Molecular Mechanisms Regulating Drug Metabolizing Enzyme Ontogeny

Abstract

The study objective was to evaluate changes in histone marks and the associated changes in chromatin structure as a mechanism controlling drug metabolizing enzyme ontogeny. The relative enrichment of histone marks was determined using chromatin immunoprecipitation with pooled chromatin isolated from 1st trimester primary human fetal or adult hepatocytes with quantitation by RT‐PCR. CYP3A4 expression increases at least 20‐fold between the third trimester and the adult (mean ± SD = 4.5 ± 2.1 versus 89.6 ± 64.0 pmol/mg microsomal protein, respectively). Evaluating the CYP3A4 CLEM4 domain located from position −11,420 to −10,883 and containing 3 USF1, 2 HNF4α, 1 HNF1α, and 1 AP1 elements revealed a 6‐ to 14‐fold enrichment of the H3K27me3 histone mark in fetal versus adult hepatocytes. In contrast, a 1.5‐ to 3‐fold enrichment of the H3K4me1 histone mark was observed in adult versus fetal hepatocytes. A 100‐fold difference in USF1 enrichment was observed in adult versus fetal hepatocytes. Similar results were observed for the presence of these same histone marks and the binding of C/EBPβ in the regulatory domain located between CYP3A4 position −5900 and −5700. Given the association of H3K27 and HeK4me1 with condensed and open chromatin, respectively, tThese data are consistent with changes in chromatin structure having a dominant role in regulating CYP3A4 ontogeny. Supported in part by PHS grant GM081344.