Lipid-lowering triterpenoids from the biotransformation of rotundic acid

Biotransformation of rotundic acid (1) yielded eleven previously undescribed metabolites (2-12) alongside five known metabolites (13-17). Among them, metabolites 2-4 possessed an 18,19-secoursane structure featuring a unique 6/6/6/6/5 pentacyclic ring system, formed by an oxidative cleavage. Structural elucidation of all metabolites was achieved through comprehensive spectral analysis. Critically, the absolute configuration of compound 2 was unambiguously determined via single-crystal X-ray diffraction. For compounds 3 and 4, absolute configurations were established using experimental and computational electronic circular dichroism analysis. Biological evaluation identified three potent inhibitors (5, 7 and 13) of FFA-induced TG accumulation in HepG2 cells. Notably, metabolites 5 and 13 featured a distinctive ring-opening modification in the E-ring, which conferred significant enhancement in lipid-lowering activity (TG clearance rates, 35.32%, 37.78%) compared to the prototype RA (TG clearance rate, 21.01%). The mechanism of the lipid-lowering effect of metabolite 5 and 13 might be its reduction in protein expression levels of sterol regulatory element binding protein 1c and stearoyl CoA desaturase 1 in lipid synthesis.