The effect of ICER on screening methods involving CRE-mediated reporter gene expression

We describe a mechanism whereby increasing levels of cAMP in Chinese hamster ovary (CHO) and other cell lines lead to a significant repression in cAMP response element (CRE)-mediated luciferase reporter gene expression. This effect was shown to be mediated by a modulatory factor located downstream of cyclic AMP (cAMP), which displayed the temporal regulation pattern of an immediate early gene. The expression of this inducible cAMP early repressor (ICER) was shown to be coincident with the time and concentration dependency of the repression of CRE-mediated luciferase gene expression on the treatment of CHO cells with forskolin. Furthermore, this phenomenon was also observed in JEG and GH3 cell lines (both previously reported to express ICER), but not in COS-7 cells, which do not express ICER. These studies suggest that, in certain cell lines, expression of ICER can be induced at pharmacologically elevated cAMP levels, leading to a potent inhibition of CRE-mediated gene expression. We therefore conclude that screening methodologies employing such CRE-linked reporter genes (particularly in high-throughput screening assays) may produce false functional responses in certain cell lines. Moreover, such effects are likely to be exacerbated in screening assays in which receptors either are overexpressed or high concentrations of potent cAMP-elevating compounds are used.