Lead inhibits meso-2,3-dimercaptosuccinic acid induced calcium transients in cultured rhesus monkey kidney cells

Lead, calcium uptake, and related genetic variants in association with renal cell carcinoma risk in a cohort of male Finnish smokers

BACKGROUND

Lead is classified as a probable human carcinogen. However, its role in renal cell cancer (RCC) has not been established. Calcium and vitamin D may off-set toxicity in vivo.

METHODS

In this nested case-control study, whole blood lead, total serum calcium, and serum 25-hydroxyvitamin D levels were measured in blood drawn prior to diagnosis among male smokers participating in the Alpha-Tocopherol, Beta-Carotene Cancer Prevention Study. Single-nucleotide polymorphisms (SNP) in five genes (CALB1, TRPV5, TRPV6, VDR, and ALAD) related to lead toxicity or calcium transport were genotyped. Logistic and linear regressions were used to determine RCC risk and time to diagnosis (respectively), adjusting for other risk factors.

RESULTS

Among 154 newly diagnosed cases and 308 matched controls, RCC was associated with higher whole blood lead [OR = 2.0; 95% confidence interval (CI), 1.0-3.9; quartile 4 (Q4) vs. Q1, P(trend) = 0.022] and CALB1 rs1800645 (P(trend) = 0.025, minor 'T' allele frequency = 0.34). Higher total serum calcium (P(trend) ≤ 0.001) was associated with reduced RCC risk. Total serum calcium and 25-hydroxyvitamin D levels did not alter the association observed with lead. Time from enrollment to RCC diagnosis was positively associated with serum calcium (P(trend) = 0.002) and 25-hydroxyvitamin D (P(trend) = 0.054) among cases.

CONCLUSIONS

Higher blood lead concentrations, below the 10 μg/dL level of concern, were associated with RCC, independent from serum calcium and CALB1 promoter polymorphism.

IMPACT

Increased risk of RCC is associated with lower serum calcium and higher whole blood lead in smokers. The clinical prognostic value of serum calcium and vitamin D in RCC should be further investigated.

The use of ESI-MS to probe the binding of divalent cations to calmodulin

Proteins have evolved with distinct sites for binding particular metal ions. This allows metalloproteins to perform a myriad of specialized tasks with conformations tailor-made by the combination of its primary sequence and the effect on this of the ligated metal ion. Here we investigate the selectivity of the calcium trigger protein calmodulin for divalent metal ions. This ubiquitous and highly abundant protein exists in equilibrium between its apo and its holo form wherein four calcium ions are bound. Amongst its many functions, calmodulin modulates the calcium concentration present in cells, but this functional property renders it a target for competition from other metal ions. We study the competition posed by four other divalent cations for the calcium binding sites in calmodulin using electrospray ionization mass spectrometry (ESI-MS). We have chosen two other group II cations Mg(2+), Sr(2+), and two heavy metals Cd(2+), Pb(2+). The ease with which each of these metals binds to apo and to holo CaM[4Ca] is described. We find that each metal ion has different properties with respect to calmodulin binding and competition with calcium. The order of affinity for apo CaM is Ca(2+) >> Sr(2+) approximately Mg(2+) > Pb(2+) approximately Cd(2+). In the presence of calcium the affinity alters to Pb(2+) > Ca(2+) > Cd(2+) > Sr(2+) > Mg(2+). Once complexes have been formed between the metal ions and protein (CaM:[xM]) we investigate whether the structural change which must accompanies calcium ligation to allow target binding takes place for a given CaM:[xM] system. We use a 20 residue target peptide, which forms the CaM binding site within the enzyme neuronal nitric-oxide synthase. Our earlier work (Shirran et al. 2005) [1] has demonstrated the particular selectivity of this system for CaM:4Ca(2+). We find that along with Ca(2+) only Pb(2+) forms complexes of the form CaM:4M(2+):nNOS. This work demonstrates the affinity for calcium above all other metals, but also warns about the ability of lead to replace calcium with apparent ease.

