Glucose enhances type IV collagen production in cultured rat glomerular mesangial cells

Nonenzymatically glycated albumin (Amadori adducts) enhances nitric oxide synthase activity and gene expression in endothelial cells

Hyperglycemia is considered to induce diabetic nephropathy through nonenzymatic glycation of proteins. Since hyperfiltration is likely to be the mechanism initiating the glomerular lesions, we investigated the effects of Amadori glucose adducts in serum albumin on the production of vasoactive mediators, including nitric oxide (NO) and eicosanoids, by endothelial cells (EC). Amadori adducts of glycated albumin induced a dose-response increase in NO synthase activity of murine endothelioma cells, up to 16.4 +/- 2.1-fold increase of basal values (P < 0.0001) at concentrations of 35 mg/ml mimicking physiological serum albumin concentration, and 4.6 +/- 0.8-fold increase at 17 mg/ml (P < 0.001). The effect was still detectable with glycated albumin 1.7 mg/ml, which approaches its estimated concentration in diabetic serum (1.6 +/- 0.3-fold increase, P < 0.05) The phenomenon was reproducible in human umbilical vein endothelial cells, though to a lesser extent, and further studies on murine EC were employed. The mRNA encoding for inducible NO synthase was overexpressed in EC incubated with Amadori adducts of glycated albumin in comparison to native albumin. Glycated albumin induced increased mRNA expression and synthesis of TNF-alpha. The stimulatory effect induced by glycated albumin on NO synthase activity was almost completely inhibited by anti TNF alpha antibodies. 3H-thymidine incorporation by EC was significantly inhibited when cells were grown in presence of glycated albumin (P < 0.001), and the phenomenon was abolished by the coincubation of the NO competitive inhibitor L-NAME. The early glycosylation products increased thromboxane production (P < 0.001), while prostaglandin E2 synthesis was unaffected. These data indicate that Amadori products of glycated albumin modulate NO synthase activity and eicosanoid balance in EC. These effects may be relevant to the hemodynamic changes in the early phases of diabetic nephropathy and in the lasting progression to sclerosis.

Role of glomerular mechanical strain in the pathogenesis of diabetic nephropathy

Glomerular rigidity limits the glomerular expansion and mesangial cell (MC) stretch induced by variations in intracapillary pressure. In tissue culture, MC stretch stimulates synthesis of extracellular matrix components (ECM). Therefore, altered glomerular rigidity in diabetes may influence ECM accumulation by modulating the glomerular distention and MC stretch associated with glomerular hypertension. An ambient of high glucose concentration per se also enhances MC formation of ECM, possibly altering the cellular response to mechanical stretch. In this study, compliance was measured in isolated perfused glomeruli from streptozotocin-injected rats at four days (4d-D), five weeks (5w-D) and six months (6m-D) after induction of diabetes. In addition, collagen metabolism induced by stretch was investigated in MC cultured in 8 and 35 mM glucose concentrations. Glomerular compliance was normal in 5w-D rats and moderately increased in 4d-D (16%) and 6m-D animals (14%). As compared to static cultures. MC stretch increased total collagen synthesis (8 mM, 50%; 35 mM, 27%) and catabolism. However, while the fraction of newly formed collagen being catabolized increased in 8 mM-stretched cultures, in 35 mM-stretched it was unchanged. This resulted in marked increase in the net collagen accumulated in the incubation medium (4 vs. 24%) and cell layer 5 vs. 15%) only in the latter. In diabetes, the largely unaltered glomerular stiffness renders hypertension-induced MC stretch unopposed. More importantly, the accumulation of ECM caused by any degree of mechanical strain is greatly aggravated in a milieu of high glucose concentration.

Hyperglycemia augments macrophage growth responses to colony-stimulating factor-1

Pathologic changes produced (or potentiated) by the diabetic state include diabetic retinopathy, nephropathy, and atherosclerosis. There is evidence that the macrophage is implicated in the pathogenesis of these lesions. One of the growth factors known to exert a profound influence on macrophage physiology is colony-stimulating factor-1 (CSF-1). CSF-1 has previously been shown to be produced by endothelial cells, mesangial cells, and vascular smooth muscle cells, and it is reasonable to suggest that the interaction of this factor on infiltrating macrophages is an event that may have pivotal importance in the formation of diabetic lesions. We report on the modulating influence of hyperglycemia on the proliferative response of mouse splenic macrophages to CSF-1. Our studies showed that hyperglycemia enhanced the growth response of such macrophages to CSF-1. The mechanism underlying this enhanced response was examined, and it was demonstrated that hyperglycemia induced a threefold increase in CSF-1 receptor (CSF-1r) mRNA expression as visualized by Northern blot analysis. These investigations provide insight into one of the molecular mechanisms potentially relevant to the genesis of diabetic lesions in the host.

Effect of an aldose reductase inhibitor on type IV collagen production by human endothelial cells cultured in high glucose

Diabetic microangiopathy is characterized by a thickening of capillary basement membranes associated with type IV collagen accumulation. An increase in type IV collagen content of the aortic wall is also observed in macroangiopathy. In order to analyse the importance of the polyol pathway in the development of the collagen metabolism alterations seen in diabetic angiopathy and their prevention by aldose reductase inhibitors, we have studied the effects of sorbinil on the high glucose-induced stimulation of type IV collagen biosynthesis in human umbilical vein endothelial cells. Primary cultures were exposed to high glucose (16.7 mmol/l), with and without 0.11 mmol/l sorbinil, for 3 or 6 days after beginning of confluence. We measured the soluble type IV collagen secreted into the culture medium and the insoluble type IV collagen accumulated in the extracellular matrix and cells, by ELISA. We also studied [14C]proline incorporation into the newly synthesized collagenous and total proteins in the culture supernatant and in the extracellular matrix and cell fraction. High glucose decreased the number of cells and increased the amount of type IV collagen in the culture supernatant and in the extracellular matrix and cell fraction. It also increased proline incorporation into the newly synthesized collagenous and total proteins in the culture supernatant and in the extracellular matrix and cell fraction. Sorbinil corrected all these high glucose-induced alterations. The corrective effects of sorbinil on the proliferation and on type IV collagen metabolism of endothelial cells cultured in high glucose may be attributed to prevention of polyol pathway dysregulation.

