Synergistic effects of heat and ET-18-OCH3 on membrane expression of hsp70 and lysis of leukemic K562 cells.

We previously reported that cell surface expression of hsp70, the major stress inducible member of the 70-kDa heat shock protein family, is inducible by nonlethal heat as well as by treatment with the membrane-interactive compound alkyl-lysophospholipid 1-octadecyl-2-methyl-rac-glycero-3-phosphocholine (ET-18-OCH3) selectively on human tumor cell lines. Plasma membrane expression of hsp70 increases selectively the sensitivity of tumor cells to lysis and, therefore, might play an important role in the antitumor immune response. Here, we demonstrate that a combined treatment consisting of sublethal heat (41.8 degrees C) and a noncytotoxic concentration of ET-18-OCH3 (25 micrograms/mL) results in a synergistic increase in the amount of cell membrane-bound hsp70 on leukemic K562 cells and on freshly isolated bone marrow of a chronic myelogeneous leukemia (CML) patient, but not on peripheral blood lymphocytes or CD34+ hematopoietic progenitor cells of healthy human individuals. Under these conditions the repopulating capacity of progenitor cells was not influenced. The increased hsp70 membrane expression on leukemic K562 cells results in a significantly increased sensitivity to lysis mediated by natural killer cells. In contrast to leukemic cells, the lysis of peripheral blood lymphocytes and CD34+ progenitor cells that lack expression of hsp70 on their plasma membrane was not negatively influenced by this treatment. A nonspecific disruption of the plasma membrane could be excluded, because treatment with a nontoxic concentration of the detergent Tween20 did not have an influence on hsp70 cell surface expression or on the sensitivity to lysis. Our findings might have further clinical implications with respect to purging of bone marrow from patients suffering from leukemia at sublethal conditions to induce a tumor-selective immune response.