Preferential inhibition by 5-bromodeoxyuridine of the synthesis of tyrosine aminotransferase in hepatoma cell cultures

Abstract

When an established line of rat hepatoma (HTC) cells is grown in culture medium containing 5-bromodeoxyuridine, there is a time-dependent 80% decrease in the activity level of tyrosine aminotransferase, a liver-specific enzyme inducible by glucocorticoids. This decrease is independent of the presence of steroid inducers, and can be reversed by replacing the 5-bromodeoxyuridine in the medium with thymidine. The presence of 5-bromodeoxyuridine has no effect on the cell growth rate for two to three generations and only slightly reduces the rates of synthesis of total cellular proteins or RNA. Other enzymes examined are not affected by growth in the analog for up to two generations. The effect on tyrosine aminotransferase appears to require incorporation of the 5-bromodeoxyuridine into cellular DNA, as indicated by dose-response studies and the results of exposure to the analog at different times during the cell cycle. The decrease in tyrosine aminotransferase activity is not caused by direct inhibition, by production of a catalytically altered enzyme, or by a change in the rate of enzyme degradation. Rather, the analog seems to act by lowering the rate of synthesis of tyrosine aminotransferase, as measured by the radio-immunochemical technique.