In This Issue

Monograph

Endocrine Disruptors and Urogenital Malformations

Exposure to xenoestrogens during pregnancy may disturb the development and function of male sexual organs. Fernandez et al.(p. 8) aimed to determine whether the combined effect of environmental estrogens measured as total effective xenoestrogen burden (TEXB) is a risk factor for male urogenital malformations. In a case–control study established at Granada University Hospital, 50 newborns with diagnosis of cryptorchidism and/or hypospadias were compared with 114 boys without malformations. TEXB and levels of 16 organochlorine pesticides were measured in placenta tissues. The study found an increased risk for male urogenital malformations related to the combined effect of environmental estrogens in placenta.

Semen Quality and Xenobiotic Activities

Semen quality in humans may be influenced by exposure to endocrine-disrupting compounds. Toft et al.(p. 15) analyzed associations between semen characteristics and serum xenoestrogen receptor (XER), xenoandrogen receptor (XAR), and aryl hydrocarbon receptor (AhR) transactivity. XER and XAR activity were measured in serum samples cleared for endogenous steroid hormones and AhR activity in raw lipophilic serum extracts free of proteins. Alteration of XER, XAR, or AhR transactivity within the range found in serum from the general European and Inuit population seems not to markedly deteriorate sperm cell concentration, motility, or morphology in adult men.

POPs and Xenoandrogenic Activity in Human Serum

Animal and in vitro studies have indicated that human male reproductive disorders can arise as a result of disrupted androgen receptor (AR) signalling by persistent organic pollutants (POPs). Krüger et al.(p. 21) compare serum xenoandrogenic activity between study groups with different POP exposures and to evaluate correlations to the POP proxy markers 2,2´,4,4´,5,5´-hexachlorobiphenyl and 1,1-dichloro-2,2-bis(p-chlorophenyl)-ethylene. Xenoandrogenic activity was determined as the effect of serum extract alone (XAR) and in the presence of the synthetic AR agonist R1881 (XARcomp) on AR transactivated luciferase activity. The differences in XARcomp serum activity between the study groups suggest differences in chemical exposure profiles, genetics, and/or lifestyle factors.

PBDEs and HBCD in Czech Fish

Brominated flame retardants—polybrominated diphenyl ethers (PBDEs) and hexabromocyclododecane (HBCD)—belong to the group of relatively “new” environmental contaminants. The occurrence of these compounds in the Czech aquatic ecosystem was first documented by Pulkrabová et al.(p. 28) within a 3-year monitoring study initiated in 2001. The major PBDE congeners and also HBCD were determined in 136 freshwater fish samples collected from several sampling sites located at three Czech rivers (Vltava, Elbe, Tichá Orlice). Without exception, the dominating congener was BDE-47. The most pronounced extent of fish contamination was found in Vltava–Klecany, downstream from the industrial agglomeration of Prague.

Brominated Flame Retardants in the North-East Atlantic

Concentrations of brominated flame retardants (BFRs) are increasing in marine ecosystems. Jenssen et al.(p. 35) characterized exposure to BFRs in animals from different trophic levels in North-East Atlantic coastal marine ecosystems along a latitudinal gradient from southern Norway to Spitsbergen, Svalbard, in the Arctic. Levels of polybrominated diphenylethers (PBDEs) and hexabromocyclododecane (HBCD) generally decreased as a function of increasing latitude, reflecting distance from release sources. The decabrominated compound BDE-209 was detected in animals from all three ecosystems. Even though the levels of PBDEs and HBCD are generally low, there are concerns about the relatively high presence of BDE-209 and HBCD.

Pre- and Postnatal Toxicity of 4-MBC in Male Rats

In previous studies, Durrer et al.(p. 42) found that the ultraviolet filter 4-methylbenzylidene camphor (4-MBC) exhibits estrogenic activity, is a preferential estrogen receptor beta ligand, and interferes with development of female reproductive organs and brain of both sexes in rats. The authors now report the effects on male development. They indicate that 4-MBC affects development of male reproductive functions and organs, with a lowest observed adverse effect level of 0.7 mg/kg. Nuclear receptor coregulators were revealed as targets for endocrine disruptors, as shown for the nuclear receptor co-repressor in the prostate and SRC-1 in the uterus. This may have widespread effects on gene regulation.

