although many contaminant - induced estrogenic actions described to date appear to be caused by receptor - mediated interactions , other responses are less clear ( mclachlan 2001 ) . several studies have provided support for the hypothesis that estrogenic actions could be due to increased aromatase activity . for example , the feminizing effect of atrazine on some species has been hypothesized to be caused by its ability to alter aromatase - induced conversion of androgens to estrogens ( crain et al . 1997 ; hayes et al . 2003 ) . several studies with mammalian or fish cell lines suggest that such a mechanism is possible ( sanderson et al . future studies must consider a mechanism by which contaminants alter steroid action not by receptor binding but rather via alterations in the synthesis of these hormones by modification in the enzymatic pathways , either by direct changes in enzyme synthesis or in enzyme activity levels ( i.e. , alterations in available cofactors ) [ for discussion see harvey and johnson ( 2002 ) ; sanderson and van den berg ( 2003 ) ] . before the presentation of the edc hypothesis , a metabolite of ddt , o , p-ddd [ 1,1-dichloro-2,2-bis(4-chlorophenyl)ethane ] , was known to disrupt adrenal steroidogenesis in some mammalian species , including humans ( benecke et al . in fact , this compound has been marketed as a drug to treat adrenal cancer and cushing s disease because of its suppressive action on adrenal steroidogenesis ( benecke et al . even with this knowledge , few investigators have examined wildlife species or humans for alterations in adrenal function in areas where ddt use is still extensive , as in many tropical regions with a high risk of malaria . not all species will respond to this compound with altered adrenal steroidogenesis ( breuner et al . furthermore , because of the action of o , p-ddd in humans , that it is a major metabolite of ddt , and that ddt is still used extensively in tropical regions , more studies are needed that examine human populations for adrenal - based end points , especially pregnant women . at least one recent study has suggested that coincident with the use of ddt and elevated dde plasma levels , a significant increase in human preterm parturition occurred ( longnecker et al . adrenal axis is central to the control of gestation length and the timing of birth in humans , and probably in eutherian mammals in general ( chrousos et al . alterations in adrenal steroidogenesis and in glucocorticoid feedback to the hypothalamo  pituitary axis are associated with disrupted gestation and birth , which suggests that chemicals capable of acting on this system are potentially important in human reproductive health . progesterone and related progestins are another group of steroid hormones with a central role in gestation maintenance in every vertebrate species studied to date ( jones and baxter 1991 ) . recent studies identified a receptor for these steroids in the cell membrane ( zhu et al . 2003a , 2003b ) , in addition to the traditional perinuclear receptor , which acts as a transcription factor ( carson - jurica et al . some species can have more than one form of the perinuclear receptor with unique tissue distributions ( custodia - lora and callard 2002 ) . very few studies have examined the action of environmental contaminants on progestin - based phenomena . various contaminants can bind the nuclear progesterone receptor ( apr ) from the american alligator ( vonier et al . although many of the chemicals tested , including most of the ddt metabolites , did not reduce [ h]r5020 binding to apr , 2,2-bis(p - chlorophenyl)ethanol ( ddoh , a ddt metabolite ) , endosulfan , alachlor , dicofol , atrazine , and kepone did ( vonier et al . pickford and morris ( 1999 ) have demonstrated that methoxychlor disrupts progesterone - induced oocyte maturation in the frog xenopus leavis . progesterone plays a major role in gestation maintenance ; thus , a disruption in progesterone signaling could also affect the frequency of spontaneous miscarriage , as described above . in short , we need to broaden our concepts of the mechanisms and end points used to assess steroid mimicry by environmental contaminants . dramatic declines in the populations of numerous bird species occurred during the decades following the wide - scale use of ddt for insect control in the united states and other countries around the world . upon examination , dde , the major metabolite of ddt was associated with altering the physiological process of eggshell formation that subsequently led to eggshell thinning and population declines of numerous avian species , particularly raptors and shorebirds [ see review by lundholm ( 1997 ) ] . these studies indicate that the supply of calcium to the eggshell gland is not impeded by dde , but rather this organochlorine contaminant disrupts calcium transport within the eggshell gland ( lundholm 1997 ) . in addition , prostaglandins ( pgs ) have been implicated in eggshell thinning because dde disrupts pg synthesis , which reduces bicarbonate transport in the duck shell gland lumen , thereby reducing calcium transport . given the important roles of various pgs during ovulation , pregnancy , and parturition in a wide array of vertebrate species ( guillette et al . 