Liver X receptor

LXRα (nuclear receptor subfamily 1, group H, member 3)
Identifiers
Symbol NR1H3
Entrez 10062
HUGO 7966
OMIM 602423
RefSeq NM_005693
UniProt Q13133
LXRβ (nuclear receptor subfamily 1, group H, member 2)
Identifiers
Symbol NR1H2
Alt. symbols UNR
Entrez 7376
HUGO 7965
OMIM 600380
RefSeq NM_007121
UniProt P55055

The liver X receptor (LXR) is a member of the nuclear receptor family of transcription factors and is closely related to nuclear receptors such as the PPARs, FXR and RXR. Liver X receptors (LXRs) are important regulators of cholesterol, fatty acid, and glucose homeostasis. LXRs were earlier classified as orphan nuclear receptors, however, upon discovery of endogenous oxysterols as ligands, they were subsequently deorphanized.

Two isoforms of LXR have been identified and are referred to as LXRα and LXRβ. The liver X receptors are classified into subfamily 1 (thyroid hormone receptor-like) of the nuclear receptor superfamily, and are given the nuclear receptor nomenclature symbols NR1H3 (LXRα) and NR1H2 (LXRβ) respectively.

LXRα and LXRβ were discovered separately between 1994-1995. LXRα isoform was independently identified by two groups and initially named RLD-1[1] and LXR,[2] whereas four groups identified the LXRβ isoform and called it UR,[3] NER,[4] OR-1,[5] and RIP-15.[6] The human LXRα gene is located on chromosome 11p11.2, while the LXRβ gene is located on chromosome 19q13.3.

Expression

While the expression of LXRα and LXRβ in various tissues somewhat overlap, the tissue distribution pattern of these two isoforms overall differ considerably. LXRα expression is restricted to liver, kidney, intestine, fat tissue, macrophages, lung, and spleen and is highest in liver, hence the name liver X receptor α (LXRα). LXRβ is expressed in almost all tissues and organs, hence the early name UR (ubiquitous receptor).[7] The different pattern of expression suggests that LXRα and LXRβ have different roles in regulating physiological function.

Structure

Crystal structure of human liver X receptor β(LXRβ) forming heterodimer with its partner retinoid X receptor α(RXRα) on its cognate element, an AGGTCA direct repeat spaced by 4 nt shows an extended X-shaped arrangement, with DNA- and ligand-binding domains crossed, in contrast to the parallel domain arrangement of other NRs that bind an AGGTCA direct repeat spaced by 1 nt. The LXRβ core binds DNA via canonical contacts and auxiliary DNA contacts that enhance affinity for the response element.[8]

Activation/ligands

LXRα and LXRβ form heterodimers with the obligate partner 9-cis retinoic acid receptor (RXR). The LXR/RXR heterodimer can be activated with either an LXR agonist (oxysterols) or a RXR agonist (9-cis-retinoic acid). Oxysterols, the oxygenated derivatives of cholesterol, such as 22(R)-hydroxycholesterol, 24(S)-hydroxycholesterol, 27-hydroxycholesterol, and cholestenoic acid, are the natural ligands for LXR.[9][10][11][12] After activation, LXR binds to LXR response element (LXRE), usually a variant of the idealized sequence AGGTCAN4AGGTCA, in the promoters of LXRs' target genes. Some synthetic LXR agonists have been developed, including nonsteroidal LXR agonists T0901317[13] and GW3965.

