Regulation of interleukin 1α secretion by inflammasomes

The crucial role of the proinflammatory cytokine interleukin 1 beta (IL-1 beta) in driving inflammatory disorders, such as Muckle-Wells syndrome and gout, has been extensively characterised. Owing to its high potency to induce inflammation the activation and secretion of IL-1 beta is tightly regulated. The sensing of various host 'dangers', including infections and metabolic deregulation, results in the formation of large protein complexes, termed inflammasomes. Formation of the inflammasomes leads to the cleavage and activation of caspase-1, which in turn proteolytically processes its substrates, including pro-IL-1 beta. Biologically active IL-1 beta is subsequently secreted by the cell. In contrast to IL-1 beta, little is known about mechanisms underlying the activation and secretion of its close homologue IL-1 alpha. Moreover, the physiological role of IL-1 alpha is still not well defined. Several studies hypothesise that IL-1 alpha serves as a danger signal, which is passively released from dying cells. However, recent studies suggest a more complex function of this cytokine. Indeed, NLRP3 inflammasome agonists such as uric acid crystal or nigericin induce IL-1 alpha cleavage and secretion, leading to the cosecretion of both IL-1 beta and IL-1 alpha. Depending on the type of NLRP3 agonist, release of IL-1 alpha is NLRP3-inflammasome/caspase-1 dependent or independent, but in both cases IL-1 alpha processing depends on calpain protease activity. Taken together, these results suggest that the promotion and progression of inflammatory diseases is not solely due to IL-1 beta but also to its close relative IL-1 alpha. This should be considered when IL-1 blockade is applied as a therapeutic strategy for diseases such as cryopyrin-associated periodic syndromes or gout.