Hepatic GATA4 regulates cholesterol and triglyceride homeostasis in collaboration with LXRs.

GATA4 is a transcription factor known for its crucial role in the development of many tissues, including the liver; however, its role in adult liver metabolism is unknown. Here, using high-throughput sequencing technologies, we identified GATA4 as a transcriptional regulator of metabolism in the liver. GATA4 expression is elevated in response to refeeding, and its occupancy is increased at enhancers of genes linked to fatty acid and lipoprotein metabolism. Knocking out GATA4 in the adult liver (Gata4LKO) decreased transcriptional activity at GATA4 binding sites, especially during feeding. Gata4LKO mice have reduced plasma HDL cholesterol and increased liver triglyceride levels. The expression of a panel of GATA4 binding genes involved in hepatic cholesterol export and triglyceride hydrolysis was down-regulated in Gata4LKO mice. We further demonstrate that GATA4 collaborates with LXR nuclear receptors in the liver. GATA4 and LXRs share a number of binding sites, and GATA4 was required for the full transcriptional response to LXR activation. Collectively, these results show that hepatic GATA4 contributes to the transcriptional control of hepatic and systemic lipid homeostasis.

Low MBOAT7 expression, a genetic risk for MASH, promotes a profibrotic pathway involving hepatocyte TAZ upregulation.

BACKGROUND AND AIMS: The common genetic variant rs641738 C>T is a risk factor for metabolic dysfunction-associated steatotic liver disease and metabolic dysfunction-associated steatohepatitis (MASH), including liver fibrosis, and is associated with decreased expression of the phospholipid-remodeling enzyme MBOAT7 (LPIAT1). However, whether restoring MBOAT7 expression in established metabolic dysfunction-associated steatotic liver disease dampens the progression to liver fibrosis and, importantly, the mechanism through which decreased MBOAT7 expression exacerbates MASH fibrosis remain unclear. APPROACH AND RESULTS: We first showed that hepatocyte MBOAT7 restoration in mice with diet-induced steatohepatitis slows the progression to liver fibrosis. Conversely, when hepatocyte-MBOAT7 was silenced in mice with established hepatosteatosis, liver fibrosis but not hepatosteatosis was exacerbated. Mechanistic studies revealed that hepatocyte-MBOAT7 restoration in MASH mice lowered hepatocyte-TAZ (WWTR1), which is known to promote MASH fibrosis. Conversely, hepatocyte-MBOAT7 silencing enhanced TAZ upregulation in MASH. Finally, we discovered that changes in hepatocyte phospholipids due to MBOAT7 loss-of-function promote a cholesterol trafficking pathway that upregulates TAZ and the TAZ-induced profibrotic factor Indian hedgehog (IHH). As evidence for relevance in humans, we found that the livers of individuals with MASH carrying the rs641738-T allele had higher hepatocyte nuclear TAZ, indicating higher TAZ activity and increased IHH mRNA. CONCLUSIONS: This study provides evidence for a novel mechanism linking MBOAT7-LoF to MASH fibrosis, adds new insight into an established genetic locus for MASH, and, given the druggability of hepatocyte TAZ for MASH fibrosis, suggests a personalized medicine approach for subjects at increased risk for MASH fibrosis due to inheritance of variants that lower MBOAT7.

Sphingosine kinases regulate ER contacts with late endocytic organelles and cholesterol trafficking.

Membrane contact sites (MCS), close membrane apposition between organelles, are platforms for interorganellar transfer of lipids including cholesterol, regulation of lipid homeostasis, and co-ordination of endocytic trafficking. Sphingosine kinases (SphKs), two isoenzymes that phosphorylate sphingosine to the bioactive sphingosine-1-phosphate (S1P), have been implicated in endocytic trafficking. However, the physiological functions of SphKs in regulation of membrane dynamics, lipid trafficking and MCS are not known. Here, we report that deletion of SphKs decreased S1P with concomitant increases in its precursors sphingosine and ceramide, and markedly reduced endoplasmic reticulum (ER) contacts with late endocytic organelles. Expression of enzymatically active SphK1, but not catalytically inactive, rescued the deficit of these MCS. Although free cholesterol accumulated in late endocytic organelles in SphK null cells, surprisingly however, cholesterol transport to the ER was not reduced. Importantly, deletion of SphKs promoted recruitment of the ER-resident cholesterol transfer protein Aster-B (also called GRAMD1B) to the plasma membrane (PM), consistent with higher accessible cholesterol and ceramide at the PM, to facilitate cholesterol transfer from the PM to the ER. In addition, ceramide enhanced in vitro binding of the Aster-B GRAM domain to phosphatidylserine and cholesterol liposomes. Our study revealed a previously unknown role for SphKs and sphingolipid metabolites in governing diverse MCS between the ER network and late endocytic organelles versus the PM to control the movement of cholesterol between distinct cell membranes.

