Heavy Metals Acting as Endocrine Disrupters

Georgescu B. et. al./Scientific Papers: Animal Science and Biotechnologies, 2011, 44 (2)Heavy Metals Acting as Endocrine DisruptersBogdan Georgescu 1 , Carmen Georgescu 2 , Stelian Dărăban 1 , Anca Bouaru 1 ,Simona Paşcalău 11 University of Agricultural Sciences and Veterinary Medicine, Faculty of Animal Husbandry and Biotechnology,400372, Cluj-Napoca, Calea Mănăştur 3, Romania2 Faculty of Medicine, University of Medicine and Pharmacy, 400349, Cluj-Napoca, Louis Pasteur 3, RomaniaAbstractLast years researches focused on several natural and synthetic compounds that may interfere with the major functionsof the endocrine system and were termed endocrine disrupters. Endocrine disrupters are defined as chemicalsubstances with either agonist or antagonist endocrine effects in human and animals. These effects may be achievedby interferences with the biosynthesis or activity of several endogenous hormones. Recently, it was demonstratedthat heavy metals such as cadmium (Cd), arsen (As), mercury (Hg), nickel (Ni), lead (Pb) and zinc (Zn) may exhibitendocrine-disrupting activity in animal experiments. Emerging evidence of the intimate mechanisms of action ofthese heavy metals is accumulating. It was revealed, for example, that the Zn atom from the Zn fingers of theestrogen receptor can be replaced by several heavy metal molecules such as copper, cobalt, Ni and Cd. By replacingthe Zn atom with Ni or copper, binding of the estrogen receptor to the DNA hormone responsive elements in the cellnucleus is prevented. In both males and females, low-level exposure to Cd interferes with the biological effects ofsteroid hormones in reproductive organs. Arsen has the property to bind to the glucocorticoid receptor thusdisturbing glucocorticoids biological effects. With regard to Hg, this may induce alterations in male and femalefertility, may affect the function of the hypothalamo-pituitary-thyroid axis or the hypothalamo-pituitary-adrenal axis,and disrupt biosynthesis of steroid hormones.Keywords: arsen, cadmium, endocrine disrupter, heavy metal, mercury, nickel.1. IntroductionEndocrine disrupters are defined as chemicalsubstances with either agonist or antagonistendocrine effects in human and animals [1]. Theseeffects may be achieved by interferences with thebiosynthesis or activity of several endogenoushormones. Recently, it was demonstrated thatheavy metals such as cadmium (Cd), arsen (As),mercury (Hg), nickel (Ni), lead (Pb) and zinc (Zn)may exhibit endocrine-disrupting activity inanimal experiments. Hence, a new class ofhormone-active substances was characterized, thatof endocrine-disrupting metals. 1Emerging evidence of the intimate mechanisms ofaction of these heavy metals is accumulating. *Corresponding author: Bogdan Georgescu, Tel:++40745283304, georgescu.bogdan63@yahoo.com.2. Arsen (As)Arsen is widely distributed in the naturalenvironment, both as pure element and withincomplex compounds. Arsen represents thetwentieth most prevalent metal in the earth crust,with a soil concentration of 1-2 ppm. This heavymetal is part of a large number of pesticideswidely used in intensive agriculture. The digestivetract and the skin represent the main ways ofcontamination with As, which is excreted throughurine, sweat and milk, in both animals and human.Milk excretion significantly contributes tocontamination of youth in animal husbandry.Likewise, breastfeeding may represent asignificant way of contamination with As inhuman. Concerning environmental pollution, itwas reported that As can accumulate ingroundwater and well water from natural sources.It has a well-known mutagenic role and, hence, it89

Georgescu B. et. al./Scientific Papers: Animal Science and Biotechnologies, 2011, 44 (2)is associated with increased disease risksincluding bladder, lung, skin and other cancers.Moreover, pilot data suggest diabetogenic,neurological and reproductive effects, likewise.Increased health risks may occur at levels as lowas 10-50 ppb, while biological effects have beenobserved in experimental animal and cell culturesystems at much lower levels.Arsen is the first metal to be linked with endocrinedisruption through binding to the glucocorticoidreceptor and interfering with glucocorticoidhormones activity in several biological processes.In fact, instead of activating the glucocorticoidreceptor, as for example organochlorine pesticidesdo after binding to the estrogen receptor, As ratherinhibits glucocorticoid receptor-mediated geneactivation. In a similar manner, this metalinterferes