Sophoraflavanone G as an ectosteric inhibitor of cathepsin K attenuates ovariectomy-induced bone loss by suppressing bone resorption.

BACKGROUND

Cathepsin K (CTSK) is a key enzyme in bone resorption, making it a promising target for osteoporosis treatment. Active-site inhibitors of CTSK are effective but have undesirable side effects, while ectosteric inhibitors may provide a safer alternative.

PURPOSE: This study investigates whether Sophoraflavanone G (SG), derived from Rhizoma Drynariae, can act as an ectosteric CTSK inhibitor to attenuate osteoporotic bone loss and explores its underlying mechanisms.

STUDY

DESIGN

SG's effects were evaluated in an ovariectomized (OVX) osteoporotic mice model, with in vitro experiments assessing SG's interaction and binding affinity with CTSK.

METHODS

Micro-CT, histology, and mechanical testing were used to evaluate bone density and strength. CTSK activity and expression were assessed by immunohistochemistry and western blotting. Cell thermal shift assays, isothermal titration calorimetry, CTSK site-specific degradation assays, molecular docking and dynamic simulation were performed to study SG's binding affinity and inhibitory effects. Biosafety, including body weight, uterine histomorphometry, and toxicity of the heart and lung, was also assessed.

RESULTS

SG improved bone mineral density, microarchitecture, and strength, primarily by inhibiting bone resorption. It inhibited CTSK's enzymatic activity with a strong binding affinity (KD: 8.49 μM) and effectively inhibited osteoclast function. CTSK site-specific assays showed SG inhibited CTSK-mediated degradation of type I collagen. Unlike odanacatib, SG did not affect gelatin or TGF-β1 degradation in fibroblasts. Biosafety assessments revealed no adverse effects.

CONCLUSION

SG acts as an ectosteric CTSK inhibitor, offering a safer alternative for postmenopausal osteoporosis treatment by selectively inhibiting bone resorption without the side effects associated with active-site inhibitors.