Target genes

Target genes of LXRs are involved in cholesterol and lipid metabolism regulation,[14] including:

Role in metabolism

The importance of LXRs in physiological lipid and cholesterol metabolism suggests that they may influence the development of metabolic disorders such as hyper lipidemia and atherosclerosis. Evidence for this idea has been observed by recent studies that linked LXR activity to the pathogenesis of atherosclerosis. LXRα knockout mice are healthy when fed with a low-cholesterol diet. However, LXRα knockout mice develop enlarged fatty livers, degeneration of liver cells, high cholesterol levels in liver, and impaired liver function when fed a high-cholesterol diet.[15] LXRβ knockout mice are unaffected by a high-cholesterol diet, suggesting that LXRα and LXRβ have separate roles. LXRs regulate fatty acid synthesis by modulating the expression of sterol regulatory element binding protein-1c (SREBP-1c).[16][17] LXRs also regulate lipid homeostasis in the brain. LXRα and LXRβ double knockout mice develop neurodegenerative changes in brain tissue.[18] LXRβ knockout mice results in adult-onset motor neuron degeneration in male mice.[19]

Potential therapeutic applications of LXR agonists

LXR agonists are effective for treatment of murine models of atherosclerosis, diabetes, anti-inflammation, and Alzheimer's disease. Treatment with LXR agonists (hypocholamide, T0901317, GW3965, or N,N-dimethyl-3beta-hydroxy-cholenamide (DMHCA)) lowers the cholesterol level in serum and liver and inhibits the development of atherosclerosis in murine disease models.[20][21][22][23] Synthetic LXR agonist GW3965 improves glucose tolerance in a murine model of diet-induced obesity and insulin resistance by regulating genes involved in glucose metabolism in liver and adipose tissue.<[24] GW3965 inhibits the expression of inflammatory mediators in cultured macrophage and inflammation in mice.[25] LXR agonists (T0901317, 22(R)-hydroxycholesterol, and 24(S)-hydroxycholesterol) were also shown to suppress the proliferation of prostate cancer and breast cancer cells[26] as well as delay progression of prostate cancer from androgen-dependent status to androgen-independent status.[27] Treatment with T0901317 decreases amyloidal beta production in an Alzheimer's disease mouse model.[28] However, both T0901317 and GW3965 have been reported to increase plasma and liver triglycerides in some mice models, indicating that T0901317 and GW3965 may not be a good candidate for a therapeutic agent. Developing new potent and effective LXR agonists without the undesirable side effects may be beneficial for clinical usage.[29] In this regard, DMHCA was reported to reduce atherosclerosis in apolipoprotein E-deficient mice without inducing hypertriglyceridemia and liver steatosis.[23] When lipogenesis is increased by pharmacological activation of the liver X receptor, hepatic VLDL production is increased 2.5-fold, and the liver produces large TG-rich VLDL particles. Interestingly, glucose induces expression of LXR target genes involved in cholesterol homeostasis like ABCA1 which is defective in Tangier disease. A common feature of many metabolic pathways is their control by retinoid X receptor (RXR) heterodimers. It is interesting to note that LXR heterodimerises with RXR. Promiscuous RXR also heterodimerises with PPAR members. PPAR-α plays a pivotal role in fatty acid catabolism in liver by upregulating the expression of numerous genes involved in mitochondrial fatty acid oxidation. Thus RXR is a common partner of two nuclear receptors acting in opposite directions with regard to fatty acid metabolism. So both LXR and PPAR-α compete for the limited pool of RXR and this dynamic equilibrium determines the direction of lipid metabolism.[30]

The hexacyclic aromatic ketones, (-)anthrabenzoxocinone and (-)bischloroanthrabenzoxocinone ((-)-BABX) derived from a Streptomyces sp. have micromolar affinity for LXR-α.[31]

