Targeting proteostasis pathways for cancer therapy

Abstract

The critical role of protein disequilibrium in driving carcinogenesis has long been recognized. Though several inhibitors of heat shock protein (HSP) family members have entered clinical trials, none of them have been approved for clinical use as a result of inevitable toxicity, leading to the identification of safer therapeutic approaches sharing a similar efficacy relevant and urgent. Through delineating the role of HSP90 inhibitors in arresting cancer hallmarks, this paper identified HSP90 inhibition as an effective therapeutic strategy capable of concomitantly targeting multiple key transformed properties of cancers via modulating cellular proteostasis. Through interrogating intrinsic connections between proteostasis and redox homeostasis, this paper proposed cold atmospheric plasma (CAP) as a possible alternative of HSP90 inhibitors with little adverse effects. This paper extended the therapeutic spectrum of HSP90 inhibitors and CAP to inflammation-driven pathologies including autoimmune diseases, as inflammation is a manifestation of failed proteostasis. These insights may conceptually advance our understandings on the driving force of cancers that can be easily extended to other disorders originated from imbalanced proteostasis and abnormal inflammation. Tools proposed here for inhibiting HSP90 including CAP and its possible synergy with HSP90 inhibitors may shift the current treatment paradigm to a new avenue in oncology and other relevant fields.

Graphical abstract

Highlights

• •Emphasizes the role of proteostasis in preventing carcinogenesis.
• •Attributes the cause of cancers to chronic inflammation.
• •Identifies intrinsic connections between proteostasis and redox homeostasis.
• •Proposes cold atmospheric plasma as a tool for maintaining proteostasis.
• •Extends the treatment spectrum of cold atmospheric plasma to autoimmune diseases.

1. Introduction

Though improved early detection, treatment options and education on keeping a healthy life style have helped control cancer mortality in recent years, there are still 2,001,140 new cancer cases and 611,720 new cancer death events estimated to occur this year in the United States according to the latest cancer statistics [1]. Drug resistance has been considered as one of the main factors leading to cancer relapse and the primary hurdle for the development of effective onco-therapeutics [2]. Among the varied factors affecting drug resistance, multidrug resistance (MDR) proteins such as MDR1 (also named P-gp), MDR-associated protein 1 (MRP1), and breast cancer resistance protein (BCRP) play significant roles [2]. For instance, the expression level of P-gp is closely associated with immune phenotypic switching, with its expression being maintained at a low level in peripheral circulating monocytes, being significantly enhanced in M2 macrophages, and being positively associated with the maturation and activation of dendritic cells (DCs) [2]. This intrinsic connection of drug resistance with immunity has led to the establishment of cancer immunotherapy that utilizes the immune system to ablate cancers [3]. Though this new mainstay in the field of oncology has gained lots of attention in the past few years, its efficacy can not be over-advocated as cancer cells can evolve multiple strategies to evade the anti-cancer immunity. Besides, most cancer immunotherapies currently available lack efficient targeting properties, rendering clinical safety another critical concern [3]. These remaining issues hurdling the march towards cancer eradication have urged us to keep exploring innovative anti-cancer strategies. One option is to find mechanistic clues from perturbed protein homeostasis (or namely proteostasis) in transformed cells.

To gain biological activities, proteins need to fold into a unique 3-dimensional structure in vivo, the process of which is challenged by the highly crowded cellular environment (with a macromolecular concentration of 300–400 mg/m [4] and constant stress situations such as heat shock and oxidative stimuli. Such a complicated molecular environment may result in unfavorable protein-protein interactions (PPIs), protein misfolding or aggregation. Thus, maintaining the cellular protein homeostasis is of vital importance to enable accurate de novo protein folding, reverse undesirable protein aggregation, and remove damaged proteins. Autophagy is one event capable of removing macromolecules and organelles for cellular homeostasis. However, its roles in the context of cancer progression and drug resistance are complicated that can be either pro-survival or pro-death, rendering therapeutic targeting of autophagy in human cancers challenging [5,6].

