New pyripyropene A derivatives, highly SOAT2-selective inhibitors, improve hypercholesterolemia and atherosclerosis in atherogenic mouse models

A rising tide lifts all MBOATs: recent progress in structural and functional understanding of membrane bound O-acyltransferases

Acylation modifications play a central role in biological and physiological processes. Across a range of biomolecules from phospholipids to triglycerides to proteins, introduction of a hydrophobic acyl chain can dramatically alter the biological function and cellular localization of these substrates. Amongst the enzymes catalyzing these modifications, the membrane bound O-acyltransferase (MBOAT) family occupies an intriguing position as the combined substrate selectivities of the various family members span all three classes of these biomolecules. MBOAT-dependent substrates are linked to a wide range of health conditions including metabolic disease, cancer, and neurodegenerative disease. Like many integral membrane proteins, these enzymes have presented challenges to investigation due to their intractability to solubilization and purification. However, over the last several years new solubilization approaches coupled with computational modeling, crystallography, and cryoelectron microscopy have brought an explosion of structural information for multiple MBOAT family members. These studies enable comparison of MBOAT structure and function across members catalyzing modifications of all three substrate classes, revealing both conserved features amongst all MBOATs and distinct architectural features that correlate with different acylation substrates ranging from lipids to proteins. We discuss the methods that led to this renaissance of MBOAT structural investigations, our new understanding of MBOAT structure and implications for catalytic function, and the potential impact of these studies for development of new therapeutics targeting MBOAT-dependent physiological processes.

Targeting Sterol O-Acyltransferase/Acyl-CoA:Cholesterol Acyltransferase (ACAT): A Perspective on Small-Molecule Inhibitors and Their Therapeutic Potential

Sterol O-acyltransferase (SOAT) is a membrane-bound enzyme that aids the esterification of cholesterol and fatty acids to cholesterol esters. SOAT has been studied extensively as a potential drug target, since its inhibition can serve as an alternative to statin therapy. Two SOAT isozymes that have discrete functions in the human body, namely, SOAT1 and SOAT2, have been characterized. Over three decades of research has focused on candidate SOAT1 inhibitors with unsatisfactory results in clinical trials. Recent research has focused on targeting SOAT2 selectively. In this perspective, we summarize the literature covering various SOAT inhibitory agents and discuss the design, structural requirements, and mode of action of SOAT inhibitors.

Molecular structures of human ACAT2 disclose mechanism for selective inhibition

Endoplasmic reticulum-localized acyl-CoA:cholesterol acyltransferases (ACAT), including ACAT1 and ACAT2, convert cholesterol to cholesteryl esters that become incorporated into lipoproteins or stored in cytosolic lipid droplets. Selective inhibition of ACAT2 has been shown to considerably attenuate hypercholesterolemia and atherosclerosis in mice. Here, we report cryogenic electron microscopy structures of human ACAT2 bound to its specific inhibitor pyripyropene A or the general ACAT inhibitor nevanimibe. Structural analysis reveals that ACAT2 has a topology in membranes similar to that of ACAT1. A catalytic core with an entry site occupied by a cholesterol molecule and another site for allosteric activation of ACAT2 is observed in these structures. Enzymatic assays show that mutations within sites of cholesterol entry or allosteric activation attenuate ACAT2 activity in vitro. Together, these results reveal mechanisms for ACAT2-mediated esterification of cholesterol, providing a blueprint to design new ACAT2 inhibitors for use in the prevention of cardiovascular disease.

Delineation of metabolic responses of Npc1-/-nih mice lacking the cholesterol-esterifying enzyme SOAT2 to acute treatment with 2-hydroxypropyl-β-cyclodextrin

