Methylation of microRNA-129-5P modulates nucleus pulposus cell autophagy by targeting Beclin-1 in intervertebral disc degeneration

MicroRNAs play an important role in the etiology and progression of many diseases, including intervertebral disc degeneration (IVDD). The miRNA miR-129-5P regulates autophagy in various cancers, but its role in human nucleus pulposus (NP) cells is unclear. The present study investigated whether miR-129-5p regulates the expression of Beclin-1 which is known to induce autophagy in NP cells by evaluating their levels in normal and degenerative disc tissues and human NP cells transfected with miR-129-5P mimic or inhibitor by quantitative real-time (qRT-)PCR, western blotting, flow cytometry, and immunofluorescence analysis. A bioinformatics analysis was used to predict the relationship between miR-129-5P and Beclin-1, which was confirmed by the dual luciferase assay. DNA methylation status was assessed by methylation-specific PCR, and the effect of demethylation on miR-129-5P level and autophagy was examined by qRT-PCR, western blotting, and flow cytometry. We found that miR-129-5P expression was downregulated while that of Beclin-1 and LC3-II was upregulated in degenerative disc NP cells. Meanwhile, autophagy was reduced in human NP cells transfected with miR-129-5P mimic, whereas the opposite result was observed upon treatment with miR-129-5P inhibitor. Bioinformatics analysis and the luciferase reporter assay revealed that Beclin-1 is a target of and is inhibited by miR-129-5P. We also found that CpG islands in the miR-129-5P promoter region were hypermethylated in degenerative as compared to normal disc tissue. Thus, miR-129-5P blocks NP cell autophagy by directly inhibiting Beclin-1, a process that is dependent on miR-129-5P promoter methylation.