Differences in cytochrome p450 enzyme expression and activity in fetal and adult tissues

Abstract

Introduction: Human cytochrome p450 (CYP) enzyme expression and activity is lower in the fetus as compared to the adult; however, limited quantitative data exists regarding the specific differences in magnitude or the degree of inducibility due to environmental factors.

Methods: We utilized a combination of in silico- and molecular-based approaches to profile and compare CYP expression/activity in human adult liver and fetal tissues. Using public datasets, we evaluated human CYP expression between: 1) placenta vs. adult livers; 2) fetal vs. adult livers; or 3) five compartments of the human placenta. We generated new experimental data, characterizing expression levels of nine CYPs in placenta/fetal liver vs. adult liver. In a subset of samples, we evaluated CYP3A4 activity. Finally, we summarized evidence of human fetal CYP expression/activity and environmental exposures during pregnancy.

Results: In silico, CYPs were predominately expressed at higher levels in the adult liver vs. fetal tissues, with a few noted exceptions. Sixty percent of CYP enzymes were expressed at nominal levels in the placenta. In wet-lab analyses, we observed significant CYP-specific differences in expression/activity between adult and fetal tissues; CYP2E1 and -3A4 were expressed significantly lower in fetal vs. adult livers, while CYP2J2 levels were similar.

Discussion: We provide a qualitative review of the expression of the CYP enzyme family in critical sites of xenobiotic distribution during human pregnancy and novel quantitative data regarding fetal CYP expression and activity during mid-gestation. Data outputs may be a resource for modeling predictions of chemical distribution and sensitivity.

Keywords: Cytochrome p450; Enzyme; Expression; Fetal; Liver; Placenta; Transcriptome.

Figure 1:. Routes of exposure and points of metabolism during human pregnancy.

Compounds are transferred from the mother to the embryo/fetus dependent on several factors such as the inherent properties of the compound, gestational age, and exposure route. Compound exposures occurring via oral exposure are absorbed through the intestinal tract and transported via portal-vein circulation to the liver, undergo first-pass metabolism, and then, enter the systemic circulation. Inhalation or dermal exposures bypass this initial step. In the earliest periods of pregnancy, compounds passively transfer from the mother to the placenta and the embryo proper. Later on in pregnancy, the embryo/fetus relies on the maternal blood supply and active transport. In humans, maternal and fetal blood do not coalesce. The placenta serves as the junction between the two units and compounds must pass through several barriers to reach the embryo/fetus.

Figure 2:. Global CYP expression profiles in human adult and fetal tissues.

Relative CYP expression (log 2 scale) in human 1^st trimester fetal and adult livers (A; GSE61279;[15]); or human term placenta and adult livers (E-MTAB-1733; B). CYPs in bold were identified to be differentially expressed between the two compartments (p<0.05; absolute FC > 1, log2). The average fold change (FC) difference between adult and fetal compartments is displayed to the right of each clustering heatmap. Expression levels of additional genes, include: X Inactive Specific Transcript (XIST), a transcript expressed higher in males versus females; Alpha-fetoprotein (AFP),a marker of liver immaturity; albumin (ALB), a marker of liver maturity; and Glycoprotein Hormones, Alpha Polypeptide (CGA), a placental hormone.

Figure 3:. Global CYP enzyme expression profiles of human placental regions.

Compartment-specific expression (RPKM) of CYPs in placental compartments (amnion, basal plate, chorion, villi, cytotrophoblasts) of 2nd trimester and term human placentas (GSE16368; [19]; A]). Median RPKM expression across all samples is displayed (right of heatmap). CYPs expressed at median levels above 1 RPKM are in bold.

Figure 4:. CYP expression in human adult livers and 2^nd trimester fetal livers.

Relative CYP RNA expression (log2) in human adult (n=9) and fetal livers (n=84). Values represent the adjusted log2 average and standard error (SE) to the mean. Asterisks (*) denote significance (t-test, p<0.05).

Figure 5:. CYP expression in human adult livers and 2^nd trimester placentas.

Relative CYP expression (log2) in human adult livers (n=9) and second trimester placentas (n=47). Asterisks (*) denote significance (t-test, p<0.05). Box plots represent the adjusted log2 average and standard error (SE) to the mean. Asterisks (*) denote significance (t-test, p<0.05).

Figure 6:. Activity of CYP3A4 in human placenta and adult/fetal livers.

Relative CYP3A4 activity (expressed as the total average rate of fluorescence units (AFU)).Asterisks (*) denote significance (t-test, p<0.05).