Control of specific gene expression in higher organisms. Expression of mammalian genes may be controlled by repressors acting on the translation of messenger RNA

Repression of interferon action: induced dedifferentiation of embryonic cells

Cells cultured from young (6-day) chicken embryos differ from those of older (13-day) embryos in having a greater susceptibility to infection by certain viruses and a considerably lesser sensitivity to the action of interferon. These circumstances parallel those observed in the intact embryo. The addition of a small percentage of cells from young embryos alters the response of cells cultured from older embryos by increasing viral susceptibility sevenfold and decreasing sensitivity to interferon 25-fold. We postulate that a repressor which inhibits the expression of interferon in older embryonic cells is elaborated by cells from young embryos.

Expression of differentiated functions in hepatoma cell hybrids. I. Tyrosine aminotransferase in hepatoma-fibroblast hybrids

The inducible enzyme tyrosine aminotransferase (TAT) has been investigated in hybrids between rat hepatoma cells and mouse fibroblasts. In the latter the TAT baseline activity is low, and in the presence of steroids does not change. By contrast, the hepatoma cells have high TAT activity, and this activity increases by a factor of 4-6 in the presence of steroids. The hybrid cells, like the fibroblasts, have low TAT activity and are not inducible. Heat inactivation curves demonstrate that the hybrid cells contain both parental forms of TAT, and therefore contain the parental genes specifying the enzyme. The presence in the hybrids of detectable rat TAT and the total absence of its inducibility suggest that a second gene is involved in the regulation of TAT inducibility, and that this gene is not expressed in the hybrids.

Glucocorticoid-induced alteration of the surface membrane of cultured hepatoma cells

Glucocorticoids induce an alteration of the surface of hepatoma tissue culture (HTC) cells as expressed by changes in cell electrophoretic, antigenic, and adhesive properties. The alteration is assayed by the increased adhesiveness of induced cells for a glass surface. The induction process has a lag period of about 3 hr and attains a plateau level after 24-30 hr when 50-80% of the steroid-treated cells are firmly adhered. Less than 10% of untreated cells adhere under the same conditions. Induction is inhibited by actinomycin D and cycloheximide, demonstrates both pH and temperature dependence, and responds to changes in steroid concentration and structure. By contrast, the attachment per se of preinduced cells is not affected by inhibitors of RNA and protein synthesis, fluctuations of temperature and pH, and the presence or absence of the hormone. When the induction process is reversed by removal of steroid or addition of actinomycin D, preinduced adhesiveness is lost with a half-life of 13-24 hr, but in the presence of cycloheximide the loss is accelerated (t(1/2) 3-5.5 hr). These results suggest that glucocorticoids induce the biosynthesis of a protein which either modifies the cell surface (an enzyme) or is incorporated into surface structures (structural protein).

Amino acid transport in hepatoma cell cultures during tyrosine aminotransferase induction

Alpha-aminoisobutyric acid transport in hepatoma cells in culture was increased by insulin but not by hydrocortisone. Both of these agents induce tyrosine aminotransferase activity in this system. The apparent increase in alpha-aminoisobutyric acid transport and tyrosine aminotransferase activity produced by glucagon is probably caused by insulin contamination. Insulin did not increase transport in this system until after tyrosine aminotransferase activity had reached maximum levels. The mechanisms underlying increased alpha-aminoisobutyric acid transport appear to differ from those for tyrosine aminotransferase induction with hydrocortisone despite their close association in previous whole animal experiments.

Posttranscriptional control of tyrosine aminotransferase synthesis by insulin

Adrenal steroid hormones induce the synthesis of tyrosine amino-transferase (TAT) in HTC cells, an established line of rat hepatoma cells in tissue culture. The addition of insulin to cells previously induced in a serum-free medium results in a rapid two- to threefold further increase in the rate of synthesis of TAT and a small increase in total amino acid incorporation. These changes do not require concomitant RNA synthesis, suggesting that insulin acts at a step in protein synthesis beyond that of gene transcription. Although the effects of insulin on HTC cells are similar to those caused by dialyzed bovine serum, evidence is presented that insulin and serum affect different aspects of TAT synthesis. Unlike the glucocorticoids, insulin does not cause the accumulation of TAT mRNA, nor the sustained induction of TAT. The continued presence of the inducing steroid is required to permit maximal expression of the insulin effect.

The appearance and disappearance of the post-transcriptional repressor of tyrosine aminotransferase synthesis during the HTC cell cycle

The synthesis of the enzyme tyrosine aminotransferase in HTC cells (an established line of rat hepatoma cells) is inducible by glucocorticoid hormones only during the latter part of G1 phase and throughout S phase in the cell generation cycle. We have earlier shown that during the first few hours of G1 phase when the enzyme cannot be induced, its synthesis is constitutive, presumably using as template, preexisting messenger RNA. Our model for tyrosine aminotransferase gene regulation in eukaryotic cells entails a specific post-transcriptional repressor which is formed only during the periods in the cell cycle when tyrosine aminotransferase is inducible. This model predicts that during the noninducible period, G2, the tyrosine aminotransferase repressor would not be present and thus tyrosine aminotransferase synthesis would be constitutive. Data are presented which confirm this prediction in further support of the model.

The relationship between glucocorticoid binding and tyrosine aminotransferase induction in hepatoma tissue culture cells

Glucocorticoid receptors with many properties suggesting their involvement in the hormonal induction of tyrosine aminotransferase have been detected in rat hepatoma tissue culture cells. These "specific receptors" approach saturation as the steroid concentrations required for maximal induction are reached. Inducer analogs influence cortisol binding to the specific receptors in direct relation to their influence on cortisol induction. The inducer association with and dissociation from specific receptors are rapid enough to account for the kinetics of induction and deinduction. Chromatographic studies suggest the specific receptors bind unaltered dexamethasone and cortisol. "Nonspecific" association of steroids with the cells also occurs and can be distinguished from specific binding.