Serum amyloid A inhibits RANKL-induced osteoclast formation

When mouse bone marrow-derived macrophages were stimulated with serum amyloid A (SAA), which is a major acute-phase protein, there was strong inhibition of osteoclast formation induced by the receptor activator of nuclear factor kappaB ligand. SAA not only markedly blocked the expression of several osteoclast-associated genes (TNF receptor-associated factor 6 and osteoclast-associated receptor) but also strongly induced the expression of negative regulators (MafB and interferon regulatory factor 8). Moreover, SAA decreased c-fms expression on the cell surface via shedding of the c-fms extracellular domain. SAA also restrained the fusion of osteoclast precursors by blocking intracellular ATP release. This inhibitory response of SAA is not mediated by the well-known SAA receptors (formyl peptide receptor 2, Toll-like receptor 2 (TLR2) or TLR4). These findings provide insight into a novel inhibitory role of SAA in osteoclastogenesis and suggest that SAA is an important endogenous modulator that regulates bone homeostasis.open

Serum Amyloid A Induces NLRP-3-Mediated IL-1β Secretion in Neutrophils

Background/Aims: Serum amyloid A (SAA) is an acute phase reactant with significant immunological activities, including effects on cytokine synthesis and neutrophil chemotaxis. Neutrophils can also release cytokines with proinflammatory properties. IL-1β is a key proinflammatory cytokine, the secretion of which is controlled by inflammasome. We investigated the proinflammatory effects of SAA in vitro in relation to the NLRP3 inflammasome in neutrophils. Methodology/Principal Findings: Human neutrophils isolated form healthy subjects were stimulated with serum amyloid A (SAA). The cellular supernatants were analyzed by western blot using anti-IL-1β or anti-caspase-1 antibodies. IL-1β or Nod-like receptor family, pyrin domain containing 3 (NLRP3) mRNA expressions were analyzed by real-time PCR or reverse transcription-PCR (RT-PCR) method. SAA stimulation induced pro-IL-1β mRNA expression in neutrophils. Furthermore, SAA engaged the caspase-1-activating inflammasome, resulting in the production of active IL-1β. SAA-induced pro-IL-1β expression was marginally suppressed by the Syk specific inhibitor, R406, and SAA-induced pro-IL-1β processing in neutrophils was prevented by R406. Furthermore, SAA-induced NLRP3 mRNA expression was completely blocked by R406. Analysis of intracellular signaling revealed that SAA stimulation activated the tyrosine kinase Syk and mitogen-activated protein kinase (MAPK). Conclusions/Significance: These results demonstrate that the innate neutrophil immune response against SAA involves a two-step activation process: an initial signal promoting expression of pro-IL-1β and a second signal involving Syk-dependent activation of the NLRP3 inflammasome and caspase-1, allowing processing of pro-IL-1β and secretion of mature IL-1β