Hepatic ChREBP orchestrates intrahepatic carbohydrate metabolism to limit hepatic glucose 6-phosphate and glycogen accumulation in a mouse model for acute Glycogen Storage Disease type Ib

OBJECTIVE

Carbohydrate Response Element Binding Protein (ChREBP) is a glucose 6-phosphate (G6P)-sensitive transcription factor that acts as a metabolic switch to maintain intracellular glucose and phosphate homeostasis. Hepatic ChREBP is well-known for its regulatory role in glycolysis, the pentose phosphate pathway, and de novo lipogenesis. The physiological role of ChREBP in hepatic glycogen metabolism and blood glucose regulation has not been assessed in detail, and ChREBP's contribution to carbohydrate flux adaptations in hepatic Glycogen Storage Disease type 1 (GSD I) requires further investigation.

METHODS

The current study aimed to investigate the role of ChREBP as a regulator of glycogen metabolism in response to hepatic G6P accumulation, using a model for acute hepatic GSD type Ib. The immediate biochemical and regulatory responses to hepatic G6P accumulation were evaluated upon G6P transporter inhibition by the chlorogenic acid S4048 in mice that were either treated with a short hairpin RNA (shRNA) directed against ChREBP (shChREBP) or a scrambled shRNA (shSCR). Complementary stable isotope experiments were performed to quantify hepatic carbohydrate fluxes in vivo.

RESULTS

ShChREBP treatment normalized the S4048-mediated induction of hepatic ChREBP target genes to levels observed in vehicle- and shSCR-treated controls. In parallel, hepatic shChREBP treatment in S4048-infused mice resulted in a more pronounced accumulation of hepatic glycogen and further reduction of blood glucose levels compared to shSCR treatment. Hepatic ChREBP knockdown modestly increased glucokinase (GCK) flux in S4048-treated mice while it enhanced UDP-glucose turnover as well as glycogen synthase and phosphorylase fluxes. Hepatic GCK mRNA and protein levels were induced by shChREBP treatment in both vehicle- and S4048-treated mice, while glycogen synthase 2 (GYS2) and glycogen phosphorylase (PYGL) mRNA and protein levels were reduced. Finally, knockdown of hepatic ChREBP expression reduced starch domain binding protein 1 (STBD1) mRNA and protein levels while it inhibited acid alpha-glucosidase (GAA) activity, suggesting reduced capacity for lysosomal glycogen breakdown.

CONCLUSIONS

Our data show that ChREBP activation controls hepatic glycogen and blood glucose levels in acute hepatic GSD Ib through concomitant regulation of glucose phosphorylation, glycogenesis, and glycogenolysis. ChREBP-mediated control of GCK enzyme levels aligns with corresponding adaptations in GCK flux. In contrast, ChREBP activation in response to acute hepatic GSD Ib exerts opposite effects on GYS2/PYGL enzyme levels and their corresponding fluxes, indicating that GYS2/PYGL expression levels are not limiting to their respective fluxes under these conditions.

Human induced pluripotent stem cell-derived liver-on-a-chip for studying drug metabolism: the challenge of the cytochrome P450 family

The liver is the primary organ responsible for the detoxification and metabolism of drugs. To date, a lack of preclinical models that accurately emulate drug metabolism by the human liver presents a significant challenge in the drug development pipeline, particularly for predicting drug efficacy and toxicity. In recent years, emerging microfluidic-based organ-on-a-chip (OoC) technologies, combined with human induced pluripotent stem cell (hiPSC) technology, present a promising avenue for the complete recapitulation of human organ biology in a patient-specific manner. However, hiPSC-derived organoids and liver-on-a-chip models have so far failed to sufficiently express cytochrome P450 monooxygenase (CYP450) enzymes, the key enzymes involved in first-pass metabolism, which limits the effectiveness and translatability of these models in drug metabolism studies. This review explores the potential of innovative organoid and OoC technologies for studying drug metabolism and discusses their existing drawbacks, such as low expression of CYP450 genes. Finally, we postulate potential approaches for enhancing CYP450 expression in the hope of paving the way toward developing novel, fully representative liver drug-metabolism models.

