Validation of CDK5 as a sensitizer hit. (A) CDK5 expression in myeloma cells. The expression of CDK5 on the Affymetrix U133A chip across the different samples in MMRC database is represented. The samples are grouped according to their diagnosis. (B) Confirming sensitizing activity using multiple CDK5 siRNAs. Control siRNA, CDK5_4, CDK5_10, and CDK5 On-TargetPlus smart pool were loaded on a 384-well plate at the final concentration of 13nM. After adding lipofectamine 2000 and KMS11 cells, the plate was incubated at 37°C for 24 hours. The vehicle and bortezomib at the indicated doses were added. The cell viability was determined at 96 hours by CellTitre-Glo luminescence. The data at each point represent the mean values of 6 wells (mean ± SD). (C-E) CDK5 kinase activity is required for bortezomib sensitization. KMS11 cell were infected with lentiviruses expressing either wild-type CDK5 or “kinase dead” dominant negative CDK5. The expression of exogenous both CDK5 proteins (C) and cell responses to bortezomib and dexamethasone were assessed and presented (D-E).