Abstract
Heat shock proteins (HSPs) are abundant cellular proteins involved with protein homeostasis. They have both constitutive and inducible isoforms, whose expression levels are further increased by stress conditions, such as temperature elevation, reduced oxygen levels, infection, inflammation and exposure to toxic substances. In these situations, HSPs exert a pivotal role in offering protection, preventing cell death and promoting cell recovery. Although the majority of HSPs functions are exerted in the cytoplasm and organelles, several lines of evidence reveal that HSPs are able to induce cell responses in the extracellular milieu. HSPs do not possess secretion signal peptides, and their secretion was subject to widespread skepticism until the demonstration of the role of unconventional secretion forms such as exosomes. Secretion of HSPs may confer immune system modulation and be a cell-to-cell mediated form of increasing stress resistance. Thus, there is a wide potential for secreted HSPs in resistance of cancer therapy and in the development new therapeutic strategies.
Highlights
Heat shock proteins (HSPs) are abundant cellular proteins involved with protein homeostasis
The nomenclature for HSPs has been diverse in the literature, which generates a lot of confusion
The surprising observation that HSPs can be actively secreted from cells was reported almost 30 years ago, when investigators discovered that cultured cells released HSPH1 and HSPA8 after a short heat shock stress [31]
Summary
Heat shock proteins (HSPs) were identified initially as proteins necessary for stress responses, such as temperature elevation and other proteotoxic stresses, preventing the damage of cellular structures, protecting essential cellular functions [1]. Endoplasmic reticulum (ER)-associated protein Mediates cross-presentation in macrophages [6]; enhances involved in stress responses promoted by hypoxia immunogenicity [7,8]; potentiates TLR9 activation [9]. HSPs function as chaperones, facilitating protein folding of client proteins They act in the co- or post-translational folding of newly synthetized proteins and on the remodeling of misfolded proteins that can be caused by heat shock and other stress conditions. They can even aid the clearance of protein aggregates and are essential for the activity of many proteins. Unlike intracellular HSPs, the functions of secreted forms are still under debate; most articles in the literature suggest that they may represent a signal for immune system modulation
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