n the prostate has been reported to be mediated by a promoter element binding the androgen receptor. No PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/22183719 comparable investigations have been reported for the kidney which, somewhat surprisingly, exhibits extremely low or nondetectable expression of PXR transcripts. Likewise, presently there is no data on CYP3A5 induction in organs other than liver and small intestine. Therefore, we investigated the determinants of the CYP3A5 expression, initially concentrating on the kidney as a model organ. To this end, we first established a two-cell line model reflecting the expression relationships of CYP3A4 and CYP3A5 in the kidney and small intestine in vivo. Our data demonstrate that the CYP3A5 expression in renal cells was enabled by the loss of a suppressing yin yang 1 -binding site from the CYP3A5 promoter. This allowed for a renal, but in all likelihood also adrenal and pulmonary CYP3A5 expression insensitive to PXR induction, as BIBW 2992 confirmed in CYP3A5 transgenic mice. The YY1 element is retained in the CYP3A4 promoter, but its effect is abrogated by PXR acting on response elements such as the xenobiotic-responsive enhancer module. The differential organ expression and induction of CYP3A4 and CYP3A5 results thus from the loss of the YY1 binding element from the CYP3A5 promoter, acting in concert with the differential organ expression of PXR, and with the higher accumulation of PXR response elements in the CYP3A4 promoter. Results Evaluation of CYP3A5 and CYP3A4 proximal promoter activities in renal and intestinal cells The conservation of the primate CYP3A5 and CYP3A4 promoters is limited to their most proximal parts. We investigated if these parts are sufficient to confer the previously reported differential expression of these genes in renal cells. To this end we applied plasmids expressing firefly luciferase under the control of CYP3A4 and CYP3A5 proximal promoter fragments of comparable lengths of 374 and 370 bp, respectively. These plasmids were transiently transfected into kidneyderived cell line MDCK.2. These cells exhibit many characteristics of tubular and collecting duct cells, which are the site of CYP3A5 expression in humans. The activity of the CYP3A5 promoter was robust, whereas that of the CYP3A4 promoter was 31-fold lower. On the other hand, the activities of these promoters were similar in the small intestinederived cell line LS174T. These findings were fully compatible with the expression relationships between CYP3A4 and CYP3A5 in the human kidney and in the small intestine in vivo. Therefore, these cell lines were taken together as a model for more detailed investigations of the determinants of the differential renal and intestinal CYP3A4 and CYP3A5 expression. Function of the 57 bp difference between the CYP3A4 and CYP3A5 promoters The most prominent difference between the proximal CYP3A5 and CYP3A4 promoter sequences is the presence of a 57 bp fragment in CYP3A4 which is absent from CYP3A5. This region is Tissue-Specific Expression 
of CYP3A5 and CYP3A4 localized upstream of the basic regulatory elements: the CCAATbox, the basic transcription element, the TATA-box, and downstream of the everted repeat separated by 6 base pairs and the nuclear factor 1 enhancer elements, which have been characterized in previous studies. Except NF1, all these elements are conserved between the CYP3A4 and CYP3A5 promoters. To determine the role of the 57 bp fragment in the absence of CYP3A4 expression in renal cells, it was deleted from the prox
