A gamma-glutamyl peptide from onion inhibits the development and activity of osteoclasts in vitro

Recently we have shown that supplementing the diet of rats with onion powder (Allium cepa L.) leads to a significant inhibition of bone resorption [1]. Gamma-glutamyl-propenyl-cysteine-sulfoxide (GPCS) was identified as the active compound by bioassay-guided fractionation [2]. In the present study we further characterized the effects of GPCS on osteoclasts (OC) in vitro. OC development was examined by culturing mouse bone marrow cells (BMC) supplemented with macrophage colony-stimulating factor (M-CSF; 30ng/ml), receptor activator of NF-κB ligand (RANKL; 5ng/ml) and varying doses of GPCS. To establish structure-activity relationship and specificity, other dipeptides were tested with the same assay, too. The effect of GPCS on the activity of OC was assessed by determining the number of actin rings in isolated rat OC attached to glass coverslips and by quantifying the number of pits formed by OC cultured on dentin slices. GPCS inhibited osteoclastogenesis from BMC at concentrations ≥1 mM (n=12; p<0.05). Sulfur containing compounds with structural similarity to GPCS reduced the number of newly formed OC (gamma-glutamyl-cysteine-ethylester: ≥0.3 mM; glycyl-cysteine: 10 mM; allyl-cysteine: 10 mM). Furthermore, compounds without sulfur (aspartyl-phenylalanine-methylester, glycyl-valine, gamma-glutamyl-glycine) did not affect osteoclastogenesis. The number of actin rings formed on glass after pre-incubation for 3h was not changed by treatment with 2 and 8 mM GPCS for 10 and 25min, respectively. When 2, 4, and 8 mM of GPCS was added to OC cultured on dentin slices, 8 mM GPCS reduced significantly resorption per OC (number of pits/OC; control: 100%±30; 8 mM GPCS: 51%±27; mean±SD, n=16; p<0.05). In conclusion, the data demonstrate that the protective effects of onion on bone may be caused at least in part by a direct effect on the cells of the OC lineage, resulting in an inhibition of osteoclastogenesis and the activity of mature OC.