Differential Regulation of IL-1β and IL-6 Release in Murine Macrophages

Asbestos and silica (exogenous danger) and adenosine triphosphate (ATP, endogenous danger-signaling molecule) synergistically increase IL-1β release from endotoxin-primed macrophage, which is mediated by NOD-like receptor protein 3 (NLRP3) inflammasome. However, the conversion of pro-IL-1β to its active form seems to depend on the macrophage cell types. In the present study, bone marrow-derived macrophages (BMM) and three murine macrophage cell lines, J774.1, J774A.1, and RAW264.7 were exposed to ATP or fibrous titanium dioxide (FTiO₂) in the presence or absence of lipopolysaccharide (LPS), and the concentrations of IL-1β and IL-6 in both cell lysates and in the culture media were measured by immunoblotting to differentiate active form of IL-1β from pro-IL-1β. IL-1β release was synergistically increased when the cells were exposed to both LPS and ATP or FTiO₂, while IL-6 was readily released by LPS alone. IL-1β released into the culture medium was pro-IL-1β in J774.1 and RAW264.7, and most of the pro-IL-1β remained inside the cells. In contrast, the active form of IL-1β was released together with pro-IL-1β from J774A.1 and BMM after the co-stimulation. J774A.1 and BMM express apoptosis-associated speck-like protein contains a carboxyl-terminal CARD (ASC) while J774.1 and RAW264.7 do not or only faintly express ASC, and accordingly, caspase-1, which converts pro-IL-1β to its active form, is activated only in J774A.1 and BMM. Collectively, the canonical inflammasome pathway is not activated in J774.1 and RAW264.7, and the apparent synergistical increase of IL-1β in the culture medium mostly reflects the leakage of pro-IL-1β from these cells.