The challenges and promise of targeting the Liver X Receptors for treatment of inflammatory disease

Abstract

The Liver X Receptors (LXRs) are oxysterol-activated transcription factors that upregulate a suite of genes that together promote coordinated mobilization of excess cholesterol from cells and from the body. The LXRs, like other nuclear receptors, are anti-inflammatory, inhibiting signal-dependent induction of pro-inflammatory genes by nuclear factor-κB, activating protein-1, and other transcription factors. Synthetic LXR agonists have been shown to ameliorate atherosclerosis and a wide range of inflammatory disorders in preclinical animal models. Although this has suggested potential for application to human disease, systemic LXR activation is complicated by hepatic steatosis and hypertriglyceridemia, consequences of lipogenic gene induction in the liver by LXRα. The past several years have seen the development of multiple advanced LXR therapeutics aiming to avoid hepatic lipogenesis, including LXRβ-selective agonists, tissue-selective agonists, and transrepression-selective agonists. Although several synthetic LXR agonists have made it to phase I clinical trials, none have progressed due to unforeseen adverse reactions or undisclosed reasons. Nonetheless, several sophisticated pharmacologic strategies, including structure-guided drug design, cell-specific drug targeting, as well as non-systemic drug routes have been initiated and remain to be comprehensively explored. In addition, recent studies have identified potential utility for targeting the LXRs during therapy with other agents, such as glucocorticoids and rexinoids. Despite the pitfalls encountered to date in translation of LXR agonists to human disease, it appears likely that this accelerating field will ultimately yield effective and safe applications for LXR targeting in humans.

Keywords: Atherosclerosis; Cholesterol; Inflammation; Liver X Receptor; Oxysterol; Reverse cholesterol transport.

Figure 1. Molecular mechanisms of LXR action

Genomic mechanisms, including gene transactivation (A) and transrepression (B), are the best characterized means of LXR activity in cells, whereas recently described non-genomic mechanisms (C) remain somewhat less well understood. In transactivation, two general models, represented by induction of the target genes ATP Binding Cassette (ABC)A1 and ABCG1, have been identified. For ABCA1, it is thought that LXR bound to promoter LXR response elements (LXREs) in the steady state tonically represses gene expression by recruiting co-repressors such as nuclear receptor co-repressor 1 (NCoR). Upon LXR ligand binding, co-repressors are shed in exchange for co-activators, driving gene expression. More recent studies suggest that for a majority of LXR targets including ABCG1, LXRs bind to LXREs only after ligand-induced activation and histone demethylation. In both of these models, LXR binds LXREs directly in the form of a heterodimer with retinoid X receptor (RXR). In the case of transrepression of pro-inflammatory genes (B), two general mechanisms of LXR action have been identified in macrophages/hepatocytes and astrocytes. In both cases, lysines in the ligand-binding domain of LXR are SUMOylated after LXR ligation. In the case of LPS-stimulated macrophages or cytokine-stimulated hepatocytes, SUMOylated LXR binds in monomeric form to a multimolecular co-repressor complex, inhibiting release of co-repressors from gene promoters, thereby blocking gene expression. In astrocytes, SUMOylated LXRs inhibit transcription by blocking the binding of signal transducer and activator of transcription 1 (STAT1) to promoters. Recently, examples of non-genomic (extranuclear) LXRβ action have been identified, as shown in panel C. In colon cancer cells, cytoplasmic LXRβ drives NLRP3- and caspase-1-dependent pyroptotic cell death by inducing pannexin-1-mediated ATP release. In platelets, LXRβ inhibits kinase signaling to degranulation and aggregation downstream of the GPVI receptor. In endothelial cells, lipid raft-localized LXRβ mediates a signaling pathway to cell migration involving estrogen receptor (ER)-α and the kinase AKT. The relative importance of genomic and non-genomic mechanisms remains poorly understood. The distinct binding partners of LXRs in these various contexts, however, creates the exciting potential for targeted, potentially cell type- and gene-specific, pharmacologic interventions. eNOS, endothelial nitric oxide synthase; GPVI, glycoprotein VI; GPS2, G-protein pathway suppressor 2; HDAC, histone deacetylase; iNOS, inducible nitric oxide synthase; P2X7, P2X purinoceptor 7; PIAS1, protein inhibitor of STAT1; SAA, serum amyloid A; SUMO, small ubiquitin-like modifier; TBLR, transducin beta-like 1X-related protein 1; UBC9, SUMO-conjugating enzyme UBC9.

Figure 2. LXRs are master regulators of reverse cholesterol transport

The in vivo pathways for disposal of excess cellular – in this case, macrophage – cholesterol from the organism, so called ‘reverse cholesterol transport,’ are depicted. Proteins in green font are encoded by LXR target genes, and are thus upregulated by LXR agonists (note, however, that cytochrome P450 family 7 subfamily A member 1 [CYP7A1] is an LXR target in mice and not humans, and cholesteryl ester transfer protein [CETP] is expressed in humans but not mice). ATP Binding Cassette (ABC)A1 in enterocytes and hepatocytes generates plasma high density lipoprotein (HDL) by lipidation of plasma apolipoprotein (apo)A-I. HDL in turn induces cholesterol efflux from macrophages via cooperative interactions with ABCA1 and ABCG1. HDL-associated cholesterol can then either be cleared via hepatocyte scavenger receptor (SR)-BI, or transferred to apoB100-containing lipoproteins via CETP in exchange for triglyceride (TG). Finally, apoB100-lipoprotein cholesterol is cleared from the plasma either by hepatocyte low density lipoprotein receptor (LDLR), or by trans-enterocyte transfer into the gut lumen, the so-called ‘trans-intestinal cholesterol excretion’ (TICE) pathway. Hepatic cholesterol that is not used to assemble very LDL (VLDL) mnplasma particles can be excreted into the biliary system (and from there into the gut lumen), either via ABCG5/8-dependent transport as free cholesterol, or as bile acids, after conversion by CYP7A1. In enterocytes, LXR upregulates ABCA1 (driving HDL production) and ABCG5/8 (promoting cholesterol efflux into gut lumen), and downregulates Niemann Pick C1 like 1 (NPC1L1) protein (reducing uptake of luminal cholesterol). Taken together, enterocyte LXR increases plasma HDL, reduces cholesterol absorption, and promotes cholesterol excretion. Hepatic LXR increases plasma HDL and promotes biliary cholesterol excretion. Hepatic LXR also induces the lipogenic genes sterol response element binding protein (SREBP)-1c, fatty acid synthase (FAS), and stearoyl coA-desaturase (SCD)1, as well as inducible degrader of the LDLR (IDOL). In rodent models, treatment with LXR agonists drives fecal excretion of macrophage-derived cholesterol (ie, RCT). HL, hepatic lipase; IDL, intermediate density lipoprotein; LPL, lipoprotein lipase.