Heat-induced alterations of nuclear protein associations and their effects on DNA repair and replication

New knowledge of nuclear structure and DNA repair pathways has provided the basis for new insight into the effects of hyperthermia on the proteins involved in these processes. The nucleus is made up of mega protein-nucleic acid complexes that conduct various nuclear functions, including DNA packing, repair, replication and transcription. Heat shocks (41-50 degrees C) cause unfolding of a number of nuclear proteins. Such unfolding changes protein associations within all of the intra-nuclear mega protein-nucleic acid complexes studied, with the exception that no alterations in the nucleosome-DNA bead and super bead complexes could be detected. This review will address heat effects on protein-nucleic acid complexes related to DNA replication and DNA repair. Heat-induced changes in DNA replication complexes can be related to the killing of S-phase cells by heat. The effects of heat on DNA repair foci, complexes involving MRE11, the nucleolus and on the complexes that anchor DNA to the nuclear matrix appear to contribute to radiosensitization as a function of increasing thermal dose. Thus, heat effects on these complexes can serve as molecular targets for the development of agents that can enhance the effectiveness of clinical thermal radiotherapy.

Role of Membrane Components in Thermal Injury of Cells and Development of Thermotolerance

Exposure of cells to hyperthermia induces a transient resistance to subsequent heat treatment. The specific mechanisms responsible for hyperthermic cell killing and thermotolerance development are not well understood. It seems that heat may induce at least two different states of thermotolerance, of which one is dependent on protein synthesis. The expression of thermotolerance may include multiple cytoplasmic and membrane components. A number of studies have indicated that membranes play an important role in governing the thermal injury of cells. It seems, therefore, that heat denatured plasma membrane proteins may be a potential target for thermal stress and a trigger for the induction of thermotolerance. The localization of heat shock proteins in the plasma membrane and the suggestion of thermal resistance in enucleate erythrocytes support this suggestion. However, a direct relationship between the plasma membrane and hyperthermic killing or development of thermotolerance has not been found.

Cellular responses to hyperthermia (40-46 degrees C): cell killing and molecular events

The goal of this review is to provide a brief introduction to the effects of hyperthermia on cellular structures and physiology. The review focuses on the effects of hyperthermia thought to contribute to the enhancement of cancer therapy namely the mechanisms of cell killing and the sensitization of cells to ionizing radiation or chemotherapeutic agents. Specifically the review addresses four topics: hyperthermia induced cell killing, mathematical models of cell killing, mechanisms of thermal effects in the hyperthermia temperature range and effects on proteins that contribute to resistance to other stresses, i.e., DNA damage. Hyperthermia has significant effects on proteins including unfolding, exposing hydrophobic groups, and aggregation with proteins not directly altered by hyperthermia. Protein aggregation has effects throughout the cell but has a significant impact within the nucleus. Changes in the associations of nuclear proteins particularly those involved in DNA replication cause the stalling of DNA replication forks and lead to the induction of DNA damage such as double strand breaks. It has long been recognized that heat has effects on plasma membrane protein distribution alters the permeability of plasma membranes resulting in a calcium spike and disrupts the mitochondrial membrane potential resulting in the change in the redox status of cells. These effects contribute to the protein unfolding effects of hyperthermia and contribute to effects observed in the nucleus. Thus heat effects on multiple cellular targets can be integrated through global effects on protein folding to affect specific end points such as cell killing and sensitization to additional stresses.

Initial characterization of heat-induced excess nuclear proteins in HeLa cells

Exposure of mammalian cells to hyperthermia is known to cause protein aggregation in the nucleus. The presence of such aggregates has been detected as the relative increase in the protein mass that is associated with nuclei isolated from heated cells. We have characterized these excess nuclear proteins from the nuclei of heated HeLa cells by two-dimensional gel electrophoresis. The abundance of cytoskeletal elements which co-purify with the nuclei did not increase with exposure to hyperthermia, indicating that these proteins are not part of the excess nuclear proteins. In contrast, several specific polypeptides become newly bound or increase in abundance in nuclei isolated from heated cells. Members of the hsp 70 family were identified as a major component of the excess nuclear proteins. Among the other excess nuclear proteins we identified ten that had apparent molecular weights of 130, 95, 75, 58, 53, 48, 46, 37, 28, and 26 kilodaltons. Since hsp 70 is mainly cytoplasmic in non-heated cells, its association with nuclei in heated cells indicates that one mechanism accounting for the heat-induced excess nuclear proteins is the movement of cytoplasmic proteins to the nucleus. We also obtained evidence that increased binding of nuclear proteins is another mechanism for this effect. No overall increase or decrease in the phosphorylation of nuclear proteins was found to be associated with such altered binding or movement from the cytoplasm to the nucleus.

Morphometric analysis of hyperthermia-induced nuclear and nucleolar size modifications in a human melanoma cell line

Although no prominent morphologic alterations were observed by light microscopy in a human melanoma cell line (M-14) exposed at 42 degrees C for 4 hr, the computerized image analysis demonstrated a significant enlargement of both nuclei and nucleoli. The mean nuclear and nucleolar areas were found to be 32% and 94% increased, respectively, in comparison with normal controls. The gaussian distribution of absolute frequency of the measures demonstrated that this nuclear and nucleolar enlargement was distributed among the whole cell population, taking place in clonogenic cells and in "sterilized" cells, ultimately doomed to die. This finding, not previously described in the context of the heat shock response, may be regarded as morphological evidence of an increase in the protein content of the nuclei of heat-treated cells, as determined by biochemical methods.