Cytoprotective and antioxidant activity of seabuckthorn (Hippophae rhamnoides L.) flavones against tert-butyl hydroperoxide-induced cytotoxicity in lymphocytes

This study was designed to determine the cytoprotective activity of flavones of seabuckthorn (Hippophae rhamnoides L.) against tert-butyl hydroperoxide (tert-BOOH), used as an oxidant to induce oxidative damage, with lymphocytes as the model system. Addition of tert-BOOH (250 microM) to the cells resulted in enhanced cytotoxicity and free radical production. The intracellular calcium levels, caspase activity, and apoptosis were significantly increased following tert-BOOH treatment. Seabuckthorn flavones at the concentration of 100 microg/mL significantly inhibited tert-BOOH-induced cytotoxicity and free radical production and also restored the antioxidant status to that of control cells. Seabuckthorn flavones also significantly restricted tert-BOOH-induced apoptosis by decreasing intracellular calcium levels and caspase activity. The extract also decreased tert-BOOH-induced formation of DNA breaks by 30%. These observations suggest that the flavones of seabuckthorn have marked cytoprotective properties, which could be attributed to the antioxidant activity.

Lead induces chondrogenesis and alters transforming growth factor-beta and bone morphogenetic protein signaling in mesenchymal cell populations

BACKGROUND

It has been established that skeletal growth is stunted in lead-exposed children. Because chondrogenesis is a seminal step during skeletal development, elucidating the impact of Pb on this process is the first step toward understanding the mechanism of Pb toxicity in the skeleton.

OBJECTIVES

The aim of this study was to test the hypothesis that Pb alters chondrogenic commitment of mesenchymal cells and to assess the effects of Pb on various signaling pathways.

METHODS

We assessed the influence of Pb on chondrogenesis in murine limb bud mesenchymal cells (MSCs) using nodule formation assays and gene analyses. The effects of Pb on transforming growth factor-beta (TGF-beta) and bone morphogenetic protein (BMP) signaling was studied using luciferase-based reporters and Western analyses, and luciferase-based assays were used to study cyclic adenosine monophosphate response element binding protein (CREB), beta-catenin, AP-1, and nuclear factor-kappa B (NF-kappaB) signaling. We also used an ectopic bone formation assay to determine how Pb affects chondrogenesis in vivo.

RESULTS

Pb-exposed MSCs showed enhanced basal and TGF-beta/BMP induction of chondrogenesis, evidenced by enhanced nodule formation and up-regulation of Sox-9, type 2 collagen, and aggrecan, all key markers of chondrogenesis. We observed enhanced chondrogenesis during ectopic bone formation in mice preexposed to Pb via drinking water. In MSCs, Pb enhanced TGF-beta but inhibited BMP-2 signaling, as measured by luciferase reporter assays and Western analyses of Smad phosphorylation. Although Pb had no effect on basal CREB or Wnt/beta-catenin pathway activity, it induced NFkappaB signaling and inhibited AP-1 signaling.

CONCLUSIONS

The in vitro and in vivo induction of chondrogenesis by Pb likely involves modulation and integration of multiple signaling pathways including TGF-beta, BMP, AP-1, and NFkappaB.

Neuroprotective effect of N-acetyl cysteine on hypoxia-induced oxidative stress in primary hippocampal culture

