br Materials and methods br Results
Materials and methods
Discussion This study demonstrates that various progestogens combined with E2 had different effects on breast cell growth. These effects had an inverse correlation with the changes of the PRA/PRB ratio. E2\'s antiapoptotic effects on MCF-7 mitomycin may be related to the expression of ERα phosphorylation. The antiapoptotic effect of E2 alone was counteracted by the combination of progestogens, especially at low concentrations of E2. This confirms the present consensus of HT in that low doses of E2 may be more suitable with respect to lowering breast cancer risk. In addition, our findings suggest that synthetic progestins combined with E2 induced more prominent changes in the expression of ERs and PRs than the progesterone plus E2. Its significance on the effects of breast cells during HT should be considered. In breast cancer cell lines, cell death induced by antiestrogen treatment  and inhibition by E2 has been reported. Like the present study, pure E2 significantly decreased the expressions of caspase-3. An inhibitory effect of E2 on caspase-3 level was also noted in the report of Somaï et al . In the current study, pure E2 induced antiapoptosis of MCF-7 cells, in which ERα expression was increased, whereas the expressions of ERβ, PRA, and PRB were not altered. This is compatible with several reports in that estrogen may induce anti-apoptotic effects in ERα-positive breast cancers ,  and that the effects may be mediated by ERα. This study has also shown that 1 nM E2 plus MPA and 10 nM E2 plus NET significantly increased caspase-3 expression and entirely abolished E2-induced augmentation of ERα expression. It is compatible with other reports that progestin can inhibit the expression of ER gene transcription , . In addition, our results also reveal that PRB expression significantly increased, whereas PRA expression did not change, in the culture with 1 nM E2 plus MPA and 10 nM E2 plus NET. This demonstrates that MPA and NET inhibiting E2-induced transcription may be triggered by PRB. Similar findings were also reported by Chalbos and Galtier , in which synthetic progestin R5020 inhibited estradiol-induced pS2 levels in MCF-7 cells whereas PRB, but not PRA, inhibited the gene transcription induced by ER. Although in the present study, significant increase of ERα expression was still noted in E2 plus CPA or P4, the decreased caspase-3 expression by E2 was entirely limited. In contrast to the increased PRB expression in E2 combined with MPA and NET, E2 plus CPA or P4 induced no change of PRB nor decreased PRA and PRB. The present study demonstrates that decreased ERα, mediated by increased PRB expression, may induce apoptosis of breast cancer cells. In the present study, the A/P ratio in the MCF-7 cell line was significantly increased by the combination of 1 nM E2 and progestogens, except P4, as well as by10 nM E2 combined with NET. The similar findings were also noted by several reports , , , ,  in which progestogens combined with E2 significantly increased the cell death to proliferation ratio, in comparison with using E2 alone. In addition, the PRA/PRB ratio was significantly reduced through combined use of 1 nM E2 with progestogens, except P4, and by the combination of 10 nM E2 and NET as well. The PRA/PRB ratio was inversely correlated with the changes of the A/P ratio. This reveals that the response of MCF-7 cells to the effects of combined E2 and progestin may be modulated via alterations in the ratio of PR-A and B expression. Hopp et al  also found that the regulated expression of PRA and B is critical to the mammary gland\'s response to progesterone. In addition, the effects of P4 plus E2 on ERs and PRs were similar to using E2 alone, but were different from using synthetic progesterones plus E2. Because the combination of E2 and P4 maintained the normal proliferation rate like the control group did, whether progesterone induces other transcriptional regulators to mediate the antiapoptosis of E2 or not should be considered.