Naro Ohashi

Transcriptional induction of Smurf2 ubiquitin ligase by TGF-β

Smad ubiquitination regulatory factor 2 (Smurf2), a ubiquitin ligase for Smads, plays critical roles in the regulation of transforming growth factor‐β (TGF‐β)‐Smad signaling via ubiquitin‐dependent degradation of Smad2 and Smad7. We found that TGF‐β stimulates Smurf2 expression. TGF‐β activated the Smurf2 promoter in a TGF‐β responsive cell lines, whereas IL‐1α, PDGF and epidermal growth factor did not. TGF‐β‐mediated Smurf2 promoter activation was inhibited by Smad7 or an activin receptor‐like kinase 5 inhibitor but not by dominant negative Smad or disruption of Smad‐binding elements in the promoter. Moreover, inhibition of the phosphatidil inositol 3 kinase (PI3K)/Akt pathway suppressed TGF‐β‐mediated Smurf2 induction. These results suggest that TGF‐β stimulates Smurf2 expression by Smad‐independent pathway such as PI3K/Akt pathway via TGF‐β receptor.

Ubiquitin-Dependent Degradation of Smad2 Is Increased in the Glomeruli of Rats with Anti-Thymocyte Serum Nephritis

The overexpression of transforming growth factor (TGF)-beta and Smad-mediated intracellular TGF-beta signaling in the kidney underlies the development of renal scarring from pathological matrix accumulation. However, changes in the Smad proteins during the progression of kidney disease are unclear. In this study, we investigated the regulation of Smad proteins in the glomeruli of rats with anti-thymocyte serum nephritis. We found that Smad2 protein decreased markedly in nephritic glomeruli, whereas no significant changes were observed in the levels of Smad3 and Smad4 proteins. In contrast, the level of Smad2 mRNA in nephritic glomeruli did not differ significantly from that in control glomeruli. Based on recent reports of the ubiquitin-mediated degradation of Smad2, we investigated the degradation and ubiquitination activity directed against Smad2 in glomerular extracts. Both the degradation and ubiquitination of Smad2 were markedly increased in glomerular extracts from rats with nephritis. We also found that Smurf2, a ubiquitin ligase for Smad2, was increased in the nephritic glomerular extracts. These data suggest that the decrease in Smad2 resulted from enhanced ubiquitin-dependent degradation of Smad2 mediated by Smurf2, and is involved in the regulation of Smad2-mediated TGF-beta signaling in nephritic glomeruli.

Prevalence of Enhanced Granular Expression of Thrombospondin Type-1 Domain-Containing 7A in the Glomeruli of Japanese Patients with Idiopathic Membranous Nephropathy

Membranous nephropathy (MN) is a leading cause of nephrotic syndrome in adults. Autoantibodies against M-type phospholipase A2 receptor (PLA2R) and thrombospondin type-1 domain-containing 7A (THSD7A), which mainly belong to the IgG4 subclass, were reported as associated antibodies for the development of MN. Although PLA2R is a major target antigen for idiopathic MN, the prevalence of MN patients seropositive for PLA2R in Japan is lower than that in other countries. In this study, we conducted immunohistochemical analysis of the presence of THSD7A and PLA2R in renal specimens of MN patients to estimate the prevalence of THSD7A/PLA2R-related idiopathic MN in Japan. Enhanced granular expression of THSD7A and PLA2R was detected in 9.1% and 52.7%, respectively, of the patients with idiopathic MN. Although none of patients with secondary MN displayed enhanced granular expression of THSD7A, 5.4% of them had enhanced granular expression of PLA2R. In conclusion, the prevalence of enhanced granular expression of THSD7A in the glomeruli of Japanese patients with idiopathic MN was higher than the prevalence of MN patients seropositive for THSD7A in USA and Europe.

