Uptake and receptor binding of dexamethasone in cultured 7800 C 1 hepatoma cells in relation to regulation of cell growth and peroxisomal β-oxidation

Abstract

1. 1. Uptake and binding of dexamethasone to glucocorticoid receptor has been studied in Morris hepatoma 7800 C 1 cells in relation to its effect on cell growth and peroxisomal β-oxidation. 2. 2. Intact cells showed saturable, specific dexamethasone binding of limited capacity and Scatchard analysis revealed one single class of binding sites with equilibrium dissociation constant ( K d) of 0.24 nM similar to other glucocorticoid receptors. However, the binding capacity of 24 fmol/mg cell protein is < 5% of previously reported values. 3. 3. Uptake of [ 3H]dexamethasone by intact cells was temperature dependent giving a linear Arrhenius plot with a calculated energy of activation of 58.5 kJ mol −1 × degree −1. 4. 4. Cytosol fractions had specific binding proteins for glucocorticoid hormones with sedimentation coefficient of ca 7S. No specific binding sites for [ 3H]dexamethasone was demonstrated in purified membrane fractions. 5. 5. Dexamethasone and the synthetic fatty acid analogue tetradecylthio acetic acid (TTA) both inhibited the growth of the 7800 C 1 cells and induced the peroxisomal acyl-CoA oxidase activity. A combination of the two compounds gave additive effects. Both these effects of dexamethasone and TTA were counteracted by insulin. 6. 6. We conclude that dexamethasone induces growth inhibition and enzyme induction by binding to functional intracellular glucocorticoid receptors. The action of dexamethasone is consistent with a dissolution in the membrane from where it diffuses passively into the cell and binds to specific receptors in an energy dependent step. 7. 6. The synergistic action of dexamethasone and TTA and the counteraction exerted by insulin are not due to changes in the dexamethasone receptor affinity or binding capacity.