Abstract 38: Mitophagy deficiency promotes osteolytic bone metastasis in breast cancer

Introduction: Metastasis of breast cancer to the bone is a common complication and occurs in 65-80% of late-stage patients. Specifically, the development of osteolytic bone lesions increases patient morbidity and mortality with debilitating skeletal complications. Mitophagy is a selective form of autophagy that removes damaged or excessive mitochondria in response to various metabolic stresses. However, the role of mitophagy in development of cancer remains largely unclear. The Janus-faced role of mitophagy during tumorigenesis leads to a novel challenge when targeting it for cancer therapies. This study focuses on the role of mitophagy deficiency in breast cancer osteolytic bone metastasis, and explores the potential of rescuing mitophagy deficiency as possible therapeutic targets for metastatic disease. Methods: Specific single-guide RNAs (sgRNAs) were used to deplete ULK1 expression in breast cancer cell lines. Mitophagy was monitored by detecting Tom20 and Hsp60. In addition, mitochondrial DNA (mtDNA) was used to further confirm the reduction of mitochondrial mass. For bone metastasis studies, luciferase-tagged tumor cells were injected into the left cardiac ventricle of anesthetized female nude mice. Metastatic progression was monitored by bioluminescence imaging (BLI). Micro-CT imaging was used to detect bone volume, cortical wall thickness, trabecular thickness and number in mice of tibia. TRAP assay was used to detect osteoclast differentiation assays. Results: We found that the core Atg protein ULK1 was not only expressed at low level in breast cancer, but also required for hypoxia-induced mitophagy. ULK1-related mitophagy deficiency promoted the formation of metastatic osteolytic lesions in experimental bone metastasis mouse model with MDA-MB-231 cells. TRAP staining revealed that mature osteoclasts significantly increased in RAW264.7 cells treated with CM from ULK1-knockout breast cancer cells. In addition, MEK inhibitor trametinib was sufficient to upregulate the expression of ULK1 and reduced the release of cytokines, which is key to osteoclast differentiation via triggering mitophagy, thereby effectively blocking breast cancer osteolytic bone metastasis. Conclusion: In conclusion, we demonstrate the functional importance of mitophagy defect in breast cancer for promoting osteolytic bone metastasis. We also provide strong preclinical evidence for using MEK-ERK pathway inhibitors to disrupt ERK kinase-involved ULK1 degradation and induce mitophagy as a breast cancer bone metastasis therapy. Citation Format: Rong Deng, Hai-Liang Zhang, Rui-Zhao Cai, Jun Tang, Xiao-Feng Zhu. Mitophagy deficiency promotes osteolytic bone metastasis in breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 38.