arsenic is a naturally occurring element that exists ubiquitously in the environment in both organic and inorganic forms combined with other elements such as oxygen , chlorine , and sulfur , as well as carbon and hydrogen ( cullen and reimer , 1989 ; who , 2000 ; iarc , 2004 ) . it is classified chemically as a metalloid , having properties intermediate between a metal and a nonmetal ; however , it is frequently referred to as a metal ( iarc , 1980 ) .
elemental arsenic is the 20 most common in the earth s crust , and is emitted to the environment as a result of volcanic and industrial activities .
mining , smelting of non - ferrous metals and burning of fossil fuels are the major anthropogenic sources of arsenic contamination of air , water , and soil .
arsenic and arsenic compounds are used in pharmaceuticals , wood preservatives , agricultural chemicals ( e.g. pesticides , insecticides , and herbicides ) , and applications in the mining , metallurgical , glass - making , and semiconductor industries ( mandal , 2002 ) .
especially , the historical usage of arsenic - containing pesticides and wood preservatives led to contamination of the agricultural land and groundwater ( who , 2001 ) .
human exposure to arsenic has become a major global health problem because the worldwide contaminations and adverse health effects . in most populations ,
the main source of arsenic exposure is the drinking water ( nrc , 1999 , 2001 ; smith et al . , 2002 ;
the current maximum contamination levels of arsenic were lowered from 50 g / l to 10 g / l by usepa and who ( iarc , 2004 ) .
arsenic in drinking water is mainly inorganic arsenic and more harmful than in food such as grains and vegetables ( akter et al .
moreover , inorganic arsenic is more toxic than organic arsenic and can exist in two major oxidation states : a trivalent form , arsenite ( as ) , and a pentavalent form , arsenate ( as ) .
biologically , arsenite is 2~10-fold more active and toxic than arsenate ( kosentt , 1994 ) . of the inorganic arsenic compounds , arsenic trioxide ( as2o3 ) , sodium arsenite ( naaso2 ) and arsenic trichloride ( ascl3 ) are the most common trivalent compounds , and arsenic pentoxide ( as2o5 ) , arsenic acid ( ash3o4 ) and arsenates [ e.g. sodium arsenate ( na2haso4 ) , lead arsenate ( pbhaso4 ) and calcium arsenate ( as2ca3o8 ) ] are most common pentavalent compounds .
the toxic effects of arsenic that are of most concern to humans are those that occur from chronic low - level exposure .
epidemiological studies have indicated that chronic exposure to arsenic is associated with increased risks of various cancers , including those of the skin , lung , bladder , kidney , and liver , as well as numerous other non - cancer illnesses including vascular and cardiovascular disease , diabetes , reproductive and developmental problems , and neurologic and cognitive problems ( abernathy et al .
conversely , arsenic has been considered as an effective chemotherapeutic agent in the treatment of certain human cancers ( douer & tallman , 2005 ) . among arsenic compounds , arsenic trioxide ( as2o3 )
has successfully been employed in the treatment of acute promyelocytic leukemia ( apl ) ( soignet et al . , 1998 ) .
it has also been shown that as2o3 efficiently induces apoptosis in malignant apl cells in vitro ( hu et al . , 1999 ) .
these observations has been confirmed that as2o3 can induce apoptosis in other leukemia cells and solid tumor cells , including gastric cancer cells , neuroblastoma cells , and prostate and ovarian carcinomas ( zhang el al .
, 1999 ; uslu et al . , 2000 ) . however , arsenic is commonly regarded as a potent carcinogen that can induce the formation of various types of solid tumors ( smith et al . , 1992 ; liu & waalkes , 2008 ) .
our previously study demonstrated that as2o3 effectively provokes cytotoxicity and apoptotic cell death in mouse testicular sertoli cells in vitro ( kim et al . , 2011 ) .
these investigations indicates that the male reproductive toxicity of arsenic may be directly associated with damage of testicular cells .
in addition , various experimental models have been developed to understand how arsenic exposure causes these diverse disease outcomes .
however , the reproductive and developmental toxicity of arsenic is poorly understood , and specific relationships between experimental and human exposures are not established .
