Synthesis and Induction of Tyrosine Aminotransferase in Synchronized Hepatoma Cells in Culture

The generation cycle of an established line of rat hepatoma cells (HTC cells) was studied using synchronized cell techniques. Dexamethasone phosphate (Dex), a synthetic adrenal corticosteriod which induces tyrosine aminotransferase (TAT) in HTC cells randomly distributed in the cell generation cycle, did not affect the durations of the various phases of the cycle. The activities of TAT and several dehydrogenases, and the rates of general protein and RNA synthesis, were studied throughout the cycle.Dex, at 10(-5)M, when added to a synchronized cell population in the latter two thirds of G1 phase or anywhere in the S phase, induces TAT. During the period made up of G2, M, and early G1, Dex does not induce TAT. However, radioisotope incorporation into specifically immunoprecipitated TAT continues during mitosis, when general protein and RNA synthesis are decreased.