Hippocampus has received a considerable attention in the recent past due to its role in a number of important functions such as learning and memory. The effect of hypoxia on neuronal cell injury especially on hippocampal cells is not well known. The aim of the present study was to characterize the biochemical changes in primary cultured hippocampal neurons during hypoxic exposure and the protective effect of N-acetyl cysteine on hypoxia-induced cytotoxicity. The hippocampal culture grown in 24-well plates was exposed to hypoxia for 3 h in a dessicator in 95% N(2), 5% CO(2) atmosphere at 37 degrees C. Later, the cells were allowed to recover for 1 h under normoxia. It was observed that there is an appreciable increase in cytotoxicity in cells exposed to hypoxia. Further, there was a significant decrease in mitochondrial membrane potential and appreciable increase in reactive oxygen species and single-strand DNA breaks in cells exposed to hypoxia compared to control. There is a significant fall in glutathione peroxidase, glutathione reductase, reduced glutathione levels, and nitric oxide in the cells exposed to hypoxia. Significant elevation in the intracellular calcium level in the cells on exposure to hypoxia was observed. Supplementation with NAC (50 microM) resulted in a significant cytoprotection, fall in ROS generation, and higher antioxidant levels similar to that of control cells. NAC also inhibited DNA strand breaks induced by hypoxia. The study indicates that NAC has significant neuroprotective activity during hypoxia in primary hippocampal culture.

Effects of dietary lead and/or dimercaptosuccinic acid exposure on regional serotonin and serotonin metabolite content in rainbow trout (Oncorhynchus mykiss)

The lead (Pb) chelator, meso-2,3-dimercaptosuccinic acid (DMSA) may be effective in reversing some of the adverse effects of Pb exposure. Pb-induced behavioral deficits observed in fish are due to disruptions in the integrative functioning of the medulla, cerebellum, and optic tectum. Pb exposure increased serotonin (5-HT) content in all three brain regions without an effect on 5-hydroxy-3-indoleacetic acid (5-HIAA). Pb exposure followed by no Pb in the diet increased 5-HT and 5-HIAA content in all three brain regions. The replacement of dietary Pb with DMSA had no effect on 5-HT and increased 5-HIAA content. DMSA increased 5-HIAA content in all three brain regions and increased 5-HT content only the optic tectum. Treatment with DMSA may be more effective than removal of Pb from the diet in reversing Pb-induced alterations in 5-HT.

Lead alters parathyroid hormone-related peptide and transforming growth factor-beta1 effects and AP-1 and NF-kappaB signaling in chondrocytes

The skeletal system is an important target for lead toxicity. One of the impacts of lead in the skeleton, the inhibition of axial bone development, is likely due to its effect on the normal progression of chondrocyte maturation that is central to the process of endochondral ossification. Since little is known about the effect of lead on chondrocyte function/maturation, its impact on (1) growth factor-induced proliferation, (2) expression of maturation-specific markers type X collagen and BMP-6, and (3) the activity of AP-1 and NF-kappaB was examined in chick growth plate and sternal chondrocyte models. Exposure to lead alone (1-30 microM) resulted in a dose-dependent inhibition of thymidine incorporation in growth plate chondrocytes. Lead also blunted the stimulation of thymidine incorporation by parathyroid hormone-related peptide (PTHrP) and transforming growth factor-beta1 (TGF-beta1), two critical regulators of chondrocyte maturation. Lead (1 and 10 microM), TGF-beta1 (3 ng/ml) and PTHrP (10(-7) M) all significantly inhibited the expression of type X collagen, a marker of chondrocyte terminal differentiation. However, when in combination, lead completely reversed the inhibition of type X collagen by PTHrP and TGF-beta1. The effect of lead on BMP-6. an inducer of terminal differentiation. was also examined. Independently, lead and TGF-beta1 were without effect on BMP-6 expression, but PTHrP significantly suppressed it. Comparatively, lead did not alter PTHrP-mediated suppression of BMP-6, but in combination with TGF-beta1. BMP-6 expression was increased 3-fold. To determine if lead effects on signaling might play a role in facilitating these events, the impact of lead on NF-kappaB and AP-1 signaling was assessed using luciferase reporter constructs in sternal chondrocytes. Lead had no effect on the AP-1 reporter, but it dose-dependently inhibited the NF-kappaB reporter. PTHrP, which signals through AP-1, did not activate the NF-kappaB reporter and did not affect inhibition of this reporter by lead. In contrast, PTHrP activation of the AP-1 reporter was dose-dependently enhanced by lead. These findings, which establish that chondrocytes are important targets for lead toxicity, suggest that the effects of lead on bone growth are derived from its impact on the modulation of chondrocyte maturation by growth factors and second messenger signaling responses.