Disturbed circadian rhythm of the intrarenal renin-angiotensin system: relevant to nocturnal hypertension and renal damage

BackgroundThe intrarenal renin-angiotensin system (RAS) plays an important role in the development of hypertension and renal damage. Disruption of diurnal blood pressure (BP) variation is an additional risk factor for renal damage. However, little is known regarding whether intrarenal RAS circadian rhythm exists or if it influences the disruption of diurnal BP and renal damage.MethodsWe investigated the circadian rhythm of urinary angiotensinogen (U-AGT) that reflects intrarenal RAS activity in 14 individuals without chronic kidney disease (CKD) and 36 CKD patients classified according to circadian BP rhythms.ResultsBP values were higher during the daytime than during the nighttime in both individuals without CKD and CKD patients. U-AGT levels were not different between the daytime and nighttime in individuals without CKD, but were significantly higher in the daytime in CKD patients (log U-AGT/creatinine: daytime, 2.39xa0±xa00.99; nighttime, 2.24xa0±xa01.06; pxa0=xa00.001). Furthermore, in CKD patients showing a riser pattern of circadian BP, U-AGT levels did not decrease during the nighttime compared with those in the daytime (log U-AGT/creatinine: daytime, 2.51xa0±xa00.65; nighttime, 2.52xa0±xa00.71; pxa0=xa00.78). Circadian fluctuation of albuminuria and proteinuria occurred parallel to that of the U-AGT levels. U-AGT levels were significantly and positively correlated with the levels of BP and circadian fluctuation of U-AGT was correlated with diurnal BP changes.ConclusionThese data suggest that the circadian rhythm of intrarenal RAS activation may lead to renal damage and hypertension, which are associated with diurnal BP variation.

Reduction of transforming growth factor-β type II receptor is caused by the enhanced ubiquitin-dependent degradation in human renal cell carcinoma

Although dysregulation of transforming growth factor‐β (TGF‐β) signaling is implicated in renal carcinogenesis, its precise mechanism is unknown in renal cell carcinoma (RCC). In our study, we investigated Smad‐mediated TGF‐β signaling pathway and its regulatory mechanisms in surgical samples from patients with RCC. We found that immunoreactivity for nuclear phosphorylated Smad2 was significantly decreased in RCC compared to normal renal tissues, thereby TGF‐β signaling was suggested to be attenuated in RCC tissues. In accordance with the result, transcriptional downregulation of Smad4 and post‐transcriptional downregulation of TGF‐β type II receptor (TβR‐II) were frequently found in RCC tissues compared to normal renal tissues. Next, to clarify the reason why the protein level of TβR‐II was decreased in RCC, we investigated the activities of degradation and ubiquitination of TβR‐II. We found that both proteasome‐mediated degradation and ubiquitination of TβR‐II were markedly enhanced in RCC tissues. Moreover, we found that the level of Smad‐ubiquitination regulatory factor 2 (Smurf2), the E3 ligase for TβR‐II, was increased in RCC tissues of the patients with higher clinical stages compared to the normal tissues and was inversely correlated with the level of TβR‐II. Our results suggest that the low TβR‐II protein level is due to augmented ubiquitin‐dependent degradation via Smurf2 and might be involved in the attenuation of TGF‐β signaling pathway in RCC.

Immunohistochemical study on caveolin-1α in regenerating process of tubular cells in gentamicin-induced acute tubular injury in rats

Caveolin-1, a principal component of caveolae, modulates growth signaling, endocytosis, and intracellular transport. We examined the expression of caveolin-1α and its relation to cell cycle and caveolin-interacting growth factor receptors in regenerating proximal tubules (PTs) after gentamicin-induced acute renal failure in rats. Caveolin-1α appeared in regenerating PTs as early as dayxa04 after last gentamicin, peaked at daysxa06 to 8, and showed cytoplasmic pattern after dayxa08. Immunoelectron microscopy revealed caveolin-1α-positive caveolae on the cell membrane and in cytoplasms in regenerating PTs at daysxa04 to 8 and caveolin-positivity confined to cytoplasms after dayxa010. The number of PT cells with proliferation markers peaked at dayxa06 and decreased afterwards as expression of cyclin-dependent kinase inhibitors increased. Platelet-derived growth factor receptor-β (PDGFR-β) and epidermal growth factor receptor (EGFR) were colocalized with caveolin-1α in proliferating PTs as early as dayxa04. Phosphorylated EGFR increased at dayxa08 and afterwards when caveolins dissociated from EGFR or decreased. In case of PDGFR-β, phosphorylation seemed to be associated with the increase and association of caveolins to the receptors. Our results suggest that transient expression of caveolin-1α in early regenerating PTs might contribute to the regenerating process of PTs through modulating growth factor receptors.