with the estrogen receptor. Experimentshave shown that As markedly suppressed estrogenreceptor-dependent gene transcription of the 17ßestradiol-induciblevitellogenin gene in chickenembryo liver [2]. In cell cultures, non-cytotoxicconcentrations of As of 2-225 ppb significantlyinhibited estradiol receptor-regulated effects inhuman breast cancer MCF-7 cells [2].3. Cadmium (Cd)The main sources of contamination with Cd arerepresented by industrial aerosols, water wastesfrom extraction mines, phosphate-based fertilizers,Cd-containing pesticides etc. It has beendocumented that in the industrial area, Cdconcentration in the air peaks to 0.03 mg Cd/ m 3 .Fodder plants are the main constituents throughwhich Cd enters the food chain. It was shown thatCd levels in fodder may attain on average 0.6ppm. Moreover, through the process ofbioaccumulation, it reaches even higher concentrationsin animal products and subproducts.Interestingly, the accumulation process is agedependent,due to the long half-time of the metals,with up to 20 years needed for Cd to becompletely metabolized in human. Significantlyhigher Cd concentrations were demonstrated inmales compared to females, in various tissues andorgans such as liver, kidney, muscle, blood, hairand wool. A particular high affinity is describedfor the kidney cortex and the testes. Milk levels ofCd are about 2-10 mcg/l, however, they increaseabout tenfold in cases of abnormal exposure. Themetal is excreted in feces and urine but also milkand bile. The free fraction is rapidly metabolized,whereas the protein-bound fraction accumulatesfor years as a metaloprotein (metallothionein).Metallothioneins are cysteine-rich, low-molecularweight proteins, that have the capacity to bindvarious heavy metals (selenium, zinc, copper,cadmium, mercury, arsen, nickel, silver) throughthe thiol group of their cysteine residue. However,it appears that the formation of metallothioneinshas no impact on heavy metals toxicity.International guidelines restrict Cd concentrationto 1 mg/l water. By Romanian guidelines, theselevels are 0,01 ppm for milk, salted fish and oils;0.05 ppm for cheese, meat and meat products;0.03 ppm for vegetables. Despite its presence invery low concentrations in water, Cd mayaccumulate in the phytoplancton and enter theaquatic trophic chain, thus bioaccumulating inmolluscs, crustaceans and fish.Several data indicate that even low-level exposureto Cd interferes with the activity of steroidhormones in both male and female reproductiveorgans. Cd disrupts steroidogenesis by interferingwith the biosynthesis of androgens, estrogens andprogesterone in vivo and in vitro experiments,thus, leading to disturbed sex differentiation andaltered gametogenesis. On the other hand, it maybind both the estrogen and androgen receptor. Inutero animal exposure to Cd induces histologicalchanges in the breast linked with breast cancer [3,4]. Recently, a Lithuanian study brought evidenceof significantly higher Cd levels in breast tissueand biological media from women with breastcancer compared to controls, thus, suggesting thatexposure to Cd could be interpreted as a potentialrisk factor for breast cancer [5].4. Mercury (Hg)Organic mercury fungicides are widely used inagriculture. In addition, mercury is used as acatalyser in industrial processes. In the form ofaerosols it is largely spread in the environment.Mercury polluted water forms sediments furthermetabolized by microorganisms into methylmercury,which is a stable but highly neurotoxicand teratogenic compound. Some organic mercuryderivates accumulate in plants by absorption fromsoil and water through the plants roots. Forexample, use of organic mercury fungicides inagriculture resulted in contamination of potatoestubercles reaching a concentration of 0.03-0.2ppm mercury. A direct correlation was foundbetween the Hg content in fodder and the90

Georgescu B. et. al./Scientific Papers: Animal Science and Biotechnologies, 2011, 44 (2)concentration of the heavy metal in tissuesoriginating from foddered animals. Animals maybe contaminated by water and fodder supply,through the skin, by inhalation of vapors andaerosols, or transplacental. Mercury compoundsare transported by blood and the lymph anddiffuse in practically all tissues but arepreferentially stored in the liver, kidney, spleen,skeleton, lymph nodes, brain and the muscles. Inpatients with chronic intoxication, mercury exertsneurotoxic, teratogenic, mutagenic and endocrinedisruptingeffects. It is metabolized and excretedthrough milk, feces, kidneys and saliva.Completely, mercury