Glucocorticoids Inhibit Apoptosis during Fibrosarcoma Development by Transcriptionally Activating Bcl-xL

Glucocorticoids influence many physiological processes, and in particular apoptosis, often with opposite effects depending on the cell type examined. We found that during fibrosarcoma development there is a strong increase in apoptosis at the tumor stage, which is repressed by dexamethasone to levels observed in normal fibroblasts. The anti-apoptotic Bcl-2 family protein Bcl-x(L) is induced by dexamethasone at the transcriptional level at all stages of fibrosarcoma development. The ligand-activated glucocorticoid receptor (GR) activates the Bcl-x promoter in transient transfection experiments, and GR binds to specific Bcl-x promoter sequences in vitro and in vivo. Furthermore, a GR antagonist abolishes this effect, indicating that Bcl-x(L) induction is mediated by GR. Importantly, exogenous Bcl-x(L) inhibits apoptosis and caspase-3 activity in fibrosarcoma cells to levels found in dexamethasone-treated fibrosarcoma cells. We conclude that Bcl-x(L) is a key target mediating the anti-apoptotic effects of glucocorticoids during fibrosarcoma development. These observations provide further understanding of the molecular basis of glucocorticoid regulation of cell death during tumorigenesis.