Ribonucleotide reductase subunit M2B deficiency leads to mitochondrial permeability transition pore opening and is associated with aggressive clinicopathologic manifestations of breast cancer.

Ribonucleotide reductase small subunit M2B (RRM2B) plays an essential role in maintaining mitochondrial homeostasis. Mitochondrial permeability transition pore (MPTP) is a key regulator of mitochondrial homeostasis. MPTP contributes to cell death and is crucial in cancer progression. RRM2B's relation to MPTP is not well known, and the role of RRM2B in cancer progression is controversial. Here, our aim was to study the role of RRM2B in regulating MPTP and the association between RRM2B and clinicopathological manifestations in breast cancer. Analysis of Rrm2b-/- mice cells found changes consistent with MPTP opening, including mitochondrial swelling and upregulation of cyclophilin D (CypD), a protein that activates MPTP opening. Silencing of RRM2B gene expression in MCF7 and KB cell lines led to MPTP opening. Accordingly, dysfunctional oxidative phosphorylation and elevated superoxide levels were also detected in RRM2B-silenced MCF7 and KB cell lines, which was consistent with the findings by gene set enrichment analysis of 159 breast cancer cases that genes involving respiratory electron transport were enriched in high-RRM2B breast cancer, and genes involving biologic oxidation were enriched in low-RRM2B breast cancers. A metabolomic study revealed that spermine levels in RRM2B-silenced MCF7 and KB cells were only 5% and 8% of control levels, respectively. Addition of exogenous spermine to RRM2B-silenced MCF7 and KB cells was able to reverse the MPTP opening induced by RRM2B deficiency. These results suggest that RRM2B may induce MPTP opening through reducing spermine levels. Immunohistochemical analysis of 148 breast cancer cases showed that RRM2B and CypD protein levels were inversely correlated in breast cancer specimens (P<0.05), so were their associated clinicopathologic parameters that high-level RRM2B expression was associated with better clinicopathological features. We conclude that RRM2B deficiency leads to MPTP opening mediated by spermine. Coupling of low RRM2B and high CypD expression is associated with aggressive manifestations of breast cancer.