Fang-Fang He

High Serum Uric Acid and Increased Risk of Type 2 Diabetes: A Systemic Review and Meta-Analysis of Prospective Cohort Studies

Objective Current evidence suggests high serum uric acid may increase the risk of type 2 diabetes, but the association is still uncertain. The aim of the study was to evaluate the association between serum uric acid and future risk of type 2 diabetes by conducting a meta-analysis of prospective cohort studies. Design and Methods We conducted a systematic literature search of the PubMed database through April 2012. Prospective cohort studies were included in meta-analysis that reported the multivariate adjusted relative risks (RRs) and the corresponding 95% confidence intervals (CIs) for the association between serum uric acid and risk of type 2 diabetes. We used both fix-effects and random-effects models to calculate the overall effect estimate. The heterogeneity across studies was tested by both Q statistic and I2 statistic. Begg’s funnel plot and Egger’s regression test were used to assess the potential publication bias. Results We retrieved 7 eligible articles derived from 8 prospective cohort studies, involving a total of 32016 participants and 2930 incident type 2 diabetes. The combined RR of developing type 2 diabetes for the highest category of serum uric acid level compared with the lowest was 1.56(95% CI, 1.39–1.76). Dose-response analysis showed the risk of type 2 diabetes was increased by 6% per 1 mg/dl increment in serum uric acid level (RR 1.06, 95% CI: 1.04–1.07). The result from each subgroup showed a significant association between serum uric acid and risk of type 2 diabetes. In sensitive analysis, the combined RR was consistent every time omitting any one study. Little evidence of heterogeneity and publication bias was observed. Conclusions Our meta-analysis of prospective cohort studies provided strong evidence that high level of serum uric acid is independent of other established risk factors, especially metabolic syndrome components, for developing type 2 diabetes in middle-aged and older people.

Calcium entry via TRPC6 mediates albumin overload-induced endoplasmic reticulum stress and apoptosis in podocytes.

Albumin, which is the most abundant component of urine proteins, exerts injurious effects on renal cells in chronic kidney diseases. However, the toxicity of albumin to podocytes is not well elucidated. Here, we show that a high concentration of albumin triggers intracellular calcium ([Ca(2+)](i)) increase through mechanisms involving the intracellular calcium store release and extracellular calcium influx in conditionally immortalized podocytes. The canonical transient receptor potential-6 (TRPC6) channel, which is associated with a subset of familial forms of focal segmental glomerulosclerosis (FSGS) and several acquired proteinuric kidney diseases, was shown to be one of the important Ca(2+) permeable ion channels in podocytes. Therefore we explored the role of TRPC6 on albumin-induced functional and structural changes in podocytes. It was found that albumin-induced increase in [Ca(2+)](i) was blocked by TRPC6 siRNA or SKF-96365, a blocker of TRP cation channels. Long-term albumin exposure caused an up-regulation of TRPC6 expression in podocytes, which was inhibited by TRPC6 siRNA. Additionally, the inhibition of TRPC6 prevented the F-actin cytoskeleton disruption that is induced by albumin overload. Moreover, albumin overload induced expression of the endoplasmic reticulum (ER) stress protein GRP78, led to caspase-12 activation and ultimately podocyte apoptosis, all of which were abolished by the knockdown of TRPC6 using TRPC6 siRNA. These results support the view that albumin overload may induce ER stress and the subsequent apoptosis in podocytes via TRPC6-mediated Ca(2+) entry.

Thioredoxin-interacting protein mediates NALP3 inflammasome activation in podocytes during diabetic nephropathy

Numerous studies have shown that the NALP3 inflammasome plays an important role in various immune and inflammatory diseases. However, whether the NALP3 inflammasome is involved in the pathogenesis of diabetic nephropathy (DN) is unclear. In our study, we confirmed that high glucose (HG) concentrations induced NALP3 inflammasome activation both in vivo and in vitro. Blocking NALP3 inflammasome activation by NALP3/ASC shRNA and caspase-1 inhibition prevented IL-1β production and eventually attenuated podocyte and glomerular injury under HG conditions. We also found that thioredoxin (TRX)-interacting protein (TXNIP), which is a pro-oxidative stress and pro-inflammatory factor, activated NALP3 inflammasome by interacting with NALP3 in HG-exposed podocytes. Knocking down TXNIP impeded NALP3 inflammasome activation and alleviated podocyte injury caused by HG. In summary, the NALP3 inflammasome mediates podocyte and glomerular injury in DN, moreover, TXNIP participates in the formation and activation of the NALP3 inflammasome in podocytes during DN, which represents a novel mechanism of podocyte and glomerular injury under diabetic conditions.

Regulation of CD2-associated protein influences podocyte endoplasmic reticulum stress-mediated apoptosis induced by albumin overload.

