Human retinal cadmium accumulation as a factor in the etiology of age-related macular degeneration

Environmental exposures to cadmium and lead as potential causes of eye diseases

Humans are exposed to cadmium and lead in various regions of the world daily due to industrial development and climate change. Increasing numbers of preclinical and clinical studies indicate that heavy metals, such as cadmium and lead, play a role in the pathogenesis of eye diseases. Excessive exposure to heavy metals such as cadmium and lead can increase the risk of impaired vision. Therefore, it is essential to better characterize the role of these non-essential metals in disease etiology and progression. This article discusses the potential role of cadmium and lead in the development of age-related eye diseases, including age-related macular degeneration, cataracts, and glaucoma. Furthermore, we discuss how cadmium and lead affect ocular cells and provide an overview of putative pathological mechanisms associated with their propensity to damage the eye.

Toxic effects of the combined cadmium and Cry1Ab protein exposure on the protective and transcriptomic responses of Pirata subpiraticus

Cadmium (Cd) pollution poses a serious threat to agricultural production and paddy field fauna. Crystalline proteins (e.g., Cry1Ab and Cry1Ac) are secreted by Bacillus thuringiensis, which can manage pests via a complicated toxic mechanism and have been widely used for pest control due to the commercialization of transgenic crops (e.g., cotton and rice) that expresses Bt insecticidal proteins. Nonetheless, studies on the effects of combined stress of Cd and Cry1Ab protein on field indicator species are limited. In the present study, we showed that spiders, Pirata subpiraticus, fed with Cd-containing flies+Cry1Ab had dramatically higher Cd accumulation than that in the spiders fed with Cd-containing flies (p < 0.05). In addition, the enrichment of Cd led to the activation of the protective mechanism by elevating the concentrations of glutathione peroxidase, glutathione S-transferase, and metallothionein in the spiders (p < 0.05). An in-depth transcriptome analysis revealed that the activities of ion metal binding proteins, transporters, and channels might play essential roles in the Cd accumulation process. More importantly, the higher Cd concentration in the combined Cd+Cry1Ab exposure prolonged developmental duration of P. subpiraticus, due to the down-regulated cuticle proteins (CPs) encoding genes involved in the molting process, which was regulated by a series of putative transcriptional factors such as ZBTB and zf-C2H2. Collectively, this integrated analysis illustrates that the combined Cd+Cry1Ab exposure increases the adverse effects of Cd stress on the growth, antioxidase, and CPs encoding genes of P. subpiraticus, thus providing a research basis and prospect for the rationality of transgenic Cry1Ab crops in the cultivation of heavy metal contaminated soil.

Soluble curcumin ameliorates motility, adhesiveness and abrogate parthanatos in cadmium-exposed retinal pigment epithelial cells

Cadmium (Cd) is a nonessential transition metal and one of the most toxic environmental pollutants. Industrial, agricultural and urban activities are the main sources of Cd environmental contamination. Multiple deleterious effects of Cd exposure were reported for different cell types and living organisms in a great number of research papers. Cd bioaccumulation hazard is mediated by the relatively long half-life of this metal in an organism. For example, in mammals its half-life lasts for about 10–30 years. Cd exposure affects many tissues. However, some of them, including the central nervous system and sensory organs, are most susceptible to its toxicity. The harmful effects of Cd could be linked to oxidative stress generation and consequently intracellular signalling disruption. Since Cd induces redox imbalance the antioxidants could be a prospective tool to ameliorate Cd cytotoxicity. In present work, we have studied the protective efficacy of soluble curcumin on Cd-caused retinal pigment epithelium (RPE) cells viability, reactive oxygen species production, adhesive and extracellular matrix proteins expression, cell migration and parthanatos level. Low dose (5 µM) of soluble curcumin ameliorated all aforementioned indices of Cd-induced cytotoxicity. Curcumin has restored the RPE cells motility as well as fibronectin and E-cadherin expression. Therefore, the modulation of RPE adhesiveness could be regarded as a cytoprotective effect of curcumin. Furthermore, Cd-caused poly(ADP-ribose) polymerase-1 (PARP-1) suppression and cleaved PARP-1 upregulation were ameliorated by curcumin exposure. Therefore, the protective effect of soluble curcumin could be related, at least partially, to the modulation of PARP activity and inhibition of parthanatos flux. The observed results have demonstrated that low doses of soluble curcumin are a promising tool to protect RPE cells against Cd-caused retinal injury.

