A Hepatic GAbp-AMPK Axis Links Inflammatory Signaling to Systemic Vascular Damage

Vitamin B2 enables regulation of fasting glucose availability

Flavin adenine dinucleotide (FAD) interacts with flavoproteins to mediate oxidation-reduction reactions required for cellular energy demands. Not surprisingly, mutations that alter FAD binding to flavoproteins cause rare inborn errors of metabolism (IEMs) that disrupt liver function and render fasting intolerance, hepatic steatosis, and lipodystrophy. In our study, depleting FAD pools in mice with a vitamin B2-deficient diet (B2D) caused phenotypes associated with organic acidemias and other IEMs, including reduced body weight, hypoglycemia, and fatty liver disease. Integrated discovery approaches revealed B2D tempered fasting activation of target genes for the nuclear receptor PPARα, including those required for gluconeogenesis. We also found PPARα knockdown in the liver recapitulated B2D effects on glucose excursion and fatty liver disease in mice. Finally, treatment with the PPARα agonist fenofibrate activated the integrated stress response and refilled amino acid substrates to rescue fasting glucose availability and overcome B2D phenotypes. These findings identify metabolic responses to FAD availability and nominate strategies for the management of organic acidemias and other rare IEMs.

Redox-dependent AMPK inactivation disrupts metabolic adaptation to glucose starvation in xCT-overexpressing cancer cells

Accelerated aerobic glycolysis is a distinctive metabolic property of cancer cells that confers dependency on glucose for survival. However, the therapeutic strategies targeting this vulnerability are still inefficient and have unacceptable side effects in clinical trials. Therefore, developing biomarkers to predict therapeutic efficacy would be essential to improve the selective targeting of cancer cells. Here, we found that the cell lines sensitive to glucose deprivation have high expression of cystine/glutamate antiporter xCT. We found that cystine uptake and glutamate export through xCT contributed to rapid NADPH depletion under glucose deprivation. This collapse of the redox system oxidized and inactivated AMPK, a major regulator of metabolic adaptation, resulting in a metabolic catastrophe and cell death. While this phenomenon was prevented by pharmacological or genetic inhibition of xCT, overexpression of xCT sensitized resistant cancer cells to glucose deprivation. Taken together, these findings suggest a novel cross-talk between AMPK and xCT for the metabolism and signal transduction and reveal a metabolic vulnerability in xCT-high expressing cancer cells to glucose deprivation.

Vesicle Induced Receptor Sequestration: Mechanisms behind Extracellular Vesicle-Based Protein Signaling

Extracellular vesicles (EVs) are fundamental for proper physiological functioning of multicellular organisms. By shuttling nucleic acids and proteins between cells, EVs regulate a plethora of cellular processes, especially those involved in immune signalling. However, the mechanistic understanding concerning the biophysical principles underlying EV-based communication is still incomplete. Towards holistic understanding, particular mechanisms explaining why and when cells apply EV-based communication and how protein-based signalling is promoted by EV surfaces are sought. Here, the authors study vesicle-induced receptor sequestration (VIRS) as a universal mechanism augmenting the signalling potency of proteins presented on EV-membranes. By bottom-up reconstitution of synthetic EVs, the authors show that immobilization of the receptor ligands FasL and RANK on EV-like vesicles, increases their signalling potential by more than 100-fold compared to their soluble forms. Moreover, the authors perform diffusion simulations within immunological synapses to compare receptor activation between soluble and EV-presented proteins. By this the authors propose vesicle-triggered local clustering of membrane receptors as the principle structural mechanism underlying EV-based protein presentation. The authors conclude that EVs act as extracellular templates promoting the local aggregation of membrane receptors at the EV contact site, thereby fostering inter-protein interactions. The results uncover a potentially universal mechanism explaining the unique structural profit of EV-based intercellular signalling.

