Loss of BRUCE reduces cellular energy level and induces autophagy by driving activation of the AMPK-ULK1 autophagic initiating axis

PLoS One. 2019 May 15;14(5):e0216553. doi: 10.1371/journal.pone.0216553. eCollection 2019.

Abstract

Autophagy is an intracellular catabolic system. It delivers cellular components to lysosomes for degradation and supplies nutrients that promote cell survival under stress conditions. Although much is known regarding starvation-induced autophagy, the regulation of autophagy by cellular energy level is less clear. BRUCE is an ubiquitin conjugase and ligase with multi-functionality. It has been reported that depletion of BRUCE inhibits starvation-induced autophagy by blockage of the fusion step. Herein we report a new function for BRUCE in the dual regulation of autophagy and cellular energy. Depletion of BRUCE alone (without starvation) in human osteosarcoma U2OS cells elevated autophagic activity as indicted by the increased LC3B-II protein and its autophagic puncta as well as further increase of both by chloroquine treatment. Such elevation results from enhanced induction of autophagy since the numbers of both autophagosomes and autolysosomes were increased, and recruitment of ATG16L onto the initiating membrane structure phagophores was increased. This concept is further supported by elevated lysosomal enzyme activities. In contrast to starvation-induced autophagy, BRUCE depletion did not block fusion of autophagosomes with lysosomes as indicated by increased lysosomal cleavage of the GFP-LC3 fusion protein. Mechanistically, BRUCE depletion lowered the cellular energy level as indicated by both a higher ratio of AMP/ATP and the subsequent activation of the cellular energy sensor AMPK (pThr-172). The lower energy status co-occurred with AMPK-specific phosphorylation and activation of the autophagy initiating kinase ULK1 (pSer-555). Interestingly, the higher autophagic activity by BRUCE depletion is coupled with enhanced cisplatin resistance in human ovarian cancer PEO4 cells. Taken together, BRUCE depletion promotes induction of autophagy by lowering cellular energy and activating the AMPK-ULK1-autophagy axis, which could contribute to ovarian cancer chemo-resistance. This study establishes a BRUCE-AMPK-ULK1 axis in the regulation of energy metabolism and autophagy, as well as provides insights into cancer chemo-resistance.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • AMP-Activated Protein Kinases / genetics
  • AMP-Activated Protein Kinases / metabolism*
  • Autophagosomes
  • Autophagy*
  • Autophagy-Related Protein-1 Homolog / genetics
  • Autophagy-Related Protein-1 Homolog / metabolism*
  • Bone Neoplasms / genetics
  • Bone Neoplasms / metabolism
  • Bone Neoplasms / pathology
  • Cell Survival
  • Energy Metabolism*
  • Female
  • Gene Expression Regulation, Neoplastic
  • Humans
  • Inhibitor of Apoptosis Proteins / genetics
  • Inhibitor of Apoptosis Proteins / metabolism*
  • Intracellular Signaling Peptides and Proteins / genetics
  • Intracellular Signaling Peptides and Proteins / metabolism*
  • Lysosomes / metabolism
  • Osteosarcoma / genetics
  • Osteosarcoma / metabolism
  • Osteosarcoma / pathology*
  • Ovarian Neoplasms / genetics
  • Ovarian Neoplasms / metabolism
  • Ovarian Neoplasms / pathology*
  • Phosphorylation
  • Signal Transduction
  • Tumor Cells, Cultured

Substances

  • BIRC6 protein, human
  • Inhibitor of Apoptosis Proteins
  • Intracellular Signaling Peptides and Proteins
  • Autophagy-Related Protein-1 Homolog
  • ULK1 protein, human
  • AMP-Activated Protein Kinases
  • PRKAA1 protein, human