Genetic deletion of hepatic NCOR1 protects from atherosclerosis by promoting alternative bile acid-metabolism and sterol excretion

BACKGROUND

The nuclear receptor corepressor 1 (NCOR1) plays an important role in the regulation of gene expression in immunometabolic conditions by connecting chromatin-modifying enzymes, coregulators and transcription factors. NCOR1 has been shown to be involved in cardiometabolic diseases. Recently, we demonstrated that the deletion of macrophage NCOR1 aggravates atherosclerosis by promoting CD36-triggered foam cell formation via PPARG derepression.

PURPOSE

Since NCOR1 modulates the function of several key regulators involved in hepatic lipid and bile acid metabolism, we hypothesized that its deletion in hepatocytes alters lipid metabolism and atherogenesis.

METHODS

To test this hypothesis, we generated hepatocyte-specific Ncor1 knockout mice on a Ldlr-/- background. Besides assessing the progression of the disease in thoracoabdominal aortae en face, we analyzed hepatic cholesterol and bile acid metabolism at expression and functional levels.

RESULTS

Our data demonstrate that liver-specific Ncor1 knockout mice on an atherosclerosis-prone background develop less atherosclerotic lesions than controls. Interestingly, under chow diet, plasma cholesterol levels of liver-specific Ncor1 knockout mice were slightly higher compared to control, but strongly reduced compared to control mice after feeding them an atherogenic diet for 12 weeks. Moreover, the hepatic cholesterol content was decreased in liver-specific Ncor1 knockout compared to control mice. Our mechanistic data revealed that NCOR1 reprograms the synthesis of bile acids towards the alternative pathway, which in turn reduce bile hydrophobicity and enhances fecal cholesterol excretion.

CONCLUSIONS

Our data suggest that hepatic Ncor1 deletion in mice decreases atherosclerosis development by reprograming bile acid metabolism and enhancing fecal cholesterol excretion.

Immune-metabolic interactions in homeostasis and the progression to NASH

The incidence of non-alcoholic fatty liver disease (NAFLD) has increased significantly over the past two decades. NAFLD ranges from simple steatosis (NAFL) to nonalcoholic steatohepatitis (NASH) and predisposes to fibrosis and hepatocellular carcinoma (HCC). The importance of the immune system in hepatic physiology and in the progression of NAFLD is increasingly recognized. At homeostasis, the liver participates in immune defense against pathogens and in tolerance of gut-derived microbial compounds. Hepatic immune cells also respond to metabolic stimuli and have a role in NAFLD progression to NASH. In this review, we discuss how metabolic perturbations affect immune cell phenotype and function in NAFL and NASH, and then focus on the role of immune cells in liver homeostasis and in the development of NASH.

Hepatocyte-specific glucose-6-phosphatase deficiency disturbs platelet aggregation and decreases blood monocytes upon fasting-induced hypoglycemia

OBJECTIVE

Glycogen storage disease type 1a (GSD Ia) is a rare inherited metabolic disorder caused by mutations in the glucose-6-phosphatase (G6PC1) gene. When untreated, GSD Ia leads to severe fasting-induced hypoglycemia. Although current intensive dietary management aims to prevent hypoglycemia, patients still experience hypoglycemic events. Poor glycemic control in GSD Ia is associated with hypertriglyceridemia, hepatocellular adenoma and carcinoma, and also with an increased bleeding tendency of unknown origin.

METHODS

To evaluate the effect of glycemic control on leukocyte levels and coagulation in GSD Ia, we employed hepatocyte-specific G6pc1 deficient (L-G6pc-/-) mice under fed or fasted conditions, to match good or poor glycemic control in GSD Ia, respectively.

