The polymorphic N terminus in human vitamin D receptor isoforms influencestranscriptional activity by modulating interaction with transcription factor IIB

The human vitamin D receptor (hVDR) is a ligand-regulated transcription fac tor that mediates the actions of the 1,25-dihydroxyvitamin D-3 hormone to e ffect bone mineral homeostasis, Employing mutational analysis, we character ized Arg-18/Arg-22, hVDR residues immediately N-terminal of the first DNA b inding zinc finger, as vital for contact with human basal transcription fac tor IIB (TFIIB). Alteration of either of these basic amino acids to alanine also compromised hVDR transcriptional activity. In contrast, an artificial hVDR truncation devoid of the first 12 residues displayed both enhanced in teraction with TFIIB and transactivation. Similarly, a natural polymorphic variant of hVDR, termed F/M4 (missing a Fokl restriction site), which lacks only the first three amino acids (including Glu-2), interacted more effici ently with TFIIB and also possessed elevated transcriptional activity compa red with the full-length (f/M1) receptor. It is concluded that the function ing of positively charged Arg-18/Arg-22 as part of an hVDR docking site for TFIIB is influenced by the composition of the adjacent polymorphic N termi nus. Increased transactivation by the F/M4 neomorphic hVDR is hypothesized to result from its demonstrated enhanced association with TFIIB. This propo sal is supported by the observed conversion of f/M1 hVDR activity to that o f F/M4 hVDR, either by overexpression of TFIIB or neutralization of the aci dic Glu-2 by replacement with alanine in f/M1 hVDR. Because the f VDR genot ype has been associated with lower bone mineral density in diverse populati ons, one factor contributing to a genetic predisposition to osteoporosis ma y be the Flf polymorphism that dictates VDR isoforms with differential TFII B interaction.