Extracellular matrix metabolism in diabetic nephropathy

Activation of bone morphogenetic protein 4 signaling leads to glomerulosclerosis that mimics diabetic nephropathy

Diabetic nephropathy (DN) is the most common cause of chronic kidney disease. We have previously reported that Smad1 transcriptionally regulates the expression of extracellular matrix (ECM) proteins in DN. However, little is known about the regulatory mechanisms that induce and activate Smad1. Here, bone morphogenetic protein 4 (Bmp4) was found to up-regulate the expression of Smad1 in mesangial cells and subsequently to phosphorylate Smad1 downstream of the advanced glycation end product-receptor for advanced glycation end product signaling pathway. Moreover, Bmp4 utilized Alk3 and affected the activation of Smad1 and Col4 expressions in mesangial cells. In the diabetic mouse, Bmp4 was remarkably activated in the glomeruli, and the mesangial area was expanded. To elucidate the direct function of Bmp4 action in the kidneys, we generated transgenic mice inducible for the expression of Bmp4. Tamoxifen treatment dramatically induced the expression of Bmp4, especially in the glomeruli of the mice. Notably, in the nondiabetic condition, the mice exhibited not only an expansion of the mesangial area and thickening of the basement membrane but also remarkable albuminuria, which are consistent with the distinct glomerular injuries in DN. ECM protein overexpression and activation of Smad1 in the glomeruli were also observed in the mice. The mesangial expansion in the mice was significantly correlated with albuminuria. Furthermore, the heterozygous Bmp4 knock-out mice inhibited the glomerular injuries compared with wild type mice in diabetic conditions. Here, we show that BMP4 may act as an upstream regulatory molecule for the process of ECM accumulation in DN and thereby reveals a new aspect of the molecular mechanisms involved in DN.

Oxidative stress in early diabetic nephropathy: fueling the fire

Diabetic nephropathy is a major microvascular complication of diabetes mellitus and the most common cause of end-stage renal disease worldwide. The treatment costs of diabetes mellitus and its complications represent a huge burden on health-care expenditures, creating a major need to identify modifiable factors concerned in the pathogenesis and progression of diabetic nephropathy. Chronic hyperglycemia remains the primary cause of the metabolic, biochemical and vascular abnormalities in diabetic nephropathy. Promotion of excessive oxidative stress in the vascular and cellular milieu results in endothelial cell dysfunction, which is one of the earliest and most pivotal metabolic consequences of chronic hyperglycemia. These derangements are caused by excessive production of advanced glycation end products and free radicals and by the subjugation of antioxidants and antioxidant mechanisms. An increased understanding of the role of oxidative stress in diabetic nephropathy has lead to the exploration of a number of therapeutic strategies, the success of which has so far been limited. However, judicious and timely use of current therapies to maintain good glycemic control, adequate blood pressure and lipid levels, along with lifestyle measures such as regular exercise, optimization of diet and smoking cessation, may help to reduce oxidative stress and endothelial cell dysfunction and retard the progression of diabetic nephropathy until more definitive therapies become available.

Oxidized LDL immune complexes stimulate collagen IV production in mesangial cells via Fc gamma receptors I and III

Diabetic nephropathy is characterized by progressive mesangial expansion. Although we have reported that circulating oxidized LDL-containing immune complexes (oxLDL-IC) are associated with abnormal levels of albuminuria, the underlying mechanisms have not been investigated. In this study, we have studied the effect of oxLDL-IC on collagen IV expression by mesangial cells. We found that oxLDL-IC markedly stimulated collagen IV expression in a concentration- and time-dependent fashion while oxLDL only had moderate effect. We also found that oxLDL-IC stimulated collagen IV expression by engaging Fc gamma receptor (FcγR) I and III, but not FcγRII, and that p38 MAPK, JNK and PKC pathways were involved in collagen IV expression. Furthermore, we found that oxLDL-IC stimulated FcγRI expression, suggesting a positive feedback mechanism involved in oxLDL-IC-stimulated collagen IV expression. Taken together, this study showed that oxLDL-IC stimulated collagen IV in mesangial cells via FcγRI and FcγRIII, and the expression of FcγRI was increased by oxLDL-IC.

The protective role of Smad7 in diabetic kidney disease: mechanism and therapeutic potential

OBJECTIVE

Although Smad3 has been considered as a downstream mediator of transforming growth factor-β (TGF-β) signaling in diabetes complications, the role of Smad7 in diabetes remains largely unclear. The current study tests the hypothesis that Smad7 may play a protective role and has therapeutic potential for diabetic kidney disease.

RESEARCH DESIGN AND METHODS

Protective role of Smad7 in diabetic kidney disease was examined in streptozotocin-induced diabetic mice that have Smad7 gene knockout (KO) and in diabetic rats given Smad7 gene transfer using an ultrasound-microbubble-mediated technique.

RESULTS

We found that mice deficient for Smad7 developed more severe diabetic kidney injury than wild-type mice as evidenced by a significant increase in microalbuminuria, renal fibrosis (collagen I, IV, and fibronectin), and renal inflammation (interleukin-1β [IL-1β], tumor necrosis factor-α [TNF-α], monocyte chemoattractant protein-1 [MCP-1], intracellular adhesion molecule-1 [ICAM-1], and macrophages). Further studies revealed that enhanced renal fibrosis and inflammation in Smad7 KO mice with diabetes were associated with increased activation of both TGF-β/Smad2/3 and nuclear factor-κB (NF-κB) signaling pathways. To develop a therapeutic potential for diabetic kidney disease, Smad7 gene was transferred into the kidney in diabetic rats by an ultrasound-microbubble-mediated technique. Although overexpression of renal Smad7 had no effect on levels of blood glucose, it significantly attenuated the development of microalbuminuria, TGF-β/Smad3-mediated renal fibrosis such as collagen I and IV and fibronectin accumulation and NF-κB/p65-driven renal inflammation including IL-1β, TNF-α, MCP-1, and ICAM-1 expression and macrophage infiltration in diabetic rats.

CONCLUSIONS

Smad7 plays a protective role in diabetic renal injury. Overexpression of Smad7 may represent a novel therapy for the diabetic kidney complication.

Endothelin-1 increases collagen accumulation in renal mesangial cells by stimulating a chemokine and cytokine autocrine signaling loop

Endothelin-1 (ET-1), a potent vasoconstrictor, has been implicated in the pathogenesis of collagen accumulation, extracellular matrix remodeling, and renal and cardiac fibrosis in diabetes. However, the mechanism by which ET-1 promotes collagen accumulation remains unclear. Here, we analyzed the gene expression profile of ET-1-stimulated mesangial cells to identify determinants of collagen accumulation. In human mesangial cells (a microvascular pericyte that secretes excess collagen in diabetic glomerulosclerosis), ET-1 increased mRNA and protein for MCP-1 (macrophage chemoattractant protein-1) and IL-6. ET-1-induced MCP-1 and IL-6 mRNAs and proteins were blocked by an ET(A) (but not ET(B)) receptor antagonist. ET-1/ET(A) receptor signaling evoked a 7.4-fold increase in collagen accumulation. Exogenous addition of either recombinant MCP-1 or IL-6 increased collagen accumulation by 3.5-fold. Co-stimulation with both MCP-1 and IL-6 did not elevate collagen accumulation further. Neither an MCP-1-neutralizing antibody nor an MCP-1 receptor antagonist inhibited ET-1-induced collagen accumulation. Similarly, neutralizing antibodies against IL-6 or the gp130 subunit of the IL-6 receptor did not attenuate ET-1-induced collagen accumulation. However, co-incubation with MCP-1- and IL-6-neutralizing antibodies inhibited ET-1-induced collagen accumulation by 52%, suggesting a robust autocrine loop wherein MCP-1 and IL-6 are redundant. Taken together, these results demonstrate that an autocrine signaling loop involving MCP-1 and IL-6 contributes to ET-1-induced collagen accumulation.