Dioxin-like and non-dioxin-like PCBs differentially regulate the hepatic proteome and modify diet-induced nonalcoholic fatty liver disease severity

Polychlorinated biphenyls (PCBs) are persistent organic pollutants associated with metabolic disruption and non-alcoholic fatty liver disease (NAFLD). Based on their ability to activate the aryl hydrocarbon receptor (AhR), PCBs are subdivided into two classes: dioxin-like (DL) and non-dioxin-like (NDL) PCBs. Previously, we demonstrated that NDL PCBs compromised the liver to promote more severe diet-induced NAFLD. Here, the hepatic effects and potential mechanisms (by untargeted liver proteomics) of DL PCBs, NDL PCBs or co-exposure to both in diet-induced NAFLD are investigated. Male C57Bl/6 mice were fed a 42% fat diet and exposed to vehicle control; Aroclor1260 (20 mg/kg, NDL PCB mixture); PCB126 (20 μg/kg, DL PCB congener); or a mixture of Aroclor1260 (20 mg/kg)+PCB126 (20 μg/kg) for 12 weeks. Each exposure was associated with a distinct hepatic proteome. Phenotypic and proteomic analyses revealed increased hepatic inflammation and phosphoprotein signaling disruption by Aroclor1260. PCB126 decreased hepatic inflammation and fibrosis at the molecular level; while altering cytoskeletal remodeling, metal homeostasis, and intermediary/xenobiotic metabolism. PCB126 attenuated Aroclor1260-induced hepatic inflammation but increased hepatic free fatty acids in the co-exposure group. Aroclor1260+PCB126 exposure was strongly associated with multiple epigenetic processes, and these could potentially explain the observed non-additive effects of the exposures on the hepatic proteome. Taken together, the results demonstrated that PCB exposures differentially regulated the hepatic proteome and the histologic severity of diet-induced NAFLD. Future research is warranted to determine the AhR-dependence of the observed effects including metal homeostasis and the epigenetic regulation of gene expression.

Hepatic Rab24 controls blood glucose homeostasis via improving mitochondrial plasticity

Non-alcoholic fatty liver disease (NAFLD) represents a key feature of obesity-related type 2 diabetes with increasing prevalence worldwide. To our knowledge, no treatment options are available to date, paving the way for more severe liver damage, including cirrhosis and hepatocellular carcinoma. Here, we show an unexpected function for an intracellular trafficking regulator, the small Rab GTPase Rab24, in mitochondrial fission and activation, which has an immediate impact on hepatic and systemic energy homeostasis. RAB24 is highly upregulated in the livers of obese patients with NAFLD and positively correlates with increased body fat in humans. Liver-selective inhibition of Rab24 increases autophagic flux and mitochondrial connectivity, leading to a strong improvement in hepatic steatosis and a reduction in serum glucose and cholesterol levels in obese mice. Our study highlights a potential therapeutic application of trafficking regulators, such as RAB24, for NAFLD and establishes a conceptual functional connection between intracellular transport and systemic metabolic dysfunction.

The role of HMGB1 in liver inflammation in obese rats

Obesity is a chronic disease that is characterized by increased body fat owing to imbalance between consumed and expended energy. Inflammation generally is accompanied by accumulation of excess lipid in adipose tissue and liver. High mobility group box-1 (HMGB1) participates in the pathogenesis of inflammatory diseases. We investigated the relation of the number of HMGB1 positive cells to body mass index (BMI), liver inflammation and the number of Kupffer cells. We divided 18 female Wistar albino rats into two groups: group 1, untreated control fed normal commercial rat diet and group 2, obese rats fed a special diet containing 40% fat. The plasma concentrations of cholesterol, glucose, superoxide dismutase enzyme (SOD) and catalase activities were measured for all animals. The numbers of hepatocytes, Kupffer cells and HMGB1 positive cells were counted using stereological methods. The mean numbers of Kupffer cells and HMGB1 positive cells were higher for group 2 than for group 1. The concentrations of plasma cholesterol and glucose levels also were higher in group 2. Plasma levels of SOD and catalase were significantly lower in group 2 compared to group 1. The number of HMGB1 cells was related directly to BMI and inflammation. The role of HMGB1 was demonstrated for the liver of the obese group. We demonstrated the relations among HMGB1, BMI, obesity and inflammation.