several epidemiologic studies have reported that arsenic exposure in utero increased spontaneous abortion and stillbirth and decreased birth weight ( ihrig et al . , 1998 ; ahmad et al . , 2001
the reproductive and developmental toxicity of arsenic have primarily been documented through murine studies , suggesting arsenic as a risk to the developing fetus . in animals ,
some experimental studies have reported that arsenic intoxication is associated with spermatotoxicity ( waalkes et al . , 2003 ; pant et al . , 2004 ) ,
inhibition of testicular steroidogenesis and reduction of the weight of the testes and accessory sex organs ( sarker et al . , 2003 ) .
arsenic exposure also has been associated with inhibition of ovarian steroidogenesis and gonadotrophins secretion ( chattopadhyay et al . , 1999 ) ,
elevation of adrenocortical steroidogenesis and plasma corticosterone level ( ghosh et al . , 1999 ) , as well as with severe metabolic disorders such as diabetes in humans ( tseng et al . , 2002 ) .
however , the actual molecular events resulting in reproductive and developmental toxicity from exposure of arsenic remain unclear .
the purpose of this paper is to review and summarize the major scientific developments on the topic of reproductive and developmental toxicity associated with arsenic exposure , integrating evidence from epidemiological and experimental studies .
arsenic is a major global health concern throughout the world widely based on its carcinogenic potential after occupational or environmental exposure ( iarc , 1987 ; nrc , 1999 ) .
arsenic is commonly present in air , water , and soil and could find their routes into the human body through inhalation , ingestion and skin absorption .
the primary route of arsenic exposure for the most population is via the ingestion of contaminated food or water .
inhalation or skin absorption of arsenic from contaminated sources is a minor exposure route for the general population ( atsdr 2007 ) .
most ingested and inhaled arsenic is well absorbed through the gastrointestinal tract and lung into the blood stream .
arsenic enters into the human body through these routes , and is distributed in a large number of organs including the lung , liver , spleen , kidney , intestine , skin , and vascular and lymphatic systems , as well as reproductive and nervous systems ( hunter et al . , 1942 ; atsdr , 2007 ) .
after absorption through lung and the gastrointestinal tract , 95 to 99% of the arsenic is located in erythrocytes , bound to the globin of hemoglobin and is then transported to the other parts of the body . about 70% of the arsenic
most arsenic absorbed into the body is converted by the liver to less toxic methylated form that is efficiently excreted in the urine ( who , 1981 ; atsdr , 1989 ) .
arsenic is a multi - site carcinogen in humans , causing tumors in a variety of tissues including lung , skin , liver , kidney , and bladder , as well as uterus and prostate ( nrc , 1999 ; waalkes et al . , 2000 , 2003 ) .
acute arsenic exposure may cause gastrointestinal tract disorders ( goebl et al . , 1990 ) , whereas chronic exposure may exert degenerative , inflammatory , and neoplastic changes of the respiratory , hematopoietic , cardiovascular , and nervous systems ( naiger & osweiler , 1989 ) .
although epidemiological data have firmly established arsenic to be a human carcinogen , animal studies are less well defined .
most laboratory animals appear to be substantially less susceptible to arsenic than humans ( naqvi et al .
it has been reported that chronic oral exposure to inorganic arsenic ( 0.05 - 0.1 mg / kg / day ) causes neurological and hematological toxicity in humans but not in monkeys , dogs , and rats exposed to arsenite or arsenate at doses of 0.72 to 2.8 mg / kg / day ( byron et al . , 1967 ) .
exposure of humans to inorganic arsenic results in the urinary excretion of inorganic arsenic and two major methylated metabolites , methyl arsenic and dimethyl arsenic ( crecelius , 1977 ; smith et al . , 1977 ; yamauchi & yamamura , 1979 ) . in many organisms , including humans , inorganic arsenic is reduced in the blood from pentavalent arsenate ( as ) to trivalent arsenite ( as ) , then taken up by hepatocytes and oxidatively methylated to monomethyl arsenic acid ( mma ) and then to dimethyl arsenic acid ( dma ) ( fig .