Alternatively, heat shock proteins (HSPs) or molecular chaperones are dominant players of the proteostasis network, which can be classified, according to their molecular weights, into large HSPs, HSP90, HSP70, HSP60, HSP4 and small HSPs families. Large HSPs are protein-protective with diversified roles such as assisting de novo protein folding and assembly, protein degradation, and cell response to cellular stress, abnormal expression of which is associated with the occurrence of, e.g., some neurological diseases and cancers (Table 1) [7]. Small HSPs contribute in stabilizing the folded states of other proteins, maintaining cells' structural integrity and protecting cells from damages. Inappropriate level of small HSPs is associated with disease progression of, e.g., neurological disorders and amyotrophic lateral sclerosis (Table 1) [8]. HSP60 proteins, being enriched in mitochondria and involved in the folding and assembly of mitochondrial proteins, play essential roles in cell metabolism and survival. Abnormal activity of HSP60 has been linked to heart disease, diabetes, and neurological disorders (Table 1) [9]. HSP70 and HSP40 proteins work together to participate in the correct folding and degradation of other proteins, as well as maintaining protein stability and regulating stress-induced cell death events. While abnormal HSP40 expression is associated with, e.g., neurological diseases, cardiovascular disorders, and cancers, dis-regulated HSP70 is linked with cataracts, Alzheimer's disease, and cancers (Table 1) [10]. Though HSP90 together with HSP70 have been widely studied given their paramount roles in cancers, HSP90 is the most abundant HSP with the expression reaching up to 4%–6% in response to stress and having approximately 600 clients being identified in mammals (Table 1) [11]. By interacting with various key proteins such as protein kinases, transcription factors and upstream signaling molecules, HSP90 proteins orchestrate signal transduction to maintain cellular proteostasis. The activity of HSP90 can be attenuated using various types of HSP90 inhibitors. Specifically, HSP90, with the participation of co-chaperones, promotes the folding and maturation of various client proteins through an adenosine triphosphate (ATP) cycle (Fig. 1), and regulates diversified biological events such as gene expression, cell cycle and proliferation [12].

Table 1.

Summarized information on heat shock protein (HSP) family members.

Family	Primary members	Location	Characteristics	Functions	Refs.
Large HSPs	HSP110 GRP170	Cytosol ER	ATP-dependent highly conserved bilobate 40 KDa nucleotide-binding domain, belonging to HSP70 superfamily. ATP-dependent nucleotide binding domain with carboxyl terminus similar to HSP110, belonging to the HSP70 superfamily.	Cochaperson of HSP70: holdase to prevent protein aggregation.	[7]
Small HSPs	HSPB1 HSPB2 HSPB3 HSPB4 HSPB5 HSPB6 HSPB7 HSPB8 HSPB9 HSPB10	Cytosol	ATP-independent large heterogeneous oligomers.	Holdase to inhibit protein aggregation and sequester misfolded proteins.	[8]
HSP60	HSP60 TRiC	Mitochondria Cytosol	ATP-dependent double ring structure containing two heptameric rings of HSP60. ATP-dependent double ring structure containing two octameric rings of TRiC.	Foldase to prevent protein aggregation.	[9]
HSP40	DNAJA DNAJB DNAJC	Cytosol Mitochondria Nucleus	J Domain containing proteins interacting with HSP70.	Cochaperon of HSP70: hold misfolded proteins, recruit HSP70, regulate HSP70 ATPase activity.	[10]
HSP70	HSPA1A/1B HSPA1L HSPA2 HSPA5 HSPA6 HSPA7 HSPA8 HSPA9 HSPA12A/12B HSPA13 HSPA14	Cytosol Nucleus ER Mitochondria	ATP-dependent conserved structure containing NTD-SBD domains; chaperon function is based on the allosteric conformation change cycle.	Cochaperon of HSP90: foldase to prevent protein aggregation; triage protein fates.	[10]
HSP90	HSP90AA HSP90AB GRP9 TRAP1	Cytosol Cytosol Cytosol/ER Mitochondria	ATP-dependent homodimers; chaperon function is based on the allosteric conformation change cycle.	Foldase for newly synthesized or misfolded proteins.	[11]

