Identification of SDPR as a metastasis suppressor in breast cancer
Metastatic dissemination of breast cancer cells represents a significant clinical obstacle to curative therapy. While some progress has been made in the understanding of metastasis, the detailed molecular mechanisms that define the various stages of the process remain elusive. A major rate limiting step in metastasis is the loss of function of metastasis suppressor genes which block a cascade of crucial steps including the loss of adhesion of primary tumor cells, intravasation into the blood and lymphatics with subsequent extravasation at distant sites, and the formation of new colonies. Our examination of gene expression profiles from a breast cancer model system consisting of cell lines with the same genetic lineage representing the benign, carcinoma in situ and the metastatic stages led to the identification of a candidate metastasis suppressor gene, serum deprivation response (SDPR). We observed that stable SDPR over-expression in highly metastatic breast cancer model cell lines significantly suppressed metastatic nodule formation in NOD/SCID mice. Furthermore, meta-analysis of pre-existing gene expression data suggests that the loss of SDPR expression significantly correlated with relapse of breast cancer in patients who underwent therapy. We found that the mechanism of SDPR function involves activation of the p53 pathway and inhibition of ERK and NF-κB signaling pathways. SDPR increased the apoptotic population, hindered growth in 3D cell culture and impaired migration. Moreover, SDPR was suppressed by promoter DNA methylation in metastatic cell line models and its expression was restored by 5-aza-2'-deoxycytidine treatment. Together, our results reveal that SDPR is a novel metastasis suppressor gene with potential value as a target for future therapeutic applications.