Abolishment of c-fos inducibility in ras-transformed mouse osteoblast cell lines.

Proto-oncogene c-fos is induced by many types of cellular stimuli, such as 12-O-tetradecanoylphorbol-13-acetate (TPA), epidermal growth factor (EGF), serum (fetal bovine), calcium ionophore A23187, and dibutyryl cAMP (But2cAMP). In this study, c-fos induction was abolished in ras-transformed mouse osteoblast cells (MC3T3). Transformants of MC3T3 were isolated after transfection with Ki or Ha murine sarcoma virus DNA. All Ki- or Ha-ras transformed MC3T3 clones examined showed exceedingly low levels of c-fos induction by all inducers, as determined by the change in amounts of c-fos mRNA or its product. Induction of other TPA-responsive genes, such as metallothionein, was not altered in some ras-transformed cells; c-myc and c-jun expression was constitutively high in all the ras-transformed clones. Nuclear extracts and gel shift assay showed that the binding activity to c-fos enhancer element (serum response element) was altered in ras-transformed cells. These results indicate that transformation with ras oncogene induces modification of c-fos enhancer binding factors and that this modification is one cause for the decrease in c-fos induction.