Osteocalcin: characterization and regulated expression of the rat gene

An osteocalcin gene was isolated from a rat genomic DNA library, and sequence analysis indicated that the mRNA is represented in 953 nucleotide segment of DNA consisting of 4 exons and 3 introns. Although the introns in the rat gene are larger, its overall organization is similar to the human gene. Analysis of the 5' flanking sequences of the rat gene shows a modular organization of the promotor as reflected by the the presence of at least 3 classes of regulatory elements. These include (1) typical sequences associated with most genes transcribed by RNA polymerase II (e.g. TATA, CAAT, AP1, AP2), (2) a series of consensus sequences for cyclic nucleotide responsive elements and several hormone receptor binding-sites (estrogen, thyroid and clusters of AG-rich putative Vitamin D responsive elements); and (3) a 24 nucleotide highly conserved sequence between the rat and human gene having a CAAT motif as a central element, designated as an "osteocalcin box." Two regulatory factors of osteocalcin gene expression have been identified. First, contained within the 600 nucleotides immediately upstream from the transcription initiation site are sequences which support Vitamin D dependent transcription of the rat osteocalcin gene. 1,25(OH)2D3 increases osteocalcin mRNA by 6-20 fold increases. In contrast, up to a 200 fold increase in osteocalcin gene expression occurs with mineralization of the extracellular matrix produced by osteoblasts. We propose osteocalcin is a bone-specific marker protein of the mature osteoblast in a mineralizing matrix.