Effect of pregnenolone-16 -carbonitrile on the hepatic ultrastructure, glycogen content and ethylmorphine n-demethylase activity in pregnant, fetal, and newborn rats

Pregnane X receptor activation remodels glucose metabolism to promote NAFLD development in obese mice

OBJECTIVE

Both obesity and exposure to chemicals may induce non-alcoholic fatty liver disease (NAFLD). Pregnane X Receptor (PXR) is a central target of metabolism disrupting chemicals and disturbs hepatic glucose and lipid metabolism. We hypothesized that the metabolic consequences of PXR activation may be modified by existing obesity and associated metabolic dysfunction.

METHODS

Wildtype and PXR knockout male mice were fed high-fat diet to induce obesity and metabolic dysfunction. PXR was activated with pregnenolone-16α-carbonitrile. Glucose metabolism, hepatosteatosis, insulin signaling, glucose uptake, liver glycogen, plasma and liver metabolomics, and liver, white adipose tissue, and muscle transcriptomics were investigated.

RESULTS

PXR activation aggravated obesity-induced liver steatosis by promoting lipogenesis and inhibiting fatty acid disposal. Accordingly, hepatic insulin sensitivity was impaired and circulating alanine aminotransferase level increased. Lipid synthesis was facilitated by increased liver glucose uptake and utilization of glycogen reserves resulting in dissociation of hepatosteatosis and hepatic insulin resistance from the systemic glucose tolerance and insulin sensitivity. Furthermore, glucagon-induced hepatic glucose production was impaired. PXR deficiency did not protect from the metabolic manifestations of obesity, but the liver transcriptomics and metabolomics profiling suggest diminished activation of inflammation and less prominent changes in the overall metabolite profile.

CONCLUSIONS

Obesity and PXR activation by chemical exposure have a synergistic effect on NAFLD development. To support liver fat accumulation the PXR activation reorganizes glucose metabolism that seemingly improves systemic glucose metabolism. This implies that obese individuals, already predisposed to metabolic diseases, may be more susceptible to harmful metabolic effects of PXR-activating drugs and environmental chemicals.

Combining Genomics To Identify the Pathways of Post-Transcriptional Nongenotoxic Signaling and Energy Homeostasis in Livers of Rats Treated with the Pregnane X Receptor Agonist, Pregnenolone Carbonitrile

Transcriptomic, proteomic, phosphoproteomic, and metabolomic analyses were combined to determine the role of pregnane X receptor (PXR) in nongenotoxic signaling and energy homeostasis in liver after rats were repeatedly orally dosed with the PXR agonist pregnenolone carbonitrile (PCN) for 7 days. Analyses of mRNAs and proteins in the supernatant, membrane, and cytosolic fractions of enlarged liver homogenates showed diverse expression profiles. Gene set enrichment analysis showed that the synchronous increase in mRNAs and proteins involved in chemical carcinogenesis and the response to drug was possibly mediated by the PXR pathway and proteasome core complex assembly was possibly mediated by the Nrf2 pathway. In addition, levels of proteins in the endoplasmic reticulum lumen and involved in the acute-phase response showed specific increase with no change in mRNA level, and those composed of the mitochondrial inner membrane showed specific decrease. The analysis of phosphorylated peptides of poly(A) RNA binding proteins showed a decrease in phosphorylation, possibly by casein kinase 2, which may be related to the regulation of protein expression. Proteins involved in insulin signaling pathways showed an increase in phosphorylation, possibly by protein kinase A, and those involved in apoptosis showed a decrease. Metabolomic analysis suggested the activation of the pentose phosphate and anaerobic glycolysis pathways and the increase of amino acid and fatty acid levels, as occurs in the Warburg effect. In conclusion, the results of combined analyses suggest that PXR's effects are due to transcriptional and post-transcriptional regulation with alteration of nongenotoxic signaling pathways and energy homeostasis.

