Abstract
Normal thyroid tissue displays a prevalent expression of ERβ than ERα, which drastically turns upside down in the initiation and progression of papillary thyroid cancer (PTC). The underlying molecular mechanism of this phenomenon remains unclear. Here, we demonstrated that ERα and ERβ were coexpressed in human thyroid tissues and cells. ERα mRNA (A-1) and ERβ mRNA (0N-1), transcribed from Promoter A of ERα gene and Promoter 0N of ERβ gene, respectively, were the major mRNA isoforms which mainly contributed to total ERα mRNA and total ERβ mRNA in human thyroid-derived cell lines and tissues. The expression levels of ERα mRNA (A-1) and total ERα mRNA were gradually increased, and those of ERβ mRNA (0N-1) and total ERβ mRNA were decreased by degree in the initiation and progression of PTC. No aberrant DNA methylation of ERα 5'-untranslated region was involved in its up-regulation; however, aberrant DNA methylation in Promoter 0N and Exon 0N of ERβ gene was found to be involved in its down-regulation in the initiation and progression of PTC. ERβ can repress ERα gene transcription via recruitment of NCoR and displacement of RNA polymerase II at the Sp1 site in ERα Promoter A-specific region in thyroid-derived cells. It is suggested that DNA methylation of CpG islands in Promoter 0N and Exon 0N of ERβ gene leads to a decreased ERβ gene expression, which attenuates its inhibitory effect on ERα gene transcription and results in an increased ERα gene expression, cell proliferation, initiation, and progression of PTC.