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Gene

Volume 575, Issue 2, Part 1, 10 January 2016, Pages 171-186
Gene

Gene wiki review
HSP90AB1: Helping the good and the bad

https://doi.org/10.1016/j.gene.2015.08.063Get rights and content

Highlights

  • This is a review on HSP90AB1 (heat shock protein 90 kDA alpha, class B, member 1), also known as HSP90beta.

  • HSP90AB1 binds to all kinds of proteins including steroid hormone receptors, transcription factors, ubiqitin ligases and kinases.

  • HSP90AB1 is a chaperone that stabilizes proteins in a properly folded structure, but is also involved in protein transport and degradation.

  • Inhibition of HSP90AB1 function is a promising therapeutic approach for a variety of diseases including cancer and inflammatory diseases.

Abstract

HSP90AB1 (heat shock protein 90 kDA alpha, class B, member 1), also known as HSP90beta, is a member of the large family of HSPs which function as molecular chaperones. Chaperones, by binding to client proteins, support proper protein folding and maintain protein stability, especially after exposure to various kinds of cellular stress. Client proteins belong to various protein families including kinases, ubiquitin ligases and transcription factors. HSP90 proteins act as dimers and bind clients with the help of co-chaperones. The co-chaperones influence many functions including client binding, ATPase activity or ATP binding of HSP90. HSPs are necessary for a large scale of cellular processes and therefore essential for cell survival. Since client proteins can be mutant proteins that would be degraded without the help of chaperones, HSPs also promote tumor formation and cancer cell proliferation. As such, they are also targets for new therapeutic approaches in cancer treatment. This review focuses on recent studies on HSP90AB1, if possible in comparison with its close homologue HSP90AA1.

Introduction

Heat shock proteins (HSP) are a large group of chaperones which are proteins that assist in protein folding, stabilize proteins and help to refold denatured proteins, processes that are dependent on hydrolysis of ATP. If proper folding is not possible, they also aid in protein degradation. The major groups of HSPs are shown in Table 1. Chaperonins form a sub-class of HSPs and are characterized by a stacked double-ring structure forming barrels (Xu et al., 1997). Inside the barrel structures, they contain hydrophobic residues for client binding (Lindquist, 1986). The prototypes of chaperonins are GroEL/GroES (large and small proteins of the GroE operon in E. coli, mutations of which affect the growth of lambda phage by interfering with assembly of its head protein E) in bacteria (Georgopoulos, CP, et al., 1973, Sternberg, N, 1973a, Sternberg, N, 1973b, Hendrix, RW, 1979, Yamamori, T and Yura, T, 1980, Fayet, O, et al., 1989) and Hsp60/Hsp10 proteins in eukaryotic cells (Johnson and Craig, 1997).

Originally, HSPs were described as proteins that were up-regulated after elevated temperatures (Lindquist, 1986). Meanwhile it is recognized that HSPs are involved in the response to all kinds of stress reactions that disturb proper protein conformation such as reactions to chemicals like ethanol, arsenite, cadmium, zinc, copper, mercury, sulfhydryl reagents, calcium ionophores, steroid hormones, chelating agents, viruses, and many more (Lindquist, 1986). Of course, HSPs are strongly induced by DNA damage since this type of stress leads to mutations that often interfere with proper protein folding (Fornace et al., 1988). Since HSPs stabilize DNA binding proteins, it is not surprising to detect genomic instability in HSP70 deficient mice (Hunt et al., 2004). Because also many undamaged proteins need assistance in folding, nearly all physiological processes require HSPs. Indeed, with the help of protein-protein interaction (PPI) studies, proteins could be identified as interaction partners that contribute to the following processes: transcription, mRNA splicing, translation, cell cycle control, DNA repair, apoptosis, intracellular transport, development, immune response, lipid and carbohydrate metabolism, cellular signaling, protein modification and many more (Gong, Y, et al., 2009, Tsaytler, PA, et al., 2009, Gano, JJ and Simon, JA, 2010, Echeverria, PC, et al., 2011, Hartson, SD and Matts, RL, 2012, Taipale, M, et al., 2014). HSPs are also involved in protein transport across membranes (e.g. of mitochondria or endoplasmic reticulum).

