Ablation of ATG4B Suppressed Autophagy and Activated AMPK for Cell Cycle Arrest in Cancer Cells.

ATG4B is a cysteine protease required for autophagy, which is a cellular catabolic pathway involved in energy balance. ATG4B expression is elevated during tumor growth in certain types of cancer, suggesting that ATG4B is an attractive target for cancer therapy. However, little is known about the mechanisms through which ATG4B deprivation suppresses the growth of cancer cells. Cancer cells were transfected with either siRNA against ATG4B or an expression vector encoding wild-type ATG4BWT or encoding catalytic mutant ATG4BC74A to determine cell cycle progression by propidium iodide staining or by BrdU incorporation assay using flow cytometry. The GFP-MAP1LC3-II puncta and protein levels in the cells were determined by immunofluorescence and immunoblotting, respectively. Knockdown of ATG4B blocked cell proliferation, particularly at the G1-S phase transition, in various cancer cells. Moreover, knockdown of ATG4B or overexpression of the ATG4BC74A catalytic mutant reduced both autophagic flux and ATP levels and increased AMP-activated protein kinase (AMPK) phosphorylation in the cancer cells. Nevertheless, knockdown of ATG4B had only a minor effect on AMPK activation and G1 phase arrest in liver kinase B1 (LKB1)-deficient or AMPK-inhibited cancer cells. These results imply that targeting ATG4B might inhibit autophagy and trigger the LKB1-AMPK energy-sensing pathway, resulting in tumor growth suppression.