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Several membrane-bound proteins with a single transmembrane domain are subjected to limited proteolysis at the cell surface. This cleavage leads to the release of their biologically active ectodomains, which can trigger different signalling pathways. In many cases, this ectodomain shedding is mediated by members of the family of a disintegrins and metalloproteinases (ADAMs). ADAM17 in particular is responsible for the cleavage of several proinflammatory mediators, growth factors, receptors and adhesion molecules. Due to its direct involvement in the release of these signalling molecules, ADAM17 can be positively and negatively involved in various physiological processes as well as in inflammatory, fibrotic and malignant pathologies. This central role of ADAM17 in a variety of processes requires strict multi-level regulation, including phosphorylation, various conformational changes and endogenous inhibitors. Recent research has shown that an early, crucial control mechanism is interaction with certain adapter proteins identified as iRhom1 and iRhom2, which are pseudoproteases of the rhomboid superfamily. Thus, iRhoms have also a decisive influence on physiological and pathophysiological signalling processes regulated by ADAM17. Their characteristic gene expression profiles, the specific consequences of gene knockouts and finally the occurrence of disease-associated mutations suggest that iRhom1 and iRhom2 undergo different gene regulation in order to fulfil their function in different cell types and are therefore only partially redundant. Therefore, there is not only interest in ADAM17, but also in iRhoms as therapeutic targets. However, to exploit the therapeutic potential, the regulation of ADAM17 activity and in particular its interaction with iRhoms must be well understood. • ADAM17 mediates ectodomain shedding of various mediators. • Thereby, ADAM17 is involved in physiological processes and several pathologies. • ADAM17 is regulated at multiple posttranslational levels. • Maturation and activity of ADAM17 require the adapter proteins iRhom1 and iRhom2. • iRhom1 and iRhom2 are only partially redundant. [ABSTRACT FROM AUTHOR]

By mediating proteolytic shedding on the cell surface the disintegrin and metalloproteinases ADAM10 and ADAM17 function as critical regulators of growth factors, cytokines and adhesion molecules. We here report that stimulation of lung epithelial A549 tumor cells with phorbol-12-myristate-13-acetate (PMA) leads to the downregulation of the surface expressed mature form of ADAM17 without affecting ADAM10 expression. This reduction could not be sufficiently explained by metalloproteinase-mediated degradation, dynamin-mediated internalization or microdomain redistribution of ADAM17. Instead, surface downregulation of ADAM17 was correlated with the presence of its mature form in exosomes. Exosomal ADAM17 release was also observed in monocytic and primary endothelial cells where it could be induced by stimulation with lipopolysaccharide. Antibody-mediated surface labelling of ADAM17 revealed that at least part of exosomal ADAM17 was oriented with the metalloproteinase domain outside and had been expressed on the cell surface. Suppression of iRHOM2-mediated ADAM17 maturation prevented surface expression and exosomal release of ADAM17. Further, deletion of the protease's C-terminus or cell treatment with a calcium chelator diminished exosomal release as well as surface downregulation of ADAM17, underlining that both processes are closely associated. Co-incubation of ADAM17 containing exosomes with cells expressing the ADAM17 substrates TGFα or amphiregulin lead to increased shedding of both substrates. This was prevented when exosomes were prepared from cells with shRNA-mediated ADAM17 knockdown. These data indicate that cell stimulation can downregulate expression of mature ADAM17 from the cell surface and induce release of exosomal ADAM17, which can then distribute and contribute to substrate shedding on more distant cells. [ABSTRACT FROM AUTHOR]