Rhee K, Bresnahan W, Hirai A, Hirai M, Thompson E A
Department of Human Biological Chemistry and Genetics, University of Texas Medical Branch, Galveston 77550-0645, USA.
Cancer Res. 1995 Sep 15;55(18):4188-95.
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.
糖皮质激素可抑制关键细胞周期调控基因的表达。G1期细胞周期蛋白基因CcnD3（编码细胞周期蛋白D3）在P1798小鼠T淋巴瘤细胞中受地塞米松抑制。糖皮质激素还可抑制细胞周期蛋白D3的催化伴侣Cdk4的表达。这两个基因的抑制导致磷酸化Rb - 1肿瘤抑制基因产物的能力下降。用SV40 T抗原表达载体进行稳定转化可防止糖皮质激素介导的细胞周期停滞，这与糖皮质激素抑制Rb - 1磷酸化的结论一致。细胞周期蛋白D3的过表达足以恢复糖皮质激素处理细胞中的Rb激酶活性。然而，细胞周期蛋白D3的过表达并不能阻止糖皮质激素对细胞增殖的抑制。用Cdk4表达载体转化的细胞，无论有无细胞周期蛋白D3表达载体，在地塞米松存在的情况下也会进入G0期停滞。糖皮质激素可抑制淋巴细胞中的c - Myc表达，c - Myc蛋白的瞬时表达可减弱糖皮质激素处理的人白血病细胞中的裂解反应（R. 图拉西、D. V. 哈伯和E. B. 汤普森，《生物化学杂志》，268: 18306 - 16312, 1993）。然而，用c - Myc表达载体稳定转染的P1798细胞对糖皮质激素介导的G0期停滞敏感。用这些转化细胞用地塞米松处理时会退出细胞周期。P1798细胞经过转化，以便在糖皮质激素存在的情况下同时表达c - Myc蛋白和细胞周期蛋白D3。这些Myc/D3细胞在地塞米松存在的情况下继续增殖，并且实际上所有这些细胞在有类固醇存在时都能够进入S期。当野生型P1798细胞在无血清培养基中用地塞米松处理时，会迅速发生凋亡性细胞死亡。Myc转化细胞和细胞周期蛋白D3转化细胞在无血清情况下用糖皮质激素处理时也会迅速死亡。T抗原转化细胞在无血清培养基中对糖皮质激素介导的凋亡具有抗性。同时表达细胞周期蛋白D3和c - Myc的双转化细胞在地塞米松存在时也对凋亡具有抗性。我们得出结论，CcnD3和c - Myc基因的抑制对于糖皮质激素介导的G0期停滞至关重要。此外，那些赋予生长停滞抗性的基因也赋予细胞死亡抗性。
