Interindividual differences in hepatic expression of CYP3A4: Relationship to genetic polymorphism in the 5 '-upstream regulatory region

Cytochrome P450 3A4, the most abundant P450 form in human liver, exhibits a very broad substrate specificity and is of great importance for drug metab olism. The interindividual difference in the hepatic expression of CYP3A4 i s considerable. In order to investigate possible genetic factor(s) causing this variation, the rate of GP-hydroxylation of testosterone in human liver microsomes prepared from 46 different human liver samples was determined a nd the 5' upstream region (+10 to -490 bp) was sequenced from genomic DNA i solated from 39 of these livers. We found a 31-fold variation of the testos terone hydroxylase activity between the samples. However, a very high seque nce homology between the CYP3A4 5'-upstream regions sequenced from the 78 d ifferent alleles was found. In fact, only three variant nucleotide exchange s were identified, all causing a -290 A-->G mutation (CYP3A4-V) in a so cal led nifedipine specific element (NFSE). The importance of this element and the polymorphism was evaluated by gel shift analysis. Competition experimen ts revealed that the binding of nuclear proteins, although having lower aff inity to the CYP3A4-V form of the element, was unspecific in nature. In acc ordance, no influence of this polymorphism was seen on the microsomal testo sterone hydroxylase activity in vitro. It is concluded that the promoter re gion of CYP3A4 is highly conserved, the only polymorphism being in the NFSE , which however does not influence the enzyme expression in liver to a sign ificant degree. This casts doubt of a previously described relationship bet ween the CYP3A4-V allele and cancer in the prostate and leukaemia. (C) 1999 Academic Press.