c-Myc and cyclin D3 (CcnD3) genes are independent targets for glucocorticoid inhibition of lymphoid cell proliferation.

Glucocorticoids inhibit the expression of critical cell cycle-regulatory genes. The G1 cyclin gene CcnD3, which encodes cyclin D3, is inhibited by dexamethasone in P1798 murine T lymphoma cells. Glucocorticoids also inhibit expression of the catalytic partner of cyclin D3, Cdk4. Inhibition of these two genes results in a decrease in the ability to phosphorylate the Rb-1 tumor suppressor gene product. Stable transformation with SV40 T antigen expression vectors prevents glucocorticoid-mediated cell cycle arrest, which is consistent with the conclusion that glucocorticoids inhibit Rb-1 phosphorylation. Overexpression of cyclin D3 suffices to restore Rb-kinase activity in glucocorticoid-treated cells. Nevertheless, overexpression of cyclin D3 does not prevent glucocorticoid inhibition of cell proliferation. Cells transformed with Cdk4 expression vectors, with or without cyclin D3 expression vectors, also undergo G0 arrest in the presence of dexamethasone. Glucocorticoids inhibit c-Myc expression in lymphoid cells, and transient expression of c-Myc protein attenuates the lytic response in glucocorticoid-treated human leukemia cells (R. Thulasi, D. V. Harbour, and E. B. Thompson, J. Biol. Chem., 268: 18306-16312, 1993). However, P1798 cells stably transfected with c-Myc expression vectors are sensitive to glucocorticoid-mediated G0 arrest. Such transformants withdraw from the cell cycle when treated with dexamethasone. P1798 cells were transformed so as to express both c-Myc protein and cyclin D3 in the presence of glucocorticoids. These Myc/D3 cells continue to proliferate in the presence of dexamethasone, and virtually all of these cells are capable of entering S phase in the presence of the steroid. Rapid apoptotic cell death occurs when wild-type P1798 cells are treated with dexamethasone in serum-free medium. Myc-transformed and cyclin D3-transformed cells also die rapidly when treated with glucocorticoids in the absence of serum. T antigen transformants are resistant to glucocorticoid-mediated apoptosis in serum-free medium. Double transformants that express both cyclin D3 and c-Myc are also resistant to apoptosis in the presence of dexamethasone. We conclude that inhibition of both CcnD3 and c-Myc genes is critical to glucocorticoid-mediated G0 arrest. Furthermore, those genes that convey resistance to growth arrest also convey resistance to cell death.