Background: Adjuvant chemotherapy is used even after eradicative surgery of primary breast cancer, particularly triple-negative one, in order to reduce tumor recurrence. However, several groups reported that anti-cancer drugs can accelerate tumor recurrence by inducing metastatic niche formation under some conditions. Hence, we examined the effects of 5-fluorouracil (5-FU), which is widely used in adjuvant chemotherapy, on lung metastasis after resection of the primary tumor arising from the injection of a mouse triple-negative breast cancer cell line, 4T1, into mammary fat pad (MFP).
Method: 4T1 cell suspensions were injected into the secondary MFP. After a visible tumor appeared, a tumor was removed. Two days after the removal, the mice received once intraperitoneally 5-FU at a dose of 50 mg/kg body weight and were sacrificed 2 weeks after 5-FU injection.
Results: Compared with saline treatment, 5-FU markedly increased the numbers and sizes of lung metastasis foci, with enhanced tumor cell proliferation and angiogenesis as evidenced by increases in Ki-67-positive cell numbers and CD31-positive areas, respectively. Moreover, 5-FU-mediated augmented lung metastasis was associated with increases in intrapulmonary neutrophil numbers and expression of neutrophilic chemokines, CXCL1 and CXCL2 in tumor cells, with few effects on intrapulmonary T cell or macrophage numbers. Furthermore, 5-FU enhanced CXCL1 and CXCL2 expression in 4T1 cells in a NF-kB-dependent manner. Finally, the administration of a neutrophil-depleting antibody, anti-Gr-1 antibody, or a CXCR2 antagonist, SB225002, significantly attenuated 5-FU-induced enhanced lung metastasis. Collectively, 5-FU treatment can induce intrapulmonary neutrophil infiltration by inducing breast cancer cells to express CXCL1 and CXCL2, thereby rather promoting tumor growth in lungs.