Oxidized low density lipoprotein enhanced RANKL expression in human osteoblast-like cells. Involvement of ERK, NFkappaB and NFAT.

Background: Receptor activator of nuclear factor kappa-light-chain-enhancer of activated B cell ligand/osteoprotegerin ratio is of crucial importance in osteoclast differentiation and thus in bone dysregulation diseases.

Methods: Receptor activator of nuclear factor kappa-light-chain-enhancer of activated B cell ligand and osteoprotegerin were determined under oxidized low density lipoprotein treatment of human osteoblast-like cells. The involvement of oxidative stress, of the extracellular signal regulated kinase and of the transcription factors nuclear factor kappa-light-chain-enhancer of activated B cells and nuclear factor of activated T cells was demonstrated.

Results: Cu(2+)-oxidized low density lipoprotein increased cell-associated and extracellular receptor activator of nuclear factor kappa-light-chain-enhancer of activated B cell ligand levels whereas osteoprotegerin levels were not affected. The increase in receptor activator of nuclear factor kappa-light-chain-enhancer of activated B cell ligand was parallel to the generation of reactive oxygen species provoked by Cu(2+)-oxidized low density lipoprotein. The lipid extract of Cu(2+)-oxidized low density lipoprotein, together with other forms of oxidized low density lipoproteins such as smooth muscle cell-oxidized low density lipoprotein and myeloperoxidase-oxidized low density lipoprotein, also induced an increase in reactive oxygen species and cell-associated receptor activator of nuclear factor kappa-light-chain-enhancer of activated B cell ligand. The effect of Cu(2+)-oxidized low density lipoprotein was prevented by the antioxidant vitamin E, and mimicked by the prooxidant compounds hydrogen peroxide and buthionine sulfoximine. Inhibitors of mitogen activated protein kinase/extracellular signal regulated kinase (PD 98059), nuclear factor kappa-light-chain-enhancer of activated B cells (Ro 106-9920) and nuclear factor of activated T cells (Vivit) reduced the effect of Cu(2+)-oxidized low density lipoprotein on receptor activator of nuclear factor kappa-light-chain-enhancer of activated B cell ligand expression. Cu(2+)-oxidized low density lipoprotein signaling was also reduced by vitamin E.

General Significance: This work describes a new molecular mechanism and elucidates the signaling pathway whereby oxidized low density lipoprotein, by means of its lipid moiety, can modulate the crosstalk between osteoblasts/osteoclasts and bone remodeling, leading to an eventual risk of osteoporosis.