Background and purpose: Radiotherapy and endocrine therapy are both important parts of adjuvant therapy for breast cancer

yet few studies have been conducted focusing on the interaction between radiation and endocrine therapy. Up to now

no conclusion has been drawn on the timing sequence of adjuvant radiation and endocrine therapy

which is indeed crucial in clinical practice. This study intended primarily to investigate the effect of concurrent or sequential exemestane combined with radiation on radiosensitivity of MCF-7 cells and its possible mechanism

and further to provide rationale for optimal clinical treatment modality. Methods: MCF-7 cells were arranged into three trial groups: the radiation group

exemestane sequenced with radiation group and exemestane followed radiation group. Radiosensitivity was evaluated by clonogenic assay

cell proliferation was measured by MTT assay

the ability to induce cell apoptosis was evaluated by DAPI staining assay

the changes of Bcl-2 and Bax were detected by Western blot. Results: Sensitive enhancement ratios (SER) were 1.51 and 1.37 in the exemestane sequenced with radiation group and exemestane followed radiation group

respectively. Compared with the radiation group

the percentage of cellular proliferation inhibition and apoptosis increased obviously in the exemestane sequenced with radiation group and exemestane followed radiation group. Exemestane combined with radiation made the Bax protein increase obviously and the Bcl-2 protein lowered significantly. Conclusion: Exemestane can enhance the radiosensitivity of MCF-7 cells

whose mechanism might be relevant to the promotion of cellular apoptosis. However

 the treatment sequence does not affect the outcome.