ADAM‐17‐independent shedding of L‐selectin

L‐selectin is expressed by leukocytes and facilitates their adhesion under flow along the walls of blood vessels. As do a variety of membrane proteins, L‐selectin undergoes ectodomain shedding. Using approaches that monitor full‐length L‐selectin in short‐term assays, it has been determined that L‐selectin shedding is defective in tumor necrosis factor α‐converting enzyme (ADAM‐17)‐deficient cells. In this study, we examined the steady‐state levels of L‐selectin on ADAM‐17‐deficient cells using a monoclonal antibody to the cytoplasmic region of L‐selectin, which allows for the detection of total L‐selectin (full‐length and the membrane‐associated cleavage fragment). We demonstrate that ADAM‐17‐deficient cells generate a 6‐kDa transmembrane fragment of L‐selectin. Although inducible L‐selctin shedding by phorbol 12‐myristate 13‐acetate stimulation was not observed by these cells in short‐term assays, basal turnover did occur, resulting in the production of soluble L‐selectin, as determined by enzyme‐linked immunosorbent assay. L‐selectin turnover was greatly increased upon ADAM‐17 reconstitution. Truncating the juxtamembrane region of L‐selectin blocked ADAM‐17‐independent shedding as did a hydroxymate metalloprotease inhibitor. Together, these findings demonstrate that a metalloprotease activity separate from ADAM‐17 can use the cleavage domain of L‐selectin. We speculate that separate proteolytic mechanisms of L‐selectin shedding may regulate distinct antiadhesive mechanisms, such as inducible shedding for the rapid dissociation of cell–cell interactions and constitutive shedding for the homeostatic maintenance of high serum levels of soluble L‐selectin, a potential adhesion buffer.