Insufficient dietary choline aggravates disease severity in a mouse model of Citrobacter rodentium-induced colitis.

Dietary choline, which is converted to phosphatidylcholine (PC) in intestinal enterocytes, may benefit inflammatory bowel disease patients who typically have reduced intestinal choline and PC. The present study investigated the effect of dietary choline supplementation on colitis severity and intestinal mucosal homoeostasis using a Citrobacter rodentium-induced colitis model. C57BL/6J mice were fed three isoenergetic diets differing in choline level: choline-deficient (CD), choline-sufficient (CS) and choline-excess (CE) for 3 weeks prior to infection with C. rodentium. The effect of dietary choline levels on the gut microbiota was also characterised in the absence of infection using 16S rRNA gene amplicon sequencing. At 7 d following infection, the levels of C. rodentium in CD mice were significantly greater than that in CS or CE groups (P < 0·05). CD mice exhibited greater damage to the surface epithelium and goblet cell loss than the CS or CE mice, which was consistent with elevated pro-inflammatory cytokine and chemokine levels in the colon. In addition, CD group exhibited decreased concentrations of PC in the colon after C. rodentium infection, although the decrease was not observed in the absence of challenge. Select genera, including Allobaculum and Turicibacter, were enriched in response to dietary choline deficiency; however, there was minimal impact on the total bacterial abundance or the overall structure of the gut microbiota. Our results suggest that insufficient dietary choline intake aggravates the severity of colitis and demonstrates an essential role of choline in maintaining intestinal homoeostasis.

The marine compound and elongation factor 1A1 inhibitor, didemnin B, provides benefit in western diet-induced non-alcoholic fatty liver disease.

Inhibition of eukaryotic elongation factor 1A1 (EEF1A1) with the marine compound didemnin B decreases lipotoxic HepG2 cell death in vitro and improves early stage non-alcoholic fatty liver disease (NAFLD) in young genetically obese mice. However, the effects of didemnin B on NAFLD in a model of long-term diet-induced obesity are not known. We investigated the effects of didemnin B on NAFLD severity and metabolic parameters in western diet-induced obese mice, and on the cell types that contribute to liver inflammation and fibrosis in vitro. Male 129S6 mice were fed either standard chow or western diet for 26 weeks, followed by intervention with didemnin B (50 µg/kg) or vehicle by intraperitoneal (i.p.) injection once every 3 days for 14 days. Didemnin B decreased liver and plasma triglycerides, improved oral glucose tolerance, and decreased NAFLD severity. Moreover, didemnin B moderately increased hepatic expression of genes involved in ER stress response (Perk, Chop), and fatty acid oxidation (Fgf21, Cpt1a). In vitro, didemnin B decreased THP-1 monocyte proliferation, disrupted THP-1 monocyte-macrophage differentiation, decreased THP-1 macrophage IL-1ß secretion, and decreased hepatic stellate cell (HSteC) proliferation and collagen secretion under both basal and lipotoxic (high fatty acid) conditions. Thus, didemnin B improves hepatic steatosis, glucose tolerance, and blood lipids in obesity, in association with moderate, possibly hormetic, upregulation of pathways involved in cell stress response and energy balance in the liver. Furthermore, it decreases the activity of the cell types implicated in liver inflammation and fibrosis in vitro. These findings highlight the therapeutic potential of partial protein synthesis inhibition in the treatment of NAFLD.

Two-Week Isocaloric Time-Restricted Feeding Decreases Liver Inflammation without Significant Weight Loss in Obese Mice with Non-Alcoholic Fatty Liver Disease.

