Dextromethorphan Modulates the Osteogenic–Adipogenic Balance in Rat Bone Marrow Mesenchymal Stem Cells

Bone marrow-derived mesenchymal stem cells (BMSCs) maintain skeletal homeostasis by balancing adipogenic and osteogenic differentiation, yet clinically used drugs that bias this fate choice and their mechanisms remain incompletely defined. Here, we investigated whether dextromethorphan (DXM), a widely used antitussive, modulated lineage commitment in rat BMSCs and interrogated candidate upstream signaling modules. Rat BMSCs were induced with adipogenic medium or osteogenic medium in the presence of DXM (30 μM). Adipogenesis and osteogenesis were quantified using Oil Red O and Alizarin Red S staining with elution-based quantification, and lineage markers were measured by RT-qPCR. Intracellular Ca2+ and ROS were analyzed using flow cytometry, and the levels of p-AKT and p-ERK were assessed through Western blotting analysis. Under adipogenic induction, DXM increased lipid droplet accumulation and the mRNA levels of Pparγ and Fabp4. Although DXM elevated Ca2+ and ROS, the chelation of intracellular Ca2+ and pharmacological inhibition of Sig-1R/PLC–IP3R signaling, redox/ROS, NMDA receptors, AKT/ERK, Kv channels, bitter taste receptor-related signaling, and mTOR did not attenuate the DXM-enhanced adipogenesis. DXM reduced p-ERK without increasing p-AKT; U0126 lowered basal adipogenesis but did not block the DXM effect. Under osteogenic induction, DXM reduced matrix mineralization and downregulated Runx2 and Bglap mRNA levels, while Wwtr1 mRNA levels were not significantly changed. DXM also partially reversed the osteogenic induction-associated reduction in Mtor mRNA. Separately, under adipogenic induction, rapamycin attenuated baseline adipogenesis but did not prevent the additional lipid accumulation induced by DXM. Collectively, DXM shifted the osteogenic–adipogenic balance toward adipogenesis through a non-canonical mechanism.