Effects of early insulin treatment on ultrastructural changes of glomeruli in diabetic rats revealed by the quick-freezing and deep-etching method

The three-dimensional ultrastructure of glomerular basement membrane (GBM) and mesangial matrix (MM) at an early stage of streptozotocin (STZ)-induced diabetes mellitus in rats was examined by the quick-freezing and deep-etching method. In diabetic rats, the GBM inner layer was diffusely enlarged and the meshwork structure not only in the GBM middle layer but also in the MM became markedly irregular due to the rupture of fine fibrils. This irregularity and enlargement of the mesh pores in diabetic rats developed during the experimental period and was significantly different from results in control rats. Insulin treatment from 1 week after STZ injection had significant effects in preventing the ultrastructural changes in the GBM and MM. It is suggested that early insulin treatment has significant effects in preventing size barrier disturbance of GBM and MM in STZ-induced diabetes.

Diabetic nephropathy and pregnancy

Knowledge of the pathogenic mechanisms of diabetic nephropathy (by which hyperglycemia, hyperfiltration, and hypertension cause the gradual development of microproteinuria, mesangial expansion, and eventual glomerular closure) provides the basis for effective treatment. Intensified glycemic control and antihypertensive therapy that is safe for the fetus are crucial for success during pregnancy. Considered outcome measures include perinatal survival, size at birth, child development, and long-term maternal renal function.

Production of type IV collagen and 72-kDa gelatinase by human endothelial cells cultured in high glucose. Effects of a protein kinase C inhibitor, GF 109203X

Since diabetic microangiopathy and macroangiopathy are characterized by type IV collagen accumulation in vascular basement membranes, it was of interest to study type IV collagen production and type IV collagenase secretion by endothelial cells (EC) cultured in high glucose and to evaluate the role of protein kinase C (PKC) activation in the alterations induced by high glucose. Primary cultures of human umbilical vein EC were exposed to high glucose concentration for 3 days at the beginning of confluence. The number of EC decreased with glucose concentration from 5 to 50 mM. At 16.7 mM glucose concentration, the amount of type IV collagen, determined by a two-step ELISA, increased in the culture supernatant and in the insoluble fraction associated with the extracellular matrix and cells; proline incorporation was more markedly elevated in the collagenous than in the total proteins of the culture supernatant and of the extracellular matrix and cell extracts. Gelatin zymography of the culture supernatant showed that EC mainly produce a 72-kDa gelatinase known to degrade type IV collagen. At 16.7 mM glucose concentration, total gelatinase activity per millilitre of culture supernatant was reduced and the 72-kDa gelatinase activity measured on the zymogram scan was lowered. When EC were exposed to 16.7 mM glucose, the specific PKC inhibitor GF 109203X corrected the increases in type IV collagen concentration and in proline incorporation into the collagenous or total proteins present in he culture supernatant or in the extract of the insoluble fraction, including the extracellular matrix and cells. Our results show that soluble and insoluble type IV collagen accumulation by EC cultured at high glucose concentration is not only associated with increased synthesis of the collagenous and total proteins but also with decreased total 72-kDa gelatinase activity in the extracellular fluid. The observed effects of GF 109203X are in favor of the involvement of PKC activation in the type IV collagen accumulation.

A quantitative ultrastructural study of juxtaglomerular arterioles in IDDM patients with micro- and normoalbuminuria

Hyalinization of juxtaglomerular arterioles is prominent in advanced diabetic nephropathy and may have important functional consequences. We studied the early stages of diabetic renal disease using kidney biopsy material from insulin-dependent diabetic patients, 8 with normal albumin excretion rate (< 15 micrograms/min) and 16 with microalbuminuria (15-200 micrograms/min). Ten living non-diabetic kidney donors served as a control group. Median duration of diabetes was 9.5 years (range 5-31) in patients with normoalbuminuria, and 12 years (7-22) in patients with microalbuminuria (p = 0.27). The tissue was sectioned systematically, 1-micron thick sections for light microscopy at 10-micron intervals, and thin sections for electron microscopy taken at 60-micron intervals. The arterioles were identified as afferent or efferent, and total profiles were photographed (magnification 7500x), providing a systematic independent sample for measurements using standard stereological methods. Patients with microalbuminuria had significantly increased arteriole parameters compared with the control group: for afferent and efferent arterioles the volume fraction of matrix/media, means and (co-efficient of variation, CV), was 0.47 (0.16) vs 0.33 (0.19) (p = 0.0009), and 0.62 (0.14) vs 0.45 (0.23) (p = 0.0004) and matrix-T, expressing amount of matrix per unit arteriolar surface, 2.38 (0.38) micron vs 1.44 (0.30) micron (p = 0.004), and 1.62 (0.28) micron vs. 1.03 (0.34) (p = 0.0009). Patients with normoalbuminuria showed no significant differences from the control group, and had lower values than microalbuminuric patients for all parameters except the afferent matrix-T. In the normoalbuminuric group a correlation was found between parameters for afferent arterioles and those for glomerular structure. In conclusion there is arteriolar accumulation of extracellular material in the early phase of diabetic nephropathy, concomitant with early glomerulopathy.

