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Chrebp Regulates Fructose-Induced Glucose Production Independently of Insulin Signaling

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Chrebp Regulates Fructose-Induced Glucose Production Independently of Insulin Signaling ChREBP regulates fructose-induced glucose production independently of insulin signaling Mi-Sung Kim, … , Michelle Lai, Mark A. Herman J Clin Invest. 2016;126(11):4372-4386. https://doi.org/10.1172/JCI81993. Research Article Endocrinology Metabolism Obese, insulin-resistant states are characterized by a paradoxical pathogenic condition in which the liver appears to be selectively insulin resistant. Specifically, insulin fails to suppress glucose production, yet successfully stimulates de novo lipogenesis. The mechanisms underlying this dysregulation remain controversial. Here, we hypothesized that carbohydrate-responsive element-binding protein (ChREBP), a transcriptional activator of glycolytic and lipogenic genes, plays a central role in this paradox. Administration of fructose increased hepatic hexose-phosphate levels, activated ChREBP, and caused glucose intolerance, hyperinsulinemia, hypertriglyceridemia, and hepatic steatosis in mice. Activation of ChREBP was required for the increased expression of glycolytic and lipogenic genes as well as glucose-6- phosphatase (G6pc) that was associated with the effects of fructose administration. We found that fructose-induced G6PC activity is a major determinant of hepatic glucose production and reduces hepatic glucose-6-phosphate levels to complete a homeostatic loop. Moreover, fructose activated ChREBP and induced G6pc in the absence of Foxo1a, indicating that carbohydrate-induced activation of ChREBP and G6PC dominates over the suppressive effects of insulin to enhance glucose production. This ChREBP/G6PC signaling axis is conserved in humans. Together, these findings support a carbohydrate-mediated, ChREBP-driven mechanism that contributes to hepatic insulin resistance. Find the latest version: https://jci.me/81993/pdf RESEARCH ARTICLE The Journal of Clinical Investigation ChREBP regulates fructose-induced glucose production independently of insulin signaling Mi-Sung Kim,1 Sarah A. Krawczyk,1 Ludivine Doridot,1 Alan J. Fowler,1 Jennifer X. Wang,2 Sunia A. Trauger,2 Hye-Lim Noh,3 Hee Joon Kang,3 John K. Meissen,4 Matthew Blatnik,4 Jason K. Kim,3 Michelle Lai,5 and Mark A. Herman1,6 1Division of Endocrinology, Beth Israel Deaconess Medical Center, Boston, Massachusetts, USA. 2Faculty of Arts and Sciences, Harvard University, Cambridge, Massachusetts, USA. 3Program in Molecular Medicine, University of Massachusetts Medical School, Worcester, Massachusetts, USA. 4Department of Pharmacokinetics, Dynamics and Metabolism, Pfizer Global Research and Development, Groton, Connecticut, USA. 5Division of Gastroenterology, Beth Israel Deaconess Medical Center, Boston, Massachusetts, USA. 6Broad Institute, Cambridge, Massachusetts, USA. Obese, insulin-resistant states are characterized by a paradoxical pathogenic condition in which the liver appears to be selectively insulin resistant. Specifically, insulin fails to suppress glucose production, yet successfully stimulates de novo lipogenesis. The mechanisms underlying this dysregulation remain controversial. Here, we hypothesized that carbohydrate- responsive element-binding protein (ChREBP), a transcriptional activator of glycolytic and lipogenic genes, plays a central role in this paradox. Administration of fructose increased hepatic hexose-phosphate levels, activated ChREBP, and caused glucose intolerance, hyperinsulinemia, hypertriglyceridemia, and hepatic steatosis in mice. Activation of ChREBP was required for the increased expression of glycolytic and lipogenic genes as well as glucose-6-phosphatase (G6pc) that was associated with the effects of fructose administration. We found that fructose-induced G6PC activity is a major determinant of hepatic glucose production and reduces hepatic glucose-6-phosphate levels to complete a homeostatic loop. Moreover, fructose activated ChREBP and induced G6pc in the absence of Foxo1a, indicating that carbohydrate-induced activation of ChREBP and G6PC dominates over the suppressive effects of insulin to enhance glucose production. This ChREBP/G6PC signaling axis is conserved in humans. Together, these findings support a carbohydrate-mediated, ChREBP-driven mechanism that contributes to hepatic insulin resistance. Introduction tor, proximal to the proposed branch points in insulin signaling, The metabolic syndrome is a cluster of disorders including obe- thereby precluding differential insulin signaling as a mechanism sity, nonalcoholic fatty liver disease (NAFLD), hypertriglyceri- to explain this paradox (7). It has also been suggested that insulin’s demia, and insulin resistance, which predisposes to the devel- acute effect of reducing hepatic glucose production (HGP) is