Trace elements exposure and risk in age-related eye diseases: a systematic review of epidemiological evidence

This systematic review aimed to evaluate existing evidence on the associations between trace elements exposure and age-related eye diseases. PubMed and Google scholar databases were searched for epidemiological and postmortem studies on the relationship between exposure to trace elements and Age-related eye diseases such as age-related macular degeneration (AMD), cataract, glaucoma and diabetic retinopathy (DR), in population groups aged 40 years and above. Available evidence suggests that cadmium (Cd) exposure may be positively associated with the risks of AMD and cataract. There is also evidence that exposure to lead (Pb) may be positively associated with higher risk of cataract and glaucoma. There is limited number of relevant studies and lack of prospective studies for most of the investigated associations. Evidence for other trace elements is weak and inconsistent, and the number of available studies is small. Likewise, there are very few relevant studies on the role of trace elements in DR. Chemical elements that affect the distribution and absorption of other trace elements have never been investigated. The suggestive but limited evidence motivates large and quality prospective studies to fully characterize the impact of exposure to trace (toxic and essential) elements on age-related eye diseases.

An inadvertent issue of human retina exposure to endocrine disrupting chemicals: A safety assessment

Endocrine disrupting chemicals (EDCs) are a group of chemical compounds that present a considerable public health problem due to their pervasiveness and associations with chronic diseases. EDCs can interrupt the endocrine system and interfere with hormone homeostasis, leading to abnormalities in human physiology. Much attention has been focused on the adverse effects EDCs have on the reproductive system, neurogenesis, neuroendocrine system, and thyroid dysfunction. The eye is usually directly exposed to the surrounding environment; however, the influences of EDCs on the eye have received comparatively little attention. Ocular diseases, such as ocular surface diseases and retinal diseases, have been implicated in hormone deficiency or excess. Epidemiologic studies have shown that EDC exposure not only causes ocular surface disorders, such as dry eye, but also associates with visual deficits and retinopathy. EDCs can pass through the human blood-retinal barrier and enter the neural retina, and can then accumulate in the retina. The retina is an embryologic extension of the central nervous system, and is extremely sensitive and vulnerable to EDCs that could be passed across the placenta during critical periods of retinal development. Subtle alterations in the retinal development process usually result in profound immediate, long-term, and delayed effects late in life. This review, based on extensive literature survey, briefly summarizes the current knowledge about the impact of representative manufactured EDCs on retinal toxicity, including retinal structure alterations and dysfunction. We also highlight the potential mechanism of action of EDCs on the retina, and the predictive retinal models of EDC exposure.

Association of plasma trace element levels with neovascular age-related macular degeneration

Although the triggers causing angiogenesis in the context of neovascular age-related macular degeneration (nAMD) are not fully understood, oxidative stress is likely involved. Oxidative stress in the eye can occur through exposure of macular tissues to sunlight and local or systemic exposure to oxidative stressors associated with environmental or lifestyle factors. Because trace elements have been implicated as regulators of oxidative stress and cellular antioxidant defense mechanisms, we hypothesized that they may play a role as a risk factor, modifying the progression toward nAMD. Herein, we determined whether levels of human plasma trace elements are different in 236 individuals with nAMD compared to 236 age-matched controls without AMD. Plasma levels of 16 trace elements including arsenic, barium, calcium, cadmium, cobalt, chromium, copper, iron, magnesium, manganese, molybdenum, lead, antimony, selenium, vanadium and zinc were measured using inductively coupled plasma mass spectrometry. Associations of trace elements with demographic, environmental and lifestyle factors and AMD-associated genetic variants were assessed. Elevated levels of barium and cadmium and reduced levels of chromium were observed in nAMD patients compared to controls. Mean plasma concentrations of barium were 1.35 μg/L (standard deviation [SD] 0.71) in nAMD and 1.15 μg/L (SD 0.63) in controls (P = 0.001). Mean levels of chromium were 0.37 μg/L (SD 0.22) in nAMD and 0.46 μg/L (SD 0.34) in controls (P = 0.001). Median levels for cadmium, which were not normally distributed, were 0.016 μg/L (interquartile range [IQR] 0.001-0.026) in nAMD and 0.012 μg/L (IQR 0.001-0.022) in controls (P = 0.002). Comparison of the Spearman's correlation coefficients between nAMD patients and controls identified a difference in correlations for 8 trace elements. Cadmium levels were associated with the smoking status (P < 0.001), while barium levels showed a trend of association with the usage of antihypertensive drugs. None of the AMD-associated genetic variants were associated with any trace element levels. In conclusion, in this case-control study we detected elevated plasma levels of barium and cadmium and reduced plasma levels of chromium in nAMD patients. An imbalance in plasma trace elements, which is most likely driven by environmental and lifestyle factors, might have a role in the pathogenesis of AMD. These trace elements may be incorporated as biomarkers into models for prediction of disease risk and progression. Additionally, population-based preventive strategies to decrease Cd exposure, especially by the cessation of smoking, could potentially reduce the burden of nAMD. Future studies are warranted to investigate whether supplementation of Cr would have a beneficial effect on nAMD.