Short-term exposure to dietary Pb and/or DMSA affects dopamine and dopamine metabolite levels in the medulla, optic tectum, and cerebellum of rainbow trout (Oncorhynchus mykiss)

Rainbow trout (Oncorhynchus mykiss) were randomly assigned to one of the following dietary exposure conditions: lead (Pb) solvent (2% nitric acid), meso-2,3-dimercaptosuccinic acid (DMSA) solvent (0.1 N NaOH), Pb, DMSA, Pb followed by Pb solvent, or Pb followed by DMSA. Medulla, cerebellum, and optic tectum homogenates were analyzed for dopamine (DA), homovanillic acid (HVA), and 3,4-dihydroxyphenylacetic acid (DOPAC). DA levels in all brain regions tended to be highest for trout exposed to dietary Pb followed by dietary DMSA. DA levels were elevated for trout exposed to dietary DMSA and Pb followed by Pb solvent. DA levels were below control levels for trout exposed to Pb only. HVA levels varied across brain regions. However, HVA levels in all brain regions tended to be elevated for trout exposed to dietary DMSA and Pb followed by Pb solvent. DOPAC levels across all brain regions were below control levels for trout dietary exposed to DMSA, Pb only, Pb followed by Pb solvent, and Pb followed by DMSA. These data indicate that Pb and/or DMSA have the potential of altering DA, HVA, and DOPAC levels in the medulla, cerebellum, and optic tectum. The animal model of short-term dietary exposure to Pb and DMSA, both alone and sequentially, to mimic dietary exposure to Pb and the oral delivery of DMSA, that our laboratory has developed, may be useful in future studies aimed at characterizing the neurobiological mechanisms by which Pb and/or DMSA alter neurotransmitter levels and behavior.

Meso-2,3-dimercaptosuccinic acid induces calcium transients in cultured rhesus monkey kidney cells

The maintenance of intracellular Ca2+ homeostasis is critical to many cellular functions that rely on the calcium ion as a messenger. While attempting to characterize the effects of lead on intracellular calcium levels ([Ca2+]i) in LLC-MK2 Rhesus Monkey kidney cells, we observed that treatment with the metal chelating drug, meso-2,3-dimer-captosuccinic acid (DMSA) evoked transient increases in [Ca2+]i. Changes in [Ca2+]i were monitored using the Ca2+ indicator dye Fura-2 and a dual wavelength fluorescence imaging system. In the presence of 2 mM extracellular Ca2+, DMSA treatment caused a concentration-dependent (15-500 microM) transient increase in [Ca2+]i returning to baseline levels within 30-60 s. Pharmacologic concentrations of DMSA (30 microM) stimulated a three-fold increase in [Ca2+]i, which was spatiotemporally comparable to Ca2+ transients induced by other calcium agonists. Depletion of inositol trisphosphate (IP3)-sensitive [Ca2+]i stores with the smooth endoplasmic reticulum calcium-ATPase (SERCA) inhibitor thapsigargin did not prevent DMSA-elicited increases in [Ca2+]i, suggesting that Ca2+ mobilized by DMSA was either extracellular or from an non-IP3 releasable Ca2+ pool. Treatment with glutathione, cysteine, or 2-mercaptoethanol caused similar but not identical calcium transients. Adenosine-5'-trisphosphate (ATP) also elicited transient increases in [Ca2+]i similar to those of DMSA. No transient increases in [Ca2+]i were elicited by DMSA or ATP in the absence of extracellular calcium. These data indicate that DMSA and other sulfhydryl compounds trigger an influx of extracellular calcium, suggesting a previously unobserved and unanticipated interaction between DMSA and the Ca2+ messenger system.