The displacement of calcium from osteocalcin at submicromolar concentrations of free lead

Ionic displacement of Ca2+ by Pb2+ in calmodulin is affected by arrhythmia-associated mutations

Lead is a highly toxic metal that severely perturbs physiological processes even at sub-micromolar levels, often by disrupting the Ca²⁺ signaling pathways. Recently, Pb²⁺-associated cardiac toxicity has emerged, with potential involvement of both the ubiquitous Ca²⁺ sensor protein calmodulin (CaM) and ryanodine receptors. In this work, we explored the hypothesis that Pb²⁺ contributes to the pathological phenotype of CaM variants associated with congenital arrhythmias. We performed a thorough spectroscopic and computational characterization of CaM conformational switches in the co-presence of Pb²⁺ and four missense mutations associated with congenital arrhythmias, namely N53I, N97S, E104A and F141L, and analyzed their effects on the recognition of a target peptide of RyR2. When bound to any of the CaM variants, Pb²⁺ is difficult to displace even under equimolar Ca²⁺ concentrations, thus locking all CaM variants in a specific conformation, which exhibits characteristics of coiled-coil assemblies. All arrhythmia-associated variants appear to be more susceptible to Pb²⁺ than WT CaM, as the conformational transition towards the coiled-coil conformation occurs at lower Pb²⁺, regardless of the presence of Ca²⁺, with altered cooperativity. The presence of arrhythmia-associated mutations specifically alters the cation coordination of CaM variants, in some cases involving allosteric communication between the EF-hands in the two domains. Finally, while wild type CaM increases the affinity for the RyR2 target in the presence of Pb²⁺, no specific pattern could be detected for all other variants, ruling out a synergistic effect of Pb²⁺ and mutations in the recognition process.

Low-Level Metal Contamination and Chelation in Cardiovascular Disease-A Ripe Area for Toxicology Research

Cardiovascular disease remains the leading cause of death worldwide. In spite of cardiovascular prevention, there is residual risk not explicable by traditional risk factors. Metal contamination even at levels previously considered safe in humans may be a potential risk factor for atherosclerosis. This review examines evidence that 2 metals, lead, and cadmium, demonstrate sufficient toxicological and epidemiologic evidence to attribute causality for atherosclerotic disease. Basic science suggests that both metals have profound adverse effects on the human cardiovascular system, resulting in endothelial dysfunction, an increase in inflammatory markers, and reactive oxygen species, all of which are proatherosclerotic. Epidemiological studies have shown both metals to have an association with cardiovascular disease, such as peripheral arterial disease, ischemic heart disease, and cardiovascular mortality. This review also examines edetate disodium-based chelation as a possible pharmacotherapy to reduce metal burden in patients with a history of cardiovascular disease and thus potentially reduce cardiovascular events.

Effects of acute oral lead exposure on the levels of essential elements of mice: a metallomics and dose-dependent study

BACKGROUND AND OBJECTIVES

Lead (Pb) has been reported to disturb the metabolism of essential elements, such as calcium (Ca), magnesium (Mg), iron (Fe) and zinc (Zn) in vivo. This study focused on the relationship between various dose of Pb and the essential elements.

METHODS

50 healthy male C57BL/6 mice underwent oral administration of 0.2 mL lead acetate trihydrate solution (0, 20, 100, 500, and 1000 mg Pb/day/kg body weight) for 3 days. The concentrations of Pb and four essential elements (Ca, Zn, Fe and Mg) in the blood, kidney, liver, bone and brain were quantified with inductively coupled plasma mass spectrometry.

RESULTS

Various doses of Pb led to significant increases in the contents of Ca, Fe and Zn in the liver, and decreased contents of Mg and Fe in the blood in a dose-dependent pattern. The Pb dose of 20 mg/kg reduced the concentration of bone Ca, which did not continue to show an obvious decline with continued increases in the oral Pb dose. Pb also caused alterations in the Mg distribution pattern, and decreased the correlation of Mg, Ca and Zn in the brain, both findings were dose-dependent. In addition to the changes in metallomics, the related oxidative stress was exacerbated, but no significant changes were detected in hepatic and renal histopathological lesions after a short period of Pb exposure.

CONCLUSIONS

This study contributes to a thorough analysis of the Pb-poisoning mechanism, and indicates that the concentrations of essential elements could be used as sensitive toxicological indicators of Pb exposure.

