Discovery of a small molecule targeting autophagy via ATG4B inhibition and cell death of colorectal cancer cells in vitro and in vivo.

Human Atg4 homologs are cysteine proteases, which play key roles in the macroautophagy/autophagy process by cleaving Atg8 homologs for conjugation to lipid membranes and for deconjugation of Atg8 homologs from membranes. Expression of ATG4B is significantly increased in colorectal cancer cells compared to normal cells, suggesting that ATG4B may be important for cancer biology. Inhibition of ATG4B may reduce the autophagy activity, thereby sensitizing cancer cells to therapeutic agents. Thus, developing specific and potent ATG4B inhibitors for research as well as for potential therapeutic uses is highly needed. In this study, we integrated screening and assays to discover a potent ATG4B inhibitor, named S130, from a noncommercial library. This chemical binds to ATG4B with strong affinity and specifically suppresses the activity of ATG4B but not other proteases. S130 did not cause the impairment of autophagosome fusion, nor did it result in the dysfunction of lysosomes. Instead, S130 might attenuate the delipidation of LC3-II on the autolysosomes to suppress the recycling of LC3-I, which normally occurs after LC3-II cleavage by ATG4B. Intriguingly, S130 induced cell death, which was accompanied with autophagy stress and could be further exacerbated by nutrient deprivation. Such cytotoxicity could be partially reversed by enhancing ATG4B activity. Finally, we found that S130 was distributed in tumor tissues in vivo and was also effective in arresting the growth of colorectal cancer cells. Thus, this study indicates that ATG4B is a potential anticancer target and S130 might be a novel small-molecule candidate for future cancer therapy.