Lipid rafts as a therapeutic target

Lipid rafts regulate the initiation of cellular metabolic and signaling pathways by organizing the pathway components in ordered microdomains on the cell surface. Cellular responses regulated by lipid rafts range from physiological to pathological, and the success of a therapeutic approach targeting “pathological” lipid rafts depends on the ability of a remedial agent to recognize them and disrupt pathological lipid rafts without affecting normal raft-dependent cellular functions. In this article, concluding the Thematic Review Series on Biology of Lipid Rafts, we review current experimental therapies targeting pathological lipid rafts, including examples of inflammarafts and clusters of apoptotic signaling molecule-enriched rafts. The corrective approaches include regulation of cholesterol and sphingolipid metabolism and membrane trafficking by using HDL and its mimetics, LXR agonists, ABCA1 overexpression, and cyclodextrins, as well as a more targeted intervention with apoA-I binding protein. Among others, we highlight the design of antagonists that target inflammatory receptors only in their activated form of homo- or heterodimers, when receptor dimerization occurs in pathological lipid rafts. Other therapies aim to promote raft-dependent physiological functions, such as augmenting caveolae-dependent tissue repair. The overview of this highly dynamic field will provide readers with a view on the emerging concept of targeting lipid rafts as a therapeutic strategy. Lipid rafts regulate the initiation of cellular metabolic and signaling pathways by organizing the pathway components in ordered microdomains on the cell surface. Cellular responses regulated by lipid rafts range from physiological to pathological, and the success of a therapeutic approach targeting “pathological” lipid rafts depends on the ability of a remedial agent to recognize them and disrupt pathological lipid rafts without affecting normal raft-dependent cellular functions. In this article, concluding the Thematic Review Series on Biology of Lipid Rafts, we review current experimental therapies targeting pathological lipid rafts, including examples of inflammarafts and clusters of apoptotic signaling molecule-enriched rafts. The corrective approaches include regulation of cholesterol and sphingolipid metabolism and membrane trafficking by using HDL and its mimetics, LXR agonists, ABCA1 overexpression, and cyclodextrins, as well as a more targeted intervention with apoA-I binding protein. Among others, we highlight the design of antagonists that target inflammatory receptors only in their activated form of homo- or heterodimers, when receptor dimerization occurs in pathological lipid rafts. Other therapies aim to promote raft-dependent physiological functions, such as augmenting caveolae-dependent tissue repair. The overview of this highly dynamic field will provide readers with a view on the emerging concept of targeting lipid rafts as a therapeutic strategy. apoA-I binding protein β-cyclodextrin cluster of apoptotic signaling molecule-enriched rafts caveolin-1 chemotherapy-induced peripheral neuropathy human immunodeficiency virus methyl-β-cyclodextrin µ-opioid receptor Niemann-Pick type C Lipid rafts play a unique role in cell physiology providing a solid platform within a membrane where macromolecular complexes can assemble without battling forces of chaos in the disorderly liquid phase of the surroundings. The abundance and functional properties of lipid rafts can change rapidly in response to changing metabolic conditions, most likely representing a fundamentally important layer of fast physiological regulation, connecting and coordinating a broad range of metabolic and signaling