Decreased Expression of ARV1 Results in Cholesterol Retention in the Endoplasmic Reticulum and Abnormal Bile Acid Metabolism*

Abstract

Endoplasmic reticulum (ER) membrane cholesterol is maintained at an optimal concentration of ϳ5 mol % by the net impact of sterol synthesis, modification, and export. Arv1p was first identified in the yeast Saccharomyces cerevisiae as a key component of this homeostasis due to its probable role in intracellular sterol transport. Mammalian ARV1, which can fully complement the yeast lesion, encodes a ubiquitously expressed, resident ER protein. Repeated dosing of specific antisense oligonucleotides to ARV1 produced a marked reduction of ARV1 transcripts in liver, adipose, and to a lesser extent, intestine. This resulted in marked hypercholesterolemia, elevated serum bile acids, and activation of the hepatic farnesoid X receptor (FXR) regulatory pathway. Knockdown of ARV1 in murine liver and HepG2 cells was associated with accumulation of cholesterol in the ER at the expense of the plasma membrane and suppression of sterol regulatory element-binding proteins and their targets. These studies indicate a critical role of mammalian Arv1p in sterol movement from the ER and in the ensuing regulation of hepatic cholesterol and bile acid metabolism.The endoplasmic reticulum (ER) 4 is the pivotal organelle with regard to cholesterol homeostasis. It is here that cholesterol is synthesized via the mevalonate pathway, sensed by the sterol regulatory element-binding protein (SREBP) cleavageactivating protein system, and neutralized by esterification (1-3). In addition, in certain cells, ER cholesterol is secreted in lipoprotein particles or hydroxylated to form bile acids. Consequently, sterol levels in the ER of all eukaryotic cells are strikingly low relative to the plasma membrane (PM) (4). Movement of sterol between these organelles is rapid; the majority of endogenously synthesized cholesterol is transported from the ER to the PM within 10 -20 min by an energy-dependent process (5). Vesicular and nonvesicular cholesterol transport pathways have been described (6, 7); however, the molecular components of these events are surprisingly obscure. The net impact of these processes is striking; any variation around the threshold concentration of ϳ5 mol % results in activation or repression of key regulators of lipid homeostasis, including the master regulators, SREBP-1 and -2 (8).In the yeast Saccharomyces cerevisiae, ARV1 encodes a key component of sterol transport from the ER to the PM and was identified by complementation of yeast mutations that confer viability dependence upon sterol esterification (9). This concept of synthetic lethality was based on the hypothesis that loss of one homeostatic pathway (e.g. sterol esterification) might be tolerated; however, removal of multiple "detoxifying" events would be lethal. Yeast with mutations in ARV1 have striking phenotypes, including an elevated ratio of subcellular sterols relative to the PM and abnormal phospholipid, sphingolipid, and glycosylphosphatidylinositol metabolism (9 -11). The yeast ARV1 gene predicts a 322-residue protein with several transmembrane domains. An ARV1...

The Putative Lipid Transporter, Arv1, Is Required for Activating Pheromone-Induced MAP Kinase Signaling in Saccharomyces cerevisiae

“…Moreover, arv1 cells harbor defects in sphingolipid metabolism (Swain et al 2002), glycosylphosphatidylinositol (GPI) biosynthesis (Kajiwara et al 2008), and may harbor defects in sterol trafficking (Tinkelenberg et al 2000;Fei et al 2008). It has been suggested that Arv1 plays a role in cholesterol trafficking from the ER to plasma membrane in mammalian cells (Tong et al 2010). In the present work we show that arv1 yeast cells are mating defective.…”

Mice lacking have reduced signs of metabolic syndrome and non-alcoholic fatty liver disease

“…Our results strongly suggest that complete loss of Arv1 protects mice from acquiring MetS and NAFLD in response to a HFD challenge. While it is still up for debate as to what is the function of ARV1 (13)(14)(15)18,19,41), evidence is accumulating that suggests it's involvement in maintaining proper sterol distribution (16,20,42,43).…”

“…Knockdown of Arv1 in HepG2 cells results in the accumulation of cholesterol in the ER and a reduction in the plasma membrane, suggesting defective sterol transport in the liver (20). Mice treated with ASOs (anti-sense oligonucleotides) to Arv1 have increases in plasma and liver bile acid levels that contain hydrophilic bile acid salts (20). The same authors observed that early loss of Arv1 caused increases in plasma PL, cholesterol, and LDL-C (low density apolipoprotein-cholesterol) levels, and a decrease in HDL-C (high density apolipoprotein-cholesterol) levels (20).…”

“…Knockdown of Arv1 in HepG2 cells results in the accumulation of cholesterol in the ER and a reduction in the plasma membrane, suggesting defective sterol transport in the liver (20). Mice treated with ASOs (anti-sense oligonucleotides) to Arv1 have increases in plasma and liver bile acid levels that contain hydrophilic bile acid salts (20).…”

“…Mice treated with ASOs (anti-sense oligonucleotides) to Arv1 have increases in plasma and liver bile acid levels that contain hydrophilic bile acid salts (20). The same authors observed that early loss of Arv1 caused increases in plasma PL, cholesterol, and LDL-C (low density apolipoprotein-cholesterol) levels, and a decrease in HDL-C (high density apolipoprotein-cholesterol) levels (20). Interestingly, human ARV1 possesses in vitro lipid binding activity, as it binds sterol intermediates (21), several fatty acid species (22), and specific phospholipids and cholesterol (H-Y…”