Gut microbiome-derived phenyl sulfate contributes to albuminuria in diabetic kidney disease.

Diabetic kidney disease is a major cause of renal failure that urgently necessitates a breakthrough in disease management. Here we show using untargeted metabolomics that levels of phenyl sulfate, a gut microbiota-derived metabolite, increase with the progression of diabetes in rats overexpressing human uremic toxin transporter SLCO4C1 in the kidney, and are decreased in rats with limited proteinuria. In experimental models of diabetes, phenyl sulfate administration induces albuminuria and podocyte damage. In a diabetic patient cohort, phenyl sulfate levels significantly correlate with basal and predicted 2-year progression of albuminuria in patients with microalbuminuria. Inhibition of tyrosine phenol-lyase, a bacterial enzyme responsible for the synthesis of phenol from dietary tyrosine before it is metabolized into phenyl sulfate in the liver, reduces albuminuria in diabetic mice. Together, our results suggest that phenyl sulfate contributes to albuminuria and could be used as a disease marker and future therapeutic target in diabetic kidney disease.

Chemical chaperon 4-phenylbutyrate protects against the endoplasmic reticulum stress-mediated renal fibrosis in vivo and in vitro.

Renal tubulointerstitial fibrosis is the common and final pathologic change of kidney in end-stage renal disease. Interesting, endoplasmic reticulum (ER) stress is known to contribute to the pathophysiological mechanisms during the development of renal fibrosis. Here, we investigated the effects of chemical chaperon sodium 4-phenylbutyrate (4-PBA) on renal fibrosis in vivo and in vitro. In a rat unilateral ureteral obstruction (UUO) model, 4-PBA mimicked endogenous ER chaperon in the kidneys and significantly reduced glucose regulated protein 78 (GRP78), CCAAT/enhancer binding protein (C/EBP) homologous protein (CHOP), activating transcription factor 4 (ATF4), and phosphorylated JNK protein expressions as well as restored spliced X-box-binding protein 1 (XBP1) expressions in the kidneys of UUO rats. 4-PBA also attenuated the increases of α-smooth muscle actin (α-SMA), connective tissue growth factor (CTGF) protein expressions, tubulointerstitial fibrosis, and apoptosis in the kidneys of UUO rats. Moreover, transforming growth factor (TGF)-ß markedly increased ER stress-associated molecules, profibrotic factors, and apoptotic markers in the renal tubular cells (NRK-52E), all of which could be significantly counteracted by 4-PBA treatment. 4-PBA also diminished TGF-ß-increased CTGF promoter activity and CTGF mRNA expression in NRK-52E cells. Taken together, our results indicated that 4-PBA acts as an ER chaperone to ameliorate ER stress-induced renal tubular cell apoptosis and renal fibrosis.

DNA methyltransferase inhibition restores erythropoietin production in fibrotic murine kidneys.

Renal erythropoietin-producing cells (REPCs) remain in the kidneys of patients with chronic kidney disease, but these cells do not produce sufficient erythropoietin in response to hypoxic stimuli. Treatment with HIF stabilizers rescues erythropoietin production in these cells, but the mechanisms underlying the decreased response of REPCs in fibrotic kidneys to anemic stimulation remain elusive. Here, we show that fibroblast-like FOXD1+ progenitor-derived kidney pericytes, which are characterized by the expression of α1 type I collagen and PDGFRß, produce erythropoietin through HIF2α regulation but that production is repressed when these cells differentiate into myofibroblasts. DNA methyltransferases and erythropoietin hypermethylation are upregulated in myofibroblasts. Exposure of myofibroblasts to nanomolar concentrations of the demethylating agent 5-azacytidine increased basal expression and hypoxic induction of erythropoietin. Mechanistically, the profibrotic factor TGF-ß1 induced hypermethylation and repression of erythropoietin in pericytes; these effects were prevented by 5-azacytidine treatment. These findings shed light on the molecular mechanisms underlying erythropoietin repression in kidney myofibroblasts and demonstrate that clinically relevant, nontoxic doses of 5-azacytidine can restore erythropoietin production and ameliorate anemia in the setting of kidney fibrosis in mice.

Autophagy induction promotes aristolochic acid-I-induced renal injury in vivo and in vitro.

Studies have found that ingestion of aristolochic acid (AA) causes nephropathy first by inducing renal tubular cell apoptosis acutely. It is currently unknown whether crosstalk between autophagy and apoptosis orchestrates the fate of tubular cells in acute AA nephropathy. We tested this hypothesis by acute administration of AA in vivo and in vitro. Autophagy was first induced in vivo through enhancing Atg5 and LC3-II expressions in kidneys of AA-I-treated rats. Punctuate LC3-GFP dots and autophagosomes were detected in this acute AA-I nephropathy rat model. We subsequently utilized normal rat renal proximal tubular epithelial cells (NRK52E) to study the autophagy mechanisms involved in acute AA-I nephropathy, with 100µM AA-I (median lethal dose 50) given in vitro. Cleavage of poly (ADP-ribose) polymerase (PARP), nuclear condensation, and fragmentation were demonstrated in the AA-I-treated NRK52E cells. Furthermore, AA-I induced Atg5 and LC3-II expressions and punctuated LC3-GFP dots. Autophagy flux by using lysosome inhibitor E64 induced the accumulation of LC3-II, which further promoted apoptosis through enhancing PARP cleavage. Inhibition of autophagy by 3-methyl adenine also led to the attenuation of AA-I-induced apoptosis, manifesting as decreased PARP cleavage, nuclei condensation, and decreased the number of cells negative for acridine orange/ethidium bromide staining. In addition, knockdown of Atg5 by short hairpin RNA attenuated LC3-II expression and PARP cleavage in NRK52E cells. Taken together, these findings suggested that the acute phase of AA-I-induced nephropathy is associated with induction of Atg5-dependent autophagy, which promotes renal tubular cell apoptosis.