Disruption of oogenesis and molting by methoprene and glyphosate in Gammarus fossarum: involvement of retinoic acid?

In the last decade, the freshwater amphipod Gammarus fossarum proved to be a promising sentinel species in active biomonitoring programs to assess the effects of environmental contamination on non-target organisms. Given that the highly conserved retinoid (RETs) metabolism supports many biological functions and is perturbed by xenobiotics and used as biomarker for vertebrates, we explored the RETs functions in the crustacean model Gammarus fossarum. More specifically, we studied the implication of all -trans retinoic acid (atRA) in the reproduction (embryo, oocyte, and juvenile production) and development (success and delay of molting) by exposing G. fossarum females to atRA and citral (CIT), a known inhibitor of RA synthesis. In parallel, we exposed gammarids to methoprene (MET) and glyphosate (GLY), two pesticides suspected to interfere with atRA metabolism and signaling and frequently found in water systems. After 14 days of exposure, atRA, CIT, and MET reduced the number of oocytes, whereas only MET caused a reduced number of embryos. After 44 days, MET and GLY showed a tendency to decrease juvenile production. The duration of the molting cycle increased following the exposures to atRA and MET, while the treatment with CIT caused a typical endocrine disruptive inverted U-shaped curve. The exposure to GLY led to increased duration of the molting cycle at the lowest concentrations and lowered molting success at the highest concentration tested. This study highlights for the first time the implication of RA in the oogenesis and molting of G. fossarum and suggests that it may be a potential mediator of MET-induced effects on these processes. This study adds to the comprehension of the reproductive and developmental control in G. fossarum and opens new research avenues to study the effects of xenobiotics on the RET system in this sentinel species. Ultimately, our study will drive the development of RET-based biomarkers for non-target aquatic invertebrates exposed to xenobiotics.

Cadmium affects autophagy in the human intestinal cells Caco-2 through ROS-mediated ERK activation

Cadmium is a toxic metal that enters the food chain. Following oral ingestion, the intestinal epithelium has the capacity to accumulate high levels of this metal. We have previously shown that Cd induces ERK1/2 activation in differentiated but not proliferative human enterocytic-like Caco-2 cells. As autophagy is a dynamic process that plays a critical role in intestinal mucosa, we aimed the present study 1) to investigate the role of p-ERK1/2 in constitutive autophagy in proliferative Caco-2 cells and 2) to investigate whether Cd-induced activation of ERK1/2 modifies autophagic activity in postconfluent Caco-2 cell monolayers. Western blot analyses of ERK1/2 and autophagic markers (LC3, SQSTM1), and cellular staining with acridine orange showed that ERK1/2 and autophagic activities both decreased with time in culture. GFP-LC3 fluorescence was also associated with proliferative cells and the presence of a constitutive ERK1/2-dependent autophagic flux was demonstrated in proliferative but not in postconfluent cells. In the latter condition, serum and glucose deprivation triggered autophagy via a transient phosphorylation of ERK1/2, whereas Cd-modified autophagy via a ROS-dependent sustained activation of ERK1/2. Basal autophagy flux in proliferative cells and Cd-induced increases in autophagic markers in postconfluent cells both involved p-ERK1/2. Whether Cd blocks autophagic flux in older cell cultures remains to be clarified but our data suggest dual effects. Our results prompt further studies investigating the consequences that Cd-induced ERK1/2 activation and the related effect on autophagy may have on the intestinal cells, which may accumulate and trap high levels of Cd under some nutritional conditions.

The retinoid metabolism of Gammarus fossarum is disrupted by exogenous all-trans retinoic acid, citral, and methoprene but not by the technical formulation of glyphosate

Over the last decade, fluctuations of retinoids (RETs), also known as vitamin A and derivatives, have proved to be useful biomarkers to assess the environmental chemical pressure on a wide variety of non-target vertebrates. This use of RET-based biomarkers is of particular interest in the non-target sentinel species Gammarus fossarum in which RETs were shown to influence crucial physiological functions. To study and probe this metabolism in this crustacean model, a UHPLC-MS/MS method was developed to 1) identify and 2) monitor several endogenous RETs in unexposed females throughout their reproductive cycle. Then, females were exposed in controlled conditions to exogenous all-trans retinoic acid (atRA) and citral (CIT), a RA synthesis inhibitor, to simulate an excess or deficiency in RA. Perturbation of vitamin A metabolism by pesticides was further studied in response to methoprene (MET), a juvenile hormone analog as well as glyphosate (GLY). The developed method allowed, for the first time in this model, the identification of RA metabolites (all-trans 4-oxo and 13-cis 4-oxo RA), RA isomers (all-trans and 13-cis RA) as well as retinaldehyde (RALD) isomers (all-trans, 11-cis, and 13-cis RALD) and showed two distinct phases in the reproductive cycle. Retinoic acid successfully increased the tissular concentration of both RA isomers and CIT proved to be efficient at perturbating the conversion from RALD to RA. Methoprene perturbed the ratios between RA isomers whereas GLY had no observed effects on the RET system of G. fossarum females. We were able to discriminate different dynamics of RET perturbations by morphogens (atRA or CIT) or MET which highlights the plausible mediation of RETs in MET-induced disorders. Ultimately, our study shows that RETs are influenced by exposure to MET and strengthen their potential to assess aquatic ecosystem chemical status.

Pesticides Inhibit Retinoic Acid Catabolism in PLHC-1 and ZFL Fish Hepatic Cell Lines

The population of yellow perch (Perca flavescens) in lake Saint-Pierre (QC, Canada) has been dramatically declining since 1995 without any sign of recovery. Previous studies have shown disrupted retinoid (vitamin A) metabolic pathways in these fish, possibly due to the influence of pesticides. Our study aimed to evaluate the impact of some herbicides and neonicotinoids on retinoic acid catabolism in the fish hepatic cell lines PLHC-1 and ZFL. We hypothesized that pesticides accelerate the catabolism of retinoic acid through oxidative stress that exacerbates the oxidation of retinoic acid. Results obtained with talarozole, a specific CYP26A1 inhibitor, and ketoconazole, a generalist inhibitor of cytochrome-P450 enzymes, revealed that CYP26A1 is mainly responsible for retinoic acid catabolism in ZFL but not PLHC-1 cells. The impacts of pesticides on retinoic acid catabolism were evaluated by incubating the cells with all-trans-retinoic acid and two herbicides, atrazine and glyphosate, or three neonicotinoids, clothianidin, imidacloprid, and thiamethoxam. Intracellular thiols and lipid peroxidation were measured following pesticide exposure. The possible causal relation between oxidative stress and the perturbation of retinoic acid catabolism was investigated using the antioxidant N-acetylcysteine. The data revealed that pesticides inhibit retinoic acid catabolism, with the involvement of oxidative stress in the case of atrazine, imidacloprid, and thiamethoxam but not with clothianidin and glyphosate. Pesticides also affected the isomerization of all-trans-retinoic acid over time, leading to an increased proportion of active isomers. These results hint at a possible perturbation of retinoic acid catabolism in fish living in pesticide-contaminated waters, as suggested by several in vivo studies. Such a disruption of retinoid metabolism is worrying, given the numerous physiological pathways driven by retinoids.

The countryside or the city: Which environment is better for the honeybee?