Lipids present in lipoproteins cleared from the circulation are processed sequentially by three major proteins within the late endosomal/lysosomal (E/L) compartment of all cells: lysosomal acid lipase (LAL), Niemann-Pick (NPC) C2 and NPC1. When all three of these proteins are functioning normally, unesterified cholesterol (UC) exits the E/L compartment and is used in plasma membrane maintenance and various pathways in the endoplasmic reticulum including esterification by sterol O-acyltransferase 2 (SOAT2) or SOAT1 depending partly on cell type. Mutations in either NPC2 or NPC1 result in continual entrapment of UC and glycosphingolipids leading to neurodegeneration, pulmonary dysfunction, splenomegaly and liver damage. To date, the most effective agent for promoting release of entrapped UC in nearly all organs of NPC1-deficient mice and cats is 2-hydroxypropyl-β-cyclodextrin (2HPβCD). The cytotoxic nature of the liberated UC triggers various defenses including suppression of sterol synthesis and increased esterification. The present studies, using the Npc1-/-nih mouse model, measured the comparative quantitative importance of these two responses in the liver versus the spleen of Npc1-/-: Soat2+/+ and Npc1-/-: Soat2-/- mice in the 24 h following a single acute treatment with 2HPβCD. In the liver but not the spleen of both types of mice suppression of synthesis alone or in combination with increased esterification provided the major defense against the rise in unsequestered cellular UC content. These findings have implications for systemic 2HPβCD treatment in NPC1 patients in view of the purportedly low levels of SOAT2 activity in human liver.

Voluhemins, new inhibitors of sterol O-acyltransferase, produced by Volutella citrinella BF-0440

New compounds, designated voluhemins A (1) and B (2), are isolated from the culture broth of the fungal strain Volutella citrinella BF-0440 along with structurally related known NK12838 (3). Spectroscopic data, including 1D and 2D NMR, elucidated their structures. Compounds 1-3 have a common indoline-diterpene core and two additional isoprenyl moieties. Compounds 1 and 3 contain a hemiaminal unit, while 2 is O-methylated 1. Their inhibitory activities toward sterol O-acyltransferase (SOAT) 1 and 2 isozymes in SOAT1- and SOAT2-expressing Chinese hamster ovary (CHO) cells show that 2 selectively inhibits the SOAT2 isozyme.

Cyp1b1 directs Srebp-mediated cholesterol and retinoid synthesis in perinatal liver; Association with retinoic acid activity during fetal development

Background

Cytochrome P450 1b1 (Cyp1b1) deletion and dietary retinol deficiency during pregnancy (GVAD) affect perinatal liver functions regulated by Srebp. Cyp1b1 is not expressed in perinatal liver but appears in the E9.5 embryo, close to sites of retinoic acid (RA) signaling.

Hypothesis

Parallel effects of Cyp1b1 and retinol on postnatal Srebp derive from effects in the developing liver or systemic signaling.

Approach

Cluster postnatal increases in hepatic genes in relation to effects of GVAD or Cyp1b1 deletion. Sort expression changes in relation to genes regulated by Srebp1 and Srebp2.Test these treatments on embryos at E9.5, examining changes at the site of liver initiation. Use in situ hybridization to resolve effects on mRNA distributions of Aldh1a2 and Cyp26a1 (RA homeostasis); Hoxb1 and Pax6 (RA targets). Assess mice lacking Lrat and Rbp4 (DKO mice) that severely limits retinol supply to embryos.

Results

At birth, GVAD and Cyp1b1 deletion stimulate gene markers of hepatic stellate cell (HSC) activation but also suppress Hamp. These treatments then selectively prevent the postnatal onset of genes that synthesize cholesterol (Hmgcr, Sqle) and fatty acids (Fasn, Scd1), but also direct cholesterol transport (Ldlr, Pcsk9, Stard4) and retinoid synthesis (Aldh1a1, Rdh11). Extensive support by Cyp1b1 is implicated, but with distinct GVAD interventions for Srebp1 and Srebp2. At E9.5, Cyp1b1 is expressed in the septum transversum mesenchyme (STM) with β-carotene oxygenase (Bco1) that generates retinaldehyde. STM provides progenitors for the HSC and supports liver expansion. GVAD and Cyp1b1-/- do not affect RA-dependent Hoxb1 and Pax6. In DKO embryos, RA-dependent Cyp26a1 is lost but Hoxb1 is sustained with Cyp1b1 at multiple sites.

Conclusion

Cyp1b1-/- suppresses genes supported by Srebp. GVAD effects distinguish Srebp1 and Srebp2 mediation. Srebp regulation overlaps appreciably in cholesterol and retinoid homeostasis. Bco1/Cyp1b1 partnership in the STM may contribute to this later liver regulation.

