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

Selectivity of cell cycle regulation of glucocorticoid receptor function

The restricted expression of some genes to distinct stages of the cell cycle is often brought about through alterations in the activity and/or abundance of specific transcription factors. Many cells have been shown to be unresponsive to glucocorticoid hormone action during the G2 phase of the mammalian cell cycle, suggesting that some activities of the glucocorticoid receptor (GR), a ligand-activated transcription factor, are subjected to cell cycle control. We show here that GR insensitivity in G2 is selective, affecting receptor-mediated transactivation from a simple glucocorticoid response element, but not repression from a composite glucocorticoid response element. Since glucocorticoid-dependent down-regulation of GR protein levels is also unaffected in G2, distinct activities of the receptor that participate in this homologous down-regulation must be operating as effectively in G2-synchronized cells as in asynchronous cells. Finally, the phosphorylation state of the GR is altered in G2-synchronized cells reflecting, in part, both site-specific phosphorylation and dephosphorylation events. These results suggest that, while GR may be a target for cell cycle regulated kinases and phosphatases, the resulting changes in receptor phosphorylation have an impact only on selected GR functions.

Regulation of the synthesis of tyrosine aminotransferase: the relationship to mRNATAT

The activity of the hepatic enzyme tyrosine aminotransferase (TAT) is the sum of many diverse regulatory factors. These include the developmental stage of the animal, the hormonal and nutritional environment of the animal (or tissue culture cell), other extrinsic and intrinsic regulatory cycles and factors (including cytoplasmic substances), and chromatin structure. Although TAT is subject to a number of post-translational modifications, alterations in catalytic activity always parallel changes in enzyme amount. In a few instances this is due to a selective change in TAT degradation, but most are due to changes in the rate of aminotransferase synthesis. Recent studies have shown that TAT synthesis is generally directly correlated with the activity, and presumably amount, of the mRNA that codes for tyrosine aminotransferase.

Glucocorticoid induction of tyrosine aminotransferase in cultured cells

For over a decade, tyrosine aminotransferase induction in tissue culture cells has been a useful model system in which to study glucocorticosteroid action. In the 1960s, the establishment in culture of rat hepatomas expressing the inducible enzyme, already known to be induced in liver in vivo, provoked a wide-ranging series of experiments. The data from these experiments have provided considerable information regarding the mechanism of action of steroids. These include the fundamental facts that the steroids act directly on the induced cell in unmetablized form, that removal of steroid results in deinduction, that induction does not require DNA synthesis or massive changes in RNA synthesis, and that cytoplasmic receptor occupancy by active steroids correlates closely with the steroids' ability to affect inductions. Studies in tissue culture cells have led to the analysis of transcriptional and posttranscriptional models attempting to explain enzyme induction. The effects on enzyme induction of nonsteroid hormones and other factors have been studied through the use of tissue culture cells. Finally, cells and clones of cell variants are being used to study enzyme induction, through biochemical analysis and cell genetic approaches, including somatic cell hybridization.

Posttranscriptional regulation of glucocorticoid-regulated functions

Relying heavily on studies of TAT regulation in cultured rat hepatoma cell lines, we have attempted in this brief review to discuss possible mechanisms for posttranscriptional regulation of glucocorticoid-sensitive enzymes and to chronicle the evidence for and against posttranscriptional mechanisms for specific enzyme induction by glucocorticoids. Initially, mechanisms were considered that would reconcile results showing sensitivity of both induction and deinduction of TAT to inhibitors of RNA synthesis with studies demonstrating first that glucocorticoids regulate the rates of specific enzyme synthesis and, then, that glucocorticoids regulate levels of enzyme-specific mRNA. Such reconciliation proved unnecessary when it was demonstrated that inhibitors of RNA synthesis such as actinomycin D were not specific for RNA synthesis, but also had effects on mRNA turnover and protein metabolism. The bulk of evidence to date establishes that glucocorticoids promote the production of enzyme-specific mRNA for the proteins whose synthesis is regulated by thses steroids. Nevertheless, there is still very little direct evidence that steroids can modulate rates of specific gene transcription. The glucocorticoid stimulation of mouse mammary tumor virus RNA production in cultured cell lines is the only example to date where such a mechanism is supported by RNA-DNA hybridization studies. Posttranscriptional actions of steroids on the turnover, processing, or extranuclear transport of specific mRNA precursors remain potential steps at which glucocorticoids might function. The rapid turnover of some glucocorticoid-regulated enzymes and their mRNAs not only ensures a rapid response to steroid addition or withdrawal, but also subjects these proteins to relatively large fluctuations upon alterations in overall protein or mRNA metabolism. Thus many of the inductions and repressions of hepatic TAT and TO by mediators other than the glucocorticoids may be attributable entirely to nonspecific mechanisms.

