Hepatic xenobiotic metabolizing enzyme and transporter gene expression through the life stages of the mouse

Abstract

Background: Differences in responses to environmental chemicals and drugs between life stages are likely due in part to differences in the expression of xenobiotic metabolizing enzymes and transporters (XMETs). No comprehensive analysis of the mRNA expression of XMETs has been carried out through life stages in any species.

Results: Using full-genome arrays, the mRNA expression of all XMETs and their regulatory proteins was examined during fetal (gestation day (GD) 19), neonatal (postnatal day (PND) 7), prepubescent (PND32), middle age (12 months), and old age (18 and 24 months) in the C57BL/6J (C57) mouse liver and compared to adults. Fetal and neonatal life stages exhibited dramatic differences in XMET mRNA expression compared to the relatively minor effects of old age. The total number of XMET probe sets that differed from adults was 636, 500, 84, 5, 43, and 102 for GD19, PND7, PND32, 12 months, 18 months and 24 months, respectively. At all life stages except PND32, under-expressed genes outnumbered over-expressed genes. The altered XMETs included those in all of the major metabolic and transport phases including introduction of reactive or polar groups (Phase I), conjugation (Phase II) and excretion (Phase III). In the fetus and neonate, parallel increases in expression were noted in the dioxin receptor, Nrf2 components and their regulated genes while nuclear receptors and regulated genes were generally down-regulated. Suppression of male-specific XMETs was observed at early (GD19, PND7) and to a lesser extent, later life stages (18 and 24 months). A number of female-specific XMETs exhibited a spike in expression centered at PND7.

Conclusions: The analysis revealed dramatic differences in the expression of the XMETs, especially in the fetus and neonate that are partially dependent on gender-dependent factors. XMET expression can be used to predict life stage-specific responses to environmental chemicals and drugs.

Figure 1. Transcriptional ontogeny of hepatic gene expression through life stages in the mouse.

A. Principal components analysis (PCA) of the livers from mice at different life stages. The PCA shows the dramatic differences between the fetal/neonatal samples and the adult. More subtle differences were observed between adult and old mice (Not shown). C57, C57BL/6J; C3H, C3H/HeJ. B. Altered gene expression through life stages in the mouse liver. Differentially expressed genes (fold change≥|±2|) were identified and clustered as detailed in the Materials and Methods. There were no changes in gene expression in the “Adult” (PND67 and 6M) by definition. The intensity scale indicates fold-changes relative to the adult controls. Red, up-regulation; green, down-regulation; black, no change.

Figure 2. Altered expression of XMETs in the male mouse liver at different life stages.

A. XMET expression at different life stages. XMET genes which exhibited significant differences in expression compared to adult animals were identified as detailed in the Materials and Methods. Genes were clustered using one-dimensional hierarchical clustering. B. Overlap in the XMET probe sets altered in the fetus (GD19) and at 24 months. C. Quantitation of the number of up- and down-regulated phase I, II and III probe sets at different life stages. DEGs, differentially expressed genes. D. Canonical pathways overrepresented by the XMET probe sets at different life stages. XMET genes described in Figure 2A were analyzed by Ingenuity Pathways Analysis. Pathways were clustered by one-dimensional clustering. Scales at the bottom indicate the −log(p-value) for all genes.

Figure 3. Life stage changes in XMET expression are partially gender-dependent.

XMETs which exhibited gender differences in gene expression in the mouse liver in a reanalysis of two studies (gender study 1 (GS1), ; GS2, [13]) were examined for expression changes across life stage. Gender-dependent gene expression was calculated as a ratio of expression in males to that in females. At each life stage the expression in males and females was compared to adult baseline (PND67 and 6M). Probe sets predominantly expressed in males are indicated in red and female-predominant probe sets are indicated in green; intensity indicates the ratio of the gender difference. The comparison shows the suppressed expression of many male-predominant probe sets and increased expression of female-predominant probe sets in the fetus and neonate compared to the adults. A. Expression of male-predominant probe sets in the fetal and neonatal liver. B. Expression of female-predominant probe sets in the fetal and neonatal liver.