1 ) ( aposhian , 1997 ; healy et al . , 1998 ; pott et al . , 2001 ; thomas et al . ,
the reduction is facilitated by reductases and the reduced form of glutathione ( gsh ) along with possibly other thiols as electron donors .
oxidative methylation is carried out by methyltransferases , and s - adenosylme - thionine ( sam ) serves as the main methyl donor ( drobna et al . , 2005 ) .
arsenate ( as ) is reduced by as reductase with glutathione ( gsh ) to yield arsenite ( as ) and glutathione disulfide ( gssg ) .
as is converted to pentavalent monomethylarsonic acid ( mma ) , a reaction catalyzed by arsenic-3-methyltransferase ( as3mt ) , with sadenosylmethione ( sam ) serving as the methyl donor ; in the process , sam is hydrolyzed to sadenosyl homocysteine ( sah ) . after the reduction of mma to mma , a second methylation step result in the synthesis of dimethylarsinic acid ( dma ) , and then it is reduced to dma .
inorganic arsenic in water is abundant in the form of arsenate ; it is negatively charged at physiological ph and slowly taken up by cells ( cohen et al . , 2006 ) .
arsenate is rapidly converted to arsenite in vivo ( vahter and envall , 1983 ; vahter & marafente , 1985 ) which is taken up by cells much more quickly than arsenate ( tseng , 2009 ) .
methylation of arsenicals facilitates their excretion from the cell and therefore was long considered a detoxification process , but recent evidence indicates that monomethylated and dimethylated arsenicals have many cyto- and genotoxic effects including increased oxidative stress ( kligerman & tennant , 2007 ) , chromosomal aberrations , and oxidative dna damage ( schwerdtle et al .
methylated trivalent metabolites are highly reactive and are more potent inhibitors of gsh reductase and thioredoxin reductase compared with arsenite or pentavalent metabolites ( styblo et al . , 1997 ; lin et al . , 1999 ) .
thioredoxin reductases catalyzes the nadph - dependent reduction of the disulfide bond in oxidized thioredoxin , which is a critical antioxidant enzyme controlling the cellular redox balance ( rossman , 2003 ) .
in experimental animals , prenatal exposure to inorganic arsenic has resulted in arsenic accumulation in fetuses , including fetal brain ( rodriguez et al . , 2002 ;
miyazaki et al . , 2005 ) , and detection of monomethyl and dimethyl arsenicals in fetuses ( hall et al . , 2007 ) . in humans , dimethylarsinic acid ( dma )
accounts for nearly 90% of all detected arsenic in the blood plasma of both the newborns and their mothers ( concha et al . , 1998 ) .
this form of arsenic accounts for 60 to 70% of the total detected arsenic in urine of the general population , whereas urine from pregnant women contained more than 90% arsenic as dma .
this suggested that arsenic methylation may be increased during pregnancy and that dma is the major form of arsenic transferred to the fetus .
the elevated evidence now suggests that arsenic exposure in utero also poses health risks to the developing fetus ( vahter , 2009 ) .
various epidemiological studies have found significant associations between prenatal arsenic exposure and adverse infant outcomes , such as infant mortality , low birth weight , and birth defects ( ahmad et al . , 2001 ; milton et al . , 2005
these health problems were most evident in individuals exposed to high - level arsenic ( iarc , 2004 ) . during pregnancy
, high - level exposure to arsenic in drinking water causes pregnancy complications , including fetal loss and premature delivery ( chakraborti et al . , 2003 ) , and low - level exposure to arsenic affects uterus and placental growth results in progeny birth weight ( hopenhayn et al . , 2003 ; rahman et al . , 2009 ) .
conclusions from these epidemiological studies are further supported by results from animal models ( ferm and carpenter , 1968 ; hood and bishop , 1972 ) .
