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      Reducing Serum Uric Acid Attenuates TGF-β 1-Induced Profibrogenic Progression in Type 2 Diabetic Nephropathy

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          Background: The pivotal role of transforming growth factor-β<sub>1</sub> (TGF-β<sub>1</sub>)-induced tubulointerstitial fibrosis in the progression of chronic kidney disease is an active topic of research. Recent evidence indicates that hyperuricemia is associated with increased TGF-β<sub>1</sub> and progressive tubulointerstitial injury. We examined the hypothesis that lowering serum uric acid attenuates TGF-β<sub>1</sub>-induced profibrogenic tubular change in type 2 diabetic nephropathy. Methods: KK-A<sup>y</sup>/Ta mice, an animal model of type 2 diabetes, were provided access to either regular drinking water or drinking water containing 10 mg/dl of allopurinol. Normal rat kidney epithelial cells were cultured and stimulated with 5 m<smlcap>M</smlcap> uric acid with or without allopurinol. Results: Type 2 diabetic mice that received allopurinol exhibited smaller increases in urinary albumin:creatinine ratio than diabetic control mice, as well as attenuated TGF-β<sub>1</sub> and Smad pathway-induced profibrogenic tubular changes in diabetic kidneys. Allopurinol attenuated TGF-β<sub>1</sub>-induced Smad pathway activation in tubular cells. These findings were related to increases in E-cadherin, and decreases in vimentin and α-smooth muscle actin. Uric acid-induced upregulation of TGF-β<sub>1</sub> depends on mitogen-activated protein kinase signaling. Conclusions: This is the first study to demonstrate that reducing serum uric acid has preventive effects against to profibrogenic progression in type 2 diabetic kidney disease. These findings suggest that lowering serum uric acid may be an effective therapeutic intervention to prevent the progression of type 2 diabetic kidney disease.

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          Most cited references 23

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          BMP-7 counteracts TGF-beta1-induced epithelial-to-mesenchymal transition and reverses chronic renal injury.

          Bone morphogenic protein (BMP)-7 is a 35-kDa homodimeric protein and a member of the transforming growth factor (TGF)-beta superfamily. BMP-7 expression is highest in the kidney, and its genetic deletion in mice leads to severe impairment of eye, skeletal and kidney development. Here we report that BMP-7 reverses TGF-beta1-induced epithelial-to-mesenchymal transition (EMT) by reinduction of E-cadherin, a key epithelial cell adhesion molecule. Additionally, we provide molecular evidence for Smad-dependent reversal of TGF-beta1-induced EMT by BMP-7 in renal tubular epithelial cells and mammary ductal epithelial cells. In the kidney, EMT-induced accumulation of myofibroblasts and subsequent tubular atrophy are considered key determinants of renal fibrosis during chronic renal injury. We therefore tested the potential of BMP-7 to reverse TGF-beta1-induced de novo EMT in a mouse model of chronic renal injury. Our results show that systemic administration of recombinant human BMP-7 leads to repair of severely damaged renal tubular epithelial cells, in association with reversal of chronic renal injury. Collectively, these results provide evidence of cross talk between BMP-7 and TGF-beta1 in the regulation of EMT in health and disease.
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            A role for uric acid in the progression of renal disease.