Proteomic profile of primary isolated rat mesangial cells in high-glucose culture condition and decreased expression of PSMA6 in renal cortex of diabetic rats

Diabetic nephropathy (DN) is one of the most important complications of diabetic patients and is characterized histologically by an accumulation of extracellular matrix (ECM) protein in the glomerular mesangium. Therefore, mesangial cells likely play an important role in the development of diabetic nephropathy. Here, we employed proteomic techniques to investigate the protein profile of rat mesangial cells under high-glucose culture conditions. Primary isolated rat glomerular mesangial cells were cultured under different concentrations of glucose (5.4 mmol.L-1 for normal control and 30 mmol.L-1 for high glucose) for 0, 8, 16, and 72 h, as well as for 25 days. Cellular total proteins were isolated from these cells and employed for two-dimensional gel electrophoresis (2-DE). Differentially expressed proteins were identified by matrix-assisted laser desorption - ionization time-of-flight mass spectrometry (MALDI-TOF-MS) and some of these proteins were documented in rat models of diabetes by Western blot. Rat mesangial cells were successfully isolated in the laboratory and their proliferation rates were significantly inhibited by high glucose. Two-dimensional gel electrophoresis analyses revealed 28 differentially expressed protein spots between the normal and high-glucose groups. After MALDI-TOF-MS analysis, all 28 protein spots were successfully identified with the peptide mass fingerprint (PMF) method. Representatively, SOD1, PCBP1 and PSMA6 were validated by Western blot analysis following protein extractions from the normal and high-glucose groups. Abundance of these proteins was consistent with that found in 2-DE. Moreover, expression of SOD1, PCBP1, and PSMA6 in renal cortex was further examined in two rat models of diabetes (streptozotocin-induced and spontaneous OLETF diabetic models). Abundance of SOD1 and PCBP1 proteins did not show any significant difference between normal control and diabetic rats. However, abundance of the PSMA6 protein was significantly reduced in the renal cortex of both STZ-induced and spontaneous OLETF diabetic rats. Proteomic analysis identified 28 differentially expressed proteins in primary isolated rat mesangial cells between normal and high glucose treatments. Expression of one identified protein was found to be consistent with expression in the renal cortex of two rat diabetic models. Therefore, identification of protein expression patterns in mesangial cells can be employed to develop a therapeutic target for treatment of diabetic nephropathy.

Cell division autoantigen 1 enhances signaling and the profibrotic effects of transforming growth factor-β in diabetic nephropathy

Cell division autoantigen 1 (CDA1) modulates cell proliferation and transforming growth factor-β (TGF-β) signaling in a number of cellular systems; here we found that its levels were elevated in the kidneys of two animal models of diabetic renal disease. The localization of CDA1 to tubular cells and podocytes in human kidney sections was similar to that seen in the rodent models. CDA1 small interfering RNA knockdown markedly attenuated, whereas its overexpression increased TGF-β signaling, modulating the expression of TGF-β, TGF-β receptors, connective tissue growth factor, collagen types I, III, IV, and fibronectin genes in HK-2 cells. CDA1 and TGF-β together were synergistic in stimulating TGF-β signaling and target gene expression. CDA1 knockdown effectively blocked TGF-β-stimulated expression of collagen genes. This was due to its ability to modulate the TGF-β type I, but not the type II, receptor, leading to increased phosphorylation of Smad3 and extracellular signal-regulated kinase mitogen-activated protein kinase. Furthermore, the Smad3 inhibitor, SIS3, markedly attenuated the activities of CDA1 in stimulating TGF-β signaling as well as gene expression of collagens I, III, and IV. Thus, our in vitro and in vivo findings show that CDA1 has a critical role in TGF-β signaling in the kidney.

Mastering a mediator: blockade of CCN-2 shows early promise in human diabetic kidney disease

In diabetes complications, CCN-2 (known originally as CTGF) has been implicated in diabetic nephropathy both as a marker and a mediator of disease. This commentary addresses CCN-2 in diabetic nephropathy, in the context of the recent publication of the first human study to inhibit CCN-2 bioactivity in diabetic kidney disease.

TGFβ enforces activation of eukaryotic elongation factor-2 (eEF2) via inactivation of eEF2 kinase by p90 ribosomal S6 kinase (p90Rsk) to induce mesangial cell hypertrophy

eEF2 phosphorylation is under tight control to maintain mRNA translation elongation. We report that TGFβ activates eEF2 by decreasing eEF2 phosphorylation and simultaneously increasing eEF2 kinase phosphorylation. Remarkably, inhibition of Erk1/2 blocked the TGFβ-induced dephosphorylation and phosphorylation of eEF2 and eEF2 kinase. TGFβ increased phosphorylation of p90Rsk in an Erk1/2-dependent manner. Inactive p90Rsk reversed TGFβ-inhibited phosphorylation of eEF2 and suppressed eEF2 kinase activity. Finally, inactive p90Rsk significantly attenuated TGFβ-induced protein synthesis and hypertrophy of mesangial cells. These results present the first evidence that TGFβ utilizes the two layered kinase module Erk/p90Rsk to activate eEF2 for increased protein synthesis during cellular hypertrophy.

Lack of {alpha}8-integrin aggravates podocyte injury in experimental diabetic nephropathy

Development of diabetic nephropathy is accompanied by changes in integrin-mediated cell-matrix interactions. The α8-integrin chain is specifically expressed in mesangial cells of the glomerulus. During experimental hypertension, α8-integrin plays a protective role in the glomerulus. We hypothesized that α8-integrin is involved in maintaining the integrity of the glomerulus in diabetic nephropathy. Experimental streptozotocin (STZ) diabetes led to an increased expression and glomerular deposition of α8-integrin. To test the functional role of α8-integrin, STZ diabetes was induced in mice with a homozygous (α8-/-) or heterozygous (α8+/-) deletion of the α8-integrin gene and in wild-type litters (α8+/+). Blood glucose and mean arterial blood pressure were not different in α8-/- and α8+/+ mice after 6 wk of diabetes. However, diabetic α8-/- mice developed significantly higher albuminuria and more glomerulosclerosis than diabetic α8+/+ mice. Moreover, in diabetic α8-/- mice, the number of glomerular cells staining positive for the podocyte markers WT-1 and vimentin were reduced more prominently than in diabetic α8+/+. The filtration barrier protein nephrin was downregulated in diabetic glomeruli with the strongest reduction observed in α8-/- mice. Taken together, α8-/- mice developed more severe glomerular lesions and podocyte damage after onset of STZ diabetes than α8+/+ mice, indicating that α8-integrin is protective for the structure and function of the glomerulus and maintains podocyte integrity during the development of diabetic nephropathy.

The combination of sirolimus and rosiglitazone produces a renoprotective effect on diabetic kidney disease in rats

AIMS

Peroxisome proliferator-activated receptor-gamma (PPAR-gamma) agonists and mammalian target of rapamycin (mTOR) inhibitors share mechanisms concerning cell growth and reduction of extracellular matrix accumulation. The purpose of this study was to evaluate the potential synergistic effect of this combination on diabetic kidney disease in rats.

MAIN METHODS

Diabetes was induced by streptozotocin in 42 male Sprague-Dawley rats. Sixteen weeks after diabetes induction, animals were divided into four groups: diabetic animals without intervention (D), diabetic animals with administration of sirolimus (D+SRL), diabetic animals with administration of rosiglitazone (D+RGT), and diabetic animals with administration of sirolimus and rosiglitazone (D+SRL+RGT).

