Dual Anti-inflammatory Actions of a Novel Retinoid X Receptor Agonist Derived from a Natural Compound in Microglial Cells

Abstract

Microglia-mediated neuroinflammation plays a critical role in the onset and progression of Alzheimer’s disease. In a previous study, we synthesized 6-hydroxy-3′-propyl-[1,1′-biphenyl]-3-propanoic acid (6OHA) based on the structure of magnaldehyde B, a natural compound that our group identified as a retinoid X receptor (RXR) agonist. However, its potential effects on inflammation in microglial cells remain unexplored. In this study, we specifically focused on the early-phase inflammatory responses to lipopolysaccharide (LPS) and evaluated the inhibitory effects of 6OHA on BV-2 microglial cells following 2h of LPS exposure. Similar to the existing RXR agonist bexarotene (Bex), 6OHA treatment (0.1 and 1µM) resulted in a dose-dependent decrease in the mRNA levels of proinflammatory mediators, including interleukin-1β (Il1b), Il6, and inducible nitric oxide synthase. However, these effects on proinflammatory mediators were effectively abolished by the RXR antagonist UVI3003. Additionally, 6OHA promoted M2 microglia polarization after 24h of treatment, as evidenced by the increased mRNA levels of the M2 marker genes arginase-1 (Arg1), C-C motif chemokine ligand 6 (Ccl6), Ccl17, and Ccl22. Notably, 6OHA induced a distinct set of M2 microglial markers compared with IL-4, a known M2 microglial inducer. Furthermore, the transcription of Arg1, a key M2 marker gene, is regulated by retinoic acid receptor/RXR heterodimers and the IL-4 signaling pathway. Collectively, 6OHA suppressed the early inflammatory responses to LPS and promoted M2 microglial polarization through a mechanism distinct from that of IL-4. Therefore, RXR agonists, including 6OHA and Bex, may exhibit dual anti-inflammatory effects and serve as novel modulators of neuroinflammation.