Several types of chaperones may act together, dependent on the type of protein or type of damage. One model suggests that client proteins (e.g. proteins in translation dependent on the stage of maturation) are first bound to Hsp70, then to chaperonins (Johnson and Craig, 1997), then to more specialized proteins of the Hsp90 group (Hartl, FU, 1996, Johnson, JL and Craig, EA, 1997). An alternative pathway, independent of HSP70 or HSP90, involves binding to CCT/TriC (chaperonin containing T-complex polypeptide/TCP-1 ring complex) proteins. This pathway is used by filamentous proteins like actin and tubulin (Johnson and Craig, 1997) but also other types of proteins. There are also multichaperone complexes like HSP70/HSP90. Complex formation in this case is mediated by the adapter protein and co-chaperone HOP (HSP organizing protein) which binds to the peptide sequence EEVD at the C-terminus of both proteins (Chen, S and Smith, DF, 1998, Scheufler, C, et al., 2000, Brinker, A, et al., 2002). Often, the client proteins remain bound to a HSP, but they may be released once they are stable on their own. Co-chaperones mediate substrate specificity, regulate activity of client proteins or recruit chaperones to specific locations in order to perform special functions like their roles in clathrin uncoating, synaptic vesicle fusion, or regulating cytoskeleton functions (Young et al., 2003b).

There are a lot of excellent reviews on the family of HSP proteins in general (Hartl, FU, 1996, Wegele, H, et al., 2004, Calderwood, SK, et al., 2006), but also on special subfamilies like HSP70 (Bukau and Horwich, 1998) or HSP90 (Csermely, P, et al., 1998, Pearl, LH and Prodromou, C, 2000, Taipale, M, et al., 2010, Erlejman, AG, et al., 2014a), their role in signaling protein movement (Pratt et al., 2004) and on co-chaperones (Pratt, WB, et al., 2004, Davies, TH and Sanchez, ER, 2005, Cioffi, DL, et al., 2011, Sivils, JC, et al., 2011, Storer, CL, et al., 2011).

This review is focused on HSP90AB1, primarily in humans. Because of overlapping functions and lack of discrimination in the past, the term HSP90 is used when it is not clear which member of the HSP90 protein family was studied.

Section snippets

Classification

Based on several suggestions (Chen, B, et al., 2005, Chen, B, et al., 2006a), lastly by Kampinga et al. (2009), the human genome organization (HUGO) gene nomenclature committee (HGNC, http://www.genenames.org/genefamilies/HSP) recognizes 5 human families of HSPs (Table 1). At the moment, all these families encompass 97 genes.

With 5 genes listed, the HSP90/HSPC family is the smallest among all human HSPs. One of them is a pseudogene (HSP90AA3P), therefore four real HSP's of this family remain (

Cloning of HSP90AB1

Human HSP90AB1 (Rebbe et al., 1987) was cloned based on homology to HSP90AA1 which was the first HSP90 to be purified, as reported in 1982 (Welch and Feramisco, 1982). Both proteins share 60% overall homology and several regions of 50 amino acids (aa) or more share greater than 90% homology (Rebbe et al., 1987). Mouse HSP90AB1 was cloned using the corresponding Drosophila cDNA as a hybridization probe (Moore, SK, et al., 1987, Hoffmann, T and Hovemann, B, 1988). It codes for a protein

Protein domains of HSP90

HSP90 is composed of five domains (Obermann, WM, et al., 1998, Chen, B, et al., 2006a):

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    N-terminal domain (NTD)

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    Linker region/Charged domain 1 (CD1)

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    middle domain (MD)

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    charged domain 2 (CD2)

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    C-terminal domain (CTD)

These main domains as well as their amino acid limits are shown in Fig. 1 for HSP90AB1 and the 732 aa translation product of HSP90AA1. Chen et al. proposed further subdomains and shifts of domain borders based on sequence conservation or variability after analysis of a large number of

Co-chaperones

As already mentioned, many HSP functions are influenced by co-chaperones. They have four major functions (Taipale et al., 2010):

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    They coordinate the interplay between HSP90 and other chaperone systems, such as HSP70

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    stimulate or inhibit ATPase activity of HSP90

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    recruit specific classes of clients and other co-chaperones and

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    have an enzymatic activity.

Some important co-chaperones are listed in Table 3 and shortly characterized in the text below. The co-chaperone binding regions on HSP90 are depicted

The chaperone cycle

After binding of the client protein to the HSP90 homodimer (open configuration), ATP binds to the NTD and induces transition of the complex to a closed ATP bound form. During proper folding, the conformation closes further and the NTDs dimerize transiently, coupled to ATP hydrolysis (Prodromou et al., 2000). Finally, the configuration opens up, the folded client protein is released and ADP dissociates (Fig. 2).

