mouse bone marrow macrophages

Role of NLRP3 Inflammasome and IL-1β in Osteoclastogenesis in the Presence and Absence of Lipopolysaccharide (LPS)

The NLRP3 inflammasome is known to promote bone resorption during inflammatory bone diseases like periodontitis by inducing the production of IL-1β. The NLRP3 inflammasome's role in physiological bone remodeling however remains to be determined. Alam et al. investigated the role of the NLRP3 inflammasome in osteoclastogenesis in the presence and absence of LPS.

To explore if the NLRP3 inflammasome regulates RANKL-induced osteoclast formation, Alam et al. measured NLRP3 components and IL-1β in mouse bone marrow macrophages (BMMs) treated with RANKL ± LPS. RANKL induced osteoclastogenesis in a dose-dependent manner with M-CSF (Fig. 1a). Treatment with RANKL + LPS increased the number of osteoclasts (Fig. 1b). Treatment with RANKL alone, but not LPS, upregulated Nlrp3 and Caspase-1 mRNA expression but not Asc or Il-1β (Fig. 1c) and increased NLRP3 and cleaved caspase-1 protein (Fig. 1d). With RANKL + LPS, all components (Nlrp3, Caspase-1, and Il-1β) were upregulated while Asc was downregulated. From these data, it can be concluded that RANKL is able to upregulate Nlrp3 and Caspase-1 but not Il-1β without LPS. Next, they analyzed whether IL-1β plays a role in BMMs. RANKL alone did not induce IL-1β in the supernatant or lysate (Fig. 2a). In the presence of LPS, IL-1β was detected in the lysate but not the supernatant (Fig. 2b). Alam et al. then tested the effect of recombinant IL-1 receptor antagonist (rIL-1ra) and determined that it did not affect osteoclast formation without LPS (Fig. 2c) while inhibiting it with LPS (Fig. 2d). These data suggests that IL-1β is required for osteoclastogenesis in the presence of LPS but not in its absence.