Rapid desensitization of PC12 cells stimulated with high concentrations ofextracellular S100

Undifferentiated PC12 cells undergo apoptosis, via a calcium-induced calciu m release mechanism, when the calcium-binding protein purified from bovine brain (native S100) is present in micromolar concentration in the medium. T his process begins when S100 binds to specific membrane binding sites and i nvolves up to 50% of the cell population. In the experiments reported here, we demonstrate that, by utilizing [H-3]S100, the S100 protein can be displ aced from its binding sites only during the first 10 min of incubation. Thi s fact is due to an internalization mechanism, having a time-course with a plateau after 10-20 min of incubation. The native form of S100 is a mixture of two different S100 isoforms: S100A1 (20%) and S100B (80%). Using confoc al microscopy and monoclonal antibodies, we demonstrated that only one of t hese isoforms, S100A1, was autoexpressed in more than 50% of the PC12 cells analysed. After cell incubation with 2 mu M native S100, S100B also appear s in PC12 cells, with a maximum presence after 10 min of incubation. This f act seems to indicate that this isoform, at least, is effectively transloca ted when stimulated with external native S100. From the data reported, it is possible to hypothesize that, in PC12 cells, a possible homeostatic mechanism is present that can counteract the effect of a continuously applied lethal stimulus (stimuli) on cell viability. (C) 1999 IBRO. Published by Elsevier Science Ltd.