title: Vitamin D-binding protein directs monocyte responses to 25-hydroxy- and 1,25-dihydroxyvitamin D.
abstract: BACKGROUND
  Serum 25-hydroxyvitamin D (25OHD) is a key factor in determining monocyte induction of the antimicrobial protein cathelicidin, which requires intracrine conversion of 25OHD to 1,25-dihydroxyvitamin D [1,25(OH)(2)D]. Both vitamin D metabolites circulate bound to vitamin D-binding protein (DBP), but the effect of this on induction of monocyte cathelicidin remains unclear.
  
  
  METHODS
  Human monocytes were cultured in medium containing 1) serum from DBP knockout (DBP(-/-)) or DBP(+/-) mice, 2) serum-free defined supplement reconstituted with DBP or albumin (control), and 3) human serum with different DBP [group-specific component [Gc]] genotypes with varying affinities for vitamin D metabolites. In each case, response to added 1,25(OH)(2)D(3) or 25OHD(3) was determined by measuring expression of mRNA for cathelicidin and 24-hydroxylase. Monocyte internalization of DBP was assessed by fluorescent tagging followed by microscopic and flow cytometric analysis of tagged DBP.
  
  
  RESULTS
  Monocytes cultured in DBP(-/-) serum showed more potent induction of cathelicidin by 25OHD(3) or 1,25(OH)(2)D(3) when compared with DBP(+/-) serum. Likewise, DBP added to serum-free medium attenuated 25OHD(3)/1,25(OH)(2)D(3) responses. Fluorescently tagged DBP showed low-level uptake by monocytes, but this did not appear to involve a megalin-mediated mechanism. Human serum containing low-affinity Gc2-1S or Gc2-2, respectively, supported 2.75-fold (P = 0.003) and 2.43-fold (P = 0.016) higher induction of cathelicidin by 25OHD relative to cells cultured with high affinity Gc1F-1F.
  
  
  CONCLUSIONS
  These data indicate that DBP plays a pivotal role in regulating the bioavailablity of 25OHD to monocytes. Vitamin D-dependent antimicrobial responses are therefore likely to be strongly influenced by DBP polymorphisms.
authors.affiliation: Orthopaedic Hospital Research Center, University of California Los Angeles, Los Angeles, CA 90095, USA.
authors.forename: Rene F
authors.initials: RF
authors.lastname: Chun
authors.forename: Anna L
authors.initials: AL
authors.lastname: Lauridsen
authors.forename: Lizabeth
authors.initials: L
authors.lastname: Suon
authors.forename: Lee A
authors.initials: LA
authors.lastname: Zella
authors.forename: J Wesley
authors.initials: JW
authors.lastname: Pike
authors.forename: Robert L
authors.initials: RL
authors.lastname: Modlin
authors.forename: Adrian R
authors.initials: AR
authors.lastname: Martineau
authors.forename: Robert J
authors.initials: RJ
authors.lastname: Wilkinson
authors.forename: John
authors.initials: J
authors.lastname: Adams
authors.forename: Martin
authors.initials: M
authors.lastname: Hewison
mesh_terms: D000704:Analysis of Variance; D000818:Animals; D023181:Antimicrobial Cationic Peptides / Q000235:genetics / Q000378:metabolism; D002478:Cells, Cultured; D005434:Flow Cytometry; D005838:Genotype; D006801:Humans; D051379:Mice; D009000:Monocytes / Q000166:cytology / Q000378:metabolism*; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D020133:Reverse Transcriptase Polymerase Chain Reaction; D014807:Vitamin D / Q000031:analogs & derivatives* / Q000378:metabolism; D014809:Vitamin D-Binding Protein / Q000378:metabolism*; D054804:Cathelicidins
chemical_list: D023181:Antimicrobial Cationic Peptides; D012333:RNA, Messenger; D014809:Vitamin D-Binding Protein; D014807:Vitamin D; C097949:1,25-dihydroxyvitamin D; C104450:25-hydroxyvitamin D; D054804:Cathelicidins