In the present study, we report the effects of honokiol, a phytochemical from Magnolia spp., on cancer cell migration capacity and the molecular mechanisms underlying these effects using breast cancer cell lines as an in vitro model. Using cell migration assays, we found that the treatment of human breast cancer cells (MCF-7) and murine mammary cancer cells (4T1) with honokiol resulted in a dose-dependent inhibition of migration of these cells, which was associated with a reduction in nitric oxide (NO) levels. The cell migration capacity was decreased in the presence of NG-nitro-L-arginine methyl ester (L-NAME), an inhibitor of NO synthase. Honokiol reduced the elevated levels of cyclic guanosine monophosphate (cGMP) in the cells, while the treatment of 4T1 cells with guanylate cyclase (GC) inhibitor 1-H-[1,2,4]oxadiaxolo[4,3-a]quinolalin-1-one (ODQ) reduced the migration of cells and the levels of cGMP. The presence of 8-bromoguanosine 3'5'-cyclic monophosphate, an analogue of cGMP, enhanced the migration of these cells, suggesting a role for GC in the migration of 4T1 cells. Honokiol also inhibited the levels of cyclooxygenase-2 (COX-2) and prostaglandin (PG) E2 in 4T1 cells. The transfection of 4T1 cells with COX-2 siRNA resulted in a reduction in cell migration. ODQ and L-NAME also decreased the levels of PGE2 in 4T1 cells suggesting a role for COX-2/PGE2 in cell migration. Moreover, honokiol inhibited the activation of nuclear factor κB (NF-κB), an upstream regulator of COX-2 and iNOS, in 4T1 cells. These results indicate that NO and COX-2 are the key targets of honokiol in the inhibition of breast cancer cell migration, an essential step in invasion and metastasis.

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