several cross - sectional studies have been reported that woman exposure of chronic arsenic in drinking water increases adverse pregnancy outcomes including premature delivery , spontaneous abortion , stillbirth and neonatal death in bangladesh ( milton et al . , 2005 ) , northeastern taiwan ( yang et al . , 2003 ) , and chile ( hopenhayn - rich et al . , 2000 ) . despite the strong association between arsenic exposure and a range of child health concerns ,
various animal studies have shown that arsenic can produce developmental toxicity , including malformation , growth retardation and fetal death ( tabocova et al . , 1996 ;
furthermore , developmental toxicity critically depends on dose , route and gestation day following the arsenic exposure ( golub et al . , 1998 ) .
fetal malformations were only reported when pregnant rats and mice were intravenous ( i.v . ) and intraperitoneal ( i.p . ) injected with inorganic arsenic at early gestation ( stump et al . , 1999 ; desesso , 2001 ) .
maternal inhalation or oral ingestion of inorganic arsenic affected fetal development and behavior , but did not cause malformations ( holson et al . , 1999 ; stump et al .
it is then transported to the liver and may undergo first - pass metabolism prior to being delivered to the uterus .
intraperitoneal injections , on the other hand , allow arsenic to be taken up by blood vessels directly and may bypass first - pass metabolism ( stump et al . , 1999 ;
desesso , 2001 ) . in pregnant rats , the maternal blood and embryonic arsenic concentration from i.p .
injected mother was significantly higher than those from the orally exposed mother ( hood et al .
inhalation is the least effective means of increasing maternal or embryonic arsenic concentrations , compared to i.p . and i.v .
intraperitoneal injection of 45 mg / kg sodium arsenate in swiss mice during gestation day ( gd ) 8 induced fetal renal and skeletal malformations without affecting maternal weights ( fascineli et al . , 2002 ) .
however , oral exposure to arsenic did not cause neural tube defects , even at maternally toxic dose levels .
oral gavage with high - dose arsenic trioxide ( 10 mg / kg / day ) in female crl : cd(sd)br rats during gd 14 to 19 decreased fetal weights , but did not induce changes in mating index , fertility index , implantation , or fetal malformation ( holson et al .
nevertheless , dimethylarsinic acid ( dma ) , an arsenic metabolite , given to pregnant mice and rats by oral gavage caused developmental toxicity ( rogers et al .
cd-1 mice were orally gavaged with dma at 200~600 mg / kg / day during gd 7 to 16 .
these mice showed lower maternal weight gain and fetal weight at 200 mg / kg / day , and a higher incidence of cleft palate at 400 mg / kg / day . similarly , during gd 7 to 16 , cd rats were orally gavaged with dma at 7.5 to 60
mg / kg / day ( rogers et al . , 1981 ) . maternal weight gain and fetal weight were decreased at > 40 mg / kg / day .
fetal mortality was increased at 50 or 60 mg / kg / day , but no fetal gross malformations were seen in these rats . in a later study , dma was given to pregnant sprague - dawley ( sd ) rats at 40 mg / kg / day by oral gavage during gd 6 to 15 .
this treatment did not induce maternal weight reduction or maternal lethality , but decreased fetal weight ( chernoff et al . ,
furthermore , postnatal developmental changes were observed when arsenic was given in the drinking water to pregnant and lactating rats and continually to the newborns ( rodriguez et al .
ingestion of drinking water with sodium arsenite ( 36.7 mg / l ) in sd rats during gd 15 to postnatal day ( pnd ) 16 induced developmental indices including delayed pinna detachement on pnd 12 and early eye opening on pnd 14 in more litters , compared to the untreated controls ( rodriguez et al .
, pinna detachment occurs between pnd 3 and 4 , and eye opening is observed on pnd 16 .
these data indicate that arsenic induces an asynchrony of the maturational processes during postnatal development .
overall , it is assumed that prenatal exposure to arsenic may cause adverse pregnancy outcomes and children s health problem , however , the exact mechanism underlying arsenic - induced developmental toxicity and a direct relationship with reproductive toxicity remains to be elucidated .
in males , arsenic may induce gonad dysfunction through declined testosterone synthesis , apoptosis and necrosis ( davila - esqueda et al .
however , the male reproductive dysfunction of arsenic exposure on human health is not well established .
recently , few epidemiologic studies have shown that arsenic exposure significantly causes infertility and low sperm quality , as well as erectile dysfunction in men ( nie et al . , 2006 ; hsieh et al . , 2008 ; meeker et al . , 2010 ) .
