The renal tubulointerstitium in diabetes mellitus

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

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

An imbalance between matrix metalloproteinase-2 and tissue inhibitor of matrix metalloproteinase-2 contributes to the development of early diabetic nephropathy

BACKGROUND

High glucose and angiotensin-II (Ang-II) levels are the known important mediators of diabetic nephropathy. However, the effects of these mediators on matrix metalloproteinase-2 (MMP-2) and on tissue inhibitor of metalloproteinase-2 (TIMP-2) in proximal tubule cells have yet to be fully examined within the context of early stage diabetic nephropathy.

METHODS

In this study, we attempted to characterize changes in MMP-2 and TIMP-2 in streptozotocin-induced diabetic rats. To further examine the molecular mechanisms involved, we evaluated the effects of high glucose (30 mM) or Ang-II on MMP-2, TIMP-2 and collagen synthesis in proximal tubule cells, and investigated whether MMP-2 and TIMP-2 are regulated via the TGF-beta1 pathway.

RESULTS

In streptozotocin-induced diabetic rats, TIMP-2 mRNA and protein levels were significantly higher than in controls. Urinary protein excretion also showed a significant positive correlation with glomerular and tubular TIMP-2 protein expressions, and a negative correlation with MMP-2 expression. In cultured cells, both high glucose and Ang-II induced significant increases in TGF-beta1, TIMP-2, and in collagen synthesis, and significant decreases in MMP-2 gene expression and activity, and thus disrupted the balance between MMP-2 and TIMP-2. Moreover, treatment with a selective angiotensin type 1 (AT1) receptor antagonist significantly inhibited Ang-II mediated changes in TGF-beta1, MMP-2, TIMP-2, and in collagen production, suggesting the role of the AT1 receptor. The addition of exogenous TGF-beta1 produced an effect similar to those of high glucose and Ang-II. Furthermore, the inhibition of TGF-beta1 protein prevented Ang-II-induced MMP-2 and TIMP-2 alterations, suggesting the involvement of a TGF-beta1 pathway.

CONCLUSIONS

High glucose or Ang-II treatment induce alterations in MMP-2 and TIMP-2 balance, which favour TIMP-2 over-activity. Moreover, Ang-II-mediated changes in the productions of MMP-2 and TIMP-2 occur via AT1 receptors and a TGF-beta1-dependent mechanism. These results suggest that an imbalance between the MMP-2 and TIMP-2, caused primarily by an increase in TIMP-2 activity, contributes to the pathogenesis of diabetic nephropathy.

Epidermal growth factor receptor tyrosine kinase-mediated signalling contributes to diabetes-induced vascular dysfunction in the mesenteric bed

In order to characterize the roles of tyrosine kinases (TKs) and epidermal growth factor receptor (EGFR) in diabetes-induced vascular dysfunction, we investigated the ability of a chronic administration of genistein, a broad-spectrum inhibitor of TKs and AG1478, a specific inhibitor of EGFR TK activity to modulate the altered vasoreactivity of the perfused mesenteric bed to common vasoconstrictors and vasodilators in streptozotocin (STZ)-induced diabetes in rats. The vasoconstrictor responses induced by norepinephrine (NE), endothelin-1 (ET-1) and angiotensin II (Ang II), were significantly increased, whereas vasodilator responses to carbachol and histamine were significantly reduced in the perfused mesenteric bed of STZ-induced diabetic rats in comparison with healthy rats. Treatment of diabetic animals with genistein or AG1478 produced a significant normalization of the altered agonist-induced vasoconstrictor and vasodilator responses without affecting blood glucose levels. In contrast, neither inhibitor had any effect on the vascular responsiveness of control (nondiabetic) animals. Treatment of diabetic animals with diadzein, an inactive analogue of genistein, did not affect the vasoconstrictor and vasodilator responses in control or diabetic animals. Phosphorylated EGFR levels were markedly raised in the mesenteric bed from diabetic animals and were normalized upon treatment with AG1478 or genistein. These data suggest that activation of TK-mediated pathways, including EGFR TK signalling are involved in the development of diabetic vascular dysfunction.

Diabetes-Induced Renal Vascular Dysfunction Is Normalized by Inhibition of Epidermal Growth Factor Receptor Tyrosine Kinase

Contribution of receptor tyrosine kinase activation to development of diabetes-induced renal artery dysfunction is not known. We investigated the ability of a chronic administration of genistein, a broad-spectrum inhibitor of tyrosine kinases (TKs), and AG1478, a specific inhibitor of epidermal growth factor receptor (EGFR) TK activity, to modulate the altered vasoreactivity of isolated renal artery ring segments to common vasoconstrictors in streptozotocin-induced diabetes. In diabetic renal artery, the vasoconstrictor responses induced by norepinephrine, endothelin-1 and angiotensin II were significantly increased. Inhibition of TKs or the EGFR pathway did not affect the agonist-induced vasoconstrictor responses in the non-diabetic control animals. However, inhibition of TKs by genistein or EGFR TK by AG1478 treatment produced a significant normalization of the altered agonist-induced vasoconstrictor responses without affecting blood glucose levels. Treatment with diadzein, an inactive analogue of genistein, did not affect the vasoconstrictor responses in the diabetic animals. Western blotting showed that phosphorylated EGFR protein levels were increased in vehicle-treated diabetic animals. In renal arteries from AG1478-treated diabetic animals, EGFR protein levels were similar to non-diabetic control animals. These data suggest that activation of TK-mediated pathways, including the EGFR TK signalling pathway, are involved in the development of diabetic vascular dysfunction in the renal artery.

The role of tyrosine kinase-mediated pathways in diabetes-induced alterations in responsiveness of rat carotid artery

1 G-protein-coupled receptor signalling, including transactivation of receptor tyrosine kinases (RTKs), has been implicated in vascular pathology. However, the role of specific RTKs in the development of diabetes-induced cardiovascular complications is not known. 2 We investigated the ability of a chronic administration of genistein, a broad-spectrum inhibitor of tyrosine kinases (TKs), AG1478, a specific inhibitor of epidermal growth factor receptor (EGFR) TK activity, and AG825, a specific inhibitor of Erb2, to modulate the altered vasoreactivity of isolated carotid artery ring segments to common vasoconstrictors and vasodilators in streptozotocin (STZ)-induced diabetes. 3 In diabetic carotid artery, the vasoconstrictor responses induced by noradrenaline (NE), endothelin-1 (ET-1), and angiotensin II (Ang II), were significantly increased whereas vasodilator responses to carbachol and histamine were significantly reduced. Inhibition of TKs, EGFR or Erb2 pathway did not affect the body weight or agonist-induced vasoconstrictor and vasodilator responses in the non-diabetic control animals. However, inhibition of TKs by genistein, EGFR TK by AG1478 or Erb2 by AG825 treatment produced a significant normalization of the altered agonist-induced vasoconstrictor responses without affecting blood glucose levels. Treatment with diadzein, an inactive analogue of genistein, did not affect the vasoconstrictor and vasodilator responses in the diabetic animals. 4 Treatment with genistein, AG1478 or AG825 resulted in a significant improvement in diabetes-induced impairment in endothelium-dependent relaxation to carbachol and histamine. 5 These data suggest that activation of TK-mediated pathways, including EGFR TK signalling and Erb2 pathway, are involved in the development of diabetic vascular dysfunction in the carotid artery.

From the periphery of the glomerular capillary wall toward the center of disease: podocyte injury comes of age in diabetic nephropathy

Nephropathy is a major complication of diabetes. Alterations of mesangial cells have traditionally been the focus of research in deciphering molecular mechanisms of diabetic nephropathy. Injury of podocytes, if recognized at all, has been considered a late consequence caused by increasing proteinuria rather than an event inciting diabetic nephropathy. However, recent biopsy studies in humans have provided evidence that podocytes are functionally and structurally injured very early in the natural history of diabetic nephropathy. The diabetic milieu, represented by hyperglycemia, nonenzymatically glycated proteins, and mechanical stress associated with hypertension, causes downregulation of nephrin, an important protein of the slit diaphragm with antiapoptotic signaling properties. The loss of nephrin leads to foot process effacement of podocytes and increased proteinuria. A key mediator of nephrin suppression is angiotensin II (ANG II), which can activate other cytokine pathways such as transforming growth factor-beta (TGF-beta) and vascular endothelial growth factor (VEGF) systems. TGF-beta1 causes an increase in mesangial matrix deposition and glomerular basement membrane (GBM) thickening and may promote podocyte apoptosis or detachment. As a result, the denuded GBM adheres to Bowman's capsule, initiating the development of glomerulosclerosis. VEGF is both produced by and acts upon the podocyte in an autocrine manner to modulate podocyte function, including the synthesis of GBM components. Through its effects on podocyte biology, glomerular hemodynamics, and capillary endothelial permeability, VEGF likely plays an important role in diabetic albuminuria. The mainstays of therapy, glycemic control and inhibition of ANG II, are key measures to prevent early podocyte injury and the subsequent development of diabetic nephropathy.