is eliminated after minimum90 days post-exposure. Previous reports indicatedthat both organic and inorganic mercurycompounds highly accumulate not only in the liverand the kidneys but also in major endocrineglands, for example, the hypothalamus, thepituitary gland, the thyroid, the testes, the ovariesand the adrenal cortex. Mercury-based compoundsdisrupt steroidogenesis, including sex hormonessynthesis, male and female fertility as well as thehypothalamic-pituitary-thyroid axis and thehypothalamic-pituitary-adrenal axis [6]. Most dataavailable indicate the fact that mercury may act asa major endocrine disrupter [7, 8].5. Nickel (Ni)The heavy metal nickel originates from naturaland artificial sources and can be found inpractically all environmental compartments: air,water, soil and living organisms. In the air it isdistributed in the form of aerosols that containvarious nickel concentrations, depending on theprimary source of metal contamination. Transportationand distribution of Ni particles betweendifferent compartments of the environment isstrongly dependent on the particles size and theclimatic conditions. Generally, the particlesoriginating from artificial resources are smallercompared to those derived from natural sources.Water contamination results by sedimentation ofmetal particles form the atmosphere, of residualindustrial waste and city waste as well as byerosion of the soil and natural rocks. In runningwater, Ni is mainly transported in precipitatedform; in lakes, it is found in ionic form,predominantly in association with organic matter.Part of Ni compounds may reach by the way ofrunning surface waters the planetary ocean. Theamount of these compounds plunged yearly intowater is estimated around 135 x 10 7 kg. Accordingto the type of the soil it contaminates, Ni showsdiverse mobility, eventually reachinggroundwater. Acid rain shows great ability tomobilize Ni from soil. The amount of metal that isabsorbed by particles from soil depends on thephysical and chemical properties of the soil,including the type of the soil, the pH, the soilhumidity and the organic matter content of thesoil. Terrestrial plants absorb the metal from soil,mainly by their roots and Ni levels above 50mg/kg dried substance are toxic for most plants.High Ni levels were reported for aquatic plants. Inunpolluted waters, Ni levels in fish may varybetween 0.02-2 mg/kg. Nevertheless, these valuesmay increase about tenfold in fish fromcontaminated water. Nickel concentrationscommonly found in the air vary between 5-35ng/m 3 , with a 0.1-0.7 μg daily humancontamination. Drinking water contains less than10 μg/l Ni, whereas its concentration in fresh foodis less than 0.5 mg/kg product. Soy, some driedvegetables, nuts and oat may contain highquantities of the metal. The daily Ni intake fromfood largely varies, in dependence of food habits,between 100-800 μg/day with a medium intake of100-300 μg/day. Pulmonary contamination isabout 2-23 μg/day. Nickel may be asorbed byrespiration, ingestion or transdermal. The mostfrequent way of contamination is the respiratoryway, whereas gastro-intestinal contamination is ofsecondary relevance. Transdermal absorption isneglijible. The absorption rate is related to thecompound solubility; as carbonyl, Ni is mostrapidly and completely absorbed in animals andhuman. It circulates bound to albumin. Thedigestive absorption rate is highly variable anddependent on diet composition. In human, theabsorption percent from water is about 27% incontrast to the absorption percent from food whichis less than 1%. All body secretions representpotential pathways for Ni excretion, includingurine, and bila, sweat, tears and milk. UnabsorbedNi is excreted through the feces. Nickel chloridecontamination by ingestion or inhalationdecreased iodine uptake by the thyroid gland.Given orally (0.5-5.0 mg/kg per day, for 2-4weeks) or by inhalation (0.05-0.5 mg/m 3 ) to rats,nickel chloride significantly decreases iodineuptake by the thyroid, the effect being morepronounced with inhaled Ni.91

