Secretion of intracellular IL-1 receptor antagonist (type 1) is dependent on P2X7 receptor activation

Abstract

Inflammatory mechanisms are critical in the arterial response to injury. Both IL-1 and the naturally occurring inhibitor of IL-1, IL-1R antagonist (IL-1ra), are expressed in the arterial wall, and in particular in the endothelium. Previous studies suggest that endothelial cells only make the intracellular type I isoform of IL-1ra (icIL-1ra1), an isoform known to lack a secretory signal peptide. It is unclear how icIL-1ra is released from the endothelial cell to act as an antagonist on cell surface IL-1 type I receptors. IL-1beta, which also lacks a secretory signal peptide, may be released by ATP stimulation of the P2X(7)R. Therefore, we examined whether icIL-1ra1 release occurs in an analogous manner, using both the mouse macrophage cell line RAW264.7 and HUVECs. P2X(7)R activation caused icIL-1ra1 release from LPS-primed RAW264.7 macrophages and from HUVECs. This release was inhibited in the absence of extracellular calcium, and attenuated by preincubation with oxidized ATP, KN62, and apyrase. Endogenous ATP release, which also facilitated release of icIL-1ra1, was detected during LPS treatment of both RAW264.7 macrophages and HUVECs. Annexin V assays showed that ATP stimulation resulted in a rapid phosphatidylserine (PS) exposure on the cell surface of RAW264.7 macrophages, and that PS-exposed microvesicles contained icIL-1ra1. However, PS flip and microvesicle shedding was not apparent in ATP-treated HUVECs. These data support a general role for the P2X(7)R in the release of leaderless cytokines into the extracellular medium, and indicate how icIL-1ra1 may act upon its extracellular target, the IL-1R.