Tubular Injury In Diabetic Nephropathy Research Articles

Rap1 Ameliorates Renal Tubular Injury in Diabetic Nephropathy

Rap1b ameliorates high glucose (HG)-induced mitochondrial dysfunction in tubular cells. However, its role and precise mechanism in diabetic nephropathy (DN) in vivo remain unclear. We hypothesize that Rap1 plays a protective role in tubular damage of DN by modulating primarily the mitochondria-derived oxidative stress. The role and precise mechanisms of Rap1b on mitochondrial dysfunction and of tubular cells in DN were examined in rats with streptozotocin (STZ)-induced diabetes that have Rap1b gene transfer using an ultrasound microbubble-mediated technique as well as in renal proximal epithelial tubular cell line (HK-2) exposed to HG ambiance. The results showed that Rap1b expression decreased significantly in tubules of renal biopsies from patients with DN. Overexpression of a constitutively active Rap1b G12V notably ameliorated renal tubular mitochondrial dysfunction, oxidative stress, and apoptosis in the kidneys of STZ-induced rats, which was accompanied with increased expression of transcription factor C/EBP-β and PGC-1α. Furthermore, Rap1b G12V also decreased phosphorylation of Drp-1, a key mitochondrial fission protein, while boosting the expression of genes related to mitochondrial biogenesis and antioxidants in HK-2 cells induced by HG. These effects were imitated by transfection with C/EBP-β or PGC-1α short interfering RNA. In addition, Rap1b could modulate C/EBP-β binding to the endogenous PGC-1α promoter and the interaction between PGC-1α and catalase or mitochondrial superoxide dismutase, indicating that Rap1b ameliorates tubular injury and slows the progression of DN by modulation of mitochondrial dysfunction via C/EBP-β–PGC-1α signaling.

Pigment epithelium-derived factor (PEDF) inhibits proximal tubular cell injury in early diabetic nephropathy by suppressing advanced glycation end products (AGEs)-receptor (RAGE) axis

Pigment epithelium-derived factor (PEDF) is a multifunctional glycoprotein with anti-angiogenic and anti-inflammatory properties, and it could block the development and progression of experimental diabetic retinopathy. However, a role for PEDF in early experimental diabetic nephropathy is not fully understood. Advanced glycation end products (AGEs) and their receptor (RAGE) axis stimulates oxidative stress generation and subsequently evokes inflammatory and fibrogenic reactions in renal tubular cells, thereby playing a role in diabetic nephropathy. Therefore, this study investigated whether PEDF could prevent AGE-elicited tubular cell injury in early diabetic nephropathy. Human proximal tubular cells were incubated with or without AGE-bovine serum albumin in the presence or absence of PEDF. Streptozotocin-induced diabetic rats were treated with or without intravenous injection of PEDF for 4 weeks. Gene expression was analyzed by quantitative real-time reverse transcription-polymerase chain reactions. Reactive oxygen species (ROS) was measured with dihydroethidium staining. PEDF or antibodies raised against RAGE inhibited the AGE-induced RAGE gene expression and subsequently reduced ROS generation, monocyte chemoattractant protein-1 (MCP-1) and transforming growth factor-β (TGF-β), fibronectin and type IV collagen mRNA levels in proximal tubular cells. RAGE gene expression, ROS generation and MCP-1 and TGF-β mRNA levels were significantly increased in diabetic kidney, which were suppressed by administration of PEDF. Our present data suggest that PEDF could play a protective role against tubular injury in diabetic nephropathy by attenuating the deleterious effects of AGEs via down-regulation of RAGE expression. Administration of PEDF may offer a promising strategy for halting the development of diabetic nephropathy.

