Search

Abstract Background Transmembrane 4 L six family member 5 (TM4SF5) translocates subcellularly and functions metabolically, although it is unclear how intracellular TM4SF5 translocation is linked to metabolic contexts. It is thus of interests to understand how the traffic dynamics of TM4SF5 to subcellular endosomal membranes are correlated to regulatory roles of metabolisms. Methods Here, we explored the metabolic significance of TM4SF5 localization at mitochondria‐lysosome contact sites (MLCSs), using in vitro cells and in vivo animal systems, via approaches by immunofluorescence, proximity labelling based proteomics analysis, organelle reconstitution etc. Results Upon extracellular glucose repletion following depletion, TM4SF5 became enriched at MLCSs via an interaction between mitochondrial FK506‐binding protein 8 (FKBP8) and lysosomal TM4SF5. Proximity labeling showed molecular clustering of phospho‐dynamic‐related protein I (DRP1) and certain mitophagy receptors at TM4SF5‐enriched MLCSs, leading to mitochondrial fission and autophagy. TM4SF5 bound NPC intracellular cholesterol transporter 1 (NPC1) and free cholesterol, and mediated export of lysosomal cholesterol to mitochondria, leading to impaired oxidative phosphorylation but intact tricarboxylic acid (TCA) cycle and β‐oxidation. In mouse models, hepatocyte Tm4sf5 promoted mitophagy and cholesterol transport to mitochondria, both with positive relations to liver malignancy. Conclusions Our findings suggested that TM4SF5‐enriched MLCSs regulate glucose catabolism by facilitating cholesterol export for mitochondrial reprogramming, presumably while hepatocellular carcinogenesis, recapitulating aspects for hepatocellular carcinoma metabolism with mitochondrial reprogramming to support biomolecule synthesis in addition to glycolytic energetics.

Summary: Transmembrane 4 L six family member 5 (TM4SF5) engages in non-alcoholic steatohepatitis (NASH), although its mechanistic roles are unclear. Genetically engineered Tm4sf5 mice fed ad libitum normal chow or high-fat diet for either an entire day or a daytime-feeding (DF) pattern were analyzed for metabolic parameters. Compared to wild-type and Tm4sf5−/− knockout mice, hepatocyte-specific TM4SF5-overexpressing Alb-TGTm4sf5−Flag (TG) mice showed abnormal food-intake behavior during the mouse-inactive daytime, increased apelin expression, increased food intake, and higher levels of NASH features. DF or exogenous apelin injection of TG mice caused severe hepatic pathology. TM4SF5-mediated abnormal food intake was correlated with peroxisomal β-oxidation, mTOR activation, and autophagy inhibition, with triggering NASH phenotypes. Non-alcoholic fatty liver disease (NAFLD) patients’ samples revealed a correlation between serum apelin and NAFLD activity score. Altogether, these observations suggest that hepatic TM4SF5 may cause abnormal food-intake behaviors to trigger steatohepatitic features via the regulation of peroxisomal β-oxidation, mTOR, and autophagy.

Abstract Transmembrane 4 L six family member 5 (TM4SF5) is involved in chronic liver disease, although its role in glucose homeostasis remains unknown. TM4SF5 deficiency caused age‐dependent glucose (in)tolerance with no link to insulin sensitivity. Further, hepatic TM4SF5 binding to GLUT1 promoted glucose uptake and glycolysis. Excessive glucose repletion caused hepatocytes to secrete small extracellular vesicles (sEVs) loaded with TM4SF5 (hep‐sEVTm4sf5), suggesting a role for sEVTm4sf5 in glucose metabolism and homeostasis. Hep‐sEVTm4sf5 were smaller than sEVControl and recruit proteins for efficient organ tropism. Liver‐derived sEVs, via a liver‐closed vein circuit (LCVC) using hepatic TM4SF5‐overexpressing (Alb‐Tm4sf5 TG) mice (liv‐sEVTm4sf5), improved glucose tolerance in Tm4sf5−/− KO mice and targeted brown adipose tissues (BATs), possibly allowing the clearance of blood glucose as heat independent of UCP1. Taken together, hep‐sEVTm4sf5 might clear high extracellular glucose levels more efficiently by targeting BAT compared with hep‐sEVControl, suggesting an insulin‐like role for sEV™4SF5 in affecting age‐related metabolic status and thus body weight (BW).

Objective: Transmembrane 4 L six family member 5 (TM4SF5) is likely involved in non-alcoholic steatohepatitis, although its roles and cross-talks with glucose/fructose transporters in phenotypes derived from high-carbohydrate diets remain unexplored. Here, we investigated the modulation of hepatic fructose metabolism by TM4SF5. Methods: Wild-type or Tm4sf5−/− knockout mice were evaluated via different diets, including normal chow, high-sucrose diet, or high-fat diet without or with fructose in drinking water (30% w/v). Using liver tissues and blood samples from the mice or hepatocytes, the roles of TM4SF5 in fructose-mediated de novo lipogenesis (DNL) and steatosis via a crosstalk with glucose transporter 8 (GLUT8) were assessed. Results: Tm4sf5 suppression or knockout in both in vitro and in vivo models reduced fructose uptake, DNL, and steatosis. Extracellular fructose treatment of hepatocytes resulted in an inverse relationship between fructose–uptake activity and TM4SF5-mediated translocalization of GLUT8 through dynamic binding at the cell surface. Following fructose treatment, TM4SF5 binding to GLUT8 transiently decreased with translocation to the plasma membrane (PM), where GLUT8 separated and became active for fructose uptake and DNL. Conclusions: Overall, hepatic TM4SF5 modulated GLUT8 localization and activity through transient binding, leading to steatosis-related fructose uptake and lipogenesis. Thus, TM4SF5 and/or GLUT8 may be promising treatment targets against liver steatosis resulting from excessive fructose consumption.

Summary: Chronic injury to hepatocytes results in inflammation, steatohepatitis, fibrosis, and nonalcoholic fatty liver disease (NAFLD). The tetraspanin TM4SF5 is implicated in fibrosis and cancer. We investigate the role of TM4SF5 in communication between hepatocytes and macrophages (MΦs) and its possible influence on the inflammatory microenvironment that may lead to NAFLD. TM4SF5 induction in differentiated MΦs promotes glucose uptake, glycolysis, and glucose sensitivity, leading to M1-type MΦ activation. Activated M1-type MΦs secrete pro-inflammatory interleukin-6 (IL-6), which induces the secretion of CCL20 and CXCL10 from TM4SF5-positive hepatocytes. Although TM4SF5-dependent secretion of these chemokines enhances glycolysis in M0 MΦs, further chronic exposure reprograms MΦs for an increase in the proportion of M2-type MΦs in the population, which may support diet- and chemical-induced NAFLD progression. We suggest that TM4SF5 expression in MΦs and hepatocytes is critically involved in modulating the inflammatory environment during NAFLD progression.