Cytochrome P 450 and Glutathione S-Transferase mRNA Expression in Human Fetal Liver Hematopoietic Stem Cells

During fetal development, the liver serves as the primary hematopoietic organ in which hematopoietic stem cells (HSC) capable of initiating long-term hematopoiesis comprise a large proportion of the hepatic cell population. Although HSC are potential targets for transplacental chemicals, little is known regarding their xenobiotic biotransformation ability. We quantitated the steady-state mRNA expression of six cytochrome P450 (P450) and 11 glutathione Stransferase (GST) isoforms in CD34 -selected HSC isolated from second trimester human fetal liver donors, genotyped donors for polymorphic hGSTM1 and hGSTT1 status, and analyzed gene expression in HSC relative to total liver from donors of similar gestational ages. Several P450 isoforms, including CYP1A1, CYP2E1, CYP3A4, and CYP3A5, were expressed at low levels in HSC (relative mRNA expression CYP3A5 > CYP1A1 > CYP2E1 > CYP3A4). CYP1A2 and CYP3A7 were not detected in HSC. The CYP3A4/5 mRNA expression in HSC was accompanied by detectable CYP3A protein and low midazolam oxidation activity. Several GST isoforms, including hGSTM1, hGSTM2, hGSTM4, and hGSTP1, were significantly higher in HSC as compared with total fetal liver. With the exception of hGSTA4, alpha class GST were not detected in HSC. GST expression in HSC was accompanied by substantial GST catalytic activity toward 1-chloro-2,4-dinitrobenzene. In summary, our data indicate that fetal liver CD34 -derived HSC constitutively express several P450 isoforms at low levels relative to total hepatic cell populations but have a higher capacity for GST conjugation reactions through mu and pi class isoforms. The functional ramifications of these observations are discussed relative to the sensitivity of human fetal HSC to transplacental chemical injury. A number of environmental chemicals and drugs have the ability to cross the placental barrier during pregnancy and interact with fetal cell targets (Shu, 1997; Alexander et al., 2001; Woodruff et al., 2004). Because the fetal liver receives a substantial proportion of maternal blood flow during pregnancy, it can be an important target for the exposure to agents that elicit cellular toxicity (Hakkola et al., 1998). However, unlike in adults, the developing fetal liver is not dominated by the presence of hepatocytes but instead by the presence of hematopoietic cells and precursors, and thus serves as the major site of multilineage hematopoiesis (Morrison et al., 1995). Accordingly, a large proportion of the hepatic fetal cell population includes hematopoietic progenitors and hematopoietic stem cells (HSC), which express primitive markers such as CD34 , CD38 , and CD33 , and that can be cell targets for maternally transferred compounds (Sancewicz-Pach et al., 1997; Moneypenny and Gallagher, 2005). It has been proposed that fetal liver HSC injury during pregnancy may underlie the development of certain hematopoietic disorders that develop after birth (Tavassoli, 1991; Shu, 1997; Alexander et al., 2001; Woodruff et al., 2004). Our laboratory has shown that CD34 HSC cultured from human fetal liver are extremely sensitive to the toxicity of certain transplacental chemicals or their metabolites, including certain pesticides and anticancer drugs (Moneypenny et al., 2006), and products of oxidative stress (Moneypenny and Gallagher, 2005). In the case of 4-hydroxynonenal, a reactive , -unsaturated aldehyde produced in vivo and in utero, the sensitivity of fetal liver HSC greatly exceeds that of other cell types (Moneypenny and Gallagher, 2005). Furthermore, we have recently shown that exposure to low levels of etoposide, a potent DNA topoisomerase II inhibitor, can cause DNA damage and genetic rearrangements in the mixed lineage leukemia (MLL) gene in cultured human fetal liver HSC (Moneypenny et al., 2006). This observation is of particular significance because MLL rearrangements are signature lesions in the infant acute leukemias that have been associated with stem cell injury during pregnancy (Armstrong et al., 2002). Collectively, these observations suggest that human fetal HSC may be relevant targets for transplacental chemicals or their metabolites dur-