Cell cycle-specific effects of glucocorticoids on phytohaemagglutinin-stimulated lymphocytes

Effects of steroid hormones and colchicine on the response of pig lymphocytes to phytohaemagglutinin (PHA) were assessed by measurement of [6-3H]thymidine incorporation. At steroid concentrations of 1 microM and below, only glucocorticoids and progesterone inhibited PHA-stimulated [6-3H]thymidine incorporation but at 100 microM inhibition was also produced by oestrogens, androgens and physiologically inactive steroids. Measurement of [6-3H]thymidine incorporation 18-24 hr, 6-12 hr or 0-6 hr after the delayed addition of the synthetic glucocorticoid, dexamethasone, to PHA-stimulated lymphocytes revealed a succession of alternating phases of sensitivity and insensitivity to the effects of the steroid which suggested that it was acting, perhaps indirectly, in a cell cycle stage-specific manner to arrest the progression of activated lymphocytes from G1 to S. Similar effects were observed with colchicine, but 100 microM 11-epicortisol inhibited [6-3H]thymidine incorporation in a non-cycle-specific manner. Glucocorticoid receptor levels in pig lymphocytes were increased 2-5-fold within 24 hr of PHA stimulation.