Thyroidal control of hepatic release and metabolism of vitamin A

The inverse relationship that exists between thyroxine and the vitamin A level of plasma has been examined in chicken. Thyroxine treatment leads to a decrease in the level of vitamin A carrier proteins, retinol-binding protein and prealbumin-2 in plasma and liver. There is an accumulation of vitamin A in the liver, with a greater proportion of vitamin A alcohol being present compared to that of control birds. In thyroxine treatment there is enhanced plasma turnover of retinol-binding protein and prealbumin-2, while their rates of synthesis are marginally increased. Amino acid supplementation partially counteracts effects of thyroxine treatment. Amino acid supplementation of thyroxine-treated birds does not alter the plasma turnover rates of retinol-binding protein and prealbumin-2 but increases substentially their rates of synthesis. The release of vitamin A into circulation is interfered with in hyperthyroidism due to inadequate availability of retinol-binding protein being caused by enhanced plasma turnover rate not compensated for by synthesis.     

Modulation of lipid metabolism and vitamin A by conjugated linoleic acid

The term conjugated linoleic acid (CLA) refers to a collection of positional and geometrical isomers of octadeca- dienoic acid with conjugated double bonds. CLA has been shown to possess several beneficial activities in different experimental models, however, out of 28 isomers only two, c9, t11 and t10, c12 have been thus far demonstrated to be biologically active.The discovery that it can be elongated and desaturated as a regular fatty acid in human and animal tissues brought a new possibility that its activity may be related to its properties as a peculiar unsaturated fatty acid. In fact, CLA is able to be incorporated in lipid classes as oleic acid, accumulating in those tissues rich in neutral lipids; to be metabolized as linoleic acid and so influencing linoleic acid desaturation and elongation; and to be beta oxidized in peroxisomes which may account for, through activation of PPARs, its ability to increase free retinol levels and influence gene expression. These activities are amplified where CLA accumulates more such as mammary and adipose tissues and may explain its peculiar beneficial properties, at relative low dietary concentrations, in these tissues. Furthermore, it has been demonstrated that CLA can be endogenously formed by delta 9 desaturation of vaccenic acid (t11 18:1) thus forming the isomer c9, t11. Either endogenously formed or through dietary intake, CLA showed to be metabolized in the same way and to exert the same biological properties. We may conclude that a regular intake of CLA, or/and vaccenic acid as its precursor, should work as an excellent preventive agent by modulating lipid metabolism in target tissues thus conferring protection against the attack of insults of different type.     

Modulation of intramacrophage iron metabolism during microbial cell invasion

The mechanisms whereby vertebrate hosts withhold iron from microbial invaders, as well as the methods used in attempts by invaders to capture the growth-essential metal, differ between host extracellular and intracellular environments. This review focuses on procedures employed by host cells and by invaders, respectively, that alter intramacrophage iron metabolism.     

Effect of vitamin A deficiency on hepatic and intestinal drug metabolizing enzymes in rats

Feeding of vitamin A-deficient diet to male weanling rats for 10 weeks caused significant reduction in the hepatic cytochrome P-450, cytochrome b5, aminopyrine N-demethylase and arylhydrocarbon hydroxylase activities. Contrary to this, the levels of these Phase I enzymes were found to be significantly elevated in all the 3 portions (proximal, middle and distal) of the intestine in deficient animals as compared to corresponding pair-fed controls. Of the Phase II enzymes studied, UDP-glucuronyltransferase showed a significant decrease whereas glutathione S-transferase showed a significant increase in vitamin A-deficient rat liver and small intestine. The study suggests that vitamin A deficiency causes an imbalance between the Phase I and phase II drug metabolizing enzyme systems which may decrease the capacity of the organism to withstand the neoplastic effects of chemical carcinogens in vitamin A deficiency.     

