The sensitivity of cells exposed in vitro to the antibiotics bleomycin or adriamycin is only mildly increased at 41°over that seen at 37°. However, at 430 a marked synergism between the effects of hyperthermia and drug is observed. This synergism can also be demonstrated to occur in solid tumors in vivo. Cells after bleo mycin exposure at 370 repair potentially lethal damage, and 430 inhibits this repair. This inhibition may in part account also for the observed sensitization of the cells to bleomycin, but not to adriamycin, since for the latter no repair can be demonstrated. However, fluorescence measurements show that at 430 much more adriamycin is able to enter the cells than at 37°. The possible implications of the results for cancer treatment are discussed.

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It is demonstrated experimentally that addition of proteins that are themselves resistant to denaturation by heat or ethanol can nonspecifically stabilize other proteins that are ordinarily highly susceptible to inactivation. It is proposed that the diffusion-limited rate with which unfolded protein molecules encounter each other and become irreversibly crosslinked is reduced in the presence ofsubstantial concentrations ofan unreactive globular protein. We suggest that one of the functions of heat shock proteins, which are synthesized in large amounts after exposure of cells to increased temperature and other forms of stress, may be to stabilize other proteins kinetically in a similarly nonspecific fashion.When exposed to sufficiently high temperatures, cells die (as measured by reproductive assay) at a rate which, after an initial lag period, may be described by a first-order decay law. Although the cause of cell death is not definitely known, various pieces of indirect evidence suggest that the irreversible loss of vital protein structure and associated function is a major factor (1, 2). When cells are pretreated by exposure to increased but nonlethal temperatures or by brief exposure to lethal temperatures followed by a period ofrecovery, the treated cells exhibit a dramatically enhanced rate of survival upon subsequent exposure to lethal temperatures. This phenomenon is referred to as "acquired thermotolerance" (3).The mechanism by which cells acquire increased tolerance to thermal stress is unknown, but attention recently has focused on the possible role of heat shock proteins (hsp), a small number ('16) of specific proteins that are transcriptionally induced and synthesized in great quantity after exposure to elevated temperature ("heat shock") (4-6). The synthesis of hsp has been studied extensively as a model of gene regulation, particularly in Drosophila (6) but also in various organisms throughout the phylogenetic tree (4,(7)(8)(9)(10)(11)(12). Recently it has been shown that the time scale for the appearance and disappearance of hsp parallels the expression ofacquired thermotolerance in various cell types (11-13) and that thermotolerance is not acquired after heat shock if synthesis of hsp is inhibited (8,12,14).The synthesis of hsp is also induced by noxious stimuli other than heat shock, including arsenite (6, 14), oxygen deprivation (4, 6), and ethanol (14). It has been demonstrated that preliminary exposure to ethanol results in subsequently acquired resistance not only to ethanol but also to thermal stress (15). Moreover, thermal pretreatment appears to result in increased tolerance to ethanol (16). These findings suggest that hsp may play a role in a generalized cellular adaptation to stress.hsp are quite widespread within the cell. Low molecular weight hsp bind predominantly to nuclear chromatin; higher molecular weight hsp are found predominantly in the cytoplasm (4,7,(17)(18)(19)(20)(21)(22). No enzymatic activity of hsp has yet been identified (6, 7).Here we propose that hsp can con...

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The mechanisms of immediate and delayed tumor cell killing by hyperthermia were investigated in EMT-6 neoplasms implanted in BALB/cKa mice. Radiofrequency electromagnetic fields were used to achieve a curative local dose of 44 degrees C for 30 minutes. The tumors were sampled sequentially, during and after heat therapy, and studied by light and elecron microscopy. Assays for cell survival, including cell cultures, were performed at various times after completion of therapy. Focal cytoplasmic swelling, rupture of the plasma membrane and peripheral migration of heterochromatin were observed 5 minutes after initiation of therapy and led to cytoplasmic fragmentation by the end of the treatment period (30 minutes). Necrosis of most cells occurred 2--6 hours after the end of treatment. At 48 hours, there were no recognizable tumor cells. A scar replaced the tumor bed 14 days later. Viable (clonogenic) tumor cells were still 2% of control levels at the end of therapy and then progressively decreased to 0.0003% at 48 hours, confirming the morphologic observations and indicating that factors other than the direct effect of heat on tumor cells contributed to complete tumor eradication. Our findings, coupled with previous studies, suggest that the immediate heat induced necrosis in this tumor occurs through the mechanisms of physical changes in the plasma membrane. The delayed (post-therapy) cell death is likely due to modifications in the environment of the tumor bed.