| Many membrane-anchored growth factors are cleaved to release soluble forms, converting signaling from an autocrine or juxtacrine mode to a paracrine mode through a phenomenon known as ectodomain shedding. Metalloproteases often regulate shedding, and one class in particular, the disintegrin family of metalloproteases (ADAMS), has been implicated as convertases. Here, we focused on the epidermal growth factor (EGF) family of ligands (EGF, transforming growth factor-alpha (TGFalpha), amphiregulin (AR), betacellulin (BTC), heparin-binding EGF (HB-EGF), epiregulin (EPR), and epigen (EPI)) and the role of the tumor necrosis factor-a converting enzyme (TACE/ADAM17) in cleavage. In addition to the membrane-releasing C-terminal cleavage event, the EGF family members are also cleaved at membrane distal, N-terminal cleavage sites that may modulate their function. Intriguingly, the cleavage sites for this family do not share a conserved sequence. However, TACE, which has seemingly broad substrate specificity, has been implicated as a possible convertase for the EGF family. Mice homozygous for a deletion in the metalloprotease domain Zn binding pocket (TACEDeltaZn/DeltaZn) exhibit perinatal lethality due to epithelial defects in multiple tissues, phenotypes similar to EGFR-null mice. TACEDeltaZn/DeltaZn mice also share phenotypes with mice lacking the individual growth factors, including delayed eyelid fusion and wavy hair and whiskers (TGFalpha) and heart valve defects (HB-EGF). Finally, a preliminary study indicated that TGFalpha shedding was impaired 10--20 fold in TACEDeltaZn/DeltaZn embryonic fibroblasts. I participated in a study demonstrating that restoration of TACE activity to TACEDeltaZn/DeltaZn fibroblasts enhanced TGFalpha, AR, and HB-EGF shedding and that TACE correctly cleaved the N- and C-terminal sites of proTGFalpha in vitro. Subsequently, I demonstrated that another metalloprotease activity could correctly cleave the C-terminal site of TGFalpha in vitro. To characterize cleavage of other EGF family members, I showed that BTC and EPR shedding were not enhanced by restoring TACE to TACEDeltaZn/DeltaZn cells, and while TACE cleaved N- and C-terminal sites of proAR and proHB-EGF in vitro , only N-terminal sites of BTC and EPR were cleaved in vitro . I also characterized the C-terminal cleavage site recognition elements required for cleavage by TACE. Thus, TACE cleaves TGFalpha, AR and HB-EGF, while other metalloproteases cleave BTC and EPR. |
