Nuclear translocation of stress protein Hsc70 during S phase in rat C6 glioma cells

The expression and the nuclear translocation of the constitutive heat shock protein 70 (Hsc70) were determined during the cell cycle in synchronized rat astrocytomic C6 glioma cells. Cells were first shifted to the G0 by serum starvation. Twelve hours after a subsequent growth stimulation by transfer to 20% newborn calf serum, about 50% of the cells entered S phase. Western blot analysis with different monoclonal antibodies showed that only the constitutively expressed and moderately stress-activated Hsc70 is induced during serum stimulation. Maximal cellular Hsc70 content (170% of the control) was observed in early to mid S phase followed by a drastic decline while cells pass through G2/M (20% of the control). Hsp70, the major heat-inducible heat shock protein in C6 cells, is not detected in either asynchronously proliferating, serum-starved or in serum-stimulated C6 cells. Analysis of the nuclear and cytoplasmic protein fractions showed a significant increase of Hsc70 translocation into the nucleus during early S phase. These results indicate a role for Hsc70 but not for Hsp70 in the process of S phase entry and/or progression in C6 cells under physiological conditions.