pathways. At the same time, as described in review articles published in this series, dysregulation of lipid rafts plays a key role in the pathogenesis of hematopoietic, neurological, inflammatory, and infectious diseases, as well as that of cancer. The emerging physiological and pathological roles of lipid rafts point to an exciting possibility to target lipid rafts for therapeutic purposes. Targeting an early step in pathogenesis has a significant advantage of addressing “a root” of the problem and mitigating diverse consequences of lipid raft pathology. For example, targeting lipid rafts in neurodegenerative diseases may simultaneously reduce amyloidogenic protein misfolding and processing as well as neuroinflammation, two key elements of pathogenesis of neurodegeneration. Targeting lipid rafts in infectious diseases can simultaneously mitigate the infection and its metabolic comorbidities. Given a key role of inflammation in a multitude of pathological processes, targeting rafts to moderate the inflammation may have a broad utility. However, targeting rafts is not without problems. Primum non nocere, “first, do no harm.” The question that inevitably comes to mind, is it really possible to target lipid rafts, an essential component in the plasma membrane organization and the platform for a multitude of physiologic processes, to achieve a therapeutic effect without significant adverse impact? Two observations indicate that this might be a realistic possibility. First, somewhat surprisingly, most raft-associated pathologies are caused by “excessive” lipid rafts: elevated raft abundance or increased raft stability, or both. Further, β-cyclodextrins (βCDs) are an effective tool to deplete cells of cholesterol and indiscriminately destroy rafts. Although at high concentrations they may be cytotoxic, when used at lower concentrations they still destroy rafts, but have remarkably few adverse effects in vitro and in vivo. This points to the existence of significant redundancy and/or backup mechanisms supporting the physiological role of rafts. Second is spatial and temporal heterogeneity of the lipid rafts in relation to their size, stability, structure, and, ultimately, function. Raft heterogeneity is determined by a repertoire of lipids and proteins in the rafts and opens, at least theoretically, a possibility to selectively target one subset of lipid rafts and not the other, one cell function and one cell type, but not all of them. The goal of this review article is to demonstrate that recent advances in understanding lipid raft regulation point to the possibility of targeting excessive or pathological lipid rafts as a viable therapeutic strategy. There are two major mechanisms that regulate dynamic remodeling of lipid rafts. One mechanism relies on the availability of lipids that are critical for raft structure, principally, cholesterol and sphingolipids. Depletion of plasma membrane cholesterol using methyl-βCD (MβCD) is a classical method to break down lipid rafts, significantly attenuating all signaling originating from rafts. Inhibition of cholesterol biosynthesis also lowers lipid raft cholesterol content and alters raft-originated signaling (1Zhuang L. Kim J. Adam R.M. Solomon K.R. Freeman M.R. Cholesterol targeting alters lipid raft composition and cell survival in prostate cancer cells and xenografts.J. Clin. Invest. 