The Complex Biology of the Aryl Hydrocarbon Receptor and Its Role in the Pituitary Gland.

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor best known for its ability to mediate the effects of environmental toxins such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD or dioxin), polycyclic aromatic hydrocarbons (PAHs), benzene, and polychlorinated biphenyls (PCBs) through the initiation of transcription of a number of metabolically active enzymes. Therefore, the AHR has been studied mostly in the context of xenobiotic signaling. However, several studies have shown that the AHR is constitutively active and plays an important role in general cell physiology, independently of its activity as a xenobiotic receptor and in the absence of exogenous ligands. Within the pituitary, activation of the AHR by environmental toxins has been implicated in disruption of gonadal development and fertility. Studies carried out predominantly in mouse models have revealed the detrimental influence of several environmental toxins on specific cell lineages of the pituitary tissue mediated by activation of AHR and its downstream effectors. Activation of AHR during fetal development adversely affected pituitary development while adult models exposed to AHR ligands demonstrated varying degrees of pituitary dysfunction. Such dysfunction may arise as a result of direct effects on pituitary cells or indirect effects on the hypothalamic-pituitary-gonadal axis. This review offers in-depth analysis of all aspects of AHR biology, with a particular focus on its role and activity within the adenohypophysis and specifically in pituitary tumorigenesis. A novel mechanism by which the AHR may play a direct role in pituitary cell proliferation and tumor formation is postulated. This review therefore attempts to cover all aspects of the AHR's role in the pituitary tissue, from fetal development to adult physiology and the pathophysiology underlying endocrine disruption and pituitary tumorigenesis.