ACTIVITY OF THE OSTEOCALCIN PROMOTER IN SKELETAL SITES OF TRANSGENIC MICE AND DURING OSTEOBLAST DIFFERENTIATION IN BONE-MARROW-DERIVED STROMAL CELL-CULTURES - EFFECTS OF AGE AND SEX

The bone-specific osteocalcin gene is a well established marker of ost eoblast activity. We have studied osteocalcin transcription in transge nic mice carrying rat osteocalcin promoter-chloramphenicol acetyltrans ferase (CAT) reporter constructs. Transgenic lines carrying each of th e 1.7-, 1.1-, 0.72-, or 0.35-kilobase promoter constructs expressed th e reporter gene in a tissue-specific manner. However, each of these co nstructs was sensitive to site of integration effects, reflected by a high frequency of nonexpressing transgenic lines. High expression of t he 1.7-kilobase promoter in osseous tissues was accompanied by low ect opic expression in the brain. Analysis of CAT expression in femurs, ca lvariae, and lumbar vertebrae of this line indicated considerable vari ability in promoter activity among individual transgenic animals. Anal ysis of the variance in CAT activity demonstrated a linkage between pr omoter activities in these distant skeletal sites. Promoter activity w as inversely correlated with age, and females exhibited severalfold hi gher activity than age-matched males. Bone marrow stromal cells from t hese animals, cultured under conditions that support osteoblast differ entiation, exhibited the expected postproliferative onset of osteocalc in promoter activity, as assessed by CAT assay. The ex vivo CAT activi ty was not dependent on the sex or the age of the donor transgenic mou se. Taken together, our results are consistent with the hypothesis tha t a common, probably humoral, factor(s) regulates osteocalcin transcri ption in distant skeletal sites. We suggest that the abundance of this factor(s) is different between males and females and among individual mice at a given time point, and that ex vivo culturing of osteoblasts reduces the variation in osteocalcin promoter activity by eliminating the physiological contribution of this factor.