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

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 2 h of LPS exposure. Similar to the existing RXR agonist bexarotene (Bex), 6OHA treatment (0.1 and 1 μM) 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 24 h 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.