HSP: heat shock protein; GRP: Glucose-Regulated Protein; ATP: adenosine triphosphate; NTD: N-terminal domain; SBD: Substrate Binding Domain; ER: endoplasmic reticulum; TRiC: TCP-1 Ring Complex; ATPase: adenosine triphosphatase; HSPA: heat shock protein family A; HSPB: heat shock protein family B; DNAJA: DnaJ (Hsp40) Homologue, Subfamily A; HSPA1A/1B: heat shock protein Ffamily A (Hsp70) Member 1A/1B; HSP90: heat shock protein 90; TRAP1: tumour necrosis factor receptor associated protein 1.

Fig. 1.

Structure and mechanism of heat shock protein 90 (HSP90). (A) Structure domains of the HSP90 protein. N-temminal domain (NTD) is the N-terminal domain which binds adenosine triphosphate (ATP), HSP90 inhibitors such as geldanamycin, and co-chaperones such as Cdc37. Middle domain (MD) is short for the middle domain that contains a catalytic arginine required for the adenosine triphosphatase (ATPase) activity. C-terminal domain (CTD) is the C-terminal domain which contains the major dimerization interface. While all three domains are involved in client protein binding, MD is the major protein binding site. (B) The ATP cycling working mode proposed for the HSP90 machinery. The HSP90 machinery transits from the "open" to the "ATP-binding" state on ATP binding; then the NTD region closes over the ATP-bound active site to enter the "closed" state that is rate-limiting. After ATP hydrolysis, inorganic phosphate (Pi) is released and the HSP90 machinery enters the "adenosine diphosphate (ADP)-binding" state. The two NTDs then dissociate to allow subsequent release of ADP and the return of HSP90 back to the "open" conformation.

This paper delineates the roles of HSP90 inhibitors in cancer control according to their effects on cancer hallmarks, and identifies critical signaling pathways relying on HSP90. Importantly, this paper bridges the gap between protein and redox homeostasis, and proposes cold atmospheric plasma (CAP) as a possible safe solution for cancer treatment via a mechanism similar to HSP90 inhibitors. In addition, this paper hypothesizes that the therapeutic value of CAP can be further extended to other inflammation-driven diseases including autoimmune disorders.

2. Basics of HSP90 inhibitors

The HSP90 family is composed of HSP90α/β, glucose-regulatory protein 94 (GRP94), and tumor necrosis factor receptor-associated protein 1 (TRAP1). Among the four paralogues, HSP90α/β reside in the cytoplasm, GRP94 is located in the endoplasmic reticulum (ER), and TRAP1 is localized in the mitochondria. HSP90 family members are ATP-dependent molecular chaperones that help promote the maturation and stability of client proteins responsible for cancer cell proliferation and cellular stress adaptation [13]. Approximately 10% proteins (including transcription factors, ubiquitin proteins, and about 60% kinase proteins in human kinome) are HSP90 clients, the maturation of which is HSP90-dependent in human proteome [14]. Many client proteins of HSP90 such as epithelial growth factor receptor (EGFR), human epithelial growth factor receptor 2 (HER2), fibroblast growth factor receptor (FGFR), and vascular endothelial growth factor receptor 2 (VEGFR2) are oncoproteins. Abnormal stabilization of these oncogenes may promote carcinogenesis by disrupting multiple pathways involved in cell proliferation, death, angiogenesis, metastasis, and drug resistance. HSP90 has been identified capable of regulating various programmed cell death (PCD) events (such as apoptosis, necroptosis, autophagy, ferroptosis, and pyropotosis), participating in sustained proliferation, helping escaping growth suppression, aiding in immune evasion, contributing in telomerase stabilization, promoting cytokine production for enhanced tumor-associated inflammation, enhancing the epithelial-mesenchymal transition (EMT) process, inducing angiogenesis, and rewiring the metabolism [15]. Under stress, HSP90 is typically up-regulated that leads to misfolding and accumulation of excessive customer proteins, perturbed proteostasis and, ultimately, cellular pathological transformation and development of a variety of refractory diseases including cancers. Thus, targeting HSP90 or simultaneously inactivating all members of the HSP90 family has been considered a promising recipe for cancer control.

