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 expression in the rat osteosarcoma cell line ROS 17/28. Our results indicate that these analogs are more transcriptionally active than 1,25-dihydroxyvitamin D-3, particularly the 24-epimer. Assessment of reporter gene chloramphenicol acetyltransferase (CAT) activity, using the vitamin 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-dihydroxyvitamin D-3, while the 24-epimer was twice as effective. 1,25-Dihydroxyvitamin D-3 also stimulated osteocalcin mRNA accumulation by 2-fold over vehicle-treated cells, 1,25-dihydroxyvitamin D-2 by 2.5-fold, and 24-epi-1,25-dihydroxyvitamin D-2 by 4-fold. Electrophoretic mobility shift assays using the osteocalcin vitamin D responsive element revealed no increase in DNA binding with either analog when compared to 1,25-(OH)2D3. Examination of CAT activity using the rat 24-hydroxylase VDRE indicated no significant difference in transcription with these compounds, suggesting that the vitamin D-2 analogs preferentially activate osteocalcin gene expression.