SELECTIVE COUPLING OF ALPHA(2)-ADRENERGIC RECEPTOR SUBTYPES TO CYCLIC-AMP-DEPENDENT REPORTER GENE-EXPRESSION IN TRANSIENTLY TRANSFECTED JEG-3 CELLS

A cAMP-dependent reporter gene has been used in transiently transfecte d human choriocarcinoma (JEG-3) cells to examine the second messenger coupling of the human alpha2-adrenergic receptor subtypes. The reporte r gene consists of a cAMP response element linked to the gene for chlo ramphenicol acetyltransferase (CAT). Plasmids encoding the alpha2-C10 (alpha2A), alpha2-C2 (alpha2B), or alpha2-C4 (alpha2C) receptor subtyp es were co-transfected with a plasmid containing the reporter gene, an d the ability of alpha2 receptor agonists to influence forskolin-stimu lated CAT expression was examined. For alpha2-C10, agonists had a biph asic effect on forskolin-stimulated CAT expression. Thus, low (nanomol ar) concentrations of agonist inhibited CAT expression by approximatel y 60%, whereas high (micromolar) concentrations reversed this inhibiti on and could even potentiate CAT expression by as much as 140%. A sign ificantly different pattern of coupling was observed for the other alp ha2 receptor subtypes. For alpha2-C4, agonists only inhibited forskoli n-stimulated CAT expression, whereas for alpha2-C2 only potentiation o f expression was seen. Each of these responses was specifically blocke d by alpha2- but not alpha1- or beta-adrenergic receptor antagonists. For alpha2-C4, the inhibition of forskolin-stimulated CAT expression w as prevented by pretreatment of the cells with pertussis toxin. This w as also true for the inhibition obtained with alpha2-C10. The potentia tion of CAT expression, however, was not prevented by pertussis toxin pretreatment in cells transfected with either alpha2-C2 or alpha2-C10. In this transient expression system, each alpha2-adrenergic receptor subtype had access to the same complement of G proteins, adenylyl cycl ase, and other second messengers. It would appear, therefore, that the potential for the activation of unique intracellular responses exists even among closely related receptor subtypes.