Calcium as a permissive factor but not an initiation factor in DNA synthesis induction in cultured rat hepatocytes by the peroxisome proliferator ciprofibrate

Toluene inhibits muscarinic receptor-mediated cytosolic Ca2+ responses in neural precursor cells

Toluene is widely used as a component in industrial solvents and many toluene-containing products are abused via inhalation. While many studies have demonstrated its inhibitory effects on neuronal activity, the effects of toluene on receptor signaling in proliferating and differentiating neural precursor cells are presently unclear. Here, using digital video microscopy and Ca2+ imaging, we investigated the effects of acute exposure to toluene on the function of muscarinic acetylcholine receptors (mAChRs) expressed in neural precursor cells. The neural precursor cells were isolatedfrom embryonic day 13 (E13) rat cortex and expanded in serum-free medium containing basic fibroblast growth factor (bFGF). We found that the acetylcholine (ACh) analog carbachol (CCh) induced a dose-dependent increase in cytosolic Ca2+, which was blocked by the muscarinic receptor antagonist atropine in a reversible manner. Toluene was added to the perfusion medium and concentrations of toluene in the medium were determined by gas chromatographic analysis. Following imaging, the cells were fixed and processed for 5-bromo-2'-deoxyuridine (BrdU, cell proliferation marker) and beta-tubulin (TuJ1, neuronal marker) immunostaining. In the 5 day culture, most cells continued to divide (BrdU+), while afew cells differentiated into young neurons (TuJ1-). The CCh-induced Ca2+ elevations in proliferating (BrdU+TuJ1-) neural precursor cells were significantly reduced by acute exposure to 0.15 mM toluene and completely blocked by 10 mM toluene. Toluene's inhibition of muscarinic receptor-mediated Ca2+ signaling was rapid, reversible and dose-dependent with an IC50 value 0.5 mM. Since muscarinic receptors mediate cell proliferation and differentiation during neural precursor cell development, these results suggest that depression of muscarinic signaling may play a role in toluene's teratogenic effect on the developing nervous system.

Effect of ethanol on muscarinic receptor-induced calcium responses in astroglia

The effects of ethanol on muscarinic receptor-mediated calcium responses were investigated in individual primary rat astrocytes and human 132 1N1 astrocytoma cells using indo-1/AM and image cytometry. After a 30-min incubation, carbachol-induced calcium responses were inhibited only at 100 or 250 mM ethanol. The effects of ethanol were more pronounced and occurred at lower concentrations with longer exposures, with significant inhibition seen at 10 mM following a 24-hr incubation. Thapsigargin- and glutamate-induced responses were unaffected by ethanol, indicating some selectivity in this inhibition. Upon removal of ethanol, inhibition of calcium responses persisted for up to 6-12 hr, with carbachol responses returning to control levels by 24 hr after washout. Ethanol exposure did not affect muscarinic-receptor binding in astrocytoma cells, but inhibited carbachol-induced IP(3) formation. Inhibition of (3)H-thymidine incorporation by ethanol also persisted upon removal of the alcohol, with a time-dependency similar to that of the calcium responses. These results indicate that ethanol inhibits muscarinic receptor-induced calcium responses in astroglia in a concentration- and duration-dependent manner. They also show that co-incubation with ethanol is not necessary for this effect, suggesting that long-term exposure to ethanol may modify, in a reversible manner, the coupling of muscarinic receptors with its effector. This effect of ethanol may play a role in ethanol's inhibition of carbachol-induced thymidine incorporation.

Effects of ethanol on calcium homeostasis in the nervous system: implications for astrocytes

Ethanol is a major health concern, with neurotoxicity occurring after both in utero exposure and adult alcohol abuse. Despite a large amount of research, the mechanism(s) underlying the neurotoxicity of ethanol remain unknown. One of the cellular aspects that has been investigated in relationship to the neuroteratogenicity and neurotoxicity of ethanol is the maintenance of calcium homeostasis. Studies in neuronal cells and other cells have shown that ethanol can alter intracellular calcium levels and affect voltage and receptor-operated calcium channels, as well as G protein-mediated calcium responses. Despite increasing evidence of the important roles of glial cells in the nervous systems, few studies exist on the potential effects of ethanol on calcium homeostasis in these cells. This brief review discusses a number of reported effects of alcohol on calcium responses that may be relevant to astrocytes' functions.

