Regulation of Cyp1a1 induction by dioxin as a function of cell cycle phase

Analyses of CYP1A1 mRNA were used to monitor the responsiveness of murine hepatoma 1c1c7 and human monocytic U937 cells in different phases of the cell cycle to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Concentrations of TCDD capable of inducing CYP1A1 were not cytostatic to either cell line. Steady-state CYP1A1 mRNA contents were reduced (45-90%) in TCDD-treated cultures arrested in G₂/M as a consequence of exposure to microtubule disrupters (Colcemid, estramustine, vinblastine) or the microtubule stabilizer Taxol, relative to TCDD-treated asynchronous 1c1c7 cultures. The accumulation of mRNAs corresponding to Nmo1, another TCDD-inducible gene of the Ah battery, was also reduced in TCDD-treated G₂/M cultures. Quantitative reverse transcriptase-polymerase chain reaction analyses of CYP1A1 heterogeneous nuclear RNA (hnRNA) revealed that Cyp1a1 transcription was suppressed in G₂/M cells. This suppression reflected neither changes in the relative content of the proteins comprising the aryl hydrocarbon receptor (AHR) complex nor a suppression of AHR activation and translocation to the nucleus. Release of 1c1c7 cultures arrested in G₂/M restored TCDD responsiveness. Centrifugal elutriation of TCDD-treated asynchronously growing U937 cells was used to prepare populations of cells in specific phases of the cell cycle. Within 3 h of TCDD exposure late G₁/early S phase cells had CYP1A1 mRNA contents ∼1.4- and 3-fold higher than the contents of asynchronous/early G₁ and G₂/M cultures, respectively. These studies suggest that the transcriptional activation of members of the Ah battery by TCDD is cell cycle-dependent, and markedly suppressed in G₂/M cells.