Sex differences in the expression of hepatic drug metabolizing enzymes

Abstract

Sex differences in pharmacokinetics and pharmacodynamics characterize many drugs and contribute to individual differences in drug efficacy and toxicity. Sex-based differences in drug metabolism are the primary cause of sex-dependent pharmacokinetics and reflect underlying sex differences in the expression of hepatic enzymes active in the metabolism of drugs, steroids, fatty acids and environmental chemicals, including cytochromes P450 (P450s), sulfotransferases, glutathione transferases, and UDP-glucuronosyltransferases. Studies in the rat and mouse liver models have identified more than 1000 genes whose expression is sex-dependent; together, these genes impart substantial sexual dimorphism to liver metabolic function and pathophysiology. Sex differences in drug metabolism and pharmacokinetics also occur in humans and are due in part to the female-predominant expression of CYP3A4, the most important P450 catalyst of drug metabolism in human liver. The sexually dimorphic expression of P450s and other liver-expressed genes is regulated by the temporal pattern of plasma growth hormone (GH) release by the pituitary gland, which shows significant sex differences. These differences are most pronounced in rats and mice, where plasma GH profiles are highly pulsatile (intermittent) in male animals versus more frequent (nearly continuous) in female animals. This review discusses key features of the cell signaling and molecular regulatory mechanisms by which these sex-dependent plasma GH patterns impart sex specificity to the liver. Moreover, the essential role proposed for the GH-activated transcription factor signal transducer and activator of transcription (STAT) 5b, and for hepatic nuclear factor (HNF) 4alpha, as mediators of the sex-dependent effects of GH on the liver, is evaluated. Together, these studies of the cellular, molecular, and gene regulatory mechanisms that underlie sex-based differences in liver gene expression have provided novel insights into the physiological regulation of both xenobiotic and endobiotic metabolism.

Fig. 1.

Regulation of rat liver CYP2C11 (male-specific) and CYP2C12 (female-specific) by plasma GH profile (A and B) and through postnatal development (C and D). A and B, Northern blots probed for the indicated liver mRNAs isolated from intact male (M) and female (F) rats, or from hypophysectomized (Hx) rats given physiological GH pulses (P) by intravenous injection six or two times per day for 7 days (A; data from Waxman et al., 1991). GH pulse treatment induces CYP2C11 in both male (top) and female (bottom) rats. Intact male rats were infused with GH continuously for 7 days using an osmotic mini-pump (B; data from Sundseth and Waxman, 1992), which suppresses CYP2C11 and induces CYP2C12. Tubulin RNA serves as a loading control. Each lane represents an individual rat liver. C and D show rat hepatic mRNA levels for CYP2C11 and CYP2C12 at various postnatal ages, as determined by quantitative polymerase chain reaction (data from Laz et al., 2007).

Fig. 2.

Liver STAT5 is activated in direct response to each plasma GH pulse in adult male rats. Plasma samples obtained from intact rats every 15 min were assayed for GH. Samples were collected from 8 to 11 AM (A) or from 8 AM to 1 PM (B), with the collection end times of each group of n = 4 rats corresponding to a peak (A) and a trough in plasma GH levels (B), as indicated. The rats were then killed (red arrow) and the livers were assayed for STAT5 DNA-binding activity (blue bar on right). Data shown are from Tannenbaum et al. (2001).

Fig. 3.

Model for direct regulation of sex-specific genes by STAT5b and HNF4α in male liver. In the model shown, GH pulse-activated STAT5 binds directly to and trans-activates its direct male-specific genes targets, which may include the family of male-specific Mup genes (Laz et al., 2009). HNF4α may also contribute to the expression of these genes. STAT5 and HNF4α may also directly bind to and repress female-specific genes, as shown. Some of the genes that serve as primary targets of STAT5 and HNF4α may also mediate the effects of these transcription factors on their downstream (i.e., indirect) targets, as shown in Fig. 4.

Fig. 4.

Model for indirect regulation of sex-specific genes by STAT5b and HNF4α in male liver. In the model shown, the effects of STAT5b and HNF4α on sex-specific liver gene expression are mediated indirectly, by male-specific transcriptional activators and repressors, which are primary targets of these two transcription factors. In an alternative formulation of this model (not shown), trans-activation of a secondary male-specific target gene could involve direct binding of STAT5b and the primary male-specific transcriptional activator. Several candidate primary target transcriptional regulators have been identified (Wauthier and Waxman, 2008).

Fig. 5.

Epigenetic regulation of sex-specific P450 genes. Female-specific Cyp genes are proposed to be repressed in male liver, and male-specific Cyp genes are proposed to be repressed in female liver by packaging in heterochromatin. Continuous GH is proposed to activate female-specific genes, such as Cyp3a genes, by a mechanism that involves the local conversion of heterochromatin to euchromatin, which enables the binding of transcription factors (TF) that activate P450 gene expression. This process could involve loss of DNA CpG methylation and/or loss of chromatin marks associated with repressed chromatin, such as histone H3 lysine 27 trimethylation, which is typically found in genes in a compact chromatin structure and is associated with a stable, inactive heterochromatic state (Cheung and Lau, 2005).

Fig. 6.

Time course for continuous GH feminization of P450 gene expression in adult male liver. Model shown is based on data presented in Holloway et al. (2006); hypothetical regulatory interactions are indicated by question marks. Female-specific repressors (Laz et al., 2007), are rapidly induced in male liver by continuous GH treatment and are proposed to down-regulate male-specific genes. The latter genes are proposed to include one or more male-specific repressors, whose down-regulation leads to de-repression of female-specific genes, such as Cyp2b9 and Cyp2b10; these Cyp2b genes require several days of continuous GH treatment for induction (Holloway et al., 2006). A distinctly longer time frame (≥7 days) is required for derepression leading to induction of the female-specific Cyp3a genes, which may involve epigenetic mechanisms, as discussed in the text and diagrammed in Fig. 5.