2005; 115: 959-968Crossref PubMed Scopus (438) Google Scholar). Enrichment of membranes with ceramides, either directly or via depletion of sphingomyelin, displaces cholesterol from rafts altering their properties (2Megha London E. Ceramide selectively displaces cholesterol from ordered lipid domains (rafts): implications for lipid raft structure and function.J. Biol. Chem. 2004; 279: 9997-10004Abstract Full Text Full Text PDF PubMed Scopus (365) Google Scholar, 3Cremesti A.E. Goni F.M. Kolesnick R. Role of sphingomyelinase and ceramide in modulating rafts: do biophysical properties determine biologic outcome?.FEBS Lett. 2002; 531: 47-53Crossref PubMed Scopus (295) Google Scholar). Monounsaturated fatty acids inhibit raft formation (4Ahmed S.N. Brown D.A. London E. On the origin of sphingolipid/cholesterol-rich detergent-insoluble cell membranes: physiological concentrations of cholesterol and sphingolipid induce formation of a detergent-insoluble, liquid-ordered lipid phase in model membranes.Biochemistry. 1997; 36: 10944-10953Crossref PubMed Scopus (613) Google Scholar), while polyunsaturated fatty acids stabilize it (5Wassall S.R. Leng X. Canner S.W. Pennington E.R. Kinnun J.J. Cavazos A.T. Dadoo S. Johnson D. Heberle F.A. Katsaras J. et al.Docosahexaenoic acid regulates the formation of lipid rafts: A unified view from experiment and simulation.Biochim. Biophys. Acta Biomembr. 2018; 1860: 1985-1993Crossref PubMed Scopus (55) Google Scholar). Thus, simple interventions acting on membrane lipids robustly modify lipid rafts and their protein cargo with consequent changes in signal transduction (6Fessler M.B. Parks J.S. Intracellular lipid flux and membrane microdomains as organizing principles in inflammatory cell signaling.J. Immunol. 2011; 187: 1529-1535Crossref PubMed Scopus (196) Google Scholar). Another mechanism regulating raft organization depends on changes in the cytoskeleton. Recent findings indicate that the structural and functional properties of lipid rafts depend upon interactions with and dynamic rearrangement of the cytoskeleton (7Head B.P. Patel H.H. Insel P.A. Interaction of membrane/lipid rafts with the cytoskeleton: impact on signaling and function: membrane/lipid rafts, mediators of cytoskeletal arrangement and cell signaling.Biochim. Biophys. Acta. 2014; 1838: 532-545Crossref PubMed Scopus (365) Google Scholar). For example, β-actin remodeling modulates raft abundance and changes their properties (8Chichili G.R. Rodgers W. Cytoskeleton-membrane interactions in membrane raft structure.Cell. Mol. Life Sci. 2009; 66: 2319-2328Crossref PubMed Scopus (189) Google Scholar). The two mechanisms are not mutually exclusive and can be used to selectively target pathological subsets of lipid rafts in one cell type or cell types harboring pathological rafts. For the purpose of this article, the definition of pathological lipid rafts is rather teleological, referring to lipid rafts in inflammatory or activated or transformed cells under pathological conditions, and to a lesser degree to their specific structural characteristics. Emerging new techniques will allow for a more detailed characterization of the composition and biophysical features of altered lipid rafts under various pathological conditions. Pathological lipid rafts serve the purpose of organizing metabolic and signaling processes leading to diseases states. We posit that operating within the framework of pathological lipid rafts, with the examples of inflammarafts and clusters of apoptotic signaling molecule-enriched rafts (CASMERs) given below, can be useful in discussing therapeutic targeting of lipid rafts. The term inflammaraft was introduced to emphasize the role of enlarged lipid rafts harboring activated receptors and adaptor molecules and serving as a scaffold to organize the cellular inflammatory response (9Miller, Y. I., J. M., Navia-Pelaez, M., Corr, and T. L., Yaksh, . Lipid rafts in glial cells: role in neuroinflammation and pain processing. J. Lipid Res. Epub ahead of print. December 20, 2019; doi:10.1194/jlr.TR119000468.Google Scholar). TLR4 is a prototypic