The mechanisms by which mammalian cells regulate their proliferative capacity are not known but events occurring in the G?1? phase of the cell cycle are thought to be of major importance. In order to facilitate the study of cell-cycle events important to cell growth control, we have cloned mouse DNA segments containing sequences whose expression is induced by mitogenic stimulation of cultured mouse embryo cells by epidermal growth factor (EGF). Our studies have shown that beta- and gamma-cytoskeletal actin genes are included in the domain of specific EGF-regulated sequences. The enhanced transcription of one of these genes (B-actin) is detectable within 10 minutes of EGF binding but is transitory and returns to basal levels within an hour. However, an inhibitor of protein synthesis, cycloheximide, was found to greatly increase both the magnitude and the duration of EGF-induced B-actin gene transcription. This effect is specific in that neither alpha-tubulin nor sarcomeric actin genes are stimulated by either EGF or cycloheximide. These data demonstrate a relationship between an important set of cytoskeletal protein genes and the response of cells to growth factors. Furthermore, these data suggest the involvement of a specific repressor of transcription in the EGF-dependent regulation of these genes. Recently, we have isolated two additional clones from a cDNA library which correspond to non-actin mRNAs which appear to be similarly regulated. This research now seeks to test several predictions which arise from our hypothesis, including the requirement for specific DNA regulatory sequences and the existence of specific, transacting transcripton factors. Knowledge of the molecular mechanisms underlying the regulation of specific gene expression by peptide growth factors will broaden our understanding of cellular growth control and may ultimately contribute to a better understanding of factors leading to both cell transformation and cell senescence. (G)
