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

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.

CHK2-FOXK axis promotes transcriptional control of autophagy programs

“…In addition, in chloroquine (CQ)–treated cells, cisplatin treatment induced further notably increased autophagosomes (yellow puncta), but not the autolysosomes (red puncta), because CQ is able to block the autophagosome-lysosome fusion (fig. S1, A and B) ( 16 , 17 ). Collectively, these results suggest that DNA damage induces autophagy, which is consistent with previous reports ( 18 – 20 ).…”

BIRC6 Is Associated with Vulnerability of Carotid Atherosclerotic Plaque

“…BIRC6 is a huge ubiquitin-conjugating E2 enzyme that regulates autophagosome-lysosome fusion and displays anti-apoptotic activity [ 31 , 32 ]. BIRC6 negatively regulates autophagy by limiting through ubiquitination the availability of MAP1LC3B, a central protein in the autophagy pathway functioning in autophagy substrate selection and autophagosome biogenesis [ 33 , 34 , 35 , 36 ]. In fact, autophagy is involved in multiple processes of carotid plaque formation and differentiation, and may constitute a protective mechanism to promote cell survival in the plaque [ 29 , 37 ].…”

Blockage of AMPK-ULK1 pathway mediated autophagy promotes cell apoptosis to increase doxorubicin sensitivity in breast cancer (BC) cells: an in vitro study

“…AMP-activated protein kinase (AMPK)-Unc-51-like kinase 1 (ULK1) signaling cascade was crucial for autophagy initiation [24][25][26], and activation of AMPK-ULK1 pathway triggered protective autophagy to promote cell survival under environmental stress, such as oxidative stress [27] and cigarette smoke exposure [28]. Aside from that, recent data indicated that AMPK-ULK1 pathway mediated autophagy involved in regulating drug resistance during cancer treatment, including cisplatin [29,30] and Dox [21].…”

“…Speci cally, Guihua Duan et al [30], Lixiao Che et al [29] and Zhenyu Liu et al [21] reported that induction of AMPK-ULK1 pathway mediated autophagy contributed to cisplatin-and Dox-resistance in ovarian cancer and BC, respectively. The above information enlightened us to speculate that AMPK-ULK1 pathway mediated protective autophagy might be the reason of Dox-resistance in BC.…”