Regulatory mechanism of gene expression of peptide (SRIF and TRH) receptors

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Estrogen (E_2) not only up-regulates the binding acitivity of Somatostatin (SRIF) receptors but also increases transcriptionally Thyrotropin-releasing hormone (TRH) receptors in the pitutary cells.The aim of this study is to determine the subtypes of SRIF receptors regulated by E_2 to elucidate the action mechanism of E_2 on the binding activity of these peptide hormone receptors at the transcriptional level, and to know the cis-elements of TRH receptor gene at 5'flanking region.The results were ; (1) E_2 up-regulated mRNA levels of SRIF recetor subtypes (SSTR1 and SSTR2) in different ways in GH_3 cells, which occurred without ongoing protein synthesis.(2) Competitive reverse transcription-polymerase chain reaction (RT-PCR) used to obtain the quantitative values of mRNA levels of 5 SRIF receptor subtypes in the pituitary primary clutured cells revealed that large amounts of SSTR2 and SSTR3 mRNA were expressed inprimary cultured cells, whereas SSTR1 and SSTR2 mRNA in GH3 cells.In primary clutured cells E_2 primarily increased the mRNA levels of SSTR2 and SSTR3, and affected weakly SSTR1 mRNA level.(3) In an attempt to understand the transciptional control of pituitary peptide receptors by E_2, we have determined the nucleotide sequence of approximately 1.1kb of 5'flanking region of the TRH receptor.Although no typical TATA box was identified, a number of promotor elements was identified, including GCF,AP1, AP2, half GRE,half TRE consensus sequences.The absence of ERE consensus sequence in the 5'flanking region of the receptor suggests that E_2 effect is mediated by the AP1 site possibly as a result of the cooperative action of E_2 receptor and a certain type of transcription factors.