Figure 4

Proteolysis plays an important role in the intracellular generation of the 69-kDa Ku86v variant protein in MM cell lines. RPMI 8226 (A) and SGH-MM5 (data not shown) MM cell lines (5.0 × 10⁶ cells/sample) were first incubated for 18 hrs with either 1× or 2× Complete™ protease inhibitor tablets (lanes 2 and 3); and then washed and checked for viability using trypan blue exclusion assay. Mock experiments in which cell lines were incubated for 18 hrs with media alone (lane 1) served as negative controls. Cell lysates were resolved by SDS-PAGE and immunoblotted using S10B1 anti-Ku86 mAb. The RPMI 8226 MM cell line s (B) was also incubated for 24 hrs with 2× Complete™ protease inhibitor tablets (lane 2), antipain (2.0 μg/mL final concentration) plus leupeptin (2.0 μg/mL final concentration) (lane 3), or aprotinin (2.0 μg/mL final concentration) plus PMSF (100 μg/mL final concentration) (lane 4) and analyzed in the same way as above. Mock experiments in which cell lines were incubated for 24 hrs with media alone (lane 1) again served as negative controls. Membranes were stripped and re-probed using anti-actin mAb (control) to confirm equal protein loading. Cell lysates were resolved by SDS-PAGE and immunoblotted using S10B1 anti-Ku86 mAb. Relative expression of 69-kDa Ku86v-DNA complexes (normalized to weakest band) was determined using image densitometry. All experiments were performed in triplicate.