Expression of inducible Hsp70 enhances the proliferation of MCF-7 breast cancer cells and protects against the cytotoxic effects of hyperthermia

Heat shock proteins (Hsps) are ubiquitous proteins that are induced followi ng exposure to sublethal heat shock, are highly conserved during evolution, and protect cells from damage through their function as molecular chaperon es. Some cancers demonstrate elevated levels of Hsp70, and their expression has been associated with cell proliferation, disease prognosis, and resist ance to chemotherapy. In this study, we developed a tetracycline-regulated gene expression system to determine the specific effects of inducible Hsp70 on cell growth and protection against hyperthermia in MCF-7 breast cancer cells. MCF-7 cells expressing high levels of Hsp70 demonstrated a significa ntly faster doubling time (39 hours) compared with nonoverexpressing contro l cells (54 hours). The effect of elevated Hsp70 on cell proliferation was characterized further by 5-bromo-2'deoxyuridine labeling, which demonstrate d a higher number of second and third division metaphases in cells at 42 an d 69 hours, respectively. Estimates based on cell cycle analysis and mean d oubling time indicated that Hsp70 may be exerting its growth-stimulating ef fect on MCF-7 cells primarily by shortening of the G(0)/G(1) and S phases o f the cell cycle. In addition to the effects on cell growth, we found that elevated levels of Hsp70 were sufficient to confer a significant level of p rotection against heat in MCF-7 cells. The results of this study support ex isting evidence linking Hsp70 expression with cell growth and cytoprotectio n in human cancer cells.