Evidence from dwarf rats that growth hormone may not regulate the sexual differentiation of liver cytochrome P450 enzymes and steroid 5 alpha-reductase

Organotypic liver culture models: meeting current challenges in toxicity testing

Prediction of chemical-induced hepatotoxicity in humans from in vitro data continues to be a significant challenge for the pharmaceutical and chemical industries. Generally, conventional in vitro hepatic model systems (i.e. 2-D static monocultures of primary or immortalized hepatocytes) are limited by their inability to maintain histotypic and phenotypic characteristics over time in culture, including stable expression of clearance and bioactivation pathways, as well as complex adaptive responses to chemical exposure. These systems are less than ideal for longer-term toxicity evaluations and elucidation of key cellular and molecular events involved in primary and secondary adaptation to chemical exposure, or for identification of important mediators of inflammation, proliferation and apoptosis. Progress in implementing a more effective strategy for in vitro-in vivo extrapolation and human risk assessment depends on significant advances in tissue culture technology and increasing their level of biological complexity. This article describes the current and ongoing need for more relevant, organotypic in vitro surrogate systems of human liver and recent efforts to recreate the multicellular architecture and hemodynamic properties of the liver using novel culture platforms. As these systems become more widely used for chemical and drug toxicity testing, there will be a corresponding need to establish standardized testing conditions, endpoint analyses and acceptance criteria. In the future, a balanced approach between sample throughput and biological relevance should provide better in vitro tools that are complementary with animal testing and assist in conducting more predictive human risk assessment.

Altered ethylbenzene-mediated hepatic CYP2E1 expression in growth hormone-deficient dwarf rats

Ethylbenzene (EB) effectively induces several hepatic P450 enzymes including CYP2E1 and CYP2B. Hypophysectomy diminishes the magnitude of EB-mediated induction of CYP2B. Although growth hormone (GH) plays a key role in sexual dimorphism of CYP2C11, its impact on EB-mediated P450 expression is still unknown. Because hypophysectomy leads to a depletion of multiple pituitary hormones besides GH, a study was designed to investigate the possible involvement of GH in EB-mediated hepatic P450 expression using GH-deficient dwarf rats as a more specific animal model. In these rats, pituitary GH was selectively reduced to about 10% of normal levels and other pituitary trophic hormones including thyroid-stimulating hormone, adrenocorticotropic hormone, luteinizing hormone, follicle-stimulating hormone, and prolactin are largely unchanged. Male control and HsdOla:DWARF-dw-4 (Harlan, UK) rats were subjected to a single ip injection of EB (10 mmol/kg). CYP2E1- and CYP2B-dependent activities, protein, and RNA levels were measured 10 and 24 h afterward. The results indicated that dwarf rats without EB exposure expressed higher CYP2E1. Although EB treatment induced CYP2E1 activity, protein, and mRNA both in controls and dwarf rats, the magnitude of the response to EB exposure was greater 10 h after the treatment in dwarf rats. Hypophysectomy also increased CYP2E1 protein induction by EB compared to intact rats. This effect was reversed by GH supplementation to hypophysectomized rats. Overall, responses of CYP2B to EB exposure in dwarf rats did not display basic differences from controls. In conclusion, the results demonstrate that (1) the suppression of CYP2B induction found in the multi-hormone-deficient HX rats is not found in the more specific GH-deficient rat model, confirming that GH does not have a major influence on CYP2B expression and (2) both hypophysectomized and GH-deficient rats show an altered inducibility of CYP2E1 after EB treatment.

Hepatitis A impairs the function of human hepatic CYP2A6 in vivo

Hepatitis virus A (HVA) is a worldwide sporadic disease but its effects on pharmacokinetics and individual drug responses have not been studied. In this study, the 7-hydroxycoumarin (7OHC) excretion test used in vivo as a bioindex of hepatic CYP2A6 activity was performed in 20, previously healthy, acute jaundice HVA patients. Volunteers with an acute HVA were treated with one p.o. administration of 5 mg coumarin (Venalot). Among the patients, 11 were children (6-10 years; two girls and nine boys), the rest (15-40 years old) consisted of two men and seven women. Urinary excretion of 7OHC was measured after overnight fasting in four fractions: 0 h before any medication (to detect if any basal 7OHC excretion exits), and after a 5-mg coumarin capsule p.o., 0-2, 2-4 and 4-8 h fractions were collected and urine volumes were recorded. Urinary excretion of 7-hydroxycoumarin occurred to a similar extent in healthy adults and children. The first 2-h 7OHC excretion was decreased by 26% (P < 0.05) and total (0-8 h) 7OHC excretion was decreased by 37% (P<0.01) among HVA-positive adults (age range 15-40 years) compared with the values obtained from healthy volunteers. In 11 HVA-positive children (age 6-10 years), the first 2-h 7OHC excretion was only 20% (P < 0.0001) and the total 7OHC excretion 28% (P < 0.0001) of the value observed in healthy controls. These results suggest that (i) an acute HVA decreases the metabolic clearance of drugs such as coumarin which are rapidly metabolised by CYP2A6 and (ii) this decrease is even more prominent in children. Such metabolic responses may be of clinical importance and may also interfere with other drug therapy in these patients.