Linking molecular targets of Cd in the bloodstream to organ-based adverse health effects

The chronic exposure of human populations to toxic metals remains a global public health concern. Although chronic Cd exposure is linked to kidney damage, osteoporosis and cancer, the underlying biomolecular mechanisms remain incompletely understood. Since other diseases could also be causally linked to chronic Cd exposure, a systems toxicology-based approach is needed to gain new insight into the underlying exposure-disease relationship. This approach requires one to integrate the cascade of dynamic bioinorganic chemistry events that unfold in the bloodstream after Cd enters with toxicological events that unfold in target organs over time. To this end, we have conducted a systematic literature search to identify all molecular targets of Cd in plasma and in red blood cells (RBCs). Based on this information it is impossible to describe the metabolism of Cd and the toxicological relevance of it binding to molecular targets in/on RBCs is elusive. Perhaps most importantly, the role that peptides, amino acids and inorganic ions, including HCO₃^-, Cl^- and HSeO₃^- play in terms of mediating the translocation of Cd to target organs and its detoxification is poorly understood. Causally linking human exposure to this metal with diseases requires a much better integration of the bioinorganic chemistry of Cd that unfolds in the bloodstream with target organs. This from a public health point of view important goal will require collaborations between scientists from different disciplines to untangle the complex mechanisms which causally link Cd exposure to disease.

Cadmium, obesity, and education, and the 10-year incidence of hearing impairment: The beaver dam offspring study

OBJECTIVES/HYPOTHESIS

To determine the 10-year incidence of hearing impairment (HI) and associated risk factors in the Beaver Dam Offspring Study (BOSS; 2004-present), a large middle-aged cohort followed for 10 years.

STUDY DESIGN

Prospective cohort study.

METHODS

Hearing thresholds were measured at baseline (2005-2008) and 5- (2010-2013) and 10-year (2015-2017) follow-up examinations. HI was defined as a pure-tone average >25 dB HL in either ear. BOSS participants free of HI at baseline with at least one follow-up examination (N = 2,065) were included. Potential risk factors evaluated included cardiovascular measures, health history, lifestyle factors, inflammatory markers, vitamins D and B12, lead, and cadmium.

RESULTS

Participants were 21 to 79 years (mean age = 47.9 years) at baseline. The 10-year cumulative HI incidence was 17.4% (95% confidence interval [CI]: 15.7-19.2) and was twice as likely in men (24.4%, 95% CI: 21.5-27.7) than in women (12.2%, 95% CI: 10.3-14.3). In a multivariable adjusted model, age (hazard ratio [HR] = 1.48, 95% CI: 1.38-1.59, per 5 years), male sex (HR = 2.47, 95% CI: 1.91-3.18), less than a college education (HR = 1.35, 95% CI: 1.02-1.79), body mass index (HR = 1.03, 95% CI: 1.01-1.05, per kg/m2 ), and higher cadmium levels (HR = 1.42, 95% CI: 1.05-1.92, quintile 5 vs. quintiles 1-4) were associated with the 10-year cumulative incidence of HI. There was no association between high lead levels, vitamins D or B12, and 10-year incidence of HI.

CONCLUSIONS

In addition to age and sex, obesity, education, and blood cadmium levels were associated with increased incidence of HI. These prospective results add to evidence that age-related HI is a multifactorial preventable disorder.

LEVEL OF EVIDENCE

2b Laryngoscope, 130:1396-1401, 2020.

Zinc Nutrition and Inflammation in the Aging Retina

Zinc is an essential nutrient for human health. It plays key roles in maintaining protein structure and stability, serves as catalytic factor for many enzymes, and regulates diverse fundamental cellular processes. Zinc is important in affecting signal transduction and, in particular, in the development and integrity of the immune system, where it affects both innate and adaptive immune responses. The eye, especially the retina-choroid complex, has an unusually high concentration of zinc compared to other tissues. The highest amount of zinc is concentrated in the retinal pigment epithelium (RPE) (RPE-choroid, 292 ± 98.5 µg g^-1 dry tissue), followed by the retina (123 ± 62.2 µg g^-1 dry tissue). The interplay between zinc and inflammation has been explored in other parts of the body but, so far, has not been extensively researched in the eye. Several lines of evidence suggest that ocular zinc concentration decreases with age, especially in the context of age-related disease. Thus, a hypothesis that retinal function could be modulated by zinc nutrition is proposed, and subsequently trialled clinically. In this review, the distribution and the potential role of zinc in the retina-choroid complex is outlined, especially in relation to inflammation and immunity, and the clinical studies to date are summarized.

Association of Cadmium and Lead Exposure With the Incidence of Contrast Sensitivity Impairment Among Middle-aged Adults

Importance

Contrast sensitivity (CS) is an important indicator of visual function that affects daily life, including mobility, visually intensive tasks, safety, and autonomy. Understanding the risk factors for CS impairment could prevent decreases in visual function.

Objective

To determine the incidence of and factors associated with CS impairment in a large cohort.

