Hepatic Carbohydrate Response Element Binding Protein Activation Limits Nonalcoholic Fatty Liver Disease Development in a Mouse Model for Glycogen Storage Disease Type 1a

Abstract

Background and Aims Glycogen storage disease (GSD) type 1a is an inborn error of metabolism caused by defective glucose‐6‐phosphatase catalytic subunit (G6PC) activity. Patients with GSD 1a exhibit severe hepatomegaly due to glycogen and triglyceride (TG) accumulation in the liver. We have shown that the activity of carbohydrate response element binding protein (ChREBP), a key regulator of glycolysis and de novo lipogenesis, is increased in GSD 1a. In the current study, we assessed the contribution of ChREBP to nonalcoholic fatty liver disease (NAFLD) development in a mouse model for hepatic GSD 1a. Approach and Results Liver‐specific G6pc–knockout (L‐G6pc−/−) mice were treated with adeno‐associated viruses (AAVs) 2 or 8 directed against short hairpin ChREBP to normalize hepatic ChREBP activity to levels observed in wild‐type mice receiving AAV8–scrambled short hairpin RNA (shSCR). Hepatic ChREBP knockdown markedly increased liver weight and hepatocyte size in L‐G6pc−/− mice. This was associated with hepatic accumulation of G6P, glycogen, and lipids, whereas the expression of glycolytic and lipogenic genes was reduced. Enzyme activities, flux measurements, hepatic metabolite analysis and very low density lipoprotein (VLDL)‐TG secretion assays revealed that hepatic ChREBP knockdown reduced downstream glycolysis and de novo lipogenesis but also strongly suppressed hepatic VLDL lipidation, hence promoting the storage of “old fat.” Interestingly, enhanced VLDL‐TG secretion in shSCR‐treated L‐G6pc−/− mice associated with a ChREBP‐dependent induction of the VLDL lipidation proteins microsomal TG transfer protein and transmembrane 6 superfamily member 2 (TM6SF2), the latter being confirmed by ChIP‐qPCR. Conclusions Attenuation of hepatic ChREBP induction in GSD 1a liver aggravates hepatomegaly because of further accumulation of glycogen and lipids as a result of reduced glycolysis and suppressed VLDL‐TG secretion. TM6SF2, critical for VLDL formation, was identified as a ChREBP target in mouse liver. Altogether, our data show that enhanced ChREBP activity limits NAFLD development in GSD 1a by balancing hepatic TG production and secretion.

The Protective Role of the Carbohydrate Response Element Binding Protein in the Liver: The Metabolite Perspective

“…One consistent finding on liver metabolic intermediates from ChREBP-deficient models irrespective of whether global, liver-selective or short-term repression is raised phosphate ester intermediates of glycolysis including glucose 6-P and phosphoenolpyruvate ( 17 , 55 , 58 , 59 ) and of the pentose phosphate pathway ( 57 ) but not UDP-glucose ( 17 , 55 ) or fructose 2,6-P 2 ( 58 ). The target genes of ChREBP include G6pc ( 23 , 30 ) and the pentose phosphate pathway enzymes ( 8 ).…”

“…Most of the studies using ChREBP knock-down models have provided evidence for a protective role for ChREBP in liver function (Figure 2), particularly during challenge with a fructosecontaining diet (52)(53)(54)(55)(56) or in other compromised metabolic states such as glucose 6-phosphatase deficiency (57). ChREBP knock-down studies have used either total ChREBP deletion (23,52,53,58) or liver-selective models generated by crossing ChREBP fl o x mice with albumin-Cre transgenic mice (LiChREBP -/-) (17,54,55) or by short-term ChREBP knockdown using antisense oligonucleotides (56) or shRNA with adenoviral vectors (57,59). While the focus of the earlier studies was on the liver, ChREBP is also important in the intestine (17) and in adipose tissue (60).…”

“…Raised hepatic cholesterol in global-ChREBP -/- ( 53 ), may be linked to endotoxemia consequent to intestinal dysfunction and inflammation as was observed in intestinal-ChREBP -/- mice ( 17 ). A further hypothesis is that ChREBP attenuates accumulation of hepatic triglycerides and diacylglycerides by promoting triglyceride secretion ( 54 , 56 , 57 ). However, the raised ALAT in the LiChREBP -/- did not associate with raised liver triglycerides or diacylglycerides ( 17 , 54 – 56 ), implicating mechanisms other than cholesterol, triglyceride or diacylglyceride accumulation in the ChREBP-mediated protection.…”

Naringin Attenuates High Fat Diet Induced Non-alcoholic Fatty Liver Disease and Gut Bacterial Dysbiosis in Mice

“…Thus, NAR may be a potential therapeutic adjuvant to improve NAFLD outcomes. Non-alcoholic fatty liver disease is defined by abnormal lipid metabolism in the liver and is the most common liver disease worldwide (Yan et al, 2014;Ipsen et al, 2018;Lei et al, 2020). NAFLD is closely associated with cardiovascular diseases, which are a main cause of NAFLD-related deaths (Petta et al, 2015;Chang et al, 2019).…”

“…Non-alcoholic fatty liver disease development and progression depends on pathologic accumulation of lipid droplets within hepatocytes (Yan et al, 2014;Lei et al, 2020). At the molecular level, sterol regulatory element binding protein 1 (Srebp1) is a key lipogenic transcription factor that directly regulates the expressions of lipid synthesis rate-limiting enzymes, including fatty acid synthase (Fas), acetyl-CoA carboxylase (Acc), and stearoyl-CoA desaturase 1 (Scd1), and lipid uptake-related genes, such as CD36, leading to hepatic lipid accumulation (Guo et al, 2018;Zhou et al, 2020).…”