For a number of years, the decline of honeybee (Apis mellifera) in North America and Europe has been the subject of much debate. Among the many factors proposed by hundreds of studies to explain this phenomenon is the hypothesis that agricultural activities using pesticides contribute to the weakness of bee colonies. Moreover, while urban beekeeping is presently booming in several cities, we do not know if this environment is more beneficial for bees than the typical, rural area. In the summer of 2018, we sampled honeybees (foragers and larvae) in rural (Laurentians) and urban (city of Montreal) areas and compared them using the following biomarkers: carotenoids, retinoids, α-tocopherol, metallothionein-like proteins (MTLPs), lipid peroxidation, triglycerides, acetylcholinesterase activity (AChE) and proteins. Pesticides, pharmaceuticals and personal care products (PPCPs) and metals were also quantified in honeybees' tissues. Our result revealed that, globally, urban foragers had higher levels of insecticides and PPCPs and that metals were in greater concentrations in urban larvae. Compared to rural foragers, urban foragers had higher concentrations of MTLPs, triglycerides, protein and AChE activity. The multifactorial analysis confirmed that insecticides, some metals and PPCPs were the most influential components in the contaminant‒biomarker relationships for both foragers and larvae.

Foraging in maize field areas: A risky business?

In Quebec, Canada, the cultivation of maize dominates the agricultural territory. This crop requires a sustained supply of fertilizers from different sources: chemical, natural or from residual materials (sludge). These amendments contain metallic trace elements, which may lead to metal-contaminated maize pollen, a possible source of prooxidants for the foraging bees. Our objective was to determine whether maize fields environment influences the oxidation processes and the accumulation of metals in bees. A few days prior to pollen shedding, beehives were installed in maize fields: one organically grown (site A) and three conventionally grown (sites B, C and D). Soil, maize pollen and bees were analyzed for metal content. Every 15days, bees were collected and analyzed for peroxidation of lipids, metallothionein-like proteins (MTLPs), proteins, retinoids and lipophilic antioxidants (carotenoids and α-tocopherol). The compound β-carotene was the most abundant in bees from all sites, followed by α-carotene, β-cryptoxanthin, α-cryptoxanthin, zeaxanthin and lutein. Retinaldehyde and retinol varied according to times and sites without demonstrating clear trends. However, significant differences between sites were noted in 13-cis-retinoic acid and two retinoic acid metabolites measured in bees, suggesting alteration in the reduction-oxidation processes. In line with these results, the level of lipid peroxidation was globally higher in sites B, C and D compared with the organic site. Higher concentrations of metals were observed in soil and pollen from the field A, but bees metal contents were equal or less than those measured in bees from other sites. Higher bee MTLP levels were measured in sites B, C and D. For most sampling times, the discriminant analysis revealed that the conditions were distinguished by the oxidation processes in bees. Our data suggest that bees foraging in conventionally grown maize fields are at risk of increased oxidative damages which can alter the fine regulation of retinoids.

Mixtures of herbicides and metals affect the redox system of honey bees

The increasing loss of bee colonies in many countries has prompted a surge of studies on the factors affecting bee health. In North America, main crops such as maize and soybean are cultivated with extensive use of pesticides that may affect non-target organisms such as bees. Also, biosolids, used as a soil amendment, represent additional sources of metals in agroecosystems; however, there is no information about how these metals could affect the bees. In previous studies we investigated the effects of environmentally relevant doses of herbicides and metals, each individually, on caged honey bees. The present study aimed at investigating the effects of mixtures of herbicides (glyphosate and atrazine) and metals (cadmium and iron), as these mixtures represent more realistic exposure conditions. Levels of metal, vitamin E, carotenoids, retinaldehyde, at-retinol, retinoic acid isomers (9-cis RA, 13-cis RA, at-RA) and the metabolites 13-cis-4-oxo-RA and at-4-oxo-RA were measured in bees fed for 10 days with contaminated syrup. Mixtures of herbicides and cadmium that did not affect bee viability, lowered bee α- and β-carotenoid contents and increased 9-cis-RA as well as 13-cis-4-oxo-RA without modifying the levels of at-retinol. Bee treatment with either glyphosate, a combination of atrazine and cadmium, or mixtures of herbicides promoted lipid peroxidation. Iron was bioconcentrated in bees and led to high levels of lipid peroxidation. Metals also decreased zeaxanthin bee contents. These results show that mixtures of atrazine, glyphosate, cadmium and iron may affect different reactions occurring in the metabolic pathway of vitamin A in the honey bee.

Diamine Oxidase from White Pea (Lathyrus sativus) Combined with Catalase Protects the Human Intestinal Caco-2 Cell Line from Histamine Damage

Diamine oxidase (DAO) administration has been proposed to treat certain gastrointestinal dysfunctions induced by histamine, an immunomodulator, signaling, and pro-inflammatory factor. However, H₂O₂ resulting from the oxidative deamination of histamine by DAO may be toxic. The purpose of this study was to investigate to which extent DAO from white pea (Lathyrus sativus), alone or in combination with catalase, may modulate histamine toxicity in the human intestinal Caco-2 cell line. The results show that histamine at concentrations higher than 1 mM is toxic to the Caco-2 cells, independently of the cell differentiation status, with a LC₅₀ of ≅ 10 mM following a 24-h exposure. Depending on its concentration, DAO increased histamine toxicity to a greater extent in differentiated cells compared to undifferentiated cultures. In the presence of catalase, the DAO-induced increase in histamine toxicity was completely abolished in the undifferentiated cells and only partially decreased in differentiated cells, showing differences in the sensitivity of Caco-2 cells to the products resulting from histamine degradation by DAO (H₂O₂, NH₃, or imidazole aldehyde). It appears that treatment of food histaminosis using a combination of vegetal DAO and catalase would protect against histamine toxicity and prevent H₂O₂-induced damage that may occur during histamine oxidative deamination.

Low dietary levels of Al, Pb and Cd may affect the non-enzymatic antioxidant capacity in caged honey bees (Apis mellifera)

Several hypotheses have been proposed to explain the abnormally high mortality rate observed in bee populations in Europe and North America. While studies based on the effects of pesticides are paramount, the metals present in agroecosystems are often overlooked. Sources of metals are linked to the nature of soils and to agricultural practices, namely the use of natural or chemical nutrients as well as residual materials from waste-water treatment sludge. The aim of this study was to investigate the effects of metals on honey bees exposed for 10 days to environmentally realistic concentrations of Al, Pb and Cd (dissolved in syrup). The monitoring of syrup consumption combined with the quantification of metals in bees revealed the following order for metal bioconcentration ratios: Cd > Pb > Al. Alpha-tocopherol, metallothionein-like proteins (MTLPs) and lipid peroxidation were quantified. When bees were exposed to increasing amounts of Cd, a marked augmentation of MTLPs levels was found. Lead (Pb) and Cd caused an increase in α-tocopherol content, while alteration of lipid peroxidation was observed only with Al exposure. These findings raise concerns about the bioavailability and the additional threat posed by metals for pollinators in agricultural areas while providing new insights for potential use of the honey bee as a sentinel species for metal exposure.