Albumin modified by Amadori glucose adducts activates mesangial cell type IV collagen gene transcription

The direct relationship between elevated glucose concentrations and accelerated protein glycation has implicated increased glycation as a potential mechanistic link between hyperglycemia and the pathogenesis of diabetic nephropathy. Albumin modified by Amadori glucose adducts has been shown to stimulate collagen secretion by mesangial cells in vitro, and to contribute to the overproduction of glomerular mesangial matrix in vivo. To delineate mechanisms responsible for these effects, we examined the influence of glycated albumin on transcriptional activation of the alpha 1 (IV) collagen gene in renal glomerular mesangial cells. These experiments used a stably transfected reporter mesangial cell line that exhibits responses to media manipulations that are directionally parallel with those of non-transformed mesangial cells, and that expresses luciferase driven by 5'-flanking and first intron regions of the alpha 1 (IV) collagen gene. In these cells, purified glycated albumin stimulated collagen IV gene transcription, whereas glucose-free albumin did not. Further, glycated albumin induced a significant increase in mesangial cell collagen IV mRNA, assessed by Northern blot analysis and quantified by calculation of the ratio of collagen IV mRNA to 18S ribosomal RNA after densitometric scanning. The stimulation of collagen gene transcription and mRNA expression were both prevented by monoclonal antibodies known to specifically recognize Amadori-modified albumin. The findings indicate that glycated albumin promotes mesangial cell transcriptional activation and mRNA expression of the alpha 1 (IV) collagen gene and further implicate increased glycated albumin in diabetes in the pathogenesis of diabetic nephropathy.

Abnormalities in protein kinase C and MAP kinase cascade in mesangial cells cultured under high glucose conditions

In order to clarify the mechanism of mesangial cell dysfunction in diabetes, we examined the activities of protein kinase C (PKC) and mitogen-activated protein kinase (MAPK), important kinases in various cellular functions, and also evaluated the isoenzymes of PKC in mesangial cells cultured under high glucose conditions. Exposure of cells to high concentrations (27.8 mM) of glucose for 5 days resulted in a significant elevation of PKC activities in the membrane fraction. MAPK was also activated in cells cultured under high glucose conditions. Of the PKC isoenzymes, the levels of PKC alpha and zeta were significantly increased in the membrane fraction after 5 days of exposure to high concentrations of glucose. These results indicate that the translocation of PKC alpha and zeta and the activation of MAPK under high glucose conditions might be underlying mechanisms of the functional disturbance of mesangial cells in diabetes.

Role of protein kinase C and cyclic AMP/protein kinase A in high glucose-stimulated transcriptional activation of collagen alpha 1 (IV) in glomerular mesangial cells

The elevated mRNA levels encoding matrix components in glomeruli isolated from streptozotocin-induced diabetic rats provide evidence that stimulation of matrix synthesis is important in early phases of diabetic glomerulopathy. We and others have demonstrated that high glucose stimulates collagen mRNA levels in short-term mesangial cell culture. To test whether transcriptional activation is operative and to gain insights into the underlying mechanisms, we studied a murine mesangial cell line stably transfected with a minigene expressing luciferase driven by 5'-flanking and first-intron regions of the alpha 1(IV) gene. High glucose stimulated luciferase activity dose and time dependently, with optimal stimulation (two-fold) achieved after 48 h in 450 mg/dL glucose (G450) versus 100 mg/dL (G100). We next tested the involvement of protein kinase C (PKC) because high glucose has been shown to stimulate de novo synthesis of diacylglycerol (DAG). Increasing PKC activity by treatment with a DAG analogue or active phorbol ester stimulated luciferase activity preferentially in G100; addition of the PKC inhibitors staurosporine or calphostin C markedly inhibited luciferase activity preferentially in G450. Thus high glucose promotes transcriptional activity of alpha 1(IV) gene through PKC activation. We also tested the involvement of protein kinase A (PKA). Intracellular cyclic AMP levels were increased two fold after 48 h in G450 versus G100, and addition of 8-Br-cAMP (0.1 mM) preferentially stimulated luciferase activity by almost three fold in G100 versus only 1.2-fold in G450. Hence, the signal-transduction mechanisms underlying the transcriptional activation of alpha 1(IV) gene in mesangial cells by high glucose are mediated by pathways involving the PKC system and possibly the cAMP/PKA system.

Overexpression of glucose transporters in rat mesangial cells cultured in a normal glucose milieu mimics the diabetic phenotype

An environment of high glucose concentration stimulates the synthesis of extracellular matrix (ECM) in mesangial cell (MC) cultures. This may result from a similar increase in intracellular glucose concentration. We theorized that increased uptake, rather than glucose concentration per se is the major determinant of exaggerated ECM formation. To test this, we compared the effects of 35 mM glucose on ECM synthesis in normal MCs with those of 8 mM glucose in the same cells overexpressing the glucose transporter GLUT1 (MCGT1). Increasing medium glucose from 8 to 35 mM caused normal MCs to increase total collagen synthesis and catabolism, with a net 81-90% increase in accumulation. MCs transduced with the human GLUT1 gene (MCGT1) grown in 8 mM glucose had a 10-fold greater GLUT1 protein expression and a 1.9, 2.1, and 2.5-fold increase in cell myo-inositol, lactate production, and cell sorbitol content, respectively, as compared to control MCs transduced with bacterial beta-galactosidase (MCLacZ). MCGT1 also demonstrated increased glucose uptake (5-fold) and increased net utilization (43-fold), and greater synthesis of individual ECM components than MCLacZ. In addition, total collagen synthesis and catabolism were also enhanced with a net collagen accumulation 111-118% greater than controls. Thus, glucose transport activity is an important modulator of ECM formation by MCs; the presence of high extracellular glucose concentrations is not necessarily required for the stimulation of matrix synthesis.