Alterations of mineralized matrix by lead exposure in osteoblast (MC3T3-E1) culture

The present study investigated the effect of lead (Pb) on bone ultrastructure and chemistry using an in vitro bone model. MC3T3-E1 preosteoblasts were differentiated and treated with lead acetate at 0.4, 2, 10, and 50 μM. No abnormalities in either cell growth or bone nodule formation were observed with the treated dose of lead acetate. However, Pb treatments could significantly increase Pb accumulation in differentiated osteoblast cultures and upregulate expression of Divalent metal transporter 1 (Dmt1) in a dose dependent manner. Pb treatments also altered the expression of osteogenic genes, including secreted phosphoprotein 1, osteocalcin, type I collagen, and osteoprotegerin. Moreover, in mineralized osteoblast cultures, Pb was found to be mainly deposited as Pb salts and oxides, respectively. Ultrastructure analysis revealed Pb localizing with calcium and phosphorus in the mineralized matrix. In mineralizing osteoblast cells, Pb was found in the intracellular calcified vesicles which is one of the bone mineralization mechanisms. Pb was also present in mineral deposits with various shapes and sizes, such as small and large globular or needle-like mineral deposits representing early to mature stages of mineral deposits. Furthermore, Pb was found more in the globular deposits than the needle shaped mineral crystals. Taken together, our observations revealed how Pb incorporates into bone tissue, and showed a close association with bone apatite.

Use of handheld X-ray fluorescence as a non-invasive method to distinguish between Asian and African elephant tusks

We describe the use of handheld X-ray fluorescence, for elephant tusk species identification. Asian (n = 72) and African (n = 85) elephant tusks were scanned and we utilized the species differences in elemental composition to develop a functional model differentiating between species with high precision. Spatially, the majority of measured elements (n = 26) exhibited a homogeneous distribution in cross-section, but a more heterologous pattern in the longitudinal direction. Twenty-one of twenty four elements differed between Asian and African samples. Data were subjected to hierarchical cluster analysis followed by a stepwise discriminant analysis, which identified elements for the functional equation. The best equation consisted of ratios of Si, S, Cl, Ti, Mn, Ag, Sb and W, with Zr as the denominator. Next, Bayesian binary regression model analysis was conducted to predict the probability that a tusk would be of African origin. A cut-off value was established to improve discrimination. This Bayesian hybrid classification model was then validated by scanning an additional 30 Asian and 41 African tusks, which showed high accuracy (94%) and precision (95%) rates. We conclude that handheld XRF is an accurate, non-invasive method to discriminate origin of elephant tusks provides rapid results applicable to use in the field.

Spatial distribution of the trace elements zinc, strontium and lead in human bone tissue

Trace elements are chemical elements in minute quantities, which are known to accumulate in the bone. Cortical and trabecular bones consist of bone structural units (BSUs) such as osteons and bone packets of different mineral content and are separated by cement lines. Previous studies investigating trace elements in bone lacked resolution and therefore very little is known about the local concentration of zinc (Zn), strontium (Sr) and lead (Pb) in BSUs of human bone. We used synchrotron radiation induced micro X-ray fluorescence analysis (SR μ-XRF) in combination with quantitative backscattered electron imaging (qBEI) to determine the distribution and accumulation of Zn, Sr, and Pb in human bone tissue. Fourteen human bone samples (10 femoral necks and 4 femoral heads) from individuals with osteoporotic femoral neck fractures as well as from healthy individuals were analyzed. Fluorescence intensity maps were matched with BE images and correlated with calcium (Ca) content. We found that Zn and Pb had significantly increased levels in the cement lines of all samples compared to the surrounding mineralized bone matrix. Pb and Sr levels were found to be correlated with the degree of mineralization. Interestingly, Zn intensities had no correlation with Ca levels. We have shown for the first time that there is a differential accumulation of the trace elements Zn, Pb and Sr in BSUs of human bone indicating different mechanisms of accumulation.

