Pannexin-1 couples to maitotoxin- and nigericin-induced interleukin-1β release through a dye uptake-independent pathway

Pannexin-1 is a recently identified membrane protein that can act as a nonselective pore permeable to dyes such as ethidium when ectopically expressed. Blockade of pannexin-1 in macrophage endogenously expressing the ATP-gated P2X(7) receptor (P2X(7)R) blocks the initial dye uptake, but not the ionic current, and also blocks processing and release of interleukin-1 beta (IL-1 beta) in response to P2X(7)R activation. These results suggest that pannexin-1 may be a hemichannel activated by the P2X(7)R to provide the conduit for dye uptake and downstream signaling to processing and release of IL-1 beta. We have pursued this hypothesis by measuring dye uptake and IL-1 beta processing and release in mouse J774 macrophage in response to P2X(7)R activation and to maitotoxin and nigericin, two agents considered to evoke IL-1 beta release via the same mechanism. The experiments were carried out over time periods during which no lactate dehydrogenase was released from cells to examine only noncytolytic pathways. P2X(7)R activation evoked dye uptake that could be separated into two components by pannexin-1 inhibition: an initial rapid phase and a slower pannexin-1-independent phase. Maitotoxin-evoked dye uptake was unaltered by pannexin-1 inhibition. Nigericin did not induce dye uptake. Inhibition of pannexin-1 blocked caspase-1 and IL-1 beta processing and release in response to all three stimuli. Thus, although pannexin-1 is required for IL-1 beta release in response to maitotoxin, nigericin, and ATP, a mechanism distinct from pannexin-1 hemichannel activation must underlie the former two processes.