Background: Abnormal DNA methylation has been detected in breast cancer (BC). Downregulation of F-box protein 31 (FBXO31) in BC and its upregulated methylation level have been initially confirmed via bioinformatic analyses. Likewise, DNA methyltransferase 3 beta (DNMT3B) is highly expressed in BC. Accordingly, this study is engineered to fathom out whether DNMT3B can affect the progression of BC (Luminal A type) by regulating FBXO31 methylation.

Methods: DNMT3B/FBXO31 expression and FBXO31 methylation level in BC were predicted by the University of Alabama at Birmingham Cancer Data Analysis Portal (UALCAN) database. 5-methylcytosine (5mC) site of FBXO31 was analyzed with RMBase database. After human BC cells were collected and transfected, viability, apoptosis and migration/invasion were tested by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) assay, flow cytometry, and Transwell assay, respectively. FBXO31 methylation level was determined by methylation-specific polymerase chain reaction (MSP). Expression levels of FBXO31, DNMT3B, and apoptosis/proliferation-associated proteins were evaluated using quantitative real-time reverse transcription polymerase chain reaction or Western blotting.

Results: DNMT3B expression was upregulated while FBXO31 expression was downregulated (p = 1.62 × 10^-12), and FBXO31 methylation level was elevated in breast invasive carcinoma (p < 1 × 10^-12). DNMT3B was highly expressed in MCF-7, MDA-MB-231 and MDA-MB-453 cells (p < 0.001). DNMT3B silencing inhibited viability, migration and invasion (p < 0.05), enhanced apoptosis (p < 0.001), upregulated expression levels of Bax and FBXO31 (p < 0.001), downregulated expression levels of B cell leukemia/lymphoma 2 (Bcl-2), Ki67 and proliferating cell nuclear antigen (PCNA) (p < 0.001), and diminished FBXO31 methylation level in MCF-7 cells. FBXO31 silencing showed the opposite effects in MCF-7 cells (p < 0.001), whereas DNMT3B silencing reversed the effects of FBXO31 silencing in MCF-7 cells (p < 0.01).

Conclusions: DNMT3B silencing alleviates malignant behaviors in Luminal A BC cells by suppressing FBXO31 methylation, indicating a novel mechanism of DNA methylation in Luminal A type BC.