Rheumatoid arthritis (RA) frequently leads to osteoporosis (OP) and increased fracture risk. The protein Klotho plays a recognized role in bone metabolism, yet its specific function in RA-associated osteoporosis (RA-OP) remains incompletely understood. This study investigated the molecular mechanisms by which Klotho maintains bone homeostasis in RA-OP patients.

We quantified Klotho levels in RA-OP patients and healthy controls and then conducted in vitro experiments using mouse embryonic osteoblast precursor cell line (MC3T3-E1) preosteoblastic cells to examine Klotho's effects on osteogenic differentiation and ferroptosis. We assessed osteogenic differentiation through runt-related transcription factor 2 (Runx2), collagen type i alpha 1 chain (Col1a1), and osteocalcin (Ocn) expression, while ferroptosis regulation was evaluated via glutathione peroxidase 4 (Gpx4) and Acyl-CoA synthetase long-chain family member 4 (Acsl4) expression. The interaction between fibroblast growth factor 23 (Fgf23) and fibroblast growth factor receptor 1 (Fgfr1) was analyzed using coimmunoprecipitation assays, with Fgf23's role examined through knockdown and overexpression experiments.

Results showed RA-OP patients had significantly reduced Klotho levels compared to controls. Klotho overexpression in MC3T3-E1 cells enhanced osteogenic differentiation and protected against ferroptosis by upregulating Gpx4. Mechanistically, Klotho facilitated Fgf23-Fgfr1 interaction and repressed nuclear factor κ (NF-κB) signaling.

Our findings demonstrate that Klotho mediates osteogenic action through the Fgf23/Fgfr1-NF-κB pathway while simultaneously protecting osteoblasts from ferroptosis, advancing our understanding of RA-OP pathophysiology and identifying Klotho as a promising therapeutic target for preventing RA-related bone loss.