Metal toxicity and opportunistic binding of Pb(2+) in proteins

Lead toxicity is associated with various human diseases. While Ca(2+) binding proteins such as calmodulin (CaM) are often reported to be molecular targets for Pb(2+)-binding and lead toxicity, the effect of Pb(2+) on the Ca(2+)/CaM regulated biological activities cannot be described by the primary mechanism of ionic displacement (e.g., ionic mimicry). The focus of this study was to investigate the mechanism of lead toxicity through binding differences between Ca(2+) and Pb(2+) for CaM, an essential intracellular trigger protein with two EF-Hand Ca(2+)-binding sites in each of its two domains that regulates many molecular targets via Ca(2+)-induced conformational change. Fluorescence changes in phenylalanine indicated that Pb(2+) binds with 8-fold higher affinity than Ca(2+) in the N-terminal domain. Additionally, NMR chemical shift changes and an unusual biphasic response observed in tyrosine fluorescence associated with C-terminal domain sites EF-III and EF-IV suggest a single higher affinity Pb(2+)-binding site with a 3-fold higher affinity than Ca(2+), coupled with a second site exhibiting affinity nearly equivalent to that of the N-terminal domain sites. Our results further indicate that Pb(2+) displaces Ca(2+) only in the N-terminal domain, with minimal perturbation of the C-terminal domain, however significant structural/dynamic changes are observed in the trans-domain linker region which appear to be due to Pb(2+)-binding outside of the known calcium-binding sites. These data suggest that opportunistic Pb(2+)-binding in Ca(2+)/CaM has a profound impact on the conformation and dynamics of the essential molecular recognition sites of the central helix, and provides insight into the molecular toxicity of non-essential metal ions.

Blood lead concentration is not altered by high-dose vitamin D supplementation in children and young adults with HIV

OBJECTIVES

Optimal vitamin D status is known to have beneficial health effects and vitamin D supplements are commonly used. It has been suggested that vitamin D supplementation may increase blood lead in children and adults with previous lead exposure. The objective was to determine the safety regarding lead toxicity during 12 weeks of high-dose vitamin D3 supplementation in children and young adults with human immunodeficiency virus (HIV).

METHODS

Subjects with HIV (8-24 years) were randomized to vitamin D3 supplementation of 4000 or 7000 IU/day and followed at 6 and 12 weeks for changes in serum 25-hydroxy vitamin D (25D) and whole-blood lead concentration. This was a secondary analysis of a larger study of vitamin D3 supplementation in children and adolescents with HIV.

RESULTS

In 44 subjects (75% African American), the baseline mean ± standard deviation serum 25D was 48.3±18.6 nmol/L. Fifty percent of subjects had baseline serum 25D <50.0 nmol/L. Serum 25D increased significantly with D3 supplementation during the 12 weeks. No subject had a whole-blood lead >5.0 μg/dL at baseline or during subsequent visits. Whole-blood lead and 25D were not correlated at baseline, and were negatively correlated after 12 weeks of supplementation (P=0.014). Whole-blood lead did not differ between those receiving 4000 and 7000 IU of vitamin D3.

CONCLUSIONS

High-dose vitamin D3 supplementation and the concomitant increased serum 25D did not result in increased whole-blood lead concentration in this sample of children and young adults living in a northeastern urban city.

Predicting Ca2+ -binding sites using refined carbon clusters

Identifying Ca(2+) -binding sites in proteins is the first step toward understanding the molecular basis of diseases related to Ca(2+) -binding proteins. Currently, these sites are identified in structures either through X-ray crystallography or NMR analysis. However, Ca(2+) -binding sites are not always visible in X-ray structures due to flexibility in the binding region or low occupancy in a Ca(2+) -binding site. Similarly, both Ca(2+) and its ligand oxygens are not directly observed in NMR structures. To improve our ability to predict Ca(2+) -binding sites in both X-ray and NMR structures, we report a new graph theory algorithm (MUG(C) ) to predict Ca(2+) -binding sites. Using carbon atoms covalently bonded to the chelating oxygen atoms, and without explicit reference to side-chain oxygen ligand co-ordinates, MUG(C) is able to achieve 94% sensitivity with 76% selectivity on a dataset of X-ray structures composed of 43 Ca(2+) -binding proteins. Additionally, prediction of Ca(2+) -binding sites in NMR structures was obtained by MUG(C) using a different set of parameters, which were determined by the analysis of both Ca(2+) -constrained and unconstrained Ca(2+) -loaded structures derived from NMR data. MUG(C) identified 20 of 21 Ca(2+) -binding sites in NMR structures inferred without the use of Ca(2+) constraints. MUG(C) predictions are also highly selective for Ca(2+) -binding sites as analyses of binding sites for Mg(2+) , Zn(2+) , and Pb(2+) were not identified as Ca(2+) -binding sites. These results indicate that the geometric arrangement of the second-shell carbon cluster is sufficient not only for accurate identification of Ca(2+) -binding sites in NMR and X-ray structures but also for selective differentiation between Ca(2+) and other relevant divalent cations.