RESULTS

We found that fasting-induced hypoglycemia in L-G6pc-/- mice decreased blood leukocytes, specifically proinflammatory Ly6Chi monocytes, compared to controls. Refeeding reversed this decrease. The decrease in Ly6Chi monocytes was accompanied by an increase in plasma corticosterone levels and was prevented by the glucocorticoid receptor antagonist mifepristone. Further, fasting-induced hypoglycemia in L-G6pc-/- mice prolonged bleeding time in the tail vein bleeding assay, with reversal by refeeding. This could not be explained by changes in coagulation factors V, VII, or VIII, or von Willebrand factor. While the prothrombin and activated partial thromboplastin time as well as total platelet counts were not affected by fasting-induced hypoglycemia in L-G6pc-/- mice, ADP-induced platelet aggregation was disturbed.

CONCLUSIONS

These studies reveal a relationship between fasting-induced hypoglycemia, decreased blood monocytes, and disturbed platelet aggregation in L-G6pc-/- mice. While disturbed platelet aggregation likely accounts for the bleeding phenotype in GSD Ia, elevated plasma corticosterone decreases the levels of proinflammatory monocytes. These studies highlight the necessity of maintaining good glycemic control in GSD Ia.

Impaired Very-Low-Density Lipoprotein catabolism links hypoglycemia to hypertriglyceridemia in Glycogen Storage Disease type Ia

Prevention of hypertriglyceridemia is one of the biomedical targets in Glycogen Storage Disease type Ia (GSD Ia) patients, yet it is unclear how hypoglycemia links to plasma triglyceride (TG) levels. We analyzed whole-body TG metabolism in normoglycemic (fed) and hypoglycemic (fasted) hepatocyte-specific glucose-6-phosphatase deficient (L-G6pc^-/- ) mice. De novo fatty acid synthesis contributed substantially to hepatic TG accumulation in normoglycemic L-G6pc^-/- mice. In hypoglycemic conditions, enhanced adipose tissue lipolysis was the main driver of liver steatosis, supported by elevated free fatty acid concentrations in GSD Ia mice and GSD Ia patients. Plasma very-low-density lipoprotein (VLDL) levels were increased in GSD Ia patients and in normoglycemic L-G6pc^-/- mice, and further elevated in hypoglycemic L-G6pc^-/- mice. VLDL-TG secretion rates were doubled in normo- and hypoglycemic L-G6pc^-/- mice, while VLDL-TG catabolism was selectively inhibited in hypoglycemic L-G6pc^-/- mice. In conclusion, fasting-induced hypoglycemia in L-G6pc^-/- mice promotes adipose tissue lipolysis and arrests VLDL catabolism. This mechanism likely contributes to aggravated liver steatosis and dyslipidemia in GSD Ia patients with poor glycemic control and may explain clinical heterogeneity in hypertriglyceridemia between GSD Ia patients.

Cholangiopathy and Biliary Fibrosis in Cyp2c70-Deficient Mice Are Fully Reversed by Ursodeoxycholic Acid

BACKGROUND AND AIMS

Bile acids (BAs) aid intestinal fat absorption and exert systemic actions by receptor-mediated signaling. BA receptors have been identified as drug targets for liver diseases. Yet, differences in BA metabolism between humans and mice hamper translation of pre-clinical outcomes. Cyp2c70-ablation in mice prevents synthesis of mouse/rat-specific muricholic acids (MCAs), but potential (patho)physiological consequences of their absence are unknown. We therefore assessed age- and gender-dependent effects of Cyp2c70-deficiency in mice.

METHODS

The consequences of Cyp2c70-deficiency were assessed in male and female mice at different ages.

