Biological relevance of decamethylcyclopentasiloxane (D5) induced rat uterine endometrial adenocarcinoma tumorigenesis: Mode of action and relevance to humans

Decamethylcyclopentasiloxane affects estradiol production in female rats but not H295R cells

Sex hormones, such as androgens and estrogens, are predominantly produced in the gonads (ovaries and testes) and adrenal cortex. Endocrine-disrupting chemicals (EDCs) are substances that mimic, block, or interfere with hormones in the endocrine systems of humans and organisms. EDCs mainly act via nuclear receptors and steroidogenesis-related enzymes. In the OECD conceptual framework for testing and assessment of EDCs, several well-known assays are used to identify the potential disruption of nuclear receptors both in vivo and in vitro, whereas the H295R steroidogenesis assay is the only assay that detects the disruption of steroidogenesis. Forskolin and prochloraz are often used as positive controls in the H295R steroidogenesis assay. Decamethylcyclopentasiloxane (D5) was suspected one of EDCs, but the effects of D5 on steroidogenesis remain unclear. To establish a short-term in vivo screening method that detects the disruption of steroidogenesis, rats in the present study were fed a diet containing forskolin, prochloraz, or D5 for 14 days. Forskolin increased plasma levels of 17β-estradiol (E2) and testosterone as well as the mRNA level of Cyp19 in both the adrenal glands and ovaries. Prochloraz induced the loss of cyclicity in the sexual cycle and decreased plasma levels of E2 and testosterone. D5 increased E2 levels and shortened the estrous cycle in a dose-dependent manner; however, potential endocrine disruption was not detected in the H295R steroidogenesis assay. These results demonstrate the importance of comprehensively assessing the endocrine-disrupting effects of chemicals on steroidogenesis in vivo.

Occurrence, time trends, and human exposure of siloxanes and synthetic musk compounds in indoor dust from Korean homes

Siloxanes and synthetic musk compounds (SMCs) have been widely used as additives in household and personal care products. Humans are easily exposed to siloxanes and SMCs originating from these products through ingestion and dermal absorption of indoor dust. In the present study, indoor dust samples were analyzed for 19 siloxanes (cyclic and linear) and 12 SMCs (polycyclic, macrocyclic, and nitro musks) to assess their occurrence, time trends over time, source, and health risks. A total of 18 siloxanes and 10 SMCs were detected in all indoor dust samples obtained from 2011⎯2021, indicating widespread and long-term contamination. Higher detection frequencies and concentrations were associated with siloxanes and SMCs with higher use and strong resistance against degradation processes. Indoor dust samples were dominated by linear siloxanes (L11-L13), 1,3,4,6,7,8-hexahydro-4,6,6,7,8,8-hexamethylcyclopenta-(g)-2-benzopyran (HHCB), musk ketone (MK), and 7-acetyl-1,1,3,4,4,6-hexamethyl-1,2,3,4-tetrahydronaphthalene (AHTN). The frequent use of household and personal care products is likely an important source of siloxane and SMC contamination in indoor environments. The concentrations of siloxanes and SMCs in indoor dust increased from 2011 to 2021, particularly, those of linear siloxanes, reflecting the impact of regulatory actions addressing cyclic siloxanes. The profiles of siloxanes remained stable throughout the study period, whereas those of SMCs shifted from nitro to polycyclic musks. The estimated daily intakes (EDIs) of siloxanes and SMCs arising from ingestion were greater than from dermal absorption of indoor dust. The EDIs of siloxanes and SMCs associated with indoor dust indicated that children are exposed to these pollutants.