The Anti-atherogenic Activity of Beauveriolide Derivative BVD327, a Sterol O-Acyltransferase 2-Selective Inhibitor, in Apolipoprotein E Knockout Mice

The fungal 13-membered cyclodepsipeptides, beauveriolides I and III, were previously reported to be atheroprotective activity in mouse models via inhibiting sterol O-acyltransferase (SOAT) activity. A total of 149 beauveriolide derivatives (BVDs) synthesized combinatorially were evaluated in in silico absorption, distribution, metabolism and excretion (ADME) analysis and inhibitory activity toward the two SOAT isozymes, SOAT1 and SOAT2. Hence, only 11 BVDs exhibited SOAT2-selective inhibition. Among these, we chose BVD327, which had the highest ADME score, for further evaluation. BVD327 administration (50 mg/kg/d, per os (p.o.)) significantly decreased atherosclerotic lesions in the aorta and heart (25.4 ± 6.9 and 20.6 ± 2.9%, respectively) in apolipoprotein E knockout (Apoe-/-) mice fed a cholesterol-enriched diet (0.2% cholesterol and 21% fat) for 12 weeks. These findings indicate that beauveriolide derivatives can be used as anti-atherosclerotic agents.

Celludinones, new inhibitors of sterol O-acyltransferase, produced by Talaromyces cellulolyticus BF-0307

New indanones, designated celludinones A ((±)-1) and B (2), were isolated from the culture broth of the fungal strain Talaromyces cellulolyticus BF-0307. The structures of celludinones were elucidated by spectroscopic data, including 1D and 2D NMR. Celludinone A was found to be a mixture of racemic isomers ((±)-1), which were isolated by a chiral column. Compounds (+)-1 and (-)-1 inhibited the sterol O-acyltransferase (SOAT) 1 and 2 isozymes in a cell-based assay using SOAT1- and SOAT2-expressing Chinese hamster ovary (CHO) cells, while 2 selectively inhibited the SOAT2 isozyme.

Studies on the Chemical Diversities of Secondary Metabolites Produced by Neosartorya fischeri via the OSMAC Method

The One Strain Many Compounds (OSMAC) method was applied to explore the chemical diversities of secondary metabolites produced by Neosartorya fischeri NRRL 181. Four pyripyropenes 1⁻4, eight steroids 5⁻11, and four prenylated indole alkaloids 12⁻15, were obtained from the fungus cultured in petri dishes containing potato dextrose agar (PDA). 1,7,11-trideacetylpyripyropene A (1) and 1,11-dideacetyl pyripyropene A (2) were obtained and spectroscopically characterized (1D, 2D NMR, and HR-ESI-MS) from a natural source for the first time. It offered a sustainable source of these two compounds, which were usually used as starting materials in preparing pyripyropene derivatives. In addition, as compared with all the other naturally occurring pyripyropenes, 1 and 2 possessed unique acetylation patterns that did not follow the established late-step biosynthetic rules of pyripyropenes. The natural occurrence of 1 and 2 in the fungus implied that the timing and order of hydroxylation and acetylation in the late-step biosynthetic pathway of pyripyropenes remained to be revealed. The isolation and identification of 1⁻15 indicated that the OSMAC method could remarkably alter the metabolic profile and enrich the chemical diversities of fungal metabolites. Compounds 1⁻4 exhibited no obvious cytotoxicity against the triple-negative breast cancer cell line MDA-MB-231 as compared with taxol.

Niemann-Pick C1-deficient mice lacking sterol O-acyltransferase 2 have less hepatic cholesterol entrapment and improved liver function

Cholesteryl esters are generated at multiple sites in the body by sterol O-acyltransferase (SOAT) 1 or SOAT2 in various cell types and lecithin cholesterol acyltransferase in plasma. Esterified cholesterol and triacylglycerol contained in lipoproteins cleared from the circulation via receptor-mediated or bulk-phase endocytosis are hydrolyzed by lysosomal acid lipase within the late endosomal/lysosomal (E/L) compartment. Then, through the successive actions of Niemann-Pick C (NPC) 2 and NPC 1, unesterified cholesterol (UC) is exported from the E/L compartment to the cytosol. Mutations in either NPC1 or NPC2 lead to continuing entrapment of UC in all organs, resulting in multisystem disease, which includes hepatic dysfunction and in some cases liver failure. These studies investigated primarily whether elimination of SOAT2 in NPC1-deficient mice impacted hepatic UC sequestration, inflammation, and transaminase activities. Measurements were made in 7-wk-old mice fed a low-cholesterol chow diet or one enriched with cholesterol starting 2 wk before study. In the chow-fed mice, NPC1:SOAT2 double knockouts, compared with their littermates lacking only NPC1, had 20% less liver mass, 28% lower hepatic UC concentrations, and plasma alanine aminotransferase and aspartate aminotransferase activities that were decreased by 48% and 36%, respectively. mRNA expression levels for several markers of inflammation were all significantly lower in the NPC1 mutants lacking SOAT2. The existence of a new class of potent and selective SOAT2 inhibitors provides an opportunity for exploring if suppression of this enzyme could potentially become an adjunctive therapy for liver disease in NPC1 deficiency. NEW & NOTEWORTHY In Niemann-Pick type C1 (NPC1) disease, the entrapment of unesterified cholesterol (UC) in the endosomal/lysosomal compartment of all cells causes multiorgan disease, including neurodegeneration, pulmonary dysfunction, and liver failure. Some of this sequestered UC entered cells initially in the esterified form. When sterol O-acyltransferase 2, a cholesterol esterifying enzyme present in enterocytes and hepatocytes, is eliminated in NPC1-deficient mice, there is a reduction in their hepatomegaly, hepatic UC content, and cellular injury.