Georgescu B. et. al./Scientific Papers: Animal Science and Biotechnologies, 2011, 44 (2)In mice, parenteral administration of nickelchloride and subsulfide results in intrauterinedeath of the fetus and severe intrauterine growthrestriction in neonates. Respiratory exposure to Nicarbonyl results in fetal death, intrauterine growthrestriction and congenital anomalies in rats andhamsters. Nickel carbonyl is associated todominant letal mutations in rats. In early studies, itwas reported that nickel chloride or nickel suphategiven parenterally to dogs and rabbits, induceinsulin resistance and hyperinsulinemia. Latter on,the authors observed that i.v. or i.p. injection inrabbits, rats or chicken or p.o. administration inrabbits resulted in an rapid peak of plasma glucoselevels which returned to normal after about 4hours. Histological examination of pancreatic cellsreported distruction of α cells and degranulationand vacuolization of ß Langerhans cells. In ratsfed with nickel acetate, marked lysis of exocrinepancreatic cells and inhibition of amylase release[9] was noticed. In hypophysectomized oradrenalectomized rats, the hyperglycaemic effectwas markedly reduced but not completelyabolished. Indeed, pretreatment with insulin willreduce nickel-induced hyperglycaemia. It appearsthat Ni combinations may inhibit the release ofprolactin, most probably due to hypothalamicdopamine agonistic effects. Other endocrineeffects related to Ni contamination were inhibitionof growth hormone secretion [9], althoughstimulatory effects on growth hormone releasefrom the pituitary were also demonstrated.6. Lead (Pb)Lead is largely found in nature, in rocks, underseveral combinations. The terrestrial crustcontains on average 1-2 ppm lead. However, dueto important pollution, lead is largely spreaded inthe environment in air, water, soil, flora and fauna.Hayfields and animal fodder cultured in thevicinity of industrial buildings using Pb in thefabrication process are at high risk to becontaminated with Pb resulted from vaporprecipitation or the sedimentation of Pb particles.Aerosols may be dispersed on a distance of 2-3km (0,1 mg/m 3 ) and even 10 km (0.01 mg/m 3 )around the contamination source. Spontaneousvegetation and fodder fields around intensivelytrafficked autorays contain 50-60-fold higherconcentrations of Pb.Contamination of the human or animal with Pb ismade by ingestion, respiration or transdermal. Ithas been shown that digestive absorption is highlyspecies specific, as from the intake about 90% isabsorbed in bovine, 1-2% in ovine and 0.8-1% inrabbits. In human, the absorption percent is about50%. In bovines, ingestion of contaminated fodderleads to a 20-fold increase of Pb levels in the liverand a 3-4-fold increase of Pb levels in milk andmeat. If fodder contains 1-6 mg/kg Pb, then themilk content persists at 0.05-0.15 mg/kg for about120 days.Lead toxicity consists of inhibition of cellularenzymes, binding of sulfhydril groups ordissociation of biological active metal ions frommetaloenzymes. Additionally, it affects themembrane stability of erythrocytes, inducesfunctional disturbances in peripheral nerves anddisturbances in the development of the skeleton.Lead is a powerful disruptor of adrenal andovarian steroidogenesis, inhibiting synthesis andactivity of progesterone, 17-hydroxyprogesterone,17,20-dihydroxyprogesterone, deoxycorticosterone,corticosterone and 21-deoxycortisol in adose-dependent manner. Interestingly, its effectson 17β-estradiol, testosterone and cortisol arebiphasic, with stimulatory effects after low-levelsexposure and inhibitory effects after high-leveexposure [10]. Lead exposure results in disturbedfertility in females, as revealed by an in vitrostudy that examined the consequences of Pbexposure on cytochrome P-450 aromatase (P-450ARO) and ß-estradiol receptors, two key proteinsin the function of the pituitary-ovarian axis. It wasshown that the activity of both P-450 ARO andER-β in the granulosa cells of the ovarian follicleswas strongly inhibited in women exposed to Pb[11]. In summary, Pb contamination may alterendocrine-regulated processes such as longevity,development, sexual receptivity, fertility andlocomotion. For example, in Drosophila, Pbinterferes with the expression of about 122 genesinvolved in locomotion. In the larvar state, thesegenes regulate the level of intracellular calciumassociated with neuronal activity at theneuromuscular jonction sinapses [12].7. Zinc (Zn)Zinc is found in several combinations both in theearth crust and vegetal and animal cells. In fact,Zn is one of the most important natural,biologically active constituents, indispensable tolife itself. In all tissues, Zn levels are 2-fold highercompared to iron levels. It was revealed that the92

