Sesquiterpenoids from Tussilago farfara: full structure elucidation, anti-diabetic and anti-inflammatory signaling pathways

Four new sesquiterpenoids (1−4), including the first reported instance of a novel 2,3-seco oplopane carbon skeleton (1), together with 19 known analogues, were isolated from the flower buds of Tussilago farfara (coltsfoot). The challenging determination of relative and absolute configurations—particularly in flexible side chains and substituents—was achieved for the first time through an integrated approach combining spectroscopic analyses, chemical derivatization, chiral gas chromatography (GC), and quantum chemical calculations. All compounds were evaluated for anti-diabetic activity using an insulin-stimulated glucose uptake model in C2C12 myotubes and for anti-inflammatory activity via a lipopolysaccharide (LPS)-induced nitric oxide (NO) inhibition assay in RAW264.7 macrophages. Six compounds significantly enhanced glucose uptake, and mechanistic investigation of compound 3 revealed activation of the insulin receptor substrate 1 (IRS-1)/protein kinase B (Akt)/glycogen synthase kinase 3β (GSK-3β) signaling pathway. Twenty-one compounds exhibited marked inhibition of NO production; among them, compounds 2 and 6 dose-dependently suppressed inducible nitric oxide synthase (iNOS) expression and nuclear factor κB (NF-κB) phosphorylation.