For proper functioning of this simplified chaperone cycle, several co-chaperones are

Mechanism of HSP90 induction

In general, heat shock generates a higher induction of HSP90AA1 compared to HSPAB1 (Ullrich et al., 1989). The first level of regulation is during gene transcription. HSF1 (heat shock factor 1), which binds to HSEs (heat shock elements), is thought to be the major transcription factor for HSPs (Ciocca et al., 2013). However, the basal level of HSP90AB1 transcripts does not depend on HSF1 since HSF1 knockout leads to no transcript reduction in a model of oocytes (Metchat et al., 2009). In

Posttranslational modifications

The tyrosine (Y) phosphorylation of Hsp90AA1 and Hsp90AB1 induced by LPS in endothelial cells is mediated by pp60src (Barabutis et al., 2013). Mass spectrometry identified Y309 as a major site of Y phosphorylation on Hsp90AA1 (and Y300 on Hsp90AB1) (Barabutis et al., 2013). Phosphorylation of Ser225 and Ser254 (counted after removal of the first methionine) of HSP90AB1 inhibited binding of AHR (arylhydrocarbon receptor) (Ogiso et al., 2004). The phosphorylation of HSP90 is inhibited by

Inhibition of HSP90

Derivatives of geldanamycin, like 17-dimethylaminoethylamino-17-demethoxy-geldanamycin (17-DMAG) or 17-AAG bind to the ATP pocket in the NTD of HSP90 (Grenert, JP, et al., 1997, Prodromou, C, et al., 1997b, Stebbins, CE, et al., 1997, Roe, SM, et al., 1999). The HSP90 inhibitor Radicicol, a macrocyclic antifungal antibiotic, also binds to the ATP binding pocket of HSP90 (Schulte, TW, et al., 1998, Roe, SM, et al., 1999). Celastrol, a triterpenoid compound that inhibits HSP90AB1 activity, alters

Inductors of HSP90(AB1)

As already described, there is a great number of stress types that are able to induce HSP protein levels. In this paragraph, some studies are summarized that show a specific induction of HSP90AB1. Infection is a basic type of stress and therefore a common reason for HSP induction. Increased HSP90AB1 has been detected after influenza infection in infected cells (Wahl et al., 2010). Bacterial challenge in the abalone Haliotis tuberculata leads to an up-regulation of HSP90AB1 (Travers et al., 2010

HSP90AB1 and protein transport

As already briefly mentioned, HSP90AB1 is necessary for the shuttle of client proteins between cytoplasm and nucleus (Galigniana, MD, et al., 2010a, Galigniana, MD, 2012). The interaction of steroid hormone receptors with Hsp90 is required for their translocation to the nucleus (Czar et al., 1997). In addition to Hsp90 homodimers, transport protein complexes consist of Hsp70, HOP, Hsp40, p23 and PP5 (Chen, MS, et al., 1996, Silverstein, AM, et al., 1997, Pratt, WB, et al., 1999, Murphy, PJ, et

HSP90 and protein degradation

A few examples of HSP90-associated protein degradation have been described. Apo(lipo)proteinB (apoB) is essential for the assembly and secretion of lipoproteins. In a cell-free system it has been shown that apoB degradation is dependent on HSP90 (Gusarova et al., 2001). The degradation of misfolded proteins can be mediated by HSP90. The VHL (von-Hippel-Lindau) tumor suppressor protein possesses ubiquitin ligase activity and is involved in the degradation of hypoxia inducible factor (HIF) (

Diseases aggravated by HSP90AB1 or its co-chaperones

Cystic fibrosis (CF, mucoviscidosis) is a genetic disorder with increased viscosity of various secretions leading to damage in the affected organs, most importantly lung and pancreas. The genetic basis is a mutation in the cAMP-regulated chloride ion channel CFTR (cystic fibrosis transmembrane conductance regulator), in most cases a deletion of Phe508. This causes a maturation defect of the protein, leaving the nucleotide binding domain 2 (NBD2) permanently in a protease-sensitive state (

Diseases alleviated by HSP90AB1

Bronchopulmonary dysplasia (BPD), a disease caused by prolonged high oxygen delivery to immature lungs involves cellular damage by oxygen free radicals. Thioredoxin-1 (Trx) is a radical scavenger stabilized by HSP90ab1 and HSP90AB2. Consequently HSP90 proteins prevent hyperoxic cell death (Floen et al., 2014).

Genetic variations

Genetic variations in HSPAB1 have been described that are likely to have an influence on stress-induced mortality, GR level, and GRE (glucocorticoid-response element) binding activity in C57BL/6 mice compared to BALB/c mice (Shen et al., 2010). Some SNPs in HSP90AB1 in laying hens are associated with longer life and higher productivity (Sun et al., 2013), but a very limited number of SNPs in human HSP90AB1 are likely to have a functional consequence (Urban et al., 2012). SNPs in HSP90AB1 in

Role in molecular evolution

Mutations can cause defects in protein folding. Their stability can often only be maintained by binding to HSPs. In addition, molecular evolution by gene duplication and subsequent progressive mutation is only conceivable with the help of a strong and versatile chaperone machinery (Rutherford, SL and Lindquist, S, 1998, Yahara, I, 1999, Jarosz, DF and Lindquist, S, 2010, Jarosz, DF, et al., 2010).

