Regulation of the rat UGT1A6 by glucocorticoids involves a cryptic glucocorticoid response element

Area of Science:

• Biochemistry
• Molecular Biology
• Pharmacology

Background:

• Glucocorticoids and polycyclic aromatic hydrocarbons (PAHs) are known to influence the expression of the UGT1A6 enzyme.
• Understanding the regulatory mechanisms of UGT1A6 induction is crucial for comprehending drug metabolism and detoxification pathways.

Purpose of the Study:

• To investigate the role of the glucocorticoid receptor (GR) in the induction of UGT1A6 by glucocorticoids.
• To elucidate the interaction between GR and the aryl hydrocarbon (Ah) receptor in regulating UGT1A6 gene expression.

Main Methods:

• Luciferase reporter assays were performed in HepG2 cells using a UGT1A6 5'-flanking region construct.
• Transfection assays involved co-expression of the glucocorticoid receptor (GR) and treatment with dexamethasone and PAHs.
• Site-directed mutagenesis and 5'-deletion analysis were employed to identify key regulatory regions.

Main Results:

• Dexamethasone (a glucocorticoid) increased basal UGT1A6 activity, while PAHs caused a more significant induction.
• The presence of dexamethasone only marginally increased PAH-dependent UGT1A6 induction, suggesting receptor interaction.
• The aryl hydrocarbon receptor response element (AhRE) region was critical for both PAH and glucocorticoid-dependent induction, with overlapping sequences for both response elements.
• Mutation of the AhRE reduced PAH induction and enhanced glucocorticoid induction, indicating cross-talk between the receptors.
• A putative glucocorticoid response element (GRE) was identified, but its mutation did not abolish glucocorticoid-dependent induction, suggesting alternative regulatory mechanisms.

Conclusions:

• The UGT1A6 gene's regulatory region contains overlapping elements for both glucocorticoid receptor and aryl hydrocarbon receptor binding.
• Interactions between these receptors, particularly at the AhRE, modulate UGT1A6 gene expression.
• A non-canonical mechanism may contribute to glucocorticoid-mediated induction of UGT1A6.