tokens: The binding of transcription factor AP - 1 and vitamin D receptor ( VDR ) to the composite AP - 1 plus vitamin - D - responsive promoter region ( AP - 1 + VDRE ) of the human osteocalcin gene was characterized in osteocalcin - producing ( MG - 63 ) and non - producing ( U2 - Os , SaOs - 2 ) human osteosarcoma cell lines .