Hypoxia perturbs aryl hydrocarbon receptor signaling and CYP1A1 expression induced by PCB 126 in human skin and liver-derived cell lines

Abstract

The aryl hydrocarbon receptor (AhR) is an important mediator of toxic responses after exposure to xenobiotics including 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and dioxin-like polychlorinated biphenyls (PCBs). Activation of AhR responsive genes requires AhR dimerization with the aryl hydrocarbon receptor nuclear translocator (ARNT), a heterodimeric partner also shared by the hypoxia-inducible factor-1α (HIF-1α) protein. TCDD-stimulated AhR transcriptional activity can be influenced by hypoxia; however, it less well known whether hypoxia interferes with AhR transcriptional transactivation in the context of PCB-mediated AhR activation in human cells. Elucidation of this interaction is important in liver hepatocytes which extensively metabolize ingested PCBs and experience varying degrees of oxygen tension during normal physiologic function. This study was designed to assess the effect of hypoxia on AhR transcriptional responses after exposure to 3,3',4,4',5-pentachlorobiphenyl (PCB 126). Exposure to 1% O2 prior to PCB 126 treatment significantly inhibited CYP1A1 mRNA and protein expression in human HepG2 and HaCaT cells. CYP1A1 transcriptional activation was significantly decreased upon PCB 126 stimulation under conditions of hypoxia. Additionally, hypoxia pre-treatment reduced PCB 126 induced AhR binding to CYP1 target gene promoters. Importantly, ARNT overexpression rescued cells from the inhibitory effect of hypoxia on XRE-luciferase reporter activity. Therefore, the mechanism of interference of the signaling crosstalk between the AhR and hypoxia pathways appears to be at least in part dependent on ARNT availability. Our results show that AhR activation and CYP1A1 expression induced by PCB 126 were significantly inhibited by hypoxia and hypoxia might therefore play an important role in PCB metabolism and toxicity.

Hypoxic Upregulation of ARNT (HIF-1β): A Cell-Specific Attribute with Clinical Implications

“…This study provided evidence for a HIF-1α-mediated inhibition of AhR signalling by sequestration of ARNT [26]. Vorrink et al [27] observed similar effects again in human hepatocellular carcinoma HepG2 cells and in the human keratinocyte HaCaT cell line. One major advantage of this study was the genuine hypoxic exposure of cells instead of stimulation with hypoxia mimetics such as cobalt chloride.…”

“…Moreover, forced ARNT expression was sufficient to overcome the inhibitory effect of hypoxia on AhR signalling. The authors concluded that ARNT is sequestered by HIF-1α in hypoxia thus limiting the availability of this transcription factor for AhR heterodimerisation [27]. Noteworthily, another report published nearly two decades ago claims the complete opposite [28].…”

Endocrine, biotransformation, and oxidative stress responses in salmon hepatocytes exposed to chemically induced hypoxia and perfluorooctane sulfonamide (PFOSA), given singly or in combination

“…Recent studies showed that AhR activation and cyp1a1 expression induced by PCB126 were significantly inhibited by hypoxia, suggesting a possible important role of hypoxia in xenobiotic metabolism (Vorrink et al 2014). Furthermore, biological processes regulated by hif-1α were negatively affected by PCB126, potentially affecting adaptive responses and cell survival in hypoxic environments (Vorrink et al 2014). In the present study, gene expression was increased after 48h while enzyme activities were affected after 24 h, showing time-related difference between mRNA and enzyme activity.…”

“…Herein, we observed time and exposure related changes of cyp1a, cyp3a and pxr mRNA, albeit PFOSA alone generally did not produce any change. Recent studies showed that AhR activation and cyp1a1 expression induced by PCB126 were significantly inhibited by hypoxia, suggesting a possible important role of hypoxia in xenobiotic metabolism (Vorrink et al 2014). Furthermore, biological processes regulated by hif-1α were negatively affected by PCB126, potentially affecting adaptive responses and cell survival in hypoxic environments (Vorrink et al 2014).…”

Deepwater Horizon Oil Spill as a Case Study for Interdisciplinary Cooperation within Developmental Biology, Environmental Sciences and Physiology

“…Consider as an example the fact that environmental oxygen levels have interactive effects with numerous contaminants including PAHs [40] [41]. It has been demonstrated that hypoxic environments appear to alter endocrine signaling and associated gene expression pathways, disrupting sex ratios and reproductive output potential [42].…”