Tm. Pereira et al., IDENTIFICATION OF A FUNCTIONAL GLUCOCORTICOID RESPONSE ELEMENT IN THECYP3A1 IGC2 GENE/, DNA and cell biology, 17(1), 1998, pp. 39-49
The rat CYP3A subfamily of cytochrome P450 consists of steroid-and dru
g-metabolizing enzymes inducible by pregnenolone 16 alpha-carbonitrile
and by supra-physiological doses of dexamethasone. The induction of C
YP3A by dexamethasone has been proposed to be mediated by a mechanism
distinct from the glucocorticoid receptor mediated response. However,
a synergistic induction of CYP3A has been observed with physiological
doses of glucocorticoids and other CYP3A inducers. We have identified
the presence of a glucocorticoid-responsive element in the CYP3A1/IGC2
gene that mediates the induction with physiological doses of glucocor
ticoids. A 219-bp dexamethasone responsive fragment of the CYP3A1/IGC2
gene localized at -2100/-1882 bp upstream of the transcription initia
tion site was identified in transfection experiments with HepG2 cells.
Maximum induction was achieved with 50-100 nM dexamethasone. DNase I
footprinting analysis revealed two glucocorticoid receptor-protected s
equences in the 5' flank of the CYP3A1/IGC2 gene. Point mutations in f
ootprint I (-1982/-1960-bp) completely abolished binding and transcrip
tion activation whereas a mutation in footprint II (-2001/-1986-bp) on
ly decreased the binding and had no effect on transcription activation
. These results led to the conclusion that the glucocorticoid response
element present in footprint I mediated the dexamethasone response in
transfection experiments with HepG2 cells. Pregnenolone 16 alpha-carb
onitrile failed to induce any transcriptional effect mediated by this
response element in the HepG2 cells.