Effect of methylxanthines on binding of the glucocorticoid receptor to DNA-cellulose and nuclei.

The binding of [3H]dexamethasone-receptor complex from rat liver cytosol to isolated nuclei or DNA-cellulose can be greatly enhanced at low temperature by the presence of theophylline. Aminophylline and caffeine can mimic this effect; however, papaverine and 1-methyl-3-isobutylxanthine, at concentrations inhibitory to phosphodiesterase, are without effect on glucocorticoid receptor binding to DNA. Furthermore, theophylline can be added when adenosine 3':5'-monophosphate-(cAMP) hydrolysis is already complete and still enhance DNA binding. These results imply that this effect of theophylline is independent of its known effect on cAMP levels. Activation by methylxanthines does not alter the sedimentation of the glucocorticoid-receptor complex in sucrose gradients but does alter the pI and in this respect brings about changes resembling those which occur upon activation by heat. Recently we have shown that pyridoxal phosphate inhibits the binding of heat-activated receptor to DNA-cellulose. Similarly, we have shown here that pyridoxal phosphate also inhibits the DNA-cellulose binding of theophylline-treated receptor. The presence of theophylline also enhances the rate of binding of [3H]dexamethasone to the receptor and increases its apparent affininty for the steroid. The data suggest that the effect of theophylline is on some cytosolic component, perhaps the receptor itself. Enhanced DNA binding as a result of exposure to theophylline at low temperature can also be demonstrated using the glucocorticoid receptor of kidney, thymus and Reuber H35 cells.