KEY FINDINGS

At a 30-day follow up, diabetic rats showed higher kidney weight, mean glomerular volume, mesangial expansion and albuminuria compared with non-diabetic rats. mTOR downstream proteins, p-T389-S6K and p-T37/46-4EBP1, were higher in diabetic than non-diabetic kidneys, whereas p-S473-AKT was not, suggesting that hyperglycemia mainly activated the mTORC1 pathway in vivo. Moreover, the catalytic subunit of protein phosphatase 2A (PP2Ac) was down-regulated in the diabetic kidney. Sirolimus inhibited the mTORC1 pathway, while the PPAR-gamma agonist rosiglitazone enhanced PP2Ac and reduced p70S6K. Both drugs were associated with a reduction in albuminuria, renal enlargement and mesangial expansion, but without any improvement in glycemic control. Sirolimus and rosiglitazone in combination down-regulated the mTORC1 pathway and over-activated PP2Ac in diabetic kidney. This effect may account for the synergistic reduction of renal hypertrophy, albuminuria and renal TGF-beta1 observed in diabetic rats treated with SRL+RGT.

SIGNIFICANCE

The combination of sirolimus and rosiglitazone is renoprotective with respect to diabetic nephropathy.

Globotriaosylsphingosine actions on human glomerular podocytes: implications for Fabry nephropathy

BACKGROUND

Transforming growth factor-β1 (TGF-β1) and the macrophage inhibitory factor receptor CD74 link the metabolic disorder with tissue injury in diabetic nephropathy. Fabry disease is an X-linked lysosomal glycosphingolipid storage disorder resulting from a deficient activity of α-galactosidase A that leads to proteinuric renal injury. However, the link between the metabolic abnormality and renal injury is poorly characterized. Globotriaosylsphingosine (lyso-Gb3) was recently identified as a bioactive molecule accumulating in Fabry disease. We hypothesized that lyso-Gb3 could modulate the release of secondary mediators of injury in glomerular podocytes and that recently described nephroprotective actions of vitamin D receptor activation in diabetic nephropathy may apply to lyso-Gb3.

METHODS

Real time RT-PCR, ELISA and Western blot were used to study the biological activity of lyso-Gb3 in cultured human podocytes and potential modulation by vitamin D receptor activation.

RESULTS

In human podocytes, lyso-Gb3 dose and time dependently increased the expression of TGF-β1, extracellular matrix proteins (fibronectin and type IV collagen) and CD74. TGF-β1 mediated lyso-Gb3 effects on extracellular matrix production. Vitamin D receptor activation with paricalcitol or calcitriol prevented the increase in TGF-β1, CD74 and extracellular matrix induced by lyso-Gb3.

CONCLUSIONS

Lyso-Gb3 may have a role in glomerular injury in Fabry disease by promoting the release of secondary mediators of glomerular injury common to diabetic nephropathy. These effects are prevented by paricalcitol, raising the issue of vitamin D receptor activation as potential adjunctive therapy in Fabry nephropathy.

Do changes in transglutaminase activity alter latent transforming growth factor beta activation in experimental diabetic nephropathy?

BACKGROUND

Diabetic nephropathy is the leading cause of end-stage kidney failure worldwide. It is characterized by excessive extracellular matrix accumulation. Transforming growth factor beta 1 (TGF-β1) is a fibrogenic cytokine playing a major role in the healing process and scarring by regulating extracellular matrix turnover, cell proliferation and epithelial mesanchymal transdifferentiation. Newly synthesized TGF-β is released as a latent, biologically inactive complex. The cross-linking of the large latent TGF-β to the extracellular matrix by transglutaminase 2 (TG2) is one of the key mechanisms of recruitment and activation of this cytokine. TG2 is an enzyme catalyzing an acyl transfer reaction leading to the formation of a stable ε(γ-glutamyl)-lysine cross-link between peptides.

METHODS

To investigate if changes in TG activity can modulate TGF-β1 activation, we used the mink lung cell bioassay to assess TGF-β activity in the streptozotocin model of diabetic nephropathy treated with TG inhibitor NTU281 and in TG2 overexpressing opossum kidney (OK) proximal tubular epithelial cells.

RESULTS

Application of the site-directed TG inhibitor NTU281 caused a 25% reduction in kidney levels of active TGF-β1. Specific upregulation of TG2 in OK proximal tubular epithelial cells increased latent TGF-β recruitment and activation by 20.7% and 19.7%, respectively, in co-cultures with latent TGF-β binding protein producing fibroblasts.

CONCLUSIONS

Regulation of TG2 directly influences the level of active TGF-β1, and thus, TG inhibition may exert a renoprotective effect by targeting not only a direct extracellular matrix deposition but also TGF-β1 activation and recruitment.

High glucose activates HIF-1-mediated signal transduction in glomerular mesangial cells through a carbohydrate response element binding protein

High glucose evokes a variety of signals in mesangial cells that alter cellular functions responsible for the development of diabetic glomerulopathy. The hypoxia-inducible factor-1alpha (HIF-1alpha) regulates cellular homeostasis under hypoxic conditions, but it also has pleiotropic effects in response to cellular stresses at normoxia. Here we determined whether HIF-1alpha has a role in the regulation of mesangial cells in hyperglycemia. In the streptozotocin-induced diabetic mouse model, glomerular mesangial cells had a significant increase in HIF-1alpha expression in the nucleus. In cultured mesangial cells, high glucose enhanced the expression of HIF-1alpha and its target genes known to be involved in the development of diabetic glomerulopathy. A glucose-responsive carbohydrate response element binding protein (ChREBP) was found to have a critical role in the transcriptional upregulation of HIF-1alpha and downstream gene expression in mesangial cells exposed to high glucose. Knockdown of HIF-1alpha or ChREBP in mesangial cells abrogated the high glucose-mediated perturbation of gene expression. Our results show that ChREBP and HIF-1alpha mediate gene regulation in mesangial cells. Further studies will be needed to find out whether these findings are relevant to the development of the diabetic nephropathy.

Pathobiology of renal-specific oxidoreductase/myo-inositol oxygenase in diabetic nephropathy: its implications in tubulointerstitial fibrosis

Renal-specific oxido-reductase/myoinositol oxygenase (RSOR/MIOX) is expressed in renal tubules. It catabolizes myo-inositol and its expression is increased in diabetic mice and in LLC-PK(1) cells under high-glucose ambience. Aldose reductase (AR) is another aldo-keto reductase that is expressed in renal tubules. It regulates the polyol pathway and plays an important role in glucose metabolism, osmolyte regulation, and ECM pathobiology via the generation of advanced glycation end products, reactive oxygen species, and activation of transforming growth factor (TGF)-beta. In view of the similarities between AR and RSOR/MIOX, the pathobiology of RSOR/MIOX and some of the cellular pathways affected by its overexpression were investigated. An increased expression of fibronectin was noted by transfection of LLC-PK(1) cells with pcDNA3.1-RSOR/MIOX. Similar changes were observed in LLC-PK(1) cells under high-glucose ambience, and they were notably lessened by RSOR/MIOX-small interfering (si) RNA treatment. The changes in tubulointerstitial fibronectin expression were also observed in the kidneys of db/db mice having high levels of RSOR. The pcDNA3.1-RSOR/MIOX transfectants had an increased NADH/NAD(+) ratio, PKC and TGF-beta activity, Raf1:Ras association, and p-ERK phosphorylation. These changes were significantly reduced by the inhibitors of PKC, aldose reductase, Ras farnesylation, and MEK1. Similar increases in various the above-noted parameters were observed under high-glucose ambience. Such changes were partially reversed with RSOR-siRNA treatment. Expression of E-cadherin and vimentin paralleled in cells overexpressing RSOR/MIOX or subjected to high-glucose ambience. These studies suggest that RSOR/MIOX modulates various downstream pathways affected by high-glucose ambience, and conceivably it plays a role in the pathobiology of tubulointerstitium in diabetic nephropathy.