Proteinuria is an exacerbating factor of chronic kidney diseases, leading to glomerulosclerosis. However, the molecular mechanisms mediating protein overload-induced podocyte injury are poorly understood. Recent studies have shown that apoptosis mediated by endoplasmic reticulum (ER) stress participated in the progression of a variety of kidney diseases. In the present study, we investigated the role of CD2-associated protein (CD2AP) in protein overload-induced ER stress and subsequent podocyte apoptosis. Conditionally immortalized mouse podocytes were cultured in vitro and treated with different concentrations of bovine serum albumin (BSA). In addition, CD2AP eukaryotic expression vector or siRNA was transfected into podocytes before exposed to BSA. Albumin endocytosis and podocyte apoptosis were visualized by confocal microscopy. The subcellular organelles were observed by transmission electron microscopy. The expressions of GRP78, caspase-12 and CD2AP were detected by RT-PCR or Western blot analysis. It was found that albumin was endocytosed by podocytes in a time-dependent manner. Accumulation of albumin in podocytes induced ER stress and apoptosis in a concentration-dependent manner as indicated by upregulation of GRP78 and caspase-12. Meanwhile, the subcellular organelles were disrupted and the expression of CD2AP was downregulated by high concentration of albumin. Transfection of CD2AP eukaryotic expression vector into podocytes increased CD2AP expression, depressed GRP78 and caspase-12 expressions, and inhibited podocyte apoptosis. In contrast, transfection of CD2AP siRNA deteriorated the above changes induced by BSA. It is concluded protein overload induces podocyte apoptosis via ER stress and CD2AP may play a crucial role in albumin overload-induced ER stress and apoptosis in podocytes.

NADPH Oxidase-Induced NALP3 Inflammasome Activation Is Driven by Thioredoxin-Interacting Protein Which Contributes to Podocyte Injury in Hyperglycemia

Diabetic nephropathy (DN) is one of the major causes of end-stage renal disease, and previously we demonstrated that NALP3 inflammasome was involved in the pathogenesis of DN. Here we investigated the mechanisms of NALP3 inflammasome activation in podocyte injury during DN. We found that, besides the activation of NALP3 inflammasome and upregulated thioredoxin-interacting protein (TXNIP), the glomerular expression of gp91phox, a subunit of NADPH oxidase, was enhanced in DN mice simultaneously. Inhibiting NADPH oxidase abrogated NALP3 inflammasome activation, and IL-1β production and eventually protected podocytes from high glucose- (HG-) induced injury. TXNIP, an inhibitor of thioredoxin, acts as a suppressor for antioxidant defense system. Our observation indicated that in HG-exposed podocytes genetic deletion of TXNIP by shRNA reversed gp91phox overexpression and alleviated the injury of podocyte. Collectively, our findings proposed that HG-induced NADPH oxidase activation was driven by TXNIP which subsequently triggered NALP3 inflammasome activation in podocytes and ultimately led to podocyte injury, and blocking TXNIP/NADPH oxidase signaling may be a promising treatment for DN.

Role of CD2-associated protein in albumin overload-induced apoptosis in podocytes.

Proteinuria is a well‐established exacerbating factor of chronic kidney diseases. However, the harmful effects of protein overload on podocytes and the underlying mechanisms are still poorly understood. In the present study, we examined the effects of high concentrations of albumin on podocytes and investigated the role of CD2AP (CD2‐associated protein) in albumin overload‐induced podocyte apoptosis. Conditionally immortalized mouse podocytes were cultured in vitro and treated with different concentrations of BSA. In addition, CD2AP eukaryotic expression vector or siRNA (small interfering RNA) was transfected into podocytes before they were exposed to BSA. Podocyte apoptosis, expressions of active caspase‐3 (p17) and CD2AP, and the distribution of F‐actin cytoskeleton were detected by flow cytometry, Western‐blot analysis and fluorescent staining respectively. It was found that exposure of podocytes to BSA induced podocyte apoptosis in a concentration‐dependent manner that was accompanied by up‐regulation of active caspase‐3, the disruption of F‐actin cytoskeleton, and decreased expression of CD2AP. Transfection of CD2AP eukaryotic expression vector into podocytes increased CD2AP expression, partially restored F‐actin distribution, blocked active caspase‐3 expression and inhibited podocyte apoptosis. In contrast, transfection of CD2AP siRNA deteriorated the above changes induced by BSA. It is concluded that protein overload induces podocyte apoptosis via the down‐regulation of CD2AP and subsequent disruption of cytoskeleton of podocytes, and CD2AP may play an important role in protein overload‐induced podocyte injury.

Actin-associated Proteins in the Pathogenesis of Podocyte Injury.

Podocytes have a complex cellular architecture with interdigitating processes maintained by a precise organization of actin filaments. The actin-based foot processes of podocytes and the interposed slit diaphragm form the final barrier to proteinuria. The function of podocytes is largely based on the maintenance of the normal foot process structure with actin cytoskeleton. Cytoskeletal dynamics play important roles during normal podocyte development, in maintenance of the healthy glomerular filtration barrier, and in the pathogenesis of glomerular diseases. In this review, we focused on recent findings on the mechanisms of organization and reorganization of these actin-related molecules in the pathogenesis of podocyte injury and potential therapeutics targeting the regulation of actin cytoskeleton in podocytopathies.

