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Ji-Eun Kim, Seung Eun Song, Yong-Woon Kim, Jong-Yeon Kim, Sung-Chul Park, Yoon-Ki Park, Suk-Hwan Baek, In Kyu Lee and So-Young Park

The present study examined whether adiponectin can inhibit palmitate-induced apoptosis, and also the associated mechanisms and signal transduction pathways in human umbilical vein endothelial cells. Cells treated with 500 μM palmitate for 48 h increased reactive oxygen species (ROS) generation and induced apoptosis. Treatment with antioxidant N-acetyl-l-cysteine (1 mM) and globular adiponectin (5 μg/ml) inhibited palmitate-induced ROS generation and apoptosis. The AMP-activated protein kinase (AMPK) activator 5-aminoimidazole-4-carboxamide-1-β-d-ribofuranoside (AICAR; 1 mM), and cAMP activators forskolin (10 μM) and cholera toxin (200 ng/ml) also displayed the same effects. The inhibitory effects of adiponectin on ROS generation and apoptosis were reversed by the AMPK inhibitor compound C (40 μM), cAMP inhibitor SQ22536 (50 μM), and protein kinase A (PKA) inhibitor H-89 (10 μM). The inhibitory effect of forskolin on palmitate-induced apoptosis was reversed by compound C, whereas the inhibitory effect of AICAR was not reversed by SQ22536 and H-89. AICAR and forskolin could not inhibit palmitate-induced apoptosis in cells treated with dominant-negative AMPK. Forskolin increased phosphorylated AMPK at both Thr-172 and Ser-485/491. These results suggest that adiponectin inhibits palmitate-induced apoptosis by suppression of ROS generation via both the cAMP/PKA and AMPK pathways. Interaction between cAMP/PKA and AMPK pathways may be involved.

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Yusuke Seino, Takashi Miki, Wakako Fujimoto, Eun Young Lee, Yoshihisa Takahashi, Kohtaro Minami, Yutaka Oiso and Susumu Seino

Glucose-induced insulin secretion from pancreatic β-cells critically depends on the activity of ATP-sensitive K+ channels (KATP channel). We previously generated mice lacking Kir6.2, the pore subunit of the β-cell KATP channel (Kir6.2 −/−), that show almost no insulin secretion in response to glucose in vitro. In this study, we compared insulin secretion by voluntary feeding (self-motivated, oral nutrient ingestion) and by forced feeding (intra-gastric nutrient injection via gavage) in wild-type (Kir6.2 + / +) and Kir6.2 −/− mice. Under ad libitum feeding or during voluntary feeding of standard chow, blood glucose levels and plasma insulin levels were similar in Kir6.2 + / + and Kir6.2 −/− mice. By voluntary feeding of carbohydrate alone, insulin secretion was induced significantly in Kir6.2 −/− mice but was markedly attenuated compared with that in Kir6.2 + / + mice. On forced feeding of standard chow or carbohydrate alone, the insulin secretory response was markedly impaired or completely absent in Kir6.2 −/− mice. Pretreatment with a muscarine receptor antagonist, atropine methyl nitrate, which does not cross the blood–brain barrier, almost completely blocked insulin secretion induced by voluntary feeding of standard chow or carbohydrate in Kir6.2 −/− mice. Substantial glucose-induced insulin secretion was induced in the pancreas perfusion study of Kir6.2 −/− mice only in the presence of carbamylcholine. These results suggest that a KATP channel-independent mechanism mediated by the vagal nerve plays a critical role in insulin secretion in response to nutrients in vivo.

