A proposed mechanism for the protective effect of dioxin against breast cancer

Although it is causative for many types of cancers, experimental and epidemiological evidence suggest that 2,3,7,8-tetrachlorodi-benzo-p-dioxin (dioxin) may in fact protect against breast cancer. The mechanism(s) for this protection remain unclear. In an attempt to further elucidate this mechanism, we performed a microarray experiment to identify genes that were modulated upon dioxin treatment. We found that dioxin downregulated the messenger RNAs for the G-protein-coupled receptor, CXCR4, as well as its unique chemokine ligand, CXCL12, in MCF-7 breast cancer cells. We demonstrated that the corresponding proteins are also downregulated by dioxin. The interaction between CXCR4 and CXCL12 plays a central role in the metastasis of breast cancer, as disruption of the CXCL12/CXCR4 axis has been shown to limit the metastasis of breast cancer cells to the lung in mice. Utilizing an in vitro chemotaxis assay, we demonstrate that dioxin specifically inhibits the migration of MCF-7 cells toward CXCL12. We also show that dioxin reduces CXCR4 under hypoxia and CXCL12 under estradiol-induced conditions in MCF-7 cells. Finally, as the CXCR4/CXCL12 axis is implicated in the progression of numerous types of cancer, we identified several other cancer cell lines in which dioxin modulates CXCR4 and CXCL12 levels. We therefore propose that one mechanism whereby dioxin may protect against breast cancer is via downregulation of CXCR4 and CXCL12, thereby inhibiting progression of the disease. Further, other nontoxic ligands for the aryl hydrocarbon receptor (selective AHR modulators) may exert their protective effects by a similar mechanism.