on activator protein-1 expression may explain the quantitative difference in insulin and phorbol ester action

Several of the complications seen in patients with both type I and type II diabetes mellitus are associated with alterations in the expression of matrix metalloproteinases. To identify the cis-acting elements that mediate the stimulatory effect of insulin on collagenase-1 (matrix metalloproteinase-1) gene transcription a series of collagenase-chloramphenicol acetyltransferase (CAT) fusion genes were transiently transfected into HeLa cells. Multiple promoter elements, including an Ets and activator protein-1 (AP-1) motif, were required for the effect of insulin. The AP-1 motif appears to be a target for insulin signaling because it is sufficient to mediate an effect of insulin on the expression of a heterologous fusion gene, whereas the data suggest that the Ets motif acts to enhance the effect of insulin mediated through the AP-1 motif. Multiple promoter elements were also required for the stimulatory effect of phorbol esters on collagenase-CAT gene transcription, and the AP-1 motif was also a target for phorbol ester signaling. However, the cis-acting elements required for the effects of insulin and phorbol esters were not identical. Moreover, phorbol esters were a much more potent inducer of collagenase-CAT gene transcription than insulin, a difference that may be explained by selective effects of insulin and phorbol esters on AP-1 expression.

Serotonin enhances the production of type IV collagen by human mesangial cells

BACKGROUND

The plasma concentration of 5-hydroxytryptamine (5-HT) in diabetic patients is higher than that in normal subjects. Since recent reports have demonstrated the presence of 5-HT2A receptor in glomerular mesangial cells, it is possible that 5-HT may be involved in the development of diabetic nephropathy through the 5-HT2A receptor in mesangial cells. Because expansion of the glomerular mesangial lesion is a characteristic feature of diabetic nephropathy, we examined the effect of 5-HT on the production of type IV collagen by human mesangial cells.

METHODS

Human mesangial cells were incubated with 5-HT with or without 5-HT receptor antagonists, protein kinase C (PKC) inhibitor or transforming growth factor-beta (TGF-beta) antibody. Type IV collagen mRNA and protein concentration in medium were measured by Northern blot analysis and enzyme-linked immunosorbent assay (ELISA), respectively. TGF-beta mRNA and bioactivity in the medium were measured by Northern blot analysis and bioassay using mink lung epithelial cells, respectively.

RESULTS

5-HT stimulated the production of type IV collagen by human mesangial cells, which was inhibited by ketanserin and sarpogrelate hydrochloride, 5-HT2A receptor antagonists, but not by ondansetron, a 5-HT3 receptor antagonist. 5-HT increased the bioactivities of both active and total TGF-beta. However, the 5-HT-enhanced production of type IV collagen was completely inhibited by an anti-TGF-beta antibody. Furthermore, a PKC inhibitor, calphostin C, inhibited the 5-HT-induced increase in type IV collagen secretion, and the activity of membrane PKC was increased by 5-HT. Phorbol ester activated type IV collagen production as well as active and total TGF-beta. Calphostin C completely inhibited the 5-HT-enhanced activity of active TGF-beta, but did not inhibit exogenous TGF-beta-induced increase in type IV collagen secretion.

CONCLUSIONS

Our results suggest that 5-HT-enhanced production of type IV collagen by human mesangial cells is mediated by activation of PKC and subsequent increase in active TGF-beta activity.

Early manifestations of nephropathy in alloxan-treated rats

An early stage of diabetic nephropathy was studied. Rat renal function was evaluated by clearance techniques, 7 or 15 days after alloxan administration (groups A7 and A15). Significant diminutions of glomerular filtration rate (inulin clearance) and p-aminohippurate clearance were observed in alloxan-treated rats. Diabetic animals presented glucosuria and enhanced water excretion. A natriuretic response was only observed in A15-rats. Arterial pressure increased along time, and enlarged lipid deposits in glomeruli and vessels of A7-kidney sections were observed. Thus, a vascular compromise at this time was suggested. To better characterize the set up of the renal dysfunction, other studies were performed in A7-group. Urinary protein excretion remained unchanged while a higher level of glycosylation of urinary proteins was observed in A7-rats. Histological studies revealed a normal general morphology in kidneys from diabetic rats. Immunohistochemical analysis in renal sections showed enlarged deposits of fibronectin in glomeruli and interstitium of alloxan-treated rats. Higher myeloperoxidase activity was observed in renal cortex from diabetic animals indicating leukocytes infiltration. These results indicated that 7 days after hyperglycemia induction, the animals presented a renal dysfunction characterized by hemodynamic alterations associated with vascular and glomerular structural impairments, without modifications in tubular function. The higher level of protein glycosylation and the inflammatory process at this early stage could be responsible for the beginning of diabetic nephropathy.

