Oxidized phospholipids inhibit canonical Wnt signaling and osteoblast differentiation via ERK activation

Accumulating lines of evidence have indicated that hyperlipidemia not only is a well-established etiological factor for cardiovascular diseases (CVD), but also plays an active role in regulating bone metabolism. As a mixture of oxidized phospholipids derived from the oxidation of low density lipoprotein (LDL), oxidized 1-palmitoyl2-arachidonoyl-sn-glycero-3-phosphorylcholine (ox-PAPC) is reported to be one of the major bioactive mediators in hyperlipidemia-associated pathological events. In this study, the effects of ox-PAPC on osteoblast differentiation and the underlying molecular mechanisms were investigated. Briefly, MC3T3-E1 murine pre-osteoblast cells were treated with Wnt-3a and/or ox-PAPC, and changes in canonical Wnt signaling pathway, ERK pathway, p38 MAPK pathway and the expression levels of bone-related genes were evaluated using real-time RT-PCR, alkaline phosphatase (ALP) activity assay, Western blot and immunofluorescent staining. We found that ox-PAPC inhibited canonical Wnt signaling, which resulted in decreased expressions of bone-related genes. Although ox-PAPC treatment activated both ERK and p38 MAPK pathways, the inhibitory effects of ox-PAPC on canonical Wnt signaling and the expressions of bone-related genes were successfully reversed only by ERK inhibitor PD98059. In conclusion, oxidized phospholipids inhibit canonical Wnt signaling pathway and osteoblast differentiation, which is mediated by the activation of the ERK pathway.