Association of volatile methylsiloxanes exposure with non-alcoholic fatty liver disease among Chinese adults

Owing to the wide use of volatile methylsiloxanes (VMSs) in various industries and consumer products, both cyclic VMSs (cVMS) and linear VMSs (lVMS) have been detected in human plasma. Experimental studies suggest that exposure to cVMSs may induce liver disease. Whereas, there is no human evidence of the potential health effects of VMSs yet. In this cross-sectional study, we evaluated the association of plasma VMSs concentrations with liver enzymes and Nonalcoholic fatty liver disease (NAFLD) among adults located in southwestern China. We used the fibrosis 4 calculator (FIB-4) as the NAFLD index and defined FIB-4≥1.45 as the NAFLD case. Among 372 participants, 45 (12.1%) of them were classified as NAFLD. Positive associations of plasma cVMSs concentrations with liver enzymes and NAFLD were observed among all participants. With per doubling increase in the total cVMSs, we observed a 1.40 (95%CI: 0.31, 2.48) increase in Alanine aminotransferase (ALT), a 1.56 (95%CI: 0.52, 2.61) increase in aspartate aminotransferase (AST) and a 0.04 (0.00, 0.09) increase in NAFLD index. A 19% increased risk of NAFLD was also found to be associated with per doubling increase in total cVMSs. In addition, positive associations of total lVMSs with ALT, AST and NAFLD were also detected when restricting our analyses to 230 participants living in industrial areas. Our study first provides epidemiological evidence on the association between VMSs and liver health, indicating more careful usage of VMSs may potentially reduce the burden of NAFLD, though more well-designed cohort studies are needed to confirm these findings.

Carcino-Evo-Devo, A Theory of the Evolutionary Role of Hereditary Tumors

A theory of the evolutionary role of hereditary tumors, or the carcino-evo-devo theory, is being developed. The main hypothesis of the theory, the hypothesis of evolution by tumor neofunctionalization, posits that hereditary tumors provided additional cell masses during the evolution of multicellular organisms for the expression of evolutionarily novel genes. The carcino-evo-devo theory has formulated several nontrivial predictions that have been confirmed in the laboratory of the author. It also suggests several nontrivial explanations of biological phenomena previously unexplained by the existing theories or incompletely understood. By considering three major types of biological development-individual, evolutionary, and neoplastic development-within one theoretical framework, the carcino-evo-devo theory has the potential to become a unifying biological theory.

Development of adverse outcome pathways relevant for the identification of substances having endocrine disruption properties Uterine adenocarcinoma as adverse outcome

Development of adverse outcome pathways (AOPs) for uterine adenocarcinoma can provide a practical tool to implement the EFSA-ECHA Guidance (2018) for the identification of endocrine disruptors in the context of Regulations (EU) No 528/2012 and (EC) No 1107/2009. AOPs can give indications about the strength of the relationship between an adverse outcome (intended as a human health outcome) and chemicals (pesticides but not only) affecting the pathways. In this scientific opinion, the PPR Panel explored the development of AOPs for uterine adenocarcinoma. An evidence-based approach methodology was applied, and literature reviews were produced using a structured framework assuring transparency, objectivity, and comprehensiveness. Several AOPs were developed; these converged to a common critical node, that is increased estradiol availability in the uterus followed by estrogen receptor activation in the endometrium; therefore, a putative AOP network was considered. An uncertainty analysis and a probabilistic quantification of the weight of evidence have been carried out via expert knowledge elicitation for each set of MIEs/KEs/KERs included in individual AOPs. The collected data on the AOP network were evaluated qualitatively, whereas a quantitative uncertainty analysis for weight of the AOP network certainty has not been performed. Recommendations are provided, including exploring further the uncertainties identified in the AOPs and putative AOP network; further methodological developments for quantifying the certainty of the KERs and of the overall AOPs and AOP network; and investigating of NAMs applications in the context of some of the MIEs/KEs currently part of the putative AOP network developed.

