Various peripheral tissues show circadian rhythmicity, which is generated at the cellular level by their own core oscillators that are composed of transcriptional/translational feedback loops involving a set of clock genes. Although the circulating levels of some adipocytokines, i.e. bioactive substances secreted by adipocytes, are on a 24-h rhythmic cycle, it remains to be elucidated whether the clock gene system works in adipose tissue. To address this issue, we investigated the daily mRNA expression profiles of the clock genes and adipocytokines in mouse perigonadal adipose tissues. In C57BL/6J mice, all transcript levels of the clock genes (Bmal1, Per1, Per2, Cry1, Cry2, and Dbp) and adipocytokines (adiponectin, resistin, and visfatin) clearly showed 24-h rhythms. On the other hand, the rhythmic expression of these genes was mildly attenuated in obese KK mice and greatly attenuated in more obese, diabetic KK-A y mice. Obese diabetes also diminished the rhythmic expression of the clock genes in the liver. Interestingly, a 2-wk treatment of KK and KK-A y mice with pioglitazone impaired the 24-h rhythmicity of the mRNA expression of the clock genes and adipocytokines despite the antidiabetic effect of the drug. In contrast, pioglitazone improved the attenuated rhythmicity in the liver. These findings suggest that the intracellular clock gene system acts in visceral adipose tissues as well as liver and is influenced by the conditions of obesity/type 2 diabetes and pioglitazone treatment.
Claudin-2 is highly expressed in tight junctions of mouse renal proximal tubules, which possess a leaky epithelium whose unique permeability properties underlie their high rate of NaCl reabsorption. To investigate the role of claudin-2 in paracellular NaCl transport in this nephron segment, we generated knockout mice lacking claudin-2 (Cldn2 −/− ). The Cldn2 −/− mice displayed normal appearance, activity, growth, and behavior. Light microscopy revealed no gross histological abnormalities in the Cldn2 −/− kidney. Ultrathin section and freezefracture replica electron microscopy revealed that, similar to those of wild types, the proximal tubules of Cldn2 −/− mice were characterized by poorly developed tight junctions with one or two continuous tight junction strands. In contrast, studies in isolated, perfused S2 segments of proximal tubules showed that net transepithelial reabsorption of Na + , Cl -, and water was significantly decreased in Cldn2 −/− mice and that there was an increase in paracellular shunt resistance without affecting the apical or basolateral membrane resistances. Moreover, deletion of claudin-2 caused a loss of cation (Na + ) selectivity and therefore relative anion (Cl -) selectivity in the proximal tubule paracellular pathway. With free access to water and food, fractional Na + and Cl -excretions in Cldn2 −/− mice were similar to those in wild types, but both were greater in Cldn2 −/− mice after i.v. administration of 2% NaCl. We conclude that claudin-2 constitutes leaky and cation (Na + )-selective paracellular channels within tight junctions of mouse proximal tubules. mouse proximal tubule | tight junction | paracellular transport | Na/Cl transport | water transport T ight junctions (TJs) are circumferential seals around cells that selectively modulate paracellular permeability between extracellular compartments (1-3). On ultrathin-section electron microscopy, TJs appear as foci where the plasma membranes of neighboring cells make complete contact (4). On freeze-fracture electron microscopy, TJs appear as a continuous and anastomosing network of intramembranous particle strands (TJ strands) (5). These strands are mainly composed of linearly polymerized integral membrane proteins called claudins with molecular masses of ∼23 kDa (2, 3, 6). The claudin gene family contains more than 20 members in humans and in mice (2, 3, 7). The expression pattern of claudins varies considerably; most cell types express more than two claudins in various combinations to constitute mosaic TJ strands.Through the formation of TJ strands, claudins are directly involved in creating a primary barrier to the paracellular diffusion of solutes and water across epithelia (8). However, TJs are not a simple barrier: the barrier varies in tightness, measured by the transepithelial electrical resistance (R T ), and charge selectivity. Furuse et al. (9) reported that, when canine claudin-2 cDNA was transfected into high-resistance Madin-Darby canine kidney (MDCK) I cells primarily expressing claudins-1 and -4, the R T decreas...
Vascular smooth muscle cell (VSMC) proliferation is a key event in the progression of arteriosclerosis. Clinical studies show that uremic toxins deteriorate the arteriosclerosis in renal failure patients. Indoxyl sulfate (IS) is a strong protein-bound uremic toxin, but the effect of IS on VSMC proliferation has not been studied. We examined the effect of IS on rat VSMC proliferation, assessed by a cell counting kit (4-[3-[4-lodophenyl]-2-4(4-nitrophenyl)-2H-5-tetrazolio-1,3-benzene disulfonate] assay) and by [(3)H]thymidine incorporation in vitro. We further evaluated a contribution of mitogen-activated protein kinase (MAPK; p44/42 MAPK) to VSMC proliferation by IS. Immunohistochemical staining was performed for VSMCs using antirat organic anion transporter (OAT)3 antibody. The mRNA expressions of platelet-derived growth factor (PDGF)-A and -C chains, and PDGF-beta receptor were evaluated by real-time PCR. IS stimulated the proliferation of VSMCs in a concentration-dependent manner and activated p44/42 MAPK. Concentration of IS needed to stimulate the proliferation of rat VSMC was about 250 microM, which is compatible with that in the serum of end-stage renal failure patients. PD98059 (10 microM), a selective inhibitor of MAPK/extracellular signal-regulated kinase, inhibited the IS-induced (250 microM) VSMC proliferation and phosphorylation of MAPK. Probenecid (0.5 mM), an inhibitor and substrate of OAT, inhibited the IS-induced (250 microM) VSMC proliferation. Rat OAT3 was detected in VSMCs. The mRNA expressions of PDGF-C chain and PDGF-beta receptor were significantly increased by IS. We conclude that IS directly stimulates rat VSMC proliferation and activates MAPK in vitro. This might be one of the mechanisms underlying the progression of atherosclerotic lesions in end-stage renal disease patients.
