Adrenocortical lipid depletion gene (ald) in AKR mice is associated with an acyl-CoA:cholesterol acyltransferase (ACAT) mutation

Glycerolipid Synthesis and Lipid Droplet Formation in the Endoplasmic Reticulum

More than 60 years ago, Eugene Kennedy and coworkers elucidated the endoplasmic reticulum (ER)-based pathways of glycerolipid synthesis, including the synthesis of phospholipids and triacylglycerols (TGs). The reactions of the Kennedy pathway were identified by studying the conversion of lipid intermediates and the isolation of biochemical enzymatic activities, but the molecular basis for most of these reactions was unknown. With recent progress in the cell biology, biochemistry, and structural biology in this area, we have a much more mechanistic understanding of this pathway and its reactions. In this review, we provide an overview of molecular aspects of glycerolipid synthesis, focusing on recent insights into the synthesis of TGs. Further, we go beyond the Kennedy pathway to describe the mechanisms for storage of TG in cytosolic lipid droplets and discuss how overwhelming these pathways leads to ER stress and cellular toxicity, as seen in diseases linked to lipid overload and obesity.

Acyl-Coenzyme A: Cholesterol Acyltransferase (ACAT) in Cholesterol Metabolism: From Its Discovery to Clinical Trials and the Genomics Era

The purification and cloning of the acyl-coenzyme A: cholesterol acyltransferase (ACAT) enzymes and the sterol O-acyltransferase (SOAT) genes has opened new areas of interest in cholesterol metabolism given their profound effects on foam cell biology and intestinal lipid absorption. The generation of mouse models deficient in Soat1 or Soat2 confirmed the importance of their gene products on cholesterol esterification and lipoprotein physiology. Although these studies supported clinical trials which used non-selective ACAT inhibitors, these trials did not report benefits, and one showed an increased risk. Early genetic studies have implicated common variants in both genes with human traits, including lipoprotein levels, coronary artery disease, and Alzheimer’s disease; however, modern genome-wide association studies have not replicated these associations. In contrast, the common SOAT1 variants are most reproducibly associated with testosterone levels.

Quantitative Trait Locus Mapping of Macrophage Cholesterol Metabolism and CRISPR/Cas9 Editing Implicate an ACAT1 Truncation as a Causal Modifier Variant

OBJECTIVE

Cholesterol metabolism is a dynamic process involving intracellular trafficking, cholesterol esterification, and cholesterol ester hydrolysis. Our objective was to identify genes that regulate macrophage cholesterol metabolism.

APPROACHES AND RESULTS

We performed quantitative trait loci mapping of free and esterified cholesterol levels and the ratio of esterified to free cholesterol in acetylated low-density lipoprotein-loaded bone marrow-derived macrophages from an AKR×DBA/2 strain intercross. Ten distinct cholesterol modifier loci were identified, and bioinformatics was used to prioritize candidate genes. The strongest locus was located on distal chromosome 1, which we named Mcmm1 (macrophage cholesterol metabolism modifier 1). This locus harbors the Soat1 (sterol O-acyltransferase 1) gene, encoding Acyl-coenzyme A:cholesterol acyltransferase 1 (ACAT1), which esterifies free cholesterol. The parental AKR strain has an exon 2 deletion in Soat1, which leads to a 33 amino acid N-terminal truncation in ACAT1. CRISPR/Cas9 editing of DBA/2 embryonic stem cells was performed to replicate the AKR strain Soat1 exon 2 deletion, while leaving the remainder of the genome unaltered. DBA/2 stem cells and stem cells heterozygous and homozygous for the Soat1 exon 2 deletion were differentiated into macrophages and loaded with acetylated low-density lipoprotein. DBA/2 stem cell-derived macrophages accumulated less free cholesterol and more esterified cholesterol relative to cells heterozygous and homozygous for the Soat1 exon 2 deletion.

CONCLUSIONS

A Soat1 deletion present in AKR mice, and resultant N-terminal ACAT1 truncation, was confirmed to be a significant modifier of macrophage cholesterol metabolism. Other Mcmm loci candidate genes were prioritized via bioinformatics.

Physiological difference in autophagic flux in macrophages from 2 mouse strains regulates cholesterol ester metabolism

OBJECTIVE

DBA/2 apoE(-/-) mice have ≈10-fold larger lesions than AKR apoE(-/-) mice. The objective of this study was to determine whether macrophages from these 2 strains had altered cholesterol metabolism that might play a role in their divergent atherosclerosis susceptibility.

