Previous Article | Next Article ![]()
Mol. Cell. Biol., Jan 1998, 525-535, Vol 18, No. 1
CY Chang and A Puga
The ligand-activated aromatic hydrocarbon receptor (AHR) dimerizes with the
AHR nuclear translocator (ARNT) to form a functional complex that
transactivates expression of the cytochrome P-450 CYP1A1 gene and other
genes in the dioxin-inducible [Ah] gene battery. Previous work from this
laboratory has shown that the activity of the CYP1A1 enzyme negatively
regulates this process. To study the relationship between CYP1A1 activity
and Ah receptor activation we used CYP1A1-deficient mouse hepatoma c37
cells and CYP1A1- and AHR-deficient African green monkey kidney CV-1 cells.
Using gel mobility shift and luciferase reporter gene expression assays, we
found that c37 cells that had not been exposed to exogenous Ah receptor
ligands already contained transcriptionally active AHR-ARNT complexes, a
finding that we also observed in wild-type Hepa-1 cells treated with
Ellipticine, a CYP1A1 inhibitor. In CV-1 cells, transient expression of AHR
and ARNT leads to high levels of AHR-ARNT-dependent luciferase gene
expression even in the absence of an agonist. Using a green fluorescent
protein-tagged AHR, we showed that elevated reporter gene expression
correlates with constitutive nuclear localization of the AHR.
Transcriptional activation of the luciferase reporter gene observed in CV-1
cells is significantly decreased by (i) expression of a functional CYP1A1
enzyme, (ii) competition with chimeric or truncated AHR proteins containing
the AHR ligand-binding domain, and (iii) treatment with the AHR antagonist
alpha-naphthoflavone. These results suggest that a CYP1A1 substrate, which
accumulates in cells lacking CYP1A1 enzymatic activity, is an AHR ligand
responsible for endogenous activation of the Ah receptor.
Copyright © 1998, American Society for Microbiology
Constitutive activation of the aromatic hydrocarbon receptor
Department of Environmental Health, University of Cincinnati Medical Center, Ohio 45267-0056, USA.
This article has been cited by other articles:
Copyright © 2009 by the American Society for Microbiology. For an alternate route to Journals.ASM.org, visit: http://intl-journals.asm.org | More Info»