The low-density lipoprotein receptor (LDLR) plays a pivotal role in clearing atherogenic circulating low-density lipoprotein (LDL) cholesterol. Here we show that the COMMD/CCDC22/CCDC93 (CCC) and the Wiskott–Aldrich syndrome protein and SCAR homologue (WASH) complexes are both crucial for endosomal sorting of LDLR and for its function. We find that patients with X-linked intellectual disability caused by mutations in CCDC22 are hypercholesterolaemic, and that COMMD1-deficient dogs and liver-specific Commd1 knockout mice have elevated plasma LDL cholesterol levels. Furthermore, Commd1 depletion results in mislocalization of LDLR, accompanied by decreased LDL uptake. Increased total plasma cholesterol levels are also seen in hepatic COMMD9-deficient mice. Inactivation of the CCC-associated WASH complex causes LDLR mislocalization, increased lysosomal degradation of LDLR and impaired LDL uptake. Furthermore, a mutation in the WASH component KIAA0196 (strumpellin) is associated with hypercholesterolaemia in humans. Altogether, this study provides valuable insights into the mechanisms regulating cholesterol homeostasis and LDLR trafficking.

Objective Recent genome-wide association studies revealed that a genetic variant in the loci corresponding to histone deacetylase 9 (HDAC9) is associated with large vessel stroke. HDAC9 expression was upregulated in human atherosclerotic plaques in different arteries. The molecular mechanisms how HDAC9 might increase atherosclerosis is not clear. Approach and Results In this study, we show that systemic and bone marrow cell deletion of HDAC9 decreased atherosclerosis in LDLr−/− mice with minimal effect on plasma lipid concentrations. HDAC9 deletion resulted upregulation of lipid homeostatic genes, downregulation of inflammatory genes, and polarization towards an M2 phenotype via increased accumulation of total acetylated H3 and H3K9 at the promoters of ABCA1, ABCG1, and PPAR-γ in macrophages. Conclusions We conclude that macrophage HDAC9 upregulation is atherogenic via suppression of cholesterol efflux and generation of alternatively activated macrophages in atherosclerosis.

The synthesis of cholesterol and fatty acids (FA) in the liver is independently regulated by SREBP-2 and SREBP-1c, respectively. Here, we genetically deleted Srebf-2 from hepatocytes and confirmed that SREBP-2 regulates all genes involved in cholesterol biosynthesis, the LDL receptor, and PCSK9; a secreted protein that degrades LDL receptors in the liver. Surprisingly, we found that elimination of Srebf-2 in hepatocytes of mice also markedly reduced SREBP-1c and the expression of all genes involved in FA and triglyceride synthesis that are normally regulated by SREBP-1c. The nuclear receptor LXR is necessary for Srebf-1c transcription. The deletion of Srebf-2 and subsequent lower sterol synthesis in hepatocytes eliminated the production of an endogenous sterol ligand required for LXR activity and SREBP-1c expression. These studies demonstrate that cholesterol and FA synthesis in hepatocytes are coupled and that flux through the cholesterol biosynthetic pathway is required for the maximal SREBP-1c expression and high rates of FA synthesis.DOI: http://dx.doi.org/10.7554/eLife.25015.001

Loss of ABCA1 activity in Tangier disease (TD) is associated with abnormal apoB lipoprotein (Lp) metabolism in addition to the complete absence of high density lipoprotein (HDL). We used hepatocyte-specific ABCA1 knock-out (HSKO) mice to test the hypothesis that hepatic ABCA1 plays dual roles in regulating Lp metabolism and nascent HDL formation. HSKO mice recapitulated the TD lipid phenotype with postprandial hypertriglyceridemia, markedly decreased LDL, and near absence of HDL. Triglyceride (TG) secretion was 2-fold higher in HSKO compared with wild type mice, primarily due to secretion of larger TG-enriched VLDL secondary to reduced hepatic phosphatidylinositol 3-kinase signaling. HSKO mice also displayed delayed clearance of postprandial TG and reduced post-heparin plasma lipolytic activity. In addition, hepatic LDLr expression and plasma LDL catabolism were increased 2-fold in HSKO compared with wild type mice. Last, adenoviral repletion of hepatic ABCA1 in HSKO mice normalized plasma VLDL TG and hepatic phosphatidylinositol 3-kinase signaling, with a partial recovery of HDL cholesterol levels, providing evidence that hepatic ABCA1 is involved in the reciprocal regulation of apoB Lp production and HDL formation. These findings suggest that altered apoB Lp metabolism in TD subjects may result from hepatic VLDL TG overproduction and increased hepatic LDLr expression and highlight hepatic ABCA1 as an important regulatory factor for apoB-containing Lp metabolism. ABCA1 (ATP-binding cassette transporter A1) is indispensable in the initial steps of high density lipoprotein (HDL)2 formation and the process of reverse cholesterol transport from peripheral tissues to the liver. ABCA1 is expressed in many cells; however, hepatocytes make the single most important contribution to plasma HDL concentration (1-3). Mutations in ABCA1 in humans cause Tangier disease (TD), an autosomal recessive disorder characterized by severe HDL deficiency, rapid plasma clearance of HDL and apoA-I, sterol deposition in tissues, and premature coronary atherosclerosis (4 -7). In addition to HDL deficiency, TD subjects have significantly elevated plasma TG and a 50% reduction in LDL cholesterol concentrations (4,8). The TG phenotype in TD disease is complicated, with most, but not all, TD subjects displaying elevated fasting or postprandial TG elevations (9). Clee et al. (8) reported an inverse relationship between dysfunctional ABCA1 alleles and plasma TG concentrations. In addition, data from case reports of 59 Tangier patients show variable TG concentrations, with mean, median, minimum, and maximum concentrations of 210, 175, 40, and 580 mg/dl, respectively (4). The underlying mechanisms for the increased plasma TG and decreased LDL concentrations in TD subjects have not been established. In one study, apoA-II enrichment of VLDL of TD subjects was proposed to result in reduced reactivity of VLDL with lipoprotein lipase (LPL) (9, 10). Another study suggested that ABCA1-dependent cholesterol efflux decreases VLDL secretion from murine hep...

Genetic variants that increase the risk of fatty liver disease (FLD) and cirrhosis have recently been identified in the proximity of membrane bound O-acyltransferase domain-containing 7 (MBOAT7).  To elucidate the link between these variants and FLD we characterized Mboat7 liver-specific knock-out mice (Mboat7-LSKO).  Chow-fed Mboat7-LSKO mice developed fatty livers and associated liver injury.  Lipidomic analysis of liver using mass spectrometry revealed a pronounced reduction in 20-carbon polyunsaturated fatty acid content in phosphatidylinositols (PIs), but not in other phospholipids. The change in fatty acid composition of PIs in these mice was associated with a marked increase in de novo lipogenesis due to activation of SREBP-1c, a transcription factor that coordinates the activation of genes encoding enzymes in the fatty acid biosynthesis pathway. Hepatic removal of both SREBP cleavage activating protein (Scap) and Mboat7 normalized hepatic triglycerides relative to Scap only hepatic knock-out showing increased SREBP-1c processing is required for Mboat7 induced steatosis.  This study reveals a clear relationship between PI fatty acid composition and regulation of hepatic fat synthesis and delineates the mechanism by which mutations in MBOAT7 cause hepatic steatosis.