Pitfall of an internal control plasmid: response of Renilla luciferase (pRL-TK) plasmid to dihydrotestosterone and dexamethasone.

The thymidine kinase promoter-Renilla luciferase reporter plasmid (pRL-TK) is commonly used as a control for transfection efficiency in the Dual-Luciferase Reporter Assay System. While investigating hormone response elements in the promoters of the androgen-dependent, epididymis-specific EP2 gene, we found that hormone treatment affected the luciferase activity of pRL-TK-transfected cells. In African Green Monkey Kidney (CV-1) cells, cotransfected transiently with a hormone-responsive promoter-firefly luciferase reporter plasmid and with pRL-TK, Renilla luciferase activity increased in response to dihydrotestosterone (DHT) and decreased in response to dexamethasone (DEX). When a thromboxane synthase promoter Renilla luciferase plasmid (pRL-TS) was used in place of pRL-TK, Renilla luciferase activity remained constant in CV-1 cells treated with DHT but decreased in CV-1 cells treated with DEX. In transfection studies, internal control plasmid expression in response to treatment must be carefully monitored to ensure proper interpretation of normalized results.