inflammatory receptor, which is dimerized in response to ligand activation, the process that requires a lipid raft microenvironment. An increased abundance of lipid rafts, for example due to deficiency of ABCA1 and ABCG1 transporters (10Yvan-Charvet L. Welch C. Pagler T.A. Ranalletta M. Lamkanfi M. Han S. Ishibashi M. Li R. Wang N. Tall A.R. Increased inflammatory gene expression in ABC transporter-deficient macrophages: free cholesterol accumulation, increased signaling via toll-like receptors, and neutrophil infiltration of atherosclerotic lesions.Circulation. 2008; 118: 1837-1847Crossref PubMed Scopus (339) Google Scholar), and the increased number of TLR4 dimers do not only reflect a ligand-induced TLR4 receptor activation event, but also indicate the permissive membrane microenvironment that supports assembly of other inflammatory receptor complexes. In this context, stimuli-mediated dimerization of TLR4 (11Wong S.W. Kwon M.J. Choi A.M.K. Kim H.P. Nakahira K. Hwang D.H. Fatty acids modulate toll-like receptor 4 activation through regulation of receptor dimerization and recruitment into lipid rafts in a reactive oxygen species-dependent manner.J. Biol. Chem. 2009; 284: 27384-27392Abstract Full Text Full Text PDF PubMed Scopus (406) Google Scholar, 12Zhu X. Owen J.S. Wilson M.D. Li H. Griffiths G.L. Thomas M.J. Hiltbold E.M. Fessler M.B. Parks J.S. Macrophage ABCA1 reduces MyD88-dependent Toll-like receptor trafficking to lipid rafts by reduction of lipid raft cholesterol.J. Lipid Res. 2010; 51: 3196-3206Abstract Full Text Full Text PDF PubMed Scopus (246) Google Scholar, 13Shridas P. Bailey W.M. Talbott K.R. Oslund R.C. Gelb M.H. Webb N.R. Group X secretory phospholipase A2 enhances TLR4 signaling in macrophages.J. Immunol. 2011; 187: 482-489Crossref PubMed Scopus (25) Google Scholar, 14Woller S.A. Choi S.H. An E.J. Low H. Schneider D.A. Ramachandran R. Kim J. Bae Y.S. Sviridov D. Corr M. et al.Inhibition of neuroinflammation by AIBP: spinal effects upon facilitated pain states.Cell Rep. 2018; 23: 2667-2677Abstract Full Text Full Text PDF PubMed Scopus (34) Google Scholar) and IFNγ receptor (15Sehgal P.B. Guo G.G. Shah M. Kumar V. Patel K. Cytokine signaling: STATS in plasma membrane rafts.J. Biol. Chem. 2002; 277: 12067-12074Abstract Full Text Full Text PDF PubMed Scopus (153) Google Scholar, 16Kim J.H. Choi D.J. Jeong H.K. Kim J. Kim D.W. Choi S.Y. Park S.M. Suh Y.H. Jou I. Joe E.H. DJ-1 facilitates the interaction between STAT1 and its phosphatase, SHP-1, in brain microglia and astrocytes: A novel anti-inflammatory function of DJ-1.Neurobiol. Dis. 2013; 60: 1-10Crossref PubMed Scopus (66) Google Scholar), association of TREM2 with the adaptor molecule DAP12 (17Poliani P.L. Wang Y. Fontana E. Robinette M.L. Yamanishi Y. Gilfillan S. Colonna M. TREM2 sustains microglial expansion during aging and response to demyelination.J. Clin. Invest. 2015; 125: 2161-2170Crossref PubMed Scopus (289) Google Scholar), and assembly of the NADPH oxidase complex (18Vilhardt F. van Deurs B. The phagocyte NADPH oxidase depends on cholesterol-enriched membrane microdomains for assembly.EMBO J. 2004; 23: 739-748Crossref PubMed Scopus (150) Google Scholar), among other inflammatory processes, lead to lipid raft clustering into larger and more stable inflammaraft units, pathological rafts. Depletion of cholesterol and/or sphingolipids from the plasma membrane disrupts inflammarafts. Thus, targeting cholesterol efflux agonists to inflammatory cells, for example via apoA-I binding protein (AIBP) (the treatment highlighted in a separate section below), could serve as a therapeutic strategy to reduce inflammation by targeting lipid rafts in a specific subset of cells. The CASMER designates a supramolecular signaling hub playing a central role in death receptor-mediated apoptosis and localizing in lipid rafts (19Gajate C. Mollinedo F. Cytoskeleton-mediated death receptor and ligand concentration in lipid rafts forms apoptosis-promoting clusters in cancer chemotherapy.J. Biol. Chem. 2005; 280: 11641-11647Abstract Full Text Full Text PDF PubMed Scopus (159) Google Scholar, 20Gajate C. Gonzalez-Camacho F. Mollinedo F. Lipid raft connection between extrinsic and intrinsic apoptotic pathways.Biochem. Biophys. Res. Commun. 