GLUCOCORTICOIDS INDUCE A G1 G0 CELL-CYCLE ARREST OF CON8 RAT MAMMARY-TUMOR CELLS THAT IS SYNCHRONOUSLY REVERSED BY STEROID WITHDRAWAL OR ADDITION OF TRANSFORMING GROWTH-FACTOR-ALPHA/

Con8 mammary tumor cells are an epithelial cell line derived from the 7,12-dimethylbenz(alpha)anthracene-induced 13762NF rat mammary adenoca rcinoma. The synthetic glucocorticoid dexamethasone suppresses the gro wth of Con8 cells, and after 5 days of treatment with this steroid, Co n8 cells undergo less than 0.5 population doublings. This growth arres t is accompanied by a 30-fold elevation in c-jun transcript levels, no change in c-fos expression, and a moderate increase in total AP-1 tra nscriptional activity. Dexamethasone inhibited DNA synthesis within on e cell cycle, and flow cytometry of propidium iodide-stained nuclei de monstrated that dexamethasone growth-suppressed cells had a DNA conten t indicative of a specific cell cycle block in either G1 or G0. Consis tent with a G1/G0 arrest of the cell cycle, dexamethasone did not prev ent Con8 cells from entering the S phase after release from synchroniz ation at the G1/S boundary by a double thymidine block. Analysis of [H -3]thymidine incorporation and autoradiography of [H-3]thymidine-label ed nuclei revealed that after either dexamethasone withdrawal or the a ddition of transforming growth factor-alpha (TGFalpha), Con8 cells syn chronously reinitiate cell cycle progression. Northern blot analysis d emonstrated that an induction of transcripts for the G1 marker genes c -myc and cyclin D1 occurs before cells enter the S-phase. After dexame thasone withdrawal, c-myc and cyclin D1 expression transiently peak at 2 and 4 h, respectively. In contrast, c-myc expression peaked at 0.5- 1 h, whereas cyclin D1 expression was induced at 2 h and maintained at a high level after the addition of TGFalpha. Our results demonstrate that glucocorticoids induce a specific block of the cell cycle progres sion of a rat mammary tumor cell, and that after synchronous progressi on through the cell cycle, the temporal expression pattern for c-myc a nd cyclin D1 is distinct for dexamethasone release vs. the addition of TGFalpha to glucocorticoid-suppressed cells.