Browsing by Author "Tao, Rongya"

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    FoxO3 transcription factor and Sirt6 deacetylase regulate LDL-cholesterol homeostasis via control of the proprotein convertase subtilisin/kexin type 9 (Pcsk9) gene expression
    (2013-08) Tao, Rongya; Xiong, Xiwen; DePinho, Ronald A.; Deng, Chu-Xia; Dong, X. Charlie; Department of Biochemistry and Molecular Biology, IU School of Medicine
    Elevated LDL-cholesterol is a risk factor for the development of cardiovascular disease. Thus, proper control of LDL-cholesterol homeostasis is critical for organismal health. Genetic analysis has identified PCSK9 (proprotein convertase subtilisin/kexin type 9) as a crucial gene in the regulation of LDL-cholesterol via control of LDL receptor degradation. Although biochemical characteristics and clinical implications of PCSK9 have been extensively investigated, epigenetic regulation of this gene is largely unknown. In this work we have discovered that Sirt6, an NAD+-dependent histone deacetylase, plays a critical role in the regulation of the Pcsk9 gene expression in mice. Hepatic Sirt6 deficiency leads to elevated Pcsk9 gene expression and LDL-cholesterol as well. Mechanistically, we have demonstrated that Sirt6 can be recruited by forkhead transcription factor FoxO3 to the proximal promoter region of the Pcsk9 gene and deacetylates histone H3 at lysines 9 and 56, thereby suppressing the gene expression. Also remarkably, overexpression of Sirt6 in high fat diet-fed mice lowers LDL-cholesterol. Overall, our data suggest that FoxO3 and Sirt6, two longevity genes, can reduce LDL-cholesterol levels through regulation of the Pcsk9 gene.
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    Inactivating hepatic follistatin alleviates hyperglycemia
    (Springer Nature, 2018-07) Tao, Rongya; Wang, Caixia; Stöhr, Oliver; Qiu, Wei; Hu, Yue; Miao, Ji; Dong, X. Charlie; Leng, Sining; Stefater, Margaret; Stylopoulos, Nicholas; Lin, Lin; Copps, Kyle D.; White, Morris F.; Biochemistry and Molecular Biology, School of Medicine
    Unsuppressed hepatic glucose production (HGP) contributes substantially to glucose intolerance and diabetes, which can be modeled by the genetic inactivation of hepatic insulin receptor substrate 1 (Irs1) and Irs2 (LDKO mice). We previously showed that glucose intolerance in LDKO mice is resolved by hepatic inactivation of the transcription factor FoxO1 (that is, LTKO mice)-even though the liver remains insensitive to insulin. Here, we report that insulin sensitivity in the white adipose tissue of LDKO mice is also impaired but is restored in LTKO mice in conjunction with normal suppression of HGP by insulin. To establish the mechanism by which white adipose tissue insulin signaling and HGP was regulated by hepatic FoxO1, we identified putative hepatokines-including excess follistatin (Fst)-that were dysregulated in LDKO mice but normalized in LTKO mice. Knockdown of hepatic Fst in the LDKO mouse liver restored glucose tolerance, white adipose tissue insulin signaling and the suppression of HGP by insulin; however, the expression of Fst in the liver of healthy LTKO mice had the opposite effect. Of potential clinical significance, knockdown of Fst also improved glucose tolerance in high-fat-fed obese mice, and the level of serum Fst was reduced in parallel with glycated hemoglobin in obese individuals with diabetes who underwent therapeutic gastric bypass surgery. We conclude that Fst is a pathological hepatokine that might be targeted for diabetes therapy during hepatic insulin resistance.
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    The inhibitory effect of ethanol on Sestrin3 in the pathogenesis of ethanol-induced liver injury
    (American Physiological Society (APS), 2014-07-01) Kang, Xinqin; Petyaykina, Kateryna; Tao, Rongya; Xiong, Xiwen; Dong, X. Charlie; Liangpunsaku, Suthat; Department of Medicine, IU School of Medicine
    Sestrins (Sesns) are a family of stress-sensitive genes that have been suggested to regulate lipid metabolism. Chronic ethanol feeding is known to cause lipid accumulation in hepatocytes. This study was designed to investigate the role of Sesn3 in the pathogenesis of alcohol-induced hepatic steatosis. We demonstrated that ethanol inhibited the expression of Sesn3 in VL-17A cells. Overexpression of Sesn3 ameliorated triglyceride accumulation; downregulation using short hairpin RNA significantly deteriorated triglyceride accumulation in these cells. The expression of Sesn3 was also reduced in mice fed with ethanol for 4 wk. Overexpression of Sesn3 prevented hepatic steatosis, whereas knockdown of Sesn3 worsened hepatic steatosis in ethanol-fed mice. Overexpression of Sesn3 significantly reduced the expression of genes encoding for lipid synthesis through AMPK pathway. Overexpression of Sesn3 augmented the effect of ethanol on phospho-p70 S6 kinase. The levels of hepatic light chain 3, a marker for autophagy, expression were significantly decreased in ethanol-fed mice after Sesn3 gene was knocked down. Our findings suggest that inhibitory effect of ethanol on Sesn3 may play an important role in the development of ethanol-induced fatty liver.