Decrease in Tumor Necrosis Factor-α Receptor-Associated Death Domain Results from Ubiquitin-Dependent Degradation in Obstructive Renal Injury in Rats

Increased expression levels of tumor necrosis factor-alpha (TNFalpha) is involved in tubulointerstitial cell proliferation and apoptosis in obstructive renal injury. Two TNFalpha receptors (TNFRs), TNFR1 and TNFR2, are known to exist. On TNFalpha binding, TNFR1 recruits TNFR-associated death domain (TRADD), an assembly platform to mediate TNFR1 signaling. We investigated postreceptor TRADD regulation in rat kidneys with unilateral ureteral obstruction (UUO). Whereas UUO was associated with increased expression levels of TNFalpha, TNFR1, TNFR2, and TRADD mRNAs, it resulted in the marked decrease of TRADD protein levels (which appeared at day 1 and persisted thereafter) and a slight decrease in TNFR1 protein levels at days 7 and 14. Both ubiquitination and degradation of TRADD were increased in UUO kidneys, degradation of TRADD was stimulated by TNFalpha in HK-2 cells, and TRADD degradation was suppressed by proteasome inhibitor. Inhibition of TNFalpha by soluble TNFR2, etanercept, reduced significantly, although transiently, tubular and interstitial cell proliferation, fibronectin expression, and apoptosis in UUO kidneys, and also suppressed TRADD degradation. These data suggest that the decrease in TRADD resulting from enhanced ubiquitin-dependent degradation is involved in obstructive renal injury. Since TRADD is not incorporated into TNFR2-mediated TNFalpha signaling, the persistent decrease in TRADD, associated with a mild decrease in TNFR1 levels, may function, at least in part, to divert TNFalpha signals toward a TNFR2-mediated pathway in UUO kidneys.

Impaired endogenous nighttime melatonin secretion relates to intrarenal renin-angiotensin system activation and renal damage in patients with chronic kidney disease.

BackgroundActivation of the intrarenal renin–angiotensin system (RAS) plays a critical role in the pathophysiology of chronic kidney disease (CKD) and hypertension. The circadian rhythm of intrarenal RAS activation leads to renal damage and hypertension, which are associated with diurnal blood pressure (BP) variation. The activation of intrarenal RAS following reactive oxygen species (ROS) activation, sympathetic hyperactivity and nitric oxide (NO) inhibition leads to the development of renal damage. Melatonin is a hormone regulating the circadian rhythm, and has multiple functions such as anti-oxidant and anti-adrenergic effects and enhancement of NO bioavailability. Nocturnal melatonin concentrations are lower in CKD patients. However, it is not known if impaired endogenous melatonin secretion is related to BP, intrarenal RAS, or renal damage in CKD patients.MethodsWe recruited 53 CKD patients and conducted 24-h ambulatory BP monitoring. urine was collected during the daytime and nighttime. We investigated the relationship among the melatonin metabolite urinary 6-sulphatoxymelatonin (U-aMT6s), BP, renal function, urinary angiotensinogen (U-AGT), and urinary albumin (U-Alb).ResultsPatients’ U-aMT6s levels were significantly and negatively correlated with clinical parameters such as renal function, systolic BP, U-AGT, and U-Alb, during both day and night. Multiple regression analyses for U-aMT6s levels were performed using age, gender, renal function, and each parameter (BPs, U-AGT or U-Alb), at daytime and nighttime. U-aMT6s levels were significantly associated with U-AGT (βxa0=xa0−0.31, pxa0=xa00.044) and U-Alb (βxa0=xa0−0.25, pxa0=xa00.025) only at night.ConclusionImpaired nighttime melatonin secretion may be associated with nighttime intrarenal RAS activation and renal damage in CKD patients.