Stimulation of the DNA binding activity of AP-1 by the peroxisome proliferator ciprofibrate and eicosanoids in cultured rat hepatocytes

Peroxisome proliferators induce hepatic peroxisome proliferation and hepatic tumors in rodents. These chemicals increase the expression of the peroxisomal beta-oxidation pathway and the cytochrome P-450 4A family, which metabolize lipids, including eicosanoids. Peroxisome proliferators also induce increased cell proliferation in vivo. However, peroxisome proliferators are only weakly mitogenic and are not comitogenic with epidermal growth factor (EGF) in cultured hepatocytes. Our earlier studies found that the peroxisome proliferator ciprofibrate is comitogenic with eicosanoids. We therefore hypothesized that the comitogenicity of the peroxisome proliferator ciprofibrate and eicosanoids may result from a synergistic increase of the DNA binding activity of AP-1. Primary rat hepatocytes were cultured on collagen gels in serum-free L-15 medium with ciprofibrate, eicosanoids, and/or growth factors. The DNA binding activity of AP-1 was determined in nuclear protein extracts by electrophoretic mobility shift assay. The DNA binding activity of AP-1 was not induced by ciprofibrate or eicosanoids alone, but the addition of eicosanoids along with ciprofibrate increased the induction of DNA binding activity of AP-1 at 30 min and 2 h after exposure. The combination of ciprofibrate and PGF2alpha blocked the inhibitory effect of transforming growth factor (TGF)-beta on the DNA binding activity of AP-1 induced by EGF. These results show that the peroxisome proliferator ciprofibrate and eicosanoids co-stimulate the DNA binding activity of AP-1 and suggest that changes in eicosanoid concentrations may modulate mitogenic signal transduction pathways by the peroxisome proliferator ciprofibrate.

Effect of rodent hepatocarcinogenic peroxisome proliferators on fatty acyl-CoA oxidase, DNA synthesis, and apoptosis in cultured human and rat hepatocytes

The effects of the rodent hepatocarcinogens clofibric acid and diprofibrate on the activity of the peroxisomal fatty acyl-CoA oxidase, DNA synthesis, and apoptosis were compared in cultured rat and human hepatocytes. Rat hepatocytes expressed a 10-fold greater level of the peroxisomal fatty acyl-CoA oxidase compared to human hepatocytes. At the highest concentration (1.0 mM), both drugs induced a two- to threefold increase in this enzyme activity in both rat and human hepatocytes. Ciprofibrate (0.1 and 0.2 mM) caused a twofold increase in DNA synthesis in rat hepatocytes, whereas clofibric acid had no effect on DNA synthesis in these cells. In contrast, increasing concentrations of both clofibric acid and ciprofibrate produced inhibition of DNA synthesis in human hepatocytes. By using the terminal transferase dUTP-biotin nick end labeling technique, it was observed that 0.1 and 0.2 mM clofibric acid and ciprofibrate suppressed transforming growth factor-beta (TGF beta)-induced apoptosis by 50% in rat hepatocytes, but they had no effect on TGF beta-induced apoptosis in human hepatocytes. Although clofibric acid and ciprofibrate diminished TGF beta-induced apoptosis, they had no effect on the basal apoptotic levels in the rat hepatocyte cultures. However, both drugs significantly increased the percent of apoptotic cells in the human hepatocyte cultures. It is concluded that primary rat and human hepatocyte cultures respond differently to peroxisome proliferators. The differences in effects on DNA synthesis and apoptosis support the hypothesis that human liver cells are refractory to peroxisome proliferator-induced hepatocarcinogenesis.

Effects of the peroxisome proliferator ciprofibrate and prostaglandin F2 alpha combination treatment on second messengers in cultured rat hepatocytes