Effects of Maternal Exposure to Decamethylcyclopentasiloxane on the Alternations in Offspring Behaviors in Mice

D5, a member of the cyclic siloxane family, is widely used in personal care products such as shampoo, cosmetics, and deodorant and as an industrial intermediate. D5 can mainly be absorbed orally or through inhalation. Through these routes, people are exposed to D5 daily. However, the risk of prenatal exposure to D5 has not been fully elucidated. In this study, the effect of D5 on neural development was established through behavioral tests on offspring mice. The result confirmed that the maternal administration of 12 mg/kg of D5 showed depression in tail suspension and decreased performance in the forced swimming test as well as an increase in repetitive activity in both the marble-burying test and grooming test compared to the vehicle group. Furthermore, the 12 mg/kg group showed a decrease in cognitive ability and social behavior in the three-chamber test. In the novel object recognition test, memory impairment and a lack of exploring ability were found in the 12 mg/kg group. In conclusion, it is suggested that maternal D5 exposure has developmental neurotoxicity and can cause behavioral disorders in the offspring of mice. Thus, the usage of D5 needs to be considered carefully.

Potential trade-off between water consumption and water quality: life cycle assessment of nonaqueous solvent dyeing

Fashion industry consumes over 60% of global fibers and attracts increasing attentions due to its environmentally polluting supply chain. In addition to natural fibers cultivation, wet processes of textile manufacturing are also important contributors to water-related impacts due to their large freshwater consumption and the production of chemicals containing wastewater. Despite of efforts made in improving efficiency of water use and wastewater treatment in textile industry, innovative 'water-free' technologies, such as nonaqueous dyeing technology using organic solvent, have been developed and demonstrated to reduce water consumption significantly. However, the potential impact on water quality by organic solvents induced in supply chain of this emerging technology remains unassessed, posing an unknown risk of its promotion. Hence, in the present study, a comprehensive life cycle assessment is applied to evaluate its full environmental impacts, including those on ecosystem and human health caused by decamethylcyclopentasiloxane (D5) as the solvent used. Further, the nonaqueous dyeing system is compared with traditional aqueous dyeing technology from both environmental and economic perspectives. Results indicate that nonaqueous dyeing system is advanced in most of environment categories except for abiotic depletion potential (ADP) and Ecotoxicity. However, scenarios analysis reveal that these findings are influenced by the loss fraction of D5 during the solvent recovery process. It is suggested that the loss fraction should be controlled below 2% o.w.f. for the nonaqueous dyeing technology to be advanced throughout all environmental categories. Nonaqueous D5 dyeing could reduce water consumption by 61.30%-79.95% and greenhouse gas emissions by 43.70% compared to the traditional system, delivering a promising contribution to China's 2060 carbon neutrality ambition. Sensitivity and uncertainty analyses are also conducted to investigate the effects of the key parameters (incl. inventory data and USEtox model inputs) and demonstrate the robustness of our assessment.

Methylsiloxanes in street dust from Hefei, China: Distribution, sources, and human exposure

Methylsiloxanes are widely found in the environment and have been of increasing concern because of their strong resistance to degradation and potential toxicity to organisms. However, little is known about the distributions of these chemicals in street dust and the associated human health risks. This study investigated three cyclic (D₄-D₆) and nine linear methylsiloxanes (L₅-L₁₃) in street dust from Hefei, China and found total concentrations in the range of 183-1030 (median, 527) ng/g dry weight. The linear congeners were dominant and represented a median of 85.3% of the total methylsiloxanes. D₅ contributed 90.0% of the total concentrations of cyclic methylsiloxanes. In this study, higher concentrations of dust methylsiloxanes were found in the industrial area relative to the other functional areas. A source assessment indicated that the linear and cyclic methylsiloxanes in the street dust were mainly from the industrial and traffic activities, respectively, in addition to important sources of the use of siloxanes-containing products. The estimated median daily intakes of total methylsiloxanes through street dust were 0.037 and 0.476 ng/kg-bw/d for adults and children, respectively, under high-exposure scenarios. More research is needed to characterize the occurrence of methylsiloxane in various exposure sources and the associated adverse effects on human health.

