Abstract: Background and purpose: Breast cancer is one of the most common carcinoma among female patients with high mortalities. Drug-resistance is the major reason that leads to chemotherapy failure in clinical practice. MCF-7/ADR is a multi-drug resistant cell line that was established on the basis of breast cancer cell line MCF-7. This research aimed to investigate the anti-apoptotic effect of c-fos in resistant breast cancer cell MCF-7/ADR, and to compare with its sensitive counterpart MCF-7. Methods: Doxorubicin with various concentrations was used to treat MCF-7 as well as its MDR- counterpart MCF-7/ADR. The growth inhibitory rate of MCF-7 and MCF-7/ADR was determined by MTT assay. Additionally, RT-PCR was used to test the expression of P-gp and c-fos mRNA in MCF-7 and MCF-7/ADR; The expression of c-fos mRNA was detected by RT-PCR after 3 μmol doxorubicin treatment; We further established cell lines that stably interfered with c-fos, named MCF-7/ADR/si-fos-8B, MCF-7/ADR/si-fos-3D. Flow cytometry and MTT assay were used to investigate the apoptosis rate and inhibitory rate in these above cells under the treatment of 5-FU, CDDP or γ-radiation. At last, RT-PCR and Western blot analysis were used to detect the expression of bax, bcl-2, puma, p53. Results: The expression of c-fos and P-gp (MDR-1) was up-regulated in MCF-7/ADR, compared with its sensitive counterpart MCF-7. Additionally, the resistant fold of MCF-7/ADR to doxorubicin was nearly 40; The expression of c-fos was gradually up-regulated after 3 μmol doxorubicin treatment; The sensitivity to drugs (5-FU and CDDP) was increased after c-fos interference while the apoptosis rate was also increased after 5-FU, CDDP and γ-radiation treatment. RT-PCR and Western blot analysis indicated that up-regulation of bax, puma, p53 after c-fos interference while the expression of bcl-2 was down-regulated. Conclusion: c-fos may act as an anti-apoptotic protein in resistant breast cancer cell line MCF-7/ADR by regulating the expression of apoptosis related proteins, and may play a vital role in the formation of multi-drug resistance phenotype.

Key words: Resistance breast cancer, c-fos, Apoptosis