Spironolactone prevents early renal injury in streptozotocin-induced diabetic rats

BACKGROUND

Glomerular and tubulointerstitial injury leads to chronic impairment of renal function, and thus, reversal of the injury may improve renal function and survival. The present study investigated whether and how mineralocorticoid receptor antagonist spironolactone ameliorates early renal injury in streptozotocin-induced diabetic rats.

METHODS

Streptozotocin (65 mg/kg, single intraperitoneal injection)- or vehicle-administered rats were used as diabetic or control rats, respectively. The streptozotocin-administered rats were treated with spironolactone (50 mg/kg/day sc) for 3 weeks. Among the 3 groups of rats, we compared renal fibrosis and renal hypertrophy, using picro-sirius red staining and immunohistochemistry of ED-1 macrophage marker, plasminogen activator inhibitor-1 (PAI-1), and transforming growth factor (TGF)-beta1.

RESULTS

Three weeks after administration of streptozotocin, rats exhibited increased collagen deposition in glomerular, tubulointerstitial, and perivascular areas in the kidney, which was completely attenuated by spironolactone treatment. In rats given streptozotocin alone, there were increases in ED-1-positive cell, PAI-1 expression, and TGF-beta1 expression in glomeruli and tubulointerstitiums, which were also suppressed by spironolactone treatment. Maximal glomerular and proximal tubular areas were not significantly different among the 3 groups. Rats given streptozotocin alone revealed an increase in proximal tubule wall-to-lumen ratio that was not influenced by treatment with spironolactone.

CONCLUSION

Streptozotocin-induced renal fibrosis, PAI-1 expression, TGF-beta1 expression, and macrophage infiltration occur via mineralocorticoid receptor, and spironolactone ameliorates renal fibrosis presumably via the inhibition of macrophage infiltration, PAI-1 expression, and TGF-beta1 expression in streptozotocin-induced early diabetic injury.

Osteopontin: correlation with interstitial fibrosis in human diabetic kidney and PI3-kinase-mediated enhancement of expression by glucose in human proximal tubular epithelial cells

AIMS

To examine the expression and localization of osteopontin (OPN), a secreted phosphoprotein implicated in the development of tubulointerstitial inflammation in various models of renal disease, in human diabetic kidneys, and to study the regulation of OPN expression in primary cultures of human renal proximal tubular epithelial cells (RPTEC).

METHODS AND RESULTS

Differential gene expression profiling through subtractive hybridization demonstrated increased renal OPN mRNA expression in a patient with diabetic nephropathy. Immunohistochemical staining of normal and diabetic human kidney samples confirmed that OPN was localized to cortical tubular, interstitial and juxtaglomerular compartments. Quantification of OPN immunostaining revealed a marked increase in the percentage of OPN-positive tubular profiles in diabetic kidneys (47 +/- 9% versus 5 +/- 3%, diabetic versus minimal change disease) that correlated strongly with the degree of cortical scarring (r2 = 0.91). Results of Northern hybridization, flow cytometry and Western blotting indicated that glucose up-regulates OPN mRNA and protein expression in primary cultures of human RPTECs. This effect was independent of the osmotic effects of glucose and independent of insulin. Finally, glucose-stimulated OPN expression was inhibited by LY294002, an inhibitor of phosphatidylinositol 3-kinase activity, in a dose-dependent manner.

CONCLUSIONS

OPN is expressed in human diabetic kidneys and regulation of OPN expression is via a glucose-mediated, phosphatidylinositol 3-kinase-dependent pathway.

Thrombospondin-1 mediates distal tubule hypertrophy induced by glycated albumin

Diabetic nephropathy is characterized by early hypertrophy in both glomerular and tubuloepithelial elements. However, no studies to date have established a direct causal link between hyperglycaemia and renal hypertrophy. Our previous studies have found that high glucose does not induce cellular hypertrophy or expression of TGF-beta1 (transforming growth factor-beta1) in distal renal tubule cells [Yang, Guh, Yang, Lai, Tsai, Hung, Chang and Chuang (1998) J. Am. Soc. Nephrol. 9, 182-193]. In the present study, we used AGEs (advanced glycation end-products) to mimic long-term hyperglycaemia. Similar to glucose, AGEs did not induce TGF-beta1 mRNA in distal renal tubule cells [MDCK (Madin-Darby canine kidney) cells]; however, TGF-beta1 bioactivity was increased significantly. This result indicated post-translational regulation. Since TSP-1 (thrombospondin-1) has been demonstrated to activate latent TGF-beta1 in a variety of systems, the following experiments were performed. We found that AGEs dose-dependently increased both intracellular and extracellular levels of TSP-1. Purified TSP-1, like AGEs, increased the cellular protein content. Furthermore, anti-TSP-1 neutralizing antibodies attenuated the AGE-induced increase in TGF-beta1 bioactivity and hypertrophy. Thus TSP-1 might mediate AGE-induced distal renal tubule hypertrophy. In addition, we observed several putative transcription factor binding sites in the TSP-1 promoter, including those for AP-1 (activator protein-1), CREB (cAMP response element binding protein), NF-kappaB (nuclear factor-kappaB), SRF (serum response factor) and HSF (heat-shock factor), by sequence mapping. We used an enhancer assay to screen possible transcription factors involved. We showed that AP-1 and CREB were specifically induced by AGEs; furthermore, TFD (transcription factor decoy) for AP-1 could attenuate the AGE-induced increases in TSP-1 levels and cellular hypertrophy. Thus regulation of TSP-1 might be critical for hyperglycaemic distal tubule hypertrophy. Furthermore, TSP-1 TFD might be a potential approach to ameliorate diabetic renal hypertrophy.

Effect of antisense TGF-beta1 oligodeoxynucleotides in streptozotocin- induced diabetic rat kidney

Transforming growth factor (TGF)-beta1 is an important fibrogenic factor that is involved in the pathogenesis of diabetic nephropathy. We evaluated the effect of circular antisense TGF-beta1 oligodeoxynucleotides (ODNs) on the TGF-beta1 expression in the rat mesangial cell culture and in streptozotocin (STZ)-induced diabetic rats. Circular antisense TGF-beta1 ODNs were found to be stable in rat serum, significantly decreasing TGF-beta1 mRNA expression compared with linear antisense ODNs in the rat mesangial cell culture. Circular antisense TGF-beta1 ODNs were introduced into the tail vein of normal rats using hemagglutinating virus of Japan (HVJ)-liposome-mediated gene transfer method and were confirmed to be delivered effectively into the kidney, liver, lungs, and spleen. To inhibit the overexpression of TGF-beta1 in diabetic kidneys, we introduced circular antisense TGF-beta1 ODNs into the STZ-induced diabetic rats. On day 13 after circular antisense TGF-beta1 ODNs injection, TGF-beta1 mRNA and protein expression markedly decreased and urinary TGF-beta1 excretion rate also dropped in the circular antisense TGF-beta1 ODNs-treated diabetic rats. These results suggest that circular antisense TGF-beta1 ODNs may be a useful tool for developing new therapeutic application for progressive diabetic nephropathy.

Down-regulation of nitric oxide synthase in the diabetic rabbit kidney: potential relevance to the early pathogenesis of diabetic nephropathy

OBJECTIVES

Nephropathy is a well-recognised complication of diabetes mellitus (DM). The aim of this study was to investigate the effect of DM on the density and distribution of nitric oxide (NO) synthase (NOS) in the rabbit kidney. Quantification of the NOS radioligand on slide-mounted sections was compared with the nitroblue tetrazolium reaction, where the intensity of the reaction varies with the nicotinamide adenine dinucleotide diaphorase (NADPH-d) activity of NOS.

MATERIALS AND METHODS

DM was induced with alloxan in six New Zealand White (NZW) rabbits. Plasma creatinine, urea and electrolytes were monitored at monthly intervals. The kidneys were removed following 6 months of DM. Transverse serial sections were cut and low-resolution autoradiography was performed using a radioligand for NOS ([(3)H]-NOARG). Histochemical localisation of NADPH-d activity was also performed. Densitometric analysis was performed on the autoradiographs and the results compared with those obtained from six age-matched control rabbits.

RESULTS

There was a significant (p < 0.01) rise in plasma creatinine levels in the diabetic rabbits, although the mean values remained within the reference range. There was a significant (p < 0.0001) down-regulation of NOS binding sites in both the cortex and medulla of the DM kidney when compared with the controls. A similar decrease in NADPH-d activity was seen in the diabetic renal cortex and medulla. In addition, NADPH-d activity also appeared to be reduced in the diabetic glomeruli when compared with controls.

CONCLUSIONS

NOS binding sites and NADPH-d activity are significantly decreased in the DM renal cortex and medulla. These changes are associated with a mild deterioration in renal function and may be an early event that could subsequently play a role in the progression of DM nephropathy. Manipulating the NO pathway during the early stages of DM nephropathy may be beneficial.

The role of renal proximal tubular cells in diabetic nephropathy

We now know that the rate of progression of diabetic nephropathy, like all progressive renal disease, correlates with the degree of corticointerstitial fibrosis. Therefore, much interest has focused on the contribution of the resident cells in the renal cortex to this process. This article reviews the evidence that the epithelial cells of the proximal tubule are major players in orchestrating events in the corticointerstitium in diabetic nephropathy. More specifically, it addresses their role in extracellular matrix turnover, generation of cytokines, and recruitment of inflammatory cells, as well as examining the concept that they are the source of the interstitial myofibroblasts, which are the principal mediators of the fibrotic process.

