Serum amyloid a inhibits osteoclast differentiation to maintain macrophage function

Serum amyloid A is an acute phase protein that is elevated under inflammatory conditions. Additionally, the serum levels of serum amyloid A are associated with the progression of inflammatory arthritis; thus, serum amyloid A might be involved in the regulation of osteoclast differentiation. In the present study, we examined the effects of serumamyloid A on osteoclast differentiation and function. When bone marrow-derived macrophages, as osteoclast precursors, were stimulated with serum amyloid A in the presence of M-CSF and receptor activator of nuclear factor-kB ligand, osteoclast differentiation and its boneresorption activity were substantially inhibited. TLR2 was important in the inhibitory effect of serum amyloid A on osteoclast differentiation, because serum amyloid A stimulated TLR2. The inhibitory effect was absent in bone marrow-derived macrophages obtained from TLR2- deficient mice. Furthermore, serum amyloid A inhibited the expression of c-Fos and nuclear factor of activated T cells c1, which are crucial transcription factors for osteoclast differentiation, but prevented downregulation of IFN regulatory factor-8, a negative regulator of osteoclast differentiation. In contrast, serumamyloid A sustained the endocytic capacity of bone marrow-derived macrophages and their ability to induce the proinflammatory cytokines, IL-6, IL-1b, and TNF-a. Taken together, these results suggest that serum amyloid A, when increased by inflammatory conditions, inhibits differentiation of macrophages to osteoclasts, likely to maintain macrophage function for host defense.