            Hyperuricemia is associated with renal disease, but it is usually considered a marker of renal dysfunction rather than a risk factor for progression. Recent studies have reported that mild hyperuricemia in normal rats induced by the uricase inhibitor, oxonic acid (OA), results in hypertension, intrarenal vascular disease, and renal injury. This led to the hypothesis that uric acid may contribute to progressive renal disease. To examine the effect of hyperuricemia on renal disease progression, rats were fed 2% OA for 6 wk after 5/6 remnant kidney (RK) surgery with or without the xanthine oxidase inhibitor, allopurinol, or the uricosuric agent, benziodarone. Renal function and histologic studies were performed at 6 wk. Given observations that uric acid induces vascular disease, the effect of uric acid on vascular smooth muscle cells in culture was also examined. RK rats developed transient hyperuricemia (2.7 mg/dl at week 2), but then levels returned to baseline by week 6 (1.4 mg/dl). In contrast, RK+OA rats developed higher and more persistent hyperuricemia (6 wk, 3.2 mg/dl). Hyperuricemic rats demonstrated higher BP, greater proteinuria, and higher serum creatinine than RK rats. Hyperuricemic RK rats had more renal hypertrophy and greater glomerulosclerosis (24.2 +/- 2.5 versus 17.5 +/- 3.4%; P < 0.05) and interstitial fibrosis (1.89 +/- 0.45 versus 1.52 +/- 0.47; P < 0.05). Hyperuricemic rats developed vascular disease consisting of thickening of the preglomerular arteries with smooth muscle cell proliferation; these changes were significantly more severe than a historical RK group with similar BP. Allopurinol significantly reduced uric acid levels and blocked the renal functional and histologic changes. Benziodarone reduced uric acid levels less effectively and only partially improved BP and renal function, with minimal effect on the vascular changes. To better understand the mechanism for the vascular disease, the expression of COX-2 and renin were examined. Hyperuricemic rats showed increased renal renin and COX-2 expression, the latter especially in preglomerular arterial vessels. In in vitro studies, cultured vascular smooth muscle cells incubated with uric acid also generated COX-2 with time-dependent proliferation, which was prevented by either a COX-2 or TXA-2 receptor inhibitor. Hyperuricemia accelerates renal progression in the RK model via a mechanism linked to high systemic BP and COX-2-mediated, thromboxane-induced vascular disease. These studies provide direct evidence that uric acid may be a true mediator of renal disease and progression.
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              Uric acid stimulates monocyte chemoattractant protein-1 production in vascular smooth muscle cells via mitogen-activated protein kinase and cyclooxygenase-2.

              Previous studies have reported that uric acid stimulates vascular smooth muscle cell (VSMC) proliferation in vitro. We hypothesized that uric acid may also have direct proinflammatory effects on VSMCs. Crystal- and endotoxin-free uric acid was found to increase VSMC monocyte chemoattractant protein-1 (MCP-1) expression in a time- and dose-dependent manner, peaking at 24 hours. Increased mRNA and protein expression occurred as early as 3 hours after uric acid incubation and was partially dependent on posttranscriptional modification of MCP-1 mRNA. In addition, uric acid activated the transcription factors nuclear factor-kappaB and activator protein-1, as well as the MAPK signaling molecules ERK p44/42 and p38, and increased cyclooxygenase-2 (COX-2) mRNA expression. Inhibition of p38 (with SB 203580), ERK 44/42 (with UO126 or PD 98059), or COX-2 (with NS398) each significantly suppressed uric acid-induced MCP-1 expression at 24 hours, implicating these pathways in the response to uric acid. The ability of both n-acetyl-cysteine and diphenyleneionium (antioxidants) to inhibit uric acid-induced MCP-1 production suggested involvement of intracellular redox pathways. Uric acid regulates critical proinflammatory pathways in VSMCs, suggesting it may have a role in the vascular changes associated with hypertension and vascular disease.

                Author and article information

                Nephron Exp Nephrol
                Cardiorenal Medicine
                S. Karger AG
                February 2013
                09 January 2013
                : 121
                : 3-4
                : e109-e121
                aDivision of Nephrology, Department of Internal Medicine, and bDepartment of Pathology, Kyung Hee University Hospital at Gangdong, College of Medicine, and cDivision of Nephrology, Department of Internal Medicine, Kyung Hee University, Seoul, Korea
                Author notes
                *Ju-Young Moon, Division of Nephrology, Department of Internal Medicine, Kyung Hee University, Hospital at Gangdong, College of Medicine, Kyung Hee University, Sangil-dong 149, Gangdong-gu, Seoul 134-727 (Korea), E-Mail jymoon@khu.ac.kr
                343567 Nephron Exp Nephrol 2012;121:e109–e121
                © 2013 S. Karger AG, Basel

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                Page count
                Figures: 8, Tables: 1, Pages: 13
                Original Paper


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