Encyclopedia of Signaling Molecules

2018 Edition
| Editors: Sangdun Choi

DNAJB6

  • Shannon E. Weeks
  • Swapnil Bawage
  • Lalita A. Shevde
  • Rajeev S. Samant
Reference work entry
DOI: https://doi.org/10.1007/978-3-319-67199-4_101779

Synonyms

 DJ4;  DnaJ;  HHDJ1;  HSJ-2;  HSJ2;  LGMD1D;  LGMD1E;  MRJ;  MSJ-1

Historical Background

Heat shock proteins were first identified in the late 1970s by Dr. Alfred Tissieres laboratory who noticed that when Drosophila cells in culture were exposed to 37 °C a new set of proteins were synthesized (Arrigo et al. 1980). These proteins have been identified as heat shock proteins (HSP) and play a role in a wide variety of biological processes. Heat shock proteins act as chaperones helping move various client proteins to different cellular compartments. They also ensure the client proteins are folded correctly, as well as aid in degradation of misfolded or damaged proteins (Mitra et al. 2009). Since then, many heat shock proteins have been identified, including DNAJB6. It was initially identified for its role in the development of the embryo and placenta (Hunter et al. 1999). Recently, however, its role in cancer and many other diseases has been investigated.

Structure and Isoforms of DNAJB6

Heat shock proteins are typically classified by their molecular weight: Hsp60, Hsp70, Hsp40/DNAJ, etc. These HSPs are then further divided into subgroups. For example, Hsp40, or DNAJ, is further divided into three subclasses: A, B, and C, which are determined based upon the similarity of their domain to the E. coli DNAJ. These proteins bind to misfolded proteins and transport them to HSP70 proteins for refolding or destruction. There are three distinct domains associated with DNAJ proteins. The first is the J domain which is highly conserved and located near the amino terminus. It has been shown to stimulate ATPase activity of HSP70. The second domain is a glycine and phenylalanine rich region, and the third domain is a cysteine-rich region that contains four zinc finger-like regions known as the C domain (Ohtsuka and Hata 2000). In DNAJ proteins, the various subclasses are named based on which of these three domains they possess. DNAJ subclass A members contain all three of the aforementioned motifs. DNAJ subclass B contains the J domain and the G/F-rich region but lacks the C domain. Finally, DNAJ subclass C possesses only the J domain (Cheetham and Caplan 1998). See Fig. 1 for illustration. As its name implies, DNAJB6 contains both the J domain as well as the G/F-rich region but lacks the C domain.
DNAJB6, Fig. 1

Schematic representation of DNAJ family subclasses

DNAJB6 is located on the distal end of the q arm of chromosome 7 (Hanai and Mashima 2003). It has several isoforms as a result of alternative splicing; isoform a (2.5 kb transcript variant I) and isoform b, which is shorter, are the most well characterized to date (1.6 kb transcript variant II). Additionally, DNAJB6b has a different 3′ coding region and a distinct 3′ untranslated region (UTR) (Bock et al. 2010). The longer isoform message (DNAJB6) has 10 exons and is 326 amino acids long, while the short isoform message (DNAJB6b) has only 8 exons and is only 241 amino acids long. Isoforms share an almost identical sequence and structure; however, DNAJB6b lack the last 95 amino acids of the carboxyl terminal of DNAJB6a. Instead, DNAJB6b contains an additional ten amino acids (KEQLLRLDNK) on the carboxyl terminal that are unique to the short isoform (Hanai and Mashima 2003). See Fig. 2 for illustration.
DNAJB6, Fig. 2

A graphic representation of DNAJB6 gene, variant I and II, and DNAJB6 isoforms

Despite the fact that DNAJB6a and DNAJB6b are splice variants of the same gene, they still have unique physiological functions. DNAJB6a contains a nuclear localization signal sequence and therefore localizes primarily in the nucleus, while DNAJB6b is found ubiquitously throughout the cell, primarily in the cytosol (Hanai and Mashima 2003). Additionally, they both play a very distinct role in various pathologies and diseases.

The Role of DNAJB6 in Pathology and Disease

DNAJB6 is involved in a wide variety of pathological states and diseases. DNAJB6 and its cochaperones, members of the HSP70 family, play a crucial role in preventing the aggregation of misfolded proteins, many of which play roles in degenerative nervous system disorders such as Alzheimer’s disease, Parkinson’s disease, and Huntington’s disease. Mutations in DNAJB6 have also been shown to cause dominantly inherited myopathies such as limb-girdle muscular dystrophy.

