ession in Africans, most of which express CYP3A5 in the kidney, perhaps at the expense of an increased risk of salt-dependent hypertension. Taken together, the PXR-independent CYP3A5 expression outside the liver and small intestine may have evolved in primates to employ this enzyme in endobiotic homeostasis protected against potentially deleterious effects of xenobiotic-driven induction. To our knowledge, this is a first evolutionary description of the mechanism uncoupling the inducible and constitutive expression in a major order Fenoterol (hydrobromide) detoxifying enzyme. Similar mechanisms may have evolved for other detoxifying proteins, many of which metabolize endobiotics. Although this work focuses on CYP3A5, some of our observations illuminate the regulation of CYP3A4, which is expressed concomitantly with CYP3A5 in the liver and small Tissue-Specific Expression of CYP3A5 and CYP3A4 intestine. Considering the ubiquitous expression of YY1, the presence of a transcriptionally repressive YY1 element in the CYP3A4 promoter seemed to be at odds with the expression of CYP3A4 in these organs. Subsequent experiments designed to resolve this contradiction suggest that the inhibitory effect of 
YY1 on CYP3A4 promoter activity is overridden, at least in smallintestinal cells, by the concerted action of one trans- and one cisacting factor. We have identified these factors using MDCK.2 cells, which normally do not support CYP3A4 expression, due to the inhibitory effect of the YY1 on its promoter. Through cotransfection of the transcriptional CYP3A regulator and xenobiotic sensor PXR, we conferred onto these cells a capability to express CYP3A4. PXR is normally expressed in the small intestine, but not in the kidney. This suggests that the expression of PXR, acting in trans, is an indispensable determinant of the CYP3A4 expression in organs such as small intestine. Besides PXR, the expression of CYP3A4 in MDCK.2 cells required the presence of the PXR-responsive, cis-acting element XREM, located in the distal part of the CYP3A4 promoter. Together with the proximal ER6 and the far-distal constitutive liver enhancer module, XREM represents the original scheme of CYP3A regulation by nuclear receptors such as PXR in placental mammals. The need to offset the inhibitory effect of YY1 may have been the force driving both the conservation of XREM and the origin of novel PXR-responsive elements outside XREM recently described in the CYP3A4 gene lineage. Conversely, the loss of XREM from the CYP3A5 gene lineage is consistent with the reduced pressure to maintain XREM, conferred by the loss of the transcriptionally repressive YY1 binding site. In support of this interpretation, the losses of the YY1 binding element and of XREM from the CYP3A5 gene lineage occurred simultaneously in evolutionary terms, since they are restricted to Haplorrhini. The XREM-mediated, CYP3A4 expression-promoting effect of PXR may have been additionally facilitated by the apparent attenuation of the YY1 inhibitory effect. This attenuation is conferred by the mutation of the YY1 consensus site core sequence PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/22183349 CAT.CAC, which is present in all Haplorrhini CYP3A genes containing this element, except the pseudogene CYP3A43. The importance of this mutation was suggested by the diminished score values and confirmed by mutagenesis. The results of this latter experiment suggest that the sequence change in the YY1 core sequence may contribute to the high expression level of CYP3A4 in humans. This mutation may contribute t