Design, Setting, and Participants

The Beaver Dam Offspring Study is an ongoing longitudinal cohort study of aging involving adults in Beaver Dam, Wisconsin. Participants who were free of CS impairment in both eyes at baseline were included (N = 1983). Baseline data collection occurred from June 8, 2005, through August 4, 2008, when the participants ranged from 21 to 84 years of age. Two follow-up examinations occurred at 5-year intervals: one was conducted between July 12, 2010, and March 21, 2013, and the other between July 1, 2015, and November 13, 2017. Data analysis was performed from November 27, 2017, to February 27, 2018.

Main Outcomes and Measures

Contrast sensitivity testing was conducted with Pelli-Robson letter sensitivity charts, and incident impairment was defined as a log CS score less than 1.55 in either eye at any follow-up examination. Cadmium and lead levels were measured in whole blood with inductively coupled plasma mass spectrometry. Associations between baseline characteristics and CS impairment incidence were examined using Cox proportional hazard models and quantified as hazard ratios (HRs) with 95% CI.

Results

Of the 1983 participants included, 1028 (51.8%) were female and 955 (48.2%) were male, with a mean (SD) age of 48 (9.3) years. The 10-year cumulative incidence of CS impairment was 24.8% (95% CI, 22.9-26.8), similar in women (24.9%) and men (24.6%), and highest in the oldest age group (65-84 years) at 66.3%. In multivariable models, cadmium level in the highest quintile (HR, 1.35; 95% CI, 1.02-1.78), older age (HR, 1.36; 95% CI, 1.25-1.47), larger waist circumference (HR, 1.06; 95% CI, 1.01-1.11), and more plaque sites (1-3 sites: HR, 1.43; 95% CI, 1.07-1.92; 4-6 sites: HR, 2.75; 95% CI, 1.26-6.05) were among the factors associated with increased risk, while male sex (HR, 0.77; 95% CI, 0.60-0.98) and any alcohol consumption (HR, 0.61; 95% CI, 0.43-0.88) were associated with decreased risk. Results were similar when smoking status replaced cadmium exposure in the models. Lead level was not associated with increased risk.

Conclusions and Relevance

This study's findings suggest that incident CS impairment was common in the 10-year follow-up, with cadmium, but not lead, exposure associated with increased risk. The associations of diminished CS with other modifiable risk factors found appear to imply that changes in behavior may reduce future incidence of CS impairment.

Blood lead and cadmium in age related macular degeneration in a Turkish urban population

PURPOSE

To evaluate the blood lead (Pb) and cadmium (Cd) levels in age related macular degeneration (AMD) in a turkish urban population.

METHODS

Blood Pb and Cd levels of 31 AMD patients and 24 age and gender matched controls with no sign of AMD were measured using dual atomic absorption spectrophotometer system (AAS). History of hypertension, diabetes mellitus, cigarette smoking, myocardial infarction and stroke were obtained from all subjects. Degree of AMD was grade 4 according to the Age-Related Eye Disease Study grading system. Median blood Pb and Cd levels were compared by using Students' t-test.

RESULTS

Demographic properties like smoking status, presence of diabetes mellitus or hypertension, cerebrovascular occlusion history, serum cholesterol and lipid levels were not significantly different between groups except history of ischemic heart disease (3.22% vs 25% in AMD and control groups respectively, p = .022). Overall in AMD group blood Pb level was 2.83 ± 0.15 μg/l and it was 2.63 ± 0.23 μg/l in control group (p = .36). The Cd level was 3.25 ± 0.20 μg/l in AMD group and 3.11 ± 0.25 μg/l in control group (p = .67). The mean Pb (2.38 ± 0.88 μg/l vs 2.91 ± 1.37 μg/l for AMD vs control, p = .61) and Cd levels (3.06 ± 1.34 μg/l vs 3.35 ± 1.26 μg/l for AMD vs control, p = .56) in current and previous smokers with AMD were not significantly different from those of the current and previous smokers in control group.

CONCLUSION

Blood Pb and Cd levels which reflect short term exposure were not significantly different in AMD patients and the control group. The difference was not significant either after involvement of previous or current smoker subjects.

Transition metals and trace elements in the retinal pigment epithelium and choroid: correlative ultrastructural and chemical analysis by analytical electron microscopy and nano-secondary ion mass spectrometry

