The Role of Host Defense Peptide Cathelicidin in Colon Tumorigenesis

Host defense peptides, such as cathelicidins (LL-37) and defensins, are important components in the innate immunity. LL-37, a human cationic host defense peptide composed of 37 residues, is the only cathelicidin described so far in humans. It plays a key role in diverse biological processes, including natural immunity, inflammation and tissue repair. Emerging evidence suggests that LL-37 is implicated in cancer development. In this regard, the expression of LL-37 is found to be dysregulated in many types of human malignancy, including lung, breast, ovarian, and gastric cancers. The expression and function of LL-37 in colon cancer, however, are still unknown. In this thesis, the roles of LL-37 and its 17-32 fragment (hereafter referred to as FK-16) in colon cancer development were investigated.;By immunohistochemical staining, it is demonstrated that the expression of LL-37 was significantly reduced in human colon cancer tissues as compared with the cancer adjacent normal tissues. Moreover, LL-37 expression was positively correlated with the number of TUNEL-positive cells. Furthermore, synthetic LL-37 induced caspase-independent apoptotic cell death in different cultured colon cancer cells. In this connection, LL-37 induced apoptosis via downregulation of Bcl-2 and upregulation of Bak and Bax in a p53-dependent manner. It also induced the upregulation and nuclear translocation of apoptosis-inducing factor (AIF) and endonuclease G (EndoG), whose targetings by siRNAs rendered the cells resistant to LL-37-induced apoptosis. Above all, the pro-apoptotic effect of LL-37 was found to be mediated through a pertussis toxin-sensitive Gαi protein-coupled receptor. Concordantly, colonic mucosa of cathelicidin-knockout mice exhibited reduced expression of p53, Bax and Bak and increased expression of Bcl-2 together with a lower basal level of apoptosis. Taken together, we demonstrated that LL-37 activates a novel signaling cascade involving the GPCR-p53-Bax/Bak/Bcl-2 axis to activate AIF/EndoG-mediated apoptosis in colon cancer cells.;Similar to the effect of LL-37 peptide, the fragment FK-16 also induced cell death in colon cancer cell lines. However, the action is different. Results demonstrated that FK-16 triggered a unique pattern of cell death characterized by initial caspase-independent apoptosis followed by autophagic cell death, the latter of which was not observed obviously in cells treated with LL-37. Treating colon cancer cells with FK-16 for 24 h induced apoptosis as evidenced by phosphatidylserine externalization, chromatin condensation and DNA fragmentation. Prolonged treatment with FK-16 induced biochemical and morphological features consistent with autophagy, including increased formation of LC3+ autophagosomes, the accumulation of acidic vesicular organelles and autolysosomes, and increased levels of LC3-I/II, Atg5 and Atg7. Knockdown of Atg5 or Atg7 partially reversed the cytotoxic effect of FK-16, suggesting that FK-16-induced autophagy was pro-death in nature. Furthermore, the novel cross-talks between apoptotic and autophagic signalings were also noted.;Collectively, the present study not only contributes to understanding the role of host defense peptide cathelicidin in tumorigenesis, but also provides pre-clinical evidence to propel the development and application of these peptides as novel therapeutic agents for the treatment of colon cancer.