RESULTS

Cyp2c70^-/- mice were devoid of MCAs and showed high abundances of chenodeoxycholic and lithocholic acids. Cyp2c70-deficiency profoundly impacted microbiome composition. Bile flow and biliary BA secretion were normal in Cyp2c70^-/- mice of both sexes. Yet, the pathophysiological consequences of Cyp2c70-deficiency differed considerably between sexes. Three-week old male Cyp2c70^-/- mice showed high plasma BAs and transaminases, which spontaneously decreased thereafter to near-normal levels. Only mild ductular reactions were observed in male Cyp2c70^-/- mice up to 8 months of age. In female Cyp2c70^-/- mice, plasma BAs and transaminases remained substantially elevated with age, gut barrier function was impaired and bridging fibrosis was observed at advanced age. Addition of 0.1% ursodeoxycholic acid to the diet fully normalized hepatic and intestinal functions in female Cyp2c70^-/- mice.

CONCLUSION

Cyp2c70^-/- mice show transient neonatal cholestasis and develop cholangiopathic features that progress to bridging fibrosis in females only. These consequences of Cyp2c70-deficiency are restored by treatment with UDCA, indicating a role of BA hydrophobicity in disease development.

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

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.

Glycogen storage disease type 1a is associated with disturbed vitamin A metabolism and elevated serum retinol levels

Glycogen storage disease type 1a (GSD Ia) is an inborn error of metabolism caused by mutations in the G6PC gene, encoding the catalytic subunit of glucose-6-phosphatase. Early symptoms include severe fasting intolerance, failure to thrive and hepatomegaly, biochemically associated with nonketotic hypoglycemia, fasting hyperlactidemia, hyperuricemia and hyperlipidemia. Dietary management is the cornerstone of treatment aiming at maintaining euglycemia, prevention of secondary metabolic perturbations and long-term complications, including liver (hepatocellular adenomas and carcinomas), kidney and bone disease (hypovitaminosis D and osteoporosis). As impaired vitamin A homeostasis also associates with similar symptoms and is coordinated by the liver, we here analysed whether vitamin A metabolism is affected in GSD Ia patients and liver-specific G6pc^−/− knock-out mice. Serum levels of retinol and retinol binding protein 4 (RBP4) were significantly increased in both GSD Ia patients and L-G6pc^−/− mice. In contrast, hepatic retinol levels were significantly reduced in L-G6pc^−/− mice, while hepatic retinyl palmitate (vitamin A storage form) and RBP4 levels were not altered. Transcript and protein analyses indicate an enhanced production of retinol and reduced conversion the retinoic acids (unchanged LRAT, Pnpla2/ATGL and Pnpla3 up, Cyp26a1 down) in L-G6pc^−/− mice. Aberrant expression of genes involved in vitamin A metabolism was associated with reduced basal messenger RNA levels of markers of inflammation (Cd68, Tnfα, Nos2, Il-6) and fibrosis (Col1a1, Acta2, Tgfβ, Timp1) in livers of L-G6pc^−/− mice. In conclusion, GSD Ia is associated with elevated serum retinol and RBP4 levels, which may contribute to disease symptoms, including osteoporosis and hepatic steatosis.

Glucose-6-Phosphate Regulates Hepatic Bile Acid Synthesis in Mice

It is well established that, besides facilitating lipid absorption, bile acids act as signaling molecules that modulate glucose and lipid metabolism. Bile acid metabolism, in turn, is controlled by several nutrient-sensitive transcription factors. Altered intrahepatic glucose signaling in type 2 diabetes associates with perturbed bile acid synthesis. We aimed to characterize the regulatory role of the primary intracellular metabolite of glucose, glucose-6-phosphate (G6P), on bile acid metabolism. Hepatic gene expression patterns and bile acid composition were analyzed in mice that accumulate G6P in the liver, that is, liver-specific glucose-6-phosphatase knockout (L-G6pc^-/- ) mice, and mice treated with a pharmacological inhibitor of the G6P transporter. Hepatic G6P accumulation induces sterol 12α-hydroxylase (Cyp8b1) expression, which is mediated by the major glucose-sensitive transcription factor, carbohydrate response element-binding protein (ChREBP). Activation of the G6P-ChREBP-CYP8B1 axis increases the relative abundance of cholic-acid-derived bile acids and induces physiologically relevant shifts in bile composition. The G6P-ChREBP-dependent change in bile acid hydrophobicity associates with elevated plasma campesterol/cholesterol ratio and reduced fecal neutral sterol loss, compatible with enhanced intestinal cholesterol absorption. Conclusion: We report that G6P, the primary intracellular metabolite of glucose, controls hepatic bile acid synthesis. Our work identifies hepatic G6P-ChREBP-CYP8B1 signaling as a regulatory axis in control of bile acid and cholesterol metabolism.

