Heat induced ‘masking’ of redox sensitive component(s) of the DNA-nuclear matrix anchoring complex

Investigations concerning the impact of consumption of hot beverages on acute cytotoxic and genotoxic effects in oral mucosa cells

Consumption of very hot beverages and foods increases the incidence of oral and esophageal cancer but the mechanisms are not known and the critical temperature is not well defined. We realized a study with exfoliated cells from the oral cavity of individuals (n = 73) that live in an area in Iran which has the highest incidence of EC worldwide. Consumption of beverages at very high temperatures is a characteristic feature of this population. We analyzed biomarkers which are (i) indicative for genetic instability (micronuclei that are formed as a consequence of chromosomal damage, nuclear buds which are a consequence of gene amplifications and binucleated cells which reflect mitotic disturbances), (ii) markers that reflect cytotoxic effects (condensed chromatin, karyorrhectic, karyolitic and pyknotic cells), (iii) furthermore, we determined the number of basal cells which is indicative for the regenerative capacity of the buccal mucosa. The impact of the drinking temperature on the frequencies of these parameters was monitored with thermometers. We found no evidence for induction of genetic damage but an increase of the cytotoxic effects with the temperature was evident. This effect was paralleled by an increase of the cell division rate of the mucosa which was observed when the temperature exceeded 60 °C. Our findings indicate that cancer in the upper digestive tract in drinkers of very hot beverages is not caused by damage of the genetic material but by an increase of the cell division rate as a consequence of cytotoxic effects which take place at temperatures over 60 °C. It is known from earlier experiments with rodents that increased cell divisions lead to tumor promotion in the esophagus. Our findings provide a mechanistic explanation and indicate that increased cancer risks can be expected when the drinking temperature of beverages exceeds 60 °C.

Nucleolin and nucleophosmin: nucleolar proteins with multiple functions in DNA repair

The nucleolus represents a highly multifunctional intranuclear organelle in which, in addition to the canonical ribosome assembly, numerous processes such as transcription, DNA repair and replication, the cell cycle, and apoptosis are coordinated. The nucleolus is further a key hub in the sensing of cellular stress and undergoes major structural and compositional changes in response to cellular perturbations. Numerous nucleolar proteins have been identified that, upon sensing nucleolar stress, deploy additional, non-ribosomal roles in the regulation of varied cell processes including cell cycle arrest, arrest of DNA replication, induction of DNA repair, and apoptosis, among others. The highly abundant proteins nucleophosmin (NPM1) and nucleolin (NCL) are two such factors that transit to the nucleoplasm in response to stress, and participate directly in the repair of numerous different DNA damages. This review discusses the contributions made by NCL and (or) NPM1 to the different DNA repair pathways employed by mammalian cells to repair DNA insults, and examines the implications of such activities for the regulation, pathogenesis, and therapeutic targeting of NPM1 and NCL.

Nucleophosmin redistribution following heat shock: a role in heat-induced radiosensitization

Cellular survival from radiation-induced DNA damage requires access to sites of damage for the assembly of repair complexes and the subsequent repair, particularly the repair of DNA double strand breaks (DSB). Hyperthermia causes changes in protein-protein/DNA interactions in the nucleus that block access to sites of DNA damage. Studies presented here indicate that the nucleolar protein, nucleophosmin (NPM), redistributes from the nucleolus following hyperthermia, increases its association with DNA, and blocks access to DNA DSBs. Reduction of NPM significantly reduces heat-induced radiosensitization, but reduced NPM level does not alter radiation sensitivity per se. NPM knockdown reduces heat-induced inhibition of DNA DSB repair. Also, these results suggest that NPM associates with nuclear matrix attachment region DNA in heat-shocked cells.

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.

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.