PDGF and TGF-beta mediate collagen production by mesangial cells exposed to advanced glycosylation end products

Deposition of type III collagen protein is increased when cultured rat mesangial cells are cultured in media containing high glucose concentrations. Possible mechanisms for this effect include the production of growth factors, such as PDGF and TGF-beta, and the formation of abnormal glucose-protein adducts called advanced glycosylation end products (AGEs). In our studies, neutralizing antibodies to PDGF and TGF-beta prevented increased type III collagen deposition by mesangial cells exposed either to high glucose media or to low glucose media containing AGEs. Daily addition of PDGF or TGF-beta stimulated type III collagen production. However, while co-incubation with the TGF-beta Ab prevented PDGF-stimulated type III collagen production, the PDGF Ab did not prevent TGF-beta-stimulated type III collagen production. Daily addition of PDGF or TGF-beta stimulated, while AGEs inhibited, mesangial cell proliferation after 96 hours. We propose that high extracellular glucose and AGEs stimulate type III collagen production by pathways that involve the intermediate formation of PDGF and TGF-beta by mesangial cells. PDGF may increase type III collagen production by stimulating the intermediate production of TGF-beta. Exposure to high glucose, AGEs, or TGF-beta also leads to impaired mesangial cell proliferation. The autocrine effects of TGF-beta and PDGF play important roles in the effects of high extracellular glucose and AGEs on cultured mesangial cells.

Effect of high glucose on formation of extracellular matrix components by cultured rat heart endothelial cells

In an attempt to define the basis for the microvascular changes observed in diabetic myocardium, a study was undertaken on the effect of elevated glucose on the synthesis by rat heart endothelial cells of the extracellular matrix components, types VI, IV and I collagen, as well as fibronectin. Confluent cultures of these cells, isolated by fluorescence-activated cell sorting after treatment with rhodamine-labelled acetylated low density lipoprotein, showed a three to fivefold enhancement in the synthesis of type VI collagen after exposure for 48 h to high glucose (20 to 30 mmol/l), as determined by immunoblot analysis. Increased production of type IV collagen and fibronectin was also observed, but the change was smaller and no effect on type I collagen was found. Measurement of mRNA levels by hybridization with cDNA probes indicated that 48-h exposure to high glucose significantly increased the level of transcripts for type VI and IV collagens but not for type I collagen. While glucose consumption by endothelial cells in high glucose doubled in the initial 24-h period, utilization returned to normal by 48 h, concomitant with a reduction in GLUT1 transcript levels, suggesting that signals for stimulation of collagen synthesis must be active during the initial period of exposure to elevated glucose levels.

Glucose entry into rat mesangial cells is mediated by both Na(+)-coupled and facilitative transporters

Since previous studies from our laboratory have demonstrated that increased glucose consumption by cultured rat mesangial cells is accompanied by an accelerated production of type IV and type VI collagen, we have now examined the manner by which glucose is transported into these cells. A progressive stimulation of glucose uptake by the mesangial cells was observed with increasing concentrations of NaCl so that at 145 mmol/l about twice as much glucose entered the cells as in its absence (substituted by choline chloride). Moreover, since phlorizin inhibited the NaCl-promoted uptake of glucose and this salt was found to increase the accumulation of alpha-methylglucoside in a manner which could not be duplicated by KCl or mannitol, both Na(+)-coupled and facilitative glucose transporters appeared to be present in the cells. Km values of 1.93 mmol/l and 1.36 mmol/l were determined for the co-transport and facilitated transport pathways, respectively, with their Vmax being 29.5 and 18.0 nmol.mg protein-1.h-1. Both uptake activities were found to be down-regulated by exposure of the cells to high glucose and furthermore the Na(+)-dependent transport could no longer be detected after about 12 passages of the cells. Hybridization of mesangial cell mRNA with cDNA probes revealed transcripts for the Na+/glucose co-transporter as well as GLUT1 and to a lesser extent GLUT4.(ABSTRACT TRUNCATED AT 250 WORDS)

Glycated albumin modified by Amadori adducts modulates aortic endothelial cell biology

Increased protein glycation has been mechanistically linked to accelerated vascular pathobiology in diabetes. To test the influence of protein modified by Amadori glucose adducts on vascular cell biology, we examined the effect of glycated albumin on replicative capacity and basement membrane collagen production by aortic endothelial cells in culture. Relative to carbohydrate-free albumin, which supported cell proliferation and Type IV collagen synthesis, glycated albumin significantly inhibited 3H-thymidine incorporation and Type IV collagen production. The glycated albumin-induced effects were prevented by monoclonal antibodies (A717) that specifically react with Amadori-modified albumin, but not by IgG that was unreactive with glycated albumin. A717 had no effect on thymidine incorporation or collagen synthesis by cells cultured in the presence of nonglycated albumin. The findings indicate that the interaction of glycated albumin with endothelial cells, which have been shown to display dose-responsive, saturable receptors, limits cell replication and triggers maladaptive biosynthetic programs, which may contribute to degenerative macrovascular disease in diabetes.

The effect of an aldose reductase inhibitor (Epalrestat) on diabetic nephropathy in rats

In order to clarify the possible contribution of the abnormal polyol pathway to the development of diabetic nephropathy, the effect of aldose reductase inhibitor on renal function and morphology was examined in streptozotocin (STZ)-induced diabetic rats. Six months after STZ injection, glomerular filtration rate and renal plasma flow showed marked decline with significant increase in nuclear-free mesangial area (MA) and relative mesangial area (RMA; MA per glomerular area) in diabetic rats. Oral administration of an aldose reductase inhibitor, Epalrestat, prevented renal hypofunction and mesangial expansion in diabetic rats without influencing the levels of blood glucose. These results suggest that the abnormal polyol pathway in diabetic rats is closely related to the development of mesangial expansion, a morphologic representative of diabetic glomerulopathy, and renal hypofunction.

