MAPK1/3 kinase-dependent ULK1 degradation attenuates mitophagy and promotes breast cancer bone metastasis.

ABSTRACT

The function of mitophagy in cancer is controversial. ULK1 is critical for induction of macroautophagy/autophagy and has a more specific role in mitophagy in response to hypoxia. Here, we show that ULK1 deficiency induces an invasive phenotype of breast cancer cells under hypoxia and increases osteolytic bone metastasis. Mechanistically, ULK1 depletion attenuates mitophagy ability during hypoxia. As a result, the accumulation of damaged, ROS-generating mitochondria leads to activation of the NLRP3 inflammasome, which induces abnormal soluble cytokines secretion, then promotes the differentiation and maturation of osteoclasts, and ultimately results in bone metastasis. Notably, phosphorylation of ULK1 by MAPK1/ERK2-MAPK3/ERK1 kinase triggers its interaction with BTRC and subsequent K48-linked ubiquitination and proteasome degradation. Also, a clearly negative correlation between the expression levels of ULK1 and p-MAPK1/3 was observed in human breast cancer tissues. The MAP2K/MEK inhibitor trametinib is sufficient to restore mitophagy function via upregulation of ULK1, leading to inhibition of NLRP3 inflammasome activation, thereby reduces bone metastasis. These results indicate that ULK1 knockout-mediated mitophagy defect promotes breast cancer bone metastasis and provide evidence to explore MAP2K/MEK- MAPK1/3 pathway inhibitors for therapy, especially in cancers displaying low levels of ULK1.Abbreviations ATG autophagy-related; Baf A1 bafilomycin A1; BTRC/ß-TrCP beta-transducin repeat containing E3 ubiquitin protein ligase; CHX cycloheximide; CM conditioned media; FBXW7/FBW7 F-box and WD repeat domain containing 7; MAPK1 mitogen-activated protein kinase 1; MTDR MitoTracker Deep Red; mtROS mitochondrial reactive oxygen species; microCT micro-computed tomography; mtROS mitochondrial reactive oxygen species; OCR oxygen consumption rate; SQSTM1 sequestosome 1; ACP5/TRAP acid phosphatase, tartrate resistant; ULK1 unc-51 like autophagy activating kinase 1.