Encyclopedia of Signaling Molecules

2018 Edition
| Editors: Sangdun Choi


Reference work entry
DOI: https://doi.org/10.1007/978-3-319-67199-4_101690


Historical Background

Heat shock proteins (Hsps) were first described by Tissières in 1974 in the salivary glands of Drosophila third instar larvae exposed to sub-lethal temperatures (temperature shift of 20–35 °C). Hsps were then discovered in all living organisms, from bacteria to human including plants (Lindquist and Craig 1988). Five families of heat shock proteins have been described: the HspH (large Hsps), 90 kDa (HspC-Hsp90), 70 kDa (HspA-Hsp70), 60 kDa (HspD-Hsp60), and the 20–30 kDa small heat shock proteins (HspB-small Hsps, sHsps). The interest in these proteins was stimulated by discovering that their expression can be triggered by many environmental stress conditions as well as by toxins known to alter the folding of proteins.This lead to the finding that Hsps are molecular chaperones whose function is to attenuate stress-induced damages in protein folding and participate in the refolding or elimination of aberrantly folded polypeptides. Cells expressing Hsps have therefore an improved ability to withstand stress and transiently develop resistance. Hsps, which are also often referred as stress proteins, are now considered as important actors of the so-called cellular stress response. The human family of small heat shock proteins is represented by ten members (HspB1–HspB10) which share a C-terminal domain (the crystallin domain), a less conserved N-terminal domain containing an hydrophobic WD/PF motif, and a flexible C-terminal tail with a IXI/V motif. In addition, the N-terminal domain of several small Hsps, including HspB1, contains serine sites that are phosphorylated under specific conditions(Kappe et al. 2003). Small Hsps are oligomeric polypeptides that display very large and heterodispersed native sizes (50–800 kDa). Three of them are stress inducible (HspB1, HspB5, and HspB8) and only four have a conserved ATP-independent chaperone activity (HspB1, HspB4, HspB5, and HspB8). HspB1, one of the most studied small Hsp, was first purified and characterized in the late 1980s (Arrigo and Welch 1987). In addition to its upregulated expression and major role in stress conditions, HspB1 is also an efficient antiapoptotic and antioxidant polypeptide and a crucial modulator of F-actin dynamics. Moreover, it is constitutively expressed in many normal and pathological tissues where it acts as a specialized chaperone (Arrigo 2012).

HspB1 Phosphorylation and Oligomerization

Human HspB1 phosphorylation occurs at the level of three serine sites (ser15, ser78, and ser82) localized in the N-terminal part of the protein while murine HspB1 (also called Hsp25) has only two phosphorylated serine sites (ser15 and ser86). The kinases responsive of HspB1 phosphorylation are the mitogen-activated protein kinases associated protein kinases (MAPK/MK2,3) that are activated by phosphorylation by MAP p38 protein kinase. In some particular conditions, HspB1 can also be phosphorylated by MK5-PRAK, PKCγ, PKD, and cGMP-dependent protein kinase. Hence, HspB1 has rather complex patterns of phosphorylation due to the many transduction pathways that activate its kinases.The oligomerization of HspB1 is a fascinating dynamic phenomenon directly linked to the physiology of the cell and to its environment. For example, in growing HeLa cancer cells where it is constitutively expressed, HspB1 displays a typical pattern of phosphorylation-oligomerization characterized by three subpopulations with different native sizes (<150 kDa, 150–400 kDa, and >400 kDa) and phosphorylated serines. Phosphoserine 15 is present only in <150 kDa oligomers while phosphoserine 78 is recovered only in the medium sized oligomers. In contrast, phosphoserine 78 is distributed between the small and large oligomers (Paul et al. 2010). However, this typical organization is very dynamic and can be transiently modified by changes in the environment, growth conditions, or differentiation. It is believed that HspB1 alpha-crystallin domain, which contains a unique cysteine residue and a beta-7 strand, is essential for the formation of dimers that can multimerize under the control of the phosphorylation-sensitive N-terminal domain (Mymrikov et al. 2010). Is phosphorylation a key factor regulating HspB1 oligomerization? It is difficult to give a straight answer to this question. Studies performed using nonphosphorylatable as well as phosphomimicry mutants revealed that in some conditions phosphorylation plays a role. For example, an intense phosphorylation usually induces the dissociation of HspB1 large oligomers but this not the case when cells are starved, confluent, exposed to reducing conditions, or growing in tumors (Bruey et al. 2000). The relation phosphorylation-oligomerization of HspB1 appears mainly regulated by the contacts between cells, such as those that exist in tissues and tumors. Hence, whatever the status of phosphorylatable serines could be, cell to cell contacts induce the formation of large oligomeric structures and small oligomers are induced by starvation. This clearly points to the role played by HspB1 as a crucial modulator of F-actin dynamics through its association with the plasma membrane cytoskeleton compartment where nucleation occurs (Pichon et al. 2004).

