CAMP-RESPONSIVE ELEMENT-MEDIATED REGULATION OF THE GENE-TRANSCRIPTIONOF THE ALPHA-1B ADRENERGIC-RECEPTOR BY THYROTROPIN

To elucidate the molecular mechanism of the stimulatory effect of thyr otropin on the gene regulation of alpha 1B adrenergic receptor in func tioning rat thyroid (FRTL-5) cells, we established a competitive rever se-transcriptase (RT) polymerase chain reaction (PCR) and nuclear run- off assay to quantify changes in mRNA levels and transcription rates. A binding assay showed that FRTL-5 cells predominantly expressed alpha 1B adrenergic receptor and that thyrotropin increased its expression sevenfold. By means of RT-PCR, we found that thyrotropin induced an 11 -fold increase in alpha 1B receptor mRNA abundance. The nuclear run-of f assay demonstrated that thyrotropin caused a ninefold increase at th e gene transcriptional level, which occurred in the presence of the pr otein synthesis inhibitor cycloheximide. The half-life of the alpha 1B receptor mRNA in cells incubated with thyrotropin for 1 h increased 1 .5-fold but returned to the original value after 12 h. Dibutyryl cAMP and forskolin mimicked the stimulatory effects of thyrotropin on the g ene transcriptional level. The 5'-flanking region of the rat alpha 1B receptor gene contained a putative cAMP responsive element (CRE) at nu cleotide -438 relative to the translation start site. The promoter ana lysis using the reporter gene indicated that the CRE motif confers the cAMP sensitivity to the transcription of the rat alpha 1B receptor ge ne. These results demonstrated that a CRE-mediated mechanism is involv ed in the transcriptional regulation of the alpha 1B receptor gene by thyrotropin without requiring new protein synthesis.