Diabetic nephropathy in Japan

According to a nation-wide survey of dialysis patients in Japan the number of diabetic patients with end-stage renal failure introduced to dialysis therapy has been increasing year by year, exceeding 6000 cases (28% of total cases) in 1991. A six-fold increase in numbers and a three-fold increase in percentages (DM/total) were observed in the past ten years. Although the medical technology and regimen in dialysis therapy have been greatly improved, the prognosis of those diabetic patients on maintenance dialysis has remained unsatisfactory. Their quality of life has been severely damaged mostly because of the impairment of visual acuity. The early detection and early introduction of effective therapy for diabetic nephropathy, therefore, may be the most imminent tasks necessary to prevent the various problems afflicting diabetic patients on end-stage renal failure.

Amelioration of diabetic nephropathy by treatment with monoclonal antibodies against glycated albumin

The pathogenesis of diabetic nephropathy is incompletely understood, but increased nonenzymatic glycation of proteins is considered an important contributory factor. Glycated albumin, which is increased in diabetic sera and is preferentially transported into the renal glomerulus, induces an increase in Type IV collagen production and a decrease in proliferative capacity by mesangial cells in culture. These effects resemble the abnormalities that characterize the glomerular mesangium in diabetes and are prevented by monoclonal antibodies that specifically react with Amadori adducts in glycated albumin. To explore the possibility that the in vitro effects of glycated albumin on mesangial cell biology pertain to the in vivo situation, we examined the effect of treatment with the A717 monoclonal antibodies on glomerular functional and structural changes in a rodent model of genetic diabetes, the db/db mouse. Weekly parenteral antibody administration reduced the elevated albumin excretion and attenuated the mesangial expansion that were observed in the untreated db/db mice that served as controls. Monoclonal antibody treatment also was shown to lower plasma concentrations of glycated albumin in diabetic mice. Thus, reducing glycated albumin concentrations and/or blocking its biologically active epitopes has a salutary influence on the pathogenesis of diabetic nephropathy. The findings indicate that glycated albumin participates in the development of the glomerular lesion in the db/db mouse, and suggest a new approach to the therapy of this complication of diabetes.

A high concentration of glucose alters the production of tPA, uPA and PAI-1 antigens from human mesangial cells

To elucidate a role of tPA, uPA and PAI-1 for the development of diabetic glomerulosclerosis, the effect of high glucose concentration on the production of both basal and thrombin-mediated tPA, uPA and PAI-1 antigens from human mesangial cells was investigated. The culture of mesangial cells in the presence of high glucose (33 mM) for 11 days resulted in an increase in the synthesis of tPA and uPA when compared with that in normal glucose concentration (5 mM). In contrast, the cells grown in high glucose produced less PAI-1 than those in normal glucose. Thrombin stimulated dose-dependently the production of tPA, uPA and PAI-1 from the cells grown in either 5 or 33 mM glucose. However, the magnitude of the increase in tPA, uPA and PAI-1 from the cells grown in high glucose was less than that in normal glucose. These results suggest that the plasmin activity in mesangial cells may increase under a high glucose condition, leading to increased proteolysis of mesangial matrix. In addition, either fibrinolysis or proteolysis mediated by thrombin may be altered by high glucose concentration. Therefore, it is postulated that the turnover of mesangial matrix may be increased in diabetic nephropathy.

Localization of fibril/microfibril and basement membrane collagens in diabetic glomerulosclerosis in type 2 diabetes

Collagen is one of the major components of the extracellular matrices of the kidney. Basement membrane collagen, type IV collagen, is the major component in normal glomeruli. Fibril and interstitial collagen such as type III collagen, type V collagen, and type VI collagen are minor components of glomerular extracellular matrices and are localized mainly in the interstitium. Diabetic glomerulosclerosis is characterized by the expansion of the glomerular mesangial matrix as well as by thickening of the glomerular basement membrane. In order to clarify the roles of these various types of collagen in the development of diabetic glomerulosclerosis, immunohistochemical studies were performed in kidney specimens from patients with Type 2 diabetes. Early glomerulosclerosis is characterized by expansion of mesangial matrix with basement membrane collagen. However, in later stages glomerulosclerosis is characterized by an increase in the minor collagen components, such as type V and type VI collagen or collagens not normally present, such as type III collagen. Mesangial cells are known to synthesize all these types of collagen. In diabetes, phenotypic change in mesangial cells might produce excess amounts of fibril and interstitial collagen such as type III, type V, and type VI collagen, thus, leading to glomerulosclerosis.

Amadori glucose adducts modulate mesangial cell growth and collagen gene expression

Diabetic serum contains increased concentrations of glycated proteins, which are preferentially transported into the renal glomerulus. We investigated effects of Amadori glucose adducts in serum proteins, the predominant form in which circulating glycated proteins exist in vivo, on glomerular mesangial cells, where the lesion of diabetic nephropathy originates. [3H]-thymidine incorporation by murine mesangial cells was significantly inhibited when cells were grown in the presence of serum glycated by incubation for four days with 28 mM glucose or when cells were cultured in microtiter plates that had been precoated with glycated serum. This effect was prevented by a monoclonal antibody immunoreactive with Amadori adducts in glycated albumin, and unreactive with other glycated serum proteins or with advanced glycation end (AGE) products. Glycated serum stimulated Type IV collagen gene expression and increased Type IV collagen secretion, an effect also prevented by monoclonal antibodies reactive with Amadori adducts in glycated albumin. The glycation-induced changes in proliferation, collagen synthesis and collagen gene expression were observed in media containing normal glucose concentration and were exaggerated in media containing high glucose concentration. The data indicate that Amadori products of glycated serum proteins induce mesangial cell abnormalities that are highly relevant to the pathogenesis of diabetic nephropathy, and that these effects are accentuated when glycated serum proteins are presented in a hyperglycemic milieu. The data also suggest that mesangial cells specifically recognize Amadori adducts in glycated albumin.

