Fig. 2. Bcl-X_L protects MEFs from ER stress-induced cell death. (A) Cell survival of MEFs overexpressing Bcl-X_L. bcl-xl was introduced into MEFs with retrovirus-mediated transduction system. Apaf-1^+/- (WT) or Apaf-1^-/- (KO) MEFs with or without bcl-xl transduction were treated with tunicamycin or thapsigargin for 24 hours. Cell survival was analyzed as described in Fig. 1A. (B) Caspase-3 activation in MEFs overexpressing Bcl-X_L. Cells as in A were analyzed for expression of GRP78 and for proteolytic activation of caspase-3. (C) Time course of ER stress-induced cell death of in Bcl-X_L-overexpressing MEFs. Cells as in A were analyzed for cell viability after treatment with either 1 µg/ml tunicamycin or 1 µM thapsigargin for indicated hours. Shown are mean ± s.d. Experiments were repeated three times with similar results. Note that the difference of kinetics of cell death in Apaf-1^-/- MEFs from Fig. 1C may be due to the infection of retrovirus.