Rap1b GTPase Ameliorates Glucose-Induced Mitochondrial Dysfunction

The role of tubular injury in diabetic nephropathy is relatively unknown, despite that apoptosis of tubular epithelial cells is commonly observed in human renal biopsies. The GTPase Ras-proximate-1 (Rap1b) is upregulated in the hyperglycemic state and is known to increase B-Raf, an antiapoptotic effector protein. In this study, the effects of high glucose on renal tubular apoptosis and the potential ability for Rap1b to ameliorate these effects were investigated. In the kidneys of diabetic mice, apoptotic tubular cells and dysmorphic mitochondria were observed, Bcl-2 expression was decreased, and Bax expression was increased. Total Rap1b expression was slightly increased, but its associated GTPase activity was significantly decreased. In vitro, high extracellular glucose led to decreased Bcl-2 expression, reduced Rap1b GTPase activity, and increased levels of both Bax and GTPase activating protein in a proximal tubular cell line (HK-2). These changes were accompanied by increased DNA fragmentation, decreased high molecular weight mitochondrial DNA, altered mitochondrial morphology and function, disrupted Bcl-2-Bax and Bcl-2-Rap1b interactions, and reduced cell survival. Overexpression of Rap1b partially prevents these abnormalities. Furthermore, the BH4 domain of Bcl-2 was found to be required for successful protein-protein interaction between Bcl-2 and Rap1b. In summary, these data suggest that Rap1b ameliorates glucose-induced mitochondrial dysfunction in renal tubular cells.

Urinary Excretion of Guanidinoacetic Acid in Rats with Diabetic Nephropathy

Urinary guanidinoacetic acid (GAA) is a sensitive marker for gentamicin nephrotoxicity in rats. This study assesses the usefulness of GAA concentrations in the diagnosis of renal tubular injury in diabetic nephropathy. Serum, urine, and renal cortex samples were obtained from rats 1, 2, and 3 weeks after streptozotocin injection (65 mg/kg body weight). Guanidinoacetic acid levels were measured by high-performance liquid chromatography. N-acetyl-β-D-glucosaminidase (NAG) activity in urine was determined by an enzymatic method. GAA levels in serum, urine, and renal cortex were significantly decreased in diabetic rats compared with those in control rats. In contrast, urinary NAG activity was significantly increased in diabetic rats. Decreases in serum, urine, and renal cortical GAA levels were attenuated by insulin treatment. These results indicate that a high serum glucose level may affect GAA synthesis in the renal cortex and that urinary GAA may be a clinically useful indicator of renal tubular injury in diabetic nephropathy.

Possible Relationship between Adiponectin and Renal Tubular Injury in Diabetic Nephropathy

Adiponectin is an adipose-derived protein which has anti-inflammatory and anti-atherogenic properties in addition to insulin-sensitizing effects. To date, the role of adiponectin in the pathogenesis of diabetic nephropathy remains unclear. The aim of the present study was to explore the relationship between adiponectin and renal tubular injury in diabetic nephropathy. We determined serum and urinary adiponectin levels in type 2 diabetic patients with normoalbuminuria (n = 19), microalbuminuria (n = 18), and overt diabetic nephropathy (n = 16), and then analyzed the correlations between serum and urinary adiponectin, urinary N-acetylglucosaminidase (NAG) as a clinical marker of renal tubular injury, urinary monocyte chemoattractant protein-1 (MCP-1) as an inflammatory marker in renal tubulointerstitium, and clinical markers of renal disease. Notably, serum and urinary adiponectin levels were significantly increased in patients with overt diabetic nephropathy compared to those with normoalbuminuria and microalbuminuria. In univariate linear regression analysis, serum adiponectin levels were positively correlated with serum creatinine (r = 0.648, P<0.0001), urinary albumin (r = 0.583, P<0.0001), urinary NAG (r = 0.406, P<0.01), urinary MCP-1 (r = 0.514, P<0.0001), and urinary adiponectin (r = 0.691, P<0.0001) levels in all diabetic patients. Urinary adiponectin levels were also positively correlated with serum creatinine (r = 0.729, P<0.0001), urinary albumin (r = 0.799, P<0.0001), urinary NAG (r = 0.701, P<0.0001), and urinary MCP-1 (r = 0.801, P<0.0001) levels in all diabetic patients. Multiple linear regression analysis showed that serum creatinine and urinary adiponectin levels were independently associated with serum adiponectin levels (r(2) = 0.522), and that serum creatinine, urinary NAG, urinary MCP-1, and serum adiponectin levels were independent determinants of urinary adiponectin levels (r(2) = 0.851). These results collectively indicate that renal insufficiency and tubular injury possibly play a contributory role in increases in serum and urinary adiponectin levels in overt diabetic nephropathy. We presume that an increase in circulating adiponectin in overt diabetic nephropathy might be a physiological response to mitigate renal tubular injury and to prevent the further progression of diabetic nephropathy through its anti-inflammatory and anti-atherogenic effects.

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

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. 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). 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. 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.