The mutation at codons 670/671 of beta-amyloid precursor protein (beta PP) dramatically elevates amyloid beta-protein (A beta) production, Since increased A beta may be responsible for the disease phenotype identified from a Swedish kindred with familial Alzheimer's disease, evaluation of the cellular mechanism(s) responsible for the enhanced A beta release may suggest potential therapies for Alzheimer's disease, In this study, we analyzed Chinese hamster ovary cells stably transfected with either wild type beta PP (beta PP-wt) or ''Swedish'' mutant beta PP (beta PP-sw) for potential differences in beta PP processing, We confirmed that increased amounts of A beta and a beta-secretase-cleaved COOH-terminally truncated soluble beta PP (beta PPs) were secreted from beta PP-sw cells, As shown previously for beta PP-wt cells, A beta was released more slowly than the secretion of beta PPs from surface-labeled beta PP-sw cells, indicating that endocytosis of cell surface beta PP is one source of A beta production, In contrast, by [S-35]methionine metabolic labeling, the rates of A beta and beta PPs release were virtually identical for both cell lines. In addition, the identification of intracellular beta PPs and A beta shortly after pulse labeling suggests that A beta is produced in the secretory pathway. Interestingly, more A beta was present in medium from beta PP-sw cells than beta PP-wt cells after either cell surface iodination or [S-35]methionine labeling, indicating that beta PP-sw cells have enhanced A beta release in both the endocytic and secretory pathways, Furthermore, a variety of drug treatments known to affect protein processing similarly reduced A beta release from both beta PP-wt and beta PP-sw cells, Taken together, the data suggest that the processing pathway for beta PP is similar for both beta PP-wt and beta PP-sw cells and that increased A beta production by beta PP-sw cells arises from enhanced cleavage of mutant beta PP by beta-secretase, the as-yet unidentified enzyme(s) that cleaves at the NH2 terminus of A beta.