Regulation of type II transforming-growth-factor-beta receptors by protein kinase C iota

TGF-beta (transforming growth factor-beta) is implicated in the pathogenesis of diabetic nephropathy. We previously demonstrated that up-regulation of type II TGF-beta receptor (TbetaRII) induced by high glucose might contribute to distal tubular hypertrophy [Yang, Guh, Yang, Lai, Tsai, Hung, Chang and Chuang (1998) J. Am. Soc. Nephrol. 9, 182-193]. We have elucidated the mechanism by using cultured Madin-Darby canine kidney cells. Enhancer assay and electrophoretic-mobility-shift assay were used to estimate the involvement of transcription factors. Western blotting and an in vitro kinase assay were used to evaluate the level and activity of protein kinase. We showed that glucose (100-900 mg/dl) induced an increase in mRNA level and promoter activity of TbetaRII (note: 'mg/dl' are the units commonly used in diabetes studies). The promoter region -209 to -177 appeared to contribute to positive transactivation of TbetaRII promoter by comparing five TbetaRII-promoter-CAT (chloramphenicol acetyl-transferase) plasmids. Moreover, the transcription factor AP-1 (activator protein 1) was significantly activated and specifically binds to TbetaRII promoter (-209 to -177). More importantly, we found that atypical PKC iota might be pivotal for high glucose-induced increase in both AP-1 binding and TbetaRII promoter activity. First, high glucose induced cytosolic translocation, activation and autophosphorylation of PKC iota. Secondly, antisense PKC iota expression plasmids attenuated high-glucose-induced increase in AP-1 binding and TbetaRII promoter activity; moreover, sense PKC iota expression plasmids enhanced these instead. Finally, we showed that antisense PKC iota expression plasmids might partly attenuate a high-glucose/TGF-beta1-induced increase in fibronectin. We conclude that PKC iota might mediate high-glucose-induced increase in TbetaRII promoter activity. In addition, antisense PKC iota expression plasmid effectively suppressed up-regulation of TbetaRII and fibronectin in hyperglycaemic distal-tubule cells.

The breakdown of preexisting advanced glycation end products is associated with reduced renal fibrosis in experimental diabetes

Renal accumulation of advanced glycation end products (AGEs) has been linked to the progression of diabetic nephropathy. Cleavage of pre-formed AGEs within the kidney by a cross-link breaker, such as ALT-711, may confer renoprotection in diabetes. STZ diabetic rats were randomized into a) no treatment (D); b) treatment with the AGE cross-link breaker, ALT-711, weeks 16-32 (DALT early); and c) ALT-711, weeks 24-32 (DALT late). Treatment with ALT-711 resulted in a significant reduction in diabetes-induced serum and renal AGE peptide fluorescence, associated with decreases in renal carboxymethyllysine and RAGE immunostaining. Cross-linking of tail tendon collagen seen in diabetic groups was attenuated only by 16 weeks of ALT-711 treatment. ALT-711, independent of treatment duration, retarded albumin excretion rate (AER), reduced blood pressure, and renal hypertrophy. It also reduced diabetes-induced increases in gene expression of transforming growth factor beta1 (TGF-beta1), connective tissue growth factor (CTGF), and collagen IV. However, glomerulosclerotic index, tubulointerstitial area, total renal collagen, nitrotyrosine, protein expression of collagen IV, and TGF-beta1 only showed improvement with early ALT treatment alone. This study demonstrates the utility of a cross-link breaker as a treatment for diabetic nephropathy and describes effects not only on renal AGEs but on putative mediators of renal injury, such as prosclerotic cytokines and oxidative stress.

ACE inhibitors improve diabetic nephropathy through suppression of renal MCP-1

OBJECTIVE

Chemokines play an important role in the pathogenesis of diabetic nephropathy. Angiotensin II induces several fibrogenic chemokines, namely monocyte chemoattractant protein-1 (MCP-1) and transforming growth factor-beta. The progression of diabetic nephropathy can be retarded by ACE inhibitors (ACEIs) in patients with type 1 and type 2 diabetes. We examined if blockade of the renin-angiotensin system lowered urinary levels of the chemokine MCP-1 and correlated urinary MCP-1 (uMCP-1) with parameters of renal function and glucose and lipid metabolism before and after 1 year of treatment with an ACE inhibitor.

RESEARCH DESIGN AND METHODS

In 22 patients with type 2 diabetes and diabetic nephropathy in stages 3-5, treatment with the ACEI lisinopril was initiated. Before treatment and after 12 months of continuous therapy, proteinuria, creatinine clearance, uMCP-1 levels, BMI, HbA(1c), and serum cholesterol were assessed.

RESULTS

Lisinopril treatment improved renal function. Proteinuria decreased from 410 +/- 662 mg per 24 h to 270 +/- 389 mg per 24 h. Creatinine clearance rose from 61 +/- 26 to 77 +/- 41 ml/min. Urinary MCP-1 levels decreased from 0.456 +/- 0.22 ng/mg creatinine to 0.08 +/- 0.096 ng/mg creatinine. The change in uMCP-1 correlated significantly (r = 0.61, P < 0.001) with the change in proteinuria. No other parameter correlated with the improvement in renal function.

CONCLUSIONS

Blockade of the renin-angiotensin system in type 2 diabetic patients with diabetic nephropathy reduces uMCP-1 levels and improves renal function. Because MCP-1 induces monocyte immigration and differentiation to macrophages, which augment extracellular matrix production and tubulointerstitial fibrosis, pharmacological reduction of angiotensin II may also exert its beneficial effects in diabetic nephropathy by downregulation of renal MCP-1.

Apoptosis of tubular epithelial cells in glycogen nephrosis during diabetes

The important problem of the fate of glycogen-accumulating clear cells in glycogen nephrosis is still unsettled. In this study, we examine whether apoptosis plays a relevant role in the development of diabetic glycogen nephrosis and explore the involvement of the Fas/Fas-L system and the activation of the caspase cascade. Diabetes was induced in rats by streptozotocin injection. Glycogen-accumulating clear cells were identified in renal tissues of hyperglycemic rats. They were found to be concentrated in the thick ascending limbs and distal tubules. Large cellular glycogen accumulations were confirmed by biochemical assays and enzyme-gold cytochemistry. Clear cells displayed apoptotic features such as Annexin V binding, nuclear TUNEL (terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick-end labeling), and the simultaneous occurrence of Fas, Annexin V, and TUNEL positivity. Western blot analysis demonstrated enhanced expression of Fas receptor/ligand and the activation of the caspase cascade in these cells because cleaved forms of the caspase-3, -8, and -9 were detected. Furthermore, active caspase-3 was located in nuclei by immunoelectron microscopy. Our results indicate that epithelial cells in thick ascending limbs and distal tubules that develop glycogen nephrosis in response to hyperglycemia undergo Fas/Fas-L mediated cell death. Thus, apoptosis could be playing a significant role in renal epithelial cell deletion during diabetes.

Glucose-induced fibronectin and collagen type III expression in renal fibroblasts can occur independent of TGF-beta1

BACKGROUND

Various renal cell types have been shown to contribute to the excessive matrix deposition observed in diabetic nephropathy. The present study examined the effect of high ambient glucose and transforming growth factor-beta1 (TGF-beta1) on matrix production by human renal fibroblasts.

METHODS

Human renal fibroblasts (TK173) were used to examine the effects of high glucose and TGF-beta1 on fibronectin and collagen type III expression. Stable transfectants were generated of TK173 cells expressing a dominant negative TGF-beta type II receptor. Matrix components were measured in enzyme-linked immunosorbent assay (ELISA) and reverse transcription-polymerase chain reaction (RT-PCR).

RESULTS

Fibronectin secretion by renal fibroblasts was increased upon exposure to high glucose, but with delayed kinetics compared to TGF-beta1-induced fibronectin. Exposure to high glucose resulted in an increased secretion of latent TGF-beta1. However, treatment with neutralizing pan-specific anti-TGF-beta antibodies could not attenuate the effects of glucose. Furthermore, collagen type III was up-regulated by high glucose, but not by TGF-beta1. Importantly, fibroblasts expressing a dominant negative TGF-beta type II receptor were defective in TGF-beta1-induced fibronectin production, whereas glucose-induced fibronectin and collagen type III were unaffected.

CONCLUSIONS

These data show that in renal fibroblasts exposure to high glucose can increase matrix production independent of endogenous TGF-beta1. Although glucose activation is accompanied by an increased production of latent TGF-beta1, which can have an important role in vivo, the data suggest involvement of alternative growth factors in the mechanism by which hyperglycemic conditions can modulate matrix accumulation in diabetic nephropathy.