HspB1 in Heat Shock Conditions

HspB1 expression is drastically upregulated when cells are exposed to heat shock or environmental conditions that damage the folding of proteins. This occurs through the activation of the transcription factor HSF-1 (Heat Shock Factor-1) which, following posttranslational modifications and homotrimer formation, migrates into the nucleus to induce a massive transcription of Hsp genes; this results in Hsps synthesis and enhanced ability of cells to resist to heat shock or other types of stress (Lindquist and Craig 1988). HspB1 accumulation occurs during a rather long time period not restricted to the duration of the stress. Indeed, it is still observed during the so-called recovery period after stress. In heat shock condition, HspB1 undergoes changes in its localization, oligomerization, and phosphorylation. Drastic challenges can redistribute HspB1 into the collapsing nucleus while, in less intense and sublethal conditions, only the oligomerization and phosphorylation of this protein are transiently modified. For example, the particular oligomerization of HspB1 is destroyed and end ups in the accumulation of highly phosphorylated small oligomers that interact with stress-induced misfolded polypepdides (Fig. 1). Thereafter, during the recovery period, larger oligomers are reformed that are differently phosphorylated than those observed in unstressed cells (Paul et al. 2010). The particular oligomers acting as storage compartment for misfolded polypeptides appear related to the holdase molecular sponges described in vitro to entrap stress-denatured client polypeptides before ATP-dependent molecular chaperones (Hsp70, Hsp90, and co-chaperones) could take care of them (Stromer et al. 2003). This storage process avoid the accumulation of damaged polypeptides that can form proteolytic resistant toxic aggregates. Unfortunately, the molecular mechanisms regulating the dynamic oligomerization of HspB1 in cells exposed to environmental challenges is still well not understood as is the precise role played by phosphorylation triggered by stress-activated p38 MAPKAP kinases 2,3.
HspB1, Fig. 1

Schematic representation of HspB1 activity in heat shock conditions. HspB1 phosphorylated small oligomers interacts with misfolded polypeptides that are subsequently stored in HspB1 large oligomers that display a specific phosphorylation pattern (reservoirs of folding intermediates). The entrapped polypeptides are then refolded by the ATP-dependent chaperone machinery or eliminated by the ubiquitin-proteasome proteolytic system. The system is transient and in cells that have recovered from the stress HspB1 oligomerization-phosphorylation is back to normal

HspB1 in Oxidative Conditions

Expression of HspB1 generates a cellular protection against oxidative stress inducers and inflammatory cytokines but does not appear upregulated in these conditions. HspB1 consolidates intracellular redox homeostasis by decreasing the intracellular level of reactive oxygen species and nitric oxide, by upholding mitochondrial membrane potential, by maintaining glutathione in its reducing form, and by decreasing iron intracellular levels (Arrigo et al. 2005) (Fig. 2). It also stimulates the activity of glucose 6-phosphate dehydrogenase (G6PD) by interacting with this enzyme (Arrigo 2013). In oxidative conditions, the presence of HspB1 therefore strongly attenuates DNA damages, protein oxidation, lipid peroxidation, and cytoskeleton architecture disruption. In response to oxidative conditions, HspB1 is highly phosphorylated and its medium and large oligomers are rapidly dissociated favoring the interaction with G6PD and irreversibly oxidized polypeptides that must be eliminated. In contrast, upon TNFα treatment, large unphosphorylated structures are rapidly formed that appear essential for the protective activity of HspB1 before phosphorylated small oligomers accumulate.
HspB1, Fig. 2

