AP-1/Stat3/PR/ErbB-2 transcriptional complex drives progestin-induced breast cancer growth. (A) and (B) Cells transfected as indicated were treated for 48 (C4HD) or 24 (T47D) h with MPA. Incorporation of [3H]thymidine was measured. Data are presented as the mean ± SD, P <0.001 for b vs. a and c vs. b. Experiments shown are representative of three. (C) Cyclin D1 protein expression in C4HD cells was analyzed by WB. (D) AP-1 activity and ErbB-2 nuclear function cooperate to drive in vivo progestin-induced growth. Left, cells (106) from each group were inoculated s.c. in mice treated with MPA and tumor volume was calculated as described in Methods. Each point represents tumor mean volume ± SEM. Right, decrease in tumor mass. (E) Tumor growth. aGrowth rates were calculated as the slopes of growth curves. Volume, percentage of growth inhibition and growth delay in tumors from the experimental groups with respect to tumors from control C4HD-p-Flag cells were calculated at Day 27. # vs. * and c vs. b for tumor volume and growth rate, P <0.001. d With respect to C4HD-p-Flag cells and f vs. e, P <0.001. g With respect to C4HD-p-Flag cells, P <0.001. (F) ChIP analysis. DNA-protein complexes were pulled down with the c-Jun antibody or with IgG and DNA was amplified by qPCR using primers indicated in Figure 5. Results are expressed as in Figure 5A and represent the average of three replicates ± SEM. For b vs. a, P <0.001. Shown is a representative sample of each tumor type. (G) Tumor lysates were analyzed by WB. C4HD cells growing in absence of MPA are shown as control: Shown are two representative samples of mice injected with the different experimental groups. See also Additional file 1: Figure 3. ChIP, chromatin immunoprecipitation; MPA, Medroxyprogesterone acetate; WB, Western blot.