Effect of protopanaxadiol derivatives in high glucose-induced fibronectin expression in primary cultured rat mesangial cells: role of mitogen-activated protein kinases and Akt

A lot of anti-diabetic agents using natural plants have been extensively studied. Ginsenosides are known to be used as a remedy for diabetes in Asian countries and American Societies. Diabetic nephropathy is a major complication of diabetes mellitus. Extracellular matrix in mesangial cells is mainly composed of fibronectin and the increase of fibronectin is a hallmark of diabetic nephropathy. Protopenaxadiol (PPD) is a major component of total ginseng. Thus, we examined the regulatory mechanism of PPD derivatives-induced preventive effect of fibronectin expression in mesangial cells cultivated under diabetic condition. In present study, ginsenoside Rb1 prevented the high glucose-induced increase of fibronectin expression in mesangial cells. Ginsenoside Rb2 and Rg3 also mildly inhibited it. However, ginsenoside Rc and Rd did not prevent the high glucose-induced increase of fibronectin expression in mesangial cells. In addition, ginsenoside Rb1 prevented high glucose-induced phosphorylation of p44/42 mitogen activated protein kinase (MAPK), p38 MAPK, JNK/SAPK, and Akt. These results suggest that ginsenoside Rb1 is the most powerful component of PPD derivatives. In conclusion, ginsenoside Rb1 prevented high glucose-induced increase of fibronectin expression via the inhibition of MAPK-Akt signaling cascade.

Urinary matrix metalloproteinase-8 and -9 activities in type 2 diabetic subjects: A marker of incipient diabetic nephropathy?

Matrix metalloproteinases (MMPs) may play a pathophysiological role in the development of diabetic nephropathy (DN). We hypothesized that urinary MMP activity in patients with type 2 diabetes mellitus (T2DM) is related to a decline in renal function. We determined MMP-2, -8 and -9 activity in 24-h urine collections in relation to risk factors for DN in T2DM patients with (UA, n=27) and without albuminuria (NA, n=48) and controls (CO, n=28). MMP-8 and -9 levels were highest in UA patients (P<0.01). Of UA patients, 93% had at least one MMP increased, compared to 78% of NA patients and 46% of CO (P=0.001). Age, diabetes duration, BMI, systolic blood pressure, fasting plasma glucose, HbA1c and renal function were determinants of MMP-8 and -9 (P<0.05). In summary, MMP-8 and -9 are highest in T2DM UA patients. MMP-9, showed the strongest associations with clinical parameters related to DN.

Pathologic classification of diabetic nephropathy

Although pathologic classifications exist for several renal diseases, including IgA nephropathy, focal segmental glomerulosclerosis, and lupus nephritis, a uniform classification for diabetic nephropathy is lacking. Our aim, commissioned by the Research Committee of the Renal Pathology Society, was to develop a consensus classification combining type1 and type 2 diabetic nephropathies. Such a classification should discriminate lesions by various degrees of severity that would be easy to use internationally in clinical practice. We divide diabetic nephropathy into four hierarchical glomerular lesions with a separate evaluation for degrees of interstitial and vascular involvement. Biopsies diagnosed as diabetic nephropathy are classified as follows: Class I, glomerular basement membrane thickening: isolated glomerular basement membrane thickening and only mild, nonspecific changes by light microscopy that do not meet the criteria of classes II through IV. Class II, mesangial expansion, mild (IIa) or severe (IIb): glomeruli classified as mild or severe mesangial expansion but without nodular sclerosis (Kimmelstiel-Wilson lesions) or global glomerulosclerosis in more than 50% of glomeruli. Class III, nodular sclerosis (Kimmelstiel-Wilson lesions): at least one glomerulus with nodular increase in mesangial matrix (Kimmelstiel-Wilson) without changes described in class IV. Class IV, advanced diabetic glomerulosclerosis: more than 50% global glomerulosclerosis with other clinical or pathologic evidence that sclerosis is attributable to diabetic nephropathy. A good interobserver reproducibility for the four classes of DN was shown (intraclass correlation coefficient = 0.84) in a test of this classification.

The emerging role of biomarkers in diabetic and hypertensive chronic kidney disease

Currently used measures to assess kidney function and injury are largely inadequate. Markers such as serum creatinine, formulas to estimate glomerular filtration rate, cystatin C, and proteinuria largely identify an underlying disease process that is well established. Thus, there has been a recent effort to identify new biomarkers that reflect kidney function, early injury, and/or repair that ultimately can relate to progression or regression of damage. Several biomarkers emerged recently that are able to detect kidney damage earlier than is currently possible with traditional biomarkers such as serum creatinine and proteinuria. Identification of urine biomarkers has proven to be beneficial in recent years because of ease of handling, stability, and the ability to standardize the various markers to creatinine or other peptides generally already present in the urine. Recent markers such as neutrophil gelatinase-associated lipocalin (NGAL), kidney injury molecule-1 (KIM-1), and podocin have garnered a lot of attention. The emergence of these and other biomarkers is largely because of the evolution of novel genomic and proteomic applications in investigations of acute kidney injury and chronic kidney disease. In this article, we focus on the applications of these biomarkers in disease.

Evaluation of podocyte lesion in patients with diabetic nephropathy: Wilms' tumor-1 protein used as a podocyte marker

INTRODUCTION

The reduction of podocyte number and density per glomerulus has been linked to the development of proteinuria and the progression of disease in patients with diabetic nephropathy (DN). However, it has been recognized that measurement of podocyte number by light microscope is quite difficult because of the complexity of both podocyte and glomerular structure, which is not suitable for clinical research. In our research institute, we used WT1 as podocyte marker to evaluate the podocyte lesion.

METHODS

In our experiment, we selected the C-terminal antibody of WT1 to stain the nuclei and the N-terminal antibody of WT1 to stain the cytoplasma of podocytes. Forty patients were enrolled with type 2 diabetes and proven to have DN by renal biopsy analysis. DN patients were classified into three groups based on the degree of proteinuria: microalbuminuria (n=10, 30-300mg/24h), overt proteinuria (n=15, 0.5-3.5g/24h), and heavy proteinuria (n=15, >3.5g/24h).

RESULTS

The results demonstrated that the podocyte number was markedly decreased in patients with DN (30-51% reduction). There was a significant negative correlation between the proteinuria and both podocyte density and number. The cover area density of podocyte cytoplasma in glomerulus was also significantly decreased in all DN patients (39-80% reduction). A significant inverse correlation was observed between the cover area density and the degree of proteinuria. The correlation coefficient (r=-0.85) was much higher than that between proteinuria and podocyte density (r=-0.56) or podocyte number (r=-0.36).

CONCLUSION

In conclusion, podocyte damage occurred in patients with DN, even in the early stage and became more dramatic during the course of proteinuria progression. WT1 staining, using the polyclonal antibody to stain the nuclei and monoclonal antibody to stain the cytoplasma of podocytes together, is a valuable alternative technique in the study of podocyte injury.

