Received for publication August 20, 2004.
Revised December 28, 2004.
Accepted for publication December 28, 2004.

Arsenite Inhibition of CYP1A1 Induction by TCDD is Independent of Cell Cycle Arrest

Abstract

We show here that arsenite (As³⁺) elicits multiple effects on gene control, such as the interruption of cell cycle control by initiating G2/M arrest as well as inhibiting the Ah receptor mediated TCDD inducible expression of CYP1A1. This raises the question as to whether As³⁺ is selectively inhibiting TCDD induction of CYP1A1 independent of cell cycle control. As³⁺ stimulated a concentration-dependent increase in G2/M phase arrest that was detected at 12.5 µM As³⁺. However, cotreatment of HepG2 cells with TCDD and concentrations of As³⁺ as low as 0.5 µM stimulated a pronounced decrease in the induction of CYP1A1 dependent EROD activity and protein, indicating that the inhibition of CYP1A1 induction by As³⁺ was considerably more sensitive than As³⁺ initiated cell cycle arrest. Low concentrations of As³⁺ also initiate a dose dependent reduction in TCDD induced mouse Cyp1a1 as well as human CYP1A1 in primary hepatocytes cultured from transgenic CYP1A1N^+/- mice. Since primary hepatocytes in culture are quiescent, these results indicate that the actions of As³⁺ on TCDD initiated induction of CYP1A1 are independent of cell cycle control. As³⁺ does not impact on Ah receptor function as evaluated by nuclear transport and binding to XRE sequences, but does reduce TCDD induced CYP1A1 mRNA, a property that is concordant with RNA polymerase II association to the gene and the reduction in transcriptional heteronuclear RNA. We conclude from these studies that interruption of CYP1A1 induced transcription by As³⁺ is not dependent upon cell cycle arrest.

Key words: Transcriptional coactivators, Promoter analysis, Cytochrome P450, Regulation - transcriptional, Ah receptor, Metals and chelators