Alzheimer’s Disease: Beta amyloid is a major component of Alzheimer’s disease (AD) pathology. It is the most common neurodegenerative disease to date. Clinically, AD is characterized by cognitive decline and memory loss. Pathologically it is characterized by the extracellular accumulation of amyloid β 1-42 (Aβ42) plaques that lead to the production of reactive oxygen species as well as intracellular hyper-phosphorylated tau aggregates (Varadarajan et al. 2000). DNAJB6 has been shown to prevent the fibrillation of Aβ42 in vitro. Recent studies have also shown that DNAJB6 plays a role in preventing the aggregation of Aβ42 intracellularly in cell lines but was unable to clear extracellular Aβ42 that had already formed (Hussein et al. 2015). DNAJB6 has been found to prevent the formation of Aβ42 fibrils by inhibiting the nucleation and growth of the fibrils by binding with high affinity to the growing fibrils. It has been proposed that the high-binding affinity of DNAJB6 to Aβ42 is due to interactions of the exposed hydrophobic residues on Aβ42 and DNAJB6 which prevent nucleation of Aβ42 from occurring (Masson et al. 2014).

Parkinson’s Disease: Parkinson’s disease (PD) is the second most common neurodegenerative disorder, second only to Alzheimer’s disease. It is characterized by bradykinesia, rigidity, and tremors. The pathological features associated with PD include deterioration of dopaminergic neurons in the substantia nigra as well as the formation of protein aggregates called Lewy bodies (Rose et al. 2011). DNAJB6b has been found in the core of these Lewy bodies in dopaminergic nerve cells as well as in astrocytes in PD patients. Alpha-synuclein is also a major contributor to the formation of these Lewy bodies and the presence of these aggregates can lead to an upregulation of heat shock proteins that can suppress the aggregation of alpha-synuclein. It is likely, based on its localization in Lewy bodies, that DNAJB6b is involved in the early formation of Lewy bodies; however, it is still unclear whether the upregulation of DNAJB6 is a cause or a result of the disease (Durrenberger et al. 2009).

Huntington’s Disease: Protein aggregations are also a critical part of Huntington’s disease pathology, which is characterized by a CAG repeat expansion in the huntingtin gene, that is causing additional polyglutamine (polyQ) repeats in the mutant huntingtin protein (The Huntington’s Disease Collaborative Research Group et al. 1993). DNAJB6 has been shown to decrease the amount of polyQ aggregates; when DNAJB6 was removed from the system, the aggregation of polyQ increased (Gillis et al. 2013). The shorter isoform (DNAJB6b) was found to be more effective over all at decreasing buildup of polyQ plaques because it is present in both the nucleus and the cytosol during cellular stress. DJANB6a is effective at preventing the aggregation of polyQ plaques located in the nucleus but is not present in the cytosol and therefore cannot prevent the aggregation of plaques in the cytosol. This indicates that DNAJB6 proteins need to be able to freely diffuse in order to prevent the formation of the polyQ plaques associated with Huntington’s disease (Hageman et al. 2010).

Limb-Girdle Muscular Dystrophy: The fact that DNAJB6 plays a protective role in many neurodegenerative diseases makes it even more intriguing that mutations in DNAJB6 present as diseases of the skeletal muscle. The most common disease caused by a DNAJB6 mutation is limb-girdle muscular dystrophy 1D and is caused by mutations in Phe89 or Phe93. Additionally, mutations of the proximal end of the G/F domain have been shown to cause proximal limb-girdle myopathy while mutations in the distal end of the G/F domain cause distal limb-girdle myopathy (Ruggieri et al. 2015). These mutations have been shown to increase the half-life of DNAJB6, which increases the toxic effect of the mutated DNAJB6 and reduces its protective antiaggregation effects. Although both isoforms have these detrimental mutations, only the small isoform, DNAJB6b, is pathological (Sarparanta et al. 2012).

