HUMAN AND MURINE OSTEOCALCIN GENE-EXPRESSION - CONSERVED TISSUE RESTRICTED EXPRESSION AND DIVERGENT RESPONSES TO 1,25-DIHYDROXYVITAMIN D-3 IN-VIVO

Human and murine osteocalcin genes demonstrate similar cell-specific e xpression patterns despite significant differences in gene locus organ ization and sequence variations in cis-acting regulatory elements. To investigate whether differences in these regulatory regions result in an altered response to 1,25-dihydroxyvitamin D-3 [1,25-(OH)(2)D-3] in vivo, we compared the response of the endogenous mouse osteocalcin gen e to a bacterial reporter gene directed by flanking regions of the hum an osteocalcin gene in transgenic mice. Transgene expression colocaliz ed with endogenous osteocalcin expression in serial sections, being de tected in osteoblasts, osteocytes and hypertrophic chondrocytes. In ca lvarial cell culture lysates from transgenic and nontransgenic mice, t he endogenous mouse osteocalcin gene did not respond to 1,25-(OH)(2)D- 3 treatment. Despite this, transgene activity was significantly increa sed in the same cells. Similarly, Northern blots of total cellular RNA and in situ hybridization studies of transgenic animals demonstrated a maximal increase in transgene expression at 6 h after 1,25-(OH)(2)D- 3 injection (23.6 +/- 3.6-fold) with a return to levels equivalent to uninjected animals by 24 h (1.2 +/- 0.1-fold). This increase in transg ene expression was also observed at 6 h after 1,25-(OH)(2)D-3 treatmen t in animals on a low calcium diet (25.2 +/- 7.7-fold) as well as in t ransgenic mice fed a vitamin D-deficient diet containing strontium chl oride to block endogenous 1,25-(OH)(2)D-3 production (7.5 +/- 0.9-fold ). In contrast to the increased transgene expression levels, neither e ndogenous mouse osteocalcin mRNA revels nor serum osteocalcin levels w ere significantly altered after 1,25-(OH)(2)D-3 injection in transgeni c or nontransgenic mice, regardless of dietary manipulations, supporti ng evidence for different mechanisms regulating the response of human and mouse osteocalcin genes to 1,25-(OH)(2)D-3. Although the cis-and t rans-acting mechanisms directing cell-specific gene expression appear to be conserved in the mouse and human osteocalcin genes, responsivene ss to 1,25-(OH)(2)D-3 is not. The mouse osteocalcin genes do not respo nd to 1,25-(OH)(2)D-3 treatment, but the human osteocalcin-directed tr ansgene is markedly upregulated under the same conditions and in the s ame cells. The divergent responses of these homologous genes to 1,25-( OH)(2)D-3 are therefore likely to be due to differences in mouse and h uman osteocalcin-regulatory sequences rather than to variation in the complement of trans-acting factors present in mouse osteoblastic cells . Increased understanding of these murine-human differences in osteoca lcin regulation may shed light on the function of osteocalcin and its regulation by vitamin D in bone physiology.