several experimental studies have been demonstrated that significant accumulation of arsenic in testes and accessory sex organs , such as epididymis , seminal vesicle , and prostate gland ( danielsson et al . , 1984 ; pant et al . , 2001 ) .
male mice exposed to sodium arsenite at 20 or 40 mg / l for 5 weeks in drinking water showed decreased epididymal sperm counts and testicular weight ( chang et al . , 2007 ) .
similarly , in male rats , daily exposure to sodium arsenite at 5 mg / l for 4 weeks in drinking water also resulted in decreased testicular weights , accessory sex organ weights , and epididymal sperm counts , as well as extensive degeneration of a wide variety of germ cells at stage vii of the spematogenic cycle ( jana et al . , 2006 ) .
furthermore , arsenic - exposed mice exhibited dose - dependent gradual reductions in seminiferous tubular diameter and various gametogenic cell populations , such as resting spermatocyte , pachytene spermatocyte , and elongated spermatid ( sanghamitra et al . ,
in addition , arsenic exposure also reduced plasma levels of testosterone and gonadotropin in male mice and rats ( sarkar et al .. 2003 ; chinoy et al . , 2004 ;
injections at 4 , 5 , or 6 mg / kg / day for 26 days . at 5 and 6 mg / kg / day , relative testicular weight , accessory sex organ weights and epididymal sperm counts were decreased .
moreover , sodium arsenite - treated rats were accompanied by decreases of plasma concentrations of luteinizing hormone ( lh ) , folliclestimulating hormone ( fsh ) , and testosterone after arsenic exposure .
these suggested that arsenic may target on the brain and/or pituitary as well as directly on the germ cells .
they might exert a direct inhibitory action on the testis , or they might affect the hypothalamic - pituitary axis causing changes in plasma concentrations of lh and fsh .
reduction of plasma lh might impair leydig cell function and result in a consequent reduction in testosterone production .
testosterone is a prerequisite for normal spermatogenesis , while fsh is also required for normal testicular function and spermatogenesis ( fig .
2 ) ( jana et al . , 2006 ) . on the other hand , a possible mechanism for decreased sperm motility might be associated with a direct binding of arsenic to sperm ( uckun et al . , 2002 ) .
nuclear chromatins of sperm have large amounts of thiol - rich protamines , and the sperm flagellum is rich in thiols .
another possible cause of the reduction in serum lh , fsh , and testosterone levels could be high serum corticosterone levels .
1988 ; hardy et al . 2005 ) , and has been reported in sodium arsenite - treated rats ( biswas et al . , 1994 ) .
luteinizing hormone ( lh ) acts directly upon the leydig cells to stimulate testosterone ( t ) production .
t and folliclestimulating hormone ( fsh ) act upon the sertoli cells within seminiferous epithelium to support development and maturation of germ cells .
in addition to spermatogenesis , in mice , cholesterol metabolism and testosterone production were affected by arsenic exposure ( chinoy et al . , 2004 ) .
male mice orally exposed to arsenic trioxide ( as2o3 ) at 0.5 mg / kg for 30 days showed increased cholesterol levels and decreased expression of 3-hydroxysteroid dehydrogenase ( hsd ) and 17-hsd , two important regulatory enzyme of steroidogenesis . in the testis ,
3-hsd mainly converts pregnenolone to progesterone , and 17-hsd converts androstenedione into testosterone . in the seminiferous tubules in the testis , cholesterol in the membrane of developing cells
these data suggest that low plasma level of testosterone after arsenic exposure may be due to low enzymatic conversion by the decreases of 3-hsd and 17-hsd , rather than a lack of the synthetic precursor cholesterol .
arsenic exposure may induce cell death or apoptosis in the testicular germ cells or somatic sertoli cells in vitro ( celino et al . , 2009 ; kim et al . , 2011 ) .
in japanese eel , low - dose ( 0.1~1 m ) of arsenic inhibits spermatogenesis via steroidogenesis suppression , while high - dose ( 100 m ) of arsenic induces oxidative stress - mediated germ cell apoptosis ( celino et al . , 2009 ) .
we previously showed that arsenic trioxide ( as2o3 ) at 10 m efficiently induces reactive oxygen species ( ros)-related cytotoxicity and apoptotic cell death in mouse tm4 sertoli cells ( kim et al .
the testicular sertoli cells interact directly with developing germ cells throughout the process of spermatogenesis .
these cells have an indispensable able in the development and movement of germ cells ( russell , 1993 ; griswold , 1998 ) .
thus , spermatogenic dysfunction of arsenic toxicity may be associated with direct damages of testicular component cells ( fig .
