Curcumin-induced promoter hypermethylation of the mammalian target of rapamycin gene in multiple myeloma cells

Curcumin, a polyphenol derived from the rhizome of Curcuma, is a potential tumor inhibitor through affecting signaling pathways and epigenetic regulation. The mammalian target of rapamycin (mTOR) gene serves a crucial role in the carcinogenesis of multiple myeloma. The curcumin-induced epigenetic regulation of mTOR, including promoter DNA methylation in multiple myeloma, has not yet been fully elucidated. In the present study, antitumor effects of curcumin were investigated in RPMI-8226 and NCI-H929 cells using an MTT assay and flow cytometry. The expression of mTOR and DNA methyltransferase proteins were determined by western blot analysis, and the methylation status of the mTOR promoter were detected by sequencing following bisulfite conversion. The results of the present study revealed that the half-maximal inhibitory concentration of curcumin was 10 µM in myeloma cells. Following curcumin treatment, the mRNA and protein expression levels of mTOR were decreased by 43.31 and 39.34% in NCI-H929 cells, respectively. The promoter of mTOR, located in chromosome 1 (chromosome position, 11262153-11263153), contains a CpG island that was hypermethylated in myeloma cells following curcumin treatment. The expression levels of DNA methyltransferase (DNMT)3a and DNMT3b were increased in curcumin-treated cells. Collectively, these results indicate that curcumin serves a role in the epigenetic regulation of mTOR expression, and that mTOR downregulation is due to promoter hypermethylation, which may be associated with DNMT3a and DNMT3b upregulation. The results of the present study contribute towards improving the understanding of curcumin treatment in multiple myeloma and provide novel insights into the molecular mechanisms underlying the epigenetic regulation of mTOR.