Effects of realistic doses of atrazine, metolachlor, and glyphosate on lipid peroxidation and diet-derived antioxidants in caged honey bees (Apis mellifera)

The decline in the population of pollinators is a worrying phenomenon worldwide. In North America, the extensive use of herbicides in maize and soya crops may affect the health of nontarget organisms like the honey bee. In this study, caged honey bees were exposed to realistic doses of atrazine, metolachlor, and glyphosate for 10 days via contaminated syrup. Peroxidation of lipids was evaluated using the thiobarbituric acid reactive substance (TBARS) test, and diet-derived antioxidants-carotenoids, all-trans-retinol (at-ROH) and α-tocopherol-were detected and quantified using reversed-phase HPLC techniques. Significant increases in syrup consumption were observed in honey bees exposed to metolachlor, and a lower TBARS value was recorded for the highest dose. No relationship was observed between the peroxidation of lipids and the levels of antioxidants. However, β-carotene, which was found to be the most abundant carotenoid, and at-ROH (derived from β-carotene) both decreased with increasing doses of atrazine and glyphosate. In contrast, metolachlor increased levels of at-ROH without any effects on β-carotene. These results show that the honey bee carotenoid-retinoid system may be altered by sublethal field-realistic doses of herbicides.

Cadmium interference with ERK1/2 and AhR signaling without evidence for cross-talk

The possibility that Cd may activate AhR indirectlyviaERK1/2 phosphorylation was tested as a function of enterocytic differentiation status in the human Caco-2 cells.

Expression of macrophage migration inhibitory factor by osteoblastic cells: protection against cadmium toxicity

Exposition to cadmium (Cd) has been linked to bone metabolism alterations and occurrence of osteoporosis. Despite its known renal toxicity which indirectly disrupts bone metabolism through impairment of vitamin D synthesis, increasing evidence argues for the direct action of Cd on bone-forming osteoblasts. Indeed, accumulation of Cd in osteoblasts and metal-induced cell death has been documented but little is known about the intracellular mechanisms of protection against this stress. In this work, we investigated the protection afforded by thiol-containing proteins against Cd cytotoxicity in MC3T3 osteoblastic cells. Viability of MC3T3 cells was reduced by Cd in a concentration-dependent manner with a LC(50) of 7.6±1.1μM. Depletion of glutathione by l-buthionine sulphoximine (BSO) increased cell sensitivity to Cd cytotoxicity, suggesting the involvement of thiol-containing peptides as a mechanism of protection. Accordingly, Cd was shown to promote progressive depletion of reduced thiol content and to stimulate the production of reactive oxygen species (ROS). Interestingly, low non cytotoxic concentrations of Cd increased the gene expression of macrophage migration inhibitory factor (MIF), also a thiol-containing protein. Inhibition of the transcription factor NFκB prevented Cd-dependent upregulation of MIF expression and consequently, increased Cd cytotoxicity in osteoblasts. Moreover, MIF deficient mouse osteoblasts were more sensitive to Cd cytotoxicity than the corresponding control cells. By gel-filtration chromatography, we demonstrated that MIF acts as a thiol-containing protein and thereby promotes Cd complexation. In accordance with its binding ability, addition of recombinant MIF to the culture medium reduced Cd cytotoxicity. Overall, upregulation of MIF expression by Cd may protect against the cytotoxicity of this metal in the osteoblasts.

Involvement of transient receptor potential melastatin-related 7 (TRPM7) channels in cadmium uptake and cytotoxicity in MC3T3-E1 osteoblasts

Exposure to cadmium (Cd) disrupts bone metabolism, causing osteoporosis. Impaired vitamin D metabolism was initially proposed as the underlying mechanism, yet recent studies argue for the direct effect of Cd on bone cells. This study aimed at characterizing (109)Cd uptake and cytotoxicity in MC3T3-E1 osteoblasts. Time-dependent accumulation of (109)Cd was observed with a 50% lethal concentration (LC(50)) of 9.6 ± 1.2 μM at 24-h. Reducing extracellular calcium (Ca) or magnesium (Mg) increased Cd cytotoxicity. The presence of Ca, Mg, zinc or gadolinium decreased (109)Cd uptake suggesting the involvement of non-selective cationic channels. The Mg-sensitive part of (109)Cd uptake increased at acidic pH, a condition known to stimulate TRPM7 channel activity. Stimulating TRPM7 channel activity by cellular Mg starvation enhanced (109)Cd uptake. Silencing TRPM7 channel expression abolished the Mg-sensitive and the Mg starvation-induced uptake indicating that TRPM7 is involved in Cd transport in osteoblasts.

Cadmium-induced hormetic effect in differentiated Caco-2 cells: ERK and p38 activation without cell proliferation stimulation

Cadmium (Cd) is a toxic metal that enters the food chain. Following oral ingestion, the intestinal epithelium may in part protect against Cd toxicity but is also a target tissue. Using human enterocytic-like Caco-2 cells, we have previously shown differences in sensitivity to Cd according to the differentiation status. The present study focuses on Cd effects on differentiated cells. Concentration and time-dependent increases in MTT (3-[4,5-dimethyl-2-thiazol-2-yl]-2,5-diphenyltetrazolium bromide assay) activity were observed in post-confluent cultures exclusively, with a twofold maximal stimulation in 21-day-old cells exposed to 10 microM Cd for 24 h. No concomitant increase in [methyl-(3)H] thymidine incorporation was noted and Cd did not modify cell distribution in the cell-cycle phases. However, Cd-induced increase in MTT activity was inhibited by cycloheximine as well as by inhibitors of ERK1/2 and p38, but not by that of JNK. Consistently, Cd increased the levels of ERK1/2 and p38 phosphorylation. Inhibition of Ras-GTP or PI3K enhanced the stimulatory effect of Cd, whereas mTOR inhibition had no effect. Inhibition of G protein-phospholipase and PKC decreased MTT stimulation. These results show a hormesis-like stimulation of Cd on MTT activity in differentiated intestinal cells exclusively. This effect is not related to cell proliferation but more likely to increased protein synthesis which involves ERK1/2 and p38 cascades and possibly PLC-beta signaling pathways. Because growth-related differentiation of intestinal cells is linked to the selective and sequential activation of MAPKs, the impacts that these Cd-induced perturbations in signaling pathways may have on intestinal functions clearly deserve to be investigated.

Capacitative calcium entry and proliferation of human osteoblast-like MG-63 cells

Adult bone tissue is continuously being remodelled and bone mass is maintained by a balance between osteoclastic bone resorption and osteoblastic bone formation. Alteration of osteoblastic cell proliferation may account in part for lack of balance between these two processes in bone loss of osteoporosis. There is calcium (Ca2+) control in numerous cellular functions; however, involvement of capacitative Ca2+ entry (CCE) in proliferation of bone cells is less well investigated.

OBJECTIVES

The study described here was aimed to investigate roles of CCE in the proliferation of osteoblast-like MG-63 cells.

MATERIALS AND METHODS

Pharmacological characterizations of CCE were undertaken in parallel, with evaluation of the expression of transient receptor potential canonical (TRPC) channels and of cell proliferation.

RESULTS

Intracellular Ca2+ store depletion by thapsigargin induced CCE in MG-63 cells; this was characterized by a rapid transient increase of intracellular Ca2+ followed by significant CCE, induced by conditions that stimulated cell proliferation, namely serum and platelet-derived growth factor. Inhibitors of store-operated Ca2+ channels (2-APB and SKF-96365) prevented CCE, while voltage-dependent Ca2+ channel blockers had no effect. Expression of various TRPC channels was shown in the cells, some having been shown to be responsible for CCE. Voltage-dependent Ca2+ channel blockers had no effect on osteoblast proliferation while thapsigargin, 2-APB and SKF-96395, inhibited it. Cell cycle analysis showed that 2-APB and SKF-96395 lengthen the S and G2/M phases, which would account for the reduction in cell proliferation.