Cardiac Myopathy: In addition to its involvement in many different neurological diseases, DNAJB6 has also been implicated in playing a role in reducing cardiomyocyte hypertrophy. The calcineurin-nuclear factor of activated T cells (NFAT) signaling pathway has been shown to be a prominent player in pathological cardiac hypertrophy. When activated, calcineurin dephosphorylates members of NFAT transcription factor family in the cytoplasm causing them to translocate into the nucleus where they upregulate the expression of several immune response genes (Molkentin 2004). The upregulation of the immune response genes includes TNFα, and increased TNFαexpression is associated with a variety of cardiac pathologies including cardiac hypertrophy. DNAJB6 has been found to form complexes with histone deacetylase4 (HADC4) which represses TNFα activity in the nucleus thus leading to a decrease in cardiac hypertrophy (Dai et al. 2005). A complete understanding of the functions and roles of DNAJB6 in cell biology is evolving. The section below summarizes the reported roles of DNAJB6 in mechanisms and signaling related to cancer biology.

The Role of DNAJB6a in Cancer

DNAJB6a has been shown to play an important role in several different kinds of cancers including breast cancer. The level of DNAJB6 found in normal tissue varies considerably depending on the type of tissue; however, the level of DNAJB6 found in each of the tissues changes when the organ is affected by cancer. No consistent pattern of up- or downregulation is evident across the various tissues, though all tissues do exhibit some level of change in the level of DNAJB6 expression when affected by cancer in comparison to normal tissue. See Table 1. This table does not account for the different isoforms of DNAJB6, only the total protein level. It has also been suggested that not only the level of DNJAB6 expression is an important component of cancer pathology but the ratio of the long isoform (DNAJB6a) to the short isoform (DNAJB6b) that plays a key role in the development and progression of cancer in various types of cancer.
DNAJB6, Table 1

Expression levels of DNAJB6 in normal and cancerous organs

Organs expressing DNAJB6

Level of DNAJB6 expression

Normal

Cancer

Breast

Undetected

Low, medium

Cervical

Low, medium

Undetected, low, medium

Colon/rectum

Medium

Low, medium

Endometrial

Medium

Undetected, low, medium

Head and neck

Low, medium

Medium

Liver

Undetected

Undetected, low, medium

Lung

Medium

Undetected, medium

Lymphoma

Undetected, low

Undetected, low, medium

Ovarian

Undetermined

Undetected, low, medium

Pancreatic

Undetected

Low, medium

Prostate

Low

Undetected, low

Renal

Medium

Undetected, low, medium

Skin

Undetected

Low, medium

Stomach

Low, medium

Low, medium, high

Testis

Medium

Medium, high

Thyroid

Medium

Undetected, low

Urinary bladder

Medium

Low, medium, high

Metastasis: DNAJB6a has been found to be significantly reduced in aggressive breast cancer, as well as in advanced grade infiltrating ductal carcinoma and other metastatic cancer types. Conversely, cells expressing DNAJB6a are more capable of suppressing metastasis and tumorigenicity. This is achieved by downregulating the production of tumor-promoting proteins such as osteopontin and upregulating breast and melanoma metastasis suppressor proteins (Mitra et al. 2008).

Unlike in breast cancer, the overexpression of DNAJB6 has been found to promote the invasion of colorectal cancer cells. Both isoforms of DNAJB6 were found to be overexpressed in 39% of colorectal tissue and this overexpression correlated directly with the depth of tumor invasion both in vitro and in vivo. Based on these findings, it is possible that expression levels DNAJB6 could be used as a prognostic marker for colorectal cancer (Zhang et al. 2015).

Epithelial to Mesenchymal Transition (EMT): DNAJB6a also plays a crucial role in maintaining the epithelial-like phenotype in cancer cells. Loss of DNAJB6a causes the cells to shift to a more mesenchymal phenotype and become more aggressive. DNAJB6a also upregulates dickopf 1 (DKK1), a known inhibitor of the Wnt signaling pathway that binds to the Wnt coreceptor LRP5/6. DNAJB6, therefore, reverses the epithelial to mesenchymal transition (EMT) by upregulating the production of DKK1 to inhibit the Wnt/β-catenin pathway (Mitra et al. 2010). Additionally, DNAJB6a chaperones for protein complex consisting of DNAJB6a-HSPA8(HSP70)-PP2A. The J domain of DNAJB6a binds to HSPA8 which recruits protein phosphatase, PP2A which dephosphorylates GSK3β leaving it in its active state. The activated GSK3 β works in concert with the degradation complex to mediate β-catenin degradation. The degradation of β catenin results in lower levels of β catenin in the cytosol and therefore less β catenin to import into the nucleus, ultimately resulting in a downregulation of Tcf/Lef-dependent transcription and reduced levels of MSX1. Reduction of MSX1 levels leads to an upregulation of DKK1 which interacts with LRP5/6 and prevents Wnt signals from binding (Menezes et al. 2012). The destabilization of β catenin also leads to a decrease in the level of OPN which is required for the expression of EMT markers (Mitra et al. 2012; Bhattacharya et al. 2012). DNAJB6a is also significantly decreased in patient-derived specimens of invasive and metastatic breast cancer as well as melanoma. Additionally, nude mice injected with DNAJB6a showed reduced tumor growth rates as well as fewer lung metastases than their untreated counterparts (Mitra et al. 2008). See Fig. 3 for an illustration of Wnt/β-catenin pathway and DNAJB6a’s role in it.
DNAJB6, Fig. 3

