The effects of tri-o-cresyl phosphate and metabolites on rat Sertoli cell function in primary culture

High Content Imaging Analyses of the Effects of Bisphenols and Organophosphate Esters on TM4 Mouse Sertoli Cells

The endocrine disruptive effects of bisphenol A (BPA) and brominated flame retardants (BDE-47) have led to restrictions to their use and increased the pressure to identify safe replacements for these chemicals. Although there is evidence that some of these alternatives may be toxic to spermatogonial and Leydig cells, little is known about the toxicity of emerging replacements on Sertoli cells, one of the major testicular cell types. We used high-content imaging to compare the effects of legacy chemicals, BPA and BDE-47, to their corresponding replacements. TM4 Sertoli cells were exposed for 48 h to each chemical (0.001-100 μM) followed by cytotoxicity and phenotypic endpoint assessment. The benchmark concentration (BMC) potency ranking for bisphenols based on cytotoxicity was BPTMC>BPM > BPAF>BPF > BPS > BPA. Human administered equivalent dose (AED) determination ranked BPS as most potent alternative replacement studied. The BMC potency ranking of BDE-47 and organophosphate esters based on cytotoxicity was TDtBPP>BDMPP>TBOEP>TDCPP>TMPP>TPHP> BDE47 > IPPP=BPDP = TCPP. Additionally, TM4 cell exposure to BDE-47 increased Calcein intensity (57.9 μM) and affected lysosomes (21.6 μM), while exposure to TPHP and TMPP resulted in cellular oxidative stress changes at BMC values as low as 0.01 μM and 0.4 μM, respectively. Overall bioactivity considerations of the chemicals on TM4 via ToxPi analyses and AED modeling further validated emerging replacements as highly potent chemicals in comparison to BPA and BDE-47. These findings demonstrate that many bisphenol and flame retardant replacements are more potent in Sertoli cells than the legacy chemical they are replacing, and that phenotypic parameter assessment is an effective tool in chemical toxicity assessment.

Effects of flame retardants on ovarian function

Flame retardants have been added to a variety of consumer products and are now found ubiquitously throughout the environment. Epidemiological, in vivo, and in vitro studies have shown that polybrominated diphenyl ether (PBDE) flame retardants may have a negative impact on human health; this has resulted in their phase-out and replacement by alternative flame retardants, such as hexabromocyclododecane (HBCDD), tetrabromobisphenol A (TBBPA), and organophosphate esters (OPEs). Evidence suggests that some of these chemicals induce ovarian dysfunction and thus may be detrimental to female fertility; however, the effects of many of these flame retardants on the ovary remain unclear. In this review, we present an overview of the effects of brominated and organophosphate ester flame retardants on ovarian function and discuss the possible mechanisms which may mediate these effects.

Inhibition of rat brain and human red cell acetylcholinesterase by thiocarbamate herbicides

Thiocarbamates are a major class of herbicides that were used extensively in the agricultural industry. Toxicological evaluation showed molinate caused reproductive impairment in male rats, whilst others produced behavioural effects at high doses. Rats dosed with molinate either as a single large oral dose of 100 mg/kg or as multiple doses of 50 mg/kg for 7 days produced inhibition of brain acetylcholinesterase (AChE). Molinate and other thiocarbamate herbicides undergo metabolism to form sulphoxides that can carbamoylate thiol's such as glutathione and proteins. We have chemically synthesised the sulphoxide and sulphone metabolites of six thiocarbamate herbicides and examined their ability to inhibit rat brain and human red cell AChE in vitro. Parent thiocarbamates were inactive, whilst the sulphoxides produced inhibition with IC₅₀'s in the 1-10 mM range, the sulphone metabolites were the most active with IC₅₀'s for molinate, pebulate, EPTC and vernolate in the μM range. Inhibition was both time- and dose-dependent with biomolecular rate constants for the inhibition of the human red cell enzyme of 0.3 × 10² and 2.0 × 10² M^-1 min^-1 for molinate sulphoxide and sulphone, respectively. No recovery of enzyme activity, with either enzyme, was seen following dilution of the inhibitor to a concentration that does not inhibit the enzyme for up to 24 h at 25°C at pH 7.4. The metabolites of these thiocarbamate herbicides are rather poor inhibitors of AChE when compared to the organophosphorus ester, paraoxon or the monomethylcarbamate, eserine. Unlike eserine the inhibition produced by the thiocarbamates is irreversible.

From the Cover: An Animal-Free In Vitro Three-Dimensional Testicular Cell Coculture Model for Evaluating Male Reproductive Toxicants

Primary testicular cell coculture model has been used to evaluate testicular abnormalities during development, and was able to identify the testicular toxicity of phthalates. However, the primary testicular cell coculture model has disadvantages in employing animals for the isolation of testicular cells, and the complicated isolation procedure leads to inconsistent results. We developed an invitro testicular coculture model from rodent testicular cell lines, including spermatogonial cells, Sertoli cells, and Leydig cells with specified cell density and extracellular matrix (ECM) composition. Using comparative high-content analysis of F-actin cytoskeletal structure between the coculture and single cell culture models, we demonstrated a 3D structure of the coculture, which created an invivo-like niche, and maintained and supported germ cells within a 3D environment. We validated this model by discriminating between reproductive toxicants and nontoxicants among 32 compounds in comparison to the single cell culture models. Furthermore, we conducted a comparison between the invitro (IC50) and invivo reproductive toxicity testing (lowest observed adverse effect level on reproductive system). We found the invitro coculture model could classify the tested compounds into 4 clusters, and identify the most toxic reproductive substances, with high concordance, sensitivity, and specificity of 84%, 86.21%, and 100%, respectively. We observed a strong correlation of IC50 between the invitro coculture model and the invivo testing results. Our results suggest that this novel invitro coculture model may be useful for screening testicular toxicants and prioritize chemicals for further assessment in the future.