1991 ; jenkin and young 2004 ) , it is surprising that few investigators have examined the disruption of this important class of hormones . pg synthesis , like steroidogenesis , is controlled by a pathway of enzymes ( helliwell et al . thus , with a wide range of chemicals capable of altering the  estrogenic milieu ,  it could be hypothesized that pg synthesis is altered in the reproductive system of any vertebrate , as described previously for the avian reproductive tract . interestingly , recent studies have suggested that pgs could work through perinuclear receptors as well as through the established membrane if this is the case , disruption at the level of transcription could occur via several mechanisms leading to different outcomes . rather , they play essential roles in the immune response , including inflammation and arthritis , cardiovascular regulation encompassing hypertension , and respiration including asthma . . moreover , pgs are evolutionarily ancient and are important in reproduction and development in invertebrates as well ( stanley and howard 1998 ) . in brief , a major research effort is needed to examine the possible linkages between the wide - ranging alterations seen in organisms living in contaminated environments and alterations in prostaglandin synthesis . many vertebrates exhibit species - specific , sexually dimorphic patterns of hepatic enzyme activity that appear to be regulated by sex steroids and/or growth hormone ( gustafsson 1994 ) . in humans and rodents , 50% or more of the drugs and pesticides currently used induce the expression of the hepatic enzyme cytochrome p450 ( cyp)3a ( gibson et al . 2002 ) . numerous steroids , including testosterone , 17-estradiol , progesterone , and androstenedione , are metabolized by cyp3a ( gibson et al . 2002 ) . it is therefore conceivable that wildlife or humans exposed to contaminated environments would exhibit elevated cyp3a activity that leads to increased clearance of such steroids as testosterone and 17-estradiol from the plasma , as reported in chickens ( chen et al . blumberg et al . ( 1998 ) first proposed the  steroid sensor hypothesis  by suggesting that the induction of hepatic enzymes responsible for the metabolism of endogenous and exogenous compounds was regulated by a nuclear receptor with broad specificity . a group of nuclear receptors ( e.g. , steroid xenobiotic receptor , sxr ) was discovered , and this hypothesis has been further supported by studies that have observed a positive relationship between sxr expression and cyp3a expression ( blumberg and evans 1998 ) . transcription of cyp3a , as well as of other hepatic enzymes involved in the biotransformation of testosterone , pharmaceutical agents , and xenobiotics , appears to be regulated , at least in part , by sxr and related nuclear transcription factors ( blumberg et al . differences in the activity of the hepatic enzymes responsible for the biotransformation of sex steroids exist among alligators collected from contaminated and relatively uncontaminated sites in florida ( gunderson et al . we have hypothesized that differences in plasma sex steroid concentrations could be caused , in part , by differences in hepatic clearance of these sex steroids , as has been demonstrated in other species ( wilson and leblanc 1998 , 2000 ) . future studies need to consider the complex interactions among contaminant exposure , hepatic enzyme induction , and regulation of circulating hormone concentrations ( e.g. , tabb et al . 2004 ) because these issues are not separate but are , in fact , tightly integrated . it has been observed relatively recently that the promoter region of the human gene for inducible nitric oxide ( no ) synthase ( inos ) has response elements for the constitutive androstane receptor ( car ) and the pregnane x receptor ( pxr ) . these orphan receptors and transcription factors specifically , pxr activation by xenobiotics appears to up - regulate inos and subsequently increases no synthesis . inflammation and apoptosis are phenomena regulated by no , as are various actions of the hypothalamo  pituitary  gonad axis . interestingly , xenobiotic effects along this axis could be explained by modulations in inos that lead to altered hormonal action of no . however , this explanation is not the only possible source of elevated no because nitrate pollutants could also alter homeostasis in no synthesis ( guillette and edwards 2005 ) . it appears that nitrate , a global pollutant of most aquatic systems , has the potential to be an endocrine - disrupting contaminant ( guillette and edwards 2005 ) . nitrate ( no3 ) and nitrite ( no2 ) have known physiological influences ( jensen 2003 ; levallois and phaneuf 1994 ; zraly et al . nitrate and nitrite have been reported for decades to be toxic in humans and animals ( avery 1999 ) . as early as 1945 , methemoglobinemia was associated with drinking nitrate - contaminated well water on farms from the midwest united states . those concerns as well as new ones continue today with increasing contamination of groundwater with nitrates ( avery 1999 ; porter et al . nitrates have been implicated as a disruptor of gonadal steroidogenesis ( panesar 1999 ; panesar and chan 2000 ; zraly et al . 1997 ) and thyroid function ( kursa et al . 2000 ; lahti et al . 1985 a recent retrospective study of our work on alligators from various lakes in florida suggests that nitrate could contribute to some of the altered endocrine parameters previously reported in juvenile animals ( guillette and edwards 2005 ) . nitrate could alter steroidogenesis by a ) conversion to nitrite and nitric oxide in the mitochondria ( meyer 1995 ; nohl et al . 1999 ) , which is the site of initial steroid synthesis ( stocco and clark 1996 ) ; b ) altering cl ion concentrations in the cell by substituting for cl in the membrane transport pump ( alrefai et al . 2001 ; panesar 1999 ) ; or c ) binding to the heme region of various cytochrome p450 enzymes associated with steroidogenesis and altering enzymatic action ( danielson 2002 ; white et al . more research is needed to examine the possible endocrine disruptive action of this ubiquitous pollutant . several exciting studies have expanded our understanding of systems capable of endocrine disruption , such as the studies documenting contaminant - induced disruption of the interspecies signaling system associated with the nitrogen fixation pathway ( fox et al . 