Understanding the localisation and abundance of structural elements, trace elements and especially transition metals like Cu and Zn in ocular tissue sections is important for physiology, and also for the characterisation of diseases related to oxidative stress like age-related macular degeneration. Transition metal abundances were investigated in an aged donor eye by nano-secondary ion mass spectrometry (nano-SIMS) elemental mapping using Cs⁺ and O^- primary ions, respectively, and correlated to their respective mole fractions investigated by analytical electron microscopy (AEM). The ultrastructure of the tissue and the elemental composition of melanosomes of the choroid and RPE, and RPE lipofuscin and melanolipofuscin granules can adequately be investigated by nano-SIMS using the secondary ion maps. Melanosomes, 0.5-1 μm in size, yield sulphur maps and maps of stored metals like calcium, sodium and copper. Lipofuscin shows especially high phosphorus signals. Elements with mole fractions of about 0.1 at%, e.g. for P and Cu, as investigated by AEM before, can be validated using simultaneous SIMS maps with an estimated lateral resolution of 66 nm with typical acquisition times of 30 minutes for each area of interest. However, Zn (0.19 at%) was not detected by SIMS. Nano-SIMS imaging of CN^-, PO₂^-, S^-, Cu^-, Ca⁺, Fe⁺ and Na⁺ ions provides excellent detection limits demonstrating the possibilities for chemical mapping with high-sensitivity trace element detection and reduced acquisition times. Quantification of nano-SIMS data was achieved by correlating mole fractions obtained by AEM to secondary ions per pixel obtained by nano-SIMS. Both methods yield the melanin type in melanosomes and trace metal storage.

Environmental exposure to cadmium-a risk for health of the general population in industrialized countries and preventive strategies

Cadmium (Cd) is a heavy metal belonging to the group of the main chemical pollutants of the natural and occupational environment in economically developed countries. The forecasts indicate that contamination of the environment with this toxic metal, and thus the exposure of the general population, will increase. Food (particularly plant products) is the main source of the general population exposure to this element. Moreover, an important, and often the main, source of intoxication with Cd is habitual tobacco smoking. Recent epidemiological studies have provided numerous evidence that even low-level environmental exposure to this toxic metal, nowadays occurring in numerous economically developed countries, creates a risk for health of the general population. The low-level lifetime exposure to this metal may lead to the damage to the kidneys, liver, skeletal system, and cardiovascular system, as well as to the deterioration of the sight and hearing. Moreover, it has been suggested that environmental exposure to this xenobiotic may contribute to the development of cancer of the lung, breast, prostate, pancreas, urinary bladder, and nasopharynx. Taking the above into account, the aim of this review article is to draw more attention to Cd as an environmental risk factor for the health of the general population and the need to undertake preventive actions allowing to reduce the risk of health damage due to a lifetime exposure to this toxic metal.

Early and intermediate age-related macular degeneration: update and clinical review

Age-related macular degeneration (AMD) is the leading cause of irreversible central vision loss in developed countries. With the aging of population, AMD will become globally an increasingly important and prevalent disease worldwide. It is a complex disease whose etiology is associated with both genetic and environmental risk factors. An extensive decline in the quality of life and progressive need of daily living assistance resulting from AMD among those most severely affected highlights the essential role of preventive strategies, particularly advising patients to quit smoking. In addition, maintaining a healthy diet, controlling other risk factors (such as hypertension, obesity, and atherosclerosis), and the use of nutritional supplements (antioxidants) are recommendable. Genetic testing may be especially important in patients with a family history of AMD. Recently, unifying criteria for the clinical classification of AMD, defining no apparent aging changes; normal aging changes; and early, intermediate, and late AMD stages, are of value in predicting AMD risk of progression and in establishing recommendations for the diagnosis, therapeutic approach, and follow-up of patients. The present review is focused on early and intermediate AMD and presents a description of the clinical characteristics and ophthalmological findings for these stages, together with algorithms for the diagnosis and management of patients, which are easily applicable in daily clinical practice.

Cadmium exposure and age-related macular degeneration

Cadmium (Cd) has been proposed as a risk factor for age-related macular degeneration (AMD), but the association between Cd exposure and AMD risk in large population studies is unknown. This study evaluated the association of Cd exposure with AMD in a large representative sample of Korean men and women. This was a cross-sectional study of 3865 Korean adults ≥ 40 years of age who participated in the Korean National Health and Nutrition Examination Survey (KNHANES) during 2008-2011. Cd concentrations in whole blood were measured by graphite-furnace atomic absorption spectrometry. The presence of AMD was determined in digital non-mydriatic fundus photographs. Cd levels were higher in participants with AMD compared with those without AMD (1.3 vs 1.1 μg/l, respectively, P<0.001). In fully adjusted models, the odds ratio for AMD comparing the highest with the lowest Cd quartiles was 1.92 (95% CI=1.08-3.39; P for trend 0.029). In restricted cubic spline models, the association between Cd and AMD was approximately linear, with no evidence of threshold effects. Blood Cd concentrations were independently associated with the prevalence of AMD. If the association is proven causal, population-based preventive strategies to decrease Cd exposure could reduce the population burden of AMD.

Five heavy metallic elements and age-related macular degeneration: Korean National Health and Nutrition Examination Survey, 2008-2011

OBJECTIVE

To investigate the association between age-related macular degeneration (AMD) and 5 heavy metallic elements (lead, mercury, cadmium, manganese, and zinc).

DESIGN

A cross-sectional study using a complex, stratified, multistage, probability cluster survey.

