RANKL-induced down-regulation of CX3CR1 via PI3K/Akt signaling pathway suppresses Fractalkine/CX3CL1-induced cellular responses in RAW264.7 cells

The receptor activator of nuclear factor-κB ligand (RANKL) is essential for osteoclast differentiation. In this study, we examined the effects of RANKL on chemokine receptor expression in osteoclast precursor cells, RAW264.7 cells. CX3CL1 (also called Fractalkine) receptor, CX3CR1 mRNA expression, was rapidly reduced by treatment with RANKL in contrast to the increased expression of CCR1 and tartrate-resistant acid phosphatase (TRAP). This reduction occurred within 12 h and was maintained for 5 days during osteoclastogenesis. Inhibitors of phosphatidylinositol 3-kinase (PI3K) and Akt, but not mitogen-activated protein kinases, restored the RANKL-induced reduction of CX3CR1 mRNA. The stability of CX3CR1 mRNA was not changed, suggesting transcriptional repression by RANKL. The down-regulation of CX3CR1 mRNA correlated with the suppression of CX3CL1-induced activation of Akt and ERK as well as chemotaxis. These results suggest a potential role for decreased CX3CL1-CX3CR1 interaction in osteoclastogenesis.