Metallothionein 1G functions as a tumor suppressor in thyroid cancer through modulating the PI3K/Akt signaling pathway

MT1G inactivation mediated by promoter methylation has been reported in thyroid cancer. However, the role of MT1G in thyroid carcinogenesis remains unclear. The aim of this study is to examine the biological functions and related molecular mechanisms of MT1G in thyroid cancer. Methylation-specific PCR (MSP) was performed to analyze promoter methylation of MT1G and its relationship with clinicopathological characteristics of papillary thyroid cancer (PTC) patients. Conventional and real-time quantitative RT-PCR assays were used to evaluate mRNA expression. The functions of ectopic MT1G expression were determined by cell proliferation and colony formation, cell cycle and apoptosis, as well as cell migration and invasion assays. MT1G expression was frequently silenced or down-regulated in thyroid cancer cell lines, and was also significantly decreased in primary thyroid cancer tissues compared with non-malignant thyroid tissues. Promoter methylation, along with histone modification, contributes to MT1G inactivation in thyroid tumorigenesis. Moreover, our data showed that MT1G hypermethylation was significantly positively associated with lymph node metastasis in PTC patients. Importantly, restoring MT1G expression in thyroid cancer cells dramatically suppressed cell growth and invasiveness, and induced cell cycle arrest and apoptosis through inhibiting phosphorylation of Akt and Rb. We have for the first time revealed that MT1G appears to be functional tumor suppressor involved in thyroid carcinogenesis mainly through modulating the phosphatidylinositol-3-kinase (PI3K)/Akt pathway and partially through regulating the activity of Rb/E2F pathway in this study