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    Sestrin 3 protein enhances hepatic insulin sensitivity by direct activation of the mTORC2-Akt signaling
    (American Diabetes Association, 2015-04) Tao, Rongya; Xiong, Xiwen; Liangpunsakul, Suthat; Dong, X. Charlie; Department of Biochemistry & Molecular Biology, IU School of Medicine
    Sestrin proteins have been implicated in multiple biological processes including resistance to oxidative and genotoxic stresses, protection against aging-related pathologies, and promotion of metabolic homeostasis; however, the underlying mechanisms are incompletely understood. Some evidence suggests that sestrins may inhibit mTORC1 (mechanistic target of rapamycin complex 1) through inhibition of RagA/B GTPases or activation of AMPK; however, whether sestrins are also involved in mTORC2 regulation and function is unclear. To investigate the functions and mechanisms of Sestrin 3 (Sesn3), we generated Sesn3 liver-specific transgenic and knockout mice. Our data show that Sesn3 liver-specific knockout mice exhibit insulin resistance and glucose intolerance, and Sesn3 transgenic mice were protected against insulin resistance induced by a high-fat diet. Using AMPK liver-specific knockout mice, we demonstrate that the Sesn3 insulin-sensitizing effect is largely independent of AMPK. Biochemical analysis reveals that Sesn3 interacts with and activates mTORC2 and subsequently stimulates Akt phosphorylation at Ser473. These findings suggest that Sesn3 can activate Akt via mTORC2 to regulate hepatic insulin sensitivity and glucose metabolism.
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    Sirt6 Regulates Insulin Secretion from the Pancreatic Beta Cells
    (Office of the Vice Chancellor for Research, 2015-04-17) Xiong, Xiwen; Wang, Gaihong; Tao, Rongya; Wu, Pengfei; Kono, Tatsuyoshi; Tong, Xin; Tersey, Sarah A.; Harris, Robert A.; Evans-Molina, Carmella; Mirmira, Raghavendra G.; Dong, X. Charlie
    Sirt6 is an NAD-dependent histone deacetylase, which is involved in multiple biological processes, including aging, DNA repair, and metabolism; however, it is unclear what its functions in pancreatic beta-cells are. The beta cells play an essential role in metabolic regulation by secreting insulin in response to an elevated glucose concentration in the circulation. To examine the role of Sirt6 in beta cells, we initially used adenovirus-mediated shRNA to knock down the Sirt6 gene expression in a mouse pancreatic beta cell line - MIN6. Knockdown of the Sirt6 gene significantly reduced glucose-stimulated insulin secretion. To further validate this phenotype in vivo, we generated pancreatic beta-cell-specific Sirt6 knockout mice (bKO) using mouse genetic approach. Indeed, the bKO mice showed remarkable impairment in both first and second phases of insulin secretion in response to a glucose load. While morphometric analyses did not reveal significant difference in islet area between wild-type and bKO mice, biochemical analysis of ATP concentrations showed a 22% decrease in bKO mouse islets relative to control wild-type islets after glucose stimulation. To assess mitochondrial function in Sirt6-deficient beta cells, we also performed Seahorse bioenergetics assays in MIN6 cells after the Sirt6 gene was knocked down. Glucose oxidation in mitochondria was decreased 20-30% in Sirt6- knockdown MIN6 cells as compared to the control cells. Since calcium signaling is critical to insulin secretion, we also measured intracellular calcium concentrations using a fluorescent imaging approach. The results showed a significant decrease in cytoplasmic calcium in the bKO islets as compared to the wild-type controls. Overall, our data demonstrate that Sirt6 plays a critical role in the regulation of pancreatic insulin secretion. This work was supported in part by the NIDDK grant R01DK091592.
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    Sirtuin 6 regulates glucose-stimulated insulin secretion in mouse pancreatic beta cells
    (Springer, 2016-01) Xiong, Xiwen; Wang, Gaihong; Tao, Rongya; Wu, Pengfei; Kono, Tatsuyoshi; Li, Kevin; Ding, Wen-Xing; Tong, Xin; Tersey, Sarah A.; Harris, Robert A.; Mirmira, Raghavendra G.; Evans-Molina, Carmella; Dong, X. Charlie; Department of Biochemistry & Molecular Biology, IU School of Medicine
    AIMS/HYPOTHESIS: Sirtuin 6 (SIRT6) has been implicated in ageing, DNA repair and metabolism; however, its function in pancreatic beta cells is unclear. The aim of this study is to elucidate the role of SIRT6 in pancreatic beta cells. METHODS: To investigate the function of SIRT6 in pancreatic beta cells, we performed Sirt6 gene knockdown in MIN6 cells and generated pancreatic- and beta cell-specific Sirt6 knockout mice. Islet morphology and glucose-stimulated insulin secretion (GSIS) were analysed. Glycolysis and oxygen consumption rates in SIRT6-deficient beta cells were measured. Cytosolic calcium was monitored using the Fura-2-AM fluorescent probe (Invitrogen, Grand Island, NY, USA). Mitochondria were analysed by immunoblots and electron microscopy. RESULTS: Sirt6 knockdown in MIN6 beta cells led to a significant decrease in GSIS. Pancreatic beta cell Sirt6 knockout mice showed a ~50% decrease in GSIS. The knockout mouse islets had lower ATP levels compared with the wild-type controls. Mitochondrial oxygen consumption rates were significantly decreased in the SIRT6-deficient beta cells. Cytosolic calcium dynamics in response to glucose or potassium chloride were attenuated in the Sirt6 knockout islets. Numbers of damaged mitochondria were increased and mitochondrial complex levels were decreased in the SIRT6-deficient islets. CONCLUSIONS/INTERPRETATION: These data suggest that SIRT6 is important for GSIS from pancreatic beta cells and activation of SIRT6 may be useful to improve insulin secretion in diabetes.