Increased laminin deposition in capillaries of the stria vascularis of quiet-aged gerbils

The distribution of laminin (LA) and type IV collagen (IV-C) in the gerbil inner ear was investigated by light and electron microscopic immunohistochemistry. Changes in protein expression were assessed from birth to old age to determine the relation of these constituents to maturation of the cochlea and development of presbyacusis. The distribution of LA paralleled that of IV-C during postnatal development, and both were visualized in the basement membrane (BM) of endothelial, epithelial and spiral ganglion cells in neonatal and young adult gerbils. Immunopositive BM underlying the stria vascularis disappeared at 8-12 days after birth coincident with the development and maturation of the strial capillaries. Immunoreactivity for LA afforded an index to the thickness of the BM and was found to increase with age only in the BM of strial capillaries. At 6 months of age, occasional strial capillaries in the apex of the cochlea showed thickening of the LA-positive BM. Abnormal deposition of LA in strial capillary BM spread to lower turns and increased in prevalence with advancing age, affecting apical and basal more than middle cochlear turns. Thickening of the capillary BM appeared to precede capillary obstruction which eventuated in complete strial atrophy. Staining for IV-C in the walls of the strial capillaries did not increase with age. The data show that LA and IV-C play important roles in postnatal development of the cochlea and that LA deposition increases with age only in the BM of strial capillaries.

The role of angiotensin II in diabetic nephropathy: emphasis on nonhemodynamic mechanisms

Several systemic or intrarenal networks of cytokines and growth factors can be modulated by the diabetic state. We summarize the status of the renin-angiotensin system in diabetes mellitus and review the evidence of its involvement in the pathogenesis of diabetic nephropathy. Particular emphasis is placed on the nonhemodynamic properties of this vasoactive agent as both a renal growth factor and a profibrogenic peptide. Antagonizing the effects of angiotensin II with converting enzyme inhibitors is an established protective strategy in the management of diabetic nephropathy even in the absence of systemic hypertension. This and other indirect evidence from experimental animal studies suggest that the intrarenal concentration of angiotensin II may be increased as a result of increased synthesis and despite enhanced breakdown, that this peptide participates in the progression of diabetic nephropathy. However, down-regulation of angiotensin type 1 (AT1)-receptors is one of the abnormalities of both tubules and glomeruli in diabetic renal disease. A heightened bioactivation of the intrarenal angiotensin II system is therefore likely but not certain. Studies in cultured proximal tubular and glomerular mesangial cells have disclosed striking similarities between the effects of high glucose-containing medium and of treatment with angiotensin II on the growth properties and the induction of cytokines in these cells. There may also exist additive effects of angiotensin II and high glucose on signal-transduction pathways, such as activation of protein kinase C, although the contractile response to angiotensin II may be blunted by high glucose in mesangial cells. An important downstream mediator of the effects of both angiotensin II and high glucose is the activation of transforming growth factor-beta that can mediate at least some of the hypertrophic and profibrotic effects of either angiotensin II or high glucose in the diabetic kidney.