Peroxisome proliferators induce hepatic peroxisome proliferation and hepatic tumors in rodents. These chemicals increase the expression of the peroxisomal beta-oxidation pathway and the cytochrome P-450 4A family, which metabolizes lipids, including eicosanoids. Peroxisome proliferators transiently induce increased cell proliferation in vivo. However, peroxisome proliferators are weakly mitogenic and are not co-mitogenic with epidermal growth factor (EGF) in cultured hepatocytes. Earlier study found that the peroxisome proliferator ciprofibrate is comitogenic with eicosanoids. In order to study possible mechanisms of the comitogenicity of peroxisome proliferator ciprofibrate and eicosanoids, we hypothesized that the co-mitogenicity may result from synergistic or additive increases of second messengers in mitogenic signal pathways. We therefore examined the effect of the peroxisome proliferator ciprofibrate, prostaglandin F2 alpha (PGF2 alpha) and the combination of ciprofibrate and PGF2 alpha with or without growth factors on the protein kinase C (PKC) activity, and inositol-1, 4, 5-triphosphate (IP3) and intracellular calcium ([Ca2+]i) concentrations in cultured rat hepatocytes. The combination of ciprofibrate and PGF2 alpha significantly increased particulate PKC activity. The combination of ciprofibrate and PGF2 alpha also significantly increased EGF, transforming growth factor-alpha (TGF-alpha) and hepatic growth factor (HGF)-induced particulate PKC activity. The combination of ciprofibrate and PGF2 alpha greatly increased [Ca2+]i. However, the increases of PKC activity and [Ca2+]i by ciprofibrate and PGF2 alpha alone were much smaller. Neither ciprofibrate or PGF2 alpha alone nor the combination of ciprofibrate and PGF2 alpha significantly increased the formation of IP3. The combination of ciprofibrate and PGF2 alpha, however, blocked the inhibitory effect of TGF-beta on particulate PKC activity and formation of IP3 induced by EGF. These results show that co-mitogenicity of the peroxisome proliferator ciprofibrate and eicosanoids may result from the increase in particulate PKC activity and intracellular calcium concentration but not from the formation of IP3.

Rodent hepatocarcinogenic peroxisome proliferators induce proliferation of rat hepatocytes in primary mixed cultures with rat liver epithelial cells

The effect of two members of the hypolipidemic medicine class of hepatocarcinogenic peroxisome proliferators on proliferation of hepatocytes in primary mixed cultures with liver epithelial cells was studied. Rat hepatocytes present in primary mixed cultures with rat liver epithelial cells were maintained for 3 months retaining their differentiated characteristics and proliferative potential. Hepatocyte clusters in mixed cultures stained positive for albumin, indicating that they retained some metabolic functions. Furthermore, in mixed cultures exposed to 0.2 mM clofibric acid or ciprofibrate for 3 months, hepatocytes were engaged in proliferation as shown by the expression of proliferating cell nuclear antigen (PCNA) and the presence of mitotic figures. This in vitro system could be useful to obtain more information about responses of liver cells during prolonged exposure to peroxisome proliferators.

Chemicals with carcinogenic activity in the rodent liver; mechanistic evaluation of human risk

A wide variety of chemicals, both naturally occurring and synthetic, have exhibited carcinogenic activity in rodent liver. Some are clearly DNA reactive whereas others produce only epigenetic effects. Hepatocarcinogens are categorized according to these properties and the characteristics of examples of both types are reviewed. DNA-reactive rodent hepatocarcinogens represent human cancer risk even at non-toxic exposures, whereas epigenetic agents pose either no risk because their effects are specific to rodents, or a risk only at high exposures at which they produce the same cellular effects in humans that are the basis for their carcinogenic activity in rodents.

Effects of extracellular calcium concentrations and calcium antagonists on vitellogenin induction by estradiol-17 beta in primary hepatocyte culture in the rainbow trout Oncorhynchus mykiss

The involvement of calcium in vitellogenin (Vg) synthesis in response to estradiol-17 beta (E2, 2 x 10(-6) M) was electrophoretically examined in primary hepatocyte culture in the rainbow trout Oncorhynchus mykiss. The relative rate of Vg synthesis was evaluated by measuring the integrated optical density of the Vg band after SDS-PAGE and expressed as a percentage of Vg to total proteins. The induction of Vg by E2 did not occur at 0 meq Ca/liter, while some proteins other than Vg (main subunit) were weakly synthesized. Vg synthesis increased in a calcium concentration-dependent way and reached the maximum at 5 meq Ca/liter in the incubation medium. The addition of bovine growth hormone (50 ng/ml) and/or ovine prolactin (100 ng/ml) had no effect on stimulating Vg synthesis by E2, regardless of the calcium concentrations in the medium. Lanthanum and verapamil (10(-4) M) markedly inhibited Vg synthesis, while diltiazem (10(-4) M) was insignificant. Reactive blue (10(-4) was also effective in inhibiting Vg synthesis to about 60% of the control. These results suggest that the synthesis of Vg, a calcium-binding protein, is more susceptible to calcium than are other proteins. Receptor-operated and verapamil-sensitive calcium channels have been proposed to be involved in calcium entry into hepatocytes in rainbow trout.

