Regulation of interleukin-6 expression in osteoblasts by oxidized phospholipids

Epidemiological evidence suggests that cardiovascular disease is associated with osteoporosis, independent of age. Bone resorptive surface is increased in mice on a high-fat diet, and osteoclastic differentiation of bone marrow preosteoclasts is promoted by oxidized phospholipids. Because osteoclastic differentiation requires cytokines produced by osteoblasts, we hypothesized that the stimulatory mechanism of oxidized phospholipids is via induction of osteoclast-regulating cytokines in osteoblasts. To investigate the effects of oxidized phospholipids on expression of such cytokines, murine calvarial preosteoblasts, MC3T3-E1, were treated with oxidized 1-palmitoyl-2-arachidonyl-sn-glycero-3-phosphocholine (ox-PAPC), an active component of oxidized lipoproteins. Results showed that ox-PAPC increased expression of interleukin-6 (IL-6) and tumor necrosis factor-alpha. IL-6 expression was also elevated in calvarial tissues from hyperlipidemic but not in wild-type mice. Ox-PAPC also induced IL-6 protein levels in both MC3T3-E1 and primary calvarial cells. Promoter-reporter assay analysis showed that ox-PAPC, but not PAPC, induced murine IL-6 promoter activity. Effects of ox-PAPC on IL-6 expression and the promoter activity were attenuated by H89, a PKA inhibitor. Analysis of deletion and mutant IL-6 promoter constructs suggested that CAAT/enhancer binding protein (C/EBP) partly mediates the ox-PAPC effects. Taken together, the data suggest that oxidized phospholipids induce IL-6 expression in osteoblasts in part via C/EBP.