The physiological effects of the flavone, apigenin on bone cells were studied. We first show that apigenin inhibits tumor necrosis factor alpha (TNFalpha)- and interferon gamma (IFNgamma)-induced secretion of several osteoclastogenic cytokines from MC3T3-E1 mouse calvarial osteoblast cell line. Ligands of the TNF receptor family constitute the most potent osteoclastic cytokines. In MC3T3-E1 cells, apigenin dose-dependently (from 5 to 20 microM) inhibits TNFalpha-induced production of the osteoclastogenic cytokines, IL-6 (interleukin-6), RANTES (regulated upon activation, normal T cell-expressed and -secreted), monocyte chemoattractant protein-1 (MCP-1) and MCP-3. In addition, apigenin inhibits IFNgamma-stimulated secretion of monokines, CXCL-9, and -10 in MC3T3-E1 cells. Next, we show that apigenin strongly inhibits differentiation of 3T3-L1 preadipocytes to adipocytes with attendant inhibition of adipocyte differentiation-induced IL-6, MCP-1, and leptin production. Inhibition of adipogenic differentiation by apigenin could be due to induction of osteogensis as it robustly upregulates mRNA levels of bone morphogenetic protein-6 (BMP-6). Finally, the presence of apigenin inhibited osteoclast differentiation from the RAW 264.7 cell line by reducing receptor activator of nuclear factor kappa ligand (RANKL)-induced expression of tartrate-resistant acid phosphatase (TRAP), RANK, and calcitonin receptor but not CCR1, resulting in the inhibition of multinucleated osteoclast formation. Similarly, apigenin inhibited expression of the osteoclast differentiation markers TRAP, RANK, and c-Fms in osteoclast precursor cells obtained from mouse bone marrow following treatment with RANKL and macrophage colony stimulating factor (MCSF). Furthermore, apigenin induced apoptosis of mature osteoclasts obtained from rabbit long bone and inhibited bone resorption. In all instances, a structurally related compound, flavone had no significant effect. These data suggest that apigenin has multiple effects on all three bone cells that could prevent bone loss in vivo.