The protective role of Sirt1 in renal damage was investigated. Sirt1 in proximal tubules (PT) was downregulated before albuminuria occurred in streptozotocin-induced or obese-type (db/db) diabetic mice. PT-specific Sirt1 transgenic (TG) and knockout (KO) mice showed prevention and aggravation of the glomerular changes occurring in diabetes, respectively, and non-diabetic KO mice exhibited albuminuria, suggesting that Sirt1 in PT affects glomerular function. Downregulation of Sirt1 and upregulation of the tight junction protein Claudin-1 by Sirt1-mediated epigenetic regulation in podocytes contributed to albuminuria. These phenomena were not observed in 5/6 nephrectomized mice. We also demonstrated retrograde interplay from PT to glomeruli using nicotinamide mononucleotide (NMN) from conditioned medium, measurement of the auto-fluorescence of photoactivatable NMN, and injection of fluorescence-labeled NMN. In human subjects with diabetes, Sirt1 and Claudin-1 levels were correlated with proteinuria level. Sirt1 in PT protects against albuminuria in diabetes through maintaining NMN concentrations around glomeruli and controlling podocyte function.

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Although endothelial dysfunction, defined as abnormal vasoreactivity, is a common early finding in individuals with type 2 diabetes, the endothelium has not been known to regulate metabolism. As PPARγ, a transcriptional regulator of energy balance, is expressed in endothelial cells, we set out to investigate the role of endothelial cell PPARγ in metabolism using mice that lack PPARγ in the endothelium and BM (γEC/BM-KO). When γEC/BM-KO mice were fed a high-fat diet, they had decreased adiposity and increased insulin sensitivity compared with control mice, despite increased serum FFA and triglyceride (TG) levels. After fasting or olive oil gavage, γEC/BM-KO mice exhibited significant dyslipidemia and failed to respond to the FFA and TG lowering effects of the PPARγ agonist rosiglitazone. BM transplantation studies, which reconstituted hematopoietic PPARγ, established that these metabolic phenotypes were due to endothelial PPARγ deficiency. We further found that the impairment in TG-rich lipoprotein metabolism in γEC/BM-KO mice was associated with fatty acid-mediated lipoprotein lipase inhibition and changes in a PPARγ-regulated endothelial cell transcriptional program. Despite their metabolic improvements, high-fat diet-fed γEC/BM-KO mice had impaired vasoreactivity. Taken together, these data suggest that PPARγ in the endothelium integrates metabolic and vascular responses and may contribute to the effects of PPARγ agonists, thus expanding what endothelial function and dysfunction may entail.

Under diabetic conditions, sodium–glucose cotransporter 2 (SGLT2) for glucose uptake in proximal tubules (PTs) increases, whereas NAD+-dependent protein deacetylase silent mating type information regulation 2 homolog 1 (Sirtuin-1; SIRT1) for PT survival decreases. Therefore, we hypothesized that increased glucose influx by SGLT2 reduces SIRT1 expression. To test this hypothesis, db/db mice with diabetes and high-glucose (HG)-cultured porcine PT LLC-PK1 cells in a two-chamber system were treated with the SGLT2 inhibitor canagliflozin. We also examined SIRT1 and SGLT2 expression in human kidney biopsies. In db/db mice, SGLT2 expression increased with concomitant decreases in SIRT1, but was inhibited by canagliflozin. For determination of the polarity of SGLT2 and SIRT1 expression, LLC-PK1 cells were seeded into Transwell chambers (pore size, 0.4 µm; Becton Dickinson, Oxford, UK). HG medium was added to either or to both of the upper and lower chambers, which corresponded to the apical and basolateral sides of the cells, respectively. In this system, the lower chamber with HG showed increased SGLT2 and decreased SIRT1 expression. Canagliflozin reversed HG-induced SIRT1 downregulation. Gene silencing and inhibitors for glucose transporter 2 (GLUT2) blocked HG-induced SGLT2 expression upregulation. Gene silencing for the hepatic nuclear factor-1α (HNF-1α), whose nuclear translocation was enhanced by HG, blocked HG-induced SGLT2 expression upregulation. Similarly, gene silencing for importin-α1, a chaperone protein bound to GLUT2, blocked HG-induced HNF-1α nuclear translocation and SGLT2 expression upregulation. In human kidney, SIRT1 immunostaining was negatively correlated with SGLT2 immunostaining. Thus, under diabetic conditions, SIRT1 expression in PTs was downregulated by an increase in SGLT2 expression, which was stimulated by basolateral HG through activation of the GLUT2/importin-α1/HNF-1α pathway.

