Dimethyl Fumarate Inhibits the Nuclear Factor B Pathway in Breast Cancer Cells by Covalent Modification of p65 Protein

In breast tumors, activation of the nuclear factor B (NFB) pathway promotes survival, migration, invasion, angiogenesis, stem cell-like properties, and resistance to therapyall phenotypes of aggressive disease where therapy options remain limited. Adding an anti-inflammatory/anti-NFB agent to breast cancer treatment would be beneficial, but no such drug is approved as either a monotherapy or adjuvant therapy. To address this need, we examined whether dimethyl fumarate (DMF), an anti-inflammatory drug already in clinical use for multiple sclerosis, can inhibit the NFB pathway. We found that DMF effectively blocks NFB activity in multiple breast cancer cell lines and abrogates NFB-dependent mammosphere formation, indicating that DMF has anti-cancer stem cell properties. In addition, DMF inhibits cell proliferation and significantly impairs xenograft tumor growth. Mechanistically, DMF prevents p65 nuclear translocation and attenuates its DNA binding activity but has no effect on upstream proteins in the NFB pathway. Dimethyl succinate, the inactive analog of DMF that lacks the electrophilic double bond of fumarate, is unable to inhibit NFB activity. Also, the cell-permeable thiol N-acetyl l-cysteine, reverses DMF inhibition of the NFB pathway, supporting the notion that the electrophile, DMF, acts via covalent modification. To determine whether DMF interacts directly with p65, we synthesized and used a novel chemical probe of DMF by incorporating an alkyne functionality and found that DMF covalently modifies p65, with cysteine 38 being essential for the activity of DMF. These results establish DMF as an NFB inhibitor with anti-tumor activity that may add therapeutic value in the treatment of aggressive breast cancers.