CONCLUSIONS

Our results indicate that CCE, likely attributed to the activation of TRPCs, might be the main route for Ca2+ influx involved in osteoblast proliferation.

Reciprocal inhibition of Cd and Ca uptake in isolated head kidney cells of rainbow trout (Oncorhynchus mykiss)

Some environmental pollutants, including cadmium (Cd) and zinc (Zn), can act as endocrine disruptors in fish, either in vivo or through a direct action on steroidogenic cells, as has been demonstrated in vitro. We have previously characterized Cd uptake in head kidney (homologue of mammalian adrenal) cells of rainbow trout (Oncorhynchus mykiss) and have provided evidence for a Cd/Ca interaction. Here, we pursued our investigation of metal competition for uptake. Our results show that inorganic speciation conditions favour Cd uptake with optimal level of accumulation for Cd2+ compared to chlorocomplexes (CdCl(n)(2-n)). Calcium uptake was studied for the first time in the fish head kidney cells and Ca was found to be less efficiently accumulated compared to Cd. A specific saturable mechanism of transport was characterized for Ca uptake but voltage-gated or La-sensitive cationic channels are unlikely to contribute appreciably. A concentration-dependent reciprocal inhibition was observed between Ca and Cd, whereas, Zn proved to inhibit Cd uptake exclusively. Additive inhibitory effect on Cd uptake was obtained with co-exposure to Ca and Zn. We conclude that Cd, but not Zn, may decrease Ca availability to the head kidney tissue. Also, Zn may partially protect against Cd toxicity but Zn would not protect against Cd-induced perturbation of Ca homeostasis.

Cadmium-induced apoptosis in rat hepatocytes does not necessarily involve caspase-dependent pathways

Cadmium (Cd) is a well-known hepatotoxic environmental pollutant. Depending on the exposure conditions, Cd may cause necrosis or apoptosis. Oxidative stress is believed to participate in Cd toxicity but the molecular signaling responsible for Cd-induced apoptosis in non-malignant liver cells still needs to be clarified. Therefore we have studied apoptosis in primary cultures of rat hepatocytes incubated with low levels of Cd for short exposure times. Studies of nuclear morphology, chromatin condensation, and oligonucleosomal DNA fragmentation demonstrate that 1-5 microM Cd induces apoptosis as early as 6-12 h with minor effects on MTT activity. A concomitant time- and concentration-dependent increase in caspase-9 and -3 activities was observed, whereas Cd did not affect caspase-8 activity as much, suggesting a minor role of the death-receptor pathway. Significant release of cytochrome c into the cytosol demonstrated the involvement of a mitochondrial-dependent apoptotic pathway. However, cell pre-treatment with caspase inhibitors (Z-VAD-fmk or Ac-DEVD-CHO) did not prevent apoptosis. Increases in the cytosolic levels of the mitochondrial apoptosis-inducing factor (AIF) were also observed: kinetics of cytochrome c and AIF release were similar. These results show that Cd-induced apoptosis in rat hepatocytes is time- and concentration-dependent. The early apoptotic events involved mitochondrial-dependent pathways but not necessarily caspase-dependent signaling.

Effects of Cu on plasma cortisol and cortisol secretion by adrenocortical cells of rainbow trout (Oncorhynchus mykiss)

Fish are exposed to multiple stressors, often acting concurrently, in their environment. To evaluate the potential of Cu to act as a chemical stressor, rainbow trout (Oncorhynchus mykiss) were exposed to Cu (30 or 80 microg/l) for 30 days in the laboratory and they were subjected to a physical stressor (1 min air exposure) before sampling. Physiological stress indicators in the whole fish as well as cortisol secretion by adrenocortical cells in vitro were measured. Fish exposed to Cu had a lower condition factor, hepatosomatic index, plasma glucose, hepatic glycogen and gill Na(+)/K(+)-ATPase activity compared to controls. Exposure to Cu did not have an effect on basal plasma cortisol (fish sampled without air exposure stress) however, the air exposure-induced increase in plasma cortisol was lower in fish exposed to Cu. Cortisol secretion stimulated by ACTH in vitro was greater in adrenocortical cells isolated from fish exposed to Cu in vivo but in vitro exposure to Cu consistently impaired cortisol secretion. Our results indicate that Cu at high concentrations disrupts cortisol secretion through a direct toxic effect on adrenocortical cells while low concentrations resulting from a 30-day exposure to environmentally relevant Cu concentrations enhances cortisol secretion in response to ACTH in vitro.

Reciprocal inhibition of Cd(2+) and Ca(2+) uptake in human intestinal crypt cells for voltage-independent Zn-activated pathways

Cadmium-Ca-Zn interactions for uptake have been studied in human intestinal crypt cells HIEC. Our results failed to demonstrate any significant cross-inhibition between Cd and Ca uptake under single metal exposure conditions. However, they revealed a strong reciprocal inhibition for a Zn-stimulated mechanism of transport. Optimal stimulation was observed under exposure conditions that favor an inward-directed Zn gradient, suggesting activation by extracellular rather than intracellular Zn. The effect of Zn on the uptake of Ca was concentration-dependent, and zinc-induced stimulation of Cd uptake resulted in a 3- and 5.8-fold increase in the K(m) and V(max) values, respectively. Neither basal nor Zn-stimulated Ca uptakes were sensitive to membrane depolarization. However, the stimulated component of uptake was inhibited by the trivalent cations Gd(3+), and La(3+) and to a lesser extent by Mg(2+) and Ba(2+). RT-PCR analysis as well as uptake measurement performed with extracellular ATP and/or suramin do not support the involvement of purinergic P2X receptor channels. Uptake and fluorescence data led to the conclusion that Zn is unlikely to trigger Ca influx in response to Ca release from thapsigargin-sensitive intracellular pools. Our data show that Zn may potentiate Cd accumulation in intestinal crypt cells through mechanism that still needs to be clarified.

In vitro bioactivity of combustion products from 12 tobacco constituents

Twelve chemical components of tobacco leaf, representing 50% of its dry weight, were individually combusted and the bioactivities of their combustion products i.e. total particulate matter (TPM) were assayed using three in vitro tests. These components included carbohydrates, amino acids, proteins, polyphenols and carboxylic acids. The mutagenic potencies were assessed with the Salmonella mutagenicity assay (S. typhimurium TA98 and TA100). The induction of chromosomal damage, determined with the micronucleus test (IVMNT), and the neutral red uptake cytotoxicity test (NRU), were conducted on V79 hamster lung fibroblast cells. The Salmonella mutagenicity test and IVMNT were conducted with and without rat liver microsomal S9 fraction. Salmonella mutagenicity data confirmed the mutagenicity of TPM samples obtained from nitrogenous compounds (amino acids and proteins). The IVMNT showed that precursors of phenols in smoke (i.e. polyphenols) exhibited significantly higher levels of toxicity compared to other tobacco components. While S9 activation amplified the Salmonella mutagenicity response to combustion products, it significantly inhibited the toxicity measured with the IVMNT. NRU data demonstrated the increasing cytotoxicity induced following longer exposure time to TPM samples from nitrogenous and phenolic components. This study is the first to characterize the toxicity of the combustion products of major tobacco constituents. Our data suggest different mechanisms of toxicity and underline the relevance of using various bioassays.

