SP1 and p23 play a crucial role in the circadian target gene induction of activated aryl hydrocarbon receptor in human breast cells.

The Aryl Hydrocarbon Receptor (AHR) is a crucial mediator of cellular responses upon exposure to environmental pollutants. Initially described as central activator in xenobiotic metabolism, recent research has unveiled additional layers of complexity in AHR function and regulation. The circadian rhythm is a fundamental regulatory process that modulates various physiological processes, including AHR activity. Our recent findings show that AHR-dependent gene induction is subject to circadian rhythmicity. While some studies suggest a circadian AHR gene transcription in various tissues, a comprehensive mechanistic understanding of the circadian AHR regulation remains elusive. This mechanistic study aimed to elucidate the circadian regulation of AHR target gene induction upon dioxin treatment in human breast cells. To acquire a more profound understanding of the intricacies of AHR regulation, we conducted a systematic analysis of the molecular co-factors and their interactions in circadian synchronized cells. Our results show circadian regulation of AHR transcriptional activity at the CYP1A1 promoter upon dioxin treatment. This appears to be orchestrated by the core clock components BMAL1/CLOCK, which directly interact with AHR in circadian synchronized cells. Additionally, we identified SP1 as an important positive and p23 as an essential negative regulator of circadian AHR activity. The understanding of these interactions is crucial for elucidating the molecular relationship between the circadian clock and cellular responses to environmental stimuli. Such knowledge is of vital importance for the application of New Approach Methods (NAMs) as part of a weight-of-evidence (WoE) approach in the next generation of risk assessments.