2009; 380: 780-784Crossref PubMed Scopus (84) Google Scholar). The aggregated rafts forming CASMERs allow for an increased complexity of recruited proteins, which include the death receptors, Fas/CD95 and TNFR1 (CD120a) (19Gajate C. Mollinedo F. Cytoskeleton-mediated death receptor and ligand concentration in lipid rafts forms apoptosis-promoting clusters in cancer chemotherapy.J. Biol. Chem. 2005; 280: 11641-11647Abstract Full Text Full Text PDF PubMed Scopus (159) Google Scholar, 21Lotocki G. Alonso O.F. Dietrich W.D. Keane R.W. Tumor necrosis factor receptor 1 and its signaling intermediates are recruited to lipid rafts in the traumatized brain.J. Neurosci. 2004; 24: 11010-11016Crossref PubMed Scopus (62) Google Scholar), and the TRAIL receptors, TRAIL-R1 (DR4) and TRAIL-R2 (DR5) (19Gajate C. Mollinedo F. Cytoskeleton-mediated death receptor and ligand concentration in lipid rafts forms apoptosis-promoting clusters in cancer chemotherapy.J. Biol. Chem. 2005; 280: 11641-11647Abstract Full Text Full Text PDF PubMed Scopus (159) Google Scholar, 22Marconi M. Ascione B. Ciarlo L. Vona R. Garofalo T. Sorice M. Gianni A.M. Locatelli S.L. Carlo-Stella C. Malorni W. et al.Constitutive localization of DR4 in lipid rafts is mandatory for TRAIL-induced apoptosis in B-cell hematologic malignancies.Cell Death Dis. 2013; 4: e863Crossref PubMed Scopus (33) Google Scholar), as well downstream signaling molecules, including FADD, procaspase-8, and procaspase-10, forming the death-inducing signaling complex (20Gajate C. Gonzalez-Camacho F. Mollinedo F. Lipid raft connection between extrinsic and intrinsic apoptotic pathways.Biochem. Biophys. Res. Commun. 2009; 380: 780-784Crossref PubMed Scopus (84) Google Scholar, 23Gajate C. Mollinedo F. Edelfosine and perifosine induce selective apoptosis in multiple myeloma by recruitment of death receptors and downstream signaling molecules into lipid rafts.Blood. 2007; 109: 711-719Crossref PubMed Scopus (241) Google Scholar, 24Gajate C. Gonzalez-Camacho F. Mollinedo F. Involvement of raft aggregates enriched in Fas/CD95 death-inducing signaling complex in the antileukemic action of edelfosine in Jurkat cells.PLoS One. 2009; 4: e5044Crossref PubMed Scopus (91) Google Scholar). It is remarkable that signaling molecules might change their regulatory features when redistributed between a raft and a non-raft microenvironment (25Mollinedo, F., and C., Gajate, . Lipid rafts as signaling hubs in cancer cell survival/death and invasion: implications in tumor progression and therapy. J. Lipid Res. Epub ahead of print. January 27, 2020; doi:10.1194/jlr.TR119000439.Google Scholar). Compared with normal cells, cancer cells contain higher levels of cholesterol, facilitating clustering of cholesterol-rich lipid rafts to form CASMERs. Thus, formation of CASMERs as a major regulatory apoptotic signaling pivot makes them another example of a distinctive subset of pathological rafts, a potential therapeutic target in cancer. However, the therapeutic strategy here would be to promote recruitment of death receptors to CASMERs rather than to disrupt CASMERs, as is highlighted with an example of edelfosine in the section below. AIBP (gene name APOA1BP, also known as NAXE) was discovered in a yeast two-hybrid screen of proteins that bind apoA-I (26Ritter M. Buechler C. Boettcher A. Barlage S. Schmitz-Madry A. Orso E. Bared S.M. Schmiedeknecht G. Baehr C.H. Fricker G. et al.Cloning and characterization of a novel apolipoprotein A-I binding protein, AI-BP, secreted by cells of the kidney proximal tubules in response to HDL or ApoA-I.Genomics. 