Regulation of the rat thyrotropin receptor gene by the methylation-sensitive transcription factor GA-binding protein.
The GA-binding protein (GABP), a transcription factor with a widespread tissue distribution, consists of two subunits, a and beta1, and acts as a potent positive regulator of various genes. The effect of GABP on transcription of the TSH receptor (TSHR) gene in rat FRTL-5 thyroid cells has now been investigated. Both deoxyribonuclease I footprint analysis and gel mobility-shift assays indicated that bacterially expressed glutathione S-transferase fusion proteins of GABP subunits bind to a region spanning nucleotides (nt) -116 to -80 of the TSHR gene. In gel mobility-shift assays, nuclear extracts of FRTL-5 cells and FRT cells yielded several specific bands with a probe comprising nt -116 to -80. Supershift assays with antibodies to GABPalpha and to GABPbeta1 showed that GABP was a component of the probe complexes formed by the nuclear extracts. Immunoblot analysis confirmed the presence of both GABP subunits in the nuclear extracts. A reporter gene construct containing the TSHR gene promoter was activated, in a dose-dependent manner, in FRTL-5 cells by cotransfection with constructs encoding both GABPalpha and GABPbeta1. Both GABP binding to and activation of the TSHR gene promoter were prevented by methylation of CpG sites at nt -93 and -85. These CpG sites were highly methylated (>82%) in FRT cells and completely demethylated in FRTL-5 cells, consistent with expression of the TSHR gene in the latter, but not the former. These results suggest that GABP regulates transcription of the TSHR gene in a methylation-dependent manner and that methylation of specific CpG sites and the methylation sensitivity of GABP contribute to the failure of FRT cells to express the endogenous TSHR gene.
Yokomori
N
N
Third Department of Internal Medicine, Yamanashi Medical University, Tamaho, Japan.
Tawata
M
M
Saito
T
T
Shimura
H
H
Onaya
T
T
D000818:Animals; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism; D001483:Base Sequence; D002457:Cell Extracts; D018899:CpG Islands; D019175:DNA Methylation; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D051268:GA-Binding Protein Transcription Factor; D005786:Gene Expression Regulation; D008969:Molecular Sequence Data; D051381:Rats; D011989:Receptors, Thyrotropin / Q000235:genetics* / Q000378:metabolism; D011994:Recombinant Proteins / Q000235:genetics / Q000378:metabolism; D013961:Thyroid Gland / Q000166:cytology; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism*; D014158:Transcription, Genetic
D001426:Bacterial Proteins; D002457:Cell Extracts; D004268:DNA-Binding Proteins; D051268:GA-Binding Protein Transcription Factor; D011989:Receptors, Thyrotropin; D011994:Recombinant Proteins; D014157:Transcription Factors