Growth factors acting via tyrosine kinase receptors induce HSP90 alpha gene expression

Hsp90 is a heat-shock protein constitutively expressed in most cells. Besides regulation by thermal stress, the expression of hsp90 is also positively regulated by developmental and mitogenic stimuli. The effect of serum and insulin on protein and hsp90 alpha-mRNA levels has been studied in the chicken hepatoma cell line DU249. The culture of cells in serum-free medium resulted in a decrease of hsp90 alpha-mRNA level. A transient increase was observed at 6-9 h after serum restimulation. The expression of hsp90 gene was also increased by insulin alone in a dose-dependent manner and was maximum between 6 and 9 h treatment. The insulin induced increase of hsp90 alpha-mRNA was suppressed by cycloheximide (10 micrograms/ml) but not by an inhibitor of DNA synthesis, demonstrating that this induction requires protein neosynthesis. In serum starved cells, other growth factors (IGF1, EGF and bFGF) showed a positive effect on hsp90 alpha-mRNA level which took place before DNA synthesis with the same time-course as that of insulin. With PDGF, the induction of hsp90 alpha-mRNA occurred earlier. The time interval between the maximum of hsp90 alpha-mRNA induction and that of DNA synthesis was the same for all growth factors studied. From these results, we conclude that growth factors acting via tyrosine kinase receptors up-regulate hsp90 alpha-mRNA level in a DNA synthesis independent manner, possibly in late G1.