APPROACH AND RESULTS

AKR and DBA/2 macrophages incubated with acetylated low-density lipoprotein resulted in higher cholesterol ester (CE) and lower free cholesterol accumulation in the DBA/2 cells. However, these strains had equivalent acetylated low-density lipoprotein uptake and cholesterol esterification activity. Cholesterol efflux from unloaded cells to apolipoprotein A-I or high-density lipoprotein was similar in the 2 strains. However, on acetylated low-density lipoprotein loading, cholesterol efflux was impaired in the DBA/2 cells, but this impairment was corrected by loading in the presence of an inhibitor of cholesterol esterification. Thus, the cholesterol efflux capabilities are similar in these strains, but there seemed to be a defect in lipid droplet-stored CE mobilization in DBA/2 cells. Lalistat 1, a specific inhibitor of lysosomal acid lipase, completely blocked the hydrolysis of lipid droplet-stored CE, implying that lipid droplet autophagy is responsible for CE turnover in these cells. CE turnover was 2-fold slower in DBA/2 versus AKR cells. Autophagic flux, estimated by a fluorescent light chain 3-II reporter and the increase in p62 levels after chloroquine treatment, was higher in AKR versus DBA/2 macrophages, which had an apparent decrease in autophagosome fusion with lysosomes. When autophagy was activated by amino acid starvation, CE levels decreased in DBA/2 cells.

CONCLUSIONS

Physiological regulation of autophagy in macrophages controls CE accumulation and may modify atherosclerosis susceptibility.

Sterol O-acyltransferase 1 (SOAT1, ACAT) is a novel target of steroidogenic factor-1 (SF-1, NR5A1, Ad4BP) in the human adrenal

CONTEXT

Steroidogenic factor-1 (SF-1, NR5A1, Ad4BP) is a master regulator of adrenal development and steroidogenesis. Defects in several known targets of SF-1 can cause adrenal disorders in humans.

OBJECTIVE

We aimed to identify novel targets of SF-1 in the human adrenal. These factors could be important regulators of adrenal development and steroidogenesis and potential candidates for adrenal dysfunction.

DESIGN

A gene discovery strategy was developed based on bidirectional manipulation of SF-1. Overexpression or knockdown of SF-1 in NCI-H295R human adrenocortical cells was used to identify a subset of positively-regulated SF-1 targets.

RESULTS

This approach identified well-established SF-1 target genes (STAR, CYP11A) and several novel genes (VSNL1, ZIM2, PEG3, SOAT1, and MTSS1). Given its role in cholesterol metabolism, sterol O-acyltransferase 1 (SOAT1, previously referred to as acyl-Coenzyme A:cholesterol acyltransferase 1, ACAT) was studied further and found to be expressed in the developing human fetal adrenal cortex. We hypothesized that impaired SOAT1 activity could result in adrenal insufficiency through reduced cholesteryl ester reserves or through toxic destruction of the adrenal cells during development. Therefore, mutational analysis of SOAT1 in a cohort of 43 patients with unexplained adrenal insufficiency was performed but failed to reveal significant coding sequence changes.

CONCLUSIONS

Our reverse discovery approach led to the identification of novel SF-1 targets and defined SOAT1 as an important factor in human adrenal steroidogenesis. SF-1-dependent up-regulation of SOAT1 may be important for maintaining readily-releasable cholesterol reserves needed for active steroidogenesis and during episodes of recurrent stress.

Hair interior defect in AKR/J mice

BACKGROUND

All AKR/J mice have a subtle defect that involves malformation of the central portion of hair fibres that is best visualized under white and polarized light microscopy.

AIMS

This study sought to characterize the clinical and ultrastructural features of the hair interior defect (HID) phenotype and to determine the chromosomal localization of the hid mutant gene locus.

METHODS

White and polarized light microscopy combined with scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to characterize the HID phenotype. Complementation testing and gene-linkage studies were performed to map the locus.

RESULTS

Using SEM, the hair-fibre structure on the surface was found to be similar to hairs obtained from normal BALB/cByJ+/+and C57BL/6 J+/+mice. There were also no differences in sulphur content. TEM revealed degenerative changes in the medulla similar to that seen by light microscopy. This autosomal recessive mutation is called HID (locus symbol: hid). We mapped the hid locus to the distal end of mouse chromosome 1. No genes reported to cause skin or hair abnormalities are known to be within this interval except for the lamin B receptor (Lbr), which had been excluded previously as the cause of the hid phenotype in AKR/J mice.

CONCLUSION

A potentially novel gene or known gene with a novel phenotype resides within this interval, which may shed light on human diseases with defects in the inner structure of the hair fibre.

Identification of three loci affecting HDL-cholesterol levels in a screen for chemically induced recessive mutations in mice

We conducted a genome-wide screen using the mutagen N-ethyl-N-nitrosourea to identify recessive mutations in genes that lead to altered lipid traits in mice. We screened 7,546 G3 mice that were of mixed C57BL/6J (B6) x C3.SW-H2(b)/SnJ (C3) genomes and identified three pedigrees with differences in plasma HDL-cholesterol. Genome scan analyses mapped three distinct loci to chromosomes 3, 4, and 7. An S1748L missense mutation was identified in ABCA1 in one pedigree with undetectable levels of HDL-cholesterol and resulted in reduced protein levels. This phenotype was completely penetrant, semi-dominant, and cosegregated with high plasma triglycerides. Mice in a second pedigree had very high levels of plasma total cholesterol and HDL-cholesterol (up to 800 mg/dl total cholesterol). Despite a high degree of phenotype lability and reduced penetrance, an I68N missense mutation was identified in the transcription factor CCAAT/enhancer binding protein alpha (C/EBPalpha). Finally, a second high HDL-cholesterol pedigree of mice, again with a highly labile phenotype and reduced penetrance, was mapped to a 7 Mb locus on chromosome 3. These results illustrate the use of a hybrid background for simultaneous screening and mapping of mutagenized pedigrees of mice and identification of three novel alleles of HDL-cholesterol phenotypes.