Cartoon of HspB1 stimulatory or inhibitory activities in apoptotic and survival pathways. Red lines are inhibitory action of HspB1 while the blue ones are indicative of a stimulatory activity. In the case of PTEN, its inhibitory action toward the PI3K-Akt pathway is abolished by HspB1 consequently of the rapid degradation of PTEN when it interacts with HspB1

HspB1 in Apoptotic Conditions

The expression of HspB1 is not upregulated by apoptotic programs. Therefore, as in oxidative conditions, a putative activity of this protein can only be detected if it is constitutively expressed, as for example in many human cells, particularly when they are of cancer origin. In that respect, we showed that HspB1 efficiently counteracts apoptosis triggered by many inducers through drastic changes in its structural organization (Mehlen et al. 1996; Paul et al. 2010). To interfere with apoptotic pathways, HspB1 targets several key polypeptides both upstream and downstream of mitochondria (Fig. 2). Major targeted upstream pathways are Bid or Daxx dependent or induced by alterations in F-actin or nucleus architecture integrity. Downstream pathways include the interaction with cytochrome c once it is released from mitochondria, impairment of the apoptosome-associated procaspase-9 activation, and interaction with effector caspase-3. The dynamic oligomerization and phosphorylation of HspB1 in cells undergoing apoptosis depends on the pathway triggered by the inducer (Paul et al. 2010). Two classes of apoptotic inducers could be defined. The first class is represented by etoposide and Fas which transiently shift HspB1 toward large oligomeric structures. The second class contains staurosporine and cytochalasin D, drugs known to induce a rapid disruption of F-actin architecture, which rapidly shift HspB1 in small oligomers before large ones are re-formed. The upstream effects, particularly at the level of F-actin, may first require small oligomers while the downstream effects require larger oligomeric structures (Bruey et al. 2000). These transient changes in oligomerization/phosphorylation, which correlate with HspB1 ability to transiently counteract the inducers effectiveness to activate caspases, suggest that this chaperone has multiple and complex strategies to negatively modulate apoptotic pathways. In addition, HspB1 is probably also efficient towards other death programs, as shown recently in the case of ferroptosis.

HspB1 in Differentiating Cells

HspB1 expression is usually transiently upregulated during the early phase of many differentiation programs where it is essential to avoid apoptosis. HspB1 probably acts against the drastic changes in protein and cytoskeletal organization occurring in differentiating cells which could sponteanously induce apoptosis (Arrigo 2005). This protecting effect is associated with drastic changes in HspB1 oligomerization and phosphorylation: from small phosphorylated oligomers to large unphosphorylated ones and then later to unphosphorylated small oligomers (Mehlen et al. 1997).

The Client Chaperone Machinery of HspB1

Constitutively expressed HspB1 has pleotropic activities required for cells to grow, rest, differentiate, or better adapt to changes in their physiology or pathological status. It is involved in many molecular mechanisms, including those which regulate cytoskeleton architecture, half-life of polypeptides, redox status, intracellular transport, and protection against spontaneous or stimulated cell death. Moreover, high levels of this protein is a common characteristic of several pathological conditions. Being secreted, it may also play important roles extracellularly. How HspB1 could be so broadly active? In that respect, it is important to note the large number of interactions of HspB1 with crucial polypeptides that have been (and are still) reported in the literature. The consequences of these interactions are modifications of the functions and/or half-lives of the interacting polypeptides and/or prevention against their aggregation potential (Arrigo 2013). So, HspB1 can act as a specialized chaperone in many different molecular mechanisms. How this is done? As described above, a key role is the dynamic oligomerization/phosphorylation organization of HspB1 which is highly sensitive to changes in the cellular physiology, being, for example, deeply modified when cells are growing, resting, differentiating, committed to apoptosis, or exposed to changes in their environment (Fig. 3). The dynamic changes in HspB1 oligomerization-phosphorylation are probably crucial factors that allow HspB1 to modulate its structural plasticity to create recognition platforms where specific polypeptides could bind. This opens new area of investigations aimed at searching the structural organizations of HspB1 required to recognize and interact with diverse client repertoires in defined cellular conditions.For example, in apoptotic cells, HspB1 probably interacts with inducer-specific polypeptides that modulate the corresponding apoptotic pathways. Moreover, our recent finding shows that, in the same human cell, specific phospho-oligomeric structures of HspB1 can interact with different client proteins (Gibert et al. 2012; Arrigo and Gibert 2013). Hence, constitutively expressed HspB1, similarly to Hsp90 (Taipale et al. 2010), is a molecular sensor linked to the physiology of the cell that could easily regulate a large repertoire of cellular functions through its chaperone/client interactome system.
HspB1, Fig. 3