Advanced glycation end products inhibit de novo protein synthesis and induce TGF-beta overexpression in proximal tubular cells

BACKGROUND

We have shown previously that OPB-9195, a novel inhibitor of advanced glycation end products (AGE), significantly prevented renal tubular injury and tubulointerstitial fibrosis in spontaneous diabetic rats. However, the molecular mechanisms underlying this have not been fully elucidated.

METHODS

Three immunochemically distinct AGE were prepared by incubating bovine serum albumin (BSA) with glucose, glyceraldehyde, or methylglyoxal. Then, the effects of AGE on human proximal tubular epithelial cells were examined. The intracellular formation of reactive oxygen species (ROS) was detected using the fluorescent probe CM-H2DCFDA. DNA synthesis was evaluated by thymidine uptake, and de novo protein synthesis was determined by [3H]leucine incorporation. Prostaglandin E2 (PGE2) and transforming growth factor-beta (TGF-beta) released into media were quantitatively analyzed in an enzyme-linked immunosorbent assay. TGF-beta gene expression was analyzed by quantitative reverse transcription-polymerase chain reaction (RT-PCR).

RESULTS

When these AGE-BSA were administered to tubular cells, each of them increased generation of intracellular ROS. All of the AGE-BSA, but not non-glycated BSA, were found to induce statistically significant decreases in de novo protein synthesis and PGE2 secretion by tubular cells. Furthermore, AGE-BSA up-regulated the levels of mRNAs for TGF-beta in tubular cells. The structural epitope designated glucose-derived AGE was found to have the greatest cytopathic effects on tubular cells. These AGE-induced inhibition of protein synthesis and PGE2 secretion as well as the up-regulation of TGF-beta mRNA were found to be completely prevented by N-acetylcysteine. Furthermore, H2O2 was shown to inhibit protein synthesis and PGE2 secretion by proximal tubular cells in a dose-dependent manner.

CONCLUSION

The results suggest that AGE inhibits de novo protein synthesis and stimulates TGF-beta mRNA expression in proximal tubular epithelial cells through overgeneration of intracellular ROS. Thus, AGE are involved in the pathogenesis of tubular injury in diabetic nephropathy.

Renoprotective effects of valsartan and enalapril in STZ-induced diabetes in rats

Effects of the angiotensin II type 1 (AT1) receptor antagonist valsartan and the angiotensin-converting enzyme (ACE) inhibitor enalapril were studied in streptozotocine (STZ)-induced diabetes in rats on the basis of microalbuminuria (Ma) and renal morphology. Five groups of Wistar rats were used, one group was the non-diabetic control, one group consisted of untreated STZ-diabetics and 3 groups of STZ-diabetics were treated with either enalapril and/or valsartan for 30 days. Blood glucose (BG) and Ma levels, body and kidney weight and glomerular size were measured. Immunohistochemical staining with an anti-transforming growth factor-beta1 (TGF-beta1) antibody was performed as well. In STZ-diabetics, BG and Ma levels were significantly increased when compared with the non-diabetic group. Although Ma levels in the valsartan-treated group was found to be higher than those in the non-diabetics group after 15 days of treatment, in all treated diabetic groups Ma levels were significantly decreased as compared with STZ-diabetics at the end of the experiment. Thickening of the glomerular and tubular basement membranes, increased mesangial matrix and glomerular size were found in the untreated diabetic group. All these changes were less in the treated groups. A significant increase in TGF-beta1 immunoreactivity was found in glomeruli of untreated STZ-diabetics as compared with non-diabetics. Again, TGF-beta1 expression was decreased in the treated groups as compared with untreated STZ-diabetics. We conclude that valsartan and enalapril have renoprotective effects in diabetic nephropathy. A combined therapy has an advantage because lower dosages of these drugs can be used. Their beneficial effects are related to a blockade of the renin-angiotensin system (RAS) and a decrease in TGF-beta1 expression in glomeruli.

Transferrinuria in type 2 diabetic patients with early nephropathy and tubulointerstitial injury

BACKGROUND: Transferrinuria is thought to be a marker for early stages of diabetic nephropathy. Transferrin has also been proposed as a mediator of tubular toxicity because the reabsorption of transferrin results in the release of reactive iron in proximal tubular cells, promoting the formation of hydroxyl radicals. We evaluated the role of urinary transferrin excretion in diabetic patients with early nephropathy by comparing tubulointerstitial injury in renal biopsy specimens. PATIENTS AND METHODS: 45 type 2 diabetic patients with normoalbuminuria (urinary albumin excretion <30 mg/24 h) or microalbuminuria (30-300 mg/24 h) were studied. All patients with microalbuminuria underwent renal biopsy, and the severity of the tubulo-interstitial lesions was determined by a semiquantitative estimate of interstitial fibrosis, tubular atrophy, and interstitial inflammatory infiltrates. Subjects were classified into group A (normoalbuminuria, n=25), group B (microalbuminuria without tubulointerstitial changes, n=11) or group C (microalbuminuria with tubulointerstitial changes, n=9). RESULTS: Urinary transferrin excretion (UTf), as well as UTf/creatinine clearance (Ccr), and transferrin clearance (CTf/Ccr), was significantly higher in groups B and C than in group A, and it was significantly higher in group C than in group B. There were no significant differences in urinary albumin excretion or mesangial expansion rate (MR% estimated by quantitative morphometric studies) between groups B and C. Although urinary beta2-microglobulin excretion was significantly higher in group C than in groups A and B, urinary N-acetyl-beta-D-glucosaminidase activity was significantly higher in groups B and C than in group A. CONCLUSIONS: Increased transferrinuria in the microalbuminuric stage may lead to the development of tubulointerstitial injuries in type 2 diabetic patients.

Nitric oxide and cGMP-dependent protein kinase regulation of glucose-mediated thrombospondin 1-dependent transforming growth factor-beta activation in mesangial cells

Excessive transforming growth factor-beta (TGF-beta) activity in hyperglycemia contributes to the development of diabetic nephropathy. Glucose stimulation of TGF-beta activity and matrix synthesis are dependent on autocrine thrombospondin 1 (TSP1) to convert latent TGF-beta to its biologically active form. The mechanisms by which glucose regulates TSP1 are not known. High glucose inhibits nitric oxide (NO) bioavailability and decreased NO increases TGF-beta activity and extracellular matrix accumulation. Yet, the impact of NO signaling on TSP1 activation of TGF-beta is unknown. We tested the role of NO signaling in the regulation of TSP1 expression and TSP1-dependent TGF-beta activity in rat mesangial cells exposed to high glucose. On exposure to 30 mm glucose, NO accumulation in the conditioned media and intracellular cGMP levels were significantly decreased. The addition of an NO donor prevented the glucose-dependent increase in TSP1 mRNA, protein, and TGF-beta bioactivity. The effects of the NO donor were blocked by ODQ (a soluble guanylate cyclase inhibitor) or Rp-8-pCPT-cGMPS (an inhibitor of cGMP-dependent protein kinase). These effects of high glucose were also reversed by the nitric-oxide synthase cofactor tetrahyrobiopterin (BH(4)). These results show that high glucose mediates increases in TSP1 expression and TSP1-dependent TGF-beta bioactivity through down-modulation of NO-cGMP-dependent protein kinase signaling.

Advanced glycation end products cause epithelial-myofibroblast transdifferentiation via the receptor for advanced glycation end products (RAGE)

Tubulointerstitial disease, a prominent phenomenon in diabetic nephropathy, correlates with decline in renal function. The underlying pathogenic link between chronic hyperglycemia and the development of tubulointerstitial injury has not been fully elucidated, but myofibroblast formation represents a key step in the development of tubulointerstitial fibrosis. RAGE, the receptor for advanced glycation end products (AGEs), induces the expression of TGF-beta and other cytokines that are proposed to mediate the transdifferentiation of epithelial cells to form myofibroblasts. Here we report specific binding of (125)I-AGE-BSA to cell membranes prepared from a rat proximal tubule cell line and show that the binding site was RAGE. AGE exposure induced dose-dependent epithelial-myofibroblast transdifferentiation determined by morphological changes, de novo alpha smooth-muscle actin expression, and loss of epithelial E-cadherin staining. These effects could be blocked with neutralizing Ab's to RAGE or to TGF-beta. Transdifferentiation was also apparent in the proximal tubules of diabetic rats and in a renal biopsy from a patient with type 1 diabetes. The AGE cross-link breaker, phenyl-4,5-dimethylthiazolium bromide (ALT 711) reduced transdifferentiation in diabetic rats in association with reduced tubular AGE and TGF-beta expression. This study provides a novel mechanism to explain the development of tubulointerstitial disease in diabetic nephropathy and provides a new treatment target.