Prolonged, isocaloric, time-restricted feeding (TRF) protocols can promote weight loss, improve metabolic dysregulation, and mitigate non-alcoholic fatty liver disease (NAFLD). In addition, 3-day, severe caloric restriction can improve liver metabolism and glucose homeostasis prior to significant weight loss. Thus, we hypothesized that short-term, isocaloric TRF would improve NAFLD and characteristics of metabolic syndrome in diet-induced obese male mice. After 26 weeks of ad libitum access to western diet, mice either continued feeding ad libitum or were provided with access to the same quantity of western diet for 8 h daily, over the course of two weeks. Remarkably, this short-term TRF protocol modestly decreased liver tissue inflammation in the absence of changes in body weight or epidydimal fat mass. There were no changes in hepatic lipid accumulation or other characteristics of NAFLD. We observed no changes in liver lipid metabolism-related gene expression, despite increased plasma free fatty acids and decreased plasma triglycerides in the TRF group. However, liver Grp78 and Txnip expression were decreased with TRF suggesting hepatic endoplasmic reticulum (ER) stress and activation of inflammatory pathways may have been diminished. We conclude that two-week, isocaloric TRF can potentially decrease liver inflammation, without significant weight loss or reductions in hepatic steatosis, in obese mice with NAFLD.

Intestinal de novo phosphatidylcholine synthesis is required for dietary lipid absorption and metabolic homeostasis.

De novo phosphatidylcholine (PC) synthesis via CTP:phosphocholine cytidylyltransferase-α (CTα) is required for VLDL secretion. To determine the precise role of de novo PC synthesis in intestinal lipid metabolism, we deleted CTα exclusively in the intestinal epithelium of mice (CTαIKO mice). When fed a chow diet, CTαIKO mice showed normal fat absorption despite a ∼30% decrease in intestinal PC concentrations relative to control mice, suggesting that biliary PC can fully support chylomicron secretion under these conditions. However, when fed a high-fat diet, CTαIKO mice showed impaired passage of FAs and cholesterol from the intestinal lumen into enterocytes. Impaired intestinal lipid uptake in CTαIKO mice was associated with lower plasma triglyceride concentrations, higher plasma glucagon-like peptide 1 and peptide YY, and disruption of intestinal membrane lipid transporters after a high-fat meal relative to control mice. Unexpectedly, biliary bile acid and PC secretion was enhanced in CTαIKO mice due to a shift in expression of bile-acid transporters to the proximal intestine, indicative of accelerated enterohepatic cycling. These data show that intestinal de novo PC synthesis is required for dietary lipid absorption during high-fat feeding and that the reacylation of biliary lyso-PC cannot compensate for loss of CTα under these conditions.

Burden of atrial fibrillation in Maori and Pacific people in New Zealand: a cohort study.

BACKGROUND: Atrial fibrillation (AF) is a major risk factor for ischaemic stroke and cardiovascular events. In New Zealand (NZ), Maori (indigenous New Zealanders) and Pacific people experience higher rates of AF compared with non-Maori/non-Pacific people. AIM: To describe a primary care population with AF in NZ. Stroke risk and medication adherence according to ethnicity are also detailed. METHODS: Electronic medical records for adults (≥20 years, n = 135 840, including 19 918 Maori and 43 634 Pacific people) enrolled at 37 NZ general practices were analysed for AF diagnosis and associated medication prescription information. RESULTS: The overall prevalence of non-valvular AF (NVAF) in this population was 1.3% (1769), and increased with age (4.4% in people ≥55 years). Maori aged ≥55 years were more likely to be diagnosed with NVAF (7.3%) than Pacific (4.0%) and non-Maori/non-Pacific people (4.1%, P < 0.001). Maori and Pacific NVAF patients were diagnosed with AF 10 years earlier than non-Maori/non-Pacific patients (median age of diagnosis: Maori = 60 years, Pacific = 61 years, non-Maori/non-Pacific = 71 years, P < 0.001). Overall, 67% of NVAF patients were at high risk for stroke (CHA2 DS2 -VASc ≥ 2) at the time of AF diagnosis. Almost half (48%) of Maori and Pacific NVAF patients aged <65 years were at high risk for stroke, compared with 22% of non-Maori/non-Pacific (P < 0.001). Irrespective of ethnic group, adherence to AF medication was suboptimal in those NVAF patients with a high risk of stroke or with stroke history. CONCLUSION: AF screening and stroke thromboprophylaxis in Maori and Pacific people could start below the age of 65 years in NZ.