Schematic representation of HspB1 interactions with client polypeptides in different physiologic conditions

HspB1 in Tumor Survival and Dissemination

In cancerous pathologies, a high level of expression of HspB1 is often observed which promotes deleterious resistance and aggressive behavior of tumor cells and consequently poor clinical outcome (Ciocca et al. 2013). In addition of being able to protect cancer cells against the apoptotic signals mediated by the immune system, HspB1 is also deeply involved in the survival of tumor cells (Fig. 2) and their dissemination into surrounding tissues as well as in the formation of metastatic colonies. It also counteracts cancer treatments, such as chemotherapy, hyperthermia, and radiation. At the molecular level, HspB1 modulates the activity of many client proteins sharing potent prooncogenic activity (Ciocca et al. 2013; Arrigo and Gibert 2014), such as: the cell survival kinase Akt, the tumor suppressor PTEN whose degradation is stimulated by HspB1, the PEA-15 molecular switch of Fas-induced apoptosis, HDAC6 which contributes to oncogenic pathways activation, Her-2/neu oncogene, β-catenin which plays a key role in in tumor cell survival, and Snail which regulates the maintenance of cancer stem cells as well as polypeptides involved in the control of genetic imbalances (haploinsufficiency). The oncogene-induced senescence pathway, a host anticancer response, as well as the cellular matrix are other important targets of HspB1. Indeed, HspB1 acts at the level of HDM2, which serves as ubiquitin ligase to target p53 for degradation, matrix metalloproteinases, and SPARC (secreted protein, acidic and rich in cysteine), a polypeptide playing an important role in cell adhesion and migration. Among several other mechanisms that are not yet elucidated an important one is the promotion of the metastatic phenotype. In that regard, HspB1 modulates the expression of prometastatic genes dependent of the STAT3/Twist signaling by playing a role in the binding of STAT3 to the Twist promoter, hence promoting epithelial-mesenchymal transition (EMT) and metastasis. Of interest, HspB1 elimination suppresses EMT signatures and induces long-term dormancy. GATA1, another transcription factor modulated by HspB1, is essential for erythroid differentiation and inactive in almost all megakaryoblastic leukemias in patients with Down syndrome. Unfortunately, the structural active forms of HspB1 that interact with these procancerous polypeptides targets are still not known. However, experiments performed in syngeneic rats bearing colon cancer cells revealed that large oligomeric structures are responsible of the tumorigenic activity of human HspB1 (Garrido et al. 1998). HspB1 phosphorylation or even an aberrant phosphorylation of HspB1 has also been suggested to be associated with cancer development and progression.


The small heat shock protein HspB1 plays an essential role in the cellular response to heat shock. This phospho-oligomeric ATP-independent molecular chaperone, whose expression is upregulated by heat shock, acts as a storage compartment for misfolded polypeptides before they are refolded by ATP-dependent chaperones. These storage compartments result of specific changes in HspB1 dynamic oligomerization and phosphorylation. Being also constitutively expressed during differentiation as well as in many human cell types, it can induce a protection against different stresses, such as apoptosis and oxidative stress. In unstressed cells, it regulates a large panel of cellular processes and plays major roles in many human pathological diseases. The large number of apparently unrelated functions assigned to constitutively expressed HspB1 results of its interactions with crucial client polypeptides that are subsequently modified in their half-life and/or activity. The dynamic changes in HspB1 oligomerization and phosphorylation that are observed in response to modifications in the cellular physiology are key factors to promote the formation of structural platforms aimed at recognizing specific client polypeptides. Hence, HspB1 is a molecular sensor that allows cells to better adapt or respond to changes in their physiology or environment.


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© Springer International Publishing AG 2018

Authors and Affiliations

  1. 1.Apoptosis, Cancer and Development Laboratory, UMR INSERM 1052-CNRS 5286, Lyon Cancer Research CenterClaude Bernard University Lyon1LyonFrance