UC Davis

The aryl hydrocarbon receptor (AhR) is a transcription factor that is activated by specific agonists, such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), inducing AhR target gene expression. The most extensively studied AhR target gene, CYP1A1, can be significantly induced via cell suspension without binding of exogenous ligand. It can also be superinduced in surface culture by cycloheximide (CHX), a protein synthesis inhibitor, to inhibit degradation of TCDD-induced AhR, thereby maintaining the AhR. The mechanism of an observed sensitization is unknown, but previous study suggested that the basis for sensitization is through epigenetic modification of chromatin. To better understand the effects of suspension, CHX, and epigenetic alterations, we examined rat CYP1A1 and three more AhR target genes: CYP1B1, COX2, and ALDH3A1. We first studied the effect of CHX on the other genes in the AhR domain and whether suspension can also achieve superinduction. Only COX2 was superinduced and suspension did not superinduce any genes. We also examined whether epigenetic modifiers, specifically, histone deacetylase inhibitors (HDACi) CAY10398 and lysine-specific demethylase 1 (LSD1) inhibitor ORY1001, can replace CHX in CYP1A1 induction and restore induction in other genes the same way as CHX. Both epigenetic modifiers superinduced CYP1A1 and ALDH3A1. We hypothesize that superinduction depends on chromatin modification, but the relevant modifier enzymes have not been identified yet.