References

  1. ↑ Apfel R, Benbrook D, Lernhardt E, Ortiz MA, Salbert G, Pfahl M (October 1994). "A novel orphan receptor specific for a subset of thyroid hormone-responsive elements and its interaction with the retinoid/thyroid hormone receptor subfamily". Mol. Cell. Biol. 14 (10): 7025–35. PMC 359232Freely accessible. PMID 7935418.
  2. ↑ Willy PJ, Umesono K, Ong ES, Evans RM, Heyman RA, Mangelsdorf DJ (May 1995). "LXR, a nuclear receptor that defines a distinct retinoid response pathway". Genes Dev. 9 (9): 1033–45. doi:10.1101/gad.9.9.1033. PMID 7744246.
  3. ↑ Song C, Kokontis JM, Hiipakka RA, Liao S (November 1994). "Ubiquitous receptor: a receptor that modulates gene activation by retinoic acid and thyroid hormone receptors". Proc. Natl. Acad. Sci. U.S.A. 91 (23): 10809–13. doi:10.1073/pnas.91.23.10809. PMC 45115Freely accessible. PMID 7971966.
  4. ↑ Shinar DM, Endo N, Rutledge SJ, Vogel R, Rodan GA, Schmidt A (September 1994). "NER, a new member of the gene family encoding the human steroid hormone nuclear receptor". Gene. 147 (2): 273–6. doi:10.1016/0378-1119(94)90080-9. PMID 7926814.
  5. ↑ Teboul M, Enmark E, Li Q, Wikström AC, Pelto-Huikko M, Gustafsson JA (March 1995). "OR-1, a member of the nuclear receptor superfamily that interacts with the 9-cis-retinoic acid receptor". Proc. Natl. Acad. Sci. U.S.A. 92 (6): 2096–100. doi:10.1073/pnas.92.6.2096. PMC 42430Freely accessible. PMID 7892230.
  6. ↑ Seol W, Choi HS, Moore DD (January 1995). "Isolation of proteins that interact specifically with the retinoid X receptor: two novel orphan receptors". Mol. Endocrinol. 9 (1): 72–85. doi:10.1210/mend.9.1.7760852. PMID 7760852.
  7. ↑ Chuu CP, Kokontis JM, Hiipakka RA, Liao S (September 2007). "Modulation of liver X receptor signaling as novel therapy for prostate cancer". J. Biomed. Sci. 14 (5): 543–53. doi:10.1007/s11373-007-9160-8. PMID 17372849.
  8. ↑ Lou X.; Toresson G.; Benod C.; Suh J. H.; Philips K. J.; Webb P.; Gustafsson JA (February 2014). "Structure of the retinoid X receptor α-liver X receptor β (RXRα-LXRβ) heterodimer on DNA". Nat. Struct. Mol. Biol. 21 (3): 277–281. doi:10.1038/nsmb.2778. PMID 24561505.
  9. ↑ Janowski BA, Willy PJ, Devi TR, Falck JR, Mangelsdorf DJ (October 1996). "An oxysterol signalling pathway mediated by the nuclear receptor LXR alpha". Nature. 383 (6602): 728–31. doi:10.1038/383728a0. PMID 8878485.
  10. ↑ Forman BM, Ruan B, Chen J, Schroepfer GJ, Evans RM (September 1997). "The orphan nuclear receptor LXRα is positively and negatively regulated by distinct products of mevalonate metabolism". Proc. Natl. Acad. Sci. U.S.A. 94 (20): 10588–93. doi:10.1073/pnas.94.20.10588. PMC 23411Freely accessible. PMID 9380679.
  11. ↑ Lehmann JM, Kliewer SA, Moore LB, Smith-Oliver TA, Oliver BB, Su JL, Sundseth SS, Winegar DA, Blanchard DE, Spencer TA, Willson TM (February 1997). "Activation of the nuclear receptor LXR by oxysterols defines a new hormone response pathway". J. Biol. Chem. 272 (6): 3137–40. doi:10.1074/jbc.272.6.3137. PMID 9013544.
