In the processing of APP, α- and β-secretase pathways compete with each other for cleaving APP. Therefore, physiologically these two secretases are likely to colocalize in the same subcellular compartments. Previously β-secretase cleavage of APP was found in the endoplasmic reticulum (ER). We herein tested whether α-secretase cleavage is also detected in the ER. We used experimental system of COS7 cells transfected with cDNA encoding human APP695, and the cell lysates and media were examined for its proteolytic products. When APP expression is concentrated in the ER by BFA-mediated transport inhibition or by using mutant APP harboring an ER-retrieval motif, α-secretase product sAPPα was accumulated in the cells. Immunofluorescence microscopy revealed that the ER-targeted APP produced intracellular accumulation of sAPPα, colocalizing with an ER marker. These results indicate that α-secretase cleavage of APP occurs in the ER. Further we examined the effects of phorbol ester PDBu, a direct activator of PKC, on the α-secretase and β-secretase cleavages of APP occurring in the ER. Treatment with PDBu of COS7 cells transfected with the ER-targeted APP increased production of sAPPα and conversely decreased production of β-secretase product sAPPβ. Thus, in the ER, α-secretase competes with β-secretase for cleaving APP and such competitive correlation might modulate the production of Aβ42 found in this compartment.
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