Relationship between cellular calcium and vitamin E metabolism during protection against cell injury

The extent of chemically induced injury to isolated hepatocytes has been previously shown to depend on the content of alpha-tocopherol in the cells, the levels of which are influenced by the concentration of extracellular calcium. Investigations into the effect of calcium on the alpha-tocopherol content of nonchemically exposed cells demonstrated that incubation of isolated hepatocytes in a calcium-deficient medium decreased cell calcium content to 10% of initial levels, and resulted in the depletion of endogenous alpha-tocopherol. This loss in alpha-tocopherol was not accounted for by alpha-tocopherylquinone formation. After supplementation of the cell incubation medium with alpha-tocopheryl succinate, the decreased cell calcium content was associated with higher levels of cellular alpha-tocopherol than in calcium-adequate cells. This was the result of greater intracellular hydrolysis of the tocopheryl ester in the calcium-depleted cells, and not an effect of extracellular calcium concentration on the uptake of alpha-tocopheryl succinate into the cells or on the extracellular hydrolysis of the ester. Uptake studies indicated a much greater achievable level of alpha-tocopherol in hepatocytes after incubation with alpha-tocopherol than with the alpha-tocopheryl ester. These data provide substantial support for the hypotheses that the content of extracellular calcium per se is not the determinant in toxic injury to hepatocytes, but that cell calcium content affects the intracellular metabolism of alpha-tocopherol and its esters, which may subsequently govern the outcome of a toxic challenge.     

Modulation of glycosaminoglycan synthesis during cell growth as observed in an embryonic chick tendon cell culture

Glycosaminoglycan synthesis during cell growth has been studied in terms of unit cell numbers, using 16-day-old embryonic chick tendon cell cultures. Hyaluronic acid production was found to be inversely proportional to the cell density, while the levels of sulfated-glycosaminoglycan synthesis remained constant. On the other hand, hyaluronic acid production remained constant during cell proliferation, though chondroitin sulfate synthesis increased rapidly during an actively growing phase of the cultured cells, and dermatan sulfate and heparan sulfate syntheses increased gradually.     

Transport and metabolism of vitamin A

The present paper is a review of up-to-date findings on vitamin A transport from intestines to target-cell and metabolism of this fat-soluble vitamin. The hypotheses of possible enzymes participation in the process of etherification/hydrolysis and oxidation/reduction of vitamin A are discussed. Furthermore, possible roles of cellular retinoid-binding proteins in the process of vitamin A transport and metabolism are reviewed.     

Modulation of host cell metabolism by Trypanosoma cruzi

Chagas disease, caused by the hemoflagellate Trypanosoma cruzi, is a complicated and devastating disease. It is hypothesized that an important target of infection may be the endothelial cell and that both the acute and chronic forms of the disease involve abnormalities in the microcirculation. Stephen Morris and colleagues suggest that endothelial cell dysfunction occurs as a consequence of amastigote-associated interference in host cell metabolism.     

Adaptation of intestinal calcium absorption: parathyroid hormone and vitamin D metabolism.

It has already been demonstrated that the adaptation of intestinal calcium absorption of rats on a low calcium diet can be eliminated by thyroparathyroidectomy plus parathyroid hormone administration. This treatment elevates intestinal and plasma levels of 1,25-dihydroxyvitamin D₃ in rats on a high calcium diet while producing no change in rats on a low calcium diet. It therefore appears likely that the modulation of intestinal calcium absorption by dietary calcium is mediated by the parathyroid glands and the renal biogenesis of 1,25-dihydroxyvitamin D₃. Changes in the other unknown vitamin D metabolite levels as a result of dietary calcium are also modified by thyroparathyroidectomy and parathyroid hormone administration, but the effect of these metabolites on intestinal calcium transport is unknown.     

Modulation of intestinal vitamin D receptor availability and calcium ATPase activity by essential fatty acids

The physiological mechanisms by which essential fatty acids (EFAs) affect calcium (Ca(2+)) retention is not clear, but suggestions have included changes in membrane fluidity, receptor modulation and induction of second messengers. The vitamin D receptor (VDR) is essential for the functioning of 1,25(OH)(2)D(3)which increases Ca(2+)absorption. Activity of the intestinal basolateral membrane (BLM) Ca(2+)ATPase correlates with the degree of Ca(2+)absorption. Therefore, changes in ATPase activity and VDR availability due to EFAs may influence calcium retention. We have investigated the effect of long-term dietary supplementation with EFAs on Ca(2+)ATPase activity (measured colourimetrically) and VDR availability (measured with the ELISA technique) after the loss of oestrogen induced by ovariectomy (OVX) in female Sprague Dawley rats. Control animals underwent anaesthesia and a surgical procedure but the ovaries were left intact (sham). Ca(2+)ATPase activity was significantly lower in OVX animals than in the intact animals (P<0.05) and following supplementation with EFAs, was significantly higher than in sham controls (P<0.05). A higher number of VDR was measured after OVX and declined due to EFA supplementation; these differences in activity of the ATPase and number of receptors could be ascribed to membrane changes due to EFA supplementation, feedback control by serum calcium or the direct influence of the EFAs.     

