Morphology of liver stellate cells and liver vitamin A content in 3,4,3',4'-tetrachlorobiphenyl-treated rats

Retinoid-xenobiotic interactions: the Ying and the Yang

The literature provides compelling evidence pointing to tight metabolic interactions between retinoids and xenobiotics. These are extensive and important for understanding xenobiotic actions in the body. Within the body, retinoids affect xenobiotic metabolism and actions and conversely, xenobiotics affect retinoid metabolism and actions. This article summarizes data that establish the importance of retinoid-dependent metabolic pathways for sustaining the body's responses to xenobiotic exposure, including the roles of all-trans- and 9-cis-retinoic acid for protecting mammals from harmful xenobiotic effects and for ensuring xenobiotic elimination from the body. This review will also consider molecular mechanisms underlying xenobiotic toxicity focusing on how this may contribute to retinoid deficiency and disruption of normal retinoid homeostasis. Special attention is paid to xenobiotic molecular targets (nuclear receptors, regulatory proteins, enzymes, and transporters) which affect retinoid metabolism and signaling.

A new approach to an unbiased estimate of the hepatic stellate cell index in the rat liver: an example in healthy conditions

Evaluation of activation and proliferation of hepatic stellate cells (HSCs) must be grounded on solid quantitative data under normal conditions. The HSC index (HSCI), number of HSCs per 1000 hepatocytes (HEP), is often used in hepatology but has been never determined using stereology. Systematically sampled sections were immunostained against glial fibrillary acidic protein and carcinoembryonic antigen, allowing unequivocal distinction of HSC and mononuclear/binuclear HEP. With the optical disector the HSCI was estimated as 109 (coefficient of error = 0.04). This work provides a sound technical basis for experiments in which the estimation of HSCI and/or simultaneous quantification of HSC and HEP are relevant.

The retinoid signaling system--a target in dioxin toxicity

This review summarizes the available data on the effects of dioxins on retinoid levels, retinoid-related enzyme activities, and toxicological endpoints that have been correlated to retinoid effects. Similarities between dioxin toxicity and retinoid deficiency as well as retinoid excess are pointed out. Several possible levels of interaction between the dioxin and the retinoid signaling pathways are discussed, including the involvement of the Ah receptor, altered retinoic acid homeostasis, and an altered set point for retinoid storage. A hypothesis for the effect of dioxins on retinoids is suggested. In this hypothesis, comprising two cascades of effects on the molecular level, the effect of dioxins on retinoic acid levels is central.

Acyl-CoA: retinol acyltransferase (ARAT) and lecithin:retinol acyltransferase (LRAT) activation during the lipocyte phenotype induction in hepatic stellate cells

We have examined retinol esterification in the established GRX cell line, representative of hepatic stellate cells, and in primary cultures of ex vivo purified murine hepatic stellate cells. The metabolism of [3H]retinol was compared in cells expressing the myofibroblast or the lipocyte phenotype, under the physiological retinol concentrations. Retinyl esters were the major metabolites, whose production was dependent upon both acyl-CoA:retinol acyltransferase (ARAT) and lecithin:retinol acyltransferase (LRAT). Lipocytes had a significantly higher esterification capacity than myofibroblasts. In order to distinguish the intrinsic enzyme activity from modulation of retinol uptake and CRBP-retinol content of the cytosol in the studied cells, we monitored enzyme kinetics in the purified microsomal fraction. We found that both LRAT and ARAT activities were induced during the conversion of myofibroblasts to lipocytes. LRAT induction was dependent upon retinoic acid, while that of ARAT was dependent upon the overall induction of the fat storing phenotype. The fatty acid composition of retinyl-esters suggested a preferential inclusion of exogenous fatty acids into retinyl esters. We conclude that both LRAT and ARAT participate in retinol esterification in hepatic stellate cells: LRAT's activity correlates with the vitamin A status, while ARAT depends upon the availability of fatty acyl-CoA and the overall lipid metabolism in hepatic stellate cells.

Vitamin A deficiency injures lung and liver parenchyma and impairs function of rat type II pneumocytes

The purpose of this research was to determine the effects of vitamin A deficiency on liver and lung morphology and type II pneumocyte function. Weanling rats were fed a retinol-adequate (control) or -deficient diet for 6 wk. Average food intakes and body weights were not different between the vitamin A-deficient and -adequate rats. Histologic examination revealed that the lungs of vitamin A-deficient rats had less collagen in the adventitia of small caliber arteries and arterioles and in the alveolar septa, which appeared thinner than that of controls. Many areas of the lungs of the same rats were also emphysematous (increased size of air spaces distal to the terminal bronchiole, with thinning and partial or total destruction of septal wall). Content of elastin also was lower in the lung parenchyma, as well as in the small arteries and arterioles, but not in the larger ones. Peribronchial collagen was not affected by the deficient diet. Scattered inflammation was observed in most of the vitamin A-deficient rats; a mild inflammatory reaction also was seen in one of the controls. Vitamin A-deficient rats also exhibited hepatocyte vacuolization and mild inflammation in the liver, specifically in the periportal tracts. Surfactant synthesis and ornithine decarboxylase activity were significantly lower in type II pneumocytes isolated from vitamin A-deficient rats. In conclusion, our data provide evidence that vitamin A deficiency produces profound morphologic alterations in liver and lung parenchyma and impairs pneumocyte function.