Ah receptor mediated toxicity : studies in vitro and in a transgenic mouse model

Abstract

The polychlorinated dibenzo-p-dioxins, dibenzofurans and biphenyls are ubiquitously present in the environment. Humans are exposed mainly through diet to continuous low levels which on average are in close proximity to those considered tolerable. The foetus and newborn infant are considered as a particular risk group. From exposure studies of laboratory animals, exposure during early life-stages leads to the most sensitive effects and the male reproductive organs are among the most susceptible target tissues for in utero and lactational TCDD exposure. The aryl hydrocarbon receptor (AhR) mediates most, if not all, of the toxic effects of dioxins. To further improve risk assessment, mechanistic understanding is required.

Previous studies of the Constitutively Activated-AhR (CA-AhR) mouse model have shown that some well-known effects of dioxins appear in the CA-AhR mouse, that the CA-AhR is expressed in numerous tissues and that the functional activity of the mutated Ah receptor corresponds to a relatively low dose of TCDD, resembling the human exposure situation.

The purpose of the present thesis work was to characterize the phenotype of the CA-AhR with regard to vital organs, reproductive organs and bone tissue, a non-reproductive tissue under hormonal regulation, and to further explore the molecular interaction of AhR and Estrogen Receptor (ER) dependent pathways. A persistent increase in the heart weight was seen only in the male CA-AhR mice possibly indicating a gender specific susceptibility to effects of an activated AhR in the heart. Bone tissue of both male and female CA-AhR mice were also shown to be affected and the effects of CA-AhR were different with regard to the two different types of bone, cortical and trabecular and females seemed to be more affected than males in contrast to the result from the heart. Several endpoints of male reproductive toxicology such as decrease in ventral prostate weight and epididymal sperm count, regarded as sensitive signs of in utero/lactational TCDD exposure were affected in the CA-AhR mice.

The weight of uterus was decreased in adult and increased in immature CA-AhR mice possibly indicating both anti-estrogenic and estrogenic properties of the CA-AhR in relation to the status of the ERs. This may be associated with the result from in vitro studies showing the AhR partner factor Arnt efficiently coactivating ER dependent transcription. Most of the effects observed in the CA-AhR mouse model in the present thesis could be considered modest indicating a relatively low activity of the CA-AhR resembling the human exposure scenario.

This thesis present data showing a novel finding of the function of Arnt and that the CA-AhR mouse can serve as a useful model for continuous low exposure to AhR ligands, where the mechanistical understanding of the most sensitive effects on early development can be further studied and which, in the future may help improving risk assessment of dioxin-like AhR ligands.