MULTIPLE LEVELS OF STEROID HORMONE-DEPENDENT CONTROL OF OSTEOCALCIN DURING OSTEOBLAST DIFFERENTIATION - GLUCOCORTICOID REGULATION OF BASAL AND VITAMIN-D STIMULATED GENE-EXPRESSION

We have examined the contribution of transcriptional mechanisms to the pleiotropic effects of glucocorticoids on basal and vitamin D stimula ted expression of the developmentally regulated bone-specific osteocal cin (OC) gene. OC expression was systematically investigated at the le vel of protein, mRNA, and newly synthesized transcripts during maturat ion of the bone cell phenotype in cultures of fetal rat calvarial-deri ved osteoblasts. Our results indicate that transcriptional control of basal and hormone-regulated OC expression predominates in immature ost eoblasts prior to matrix mineralization. However, in mature osteoblast s OC expression is controlled primarily by posttranscriptional mechani sms reflected by elevated mRNA levels with a decline in transcription. Vitamin D, alone or in combination with Dex, is a significant factor contributing to mRNA stabilization in mature osteoblasts with a minera lized extracellular matrix. Transcriptional modifications in response to Dex are reflected by quantitative differences between proliferating and mature osteoblasts in the formation of glucocorticoid receptor bi nding complexes at the proximal OC glucocorticoid response element. Vi tamin D and glucocorticoid receptor mRNA levels are significantly high er in mature osteoblasts than in early stage bone cells. However, rece ptor complexes do not appear to be rate limiting in proliferating oste oblasts when the OC gene is not transcribed. Our results indicate (1) developmental stage-specific effects of steroid hormone on transcripti onal regulation of bone expressed genes, and (2) inverse relationships between levels of transcription and cellular representation of mRNA w ith OC message stabilized in mature osteoblasts. (C) 1998 Wiley-Liss, Inc.