The PS inhibitors DAPT and DupE inhibit the fusion of macrophages and prolong the induced expression of PS2. (A) Rat alveolar macrophages were cultured in fusogenic conditions and treated with increasing concentrations of DAPT or DupE for 3 days. Whereas control cells fused efficiently into multinucleated macrophages, as reflected by the extensive plasma membrane of multinucleate macrophages (^*), DAPT and DupE inhibited fusion of macrophages dose dependently, as illustrated by the lack of plasma membrane extension (bar represents 1 mm; ^*P < .001 versus control; SD; n = 6). (B) Rat alveolar macrophages were cultured in fusogenic conditions in 96-well dishes for the indicated times in the absence or presence of 10 μM DAPT, 3 μM DupE, or 50 nM JLK2. Cells were subjected to Western blotting analysis (0.2 × 10⁶ cells/lane) using antibodies directed against PS1, PS2, or GAPDH. Note that PS1 and PS2 migrated as 23-kDa cleaved amino-terminal fragments, whereas intact PS2 ran as an intact protein of 50 kDa. Also, note the lower molecular weight band of about 15 kDa that is detected in all samples, but at time zero only.