Design and Synthesis of A-Ring Simplified Pyripyropene A Analogues as Potent and Selective Synthetic SOAT2 Inhibitors

Currently, pyripyropene A, which is isolated from the culture broth of Aspergillus fumigatus FO-1289, is the only compound known to strongly and selectively inhibit the isozyme sterol O-acyltransferase 2 (SOAT2). To aid in the development of new cholesterol-lowering or anti-atherosclerotic agents, new A-ring simplified pyripyropene A analogues have been designed and synthesized based on total synthesis, and the results of structure-activity relationship studies of pyripyropene A. Among the analogues, two A-ring simplified pyripyropene A analogues exhibited equally efficient SOAT2 inhibitory activity to that of natural pyripyropene A. These new analogues are the most potent and selective SOAT2 inhibitors to be used as synthetic compounds and attractive seed compounds for the development of drug for dyslipidemia, including atherosclerotic disease and steatosis.

Impact of loss of SOAT2 function on disease progression in the lysosomal acid lipase-deficient mouse

Although only a small proportion of cholesterol in the body is esterified, in several diseases marked expansion of the esterified cholesterol (EC) pool occurs. These include Wolman disease (WD) and Cholesteryl Ester Storage Disease (CESD) which both result from mutations in LIPA, the gene that encodes lysosomal acid lipase (LAL). The respective contributions that our three cholesterol esterifying enzymes make to EC production, especially in disorders like CESD, are not well defined. The current studies represent a detailed exploration of our earlier findings in young male LAL-deficient mice also missing sterol O-acyltransferase 2 (SOAT2, also called ACAT2). Here we show that, even as they aged, male and female Lal-/-: Soat2- /- mice, compared to Lal-/-: Soat2+/+ littermates, had appreciably less hepatomegaly as well as a marked reduction in the level of sequestration of EC, in liver transaminase activities, and in hepatic mRNA expression levels for markers of inflammation. Loss of SOAT2 function also dramatically curtailed EC entrapment in the small intestine of the LAL-deficient mice. Together, these data imply that SOAT2 inhibition, if applied concurrently with enzyme replacement therapy for LAL deficiency, may blunt the re-esterification of newly released unesterified cholesterol thereby improving clinical outcomes.

Quantitation of the rates of hepatic and intestinal cholesterol synthesis in lysosomal acid lipase-deficient mice before and during treatment with ezetimibe

Esterified cholesterol (EC) and triglycerides, contained within lipoproteins taken up by cells, are hydrolysed by lysosomal acid lipase (LAL) in the late endosomal/lysosomal (E/L) compartment. The resulting unesterified cholesterol (UC) is transported via Niemann-Pick type C2 and C1 into the cytosolic compartment where it enters a putative pool of metabolically active cholesterol that is utilized in accordance with cellular needs. Loss-of-function mutations in LIPA, the gene encoding LAL, result in dramatic increases in tissue concentrations of EC, a hallmark feature of Wolman disease and cholesteryl ester storage disease (CESD). The lysosomal sequestration of EC causes cells to respond to a perceived deficit of sterol by increasing their rate of cholesterol synthesis, particularly in the liver. A similar compensatory response occurs with treatments that disrupt the enterohepatic movement of cholesterol or bile acids. Here we measured rates of cholesterol synthesis in vivo in the liver and small intestine of a mouse model for CESD given the cholesterol absorption inhibitor ezetimibe from weaning until early adulthood. Consistent with previous findings, this treatment significantly reduced the amount of EC sequestered in the liver (from 132.43±7.35 to 70.07±6.04mg/organ) and small intestine (from 2.78±0.21 to 1.34±0.09mg/organ) in the LAL-deficient mice even though their rates of hepatic and intestinal cholesterol synthesis were either comparable to, or exceeded those in matching untreated Lal-/- mice. These data reveal the role of intestinal cholesterol absorption in driving the expansion of tissue EC content and disease progression in LAL deficiency.

