Heat shock protein 27 (Hsp27) also known as heat shock protein beta-1 (HSPB1) is a protein that in humans is encoded by the HSPB1 gene. [5] [6]
Hsp27 is a chaperone of the sHsp (small heat shock protein) group among α-crystallin, Hsp20, and others. The common functions of sHsps are chaperone activity, thermotolerance, inhibition of apoptosis, regulation of cell development, and cell differentiation. They also take part in signal transduction.
sHsps have some structural features in common: Very characteristic is a homologous and highly conserved amino acid sequence, the so-called α-crystallin domain near the C-terminus. These domains consist of 80 to 100 residues with sequence homology between 20% and 60% and fold into β-sheets, which are important for the formation of stable dimers. [7] [8] Hsp27 is rather unique among sHsps in that its α-crystallin domain contains a cysteine residue at its dimer interface, which can become oxidized to form a disulfide bond that covalently links the dimer. [9] The N-terminus consists of a less conserved region, the so-called WD/EPF domain, followed by a short variable sequence with a rather conservative site near the end of this domain. The C-terminal region of sHsps consists of the above mentioned α-crystallin domain, followed by a variable sequence with high motility and flexibility. [10] Despite relatively low levels of global sequence conservation in the C-terminal region, many sHsps contain a locally conserved Ile-Xxx-Ile/Val (IxI/V) motif that plays a role in regulating the assembly of oligomers. [11] It is highly flexible and polar because of its negative charges. [12] Probably it functions as a mediator of solubility for hydrophobic sHsps and it stabilizes the protein and protein/substrate complexes. This was shown by elimination of the C-terminal tail in Hsp27Δ182-205 [13] and in Hsp25Δ18. [14] In the case of Hsp27, the IxI/V motif corresponds to 181-Ile-Pro-Val-183, and this region of the protein plays a critical role, as the mutation of the central Pro residue causes the hereditary motor neuropathy Charcot-Marie-Tooth disease. [15]
Hsp27 forms large, dynamic oligomers with an average mass near 500 kDa in vitro. [16] The N-terminus of Hsp27, with its WD/EPF-region, is essential for the development of these large oligomers. [17] [18] Hsp27-oligomers consist of stable dimers, which are formed by two α-crystallin-domains of neighboring monomers, [16] [11] which was first shown in crystal structures of the proteins MjHSP16.5 from Methanocaldococcus jannaschii [7] and wheat Hsp16.9. [8] Therefore the first step in the oligomeric process involves dimerization of the α-crystallin domain. In metazoans, dimerization by α-crystallin domains proceeds through the formation of a long β-strand at the interface. The amino acid sequences in this region, however, are predicted to be disordered [19] Indeed, the α-crystallin domain of Hsp27 partially unfolds in its monomeric state and is less stable than the dimer. [20]
The oligomerization of Hsp27 is a dynamic process: There is a balance between stable dimers and oligomers (up to 800 kDa) consisting of 16 to 32 subunits and a high exchange rate of subunits. [18] [21] [22] The oligomerization depends on the physiology of the cells, the phosphorylation status of Hsp27 and the exposure to stress. Stress induces an increase of expression (after hours) and phosphorylation (after several minutes) of Hsp27. Stimulation of the p38 MAP kinase cascade by differentiating agents, mitogens, inflammatory cytokines such as TNFα and IL-1β, hydrogen peroxide and other oxidants, [23] leads to the activation of MAPKAP kinases 2 and 3 which directly phosphorylate mammalian sHsps. [22] The phosphorylation plays an important role for the formation of oligomers in exponentially growing cells in vitro, but the oligomerization in tumor cells growing in vivo or growing at confluence in vitro is dependent on cell-cell contact, but not on the phosphorylation status. [24] Furthermore, it was shown that HSP27 contains an Argpyrimidine modification. [25]
In all probability, the oligomerization status is connected with the chaperone activity: aggregates of large oligomers have high chaperone activity, whereas dimers and monomers have relatively higher chaperone activity. [16] [20] [11]
Hsp27 appears in many cell types, especially all types of muscle cells. It is located mainly in the cytosol, but also in the perinuclear region, endoplasmatic reticulum, and nucleus. It is overexpressed during different stages of cell differentiation and development. This suggests an essential role for Hsp27 in the differentiation of tissues.
An affinity of high expression levels of different phosphorylated Hsp27 species and muscle/neurodegenerative diseases and various cancers was observed. [26] High expression levels possibly are in inverse relation with cell proliferation, metastasis, and resistance to chemotherapy. [27] High levels of Hsp27 were also found in sera of breast cancer patients; [28] therefore Hsp27 could be a potential diagnostic marker.
