ObjectiveTo investigate the effects of Zuoguiwan on osteoclast activation induced by breast cancer and its mechanism. MethodsTo simulate breast cancer-induced osteoclastic bone metastasis， RAW264.7 cells were cultured in conditioned medium containing 50% supernatant of MDA-MB-231 breast cancer cells. The dosages of Zuoguiwan used in the experiment were sera containing 5% and 10% Zuoguiwan. Tartrate-resistant acid phosphatase （TRAP） staining was used to detect osteoclast activation. Enzyme-linked immunosorbent assay （ELISA） was used to measure Cathepsin K secretion from RAW264.7 cells. Real-time quantitative polymerase chain reaction （PCR） was used to detect the mRNA expression levels of osteocalcin （OCN） and bone sialoprotein （BSP）. Immunoprecipitation was employed to detect the interaction between Runt-related transcription factor 2 （Runx2） and core binding factor β subunit （CBF-β）. Western blot was used to assess the protein expression of Runx2， phosphorylated Runx2 （p-Runx2）， extracellular signal-regulated kinases 1/2 （ERK1/2）， p-ERK1/2， p38 mitogen-activated protein kinase （MAPK）， p-p38 MAPK， and CBF-β. ResultsCompared with the blank group， the MDA-MB-231 cell supernatant group showed a significant increase in TRAP-positive cell counts and Cathepsin K secretion. Meanwhile， the expression levels of p-Runx2， Runx2-CBF-β interaction， BSP and OCN mRNA， p-p38 MAPK， and p-ERK1/2 proteins were significantly decreased （P<0.01）. Compared with the MDA-MB-231 cell supernatant group， Zuoguiwan-containing sera significantly reduced TRAP-positive cell counts and Cathepsin K secretion （P<0.01）， significantly increased p-Runx2， BSP and OCN mRNA expression， as well as p-p38 MAPK and p-ERK1/2 protein levels， and promoted the interaction between Runx2 and CBF-β （P<0.01）. No significant change in Runx2 expression was observed. Compared to the blank group， the BVD-523 group showed significantly lower expression of p-p38 MAPK and p-ERK1/2 proteins （P<0.01）. Compared with the BVD-523 group， both low and high concentration Zuoguiwan-containing sera groups showed significantly higher p-p38 MAPK expression （P<0.01）， and the high concentration Zuoguiwan group also exhibited a significant increase in p-ERK1/2 expression （P<0.01）， while no statistical difference was found in the low-dose group. ConclusionZuoguiwan inhibits osteoclast activation by inducing phosphorylation of the key transcriptional regulator Runx2 in intra-osteoclast bone formation， and this process is closely associated with the activation of the p38 MAPK/ERK signaling pathway.