Characterization of cadmium uptake in human intestinal crypt cells HIEC in relation to inorganic metal speciation

Cadmium (Cd) uptake was studied under inorganic exposure conditions in normal human intestinal crypt cells HIEC. The uptake time course of 0.3 microM Cd in a serum-free chloride medium was analyzed according to a first order equation with rapid initial (U0) and maximal (Umax) accumulation values of 14.1+/-1.4pmol/mgprotein and 41.4+/-2.0 pmol/mgprotein, respectively. The presence of a 300-fold excess of unlabeled Cd dramatically decreased tracer uptake, showing the involvement of specific mechanism(s) of transport. Our speciation studies revealed the preferential uptake of the free ion Cd2+, but also suggested that CdCln(2-n) species may contribute to Cd accumulation. Specific mechanisms of transport of very high and similar affinity (Km approximately 5 microM) have been characterized under both chloride and nitrate exposure conditions, but a two-fold higher capacity (Vmax) was estimated in the nitrate medium used to increase [Cd2+] over chlorocomplex formation. A clear inhibition of 109Cd uptake was observed at external acidic pH under both exposure media. An La-inhibitible 46% increase in 109Cd uptake was obtained in nominally Ca-free nitrate medium, whereas Zn provided additional inhibition. These results show different kinetic parameters for Cd uptake as a function of inorganic metal speciation. Cd2+ uptake would not involve the H+-coupled symport NRAMP2 but would be related instead to the Ca and/or Zn pathways. Because proliferative crypt cells play a critical role in the renewal process of the entire intestinal epithelium, studies on the impact of Cd on HIEC cell functions clearly deserve further investigation.

Reciprocal inhibition of Cd and Pb sulfocomplexes for uptake in Caco-2 cells

Cadmium-lead interactions for uptake were studied in the TC7 clone of human enterocytic-like Caco-2 cells as a function of inorganic metal speciation. We have previously shown that Cd uptake in these cells involves both the free cation Cd2+ and chlorocomplex (CdCln(2-n)) species. Here we show 1.9 times higher uptake levels for 109CdCln(2-n) compared to 210PbCln(2-n). Reciprocal inhibitions of chlorocomplexes were observed with a much higher inhibitory effect of Cd compared to Pb. Replacing Cl- by NO3- increased both the level of aquo ion 109Cd2+ and 109Cd accumulation. In contrast, higher levels of 210Pb2+ did not favor 210Pb uptake. For both metals, higher uptake data were recorded in the presence of SO4(2-), leading to sulfocomplex formation, compared with Cl-. Reciprocal inhibitions were minimal at high-cation levels but were significant and comparable in the presence of sulfo-complexes. We conclude that, in addition to Cd2+ (but not Pb2+), sulfocomplexes of both metals would preferentially be taken up compared to chlorocomplexes. NRAMP2 is not involved in Pb2+ uptake, and the NRAMP2-mediated Cd2+ uptake is insensitive to Pb. Uptake of Pb chlorocomplexes could involve specific mechanisms but of very low affinity, whereas uptake of Pb sulfocomplexes occurs with high affinity.

Kinetics of the early subcellular distribution of cadmium in rat hepatocytes

The kinetics of the early subcellular distribution of cadmium (Cd) was characterized in primary cultures of rat hepatocytes exposed to 10, 50 and 100 microM Cd in a serum-free WME medium for 10, 30 or 60 min. Our results demonstrate a time- and concentration-dependent increase in Cd content with the highest metal concentration measured in the cytosol, whereas the lowest was observed in the mitochondria. With the exception of early localization in the plasma membrane, Cd concentrations in fractions were characterized by the following decreasing order of magnitude: cytosol > low density molecules approximately nuclei > lysosomes approximately mitochondria. We also found evidence for: (i) a two-step process for Cd distribution in the nuclei and mitochondria; and (ii) a time-dependent 'slow' process of transfer from the plasma membrane to the cytosol. Saturation in Cd uptake was observed at 50 microM in most cell fractions at 10 and 30 min, except for the plasma membrane. The lack of apparent saturation for Cd accumulation at 60 min was not related to an increase in metallothionein synthesis. Altogether, our data provide insights into the dynamics of transfer between intracellular compartments, and allow a better identification of the organelles that are the most subjected to Cd toxicity for early exposure conditions.

Multiple effects of mercury on cell volume regulation, plasma membrane permeability, and thiol content in the human intestinal cell line Caco-2

In a previous study, we characterized Cd-Hg interactions for uptake in human intestinal Caco-2 cells. We pursued our investigations on metal uptake from metal mixtures, focusing on the effects of Hg on cellular homeostasis. A 4-fold higher equilibrium accumulation value of 0.3 micromol/L (203)Hg was measured in the presence of 100 micromol/L unlabeled Hg in the serum-free exposure medium without modification in the initial uptake rate. This phenomenon was eliminated at 4 degrees C. Mercury induced an increase in tritiated water and [(3)H]mannitol uptakes for exposure times greater than 20 min. Incubations for 20 min and 30 min with 100 micromol/L Hg and 2 mmol/L N-ethylmaleimide (NEM) resulted in a 34% and 50% reductions in cellular thiol staining, respectively, with additive effects. Lactate dehydrogenase leakage and live/dead assays confirmed the maintenance of cell membrane integrity in Hg- or NEM-treated cells. We conclude that Hg may alter membrane permeability and increase cell volume without any loss in cell viability. This phenomenon is sensitive to temperature and could involve Hg interaction with membrane thiols, possibly related to solute transport. During metal uptake from metal mixtures, Hg may thus promote the uptake of other toxic metals by increasing cell volume and consequently cell capacity.

Cadmium uptake in isolated adrenocortical cells of rainbow trout and yellow perch

Cadmium uptake was studied in isolated adrenocortical cells of rainbow trout (Oncorhynchus mykiss) and yellow perch (Perca flavescens) to test the hypothesis that the greater sensitivity of trout cells to Cd-induced disruption of cortisol secretion observed in previous studies is correlated to higher level of metal accumulation. There was no evidence for interspecies differences in accumulation level, and a specific transport mechanism of similar affinity has been characterized in both fish species. However, inhibition of Cd uptake by calcium was observed in rainbow trout exclusively. The free metal ion Cd(2+) and chlorocomplexes CdCl(n)(2-n) both contribute to Cd accumulation with different level of contribution between fish species. We conclude that interspecies differences in sensitivity to Cd endocrine disrupting effect are not necessarily related to different levels of metal accumulation but would rather be linked to transport pathways and metal speciation. Cadmium/calcium competition for uptake could be a determinant of the early Cd-induced impaired cortisol secretion in trout but not perch cells.