The amelioration of renal damage in Skp2-deficient mice canceled by p27 Kip1 deficiency in Skp2-/- p27-/- mice.

SCF-Skp2 E3 ubiquitin ligase (Skp2 hereafter) targets several cell cycle regulatory proteins for degradation via the ubiquitin-dependent pathway. However, the target-specific physiological functions of Skp2 have not been fully elucidated in kidney diseases. We previously reported an increase in Skp2 in progressive nephropathy and amelioration of unilateral ureteral obstruction (UUO) renal injury associated with renal accumulation of p27 in Skp2−/− mice. However, it remains unclear whether the amelioration of renal injury in Skp2−/− mice is solely caused by p27 accumulation, since Skp2 targets several other proteins. Using Skp2−/−p27−/− mice, we investigated whether Skp2 specifically targets p27 in the progressive nephropathy mediated by UUO. In contrast to the marked suppression of UUO renal injury in Skp2−/− mice, progression of tubular dilatation associated with tubular epithelial cell proliferation and tubulointerstitial fibrosis with increased expression of collagen and α-smooth muscle actin were observed in the obstructed kidneys in Skp2−/−p27−/− mice. No significant increases in other Skp2 target proteins including p57, p130, TOB1, cyclin A and cyclin D1 were noted in the UUO kidney in Skp2−/− mice, while p21, c-Myc, b-Myb and cyclin E were slightly increased. Contrary to the ameliorated UUO renal injure by Skp2-deficiency, the amelioration was canceled by the additional p27-deficiency in Skp2−/−p27−/− mice. These findings suggest a pathogenic role of the reduction in p27 targeted by Skp2 in the progression of nephropathy in UUO mice.

Alogliptin improves steroid-induced hyperglycemia in treatment-naïve Japanese patients with chronic kidney disease by decrease of plasma glucagon levels

Background Chronic kidney disease (CKD) is a risk factor for end-stage renal failure and cardiovascular disease, and a strategy to counteract CKD must be established. CKD caused by immunological abnormalities is treated by steroids, frequently resulting in steroid diabetes. Although insulin is the most effective drug against steroid diabetes, administering it to patients can be difficult. Dipeptidyl peptidase-4 (DPP-4) inhibitors were developed for diabetes mellitus with a new mechanism of action. However, their efficacies and mechanisms of action for steroid diabetes are unclear. Material/Methods We studied 11 CKD patients treated with steroids admitted to our hospital (3 men and 8 women; age, 66.0±15.9 years). DPP-4 inhibitor alogliptin was administered for steroid diabetes. Levels of markers related to glucose metabolism were measured before alogliptin treatment and after alogliptin treatment, before the prednisolone dose was reduced. Results Alogliptin treatment significantly increased plasma glucagon-like peptide-1 (GLP-1) levels from 1.16±1.71 pmol/L to 4.48±1.53 pmol/L and significantly reduced levels of plasma glucose recorded 2 h after lunch and hemoglobin A1c (HbA1c). No significant differences were seen in insulin secretory ability of homeostasis model assessment (HOMA) (HOMA-β) and insulin resistance index of HOMA (HOMA-R) before and after alogliptin treatment. In contrast, alogliptin treatment significantly decreased plasma glucagon levels, from 116.1±38.7 pg/mL to 89.6±17.3 pg/mL. Moreover, there were significant correlations among HbA1c, GLP-1, and glucagon levels. Conclusions Alogliptin improves steroid-induced hyperglycemia by decrease of glucagon levels through an increase in plasma GLP-1 levels.