Abstract: Objective: To investigate the effects of antimicrobial peptide LL-37 on the tumor growth and apoptosis of the mice with colon cancer, and to elucidate the possible molecular mechanism of its anti-tumor effect. Methods: The LL-37 over-expression colon cancer HT-29 cells were constructed, and the expression levels of LL-37 mRNA and protein in the HT-29 cells were detected by qRT-PCR and Western blotting methods. A total of 30 BALB/c mice were randomly divided into control group (given the uninfected HT-29 cells), empty vector group (given the HT-29 cells infected with empty plasmid), LL-37 over-expression group (given the HT-29 cells infected with LL-37 over-expression vector), AMPK inhibitor group[given the HT-29 cells infected with empty vector, and then injected with 2 mg·kg^-1 Dorsomorphin (Dor) in the tail vein] and combination group (given the HT-29 cells infected with LL-37 over-expression vector, and then injected with 2 mg·kg^-1 Dor in the tail vein),and there were 6 mice in each group. The colon cancer HT-29 cells were used to replicate the colon cancer models of the mice. The tumor volumes of the mice in various groups were detected and the growth curve was drawn. After infected for 24 d, the weights of the tumor of the mice in various groups were measured and the inhibitory rates of the tumor were calculated. TUNEL staining was used to detect the apoptosis of tumor cells in various groups. Western blotting method was used to detect the expression levels of autophagy-related proteins Beclin1, ATG5, LC3Ⅰ, LC3Ⅱ and AMPK/mTOR pathway-related proteins AMPK, p-AMPK, mTOR and p-mTOR in tumor tissue of the mice in various groups. Results: Compared with control group and empty vector group, the expression levels of LL-37 mRNA and protein in the HT-29 cells of the mice in over-expression group were significantly increased(P<0.01),the weight and volume of the tumor were significantly decreased (P<0.05), the inhibitory rate of tumor was increased(P<0.05), the number of apoptotic cells was increased, the expression levels of autophage-related proteins Beclin1 and ATG5 in tumor tissue and the ratios of LC3Ⅱ/LC3Ⅰ and p-AMPK/AMPK were increased(P<0.05), while the ratio of p-mTOR/mTOR was significantly decreased (P<0.05).Compared with empty vector group,the weight and volume of the tumor of the mice in AMPK inhibitor group were increased (P<0.05), the inhibitory rate of the tumor was decreased (P<0.05), the number of apoptotic cells was decreased, the expression levels of autophagy-related proteins Beclin1 and Atg5,the ratios of LC3Ⅱ/LC3Ⅰ and p-AMPK/AMPK were decreased (P<0.05), and the ratio of p-mTOR/mTOR was increased (P<0.05).Compared with LL-37 over-expression group, the expression levels of Beclin1, ATG5,the ratios of LC3Ⅱ/LC3Ⅰ and p-AMPK/AMPK in tumor tissue of the mice in combination group were significantly decreased (P< 0.05 or P<0.01), and the ratio of p-mTOR/mTOR was significantly increased (P<0.01). Conclusion: The antimicrobial peptide LL-37 can activate the AMPK/mTOR signaling pathway and promote the autophagy of the cells, thereby inhibit the tumor growth in the colon cancer HT-29-bearing mice.

Key words: antimicrobial peptide LL-37, AMPK/mTOR signaling pathway, autophagy, colon neoplasms