inde- opment of type 2 diabetes and cardiovascular disease (1). This pendent of hepatic insulin signaling altogether and instead depends syndrome is also characterized by a pathogenic paradox in which on suppression of adipose lipolysis and delivery of fatty-acid sub- the liver appears selectively insulin resistant: hyperinsulinemia is strate to the liver (8, 9). However, this contrasts with other studies unable to appropriately suppress glucose production, but the liver indicating that insulin’s acute effect of reducing glucose production remains sensitive to insulin’s ability to stimulate de novo lipogen- is mediated through insulin’s actions directly on the liver (10, 11). esis (DNL) (2). These studies do not address the observation that hyperinsulinemia Despite extensive investigation, the mechanisms accounting fails to suppress hepatic glucose-6-phosphatase (G6PC) expression for this apparent paradox remain controversial. Investigation at in insulin-resistant humans (12), a condition where increased DNL the level of hepatic insulin signaling suggested that different arms in association with steatosis and hypertriglyceridemia is readily of the insulin signaling cascade might be differentially sensitive apparent (13, 14). Understanding the signaling mechanism regulat- to insulin (3–6). For instance, insulin signaling to inhibit the tran- ing G6pc in this context is important, as increased hepatic G6PC is scription factor forkhead box O1a (FOXO1A), which transactivates sufficient to cause glucose intolerance and insulin resistance (15). gluconeogenic enzyme gene expression, might be impaired, while Another mechanism that may contribute to the paradox is insulin-mediated stimulation of a mechanistic target of rapamycin that glucose or gluconeogenic substrate delivered to the liver can (MTOR)/sterol regulatory element–binding transcription factor 1 activate hepatic DNL independently of insulin signaling (16–19). (SREBPF1 or SREBP1) signaling axis, which upregulates a DNL gene Carbohydrate-responsive element-binding protein (ChREBP, also program, remains intact (5). However, others reported that defects known as MLXIPL) is a master transcriptional regulator of glyco- in hepatic insulin signaling occur at the level of the insulin recep- lytic and lipogenic genes and is activated by carbohydrate (CHO) metabolites in key metabolic tissues including liver. In insulin-resis- tant states, when glucose disposal is impaired in peripheral tissues Authorship note: M.S. Kim and S.A. Krawczyk contributed equally to this work. such as skeletal muscle and adipose tissue, shunting of glucose to Conflict of interest: J.K. Meissen and M. Blatnik are employees of Pfizer Inc. Submitted: March 23, 2015; Accepted: August 18, 2016. the liver might activate ChREBP and enhance DNL. For instance, Reference information: J Clin Invest. 2016;126(11):4372–4386. doi:10.1172/JCI81993. genetically ablating Glut4, the insulin-responsive glucose transport- 4372 jci.org Volume 126 Number 11 November 2016 The Journal of Clinical Investigation RESEARCH ARTICLE Figure 1. High-fructose feeding induces metabolic disease. (A) Body weight and (B) fat pad weights (PG, perigonadal; SC, subcutaneous; BAT, brown adipose tissue) were measured in WT male mice fed chow, high-dextrose, or high-fructose diet for 9 weeks (n = 8 per group). (C) Glycemia and (D) serum insulin were measured in the ad libitum–fed state. (E) Serum triglyceride (TG) levels were measured after an overnight fast followed by 3 hours refeeding with chow, glucose, or fructose diet. (F) Hepatic TG levels and representative H&E-stained liver sections. Images were obtained at ×20 magnification. (G) Glucose tolerance test and (H) glycerol tolerance test with incremental areas under the curve (Δ AUC) for this cohort. P values were obtained by 1-way ANOVA. *P < 0.05 versus chow; **P < 0.05 versus all others. Values are the mean ± SEM. er, from skeletal muscle and adipose tissue enhances hepatic DNL It has recently been hypothesized that hepatocyte CHO (20). Additionally, siRNA-mediated knockdown of ChREBP in metabolites (hexose- or triose-phosphates) might not only stimu- ob/ob mice decreases hepatic DNL in the setting of persistent hyper- late ChREBP and activate DNL, but might also serve as a signal insulinemia (21). Thus, hepatic DNL may be regulated by increased to enhance HGP (17, 18). This hypothesis derives in part from the substrate delivery independently of insulin signaling. However, counterintuitive observation that while ChREBP is known to stim- whether increasing intrahepatic CHO metabolites might also signal ulate glycolysis through transactivation of glycolytic genes (22), it to increase glucose production has not been fully explored. may also transactivate expression of G6pc encoding the enzyme jci.org Volume 126 Number 11 November 2016 4373 RESEARCH ARTICLE The Journal of Clinical Investigation
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