RSK2 knockdown inhibited myeloma growth and enhanced lenalidomide-mediated myeloma cytotoxicity. (A) JJN3 (left), MM1.S (middle), and H929 (right) cells were infected with control virus (NT) and RSK2 shRNAs (#52, #54, and #55) lentivirus, and immunoblotting was performed at 48 or 72 hours after virus infection to confirm RSK2 knockdown by shRNAs. (B) Cell viability was measured by MTT assay at day 6 after infection. The data from cells infected with RSK2 shRNAs were normalized to NT control. (C) Escalating doses of lenalidomide were added at 24 hours after virus infection and cell viability was measured at day 5 after treatment (the data from each treatment were normalized to vehicle-treated control). shRNA (#52) in H929 cells and shRNA (#55) in MM1.S cells alone were lethal to most of those cells and we were not able to evaluate drug response in them.