Fig. 2. MKK7, but not MKK4, is involved in the SAPK activation and cell death induced by serum withdrawal (SW) and C₂-ceramide. (A) The U937 cells were exposed to a serum-depleted medium or 0.06 mM C₂-ceramide. At the end of the indicated incubation periods, the MKK7 activity was analyzed (top). U937/pcDNA and U937/MKK7 cells received the same treatments for 30 minutes. SAPK activities in the treated and untreated cells were compared (bottom). (B) U937 cells were treated with the indicated stimuli for 15 minutes, and MKK4 activity was measured (top). U937/pcDNA and U937/MKK4 cells received the indicated treatments for 30 minutes, and the SAPK activity was compared (bottom). (C) The U937/pcDNA, U937/MKK4 and U937/MKK7 cells were treated with either the serum-depleted medium or C₂-ceramide for 48 hours. The viability of the treated and untreated cells was compared by flow cytometry.