Stimulation of collagen gene expression and protein synthesis in murine mesangial cells by high glucose is mediated by autocrine activation of transforming growth factor-beta

Previous investigations have demonstrated that growing mesangial cells in high glucose concentration stimulates extracellular matrix synthesis and also increases the expression of TGF-beta. We tested whether the stimulation of extracellular matrix production is mediated by autocrine activation of TGF-beta, a known prosclerotic cytokine. Addition of neutralizing anti-TGF-beta antibody, but not normal rabbit IgG, significantly reduced the high glucose-stimulated incorporation of 3[H]proline. Denaturing SDS-PAGE revealed that mainly collagen types I and IV were stimulated by high (450 mg/dl) D-glucose. This high glucose-mediated increase in collagen synthesis was reduced by the anti-TGF-beta antibody. Treatment of mesangial cells grown in normal (100 mg/dl) D-glucose with 2 ng/ml recombinant TGF-beta 1 mimicked the effects of high glucose. Furthermore, the anti-TGF-beta antibody significantly reduced the increase in mRNA levels encoding alpha 2(I) and alpha 1(IV) collagens induced by high glucose. Thus, the high glucose-stimulated increase of collagen production in mesangial cells is mediated, at least in part, by autocrine TGF-beta activation. We postulate that the interception of the glomerular activity of TGF-beta may be an effective intervention in the management of diabetic nephropathy.

The extracellular matrix in diabetic nephropathy

In the subgroup of diabetic patients who are destined to develop the full spectrum of the clinical syndrome of diabetic nephropathy, the kidney is afflicted with a series of distinct structural lesions principally involving the extracellular matrices. Diabetic nephropathy is characterized by hypertrophy of both glomerular and tubular elements, progressive accumulation of extracellular matrix components in the glomerular mesangium, and thickening of the glomerular and tubular basement membranes. Albeit less well recognized, progressive tubulointerstitial fibrosis is also a feature of the syndrome. Irrespective of pathogenetic mechanisms (be they metabolic, hemodynamic, or genetic), the structural changes involving the renal extracellular matrix compartments are believed to be the basis for the appearance of overt dysfunction, namely, proteinuria, hypertension, and renal failure. Therefore, a full understanding of the mechanisms that culminate in irreversible kidney failure requires a closer inspection of the status of the extracellular matrix in diabetes. This review outlines the different structural changes that typically occur during the course of the disease. Both glomerular and tubulointerstitial changes are reviewed. Valuable structural-functional correlations have been derived from examining kidney specimens obtained from patients with a wide spectrum of disease stages. Experimental animal models, supplanted with recent investigations in tissue culture on the effect of high ambient glucose levels, have increased our understanding of the cellular mechanisms that underlie the disordered matrix composition. Alterations in the metabolism of the collagens, proteoglycans, and other matrix constituents are reviewed.(ABSTRACT TRUNCATED AT 250 WORDS)

Alteration of mesangial response to ANP and angiotensin II by glucose

To test the hypothesis that the function of glomerular mesangial cells is impaired in diabetes, we examined the responsiveness of mesangial cells cultured under high concentrations of glucose to atrial natriuretic peptide (ANP1) and angiotensin II (Ang II). The ANP-induced accumulation of cGMP was enhanced in mesangial cells cultured under high glucose conditions, possibly due to the activation of particulate guanylate cyclase. Ang II action in mesangial cells was evaluated by measuring the ability of Ang II to inhibit ANP-induced cGMP accumulation through both activating phosphodiesterase (initial phase) and inhibiting guanylate cyclase (maintenance phase). The inhibition of both ANP-induced cellular cGMP accumulation and particulate guanylate cyclase activity by Ang II was significantly reduced in mesangial cells cultured under high concentrations of glucose. Moreover, in the cells exposed to high concentrations of glucose, both basal and Ang II-stimulated levels of inositol 1,4,5-trisphosphate (IP3) were significantly reduced. These results indicate that, in high glucose conditions, the actions of ANP and Ang II are modulated differently, resulting in the impairment of contractile responsiveness of mesangial cells.

Effects of glycated albumin on mesangial cells: evidence for a role in diabetic nephropathy

Nonenzymatically glycated proteins are preferentially transported across the glomerular filtration barrier, and the glomerular mesangium in diabetes is bathed with serum containing increased concentrations of glycated albumin. We investigated effects of glycated albumin on mesangial cells, which are involved in diabetic nephropathy. [3H]-thymidine incorporation was significantly inhibited when murine mesangial cells were grown in culture media containing human serum that had been nonenzymatically glycated by incubation for 4 days with 28 mM glucose. This inhibition was reversed when monoclonal antibodies that selectively react with Amadori products of glycated albumin were added to the culture media. Purified glycated albumin containing Amadori adducts of the glycation reaction induced significant inhibition of thymidine incorporation and stimulation of Type IV collagen secretion compared with cells cultured in the presence of purified nonglycated albumin. These changes were prevented when monoclonal antibodies specifically reactive with fructosyl-lysine epitopes in glycated albumin were added to the cultures. The antibodies had no effect on growth or collagen production in the presence of nonglycated albumin. The results provide the first evidence directly implicating Amadori adducts in glycated albumin in the pathogenesis of diabetic nephropathy, which is characterized by decreased cellularity in association with expansion of the mesangial matrix.