3 ) . arsenic may inhibit spermatogenesis and sperm maturation following certain key molecular mechanisms including reductions of testosterone and gonadotrophins lh and fsh , disruptions of steroidogenic enzymes 3-hsd and 17-hsd , negative regulations of lh and fsh by increased corticosterone , decreases of sperm motility and viability following the direct binding of arsenic ( as ) to sperm , and direct damages or apoptosis of testicular component germ cells or sertoli cells .
lh , luteinizing hormone ; fsh , follicle - stimulating hormones ; 3-hsd , 3-hydroxysteroid dehydrogenase ; 17-hsd , 3-hydroxysteroid dehydrogenase .
these toxic effects are influenced by the forms , sources , and routes , as well as doses and periods of arsenic exposure . in vivo studies demonstrated that inorganic arsenic , such as sodium arsenite , arsenic trioxide , and dimethyl arsinic acid , an arsenic metabolite , causes reproductive and developmental toxicity .
injection routes causes fetal malformation ; in contrast , oral and inhalational exposure to inorganic arsenic affects fetal development , including growth retardation and fetal death . in drinking water , oral exposure to inorganic arsenic causes dysfunctions of spermatogenesis , reductions of testosterone and gonadotrophins , and disruptions of steroidogenesis .
however , the reproductive and developmental toxicity of arsenic is poorly understood , and the molecular mechanism of arsenic - induced reproductive toxicity remains unclear .
therefore , we further investigated some of the possible mechanisms that are affected by arsenic causing reproductive toxicity . the crucial mechanisms of arsenic - induced reproductive toxicity may be associated with hormonal regulation and function , binding to sperm , and regulation of steroidogenesis , as well as direct effects of testicular component cells .
 arsenic is a toxic metalloid that exists ubiquitously in the environment , and affects global health problems due to its carcinogenicity . in most populations , 
 the main source of arsenic exposure is the drinking water . in drinking water , 
 chronic exposure to arsenic is associated with increased risks of various cancers including those of skin , lung , bladder , and liver , as well as numerous other non - cancer diseases including gastrointestinal and cardiovascular diseases , diabetes , and neurologic and cognitive problems . 
 recent emerging evidences suggest that arsenic exposure affects the reproductive and developmental toxicity . 
 prenatal exposure to inorganic arsenic causes adverse pregnancy outcomes and children s health problems . 
 some epidemiological studies have reported that arsenic exposure induces premature delivery , spontaneous abortion , and stillbirth . in animal studies , 
 inorganic arsenic also causes fetal malformation , growth retardation , and fetal death . 
 these toxic effects depend on dose , route and gestation periods of arsenic exposure . in males , 
 inorganic arsenic causes reproductive dysfunctions including reductions of the testis weights , accessory sex organs weights , and epididymal sperm counts . 
 in addition , inorganic arsenic exposure also induces alterations of spermatogenesis , reductions of testosterone and gonadotrophins , and disruptions of steroidogenesis . 
 however , the reproductive and developmental problems following arsenic exposure are poorly understood , and the molecular mechanism of arsenic - induced reproductive toxicity remains unclear . 
 thus , we further investigated several possible mechanisms underlying arsenic - induced reproductive toxicity . 
INTRODUCTION
ARSENIC EXPOSURE TO HUMAN
BIOCHEMICAL METABOLISM OF ARSENICALS
DEVELOPMENTAL TOXICITY OF ARSENIC
REPRODUCTIVE TOXICITY OF ARSENIC
CONCLUSION