Cell cycle regulation of the chicken hsp90 alpha expression

The heat-shock protein of 90 kDa (hsp90), constitutively expressed in most cells, is up-regulated by thermal stress and by developmental and mitogenic stimuli. When the serum-starved chicken hepatoma cell line DU249 is stimulated by serum, insulin, or growth factors acting via tyrosine kinase receptors, a transient induced expression of the hsp90 alpha gene takes place at both the messenger RNA and the protein synthesis level. This response is protein synthesis dependent and DNA synthesis independent. The maximum level of hsp90 alpha mRNA accumulation always precedes the maximum level of thymidine incorporation, suggesting a possible link between cell cycle and hsp90 alpha regulation (Jérôme, V., J. Léger, J. Devin, E.E. Baulieu, and M. G. Catelli. Growth Factors 4:317-327, 1991). Herein, we examine the subcellular distribution of hsp90 and the cell cycle-dependent regulation of the hsp90 alpha mRNA level. We show that, in contrast to hsp70, the 35S metabolically-labeled hsp90, which accumulates in the cytosoluble fraction 6 to 8 h after serum treatment, is not preferentially translocated to the nuclear compartment, although a small fraction is always present in the nucleus. We also demonstrated that serum- or insulin-induced accumulation of hsp90 alpha mRNA results from an activation of gene transcription and that hsp90 alpha promotor activity, which is low in quiescent DU249 cells, is induced approximately fivefold after serum stimulation. Finally, in cell culture synchronized by nocodazole or aphidicholin, hsp90 alpha mRNA accumulation seems an event specific to G1/S transition.