Possible existence of a CYP3A protein in liver microsomes from female rats

CYP3A proteins are P450 monooxygenases involved in the metabolism of steroids, retinoic acid and several important drugs. In rats, the number of CYP3A genes and proteins, and therefore important aspects of their inducibility, developmental regulation and sex specificity are not known for certain. Using triacetyloleandomycin-metabolite complex formation, testosterone hydroxylase assays and immunoblots from peptide maps, we obtained results suggesting that in liver microsomes from mature rats, at least three, possibly four CYP3A proteins are expressed: one is present in females, another is male-specific, and one or two additional CYP3A proteins are inducible by phenobarbital, steroids, and triacetyloleandomycin.

Effects of caloric restriction on rodent drug and carcinogen metabolizing enzymes: implications for mutagenesis and cancer

Caloric restriction in rodents results in increased longevity and a decreased rate of spontaneous and chemically induced neoplasia. The low rates of spontaneous neoplasia and other pathologies have made calorically restricted rodents attractive for use in chronic bioassays. However, caloric restriction also alters hepatic drug metabolizing enzyme (DME) expression and so may also alter the biotransformation rates of test chemicals. These alterations in DME expression may be divided into two types: (1) those that are the direct result of caloric restriction itself and are detectable from shortly after the restriction is initiated; (2) those which are the result of pathological conditions that are delayed by caloric restriction. These latter alterations do not usually become apparent until late in the life of the organism. In rats, the largest direct effect of caloric restriction on liver DMEs is an apparent de-differentiation of sex-specific enzyme expression. This includes a 40-70% decrease in cytochrome P450 2C11 (CYP2C11) expression in males and a 20-30% reduction of corticosterone sulfotransferase activity in females. Changes in DME activities that occur late in life in calorically restricted rats include a stimulation of CYP2E1-dependent 4-nitrophenol hydroxylase activity and a delay in the disappearance of male-specific enzyme activities in senescent males. It is probable that altered DME expression is associated with altered metabolic activation of chemical carcinogens. For example the relative expression of hepatic CYP2C11 in ad libitum-fed or calorically restricted rats of different ages is closely correlated with the amount of genetic damage in 2-acetylaminofluorene- or aflatoxin B1-pretreated hepatocytes isolated from rats of the same age and caloric intake. This suggests that altered hepatic drug and carcinogen metabolism in calorically restricted rats can influence the carcinogenicity of test chemicals.

Interpulse interval in circulating growth hormone patterns regulates sexually dimorphic expression of hepatic cytochrome P450

Plasma growth hormone (GH) profiles are sexually differentiated in many species and regulate the sex-dependence of peripubescent growth rates and liver function, including steroid hydroxylase cytochrome P450 expression, by mechanisms that are poorly understood. By use of an external pump to deliver to hypophysectomized rats pulses of rat GH of varying frequency and amplitude, a critical element for liver discrimination between male and female GH patterns was identified. Liver expression of the male-specific steroid 2 alpha (or 16 alpha)-hydroxylase P450, designated CYP2C11, was stimulated by GH at both physiological and nonphysiological pulse amplitudes, durations, and frequencies, provided that an interpulse interval of no detectable GH was maintained for at least 2.5 hr. This finding suggests that hepatocytes undergo an obligatory recovery period after stimulation by a GH pulse. This period may be required to reset a GH-activated intracellular signaling pathway or may relate to the short-term absence of GH receptors at the hepatocyte surface after a cycle of GH binding and receptor internalization. These requirements were distinguished from those necessary for the stimulation by GH of normal male growth rates in hypophysectomized rats, indicating that different GH responses and, perhaps, different GH-responsive tissues recognize distinct signaling elements in the sexually dimorphic patterns of circulating GH.