Fig. 5. 5-HT potentiates induction of AhR target genes by ligands which are CYP1A1 substrates. Caco-2 cells were plated at low density and allowed to differentiate for 10-14 d in medium containing 10% serum before treatments were performed. (A and B) Cells were treated with vehicle (veh) or 5 nM FICZ in the presence of vehicle, 5-HT (10 μM), or ɑ-NF (1 μM) for 12 h or 24 h in TRP-free and serum-free medium. Data represent the relative expression of CYP1A1 (A) or CYP1B1 (B) mRNA quantified by qPCR as compared to time-matched vehicle cells (n = 3). (C) Cells were also treated with vehicle or 5 nM FICZ in the presence of vehicle, 5-HT (5 μM), ɑ-NF (5 μM), or 5-HT + ɑ-NF in combination (both 5 μM) for 16 h in TRP-free and serum-free medium (n = 4). Data analyzed by 1-way ANOVA followed by Tukey's multiple comparisons test. (D and E) Cells were treated with vehicle (veh), FICZ (10 nM), or TCDD (10 nM) for 24 h in the presence of absence of 5-HT (1 - 1000 μM) in serum-free and TRP-free medium. (D) Relative CYP1A1 mRNA induction after FICZ or TCDD alone is expressed as fold-change over vehicle and (E) CYP1A1 mRNA expression in the presence of FICZ or TCDD with increasing concentrations of 5-HT expressed as percentage of induction without 5-HT. CYP1A1 mRNA was quantified by qPCR (n = 3-4). Data analyzed by Student's unpaired t-test compared to vehicle treated cells (D) or 1-way ANOVA followed by Dunnett's multiple comparisons test (E). *P<0.05, **P<0.01, ***P<0.001, ****P<0.0001. n.s. = not significant.