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Eun Young Lee, Shuji Kaneko, Promsuk Jutabha, Xilin Zhang, Susumu Seino, Takahito Jomori, Naohiko Anzai and Takashi Miki

Oral ingestion of carbohydrate triggers glucagon-like peptide 1 (GLP1) secretion, but the molecular mechanism remains elusive. By measuring GLP1 concentrations in murine portal vein, we found that the ATP-sensitive K+ (KATP) channel is not essential for glucose-induced GLP1 secretion from enteroendocrine L cells, while the sodium-glucose co-transporter 1 (SGLT1) is required, at least in the early phase (5 min) of secretion. By contrast, co-administration of the α-glucosidase inhibitor (α-GI) miglitol plus maltose evoked late-phase secretion in a glucose transporter 2-dependent manner. We found that GLP1 secretion induced by miglitol plus maltose was significantly higher than that by another α-GI, acarbose, plus maltose, despite the fact that acarbose inhibits maltase more potently than miglitol. As miglitol activates SGLT3, we compared the effects of miglitol on GLP1 secretion with those of acarbose, which failed to depolarize the Xenopus laevis oocytes expressing human SGLT3. Oral administration of miglitol activated duodenal enterochromaffin (EC) cells as assessed by immunostaining of phosphorylated calcium–calmodulin kinase 2 (phospho-CaMK2). In contrast, acarbose activated much fewer enteroendocrine cells, having only modest phospho-CaMK2 immunoreactivity. Single administration of miglitol triggered no GLP1 secretion, and GLP1 secretion by miglitol plus maltose was significantly attenuated by atropine pretreatment, suggesting regulation via vagal nerve. Thus, while α-GIs generally delay carbohydrate absorption and potentiate GLP1 secretion, miglitol also activates duodenal EC cells, possibly via SGLT3, and potentiates GLP1 secretion through the parasympathetic nervous system.

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Jung-Min Koh, Young-Sun Lee, Chang-Hyun Byun, Eun-Ju Chang, Hyunsoo Kim, Yong Hee Kim, Hong-Hee Kim and Ghi Su Kim

Growing evidence has shown a biochemical link between increased oxidative stress and reduced bone density. Although α-lipoic acid (α-LA) has been shown to act as a thiol antioxidant, its effect on bone cells has not been determined. Using proteomic analysis, we identified six differentially expressed proteins in the conditioned media of α-LA-treated human bone marrow stromal cell line (HS-5). One of these proteins, receptor activator of nuclear factor κB ligand (RANKL), was significantly up-regulated, as confirmed by immunoblotting with anti-RANKL antibody. ELISA showed that α-LA stimulated RANKL production in cellular extracts (membranous RANKL) about 5-fold and in conditioned medium (soluble RANKL) about 23-fold, but had no effect on osteoprotegerin (OPG) secretion. Despite increasing the RANKL/OPG ratio, α-LA showed a dose-dependent suppression of osteoclastogenesis, both in a coculture system of mouse bone marrow cells and osteoblasts and in a mouse bone marrow cell culture system, and reduced bone resorption in a dose-dependent manner. In addition, α-LA-induced soluble RANKL was not inhibited by matrix metalloprotease inhibitors, indicating that soluble RANKL is produced by α-LA without any posttranslational processing. In contrast, α-LA had no significant effect on the proliferation and differentiation of HS-5 cells. These results suggest that α-LA suppresses osteoclastogenesis by directly inhibiting RANKL–RANK mediated signals, not by mediating cellular RANKL production. In addition, our findings indicate that α-LA-induced soluble RANKL is not produced by shedding of membranous RANKL.

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Yeon-Ju Lee, Se-Hui Jung, JongYun Hwang, Sohee Jeon, Eun-Taek Han, Won Sun Park, Seok-Ho Hong, Young-Myeong Kim and Kwon-Soo Ha

