The impact of the aryl hydrocarbon receptor and its DNA-binding partner, ARNT, on p53-mediated apoptosis

TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin) is an environmental contaminant that has been shown to promote tumor growth within laboratory animals and its exposure to human populations has been linked to an overall increase in most cancers. While the tumor promoter ability of TCDD requires its activation of the aryl hydrocarbon receptor (AhR), the mechanism by which this receptor mediates tumor promotion is unclear. ARNT, the DNA-binding partner of AhR, has been previously determined to form homodimers in vivo and bind the same DNA response element recognized by the proto-oncogene c-Myc, CACGTG. In testing the idea of response element competition between ARNT and c-Myc, it was serendipitously discovered that ARNT over-expression enhanced c-Myc transactivation, including that of the human p53 promoter. Further characterization determined that the dimerization motifs of ARNT, the HLH (helix-loop-helix) and PAS (Per, ARNT, and Sim) domains, were necessary and sufficient for this effect. Surprisingly the effect did not require the ability of ARNT to bind DNA. While performing studies that tested the effects of TCDD and AhR activation on the p53-signaling pathway, it was discovered that pifithrin-alpha (PFTa), a p53 chemical inhibitor, is a potent AhR ligand (KI = 19.1nM). While both TCDD and PFTa pre-treatment of HepG2 cells inhibited apoptosis induction by chemotherapeutic agents, TCDD only slightly blocked UVC-induced apoptosis, suggesting that the primary effect of AhR activation is not due to inhibition of the p53-mediated signaling pathway but most likely through xenobiotic-metabolizing enzyme induction. PFTa, whose mechanism is poorly understood, inhibited p53-mediated gene transactivation as well as UVC-induced apoptosis independent of AhR and ARNT. Interestingly, AhR- and ARNT-deficient cells were discovered to be more sensitive to UVC-induced apoptosis, suggesting an endogenous role of the AhR signaling pathway in regulating cell stress response. In summary, the novel research findings were: (1) ARNT enhances c-Myc transactivation through its HLH and PAS domains; (2) pifithrin-alpha, a pharmacological inhibitor of p53, is a potent AhR ligand; and (3) cells deficient in AhR signaling are hypersensitive in apoptotic response. These findings give insight into the role of AhR and ARNT in regulating cell fate and tumor promotion.