Proteasome inhibitors down-regulate Btk expression both in B-cell lines and primary B cells. (A) A20 cells were treated with different concentrations of MG132 for 16 hours. Cells were lysed and subjected to Western blot analysis. (B) Nalm6 pre-B cells were treated with ALLN (150 μM), MG-132 (10 μM), lactacystin (10 μM), or DMSO alone for 16 hours. Cells were lysed and subjected to Western blot analysis. (C) Purified splenic B cells from C57BL/6 wild-type mice were cultured in the absence or presence of 10 μM MG132 for 5, 10, and 15 hours as indicated in the figure. Total cell lysates were processed for immunoblotting analysis for Btk and actin.