CYP3A4 substrate activation varies amongst different animal species.
Certain ligands activate human PXR, which promotes CYP3A4 transcription, while showing no activation in other species.
For instance, mouse PXR is not activated by rifampicin and human PXR is not activated by pregnenalone 16α-carbonitrile[24] In order to facilitate study of CYP3A4 functional pathways in vivo, mouse strains have been developed using transgenes in order to produce null/human CYP3A4 and PXR crosses.
Although humanized hCYP3A4 mice successfully expressed the enzyme in their intestinal tract, low levels of hCYP3A4 were found in the liver.
This effect has been attributed to CYP3A4 regulation by the growth hormone signal transduction pathway.
In addition to providing an in vivo model, humanized CYP3A4 mice (hCYP3A4) have been used to further emphasize gender differences in CYP3A4 activity.