Advanced glycation end products cause epithelial-myofibroblast transdifferentiation via the receptor for advanced glycation end products (RAGE)

Tubulointerstitial disease, a prominent phenomenon in diabetic nephropathy, correlates with decline in renal function. The underlying pathogenic link between chronic hyperglycemia and the development of tubulointerstitial injury has not been fully elucidated, but myofibroblast formation represents a key step in the development of tubulointerstitial fibrosis. RAGE, the receptor for advanced glycation end products (AGEs), induces the expression of TGF-beta and other cytokines that are proposed to mediate the transdifferentiation of epithelial cells to form myofibroblasts. Here we report specific binding of (125)I-AGE-BSA to cell membranes prepared from a rat proximal tubule cell line and show that the binding site was RAGE. AGE exposure induced dose-dependent epithelial-myofibroblast transdifferentiation determined by morphological changes, de novo alpha smooth-muscle actin expression, and loss of epithelial E-cadherin staining. These effects could be blocked with neutralizing Ab's to RAGE or to TGF-beta. Transdifferentiation was also apparent in the proximal tubules of diabetic rats and in a renal biopsy from a patient with type 1 diabetes. The AGE cross-link breaker, phenyl-4,5-dimethylthiazolium bromide (ALT 711) reduced transdifferentiation in diabetic rats in association with reduced tubular AGE and TGF-beta expression. This study provides a novel mechanism to explain the development of tubulointerstitial disease in diabetic nephropathy and provides a new treatment target.

Loss of tubular bone morphogenetic protein-7 in diabetic nephropathy

Bone morphogenetic protein-7 (BMP7), a member of the transforming growth factor-beta (TGF-beta) superfamily of cytokines, is highly expressed in renal tubules and generally promotes maintenance of epithelial phenotype. It was examined whether, during the evolution of experimental diabetic nephropathy, the renal expression of BMP7 and BMP7 receptors declines, and the hypothesis that loss of BMP7 activity is profibrogenic in proximal tubular cells was tested. Moreover, in vitro studies in cultured proximal tubular cells were performed to examine putative mechanisms that cause these changes. At 15 wk of streptozotocin-induced diabetes, renal expression of BMP7 is declined by about half, and it decreased further by 30 wk to <10% of timed controls. Renal expression of the high-affinity BMP type II receptor and the type I receptor Alk2 (activin receptor-like kinase-2) decreased. Alk3 tended to decrease, but Alk6 remained unchanged. During the evolution of diabetic nephropathy, the secreted BMP antagonist gremlin increased substantially. In cultured tubular cells, TGF-beta reduced BMP7 and Alk3 expression and increased gremlin but did not interrupt BMP7-induced activation of smad5 or Erk1 and -2. In contrast, BMP7 did not alter TGF-beta expression. Neutralization of endogenous BMP7 in cultured proximal tubular cells raised the expression of fibronectin and tended to increase collagen alpha(1) III mRNA levels. In conclusion, in experimental diabetic nephropathy, renal tubular BMP7 and some of its receptors decreased and gremlin, a secreted BMP antagonist, increased. Some, but not all, of these changes are explained by increased TGF-beta. The loss of BMP7 activity per se is profibrogenic in tubular cells.

Role of immunocompetent cells in nonimmune renal diseases

Renal infiltration with macrophages and monocytes is a well-recognized feature of not only immune, but also nonimmune kidney disease. This review focuses on the investigations that have shown accumulation of immunocompetent cells in experimental models of acute and chronic ischemia, protein overload, hypercholesterolemia, renal ablation, obstructive uropathy, polycystic kidney disease, diabetes, aging, murine hypertension, and nephrotoxicity. We examine the mechanisms of infiltration of immunocompetent cells and their participation in the self-perpetuating cycle of activation of the angiotensin system, generation of reactive oxygen species, and further recruitment of monocytes and lymphocytes. We also discuss the possibility of antigen-dependent and antigen-independent mechanisms of immune cell activation in these animal models. Finally, we review the recent studies in which suppression of cellular immunity with mycophenolate mofetil has proven beneficial in attenuating or preventing the progression of renal functional and histologic damage in experimental conditions of nonimmune nature.

Increases in renal epsilon-(gamma-glutamyl)-lysine crosslinks result from compartment-specific changes in tissue transglutaminase in early experimental diabetic nephropathy: pathologic implications

Diabetic nephropathy (DN) is characterized by an early, progressive expansion and sclerosis of the glomerular mesangium leading to glomerulosclerosis. This is associated with parallel fibrosis of the renal interstitium. In experimental renal scarring, the protein cross-linking enzyme, tissue transglutaminase (tTg), is up-regulated and externalized causing an increase in its crosslink product, epsilon-(gamma-glutamyl)-lysine, in the extracellular space. This potentially contributes to the extracellular matrix (ECM) accumulation central to tissue fibrosis by increasing deposition and inhibiting breakdown. We investigated if a similar mechanism may contribute to the ECM expansion characteristic of DN using the rat streptozotocin model over 120 days. Whole kidney epsilon-(gamma-glutamyl)-lysine (HPLC analysis) was significantly increased from Day 90 (+337%) and peaked at Day 120 (+650%) (p < 0.05). Immunofluorescence showed this increase to be predominantly extracellular in the peritubular interstitial space, but also in individual glomeruli. Total kidney transglutaminase (Tg) was not elevated. However, using a Tg in situ activity assay, increased Tg was detected in both the extracellular interstitial space and glomeruli by Day 60, with a maximal 53% increase at Day 120 (p < 0.05). Using a specific anti-tTg antibody, immunohistochemistry showed a similar increase in extracellular enzyme in the interstitium and glomeruli. To biochemically characterize glomerular changes, glomeruli were isolated by selective sieving. In line with whole kidney measurement, there was an increase in glomerular epsilon-(gamma-glutamyl) lysine (+361%); however, in the glomeruli this was associated with increases in Tg activity (+228%) and tTg antigen by Western blotting (+215%). Importantly, the ratio of glomerular epsilon-(gamma-glutamyl) lysine to hydroxyproline increased by 2.2-fold. In DN, changes in the kidney result in increased translocation of tTg to the extracellular environment where high Ca(2+) and low GTP levels allow its activation. In the tubulointerstitium this is independent of increased tTg production, but dependent in the glomerulus. This leads to excessive ECM cross-linking, contributing to the renal fibrosis characteristic of progressive DN.

Role of receptor for advanced glycation end-product (RAGE) and the JAK/STAT-signaling pathway in AGE-induced collagen production in NRK-49F cells

Advanced glycation end-product (AGE) is important in the pathogenesis of diabetic nephropathy (DN), and captopril (an angiotensin converting enzyme inhibitor) is effective in treating this disorder. We have shown that the Janus kinase (JAK)/signal transducers and activators of transcription (STAT) cascade is responsible for AGE-induced mitogenesis in NRK-49F (normal rat kidney fibroblast) cells, but its role in renal fibrosis in DN remains unknown. Therefore, we have sought to determine whether JAK/STAT is involved in AGE-regulated collagen production in NRK-49F cells. We found that AGE time (1-7 days) and dose (10-200 microg/ml)-dependently increased collagen production in these cells. Additionally, AGE increased RAGE (receptor for AGE) protein expression. AGE-induced RAGE expression was dose-dependently inhibited by antisense RAGE oligodeoxynucleotide (ODN) and captopril. AGE-induced type I collagen production and JAK2-STAT1/STAT3 activation were decreased by AG-490 (a specific JAK2 inhibitor), antisense RAGE ODN and captopril. Meanwhile, STAT1 and STAT3 decoy ODNs also suppressed the induction of collagen by AGE. We concluded that RAGE and the JAK2-STAT1/STAT3 pathway were involved in AGE-induced collagen production in NRK-49F cells. Furthermore, captopril was found to reverse AGE-induced collagen production, probably by attenuating RAGE expression and JAK2-STAT1/STAT3 activities.

Effects of dietary supplementation of alpha-lipoic acid on early glomerular injury in diabetes mellitus

Antioxidants, in particular vitamin E (VE), have been reported to protect against diabetic renal injury. alpha-Lipoic acid (LA) has been found to attenuate diabetic peripheral neuropathy, but its effects on nephropathy have not been examined. In the present study, parameters of glomerular injury were examined in streptozotocin diabetic rats after 2 mo on unsupplemented diets and in diabetic rats that received the lowest daily dose of dietary LA (30 mg/kg body wt), VE (100 IU/kg body wt), or vitamin C (VC; 1 g/kg body wt), which detectably increased the renal cortical content of each antioxidant. Blood glucose values did not differ among the diabetic groups. At 2 mo, inulin clearance, urinary albumin excretion, fractional albumin clearance, glomerular volume, and glomerular content of immunoreactive transforming growth factor-beta (TGF-beta) and collagen alpha1 (IV) all were significantly increased in unsupplemented D compared with age-matched nondiabetic controls. With the exception of inulin clearance, LA prevented or significantly attenuated the increase in all of these glomerular parameters in D, as well as the increases in renal tubular cell TGF-beta seen in D. At the dose used, VE reduced inulin clearance in D to control levels but failed to alter any of the other indices of glomerular injury or to suppress renal tubular cell TGF-beta in D. VC suppressed urinary albumin excretion, fractional albumin clearance, and glomerular volume but not glomerular or tubular TGF-beta or glomerular collagen alpha1 (IV) content. LA but not VE or VC significantly increased renal cortical glutathione content in D. These data indicate that LA is effective in the prevention of early diabetic glomerular injury and suggest that this agent may have advantages over high doses of either VE or VC.

