Essential role for Ca2+ in regulation of IL-1β secretion by P2X7 nucleotide receptor in monocytes, macrophages, and HEK-293 cells

Interleukin (IL)-1beta is a proinflammatory cytokine that elicits the majority of its biological activity extracellularly, but the lack of a secretory signal sequence prevents its export via classic secretory pathways. Efficient externalization of IL-1beta in macrophages and monocytes can occur via stimulation of P2X(7) nucleotide receptors with extracellular ATP. However, the exact mechanisms by which the activation of these nonselective cation channels facilitates secretion of IL-1beta remain unclear. Here we demonstrate a pivotal role for a sustained increase in cytosolic Ca2+ to potentiate secretion of IL-1beta via the P2X(7) receptors. Using HEK-293 cells engineered to coexpress P2X(7) receptors with mature IL-1beta ( mIL-1beta), we show that activation of P2X(7) receptors results in a rapid secretion of mIL-1beta by a process(es) that is dependent on influx of extracellular Ca2+ and a sustained rise in cytosolic Ca2+. Moreover, reduction in extracellular Ca2+ attenuates similar to90% of P2X(7) receptor-ediated IL-1beta secretion but has no effect on enzymatic processing of precursor IL-1beta ( proIL-1beta) to mIL-1beta by caspase-1. Similar experiments with THP-1 human monocytes and Bac1.2F5 murine macrophages confirm the unique role of Ca2+ in P2X(7) receptor-mediated secretion of IL-1beta. In addition, we report that cell surface expression of P2X(7) receptors in the absence of external stimulation also results in enhanced release of IL-1beta and that this can be repressed by inhibitors of P2X(7) receptors. We clarify an essential role for Ca2+ in ATP-induced IL-1beta secretion and indicate an additional role of P2X(7) receptors as enhancers of the secretory apparatus by which IL-1beta is released.