Differential modulation of mitogenic and metabolic actions of insulin-like growth factor I in rat glomerular mesangial cells in high glucose culture

In order to explore the possible contribution of insulin-like growth factor I to the development of diabetic nephropathy, the effect of glucose on the mitogenic and metabolic actions of insulin-like growth factor I in cultured rat glomerular mesangial cells was examined. The stimulation of [3H]-thymidine incorporation by insulin-like growth factor I in the cells exposed to high concentrations (55 mmol/l) of glucose (4.6 +/- 1.3 fold stimulation) was significantly suppressed as compared with that in the cells cultured in 11 mmol/l glucose (17.5 +/- 0.8 fold). In contrast, [3H]-amino-isobutylic acid uptake into the mesangial cells was significantly enhanced by glucose (2.03 +/- 0.03 nmol.mg protein-1. 15 min-1 at 55 mmol/l glucose vs 0.59 +/- 0.01 at 11 mmol/l glucose), while 2-deoxyglucose uptake remained unchanged. [125I]-insulin-like growth factor I binding was slightly but significantly increased in the cells exposed to high concentrations of glucose. Thus, glucose may modulate the mitogenic and metabolic actions of insulin-like growth factor I differently in cultured mesangial cells probably at the post-insulin-like growth factor I receptor level. These results may indicate that the differential modulation of the actions of insulin-like growth factor I by glucose could result in the increase in amino acid uptake and decrease in the cell proliferation in the mesangial cells, possibly leading to enhanced mesangial matrix synthesis with a relatively small increase in mesangial cell volume as seen in diabetic nephropathy.

Alterations of glomerular matrix proteins in the pathogenesis of diabetic nephropathy

Diabetic late complications are characterized by morphological and biochemical alterations of the extracellular matrix. In particular, longstanding diabetes causes quantitative and qualitative changes in basement membrane structure of retinal and renal capillaries. Immunohistochemical investigations of diabetic kidneys with diffuse glomerulosclerosis show increased collagen type IV deposition in the mesangial matrix and decreased heparan sulfate proteoglycan content in the mesangial matrix and glomerular basement membrane as well. In nodular glomerulosclerosis normal basement membrane components are decreased or absent while the occurrence of collagen type III in this stage has been interpreted as an irreversible alteration of the glomerular structure. These changes seem to be the underlying cause for the alterations in renal functions like persistent albuminuria and proteinuria. Increased intra- and extracellular levels of glucose and its derivatives are thought to be responsible for diabetic tissue dysfunction although there are reports on possible genetic defects causing increased susceptibility to develop diabetic nephropathy. Recent results, however, focus on the role of glucose-induced cytokine secretion as mediator for altered metabolism of glomerular matrix proteins. In vitro studies with cultured kidney cells have shown that the glucose-induced dysregulation of the basement membrane synthesis may be mediated by a glucose dependent activation of protein kinase C. Alternatively or synergistically, the formation of AGE products formed after prolonged exposure of matrix proteins to elevated glucose may also lead to cytokine secretion subsequently inducing synthesis of extracellular matrix proteins. Studies in experimental animals confirm the diabetes induced dysregulation of the synthesis of extracellular matrix components on the molecular level.

High glucose-induced proliferation in mesangial cells is reversed by autocrine TGF-beta

We investigated the effects of glucose concentration in serum-free media on the proliferative growth response of a cultured murine mesangial cell line. Raising the ambient D-glucose concentration from 100 mg/dl to 450 mg/dl stimulated cell proliferation after 24 to 48 hours but had a growth inhibitory effect after 72 to 96 hours of incubation. This biphasic proliferative response to high glucose concentration was not mediated by the elevated osmolarity of the medium and did not occur when L-glucose was used. The early phase of glucose-induced proliferation was associated with increased expression of the immediate early genes c-myc and egr-1 as well as with induction of the S-phase related proliferating nuclear cell antigen (PCNA). Several lines of evidence indicated that the late phase of glucose-induced growth inhibition was mediated by the bioactivation of endogenous transforming growth factor beta (TGF-beta). Neutralizing antibody against TGF-beta prevented the late inhibitory effects of glucose on proliferation. On the other hand, exogenous TGF-beta (1 ng/ml) significantly inhibited basal proliferation in mesangial cells. Furthermore, Northern blot analysis revealed that TGF-beta 1 mRNA was induced by 450 mg/dl glucose in the medium after 48 to 72 hours, but not after 24 hours. Cell cycle analysis demonstrated that mesangial cells incubated in high glucose for 24 hours have a higher percentage of cells in the S-G2 phase of the cell cycle compared with cells grown in normal glucose concentration. After 48 hours of culture in elevated glucose concentration, the percentage of cells in S-G2 phase was decreased, and became comparable to that of cells in normal glucose concentration. However, the addition of neutralizing anti-TGF-beta antibody stimulated the progression of cells towards S-G2 in high glucose medium after 48 hours. The findings of this study demonstrate a biphasic growth response of mesangial cells when they were cultured in high glucose concentration; initially there was a transient stimulation of replication for 24 to 48 hours followed by a sustained inhibition after longer incubation periods. This inhibition may be mediated by the glucose-induced synthesis and/or bioactivation of TGF-beta which can inhibit proliferation of mesangial cells in an autocrine fashion.

Intraglomerular pressure and mesangial stretching stimulate extracellular matrix formation in the rat

To define the interplay of glomerular hypertension and hypertrophy with mesangial extracellular matrix (ECM) deposition, we examined the effects of glomerular capillary distention and mesangial cell stretching on ECM synthesis. The volume of microdissected rat glomeruli (Vg), perfused ex vivo at increasing flows, was quantified and related to the proximal intraglomerular pressure (PIP). Glomerular compliance, expressed as the slope of the positive linear relationship between PIP and Vg was 7.68 x 10(3) microns 3/mmHg. Total Vg increment (PIP 0-150 mmHg) was 1.162 x 10(6) microns 3 or 61% (n = 13). A 16% increase in Vg was obtained over the PIP range equivalent to the pathophysiological limits of mean transcapillary pressure difference. A similar effect of renal perfusion on Vg was also noted histologically in tissue from kidneys perfused/fixed in vivo. Cultured mesangial cells undergoing cyclic stretching increased their synthesis of protein, total collagen, and key components of ECM (collagen IV, collagen I, laminin, fibronectin). Synthetic rates were stimulated by cell growth and the degree of stretching. These results suggest that capillary expansion and stretching of mesangial cells by glomerular hypertension provokes increased ECM production which is accentuated by cell growth and glomerular hypertrophy. Mesangial expansion and glomerulosclerosis might result from this interplay of mechanical and metabolic forces.