All HSP90 homologues share a common structure that contains three domains, i.e., nucleotide-binding domain at the N-terminal domain (NTD), a middle domain (MD), and a C-terminal domain (CTD) that drives the formation of HSP90 homodimers and contains an Met-Glu-Glu-Val-Asp (MEEVD) motif for co-chaperone binding (Fig. 1). HSP90 switches from an NTD “open” mode to a “ATP-binding” mode on ATP binding; unmature client proteins are loaded to this ATP-binding form of HSP90 at the MD domain that induces NTD dimerization and formation of the “closed” mode; ATP hydrolysis triggers conformation alteration of the complex, leading to the release of mature client proteins and the formation of a “adenosine diphosphate (ADP)-binding” form of HSP90; lastly, ADP is released from HSP90, turning the conformation of HSP90 back to the “open” mode (Fig. 1).

Many natural and synthetic compounds with different chemical structures and HSP90 binding sites have been identified or synthesized as HSP90 inhibitors. Inhibitors targeting different domains of the HSP90 protein function through different mechanisms. NTD inhibitors act by disrupting interactions between ATP and ATP-binding pockets that lead to proteasome degradation of the client proteins; CTD inhibitors destabilize the chaperone complex by inducing co-chaperone release that leads to client protein degradation; MD inhibitors directly or isomerically disrupt interactions between HSP90 and client proteins [16].

HSP90 inhibitors have certain levels of selectivity against transformed cells given the high level and activity of HSP90 in cancers. HSP90 expression in tumor cells was 2–10 times that of healthy cells, and increased in the plasma membrane and extracellular space of transformed cells [17]. Specifically, elevated HSP90 level was detected in the serum of diverse types of cancers including, e.g., liver cancer, lung cancer, advanced colorectal cancer, hepatocellular carcinoma and acute myeloid leukemia, suggesting its use for early diagnosis [18]. Also, HSP90 appears in a latent state in normal tissues, but presents in the form of polymeric complexes in tumor cells, the later of which enables it with a high adenosine triphosphatase (ATPase) activity. In addition, activated HSP90 phosphorylation was typically associated with promoted carcinogenesis and enhanced cell vulnerable to HSP90 inhibition. For instance, HSP90 phosphorylation at Thr115 in its NTD stabilized its interactions with the client protein activator of HSP90 ATPase activity 1 (AHA1) and conferred increased sensitivity of renal cell carcinoma to HSP90 NTD inhibitors SNX-2112 and ganetespib [19]; HSP90 phosphorylation at Tyr313 in the MD led to an over 7-fold increase in its binding affinity with AHA1 [20]; and phosphorylation of CTD residues at Thr725 and Ser726 of HSP90α as well as Ser718 of HSP90β regulated binding to co-chaperones for tumour-promotive activity [21]. These, collectively, empower cancer cells with a high sensitive to HSP90 inhibition.