  12. ↑ Song C, Liao S (November 2000). "Cholestenoic acid is a naturally occurring ligand for liver X receptor alpha". Endocrinology. 141 (11): 4180–4. doi:10.1210/en.141.11.4180. PMID 11089551.
  13. ↑ Schultz JR, Tu H, Luk A, Repa JJ, Medina JC, Li L, Schwendner S, Wang S, Thoolen M, Mangelsdorf DJ, Lustig KD, Shan B (November 2000). "Role of LXRs in control of lipogenesis". Genes Dev. 14 (22): 2831–8. doi:10.1101/gad.850400. PMC 317060Freely accessible. PMID 11090131.
  14. ↑ Edwards PA, Kennedy MA, Mak PA (April 2002). "LXRs; oxysterol-activated nuclear receptors that regulate genes controlling lipid homeostasis". Vascul. Pharmacol. 38 (4): 249–56. doi:10.1016/S1537-1891(02)00175-1. PMID 12449021.
  15. ↑ Peet DJ, Turley SD, Ma W, Janowski BA, Lobaccaro JM, Hammer RE, Mangelsdorf DJ (May 1998). "Cholesterol and bile acid metabolism are impaired in mice lacking the nuclear oxysterol receptor LXR alpha". Cell. 93 (5): 693–704. doi:10.1016/S0092-8674(00)81432-4. PMID 9630215.
  16. ↑ Yoshikawa T, Shimano H, Amemiya-Kudo M, Yahagi N, Hasty AH, Matsuzaka T, Okazaki H, Tamura Y, Iizuka Y, Ohashi K, Osuga J, Harada K, Gotoda T, Kimura S, Ishibashi S, Yamada N (May 2001). "Identification of Liver X Receptor-Retinoid X Receptor as an Activator of the Sterol Regulatory Element-Binding Protein 1c Gene Promoter". Mol. Cell. Biol. 21 (9): 2991–3000. doi:10.1128/MCB.21.9.2991-3000.2001. PMC 86928Freely accessible. PMID 11287605.
  17. ↑ Repa JJ, Liang G, Ou J, Bashmakov Y, Lobaccaro JM, Shimomura I, Shan B, Brown MS, Goldstein JL, Mangelsdorf DJ (November 2000). "Regulation of mouse sterol regulatory element-binding protein-1c gene (SREBP-1c) by oxysterol receptors, LXRα and LXRβ". Genes Dev. 14 (22): 2819–30. doi:10.1101/gad.844900. PMC 317055Freely accessible. PMID 11090130.
  18. ↑ Wang L, Schuster GU, Hultenby K, Zhang Q, Andersson S, Gustafsson JA (October 2002). "Liver X receptors in the central nervous system: From lipid homeostasis to neuronal degeneration". Proc. Natl. Acad. Sci. U.S.A. 99 (21): 13878–83. doi:10.1073/pnas.172510899. PMC 129791Freely accessible. PMID 12368482.
  19. ↑ Andersson S, Gustafsson N, Warner M, Gustafsson JA (March 2005). "Inactivation of liver X receptor β leads to adult-onset motor neuron degeneration in male mice". Proc. Natl. Acad. Sci. U.S.A. 102 (10): 3857–62. doi:10.1073/pnas.0500634102. PMC 553330Freely accessible. PMID 15738425.
  20. ↑ Alberti S, Schuster G, Parini P, Feltkamp D, Diczfalusy U, Rudling M, Angelin B, Björkhem I, Pettersson S, Gustafsson JA (March 2001). "Hepatic cholesterol metabolism and resistance to dietary cholesterol in LXRβ-deficient mice". J. Clin. Invest. 107 (5): 565–73. doi:10.1172/JCI9794. PMC 199420Freely accessible. PMID 11238557.
  21. ↑ Joseph SB, McKilligin E, Pei L, Watson MA, Collins AR, Laffitte BA, Chen M, Noh G, Goodman J, Hagger GN, Tran J, Tippin TK, Wang X, Lusis AJ, Hsueh WA, Law RE, Collins JL, Willson TM, Tontonoz P (May 2002). "Synthetic LXR ligand inhibits the development of atherosclerosis in mice". Proc. Natl. Acad. Sci. U.S.A. 99 (11): 7604–9. doi:10.1073/pnas.112059299. PMC 124297Freely accessible. PMID 12032330.
