Constitutive shedding of the amyloid precursor protein ectodomain is up-regulated by tumour necrosis factor-alpha converting enzyme

The amyloid precursor protein (A-PP) of Alzheimer's disease is a transmembr ane protein that is cleaved within its extracellular domain, liberating a s oluble N-terminal fragment (sAPP alpha). Putative mediators of this process include three members of the ADAM (a disintegrin and metalloprotease) fami ly, ADAM9, ADAM10 and ADAM17/TACE (tumour necrosis factor-alpha converting enzyme). Tumour necrosis factor-alpha protease inhibitor (TA-PI-1), an inhi bitor of ADAMs, reduced constitutive and muscarinic receptor-stimulated sAP P alpha release in HEK-293 cells stably expressing M3 muscarinic receptors. However, the former was less sensitive to TAPI-1 (IC50 = 8.09 muM) than th e latter (IC50 = 3.61 muM), suggesting that these processes may be mediated by different metalloproteases. Constitutive sAPP alpha release was increas ed several-fold in cells transiently transfected with TACE, and this increa se was proportional to TACE expression. In contrast, muscarinic-receptor-ac tivated sAPP alpha release was not altered in TACE transfectants. TACE-depe ndent constitutive release of co-transfected APP(695) was inhibited by TAPI -1 with an IC50 of 0.92 muM, a value significantly lower than the IC(50)s f or inhibition of either constitutive or receptor-regulated sAPP alpha shedd ing mediated by endogenous secretases. The results indicate that TACE is ca pable of catalysing constitutive alpha -secretory cleavage of APP, but it i s likely that additional members of the ADAM family mediate endogenous cons titutive and receptor-coupled release of sAPP alpha in HEK-293 cells.