Primulagenin A is a potent inverse agonist of the nuclear receptor RAR-related orphan receptor gamma (RORγ)

Throughout history, herbal medicines and natural products have played a crucial role as therapeutics for humans, yet their molecular mechanisms of action often remain elusive. Here, we ask whether primulagenin A (PGA) from the traditionally used herbal substance Primula root, acts via the nuclear receptor ROR{gamma}, a key regulator of pro-inflammatory Th17 cells, which are linked to autoimmune diseases like psoriasis. Luciferase assays revealed a high potency (IC50 ~100 nM) and efficacy (Imax ~ 90%) of PGA as an inverse agonist of ROR{gamma}. To ensure sufficient supply, we established methods to isolate and synthesize PGA. Its binding to the human ROR{gamma} ligand binding domain was confirmed by nano differential scanning fluorimetry, and a structure-activity relationship was proposed by docking and site-directed mutagenesis. PGA downregulated ROR{gamma} target gene expression and inhibited murine and human Th17 differentiation in a concentration-dependent manner. It also reduced the proportion of IL-17A-producing Th17 cells. In this work, we identify PGA as a new, potent, and efficacious inverse agonist of ROR{gamma}, with potential for modulating immune responses in inflammatory and autoimmune diseases.