Gene expression profiling of drug metabolism and toxicology markers using a low-density DNA microarray

DNA microarrays are useful tools to study changes of gene expression in response to a treatment with drugs. Here, we describe the optimization of conditions for the cDNA synthesis and hybridization protocols to be used for a low-density DNA microarray called 'Rat HepatoChips.' This DNA microarray with 59 carefully selected genes could be used to study changes in gene expression levels due to a treatment with xenobiotic. These 59 genes (including 8 housekeeping genes) have been selected among potential toxic markers involved in basic cellular processes and drug metabolism related genes. Using the optimized conditions, the results were shown to be reproducible, with 6% variation between the duplicated spots and 10% between arrays. Conditions were optimized to allow quantification with a dynamic range of four log units. In order to demonstrate the major advantage of these tool for studying gene expression, samples of control rat liver were compared with those of animals dosed with phenobarbital (PB) or pregnenolone-16 alpha-carbonitrile (PCN), two compounds well known to induce cytochrome P450 isoforms of 2B and 3A subfamilies, respectively. This microarray has shown that other genes apart from the corresponding CYP P450 genes have been changed due to PB and PCN treatment. Apoptosis-related genes have shown to be changed due to PB and PCN treatment, which confirms results from previous work.

Inhibition of gap-junctional-intercellular communication in intact rat liver by nongenotoxic hepatocarcinogens

Many nongenotoxic hepatocarcinogens can induce cell proliferation, and inhibit apoptosis and gap-junctional-intercellular communication (GJIC). GJIC, the movement of small molecules (less than 1.2 kD) through membrane channels, is important in regulating cellular homeostasis and differentiation. The inhibition of hepatic GJIC can increase cell proliferation and possibly, inhibit apoptosis. In this study, the relationship between hepatic GJIC, proliferation, and apoptosis was examined in rats treated for 7 days with tumor-promoting doses of the nongenotoxic hepatocarcinogens phenobarbital (PB; 800 ppm), pregnenolone-16alpha-carbonitrile (PCN; 1000 ppm), and Aroclor 1254 (PCB; 100 ppm). In addition, 3-methylcholanthrene (3MC) was included as a negative control. PB, PCN, and PCB increased parenchymal-cell proliferation and inhibited hepatic apoptosis, while no alteration in these growth parameters was observed in 3MC-treated rats. GJIC, as measured by fluorescent-dye transfer through intact liver, was decreased nearly 50% by PB, PCN, and PCB, yet no effect on GJIC was observed in liver from 3MC-treated rats. These data indicate that compounds that inhibit GJIC in liver may be nongenotoxic hepatocarcinogens, which occurs simultaneously during increased cell proliferation and inhibited apoptosis.

Environmental influences on human foetal and placental xenobiotic metabolism

The human foetus is more capable of metabolizing xenobiotics than foetuses of common laboratory animal species. However, xenobiotic metabolism in animal foetuses is inducible by the exposure of the mother to various inducers during late pregnancy. Xenobiotic metabolism in neonates is more easily inducible than in foetal animals. With respect to the human foetus at mid-pregnancy, the hepatic enzyme systems do not seem to be readily inducible by exoaenous inducers, whereas the placental monooxygenase system is almost totaly dependent on maternal cigarette smoking. In the human newborn, indirect evidence points to the possibility of induction by potential inducers. The ontogenetic development of xenobiotic metabolism is probably regulated by endogenous hormones. It is possible that environmental factors may effect these normal regulatory and "imprinting" phenomena and thus lead to permanent disturbances in xenobiotic metabolism.

[Effects of the combination of a pesticide, lindane, and a sulfur depleted diet on pregnancy in the rat]

Effects of assocation of lindane ingestion and sulfur depletion on pregnancy in the rat. Sulfoconjugation stands as one of the main processes of foreign compound detoxication in mammals. When feeding conditions are adequate, sulfate ions necessary to form sulfoconjugates are provided by the sulfur aminoacids of the diet. The feeding to pregnant rats of a diet containing lindane, when sulfur aminoacids are limiting factors, should lead to competition between their utilisation for anabolic or detoxifying processes. The balance between stimulation of glucuroconjugation and sulfoconjugation seems to depend on which of the 2 compounds necessary for detoxification is the most limiting factor. Glucuroconjugation spares sulfur aminoacids for foetal growth; sulfoconjugation spares glucose for foetal utilisation. The foetal needs appear to modulate the orientation of the maternal detoxication processes; foetal anabolism is predominent over detoxication.