Co-isolation of heat stress and cytoskeletal proteins with plasma membrane proteins

Previous reports have suggested the possible existence of a plasma-cell-membrane function associated with some heat stress proteins (HSPs). To investigate the effect of hyperthermia on plasma membrane proteins, rat mammary tumour clone C (MTC) cells were heated at 42 degrees C for 1 h. Their surface proteins were (1) labelled with [3H]leucine, (2) biotinylated, (3) affinity isolated with streptavidin-agarose beads under denaturing or non-denaturing conditions, and (4) analysed by one- and two-dimensional polyacrylamide-gel electrophoresis and protein blotting under denaturing conditions. Affinity isolation of biotinylated proteins enriched for a protein subfraction believed to be membrane-associated. Several proteins analogous to HSP or their heat-stress cognates (HSC) were present with these biotinylated protein subfractions in control or heated cells. The major and most consistent feature of affinity isolates from heated cells was the presence of a small fraction of the induced 68-kD HSP. The 112-, 90-, 70- and 22-kD HSC/HSP were also present in small amounts in affinity isolates of control cells, and the fraction increased in heated cells. Several structural proteins, including actin and the tubulins were present in the same affinity isolates. Protein blotting experiments indicated that none were exposed on the exterior of the plasma-cell membrane or biotinylated and thus none were exposed on the exterior of the plasma-cell membrane or biotinylated intracellularly through membrane damage. These results suggest that small fractions of several HSC are located at or near the cytoplasmic face of the plasma membrane along with cytoskeletal proteins, and that additional submembranous localization of HSP occurs after heat stress and may be part of the processes associated with membrane damage or cellular responses to heat. Further studies will be directed at establishing the relationships between these proteins and the role, if any, of the changes associated with heat stress.

DNase I sensitivity of nuclear DNA measured by flow cytometry

The DNase I digestion kinetics of DNA in isolated nuclei (from HeLa or murine mammary carcinoma, 67 cells) were assayed flow cytometrically by measuring the changes in ethidium bromide (EtBr) fluorescence following various digestion time intervals. The DNase I digestion curve was characterized by an initial 25-30% increase in fluorescence upon addition of the enzyme, a rapid reduction in fluorescence to approximately 50-55% in 30 minutes, and a limit digest of 45-50% beyond 45 minutes. Throughout digestion, the DNA histogram retained its characteristic bimodal shape, showing that histogram rearrangement was not responsible for the changes in EtBr fluorescence. Irradiation with 5 X 10(6) rads (137Cs-gamma-rays) or exposure to 50 mM EDTA caused an increase in EtBr fluorescence similar to that caused by DNase I, suggesting that DNA nicking and/or chromatin loosening were responsible for this increase. Residual DNA assayed by the solubilization of 14C-TdR (thymidine)-labeled DNA indicated a similar kinetic pattern without the initial increase. However, at the limit digest, the fraction of DNA remaining trichloroacetic acid (TCA) insoluble (10%) was smaller than that measured by loss of EtBr fluorescence (50% of initial, 40% of maximum). Part of this difference was due to the presence of TCA soluble DNA trapped within the nuclear matrix (15-20%). This trapped DNA was released when the digested nuclei were exposed to 0.5-1.0 M NaCl just prior to EtBr staining. Exposure of HeLa cells to three agents that are believed to cause changes in chromatin structure resulted in alterations in the DNase I digestion kinetics measured flow cytometrically.(ABSTRACT TRUNCATED AT 250 WORDS)

Hyperthermia in the treatment of cancer. Perspectives on its promise and its problems

The potential for the use of hyperthermia in the treatment of cancer is based on a strong and compelling biologic rationale. In the laboratory it has been shown in quantitative assays both in vitro and in vivo that (1) hyperthermia is cytotoxic to tumor cells as a function of time at temperatures above 42 degrees C; (2) cytotoxicity is relatively high for radioresistant S-phase cells and for cells that are nutritionally deprived and acidotic, conditions one might expect in regions of tumors containing large numbers of radioresistant hypoxic cells; and (3) heat is a radiosensitizer and a chemosensitizer. Clinical study is hampered by less than optimal physical heating methods and the need for invasive thermometry. Ultrasonic and electromagnetic approaches each have limitations and advantages. In spite of technical limitations, efficacy has been shown for superficial tumor sites treated by local hyperthermia and Phase I studies are underway investigating the more complicated problem of deep regional hyperthermia. Although whole body hyperthermia has the attractive capability of treating metastatic as well as more localized cancer, it is toxic therapy and its role in treatment remains undefined. Research advances in equipment design and treatment optimization are needed; however, there are studies underway utilizing existing methods and rationale which should further clarify the potential clinical usefulness of regional hyperthermia in combined modality approaches to cancer therapy.

The effects of alcohols, procaine and hyperthermia on the protein content of nuclei and chromatin

Since previous studies suggest that heat-induced nuclear changes correlate with cell killing we have extended these observations by measuring nuclear and chromatin protein content in HeLa cells heat sensitized by agents which are believed to cause membrane damage. Specifically, we have investigated the effects of alcohol (C2-C5) and procaine. Exposure of HeLa cells to alcohol concentrations greater than 1 M ethanol or 0.2 M butanol for 30 min causes a measurable increase in the protein content of both nuclei and chromatin. When cells were heated (45 degrees C) in the presence of alcohol, the increase in nuclear and chromatin protein content was significantly greater than that for heat alone at concentrations above 0.25 M ethanol and 0.07 M butanol. In addition, the presence of 0.41 M ethanol causes a two-fold increase, over heat alone, in the amount of protein absorbed to chromatin when cells are heated at 45 degrees C from 0 to 60 min. Similar effects were observed with procaine. Thus, alcohol or procaine alone can cause an increase in chromatin protein content and can act synergistically with heat to cause a larger increase. These results suggest that membrane damage may cause a larger increased protein content of chromatin and thereby lead to cell death.