A cAMP-dependent reporter gene has been used in transiently transfected human choriocarcinoma (JEG-3) cells to examine the second messenger coupling of the human α2-adrenergic receptor subtypes. The reporter gene consists of a cAMP response element linked to the gene for chloramphenicol acetyltransferase (CAT). Plasmids encoding the α2-C10 (α(2A)), α2-C2 (α(2B)), or α2-C4 (α(2C)) receptor subtypes were co-transfected with a plasmid containing the reporter gene, and the ability of α2 receptor agonists to influence forskolin-stimulated CAT expression was examined. For α2-C10, agonists had a biphasic effect on forskolin-stimulated CAT expression. Thus, low (nanomolar) concentrations of agonist inhibited CAT expression by ~60%, whereas high (micromolar) concentrations reversed this inhibition and could even potentiate CAT expression by as much as 140%. A significantly different pattern of coupling was observed for the other α2 receptor subtypes. For α2-C4, agonists only inhibited forskolin-stimulated CAT expression, whereas for α2-C2 only potentiation of expression was seen. Each of these responses was specifically blocked by α2- but not α1- or β-adrenergic receptor antagonists. For α2-C4, the inhibition of forskolin- stimulated CAT expression was prevented by pretreatment of the cells with pertussis toxin. This was also true for the inhibition obtained with α2- C10. The potentiation of CAT expression, however, was not prevented by pertussis toxin pretreatment in cells transfected with either α2-C2 or α2-C10. In this transient expression system, each α2-adrenergic receptor subtype had access to the same complement of G proteins, adenylyl cyclase, 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.
|Original language||English (US)|
|Number of pages||8|
|State||Published - 1993|
ASJC Scopus subject areas
- Molecular Medicine