  22. ↑ Song C, Hiipakka RA, Liao S (June 2001). "Auto-oxidized cholesterol sulfates are antagonistic ligands of liver X receptors: implications for the development and treatment of atherosclerosis". Steroids. 66 (6): 473–9. doi:10.1016/S0039-128X(00)00239-7. PMID 11182136.
  23. 1 2 Kratzer A, Buchebner M, Pfeifer T, Becker TM, Uray G, Miyazaki M, Miyazaki-Anzai S, Ebner B, Chandak PG, Kadam RS, Calayir E, Rathke N, Ahammer H, Radovic B, Trauner M, Hoefler G, Kompella UB, Fauler G, Levi M, Levak-Frank S, Kostner GM, Kratky D (February 2009). "Synthetic LXR agonist attenuates plaque formation in apoE-/- mice without inducing liver steatosis and hypertriglyceridemia". J. Lipid Res. 50 (2): 312–26. doi:10.1194/jlr.M800376-JLR200. PMC 2636920Freely accessible. PMID 18812595.
  24. ↑ Laffitte BA, Chao LC, Li J, Walczak R, Hummasti S, Joseph SB, Castrillo A, Wilpitz DC, Mangelsdorf DJ, Collins JL, Saez E, Tontonoz P (April 2003). "Activation of liver X receptor improves glucose tolerance through coordinate regulation of glucose metabolism in liver and adipose tissue". Proc. Natl. Acad. Sci. U.S.A. 100 (9): 5419–24. doi:10.1073/pnas.0830671100. PMC 154360Freely accessible. PMID 12697904.
  25. ↑ Joseph SB, Castrillo A, Laffitte BA, Mangelsdorf DJ, Tontonoz P (February 2003). "Reciprocal regulation of inflammation and lipid metabolism by liver X receptors". Nat. Med. 9 (2): 213–9. doi:10.1038/nm820. PMID 12524534.
  26. ↑ Fukuchi J, Kokontis JM, Hiipakka RA, Chuu CP, Liao S (November 2004). "Antiproliferative effect of liver X receptor agonists on LNCaP human prostate cancer cells". Cancer Res. 64 (21): 7686–9. doi:10.1158/0008-5472.CAN-04-2332. PMID 15520170.
  27. ↑ Chuu CP, Hiipakka RA, Kokontis JM, Fukuchi J, Chen RY, Liao S (July 2006). "Inhibition of tumor growth and progression of LNCaP prostate cancer cells in athymic mice by androgen and liver X receptor agonist". Cancer Res. 66 (13): 6482–6. doi:10.1158/0008-5472.CAN-06-0632. PMID 16818617.
  28. ↑ Koldamova RP, Lefterov IM, Staufenbiel M, Wolfe D, Huang S, Glorioso JC, Walter M, Roth MG, Lazo JS (February 2005). "The liver X receptor ligand T0901317 decreases amyloid beta production in vitro and in a mouse model of Alzheimer's disease". J. Biol. Chem. 280 (6): 4079–88. doi:10.1074/jbc.M411420200. PMID 15557325.
  29. ↑ Im SS, Osborne TF (April 2011). "Liver x receptors in atherosclerosis and inflammation". Circulation Research. 108 (8): 996–1001. doi:10.1161/CIRCRESAHA.110.226878. PMC 3082200Freely accessible. PMID 21493922.
  30. ↑ Sanal MG (2008). "The blind men 'see' the elephant-the many faces of fatty liver disease". World J. Gastroenterol. 14 (6): 831–44. doi:10.3748/wjg.14.831. PMC 2687050Freely accessible. PMID 18240340.
  31. ↑ Herath KB, Jayasuriya H, Guan Z, Schulman M, Ruby C, Sharma N, MacNaul K, Menke JG, Kodali S, Galgoci A, Wang J, Singh SB (September 2005). "Anthrabenzoxocinones from Streptomyces sp. as liver X receptor ligands and antibacterial agents". J. Nat. Prod. 68 (9): 1437–40. doi:10.1021/np050176k. PMID 16180833.
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