Evaluation of glomerular lesion and abnormal urinary findings in OLETF rats resulting from a long-term diabetic state

Otsuka Long-Evans Tokushima Fatty (OLETF) rats have been established as an animal model in which non-insulin-dependent diabetes mellitus develops spontaneously. We examined the renal histopathology and the urinary findings serially in OLETF rats and compared these findings with findings in age-matched Long-Evans Tokushima Otsuka (LETO) rats as a control strain. OLETF rats showed higher blood glucose levels than did LETO rats from 18 weeks of age, and hemoglobin A1c levels became higher in OLETF rats than in LETO rats from 22 weeks of age. Accompanying the development of hyperglycemia was an increase in the amount of albuminuria in OLETF rats from 18 weeks of age. The initial histopathologic change found in OLETF rats was an increase in glomerular area, and mesangial expansion started to develop from 22 weeks of age. Mesangial lesions progressed to mesangial sclerosis, and exudative lesions were found in OLETF rats from 36 weeks of age. The anionic charge of glomerular basement membrane (GBM), measured by polyethyleneimine grain density, demonstrated that the lower grain density in OLETF rats when compared with that in LETO rats became more evident with an increase in the amount of albuminuria. Therefore, the defect in the charge-selective property found in OLETF rats might be one of the causes of albuminuria. The GBM became thickened in elderly OLETF rats as compared with that in age-matched LETO rats. Disturbances in the selectivity of urinary protein, as determined by the clearance ratio of immunoglobulin G to transferrin, were found to accompany the thickening of GBM in OLETF rats. We consider that both the loss of the charge-selective property and massive albuminuria might be the causes of GBM thickening, through a clogging mechanism, and that GBM thickening might in turn produce the loss of size selectivity. Given these findings, we consider the OLETF strain of rats to be an interesting animal model for studying the relationship between diabetes and renal involvement, because the glomerular abnormalities and massive albuminuria found in OLETF rats were results of a long-term diabetic state.

Inhibition of diabetic nephropathy by a GH antagonist: a molecular analysis

Streptozotocin-treated C57B1/SJL mice developed glomerular hypertrophy and light microscopic lesions mimicking human diabetic glomerulosclerosis. In contrast, there were no glomerular hypertrophy and lesions in diabetic mice transgenic (TG) for a mutated growth hormone (bGH-G119K) that competes with native endogenous GH and results in dwarfism. We examined the molecular events underlying these findings. The non-transgenic (non-TG) diabetic mouse glomeruli had an increase in mRNA coding for alpha 1IV collagen, laminin B1, TGF-beta 1, 72 kDa collagenase, and TIMP-3. In contrast, glomerular type IV collagen and laminin B1 mRNA levels were normal in diabetic TG dwarf mice. However, the 72 kDa gelatinase, TIMP-3, and TGF-beta 1 mRNAs were elevated in the diabetic dwarfs. Type IV collagen and laminin accumulated in the glomeruli of diabetic non-TG, but not of diabetic dwarf mice, by immunofluorescence microscopy, confirming the mRNA data. GH binding protein mRNA levels were comparable in glomeruli from dwarf and non-TG mice, both diabetic and non-diabetic. We did not detect GH receptor mRNA in glomeruli. These data suggest that diabetic glomerulosclerosis is associated with an increase in type IV collagen and laminin synthesis, and that these changes do not occur in mice transgenic for bGH119K, a functional antagonist of GH. The increase of 72 kDa gelatinase, TIMP-3 and TGF-beta 1 mRNAs, independent of GH, suggested that these changes induced by hyperglycemia were not sufficient for the induction of glomerulosclerosis.

Phenotypic changes of the mesangium in diabetic nephropathy

In diabetic nephropathy there is accumulation of matrix proteins. Overproduction of these matrix proteins considered to be due to the phenotypic change of mesangial cell. In order to detect the phenotypic change of the mesangial cell, renal biopsy specimens from patients with diabetic nephropathy were stained with antibodies against various types of collagens and contractile-associated protein, caldesmon. Type III collagen was not stained in the glomerulus and type VI collagen showed mesangial pattern from normal controls. In diabetes, mesangial staining of type III collagen and increases in type VI collagen were observed in the mesangium. Increased mesangial staining of caldesmon was noted in the glomerulus from diabetic nephropathy in contrast to only vessel staining from normal controls. These results indicate that phenotypic changes are noted in the mesangium in diabetes. Expression of contractile-associated protein such as caldesmon, would serve as a useful marker to predict glomerulosclerosis.