The main function of Hsp27 is to provide thermotolerance in vivo, cytoprotection, and support of cell survival under stress conditions. More specialized functions of Hsp27 are manifold and complex. In vitro it acts as an ATP-independent chaperone by inhibiting protein aggregation and by stabilizing partially denatured proteins, which ensures refolding by the Hsp70-complex. Hsp27 is also involved in the apoptotic signalling pathway. Hsp27 interacts with the outer mitochondrial membranes and interferes with the activation of cytochrome c/Apaf-1/dATP complex and therefore inhibits the activation of procaspase-9. [26] The phosphorylated form of Hsp27 inhibits Daxx apoptotic protein and prevents the association of Daxx with Fas and Ask1. [29] Moreover, Hsp27 phosphorylation leads to the activation of TAK1 and TAK1-p38/ERK pro-survival signaling, thus opposing TNF-α-induced apoptosis. [30]
A well documented function of Hsp27 is the interaction with actin and intermediate filaments. It prevents the formation of non-covalent filament/filament interactions of the intermediate filaments and protects actin filaments from fragmentation. It also preserves the focal contacts fixed at the cell membrane. [26]
Another function of Hsp27 is the activation of the proteasome. It speeds up the degradation of irreversibly denatured proteins and junkproteins by binding to ubiquitinated proteins and to the 26S proteasome. Hsp27 enhances the activation of the NF-κB pathway, that controls a lot of processes, such as cell growth and inflammatory and stress responses. [31] The cytoprotective properties of Hsp27 result from its ability to modulate reactive oxygen species and to raise glutathione levels.
Probably Hsp27 – among other chaperones – is involved in the process of cell differentiation. [32] Changes of Hsp27 levels were observed in Ehrlich ascite cells, embryonic stem cells, normal B-cells, B-lymphoma cells, osteoblasts, keratinocytes, neurons etc. The upregulation of Hsp27 correlates with the rate of phosphorylation and with an increase of large oligomers. It is possible that Hsp27 plays a crucial role in the termination of growth.
At least 12 disease-causing mutations in this gene have been discovered. [33] Heritable mutations in HSPB1 cause distal hereditary motor neuropathies and the motor neuropathy Charcot-Marie-Tooth disease. [34] There are missense mutations throughout the amino acid sequence of Hsp27, and most disease-causing mutations present with adult-onset symptoms. [34] One of the more severe Hsp27 mutants is the Pro182Leu mutant, which manifests symptomatically in the first few years of life and was additionally demonstrated in a transgenic mouse model. [34] [35] The genetic basis of these diseases is typically autosomal dominant, meaning that only one allele contains a mutation. Since the wild-type HSPB1 gene is also expressed alongside the mutated allele, the diseased cells contain a mixed populations of wild-type and mutant Hsp27, and in vitro experiments have shown that the two proteins can form heter-oligomers. [36]
Notably, phosphorylated Hsp27 increases human prostate cancer (PCa) cell invasion, enhances cell proliferation, and suppresses Fas-induced apoptosis in human PCa cells. Unphosphorylated Hsp27 has been shown to act as an actin capping protein, preventing actin reorganization and, consequently, cell adhesion and motility. OGX-427, which targets HSP27 through an antisense mechanism, is currently undergoing testing in clinical trials. [37]
Protein kinase C-mediated HSPB1 phosphorylation protects against ferroptosis, an iron-dependent form of non-apoptotic cell death, by reducing iron-mediated production of lipid reactive oxygen species. These novel data support the development of Hsp-targeting strategies and, specifically, anti-HSP27 agents for the treatment of ferroptosis-mediated cancer. [38]
Hsp27 has been shown to interact with:
In molecular biology, molecular chaperones are proteins that assist the conformational folding or unfolding of large proteins or macromolecular protein complexes. There are a number of classes of molecular chaperones, all of which function to assist large proteins in proper protein folding during or after synthesis, and after partial denaturation. Chaperones are also involved in the translocation of proteins for proteolysis.
Heat shock proteins (HSP) are a family of proteins produced by cells in response to exposure to stressful conditions. They were first described in relation to heat shock, but are now known to also be expressed during other stresses including exposure to cold, UV light and during wound healing or tissue remodeling. Many members of this group perform chaperone functions by stabilizing new proteins to ensure correct folding or by helping to refold proteins that were damaged by the cell stress. This increase in expression is transcriptionally regulated. The dramatic upregulation of the heat shock proteins is a key part of the heat shock response and is induced primarily by heat shock factor (HSF). HSPs are found in virtually all living organisms, from bacteria to humans.
In anatomy, a crystallin is a water-soluble structural protein found in the lens and the cornea of the eye accounting for the transparency of the structure. It has also been identified in other places such as the heart, and in aggressive breast cancer tumors. Since it has been shown that lens injury may promote nerve regeneration, crystallin has been an area of neural research. So far, it has been demonstrated that crystallin β b2 (crybb2) may be a neurite-promoting factor.
The 70 kilodalton heat shock proteins are a family of conserved ubiquitously expressed heat shock proteins. Proteins with similar structure exist in virtually all living organisms. Intracellularly localized Hsp70s are an important part of the cell's machinery for protein folding, performing chaperoning functions, and helping to protect cells from the adverse effects of physiological stresses. Additionally, membrane-bound Hsp70s have been identified as a potential target for cancer therapies and their extracellularly localized counterparts have been identified as having both membrane-bound and membrane-free structures.
