Chronic high-fat feeding and prolonged fasting in liver-specific ANGPTL4 knockout mice

Obesity is associated with dyslipidemia, ectopic lipid deposition, and insulin resistance. In mice, the global or adipose-specific loss of function of the protein angiopoietin-like 4 (ANGPTL4) leads to decreased plasma triglyceride levels, enhanced adipose triglyceride uptake, and protection from high-fat diet (HFD)-induced glucose intolerance. ANGPTL4 is also expressed highly in the liver, but the role of liver-derived ANGPTL4 is unclear. The goal of this study was to determine the contribution of hepatocyte ANGPTL4 to triglyceride and glucose homeostasis in mice during a high-fat diet challenge. We generated hepatocyte-specific ANGPTL4 deficient (Angptl4^LivKO) mice, fed them a 60% kcal/fat diet (HFD) for 6 mo and assessed triglyceride, liver, and glucose metabolic phenotypes. We also explored the effects of prolonged fasting on Angptl4^LivKO mice. The loss of hepatocyte-derived ANGPTL4 led to no major changes in triglyceride partitioning or lipoprotein lipase activity compared with control mice. Interestingly, although there was no difference in fasting plasma triglyceride levels after a 6 h fast, after an 18-h fast, normal chow diet-fed Angptl4^LivKO mice had lower triglyceride levels than control mice. On a HFD, Angptl4^LivKO mice initially showed no difference in glucose tolerance and insulin sensitivity, but improved glucose tolerance emerged in these mice after 6 mo on HFD. Our data suggest that hepatocyte ANGPTL4 does not directly regulate triglyceride partitioning, but that loss of liver-derived ANGPTL4 may be protective from HFD-induced glucose intolerance and influence plasma triglyceride (TG) metabolism during prolonged fasting.NEW & NOTEWORTHY1) Angiopoietin-like 4 deficiency in hepatocytes (Angptl4^LivKO) does not improve triglyceride phenotypes during high-fat feeding. 2) Angptl4^LivKO mice have improved glucose tolerance after chronic high-fat diet. 3) Angptl4^LivKO mice have decreased fasting plasma triglyceride levels after an 18-h fast, but not after a 6-h fast.

A large-scale genome-lipid association map guides lipid identification

Despite the crucial roles of lipids in metabolism, we are still at the early stages of comprehensively annotating lipid species and their genetic basis. Mass spectrometry-based discovery lipidomics offers the potential to globally survey lipids and their relative abundances in various biological samples. To discover the genetics of lipid features obtained through high-resolution liquid chromatography-tandem mass spectrometry, we analysed liver and plasma from 384 diversity outbred mice, and quantified 3,283 molecular features. These features were mapped to 5,622 lipid quantitative trait loci and compiled into a public web resource termed LipidGenie. The data are cross-referenced to the human genome and offer a bridge between genetic associations in humans and mice. Harnessing this resource, we used genome-lipid association data as an additional aid to identify a number of lipids, for example gangliosides through their association with B4galnt1, and found evidence for a group of sex-specific phosphatidylcholines through their shared locus. Finally, LipidGenie's ability to query either mass or gene-centric terms suggests acyl-chain-specific functions for proteins of the ABHD family.

Impaired skeletal muscle mitochondrial pyruvate uptake rewires glucose metabolism to drive whole-body leanness

Metabolic cycles are a fundamental element of cellular and organismal function. Among the most critical in higher organisms is the Cori Cycle, the systemic cycling between lactate and glucose. Here, skeletal muscle-specific Mitochondrial Pyruvate Carrier (MPC) deletion in mice diverted pyruvate into circulating lactate. This switch disinhibited muscle fatty acid oxidation and drove Cori Cycling that contributed to increased energy expenditure. Loss of muscle MPC activity led to strikingly decreased adiposity with complete muscle mass and strength retention. Notably, despite decreasing muscle glucose oxidation, muscle MPC disruption increased muscle glucose uptake and whole-body insulin sensitivity. Furthermore, chronic and acute muscle MPC deletion accelerated fat mass loss on a normal diet after high fat diet-induced obesity. Our results illuminate the role of the skeletal muscle MPC as a whole-body carbon flux control point. They highlight the potential utility of modulating muscle pyruvate utilization to ameliorate obesity and type 2 diabetes.

Abstract P188: Adipose-Specific Deletion of Angiopoietin-like 4 Protects Against High Fat Diet Induced Dyslipidemia