2002; 79: 693-702Crossref PubMed Scopus (63) Google Scholar) and shown to promote cholesterol efflux from endothelial cells, macrophages, and microglia to apoA-I and/or HDL (14Woller S.A. Choi S.H. An E.J. Low H. Schneider D.A. Ramachandran R. Kim J. Bae Y.S. Sviridov D. Corr M. et al.Inhibition of neuroinflammation by AIBP: spinal effects upon facilitated pain states.Cell Rep. 2018; 23: 2667-2677Abstract Full Text Full Text PDF PubMed Scopus (34) Google Scholar, 27Fang L. Choi S.H. Baek J.S. Liu C. Almazan F. Ulrich F. Wiesner P. Taleb A. Deer E. Pattison J. et al.Control of angiogenesis by AIBP-mediated cholesterol efflux.Nature. 2013; 498: 118-122Crossref PubMed Scopus (134) Google Scholar, 28Zhang M. Li L. Xie W. Wu J.F. Yao F. Tan Y.L. Xia X.D. Liu X.Y. Liu D. Lan G. et al.Apolipoprotein A-1 binding protein promotes macrophage cholesterol efflux by facilitating apolipoprotein A-1 binding to ABCA1 and preventing ABCA1 degradation.Atherosclerosis. 2016; 248: 149-159Abstract Full Text Full Text PDF PubMed Scopus (56) Google Scholar, 29Choi S.H. Wallace A.M. Schneider D.A. Burg E. Kim J. Alekseeva E. Ubags N.D. Cool C.D. Fang L. Suratt B.T. et al.AIBP augments cholesterol efflux from alveolar macrophages to surfactant and reduces acute lung inflammation.JCI Insight. 2018; 3: 120519Crossref PubMed Scopus (24) Google Scholar). AIBP also binds to TLR4 (14Woller S.A. Choi S.H. An E.J. Low H. Schneider D.A. Ramachandran R. Kim J. Bae Y.S. Sviridov D. Corr M. et al.Inhibition of neuroinflammation by AIBP: spinal effects upon facilitated pain states.Cell Rep. 2018; 23: 2667-2677Abstract Full Text Full Text PDF PubMed Scopus (34) Google Scholar). Surface expression of TLR4, which is localized to inflammarafts, is rapidly increased in activated cells, for example, in macrophages stimulated with LPS (14Woller S.A. Choi S.H. An E.J. Low H. Schneider D.A. Ramachandran R. Kim J. Bae Y.S. Sviridov D. Corr M. et al.Inhibition of neuroinflammation by AIBP: spinal effects upon facilitated pain states.Cell Rep. 2018; 23: 2667-2677Abstract Full Text Full Text PDF PubMed Scopus (34) Google Scholar), until TLR4 dimers are internalized via endocytosis (30Zanoni I. Ostuni R. Marek L.R. Barresi S. Barbalat R. Barton G.M. Granucci F. Kagan J.C. CD14 controls the LPS-induced endocytosis of Toll-like receptor 4.Cell. 2011; 147: 868-880Abstract Full Text Full Text PDF PubMed Scopus (618) Google Scholar). The increased TLR4 expression increases binding of recombinant AIBP to activated inflammatory cells, and this leads to enhanced cholesterol efflux and reduced abundance of inflammarafts (14Woller S.A. Choi S.H. An E.J. Low H. Schneider D.A. Ramachandran R. Kim J. Bae Y.S. Sviridov D. Corr M. et al.Inhibition of neuroinflammation by AIBP: spinal effects upon facilitated pain states.Cell Rep. 2018; 23: 2667-2677Abstract Full Text Full Text PDF PubMed Scopus (34) Google Scholar). The TLR4 binding affords selectivity to an AIBP mode of action: recombinant AIBP has little effect on nonactivated cells, while reversing pathological changes in lipid rafts back to the levels observed in nonactivated cells. A single intrathecal dose of AIBP reverses tactile allodynia (pain response to a light touch) in mouse models of chemotherapy-induced peripheral neuropathy (CIPN) and arthritis, with the therapeutic effect lasting as long as over 2 months in the CIPN