DEVELOPMENTAL EXPRESSION AND ACTIVITIES OF SPECIFIC FOS AND JUN PROTEINS ARE FUNCTIONALLY RELATED TO OSTEOBLAST MATURATION - ROLE OF FRA-2 AND JUN-D DURING DIFFERENTIATION

Developmental studies of oncogene expression implicate the Fos and Jun family of transcription Factors in the regulation of bone growth and differentiation. Promoters of many developmentally regulated genes, in cluding osteocalcin, a marker of osteoblast differentiation, contain A P-1 sites that bind Fos/Jun dimers. Here, we demonstrate that the sele ctive expression of fos- and jun-related genes is functionally related to the stage of osteoblast growth and differentiation in vitro. Durin g osteoblast proliferation, nuclear protein levels of all seven activa ting protein-1 (AP-1) members are maximal. Subsequently, during the pe riod of extracellular matrix maturation, levels decline. In fully diff erentiated osteoblasts, Fra-2 and (to a lesser extent) Jun D are the p rincipal AP-1 members detectable by Western blot analysis. AFP-1 compl ex composition and binding activity also exhibit developmental changes . All Fos and Jun family members are involved in AP-1 complex formatio n in proliferating cells, whereas Fra-2 and Jun D predominate in AP-1 complexes in differentiated osteoblasts. Overexpression of Fos and Jun family members in ROS 17/2.8 cells markedly affects the expression of an osteocalcin promoter-chloramphenicol acetyltransferase construct. Coexpression of only one AP-1 pair, Fra-2 and Jun D, stimulated report er expression, whereas coexpression of other AP-1 pairs down-regulated expression (i.e. c-jun and any Fos family member) or had no effect (i .e. Fra-1 and Jun B). Promoter deletion analyses indicate that these e ffects are site specific. Consequential effects of Fra-2 on osteoblast differentiation are further demonstrated by antisense studies in whic h osteoblast differentiation and the development of a bone tissue-like organization were suppressed. Consistent with recent findings suggest ing that AP-1 complex composition can selectively regulate gene transc ription, our findings demonstrate that differential expression of Fos and Jun family members could play a role in the developmental regulati on of bone-specific gene expression and, as a result, may be functiona lly significant for osteoblast differentiation.