Autophagy is a critical cytoprotective mechanism against stress, which is initiated by the protein kinase Unc-51-like kinase 1 (ULK1) complex. Autophagy plays a role in both inhibiting the progression of diseases and facilitating pathogenesis, so it is critical to elucidate the mechanisms regulating individual components of the autophagy machinery under various conditions. Here, we examined whether ULK1 complex component autophagy-related protein 101 (ATG101) is downregulated via ubiquitination, and whether this in turn suppresses autophagy activity in cancer cells. Knockout of ATG101 in cancer cells using CRISPR resulted in severe growth retardation and lower survival under nutrient starvation. Transfection of mutant ATG101 revealed that the C-terminal region is a key domain of ubiquitination, while co-immunoprecipitation and knockdown experiments revealed that HECT, UBA and WWE domain containing E3 ubiquitin protein ligase 1(HUWE1) is a major E3 ubiquitin ligase targeting ATG101. Protein levels of ATG101 was more stable and the related-autophagy activity was higher in HUWE1-depleted cancer cells compared to wild type (WT) controls, indicating that HUWE1-mediated ubiquitination promotes ATG101 degradation. Moreover, enhanced autophagy in HUWE1-depleted cancer cells was reversed by siRNA-mediated ATG101 knockdown. Stable ATG101 level in HUWE1-depleted cells was a strong driver of autophagosome formation similar to upregulation of the known HUWE1 substrate WD repeat domain, phosphoinositide interacting 2 (WIPI2). Cellular survival rates were higher in HUWE1-knockdown cancer cells compared to controls, while concomitant siRNA-mediated ATG101 knockdown tends to increase apoptosis rate. Collectively, these results suggest that HUWE1 normally serves to suppress autophagy by ubiquitinating and triggering degradation of ATG101 and WIPI2, which in turn represses the survival of cancer cells. Accordingly, ATG101-mediated autophagy may play a critical role in overcoming metabolic stress, thereby contributing to the growth, survival, and treatment resistance of certain cancers.
Keywords: E3 ubiquitin ligase; HECT, UBA and WWE domain containing E3 ubiquitin protein ligase 1(HUWE1); Unc-51-like kinase 1(ULK1); WD repeat domain, phosphoinositide interacting 2 (WIPI2); autophagy; autophagy-related gene 101(ATG101); cancer; mitophagy; ubiquitination.