Supplementation with Lactobacillus plantarum WCFS1 Prevents Decline of Mucus Barrier in Colon of Accelerated Aging Ercc1-/Δ7 Mice

Although it is clear that probiotics improve intestinal barrier function, little is known about the effects of probiotics on the aging intestine. We investigated effects of 10-week bacterial supplementation of Lactobacillus plantarum WCFS1, Lactobacillus casei BL23, or Bifidobacterium breve DSM20213 on gut barrier and immunity in 16-week-old accelerated aging Ercc1^−/Δ7 mice, which have a median lifespan of ~20 weeks, and their wild-type littermates. The colonic barrier in Ercc1^−/Δ7 mice was characterized by a thin (< 10 μm) mucus layer. L. plantarum prevented this decline in mucus integrity in Ercc1^−/Δ7 mice, whereas B. breve exacerbated it. Bacterial supplementations affected the expression of immune-related genes, including Toll-like receptor 4. Regulatory T cell frequencies were increased in the mesenteric lymph nodes of L. plantarum- and L. casei-treated Ercc1^−/Δ7 mice. L. plantarum- and L. casei-treated Ercc1^−/Δ7 mice showed increased specific antibody production in a T cell-dependent immune response in vivo. By contrast, the effects of bacterial supplementation on wild-type control mice were negligible. Thus, supplementation with L. plantarum – but not with L. casei and B. breve – prevented the decline in the mucus barrier in Ercc1^−/Δ7 mice. Our data indicate that age is an important factor influencing beneficial or detrimental effects of candidate probiotics. These findings also highlight the need for caution in translating beneficial effects of probiotics observed in young animals or humans to the elderly.

Interaction of mouse splenocytes and macrophages with bacterial strains in vitro: the effect of age in the immune response

Probiotics influence the immune system, both at the local and systemic level. Recent findings suggest the relation between microbiota and the immune system alters with age. Our objective was to address direct effects of six bacterial strains on immune cells from young and aged mice: Lactobacillus plantarum WCFS1, Lactobacillus casei BL23, Lactococcus lactis MG1363, Bifidobacterium breve ATCC15700, Bifidobacterium infantis ATCC15697, and Akkermansia muciniphila ATCC BAA-835. We used splenocytes and naïve or interferon-γ-stimulated bone marrow-derived macrophages (BMDM) as responder populations. All tested bacterial strains induced phenotypic and cytokine responses in splenocytes and BMDM. Based on magnitude of the cellular inflammatory response and cytokine profiles, two subgroups of bacteria were identified, i.e. L. plantarum and L. casei versus B. breve, B. infantis, and A. muciniphila. The latter group of bacteria induced high levels of cytokines produced under inflammatory conditions, including tumour necrosis factor (TNF), interleukin (IL)-6 and IL-10. Responses to L. lactis showed features of both subgroups. In addition, we compared responses by splenocytes and BMDM derived from young mice to those of aged mice, and found that splenocytes and BMDM derived from aged mice had an increased IL-10 production and dysregulated IL-6 and TNF production compared to young immune cells. Overall, our study shows differential inflammatory responses to distinct bacterial strains, and profound age-dependent effects. These findings, moreover, support the view that immune environment importantly influences bacterial immune effects.