Programmed cell death (apoptosis) is of central importance in many age-related degenerative processes. Interactions between cells and their extracellular matrix are critical to many cellular processes including differentiation, and survival. Loss of contact with extracellular matrix can induce apoptosis. We demonstrate apoptosis in PC12 cells deprived of extracellular matrix adhesion and we report that nerve growth factor (NGF) treatment greatly accelerates this process. Plating PC12 cells on agarose-coated dishes effectively blocks cell adherence. During the initial 24 hours of culture, 30% of the cells grown on agarose undergo apoptosis while cells plated on collagen-coated surfaces maintain nearly 100% viability. Apoptosis of adhesion-blocked cells continued over the next 72 hours. Addition of NGF paradoxically accelerates apoptosis in nonadherent cells while inducing proliferation and differentiation of adherent cells. Both NGF and fibroblast growth factor induced apoptosis while epidermal growth factor (which induces proliferation but not neuronal differentiation of PC12 cells) was far less potent. We investigated the nature of the signaling associated with NGF in nonadherent PC12 cells. Immunoblotting with anti-phosphotyrosine antibodies revealed striking differences in the pattern of tyrosine phosphorylation induced by NGF in adherent as compared to nonadherent cells, suggesting that NGF signaling is altered in nonadherent PC12 cells.
