Transcriptional down-regulation of the human alpha 2C-Adrenergic receptor by cAMP

The heterologous regulation of the alpha 2C-adrenergic receptor ( alpha 2C- AR) was investigated in the HepG2 cell line. Binding of [H-3]MK912 (alpha 2 -antagonist) to membranes from cells submitted to various treatments showed that exposure to insulin, phorbol 12-myristate 13-acetate, or dexamethason e did not affect receptor density. On the other hand, treatment with forsko lin resulted in a large reduction of alpha 2C-AR number. The effect of fors kolin was mimicked by 8-br-cAMP and was abolished by the protein kinase A i nhibitor, H89. The action of cAMP was slow (t(1/2) = 23 h), dose-dependent, and additive to the receptor down-regulation elicited by the alpha 2-agoni st, UK14304. Furthermore, the diminution of receptor was not caused by an i ncreased rate of its degradation but resulted from a decrease in the steady state amounts of alpha 2C4-mRNA. As assessed by experiments in the presenc e of actinomycin D, the stability of alpha 2C4-mRNA was not affected by 8-b r-cAMP or forskolin. By contrast, the activity of a luciferase construct co ntaining the entire promoter region of the alpha 2C4 gene (1.9 kilobase pai rs) was inhibited, indicating that the primary mechanism of action of the t wo compounds is at the transcriptional level. Deletions in the 5'-end of th is construct showed that the elements responsible for cAMP responsiveness l ie within a 242-base-pair fragment of the gene promoter (nucleotides -236/6 relative to transcription start). Band-shift experiments indicated that n uclear factors bind to this region in a cAMP-dependent manner. The determin ation of the actual cis- and trans-acting elements involved will be the obj ect of future investigation, but the present study provides evidence for tr anscriptional regulation of human alpha 2C-AR by cAMP.