Mapping, genetic isolation, and characterization of genetic loci that determine resistance to atherosclerosis in C3H mice

OBJECTIVE

C3H/HeJ (C3H) mice are extremely resistant to atherosclerosis. To identify the genetic factors involved in lesion initiation, we studied a cross between C3H and the susceptible strain C57BL/6J (B6) on a hyperlipidemic (apolipoprotein E-null) background.

METHODS AND RESULTS

Whereas a previous cross in mice fed a Western diet for 16 weeks revealed a very complex inheritance pattern with many significant lesion QTLs, the present cross, on a chow diet, revealed a single major locus on chromosome 9 (lod=5.0, Ath29*), and a suggestive locus on chromosome 4 (lod=2.6, Ath8). QTLs for plasma HDL, total cholesterol, and triglyceride levels were found on chromosome 1 over the ApoA2 gene. Neither of the lesion QTLs were associated with differences in plasma lipid levels or other systemic risk factors, consistent with the concept that genetic factors affecting cellular functions of the vessel wall are important determinants of atherosclerosis susceptibility. We generated a congenic strain for Ath29 and confirmed its contribution to lesion development. Toll-like receptor 4 (Tlr4), the lipopolysaccharide (LPS) receptor, is located in the Ath8 region and is known to be defective in C3H/HeJ mice. We constructed a congenic strain carrying a normal Tlr4 gene on the C3H Apoe-null background and found that the defective Tlr4 does not contribute significantly to lesion resistance during early lesion development.

CONCLUSIONS

We identified one major QTL on chromosome 9, Ath29, for early lesion development in the BXH ApoE(-/-) cross fed on a chow diet and confirmed its contribution in congenic mice. We have also determined that Tlr4 on the C3H ApoE(-/-) background does not contribute to early lesion development. *Ath29 is referred to as Ath22 in Su et al 2006.

Approaches to lipid metabolism gene identification and characterization in the postgenomic era

The availability of genomic resources has already had a tremendous impact on biomedical research. In this review, we describe how whole genome sequence and high-throughput functional genomics projects have facilitated the identification and characterization of important genes in lipid metabolism and disease. We review key approaches and lipid genes identified in the first years of this century and discuss how genomic resources are likely to streamline gene identification and functional characterization in the future.

Conservation of eukaryotic sterol homeostasis: new insights from studies in budding yeast

The model eukaryote Saccharomyces cerevisiae (budding yeast) has provided significant insight into sterol homeostasis. The study of sterol metabolism in a genetically amenable model organism such as yeast is likely to have an even greater impact and relevance to human disease with the advent of the complete human genome sequence. In addition to definition of the sterol biosynthetic pathway, almost to completion, the remarkable conservation of other components of sterol homeostasis are described in this review.

Recombinant acyl-CoA:cholesterol acyltransferase-1 (ACAT-1) purified to essential homogeneity utilizes cholesterol in mixed micelles or in vesicles in a highly cooperative manner

Acyl-coenzyme A:cholesterol acyltransferase (ACAT) is an integral membrane protein located in the endoplasmic reticulum. It catalyzes the formation of cholesteryl esters from cholesterol and long-chain fatty acyl coenzyme A. The first gene encoding the enzyme, designated as ACAT-1, was identified in 1993 through an expression cloning approach. We isolated a Chinese hamster ovary cell line that stably expresses the recombinant human ACAT-1 protein bearing an N-terminal hexahistidine tag. We purified this enzyme approximately 7000-fold from crude cell extracts by first solubilizing the cell membranes with the zwitterionic detergent 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate, then proceeding with an ACAT-1 monoclonal antibody affinity column and an immobilized metal affinity column. The final preparation is enzymologically active and migrates as a single band at 54 kDa on SDS-polyacrylamide gel electrophoresis. Pure ACAT-1 dispersed in mixed micelles containing sodium taurocholate, phosphatidylcholine, and cholesterol remains catalytically active. The cholesterol substrate saturation curves of the enzyme assayed either in mixed micelles or in reconstituted vesicles are both highly sigmoidal. The oleoyl-coenzyme A substrate saturation curves of the enzyme assayed under the same conditions are both hyperbolic. These results support the hypothesis that ACAT is an allosteric enzyme regulated by cholesterol.

Acyl CoA:cholesterol acyltransferase genes and knockout mice

Acyl coenzyme A:cholesterol acyltransferase (ACAT) (EC 2.3.1.26) is an enzyme, located in the endoplasmic reticulum of many types of cells, that catalyzes cholesterol ester formation from cholesterol and fatty acyl CoA substrates. Sterol esterification by ACAT or homologous enzymes is conserved in evolution dating back to yeast. The recent cloning of a human cDNA for ACAT, together with genome sequencing projects, has led to the identification of an ACAT gene family and provided molecular tools for determining ACAT's functions in vivo. Summarized here is the current knowledge concerning the molecular genetics of ACAT.