Hypertriglyceridemia is a risk factor for the development of hypertension and diabetes. Plasma triglycerides (TGs) are hydrolyzed by lipoprotein lipase (LPL) to release free fatty acids for uptake into tissues. A circulating inhibitor of LPL is angiopoietin-like 4 (A4). A4 is expressed in liver and adipose and is present in circulation. In both humans and mice, A4 deficiency reduces plasma TGs. We previously reported that A4 deficient mice clear more TGs from plasma to adipose tissue. However, it is not clear if this effect is mediated by circulating ANGPTL4 or from local, adipose-derived A4. In this study we sought to characterize the effects of adipose- or liver-specific deletion of A4 on lipid homeostasis. Liver-specific or adipose-specific A4 knockout mice (A4-LivKO/AdipoKO) were generated by crossing A4 floxed mice (A4-fl/fl) with albumin-cre mice or adiponectin-cre mice, respectively. At 8 weeks of age, mice were randomized to either a normal chow diet (NCD) or a high fat diet (60% fat/kCal) for 12 weeks. TG levels (mg/dL) were significantly decreased in A4-AdipoKO mice at baseline (90±4) vs. A4-fl/fl (132±5). No differences were seen in A4-LivKO vs. A4-fl/fl mice (132±3 vs 130±4). After 12 weeks on diets, the A4-AdipoKO mice on NCD (92±6) and HFD (73±6) maintained lower plasma TG levels than A4-fl/fl mice (145±12) on NCD. No differences were seen in TG levels between A4-LivKO mice on HFD or NCD compared to A4-fl/fl mice. TG uptake was increased into adipose tissue of HFD fed A4-AdipoKO [(%-injected dose/mg tissue) (epididymal (eWAT) 0.82± 0.2 and brown adipose tissue (BAT) 1.74± 0.6)] compared to floxed controls on HFD (eWAT 0.44± 0.3 and BAT 0.93± 0.9). Liver-specific loss of ANGPTL4 had no differences in TG uptake from floxed controls on HFD. Overall, we anticipate that identifying tissue-specific contributions will be valuable in informing therapeutic approaches for treating hypertriglyceridemia.

Novel GPIHBP1-independent pathway for clearance of plasma TGs in Angptl4-/-Gpihbp1-/- mice

Mice lacking glycosylphosphatidylinositol-anchored HDL-binding protein 1 (GPIHBP1) are unable to traffic LPL to the vascular lumen. Thus, triglyceride (TG) clearance is severely blunted, and mice are extremely hypertriglyceridemic. Paradoxically, mice lacking both GPIHBP1 and the LPL regulator, angiopoietin-like 4 (ANGPTL4), are far less hypertriglyceridemic. We sought to determine the mechanism by which Angptl4^-/-Gpihbp1^-/- double-knockout mice clear plasma TGs. We confirmed that, on a normal chow diet, plasma TG levels were lower in Angptl4^-/-Gpihbp1^-/- mice than in Gpihbp1^-/- mice; however, the difference disappeared with administration of a high-fat diet. Although LPL remained mislocalized in double-knockout mice, plasma TG clearance in brown adipose tissue (BAT) increased compared with Gpihbp1^-/- mice. Whole lipoprotein uptake was observed in the BAT of both Gpihbp1^-/- and Angptl4^-/-Gpihbp1^-/- mice, but BAT lipase activity was significantly higher in the double-knockout mice. We conclude that Angptl4^-/-Gpihbp1^-/- mice clear plasma TGs primarily through a slow and noncanonical pathway that includes the uptake of whole lipoprotein particles.

ANGPTL8 promotes the ability of ANGPTL3 to bind and inhibit lipoprotein lipase

OBJECTIVE

Several members of the angiopoietin-like (ANGPTL) family of proteins, including ANGPTL3 and ANGPTL8, regulate lipoprotein lipase (LPL) activity. Deficiency in either ANGPTL3 or ANGPTL8 reduces plasma triglyceride levels and increases LPL activity, whereas overexpression of either protein does the opposite. Recent studies suggest that ANGPTL8 may functionally interact with ANGPTL3 to alter clearance of plasma triglycerides; however, the nature of this interaction has remained elusive. We tested the hypothesis that ANGPTL8 forms a complex with ANGPTL3 and that this complex is necessary for the inhibition of vascular LPL by ANGPTL3.

METHODS

We analyzed the interactions of ANGPTL3 and ANGPTL8 with each other and with LPL using co-immunoprecipitation, western blotting, lipase activity assays, and the NanoBiT split-luciferase system. We also used adenovirus injection to overexpress ANGPTL3 in mice that lacked ANGPTL8.

RESULTS

We found that ANGPTL3 or ANGPTL8 alone could only inhibit LPL at concentrations that far exceeded physiological levels, especially when LPL was bound to its endothelial cell receptor/transporter GPIHBP1 (glycosylphosphatidylinositol-anchored high-density lipoprotein binding protein 1). Physical interaction was observed between ANGPTL3 and ANGPTL8 when the proteins were co-expressed, and co-expression with ANGPTL3 greatly enhanced the secretion of ANGPTL8. Importantly, ANGPTL3-ANGPTL8 complexes had a dramatically increased ability to inhibit LPL compared to either protein alone. Adenovirus experiments showed that 2-fold overexpression of ANGPTL3 significantly increased plasma triglycerides only in the presence of ANGPTL8. Protein interaction assays showed that ANGPTL8 greatly increased the ability of ANGPTL3 to bind LPL.

CONCLUSIONS

Together, these data indicate that ANGPTL8 binds to ANGPTL3 and that this complex is necessary for ANGPTL3 to efficiently bind and inhibit LPL.