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Low-density lipoprotein receptor-related protein 6 is a protein that in humans is encoded by the LRP6gene.[1][2] LRP6 is a key component of the LRP5/LRP6/Frizzled co-receptor group that is involved in canonical Wnt pathway.
LRP6 is a transmembrane low-density lipoproteinreceptor that shares a similar structure with LRP5. In each protein, about 85% of its 1600-amino-acid length is extracellular. Each has four β-propeller motifs at the amino terminal end that alternate with four epidermal growth factor (EGF)-like repeats. Most extracellular ligands bind to LRP5 and LRP6 at the β-propellers. Each protein has a single-pass, 22-amino-acid segment that crosses the cell membrane and a 207-amino-acid segment that is internal to the cell.[3]
Function
LRP6 acts as a co-receptor with LRP5 and the Frizzled protein family members for transducing signals by Wnt proteins through the canonical Wnt pathway.[3]
Interactions
Canonical WNT signals are transduced through Frizzled receptor and LRP5/LRP6 coreceptor to downregulate GSK3beta (GSK3B) activity not depending on Ser-9 phosphorylation.[4] Reduction of canonical Wnt signals upon depletion of LRP5 and LRP6 results in p120-catenin degradation.[5]
LRP6 is regulated by extracellular proteins in the Dickkopf (Dkk) family (like DKK1[6]), sclerostin, R-spondins and members of the cysteine-knot-type protein family.[3]
Clinical significance
Loss-of-function mutations or LRP6 in humans lead to increased plasma LDL and triglycerides, hypertension, diabetes and osteoporosis.[3] Similarly, mice with a loss-of-function Lrp6 mutation have low bone mass.[7] LRP6 is critical in bone's anabolic response to parathyroid hormone (PTH) treatment, whereas LRP5 is not involved.[7] On the other hand, LRP6 does not appear active in mechanotransduction (bone's response to forces), while LRP5 is critical in that role.[7]
Sclerostin, one of the inhibitors of LRP6, is a promising osteocyte-specific Wnt antagonist in osteoporosis clinical trials.[8][9]
References
↑Brown SD, Twells RC, Hey PJ, Cox RD, Levy ER, Soderman AR, Metzker ML, Caskey CT, Todd JA, Hess JF (1998). "Isolation and characterization of LRP6, a novel member of the low density lipoprotein receptor gene family". Biochem. Biophys. Res. Commun. 248 (3): 879–88. doi:10.1006/bbrc.1998.9061. PMID9704021.
↑Katoh M, Katoh M (2006). "Cross-talk of WNT and FGF signaling pathways at GSK3beta to regulate beta-catenin and SNAIL signaling cascades". Cancer Biol. Ther. 5 (9): 1059–64. doi:10.4161/cbt.5.9.3151. PMID16940750.
↑Semënov MV, Tamai K, Brott BK, Kühl M, Sokol S, He X (2001). "Head inducer Dickkopf-1 is a ligand for Wnt coreceptor LRP6". Curr. Biol. 11 (12): 951–61. doi:10.1016/s0960-9822(01)00290-1. PMID11448771.
↑Baron R, Kneissel M (February 2013). "WNT signaling in bone homeostasis and disease: from human mutations to treatments". Nature Medicine. 19 (2): 179–192. doi:10.1038/nm.3074. PMID23389618.
He X, Semenov M, Tamai K, Zeng X (2004). "LDL receptor-related proteins 5 and 6 in Wnt/beta-catenin signaling: arrows point the way". Development. 131 (8): 1663–77. doi:10.1242/dev.01117. PMID15084453.
Hillier LD, Lennon G, Becker M, et al. (1997). "Generation and analysis of 280,000 human expressed sequence tags". Genome Res. 6 (9): 807–28. doi:10.1101/gr.6.9.807. PMID8889549.
Baens M, Wlodarska I, Corveleyn A, et al. (1999). "A physical, transcript, and deletion map of chromosome region 12p12.3 flanked by ETV6 and CDKN1B: hypermethylation of the LRP6 CpG island in two leukemia patients with hemizygous del(12p)". Genomics. 56 (1): 40–50. doi:10.1006/geno.1998.5685. PMID10036184.
Tamai K, Semenov M, Kato Y, et al. (2000). "LDL-receptor-related proteins in Wnt signal transduction". Nature. 407 (6803): 530–5. doi:10.1038/35035117. PMID11029007.
Mao B, Wu W, Li Y, et al. (2001). "LDL-receptor-related protein 6 is a receptor for Dickkopf proteins". Nature. 411 (6835): 321–5. doi:10.1038/35077108. PMID11357136.
Semënov MV, Tamai K, Brott BK, et al. (2001). "Head inducer Dickkopf-1 is a ligand for Wnt coreceptor LRP6". Curr. Biol. 11 (12): 951–61. doi:10.1016/S0960-9822(01)00290-1. PMID11448771.
Li L, Mao J, Sun L, et al. (2002). "Second cysteine-rich domain of Dickkopf-2 activates canonical Wnt signaling pathway via LRP-6 independently of dishevelled". J. Biol. Chem. 277 (8): 5977–81. doi:10.1074/jbc.M111131200. PMID11742004.
Caricasole A, Ferraro T, Iacovelli L, et al. (2003). "Functional characterization of WNT7A signaling in PC12 cells: interaction with A FZD5 x LRP6 receptor complex and modulation by Dickkopf proteins". J. Biol. Chem. 278 (39): 37024–31. doi:10.1074/jbc.M300191200. PMID12857724.
Zilberberg A, Yaniv A, Gazit A (2004). "The low density lipoprotein receptor-1, LRP1, interacts with the human frizzled-1 (HFz1) and down-regulates the canonical Wnt signaling pathway". J. Biol. Chem. 279 (17): 17535–42. doi:10.1074/jbc.M311292200. PMID14739301.
Wang X, Adhikari N, Li Q, Hall JL (2005). "LDL receptor-related protein LRP6 regulates proliferation and survival through the Wnt cascade in vascular smooth muscle cells". Am. J. Physiol. Heart Circ. Physiol. 287 (6): H2376–83. doi:10.1152/ajpheart.01173.2003. PMID15271658.
Li Y, Lu W, He X, et al. (2005). "LRP6 expression promotes cancer cell proliferation and tumorigenesis by altering beta-catenin subcellular distribution". Oncogene. 23 (56): 9129–35. doi:10.1038/sj.onc.1208123. PMID15516984.
Semënov M, Tamai K, He X (2005). "SOST is a ligand for LRP5/LRP6 and a Wnt signaling inhibitor". J. Biol. Chem. 280 (29): 26770–5. doi:10.1074/jbc.M504308200. PMID15908424.
Li Y, Chen J, Lu W, et al. (2006). "Mesd binds to mature LDL-receptor-related protein-6 and antagonizes ligand binding". J. Cell Sci. 118 (Pt 22): 5305–14. doi:10.1242/jcs.02651. PMID16263759.
Mi K, Dolan PJ, Johnson GV (2006). "The low density lipoprotein receptor-related protein 6 interacts with glycogen synthase kinase 3 and attenuates activity". J. Biol. Chem. 281 (8): 4787–94. doi:10.1074/jbc.M508657200. PMID16365045.
