Aryl hydrocarbon receptor is regulated via multiple mechanisms in human keratinocytes

Aryl hydrocarbon receptor (AhR) is a basic helix-loop-helix transcription factor activated by polycyclic aromatic hydrocarbons of synthetic and natural origin. While a number of novel AhR ligands have been recently identified, little is known about their possible influence on AhR levels and stability. We used western blot, qRT-PCR and immunocytochemistry to determine the effects of AhR ligands on AhR expression in N-TERT (N-TERT1) immortalized human keratinocytes, and immunohistochemistry to assess patterns of AhR expression in human and mouse skin and skin appendages. While AhR was highly expressed in cultured keratinocytes and in the skin, it was found primarily in the cytoplasm, but not in the nucleus, suggesting its inactivity. At the same time, treatment of N-TERT cells with proteasomal inhibitor MG132 and eventual inhibition of AhR degradation resulted in nuclear AhR accumulation. Treatment of keratinocytes with AhR ligands such as TCDD, FICZ, caused near-complete disappearance of AhR, and treatment with I3C resulted in substantially diminished level of AhR possibly due to ligand-induced AhR degradation. The AhR decay was blocked by proteasome inhibition, indicating degradation-based mechanism of regulation. Additionally, AhR decay was blocked by ligand-selective AhR antagonist CH223191, implying substrate-induced mechanism of degradation. Furthermore, degradation of AhR was blocked in N-TERT cells with knockdown of AhR dimerization partner ARNT (HIF1β), suggesting that ARNT is required for AhR proteolysis. However, addition of hypoxia mimetics (HIF1 pathway activators) CoCl₂ and DMOG had only minor effects on degradation of AhR. Additionally, inhibition of HDACs with Trichostatin A resulted in enhanced expression of AhR in both untreated and ligand-treated cells. These results demonstrate that in immortalized epidermal keratinocytes AhR is primarily regulated post-translationally via proteasome-mediated degradation, and suggest potential means to manipulate AhR levels and signaling in the skin. Overall, the AhR is regulated via multiple mechanisms, including proteasomal ligand- and ARNT-dependent degradation, and transcriptional regulation by HDACs, implying complex system of balancing its expression and protein stability.