Test systems to identify reproductive toxicants

Toxicological assessment of industrial solvents using human cell bioassays: assessment of short-term cytotoxicity and long-term genotoxicity potential

There is an increasing demand for simple toxicological screening methods to assess the human health risk associated with exposure to environmental toxicants. Such screening tools should allow for risk evaluation in terms of both short-term/acute toxicity and longer-term genetic damage, which may lead to mutagenicity and carcinogenicity. We employed a battery of human cell bioassays using the human hepatoma cell-line, HepG2, to assess the cytotoxic and genotoxic potential of environmental pollutants. Here, we demonstrate direct application of these human cell bioassays to the toxicological assessment of a number of industrial solvents that are in common use worldwide. HepG2 cells were exposed to various solvents at concentrations ranging from 25 to 500 ppm. The cells were then analysed using specific protocols for four different adverse effects: cell death/acute cytotoxicity using a neutral red uptake assay, altered enzyme function (often an indicator of cell stress) using the ethoxyresorufin O-deethylase (EROD) bioassay, DNA single strand breaks (SSB), and DNA repair induction, which evaluates mutagenic activity. Using the positive controls, linear dose-response curves were achieved for all four bioassays. The high sensitivity of the tests allowed for environmentally meaningful assessments, and precision studies showed excellent reproducibility for all four bioassays. Comparing the results of the four bioassays on each of 16 industrial solvents allowed for ranking of the anticipated relative human toxicity of these solvents, which were comparable with data from standard toxicity tests and human occupational data. Overall, the study clearly supports the application of the HepG2 cell bioassay system for rapid toxicological screening of many candidate toxicants for both short- and long-term toxicity potential.

Genetic loss or pharmacological blockade of testes-expressed taste genes causes male sterility

TAS1R taste receptors and their associated heterotrimeric G protein gustducin are involved in sugar and amino acid sensing in taste cells and in the gastrointestinal tract. They are also strongly expressed in testis and sperm, but their functions in these tissues were previously unknown. Using mouse models, we show that the genetic absence of both TAS1R3, a component of sweet and amino acid taste receptors, and the gustducin α-subunit GNAT3 leads to male-specific sterility. To gain further insight into this effect, we generated a mouse model that expressed a humanized form of TAS1R3 susceptible to inhibition by the antilipid medication clofibrate. Sperm formation in animals without functional TAS1R3 and GNAT3 is compromised, with malformed and immotile sperm. Furthermore, clofibrate inhibition of humanized TAS1R3 in the genetic background of Tas1r3(-/-), Gnat3(-/-) doubly null mice led to inducible male sterility. These results indicate a crucial role for these extraoral "taste" molecules in sperm development and maturation. We previously reported that blocking of human TAS1R3, but not mouse TAS1R3, can be achieved by common medications or chemicals in the environment. We hypothesize that even low levels of these compounds can lower sperm count and negatively affect human male fertility, which common mouse toxicology assays would not reveal. Conversely, we speculate that TAS1R3 and GNAT3 activators may help infertile men, particularly those that are affected by some of the mentioned inhibitors and/or are diagnosed with idiopathic infertility involving signaling pathway of these receptors.

Implications of the lack of accuracy of the lifetime rodent bioassay for predicting human carcinogenicity

The NTP lifetime rodent bioassay (LRB) is the "gold standard" for predicting human carcinogenicity. Unfortunately, little attempt has been made to validate it against human carcinogenicity. Here we show that the extremely limited data available do not support either of the two common interpretations of LRB results. If a risk-avoidance interpretation is used where any positive result in a sex/species combination is considered positive, 9 of the 10 known human carcinogens tested are positive, but an implausible 22% of all chemicals are positive. If a less risk averse interpretation is used where only chemicals positive in both rats and mice are considered positive, only 3 of the 6 known human carcinogens tested are positive. In either interpretation, some known human carcinogens are not positive in the LRB, potentially allowing widespread human exposure to misidentified chemicals. Improving the predictive accuracy of the LRB and other tests for human carcinogenicity requires that test results be validated against the known human carcinogenicity of chemicals. This will require redirecting available resources from screening chemicals to validating carcinogenicity tests as well as a substantial investment in epidemiology to identify more known human carcinogens and presumed human non-carcinogens.

Bioassay of steroid hormone agonist and antagonist activities of anti-androgens on mammary gland, seminal vesicles and spleen of male mice

Young intact and adult castrated outbred C3H/Di male mice were used to characterize steroid hormone agonist and antagonist activities of anti-androgens by bioassay. Animals were injected subcutaneously with flutamide (Flut), chlormadinone acetate (CMA), cyproterone acetate (CA) or Casodex (Cas) alone or simultaneously with oestradiol (E), E plus progesterone (Prog) or norethindrone acetate (NA; a steroid exhibiting progestational and oestrogenic activities) and testosterone (T) for 15 days. Mammary gland growth was not affected with anti-androgen alone. However, all anti-androgens decreased seminal vesicle weights in intact males. In E (0.01 microg day(-1))-treated intact males, mammary growth was stimulated by CMA (progestational activity) and inhibited by CA. The inhibitory effect of CA on mammary growth (glucocorticoid activity) was overcome with high dose of E (0.1 microg day(-1)). When seminal vesicles weights were decreased with a moderate dose of E (0.01 microg day(-1)) anti-androgens injected simultaneously acted synergistically with E and decreased seminal vesicles weights more than E alone. However, in animals overloaded with E (0.1 microg day(-1)), anti-androgen CA was unable to decrease seminal vesicles weights. E (0.01 or 0.05 microg day(-1)) or E + Prog (500 or 1000 microg day(-1)) or NA (12.5 to 50 microg day(-1)) stimulated mammary growth was inhibited by T at doses 20-200 microg day(-1) and these effects were decreased or abolished by simultaneous application of Flut, CMA or Cas in both young intact and adult castrated males. In the same animals, the seminal vesicles weights were increased by T and decreased by anti-androgens. The effects of higher doses of T (300 microg day(-1)) were not inhibited by anti-androgens both in the mammary gland and seminal vesicles. Spleen weights were not consistently affected with Flut, CMA or Cas, but decreased with CA by dose dependent manner. These results demonstrated that anti-androgenic activities could be detected not only on seminal vesicle but also on the mammary gland. Our model system also detected a glucocorticoid activity of CA and progestational activity of CMA.

Application of toxicogenomics to drug development

The application of advanced modern biomedical and chemical research technologies in the pharmaceutical industry has led to a significant increase in the number of potential drug targets and lead candidates. Whereas the drug discovery process is enhanced significantly, the failure rate of new compounds due to toxicity remains very high. The pharmaceutical industry is setting high hopes on the new discipline of toxicogenomics to revolutionize the process of drug toxicity assessment by reducing the bottleneck of new drug candidates and minimizing late-stage developmental failures. Toxicogenomics is expected to facilitate the efficient screening of new compounds at an early stage, resulting in significant savings of time and cost associated with new drug development. In this review, a general description of the new discipline of toxicogenomics and its potential impact on the safety assessment of new drugs in the pharmaceutical industry is provided. An overview of the key issues and questions that are confronting investigators in this field today is also given as well as a prospective view of the future of this new discipline.