Modulation in the levels and localization of plasminogen activator in differentiating neuroblastoma cells

Abstract

Variations in the specific activity and localization of plasminogen activator (PA) were examined prior to and during the differentiation of mouse neuroblastoma and rat glioma cells in culture, in an attempt to reveal whether PA is actively involved in neurite extension and development in these cells. Following differentiation induced by dibutyryl cyclic AMP, the level of PA increased by 10 to 20-fold in neuroblastoma N18TG-2 cells and in the hybrid cell line NG108-15, obtained by fusion of mouse neuroblastoma N18TG-2 and rat glioma C6BU-1 cells, but was very low in the glioma parental cells throughout differentiation. Plasminogen-dependent proteolysis was found not to be an obligatory requirement for the differentiation of the hybrid cells in culture, since the extension of neurites, the appearance of toxin-activated Rb +-efflux and the eletrical properties were largely unaffected by eliminating and blocking this activity. Confluent cells from several clones derived from mouse neuroblastoma C1300 cells displayed significant intracellular and secreted PA activities, and the axon-minus neuroblastoma NIA-103 cells were found to secrete the highest amounts of PA. Upon gel electrophoresis, all mouse neuroblastoma cell lines examined displayed a major molecular form of active PA with an apparent molecular weight of 80,000 and a minor one of 50,000. Only the larger enzyme form was expressed by the rat glioma or the hybrid cells, and the electrophoretic migration pattern of the active enzyme species remained unchanged upon differentiation of the various cell lines. Distribution of PA activity on the surface of differentiating neuroblastoma N1E-115 cells and hybrid NG108-15 cells was visualized by high resolution autoradiography of [ 125I]fibrin degradation products, cross-linked to the cell surface. Particularly intense labeling was observed along membranes of cell bodies in both developing and mature cells, around branching zones, along individual neurite extensions and in part, but not all, of the growth cones. These findings suggest that PA plays a role in the metabolism of mature cells of neuronal origin, and imply its involvement in the development and/or the functions of specific neuronal processes and growth cones.