Sex- and dose-dependent effects of neonatally administered aspartate on the ultradian patterns of circulating growth hormone regulating hexobarbital metabolism and action

Constitutive and inducible hepatic cytochrome P450 isoforms in senescent male and female rats and response to low-dose phenobarbital

Numerous studies, usually limited to male rodents, have reported an inverse relationship between the age of the animal and the activities of various multi-cytochrome P450-dependent drug-metabolizing enzymes. It has been suggested that the aging-induced decline in hepatic drug-metabolizing capacity is solely a male phenomenon. That is, whereas the levels of male-specific isoforms of P450 decline with senescence, the female-dependent isoforms remain unchanged in females and even increase in male liver. In addition to their baseline activities, induction levels of hepatic monooxygenases have also been reported to decrease with aging. To examine aging- and sex-dependent effects on drug metabolism at a more molecular level, we measured the expression (mRNA, protein, and/or catalytic activity) of a near dozen constitutive and inducible isoforms of P450 in 5-and 23-month-old male and female Sprague-Dawley rats. Moreover, we investigated the induction effects of low concentrations of phenobarbital known to reveal gender differences and the threshold sensitivities of both constitutive and inducible isoforms. With the exception of male-specific CYP2C11 (whose expression declined approximately 70% in aged male rats), we observed little senescence-associated reduction in either preinduction or induction levels of CYP2B1, CYP2B2, CYP3A1, CYP3A2, CYP2C6, CYP2C7, CYP2C12, and CYP2C13 in either male or female rats. Moreover, the sexually dimorphic expression levels apparent at 5 months of age persisted in the old rats.

Possible pathomechanism of autoimmune hepatitis

The hepatotropic viruses, measles, and herpesviruses as well as different drugs were repeatedly shown to act presumably as a trigger in patients with autoimmune hepatitis (AI-H). On the other hand, it is known that viral infections stimulate interferon production, which inactivates the cytochrome P-450 enzymes involved in the metabolism of several endogenous substances and exogenous environmental agents. Moreover, it was reported that several cytokines, including interferons, as well as transforming growth factor beta1 and human hepatocyte growth factor, which are abundantly produced and released in the body during infections, also downregulated expression of major cytochrome P-450 and/or other biotransformation enzymes. It seems that all these factors, in addition to individual immune response and the nature and amount of the neoantigen(s) produced, impair the equilibrium of bioactivation and detoxication pathways, thus leading to the development of AI-H in a genetically predisposed person continually exposed to harmful environmental factor(s). Possible increased/decreased density of lysine residues at position D-related human leukocyte antigen locus (DR)beta71 of the antigen-binding groove may affect the eventual steroid-sparing effect of this critical amino acid at the cellular level. In addition, some food additives, such as monosodium glutamate (MSG) and/or aspartame regularly consumed in excessive amounts, may eventually disturb the delicate balance between a positively charged amino acid residue at position DRbeta71 (lysine or arginine) and a negatively charged amino acid residue at position P4 on the antigenic peptide (glutamic acid or aspartic acid). This may favor formation of a salt bridge between these amino acid residues within the hypervariable region 3 on the alpha-helix of the DRbeta polypeptide and facilitate autoantigen presentation and CD4 T-helper cell activation. MSG and aspartate may also depress serum concentrations of growth hormone, which downregulate the activity of several cytochrome P-450 hepatic and other drug-metabolizing enzymes, thus increasing sensitivity to some environmental agents and possibly influencing efficacy of treatment regimens and final outcome of patients with type 1 AI-H.

Prevention of latently expressed CYP2C11, CYP3A2, and growth hormone defects in neonatally monosodium glutamate-treated male rats by the N-methyl-D-aspartate receptor antagonist dizocilpine maleate

Neonatal administration of monosodium glutamate (MSG) can produce latently expressed defects in drug metabolism and growth hormone secretion as well as stunted growth and obesity. Instead of secreting growth hormone in the masculine episodic profile, plasma hormone levels are generally undetectable in affected adult male rats. Moreover, male-specific isoforms of cytochrome P450 (P450; e.g., CYP2C11 and CYP3A2), whose combined levels comprise the bulk of the total hepatic P450 in adult male rats, are similarly undetectable in these animals. Since "signaling elements" in the masculine episodic growth hormone profile are solely responsible for the elevated characteristic male-like expression levels of CYP2C11 and CYP3A2, suppression of the isoforms in the MSG-treated rats appeared to be caused by the simple absence of the hormone from the circulation. However, the reported failures of restored physiologic masculine growth hormone profiles to correct the P450 defects suggested the occurrence of direct MSG-induced liver damage independent of the well known hypothalamic lesions produced by the amino acid. Concurrent administration of dizocilpine maleate (MK-801), a selective and highly potent noncompetitive N-methyl-D-aspartate receptor antagonist of glutamate, completely prevented the adverse effects of neonatal MSG treatment on P450 expression, growth hormone secretion, and growth parameters, indicating that the amino acid-induced defects are solely a result of neuronal (i.e., hypothalamic) damage produced at the time of MSG exposure. The irreversibility of the P450 damage is described as resulting from secondary defects initially induced by the neuronal lesions.

