Osteoclasts counteract lenalidomide-induced DKK1 up-regulation. Five primary myeloma plasma cell samples were cultured with or without osteoclasts for 2 days in the absence (orange and violet bars, respectively) or in the presence of lenalidomide (blue and red bars, respectively). (A) DKK1 relative expression was evaluated by real-time PCR; (B) c-Jun binding activity was evaluated in nuclear extracts. In panels A and B each sample is expressed as the mean ± SEM of 2 duplicates. (C) The level of JNK pathway activation was determined by Western blot analysis of JNK phosphorylated isoforms in cytosolic extracts of 3 representative samples. Number 1 represents samples cultured without osteoclasts; number 2, samples treated with lenalidomide; number 3, samples cultured with osteoclasts; and number 4, samples cultured with osteoclasts in the presence of lenalidomide.