Analysis of the induction and deinduction of tyrosine aminotransferase in enucleated HTC cells

Anucleate HTC cells have been used to determine the importance of the nucleus in the regulation of the intracellular levels of tyrosine aminotransferase (TAT) in hepatoma tissue culture (HTC) cells. In the absence of the nucleus, neither the induction of the enzyme by dexamethasone nor its deinduction upon removal of the hormone occurs. Degradation of the enzyme takes place when protein synthesis is inhibited in anucleates by cycloheximide. Therefore, the maintenance of induced levels of enzyme activity after dexamethasone withdrawal from pre-induced anucleates suggest that the nucleus is required for the inactivation of the TAT mRNA.

Regulation of tyrosine aminotrasferase activity in two liver-derived permanent cell lines

The regulation of tyrosine aminotransferase (TAT) activity has been examined in two liver-derived heteroploid cell lines. One (hepatoma tissue culture cells [HTC]) was derived from a hepatoma, the other (rat liver culture cells [RLC]) was derived from normal liver. The two cell lines show the following striking similarities in the control of this specific protein: (a) The kinetics of TAT induction by dexamethasone phosphate (DxP) are similar in randomly growing cells of both lines; (b) During mitosis and early G(1) phase of the cell cycle TAT activity cannot be induced by DxP in either cell line; (c) 2-3 h into G(1), when both lines become sensitive to inducer, basal enzyme activity declines to a new steady-state level; (d) Preinduced cells collected in mitosis show approximately twice the level of TAT activity as fully induced, randomly growing cultures and this activity is maintained in early G(1) with or without the inducer; and (e) Inhibition of RNA synthesis by 5 microg/ml of actinomycin D in preinduced, synchronized cells allows TAT activity to remain at constitutive levels throughout G(1), even in the absence of inducer. These results are presented in support of a previously described model which states that glucocorticoid hormones exert posttranscriptional control of the synthesis of specific proteins in mammalian cells.

Relationship of phenotypic expression of blood group H to changes in growth kinetics of cultured primary and transformed epithelioid cells

Population studies of continuously cultured primary amnion cells from appropriate donors and of HeLa cells have established that the H- cell behaves as a stem cell which commonly divides into a like cell and a differentiated H+ type. Exfoliated H+ cells, anucleate or with small or pyknotic nuclei and impaired membrane function, constitute the terminal cell in this process. Stem and differentiated populations have been identified and enumerated a) during the phase of exponential growth in monolayer culture, b) in the course of serial transfers of established and primary cultures, c) in chemically synchronized cells and d) in nutritionally deprived cells. Under the experimental conditions, the rate of phenotypic expression of group H was essentially the same in primary amnion cells from group O donors and in HeLa cells during the phase of exponential growth. The quantitative experiments on H+/H- cell populations suggested that the phenotypic expression of blood group H was adversely affected by altered cell growth which came about by limiting the potential of H- stem cells for differentiation. This occurred following multiple passages of primary cell cultures, or under conditions of nutritional inadequacy in HeLa cells. Studies on chemically synchronized cells indicated that following exposure to excessive thymidine, the predominant cell doubling pattern was reflected as mixed H+/H- progeny. Limitations imposed upon the ability of stem cells to differentiate are probably expressed either as slowly cycling, noncycling or nonviable cells and compensation by surviving stem cells may be provided for by changes in the cell doubling pattern. Since L-fucose is the immunodeterminant sugar for blood group H, it is proposed that a portion of the cellular DNA codes for a fucosyl transferase enzyme responsible for attaching this sugar to a membrane acceptor molecule. Disturbances of cell growth may interrupt this pathway at one or more points.