Resumen de Assessing the carcinogenic risk of water disinfection by-products: Use of in vitro models to evaluate halonitromethanes and haloacetic acids /
Disinfection by-products (DBPs) are all those chemical species that are formed during the disinfection of water. To date, more than 600 species of DBPs have been identified in chemically-disinfected waters all over the world. Although epidemiological studies show a clear link between cancer, especially bladder cancer, and DBPs exposure, none of the individual species alone has shown sufficient carcinogenic potency to account for the increased cancer risk.
To date, several different models and protocols have been used, attempting to establish the specific genotoxic and carcinogenic properties of individual DBP species. Still, most in vitro hazard evaluation has focused on acute exposures to relatively high concentrations of these chemicals, followed by an extrapolation to low concentrations. Thus, current assessment methods led to results that do not reflect real exposure scenarios. Additionally, the lack of a more systematic methodology to assess the carcinogenic potential of DBPs generated gaps in the literature that must be resolved.
In this Thesis we propose the application of an in vitro carcinogenic risk assessment methodology based on the evaluation of a wide range of cell transformation markers, whereby we analyse the carcinogenic potential and long-term effects of two DBPs groups, halonitromethanes (HNMs) and haloacetic acids (HAAs). The principal advantage of such methodology is the possibility to analyse prolonged exposures to more realistic concentrations of these groups, thus, setting a more realistic exposure model.
Results from our first study indicate that HNMs are not capable of inducing tumoral transformation in human pulmonary cells after an 8 weeks’ exposure, nor to prompt exposed cells to induce or enhance tumoral growth. The evaluation of fibroblasts grown in contact with the CM of exposed cells, emulating the tumoral stroma, showed that, in the exposure period, stromal cells did not acquire the capacity to induce tumoral growth.
Evaluation of MMPs secretion determined that neither exposed cells nor stromal cells could secrete these proteinases that would facilitate tumoral migration and metastasis.
Our second study showed that although all three selected mono-HAAs appeared capable of inducing oxidative DNA damage after acute exposures, exposures to non-cytotoxic concentrations do not appear to be genotoxic after 8 weeks of exposure. On the contrary, cells long-term exposed to non-cytotoxic concentrations of the selected HAAs developed resistance to DNA-oxidising agents. In vitro long-term carcinogenicity assessment of HAAs proved them unable to initiate tumorigenic transformation in urothelial cells nor in cells emulating the tumoral stroma. In addition, cells were not capable of inducing nor enhancing tumoral growth in any case.
Altogether, we conclude that the use of cell transformation assays, based on prolonged exposures to low concentrations, emulating a more realistic exposure scenario, is a more consistent manner to assess the carcinogenicity of DBPs. Additionally, analyses performed in this Thesis suggest that the increased cancer incidence linked to DBPs exposure cannot be attributed to HNMs nor HAAs.
