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

Duo You; Danfeng Du; Xueke Zhao; Xinmin Li; Minfeng Ying; Xun Hu

doi:10.21147/j.issn.1000-9604.2021.03.03

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

Chin J Cancer Res. 2021 Jun 30;33(3):308-322. doi: 10.21147/j.issn.1000-9604.2021.03.03.

Authors

Duo You¹, Danfeng Du², Xueke Zhao², Xinmin Li³, Minfeng Ying¹, Xun Hu¹

Affiliations

¹ Cancer Institute (Key Laboratory for Cancer Intervention and Prevention, China National Ministry of Education, Zhejiang Provincial Key Laboratory of Molecular Biology in Medical Sciences), The Second Affiliated Hospital, Zhejiang University School of Medicine, Hangzhou 310000, China.
² State Key Laboratory of Esophageal Cancer Prevention & Treatment and Henan Key Laboratory of Esophageal Cancer Research of the First Affiliated Hospital, Zhengzhou University, Zhengzhou 450000, China.
³ Department of Pathology, Women and Infants Hospital of Zhengzhou, Zhengzhou 450000, China.

PMID: 34321828
PMCID: PMC8286887
DOI: 10.21147/j.issn.1000-9604.2021.03.03

Abstract

Objective: α-ketoglutarate (α-KG) is the substrate to hydroxylate collagen and hypoxia-inducible factor-1α (HIF-1α), 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-1α by depleting α-KG levels. We hypothesized that mitochondrial malic enzyme 2 (ME2) might also affect HIF-1α expression via modulating α-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 α-KG and HIF-1α 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 metastasisin vivo. In the tumors formed by ME2 knockout cells, the levels of α-KG were significantly increased and collagen hydroxylation level did not change significantly but HIF-1α protein expression was significantly decreased, compared to the control samples. In cell culture, cells with ME2 knockout or knockdown demonstrated significantly higher α-KG levels but significantly lower HIF-1α protein expression 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. Additionally, treatment with malate significantly decreased 4T1 breast cancer lung metastasis. ME2 expression was associated with HIF-1α 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.

Keywords: Malic enzyme 2; breast cancer; hypoxia-inducible factor-1α; malate; metastasis; α-ketoglutarate.