Coordinate regulation of adenylate cyclase, protein kinase, and specific enzyme synthesis by cholera toxin in hormonally unresponsive hepatoma cells

Inhibition of growth and induction of enzyme activities in a clonal human hepatoma cell line (Li-7A): comparison of the effects of epidermal growth factor and an anti-epidermal growth factor receptor antibody

A cloned human hepatoma cell line (Li-7A), possessing epidermal growth factor (EGF) receptors numbering in the range of 10-20 pmol/10(6) cells, was inhibited in its growth by EGF as well as an antagonist monoclonal antibody (MoAb) to the EGF receptor. The mode of action of the two ligands of EGF receptors appeared to be different as indicated by the following results: 1) EGF induced marked alteration in cell morphology, whereas the antibody did not; 2) cellular protein accumulated in the EGF-treated cells but not in the antibody treated cells; and 3) ectoATPase activities were greatly enhanced in Li-7A cells treated with EGF and cholera toxin but were unaffected in cells treated with antibody and cholera toxin. The last result also suggests that expression of ectoATPase activities is under the regulation of both EGF and cholera toxin. Li-7A cells provide an additional valuable experimental system for the study of EGF action, as well as the interactive effects of EGF and cholera toxin. The enrichment of the ATPase activities in the EGF-cholera toxin-treated cells can be exploited for the detailed study and isolation of these enzymes and elucidation of their physiological functions.

Hormonal regulation of carbamoyl-phosphate synthetase I synthesis in primary cultured hepatocytes and Reuber hepatoma H-35. Defective regulation in hepatoma cells

Regulation of carbamoyl-phosphate synthetase I (CPS) synthesis by various hormones was compared in primary cultured hepatocytes from adult rat and in Reuber hepatoma H-35 by pulse labeling of the cells with [35S]methionine. CPS synthesis in hepatocytes was stimulated 8-fold and 5-fold by dexamethasone and glucagon respectively. CPS synthesis in hepatocytes was synergically (about 50-fold) stimulated by a combination of dexamethasone and glucagon. Less synergic stimulation was observed by combining dexamethasone with N6, O2'-dibutyryladenosine 3',5'-monophosphate (dibutyryl-cAMP) or with isoproterenol. The basal level of CPS synthesis in hepatoma cells was higher than that in hepatocytes. CPS synthesis in hepatoma cells was stimulated by dexamethasone and dibutyryl-cAMP but the extent was only 3-fold and 1.8-fold respectively. The synergic effect of combination of dexamethasone and dibutyryl-cAMP was not observed in hepatoma cells. Neither glucagon nor isoproterenol exhibited an appreciable effect on CPS synthesis in hepatoma cells. Insulin and epinephrine suppressed CPS synthesis both in hepatocytes and hepatoma cells. The effect of epinephrine was indicated to be through alpha-adrenergic receptors. The effects of insulin and epinephrine were additive on CPS synthesis both in hepatocytes and hepatoma cells.

Epidermal growth factor inhibits growth while increasing the expression of an ecto-Ca2+-ATPase of a human hepatoma cell line

We have obtained a cloned cell line (Li-7A) from primary cultures of a human hepatoma xenograft (Li-7). Li-7A was able to grow in the absence of serum. Growth was stimulated 0-3 fold by addition of newborn calf serum, but was inhibited in DME/F12 media containing nine growth factors. The ectoMg2+-ATPase was 1.5-2 fold higher than the ectoCa2+-ATPase activity in cells grown in media with or without serum. In cells grown in media supplemented with the nine factors, the ectoCa2+-ATPase activity exceeded the ectoMg2+-ATPase, and there was also a 5-10 fold increase in its specific activity. Inhibition of growth was due to epidermal growth factor alone. The increased expression of the ectoCa2+-ATPase was absolutely dependent on EGF, but also required hydrocortisone and cholera toxin. The characteristics of Li-7A cells make it a suitable system for studying both the mechanism of action of EGF and plasma membrane ATPases.

Hormonal alteration of methotrexate and folate polyglutamate formation in cultured hepatoma cells

Glutamylation of the antifolate methotrexate in H35 hepatoma cells was stimulated by physiologic concentrations of insulin and dexamethasone. At saturating concentrations of the hormone a 2.7-fold stimulation could be obtained with insulin (65 nM, 16-h exposure) and a 1.8-fold stimulation with dexamethasone (100 nM, 16-h exposure). The increases in glutamylation caused by the hormones were not additive, and both were inhibited by actinomycin D and cycloheximide. N6,O2'-dibutyryl cAMP and theophylline caused a modest reduction of glutamylation in control and dexamethasone-treated cultures, but repressed the stimulation caused by insulin by approximately one-third. Enhancement of synthesis by dexamethasone and insulin was associated with increases in the tri-, tetra-, and pentaglutamate derivatives of methotrexate, with little change in intracellular methotrexate and methotrexate diglutamate. When the conversion of folinic acid into the folylpolyglutamate pool was examined in folate-depleted H35 cells, insulin and dexamethasone had similar effects. The results suggest that these hormones play a role in the glutamylation of the folate coenzymes in a liver-derived transformed cell line in culture and that these effects are also reflected in the interaction of the cells with antifolates such as methotrexate.

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.

Differences in rates of tyrosine aminotransferase deinduction with cyclic AMP and glucocorticoids

As an indirect test of the possibility that cyclic AMP produces its stimulatory effects on the synthesis of tyrosine aminotransferase (L-tyrosine:2-oxoglutarate aminotransferase, EC 2.6.1.5) by stabilizing its mRNA, the kinetics of decline in the rate of synthesis of this enzyme was measured in rat hepatoma cells following inducer removal or addition of RNA synthesis inhibitors. In all cases in which cells were previously exposed to cyclic AMP, the decline in enzyme synthesis after removal of inducer or addition of inhibitors of RNA synthesis was 4 to 5 times more rapid than in cells exposed to a glucocorticoid (dexamethasone) which also induces the aminotransferase. Thus, it seems unlikely that cyclic AMP had been acting by stabilizing the mRNA that directs the synthesis of the aminotransferase. Possible explanations for these paradoxical results are suggested. A more rapid decline in enzyme synthesis was also seen in cells induced with both cyclic AMP and glucocorticoid, suggesting direct interaction between the two inducers.