Cysteamine (an aminothiol), which is derived from coenzyme A degradation and metabolized into taurine, has beneficial effects against cystinosis and neurodegenerative diseases; however, its role in diabetic complications is unknown. Thus, we sought to determine the preventive effect of cysteamine against hyperglycemia-induced vascular leakage in the retinas of diabetic mice. Cysteamine and ethanolamine, the sulfhydryl group-free cysteamine analogue, inhibited vascular endothelial growth factor (VEGF)-induced stress fiber formation and vascular endothelial (VE)-cadherin disruption in endothelial cells, which play a critical role in modulating endothelial permeability. Intravitreal injection of the amine compounds prevented hyperglycemia-induced vascular leakage in the retinas of streptozotocin-induced diabetic mice. We then investigated the potential roles of reactive oxygen species (ROS) and transglutaminase (TGase) in the cysteamine prevention of VEGF-induced vascular leakage. Cysteamine, but not ethanolamine, inhibited VEGF-induced ROS generation in endothelial cells and diabetic retinas. In contrast, VEGF-induced TGase activation was prevented by both cysteamine and ethanolamine. Our findings suggest that cysteamine protects against vascular leakage through inhibiting VEGF-induced TGase activation rather than ROS generation in diabetic retinas.

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Eun Young Lee, Xilin Zhang, Junki Miyamoto, Ikuo Kimura, Tomoaki Tanaka, Kenichi Furusawa, Takahito Jomori, Kosuke Fujimoto, Satoshi Uematsu and Takashi Miki

Mechanisms of carbohydrate-induced secretion of the two incretins; glucagon-like peptide 1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are considered to be mostly similar. However, we found that mice exhibit opposite secretory responses in response to co-administration of maltose plus an α-glucosidase inhibitor miglitol (maltose/miglitol); stimulatory for GLP-1, as reported previously, but inhibitory for GIP. Gut microbiota was shown to be involved in maltose/miglitol-induced GIP suppression, as the suppression was attenuated in antibiotics (Abs)-treated mice and abolished in germ free mice. In addition, maltose/miglitol administration increased plasma levels of short chain fatty acids (SCFAs), carbohydrate-derived metabolites, in the portal vein. GIP suppression by maltose/miglitol was not observed in mice lacking a SCFA receptor Ffar3, but it was normally seen in Ffar2-deficient mice. Similarly to maltose/miglitol administration, co-administration of glucose plus a sodium glucose transporter inhibitor phloridzin (glucose/phloridzin) induced GIP suppression, which was again cancelled by Abs treatment. In conclusion, oral administration of carbohydrates with α-glucosidase inhibitors suppress GIP secretion through a microbiota/SCFA/FFAR3 pathway.

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Jung-Yoon Heo, Ji-Eun Kim, Yongwook Dan, Yong-Woon Kim, Jong-Yeon Kim, Kyu Hyang Cho, Young Kyung Bae, Seung-Soon Im, Kwang-Hyeon Liu, In-Hwan Song, Jae-Ryong Kim, In-Kyu Lee and So-Young Park

Clusterin is a secretory glycoprotein that is involved in multiple physiopathological processes, including lipid metabolism. Previous studies have shown that clusterin prevents hepatic lipid accumulation via suppression of sterol regulatory element-binding protein (SREBP) 1. In this study, we examined the role of clusterin in renal lipid accumulation in clusterin-knockout mice and NRK52e tubular epithelial cells. Clusterin deficiency increased the expression of SREBP1 and its target genes and decreased malonyl-CoA decarboxylase protein levels in the kidney. Expression of the endocytic receptor, megalin, and scavenger receptor class A was increased in clusterin-deficient mice. Functional analysis of lipid metabolism also revealed that lipid uptake and triglyceride synthesis were increased and fatty acid oxidation was reduced, leading to increased lipid accumulation in clusterin-deficient mice. These phenomena were accompanied by mesangial expansion, fibrosis and increased urinary protein-to-creatinine ratio. High-fat feeding aggravated these clusterin deficiency-induced pathological changes. Clusterin knockdown in NRK52e cells increased lipogenic gene expression and lipid levels, whereas overexpression of clusterin by treatment with adenovirus or recombinant clusterin protein suppressed lipogenic gene expression and lipid levels. Transforming growth factor-beta 1 (TGFB1) expression increased in the kidney of clusterin-deficient mice and suppression of TGFB1 in NRK52e cells suppressed lipid accumulation. These results suggest that clusterin deficiency induces renal lipid accumulation by dysregulating the expression of lipid metabolism-related factors and TGFB1, thereby leading to chronic kidney disease. Hence, clusterin may serve as a therapeutic target for lipid-induced chronic kidney disease.

