Heat Shock Proteins (HSPs)

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Heat Shock Proteins (HSPs)

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Heat Shock Proteins (HSPs) Background

Heat shock proteins (HSPs) are small polypeptide groups that are classified into families based on molecular weight. They are expressed by cells in the body as a protection against harmful environmental conditions. HSPs are expressed in situations where cell survival may be compromised. The stimulus for HSP expression includes, but is not limited to: temperature, free radicals, anoxia, bacteria, endotoxin, and viruses.

Heat shock proteins are involved in many chaperone functions. Their expression can be constitutive or inducible depending on the family member. Constitutively expressed members are present in all cell compartments and appear to assist in proper folding and assembly of polypeptide of newly synthesized proteins. Inducible forms were originally described following heat stress; however, a variety of cellular stresses will lead to their induction as part of an orchestrated stress response. Messenger RNAs encoding for HSPs are found in normal conditions, but during stress, transcription of HSP genes occurs rapidly.

HSPs also function as a potential cytoprotective protein. Under stressful conditions such as cytotoxic injury, heat shock, oxidative stress, radiation, viral infection, and chemical exposure, HSPs have long been known to serve as protein chaperones in the sense that they assist in protein folding and the correct attainment of functional three-dimensional configuration, while preventing incorrect folding and protein aggregation.

While recent studies that highlight the critical functions of Heat Shock Proteins in various cellular processes have made them an increasing popular subject of interest to medical specialists in a wide variety of fields, including infectious disease, immunology, oncology, and autoimmunity, heat shock proteins are relatively unstudied in the field of endodontics. There are only two articles available in the endodontic literature that deal with HSPs and they both study HSP27. HSP27 is expressed in many cell types and tissues at specific stages of development and differentiation. Overexpressed HSP27 protects against apoptotic cell death triggered by various stimuli, including hyperthermia and oxidative stress. The first article studied the role of HSP27 in the pathogenesis of periapical lesions by using immunohistochemistry techniques. An overexpression of HSP27 was observed in sixteen human periapical lesions when local immune cells where present. According to the authors, HSP27 may play several roles in periapical lesions that include contributing to the migration of epithelial cell rests and to an increased resistance to necrotic and apoptotic cell death. The second article investigated the relationship between transforming growth factor β1 (TGFβ1) and HSP 27 activation. The authors reported that HSP27 was required for TGFβ1 induced cell migration.

Heat Shock Proteins may activate the immune response but also dampen the inflammatory pathways by presenting antigens to the immune system. HSPs have been linked to the therapy of both cancer and inflammatory diseases, approaches that utilize contrasting immune properties of these proteins. It would appear that HSP family members HSP60 and HSP70, whether from external sources or induced locally during inflammation, can be processed by antigen-presenting cells and that HSP-derived epitopes then activate regulatory T cells to suppress inflammatory disease. The HSP70 family constitutes the most conserved and best studied class of HSPs. Human cells contain several HSP70 family members. Under normal conditions, HSP70 proteins function as ATP-dependent molecular chaperones by assisting the folding of newly synthesized polypeptides, the assembly of multiprotein complexes, and the transport of proteins across cellular membranes. HSP70 also reduces or blocks caspase activation and suppresses mitochondrial damage and nuclear fragmentation.

Prominent members of the HSP90 family of proteins are HSP90α and HSP90β. These two HSP90 isoforms are essential for the viability of eukaryotic cells. They are rather abundant constitutively, make up 1- 2% of cytosolic proteins, and can be further stimulated in their expression level by stress. HSP90 seems to have different molecular partners depending on the cell death stimuli. It should be noted that most studies do not differentiate between the α and β isoforms of HSP90.

HSP110 is one of major heat shock proteins of eukaryotic cells and is a diverged relative of the HSP70 family. Based on strong inducibility, quantity, and presence in many cell types, HSP110 has also been recognized for the last two decades as a major heat shock protein, specifically in mammalian cells. HSP110 exists in parallel with the HSP70s in the cytoplasm and nucleus in diverse organisms, argues strongly for related but differential functions and properties for these two major stress protein groups. The overexpression of HSP110 has been shown to confer cellular heat resistance, and prevention of protein aggregation keeping denatured protein in a folding-competent state.

Extracellular HSPs can also stimulate the release of tumor necrosis factor alpha (TNF-α), interleukins - 1β, -6 and -12, nitric oxide, as well as chemokines by monocytes/macrophages, which may serve as endogenous danger signals to alert the host defense system through their cytokine-like function contributing to lesion development/expansion.

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