Potassium Regulates IL-1β Processing Via Calcium-Independent Phospholipase A2

Abstract

We report that potassium leakage from cells leads to activation of the Ca2+-independent phospholipase A2 (iPLA2), and the latter plays a pivotal role in regulating the cleavage of pro-IL-1β by the IL-converting enzyme caspase-1 in human monocytes. K+ efflux led to increases of cellular levels of glycerophosphocholine, an unambiguous indicator of phospholipase A2 activation. Both maturation of IL-1β and formation of glycerophosphocholine were blocked by bromoenol lactone, the specific iPLA2 inhibitor. Bromoenol lactone-dependent inhibition of IL-1β processing was not due to perturbation of the export machinery for pro-IL-1β and IL-1β or to caspase-1 suppression. Conspicuously, activation of Ca2+-dependent phospholipase A2 did not support but rather suppressed IL-1β processing. Thus, our findings reveal a specific role for iPLA2 activation in the sequence of events underlying IL-1β maturation.

Ca2+ Stores and Ca2+ Entry Differentially Contribute to the Release of IL-1β and IL-1α from Murine Macrophages

“…6), and the purely metabotropic stimulus platelet-activating factor (data not shown) failed to induce IL-1␤ release. In addition, treatment of the cells with ionomycin correlated with a high level of cell death that was accompanied by the release of only pro-IL-1␤, and is consistent with previous studies (14,16).…”

“…These studies suggest that K ϩ movement or local concentration influences the activity of caspase-1 directly or indirectly (15). In addition, K ϩ leakage from human monocytes activates Ca 2ϩ -independent phospholipase A 2 , which is essential for IL-1␤ maturation and release, while activation of Ca 2ϩ -dependent phospholipase A 2 by the Ca 2ϩ ionophore A23187 inhibits IL-1␤ maturation (16). Taken together, these data suggest that maturation and release of IL-1␤ are dependent on K ϩ efflux from the cell, and that Ca 2ϩ has inhibitory effects on IL-1 secretion.…”

ATP is released by monocytes stimulated with pathogen-sensing receptor ligands and induces IL-1β and IL-18 secretion in an autocrine way

“…The entire set of PRRs is likely redundant, with extensive cross-talk and possibly positive or negative feedback controls (3). The idea of a large number of different sensors converging on relatively few signaling pathways, at least for a common inflammatory response such as IL-1␤ secretion, is supported by the observation that the same biochemical events previously identified as mediators of ATPinduced secretion [K ϩ eff lux (26), activation of calciumindependent (27) and calcium-dependent (9) PLA 2 , deacetylation events (28)] are involved in the secretion of mature IL-1␤ driven by engagement of all of the PRRs tested.…”

“…As shown in Fig. 2A, the calcium-dependent PLA 2 blocker arachidonyl trif luoromethylketone (AACOCF 3 ) or the calciumindependent PLA 2 inhibitor bromoenol lactone (BEL) (9,27) significantly prevented IL-1␤ secretion induced by all of the stimuli tested, in the absence of any sign of cell toxicity (data not shown). A significant inhibition of IL-1␤ secretion was also obtained with the HDAC inhibitors trichostatin A or ITF2357 (Fig.…”

“…Also, in LPS-activated primary human monocytes exogenous ATP strongly accelerates IL-1␤ processing and secretion through a series of events (25), including efflux of K ϩ from the cell (25,26), Ca 2ϩ influx, and activation of phospholipases (9,25,27). However, LPS alone is sufficient to induce secretion of IL-1␤, although at a low extent and with slow kinetics (5,28,29).…”