2001 ) , thyroid hormone  dependent gene expression during metamorphosis in amphibians ( crump et al . these examples are just a few of the natural systems that require further research efforts . although regulatory biology and most research funding will continue to focus on a few model systems and mechanisms , a broad basic research agenda is needed if we are to move the field forward . from my perspective , research in endocrine disruption represents less a new field , but rather more an integration of many fields , including biology , chemistry , epidemiology , and atmospheric and earth sciences . it is the new synthesis of science by which we will begin to understand the complexity of the world around us and our impact on it . new understanding of the effect of contamination has been gained because comparative endocrinologists , developmental biologists , ecologists and evolutionary biologists have exchanged ideas , knowledge , and hypotheses with physicians , epidemiologists , toxicologists and biochemists . understanding the complex interactions between the plethora of environmental contaminants and the organisms living in contaminated environments will require many approaches . the most important is a broad , creative approach to the science and its interpretation . it appears that nitrate , a global pollutant of most aquatic systems , has the potential to be an endocrine - disrupting contaminant ( guillette and edwards 2005 ) . nitrate ( no3 ) and nitrite ( no2 ) have known physiological influences ( jensen 2003 ; levallois and phaneuf 1994 ; zraly et al . nitrate and nitrite have been reported for decades to be toxic in humans and animals ( avery 1999 ) . as early as 1945 , methemoglobinemia was associated with drinking nitrate - contaminated well water on farms from the midwest united states . those concerns as well as new ones continue today with increasing contamination of groundwater with nitrates ( avery 1999 ; porter et al . nitrates have been implicated as a disruptor of gonadal steroidogenesis ( panesar 1999 ; panesar and chan 2000 ; zraly et al . 1997 ) and thyroid function ( kursa et al . 2000 ; lahti et al . 1985 a recent retrospective study of our work on alligators from various lakes in florida suggests that nitrate could contribute to some of the altered endocrine parameters previously reported in juvenile animals ( guillette and edwards 2005 ) . nitrate could alter steroidogenesis by a ) conversion to nitrite and nitric oxide in the mitochondria ( meyer 1995 ; nohl et al . 1999 ) , which is the site of initial steroid synthesis ( stocco and clark 1996 ) ; b ) altering cl ion concentrations in the cell by substituting for cl in the membrane transport pump ( alrefai et al . 2001 ; panesar 1999 ) ; or c ) binding to the heme region of various cytochrome p450 enzymes associated with steroidogenesis and altering enzymatic action ( danielson 2002 ; white et al . more research is needed to examine the possible endocrine disruptive action of this ubiquitous pollutant . several exciting studies have expanded our understanding of systems capable of endocrine disruption , such as the studies documenting contaminant - induced disruption of the interspecies signaling system associated with the nitrogen fixation pathway ( fox et al . 2001 ) , thyroid hormone  dependent gene expression during metamorphosis in amphibians ( crump et al . these examples are just a few of the natural systems that require further research efforts . although regulatory biology and most research funding will continue to focus on a few model systems and mechanisms , a broad basic research agenda is needed if we are to move the field forward . from my perspective , research in endocrine disruption represents less a new field , but rather more an integration of many fields , including biology , chemistry , epidemiology , and atmospheric and earth sciences . it is the new synthesis of science by which we will begin to understand the complexity of the world around us and our impact on it . new understanding of the effect of contamination has been gained because comparative endocrinologists , developmental biologists , ecologists and evolutionary biologists have exchanged ideas , knowledge , and hypotheses with physicians , epidemiologists , toxicologists and biochemists . understanding the complex interactions between the plethora of environmental contaminants and the organisms living in contaminated environments will require many approaches . the most important is a broad , creative approach to the science and its interpretation .
descriptions of endocrine disruption have largely been associated with wildlife and driven by observations documenting estrogenic , androgenic , antiandrogenic , and antithyroid actions . these actions , in response to exposure to ecologically relevant concentrations of various environmental contaminants , have now been established in numerous vertebrate species . however , many potential mechanisms and endocrine actions have not been studied . 
 for example , the ddt [ 1,1,1-trichloro-2,2-bis(p - chlorophenyl)ethane ] metabolite , p , p-dde [ 1,1-dichloro-2,2-bis(p - chlorophenyl)ethylene ] is known to disrupt prostaglandin synthesis in the uterus of birds , providing part of the explanation for ddt - induced egg shell thinning . 
 few studies have examined prostaglandin synthesis as a target for endocrine disruption , yet these hormones are active in reproduction , immune responses , and cardiovascular physiology . 
 future studies must broaden the basic science approach to endocrine disruption , thereby expanding the mechanisms and endocrine end points examined . 
 this goal should be accomplished even if the primary influence and funding continue to emphasize a narrower approach based on regulatory needs . without this broader approach , research into endocrine disruption will become dominated by a narrow dogma , focusing on a few end points and mechanisms .