Sirenin and bisabolane sesquiterpenoids from two marine isolates of Penicillium chermesinum with anti-osteoporotic activity by modulating NOD-like receptor signaling pathway

Nine unreported (chermesirenins A–I, 1–9) and one known (eupenicisirenin C, 10) sirenin sesquiterpenoids as well as two unreported bisabolane-type sesquiterpenoids (chermebisabolins A and B, 11 and 12) were isolated from two isolates of marine-sourced fungal strains Penicillium chermesinum EN-480 and AS-400, which were obtained as endophytic and endozoic fungi from the marine red alga Pterocladiella tenuis and the marine chiton Liolophura japonica, respectively. The sirenin sesquiterpenoids (1–9) featured with unique 6/3 dicyclic ring system while compounds 8 and 9 are nor-sirenin analogs by degrading the C-12 methyl group. Their structures including stereochemical configurations were confirmed through NMR interpretation, ECD analysis, Mosher's method, X-ray crystallographic diffraction, and quantum chemical calculations. The anti-osteoporotic activity evaluation in the zebrafish osteoporosis model revealed that chermesirenin A (1) could significantly alleviate the reduction of bone fluorescence area and fluorescence density in prednisolone-treated zebrafish. Furthermore, compound 1 could down-regulate the expression of the genes associated with prednisolone-induced osteoclastogenesis, ctsk and tnfrsf11b, to improve bone formation status. Exploration on the mechanism through the transcriptomic analysis showed that compound 1 might act on the NOD-like receptor (NLR) signaling pathway involved in inflammatory responses and bone metabolism regulation, which has not been reported among previous investigations on sesquiterpenoids. These results demonstrated that compound 1 deserved further development as a potential candidate towards the therapy of osteoporotic disease.