Suppressed translation and ULK1 degradation as potential mechanisms of autophagy limitation under prolonged starvation

Abstract

Macroautophagy/autophagy is a well-organized process of intracellular degradation, which is rapidly activated under starvation conditions. Recent data demonstrate a transcriptional upregulation of several autophagy genes as a mechanism that controls autophagy in response to starvation. Here we report that despite the significant upregulation of mRNA of the essential autophagy initiation gene ULK1, its protein level is rapidly reduced under starvation. Although both autophagic and proteasomal systems contribute to the degradation of ULK1, under prolonged nitrogen deprivation, its level was still reduced in ATG7 knockout cells, and only initially stabilized in cells treated with the lysosomal or proteasomal inhibitors. We demonstrate that under starvation, protein translation is rapidly diminished and, similar to treatments with the proteosynthesis inhibitors cycloheximide or anisomycin, is associated with a significant reduction of ULK1. Furthermore, it was found that inhibition of the mitochondrial respiratory complexes or the mitochondrial ATP synthase function that could also take place in the absence of substrates, promote upregulation of ULK1 mRNA and protein expression in an AMPK-dependent manner in U1810 lung cancer cells growing in complete culture medium. These inhibitors could also drastically increase the ULK1 protein in U1810 cells with knockout of ATG13, where the ULK1 expression is significantly diminished. However, such upregulation of ULK1 protein is negligible under starvation conditions, further signifying the contribution of translation and suggesting that transcriptional upregulation of ULK1 protein will be diminished under such conditions. Thus, we propose a model where inhibition of protein translation, together with the degradation systems, limit autophagy during starvation.

Hepatitis C virus triggers Golgi fragmentation and autophagy through the immunity-related GTPase M

“…As discussed above, we propose that these findings may indicate that autophagy is reduced in cells deprived of IRGM. Another study reported that ULK1 levels were reduced upon starvation, presumably as a mechanism to restrain prolonged autophagy (30). Collectively, our results suggest that IRGM regulates ULK activation and ULK-mediated autophagy upon HCV infection.…”

A Review of ULK1-Mediated Autophagy in Drug Resistance of Cancer

“…Matured autophagosomes are degraded by autolysosomes in a manner dependent on small GTPase RAB7, CCZ1-MON1, HOPS complex, syntaxin-17, and other SNARE proteins [86,87]. In mammalian cells, ULK1 binds to ATG8 at the interacting motif/LC3-interacting region and, then, is transferred to the lysosome for degradation [88][89][90]. The degradation of ULK1 not only involves lysosomes and proteasomes but also the transcription of inhibitory proteins.…”

MicroRNA-93 Regulates Hypoxia-Induced Autophagy by Targeting ULK1

“…ULK1 protein level can be regulated by a variety of stimuli such as nutrient-depleted condition, energy deprivation, and hypoxia [ 14 – 16 ]. In the initial stage of nutrient-depletion condition, the elevated transcription of ULK1 is immediately induced, but after prolonged starvation, the ULK1 protein level is reduced because of the suppressed translation as well as the increased degradation of ULK1, which limits further autophagy induction [ 17 – 19 ]. ULK1 level is significantly increased in response to inhibition of mitochondrial respiratory complexes in cell treated with inhibitors of the mitochondrial complex I (rotenone), complex II (thenoyltrifluoroacetone/TTFA), or complex III (antimycin A or myxothiazol) [ 17 ].…”

“…In the initial stage of nutrient-depletion condition, the elevated transcription of ULK1 is immediately induced, but after prolonged starvation, the ULK1 protein level is reduced because of the suppressed translation as well as the increased degradation of ULK1, which limits further autophagy induction [ 17 – 19 ]. ULK1 level is significantly increased in response to inhibition of mitochondrial respiratory complexes in cell treated with inhibitors of the mitochondrial complex I (rotenone), complex II (thenoyltrifluoroacetone/TTFA), or complex III (antimycin A or myxothiazol) [ 17 ]. Previously, we also found that ULK1 level is induced in cells subjected to 1% oxygen stress condition or by mitochondrial depolarized drug, FCCP, indicating that the ULK1 level is regulated at both transcriptional and translational levels and is more complicatedly controlled than previously thought [ 15 , 16 ].…”