Rho-kinase pathway is involved in the pathogenesis of renal injury. Furthermore, the inhibition of Rho-kinase may constitute a therapeutic strategy for the treatment of renal injury in part through the p27kip1 up-regulation and the subsequent inhibition of cell proliferation and macrophage recruitment.

Abstract-Although peroxisome proliferator-activated receptor ␥ (PPAR␥) ligands have an antihypertensive effect in vivo, the precise mechanism has not been fully elucidated. We examined their effects on Rho/Rho kinase pathway, a key regulator of vascular tone. In cultured rat aortic smooth muscle cells (RASMC), Rho kinase stimulated by angiotensin II was suppressed by the pretreatment with pioglitazone and troglitazone, and these effects were explained by the inhibition of the Rho translocation to the cell membrane. We evaluated the role of Vav, a GTP/GDP exchange factor upregulating Rho kinase activity, and Src homology region 2-containing protein tyrosine phosphatase-2 (SHP-2), a protein tyrosine phosphatase that dephosphorylated Vav and subsequently inactivated Rho kinase. Both pioglitazone and troglitazone upregulated SHP-2, particularly in the cytosolic fraction, and the SHP-2-bound Vav, and reduced the phosphorylation of Vav. Furthermore, 4-week treatment with pioglitazone lowered systolic blood pressure in spontaneously hypertensive rats (SHR) and suppressed the Rho/Rho kinase activity in aortic tissues isolated from SHR. Consistently, the expression of SHP-2 was upregulated in vascular tissues from pioglitazone-treated SHR. The phosphorylated Vav was increased in SHR, compared with that in normotensive Wistar-Kyoto rats (WKY), which was mitigated by pioglitazone. Finally, both basal and angiotensin II-stimulated levels of Rho kinase activity were greater in RASMC from SHR than those from WKY, and the enhanced Rho kinase activity was blocked by pioglitazone or troglitazone in both strains. Collectively, PPAR␥ ligands inhibit the Rho/Rho kinase pathway through upregulation of cytosolic SHP-2 expression and inactivation of Vav, and may contribute to the hemodynamic, in addition to metabolic, action in hypertensive metabolic syndrome.

Nicotinamide N-methyltransferase (NNMT) catalyses the reaction between nicotinamide (NAM) and S-adenosylmethionine to produce 1-methylnicotinamide and S-adenosylhomocysteine. Recently, this enzyme has also been reported to modulate hepatic nutrient metabolism, but its role in the liver has not been fully elucidated. We developed transgenic mice overexpressing NNMT to elucidate its role in hepatic nutrient metabolism. When fed a high fat diet containing NAM, a precursor for nicotinamide adenine dinucleotide (NAD)+, these NNMT-overexpressing mice exhibit fatty liver deterioration following increased expression of the genes mediating fatty acid uptake and decreased very low-density lipoprotein secretion. NNMT overactivation decreased the NAD+ content in the liver and also decreased gene activity related to fatty acid oxidation by inhibiting NAD+–dependent deacetylase Sirt3 function. Moreover, the transgenic mice showed liver fibrosis, with the induction of inflammatory and fibrosis genes. Induced NNMT expression decreased the tissue methylation capacity, thereby reducing methylation of the connective tissue growth factor (CTGF) gene promoter, resulting in increased CTGF expression. These data indicate that NNMT links the NAD+ and methionine metabolic pathways and promotes liver steatosis and fibrosis. Therefore, targeting NNMT may serve as a therapeutic strategy for treating fatty liver and fibrosis.

Rho-kinase plays an important role in hypertension and is reported to interfere with insulin signaling through serine phosphorylation of insulin receptor substrate-1 (IRS-1) in cultured vascular smooth muscle cells. We therefore examined the role of Rho-kinase in the development of insulin resistance in Zucker obese rats. In skeletal muscles and aortic tissues of Zucker obese rats, activation of RhoA/Rho-kinase was observed. Long-term Rho-kinase inhibition by 4 wk treatment with fasudil (a Rho-kinase inhibitor) not only reduced blood pressure but corrected glucose and lipid metabolism, with improvement in serine phosphorylation of IRS-1 and insulin signaling in skeletal muscles. Direct visualization of skeletal muscle arterioles with an intravital CCD videomicroscope demonstrated that both acetylcholine- and sodium nitroprusside-induced vasodilations were blunted, which were restored by the fasudil treatment. Furthermore, both fasudil and Y-27632 prevented the serine phosphorylation of IRS-1 induced by insulin and/or tumor necrosis factor-alpha in skeletal muscle cells. Collectively, Rho-kinase is responsible for the impairment of insulin signaling and may constitute a critical mediator linking between metabolic and hemodynamic abnormalities in insulin resistance.