Ketone body 3‐hydroxybutyrate mitigates apoptosis and enhances osteogenesis in bone organoid construction via the <scp>cAMP</scp> / <scp>PKA</scp> / <scp>CREB</scp> signaling pathway

Bone organoids offer potential for bone regeneration and in vitro organ models. Current limitations in bone organoid culture systems include low efficiency in construction and functionality, as well as increased apoptosis in prolonged cultures of larger sizes. The ketone body 3-hydroxybutyrate (3HB), synthesized in the liver, addresses these challenges effectively. Our findings suggest that 3HB increases intracellular calcium ion (Ca²⁺) levels in human bone marrow-derived mesenchymal stem cells (hBMSCs) by activating the hydroxycarboxylic acid receptor 2 (HCAR2). This activation initiates the cAMP/PKA/CREB pathway, which elevates the expression of anti-apoptotic genes such as myeloid cell leukemia 1 (MCL1) and B cell lymphoma 2-related protein A1 (BCL2A1), thereby reducing apoptosis. Furthermore, this pathway boosts the expression of osteogenic proteins, including Runt-related transcription factor 2 (RUNX2) and bone morphogenetic protein 2 (BMP2), facilitating osteogenesis in bone organoids. Consequently, 3HB may enhance the construction of bone organoids that are more mature, larger, and have longer viability.