Resumen de Carcinogènesi de l'arsènic i mecanismes associats després de (co)exposicions ambientalment rellevants a llarg termini
Arsenic is a ubiquitously distributed environmental pollutant with well-known cytotoxic, genotoxic, and carcinogenic effects. Large human populations are subjected to sustained arsenic exposure mainly via the intake of contaminated water in arsenic-rich areas. Although extensive epidemiological data has linked this exposure to an increase in the incidence of several types of cancers, the mechanisms leading to arsenic-associated carcinogenesis remain incompletely characterized. In this Thesis we have performed extended in vitro studies with the aim to gain new insight into the mechanisms of action leading to arsenic-derived effects, highly valuable for an in-depth risk assessment.
Among the described mechanisms of arsenic carcinogenesis, arsenic-induced oxidative stress and genotoxicity remains the most explored up to date. The high levels of reactive oxygen species generated during arsenic biotransformation play a crucial role in its transforming potential. Hence, the first study reported in this Thesis, confirmed that oxidative and chromosomal DNA damage progressively accumulate in cells chronically exposed to arsenic. This increment was observed up to the cells’ transformation point, and the DNA damage decreased rapidly thereafter. The arsenic-metabolizing enzyme AS3MT underwent expression changes like those of DNA damage levels and, importantly, its inhibition reduced the arsenic-induced genotoxicity. On the other hand, the stress-protective protein MTH1 was stimulated after the transformation point and its knockdown remarkably increased the levels of DNA damage and decreased the aggressiveness of the oncogenic phenotype. This demonstrates that As3mt gene function contributes to the genotoxic effects before the arsenic-induced transformation, while Mth1 prevents DNA damage fixation and allows the progression of the oncogenic phenotype.
Arsenic also induces oncogenic effects by activating stress-related signaling pathways. Hence, another point of interest of this Thesis was to assess the role of FRA1 in arsenic-induced oncogenesis. Fra1 is frequently overexpressed in tumor tissue and, accordingly, we described the progressive stimulation of its expression during the cell transformation process induced by chronic arsenic exposure. The levels of upstream FRA1 activators were monitored at the same time-points and ERK, p38, and RAS were pinpointed as potential drivers of arsenic-associated FRA1 stimulation. In turn, FRA1 overexpression potentially leads to the observed altered expression in downstream target genes such as Pten, Pdcd4, Tgfβ1, Zeb, and Twist. Further, we found that FRA1 knockdown, under chronic arsenic exposure settings, elicits a remarkable impact diminishing the features relative to cells' oncogenic phenotype. Thus, these findings demonstrate the essential role of FRA1 in the aggressiveness of the in vitro transformed phenotype induced by long-term arsenic exposure.
The characterization of new mechanisms associated to arsenic carcinogenesis is of utmost importance to obtain accurate risk estimations; however, other aspects of the exposure must be considered as well. Chemical safety research has largely proceeded through a material-by-material approach which results in an incomplete picture of contamination, human exposure and potential hazard effects. The increasing number of environmental pollutants calls for a more comprehensive hazard assessment in which the effects and behavior of mixtures of contaminants are considered. In this context, one of the objectives of this Thesis was to evaluate the impact of the long-term co-exposure to arsenic and micro- and nanoplastics (MNPLs). Remarkably, our third study demonstrated that 12-weeks of arsenic and polystyrene nanoparticles (PSNPs) combined exposure was able to enhance the cancer-like features of the cells’ transformed phenotype induced and characterized in the previous studies. In addition, our data show that arsenic-induced DNA damage was also promoted after the co-exposure. Further, we have demonstrated that both pollutants physically interact. Thus, this study brings out the need to further explore the long-term effects of emergent contaminants, such as MNPLs, and to consider co-exposures and complex mixtures when assessing their potential hazardous effects.