model. This remarkable therapeutic effect is accompanied by no adverse effects of AIBP on motor or sensory function in mice (14Woller S.A. Choi S.H. An E.J. Low H. Schneider D.A. Ramachandran R. Kim J. Bae Y.S. Sviridov D. Corr M. et al.Inhibition of neuroinflammation by AIBP: spinal effects upon facilitated pain states.Cell Rep. 2018; 23: 2667-2677Abstract Full Text Full Text PDF PubMed Scopus (34) Google Scholar). Inhaled AIBP reduces LPS-induced acute lung injury in mice (29Choi S.H. Wallace A.M. Schneider D.A. Burg E. Kim J. Alekseeva E. Ubags N.D. Cool C.D. Fang L. Suratt B.T. et al.AIBP augments cholesterol efflux from alveolar macrophages to surfactant and reduces acute lung inflammation.JCI Insight. 2018; 3: 120519Crossref PubMed Scopus (24) Google Scholar) and AAV-mediated sustained expression of secreted AIBP reduces hyperlipidemia and atherosclerosis in Ldlr−/− mice fed a Western type diet (31Schneider D.A. Choi S.H. Agatisa-Boyle C. Zhu L. Kim J. Pattison J. Sears D.D. Gordts P. Fang L. Miller Y.I. AIBP protects against metabolic abnormalities and atherosclerosis.J. Lipid Res. 2018; 59: 854-863Abstract Full Text Full Text PDF PubMed Scopus (29) Google Scholar, 32Zhang M. Zhao G.J. Yao F. Xia X.D. Gong D. Zhao Z.W. Chen L.Y. Zheng X.L. Tang X.E. Tang C.K. AIBP reduces atherosclerosis by promoting reverse cholesterol transport and ameliorating inflammation in apoE(-/-) mice.Atherosclerosis. 2018; 273: 122-130Abstract Full Text Full Text PDF PubMed Scopus (31) Google Scholar) and human immunodeficiency virus (HIV) replication in humanized mice (33Dubrovsky L. Ward A. Choi S-H. Pushkarsky T. Brichacek B. Vanpouille C. Adzhubei A.A. Mukhamedova N. Sviridov D. Margolis L. et al.Inhibition of HIV replication by apolipoprotein A-I binding protein targeting the lipid rafts.MBio. 2020; 11: e02956-19Crossref PubMed Scopus (22) Google Scholar). Although experimental data provide evidence for TLR4-mediated targeting of AIBP to inflammatory cells (14Woller S.A. Choi S.H. An E.J. Low H. Schneider D.A. Ramachandran R. Kim J. Bae Y.S. Sviridov D. Corr M. et al.Inhibition of neuroinflammation by AIBP: spinal effects upon facilitated pain states.Cell Rep. 2018; 23: 2667-2677Abstract Full Text Full Text PDF PubMed Scopus (34) Google Scholar), other components of inflammarafts may mediate this targeting as well, depending on the cell type and specific pathologic conditions. By the virtue of affecting lipid raft composition and abundance, in addition to TLR4 dimerization (14Woller S.A. Choi S.H. An E.J. Low H. Schneider D.A. Ramachandran R. Kim J. Bae Y.S. Sviridov D. Corr M. et al.Inhibition of neuroinflammation by AIBP: spinal effects upon facilitated pain states.Cell Rep. 2018; 23: 2667-2677Abstract Full Text Full Text PDF PubMed Scopus (34) Google Scholar), AIBP likely inhibits other receptors, enzymes, and channels localized to inflammarafts, but this hypothesis needs experimental validation. LXR is a transcriptional regulator of ABCA1 and ABCG1 (among other genes), and, in the presence of an agonist, it significantly stimulates expression and abundance of these cholesterol transporters. ABCA1 is a key regulator of both cholesterol availability and actin polymerization and regulates the abundance of lipid rafts through both mechanisms. The “lipid” mechanism relies on the central role of ABCA1 and ABCG1 in cholesterol efflux. Thus, reduced abundance of ABCA1 increases the amount of cellular cholesterol potentiating formation of lipid rafts and vice versa (34Lai L. Azzam K.M. Lin W-C. Rai P. Lowe J.M. Gabor K.A. Madenspacher J.H. Aloor J.J. Parks J.S. Näär A.M. et al.MicroRNA-33 regulates the innate immune response via ATP binding cassette transporter-mediated remodeling of membrane