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Abstract
Life Cycles and Heat Shock Proteins
Organisms can encounter wide fluctuations in temperatures of their surrounding throughout their life cycle.
For example the bacterium E. coli cycles between the 37°C of the mamalian gut and the <20°C of waste water. Dimorphic fungi (e.g., Histocapsulatum) and the parasitic protozoa Leishmania major and Trypanosoma experience similar temperature changes between hosts. Not surprisingly, these organisms induce HSPs when they initially encounter the higher temperatures[51].
Presumably the HSPs help the cell overcome temperature-induced damage in the same way they help other stressed cells whose life style shows less temperature variations.
What is striking, however, and should be borne in mind when speculating about HSP function are the observations that several HSPs are induced during normal cellular development, in the absence of temperature stress (of course, there could be another kind of stress operating during these stages).
2.4 Virus Infections and HSP Synthesis:
Bacteriophage lambda actually uses several of the E. coli HSPs for its replication. Several eukaryotic cell DNA viruses, i.e., adenovirus, herpes virus, simian virus, and polyoma viruses, activate synthesis of HSP70 early in infection.
Newcastle disease virus, an RNA virus, induces HSP70 in infected chicken cells. As in the case of bacteriophage, the HSPs may be used by the virus replication system or their induction may simply reflect a stress from the infection. The fact that the adenovirus A gene product is an inducer of cell HSP70 argues for a positive role of this protein in the virus life cycle and to over counter infection.[51].
2.5 Immunological roles of heat shock proteins in cancer:
Peptide carrier function depending on their intra or extracellular localization, HSPs mediate dual functions. On the one hand, upregulated intracellular HSP levels protect tumor cells from lethal damage induced by environmental or physiological stress.
On the other hand, HSPs with molecular weights 70 and 90 kDa have been identified as being key regulators of the host immune system.
The immunological roles of extracellularly located and membrane bound HSPs as potent stimulators of immune responses against cancer, following cross-presentation of HSP-chaperoned peptides on MHC class I molecules an antigen-specific CD8+ T cell response is initiated in tumor cells[52].