Vitamin A metabolism in the human intestinal Caco-2 cell line

The human intestinal Caco-2 cell line, described as enterocyte-like in a number of studies, was examined for its ability to carry out the metabolism of vitamin A normally required in the absorptive process. Caco-2 cells contained cellular retinol-binding protein II, a protein which is abundant in human villus-associated enterocytes and may play an important role in the absorption of vitamin A. Microsomal preparations from Caco-2 cells contained retinal reductase, acyl-CoA-retinol acyltransferase (ARAT), and lecithin-retinol acyltransferase (LRAT) activities, which have previously been proposed to be involved in the metabolism of dietary vitamin A in the enterocyte. When intact Caco-2 cells were provided with beta-carotene, retinyl acetate, or retinol, synthesis of retinyl palmitoleate, oleate, palmitate, and small amounts of stearate resulted. However, exogenous retinyl palmitate or stearate was not used by Caco-2 cells as a source of retinol for ester synthesis. While there was a disproportionate synthesis of monoenoic fatty acid esters of retinol in Caco-2 cells compared to the retinyl esters typically found in human chylomicrons or the esters normally synthesized in rat intestine, the pattern was consistent with the substantial amount of unsaturated fatty acids, particularly 18:1 and 16:1, found in the sn-1 position of Caco-2 microsomal phosphatidylcholine, the fatty acyl donor for LRAT. Both ARAT and LRAT have been proposed to be responsible for retinyl ester synthesis in the enterocyte.(ABSTRACT TRUNCATED AT 250 WORDS)     

Corneal epithelial cell fate is maintained during repair by Notch1 signaling via the regulation of vitamin A metabolism

Integrity and preservation of a transparent cornea are essential for good vision. The corneal epithelium is stratified and nonkeratinized and is maintained and repaired by corneal stem cells. Here we demonstrate that Notch1 signaling is essential for cell fate maintenance of corneal epithelium during repair. Inducible ablation of Notch1 in the cornea combined with mechanical wounding show that Notch1-deficient corneal progenitor cells differentiate into a hyperplastic, keratinized, skin-like epithelium. This cell fate switch leads to corneal blindness and involves cell nonautonomous processes, characterized by secretion of fibroblast growth factor-2 (FGF-2) through Notch1(-/-) epithelium followed by vascularization and remodeling of the underlying stroma. Vitamin A deficiency is known to induce a similar corneal defect in humans (severe xerophthalmia). Accordingly, we found that Notch1 signaling is linked to vitamin A metabolism by regulating the expression of cellular retinol binding protein 1 (CRBP1), required to generate a pool of intracellular retinol.     

Effects of beta-carotene and vitamin A on oval cell proliferation and connexin 43 expression during hepatic differentiation in the rat(1)

The effects of β-carotene and vitamin A administrations were evaluated in an in vivo model of hepatic cell differentiation. For this purpose, male Wistar rats received β-carotene (70 mg/kg of body weight), vitamin A (10 mg/kg of body weight) or corn oil (control group), by gavage and at every other day during the entire experimental period. After 4 consecutive weeks of treatment, the animals were submitted to the AAF/PH model of hepatic cell differentiation (6 × 20 mg of AAF [2-acetylaminofluorene]/kg of body weight and partial hepatectomy) and killed on different days following the surgery (until day 16 after hepatectomy). Liver samples were collected for determination of β-carotene, retinol and retinyl palmitate concentrations, for histopathological (hematoxilin-eosin) examination, for immunohistochemical detection of glutathione S-transferase, as well as for the evaluation of connexin 43 (a structural protein of gap junctions of oval cells) expression by northern blot analysis. Compared to controls, the oval cell proliferation peaks (observed by histopathological examination and immunohistochemistry) and connexin 43 expression peaks, were postponed to later days after hepatectomy, in a similar way in β-carotene and vitamin A treated animals. Compared to the other experimental groups, the vitamin A treated group showed an increase in connexin 43 expression. It was concluded that β-carotene and vitamin A modulated oval cell proliferation and connexin 43 expression, delaying both events. These findings suggest that β-carotene and vitamin A can modulate the hepatic differentiation process in vivo.     

