Abstract
It is still unclear how glucocorticoids (GCs) induce apoptosis of thymocytes and T lymphoma cells. Emergence of GC-resistant lymphoma cells is a major obstacle in GC therapy, emphasizing the need for novel strategies that maintain the sensitivity of lymphoma cells to the proapoptotic effects of GC. We have undertaken a kinome study to elucidate the signal transduction pathways involved in mediating GC-induced apoptosis. Our study shows that glycogen synthase kinase (GSK3) plays a central role in promoting GC-induced apoptosis. In the absence of a ligand, GSK3alpha, but not GSK3beta, is sequestered to the glucocorticoid receptor (GR). Exposure to GCs leads to dissociation of GSK3alpha from GR and subsequent interaction of GSK3alpha and GSK3beta with the proapoptotic Bim protein, an essential mediator of GC-induced apoptosis. Chemical inhibition of GSK3 by SB216763, BIO-Acetoxime, or LiCl and GSK3 inhibition using a dominant-negative mutant of GSK3 impede this cell death process, indicating that GSK3 is involved in transmitting the apoptotic signal. GC resistance in lymphoma cells can be relieved by inhibiting the phosphatidylinositol-3 kinase-Akt survival pathway, which inactivates GSK3. Notch1, a transcription factor frequently activated in T acute lymphoblastic leukemia cells, confers GC resistance through activation of Akt. Altogether, this study illuminates the link connecting upstream GR signals to the downstream mediators of GC-induced apoptosis. Our data suggest that targeting protein kinases involved in GSK3 inactivation should improve the outcome of GC therapy.