Cadmium uptake in rat hepatocytes in relation to speciation and to complexation with metallothionein and albumin

Cadmium (Cd) uptake has been studied in primary cultures of rat hepatocytes focusing on the impact of inorganic and organic speciation. Uptake time-course studies over a 60-min exposure to 0.3 microM (109)Cd revealed a zero-time uptake and a slower process of accumulation which proceeds within minutes. (109)Cd uptake showed saturation kinetics (K(m) = 3.5 +/- 0.8 microM), and was highly sensitive to inhibition by Zn and Hg. There was no evidence for sensitivity to the external pH nor for any preferential transport of the free cation Cd(2+) over CdCl(n) (2-n) chloro-complexes. According to the assumption that only inorganic metal species are available, metal uptake decreased upon albumin (BSA) addition to the exposure media. In contrast, higher levels of (109)Cd accumulation were obtained under optimal conditions for Cd complexation by MT. Comparison among uptake data obtained under inorganic and organic conditions revealed that Cd-MT would be taken up 0.4 times as rapidly as Cd(inorg). We conclude that uptake of Cd in rat hepatocytes involves specific transport mechanism(s) subjected to Zn or Hg interactions. Uptake of inorganic Cd is not proportional to the levels of free Cd(2+) and does not involve the divalent cation transporter DCT1 nor the co-transporter Fe(2+)-H(+) NRAMP2. We found Cd-MT but not Cd-BSA to be available for the liver cells, and have estimated a binding affinity four orders of magnitude higher for Cd complexation with MT compared to BSA; MT may have a significant role in Cd delivery to the liver.

Cadmium-Induced Mitochondrial Membrane-Potential Dissipation Does Not Necessarily Require Cytosolic Oxidative Stress: Studies Using Rhodamine-123 Fluorescence Unquenching

The impact of cadmium on the cellular redox state and mitochondrial membrane potential (psi(m)) has been studied by monitoring dichlorofluorescein (DCF), CMXRos (dichlorodihydrofluorescein diacetate, chloromethyl-X-rosamine), and Rh-123 fluorescence in 5-day-old TC7 cells, a highly differentiated clone of the human intestinal Caco-2 cell line. Flow cytometry analyses, using DCFH oxidation to DCF, clearly revealed that a 30-min incubation to 50 microM cadmium (Cd) is sufficient to induce reactive oxygen species (ROS) formation; this effect was completely eliminated by the presence of 50 mM mannitol for the 30-min incubation period, but mannitol only partially scavenged ROS for the longer period of time studied. Imaging studies using fluorescence video microscopy revealed a parallel increase in (DCF) fluorescence in the nuclear and cytoplasmic regions as soon as Cd was added to the exposure medium. Flow cytometry analyses monitoring CMXRos fluorescence clearly showed that Cd also leads to psi(m) disruption, but, contrary to what was observed for ROS formation, mannitol was completely inefficient in preventing this effect. Further investigation using fluorescence microscopy and Rh-123 fluorescence unquenching revealed that although mannitol did not protect against Cd-induced dissipation of psi(m), it considerably delayed the process. We found that Rh-123 unquenching, occurring during probe redistribution, is a suitable tool to monitor the decrease of psi(m). We conclude that Cd rapidly induces ROS formation, mainly hydroxyl radical species OH(*), as well as the loss of psi(m). However, psi(m) dissipation does not necessarily require cellular OH(*) and may occur in the absence of apparent oxidative injury.

Different transport mechanisms for cadmium and mercury in Caco-2 cells: inhibition of Cd uptake by Hg without evidence for reciprocal effects

Cadmium/Hg interactions have been studied in the TC7 clone of the enterocytic-like Caco-2 cells to test the hypothesis that these metals may compete for intestinal transport. Comparison of the kinetic parameter values for 203Hg(II) and 109Cd(II) uptake in a serum-free medium revealed that Hg is accumulated much more rapidly and to higher concentrations. The very rapid uptake/binding step and the initial uptake rate of 109Cd were both significantly inhibited by an excess of unlabeled Cd or Hg (apparent K(i) for Hg of 9.3 +/- 1.2 microM) without reciprocal effects. 109cadmium uptake was highly sensitive to temperature and a significant fraction of accumulation (12%) was EDTA extractable. 203Hg uptake remained insensitive to temperature or the EDTA washing procedure. However, the uptake of both tracers was half-decreased when an excess of the respective unlabeled metal was added in the stop solution, suggesting an exchange mechanism for adsorption. Cell pretreatment with N-ethylmaleimide (NEM) led to a 30% decrease or a 73% increase in the 3-min specific transport of 109Cd when NEM was still present in or removed from the uptake medium, respectively. NEM had no effect on 203Hg uptake. Overall our results suggest the involvement of a saturable specific mechanism for Cd, which is highly sensitive to inhibition by Hg and NEM under some conditions, and a nonspecific passive diffusion for Hg. The Hg- or NEM-induced inhibition of Cd uptake likely involves a thiol-mediated reaction, but our results suggest that NEM pretreatment may activate other cellular mechanisms leading to a stimulatory effect.

Cadmium transport through type II alveolar cell monolayers: contribution of transcellular and paracellular pathways in the rat ATII and the human A549 cells

Cadmium (CD) transport in alveolar type II (ATII) cells has been studied using two in vitro models widely used to investigate lung function: primary cultures of rat ATII cells and the human cell line A549. Nonlinear regression analyses of the uptake time-course of (109)Cd revealed: a zero-time accumulation, a fast process of accumulation which proceeds within minutes, and a much slower process which takes hours. This three-step mechanism was characterized by different parameter values under dishes-or filter-growth conditions. A higher initial uptake rate (v(i)) and equilibrium accumulation (A(max)) of (109)Cd were found in the rat ATII cells; these differences were not related to a higher level of adsorption onto the external surface of the cell membrane. Specific transport systems of similar capacity but different affinity (threefold higher in rat cells) were characterized. A significant transepithelial transport of (109)Cd, with similar P(coeff) in both cell models, could not be exclusively related to cellular metal release. Results on 3H-mannitol permeability together with (109)Cd efflux data strongly suggest a greater contribution of the paracellular pathways in Cd transport through A549 cell monolayers. These differences in transport properties between the two lung cell models may modify the dose-response curve for Cd toxicity.

Evidence for cadmium uptake through Nramp2: metal speciation studies with Caco-2 cells

The specific uptake of 0.3 microM (109)Cd by the TC7 clone of the human enterocytic-like Caco-2 cells increased 4-fold as the pH(out) was lowered from 7.5 to 5.5; the stimulatory effect of acidic media being more pronounced when the level of the free ion (109)Cd(2+), relative to total (109)Cd, was increased. The initial uptake rate was 12-fold higher under conditions, optimizing (109)Cd(2+) accumulation over that of (109)CdCl(2-n)(n) (NO(-)(3)/pH(out) 5.5); a saturable system of transport has been characterized (K(m) = 1.1 +/- 0.1 microM, V(max) = 87 +/- 3 pmol/3 min/mg protein). An excess of Fe(2+) failed to affect (109)Cd uptake when the pH(out) was 7.4, whereas a strong inhibition was observed under NO(-)(3)/pH(out) 5.5 conditions. In contrast, the maximal inhibitory effect of Zn(2+) was observed under Cl(-)/pH(out) 7.4 conditions. This results strongly suggest that Fe(2+) may compete with Cd(2+) for Nramp2, whereas Zn and CdCl(2-n)(n) compete for another system of transport that has yet to be identified.

