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1 on
Expression of Aryl Hydrocarbon Receptor and Genes of Ah
Gene Battery: Clues for Independent Down-Regulation in A549 Cells
Medical Institute of Environmental Hygiene,
Heinrich-Heine-University of Düsseldorf, Department of
Toxicology, 40225 Düsseldorf, Germany (O.D., R.S., C.V., J.A.),
and
Institute of Toxicology, University of Tübingen,
Wilhelmstra An inhibitory effect on both constitutive and inducible expression of
cytochrome P450 isoenzymes has been shown for different cytokines and
growth factors. We previously described an inhibition of
2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced
CYP1A1 mRNA and enzyme activity by transforming growth
factor-
e 56, 72074 Tübingen, Germany (P.M.)
1 (TGF-1) in human lung cancer
A549 cells. In the present study, we report that not only TCDD-induced
expression of CYP1A1 but also basal mRNA expression of
CYP1A1, CYP1B1, and aryl hydrocarbon receptor (AHR) was
down-regulated by TGF-1 in cells not treated with TCDD. In contrast, mRNA expression of the AHR partner protein Arnt (aryl hydrocarbon receptor nuclear translocator) was not influenced. Furthermore, TCDD-induced expression of CYP1B1 and NMO-1 was inhibited, and the IC50 values of 5-10 pM
TGF-1 were in the same range as observed for inhibition
of CYP1A1 and AHR mRNA expression. Transfection studies with a plasmid
containing a luciferase reporter gene under control of two
dioxin-responsive elements indicate an effect on AHR protein
expression. Results of time-course studies revealed a parallel
inhibition of AHR and CYP1 mRNA expression, indicating that
TGF-1 is a direct negative regulator of transcription of these genes. The treatment of cells with cycloheximide led to a
superinduction of TCDD-induced CYP1A1 and CYP1B1 mRNA expression and
abolished the inhibitory effect of TGF-1 on basal as
well as TCDD-induced CYP1 and AHR mRNA expression. TGF-1
seems not to influence the stability of AHR mRNA. The results suggest
that TGF-1 induces rapid transcription and translation
of an as-yet-unknown negative regulatory factor or factors that may
directly regulate expression of AHR and genes of Ah gene
battery.
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