**2. Secretion and uptake of extracellular heat shock proteins**

When cells are exposed to a stressor, which includes but is not limited to heat shock, osmotic stress, exposure to heavy metals, hypoxia, ischemia or pathogens, these release signalling molecules in order to alert the rest of the system that a stressful condition is being experienced in some part of the organism and which might potentially lead to a situation of systemic damage. Among the stress signals which can be released by cells in response to an incidence of cellular stress are HSPs and other components of the chaperone (**Figure 1**). It is worth noting that most HSPs lack the consensus signal required for secretion via the classical endoplasmic reticulum (ER)-Golgi pathway [3, 18]. So far, it appears that the secretion of HSPs is achieved via a number of alternative pathways; however, these are still not well defined. Presently, the HSP release mechanisms identified are (but might not be limited to) processes via:

**i.** Cell lysis—where the process can be the result of a physiologically regulated release of cytokines or necrosis resulting from a pathological condition. Extracellular HSP70 has been suggested to be released into circulation under a variety of pathological conditions which cause widespread cell death as well as the following necrosis of tumour cells [19].

**v.** CD14—a co-receptor for TLR4 activation, which was found to be also required for HSP70

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http://dx.doi.org/10.5772/intechopen.79737

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**vi.** Scavenger receptors (SR)—a family of receptors currently classified into ten subclasses (A–J) based on structure and biological function [39]. At least three SRs bind to and internalise HSPs, namely, lectin-like oxidised LDL receptor 1 (LOX-1), scavenger receptor expressed by endothelial cell 1 (SREC-1) and fasciclin and EGF-like, laminin-type EGF-like and link domain-containing scavenger receptor 1(FEEL-1)/common lymphatic endothelial and vascular endothelial receptor 1 (CLEVER-1), with HSP70 binding to all three, HSP60 binding to LOX-1, HSP90 binding to LOX-1 and SREC-1 and calreticulin binding SREC-1 but not LOX-1 [38, 40–43]. Furthermore, scavenger receptor-A (SR-A) can bind to and internalise HSP90 and calreticulin as well as HSP110 and GRP170 [42, 44]. The sialic acid-binding immunoglobulin-type lectin (Siglec) receptors Siglec-5 and Siglec-14

**Figure 1.** Heat shock proteins (HSPs) are exported into the extracellular space and general circulation via a number of different processes including cell lysis, secretory vesicles, lysosomal endosomes or export vesicles. Once these extracellular HSPs reach the target tissues, they bind to a variety of receptors, which initiate an alarm response. When these extracellular HSPs are collected from patients with chronic diseases and quantified, they can have diagnostic or prognostic value.

induction of cytokines [35].

have also been found to bind to HSP70 [45, 46].


Once in the extracellular space or general circulation, these HSPs can stimulate a wide range of cell types. However, similar to the secretion mechanisms, the recognition and uptake of HSPs by cells, as well as the role that extracellular HSPs play in cell activation, are poorly understood. HSPs have been reported to bind to a wide variety of receptors on target cells, among which are:


**v.** CD14—a co-receptor for TLR4 activation, which was found to be also required for HSP70 induction of cytokines [35].

**i.** Cell lysis—where the process can be the result of a physiologically regulated release of cytokines or necrosis resulting from a pathological condition. Extracellular HSP70 has been suggested to be released into circulation under a variety of pathological conditions which cause widespread cell death as well as the following necrosis of tumour cells [19]. **ii.** Endolysosomal pathway—where the HSP is translocated into lysosomes and instead of being degraded is translocated out of the cell via endocytosis. HSP27 (dephosphorylated at S15 and S82) [20] and HSP70 [21] have been shown to enter endolysosomes, which are then secreted extracellularly in an ATP-dependent manner, from both tumuor cells and macrophages possibly via some pathway analogous to the ATP-binding cassette (ABC)

**iii.** Exosomal pathway—where the HSP is contained in secretory vesicles (exosome lumen) which rupture or are lysed once present in the extracellular space. A number of HSPs have been detected within extracellular vesicles including HSP27, HSP70, HSC70, GRP75,

**iv.** Inclusion in the exosomal membrane—where the HSP is inserted into the membrane of the secretory vesicles rather than being in the lumen. The isolation of HSP70-containing vesicles, derived from the plasma membrane, indicates that the surface of the vesicle can

**v.** Secretory-like granules—where the vesicles used to transport the HSP are neither lipid bodies, nor endosomes, or lysosomes. Tumour cells were found to release HSP70 in structures that were only positive for chromogranin A, which is a marker of secretory granules [28].

Once in the extracellular space or general circulation, these HSPs can stimulate a wide range of cell types. However, similar to the secretion mechanisms, the recognition and uptake of HSPs by cells, as well as the role that extracellular HSPs play in cell activation, are poorly understood. HSPs have been reported to bind to a wide variety of receptors on target cells, among which are:

**i.** Low-density lipoprotein (LDL) receptor-related protein 1 (LRP1; CD91)—a receptor involved in receptor-mediated endocytosis, which is found on numerous cell types including antigen-presenting cells (APCs), known to bind to HSP70, HSP90 and calreti-

**ii.** CD40—a member of the tumour necrosis factor (TNF) receptor family that is essential in mediating a broad variety of immune and inflammatory responses and can bind to

**iii.** C-C chemokine receptor type 5 (CCR5; CD195)—a receptor on white blood cells involved in the process by which T cells are attracted to target areas via cytokines, which has also

**iv.** Toll-like receptors (TLRs)—of the ten TLR receptors found in humans, only TLR2 and TLR4 are so far known to act as HSP receptors. They are known to bind to HSP60, HSP70 and HSP90 [34–37]. It has been suggested that TLR activation by HSP is most likely not the result of a direct binding of HSP70 to these receptors but rather either a low affinity interaction or

a secondary activation involving the prior binding of HSP to another receptor [38].

transport system [21].

104 Liquid Biopsy

GRP78 and HSP90 [22–25].

culin [29, 30].

HSP70 [31, 32].

be used as an export system [17, 26, 27].

been shown to bind to mycobacterial HSP70 [33].

**vi.** Scavenger receptors (SR)—a family of receptors currently classified into ten subclasses (A–J) based on structure and biological function [39]. At least three SRs bind to and internalise HSPs, namely, lectin-like oxidised LDL receptor 1 (LOX-1), scavenger receptor expressed by endothelial cell 1 (SREC-1) and fasciclin and EGF-like, laminin-type EGF-like and link domain-containing scavenger receptor 1(FEEL-1)/common lymphatic endothelial and vascular endothelial receptor 1 (CLEVER-1), with HSP70 binding to all three, HSP60 binding to LOX-1, HSP90 binding to LOX-1 and SREC-1 and calreticulin binding SREC-1 but not LOX-1 [38, 40–43]. Furthermore, scavenger receptor-A (SR-A) can bind to and internalise HSP90 and calreticulin as well as HSP110 and GRP170 [42, 44]. The sialic acid-binding immunoglobulin-type lectin (Siglec) receptors Siglec-5 and Siglec-14 have also been found to bind to HSP70 [45, 46].

**Figure 1.** Heat shock proteins (HSPs) are exported into the extracellular space and general circulation via a number of different processes including cell lysis, secretory vesicles, lysosomal endosomes or export vesicles. Once these extracellular HSPs reach the target tissues, they bind to a variety of receptors, which initiate an alarm response. When these extracellular HSPs are collected from patients with chronic diseases and quantified, they can have diagnostic or prognostic value.