BackgroundOptimal treatment for nonalcoholic steatohepatitis (NASH) has not yet been established, particularly for individuals without diabetes. We examined the effects of metformin, commonly used to treat patients with type 2 diabetes, on liver pathology in a non-diabetic NASH mouse model.Methodology/Principal FindingsEight-week-old C57BL/6 mice were fed a methionine- and choline-deficient plus high fat (MCD+HF) diet with or without 0.1% metformin for 8 weeks. Co-administration of metformin significantly decreased fasting plasma glucose levels, but did not affect glucose tolerance or peripheral insulin sensitivity. Metformin ameliorated MCD+HF diet-induced hepatic steatosis, inflammation, and fibrosis. Furthermore, metformin significantly reversed hepatic steatosis and inflammation when administered after the development of experimental NASH.Conclusions/SignificanceThese histological changes were accompanied by reduced hepatic triglyceride content, suppressed hepatic stellate cell activation, and the downregulation of genes involved in fatty acid metabolism, inflammation, and fibrogenesis. Metformin prevented and reversed steatosis and inflammation of NASH in an experimental non-diabetic model without affecting peripheral insulin resistance.
The transient receptor vanilloid-4 (TRPV4) is a mechanosensitive, swell-activated cation channel that is abundant in the renal distal tubules. Immunolocalization studies, however, present conflicting data as to whether TRPV4 is expressed along the apical and/or basolateral membranes. To disclose the role of TRPV4 in flow-dependent K(+) secretion in distal tubules in vivo, urinary K(+) excretion and net transports of K(+) and Na(+) in the cortical collecting duct (CCD) were measured with an in vitro microperfusion technique in TRPV4(+/+) and TRPV4(-/-) mice. Both net K(+) secretion and Na(+) reabsorption were flow dependently increased in the CCDs isolated from TRPV4(+/+)mice, which were significantly enhanced by a luminal application of 50 microM 4alpha-phorbol-12,13-didecanoate (4alphaPDD), an agonist of TRPV4. No flow dependence of net K(+) and Na(+) transports or effects of 4alphaPDD on CCDs were observed in TRPV4(-/-) mice. A basolateral application of 4alphaPDD had little effect on these ion transports in the TRPV4(+/+) CCDs, while the luminal application did. Urinary K(+) excretion was significantly smaller in TRPV4(-/-) than in TRPV4(+/+) mice when urine production was stimulated by a venous application of furosemide. These observations suggested an essential role of the TRPV4 channels in the luminal or basolateral membrane as flow sensors in the mechanism underlying the flow-dependent K(+) secretion in mouse CCDs.
Recent studies have demonstrated relationships between the dysfunction of circadian clocks and the development of metabolic abnormalities, but the chicken-and-egg question remains unresolved. To address this issue, we investigated the cause-effect relationship in obese, diabetic ob/ob mice. Compared with control C57BL/6J mice, the daily mRNA expression profiles of the clock and clock-controlled genes Clock, Bmal1, Cry1, Per1, Per2, and Dbp were substantially dampened in the liver and adipose tissue, but not the hypothalamic suprachiasmatic nucleus, of 10-wk-old ob/ob mice. Four-week feeding of a low-calorie diet and administration of leptin over a 7-d period attenuated, to a significant and comparable extent, the observed metabolic abnormalities (obesity, hyperglycemia, hyperinsulinemia, and hypercholesterolemia) in the ob/ob mice. However, only leptin treatment improved the impaired peripheral clocks. In addition, clock function, assessed by measuring levels of Per1, Per2, and Dbp mRNA at around peak times, was also reduced in the peripheral tissues of 3-wk-old ob/ob mice without any overt metabolic abnormalities. Collectively these results indicate that the impairment of peripheral clocks in ob/ob mice does not result from metabolic abnormalities but may instead be at least partially caused by leptin deficiency itself. Further studies are needed to clarify how leptin deficiency affects peripheral clocks.
Aim/hypothesis Recent studies have demonstrated relationships between circadian clock function and the development of metabolic diseases such as type 2 diabetes. We investigated whether the peripheral circadian clock is impaired in patients with type 2 diabetes. Methods Peripheral leucocytes were obtained from eight patients with diabetes and six comparatively young nondiabetic volunteers at 09:00, 15:00, 21:00 and 03:00 hours (study 1) and from 12 male patients with diabetes and 14 agematched men at 09:00 hours (study 2). Transcript levels of clock genes (CLOCK, BMAL1 [also known as ARNTL], PER1, PER2, PER3 and CRY1) were determined by real-time quantitative PCR.Results In study 1, mRNA expression patterns of BMAL1, PER1, PER2 and PER3 exhibited 24 h rhythmicity in the leucocytes of all 14 individuals. The expression levels of these mRNAs were significantly (p<0.05) lower in patients with diabetes than in non-diabetic individuals at one or more time points. Moreover, the amplitudes of mRNA expression rhythms of PER1 and PER3 genes tended to diminish in patients with diabetes. In study 2, leucocytes obtained from patients with diabetes expressed significantly (p<0.05) lower transcript levels of BMAL1, PER1 and PER3 compared with leucocytes from control individuals, and transcript expression was inversely correlated with HbA 1c levels (ρ=−0.47 to −0.55, p<0.05). Conclusions/interpretation These results suggest that rhythmic mRNA expression of clock genes is dampened in peripheral leucocytes of patients with type 2 diabetes. The impairment of the circadian clock appears to be closely associated with the pathophysiology of type 2 diabetes in humans.
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