PARTICIPANTS

Participants of the Korean National Health and Nutrition Examination Survey from 2008 to 2011.

METHODS

Using a standardized protocol, AMD was determined by fundus photograph grading. Blood concentrations of lead, mercury, cadmium, manganese, and zinc were measured. Associations between AMD and these 5 elements were estimated using logistic regression analyses (LRAs). The distributions of the 5 metallic elements in blood were analyzed, and the same set of LRAs estimating the association between AMD and logarithmic-transformed blood concentrations of the 5 elements were also conducted.

MAIN OUTCOME MEASURES

Association between AMD and 5 heavy metals.

RESULTS

Lead was positively associated with both early AMD and late AMD in all LRAs. Mercury and cadmium also had a positive association with late AMD in all LRAs, but not with early AMD. In contrast, manganese and zinc had an inverse association with late AMD in all LRAs. Manganese and zinc were not associated with early AMD. Using logarithmic-transformed blood concentrations for each metallic element, the LRAs showed similar results compared with those of the LRAs using nontransformed blood concentrations, despite the skewed distribution of these metallic elements in the blood.

CONCLUSIONS

This study suggests that the toxic heavy metals (lead, mercury, and cadmium) may negatively influence late AMD, whereas essential heavy metals (manganese and zinc) may favorably influence late AMD. Lead may widely affect the pathogenesis of both early and late AMD.

Environmental cadmium and lead exposures and age-related macular degeneration in U.S. adults: the National Health and Nutrition Examination Survey 2005 to 2008

Age-related macular degeneration (AMD) is a complex disease resulting from the interplay of genetic predisposition and environmental exposures, and has been linked to oxidative stress and inflammatory mechanisms. Lead and cadmium can accumulate in human retinal tissues and may damage the retina through oxidative stress, and may thereby play a role in the development of AMD. We examined associations between blood lead, blood cadmium, and urinary cadmium concentrations and the presence of AMD in 5390 participants aged 40 years and older with blood lead and blood cadmium measures and a subsample of 1548 with urinary cadmium measures in the 2005-2008 National Health and Nutrition Examination Surveys. AMD was identified by grading retinal photographs with a modification of the Wisconsin Age-Related Maculopathy Grading System. The weighted prevalence of AMD was 6.6% (n=426). Controlling for age, gender, race/ethnicity, education and body mass index, adults in the highest blood cadmium quartile had higher odds of AMD compared to the lowest quartile (odds ratio [OR], 1.56; 95% CI, 1.02-2.40), with a significant trend across quartiles (p-trend=0.02). After further adjustment for pack-years of cigarette smoking, estimates were somewhat attenuated (OR, 1.43; 95% CI, 0.91-2.27; p-trend=0.08). Similar associations were found with urinary cadmium. The association between urinary cadmium and AMD was stronger in non-Hispanic whites (NHW) than in non-Hispanic blacks (NHB) (OR, 3.31; 95% CI, 1.37-8.01 for levels above versus below the median among NHW; OR,1.45; 95% CI, 0.40-5.32 for levels above versus below the median among NHB; p-interaction=0.03). We found no association between blood lead levels and AMD. Higher cadmium body burden may increase risk of AMD, particularly among non-Hispanic white individuals; however, additional studies are needed before firm conclusions can be drawn.

Role of antioxidant enzymes and small molecular weight antioxidants in the pathogenesis of age-related macular degeneration (AMD)

Cells in aerobic condition are constantly exposed to reactive oxygen species (ROS), which may induce damage to biomolecules, including proteins, nucleic acids and lipids. In normal circumstances, the amount of ROS is counterbalanced by cellular antioxidant defence, with its main components—antioxidant enzymes, DNA repair and small molecular weight antioxidants. An imbalance between the production and neutralization of ROS by antioxidant defence is associated with oxidative stress, which plays an important role in the pathogenesis of many age-related and degenerative diseases, including age-related macular degeneration (AMD), affecting the macula—the central part of the retina. The retina is especially prone to oxidative stress due to high oxygen pressure and exposure to UV and blue light promoting ROS generation. Because oxidative stress has an established role in AMD pathogenesis, proper functioning of antioxidant defence may be crucial for the occurrence and progression of this disease. Antioxidant enzymes play a major role in ROS scavenging and changes of their expression or/and activity are reported to be associated with AMD. Therefore, the enzymes in the retina along with their genes may constitute a perspective target in AMD prevention and therapy.