Inhibition of nitric oxide synthase activity by early and advanced glycation end products in cultured rabbit proximal tubular epithelial cells

Nitric oxide (NO) is important in the regulation of renal tubular function. We have investigated whether glycated proteins could impair the NO production by examining the effects of Amadori products (AP-BSA) and advanced glycation end products (AGE-BSA) on primary cultures of rabbit proximal tubular epithelial (PTE) cells. Nitric oxide synthase activity was assessed by measurement of the conversion of L-arginine to L-citrulline and by production of NO, after short-term (30 min) or long-term (1 or 3 days) incubation. Short incubations of PTE cells with either 200 microg/ml AP-BSA or 40 microg/ml AGE-BSA significantly decreased NO production. AP-BSA (3000 microg/ml) inhibited the Ca(2+)-dependent NOS activity even though above 50 microg/ml it increased Ca(2+)-independent NOS activity. In contrast, 40 microg/ml AGE-BSA inhibited both isoforms of NOS. Longer incubations with 200 microg/ml AP-BSA or 250 microg/ml AGE-BSA decreased NO release and inhibited Ca(2+)-dependent and -independent NOS activities. APs did not affect NO release by S-nitroso-N-acetyl-penicillamine (SNAP), while 250 microg/ml AGEs decreased it. After 3 days incubation, glycation products had no effect on the NOS cell content. Cell viability and proliferation were not modified under these experimental conditions, suggesting that the fall in NO production was not due to there being fewer cells. These data indicate that APs and AGEs directly inhibit NOS activity, and additionally that AGEs quench released NO. Thus, both types of glycated proteins alter the production of NO by PTE cells and could participate in the renal tubule dysfunction associated with aging and diabetes.

Glucose stimulation of transforming growth factor-beta bioactivity in mesangial cells is mediated by thrombospondin-1

Glucose is a key factor in the development of diabetic complications, including diabetic nephropathy. The development of diabetic glomerulosclerosis is dependent on the fibrogenic growth factor, transforming growth factor-beta (TGF-beta). Previously we showed that thrombospondin-1 (TSP-1) activates latent TGF-beta both in vitro and in vivo. Activation occurs as the result of specific interactions of latent TGF-beta with TSP-1, which potentially alter the conformation of latent TGF-beta. As glucose also up-regulates TSP-1 expression, we hypothesized that the increased TGF-beta bioactivity observed in rat and human mesangial cells cultured with high glucose concentrations is the result of latent TGF-beta activation by autocrine TSP-1. Glucose-induced bioactivity of TGF-beta in mesangial cell cultures was reduced to basal levels by peptides from two different sequences that antagonize activation of latent TGF-beta by TSP, but not by the plasmin inhibitor, aprotinin. Furthermore, glucose-dependent stimulation of matrix protein synthesis was inhibited by these antagonist peptides. These studies demonstrate that glucose stimulation of TGF-beta activity and the resultant matrix protein synthesis are dependent on the action of autocrine TSP-1 to convert latent TGF-beta to its biologically active form. These data suggest that antagonists of TSP-dependent TGF-beta activation may be the basis of novel therapeutic approaches for ameliorating diabetic renal fibrosis.

Follow-up study of glomerular dimensions and cortical interstitium in microalbuminuric type 1 diabetic patients with or without antihypertensive treatment

BACKGROUND

A decrease in urinary albumin excretion is regularly seen with antihypertensive treatment in patients with diabetic nephropathy. Our study concerns structural data obtained by light microscopy in baseline and follow-up biopsies in antihypertensive treated patients and in a reference group.

METHODS

Microalbuminuric type 1 diabetic patients with diabetes duration of 6-16 years were studied. Two groups, allocated to treatment with either angiotensin-converting enzyme-inhibitor (group 1, n=6) or beta-blocker (group 2, n=6) after the baseline biopsy, were studied in parallel, whereas the reference group (group 3, n=9), without antihypertensive treatment, was part of a previously completed study. The renal plastic-embedded biopsies were serially sectioned (1 microm), the sections being used for determining glomerular volume, vascular pole area, and interstitial space expressed as fraction of tubular cortex.

RESULTS

A significant increase in glomerular volume (P=0.04) was seen in group 3 only. Vascular pole area (VPA) and VPA relative to calculated glomerular surface did not show significant changes in any of the groups, only a tendency to increase in VPA in group 3 (P=0.051). The increase in VPA correlated with systolic blood pressure during the study period (r=0.49, P=0.03). Glomerular volume did not correlate with HbA(1C), current diabetic glomerulopathy, or ensuing worsening of glomerulopathy. In group 3 every case showed an increase in interstitium (P=0.0009), group 2 showed a decrease (P=0.03), and group 1 showed no change. Increase in interstitial fractional volume correlated with diastolic blood pressure during the study (r=0.54, P=0.01).

CONCLUSIONS

In early microalbuminuria, type 1 diabetic patients show glomerular growth, probably compensatory to the developing glomerulopathy. The increase in interstitial volume fraction, demonstrable in early nephropathy, is further augmented over a few years, but is arrested by antihypertensive treatment.

Cell cycle regulation in diabetic nephropathy

Cell cycle regulation in diabetic nephropathy. Renal hypertrophy is one of the earliest abnormalities of diabetic nephropathy. Although selected cell populations. such as tubulointerstitial fibroblasts, may undergo sustained proliferation in the diabetic environment, most renal cells such as mesangial cells are arrested in the G1-phase of the cell cycle after actively leaving G0-phase and some self-limited early proliferation. High glucose, transforming growth factor-beta (TGF-beta), angiotensin II, and probably other factors induce inhibitors of cyclin-dependent kinases (CDK) including p21Cip1 and p27KiP1. These CDK-inhibitors bind to and inactivate G1-phase cyclin/CDK complexes. The consequence is a lack in kinase activity, underphosphorylation of the retinoblastoma gene protein, and a failure to initiate the G1-S-phase transit. The half-life of CDK-inhibitors may also be increased by serine phosphorylation mediated through activated MAP kinases. Treatment of diabetic rats with angiotensin-converting enzyme inhibitors attenuates glomerular hypertrophy and abolishes the glomerular expression of the CDK-inhibitors p16INK4 and p27KiP1, thus indicating that the cell cycle arrest can be therapeutically influenced. Cell cycle proteins may also be involved in these molecular events, leading to a limited degree of tubular apoptosis, which is a feature of diabetic nephropathy. Although not definitively proven, accumulating evidence suggests that early hypertrophy of renal cells may act as pacemaker for subsequent irreversible structural changes, such as glomerulosclerosis and tubulointerstitial fibrosis. Therefore, a better understanding of altered processes of cell cycle regulation is necessary to develop novel therapeutic strategies to prevent diabetic nephropathy. The recent observation that glomerular hypertrophy and proteinuria do not develop in diabetic p21CiP1 knockout mice indicates that this approach is feasible.

Age-related progressive renal fibrosis in rats and its prevention with ACE inhibitors and taurine

Our previous studies demonstrated an increased reactive oxygen species (ROS) production, as well as transforming growth factor-beta1 (TGF-beta1) expression in the rat kidney with aging. In the present study, we examined the effect of aging on extracellular matrix (ECM) accumulation and the effects of treatment with angiotensin-converting enzyme inhibitors (captopril and lisinopril) and taurine, an antioxidant amino acid. Age-related increases in types I and IV collagen and fibronectin mRNA expression were found at 24 and 30 mo of age. In contrast, type III collagen only increased in 30-mo-old rats. Captopril-, lisinopril-, and taurine-treated animals showed a statistically significant decrease in ECM protein expression at both ages. Moreover, treatment with taurine reduced the TGF-beta1 mRNA levels in 24- and 30-mo-old rats by 40%. Taurine also completely blocked increases in type I and type IV collagen expression in mesangial cells in response to TGF-beta1. Our results demonstrate a protective role from both converting enzyme inhibitors and taurine in the age-related progressive renal sclerosis. In addition, taking into account that taurine is considered as an antioxidant amino acid, present data suggest a role for ROS in age-related progressive renal fibrosis, perhaps through interactions with the TGF-beta1 pathway.

Type IV collagen as an early marker for diabetic nephropathy in non-insulin-dependent diabetes mellitus

We measured urinary albumin (U-Alb) and type IV collagen (uIV.C) in spot urine collected from 82 patients with non-insulin-dependent diabetes mellitus (NIDDM) and 205 controls. Eighty-two NIDDM patients that had no increased excretion of either U-Alb or uIV.C were observed for 6 months. Prevalence of increased excretion of U-Alb and uIV.C at 6 months in these patients were 32.9%, and 62.2%, respectively. Increased excretion of uIV.C was detected in 27 patients without microalbuminuria. chi(2) analysis suggested that uIV.C was more sensitive than U-Alb, and that hypertension enhanced increased excretion of both U-Alb and uIV.C. uIV.C was significantly correlated (P<0.01) with U-Alb but not glycosylated hemoglobin A1C (HbA1C) in NIDDM patients. Taken together, uIV.C may be a useful marker for early diabetic nephropathy.