The critical role of phosphatidylcholine and phosphatidylethanolamine metabolism in health and disease.

Phosphatidylcholine (PC) and phosphatidylethanolamine (PE) are the most abundant phospholipids in all mammalian cell membranes. In the 1950s, Eugene Kennedy and co-workers performed groundbreaking research that established the general outline of many of the pathways of phospholipid biosynthesis. In recent years, the importance of phospholipid metabolism in regulating lipid, lipoprotein and whole-body energy metabolism has been demonstrated in numerous dietary studies and knockout animal models. The purpose of this review is to highlight the unappreciated impact of phospholipid metabolism on health and disease. Abnormally high, and abnormally low, cellular PC/PE molar ratios in various tissues can influence energy metabolism and have been linked to disease progression. For example, inhibition of hepatic PC synthesis impairs very low density lipoprotein secretion and changes in hepatic phospholipid composition have been linked to fatty liver disease and impaired liver regeneration after surgery. The relative abundance of PC and PE regulates the size and dynamics of lipid droplets. In mitochondria, changes in the PC/PE molar ratio affect energy production. We highlight data showing that changes in the PC and/or PE content of various tissues are implicated in metabolic disorders such as atherosclerosis, insulin resistance and obesity. This article is part of a Special Issue entitled: Membrane Lipid Therapy: Drugs Targeting Biomembranes edited by Pablo V. Escribá.

Cystathionine beta-synthase deficiency alters hepatic phospholipid and choline metabolism: Post-translational repression of phosphatidylethanolamine N-methyltransferase is a consequence rather than a cause of liver injury in homocystinuria.

Classical homocystinuria (HCU) due to inactivating mutation of cystathionine ß-synthase (CBS) is a poorly understood life-threatening inborn error of sulfur metabolism. A previously described cbs-/- mouse model exhibits a semi-lethal phenotype due to neonatal liver failure. The transgenic HO mouse model of HCU exhibits only mild liver injury and recapitulates multiple aspects of the disease as it occurs in humans. Disruption of the methionine cycle in HCU has the potential to impact multiple aspect of phospholipid (PL) metabolism by disruption of both the Kennedy pathway and phosphatidylethanolamine N-methyltransferase (PEMT) mediated synthesis of phosphatidylcholine (PC). Comparative metabolomic analysis of HO mouse liver revealed decreased levels of choline, and choline phosphate indicating disruption of the Kennedy pathway. Alterations in the relative levels of multiple species of PL included significant increases in PL degradation products consistent with enhanced membrane PL turnover. A significant decrease in PC containing 20:4n6 which primarily formed by the methylation of phosphatidylethanolamine to PC was consistent with decreased flux through PEMT. Hepatic expression of PEMT in both the cbs-/- and HO models is post-translationally repressed with decreased levels of PEMT protein and activity that inversely-correlates with the scale of liver injury. Failure to induce further repression of PEMT in HO mice by increased homocysteine, methionine and S-adenosylhomocysteine or depletion of glutathione combined with examination of multiple homocysteine-independent models of liver injury indicated that repression of PEMT in HCU is a consequence rather than a cause of liver injury. Collectively, our data show significant alteration of a broad range of hepatic PL and choline metabolism in HCU with the potential to contribute to multiple aspects of pathogenesis in this disease.

Flow-Mediated Modulation of the Endothelial Cell Lipidome.

