Mitochondrial malic enzyme 2 promotes breast cancer metastasis via stabilizing HIF-1α under hypoxia

Objective: alpha-ketoglutarate (alpha-KG) is the substrate to hydroxylate collagen and hypoxia-inducible factor-1 alpha (HIF-1 alpha), which are important for cancer metastasis. Previous studies have shown that the upregulation of collagen prolyl 4-hydroxylase in breast cancer cells stabilizes the expression of HIF-1 alpha by depleting alpha-KG levels. We hypothesized that mitochondrial malic enzyme 2 (ME2) might also affect HIF-1 alpha expression via modulating alpha-KG levels in breast cancer cells. Methods: We evaluated ME2 protein expression in 100 breast cancer patients using immunohistochemistry and correlated with clinicopathological indicators. The effect of ME2 knockout on cancer metastasis was evaluated using an orthotopic breast cancer model. The effect of ME2 knockout or knockdown on the levels of alpha-KG and HIF-1 alpha proteins in breast cancer cell lines was determined both in vitro and in vivo. Results: ME2 was found to be upregulated in the human breast cancerous tissues compared with the matched precancerous tissues (P<0.001). The elevated expression of ME2 was associated with a poor prognosis (P=0.019). ME2 upregulation was also related to lymph node metastasis (P=0.016), pathological staging (P=0.033), and vascular cancer embolus (P=0.014). Also, ME2 knockout significantly inhibited lung metastasis in vivo. In the tumors formed by ME2 knockout cells, the levels of alpha-KG were significantly increased and collagen hydroxylation level did not change significantly but HIF-1 alpha protein expression was significantly decreased, compared to the control samples. In cell culture, cells with ME2 knockout or knockdown demonstrated significantly higher alpha-KG levels but significantly lower HIF-1 alpha protein expression than control cells under hypoxia. Exogenous malate and alpha-KG exerted similar effect on HIF-1 alpha in breast cancer cells to ME2 knockout or knockdown. Additionally, treatment with malate significantly decreased 4T1 breast cancer lung metastasis. ME2 expression was associated with HIF-1 alpha levels in human breast cancer samples (P=0.008). Conclusions: Our results 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.