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Tyrosine Aminotransferase Gene Expression in a Temperature-sensitive Adult Rat Liver Cell Line

National Institute of Child Health and Human Development, NIH, Bethesda, Maryland 20892 [J. Y. C.], and Department of Physiology, University of Western Australia, Nedlands, Western Australia 6009, Australia [G. C. T. Y.]

Regulation of tyrosine aminotransferase gene expression was studied in an adult rat hepatocyte line (RALA255-10G) which is temperature sensitive for the maintenance of the differentiated liver phenotype. Glucocorticoid hormones such as cortisol were necessary for expression of the aminotransferase gene. In the absence of these steroids, enzyme synthesis, activity, and mRNA accumulation were virtually abolished. In the presence of cortisol at 33°C, RALA255-10G cells showed characteristics of malignant transformation and contained little tyrosine amino-transferase activity, synthesized low levels of this enzyme, and produced low levels of enzyme mRNA. At 40°C, cells maintained in the presence of cortisol regained the normal, differentiated phenotype, and tyrosine aminotransferase synthesis and mRNA accumulation were greatly increased. This increase in aminotransferase synthesis paralleled the increase in the enzyme mRNA. However, after a temperature shift-up, tyrosine aminotransferase activity was increased only for the first 2 days, probably due to thermal inactivation of this enzyme at 40°C. Dibutyryl cAMP alone was not sufficient to induce expression of the tyrosine aminotransferase gene, but it enhanced the induction caused by cortisol. Immunocytochemical studies revealed that the enhanced expression of the tyrosine aminotransferase gene at 40°C and in the presence of cortisol or cortisol plus dibutyryl cAMP resulted from an increase in both the number of cells producing this enzyme and the quantity of tyrosine aminotransferase synthesized per cell.

¹ To whom requests for reprints should be addressed, at Building 10, Room 8C429, NIH, Bethesda, MD 20892.

² Supported by the National Health and Medical Research Council of Australia and The Raine Centre for the Study of Perinatal and Developmental Biology.

Received 3/11/87. Revised 6/29/89. Accepted 7/13/87.

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