The luminal surface of the endothelium is exposed to dynamic blood flow patterns that are known to affect endothelial cell phenotype. While many studies have documented the phenotypic changes by gene or protein expression, less is known about the role of blood flow pattern on the endothelial cell (EC) lipidome. In this study, shotgun lipidomics was conducted on human aortic ECs (HAECs) exposed to unidirectional laminar flow (UF), disturbed flow (DF), or static conditions for 48 hrs. A total of 520 individual lipid species from 17 lipid subclasses were detected. Total lipid abundance was significantly increased for HAECs exposed to DF compared to UF conditions. Despite the increase in the total lipid abundance, HAECs maintained equivalent composition of each lipid subclass (% of total lipid) under both DF and UF. However, by lipid composition (% of total subclass), 28 lipid species were significantly altered between DF and UF. Complimentary RNA sequencing of HAECs exposed to UF or DF revealed changes in transcripts involved in lipid metabolism. Shotgun lipidomics was also performed on HAECs exposed to pro-inflammatory agonists lipopolysaccharide (LPS) or Pam3CSK4 (Pam3) for 48 hrs. Exposure to LPS or Pam3 reshaped the EC lipidome in both unique and overlapping ways. In conclusion, exposure to flow alters the EC lipidome and ECs undergo stimulus-specific lipid reprogramming in response to pro-inflammatory agonist exposure. Ultimately, this work provides a resource to profile the transcriptional and lipidomic changes that occur in response to applied flow that can be accessed by the vascular biology community to further dissect and extend our understanding of endothelial lipid biology.

Flow-mediated modulation of the endothelial cell lipidome.

The luminal surface of the endothelium is exposed to dynamic blood flow patterns that are known to affect endothelial cell phenotype. While many studies have documented the phenotypic changes by gene or protein expression, less is known about the role of blood flow pattern on the endothelial cell (EC) lipidome. In this study, shotgun lipidomics was conducted on human aortic ECs (HAECs) exposed to unidirectional laminar flow (UF), disturbed flow (DF), or static conditions for 48 h. A total of 520 individual lipid species from 17 lipid subclasses were detected. Total lipid abundance was significantly increased for HAECs exposed to DF compared to UF conditions. Despite the increase in the total lipid abundance, HAECs maintained equivalent composition of each lipid subclass (% of total lipid) under DF and UF. However, by lipid composition (% of total subclass), 28 lipid species were significantly altered between DF and UF. Complimentary RNA sequencing of HAECs exposed to UF or DF revealed changes in transcripts involved in lipid metabolism. Shotgun lipidomics was also performed on HAECs exposed to pro-inflammatory agonists lipopolysaccharide (LPS) or Pam3CSK4 (Pam3) for 48 h. Exposure to LPS or Pam3 reshaped the EC lipidome in both unique and overlapping ways. In conclusion, exposure to flow alters the EC lipidome and ECs undergo stimulus-specific lipid reprogramming in response to pro-inflammatory agonist exposure. Ultimately, this work provides a resource to profile the transcriptional and lipidomic changes that occur in response to applied flow that can be accessed by the vascular biology community to further dissect and extend our understanding of endothelial lipid biology.

Lipid- and protein-directed photosensitizer proximity labeling captures the cholesterol interactome.

The physical properties of cellular membranes, including fluidity and function, are influenced by protein and lipid interactions. In situ labeling chemistries, most notably proximity-labeling interactomics are well suited to characterize these dynamic and often fleeting interactions. Established methods require distinct chemistries for proteins and lipids, which limits the scope of such studies. Here we establish a singlet-oxygen-based photocatalytic proximity labeling platform (POCA) that reports intracellular interactomes for both proteins and lipids with tight spatiotemporal resolution using cell-penetrant photosensitizer reagents. Using both physiologically relevant lipoprotein-complexed probe delivery and genetic manipulation of cellular cholesterol handling machinery, cholesterol-directed POCA captured established and unprecedented cholesterol binding proteins, including protein complexes sensitive to intracellular cholesterol levels and proteins uniquely captured by lipoprotein uptake. Protein-directed POCA accurately mapped known intracellular membrane complexes, defined sterol-dependent changes to the non-vesicular cholesterol transport protein interactome, and captured state-dependent changes in the interactome of the cholesterol transport protein Aster-B. More broadly, we find that POCA is a versatile interactomics platform that is straightforward to implement, using the readily available HaloTag system, and fulfills unmet needs in intracellular singlet oxygen-based proximity labeling proteomics. Thus, we expect widespread utility for POCA across a range of interactome applications, spanning imaging to proteomics.

