The combined treatment of 4T1 cells pre-treated with 3- MA, an inhibitor of autophagy, resulted in a significant reduction in cytotoxicity. In addition, although DNA damage plays an important role in the induction of autophagy, the exact mechanisms by which DNA damage triggers autophagy are unclear. Thus, more studies are needed to clarify the relationship between DNA damage and autophagy. Recent evidence shows that one of the mechanisms whereby IR activates ER stress is the induction of DNA damage. Thus, DSBs may serve as one link between IR and ER stress activation. Our TWS119 results showed that IRE1a and the phosphorylation of eIF2a increased with the combined treatment, indicating that the combined treatment could induce ER stress in 4T1 cells. Similar to our finding, occurrence of ER stress has been reported to be associated with the induction of autophagy and upregulation of autophagic marker LC3. In mammalian cells, ER stress has been shown to facilitate the formation of autophagosomes, and induction of autophagy enables the removal of toxic misfolded proteins. Healy et al. indicated that the growth arrest and DNA damage-inducible protein could serve as a main mediator of ER stress-induced cell death. Therefore, DNA damage could induce ER stress and consequently autophagy. A better understanding of the signaling pathways controlling autophagy, DNA damage and ER stress will hopefully open new possibilities for the treatment of numerous cancers. The majority of deaths in breast cancer patients are from metastases rather than primary tumors, and studies have shown that breast cancer preferentially metastasizes to the lung, liver, and bones. With up to 20% of these patients likely to develop metastatic disease, the identification and implementation of more effective therapies is a high priority. In the present study, we demonstrated for the first time that SAHA significantly delayed lung metastases in an in vivo experimental metastasis animal model. Our results showed that SAHA decreased lung weight and tumor volume compared to the control treatment. Furthermore, a significant decrease in the number and size of tumors was observed in the lung sections from animals treated with SAHA. Mechanistically, our cellular data indicated that SAHA inhibited breast cancer cell migration through inhibiting the activity of MMP-9. MMPs have been implicated as possible mediators of invasion and metastasis in some cancers. Among the human MMPs, MMP-9 is the key enzyme that degrades type IV collagen. In addition, aberrant overexpression of MMP-9 has been found to be associated with an increased invasive potential in breast cancer cells. Investigation into which additional metastasis-related proteins are involved in SAHA-induced migration inhibition is worth further analysis. To our knowledge, this is the first report demonstrating that SAHA enhances the radiation response in a 4T1-luc orthotopic mouse model. In our animal model, SAHA plus IR showed better efficacy over individual treatments in delaying the growth of tumors. Furthermore, the combined treatment induced stronger cytotoxicity in breast cancer cells. One of the mechanisms whereby SAHA inhibited the cell��s capacity to repair IR-induced DNA damage by affecting the DNA repair pathways, could contribute to this combined NVP-BEZ235 effect. Induction of autophagy and ER stress could also be involved in the underlying mechanisms. In addition, we further found that SAHA inhibited the invasion and migration of breast cancer cells by inhibiting the activity of MMP- 9 in vitro. Accordingly, in an in vivo experimental metastasis mouse model, SAHA significantly inhibited lung metastasis at noncytotoxic concentrations.