Inhalation toxicity of cyclic semi-volatile methylsiloxanes: Disentangling the conundrum of phase-specific adaptations from adverse outcomes

This paper compares the phase-specific inhalation toxicity of the cyclic semi-volatile methylsiloxanes (cVMSs) D4, D5 and D6. The objectives of this paper are to re-analyze information from acute to chronic inhalation studies on rats with these cVMSs to identify the unifying principles of phase-specific toxicity at the portal-of-entry and if they depend on acute, acute-on-chronic or chronic mechanisms. This re-analysis supports the hypothesis that concentrations must be high enough to exceed the vapor saturation at any given temperature for stabilizing the aerosol phase and evoking phase-specific effects at sites of the respiratory tract susceptible to the cVMSs-specific physicochemical properties amphiphilicity and surface tension. In summary, the portal-of-entry effects and related findings appear to be acute in nature and specific to liquid aerosol. The repeated inhalation exposure studies with D4 and D5 up to two years in duration did not reveal chronic aggravations of portal of entry outcomes. Findings at a pulmonary location where amphiphilic surfactant molecules are present appear to be caused by the acute adaptation to deposited dose. Such outcome should better be described as a high-dose liquid aerosol phenomenon imparted by the physicochemical properties "liquid" and "hydrophobic". This calls for a phase-specific human risk characterization of cVMSs.

Evaluation of Cystic Endometrial Hyperplasia and the Normal Estrous Cycle in Longitudinal Sections of Uterus from Female Harlan Sprague-Dawley Rats

The National Toxicology Program (NTP) has recently introduced the practice of examining longitudinal histological sections of the female rodent uterus to improve the identification of non-neoplastic lesions, preneoplastic lesions, and uterine tumors. This practice has created a need for reference material that includes normal histology, spontaneous lesions, and inducible lesions in longitudinal as well as transverse sections of the body of the uterus, uterine horns, cervix and vagina. Using 3 archived NTP reproductive and developmental toxicity studies, the authors reviewed longitudinal and transverse sections of uteri from female Hsd:Sprague Dawley SD® (Hsd:SD) rats for cystic endometrial hyperplasia (CEH). The purposes of this review were to (1) evaluate if existing criteria for CEH in transverse uterine sections could be applied to longitudinal sections to develop diagnostic features of CEH in longitudinal uterine sections of rat uterus and (2) create an atlas of the normal estrous cycle phases in longitudinal sections of young and mature adult Hsd:SD rat uteri. The information provided in this original article should help facilitate the examination of longitudinal sections of the uterus in future commercial and governmental rodent studies.

Biocatalytic solutions to cyclomethicones problem in cosmetics

Silicones are polymers that have properties of great interest for cosmetic and personal care industry, especially D₄ (octamethylcyclotetrasiloxane) and D₅ (decamethylcyclopentasiloxane), generically named cyclomethicones. However, different studies show the hypothesis that the use of these products is harmful and, therefore, several countries have adopted legislative actions to limit their use in personal care and cosmetic products. Faced with this new situation, different cosmetic companies began to look for different alternatives to cyclomethicones, mainly: organomodified silicones or esters. The former are based on the addition of functional groups to the main chain of silicon and oxygen. The latter represent a totally different line, since their functional group is the ester, but they have properties similar to silicones. Esters, especially branched-chain ones are currently the more promising alternative to replace cyclomethicones in cosmetic formulations. Nowadays, most of them are obtained by chemical reactions that require high temperatures and non-selective /contaminant catalysts that lead to low-quality final products. As an alternative, biocatalytic synthesis occurs always at mild operation conditions supplying ultra-pure, odor, and colorless products with less wastes and side reactions. Therefore, biocatalysis is a valid and environmentally sustainable option for the synthesis of silicone-substitute esters used in cosmetic formulations.