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Mahendra Prasad Bhatt, Yeon-Ju Lee, Se-Hui Jung, Yong Ho Kim, Jong Yun Hwang, Eun-Taek Han, Won Sun Park, Seok-Ho Hong, Young-Myeong Kim and Kwon-Soo Ha

C-peptide exerts protective effects against diabetic complications; however, its role in inhibiting hyperglycemic memory (HGM) has not been elucidated. We investigated the beneficial effect of C-peptide on HGM-induced vascular damage in vitro and in vivo using human umbilical vein endothelial cells and diabetic mice. HGM induced apoptosis by persistent generation of intracellular ROS and sustained formation of ONOO and nitrotyrosine. These HGM-induced intracellular events were normalized by treatment with C-peptide, but not insulin, in endothelial cells. C-peptide also inhibited persistent upregulation of p53 and activation of mitochondrial adaptor p66shc after glucose normalization. Further, C-peptide replacement therapy prevented persistent generation of ROS and ONOO in the aorta of diabetic mice whose glucose levels were normalized by the administration of insulin. C-peptide, but not insulin, also prevented HGM-induced endothelial apoptosis in the murine diabetic aorta. This study highlights a promising role for C-peptide in preventing HGM-induced intracellular events and diabetic vascular damage.

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Seul Gi Kang, Hyon-Seung Yi, Min Jeong Choi, Min Jeong Ryu, Saetbyel Jung, Hyo Kyun Chung, Joon Young Chang, Yong Kyung Kim, Seong Eun Lee, Hyeon-Woo Kim, Hoil Choi, Dong Seok Kim, Ju Hee Lee, Koon Soon Kim, Hyun Jin Kim, Chul-Ho Lee, Yuichi Oike and Minho Shong

Recent studies revealed that the inhibition of mitochondrial oxidative phosphorylation (OXPHOS) is coupled with the mitochondrial unfolded protein response, thereby stimulating the secretion of non-cell autonomous factors, which may control systemic energy metabolism and longevity. However, the nature and roles of non-cell autonomous factors induced in adipose tissue in response to reduced OXPHOS function remain to be clarified in mammals. CR6-interacting factor 1 (CRIF1) is an essential mitoribosomal protein for the intramitochondrial production of mtDNA-encoded OXPHOS subunits. Deficiency of CRIF1 impairs the proper formation of the OXPHOS complex, resulting in reduced function. To determine which secretory factors are induced in response to reduced mitochondrial OXPHOS function, we analyzed gene expression datasets in Crif1-depleted mouse embryonic fibroblasts. Crif1 deficiency preferentially increased the expression of angiopoietin-like 6 (Angptl6) and did not affect other members of the ANGPTL family. Moreover, treatment with mitochondrial OXPHOS inhibitors increased the expression of Angptl6 in cultured adipocytes. To confirm Angptl6 induction in vivo, we generated a murine model of reduced mitochondrial OXPHOS function using adipose tissue-specific Crif1-deficient mice and verified the upregulation of Angptl6 and fibroblast growth factor 21 (Fgf21) in white adipose tissue. Treatment with recombinant ANGPTL6 protein increased oxygen consumption and Pparα expression through the extracellular signal-regulated kinase/mitogen-activated protein kinase pathway in cultured adipocytes. Furthermore, the ANGPTL6-mediated increase in Pparα expression resulted in increased FGF21 expression, thereby promoting β-oxidation. In conclusion, mitochondrial OXPHOS function governs the expression of ANGPTL6, which is an essential factor for FGF21 production in adipose tissue and cultured adipocytes.