Protective concentrations of IL-6 do not activate RAF-1. 8226 and AF-10 MM cells were cultured with or without IL-6 (1,000 U/mL) or PMA (500 nmol/L) for 5 minutes. RAF-1 was then immunoprecipitated and tested for enzymatic activity against the substrate GST-MEK in an in vitro kinase assay. Similar results, ie, a lack of IL-6–dependent activation of RAF-1 in UCLA #1 cells was also shown (not shown). Bottom panel shows Coomassie blue staining of the gel.