Adsorptive performance of coal-based magnetic activated carbon for cyclic volatile methylsiloxanes from landfill leachate

Bituminous coal-based magnetic activated carbon (MAC) was prepared, characterized, and used successfully for removal of cyclic volatile methylsiloxanes (cVMSs) from treated landfill leachate. Batch adsorption studies were performed at different adsorption dosages and contact times. With adsorptive dosage of 0.75 g/L and contact time of 60 min, the removal efficiencies achieved by MAC for octamethylcyclotetrasiloxane (D₄), decamethylcyclopentasiloxane (D₅), and dodecamethylcyclohexasiloxane (D₆) are 100, 82.8, and 71.4%, respectively. The specific magnetization coefficients of MAC before and after adsorption are 4.6 × 10^-7 and 5.2 × 10^-7 m³/kg, between 1.26 × 10^-7 and 3.8 × 10^-5 m³/kg, which suggests that MAC can be recycled by the high-intensity magnetic separators.

Biological relevance of effects following chronic administration of octamethylcyclotetrasiloxane (D4) in Fischer 344 rats

Octamethylcyclotetrasiloxane (D4) is a cyclic siloxane primarily used as a monomer or intermediate in the production of silicone polymers resulting in potential exposure of workers, and potential low level inhalation or dermal exposure for consumers and the general public. Following a two-year inhalation toxicity study with D4 in rats, increases in uterine endometrial cystic hyperplasia and adenomas were observed at the highest concentration of D4 administered (700ppm). No other neoplasms were increased with D4 treatment. In addition, chronic inhalation exposure of rats to D4 induced changes in relative liver and kidney weights, and produced a chronic nephropathy. This manuscript examines the biological relevance and possible modes of action for the effects observed in the F344 rat following chronic inhalation exposure to D4. D4 is not genotoxic and appears to exert its effects through a nongenotoxic mode of action. An alteration in the estrous cycle in the aging F344 rat was the most likely mode of action for the observed uterine effects following chronic inhalation exposure. Data support the conclusion that D4 acts indirectly via a dopamine-like mechanism leading to alteration of the pituitary control of the estrous cycle in aging F344 rats with a decrease in progesterone and an increase in the estrogen/progesterone ratio most likely induced by a decrease in prolactin concentration. D4 also inhibited the pre-ovulatory LH surge causing a delay in ovulation, persistent follicles and thus a prolonged exposure to elevated estrogen in the adult Sprague Dawely rat. A lengthening of the estrous cycle in the F344 rat with an increase in endogenous estrogen was also induced by D4 inhalation. Although the mode of action responsible for induction of uterine adenomas in the female F344 rat has not been clearly confirmed, the subtlety of effects on the effects of D4 on cyclicity may prevent further assessment and definition of the mode of action. The occurrence of uterine endometrial adenoma in the rat is not relevant for human risk characterization because (1) there are differences in ovulatory cycle regulation in rats compared to humans, (2) cystic hyperplasia without atypia in women is not a cancer precursor, and (3) there is no endometrial lesion in women that is directly analogous to endometrial adenoma in the rat. The effects of D4 on liver are due to a phenobarbital-like mechanism that results in induction of cytochrome P450 and other enzymes of xenobiotic biotransformation. The liver effects are adaptive and not adverse. Kidney findings included chonic progressive nephropathy, a rat lesion that has no counterpart in the human and that should not be used in human risk assessment.

A high dose mode of action for tetrabromobisphenol A-induced uterine adenocarcinomas in Wistar Han rats: A critical evaluation of key events in an adverse outcome pathway framework

