Oxidized low density lipoprotein inhibits phosphate signaling and phosphate-induced mineralization in osteoblasts. Involvement of oxidative stress.

BACKGROUND: It is well admitted that oxidized LDL (OxLDL) plays a major role in the generation and progression of atherosclerosis. Since atherosclerosis is often accompanied by osteoporosis, the effects of OxLDL on phosphate-induced osteoblast mineralization were investigated.
METHODS: Calcium deposition, expression of osteoblast markers and inorganic phosphate (Pi) signaling were determined under OxLDL treatment.
RESULTS: OxLDL, within the range of 10-50 μg protein/ml, inhibited Pi-induced UMR106 rat osteoblast mineralization. In parallel, the expression of Cbfa1/Runx2 transcription factor was decreased, and the intracellular level of the osteoblast marker osteopontin (OPN) was reduced. The extracellular level of another marker, receptor activator of nuclear factor kappa B ligand (RANKL), was also diminished. OxLDL inhibited Pi signaling via ERK/JNK kinases and AP1/CREB transcription factors. OxLDL triggered the generation of reactive oxygen species (ROS), either in the absence or presence of Pi. Furthermore, the effects of OxLDL on Pi-induced mineralization, generation of ROS and extracellular level OPN were reproduced by the lipid extract of the particle, whereas the antioxidant vitamin E prevented them.
CONCLUSIONS: This work demonstrates that OxLDL, by generation of an oxidative stress, inhibits of Pi signaling and impairs Pi-induced osteoblast differentiation. GENERAL SIGNIFICANCE: This highlights the role of OxLDL in bone remodeling and in degenerative disorders other than atherosclerosis, especially in osteoporosis.