INSIGHT ON ANGPTL3 IMPACT ON HEPATIC METABOLISM AND FUNCTION

Background and aim Angiopoietin-like protein 3 (ANGPTL3) is a hepatokine, that inhibits lipoprotein and endothelial lipase with ANGPTL8, sparing the hydrolyzation of VLDLs. Its deficiency leads to higher lipases activation, resulting into hypolipidemia and protection from cardiometabolic risk. While the role of circulating ANGPTL3 on lipases has been well established, its effect on hepatic homeostasis and metabolism is still unclear. This work aims to unravel the autocrine effects of ANGPTL3 within the liver and its systemic impact on peripheral organs. The liver plays a key role in systemic metabolism, substrates management and partitioning, xenobiotics clearance, energy production and lipoproteins synthesis. Absence of hepatic ANGPTL3 - master regulator of lipids partitioning - may therefore alter essential liver functions. Methods Angptl3 full knockout (Angptl3KO) and wild-type mice were utilized. In vivo observations included indirect calorimetry, biochemical profiling of glucose lipids and glucose levels and metabolic challenges comprised insulin, pyruvate/lactate and triglycerides (TG) tolerance tests. After 16-18 weeks on either standard chow diet or high-fat (HFD) diet, mice were sacrificed for blood and organs collection. Ex vivo analyses included transcriptomic analysis of liver, skeletal muscle and heart, as well as metabolomic analysis on liver samples. Spatial gene expression profiling of the liver was evaluated. Obesity development was assessed through histology and immunophenotyping of blood, liver and bone marrow through flow cytometry. Results Angptl3KO mice exhibited persistent hypolipidemia regardless of feeding status, with higher TG clearance and a reduced hepatic lipoprotein production, without signs of hepatic steatosis. Compared to controls, Angptl3KO mice on a chow diet showed altered basal metabolism and a higher susceptibility to fasting, as highlighted by higher gluconeogenesis and as supported by transcriptomic and metabolomic data. This hypercatabolic phenotype is driven by activation of metabolic stress pathways (cAMP, PPARα, FOXO, NFIL3) leading to biomolecular adaptations reflected in spatially altered gene expression. These changes involved hepatic and extrahepatic signalling pathways, including thyroid hormones signalling, autophagy and bile acids metabolism. Transcriptomic analyses of the skeletal muscle revealed a preference for oxidative phosphorylation (OxPHOS), while the heart adapted to altered substrate availability. On chronic HFD, Angptl3KO mice maintained hypolipidemia, accelerated TG clearance and reduced hepatic lipoprotein production, but showed similar degree of hepatic steatosis versus controls. Notably, the hyper catabolic phenotype is rescued. Obesity onset appears delayed in Angptl3KO mice, and differences were found when immunophenotyping the blood, liver and bone marrow of these animals, suggesting a higher propensity of Angptl3KO mice in sustaining a functional immune response. Discussion Angptl3KO mice constantly display hypolipidemia, not only imputable to increased lipases activity but also to intracellular and intrahepatic mechanisms. Hepatic lipoprotein synthesis is reduced, and mice on chow diet do not display hepatic steatosis, while hypercatabolic phenotype is exacerbated at fasting. This feature is lost when Angptl3KO mice are chronically fed an HFD, despite hypolipidemia is persisting, suggesting an intracellular mechanism of ANGPTL3 that is fasting dependent. Although Angptl3KO mice present delayed obesity onset, they show sign of insulin resistance and glucose intolerance, suggesting a potential beneficial role of Angptl3 in weight control. Conclusion ANGPTL3 is a master regulator of lipid and glucose metabolism, of liver responses during fasting and obesity and has an impact in the periphery, altering oxidative and adipose tissue biology and function.