Organophosphate-induced changes in the PKA regulatory function of Swiss Cheese/NTE lead to behavioral deficits and neurodegeneration

Organophosphate-induced delayed neuropathy (OPIDN) is a Wallerian-type axonopathy that occurs weeks after exposure to certain organophosphates (OPs). OPs have been shown to bind to Neuropathy Target Esterase (NTE), thereby inhibiting its enzymatic activity. However, only OPs that also induce the so-called aging reaction cause OPIDN. This reaction results in the release and possible transfer of a side group from the bound OP to NTE and it has been suggested that this induces an unknown toxic function of NTE. To further investigate the mechanisms of aging OPs, we used Drosophila, which expresses a functionally conserved orthologue of NTE named Swiss Cheese (SWS). Treating flies with the organophosporous compound tri-ortho-cresyl phosphate (TOCP) resulted in behavioral deficits and neurodegeneration two weeks after exposure, symptoms similar to the delayed effects observed in other models. In addition, we found that primary neurons showed signs of axonal degeneration within an hour after treatment. Surprisingly, increasing the levels of SWS, and thereby its enzymatic activity after exposure, did not ameliorate these phenotypes. In contrast, reducing SWS levels protected from TOCP-induced degeneration and behavioral deficits but did not affect the axonopathy observed in cell culture. Besides its enzymatic activity as a phospholipase, SWS also acts as regulatory PKA subunit, binding and inhibiting the C3 catalytic subunit. Measuring PKA activity in TOCP treated flies revealed a significant decrease that was also confirmed in treated rat hippocampal neurons. Flies expressing additional PKA-C3 were protected from the behavioral and degenerative phenotypes caused by TOCP exposure whereas primary neurons were not. In addition, knocking-down PKA-C3 caused similar behavioral and degenerative phenotypes as TOCP treatment. We therefore propose a model in which OP-modified SWS cannot release PKA-C3 and that the resulting loss of PKA-C3 activity plays a crucial role in developing the delayed symptoms of OPIDN but not in the acute toxicity.

Pathologic effects of butylated triphenyl phosphate-based hydraulic fluid and tricresyl phosphate on the adrenal gland, ovary, and testis in the Fischer-344 rat

Triaryl phosphates including tricresyl phosphate (TCP) and butylated triphenyl phosphates (BTPs) are used in the commercial manufacture of plastics, lubricants, and hydraulic fluids. Recent reports implicate these compounds as endocrine and reproductive toxicants in rodents. The objectives of this study were to develop and characterize a rat model to investigate the mechanism(s) of toxicity of triaryl phosphate-based hydraulic fluids and to elucidate potential mechanistic pathways of toxicity through studies of structural/functional relationships. Groups of male and female rats received daily oral doses of either sesame oil alone or 1.7 g/kg of BTP or 0.4 g/kg TCP in sesame oil vehicle or 2.8 g/kg neat BTP for 20, 40, and 60 days. Light microscopic, morphometric, ultrastructural, and histochemical studies revealed hypertrophy and cholesteryl lipidosis of adrenocortical (both sexes) and ovarian interstitial cells that were progressive with duration of exposure. Minimal degeneration was observed in the adrenal cortex and ovary. TCP caused the most severe lesions in both the adrenal gland and ovary, but the morphologic and histochemical changes produced were similar for both compounds, suggesting a common mechanism of toxicity. Decreased testicular weight and degeneration of seminiferous tubules were detected only in TCP-treated rats. The Fischer-344 rat model using TCP and BTP administered by gavage is a valuable system to study mechanisms of endocrine and reproductive toxicity induced by triaryl phosphate-based hydraulic fluids.

Sensitivity of Sertoli and Leydig cells to xenobiotics in in vitro models

Different chemicals are known to cause testicular damage in the human male and experimental animals. However, the ability to assess the potential and mechanism of action leading to chemically-induced damage in men has been hampered by a lack of good predictive models. Although many of these chemicals were found to impair reproductive capacity in various laboratory animals, only some have caused reproductive damage in men. Mammalian spermatogenesis takes places within the avascular seminiferous tubules of the testis. Specialized tight junctions, which form between adjacent Sertoli cells at the time of puberty, divide the tubular space into the basal and adluminal compartments, and create a "blood-testis" barrier that restricts passage of substances and ions from the circulation. Thus, the completion of meiosis and post-meiotic germ cell differentiation, which take place in the adluminal compartment, are isolated from circulating substances unable to cross the blood-testis barrier. It seems feasible, therefore, that damage to the germ cells induced by testicular toxicants may be mediated through other cells in the testis such as the Sertoli, peritubular, or Leydig cells. A recently developed two-compartment system for culture of testicular cells can simulate, to some degree, the normal physiologic conditions. In principle, Sertoli cells isolated from mammalian testes are cultured on a permeable support (that is millipore filter) between two fluid compartments. They form a highly polarized epithelial layer with characteristic tight junctions that restrict the passage of substances between the two compartments, in analogy to the blood-testis barrier. We believe this system provides an excellent in vitro model for determining the ability of chemicals to: a) alter the permeability of the blood-testis barrier, b) impair the secretory function of Sertoli cells, or c) affect their viability, all of which could indirectly affect the germ cells. We have utilized this system for examining the effects of cadmium chloride (CdCl2) and other toxic substances known to affect the testis. The Leydig cell toxicity was investigated in testicular perfusion system or cultures of isolated Leydig cells.