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

The amyloid precursor protein (A-PP) of Alzheimer's disease is a transmembrane protein that is cleaved within its extracellular domain, liberating a soluble N-terminal fragment (sAPP alpha). Putative mediators of this process include three members of the ADAM (a disintegrin and metalloprotease) family, ADAM9, ADAM10 and ADAM17/TACE (tumour necrosis factor-alpha converting enzyme). Tumour necrosis factor-alpha protease inhibitor (TA-PI-1), an inhibitor of ADAMs, reduced constitutive and muscarinic receptor-stimulated sAPP 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 the latter (IC50 = 3.61 muM), suggesting that these processes may be mediated by different metalloproteases. Constitutive sAPP alpha release was increased several-fold in cells transiently transfected with TACE, and this increase was proportional to TACE expression. In contrast, muscarinic-receptor-activated sAPP alpha release was not altered in TACE transfectants. TACE-dependent 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 for inhibition of either constitutive or receptor-regulated sAPP alpha shedding mediated by endogenous secretases. The results indicate that TACE is capable of catalysing constitutive alpha -secretory cleavage of APP, but it is likely that additional members of the ADAM family mediate endogenous constitutive and receptor-coupled release of sAPP alpha in HEK-293 cells.