The role of insulin-like growth factor 1 (IGF-1) in preventing apoptosis was examined in differentiated PC12 cells. Induction of differentiation was achieved using nerve growth factor, and apoptosis was provoked by serum withdrawal. After 4-6 h of serum deprivation, apoptosis was initiated, concomitant with a 30% decrease in cell number and a 75% decrease in MTT activity. IGF-1 was capable of preventing apoptosis at concentrations as low as 10-9 M and as early as 4 h. The phosphatidylinositol 3' (PI3')-kinase inhibitors wortmannin (at concentrations of 10-8 M) and LY294002 (10-6 M) blocked the effect of IGF-1. The pp70 S6 kinase (pp70(S6K)) inhibitor rapamycin (10-8 M) was, however, less effective in blocking IGF-1 action. Moreover, stable transfection of a dominant-negative p85 (subunit of PI3'- kinase) construct in PC12 cells enhanced apoptosis provoked by serum deprivation. Interestingly, in the cells overexpressing the dominant-negative p85 protein, IGF-1 was still capable of inhibiting apoptosis, suggesting the existence of a second pathway involved in the IGF-1 effect. Blocking the mitogen-activated protein kinase pathway with the specific mitogen-activated protein kinase/extracellular-response kinase kinase inhibitor PD098059 (10- 3 M) inhibited the IGF-1 effect. When wortmannin and PD098059 were given together, the effect was synergistic. The results presented here suggest that IGF-1 is capable of preventing apoptosis by activation of multiple signal transduction pathways.