Gefitinib (Iressa) is a specific and effective epidermal growth factor receptor 
inhibitor. An understanding of the downstream cellular targets of gefitinib will 
allow the discovery of biomarkers for predicting outcomes and monitoring 
anti-epidermal growth factor receptor therapies and provide information for 
overcoming gefitinib resistance. In this study, we investigated the role and 
regulation of FOXM1 in response to gefitinib treatment in breast cancer. Using 
the gefitinib-sensitive breast carcinoma cell lines BT474 and SKBR3 as well as 
the resistant lines MCF-7, MDA-MB-231, and MDA-MB-453, we showed that gefitinib 
represses the expression of the transcription factor FOXM1 in sensitive, but not 
resistant, cells. FOXM1 repression by gefitinib is associated with FOXO3a 
activation and is mediated at the transcriptional level and gene promoter level. 
These results were verified by immunohistochemical staining of biopsy samples 
from primary breast cancer patients obtained from a gefitinib neoadjuvant study. 
We also showed that ectopic expression of an active FOXO3a represses FOXM1 
expression, whereas knockdown of FOXO3a expression using small interfering RNA 
can up-regulate FOXM1 and its downstream targets polo-like kinase, cyclin B1, 
and CDC25B and rescue sensitive BT474 cells from gefitinib-induced cell 
proliferative arrest. These results suggest that gefitinib represses FOXM1 
expression via FOXO3a in breast cancer. We further showed that overexpression of 
a wild-type FOXM1 or a constitutively active FOXM1, DeltaN-FOXM1, abrogates the 
cell death induced by gefitinib, indicating that FOXM1 has a functional role in 
mediating the gefitinib-induced proliferative arrest and in determining 
sensitivity to gefitinib. In summary, our study defined FOXM1 as a cellular 
target and marker of gefitinib activity in breast cancer.