Objective      To determine whether macrophage participates in secondary chemoresistance to 5-fluorouracil (5-FU) in colorectal cancer cell line CT26 and explore the underlying mechanism.       Methods      CT26 cells were cultured in normal medium (NM)

macrophage-conditioned medium (CM) or 5-FU-treated macrophage-conditioned medium [CM(5-FU)]

followed by 10 μmol/L 5-FU treatment. CCK-8 assay was used to detect cell proliferation

PI/Annexin-V assay was used to detect cell apoptosis

and Western blotting was used to detect the expression of apoptosis related proteins. Furthermore

subcutaneous tumor-bearing mouse models were established. Growth of xenografts was observed after 5-FU treatment in different culture media.       Results      Compared with the non-treatment control group

5-FU treatment induced apoptosis in CT26 cells (P0.05) and up-regulated expression of p-JNK and cleaved caspase-3 (P0.05). P-JNK inhibitor could significantly inhibit the activation of caspase-3 signaling pathway induced by 5-FU treatment in the xenografts. P-JNK-dependent caspase-3 signaling pathway in the cells cultured with CM(5-FU) was significantly inhibited

and the resistance of CT26 cells to 5-FU was induced.       Conclusion      5-FU inhibits the growth of CT26 cells by activating p-JNK-dependent caspase-3 pathway. Macrophage treated with 5-FU can induce secondary resistance to 5-FU in CT26 cells. The culture supernatant of 5-FU-induced macrophage can inhibit p-JNK/caspase-3 signaling activated by 5-FU in CT26 cells.