Yanqing Zhu

Adiponectin regulates albuminuria and podocyte function in mice

Increased albuminuria is associated with obesity and diabetes and is a risk factor for cardiovascular and renal disease. However, the link between early albuminuria and adiposity remains unclear. To determine whether adiponectin, an adipocyte-derived hormone, is a communication signal between adipocytes and the kidney, we performed studies in a cohort of patients at high risk for diabetes and kidney disease as well as in adiponectin-knockout (Ad(-/-)) mice. Albuminuria had a negative correlation with plasma adiponectin in obese patients, and Ad(-/-) mice exhibited increased albuminuria and fusion of podocyte foot processes. In cultured podocytes, adiponectin administration was associated with increased activity of AMPK, and both adiponectin and AMPK activation reduced podocyte permeability to albumin and podocyte dysfunction, as evidenced by zona occludens-1 translocation to the membrane. These effects seemed to be caused by reduction of oxidative stress, as adiponectin and AMPK activation both reduced protein levels of the NADPH oxidase Nox4 in podocytes. Ad(-/-) mice treated with adiponectin exhibited normalization of albuminuria, improvement of podocyte foot process effacement, increased glomerular AMPK activation, and reduced urinary and glomerular markers of oxidant stress. These results suggest that adiponectin is a key regulator of albuminuria, likely acting through the AMPK pathway to modulate oxidant stress in podocytes.

Pirfenidone Is Renoprotective in Diabetic Kidney Disease

Although several interventions slow the progression of diabetic nephropathy, current therapies do not halt progression completely. Recent preclinical studies suggested that pirfenidone (PFD) prevents fibrosis in various diseases, but the mechanisms underlying its antifibrotic action are incompletely understood. Here, we evaluated the role of PFD in regulation of the extracellular matrix. In mouse mesangial cells, PFD decreased TGF-beta promoter activity, reduced TGF-beta protein secretion, and inhibited TGF-beta-induced Smad2-phosphorylation, 3TP-lux promoter activity, and generation of reactive oxygen species. To explore the therapeutic potential of PFD, we administered PFD to 17-wk-old db/db mice for 4 wk. PFD treatment significantly reduced mesangial matrix expansion and expression of renal matrix genes but did not affect albuminuria. Using liquid chromatography with subsequent electrospray ionization tandem mass spectrometry, we identified 21 proteins unique to PFD-treated diabetic kidneys. Analysis of gene ontology and protein-protein interactions of these proteins suggested that PFD may regulate RNA processing. Immunoblotting demonstrated that PFD promotes dosage-dependent dephosphorylation of eukaryotic initiation factor, potentially inhibiting translation of mRNA. In conclusion, PFD is renoprotective in diabetic kidney disease and may exert its antifibrotic effects, in part, via inhibiting RNA processing.

Uncoupling of ER-mitochondrial calcium communication by transforming growth factor-β

Transforming growth factor-beta (TGF-beta) has been implicated as a key factor in mediating many cellular processes germane to disease pathogenesis, including diabetic vascular complications. TGF-beta alters cytosolic [Ca2+] ([Ca2+]c) signals, which in some cases may result from the downregulation of the IP3 receptor Ca2+ channels (IP3R). Ca2+ released by IP3Rs is effectively transferred from endoplasmic reticulum (ER) to the mitochondria to stimulate ATP production and to allow feedback control of the Ca2+ mobilization. To assess the effect of TGF-beta on the ER-mitochondrial Ca2+ transfer, we first studied the [Ca2+]c and mitochondrial matrix Ca2+ ([Ca2+]m) signals in single preglomerular afferent arteriolar smooth muscle cells (PGASMC). TGF-beta pretreatment (24 h) decreased both the [Ca2+]c and [Ca2+]m responses evoked by angiotensin II or endothelin. Strikingly, the [Ca2+]m signal was more depressed than the [Ca2+]c signal and was delayed. In permeabilized cells, TGF-beta pretreatment attenuated the rate but not the magnitude of the IP(3)-induced [Ca2+]c rise, yet caused massive depression of the [Ca2+]m responses. ER Ca2+ storage and mitochondrial uptake of added Ca2+ were not affected by TGF-beta. Also, TGF-beta had no effect on mitochondrial distribution and on the ER-mitochondrial contacts assessed by two-photon NAD(P)H imaging and electron microscopy. Downregulation of both IP3R1 and IP3R3 was found in TGF-beta-treated PGASMC. Thus, TGF-beta causes uncoupling of mitochondria from the ER Ca2+ release. The sole source of this would be suppression of the IP3R-mediated Ca2+ efflux, indicating that the ER-mitochondrial Ca2+ transfer depends on the maximal rate of Ca2+ release. The impaired ER-mitochondrial coupling may contribute to the vascular pathophysiology associated with TGF-beta production.

Renal type I inositol 1,4,5-trisphosphate receptor is reduced in streptozotocin-induced diabetic rats and mice

The mechanisms underlying glomerular hypertrophy and hyperfiltration in diabetes remain unclear. We have previously demonstrated that the cytokine transforming growth factor-β1 (TGF-β1) is increased in early diabetic kidney disease and TGF-β1 inhibits the expression of the inositol 1,4,5-trisphosphate (IP3)-gated calcium channel, the type I IP3 receptor (IP3R), in mesangial cells. To test the hypothesis that reduced type I IP3R may be important in diabetic kidney disease, we evaluated type I IP3R expression in the kidney of streptozotocin-induced diabetic rats and mice. Two-week-old diabetic rats have decreased renal type I IP3R protein and mRNA levels. Immunostaining of normal rat kidney demonstrated presence of type I IP3R in glomerular and vascular smooth muscle cells, whereas diabetic rats had reduced staining in both compartments. Reduction of type I IP3R also occurred in parallel with renal hypertrophy, increased creatinine clearance, and increased renal TGF-β1 expression in the diabetic rats. Two-week-old diabetic mice also had reduced renal type I IP3R protein and mRNA expression in association with renal hypertrophy and increased TGF-β1 mRNA expression. These findings demonstrate that there is reduced type I IP3R in glomerular and vascular smooth muscle cells in the diabetic kidney, which may contribute to the altered renal vasoregulation and renal hypertrophy of diabetes.The mechanisms underlying glomerular hypertrophy and hyperfiltration in diabetes remain unclear. We have previously demonstrated that the cytokine transforming growth factor-beta1 (TGF-beta1) is increased in early diabetic kidney disease and TGF-beta1 inhibits the expression of the inositol 1,4,5-trisphosphate (IP3)-gated calcium channel, the type I IP3 receptor (IP3R), in mesangial cells. To test the hypothesis that reduced type I IP3R may be important in diabetic kidney disease, we evaluated type I IP3R expression in the kidney of streptozotocin-induced diabetic rats and mice. Two-week-old diabetic rats have decreased renal type I IP3R protein and mRNA levels. Immunostaining of normal rat kidney demonstrated presence of type I IP3R in glomerular and vascular smooth muscle cells, whereas diabetic rats had reduced staining in both compartments. Reduction of type I IP3R also occurred in parallel with renal hypertrophy, increased creatinine clearance, and increased renal TGF-beta1 expression in the diabetic rats. Two-week-old diabetic mice also had reduced renal type I IP3R protein and mRNA expression in association with renal hypertrophy and increased TGF-beta1 mRNA expression. These findings demonstrate that there is reduced type I IP3R in glomerular and vascular smooth muscle cells in the diabetic kidney, which may contribute to the altered renal vasoregulation and renal hypertrophy of diabetes.