Glucocorticoids regulate plasma membrane potential during rat thymocyte apoptosis in vivo and in vitro

Glucocorticoids induce a series of profound biochemical changes in thymocyt es that initiate apoptosis; however, the pathways beyond receptor transacti vation that lead to this form of cell death are not fully understood. In th is study, we report a novel site of action for glucocorticoids at the site of the plasma membrane. Specifically, we find that glucocorticoids induce t he loss of plasma membrane potential both in vivo and in vitro. The glucoco rticoid-induced loss of plasma membrane potential in cultured primary isola ted rat thymocytes was both dose and time dependent. Other steroid hormones , including progesterone, estrogen, and testosterone, fail to alter the dep olarization state of the thymocyte plasma membrane. Interestingly, other no nsteroid stimuli that also activate apoptosis in thymocytes also lead to ce llular depolarization. In contrast, HeLa cells, which contain functional gl ucocorticoid receptors but do not die in response to hormone, do not alter their plasma membrane potential in response to glucocorticoids, indicating a strong association between depolarization and apoptosis. Furthermore, the ability of glucocorticoids to depolarize the plasma membrane of thymocytes required the interaction of glucocorticoids with their cognate receptor, b ecause RU486 failed to depolarize thymocytes and antagonized the effect of glucocorticoids. Finally, experiments using inhibitors of transcription and translation indicated that the loss of plasma membrane potential in thymoc ytes following glucocorticoid treatment required de novo gene expression. T he results of these studies establish that the loss of plasma membrane pote ntial is an early important feature of glucocorticoid-induced apoptosis of thymocytes.