Loss of subcellular lipid transport due to ARV1 deficiency disrupts organelle homeostasis and activates the unfolded protein response.

The ARV1-encoded protein mediates sterol transport from the endoplasmic reticulum (ER) to the plasma membrane. Yeast ARV1 mutants accumulate multiple lipids in the ER and are sensitive to pharmacological modulators of both sterol and sphingolipid metabolism. Using fluorescent and electron microscopy, we demonstrate sterol accumulation, subcellular membrane expansion, elevated lipid droplet formation, and vacuolar fragmentation in ARV1 mutants. Motif-based regression analysis of ARV1 deletion transcription profiles indicates activation of Hac1p, an integral component of the unfolded protein response (UPR). Accordingly, we show constitutive splicing of HAC1 transcripts, induction of a UPR reporter, and elevated expression of UPR targets in ARV1 mutants. IRE1, encoding the unfolded protein sensor in the ER lumen, exhibits a lethal genetic interaction with ARV1, indicating a viability requirement for the UPR in cells lacking ARV1. Surprisingly, ARV1 mutants expressing a variant of Ire1p defective in sensing unfolded proteins are viable. Moreover, these strains also exhibit constitutive HAC1 splicing that interacts with DTT-mediated perturbation of protein folding. These data suggest that a component of UPR induction in arv1Δ strains is distinct from protein misfolding. Decreased ARV1 expression in murine macrophages also results in UPR induction, particularly up-regulation of activating transcription factor-4, CHOP (C/EBP homologous protein), and apoptosis. Cholesterol loading or inhibition of cholesterol esterification further elevated CHOP expression in ARV1 knockdown cells. Thus, loss or down-regulation of ARV1 disturbs membrane and lipid homeostasis, resulting in a disruption of ER integrity, one consequence of which is induction of the UPR.

Sterol homeostasis in the budding yeast, Saccharomyces cerevisiae.

Lipid related diseases, such as obesity, type 2 diabetes, and atherosclerosis are epidemics in developed civilizations. A common underlying factor among these syndromes is excessive subcellular accumulation of lipids such as cholesterol and triglyceride. The homeostatic events that govern these metabolites are understood to varying degrees of sophistication. We describe here the utilization of a genetically powerful model organism, budding yeast, to identify and characterize novel aspects of sterol and lipid homeostasis.

Identification of two novel human acyl-CoA wax alcohol acyltransferases: members of the diacylglycerol acyltransferase 2 (DGAT2) gene superfamily.

The esterification of alcohols such as sterols, diacylglycerols, and monoacylglycerols with fatty acids represents the formation of both storage and cytoprotective molecules. Conversely, the overproduction of these molecules is associated with several disease pathologies, including atherosclerosis and obesity. The human acyl-CoA:diacylglycerol acyltransferase (DGAT) 2 gene superfamily comprises seven members, four of which have been previously implicated in the synthesis of di- or triacylglycerol. The remaining 3 members comprise an X-linked locus and have not been characterized. We describe here the expression of DGAT2 and the three X-linked genes in Saccharomyces cerevisiae strains virtually devoid of neutral lipids. All four gene products mediate the synthesis of triacylglycerol; however, two of the X-linked genes act as acyl-CoA wax alcohol acyltransferases (AWAT 1 and 2) that predominantly esterify long chain (wax) alcohols with acyl-CoA-derived fatty acids to produce wax esters. AWAT1 and AWAT2 have very distinct substrate preferences in terms of alcohol chain length and fatty acyl saturation. The enzymes are expressed in many human tissues but predominate in skin. In situ hybridizations demonstrate a differentiation-specific expression pattern within the human sebaceous gland for the two AWAT genes, consistent with a significant role in the composition of sebum.

The major sites of cellular phospholipid synthesis and molecular determinants of Fatty Acid and lipid head group specificity.

Phosphatidylcholine and phosphatidylethanolamine are the two main phospholipids in eukaryotic cells comprising ~50 and 25% of phospholipid mass, respectively. Phosphatidylcholine is synthesized almost exclusively through the CDP-choline pathway in essentially all mammalian cells. Phosphatidylethanolamine is synthesized through either the CDP-ethanolamine pathway or by the decarboxylation of phosphatidylserine, with the contribution of each pathway being cell type dependent. Two human genes, CEPT1 and CPT1, code for the total compliment of activities that directly synthesize phosphatidylcholine and phosphatidylethanolamine through the CDP-alcohol pathways. CEPT1 transfers a phosphobase from either CDP-choline or CDP-ethanolamine to diacylglycerol to synthesize both phosphatidylcholine and phosphatidylethanolamine, whereas CPT1 synthesizes phosphatidylcholine exclusively. We show through immunofluorescence that brefeldin A treatment relocalizes CPT1, but not CEPT1, implying CPT1 is found in the Golgi. A combination of coimmunofluorescence and subcellular fractionation experiments with various endoplasmic reticulum, Golgi, and nuclear markers confirmed that CPT1 was found in the Golgi and CEPT1 was found in both the endoplasmic reticulum and nuclear membranes. The rate-limiting step for phosphatidylcholine synthesis is catalyzed by the amphitropic CTP:phosphocholine cytidylyltransferase alpha, which is found in the nucleus in most cell types. CTP:phosphocholine cytidylyltransferase alpha is found immediately upstream cholinephosphotransferase, and it translocates from a soluble nuclear location to the nuclear membrane in response to activators of the CDP-choline pathway. Thus, substrate channeling of the CDP-choline produced by CTP:phosphocholine cytidylyltransferase alpha to nuclear located CEPT1 is the mechanism by which upregulation of the CDP-choline pathway increases de novo phosphatidylcholine biosynthesis. In addition, a series of CEPT1 site-directed mutants was generated that allowed for the assignment of specific amino acid residues as structural requirements that directly alter either phospholipid head group or fatty acyl composition. This pinpointed glycine 156 within the catalytic motif as being responsible for the dual CDP-alcohol specificity of CEPT1, whereas mutations within helix 214-228 allowed for the orientation of transmembrane helices surrounding the catalytic site to be definitively positioned.