Vitamin D-binding protein directs monocyte responses to 25-hydroxy- and 1,25-dihydroxyvitamin D

J Clin Endocrinol Metab. 2010 Jul;95(7):3368-76. doi: 10.1210/jc.2010-0195. Epub 2010 Apr 28.

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.

Conclusion: 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.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Analysis of Variance
  • Animals
  • Antimicrobial Cationic Peptides / genetics
  • Antimicrobial Cationic Peptides / metabolism
  • Cells, Cultured
  • Flow Cytometry
  • Genotype
  • Humans
  • Mice
  • Monocytes / cytology
  • Monocytes / metabolism*
  • RNA, Messenger / genetics
  • RNA, Messenger / metabolism
  • Reverse Transcriptase Polymerase Chain Reaction
  • Vitamin D / analogs & derivatives*
  • Vitamin D / metabolism
  • Vitamin D-Binding Protein / metabolism*

Substances

  • Antimicrobial Cationic Peptides
  • RNA, Messenger
  • Vitamin D-Binding Protein
  • Vitamin D
  • ropocamptide
  • 1,25-dihydroxyvitamin D
  • 25-hydroxyvitamin D