Renal effects of plant-derived oleanolic acid in streptozotocin-induced diabetic rats

Previous studies from our laboratories indicate that the anti-diabetic effects of Syzygium cordatum (Hochst.) [Myrtaceae] leaf extract in streptozotocin-induced diabetic rats may be attributed in part to mixtures of triterpenes, oleanolic acid (3ss-hydroxy-olea-12-en-28-oic acid, OA) and ursolic acid (3ss -hydroxyl-urs-12-en-28-oic acid, UA). For the bioactive compounds to have potential in diabetes management, they should alleviate or prevent complications of diabetes mellitus, kidney function, and cardiovascular disorders. This study was, therefore, designed to assess whether S. cordatum leaf derived OA influenced renal function evaluated by the ability to increase urinary Na(+) outputs parameters and creatinine clearance (Ccr) of streptozotocin (STZ)-induced diabetic rats. Extraction and fractionation of S. cordatum powdered leaf ethyl acetate-solubles (EAS) yielded mixtures of OA/UA and methyl maslinate/methyl corosolate. Recrystallization of OA/UA mixture using ethanol afforded OA, the structure of which was confirmed by NMR spectroscopy ((1)H & (13)C). Acute effects of OA on kidney function and mean arterial blood pressure (MAP) were investigated in anesthetized rats challenged with hypotonic saline after a 3.5-h equilibration for 4h of 1 h control, 1.5 h treatment, and 1.5 h recovery periods. OA was added to the infusate during the treatment period. Chronic effects of OA were studied in individually caged rats treated twice daily with OA (60 mg/kg, p.o.) for five weeks. By comparison with respective control animals administration, OA significantly increased Na(+) excretion rates of non-diabetic and STZ-induced diabetic rats without affecting urine flow, K(+) and Cl(-) rates. At the end of five weeks, OA treatment significantly (p < 0.05) increased Ccr in non-diabetic (2.88 +/- 0.14 vs. 3.71 +/- 0.30 ml/min) and STZ-diabetic rats (1.81 +/- 0.32 vs. 3.07 +/- 0.16 ml/min) with concomitant reduction of plasma creatinine concentration (n = 6 in all groups). OA also caused significant decreases in MAP in non-diabetic and STZ-induced diabetic rats. These findings suggest that OA may have beneficial effects on some processes associated with renal derangement of STZ-induced diabetic rats.

Diabetes induces stromal remodelling and increase in chondroitin sulphate proteoglycans of the rat ventral prostate

Extracellular matrix (ECM) remodelling is an important process involved in prostate cancer progression. Alterations in ECM caused by diabetes in different tissues such as kidney is well described; however, it is poorly investigated in prostate. The aim of this study was to evaluate changes in ECM of rat prostate showing gland atrophy caused by diabetes and their implications in development of malignant lesions. Diabetes was induced in Wistar rats using alloxan (45 mg/kg bw). After 90 days of diabetes onset, animals were killed and ventral prostate was removed and prepared for light microscopy following immunoreaction for fibronectin, chondroitin sulphate and Picrossirius staining for collagen fibres. Proteoglycans (PG) were identified at transmission electron microscopy after fixation with Cuprolinic Blue. Diabetes led to a thickening of 25% in the acinar basement membrane accompanied by increase and disorganization of its proteoglycans (P1). Three additional populations of prostatic stromal PGs were identified: collagen fibril linked (P2) and interstitial (P3) and (P4) PGs. Diabetes increased P3 and mainly P4 which had higher dimension and accumulated around the smooth muscle cells. In addition, an increase in chondrotin sulphate (33%, mainly in sites where P4 were noted) and collagen (44%) was noted in diabetic rats, whereas fibronectin did not change. Atrophic changes observed in rat ventral prostate after diabetes are accompanied by stromal remodelation related to increase in collagen and chondroitin sulphate proteoglycans. Thus, diabetes can promote a stromal microenvironment rich in elements that could favour cell migration, proliferation and pathological process.

High ambient glucose induces angiotensin-independent AT-1 receptor activation, leading to increases in proliferation and extracellular matrix accumulation in MES-13 mesangial cells

Diabetic nephropathy is associated with mesangial ECM (extracellular matrix) accumulation. We have shown that AT-1R [Ang II (angiotensin II) type I receptor] signalling induces ECM proteins via transactivation of PI3K (phosphoinositide 3-kinase) in mesangial cells. In the present study, we examined the mechanisms underlying the effect of high ambient glucose on cell proliferation and ECM expansion in a mesangial context. High glucose induced increases in PI3K activity, proliferation and ECM accumulation in mesangial cells. These effects were abrogated by losartan, an AT-1R antagonist, but not by [Sar1,Thr8]-Ang II (Sar is sarcosine), an inactive analogue of Ang II, or by a neutralizing antibody against Ang I/II. Overexpression of a constitutively active PI3Kα or AT-1R alone was sufficient to induce similar changes by high glucose. In contrast, overexpression of an inactive AT-1R lowered the basal levels and rendered the cells non-responsive to high glucose. Moreover, cells overexpressing wild-type AT-1R had enhanced sensitivity to acute Ang II stimulation. These cells, however, did not respond to conditioned medium obtained from mesangial cells cultured in high glucose. We further demonstrated that iAng (intracellular Ang II) can be induced by high glucose but only under certain conditions. Efficient suppression of iAng by short hairpin RNA against angiotensinogen, however, did not affect high glucose-induced effects on MES-13 cells. These results suggest that high ambient glucose induces activation of AT-1R in an Ang II-independent manner to transactivate PI3K, resulting in proliferation and ECM accumulation in mesangial cells.

EGFR-PLCgamma1 signaling mediates high glucose-induced PKCbeta1-Akt activation and collagen I upregulation in mesangial cells

Glomerular matrix accumulation is a hallmark of diabetic nephropathy. We have recently shown that epidermal growth factor receptor (EGFR) transactivation mediates high glucose (HG)-induced collagen I upregulation through PI3K-PKCbeta1-Akt signaling in mesangial cells (MC). Phospholipase Cgamma1 (PLCgamma1) interacts with activated growth factor receptors and activates classic PKC isoforms. We thus studied its role in HG-induced collagen I upregulation in MC. Primary rat MC were treated with HG (30 mM) or mannitol as osmotic control. Protein kinase activation was assessed by Western blotting and collagen I upregulation by Northern blotting. Diabetes was induced in rats by streptozotocin. HG treatment for 1 h led to PLCgamma1 membrane translocation and Y783 phosphorylation, both indicative of its activation. Mannitol was without effect. PLCgamma1 Y783 phosphorylation was also seen in cortex and glomeruli of diabetic rats. HG induced a physical association between EGFR and PLCgamma1 as identified by coimmunoprecipitation. PLCgamma1 activation required EGFR kinase activity since it was prevented by the EGFR inhibitor AG1478 or overexpression of kinase-inactive EGFR (K721A). Phosphoinositide-3-OH kinase inhibition also prevented PLCgamma1 activation. HG-induced Akt S473 phosphorylation, effected by PKCbeta1, was inhibited by the PLCgamma inhibitor U73122. PLCgamma1 inhibition or downregulation by small interference RNA also prevented HG-induced collagen I upregulation. Our results indicate that EGFR-PLCgamma1 signaling mediates HG-induced PKCbeta1-Akt activation and subsequent collagen I upregulation in MC. Inhibition of EGFR or PLCgamma1 may provide attractive therapeutic targets for the treatment of diabetic nephropathy.

Rosiglitazone prevents high glucose-induced vascular endothelial growth factor and collagen IV expression in cultured mesangial cells

Peroxisome proliferator-activated receptor (PPARγ), a ligand-dependent transcription factor, negatively modulates high glucose effects. We postulated that rosiglitazone (RSG), an activator of PPARγ prevents the upregulation of vascular endothelial growth factor (VEGF) and collagen IV by mesangial cells exposed to high glucose. Primary cultured rat mesangial cells were growth-arrested in 5.6 mM (NG) or 25 mM D-glucose (HG) for up to 48 hours. In HG, PPARγ mRNA and protein were reduced within 3 h, and enhanced ROS generation, expression of p22^phox, VEGF and collagen IV, and PKC-ζ membrane association were prevented by RSG. In NG, inhibition of PPARγ caused ROS generation and VEGF expression that were unchanged by RSG. Reduced AMP-activated protein kinase (AMPK) phosphorylation in HG was unchanged with RSG, and VEGF expression was unaffected by AMPK inhibition. Hence, PPARγ is a negative modulator of HG-induced signaling that acts through PKC-ζ but not AMPK and regulates VEGF and collagen IV expression by mesangial cells.

