Cyclin K interacts with and inhibits the transcriptional activity of STAT3.
(A) COS cells cotransfected with STAT3 and flagged cyclin K (CK)–expressing vectors were serum starved for 24 hours and stimulated for 30 minutes with OSM (10 ng/mL; lanes 1-3) or left untreated (lane 4). Nuclear extracts were immunoprecipitated with the appropriate antibodies, and the resulting precipitate was examined by Western blot probed with polyclonal antibodies directed against STAT3 proteins (top) or its Tyr705 phosphorylated form (bottom). (B) COS, A375, and HepG2 cells were cotransfected with a luciferase reporter gene containing 2 copies of a STAT3 consensus binding site linked to a minimal thymidine kinase promoter (Tk) together with vectors expressing STAT3 in the presence or absence of a vector encoding for cyclin K. Following transfection, cells were serum starved for 15 hours and left untreated or stimulated for 6 hours with OSM (10 ng/mL). Error bars represent SD.
