Mercury uptake by LLC-PK1 cells: dependence on temperature and membrane potential

Mercury induced haemocyte alterations in the terrestrial snail Cantareus apertus as novel biomarker

The aim of the present work was to study the response of a suite of cellular and biochemical markers in the terrestrial snail Cantareus apertus exposed to mercury in view of future use as sensitive tool suitable for mercury polluted soil monitoring and assessment. Besides standardized biomarkers (metallothionein, acetylcholinesterase, and lysosomal membrane stability) novel cellular biomarkers on haemolymph cells were analyzed, including changes in the spread cells/round cells ratio and haemocyte morphometric alterations. The animals were exposed for 14 days to Lactuca sativa soaked for 1h in HgCl2 solutions (0.5 e 1 μM). The temporal dynamics of the responses were assessed by measurements at 3, 7 and 14 days. Following exposure to HgCl2 a significant alteration in the relative frequencies of round cells and spread cells was evident, with a time and dose-dependent increase of the frequencies of round cells with respect to spread cells. These changes were accompanied by cellular morphometric alterations. Concomitantly, a high correspondence between these cellular responses and metallothionein tissutal concentration, lysosomal membrane stability and inhibition of AChE was evident. The study highlights the usefulness of the terrestrial snail C. apertus as bioindicator organism for mercury pollution biomonitoring and, in particular, the use of haemocyte alterations as a suitable biomarker of pollutant effect to be included in a multibiomarker strategy.

Characterization of the intestinal absorption of inorganic mercury in Caco-2 cells

The main form of mercury exposure in the general population is through food. Intestinal absorption is therefore a key step in the penetration of mercury into the systemic circulation, and should be considered when evaluating exposure risk. Many studies have investigated the transport of mercury species in different cell lines, though the mechanisms underlying their intestinal absorption are not clear. This study evaluates the accumulation and transport of Hg(II), one of the mercury species ingested in food, using Caco-2 cells as intestinal epithelium model with the purpose of clarifying the mechanisms involved in its absorption. Hg(II) shows moderate absorption, and its transport fundamentally takes place via a carrier-mediated transcellular mechanism. The experiments indicate the participation of an energy-dependent transport mechanism. In addition, H(+)- and Na(+)-dependent transport is also observed. These data, together with those obtained from inhibition studies using specific substrates or inhibitors of different transporter families, suggest the participation of divalent cation and amino acid transporters, and even some organic anion transporters, in Hg(II) intestinal transport. An important cellular accumulation of up to 51% is observed - a situation which in view of the toxic nature of this species could affect intestinal mucosal function. This study contributes new information on the mechanisms of transport of Hg(II) at intestinal level, and which may be responsible for penetration of this mercurial form into the systemic circulation.

Assimilation of elements and digestion in grass shrimp pre-exposed to dietary mercury

Grass shrimp Palaemonetes pugio were fed mercury (Hg)-contaminated oligochaetes for 15 days and analyzed for Hg, cadmium (Cd), and carbon assimilation efficiencies (AE) as well as toxicological end points related to digestion. Disproportionate increases in stable Hg concentrations in shrimp did not appear to be related to partitioning to trophically available Hg in worms. Hg AE by pre-exposed shrimp reached a plateau (approximately 53 %), whereas Cd AE varied (approximately 40-60 %) in a manner that was not dose-dependent. Carbon AE did not differ among treatments (approximately 69 %). Gut residence time was not impacted significantly by Hg pre-exposure (grand median approximately 465 min), however, there was a trend between curves showing percentages of individuals with markers in feces over time versus treatment. Feces-elimination rate did not vary with dietary pre-exposure. Extracellular protease activity varied approximately 1.9-fold but did not exhibit dose-dependency. pH increased over the range of Hg pre-exposures within the anterior (pH approximately 5.33-6.51) and posterior (pH approximately 5.29-6.25) regions of the cardiac proventriculus and Hg assimilation exhibited a negative relationship to hydrogen ion concentrations. The results of this study indicate that previous Hg ingestion can elicit post-assimilatory impacts on grass shrimp digestive physiology, which may, in turn, influence Hg assimilation during subsequent digestive cycles.

