Hydroxyl safflower yellow A regulates bone-fat balance in osteoporosis by SphK1/S1P/S1PR signaling pathway

Osteoporosis is hallmarked by marrow adiposity, whereas the involvement of sphingosine kinase-1(SphK1)/sphingosine 1-phosphate (S1P)/sphingosine 1-phosphate receptors(S1PR) mediated signaling in adipocyte/osteoblast lineage commitment remains elusive. While Hydroxysafflor yellow A (HSYA) attenuates estrogen deficiency-induced bone loss, its pharmacological mechanisms remain incompletely elucidated. Our investigations in ovariectomized (OVX) murine models revealed that SphK1 ablation diminished osteoblast-specific markers (Procollagen type I N-terminal propeptide [PINP], Osteocalcin [OCN], Osteoprotegerin [OPG]), disrupted trabecular microarchitecture, and exacerbated adipose conversion through suppression of SphK1/S1PR2 coupled with Peroxisome proliferator-activated receptor gamma (PPARγ) upregulation. Pharmacological HSYA administration normalized serum estradiol and S1P concentrations, reactivated SphK1/S1P/S1PR2 axis activity, and stimulated osteogenic differentiation (Runt-related transcription factor 2 [RUNX2], Bone Morphogenetic Protein 2 [BMP2]) while concurrently inhibiting adipogenic pathways (PPARγ, CCAAT/Enhancer Binding Protein Alpha [C/EBP-α], Fatty Acid Binding Protein 4[FABP4]), leading to enhanced bone structural integrity. SphK1-deficient OVX mice displayed hypoesrogenemia, diminished bone trabecular volumetric density, and increased marrow adipocyte infiltration, concomitant with transcriptional downregulation of RUNX2 and upregulation of PPARγ. HSYA intervention reversed these pathophysiological alterations, elevating S1P levels and SphK1/S1PR2 expression, decreased S1PR1/3 protein expression and downstream PPARγ suppression, while activating S1PR2. Mechanistic analyses demonstrated that HSYA regulated mesenchymal stem cell fate determination through SphK1/S1P/S1PR2 axis modulation, evidenced by PPARγ suppression and RUNX2 activation. This study identifies the SphK1/S1PR2 axis as a critical therapeutic target and establishes HSYA as a promising therapeutic agent for osteoporosis by orchestrating mesenchymal stem cell differentiation toward osteogenesis.