Effects of Endocrine Disruptors on DHEA Sulfation

Sulfation plays an important role in detoxification and in the control of steroid activity. Studies have shown that the conversion of dehydroepiandrosterone (DHEA) to DHEA-sulfate is involved in learning and the memory process. The effects of plasticizers and related compounds were evaluated kinetically against human DHEA sulfotransferase and by reverse transcriptase–polymerase chain reaction against several enzymes involved in the synthesis of the sulfotransferase cofactor adenosine 3´-phosphate 5´-phosphosulfate (PAPS). Harris et al.(p. 51) suggest that environmental contaminants may exert effects on neuronal function both by direct inhibition of sulfotransferase enzymes and by interrupting the supply of PAPS, which has wider implications for endocrine disruption and xenobiotic metabolism.

DBP Dose Sensitivity and Testicular Dysgenesis

Fetal exposure of male rats to di(n-butyl) phthalate (DBP) induces reproductive disorders similar to those in human testicular dysgenesis syndrome (TDS). Humans are widely exposed to DBP, but at much lower levels than those causing adverse effects in rats. Mahood et al.(p. 55) evaluated end points that are relevant to human TDS, and compared their dose sensitivity. The fetal end points evaluated can be objectively quantified and may prove helpful in evaluating the health risk of exposure to DBP and other phthalates, as well as identifying DBP-sensitive fetal events that have adult consequences/end points that are identifiable in human TDS.

Effects of Genistein in Reproductive Organs

The isoflavone genistein (GEN) is found in soy and red clover. The estrogenic activity of GEN is known, and it is widely advertised as a phytoestrogen used to alleviate climacteric complaints and other postmenopausal disorders. Knowledge of effects of long-term administration of GEN is scarce, and effects in the uterus and mammary gland after long-term administration have not been studied. The uterus and mammary gland are known to be negatively influenced by estrogens used in hormone therapy. According to Rimoldi et al.(p. 62), 17BETA-estradiol and GEN share many effects in the studied organs, particularly in the vagina, uterus, and mammary gland but not in the hypothalamo/pituitary unit.

Endocrine-Disruption Potential of BPA, BPA-DM, nNP, and nOP

An array of environmental compounds possesses endocrine disruption (ED) potential. Bisphenol A (BPA) and BPA dimethacrylate (BPA-DM) are monomers used in the plastic industry and as dental sealants. Alkylphenols such as 4-n-nonylphenol (nNP) and 4-n-octylphenol (nOP) are widely used as surfactants. According to Bonefeld-Jørgensen et al.(p. 69)in vitro data indicate that the four industrial compounds have ED potentials and the effects can be mediated via several cellular pathways, including the two sex steroid hormone receptors (estrogen receptor and androgen receptor), aromatase activity converting testosterone to estrogen, and aryl hydrocarbon receptor (AhR); AhR isinvolved in syntheses of steroids and metabolism of steroids and xenobiotic compounds.

Endocrine Disruptors and Thyroid Hormone Axis

There is growing evidence that, apart from the reproductive system, the hypothalamic–pituitary–thyroid (HPT) axis is a target of endocrine-disrupting compounds (EDCs). This is not adequately reflected in current screening and assessment procedures, which only determine general parameters of thyroid function. Schmutzler et al.(p. 77) used several in vitro and ex vivo assays to identify suitable biomarkers for antithyroid action using a panel of EDCs. As the effects do not reflect classical mechanisms of hormone-dependent regulation and feedback, the authors suppose multitarget and multimodal actions of EDCs on the HPT axis. These complex effects require a diversified approach for screening, evaluation, and risk assessment of potential antithyroid compounds.