Dose-dependent inorganic mercury absorption by isolated perfused intestine of rainbow trout, Oncorhynchus mykiss, involves both amiloride-sensitive and energy-dependent pathways

The trophic transfer and nutritional toxicity of mercury (Hg) in aquatic food chains is well known, but there is limited information on the mechanism of mercury uptake across the gut. In this study, isolated whole gut sacs from rainbow trout were used to identify the regions of the gut involved in Hg absorption, and then perfused intestines were used to investigate Hg uptake. Exposure of whole gut sacs to 100 micromol l(-1) Hg as HgCl2 in the luminal solution caused Hg accumulation primarily in the mucosa (78% or more), with the intact mid and hind gut supporting 59% of the accumulated Hg. Luminal exposure to [Hg] between 0 and 100 micromol l(-1) for 4 h in perfused trout intestines showed a non-linear dose-dependent accumulation with a maximum Hg uptake rate of about 103 nmol g(-1) h(-1), and suggests carrier mediated transport into the gut cells and the blood. Additions of 2 mmol l(-1) amiloride depressed Hg accumulation by the mid and hind gut by 40-50%, whilst additions of the Ca chelator 1 mmol l(-1) EGTA increased Hg levels in the tissue. Symmetrical additions of 10 mmol l(-1) cyanide did not prevent tissue accumulation of Hg, but caused a 3.4-fold decline in net Hg flux to the serosal compartment. We conclude that Hg absorption across the gut is partly carrier mediated and involves both amiloride sensitive, and energy-dependent pathways.

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.

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.

Mercury accumulation and flux across the gills and the intestine of the blue crab (Callinectes sapidus)

This paper details the results of perfusion experiments examining the accumulation of inorganic and methylmercury (Hg and MMHg) into the gill and intestine tissue of the blue crab, Callinectes sapidus. Additionally, the flux across the tissue to an internal medium, representative of crab tissue or haemolymph, during the perfusion was also measured. The accumulation and transfer processes were studied for each form by exposing the organs to a wide range of Hg and MMHg water concentrations, as well as a mixture of the two Hg forms. Experiments were also performed at different temperatures and in the presence of a metabolic inhibitor to assess the accumulation mechanisms. While the Hg levels bioaccumulated in the two organs were of the same order, the fluxes of Hg from the tissue to the internal medium were slightly higher in the intestine than in the gill. At low external concentrations, the uptake was very similar for both Hg forms, but as exposure pressure increased, inorganic Hg uptake slowed whereas MMHg uptake increased linearly. The results from the perfusion experiments with a mixture of inorganic Hg and MMHg show that while these two forms of Hg do share common uptake pathways, there is also independent uptake. The temperature and inhibition experiments with ouabain, a Na(+)K(+)ATPase inhibitor, show that accumulation is at least partially energy dependent. Overall, the results suggest that there is more than one mechanism of accumulation for both Hg forms. Finally, as accumulation of Hg and MMHg into these tissues was similar, these results contrast with the literature assertion that the enhanced bioaccumulation of MMHg over inorganic Hg is a result of MMHg being more readily transported across the gut membrane.

Mercuric chloride enhances immunoglobulin E-dependent mediator release from human basophils

Mercuric chloride (HgCl2) is an industrial agent known to cause autoimmune disorders and induce IgE synthesis, which plays a crucial role in the manifestation of allergic diseases. In rodents, the immunomodulatory effects of HgCl2 have been shown to involve the enhancement of mast cell-derived IL-4 secretion, which facilitates both Th2-lymphocyte development and IgE production. In humans, rapid allergen-dependent release of IL-4 and the related cytokine IL-13 from histamine-containing cells occurs primarily in basophils, along with other proinflammatory mediators such as histamine and LTC4. In this study, we therefore investigated the effects of HgCl2 on the release of the above basophil mediators, either due to the compound alone or in conjunction with IgE-dependent stimulation. HgCl2 (10(-9) to 10(-6) M) did not induce mediator secretion alone but significantly enhanced the release of histamine, LTC4, IL-4, and IL-13 caused by anti-IgE. Higher concentrations of HgCl2 (10(-5) to 10(-3) M) strikingly reduced cell viability; however, toxicity varied depending on cell density and incubation time. Removal of HgCl2 following a short incubation with basophils did not reverse the potentiating effects on basophil mediator secretion to anti-IgE and the concentration of free mercury in the supernatants significantly diminished by up to 20% after incubation with the cells, indicating irreversible Hg binding to cells. By upregulating IgE-dependent human basophil mediator release, our results clearly indicate that HgCl2 potentially exacerbates allergic disorders and promotes a Th2-cytokine profile.

gamma-Glutamyl transpeptidase and l-cysteine regulate methylmercury uptake by HepG2 cells, a human hepatoma cell line