Functional, biochemical, and molecular investigations of renal kallikrein-kinin system in diabetic rats

A reduction of renal kallikrein has been found in non-insulin-treated diabetic individuals, suggesting that an impaired renal kallikrein-kinin system (KKS) contributes to the development of diabetic nephropathy. We analyzed relevant components of the renal KKS in non-insulin-treated streptozotocin (STZ)-induced diabetic rats. Twelve weeks after a single injection of STZ, rats were normotensive and displayed hyperglycemia, polyuria, proteinuria, and reduced glomerular filtration rate. Blood bradykinin (BK) levels and prekallikrein activity were significantly increased compared with controls. Renal kallikrein activity was reduced by 70%, whereas urinary BK levels were increased up to threefold. Renal kininases were decreased as indicated by a 3-fold reduction in renal angiotensin-converting enzyme activity and a 1.8-fold reduction in renal expression of neutral endopeptidase 24.11. Renal cortical expression of kininogen and B2 receptors was enhanced to 1.4 and 1. 8-fold, respectively. Our data suggest that increased urinary BK levels found in severely hyperglycemic STZ-diabetic rats are related to increased filtration of components of the plasma KKS and/or renal kininogen synthesis in combination with decreased renal kinin-degrading activity. Thus, despite reduced renal kallikrein synthesis, renal KKS is activated in the advanced stage of diabetic nephropathy.

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

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

High glucose stimulates proliferation and collagen type I synthesis in renal cortical fibroblasts: mediation by autocrine activation of TGF-beta

Renal tubular epithelial cells and interstitial fibroblasts are active participants in tubulointerstitial fibrosis, the best correlate of decreased glomerular filtration in diabetic nephropathy. It was reported previously that high ambient glucose stimulates transforming growth factor-beta (TGF-beta) mRNA and bioactivity, promotes cellular hypertrophy, and increases collagen synthesis in proximal tubular cells. This study evaluates the effects of high glucose and TGF-beta on the behavior of murine renal cortical fibroblasts (TFB) in culture. High glucose (450 mg/dl) significantly increased [3H]-thymidine incorporation (by 60 to 80% after 24 to 72 h) and cell number, without significantly increasing cell death when compared with normal glucose (100 mg/dl). There also was a transient increase in the mRNA of the c-myc and egr-1 early-response genes. Exogenous TGF-beta1 was promitogenic rather than antiproliferative in contrast to other renal cell types. Northern blot analysis demonstrated constitutive expression of TGF-beta1, -beta2, and -beta3 transcripts. Exposure to high glucose increased all three TGF-beta isoforms in a time-dependent manner. High glucose as well as exogenous TGF-beta1 also increased [3H]-proline incorporation, alpha2(I) collagen mRNA, and type I collagen protein (measured by immunoassay). Treatment with a neutralizing pan-selective monoclonal anti-TGF-beta antibody markedly attenuated the stimulation by high ambient glucose of thymidine incorporation, TGF-beta1 mRNA, and type I collagen mRNA and protein levels. It is concluded that high ambient glucose and exogenous TGF-beta1 share similar actions on renal fibroblasts. Moreover, the stimulation of cell proliferation and collagen type I synthesis in these cells by high ambient glucose are mediated by activation of an autocrine TGF-beta system.

Molecular mechanisms of diabetic renal hypertrophy

Altered growth of renal cells is one of the early abnormalities detected after the onset of diabetes. Cell culture studies whereby renal cells are exposed to high glucose concentrations have provided a considerable amount of insight into mechanisms of growth. In the glomerular compartment, there is a very early and self-limited proliferation of mesangial cells with subsequent hypertrophy, whereas proximal tubular cells primarily undergo hypertrophy. There is overwhelming evidence from in vivo and cell culture studies that induction of the transforming growth factor-beta (TGF-beta) system mediates the actions of high ambient glucose and that this system is pivotal for the hypertrophy of mesangial and tubular cells. Other factors such as hemodynamic forces, protein glycation products, and several mediators (for example, angiotensin II, endothelin-1, thromboxane, and platelet-derived growth factor) may further amplify the synthesis of TGF-beta and/or the expression of its receptors in the diabetic state. Cellular hypertrophy can be characterized by cell cycle arrest in the G1 phase. The molecular mechanism arresting mesangial cells in the G1 phase of the cell cycle is the induction of cyclin-dependent kinase (CdK) inhibitors such as p27Kip1 and p21, which bind to and inactivate cyclin-CdK complexes responsible for G1-phase exit. High-glucose-induced activation of protein kinase C and stimulated TGF-beta expression appear to be essential for stimulated expression of p27Kip1. In addition, a decreased turnover of protein caused by the inhibition of proteases contributes to hypertrophy. The development of irreversible renal changes in diabetes mellitus such as glomerulosclerosis and tubulointerstitial fibrosis is always preceded by the early hypertrophic processes in the glomerular and the tubular compartments. It may still be debated whether diabetic renal hypertrophy will inevitably lead to irreversible fibrotic changes in the absence of other factors such as altered intraglomerular hemodynamics and genetic predisposition. Nevertheless, understanding cellular growth on a molecular level may help design a novel therapeutic approach to prevent or treat diabetic nephropathy effectively.

Effect of glucose on stress-activated protein kinase activity in mesangial cells and diabetic glomeruli

BACKGROUND

We have reported that hyperglycemia increases c-jun mRNA levels in isolated glomeruli of diabetic rats. The transcriptional activity of c-jun can be modified by phosphorylation of serine residues in the regulatory domain of the protein by stress-activated protein kinases (SAPKs), but the effect of high glucose concentrations on SAPK expression and activity is unknown. Accordingly, we studied p42/44 MAPK, p38 MAPK, and SAPK expression and activity in primary mesangial cells exposed to high glucose concentrations, as well as SAPK expression and activity in glomeruli of normal and streptozotocin-induced diabetic rats.

METHODS

Mesangial cells were incubated in 40 mM glucose for 30 and 60 minutes and 6, 12, 24, and 48 hours, whereas glomeruli of streptozotocin-induced diabetic rats were isolated one day and one and two weeks after the onset of hyperglycemia (blood glucose levels more than 15 mmol/liter), and were compared with age-matched normal rats. Cell lysates were subjected to Western blot analysis of SAPK and phosphorylated SAPK and an in vitro SAPK assay using recombinant c-jun.

RESULTS

Western blot analysis revealed that SAPK was expressed, but unphosphorylated, in unstimulated mesangial cells and whole glomerular lysates from normal rats. In accord with these observations, no SAPK activity was detected in lysates from mesangial cells or whole glomeruli from normal rats, although mesangial cell SAPK activity was readily induced in vitro by sorbitol. High glucose concentrations did not increase SAPK activity or lead to detectable phosphorylated SAPK either in vitro or in vivo. In contrast, short-term exposure to 40 mM of glucose activated both p42/44 MAPK and p38 MAPK.

CONCLUSIONS

We conclude that high glucose concentrations do not activate SAPK in primary cultured mesangial cells or in diabetic glomeruli during the early phase of diabetic renal hypertrophy.

Vascular cell adhesion molecule-1 expression in the renal interstitium of diabetic KKAy mice

To investigate the mechanism of interstitial inflammation in diabetic nephropathy, we used spontaneously diabetic KKAy mice. Twelve KKAy mice were divided into two groups; six mice were fed standard mouse chow ad libitum and six mice were placed on a diet (i.e. they received the same amount of chow as six control C57BL mice). Diabetic KKAy mice developed hypercholesterolemia and albuminuria. Animals were killed at 16 weeks of age and renal tissues were immunostained for vascular cell adhesion molecule-1 (VCAM-1). In diabetic KKAy mice, the renal interstitium was infiltrated by monocytes, lymphocytes, plasma cells, and other cells. The walls of venules near the infiltrating cells were more intensely stained for VCAM-1 when compared with other sites. In contrast, the VCAM-1 staining of arterioles and peritubular capillaries was not significantly increased. There was weak VCAM-1 staining of the infiltrating cells, including lymphocytes, monocytes, and other cells. Electron microscopy demonstrated immunolabeling for VCAM-1 on the cell surface and in the cytoplasm of both infiltrating cells and vascular endothelial cells. In KKAy mice placed on a diet, there was less staining for VCAM-1 and cellular infiltration was also decreased. Thus, increased expression of VCAM-1 by the endothelial cells of venules and VCAM-1 expression by infiltrating cells were demonstrated in the interstitium of kidneys from diabetic mice. These results suggest that increased expression of VCAM-1 by endothelial cells and infiltrating cells contributes to interstitial inflammation in diabetic nephropathy.