Shunts, channels and lipoprotein endosomal traffic: a new model of cholesterol homeostasis in the hepatocyte

The liver directs cholesterol metabolism in the organism. All the major fluxes of cholesterol within the body involve the liver: dietary cholesterol is directed to the liver; cholesterol from peripheral cells goes to the liver; the liver is a major site of cholesterol synthesis for the organism; cholesterol is secreted from the liver within the bile, within apoB lipoproteins and translocated to nascent HDL. The conventional model of cholesterol homeostasis posits that cholesterol from any source enters a common, rapidly exchangeable pool within the cell, which is in equilibrium with a regulatory pool. Increased influx of cholesterol leads rapidly to decreased synthesis of cholesterol. This model was developed based on in vitro studies in the fibroblast and validated only for LDL particles. The challenges the liver must meet in vivo to achieve cholesterol homeostasis are far more complex. Our model posits that the cholesterol derived from three different lipoproteins endosomes has three different fates: LDL-derived cholesterol is largely recycled within VLDL with most of the cholesterol shunted through the hepatocyte without entering the exchangeable pool of cholesterol; high density lipoprotein-derived CE is transcytosed into bile; and chylomicron remnant-derived cholesterol primarily enters the regulatory pool within the hepatocyte. These endosomal channels represent distinct physiological pathways and hepatic homeostasis represents the net result of the outcomes of these distinct channels. Our model takes into account the distinct physiological challenges the hepatocyte must meet, underlie the pathophysiology of many of the apoB dyslipoproteinemias and account for the sustained effectiveness of therapeutic agents such as statins.

Sterol O-Acyltransferase 2 Contributes to the Yolk Cholesterol Trafficking during Zebrafish Embryogenesis

To elucidate whether Sterol O-acyltransferase (Soat) mediates the absorption and transportation of yolk lipids to the developing embryo, zebrafish soat1 and soat2 were cloned and studied. In the adult zebrafish, soat1 was detected ubiquitously while soat2 mRNA was detected specifically in the liver, intestine, brain and testis. Whole mount in situ hybridization demonstrated that both soat1 and soat2 expressed in the yolk syncytial layer, hatching gland and developing cardiovascular as well as digestive systems, suggesting that Soats may play important roles in the lipid trafficking and utilization during embryonic development. The enzymatic activity of zebrafish Soat2 was confirmed by Oil Red O staining in the HEK293 cells overexpressing this gene, and could be quenched by Soat2 inhibitor Pyripyropene A (PPPA). The zebrafish embryos injected with PPPA or morpholino oligo against soat2 in the yolk showed significantly larger yolk when compared with wild-type embryos, especially at 72 hpf, indicating a slower rate of yolk consumption. Our result indicated that zebrafish Soat2 is catalytically active in synthesizing cholesteryl esters and contributes to the yolk cholesterol trafficking during zebrafish embryogenesis.

Sterol O-Acyltransferase 2-Driven Cholesterol Esterification Opposes Liver X Receptor-Stimulated Fecal Neutral Sterol Loss

Statin drugs have proven a successful and relatively safe therapy for the treatment of atherosclerotic cardiovascular disease (CVD). However, even with the substantial low-density lipoprotein (LDL) cholesterol lowering achieved with statin treatment, CVD remains the top cause of death in developed countries. Selective inhibitors of the cholesterol esterifying enzyme sterol-O acyltransferase 2 (SOAT2) hold great promise as effective CVD therapeutics. In mouse models, previous work has demonstrated that either antisense oligonucleotide (ASO) or small molecule inhibitors of SOAT2 can effectively reduce CVD progression, and even promote regression of established CVD. Although it is well known that SOAT2-driven cholesterol esterification can alter both the packaging and retention of atherogenic apoB-containing lipoproteins, here we set out to determine whether SOAT2-driven cholesterol esterification can also impact basal and liver X receptor (LXR)-stimulated fecal neutral sterol loss. These studies demonstrate that SOAT2 is a negative regulator of LXR-stimulated fecal neutral sterol loss in mice.