Trolox inhibits osteolytic bone metastasis of breast cancer through both PGE2-dependent and independent mechanisms.

Bone is a preferred site of metastasis from breast cancer, and increased osteoclast activity is implicated in breast cancer outgrowth in the bone microenvironment. Our previous observation of an anti-osteoclastic activity of Trolox, a vitamin E analog, led us to investigate whether Trolox inhibits bone metastasis and osteolysis caused by breast cancer. Administration of Trolox markedly inhibited osteolytic bone metastasis in an experimental metastasis model by intracardiac injection of 4T1 breast cancer cells. Trolox inhibited proliferation of 4T1 cells in the bone marrow but not in the mammary fat pad. In addition, Trolox could reduce tumor burden, osteolysis, and prostaglandin E2 (PGE2) production induced by direct inoculation of 4T1 cells into the marrow cavity of the tibia. Trolox decreased the migratory and invasive activities of 4T1 cells via PGE2-dependent and independent mechanisms. It also inhibited the ability of 4T1 cells to stimulate the expression of receptor activator of nuclear factor-κB ligand (RANKL), a key cytokine for osteoclast differentiation factor, in osteoblasts. In addition, Trolox suppressed RANKL expression in osteoblasts induced by soluble factors from 4T1 cells. Furthermore, Trolox suppressed 4T1 cell-induced osteoclast differentiation in the co-culture of bone marrow cells and osteoblasts via both PGE2-dependent and independent mechanisms. Taken together, these results suggest that Trolox inhibits breast cancer cell-induced osteoclast differentiation and the invasive behavior of cancer cells through PGE2-dependent and independent mechanisms, thereby suppressing osteolytic bone metastasis of breast cancer.