Peroxisome proliferators and peroxisome proliferator activated receptors (PPARs) as regulators of lipid metabolism

Peroxisome proliferation (PP) in mammalian cells, first described 30 years ago, represents a fascinating field of modern research. Major improvements made in its understanding were obtained through basic advances that have opened up new areas in cell biology, biochemistry and genetics. A decade after the first report on PP, a new metabolic pathway (peroxisomal beta-oxidation) and its inducibility by peroxisome proliferators were discovered. More recently, a new type of nuclear receptor, the peroxisome proliferator-activated receptor (PPAR), has been described. The first PPAR was discovered in 1990. Since then, many other PPARs have been characterized. This original class of nuclear receptors belongs to the superfamily of steroid receptors. With activation of cell signal transduction pathways, the occurrence of PPARs provides, for the first time, a coherent explanation of mechanisms by which PP is triggered. Nevertheless, although many compounds or metabolites are capable of activating PPARs, the natural direct ligands of these receptors have not been, up to now, clearly identified, with, however, the exception of 15-deoxy-12,14-prostaglandin J2 which is the ligand of PPAR gamma 2 while leukotrien LTB4 binds PPAR alpha. At this stage, the hypothesis of some orphan PPARs (ie receptors without known ligand) can not be ruled out. Despite these relatively restrictive aspects, the mechanisms by which activation of PPARs leads to PP become clear; also, coherent hypotheses among which a scenario involving receptor phosphorylation or a heat shock protein (ie HSP 72) can be proposed to explain how PPARs would be activated. The aim of this note is to review recent developments on PPARs, to present members up to now recognized to belong to the PPAR family, their characterization, functions, regulation and mechanisms of activation as well as their involvement in lipid metabolism regulation such as control of beta-oxidation, ketogenesis, fatty acid synthesis and lipoprotein metabolism. As an introducing section, a brief review of the major events between the first report of PP in mammals and the discovery of the first PPAR is given. Another section is devoted to current hypotheses on mechanisms responsible for PPAR activation and PP induction. Rather than an exhaustive presentation of cellular alterations accompanying PP induction, a dynamic overview of the lipid metabolism is provided. By assessing the biological significance of this organellar proliferative process, the reader will be led to conclude that the discovery of PPARs and related gene activation through peroxisome proliferator responsive element (PPRE) makes PP induction one of the most illustrative examples of control that occurs in lipid metabolism.

Comitogenicity of eicosanoids and the peroxisome proliferator ciprofibrate in cultured rat hepatocytes

Several hypolipidemic drugs and environmental contaminants induce hepatic peroxisome proliferation and hepatic tumors when administered to rodents. These chemicals increase the expression of the peroxisomal beta-oxidation pathway and the cytochrome P-450 4A family, which metabolize lipids, including eicosanoids and their precursor fatty acids. We previously found that the peroxisome proliferator ciprofibrate decreases the level of eicosanoids in the liver and in cultured hepatocytes. In this study, we examined the effect of prostaglandins E2 and F2 alpha (PGE2 and PGF2 alpha), leukotriene C4 (LTC4) and the peroxisome proliferator ciprofibrate on DNA synthesis in cultured hepatocytes. Primary rat hepatocytes were cultured on collagen gels in serum-free L-15 medium with varying concentrations of eicosanoids and ciprofibrate, and the absence or presence of growth factors. Ciprofibrate lowered hepatocyte eicosanoid concentrations; the addition of eicosanoids restored their levels. After a 48-h exposure with [3H]-thymidine, DNA synthesis was determined by measuring [3H]-thymidine incorporation into DNA. The addition of PGE2, PGF2 alpha, and LTC4 to cultures along with ciprofibrate increased DNA synthesis, whereas treatment with ciprofibrate or eicosanoids alone resulted in a much smaller increase. The addition of epidermal growth factor (EGF) to the eicosanoid-ciprofibrate combination increased DNA synthesis more than EGF or the eicosanoid-ciprofibrate combination alone. The PGF2 alpha-ciprofibrate combination also was comitogenic with transforming growth factor-alpha and hepatocyte growth factor. The addition of both ciprofibrate and prostaglandins also blocked the growth inhibitory effect of transforming growth factor-beta on DNA synthesis induced by EGF. These results show that the eicosanoids PGE2, PGF2 alpha, and LTC4 are comitogenic with the peroxisome proliferator ciprofibrate in cultured rat hepatocytes.