Regulation of c-jun and jun-B by progestins in T-47D human breast cancer cells

Abstract

To investigate further the molecular mechanisms of progestin regulation of human breast cancer cell growth, we studied the effect of progestins on expression of the protooncogene c-jun and other members of the jun family, jun-B and jun-D, in T-47D human breast cancer cells. The progestin medroxyprogesterone acetate (MPA) increased c-jun mRNA levels in a time- and dose-dependent fashion. Maximal effects were seen after 3 h of treatment with 10-100 nM MPA. Under these conditions, the c-jun mRNA was increased 5.4-fold above the control level. Although the c-jun mRNA level was increased by cycloheximide alone, a further 2.4-fold increase was seen when the cells were treated with MPA in the presence of cycloheximide. The p39 c-jun protein was also increased 3.8-fold by this treatment. Maximum levels of p39 c-jun protein were achieved 9 h after treatment, and this level was maintained for at least 24 h. Dexamethasone and dihydrotestosterone did not increase the p39 c-jun protein level under these conditions. However, MPA treatment of T-47D cells resulted in a 55% decrease in overall AP-1 activity, as measured by transient transfection of an AP-1-regulated chloramphenicol acetyltransferase reporter gene. These effects were all reversible by cotreatment with a 10-fold higher concentration of the antiprogestin RU 486. MPA decreased jun-B mRNA levels 50% 1 h after treatment in T-47D cells. Thereafter, jun-B mRNA levels increased and became elevated (2.5- to 3.5-fold) 12, 24, and 48 h after treatment compared to the untreated control values. MPA had little, if any, effect on jun-D mRNA levels in T-47D cells. The data support the conclusion that progestins specifically alter the expression and activity of components that can form AP-1 transcription complexes in T-47D cells. These effects may play a role in progestin-induced growth modulation in human breast cancer cells.