Cadmium uptake by Caco-2 cells: effects of Cd complexation by chloride, glutathione, and phytochelatins

Short-term cadmium uptake by the highly differentiated TC7 clone of enterocytic-like Caco-2 cells was studied as a function of Cd speciation. For low metal concentrations and with a constant free [Cd(2+)] = 43 nM, initial uptake rates of (109)Cd increased linearly as a function of increasing concentration of chlorocomplexes (Sigma[(109)CdCl(2-n)(n)]) over the range from 0 to 250 nM. When normalized as a function of the metal concentration, the absorption rate for the chlorocomplexes was less than that estimated for uptake of the free Cd(2+) cation. Metal absorption decreased upon organic ligand addition in the exposure media, but much less than predicted from the assumption that only inorganic metal species would be transported. Under exposure conditions where the concentration of each of the inorganic species was kept constant, (109)Cd uptake increased with increasing concentrations of cadmium glutathione ((109)Cd-GSH) or phytochelatin ((109)Cd-hmPC(3)) complexes. A specific system of very high affinity but low capacity has been characterized for (109)Cd-GSH transport, whereas accumulation data increased linearly with (109)Cd-hmPC(3) up to 6 microM. Comparison among uptake data for 0.3 microM inorganic (109)Cd, (109)Cd-GSH, or (109)Cd-hmPC(3) yields the following accumulation ratios: Cd-GSH/Cd(inorg) = 0.2; Cd-hmPC(3)/Cd(inorg) = 0.5. These results clearly show that Cd(2+) is not the exclusive metal species participating in Cd absorption, though, for comparable Cd concentrations, its contribution to transport would be more important than that of other species. Cadmium bound to thiol-containing peptides may be absorbed via transport systems that differ from those involved in absorption of the inorganic metal species.

Evidence for an intracellular barrier to cadmium transport through Caco-2 cell monolayers

109Cd transport was studied in the highly differentiated TC7 clone of the enterocytic-like Caco-2 cells grown on filters. Accumulation curves for 0.3 microM 109Cd over 12 h from the apical (AP) or the basal (BL) sides revealed a three-step mechanism involving: 1) a zero-time accumulation Ao; 2) a fast process Af(t1/2 < or = 10 min); and 3) a slow process of uptake As (5 h < or = t1/2 < or = 10 h) responsible for the major cellular levels of 109Cd. The relative contribution of adsorption to total accumulation is greater for short exposure times (< or = 35%), but is no longer significant after the exposure times needed to reach equilibrium. Transepithelial transport was less than 4% of the cellular level at 12 h. A negligible but specific binding onto the BL surface of the filters was characterized. Saturable systems of accumulation with comparable affinities (Km = 2.5+/-0.5 and 5.4+/-0.4 microM) but distinct capacities (Vmax = 8.9+/-1.2 and 312+/-22 pmol/min/mg protein) were identified at the AP and BL cell membranes, respectively. Efflux studies revealed that Cd accumulation is only partially reversible, with an exclusive metal release at the same side. A 2-h exposure on both sides simultaneously failed to demonstrate any competition for cellular accumulation: uptake was additive relative to AP and BL uptake values. These data suggest that Af leads to an accumulation of loosely bound Cd, whereas As represents irreversible intracellular binding processes. We conclude that Cd transport occurs exclusively by a transcellular route and that saturation of the intracellular high-capacity binding sites is the rate-limiting step in Cd absorption.

Caco-2 cell line used as an in vitro model to study cadmium accumulation in intestinal epithelial cells

109Cd uptake was studied using the highly differentiated TC7 clone of Caco-2 cells as a model of human enterocyte function. Intracellular accumulation of 0.3 microM 109Cd involved a rapid and a slow uptake phase, which resulted in complete equilibration (t(1/2) = 17.3 +/- 1.3 min) with an apparent in-to-out distribution ratio (alphae) of 11.6 +/- 0.8. The amplitude of the rapid phase (U0) and the rate of the slow phase (V) were similarly reduced in the less differentiated PF11 clone, but comparable alphae values were observed at equilibrium. In both clones, the t(1/2) and alphae values increased and decreased, respectively, upon addition of unlabeled Cd to the uptake media. In TC7 cells, 109Cd uptake at 1 min (U1) was unaffected by Ca concentrations four order of magnitude in excess, but both U0 and V demonstrated similar sensitivities to unlabeled Cd, Zn and sulfhydryl-reactive agents. Only U0 disappeared when EDTA was present in the wash solutions. U1 showed saturation kinetics and the data were found compatible with a model assuming rapid initial Cd binding and transport through a unique transport protein (Km = 3.8 +/- 0.7 microM). Cd efflux kinetics demonstrated partial reversibility in EDTA-containing solutions, suggesting that the taken up Cd might be both tightly and loosely bound to intracellular binding sites. However, the displacement of 109Cd measured at 65 min failed to reveal this heterogeneity: the data were found compatible with a model equation assuming the presence of one class of high-capacity high-affinity binding sites. We conclude that a slow-transport fast-intracellular binding mechanism of Cd uptake best accounts for these results and that Cd transport most likely involves a carrier-type of protein unrelated to Ca absorption.

Triiodothyronine stimulates the expression of sucrase-isomaltase in Caco-2 cells cultured in serum-free medium

In a previous study we have shown that triiodothyronine (T3) added to a serum-free medium supplemented with insulin, transferrin, and selenous acid (ITS) can stimulate Caco-2 cell differentiation. In this study we have focused on the effects of T3 on sucrase activity. The results obtained demonstrate that T3 (50 nM) does not change Caco-2 cell proliferation but enhances sucrase activity from 50 to 80%. Similar increases were observed whether or not insulin was present in the culture medium, showing that there was no synergistic effect between T3 and insulin on sucrase activity. Moreover, T3 acts specifically during the differentiation period since addition of T3 to the defined TS medium before confluency is reached does not stimulate sucrase activity. Sucrase kinetic parameters were evaluated for the first time in Caco-2 cells under various culture conditions. The presence of a single enzyme was verified, with a Km of about 7 mM and a Vmax around 20 nmol of substrate hydrolyzed min-1 mg-1 of protein. Our results showed that T3 did not change the enzyme's affinity for sucrose but doubled the Vmax. Moreover, immunoblotting using anti-sucrase-isomaltase (SI) antibodies revealed an approximately twofold increase in the relative amount of SI immunoreactive protein in T3-stimulated cells compared to untreated cells. Results obtained by both Northern hybridization and RT-PCR amplification showed a significant increase in SI mRNA contents. These results suggest that T3 acts primarily on sucrase expression at the mRNA level.

Ornithine decarboxylase activity is associated with proliferation but not with T3-induced differentiation of Caco-2 cells

Ornithine decarboxylase (ODC) activity and polyamine (putrescine, spermidine, spermine) concentrations were measured in parallel in enterocyte-like Caco-2 cells maintained under various culture conditions. ODC activity was maximal at the beginning of the exponential growth phase, decreasing dramatically thereafter to a negligible level at confluency (day 9). Kinetic studies performed on day 3 revealed the presence of a single enzyme with a Km around 200 microM and a Vmax of about 2 nmol CO2 released/h/mg protein. Similar values were obtained in both serum-supplemented and transferrin/selenium (TS)-defined culture media, indicating that ODC kinetic parameters are not affected by any factors present in serum. Polyamine concentrations were maximal on day 5. By day 9, they returned to initial levels and remained at these fairly high values until day 21. Since we have previously shown (Jumarie and Malo, 1994, in Vitro Cell. Dev. Biol., 30A:753-760) that triiodothyronine (T3) stimulates differentiation but not proliferation of Caco-2 cells maintained in TS-defined medium, we investigated if it induces differentiation by a polyamine-dependent mechanism. Short- and long-term measurements revealed similar ODC activity and polyamine levels whether T3 was present or not in the culture medium. These results clearly demonstrate that polyamine synthesis is more likely to be associated with Caco-2 cell proliferation, and that the T3 effect on Caco-2 cell differentiation does not involve polyamine biosynthesis. Moreover, our data show that ODC activity is not solely regulated by intracellular polyamine concentration.