The heat shock response (HSR) is a cell stress response that increases the number of molecular chaperones to combat the negative effects on proteins caused by stressors such as increased temperatures, oxidative stress, and heavy metals. In a normal cell, proteostasis must be maintained because proteins are the main functional units of the cell. Many proteins take on a defined configuration in a process known as protein folding in order to perform their biological functions. If these structures are altered, critical processes could be affected, leading to cell damage or death. The heat shock response can be employed under stress to induce the expression of heat shock proteins (HSP), many of which are molecular chaperones, that help prevent or reverse protein misfolding and provide an environment for proper folding.
Heat shock 70 kDa protein 8 also known as heat shock cognate 71 kDa protein or Hsc70 or Hsp73 is a heat shock protein that in humans is encoded by the HSPA8 gene on chromosome 11. As a member of the heat shock protein 70 family and a chaperone protein, it facilitates the proper folding of newly translated and misfolded proteins, as well as stabilize or degrade mutant proteins. Its functions contribute to biological processes including signal transduction, apoptosis, autophagy, protein homeostasis, and cell growth and differentiation. It has been associated with an extensive number of cancers, neurodegenerative diseases, cell senescence, and aging.
Heat shock 70 kDa protein 1, also termed Hsp72, is a protein that in humans is encoded by the HSPA1A gene. As a member of the heat shock protein 70 family and a chaperone protein, it facilitates the proper folding of newly translated and misfolded proteins, as well as stabilize or degrade mutant proteins. In addition, Hsp72 also facilitates DNA repair. Its functions contribute to biological processes including signal transduction, apoptosis, protein homeostasis, and cell growth and differentiation. It has been associated with an extensive number of cancers, neurodegenerative diseases, cell senescence and aging, and inflammatory diseases such as Diabetes mellitus type 2 and rheumatoid arthritis.
Heat shock protein HSP 90-alpha is a protein that in humans is encoded by the HSP90AA1 gene.
Heat shock factor 1 (HSF1) is a protein that in humans is encoded by the HSF1 gene. HSF1 is highly conserved in eukaryotes and is the primary mediator of transcriptional responses to proteotoxic stress with important roles in non-stress regulation such as development and metabolism.
Alpha-crystallin B chain is a protein that in humans is encoded by the CRYAB gene. It is part of the small heat shock protein family and functions as molecular chaperone that primarily binds misfolded proteins to prevent protein aggregation, as well as inhibit apoptosis and contribute to intracellular architecture. Post-translational modifications decrease the ability to chaperone. Mutations in CRYAB cause different cardiomyopathies, skeletal myopathies mainly myofibrillar myopathy, and also cataracts. In addition, defects in this gene/protein have been associated with cancer and neurodegenerative diseases such as Alzheimer's disease and Parkinson's disease.
Heat shock protein HSP 90-beta also called HSP90beta is a protein that in humans is encoded by the HSP90AB1 gene.
Heat shock protein beta-8 is a protein that in humans is encoded by the HSPB8 gene.
Heat shock protein beta-2 is a protein that in humans is encoded by the HSPB2 gene.
Heat shock 70 kDa protein 1L is a protein that in humans is encoded by the HSPA1L gene on chromosome 6. As a member of the heat shock protein 70 (Hsp70) family and a chaperone protein, it facilitates the proper folding of newly translated and misfolded proteins, as well as stabilize or degrade mutant proteins. Its functions contribute to biological processes including signal transduction, apoptosis, protein homeostasis, and cell growth and differentiation. It has been associated with an extensive number of cancers, neurodegenerative diseases, cell senescence and aging, and Graft-versus-host disease.
Heat shock protein beta-6 (HSPB6) is a protein that in humans is encoded by the HSPB6 gene.
Alpha-crystallin A chain is a protein that in humans is encoded by the CRYAA gene.
The heat shock protein Hsp20 family, also known as small heat shock proteins (sHSPs), is a family of heat shock proteins.
Protein moonlighting is a phenomenon by which a protein can perform more than one function. It is an excellent example of gene sharing.
Heat shock protein beta-3 (HspB3) also known as heat shock 27kDa protein 3 is a protein that in humans is encoded by the HSPB3 gene.
Hsp104 is a heat-shock protein. It is known to reverse toxicity of mutant α-synuclein, TDP-43, FUS, and TAF15 in yeast cells. Conserved in prokaryotes (ClpB), fungi, plants and aswell as animal mitochondria, there is yet to see hsp104 in multicellular animals. Hsp104 is classified as a. AAA+ ATPases and a subgroup of Hsp100/Clp, because of the usage of Atp hydrolysis for structural modulation of other proteins. Hsp104 is not needed for normal cell growth but when exposed to stress there is an increase amount. Removing the aggregates without the hsp104 is insufficient there highlighting the importance of this heat shock protein and its interactions.