Cancer cells often trigger an inflammatory process, which in some cases may be driven by the presence of lysyl-tRNA synthetase (KRS) in the medium. Kim et al. (2017. J. Cell Biol.https://doi.org/10.1083/jcb.201605118) now demonstrate that cleavage of the KRS by caspase-8 inside cells triggers its interaction with syntenin and its release in inflammatory exosomes.
Aminoacyl-tRNA synthetases (ARSs) are intriguing enzymes. Intracellularly, they catalyze the covalent attachment of amino acids to tRNAs and are key regulators of protein translation. However, these housekeeping enzymes have many other tricks up their sleeves. In the cell, several ARSs are also able to regulate gene expression at the level of transcription, splicing, and translation but via noncatalytic and unique mechanisms. ARSs also have different functions in the extracellular space, where they can elicit cytokine signaling responses that control angiogenesis, induce immune and proinflammatory gene expression programs, and trigger cell migration or apoptosis. The response elicited by particular ARSs is specific to the target cells (Son et al., 2014). For instance, when present in the extracellular medium, lysyl-tRNA synthetase (KRS) binds to macrophages and monocytes and activates MAPK signaling pathways that induce macrophage migration and TNF production (Park et al., 2005). How ARSs are released to the extracellular medium to carry out these activities is unknown. ARSs do not contain a signal peptide, and pharmacological agents blocking secretion through the secretory pathway have no effect on the amount of ARS in the medium. For a while, the presence of ARSs in the extracellular medium was thus thought to be caused by their passive release from cells that have undergone necrosis. In this issue, Kim et al. reinvestigate how KRS is released from cancer cells and find that this occurs through a caspase-8– and syntenin-dependent incorporation of KRS in exosomes.