Low-Dose Testing of Endocrine Disruptors

The endocrine disruptor field has difficulties in reproducing various claims of effects at unusually low doses. Variations in control responses from experiment to experiment and problems with observing effects in positive controls have been identified as possible explanations. Scholze and Kortenkamp(p. 84) argue that both of these viewpoints fail to take sufficient account of the problems that exist in estimating low effects and low-effect doses. Low-dose estimates such as the no observed adverse effect levels are dependent on the specific experimental conditions used for testing. Below the statistical detection limit of the experiment, the presence of effects can neither be proven nor ruled out.

Low-Dose Effects of Xenoestrogens in the E-Screen

Effects induced by endocrine-disrupting chemicals may occur at dose levels lower than normally tested. Silva et al.(p. 91) illustrate the difficulties that may be encountered during the estimation of low doses in vivo. High statistical power is required when dose–response curves are shallow. Using large sample sizes and numerous repeats, the power of the E-Screen was sufficiently high to measure effect magnitudes around 1–2%. The lack of such resources may result in statistical detection limits that are considerably higher. If this coincides with shallow dose–response curves in the low-effect range, the resolving power of in vivo assays may seriously constrain low-dose testing.

Review of Endocrine-Disrupting Chemical Mixture Effects

Good evidence has become available to show that the combined effects of endocrine disruptors (EDs) belonging to the same category (e.g., estrogenic, antiandrogenic, or thyroid-disrupting agents) can be predicted by using dose addition. Combinations of EDs are able to produce significant effect, even when each chemical is present at low doses that individually do not induce observable effects. However, comparatively little is known about mixtures composed of chemicals from different classes of EDs. Kortenkamp(p. 98) argues that the accumulated evidence seriously undermines continuation with the customary chemical-by-chemical approach to risk assessment for EDs. Instead, we should seriously consider group-wise regulation of classes of EDs.

Low-Level Exposure to Multiple Chemicals

A key question in the risk assessment of exposures to multiple chemicals is whether mixture effects occur when chemicals are combined at low doses that individually do not induce observable effects. However, a systematic evaluation of experimental studies addressing this issue is missing. According to Kortenkamp et al.(p. 106) the widely held view that mixtures of dissimilarly acting chemicals are “safe” at levels below no observed adverse effects levels (NOAELs) is not supported. This view is based on the erroneous assumption that NOAELs can be equated with zero-effect levels. Thus, on the basis of published evidence, it is difficult to rule out the possibility of mixture effects from low-dose multiple exposures.

Estrogenic Mixture Effects in Marine Fish

The potential impact of natural and synthetic estrogens on aquatic ecosystems has attracted considerable attention because it is currently accepted that their joint effects are more severe when they are present in mixtures. There is little information about the combined effects of estrogenic chemicals on marine biota. In 14-day tests with juvenile sea bass, Correia et al.(p. 115) analyzed singly and in combination the estrogenic activity of estradiol, ethynylestradiol, and bisphenol A using vitellogenin induction as an end point. The study demonstrates the potential hazard of these compounds by their ability to act together in an additive manner.

Antiandrogen Mixture: Anogenital Distance and Nipple Retention

Hass et al.(p. 122) assessed whether the joint effects of three androgen receptor antagonists (vinclozolin, flutamide, and procymidone) on male sexual differentiation after in utero and postnatal exposure can be predicted based on dose–response data of the individual chemicals. Effects of a mixture of similarly acting antiandrogens can be predicted fairly accurately based the potency of the individual mixture components by using the dose-addition concept. Exposure to antiandrogens, which individually appears to exert only small effects, may induce marked responses in concert with, possibly unrecognized, similarly acting chemicals.

Binary Mixtures Enhance Micronucleus Formation

Within mixtures, interactions between different xenobiotics may occur to give rise to additive, synergistic, inhibitory, and/or stimulatory effects in target cells. The role that xenobiotics individually or in mixtures, and at environmental concentrations, play in the etiology of common human diseases often remains obscure. Following exposure to binary mixtures of benzo[a]pyrene plus lindane or 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine plus lindane, a 10-fold increase in micronucleus formation resulted; these test agents individually induced 2- to 5-fold increases. Hewitt et al.(p. 129) suggest that low-dose treatments (i.e., close to environmental levels) may increase DNA damage while influencing survival in exposed cells and that these effects may be dependent upon the endocrine activity of test agents.