Pinoresinol diglucoside alleviates hindlimb unloading-induced bone loss in mice

Microgravity-induced bone loss has long been a critical issue in space exploration. While countermeasures have been suggested, none have achieved the desired effects. Pinoresinol diglucoside (PDG), an effective constituent of the medicinal herb Eucommia ulmoidis Oliv, was reported to increase peak bone mass in growing rats and mitigate dexamethasone-induced osteoporosis. However, it is unknown if it affects microgravity-induced bone loss. Here, PDG effects were investigated in cultured rat calvarial osteoblasts exposed to simulated microgravity (SMG) and in hindlimb-unloaded mice. PDG (1 × 10⁶ mol/L) prevented SMG-induced osteoblast osteogenesis reduction and oxidative stress (increased levels of oxidative markers and decreased activities of antioxidant enzymes). It alleviated cell proliferation suppression, apoptosis and cell cycle arrest. In hindlimb-suspended mice orally administrated with PDG for 21 days, unloading-induced reduction of femoral bone mineral density, deterioration of bone microstructure and strength, and reduction of osteogenic potential of bone marrow stromal cells were significantly reversed at three dosages tested. It alleviated unloading-induced bone loss (decreased serum bone formation marker but increased resorption marker levels), osteocyte apoptosis (changes in TUNEL staining and apoptosis marker expression), oxidative damages (increased serum 8-isoPGF2α and 8-OHdG levels), inflammation (increased serum inflammatory cytokine levels), and increased bone resorption signal (bone and serum RANKL/OPG expression ratio). These results demonstrated that PDG effectively counteracted hindlimb unloading-induced bone loss by inhibiting osteocyte apoptosis, preventing oxidative stress and inflammation, preserving bone formation and inhibiting resorption. PDG is a good candidate to be further tested for its potential use in preventing spaceflight-induced bone loss and disuse osteoporosis.