Increased proteasomal workload in differentiating B lymphocytes is associated with higher apoptotic sensitivity to PIs. (A) Increased rapid protein degradation in LPS-stimulated B cells. I.29μ⁺ cells stimulated with LPS for 0 or 3 days were pulsed for 30 minutes with ³⁵S aminoacids and chased for the indicated times, with or without MG-132 (2 μM). The data indicate the percentage of TCA-insoluble radioactivity, the disappearance of which was inhibited by MG-132 at any given timepoint, relative to the total radioactivity present at the end of the pulse (*P < .05). (B) Increased RDPs in LPS-stimulated B cells. I.29μ⁺ cells cultured for 0 or 3 days with LPS were pulsed for 10 minutes and chased for 15 minutes, with or without MG-132. The bars show the percentage of radioactive polypeptides degraded by proteasomes during the chase (*P < .05). (C) Increased apoptotic sensitivity in LPS-stimulated I.29μ⁺ cells. Cells were stimulated for 3 days with LPS or left untreated, and then treated for 5 hours with the indicated concentrations of bortezomib (Btz). The proportion of apoptotic (annexin V⁺ propidium iodide⁻) cells was measured by FACS analysis. One of 3 representative experiments is shown.