Serum-free culture of Japanese quail kidney cells regulation of vitamin D metabolism

Cells obtained from male quail kidneys by digestion with collagenase and hyaluronidase were plated and maintained in a chemically defined, serum-free medium. Culture dishes (35 mm) were inoculated with 1.5 · 10⁶ cells which became confluent in 5 days. The cells maintained an epithelial-like morphology over the entire culture period. During a 2 h incubation the cells metabolized 25–30% of the 10 nM 25-hydroxyvitamin D-3 (25-OH-D-3) provided. Seven metabolites were chromotographically separated on Sephadex LH-20. Three have been identified as 1α,25-dihydroxyvitamin D-3 (1,25(OH)₂D-3), 24,25-dihydroxyvitamin D-3 (24,25(OH)₂D-3) and 1α,24,25-trihhydroxyvitamin D-3 (1,24,25(OH)₃D-3). The activities of the 25-OH-D-3:1α- and 24-hydroxylases increased eight times faster than the cell number in 5 days. Preincubation of the cells with 10 nM 25-OH-D-3 or 1,25(OH)₂D-3 decreased 1,25(OH)₂D-3 synthesis, and increased both 24,25(OH)₂D-3 and metabolite IV synthesis. The decrease in 25-OH-D-3:1α-hydroxylase activity required a 2 h preincubation with 25-OH-D-3, while stimulation of 25-OH-D-3:24-hydroxylase activity and metabolite IV production required a 6 h preincubation. Incubations of cells for 1 h with parathyroid hormone resulted in a 30-fold increase in cyclic AMP in the medium. A 6 h preincubation with parathyroid hormone decreased 24,25-(OH)₂D-3 synthesis 50% relative to control cells. These results demonstrate the amenability of this system for studying the regulation of 25-OH-D-3 metabolism, as well as its use for other in vitro studies on renal cell function in a chemically defined culture system.     

Modulation of epithelial cell proliferation in culture by dissolved oxygen

Modulation of epithelial cell proliferation by the dissolved oxygen concentration (P_O2) of the growth medium was assessed with primary human foreskin epithelium and a continuous monkey kidney epithelial cell line (LLC-MK₂). Direct measurement of the growth medium P_O2 provides the first quantitative evaluation of epithelial cell proliferation as a function of P_O2 provides the first quantitative evaluation of epithelial cell proliferation as a function of P_O2. Sustained proliferation of LLC-MK₂ cells occurs in serum-free medium equilibrated with a gas phase containing 18% or 30% O₂ v/v. Mid-logarithmic phase cultures rapidly consume dissolved oxygen; this results in a 60–70 mm Hg decline in P_O2 and leads to a stable growth medium P_O2 between 70 and 100 mm Hg, well above anoxic values. In contrast, if culture medium is equilibrated with a gas phase containing 0% or 1% O₂ v/v to yield a growth medium P_O2 ～ 20–40 mm Hg, proliferation of LLC-MK₂ and primary foreskin epithelial cells is retarded, and LLC-MK₂ cells use little dissolved oxygen. Gentle, continuous rocking to prevent diffusion gradient formation enhances proliferation slightly at the higher P_O2, but neither periodic fluid renewals nor continued rocking stimulates cells retarded by a lowered oxygen concentration to resume proliferation. The data collectively demonstrate that epithelial cell proliferation requires a P_O2 > 40 mm Hg, and threshold requirements are probably closer to 70 mm Hg. Glycolysis continues at a P_O2 insufficient for proliferation, but more lactic acid accumulates in actively proliferating cultures than in cultures equilibrated with 0% oxygen. We conclude that epithelial cells in vitro both consume more oxygen and require a higher P_O2 for continued proliferation, and that the oxygen requirement for epithelial cell proliferation exceeds that of a comparable population of fibroblasts for which low oxygen may enhance survival and proliferation.