Glomerular structural changes in type 1 (insulin-dependent) diabetes mellitus: causes, consequences, and prevention

Diabetic nephropathy is caused primarily by advanced glomerulopathy, the renal expression of diabetic microangiopathy. With stereological methods a quantitative description of the structural changes is achieved. The glomerulopathy is characterized by an increase in basement membrane material: thickening of the capillary wall and an increase in mesangial volume relative to glomerular volume, comprising increase in matrix. Among groups of patients conformity between renal function stage and structure exists. The parameters measuring glomerulopathy are normal at the onset of diabetes; patients with normoalbuminuria may show slight basement membrane thickening, or normal parameters; the microalbuminuric group shows a measurable, but moderate glomerulopathy; patients with overt nephropathy have advanced lesions; at this stage heterogeneity among glomeruli makes the estimates weaker. Recent data indicate that the changes in peripheral basement membrane and in mesangial matrix develop in concert and both contribute to the early stage of glomerulopathy in patients with microalbuminuria. As to the consequences of the structural changes the mechanism of albuminuria is not clear. It is suggested that the early glomerulopathy entails other structural modifications, including formation of new vessels which may be the site of leakage. The marked deviations in glomerular filtration rate correspond well with estimates of filtration surface area: in the early hyperfunction state it is increased; in advanced nephropathy it is decreased, due to advanced glomerulopathy in conjunction with glomerular occlusion. The diabetic state is the necessary condition for the glomerulopathy. In relating structural changes to presumed contributing causes no supporting evidence of a relationship with glomerular hyperfunction or hypertrophy was observed. The structural parameters may be useful tools in clinical trials aiming at arresting the development of glomerulopathy, and thereby providing a prevention of diabetic nephropathy.

Gene expression of metalloproteinase and its inhibitor in mesangial cells exposed to high glucose

To clarify the roles of metalloproteinases and their inhibitor (TIMP) in diabetic glomerulopathy, we studied the effect of a high glucose concentration on the gene expression of metalloproteinase transin and TIMP as well as collagen type IV and laminin in cultured rat mesangial cells (MCs). In the high glucose group, collagen type IV, laminin, and TIMP mRNA levels were all elevated in a concentration-dependent manner, whereas transin expression was suppressed. Osmotic control of high glucose with mannitol selectively stimulated TIMP expression. We hypothesize that high glucose decreases matrix-degrading activity as well as increases matrix productivity in MCs.

Elevated glucose decreases the content of a basement membrane associated heparan sulphate proteoglycan in proliferating cultured porcine mesangial cells

The effect of elevated glucose concentrations on the synthesis of basement membrane components was investigated in proliferating cultured porcine mesangial cells. Basement membrane associated heparan sulphate proteoglycan was determined by enzyme immunoassay with a specific antiserum recognizing the core protein of the heparan sulphate proteoglycan. When cells were exposed to increasing glucose concentrations up to 40 mmol/l, the heparan sulphate proteoglycan content was concomitantly decreased to 53% when compared to cells cultured under normal glucose concentrations or in the presence of 40 mmol/l sorbitol. The fibronectin content was essentially unchanged under these conditions. No significant effect of insulin on these basement membrane components was found. The results indicate that hyperglycaemia may be responsible for a decrease of mesangial heparan sulphate proteoglycan content in diabetes mellitus. This supports the view that loss of heparan sulphate proteoglycan may be an important step or even an initial event of mesangial alterations in diabetic glomerulopathy.

Metabolic actions of insulin-like growth factor I in cultured glomerular mesangial cells

Glomerular mesangial cells in culture have been reported to possess a considerable number of receptors specific to insulin-like growth factor I (IGF-I), with very small number of receptors specific to insulin. To explore acute metabolic effects of IGF-I on mesangial cells, uptake of glucose and amino acid was measured in the presence of IGF-I or insulin. IGF-I stimulated D-[U-14C]glucose incorporation, 2-deoxy[1-3H]glucose uptake and alpha-[methyl-3H]aminoisobutyric acid (AIB) uptake into cultured mesangial cells by 139.8% +/- 2.1%, 116.6% +/- 1.7%, and 214.9% +/- 12.8% (percent of basal), respectively. Similar maximal stimulation was also induced by insulin, while the ED50 of IGF-I to stimulate these uptake systems (9.98 +/- 2.36, 3.45 +/- 1.86, and 3.35 +/- 0.40 ng/mL, respectively) was significantly lower than that of insulin (120.8 +/- 28.5, 61.8 +/- 7.7, and 76.3 +/- 17.5, respectively). These results indicate that, in cultured glomerular mesangial cells, IGF-I induces acute metabolic effects, possibly through its own receptors.

Glucose-induced overproduction of type IV collagen in cultured glomerular mesangial cells

Type IV collagen production by cultured rat glomerular mesangial cells was evaluated quantitatively by measuring type IV collagen secreted into culture media and associated with the cells using enzyme-linked immunosorbent assay (ELISA). The majority of type IV collagen was secreted into culture media; type IV collagen increased with cell growth in the early log phase and decreased in the late log phase and after cofluency. By exposing the cells to high concentrations of glucose (27.8 mM), both secreted and cell-associated type IV collagens increased significantly compared with the cells cultured under normal glucose concentrations (5.6 mM) or under equivalent concentrations of mannitol, resulting in a significant increase in total type IV collagen accumulation.