8‐Br‐cAMP inhibits the transient expression of firefly luciferase

The genes for firefly luciferase and chloramphenicol acetyltransferase (CAT) were used as reporter genes to explore the activation of heterologous promoters by 8‐Br‐cAMP. Cells were transfected with a CAT gene/tyrosine hydroxylase promoter, which contains a cAMP response element. Extracts from cells treated with 8‐Br‐cAMP had 340% more enzyme activity than untreated cells. In contrast, treated cells transfected with a tyrosine hydroxylase/luciferase construct had 30% less activity than control cells. Simian virus and rous sarcoma virus promoters/luciferase constructs also had lower activities in cells treated with 8‐Br‐cAMP than untreated cells. The inhibition of luciferase enzyme activity by cAMP appears to be posttranscriptional since both luciferase and CAT RNA levels were similarly increased in cells treated with 8‐Br‐cAMP or 1‐methyl‐3‐isobutylmethylxanthine. The lower level of luciferase activity was not due to simple allosteric inhibition. We conclude that constructs using the firefly luciferase as a reporter gene are unsuitable for studying the effects of cAMP on the regulation of promoters.