Albumin Modulates the Production of Matrix Metalloproteinases-2 and -9 in Podocytes

To investigate the effects of albumin on the production of matrix metalloproteinases-2 (MMP-2) and matrix metalloproteinases-9 (MMP-9) in podocytes. Podocytes were treated with bovine serum albumin (BSA) at the concentration of 0.1, 0.5, 1, 2 g/L, respectively. Conditioned media were harvested 12, 24, 48 and 72 h after the treatment. The expression of MMP-2 and MMP-9 was assayed by gelatin zymography, RT-PCR and Western blotting analysis. Our results showed that in comparison with the control group, BSA increased the expression of MMP-2 and MMP-9 mRNA and protein in a dose- and time-dependent manner (P<0.05). Meanwhile, the enzymatic activities of MMP-2 and MMP-9 in the culture supernatants of podocytes were also increased (P<0.05). It is concluded that albumin up-regulated the expression of MMP-2 and MMP-9 at gene and protein levels in a time- and dose-dependent manner.SummaryTo investigate the effects of albumin on the production of matrix metalloproteinases-2 (MMP-2) and matrix metalloproteinases-9 (MMP-9) in podocytes. Podocytes were treated with bovine serum albumin (BSA) at the concentration of 0.1, 0.5, 1, 2 g/L, respectively. Conditioned media were harvested 12, 24, 48 and 72 h after the treatment. The expression of MMP-2 and MMP-9 was assayed by gelatin zymography, RT-PCR and Western blotting analysis. Our results showed that in comparison with the control group, BSA increased the expression of MMP-2 and MMP-9 mRNA and protein in a dose- and time-dependent manner (P<0.05). Meanwhile, the enzymatic activities of MMP-2 and MMP-9 in the culture supernatants of podocytes were also increased (P<0.05). It is concluded that albumin up-regulated the expression of MMP-2 and MMP-9 at gene and protein levels in a time- and dose-dependent manner.

Tumor Necrosis Factor-Alpha and 8-Hydroxy-2'-Deoxyguanosine are Associated with Elevated Urinary Angiopoietin-2 Level in Type 2 Diabetic Patients with Albuminuria.

Background/Aims: We previously showed that urine and serum Angiopoietin-2 (Ang-2) levels increased significantly with the degree of albuminuria in diabetes patients, but the reasons remain unclear. Consequently we aimed to determine whether there was an association between Ang-2, inflammatory cytokines (TNF-α and IL-18) and reactive oxygen species (8-OHdG and SOD) in type 2 diabetes patients with albuminuria. Methods: This retrospective study evaluated 113 patients with type 2 diabetes and normoalbuminuria, microalbuminuria, or macroalbuminuria and 30 healthy controls. Serum and urine TNF-α, IL-18 and 8-OHdG levels were measured by ELISA. Superoxide dismutase (SOD) activity was determined by spectrophotometry. Results: Serum and urine TNF-α, IL-18 and 8-OHdG levels increased significantly with the degree of albuminuria, and were positively correlated with increased Ang-2. In contrast, SOD activity decreased with the degree of albuminuria and was negatively correlated with Ang-2. Multivariable linear regression analysis revealed that serum Ang-2 level was independently associated with serum levels of TNF-α (P<0.001), 8-OHdG (P=0.001), and IL-18 (P=0.003). Urinary Ang-2 level was independently associated with urinary TNF-α (P<0.001) and 8-OHdG (P=0.004) levels. Conclusion: TNF-α and 8-OHdG are associated with elevated urinary Angiopoietin-2 levels in type 2 diabetic patients with albuminuria.

MAD2B contributes to podocyte injury of diabetic nephropathy via inducing cyclin B1 and Skp2 accumulation

It is well documented that mitotic arrest deficiency (MAD)2B can inhibit the anaphase-promoting complex/cyclosome (APC/C) via cadherin (Cdh)1 and, consequently, can destroy the effective mitotic spindle checkpoint control. Podocytes have been observed to rapidly detach and die when being forced to bypass cell cycle checkpoints. However, the role of MAD2B, a cell cycle regulator, in podocyte impairment of diabetic nephropathy (DN) is unclear. In the present study, we investigated the significance of MAD2B in the pathogenesis of DN in patients, an animal model, and in vitro podocyte cultures. By Western blot and immunohistochemistry analyses, we found that MAD2B was evidently upregulated under high glucose milieu in vivo and in vitro, whereas Cdh1 was inhibited with high glucose exposure. Overexpression of MAD2B in podocytes by plasmid DNA transfection suppressed expression of Cdh1 and triggered the accumulation of cyclin B1 and S phase kinase-associated protein (Skp)2, two key molecules involving in cell cycle regulation, and the subsequent podocyte insult. In contrast, MAD2B deletion alleviated the high glucose-induced reduction of Cdh1 as well as the elevation of cyclin B1 and Skp2, which rescued the podocyte from damage. Taken together, our data demonstrate that MAD2B may play an important role in high glucose-mediated podocyte injury of DN via modulation of Cdh1, cyclin B1, and Skp2 expression.