HSP90AB1 client proteins

Large scale protein-protein interaction studies revealed many new client proteins (Tsaytler, PA, et al., 2009, Taipale, M, et al., 2014). Although the spectrum of interaction partners can be well described, they cannot be easily predicted based on protein structures. Therefore, it remains largely enigmatic why some proteins are clients of HSPs and others not. However, interaction of HSP90 chaperone and its Cdc37 co-chaperone with kinases depends on the thermal stability of the kinase domain (

Diseases with down-regulated HSP90

Hsp90ab1, expressed in oligodendrocyte precursor cells, has been shown to be down-regulated in a rat model of acute spinal cord injury (ASCI) (Zhou et al., 2014). HIV virus causes a progressive decline in CD4 + T-cells. This involves an aging-like phenotype with decline of telomerase activity. This is, at least partly, caused by a reduction of HSP90AB1 which is mediated by the HIV-protein Tat (Comandini et al., 2013). In a rat model of radiation-induced fibrosing alveolitis, it has been shown

Diseases with up-regulated HSP90

HSP90 is increased in systemic sclerosis (SSc) of the skin. Inhibition of Hsp90 by 17-DMAG inhibited canonical TGF-beta signaling and completely prevented the stimulatory effects of TGF-beta on collagen synthesis and myofibroblast differentiation (Tomcik et al., 2014). Thus, HSP is pro-fibrotic in this model. Conversely, HSP90 inhibitors repress TGFbeta1 signaling (Noh et al., 2012) by a mechanism dependent on Smurf2 (SMAD-specific E3 ubiquitin ligase 1)-mediated degradation of TGFbetaRII. This

Miscellaneous roles of HSP90 in diseases

Huntington's disease, an autosomal dominant disease, is caused by increased amounts of repetitive trinucleotide repeats (CAG) in the vicinity of promoters which leads to precipitation of polyglutamin protein. As a consequence, massive loss of interneurons in the caudate nucleus occurs, especially affecting GABA- and cholinergic neurons. The trinucleotide expansion comprises up to 100 repeats whereas 11–35 repeats are normal. Inhibition of cytoplasmic HSP90 supports clearance of Huntingtin

DNA damage recognition/DNA repair/Free radicals

The tumor suppressor protein p53 is normally degraded after binding to the E3 ubiquitin ligase MDM2 (mouse double minute 2 homolog). Mutant p53 is commonly overexpressed in cancer cells. HSP90AB1 binds to mutant p53 in cancer cells and stabilizes it (Sepehrnia et al., 1996). P53 binds with its DNA binding domain to the middle and CTD of HSP90AB1 (Muller et al., 2004). It has been demonstrated that the complex is only stable in the presence of HSC70, HSP40, HOP and ATP (King et al., 2001).

Cancer

HSPs are frequently up-regulated in cancer. One way of promoting cancer cell survival and proliferation is by stabilizing proteins with activating mutations, and other cancer promoting proteins that would otherwise be degraded. A systematic analysis of the expression of HSP90AA1 and HSP90AB1 proteins, their co-chaperones (Aha1, Cdc37, p23, Tpr2) and the Hsp90 dependent transcription factor HSF1 in 17 cancer types suggested that the overexpression is tumor-specific and obviously random. However,

Miscellaneous functions

In a study of 55 women with primary ovarian failure and 65 women with infertility, about 30% had anti-ovarian antibodies (AOA) (Pires et al., 2007). None of the 60 control women had AOA. Using chromatography and mass spectrometry, the predominant antigen could be identified as HSP90AB1 (Pires and Khole, 2009).

Concluding remarks

Since HSP90AB1 is involved in stabilization and transport of all types of proteins, it belongs to the proteins with most universal impact on biological functions. Because it is an ubiquitous protein, its importance for these various processes is often not highly enough estimated. Its overexpression in cancers together with its stabilizing function of proteins that promote essential functions of tumor cells make HSP inhibition a promising strategy for tumor treatment. Many clinical trials are

Acknowledgments

This review and the corresponding Gene Wiki article are written as part of the Gene Wiki Review series — a series resulting from a collaboration between the journal GENE and the Gene Wiki Initiative. The Gene Wiki Initiative is supported by National Institutes of Health (GM089820). Additional support for Gene Wiki Reviews is provided by Elsevier, the publisher of GENE. The corresponding Gene Wiki entry for this review can be found here: https://en.wikipedia.org/wiki/HSP90AB1.

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