Hsp27

Heat shock protein 27 (Hsp27) also known as heat shock protein beta-1 (HSPB1) is a is_associated_with::protein that in humans is encoded by the HSPB1 is_associated_with::gene.

Hsp27 is a chaperone of the is_associated_with::sHsp (small is_associated_with::heat shock protein) group among is_associated_with::ubiquitin, α-is_associated_with::crystallin, is_associated_with::Hsp20 and others. The common functions of sHsps are chaperone activity, thermotolerance, inhibition of is_associated_with::apoptosis, regulation of cell development, and cell differentiation. They also take part in is_associated_with::signal transduction.

Structure
sHsps have some structural features in common: Very characteristic is a homologous and highly conserved amino acid sequence, the so-called α-crystallin-domain at the is_associated_with::C-terminus. These sequences consist of 80 to 100 residues with a homology between 20% and 60% and form β-sheets, which are important for the formation of stable dimers.

The is_associated_with::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 C-terminus of this domain. The C-terminal part of the sHsps consists of the above mentioned α-crystallin domain, followed by a variable sequence with high motility and flexibility.

This C-terminal tail appears in many mammalian sHsps (e.g. mouse Hsp25, αA-crystallin) and has no homology. It is highly flexible and polar because of its negative charges. 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 and in Hsp25Δ18.

Oligomerization
The N-terminus with its WD/EPF-region is essential for the development of high molecular is_associated_with::oligomers, which exclusively have chaperone activity in vitro. Hsp27-oligomers probably consist of stable dimers, which are formed by two α-crystallin-domains of neighbouring is_associated_with::monomers, which was shown with the proteins MjHSP16.5 from Methanocaldococcus jannaschii and wheat Hsp16.9. The stable dimers aggregate to tetramers and finally form unstable oligomers.

The oligomerization of Hsp27 is a dynamic process: There is a balance between stable dimers respectively tetramers and instable oligomers (up to 800 kDa) consisting of 16 to 32 subunits and a high exchange rate of subunits. The oligomerization depends on the physiology of the cells, the is_associated_with::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 is_associated_with::MAP kinase cascade by differentiating agents, is_associated_with::mitogens, inflammatory is_associated_with::cytokines such as TNFα and IL-1β, is_associated_with::hydrogen peroxide and other is_associated_with::oxidants, leads to the activation of MAPKAP kinases 2 and 3 which directly phosphorylate mammalian sHsps. The phosphorylation plays an important role for the formation of oligomers in exponentially growing cells in vitro, but the oligomerization in is_associated_with::tumor cells growing in vivo or growing at confluence in vitro is dependent on cell-cell contact, but not on the phosphorylation status. Furthermore, it was shown that HSP27 contains an Argpyrimidine modification.

In all probability, the oligomerization status is connected with the chaperone activity: aggregates of large oligomers have high chaperone activity, whereas dimers have no chaperone activity. Therefore it is clear, that a formation of large aggregates takes place under heat shock.

Cellular Localization
Hsp27 appears in many is_associated_with::cell types, especially all types of is_associated_with::muscle cells. It is located mainly in the is_associated_with::cytosol, but also in the perinuclear region, is_associated_with::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/is_associated_with::neurodegenerative diseases and various is_associated_with::cancers was observed. High expression levels possibly are in inverse relation with is_associated_with::cell proliferation, is_associated_with::metastasis, and resistance to is_associated_with::chemotherapy. High levels of Hsp27 were also found in sera of is_associated_with::breast cancer patients; therefore Hsp27 could be a potential diagnostic marker.

Function
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 is_associated_with::Hsp70-complex.

Hsp27 is also involved in the apoptotic signalling pathway. Hsp27 interacts with the outer mitochondrial membranes and interferes with the activation of is_associated_with::cytochrome c/is_associated_with::Apaf-1/dATP complex and therefore inhibits the activation of procaspase-9. The phosphorylated form of Hsp27 inhibits Daxx apoptotic protein and prevents the association of Daxx with Fas and Ask1. Moreover, Hsp27 phosphorylation leads to the activation of TAK1 and TAK1-p38/ERK pro-survival signaling, thus opposing TNF-α-induced apoptosis.

A well documented function of Hsp27 is the interaction with is_associated_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 is_associated_with::cell membrane.

Another function of Hsp27 is the activation of the is_associated_with::proteasome. It speeds up the degradation of irreversibly denatured proteins and junkproteins by binding to is_associated_with::ubiquitinated proteins and to the 26S proteasome. Hsp27 enhances the activation of the is_associated_with::NF-κB pathway, that controls a lot of processes, such as cell growth and inflammatory and stress responses. The cytoprotective properties of Hsp27 result from its ability to modulate is_associated_with::reactive oxygen species and to raise is_associated_with::glutathione levels.

Probably Hsp27 – among other chaperones – is involved in the process of cell differentiation. Changes of Hsp27 levels were observed in is_associated_with::Ehrlich ascite cells, is_associated_with::embryonic stem cells, normal is_associated_with::B-cells, B-is_associated_with::lymphoma cells, is_associated_with::osteoblasts, is_associated_with::keratinocytes, is_associated_with::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.

Clinical significance
Hsp70 member proteins, including Hsp72, inhibit apoptosis by acting on the caspase-dependent pathway and against apoptosis-inducing agents such as tumor necrosis factor-α (TNFα), staurosporin, and doxorubicin. This role leads to its involvement in many pathological processes, such as oncogenesis, neurodegeneration, and senescence. In particular, overexpression of HSP72 has been linked to the development some cancers, such as hepatocellular carcinoma, gastric cancers, colonic tumors, breast cancers, and lung cancers, which led to its use as a prognostic marker for these cancers. 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.

Elevated Hsp70 levels in tumor cells may increase malignancy and resistance to therapy by complexing, and hence, stabilizing, oncofetal proteins and products and transporting them into intracellular sites, thereby promoting tumor cell proliferation. As a result, tumor vaccine strategies for Hsp70s have been highly successful in animal models and progressed to clinical trials. Alternatively, overexpression of Hsp70 can mitigate the effects of neurodegenerative diseases, such as Alzheimer’s disease, Parkinson’s disease, Huntington’s corea, and spinocerebellar ataxias, and aging and cell senescence, as observed in centenarians subjected to heat shock challenge. 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.

Interactions
Hsp27 has been shown to interact with:


 * is_associated_with::ASK1,
 * is_associated_with::CRYAA,
 * is_associated_with::CRYAB,
 * is_associated_with::CRYBB2,
 * is_associated_with::HNRPD,
 * is_associated_with::HSPB8,
 * MK2,
 * TAK1, and
 * is_associated_with::TGFB1I1.