FIG. 6.

C/EBPβ activation by serum growth factors. (A) L cells were transfected with WT C/EBPβ (lanes 1 to 5) or S273A (lanes 6 to 10), serum starved overnight, and restimulated with 15% FBS for the indicated times. Equivalent C/EBPβ expression at each time point was verified by immunoblotting (bottom). (B) Kinetics of RSK2 activation in serum-stimulated cells. The extracts described in panel A were immunoprecipitated with RSK2 antibody and assayed for autophosphorylation by incubation with [γ-³²P]ATP. RSK2 activity in Ras^V12-transfected cells is shown in lane 6. (C) L cells were transfected with C/EBPβ vector with or without dn-RSK1. The cells were serum starved for 16 h and switched to complete growth medium containing 15% FBS, and extracts were prepared at the indicated time points.