Poly aromatic hydrocarbon (PAH) compounds are pervasive environmental contaminates. PAHs are by-products of industrial processes such as, paper bleaching, pesticide production, fossil fuel combustion, and waste incineration. These compounds are highly stable and lipophilic, leading to their bio-accumulation in soil and the food chain. Exposure to PAHs elicits toxic responses in mammals. Some of these responses include chloracne, liver hyperplasia, and immune system suppression. Moreover, increased risk of diabetes, cancer, infertility, and birth defects correlate with PAH exposure. There are several families of compounds classified as PAHs. Two of the more infamous PAH groups are polychlorinated biphenyls (PCBs) and polychlorinated dibenzo-p-dioxins (PCDDs). 2,3,7,8 tetrachlorodibenzo-p-dioxin (TCDD) is the hallmark dioxin congener and one of the most toxic PAHs known. Toxic responses to PAHs are mediated through the aryl hydrocarbon receptor (AHR). The AHR is a ligand activated member of the PER-ARNT-SIM (PAS) superfamily of transcription factors. PAS protein family members act as environmental sensors. PAHs bind the AHR, instigating its translocation to the nucleus, heterodimerization, and activation of its ability to regulate the transcription of genes. Though AHR gene regulation is well characterized, a complete understanding of the receptor mediated mechanisms underlying dioxin toxicity is lacking. This project focused on AHR protein interactions and potential roles these interactions have in toxic responses to TCDD exposures. The first aim was to establish the AHR protein interaction network (AHR-PIN). Using tandem affinity purification (TAP) and mass spectrometry we have established AHR-PINs in the presence and absence of TCDD. Changes in the PIN were observed between the dosed and vehicle treated samples. In addition, flux in the network dependent on length of exposure was observed. There were a number of protein hits identified in the PINs that are of interest. These proteins include Smarcad (a DNA helicase), Cfcl and Alcam (immune response proteins), and Arf-GAP. Finally, an interaction between the AHR and ATP5α1, a subunit of the ATP synthase complex which controls cellular energy homeostasis, is of particular interest. This interaction is the first to link the AHR to mitochondrial energy production. The second aim of this project was to establish functional relevance associated with the identified protein interactions. To that end, further investigation revealed a TCDD induced AHR dependent hyperpolarization of the mitochondrial inner membrane. This novel function may provide insight into the underlying mechanisms of TCDD induced toxicity. Mitochondrial dysregulation could play a key role in wasting syndrome and metabolic diseases. The findings establish the dynamic nature of the AHR-PIN and identify a potentially novel function of the AHR in cellular energy homeostasis.
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