Induction of a receptor-mediated genomic DNA fragmentation in rat thymus following administration of glucocorticoids with different biopotencies: an approach by a cell-free system

To further understand a receptor-mediated apoptosis in rat thymus, we undertook experiments to examine the relationship between the induction of genomic DNA fragmentation and the depletion and the replenishment of cytosolic glucocorticoid receptor in rat thymus. Following administration of dexamethasone and prednisolone which had been known as synthetic glucocorticoids with different biological activities, we observed the fragmentation of thymus DNA in a dose-dependent manner. The time course and the extent of DNA fragmentation induced by these glucocorticoids were closely related to the degree of receptor depletion and the length of the depletion period. Relative biopotencies of dexamethasone and prednisolone estimated by their abilities to induce the DNA fragmentation were approximately 50:1. Administration of dexamethasone in combination with 2-deoxy-D-glucose, a potent inhibitor of the glycolytic pathway, significantly decreased the ability of dexamethasone to induce the DNA fragmentation, suggesting that a receptor-mediated DNA fragmentation by glucocorticoids is an ATP-dependent process. As an attempt to elucidate the molecular mechanism of DNA fragmentation in vivo, we reconstituted a cell-free system of thymus nuclei and cytosol fraction. Using the thymus cytosol fraction from dexamethasone-treated rats, we demonstrated for the first time an ATP-dependent fragmentation of nuclear DNA into nucleosomal units in vitro. Toward further understanding of the biochemical process of glucocorticoid-induced apoptosis in the thymocyte, the cell-free system reconstituted in the present study might provide a useful model system.