<unk>  and dioxin-like compounds act via a specific receptor present in all cells: the aryl hydrocarbon (AH) receptor. This receptor is a transcription factor which is involved in the expression of genes; it has been shown that high doses of<unk>  either increase or decrease the expression of several hundred genes in rats. Genes of enzymes activating the breakdown of foreign and often toxic compounds are classic examples of such genes (enzyme induction).<unk>  increases the enzymes breaking down, e.g., carcinogenic polycyclic hydrocarbons such as benzo(a)pyrene. These polycyclic hydrocarbons also activate the AH receptor, but less than<unk>  and only temporarily. Even many natural compounds present in vegetables cause some activation of the AH receptor. This phenomenon can be viewed as adaptive and beneficial, because it protects the organism from toxic and carcinogenic substances. Excessive and persistent stimulation of AH receptor, however, leads to a multitude of adverse effects. The physiological function of the AH receptor has been the subject of continuous research. One obvious function is to increase the activity of enzymes breaking down foreign chemicals or normal chemicals of the body as needed. There seem to be many other functions, however, related to the development of various organs and the immune systems or other regulatory functions. The AH receptor is phylogenetically highly conserved transcription factor with a history of at least 600 million years and is found in all vertebrates. Its ancient analogs are important regulatory proteins even in more primitive species. In fact, knock-out animals with no AH receptor are prone to illness and developmental problems. Taken together, this implies the necessity of a basal degree of AH receptor activation to achieve normal physiological function.
2,3,7,8-Tetrachlorodibenzodioxin
Alpha-Hexachlorocyclohexane
Beta-Hexachlorocyclohexane
1,4-Dioxin
Polychlorinated dibenzodioxins
Coal tar
N-Nitrosodimethylamine
Polychlorinated biphenyl