Preferential lipolysis of DGAT1 over DGAT2 generated triacylglycerol in Huh7 hepatocytes

Hepatic steatosis is defined by accumulation of neutral lipids in lipid droplets (LDs) including triacylglycerol (TG) and steryl esters. Two distinct diacylglycerol acyltransferases (DGAT1 and DGAT2) catalyze synthesis of TG in hepatocytes. TG formed through either DGAT1 or DGAT2 appears to be preferentially directed to distinct intercellular fates, such as fatty acid production for oxidation or very-low density lipoprotein assembly, respectively. Because of the preferential use of TG generated by DGAT1 and DGAT2, we hypothesized that targeting/association of lipolytic machinery to LDs would differ depending on whether the TG stores were generated through DGAT1 or DGAT2 activities. Inhibition of DGAT1 or DGAT2 in human hepatoma cells (Huh7) incubated with oleic acid resulted in only a small change in TG accretion suggesting that the two DGATs can compensate for each other in fatty acid esterification. This compensation was not accompanied by changes in DGAT1 or DGAT2 mRNA expression. DGAT1 inhibition (TG synthesized by DGAT2) resulted in large LDs, whereas DGAT2 inhibition (TG synthesized by DGAT1) caused the accumulation of numerous small LDs. Oleic acid treatment increased mRNA and protein expression of the LD-associated protein PLIN2 but not PLIN5 or the lipase ATGL and its activator ABHD5/CGI-58. Inactivation of DGAT1 or DGAT2 did not alter expression (mRNA or protein) of ATGL, ABHD5/CGI-58, PLIN2 or PLIN5, but inactivation of both DGATs increased PLIN2 abundance despite a dramatic reduction in the number of LDs. ATGL localized preferentially to DGAT1- made LDs rather than to DGAT2-made LDs, and TG in these LDs was preferentially used for fatty acid (FA) oxidation. A combination of DGAT2 inhibitor and the pan lipase inhibitor E600 resulted in large LDs, suggesting that the small size of DGAT1-made LDs is due to a lipolytic process.

Vaccine antigen, Factor H binding protein, is typically a non-lipidated precursor that localises to the meningococcal surface by Slam

Meningococcal surface lipoprotein, Factor H binding protein (FHbp), is the sole antigen of the Trumenba vaccine (Pfizer) and one of four antigens of the Bexsero vaccine (GSK) targetingNeisseria meningitidisserogroup B isolates. Lipidation of FHbp is assumed to occur for all isolates and its surface localisation is conducted by surface lipoprotein assembly modulator, Slam.We show in 91% of a collection of UK isolates (1742/1895) non-synonymous single nucleotide polymorphisms (SNPs) in the signal peptide of FHbp. A single SNP, common to all, alters a polar amino acid that abolishes processing, including lipidation and signal peptide cleavage. Rather than the toxic accumulation of the precursor in the periplasm as expected from disrupting the canonical processing pathway, remarkably the FHbp precursor is translocated to the outer membrane and surface-localised by Slam. Thus we show Slam is not lipoprotein-specific. In a panel of isolates expressing precursor FHbp at the surface, we investigated their binding to human factor H and their susceptibility to antibody-mediated killing. Our findings have implications for Trumenba and Bexsero and provide key insights for lipoprotein-based vaccines in development.

Choline Supplementation in Cystic Fibrosis—The Metabolic and Clinical Impact

Background: Choline is essential for the synthesis of liver phosphatidylcholine (PC), parenchymal maintenance, bile formation, and lipoprotein assembly to secrete triglycerides. In choline deficiency, the liver accretes choline/PC at the expense of lung tissue, thereby impairing pulmonary PC homoeostasis. In cystic fibrosis (CF), exocrine pancreas insufficiency results in impaired cleavage of bile PC and subsequent fecal choline loss. In these patients, the plasma choline concentration is low and correlates with lung function. We therefore investigated the effect of choline supplementation on plasma choline/PC concentration and metabolism, lung function, and liver fat. Methods: 10 adult male CF patients were recruited (11/2014–1/2016), and orally supplemented with 3 × 1 g choline chloride for 84 (84–91) days. Pre-/post-supplementation, patients were spiked with 3.6 mg/kg [methyl-D9]choline chloride to assess choline/PC metabolism. Mass spectrometry, spirometry, and hepatic nuclear resonance spectrometry served for analysis. Results: Supplementation increased plasma choline from 4.8 (4.1–6.2) µmol/L to 10.5 (8.5–15.5) µmol/L at d84 (p < 0.01). Whereas plasma PC concentration remained unchanged, D9-labeled PC was decreased (12.2 [10.5–18.3] µmol/L vs. 17.7 [15.5–22.4] µmol/L, p < 0.01), indicating D9-tracer dilution due to higher choline pools. Supplementation increased Forced Expiratory Volume in 1 second percent of predicted (ppFEV1) from 70.0 (50.9–74.8)% to 78.3 (60.1–83.9)% (p < 0.05), and decreased liver fat from 1.58 (0.37–8.82)% to 0.84 (0.56–1.17)% (p < 0.01). Plasma choline returned to baseline concentration within 60 h. Conclusions: Choline supplementation normalized plasma choline concentration and increased choline-containing PC precursor pools in adult CF patients. Improved lung function and decreased liver fat suggest that in CF correcting choline deficiency is clinically important. Choline supplementation of CF patients should be further investigated in randomized, placebo-controlled trials.

Kinesin-dependent mechanism for controlling triglyceride secretion from the liver

Despite massive fluctuations in its internal triglyceride content, the liver secretes triglyceride under tight homeostatic control. This buffering function is most visible after fasting, when liver triglyceride increases manyfold but circulating serum triglyceride barely fluctuates. How the liver controls triglyceride secretion is unknown, but is fundamentally important for lipid and energy homeostasis in animals. Here we find an unexpected cellular and molecular mechanism behind such control. We show that kinesin motors are recruited to triglyceride-rich lipid droplets (LDs) in the liver by the GTPase ARF1, which is a key activator of lipolysis. This recruitment is activated by an insulin-dependent pathway and therefore responds to fed/fasted states of the animal. In fed state, ARF1 and kinesin appear on LDs, consequently transporting LDs to the periphery of hepatocytes where the smooth endoplasmic reticulum (sER) is present. Because the lipases that catabolize LDs in hepatocytes reside on the sER, LDs can now be catabolized efficiently to provide triglyceride for lipoprotein assembly and secretion from the sER. Upon fasting, insulin is lowered to remove ARF1 and kinesin from LDs, thus down-regulating LD transport and sER–LD contacts. This tempers triglyceride availabiity for very low density lipoprotein assembly and allows homeostatic control of serum triglyceride in a fasted state. We further show that kinesin knockdown inhibits hepatitis-C virus replication in hepatocytes, likely because translated viral proteins are unable to transfer from the ER to LDs.