title: LGR6 is a high affinity receptor of R-spondins and potentially functions as a tumor suppressor.
abstract: BACKGROUND
  LGR6 (leucine-rich repeat containing, G protein-coupled receptor 6) is a member of the rhodopsin-like seven transmembrane domain receptor superfamily with the highest homology to LGR4 and LGR5. LGR6 was found as one of the novel genes mutated in colon cancer through total exon sequencing and its promoter region is hypermethylated in 20-50% of colon cancer cases. In the skin, LGR6 marks a population of stem cells that can give rise to all cell lineages. Recently, we and others demonstrated that LGR4 and LGR5 function as receptors of R-spondins to potentiate Wnt/β-catenin signaling. However, the binding affinity and functional response of LGR6 to R-spondins, and the activity of colon cancer mutants of LGR6 have not been determined.
  
  
  RESULTS
  We found that LGR6 also binds and responds to R-spondins 1-3 with high affinity to enhance Wnt/β-catenin signaling through increased LRP6 phosphorylation. Similar to LGR4 and LGR5, LGR6 is not coupled to heterotrimeric G proteins or to β-arrestin following R-spondin stimulation. Functional and expression analysis of three somatic mutations identified in colon cancer samples indicates that one mutant fails to bind and respond to R-spondin (loss-of-function), but the other two have no significant effect on receptor function. Overexpression of wild-type LGR6 in HeLa cells leads to increased cell migration following co-treatment with R-spondin1 and Wnt3a when compared to vector control cells or cells overexpressing the loss-of-function mutant.
  
  
  CONCLUSIONS
  LGR6 is a high affinity receptor for R-spondins 1-3 and potentially functions as a tumor suppressor despite its positive effect on Wnt/β-catenin signaling.
authors.affiliation: The Brown Foundation Institute of Molecular Medicine and Texas Therapeutics Institute, University of Texas Health Science Center at Houston, Houston, Texas, United States of America.
authors.forename: Xing
authors.initials: X
authors.lastname: Gong
authors.forename: Kendra S
authors.initials: KS
authors.lastname: Carmon
authors.forename: Qiushi
authors.initials: Q
authors.lastname: Lin
authors.forename: Anthony
authors.initials: A
authors.lastname: Thomas
authors.forename: Jing
authors.initials: J
authors.lastname: Yi
authors.forename: Qingyun
authors.initials: Q
authors.lastname: Liu
mesh_terms: D002465:Cell Movement / Q000235:genetics; D003110:Colonic Neoplasms / Q000235:genetics / Q000378:metabolism; D019204:GTP-Binding Proteins / Q000235:genetics / Q000378:metabolism; D006367:HeLa Cells; D006801:Humans; D036341:Intercellular Signaling Peptides and Proteins / Q000235:genetics / Q000378:metabolism; D009154:Mutation; D011485:Protein Binding; D043562:Receptors, G-Protein-Coupled / Q000235:genetics* / Q000378:metabolism*; D015398:Signal Transduction; D019699:Thrombospondins / Q000235:genetics* / Q000378:metabolism*; D025521:Tumor Suppressor Proteins / Q000235:genetics* / Q000378:metabolism*; D051153:Wnt Proteins / Q000235:genetics / Q000378:metabolism; D051176:beta Catenin / Q000235:genetics / Q000378:metabolism
chemical_list: C501126:DKK1 protein, human; D036341:Intercellular Signaling Peptides and Proteins; C417544:LGR6 protein, human; D043562:Receptors, G-Protein-Coupled; D019699:Thrombospondins; D025521:Tumor Suppressor Proteins; D051153:Wnt Proteins; D051176:beta Catenin; D019204:GTP-Binding Proteins