Tat Induction Research Articles

Study on the role of endogenous polyamines in glucagon, isoproterenol or serum-mediated induction of tyrosine aminotransferase in cultured heart cells

In confluent and serum-starved embryonic heart cell cultures, the addition of serum (10%), glucagon (GLU, 0.1 microM) or isoproterenol (ISO, 10 microM), causes the onset of ornithine decarboxylase (ODC) activity, with a maximum after 5-6 hr. This is paralleled by polyamine accumulation and by the induction of TAT, which, in the case of GLU and ISO, exhibits maximal activity at 4-3 hr respectively, followed by a net decline. Cyclic AMP (cAMP) also accumulates after exposure to GLU or ISO. However, under different conditions of ODC inhibition, serum fails to induce TAT, thus supporting a relevant role of cellular polyamines in serum action. Conversely, cAMP and TAT responses to GLU or ISO are markedly improved under prevention of polyamine accumulation, which also leads to a longer lasting TAT inducibility. The suggestion is made that polyamines are not required in the cAMP-dependent mechanism of TAT induction, but rather in the restoration of the basal activity of the enzyme.

Insulin inhibition of tyrosine aminotransferase induction by dexamethasone is species-related

1. 1. Dexamethasone induces TAT in the embryonic chick liver, but this effect is not blocked by prior administration of insulin at a dose sufficient to increase the hepatic protein/DNA ratio. 2. 2. Similarly, insulin does not block dexamethasone induction of TAT in chick embryo hepatocytes in vitro. 3. 3. In contrast, insulin reduces TAT induction by dexamethasone in fetal rat hepatocytes by 35–40%. 4. 4. No significant differences in insulin binding were noted between chick and rat hepatocytes.

Relations between steroid-cell contact, steroid-binding and induction of tyrosine aminotransferase

The induction of tyrosine aminotransferase by a variety of steroids was studied in cells from a hepatoma tissue culture (HTC). We have defined a class of steroids that induce TAT synthesis to a higher level than optimal inducers described earlier; these are called supra-inducers. When TAT induction is compared with the binding of the steroids to the cytoplasmic receptor or to their binding in the whole cell, a good correlation between binding in vivo of the hormone and its induction capacity can be established, whereas such a correlation was not systematically observed in vitro. A very short exposure of HTC cells to either dexamethasone or corticosterone is sufficient to induce TAT. When the inducer is removed from the culture medium a few minutes after its administration, the intracellular hormone concentration decreases very rapidly but TAT will be synthesized at its maximal rate. Thus the hormones behave as a starting signal for the optimal synthesis of the enzyme and their presence in the culture medium is not necessary throughout the entire induction period.

DNA methylation of liver and HTC cells during corticosteroid induction

The status of DNA methylation, as measured by m 5C 2 2 Abbreviations used are: m 5C, 5-methylcytosine; HPLC, high pressure liquid chromatography; HTC, hepatoma tissue culture; TAT, tyrosine aminotransferase content of nuclear DNA, was examined during corticosteroid mediated TAT induction in rat liver and in dividing and nondividing HTC cells. The m 5C content, determined by HPLC, was not significantly altered in HTC cells during TAT induction whether the cells were in the logarithmic or stationary growth phase. In the liver of adrenalectomized rats where the range of corticosteroid effects is greater than in HTC cells, a small but significant decline in genomic levels of m 5C was detected between 1 to 8 hr post-induction. The alterations in DNA methylation did not fluctuate during induction by more than 8% in the liver or 7.5% in HTC cells. These results demonstrate that no gross change or elevation in m 5C content is detected in two, different, hormonally responsive hepatocellular systems during gene activation.

Antiglucocorticoids: in vivo assay and evaluation of cortexolone, progesterone, and 6-beta-bromoprogesterone.

In vitro antiglucocorticoids (cortexolone and progesterone) were evaluated as in vivo antagonists of dexamethasone-induced increases in liver tyrosine amino transferase (TAT; EC 2.6.1.5), tryptophan oxygenase (TPO; EC 1.13.1.12), and glycogen deposition. Cortexolone antagonized the TPO and glycogen responses to dexamethasone in the liver of adrenalectomized rats but did not significantly influence the induced TAT activity. Progesterone, although toxic at levels approaching those used for cortexolone, was capable of antagonizing the glycogen increase. A new antagonist, 6 beta-bromoprogesterone, was found to be nontoxic and was more potent than cortexolone in blocking the TPO and glycogen responses. These results demonstrate that in vivo antiglucocorticoid activity can be evaluated and suggested significant differences between the sensitivity of TAT induction and that of glycogen or TPO.

Unlinked control of multiple glucocorticoid-induced processes in HTC cells

HTC cell variants chosen for their lack of tyrosine aminotransferase (EC 2.6.1.5) (TAT) induction by glucocorticoids were tested for interrelated effects on other glucocorticoid responses: TAT induction by dibutyryl cyclic AMP (dBcAMP) ± dexamethasone, glutamine synthetase (GS) induction, cyclic nucleotide phosphodiesterase (PDE) suppression, inhibition of α-aminoisobutyric acid (AIB) uptake, inhibition of plasminogen activator (PA), and induction of mouse mammary tumor virus (MTV). Loss of TAT induction by steroid was accompanied by loss of TAT induction by dBcAMP and of PDE suppression by steroid. In addition, subclones of MTV-infected cells were examined for the effect of the virus on glutamine synthetase (GS) and TAT induction. The virus had no effect on their induction in wild-type cells and no effect on GS induction in the variants. One MTV-infected subclone from a TAT variant, however, showed significant return of TAT induction.

Enhancement of hepatic tyrosine aminotransferase induction in the rat by 5-fluorouracil

5-Fluorouracil (5-FU) treatment with large doses in adrenal-intact rats inhibited glucocorticoid induction of tryptophan oxygenase (TPO, EC 1.13.1.12) while simultaneously enhancing induction of hepatic tyrosine aminotransferase (TAT, EC 2.6.1.5). 5-FU treatment alone stimulated hepatic TAT several-fold in intact rats, and less so in adrenalectomized rats, while enhanced induction of TAT by 5-FU with glucocorticoids occurred equally well in adrenalectomized rats. Enhanced induction of TAT in 5-FU treated rats was not obtained if the primary inducer of TAT was insulin. The normal apparent half-life in vivo of glucocorticoid-induced TAT was 1.9 hr; this was increased to 3.9 hr by 5-FU cotreatment. These data support the likelihood that 5-FU supports TAT induction by altering the turnover in vivo of glucocorticoid-induced TAT.