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 (1)H NMR structure of bovine Pb(2+)-osteocalcin and implications for lead toxicity

Structural information on the effect of Pb(2+) on proteins under physiologically relevant conditions is largely unknown. We have previously shown that low levels of lead increased the amount of osteocalcin bound to hydroxyapatite (BBA 1535:153). This suggested that lead induced a more compact structure in the protein. We have determined the 3D structure of Pb(2+)-osteocalcin (49 amino acids), a bone protein from a target tissue, using (1)H 2D NMR techniques. Lead, at a stoichiometry of only 1:1, induced a similar fold in the protein as that induced by Ca(2+) at a stoichiometry of 3:1. The structure consisted of an unstructured N-terminus and an ordered C-terminal consisting of a hydrophobic core (residues 16-49). The genetic algorithm-molecular dynamics simulation predicted the lead ion was coordinated by the Gla 24 and Gla 21 residues. It is proposed that mineral binding occurs via uncoordinated Gla oxygen ions binding to calcium in hydroxyapatite. A comparison of Pb(2+)- and Ca(2+)-osteocalcin suggests Pb(2+), at a lower stoichiometry, may induce similar conformational changes in proteins and subsequent molecular processes normally controlled by calcium alone. This may contribute to a molecular mechanism of lead toxicity for calcium binding proteins. Lead exposure may alter the amount of mineral bound osteocalcin and contribute to abnormal bone remodeling.

The effect of lead on calcium release activated calcium influx in primary cultures of human osteoblast-like cells

To further understand the significance of bone as a target tissues of lead toxicity, as well as a reservoir of systemic lead, it is necessary to define the effect of lead on the calcium release activated calcium influx (CRACI) in primary cultures of human osteoblast-like cells (OLC). Pb2+ inhibited the immediate CRACI dose-dependent manner. Influx of Pb2+ into human OLC was increased dose-dependent manner. The present study demonstrates that the interference of Pb2+ with CRACI of human OLC is at least twofold: (1) the initiation of CRACI, i.e., the measurable influx of Ca2+ upon Ca2+ readdition, is partially inhibited by Pb2+ and (2) the influx of Pb2+ was enhanced after CRACI had been induced.

Lead exposure and periodontitis in US adults

BACKGROUND AND OBJECTIVE

Lead is known to have significant effects on bone metabolism and the immune system. This study tested the hypothesis that lead exposure affects periodontitis in adults.

MATERIAL AND METHODS

This study used the data from the Third National Health and Nutrition Examination Survey (NHANES III, 1988-94). It analyzed data from 2500 men and 2399 women, 20-56 yr old, who received complete periodontal examination. Periodontitis was defined as the presence of > 20% of mesial sites with >or= 4 mm of attachment loss. Lead exposure was grouped into three categories: < 3; 3-7; and > 7 microg/dL. Covariates were cotinine levels, poverty ratio, race/ethnicity, education, bone mineral density, diabetes, calcium intake, dental visit, and menopause (for women). All analyses were performed separately for men and women and considering the effect design. Univariate, bivariate, and stratified analysis was followed by multivariable analysis by estimating prevalence ratios through poisson regression.

RESULTS

After adjustment for confounders, the prevalence ratios, comparing those with a lead blood level of > 7 microg/dL to those with a lead blood level of < 3 microg/dL was 1.70 (95% confidence interval (CI): 1.02, 2.85) for men and 3.80 (95% CI: 1.66, 8.73) for women.

CONCLUSION

The lead blood level was positively and statistically associated with periodontitis for both men and women. Considering the public health importance of periodontitis and lead exposure, further studies are necessary to confirm this association.

Lead accumulation in tidemark of articular cartilage

OBJECTIVE

Determination of the spatial distribution of the toxic element lead (Pb) and other trace elements in normal articular cartilage and subchondral bone from adult humans with no history of work-related exposure to Pb.

METHODS

Four macroscopically normal femoral heads and three patellas were harvested from randomly selected forensic autopsies. All subjects died of acute illnesses, had no history of work-related exposure to Pb and had no metabolic bone disease. The elemental distribution of lead (Pb) together with zinc (Zn), strontium (Sr) and calcium (Ca) in the chondral and subchondral region was detected using high resolution synchrotron radiation induced micro X-ray fluorescence (SR mu-XRF) analysis. SR mu-XRF line scans in conventional and SR mu-XRF area scans in confocal geometry were correlated to backscattered electron (BE) images visualizing the mineralized tissue.

RESULTS

In all samples, we found a highly specific accumulation of Pb in the tidemark, the transition zone between calcified and non-calcified articular cartilage. Pb fluorescence intensities in the tidemark, which is thought to be a metabolically active mineralization front, were 13-fold higher when compared to subchondral bone. Pb intensities in the subchondral region were strongly correlated with Zn, but were distinctly different from Ca and Sr.