Mechanisms of methylmercury (MeHg) and inorganic mercury (Hg) uptake were examined in HepG2 cells, a human hepatoma-derived cell line. MeHg uptake was faster when it was present as the l-cysteine complex, as compared to the glutathione (GSH), CysGly, gamma-GluCys, d-cysteine, N-acetylcysteine, l-penicillamine, or albumin complexes. Uptake of MeHg-l-cysteine was independent of Na(+), stereoselective, and was inhibited by the amino acid transport system l substrates l-leucine, l-valine, and l-phenylalanine (5 mM). Moreover, [(3)H]l-leucine uptake was inhibited by MeHg-l-cysteine, suggesting that MeHg-l-cysteine is transported into HepG2 cells by an l-type amino acid carrier. Uptake of MeHg as the GSH complex (MeHg-SG) was dependent on the extracellular GSH concentration, and was diminished when cellular gamma-glutamyl transpeptidase activity was inhibited. Inorganic mercury uptake was slower than that of MeHg, but was also sensitive to the type of thiol ligand present. These findings demonstrate that mercury uptake by HepG2 cells is dependent on the chemical structure of the mercury compound, the thiol ligand, and the activity of gamma-glutamyl transpeptidase. gamma-Glutamyl transpeptidase appears to play a key role in the disposition of MeHg-SG by facilitating the formation of MeHg-l-cysteine, which is readily transported into the cells on an amino acid-type carrier.

Transport of toxic heavy metals across cell membranes

Membrane transport of nonessential toxic heavy metals (type D heavy metals) not only controls their access to intracellular target sites but also helps determine their uptake, distribution, and excretion from the body. The critical role of membranes in the toxicology of class D metals has attracted the attention of many investigators, and extensive information has been collected on the mechanism(s) of metal transfer across membranes. Characteristics of metal transport in different cells, or even on opposite sides of the same cell, or under different physiological conditions, are not identical, and no unitary hypothesis has been formulated to explain this process in all cells. However, it seems possible that the mechanisms proposed for different cells represent variations on a few common themes.

Further analysis of cadmium uptake from apical membrane of LLC-PK1 cells via inorganic anion exchanger

This study was undertaken to analyze Cd uptake via the inorganic anion exchanger (HCO3-/Cl-) by LLC-PK1 cells cultured on permeable membranes. LLC-PK1 cells were incubated at 37 degrees with 1 microM CdCl2 added to the apical medium, and Cd accumulation in the cells was fractionated into a membrane-bound (non-internalized) Cd fraction and an internalized Cd fraction using ethyleneglycol-bis-(beta-aminoethylether)-N,N,N',N'-tetraace tic acid (EGTA), a chelating agent for Cd. Incubation at a lower temperature (4 degrees) significantly decreased the membrane-bound Cd fraction, and drastically decreased the internalized Cd fraction. Addition of NaHCO3 (a stimulator of Cd uptake via inorganic anion exchanger) to the apical medium significantly increased both membrane-bound and internalized Cd fractions, and this increase was greater for the internalized fraction. Pretreatment of cells with 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), a specific inhibitor of inorganic anion exchangers, significantly decreased the internalized Cd fraction without changing the membrane-bound Cd fraction. Addition of NaHCO3 did not effect both Cd fractions in DIDS-pretreated cells. These results suggest that most of Cd binds non-specifically to the apical membrane surface and then is internalized via simple diffusion and some Cd specifically binds to the inorganic anion exchanger and is internalized in a metabolism-dependent manner.

Cadmium uptake from apical membrane of LLC-PK1 cells via inorganic anion exchanger

Effects of Na+, Cl-, OH- and HCO3-, and an inorganic anion exchange inhibitor (HCO3-/Cl-), 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid, on Cd uptake from the apical membrane of LLC-PK1 cells were investigated to elucidate the mechanism of Cd uptake by these cells. Furthermore, the initial uptake of Cd incubated at different temperatures was analyzed by using the Arrhenius plot to test if Cd is taken up by the cells via the carrier-mediated process. The cells were incubated with 1 microM CdCl2 for 30 min. at 37 degrees in phosphate buffer containing NaCl at pH 5.5 or 7.4. Cd accumulation by the cells at pH 7.4 was about 5 times higher than at pH 5.5. Replacement of NaCl in the phosphate buffer with KCl or mannitol decreased the Cd accumulation at pH 7.4 about 80%, but had no effect at pH 5.5. The addition of 2 mM NaHCO3 to the pH 7.4 buffer containing NaCl significantly increased the Cd accumulation, and pretreatment of the cells with the inorganic anion exchange inhibitor abolished this effect of NaHCO3 on Cd accumulation. The cells were incubated for 10 min. at different temperatures with 1 microM CdCl2 in the phosphate buffer containing NaCl at pH 7.4 in the presence or absence of 2 mM NaHCO3 or at pH 5.5 in the absence of NaHCO3. In all cases, the Arrhenius plots of Cd accumulation were nonlinear. The breakpoint was observed at about 30 degrees in the Cd accumulation at pH 7.4 in the presence of NaHCO3, suggesting the involvement of a carrier-mediated process. This breakpoint was not as clear in the Cd accumulation at pH 7.4 in the absence of NaHCO3 and not observed at all at pH 5.5. These findings suggested that Cd is partially taken up from the apical membrane of LLC-PK1 cells via the inorganic anion exchanger and the uptake of Cd is more efficient at alkaline pH and in the presence of Na+, Cl- and HCO3-.