T cell cholesterol transport is a metabolic checkpoint that links intestinal immune responses to dietary lipid absorption.

The intrinsic pathways that control membrane organization in immune cells and the impact of such pathways on cellular function are not well defined. Here we report that the non-vesicular cholesterol transporter Aster-A links plasma membrane (PM) cholesterol availability in T cells to immune signaling and systemic metabolism. Aster-A is recruited to the PM during T-cell receptor (TCR) activation, where it facilitates the removal of newly generated "accessible" membrane cholesterol. Loss of Aster-A leads to excess PM cholesterol accumulation, resulting in enhanced TCR nano-clustering and signaling, and Th17 cytokine production. Finally, we show that the mucosal Th17 response is restrained by PM cholesterol remodeling. Ablation of Aster-A in T cells leads to enhanced IL-22 production, reduced intestinal fatty acid absorption, and resistance to diet-induced obesity. These findings delineate a multi-tiered regulatory scheme linking immune cell lipid flux to nutrient absorption and systemic physiology.

Aster-B-dependent estradiol synthesis protects female mice from diet-induced obesity.

Aster proteins mediate the nonvesicular transport of cholesterol from the plasma membrane (PM) to the endoplasmic reticulum (ER). However, the importance of nonvesicular sterol movement for physiology and pathophysiology in various tissues is incompletely understood. Here we show that loss of Aster-B leads to diet-induced obesity in female but not in male mice, and that this sex difference is abolished by ovariectomy. We further demonstrate that Aster-B deficiency impairs nonvesicular cholesterol transport from the PM to the ER in ovaries in vivo, leading to hypogonadism and reduced estradiol synthesis. Female Aster-B-deficient mice exhibit reduced locomotor activity and energy expenditure, consistent with established effects of estrogens on systemic metabolism. Administration of exogenous estradiol ameliorates the diet-induced obesity phenotype of Aster-B-deficient female mice. These findings highlight the key role of Aster-B-dependent nonvesicular cholesterol transport in regulating estradiol production and protecting females from obesity.

Mechanosensitive membrane domains regulate calcium entry in arterial endothelial cells to protect against inflammation.

Endothelial cells (ECs) in the descending aorta are exposed to high laminar shear stress, and this supports an antiinflammatory phenotype. High laminar shear stress also induces flow-aligned cell elongation and front-rear polarity, but whether these are required for the antiinflammatory phenotype is unclear. Here, we showed that caveolin-1-rich microdomains polarize to the downstream end of ECs that are exposed to continuous high laminar flow. These microdomains were characterized by high membrane rigidity, filamentous actin (F-actin), and raft-associated lipids. Transient receptor potential vanilloid (TRPV4) ion channels were ubiquitously expressed on the plasma membrane but mediated localized Ca2+ entry only at these microdomains where they physically interacted with clustered caveolin-1. These focal Ca2+ bursts activated endothelial nitric oxide synthase within the confines of these domains. Importantly, we found that signaling at these domains required both cell body elongation and sustained flow. Finally, TRPV4 signaling at these domains was necessary and sufficient to suppress inflammatory gene expression and exogenous activation of TRPV4 channels ameliorated the inflammatory response to stimuli both in vitro and in vivo. Our work revealed a polarized mechanosensitive signaling hub in arterial ECs that dampened inflammatory gene expression and promoted cell resilience.

Gender differences in cardiovascular disease risk management for Pacific Islanders in primary care.

OBJECTIVES: To assess gender differences in cardiovascular disease risk (CVR) assessment and management for Pacific people in New Zealand. METHODS: New Zealand guidelines indicate CVR assessment from age 35 years for Pacific men and from age 45 years for Pacific women. Using general practice electronic medical records from 16 practices in New Zealand, the rate of CVR screening, treatment patterns and physiological measures for high-CVR (≥15% five-year) patients were assessed for Pacific patients ≥20 years of age by gender. RESULTS: Records for 10 863 Pacific patients showed a higher proportion of indicated women screened for CVR (65 vs 56%), but a lower proportion of assessed women with high CVR (28% for Pacific women vs 40% for Pacific men). Many of these high-CVR patients had physiological measures well above desirable levels based on their most recent readings. In the high-CVR group, women had similar CVR levels to men, but higher systolic blood pressure and HbA1c level, and a higher proportion of women were treated with antihypertensive and oral antidiabetic medication. There were substantial levels of poor medication adherence, particularly for cholesterol-lowering medication. Women and men were equally likely to adhere to treatment. Those adhering to relevant medications had lower blood pressure, total-to-HDL cholesterol ratio and HbA1c than non-adherers. CONCLUSIONS: Pacific men were less likely than Pacific women to have their CVR assessed when indicated, more likely once assessed to have high CVR and equally likely to adhere to treatment. Medication adherence was associated with better control of risk factors and should be further promoted in this population.

