Surgery: Current Research

Mechanistic Insights and Translational Relevance of Targeting HSP90 as a Novel Therapy for Cancer

^*Correspondence: Elena Johnson, Editorial Office, Surgery: Current Research, Belgium, Email:

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Introduction

In the crowded environment of a single cell, efficient and precise management of the cellular protein pool is essential for homeostasis. One of the heat shock protein subgroups is HSP90. HSP90 shields cells from negative effects when it is functionally increased in response to environmental stress. HSP90, which makes up between 1% and 2% of all cellular proteins when cells are not under stress, is universally expressed across all species and is evolutionarily conserved. In a pressured state, it can be raised even higher to around 4%-6%. HSP90 has been compared to "molecular glue" because of its abundance and adhesion capabilities. The N-Terminal Domain (NTD) of HSP90, which has ATPase activity, the Middle Domain (MD), which binds to the client protein, and the primary dimerization C-Terminal Domain (CTD) make up the protein's structure. To maintain its "closed" shape for the binding of client proteins, HSP90 uses ATP. The immature client proteins continue to fold, which is followed by the release of energy via ATP hydrolysis. After that, HSP90 transforms into a "open" conformation and releases the matured product. The complex regulation of ATP hydrolysis rates and certain conformational states also calls for co-chaperones. In eukaryotic cells, over 20 co-chaperones of HSP90 have been identified. These cochaperones primarily affect HSP90's molecular activity in four ways: Coordinate the interaction between HSP90 and other chaperone systems, such as HSP70; promote or inhibit the ATPase activity of HSP90; attract particular classes of clients to HSP90; and contribute to various parts of the chaperone cycle through their enzymatic activities.

The HSP90-HSP70-HSP40 complex works cooperatively and successively with the co-chaperones with the TPR domain (i.e., HOP) to mature client proteins. Separately, co-chaperones that increase ATPase activity are thought to be HSP90 conformational cycle activators, whereas those that decrease ATPase activity are more likely to be involved in client loading or the development of mature HSP90 complexes. In order to determine HSP90's dependence, recently comprehensively screened 2156 clones, including protein kinases, transcription factors, and E3 ligases. They discovered that more than 400 client proteins rely on the HSP90- regulated protein folding mechanism to achieve and maintain their active conformations. The HSP90 client proteins also control a wide range of physiological processes, including signal transduction, protein trafficking, chromatin remodeling, autophagy, and cell proliferation and survival, according to a number of studies. In addition, a lot of HSP90 client proteins are frequently overexpressed or mutated in cancer cells.

Subcellular localization of HSP90

On an evolutionary scale, HSP90 is a highly conserved molecular chaperone that is constitutively expressed in the majority of organs and tissues. With its ATPase activity, HSP90 aids in the correct folding, intracellular distribution, and proteolytic turnover of numerous important regulators of cellular homeostasis. Under normal circumstances, HSP90 is primarily found in the cytoplasm, which is also where proteins with abnormal folds are made and polypeptides are generated. HSP90 was discovered to be expressed "everywhere," including the nucleus, mitochondrion, and plasma membrane, as well as being released into the extracellular matrix, utilizing an in vivo PET tracer or in vitro organelle fractionation experiment. Differential subcellular HSP90 expression may play specific roles in a variety of biological processes, particularly cancer. It's interesting to note that HSP90 expression patterns differ between healthy cells and malignant cells, with differences only occurring in the mitochondria or extracellular matrix.

Cytosolic localization of HSP90

The cytosol component expressed HSP90, a crucial homodimer chaperone machinery. It discovered that the C-terminal half of HSP90 (amino acids between 333 and 664) is important for the cytoplasmic localization by creating several shortened versions of HSP90 and utilizing confocal microscopy. However, NLS-HSP90 was preferentially produced in the nucleus by fusing with the Nuclear Localization Signal sequence (NLS). The bulk of HSP90 in these investigations was likewise primarily confined to the cytoplasm. Perhaps the cytosolic HSP90 provides a co-chaperone buffering system that significantly aids in the folding and refolding of important oncogenic drivers and prosurvival regulators. The term "molecular glue" has also been used to describe cytosolic HSP90, emphasizing both its abundance and its buffering capabilities. The majority of HSP90's client proteins are found in the cytoplasm, where they perform a number of activities including metabolic rewiring, signal transmission, post-transcriptional modification, and cytoskeleton remodeling. Actin filament bundling behavior was recently revealed to be modulated by HSP90, and this dynamic interaction between HSP90 and nearly all cytoplasmic filamentous structures highlights this role. Recently, it was discovered that HSP90 transiently crosslinks actin filaments in vitro. Additionally linked to tubulin, HSP90 likely guards against heat denaturation of the protein. Similarly, it has been demonstrated that HSP90 guards against heat stress in myosin. HSP90 therefore has a functional role in the control of the cytoskeleton.

Nuclear localization of HSP90

The nucleus of normal cells showed a low expression of HSP90 (5% -10% of total cellular HSP90). Interestingly, it was discovered that HSP90 and its cochaperones concentrate in the nucleus of dormant Saccharomyces cerevisiae cells with the help of the importin system, which was strengthened during periods of relative metabolic inactivity.