Sex-dependent metabolism of xenobiotics

Sex-dependent differences in xenobiotic metabolism have been most extensively studied in the rat. Because sex-dependent differences are most pronounced in rats, this species quickly became the most popular animal model to study sexual dimorphisms in xenobiotic metabolism. Exaggerated sex-dependent variations in metabolism by rats may be the result of extensive inbreeding and/or differential evolution of isoforms of cytochromes P450 in mammals. For example, species-specific gene duplications and gene conversion events in the CYP2 and CYP3 families have produced different isoforms in rats and humans since the species division over 80 million years ago. This observation can help to explain the fact that CYP2C is not found in humans but is a major subfamily in rats (Table 11). Animal studies are used to help determine the metabolism and toxicity of many chemical agents in an attempt to extrapolate the risk of human exposure to these agents. One of the most important concepts in attempting to use rodent studies to identify sensitive individuals in the human population is that human cytochromes P450 differ from rodent cytochromes P450 in both isoform composition and catalytic activities. Xenobiotic metabolism by male rats can reflect human metabolism when the compound of interest is metabolized by CYP1A or CYP2E because there is strong regulatory conservation of these isoforms between rodents and humans. However, problems can arise when rats are used as animal models to predict the potential for sex-dependent differences in xenobiotic handling in humans. Information from countless studies has shown that the identification of sex-dependent differences in metabolism by rats does not translate across other animal species or humans. The major factor contributing to this observation is that CYP2C, a major subfamily in rats, which is expressed in a sex-specific manner, is not found in humans. To date, sex-specific isoforms of cytochromes P450 have not been identified in humans. The lack of expression of sex-dependent isoforms in humans indicates that the male rat is not an accurate model for the prediction of sex-dependent differences in humans. Differences in xenobiotic metabolism among humans are more likely the consequence of intraindividual variations as a result of genetics or environmental exposures rather than from sex-dependent differences in enzyme composition. A major component of the drug discovery and development process is to identify, at as early a stage as possible, the potential for toxicity in humans. Earlier identification of individual differences in xenobiotic metabolism and the potential for toxicity will be facilitated by improving techniques to make better use of human tissue to prepare accurate in vitro systems such as isolated hepatocytes and liver slices to study xenobiotic metabolism and drug-induced toxicities. Accurate systems should possess an array of bioactivation enzymes similar to the in vivo expression of human liver. In addition, the compound concentrations and exposure times used in these in vitro test systems should mimic those achieved in the target tissues of humans. Consideration of such factors will allow the development of compounds with improved efficacy and low toxicity at a more efficient rate. The development of accurate in vitro systems utilizing human tissue will also aid in the investigation of the molecular mechanisms by which the CYP genes are regulated in humans. Such studies will facilitate the study of the basis for differences in expression of isoforms of CYP450 in humans.

Involvement of growth hormone as a regulating factor in sex differences of mouse hepatic aldehyde oxidase

The participation of circulating growth hormone (GH) as a regulator of sex differences in hepatic aldehyde oxidase (AO) activity in ddy mice was examined. The 2- to 3-fold higher activities in adult male mice compared with adult female mice were decreased to the female levels by neonatal pretreatment with monosodium glutamate (MSG) or monosodium aspartate (MSA), either of which is known to reduce circulating GH levels. A decline of the activities in the MSG-treated male mice was restored nearly to the male control levels by subsequent injections of human GH every 12 hr for 7 days. These changes in AO activities in male mice caused by the excitotoxic amino acids were not observed in females. Hypophysectomy markedly decreased hepatic AO activities in male mice and partially in female mice. The activities in hypophysectomized male mice were restored again to levels similar to the control males by intermittent injections of human GH. Administration of testosterone propionate (TP) significantly increased the activities of hepatic AO in intact female mice, but not in MSA-treated or hypophysectomized females. On the other hand, the AO activities in adult male mice were decreased partially by the administration of estradiol benzoate. These results indicate that the pituitary GH is involved as one of the major regulatory factors of sex differences in the activities of hepatic AO in mice and TP also contributes to maintaining the higher activity in male mice mainly through the hypothalamus-pituitary system.