TBBPA is a non-genotoxic flame retardant used to improve fire safety in a wide variety of consumer products. Estimated human exposures to TBBPA are very low (<0.000084 mg/kg-day), relative to the doses (500 and 1000 mg/kg-day of TBBPA) administered in a recent bioassay that resulted in uterine tumors in Wistar Han rats following chronic exposure. As part of an effort to characterize the relevance of the uterine tumors to humans, data and biological knowledge relevant to the progression of events associated with TBBPA-induced uterine tumors in female rats were organized in an adverse outcome pathway (AOP) framework. Based on a review of possible MOAs for chemically induced uterine tumors and available TBBPA data sets, a plausible molecular initiating event (MIE) was the ability of TBBPA to bind to and inhibit estrogen sulfotransferases, the enzymes responsible for sulfation of estradiol. Subsequent key events in the AOP, including increased bioavailability of unconjugated estrogens in uterine tissue, would occur as a result of decreased sulfation, leading to a disruption in estrogen homeostasis, increased expression of estrogen responsive genes, cell proliferation, and hyperplasia. Available data support subsequent key events, including generation of reactive quinones from the metabolism of estrogens, followed by DNA damage that could contribute to the development of uterine tumors. Uncertainties associated with human relevance are highlighted by potential strain/species sensitivities to development of uterine tumors, as well as the characterization of a dose-dependent MIE. For the latter, it was determined that the TBBPA metabolic profile is altered at high doses (such as those used in the cancer bioassay), and thus an MIE that is only operative under repeated high dose, administration. The MIE and subsequent key events for the development of TBBPA-induced uterine tumors are not feasible in humans given differences in the kinetic and dynamic factors associated with high dose exposures in rats relative to human exposure levels to TBBPA.

Dose- and time-dependent changes in tissue levels of tetrabromobisphenol A (TBBPA) and its sulfate and glucuronide conjugates following repeated administration to female Wistar Han Rats

Tetrabromobisphenol A (TBBPA), a nongenotoxic flame retardant, causes uterine tumors in female rats. A proposed mode of action (MoA) for these tumors involves an increase in the bioavailability of estradiol as a result of TBBPA inhibiting estrogen sulfotransferases (ES), the enzymes responsible for inactivating and enhancing the elimination of estradiol. The objective of this study was to evaluate the effect of dose and repeated administration of TBBPA on the level of TBBPA, TBBPA-glucuronide (GA) and TBBPA-sulfate (S) conjugates in plasma, liver and uterus of female Wistar Han rats administered TBBPA (50, 100, 250, 500 or 1000 mg/kg) for 28 consecutive days. In accordance with this objective, TBBPA sulfation was used as a surrogate for evaluating the potential for estradiol sulfation to be limited at high dose levels of TBBPA. Blood samples were collected at 4 and 8 h post-dosing on study day 7, 14, and 28, while liver and uterus were collected at the same time points following 28 days of dosing. Tissue samples were analyzed for TBBPA, TBBPA-GA and TBBPA-S by LC–MS/MS. A dose-related increase in the concentration of all three analytes occurred in plasma (day 7, 14, and 28) as well as liver and uterus tissue (day 28) at both 4 and 8 h post dose. The plasma concentration of TBBPA-GA and TBBPA-S was higher in animals dosed for 28 days compared to those dosed for 7 or 14 days showing an increase in systemic circulation of these conjugates with repeated administration. The balance of these conjugates was also different in tissues with TBBPA-S > TBBPA-GA at high doses in the liver and TBBPA-GA > TBBPA-S in both plasma and uterus. In all three tissues the ratio of TBBPA-S/TBBPA-GA showed a decreasing trend with dose, suggesting that at high TBBPA dose levels sulfation of TBBPA becomes limited. This effect was most apparent in the liver and plasma at 28 days of administration. Together these data show that administration of high doses of TBBPA associated with the induction of uterine tumors, results in a disruption in the balance of conjugates reflected by a decrease in the TBBPA-S/TBBPA-GA ratio. A limitation in the sulfation of TBBPA in vivo supports in vitro data defining TBBPA as an inhibitor of ES activity, thus providing further support that the proposed MoA occurs under conditions of high dose, chronic TBBPA administration to Wistar Han rats. Given that the uterine tumors observed in rats (250–1000 mg/kg-day) only occur at very high doses that perturb homeostatic control, it is unlikely such effects would occur in humans given that current TBBPA exposure levels are approximately eight orders of magnitude lower than these doses that are associated with exceeding the capacity of conjugation pathways in animal studies.