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Background: α-ketoglutarate (α-KG) is the substrate to hydoxylate collagen and hypoxia-inducible factor-1α (HIF-1α), which are important for cancer metastasis. Previous studies showed that upregulation of collagen prolyl 4-hydroxylase in breast cancer cells stabilizes HIF-1α via depleting α-KG in breast cancer cells. We propose that mitochondrial malate enzyme 2 (ME2) may also affect HIF-1α via modulating α-KG level in breast cancer cells. Methods: ME2 protein expression was evaluated by immunohistochemistry on 100 breast cancer patients and correlated with clinicopathological indicators. The effect of ME2 knockout on cancer metastasis was evaluated by an orthotopic breast cancer model. The effect of ME2 knockout or knockdown on the levels of α-KG and HIF-1α protein in breast cancer cell lines (4T1 and MDA-MB-231) was determined in vitro and in vivo.Results: The high expression of ME2 was observed in the human breast cancerous tissues compared to the matched precancerous tissues (P=0.000). The breast cancer patients with a high expression of ME2 had an inferior survival than the patients with low expression of ME2 (P=0.019). ME2 high expression in breast cancer tissues was also related with lymph node metastasis (P=0.016), pathological staging (P=0.033) and vascular cancer embolus (P=0.014). In a 4T1 orthotopic breast cancer model, ME2 knockout significantly inhibited lung metastasis. In the tumors formed by ME2 knockout 4T1 cells, α-KG level significantly increased, collagen hydroxylation level did not change significantly, but HIF-1α protein level significantly decreased, in comparison to control. In cell culture, ME2 knockout or knockdown cells demonstrated a significantly higher α-KG level but significantly lower HIF-1α protein level than control cells under hypoxia. Exogenous malate and α-KG exerted similar effect on HIF-1α in breast cancer cells to ME2 knockout or knockdown. Treatment with malate significantly decreased 4T1 breast cancer lung metastasis. ME2 expression was associated with HIF-1α level in human breast cancer samples (P=0.027).Conclusion: We provide evidence that upregulation of ME2 is associated with a poor prognosis of breast cancer patients and propose a mechanistic understanding of a link between ME2 and breast cancer metastasis.