A simplified diagram of the role of DNAJB6 in the Wnt/β-catenin pathway

Regulation: Although little is currently known about how DNAJB6 expression levels are regulated, a knowledge-based screen of miRNAs identifies hsa-miR-632 (miR-632) that was found to target the coding region of DNAJB6. Exogenous miR-632 expression was found to downregulate levels of DNAJB6 causing significantly increased invasive ability in cell populations. Additionally, silencing of miR-623 was found to reverse the invasive ability of the cells and promote an epithelial like phenotype (Mitra et al. 2012). This could potentially be developed as a therapeutic target for the treatment of breast cancer.

The Role of DNAJB6b in Cancer

Despite the fact that DNAJB6a and DNAJB6b are two splice variants from the same gene, they have remarkably different roles in cancer. DNAJB6b is predominantly found in the cytosol unlike DNAJB6a which is located primarily in the nucleus. Additionally, DNAJB6b lacks the nuclear localization sequence that is found on DNAJB6a; however, during times of cellular stress, such as hypoxia, DNAJB6b does translocate into the nucleus. Currently, little is known about the role of DNAJB6b in cancer. While DNAJB6a suppresses metastasis and tumor growth, DNAJB6b has been shown to promote an increase in contact-dependent growth and accelerated proliferation rates as well as a shift to a more invasive morphology when localized to the nucleus as a result of heat shock or hypoxia (Andrews et al. 2012). One possible mechanism for this translocation is that a pathological increase in cell number causes the formation of a tumor that will result in increased oxygen consumption. The vessels stimulated by angiogenesis are functionally abnormal and therefore unable to supply an adequate amount of oxygen to the tumor cells, which creates a hypoxic environment and leads to the import of DNAJB6b into the nucleus where it could promote further tumor growth (Krishnamachary et al. 2003). This hypothesis seems logical when DNAJB6b localization to the nucleus is looked at in the context of hypoxia, which promotes tumor growth and metastasis as well as drug resistance (Comerford et al. 2002). More investigation is needed in order to gain a more complete understanding of the role of DJANB6b in cancer.

Summary

DNAJB6 is a HSP40 this has two splice variants that play very individual roles in a variety of different diseases including cancer. It has been shown to be beneficial in preventing and/or clearing protein aggregates that are common in many neurodegenerative pathologies. Mutations in DNAJB6 lead to limb-girdle muscular dystrophy 1D as well as proximal muscular dystrophy. The long isoform DNAHB6a is primarily localized in the nucleus and is absent in aggressive or late stage cancers and has been shown to inhibit tumor growth and metastasis through its involvement in the Wnt/β-catenin pathway. Inversely, DNAJB6b, which is normally found in the cytosol, has been found to promote metastasis, invasiveness, and tumor growth when it localizes to the nucleus. It is crucial to gain a deeper understanding of DNAJB6 and the role of both its splice variant in diseases such as cancer. This knowledge could lead to a better understanding of signaling pathways underlying tumor metastasis or possibly uncover a novel therapeutic target to better treat malignant diseases.

Notes

Acknowledgments

NIH R01CA194048 grant to R.S.S.

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Copyright information

© Springer International Publishing AG 2018

Authors and Affiliations

  • Shannon E. Weeks
    • 1
  • Swapnil Bawage
    • 1
  • Lalita A. Shevde
    • 1
  • Rajeev S. Samant
    • 1
  1. 1.Division of Molecular and Cellular Pathology, Department of PathologyUniversity of Alabama at BirminghamBirminghamUSA