Effects of cadmium, copper, and zinc on βAPP processing and turnover in COS-7 and PC12 cells

The effects of cadmium, copper, and zinc on βAPP metabolism were investigated in COS-7 and PC12 cells. Cadmium chloride (CdCl 2 ) increased βAPP steady-state protein levels and decreased βAPP posttranslational processing. These changes were not accompanied by alterations in βAPP mRNA levels or splicing. In addition, cytosolic α-actin and G3PDH levels were not affected. Further, neither zinc (ZnCl 2 ) nor copper (CuSO 4 ) altered βAPP levels or affected its normal processing. Pulse-chase studies revealed that the rate of βAPP maturation decreased twofold in the presence of 25 μ M CdCl 2 compared to untreated controls. βAPP secretion from the cell also dramatically slowed. These two factors result in the accumulation of partially processed βAPP inside cells. The presence of CdCl 2 also decreased the amount of an 8-kDa βAPP C-terminal fragment, indicating that the cellular compartment in which βAPP accumulates is not accessible to α-secretase. Studies using Brefeldin A suggest that this compartment may be the cis or medial Golgi. However, Aβ production was proportionately increased. These data show that CdCl 2 can modulate the βAPP cleavage to favor Aβ. Finally, βAPP mis -metabolism was shown to be unrelated to the hsp70 induction elicited by CdCl 2 ; both heat shock and CuSO 4 induced hsp70 but had no effect on steady-state levels of βAPP, although heat shock did slow βAPP maturation. These data indicate that hsp70 alone cannot chaperone βAPP through an alternate processing pathway leading to Aβ production.