Promoter methylation and downregulation of SLC22A18 are associated with the development and progression of human glioma

Background: Downregulation of the putative tumor suppressor gene SLC22A18 has been reported in a number of human cancers. The aim of this study was to investigate the relationship between SLC22A18 downregulation, promoter methylation and the development and progression of human glioma. Method: SLC22A18 expression and promoter methylation was examined in human gliomas and the adjacent normal tissues. U251 glioma cells stably overexpressing SLC22A18 were generated to investigate the effect of SLC22A18 on cell growth and adherence in vitro using the methyl thiazole tetrazolium assay. Apoptosis was quantified using flow cytometry and the growth of SLC22A18 overexpressing U251 cells was measured in an in vivo xenograft model. Results: SLC22A18 protein expression is significantly decreased in human gliomas compared to the adjacent normal brain tissues. SLC22A18 protein expression is significantly lower in gliomas which recurred within six months after surgery than gliomas which did not recur within six months. SLC22A18 promoter methylation was detected in 50% of the gliomas, but not in the adjacent normal tissues of any patient. SLC22A18 expression was significantly decreased in gliomas with SLC22A18 promoter methylation, compared to gliomas without methylation. The SLC22A18 promoter is methylated in U251 cells and treatment with the demethylating agent 5-aza-2-deoxycytidine increased SLC22A18 expression and reduced cell proliferation. Stable overexpression of SLC22A18 inhibited growth and adherence, induced apoptosis in vitro and reduced in vivo tumor growth of U251 cells. Conclusion: SLC22A18 downregulation via promoter methylation is associated with the development and progression of glioma, suggesting that SLC22A18 is an important tumor suppressor in glioma.