Lack of type VIII collagen in mice ameliorates diabetic nephropathy

OBJECTIVE

Key features of diabetic nephropathy include the accumulation of extracellular matrix proteins. In recent studies, increased expression of type VIII collagen in the glomeruli and tubulointerstitium of diabetic kidneys has been noted. The objectives of this study were to assess whether type VIII collagen affects the development of diabetic nephropathy and to determine type VIII collagen-dependent pathways in diabetic nephropathy in the mouse model of streptozotocin (STZ)-induced diabetes.

RESEARCH DESIGN AND METHODS

Diabetes was induced by STZ injections in collagen VIII-deficient or wild-type mice. Functional and histological analyses were performed 40 days after induction of diabetes. Type VIII collagen expression was assessed by Northern blots, immunohistochemistry, and real-time PCR. Proliferation of primary mesangial cells was measured by thymidine incorporation and direct cell counting. Expression of phosphorylated extracellular signal-regulated kinase (ERK1/2) and p27(Kip1) was assessed by Western blots. Finally, Col8a1 was stably overexpressed in mesangial cells.

RESULTS

Diabetic wild-type mice showed a strong renal induction of type VIII collagen. Diabetic Col8a1(-)/Col8a2(-) animals revealed reduced mesangial expansion and cellularity and extracellular matrix expansion compared with the wild type. These were associated with less albuminuria. High-glucose medium as well as various cytokines induced Col8a1 in cultured mesangial cells. Col8a1(-)/Col8a2(-) mesangial cells revealed decreased proliferation, less phosphorylation of Erk1/2, and increased p27(Kip1) expression. Overexpression of Col8a1 in mesangial cells induced proliferation.

CONCLUSIONS

Lack of type VIII collagen confers renoprotection in diabetic nephropathy. One possible mechanism is that type VIII collagen permits and/or fosters mesangial cell proliferation in early diabetic nephropathy.

Effects of glutamine supplementation on kidney of diabetic rat

Glutamine is the most important donor of NH(3) in kidney playing an important role in acid-base buffering system. Besides this effect, glutamine presents many other relevant functions in the whole body, such as a precursor of arginine in adult and neonates. In addition to these effects, some studies have shown that glutamine can potentiate renal disease. In the present study, the effect of short-term treatment (15 days) with glutamine on control and diabetic rats was investigated. Using biochemical, histological and molecular biology analysis from control and diabetic rats we verified that glutamine supplementation increase in pro-inflammatory interleukins (IL)-1beta and IL-6 content in renal cortex and induce alteration in glomerular characteristics. This study showed that short-term treatment with glutamine in association with increased glucose levels could cause important alterations in glomerular morphology that may result in fast progression of kidney failure.

Regulation of mRNA translation in renal physiology and disease

Translation, a process of generating a peptide from the codons present in messenger RNA, can be a site of independent regulation of protein synthesis; it has not been well studied in the kidney. Translation occurs in three stages (initiation, elongation, and termination), each with its own set of regulatory factors. Mechanisms controlling translation include small inhibitory RNAs such as microRNAs, binding proteins, and signaling reactions. Role of translation in renal injury in diabetes, endoplasmic reticulum stress, acute kidney injury, and, in physiological adaptation to loss of nephrons is reviewed here. Contribution of mRNA translation to physiology and disease is not well understood. Because it is involved in such diverse areas as development and cancer, it should prove a fertile field for investigation in renal science.

Urinary protein profiles in a rat model for diabetic complications

Diabetes mellitus is estimated to affect approximately 24 million people in the United States and more than 150 million people worldwide. There are numerous end organ complications of diabetes, the onset of which can be delayed by early diagnosis and treatment. Although assays for diabetes are well founded, tests for its complications lack sufficient specificity and sensitivity to adequately guide these treatment options. In our study, we employed a streptozotocin-induced rat model of diabetes to determine changes in urinary protein profiles that occur during the initial response to the attendant hyperglycemia (e.g. the first two months) with the goal of developing a reliable and reproducible method of analyzing multiple urine samples as well as providing clues to early markers of disease progression. After filtration and buffer exchange, urinary proteins were digested with a specific protease, and the relative amounts of several thousand peptides were compared across rat urine samples representing various times after administration of drug or sham control. Extensive data analysis, including imputation of missing values and normalization of all data was followed by ANOVA analysis to discover peptides that were significantly changing as a function of time, treatment and interaction of the two variables. The data demonstrated significant differences in protein abundance in urine before observable pathophysiological changes occur in this animal model and as function of the measured variables. These included decreases in relative abundance of major urinary protein precursor and increases in pro-alpha collagen, the expression of which is known to be regulated by circulating levels of insulin and/or glucose. Peptides from these proteins represent potential biomarkers, which can be used to stage urogenital complications from diabetes. The expression changes of a pro-alpha 1 collagen peptide was also confirmed via selected reaction monitoring.

Relevance of insulin-like growth factor 2 in the etiopathophysiology of diabetic nephropathy: possible roles of phosphatase and tensin homolog on chromosome 10 and secreted protein acidic and rich in cysteine as regulators of repair

BACKGROUND

Diabetic nephropathy (DN) is a devastating complication of diabetes, the exact molecular pathophysiology of which is not well established. Hyperglycemia increases insulin-like growth factors (IGFs), especially IGF2, which acts via the IGF1 receptor present on renal cells. Elevated glucose levels damage the kidney, which is repaired by modulators such as secreted protein acidic and rich in cysteine (SPARC). Hence, it was hypothesized that IGF2 and SPARC may have an important role in the etiology of DN.

METHODS

Human renal biopsies, histopathologically categorized as normal, early Type 2 diabetes mellitus (T2DM), or established DN, were analyzed for the localization and expression of IGF2, its negative regulator phosphatase and tensin homolog on chromosome 10 (PTEN), and SPARC.

RESULTS

Expression of IGF2, PTEN, and SPARC was increased in renal biopsies from T2DM patients compared with normal samples. Although IGF2 protein was increased in biopsies from DN patients, PTEN and SPARC levels were decreased. Real-time reverse transcription-polymerase chain reaction indicated that transcript levels of IGF2 and PTEN were greater than those of β-actin in all human renal biopsy samples.

CONCLUSION

The results suggest the following molecular etiopathophysiology of DN: (i) hyperglycemia upregulates IGF2, which initiates PTEN, a regulator of IGF2 signaling; (ii) loss of this IGF2-PTEN feedback loop causes changes that are characteristic of DN; and (iii) lowered expression of the repair modulator SPARC results in the development and/or progression of DN. Hence, targeting relevant modulators, such as like IGF2, PTEN, and SPARC, may be important in the management of DN.

Effect of activin A on tubulointerstitial fibrosis in diabetic nephropathy

AIM

The effect of activin A on tubulointerstitial fibrosis in diabetic nephropathy (DN) using streptozotocin (STZ)-induced diabetic rats and high glucose-cultured HK-2 cells was investigated.

METHODS

Male Wistar rats were randomized into a normal control group (NC) and diabetes mellitus group (DM). Diabetes was induced by i.p. injection of STZ. Six rats were respectively killed 4, 8, 12 and 16 weeks after model establishment in each group. The changes of kidney weight/bodyweight (KW/BW), urine albumin excretion rate (AER) and creatinine clearance rate (Ccr) were determined. The morphology of tubulointerstitium was observed by light microscopy. Further biochemical analysis was provided using immunohistochemistry and real-time polymerase chain reaction. The different parameters in high glucose-cultured HK-2 cells were monitored by western blotting or enzyme-linked immunosorbent assay (ELISA) and the intervention of rh-follistatin on them was investigated.