Alkaline phosphatase and peptidase activities in Caco-2 cells: differential response to triiodothyronine

Caco-2 cell human colon adenocarcinoma cell line was used to study the hormonal regulation of small intestinal epithelial cell differentiation. We had previously shown that insulin-transferrin-selenium and triiodothyronine (5 x 10(-8) M)-supplemented medium can best replace serum after 2 days of culture for both the maintenance and differentiation of Caco-2 cells. The present study demonstrates that precoating petri dishes with complete serum allows the growth and differentiation of Caco-2 cells seeded directly in serum-free medium. On the other hand, precoating with dialyzed serum inhibits alkaline phosphatase and dipeptidyl-dipeptidase IV activities by more than 50%. The results obtained with complete serum-precoated culture plates indicate that there is no synergy between insulin and triiodothyronine because cells maintained in transferrin-selenium and triiodothyronine-supplemented medium, with or without insulin, express comparable enzyme activities. Moreover, large increases in alkaline phosphatase and dipeptidyl-dipeptidase IV activities were observed when triiodothyronine was added to the culture medium by the time confluency was reached. In contrast, gamma-glutamyltransferase was lowered to a greater extent when triiodothyronine was present from the beginning of culture. These findings show that triiodothyronine preferentially stimulates alkaline phosphatase and dipeptidyl-dipeptidase IV activities during the differentiation period whereas it selectively inhibits gamma-glutamyltransferase during the proliferation phase. Triiodothyronine acts in a dose-dependent manner.

Polarized distribution of neutral endopeptidase 24.11 at the cell surface of cultured human intestinal epithelial Caco-2 cells

The human colon cancer cell line Caco-2 undergoes spontaneous enterocytic differentiation during growth, and expresses a number of brush-border-membrane-associated hydrolases typical of a differentiated phenotype. Among these are alkaline phosphatase, dipeptidyl peptidase IV and sucrase-isomaltase (sucrase, EC 3.2.1.48). Neutral endopeptidase 24.11 [EC 3.4.24.11, neprilysin (NEP)] is another abundant protease of normal enterocytes but its presence in Caco-2 cells has not been fully documented yet. In this paper, we show that Caco-2 cell extracts hydrolyse tritiated [D-Ala2Leu5]enkephalin with a Km of 180 microM, very close to the value obtained for the NEP present in the rabbit kidney (118 microM). Western-blot analysis of brush-border membranes purified from post-confluent cells revealed a protein with an apparent molecular mass of 94000 Da similar to that of the rabbit kidney NEP. The amount of enzyme in cell extracts increased as a function of the age of the culture, indicating that NEP expression is correlated with the degree of cell differentiation as is also the case for sucrase and dipeptidylpeptidase IV (DPP-IV). Binding of a radiolabelled antibody to Caco-2 cell monolayers grown on semi-permeable filters indicated that 95% of NEP molecules present at the cell surface are on the apical side. Immunocytochemical and flow cytometric analysis of intact and permeabilized cells were also used to investigate the presence of NEP and DPP-IV at the surface of Caco-2 cells. Whereas DPP-IV staining appeared to be homogeneous throughout the entire cell population, NEP-related fluorescence exhibited a bimodal distribution which indicates an uneven expression of the protein at the cell surface. Permeabilization of monolayers with saponin before staining restored a labelling pattern for NEP similar to the one obtained for DPP-IV. This suggests that although DPP-IV and NEP follow similar patterns of expression when enzymic activities are measured on whole-cell extracts, targeting of these brush-border proteins to the cell surface appears to be regulated in different ways.

Caco-2 cells cultured in serum-free medium as a model for the study of enterocytic differentiation in vitro

Caco-2 cells, which express spontaneous enterocytic differentiation at confluency, is one of the most relevant in vitro models for the study of differentiation and regulation of intestinal functions. However, these cells are normally cultured in the presence of 15-20% serum which renders extremely complex the identification of the factors involved in the regulation of both proliferation and differentiation. This study has been devoted to the establishment of chemically defined culture conditions which can sustain growth and differentiation of Caco-2 cells. The replacement of serum by ITS (insulin, transferrin, and selenium) allowed for normal structural and functional differentiation of cells as revealed by the establishment of cell polarity and the expression of brush-border membrane enzyme markers (sucrase, maltase, lactase, alkaline phosphatase, gamma-glutamyltransferase, aminopeptidase N, and dipeptidyl-dipeptidase IV), although the levels of sucrase activity were lower in ITS-supplemented medium. Coating petridishes with either type IV collagen or basement membrane proteins (Matrigel) did not improve the differentiation of cells, brush-border membrane enzyme activities being, in fact, lower when the cells were grown on these substrata. When triiodothyronine (T3, 5 x 10(-8) M) was added to the ITS-supplemented medium, disaccharidase and alkaline phosphatase activities were significantly increased while gamma-glutamyltransferase activity was diminished by T3 and stimulated by epidermal growth factor (1.6 x 10(-6) M). On the other hand, hydrocortisone (HC, 10(-6) M) did not modify disaccharidase and peptidase activities. These data clearly show that Caco-2 cells can be maintained in serum-free medium and that this system allows the study of the factors involved in the regulation of the differentiation of enterocyte in vitro.

Ionic channels induced by surfactin in planar lipid bilayer membranes

Surfactin is a lipopeptide produced by certain strains of Bacillus subtilis and has potent surface activity. Here, we present the first results showing that ion-conducting pores can be formed by surfactin in artificial lipid membranes. With a low aqueous concentration of surfactin (1 microM) and a restricted membrane area (5.10(-5) cm2) we observed conductance jumps that indicate the formation of individual ionic channels in the presence of K+, Rb+, Cs+, Na+ or Li+ chlorides. Although for every salt concentration (Ci), the distribution in amplitude of the conductance steps (lambda i) may be rather broad, there is always a step amplitude which is more frequent than the others. In addition, the channels corresponding to this most frequent step amplitude are the longest in duration. For Ci = 1 M, the cationic selectivity sequence deduced from these most frequent events is K+ greater than Rb+ greater than Na+ greater than Cs+ = Li+ with respective values for lambda Mi: 130, 110, 80 and 30 pS. In KCl solutions lambda MKCl increases as a function of Ci for low Ci, and shows a plateau for Ci greater than 0.5 M. When measured on larger area membranes (10(-2)cm2) with 1 M solutions of the monovalent salts KCl, NaCl, RbCl and CsCl or the divalent salt CaCl2, the macroscopic low voltage conductance (G0) increases with a slope of 2 on a log-log plot as a function of surfactin concentration. These results demonstrate that surfactin produces selective cationic channels in lipid bilayer membranes and suggest that at higher salt concentration, a dimer is involved in this functional channel-forming process.