Perindopril ameliorates glomerular and renal tubulointerstitial injury in the SHR/N-corpulent rat

We compared the effects of long-term treatment with the angiotensin-converting enzyme inhibitor perindopril and triple therapy (hydrochlorothiazide, reserpine, and hydralazine) on the metabolic and renal features in the SHR/N-corpulent (cp) rat, a genetic model of non-insulin-dependent diabetes mellitus and hypertension. Obese male SHR/N-cp rats (4 to 6 weeks old) were fed a 54% carbohydrate diet containing 18% sucrose and 36% starch. After 2 months on the diet, rats were assigned to one of three groups: one group (n=8) received perindopril (PE); the second group (n=8) received triple therapy (TT); and the third group (n=8) did not receive therapy. Treatment was maintained for 3 to 4 months. Body weight, food intake, and fasting levels of serum glucose and insulin did not differ among the three groups. Control rats exhibited progressive proteinuria in parallel with the rise in systolic blood pressure (SBP). Both PE and TT equally lowered SBP to normal levels and reduced proteinuria in treated rats. However, the reduction of proteinuria was greater and more sustained with PE than with TT (P<.05), whereas the effect of TT on proteinuria was delayed. Plasma renin activity was increased in PE and TT rats compared with control rats (P<.02). Semiquantitative analysis of renal lesions showed that the percentage of glomeruli with mesangial expansion and sclerosis and the tubulointerstitial score (an index of severity of tubulointerstitial lesions, namely tubular atrophy, inflammatory cellular infiltrates, and interstitial fibrosis) was reduced in both PE and TT rats. However, the reduction of glomerulosclerosis and tubulointerstitial lesions was greater in PE than in TT rats (P<.01). The percentage of glomerular sclerosis was positively correlated with the severity score of tubulointerstitial lesions (r=.60, P<.01). We conclude that PE is more effective than TT in halting the progression of proteinuria in the SHR/N-cp rat with non-insulin-dependent diabetes mellitus and hypertension. The antiproteinuric effect of PE is associated with significant reduction in glomerulosclerosis and tubulointerstitial lesions, independent of the effect of treating hypertension.

Intrarenal hemodynamic abnormalities in diabetic nephropathy measured by duplex Doppler sonography

Intrarenal hemodynamics were studied by duplex Doppler sonography in 112 inpatients with type II diabetes mellitus (DM; 65 males, 47 females, 58 +/- 13 years old). The resistive index (RI) and pulsatility index (PI) of the interlobar arteries were calculated. The patients were divided into four groups: group I consisted of patients with urinary albumin excretion (UAE) < 20 micrograms/min (N = 42), group II with 20 < or = UAE < 200 (N = 28), group III with UAE > or = 200 (N = 25), and group IV with serum creatinine > or = 1.5 mg/dl (N = 17). Both RI and PI values in groups II, III, and IV were significantly higher than those in the controls (age- and sex-matched healthy persons, N = 37; P < 0.001), and those in group IV were significantly higher than those in groups I, II, and III (P < 0.0001). Multiple regression analysis revealed that RI values in DM patients were significantly affected by creatinine clearance, age, and duration of diabetes (R2 = 0.554, P < 0.0001). When intima-medial thickness (IMT) of the femoral and carotid arteries were measured by B-mode ultrasonography, RI values were significantly correlated with both the femoral and carotid arterial IMT. These results demonstrate that intrarenal hemodynamic abnormalities are present in type II DM patients with nephropathy, and that intrarenal hemodynamics are affected by decreased glomerular function and also probably by advanced arteriosclerosis.

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.

Fibronectin production by human tubular cells: the effect of apical protein

In progressive renal disease the degree of renal failure correlates with interstitial scarring and the rate of progression correlates with the degree of proteinuria. This has led to the hypothesis that proteinuria may cause interstitial scarring. Human tubular cells (HTC) grown on permeable membrane supports were characterized to be predominantly of proximal tubular origin. HTC produce the matrix protein fibronectin in a polarised fashion the ratio of basolateral to apical secretion being 2.9 +/- 0.2 at 48 hours. The addition of serum proteins (1.0 mg/ml) to the apical medium resulted in increased basolateral secretion of fibronectin, 2.62 +/- 0.23-fold after 24 hours and 2.40 +/- 0.16-fold after 48 hours. Serum fractionation revealed that the stimulant to fibronectin production had a molecular weight 40 to 100 kDa. Platelet derived growth factor secretion was also stimulated to apical exposure to serum but transforming growth factor beta secretion was not detected. Addition of neutralizing anti-PDGF antibodies did not decrease fibronectin secretion. The activity of serum was not reproduced by albumin or by transferrin. Exposure of HTC to serum resulted in increased release of lactate dehydrogenase, suggesting a degree of cytotoxicity. This evidence could provide a mechanism for the link between proteinuria and interstitial scarring.

Effect of insulin on urinary phosphate excretion in type II diabetes mellitus with or without renal insufficiency

We investigated the effect of insulin on urinary excretion of phosphate in type II diabetes mellitus (DM) with respect to the absence or presence of renal insufficiency. A euglycemic-hyperinsulinemic clamp was performed in 37 type II DM patients. Subjects were divided into two groups: group A consisted of patients with serum creatinine levels less than 1.5 mg/dL (n = 22), and group B consisted of patients with serum creatinine levels of 1.5 mg/dL or greater (n = 15). Blood and urine samples were collected at the beginning and end of the clamp, and urinary excretion of phosphate was evaluated by calculating fractional excretion (FE-P). Tissue sensitivity to insulin in the whole body was expressed as the glucose infusion rate (M value) and that divided by steady-state plasma insulin levels (M/I ratio) during the last 30 minutes of the clamp. FE-P in group A patients significantly decreased during the clamp (from 9.46 +/- 0.67% before the clamp to 7.12 +/- 0.73% after the clamp, P < .004), whereas FE-P in group B patients did not change significantly during the clamp. The percent decrease of FE-P (decrease of FE-P during the clamp divided by FE-P before the clamp) in group A patients was significantly higher than in group B patients (22.5 +/- 7.0% and 2.5 +/- 5.1 %, respectively, P < .04). In all 37 patients, the percent decrease of FE-P was negatively correlated with blood urea nitrogen ([BUN] r = -.36, P < .05), serum creatinine (r = -.34, P < .05), and serum beta2-microglobulin (r = -.44, P < .01) and positively correlated with creatinine clearance (r = .570, P < .004), but it was not correlated with the M value or M/I ratio. These results showed that the kidneys of diabetic patients with renal insufficiency are insulin-insensitive in terms of phosphate transport, and the insulin insensitivity is related to the glomerular filtration rate but not to systemic insulin insensitivity. The percent decrease of FE-P on clamp study could be useful for assessing the insulin insensitivity of the kidney, which probably occurs primarily in the renal tubules.

Serum type III, IV collagens and TIMP in patients with type II diabetes mellitus

We investigated serum levels of type III procollagen aminopeptide (CIII), 7S type IV collagen (CIV), and tissue inhibitor of metalloproteinase (TIMP) in 33 patients with type II diabetes mellitus (DM) without uremia (serum creatinine less than 1.5 mg/dl). The patients were divided into three groups based on measurement of the urinary albumin excretion (UAE) index obtained during two morning outpatient clinic visits: non-proteinuric patients (n = 11), UAE index less than 2.26 mg/mmol Cr; patients with microalbuminuria (n = 15), UAE index of 2.26 - 22.6 mg/mmol Cr; and patients with proteinuria (n = 7), UAE index more than 22.6 mg/mmol Cr. Serum levels of CIV and TIMP in patients with microalbuminuria and proteinuria were significantly higher than non-proteinuric patients (ANOVA, p <0.05). Serum levels of CIII in patients with proteinuria were significantly higher than those in non-proteinuric patients (p < 0.05). There was a significant positive correlation between CIV and TIMP (r = 0.502, p < 0.003), but no correlation was observed between CIII and TIMP. These results demonstrated that serum CIII and CIV increases as diabetic nephropathy progresses in terms of increasing proteinuria in type II DM patients, suggesting feasibility and usefulness of measuring serum CIV and CIII in assessing diabetic nephropathy. The increase in TIMP may be, at least in part, a possible cause for the increase in serum CIV in type II DM patients.

Synthesis of type III collagen and type IV collagen by tubular epithelial cells in diabetic nephropathy

Overproduction of extracellular matrix (ECM) is considered to be primarily responsible for both glomerular and tubulointerstitial (TI) changes in diabetic nephropathy (DN). To clarify the possible role of the collagens in TI damage in DN, type III interstitial collagen and type IV basement membrane collagen were studied in 10 cases of DN and 10 control cases by immunohistochemistry and in situ hybridization techniques. In control cases, no immunostaining for type III collagen was found in the renal tubules, while strongly positive in the adjacent interstitium. On the other hand, type IV collagen was found weakly in the tubular basement membrane (TBM) in control cases. In DN, increased immunostaining for both type III and type IV collagens were found in the damaged tubulointenstitium (TI). To determine the sources of these collagens in TI damage, non-radioactive in situ hybridization was performed utilizing thymine-thymine (T-T) dimerized synthetic oligonucleotides complementary to either human pro alpha 1 (III) chain or pro alpha 1 (IV) chain mRNA as probe. In normal tubules, tubular epithelial cells were not uniformly but persistently positive for pro alpha 1 (IV) mRNA. Meanwhile, no specifically detectable positive hybridization signals for pro alpha 1 (III) mRNA was found in the normal tubular epithelial cells. Accelerated synthesis of both type III and type IV collagens by tubular epithelial cells was noted in TI damage in DN. From the results we concluded that excessive synthesis of both type III and type IV collagens by tubular epithelial cells might significantly contribute to the TI damage found in DN.