CONCLUSIONS

The finding of the highly specific accumulation of lead in the tidemark of human articular cartilage is novel. However at this point, the exact mechanisms of the local Pb accumulation as well as its clinical implications are unknown.

The effects of dietary calcium during lactation on lead in bone mobilization: implications for toxicology

Under a normal 1.0% calcium (Ca) concentration in the diet during and after chronic lead (Pb) intoxication, there was a significant increment in the concentration of Pb in the blood, kidney, liver and brain during lactation, parallel to a decrement of Pb in the bone. The increment of Pb in the brain was accompanied by an enhanced lipid oxidation (increase in conjugated dienes). During lactation, on the first 14 days, when dietary Ca was reduced to 0.05%, bone Ca concentration was decreased by 15%, bone resorption measured as acid phosphatase activity in plasma increased three times and Pb bone concentration dropped by 30%. Under a 0.05% Ca in the diet in the nonlactating rats, Ca in the bone decreases also, but there were neither increments in bone resorption nor Pb efflux from the bone. These results suggested that Pb efflux in the bone was related to bone resorption during lactation. Interestingly, when dietary Ca was enhanced to 2.5% in lactating rats, Ca concentration in the bone increased by 21%, but resorption did not decrease and Pb bone concentration decreased by 28%, enhancing toxicity. In the control Pb-exposed nonlactating rats under a 2.5% Ca in the diet, Ca concentration in the bone was increased (18%), and Pb concentration in the bone was unaltered. Since Ca metabolism changes drastically in humans during pregnancy and lactation, and it is likely that bone Pb is mobilized and transferred to the more available compartments of the maternal circulation, the increment in daily intake of Ca during lactation could enhance Pb efflux from the bone.

The effect of Pb(2+) on the structure and hydroxyapatite binding properties of osteocalcin

Lead toxicity is a major environmental health problem in the United States. Bone is the major reservoir for body lead. Although lead has been shown to impair bone metabolism in animals and at the cellular level, the effect of Pb(2+) at the molecular level is largely unknown. We have used circular dichroism (CD), and a hydroxyapatite binding assay to investigate the effect of Pb(2+) on the structure and mineral binding properties of osteocalcin, a noncollagenous bone protein. The CD data indicate Pb(2+) induces a similar structure in osteocalcin as Ca(2+) but at 2 orders of magnitude lower concentration. These results were explained by the more than 4 orders of magnitude tighter binding of Pb(2+) to osteocalcin (K(d)=0.085 microM) than Ca(2+) (K(d)=1.25 mM). The hydroxyapatite binding assays show that Pb(2+) causes an increased adsorption to hydroxyapatite, similar to Ca(2+), but at 2-3 orders of magnitude lower concentration. Low Pb(2+) levels (1 microM) in addition to physiological Ca(2+) levels (1 mM) caused a significant (40%) increase in the amount of mineral bound osteocalcin as compared to 1 mM Ca(2+) alone. These results suggest a molecular mechanism of Pb(2+) toxicity where low Pb(2+) levels can inappropriately perturb Ca(2+) regulated processes. In-vivo, the increased mineral bound osteocalcin could play a role in the observed low bone formation rates and decreased bone density observed in Pb(2+)-intoxicated animals.

Alteration of rat submandibulary gland secretion of protein, calcium and N-acetyl-beta-D-glucosaminidase activity by lead

1. Effects of various doses of long-term lead treatment (0.01%, 0.04% and 0.05%) on rat submandibular saliva were investigated in this study. 2. Both submandibular ducts were cannulated intraorally with polyethylene tubes and saliva was collected from anesthetized lead treated and control rats using pilocarpine as secretagogue. 3. Saliva protein concentration was found to be reduced in lead (0.04%)- and (0.05%)-treated groups. 4. Saliva calcium concentration had a significant reduction only in the lead (0.05%)-treated group. 5. The secretion of the lysosomal enzyme, N-acetyl-beta-D-glucosaminidase (NAG) in saliva decreased significantly in the lead (0.04%)- and (0.05)-treated groups. Specific activity of the enzyme showed an increase in these treated groups. 6. Good correlations were found between saliva protein concentration and NAG activity, saliva protein and calcium concentrations and calcium concentration and NAG activity. 7. There was a correlation between blood and submandibular saliva lead levels, and the saliva/blood ratio was approximately constant for all treated groups. 8. With respect to the ability of lead to substitute for calcium in several intracellular regulatory events, explanation for these alterations in submandibular saliva composition can be made.