Cholesterol Transport to the Endoplasmic Reticulum.

Most cholesterol in mammalian cells is stored in the plasma membrane (PM). Cholesterol transport from the PM to low-sterol regulatory regions of the endoplasmic reticulum (ER) controls cholesterol synthesis and uptake, and thereby influences the rates of cholesterol flux between tissues of complex organisms. Cholesterol transfer to the ER is also required for steroidogenesis, oxysterol and bile acid synthesis, and cholesterol esterification. The ER-resident Aster proteins (Aster-A, -B, and -C) form contacts with the PM to move cholesterol to the ER in mammals. Mice lacking Aster-B have low adrenal cholesteryl ester stores and impaired steroidogenesis because of a defect in cholesterol transport from high-density lipoprotein (HDL) to the ER. This work reviews the molecular characteristics of Asters, their role in HDL- and low-density lipoprotein (LDL)-cholesterol movement, and how cholesterol transferred to the ER is utilized by cells. The roles of other lipid transporters and of membrane lipid organization in maintaining aspects of cholesterol homeostasis are also highlighted.

Polarized Mechanosensitive Signaling Domains Protect Arterial Endothelial Cells Against Inflammation.

Endothelial cells (ECs) in the descending aorta are exposed to high laminar shear stress, which supports an anti-inflammatory phenotype that protects them from atherosclerosis. High laminar shear stress also supports flow-aligned cell elongation and front-rear polarity, but whether this is required for athero-protective signaling is unclear. Here, we show that Caveolin-1-rich microdomains become polarized at the downstream end of ECs exposed to continuous high laminar flow. These microdomains are characterized by higher membrane rigidity, filamentous actin (F-actin) and lipid accumulation. Transient receptor potential vanilloid-type 4 (Trpv4) ion channels, while ubiquitously expressed, mediate localized Ca 2+ entry at these microdomains where they physically interact with clustered Caveolin-1. The resultant focal bursts in Ca 2+ activate the anti-inflammatory factor endothelial nitric oxide synthase (eNOS) within the confines of these domains. Importantly, we find that signaling at these domains requires both cell body elongation and sustained flow. Finally, Trpv4 signaling at these domains is necessary and sufficient to suppress inflammatory gene expression. Our work reveals a novel polarized mechanosensitive signaling hub that induces an anti-inflammatory response in arterial ECs exposed to high laminar shear stress.

Hepatic nonvesicular cholesterol transport is critical for systemic lipid homeostasis.

In cell models, changes in the 'accessible' pool of plasma membrane (PM) cholesterol are linked with the regulation of endoplasmic reticulum sterol synthesis and metabolism by the Aster family of nonvesicular transporters; however, the relevance of such nonvesicular transport mechanisms for lipid homeostasis in vivo has not been defined. Here we reveal two physiological contexts that generate accessible PM cholesterol and engage the Aster pathway in the liver: fasting and reverse cholesterol transport. During fasting, adipose-tissue-derived fatty acids activate hepatocyte sphingomyelinase to liberate sequestered PM cholesterol. Aster-dependent cholesterol transport during fasting facilitates cholesteryl ester formation, cholesterol movement into bile and very low-density lipoprotein production. During reverse cholesterol transport, high-density lipoprotein delivers excess cholesterol to the hepatocyte PM through scavenger receptor class B member 1. Loss of hepatic Asters impairs cholesterol movement into feces, raises plasma cholesterol levels and causes cholesterol accumulation in peripheral tissues. These results reveal fundamental mechanisms by which Aster cholesterol flux contributes to hepatic and systemic lipid homeostasis.