Georgescu B. et. al./Scientific Papers: Animal Science and Biotechnologies, 2011, 44 (2)Zn atom from the Zn fingers of the estrogenreceptor can be replaced by several heavy metalmolecules such as copper, cobalt, Ni and Cd. Byreplacing the Zn atom with Ni or copper, bindingof the estrogen receptor to the DNA-hormoneresponsiveelements in the cell nucleus isprevented.Zinc contamination results from industrial smoke,with the most relevant compounds represented byZn chloride, Zn chromate, Zn phosphur, Znsulphate and Zn oxide. Contamination is alsopossible by use of zincate containers to heat milkand foods. Moreover, Zn is an important substanceused in the fabrication process of severalpesticides. It may enter the body either by enteralor respiratory way. It is easily absorbed in alltissues and rapidly diffuses. Zinc is excreted byfeces, the bila etc.; it is co-secreted with insulin bythe pancreas. In the presence of As, Zn toxiceffects are 3-4-fold increased.The maximum admitted limit of Zn in our countryis of 25 ppm in vegetables, 40 ppm in dry beans,50 ppm in meat, meat subproducts and conserves,30 ppm in eggs, 5 ppm in milk, beverage, oil andtomato juice. Besides direct toxic effects, Zn saltsact as endocrine disrupters. In a recent study,copper, Cd, Pb, Hg and Zn at 95.4 pM-1 mM,alone or in combination with the natural estrogen,17ß-estradiol, have been tested using the yeastestrogen screen, an estrogen receptor-dependenttranscriptional expression assay. No direct transactivationof the estrogen-responsive elementcould be measured with any of the concentrationof the metals tested. However, Zn, Cd and copperwere able to potentiate the estradiol-inducedresponse in a dose-dependent manner, thusindicating that Zn can act as a potential endocrinedisrupter by modulating the estrogenic activity ofendogenous hormones (xenoestrogen) [13].ConclusionsBesides several man-made chemicals, most ofthem major components of widely used pesticides,recent studies strongly suggest that some heavymetals may exert endocrine-disrupting activities inanimals and human. Of these metals, Zn, Pb andHg and As interfere with sex hormones andadrenal cortex hormones steroidogenesis to alterreproduction and sex differentiation, Cd isinvolved as a risk factor for breast cancer and Niappears to induce pancreatic cell lesions, possiblyincreasing the risk of diabetes mellitus.Nevertheless, further studies are needed toestablish a dose-effect relationship. Futurestrategies in agriculture, animal husbandry andenvironmental health should aim limitation of theendocrine disruption phenomenon induced byheavy metals.References1.Georgescu, C., Georgescu, B., Duncea, I.,Xenoestrogenii: implicatii clinice si metode de evaluarecalitativa si cantitativa, Clujul Medical, 2006, vol.LXXIX, 1, 7-122.Davey, J. C., Bodwell, J. E., Gosse, J. A., Hamilton,J. W., Arsenic as an endocrine disruptor: Effects ofArsenic on estrogen receptor–mediated gene expressionin vivo and in cell culture, Toxicol Sci, 2007, 98, 75-863.Masufumi, T., Schin’Ichi, Y., New Aspects ofCadmium as Endocrine Disruptor, Environ Sci, 2006,13, 107-1164. Heilier, J. F., Donnez, J., Verougstraete, V., Donnez,O., et al., Cadmium, lead and endometriosis, Int ArchOccup Environ Health, 2006, 80, 149-1535. Strumylaite, L., Bogusevicius, A., Abdrachmanovas,O., Baranauskiene, D., Kregzdyte, R., Pranys, D.,Poskiene, L., Cadmium concentration in biologicalmedia of breast cancer patients, Breast Cancer ResTreat, 2011, 125, 511-5176.Tan, S., Mahaffey, K., Evidence for mercurty as anendocrine disrupter: an overview of the literature, 2003,http://www.scribd.com/doc/17905747. Darbre, P. D., Metalloestrogens: an emerging classof inorganic xenoestrogens with potential to add to theoestrogenic burden of the human breast, J ApplToxicol, 2006, 26, pp. 149-1538. Frynn-Aikins, K., Gallagher, E., Ruessler, S., et al.,An evaluation of methyl mercury as and endocrinedisrupter in Largemeouth Bass, SESC Webmaster,2011, http://www. fl. biology. usgs. gov.9.Dormer, R., L., Kerbey, A. L., McPherson, M.,Manley, S. et al., The effect of nickel on secretorysystems. Studies on the release of amylase, insulin andgrowth hormone, Biochem. J, 1973, 140, 135-14210.Chaube, R., Mishra, S., Rahil Singh, K., In vitroeffects of lead nitrate on steroid profiles in the postvitellogenicovary of the catfish Heteropneustesfossilis, Toxicol in vitro, 2010, 24, 1899-190411.Taupeau, C., Poupon, J., Treton, D., et al., Lead(Pb2+) reduces mRNA and protein levels ofcytochrome P-450 aromatase and estrogen receptorbetain human ovarian granulosa cells, Biol Reprod,2003, 68, 1982-198812. Hirsch, H. V., Possidente, D., Possidente, B., Pb 2+ :an endocrine disruptor in Drosophila?, Behaviour andPhysiology, 2010, 99, 254-25913. Denier, X., Hill, E. M., et al., Estrogenic activity ofcadmium, copper and zinc in the yeast estrogen screen,Toxicol in vitro, 2009, 23, 569-57393