Selective AhR Modulation by Tapinarof Reprograms Microglial Inflammation through the AhR-Nrf2 Axis

Neuroinflammation is a key contributor to neurodegenerative disease progression, driven by chronic microglial activation. The aryl hydrocarbon receptor (AhR) has emerged as an important regulator of inflammatory and antioxidant responses in microglia; however, AhR activation can produce divergent outcomes depending on the activating ligand. This study aimed to compare the effects of tapinarof, a bacteria-derived, nonsteroidal small-molecule therapeutic AhR agonist with immunomodulatory and antioxidant activity, with those of the classical environmental ligand benzo[a]pyrene (B[a]P) on inflammatory and antioxidant signaling in activated microglial cells. Using lipopolysaccharide (LPS)-stimulated SIM-A9 microglia, cell viability, nitric oxide production, time- and concentration-dependent gene expression, cytokine secretion, and pathway-specific knockdown of AhR and nuclear factor erythroid 2-related factor 2 (Nrf2) were assessed. Our results showed that tapinarof significantly reduced LPS-induced nitrite production and suppressed key pro-inflammatory genes while promoting selected Nrf2-associated antioxidant markers. In contrast, B[a]P did not attenuate nitric oxide production, showed weaker inhibition of inflammatory gene expression, and induced cytotoxicity at high concentration despite activating canonical AhR targets. Gene-silencing experiments showed that the effects of tapinarof were partly AhR-dependent and involved functional crosstalk with Nrf2. In conclusion, the present study demonstrates that AhR activation is ligand- and context-dependent in activated microglia and supports further evaluation of selective AhR modulation as an approach for limiting microglial inflammatory responses.