microdomains.J. Biol. Chem. 2016; 291: 19651-19660Abstract Full Text Full Text PDF PubMed Scopus (43) Google Scholar). The same mechanism is probably also responsible for the increased abundance of lipid rafts in ABCG1- or ABCA1/ABCG1-deficient macrophages (10Yvan-Charvet L. Welch C. Pagler T.A. Ranalletta M. Lamkanfi M. Han S. Ishibashi M. Li R. Wang N. Tall A.R. Increased inflammatory gene expression in ABC transporter-deficient macrophages: free cholesterol accumulation, increased signaling via toll-like receptors, and neutrophil infiltration of atherosclerotic lesions.Circulation. 2008; 118: 1837-1847Crossref PubMed Scopus (339) Google Scholar). The “cytoskeleton” mechanism relies on the ability of ABCA1 to activate the small GTPase Cdc42, which stimulates polymerization of actin (35Nofer J-R. Remaley A.T. Feuerborn R. Wolinnska I. Engel T. von Eckardstein A. Assmann G. Apolipoprotein A-I activates Cdc42 signaling through the ABCA1 transporter.J. Lipid Res. 2006; 47: 794-803Abstract Full Text Full Text PDF PubMed Scopus (60) Google Scholar, 36Nofer J-R. Feuerborn R. Levkau B. Sokoll A. Seedorf U. Assmann G. Involvement of Cdc42 Signaling in ApoA-I-induced Cholesterol Efflux.J. Biol. Chem. 2003; 278: 53055-53062Abstract Full Text Full Text PDF PubMed Scopus (59) Google Scholar, 37Kheirollah A. Nagayasu Y. Ueda H. Yokoyama S. Michikawa M. Ito J. Involvement of cdc42/Rho kinase in ApoA-I-mediated cholesterol efflux through interaction between cytosolic lipid-protein particles and microtubules in rat astrocytes.J. Neurosci. Res. 2014; 92: 455-463Crossref PubMed Scopus (11) Google Scholar), with a subsequent negative effect on raft abundance (8Chichili G.R. Rodgers W. Cytoskeleton-membrane interactions in membrane raft structure.Cell. Mol. Life Sci. 2009; 66: 2319-2328Crossref PubMed Scopus (189) Google Scholar). The connection between ABCA1 and rafts is reciprocal: ABCA1 determines the abundance of rafts (38Landry Y.D. Denis M. Nandi S. Bell S. Vaughan A.M. Zha X. ATP-binding cassette transporter A1 expression disrupts raft membrane microdomains through its ATPase-related functions.J. Biol. Chem. 2006; 281: 36091-36101Abstract Full Text Full Text PDF PubMed Scopus (184) Google Scholar); but at the same time, activity and stability of ABCA1 is determined by the abundance of rafts (39Klappe K. Hummel I. Hoekstra D. Kok J.W. Lipid dependence of ABC transporter localization and function.Chem. Phys. Lipids. 2009; 161: 57-64Crossref PubMed Scopus (95) Google Scholar). LXR agonists have been shown to reduce the abundance of lipid rafts in vitro and in vivo (40Noghero A. Perino A. Seano G. Saglio E. Sasso G.L. Veglio F. Primo L. Hirsch E. Bussolino F. Morello F. Liver X receptor activation reduces angiogenesis by impairing lipid raft localization and signaling of vascular endothelial growth factor receptor-2.Arterioscler. Thromb. Vasc. Biol. 2012; 32: 2280-2288Crossref PubMed Scopus (58) Google Scholar, 41Sun Y. Yao J. Kim T-W. Tall A.R. Expression of liver X receptor target genes decreases cellular amyloid {beta} peptide secretion.J. Biol. Chem. 2003; 278: 27688-27694Abstract Full Text Full Text PDF PubMed Scopus (165) Google Scholar, 42Ramezani A. Dubrovsky L. Pushkarsky T. Sviridov D. Karandish S. Raj D.S. Fitzgerald M.L. Bukrinsky M. Stimulation of liver X receptor has potent anti-HIV effects in a humanized mouse model of HIV infection.J. Pharmacol. Exp. Ther. 2015; 354: 376-383Crossref PubMed Scopus (13) Google Scholar). Given that LXR regulates the expression of many genes and is involved in regulation of multiple pathways, selectivity of the effect of LXR agonists on lipid rafts and the contribution of raft-dependent effects to overall outcome are difficult to ascertain. The ability of LXR agonists