TRANSCRIPTIONAL CONTROL OF THE OSTEOCALCIN GENE BY 1,25-DIHYDROXYVITAMIN D-2 AND ITS 24-EPIMER IN RAT OSTEOSARCOMA CELLS

The effects of two vitamin D analogs, 1,25-dihydroxyvitamin D-2 and 24 -epi-1,25-dihydroxyvitamin D-2, were examined on osteocalcin gene expr ession in the rat osteosarcoma cell line ROS 17/2.8. Our results indic ate that these analogs are more transcriptionally active than 1,25-dih ydroxyvitamin D-3, particularly the 24-epimer. Assessment of reporter gene chloramphenicol acetyltransferase (CAT) activity, using the vitam in D responsive element (VDRE) derived from the human osteocalcin gene promoter, revealed that both analogs stimulated CAT activity 5- to 10 -fold. 1,25-Dihydroxyvitamin D-2 was slightly more active than 1,25-di hydroxyvitamin D-3, while the 24-epimer was twice as effective. 1,25-D ihydroxyvitamin D-3 also stimulated osteocalcin mRNA accumulation by 2 -fold over vehicle-treated cells, 1,25-dihydroxyvitamin D-2 by 2.5-fol d, and 24-epi-1,25-dihydroxyvitamin D-2 by 4-fold. Electrophoretic mob ility shift assays using the osteocalcin vitamin D responsive element revealed no increase in DNA binding with either analog when compared t o 1,25-(OH)(2)D-3. Examination of CAT activity using the rat 24-hydrox ylase VDRE indicated no significant difference in transcription with t hese compounds, suggesting that the vitamin D-2 analogs preferentially activate osteocalcin gene expression.