The nuclear localization sequence is a recognized traditional or alternative nuclear import and export signal, and the HSP90 protein contains sequences that are homologous to these signals. It is significant because HSP90 is linked to a variety of nuclear chaperone clients, such as transcription factors, histone, nucleic acids, and histone modifications. Additionally, HSP90 controls a variety of biological processes in the nucleus, such as RNA synthesis, processing, and other telomerase activities. Steroid receptors, kinases, and FKBP52 control HSP90's nuclear translocation. HSP90 interacts with zinc finger proteins, helix-loop-helix proteins, MyoD1, E12, HIF1, HSF1, and the glucocorticoid receptor. When exposed to a high temperature, HSP90 was found in the nuclear membrane, as demonstrated by the transfection of 3T3 cells with HSP90 coupled with EGFP. The plasma membrane allows a large number of tiny chemical molecules to reach the cytoplasm, but it has been demonstrated that none of these molecules can penetrate the nucleus, limiting their effectiveness. However, HSP90 could be translocated from cytoplasm to nucleus, which protects cancer cells from therapeutic pressure. This shows that HSP90 inhibitors that are nuclear-directed should be taken into account.

Mitochondrial localization of HSP90

HSP90 is highly expressed in tumoral mitochondria but hardly expressed in normal tissues, the mitochondrial expression of HSP90 was unknown. On the back of this groundbreaking breakthrough, a significant amount of research was conducted with the goal of creating HSP90 organelle-specific small molecule medicines. These inhibitors work together to precipitate a rapid collapse of mitochondrial integrity and death that would only happen in tumor cells. Pancreatic cancer, breast cancer, colon cancer, NSCLC, melanoma, glioblastoma, prostate cancer, lymphoma, and leukemia are just a few cancer types that have been proven to respond favorably to a number of inhibitors that directly target the mitochondrion. By examining the metabolic network in the tumoral mitochondrion that is regulated by HSP90 , mitochondrial HSP90 (hereafter mtHSP90), but not cytosolic HSP90 , binds and stabilizes the electron transport chain complex II subunit Succinate Dehydrogenase-B (SDHB), which keeps cellular respiration under low-nutrient conditions and aids in HIF1 -mediated tumor. The dynamic interaction between mitochondrial HSP90 and SDHB folding intermediates was also confirmed by cryo-EM data. It is important to note that while TRAP1 and HSP90 share 60% of their sequence and have the same NTD, ND, and CTD domains, they have been referred to be different forms of HSP90 , namely mitochondrial HSP90 . TRAP1 was almost completely undetectable in normal tissues but consistently raised in primary tumors.

Furthermore, Cyclophilin D , a mitochondrial residential protein that protects cells from apoptosis and maintains mitochondrial integrity, was one of the crucial client proteins of TRAP1. Additionally, the HSP90 and TRAP1 crystal structures offer additional chemical insights that can be used to create brand-new inhibitors. TARP1 is a promising target for the creation of new cancer treatments when taken collectively.

Membrane and extracellular localization of HSP90

Since HSP90 is highly expressed inside of cells, it was initially thought of as an artefact until it was discovered on the cell surface by a functional screening. After cautious verification, HSP90 is no longer considered as being exclusively localized within the cell. Additionally, HSP90 can secrete itself into the extracellular matrix. The malignant stage of the tumor correlates with the cell surface expression of HSP90 , which is higher on cancer cells than on normal cells.The first evidence for the detection of HSP90 in the extracellular matrix was implicated in 1986. Extracellular HSP90 (hence referred to as eHSP90) may function without ATP since the extracellular environment has low levels of ATP due to the absence of an energy source. Environmental stressors and growth factors can cause eHSP90 to secrete, and post-translational chaperone changes like phosphorylation and acetylation can have an impact. Recent studies have also revealed that invasive cancer cells secrete HSP90 via exosomes, which interacts with plasmin to increase their invasiveness. Cancer cells release both HSP90 and HSP90 to interact with MMP2 and MMP9 and increase the invasiveness of tumor cells. Similarly, eHSP90 was found in normal cells only in reaction to stress, while cancer cells consecutively release HSP90 . Interestingly, eHSP90 promotes the downstream signal transduction linked to tumor development and metastasis, which is similar to the EMT phenotype, by interacting with a number of receptors, including EGFR/HER2/LPR1. Furthermore, across a variety of cancer types, eHSP90 expression is associated with a rise in the risk of metastasis and a decline in the immune response. Cancer cells release both HSP90 and HSP90 in order to interact with MMP2 and MMP9 and increase the invasiveness of tumor cells. Similar to how cancer cells consistently release HSP90 , normal cells only show signs of eHSP90 in response to stress. Interestingly, eHSP90 promotes the downstream signal transduction linked to tumor development and metastasis, which is similar to the EMT phenotype, by interacting with a number of receptors like EGFR/HER2/LPR1. Additionally, in numerous cancer types, eHSP90 expression, is correlated with a decline in the immune response and an increase in the metastatic potential.