RESULTS

Compared with the NC group, there was marked enlargement in the levels of KW/BW, AER, Ccr and interstitial fibrosis index, and the production of P-Smad2/3 and fibronectin in the DM group from 8 to 16 weeks. Activin betaA, mainly located in tubular epithelial cells, was significantly higher in the DM group than that in the NC group throughout the study periods. Follistatin was abundant in the NC group, but was diminished gradually in the DM group. High glucose may facilitate the synthesis of activin betaA, transforming growth factor (TGF)-beta, P-Smad2/3 and fibronectin in HK-2 cells while rh-follistatin inhibited them except TGF-beta.

CONCLUSION

Activin A is involved in tubulointerstitial fibrosis in DN by inducing the production of fibronectin through Smad signal pathway.

Connective tissue growth factor(CCN2), a pathogenic factor in diabetic nephropathy. What does it do? How does it do it?

Connective tissue growth factor (CTGF/CCN2) is a member of the CCN family of matricellular proteins. Its expression is induced by a number of factors including TGF-β. It has been associated with fibrosis in various tissues including the kidney. Diabetic nephropathy (DN) develops in about 30% of patients with diabetes and is characterized by thickening of renal basement membranes, fibrosis in the glomerulus (glomerulosclerosis), tubular atrophy and interstitial fibrosis, all of which compromise kidney function. This review examines changes in CTGF expression in the kidney in DN, the effects they have on glomerular mesangial and podocyte cells and the tubulointerstitium, and how these contribute to driving fibrotic changes in the disease. CTGF can bind to several other growth factors modifying their function. CTGF is also able to interact with receptors on cells, including integrins, tyrosine receptor kinase A (TrkA), low density lipoprotein receptor-related protein (LRP) and heparan sulphate proteoglycans. These interactions, the intracellular signalling pathways they activate, and the cellular responses evoked are reviewed. CTGF also induces the expression of chemokines which themselves have pharmacological actions on cells. CTGF may prompt some responses by acting through several different mechanisms, possibly simultaneously. For example, CTGF is often described as an effector of TGF-β. It can promote TGF-β signalling by binding directly to the growth factor, promoting its interaction with the TGF-β receptor; by triggering intracellular signalling on binding the TrkA receptor, which leads to the transcriptional repression of Smad7, an inhibitor of the TGF-β signalling pathway; and by binding to BMP-7 whose own signalling pathway opposing TGF-β is inhibited, leading to enhanced TGF-β signalling.

Advanced oxidation protein products induce mesangial cell perturbation through PKC-dependent activation of NADPH oxidase

Mesangial deposition of extracellular matrix (ECM) is a hallmark of several glomerular diseases including diabetic nephropathy. Accumulation of advanced oxidation protein products (AOPPs) has been found in diabetes and chronic kidney disease and linked to mesangial ECM deposition and progressive glomerulosclerosis in these disorders. Although emerging evidence implicates AOPPs as the renal pathogenic factors, the underlying mechanisms have not been investigated. Here, using cultured rat mesangial cells (MCs) as a model, we identify AOPPs as the important mediators for activation of MC NADPH oxidase. Exposure of MCs to AOPPs, through membrane-associated phosphorylation of PKCα, induced rapid phosphorylation of cytosolic p47phox and its membrane translocation, enhanced interaction of p47phox with the membrane components p22phox and Nox4, and increased expression of these key regulatory subunits of NADPH oxidase. Challenge with AOPPs triggered cytosolic superoxide generation, resulting in upregulation of fibronectin and collagen IV genes and proteins and overexpression of TGF-β1 via a PKC-NADPH oxidase-dependent pathway, as these downstream events were blocked by the inhibitors of PKC, inhibitors of NADPH oxidase, or the cytosolic superoxide scavenger. These data provide new information for understanding the molecular basis underlying AOPP-induced MC perturbation and might be a central step toward development of new interventions.

Matrix metalloproteinases: their potential role in the pathogenesis of diabetic nephropathy

Matrix metalloproteinases (MMPs), a family of proteinases including collagenases, gelatinases, stromelysins, matrilysins, and membrane-type MMPs, affect the breakdown and turnover of extracellular matrix (ECM). Moreover, they are major physiologic determinants of ECM degradation and turnover in the glomerulus. Renal hypertrophy and abnormal ECM deposition are hallmarks of diabetic nephropathy (DN), suggesting that altered MMP expression or activation contributes to renal injury in DN. Herein, we review and summarize recent information supporting a role for MMPs in the pathogenesis of DN. Specifically, studies describing dysregulated activity of MMPs and/or their tissue inhibitors in various experimental models of diabetes, including animal models of type 1 or type 2 diabetes, clinical investigations of human type 1 or type 2 diabetes, and kidney cell culture studies are reviewed.

Novel mechanisms of protein synthesis in diabetic nephropathy--role of mRNA translation

Ambient protein levels are affected by both synthesis and degradation. Synthesis of a protein is regulated by transcription and messenger RNA (mRNA) translation. Translation has emerged as an important site of regulation of protein expression during development and disease. It is under the control of distinct factors that regulate initiation, elongation and termination phases. Regulation of translation occurs via signaling reactions, guanosine diphosphate-guanosine triphosphate binding and by participation of non-coding RNA species such as microRNA. Recent work has revealed an important role for translation in hypertrophy, matrix protein synthesis, elaboration of growth factors in in vivo and in vitro models of diabetic nephropathy. Studies of translation dysregulation in diabetic nephropathy have enabled identification of novel therapeutic targets. Translation of mRNA is a fertile field for exploration in investigation of kidney disease.

New insights into the mechanisms of fibrosis and sclerosis in diabetic nephropathy

Progression of diabetic nephropathy (DN) is manifested by gradual scarring of both the renal glomerulus and tubulointerstitial region. Over the past several years, the general understanding of the pathogenic factors that lead to renal fibrosis in DN has expanded considerably. In this review, some of the important factors that appear to be involved in driving this fibrosing process are discussed, with special emphasis on newer findings and insights. It is now clear that multiple cell types in the kidney contribute to progressive fibrosis in DN. New concepts about bradykinin, TGF-beta and eNOS signaling as well as JAK/STAT activation and the central role of inflammation in both glomerular and tubulointerstitial fibrosis are discussed.

Fibrosis in diabetes complications: pathogenic mechanisms and circulating and urinary markers

Diabetes mellitus is characterized by a lack of insulin causing elevated blood glucose, often with associated insulin resistance. Over time, especially in genetically susceptible individuals, such chronic hyperglycemia can cause tissue injury. One pathological response to tissue injury is the development of fibrosis, which involves predominant extracellular matrix (ECM) accumulation. The main factors that regulate ECM in diabetes are thought to be pro-sclerotic cytokines and protease/anti-protease systems. This review will examine the key markers and regulators of tissue fibrosis in diabetes and whether their levels in biological fluids may have clinical utility.

