INSULIN-LIKE GROWTH-FACTOR-1 INHIBITS APOPTOSIS USING THE PHOSPHATIDYLINOSITOL 3'-KINASE AND MITOGEN-ACTIVATED PROTEIN-KINASE PATHWAYS

The role of insulin-like growth factor 1 (IGF-1) in preventing apoptos is was examined in differentiated PC12 cells, Induction of differentia tion was achieved using nerve growth factor, and apoptosis was provoke d by serum withdrawal. After 4-6 h of serum deprivation, apoptosis was initiated, concomitant with a 30% decrease in cell number and a 75% d ecrease 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 phosphatidy linositol 3' (PI3')-kinase inhibitors wortmannin (at concentrations of 10(-8) M) and LY294002 (10(-6) M) blocked the effect of IGF-1. The pp 70 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 o f a second pathway involved in the IGF-1 effect. Blocking the mitogen- activated protein kinase pathway with the specific mitogen-activated p rotein kinase/extracellular-response kinase kinase inhibitor PD098059 (10(-5) M) inhibited the IGF-1 effect, When wortmannin and PD098059 me re given together, the effect was synergistic. The results presented h ere suggest that IGF-1 is capable of preventing apoptosis by activatio n of multiple signal transduction pathways.