The antiaging approach for the treatment of dry eye

Dry eye is one of the most common eye disorders affecting millions of people. It causes ocular irritation or discomfort, and decreases functional vision, causing a dramatic deterioration in the quality of life. Although new treatments such as the P2Y2 agonist or cyclosporine eye drops have been developed and a certain level of patient satisfaction can now be obtained, no fundamental treatment has been developed. Currently, there is no therapy available to recover lacrimal function to its normal status. Recent progress in the understanding of aging has laid the foundations for a new way of thinking about intervention of the aging process. Because dry eye is accelerated by aging, a useful approach for the prevention or treatment of dry eye may be to interfere with the aging process. In the scientific community, there is a global consensus that calorie restriction can extend the life span of various kinds of animals, establishing an intervention to aging. Another important hypothesis believed to be involved in aging is the free radical theory. According to these theories, the aging process may be managed by controlling levels of calories or reactive oxygen species. In this review, these 2 important aging theories, calorie restriction and free radical aging, are examined, and we discuss how to apply these theories to the prevention and treatment of dry eye.

Emerging roles of cadmium and heme oxygenase in type-2 diabetes and cancer susceptibility

Many decades after an outbreak of severe cadmium poisoning, known as Itai-itai disease, cadmium continues to pose a significant threat to human health worldwide. This review provides an update on the effects of this environmental toxicant cadmium, observed in numerous populations despite modest exposure levels. In addition, it describes the current knowledge on the link between heme catabolism and glycolysis. It examines novel functions of heme oxygenase-2 (HO-2) that protect against type 2-diabetes and obesity, which have emerged from diabetic/obese phenotypes of the HO-2 knockout mouse model. Increased cancer susceptibility in type-2 diabetes has been noted in several large cohorts. This is a cause for concern, given the high prevalence of type-2 diabetes worldwide. A lifetime exposure to cadmium is associated with pre-diabetes, diabetes, and overall cancer mortality with sex-related differences in specific types of cancer. Liver and kidney are target organs for the toxic effects of cadmium. These two organs are central to the maintenance of blood glucose levels. Further, inhibition of gluconeogenesis is a known effect of heme, while cadmium has the propensity to alter heme catabolism. This raises the possibility that cadmium may mimic certain HO-2 deficiency conditions, resulting in diabetic symptoms. Intriguingly, evidence has emerged from a recent study to suggest the potential interaction and co-regulation of HO-2 with the key regulator of glycolysis: 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase 4 (PFKFB4). HO-2 could thus be critical to a metabolic switch to cancer-prone cells because the enzyme PFKFB and glycolysis are metabolic requirements for cell proliferation and resistance to apoptosis.

Cadmium intake and systemic exposure in postmenopausal women and age-matched men who smoke cigarettes

Mean blood cadmium (B-Cd) concentrations are two- to threefold higher in smokers than in nonsmokers. The basis for this phenomenon is not well understood. We conducted a detailed, multifaceted study of cadmium exposure in smokers. Groups were older smokers (62±4 years, n = 25, 20% male) and nonsmokers (62±3 years, n = 16, 31% male). Each subject's cigarettes were machine smoked, generating individually paired measures of inhaled cadmium (I-Cd) versus B-Cd; I-Cd and B-Cd were each evaluated three times, at monthly intervals. Urine cadmium (U-Cd) was analyzed for comparison. In four smokers, a duplicate-diet study was conducted, along with a kinetic study of plasma cadmium versus B-Cd. Female smokers had a mean B-Cd of 1.21ng Cd/ml, with a nearly 10-fold range (0.29-2.74ng Cd/ml); nonsmokers had a lower mean B-Cd, 0.35ng Cd/ml (p < 0.05), and narrower range (0.20-0.61ng Cd/ml). Means and ranges for males were similar. Estimates of cadmium amounts inhaled daily for our subjects smoking ≥ 20 cigarettes/day were far less than the 15 µg Cd reported to be ingested daily via diet. This I-Cd amount was too low to alone explain the 3.5-fold elevation of B-Cd in our smokers, even assuming greater cadmium absorption via lungs than gastrointestinal tract; cadmium accumulated in smokers' lungs may provide the added cadmium. Finally, B-Cd appeared to be linearly related to I-Cd values in 75% of smokers, whereas 25% had far higher B-Cd, implying a possible heterogeneity among smokers regarding circulating cadmium concentrations and potentially cadmium toxicity.

Cadmium, environmental exposure, and health outcomes

We provide an update of the issues surrounding health risk assessment of exposure to cadmium in food. Bioavailability of ingested cadmium has been confirmed in studies of persons with elevated dietary exposure, and the findings have been strengthened by the substantial amounts of cadmium accumulated in kidneys, eyes, and other tissues and organs of environmentally exposed individuals. We hypothesized that such accumulation results from the efficient absorption and systemic transport of cadmium, employing multiple transporters that are used for the body's acquisition of calcium, iron, zinc, and manganese. Adverse effects of cadmium on kidney and bone have been observed in environmentally exposed populations at frequencies higher than those predicted from models of exposure. Population data raise concerns about the validity of the current safe intake level that uses the kidney as the sole target in assessing the health risk from ingested cadmium. The data also question the validity of incorporating the default 5% absorption rate in the threshold-type risk assessment model, known as the provisional tolerable weekly intake (PTWI), to derive a safe intake level for cadmium.