High glucose activates PKC-zeta and NADPH oxidase through autocrine TGF-beta1 signaling in mesangial cells

Conversion of normally quiescent mesangial cells into extracellular matrix-overproducing myofibroblasts in response to high ambient glucose and transforming growth factor (TGF)-beta(1) is central to the pathogenesis of diabetic nephropathy. Previously, we reported that mesangial cells respond to high glucose by generating reactive oxygen species (ROS) from NADPH oxidase dependent on protein kinase C (PKC) -zeta activation. We investigated the role of TGF-beta(1) in this action of high glucose on primary rat mesangial cells within 1-48 h. Both high glucose and exogenous TGF-beta(1) stimulated PKC-zeta kinase activity, as measured by an immune complex kinase assay and immunofluorescence confocal cellular imaging. In high glucose, Akt Ser473 phosphorylation appeared within 1 h and Smad2/3 nuclear translocation was prevented with neutralizing TGF-beta(1) antibodies. Neutralizing TGF-beta(1) antibodies, or a TGF-beta receptor kinase inhibitor (LY364947), or a phosphatidylinositol 3,4,5-trisphosphate (PI3) kinase inhibitor (wortmannin), prevented PKC-zeta activation by high glucose. TGF-beta(1) also stimulated cellular membrane translocation of PKC-alpha, -beta(1), -delta, and -epsilon, similar to high glucose. High glucose and TGF-beta(1) enhanced ROS generation by mesangial cell NADPH oxidase, as detected by 2,7-dichlorofluorescein immunofluorescence. This response was abrogated by neutralizing TGF-beta(1) antibodies, LY364947, or a specific PKC-zeta pseudosubstrate peptide inhibitor. Expression of constitutively active PKC-zeta in normal glucose caused upregulation of p22(phox), a likely mechanism of NADPH oxidase activation. We conclude that very early responses of mesangial cells to high glucose include autocrine TGF-beta(1) stimulation of PKC isozymes including PI3 kinase activation of PKC-zeta and consequent generation of ROS by NADPH oxidase.

MicroRNA-377 is up-regulated and can lead to increased fibronectin production in diabetic nephropathy

Intrinsic glomerular cells in a diabetic milieu have transcriptional activation of genes that influence the development of diabetic nephropathy. The cellular repertoire of microRNAs can regulate translation of these expressed genes into proteins. Fibronectin is a key matrix protein accumulated in excess in diabetic nephropathy. Here, we exposed cultured human and mouse mesangial cells to high glucose and transforming growth factor-beta to simulate the diabetic milieu. In these conditions in vitro, as well as in mouse diabetic nephropathy models in vivo, microRNA-377 was consistently up-regulated relative to controls. Through a combination of computational and biological approaches, we identified relevant miR-377 target genes. Although fibronectin was induced by miR-377, it was not a direct target of miR-377. However, miR-377 led to reduced expressions of p21-activated kinase and superoxide dismutase, which enhanced fibronectin protein production. Thus, overexpression of miR-377 in diabetic nephropathy indirectly leads to increased fibronectin protein production; as such, miR-377 can have a critical role in the pathophysiology of this prevalent human disease.

RhoA/Rho-kinase contribute to the pathogenesis of diabetic renal disease

OBJECTIVE

Accumulation of glomerular matrix proteins is central to the pathogenesis of diabetic nephropathy, with resident mesangial cells (MCs) known to upregulate matrix protein synthesis in response to high glucose. Because activation of the GTPase RhoA has been implicated in matrix upregulation, we studied its role in induction of the matrix protein fibronectin in diabetic MCs and in vivo in diabetic nephropathy.

RESEARCH DESIGN AND METHODS

Glucose (30 mmol/l)-induced RhoA/Rho-kinase, AP-1 activation, and fibronectin upregulation were assessed by immunoblotting, luciferase, electrophoretic mobility shift assay, enzyme-linked immunosorbent assay, real-time PCR, Northern blots, and immunofluorescence. Streptozotocin-induced diabetic rats were treated with the rho-kinase inhibitor fasudil, which was compared with enalapril, and functional and pathologic parameters were assessed.

RESULTS

Glucose led to RhoA and downstream Rho-kinase activation. Mannitol was without effect. Activity of the transcription factor AP-1, increased in diabetic MCs and kidneys, is important in the profibrotic effects of glucose, and this was dependent on Rho-kinase signaling. Upregulation of fibronectin by glucose, shown to be mediated by activator protein-1 (AP-1), was prevented by Rho-kinase inhibition. RhoA siRNA and dominant-negative RhoA also markedly attenuated fibronectin upregulation by high glucose. Applicability of these findings were tested in vivo. Fasudil prevented glomerular fibronectin upregulation, glomerular sclerosis, and proteinuria in diabetic rats, with effectiveness similar to enalapril.

CONCLUSIONS

High glucose activates RhoA/Rho-kinase in MCs, leading to downstream AP-1 activation and fibronectin induction. Inhibition of this pathway in vivo prevents the pathologic changes of diabetic nephropathy, supporting a potential role for inhibitors of RhoA/Rho in the treatment of diabetic renal disease.

IHG-1 amplifies TGF-beta1 signaling and is increased in renal fibrosis

Induced in high glucose-1 (IHG-1) is an evolutionarily conserved gene transcript upregulated by high extracellular glucose concentrations, but its function is unknown. Here, it is reported that the abundance of IHG-1 mRNA is nearly 10-fold higher in microdissected, tubule-rich renal biopsies from patients with diabetic nephropathy compared with control subjects. In the diabetic nephropathy specimens, in situ hybridization localized IHG-1 to tubular epithelial cells along with TGF-beta1 and activated Smad3, suggesting a possible role in the development of tubulointerstitial fibrosis. Supporting this possibility, IHG-1 mRNA and protein expression also increased with unilateral ureteral obstruction. In the HK-2 proximal tubule cell line, overexpression of IHG-1 increased TGF-beta1-stimulated expression of connective tissue growth factor and fibronectin. IHG-1 was found to amplify TGF-beta1-mediated transcriptional activity by increasing and prolonging phosphorylation of Smad3. Conversely, inhibition of endogenous IHG-1 with small interference RNA suppressed transcriptional responses to TGF-beta1. In summary, IHG-1, which increases in diabetic nephropathy, may enhance the actions of TGF-beta1 and contribute to the development of tubulointerstitial fibrosis.

LR-90 prevents dyslipidaemia and diabetic nephropathy in the Zucker diabetic fatty rat

AIMS/HYPOTHESIS

Previous studies have shown that LR-90, a new inhibitor of AGE formation, prevented the development of experimental type 1 diabetic nephropathy. In this study, we examined the effects of LR-90 in the Zucker diabetic fatty (ZDF) rat, a model of type 2 diabetes and metabolic syndrome, and investigated the mechanisms by which it may protect against renal injury.

METHODS

Male ZDF rats were treated without or with LR-90 from age 13 to 40 weeks. Metabolic and kidney functions and renal histology were evaluated. AGE accumulation and the production of the receptor for AGE (AGER) were measured. Profibrotic growth factors, extracellular matrix proteins and intracellular signalling pathways associated with glomerular and tubular damage were also analysed.

RESULTS

LR-90 dramatically reduced plasma lipids in ZDF rats, with only modest effects on hyperglycaemia. Renal AGE, AGER and lipid peroxidation were all attenuated by LR-90. LR-90 significantly retarded the increase in albuminuria and proteinuria. This was associated with reduction in glomerulosclerosis and tubulointerstitial fibrosis, concomitant with marked inhibition of renal overproduction of TGF-beta1, connective tissue growth factor, fibronectin and collagen IV. Additionally, LR-90 downregulated the activation of key mitogen-activated protein kinases (MAPKs) and nuclear factor kappa B (NF-kappaB) in the renal cortex.

CONCLUSIONS/INTERPRETATION

These results support our earlier studies on the renoprotective effects of LR-90 on type 1 diabetic nephropathy and provide further evidence that LR-90, an AGE inhibitor with pleiotrophic effects, may also be beneficial for the prevention of type 2 diabetic nephropathy, where multiple risk factors, such as hyperglycaemia, dyslipidaemia, obesity, insulin resistance and hypertension, contribute to renal injury.