Cell cycle variation of Hsp70 levels in HeLa cells at 37°C and after a heat shock

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Abstract
The expression of the 72 kD inducible heat shock protein (hsp72) has been reported to be cell cycle associated in unheated, synchronized HeLa cells. In this study, flow cytomerty was used to investigate hsp72 levels through the cell cycle in HeLa cells by dual labeling with propidium iodide and antibodies against hsp72. The entire cell cycle distribution of hsp72 could be measured in a single sample of asynchronously growing cells. For unheated cells, the level of hsp72 increased about 30% from G1 to S phase, with about a 65% increase in G2/M, probably due to cell size differences. Neither mitotic selection nor serum stimulation induced a higher level of hsp72 than in the control cells. Western blot analysis of hsp72 from Hoechst‐stained cells sorted from G1, mid‐S, or G2/M showed that G1 cells had the lowest level of hsp72, with about a 30% increase in S phase and a 60% increase in G2/M, in good agreement with the flow cytometry results. These data conflict with previous reporty by other laboratories which showed a 3‐fold higher level of hsp72 in S phase than in G1 or G2. In contrast, heat shock (both acute and chronic) led to a non‐uniform increase in hsp72 through the cell cycle. Most cells in mid S phase had high levels of hsp72, and a larger range in the levels of hsp72 were found in G1 and late S/G2/M phase cells.