Cadmium-induced induction of cell death in human lens epithelial cells: implications to smoking associated cataractogenesis

Cadmium is reported to accumulate in human eye tissues suggesting its implication in diverse ocular pathology. Using an in vitro cell culture model we investigated the effects of cadmium on human lens epithelial cells (HLECs) (HLE-B3). We observed cadmium-induced dose- as well as time-dependent decline in HLECs viability which was exacerbated significantly upon reduction of intracellular glutathione levels by buthionine sulfoximine (BSO). There was a dose-dependent significant increase in lactate dehydrogenase (LDH) release from HLECs suggesting cadmium-induced alteration of membrane integrity as well as necrotic cell death. The decline in cell viability was also due to apoptosis of the HLECs as determined by quantifying % apoptotic cells as well as PARP cleavage. Moreover, release of apoptosis inducing factor (AIF) into the cytosol was also detected. Cadmium was also observed to increase oxidative stress, lipid peroxidation and activation of MAPK pathway in HLECs. Antioxidants like N-acetylcysteine (NAC) and alpha-Tocopherol significantly prevented cadmium-induced toxicity in HLECs. Our findings suggest that cadmium-induced elevated oxidative stress as well as activation of MAPK signaling cascade eventually led to cell death of HLECs through apoptosis as well as necrosis. The loss of HLECs by cadmium could possibly explain its implication in cataract development particularly associated with smoking.

Cadmium, environmental exposure, and health outcomes

OBJECTIVES

We provide an update of the issues surrounding health risk assessment of exposure to cadmium in food.

DATA SOURCES

We reviewed epidemiologic studies published between 2004 and 2009 concerning the bioavailability of cadmium in food, assessment of exposure, and body burden estimate, along with exposure-related effects in nonoccupationally exposed populations.

DATA EXTRACTION AND SYNTHESIS

Bioavailability of ingested cadmium has been confirmed in studies of persons with elevated dietary exposure, and the findings have been strengthened by the substantial amounts of cadmium accumulated in kidneys, eyes, and other tissues and organs of environmentally exposed individuals. We hypothesized that such accumulation results from the efficient absorption and systemic transport of cadmium, employing multiple transporters that are used for the body's acquisition of calcium, iron, zinc, and manganese. Adverse effects of cadmium on kidney and bone have been observed in environmentally exposed populations at frequencies higher than those predicted from models of exposure. Increasing evidence implicates cadmium in the risk of diseases that involve other tissues and organ systems at cadmium concentrations that do not produce effects on bone or renal function.

CONCLUSIONS

Population data raise concerns about the validity of the current safe intake level that uses the kidney as the sole target in assessing the health risk from ingested cadmium. The data also question the validity of incorporating the default 5% absorption rate in the threshold-type risk assessment model, known as the provisional tolerable weekly intake (PTWI), to derive a safe intake level for cadmium.

Excess lead in the neural retina in age-related macular degeneration

PURPOSE

To measure lead and cadmium in retinal tissues of human donor eyes with and without age-related macular degeneration (AMD).

DESIGN

Laboratory investigation.

METHODS

Lead and cadmium concentrations in retinal tissues (neural retina and retinal pigment epithelium [RPE]-choroid complex) in 25 subjects with AMD (50 donor eyes) and 36 normal subjects (72 donor eyes) were determined by using inductively coupled plasma-mass spectrometry. Severity of AMD was graded by using color fundus photographs and the Minnesota Grading System. Differences in metal concentrations were compared by using Wilcoxon rank-sum tests.

RESULTS

The neural retinas of subjects with AMD had increased lead concentrations (median, 12.0 ng/g; 25% to 75% interquartile range, 8 to 18 ng/g; n = 25) compared with normal subjects (median, 8.0 ng/g; 25% to 75% interquartile range, 0 to 11 ng/g; P = .04; n = 36). There was no difference in lead concentration in the RPE-choroid complex between subjects with AMD (median, 198 ng/g; 25% to 75% interquartile range, 87 to 381 ng/g) and normal subjects (median, 172 ng/g; 25% to 75% interquartile range, 100 to 288 ng/g; P = .25). Cadmium concentration in the neural retina (median, 0.9 microg/g; 25% to 75% interquartile range, 0.7 to 1.8 microg/g) and RPE-choroid complex (median, 2.2 microg/g; 25% to 75% interquartile range, 1.8 to 3.7 microg/g) in subjects with AMD was not different from concentrations in the neural retina (median, 0.9 microg/g; 25% to 75% interquartile range, 0.7 to 1.4 microg/g; P = .32) and RPE-choroid complex (median, 1.5 microg/g; 25% to 75% interquartile range, 0.9 to 2.5 microg/g; P = .12) of normal subjects.

CONCLUSIONS

AMD is associated with excess lead in the neural retina, and this relationship suggests that metal homeostasis in AMD eyes is different from normal.