HSP90 inhibitors can be "natural" or "synthetic". Most natural HSP90 inhibitors come from geldanamycin and radicicol [22]. Geldanamycin is an ansamycin-derivative benzoquinone that binds to the ATP binding "pocket" of HSP90 with a higher affinity than nucleotides. Radicicol is a macrocyclic antifungal antibiotic that destabilizes the client proteins of HSP90 by binding to the NTD of HSP90 [23]. As the benzoquinone moiety is highly hepatotoxic [24] and radicicol is unstable in vivo, derivatives of geldanamycin such as 17-allylamino-17-demethoxygeldanamycin (17-AAG) [25], 17-dimethylaminoethylamino-17-demethoxygeldanamycin (17-DMAG) [26], Retaspimycin Hydrochloride (IPI-504) [27] and derivatives of radicicol, such as VER-52296 (NVP-AUY922), have been established and have entered clinical trials [27]. Ganetespib, considered the second generation of HSP90 inhibitor, is a resorcinol-containing triazolone heterocyclic HSP90 inhibitor, which was found to have a 20-fold greater potency but reduced hepatotoxicity over 17-DMAG in treating hematologic and solid cancer cells [28]. Synthetic HSP90 inhibitors are mostly purine-based compounds including the PU series and SNX series that convey comparable efficacies with their natural peers. For instance, PU-H71, a member of the PU series attenuated triple negative breast cancer progression in vivo [29]; and SNX-7081, a small oral molecule from the SNX series, demonstrated a higher potency than 17-AAG in treating chronic lymphocytic leukemia [30]. Synthetic HSP90 inhibitors SNX-5422 and XL888 are benzamide-based compounds. SNX-5422 was shown promising in treating ibrutinib resistant chronic lymphocytic leukemia tumor cells in vivo [31]. XL-888 effectively induced the apoptosis and inhibited the migration of chemo-resistant neuroblastoma cells preclinically [32].

Most HSP90 inhibitors so far identified or synthesized target the ATP pocket in the NTD of HSP90, with four classical scaffolds, i.e., ansamycin, resorcinol, purine, and benzamide [33]. Among these inhibitors, over 20 have reached the clinical stage, most of which are in the phases I/II. Several clinical investigations have already reported promising results. For instance, IPI-504, an ansamycin-based inhibitor, demonstrated its clinical activity in treating non-small cell lung cancer patients especially those carrying anaplastic lymphoma kinase (ALK) rearrangements in a prospective non-randomized multi-center phase II trial including 76 patients (NCT00431015) [34]. In a larger phase II study where 153 patients were recruited, the resorcinol-based inhibitor AUY922 demonstrated its efficacy in treating advanced non-small cell lung cancers, particularly among patients carrying ALK rearrangements and EGFR mutations (NCT01124864) [35]. Another resorcinol-based inhibitor KW-2478 was used as a monotherapy for treating B-cell malignancies in a phase I multi-center clinical trial including 27 patients (NCT00457782); as the results, 96% patients showed stable disease and 5 patients remained progression-free for over 6 months [36]. In a phase II study examining the efficacy of the purine-based inhibitor BIIB021 in treating gastrointestinal stromal tumors refractory to imatinib and sunitinib where 23 patients were involved, BIIB021 achieved objective remission with mild-to-moderate adverse events [37]. In a phase I open-label multi-center trial evaluating the benzamide-based inhibitor SNX-5422 involving 20 patients carrying advanced non-small cell lung cancers and 3 having small cell lung cancers, combined use of SNX-5422, carboplatin and paclitaxel followed by maintenance SNX-5422 therapy was shown effective and well-tolerated especially for oncogene-driven non-small cell lung cancers [38]. Despite these encouraging results, none of these drugs has been approved by the U.S. Food & Drug Administration (FDA) for clinical use due to their relatively high toxicity, poor pharmacokinetics, and/or lack of sufficient clinical efficacy. Among the adverse effects reported, ophthalmotoxicity and cardiotoxicity have been associated with the ansamycin-based 17-DMAG in treating refractory solid tumors according to a phase I dose-escalation trial [39], and the benzamide-based SNX-5422 in treating patients carrying refractory solid tumor malignancies and lymphomas according to another phase I study [40]. The resorcinol-based ganetespib displayed many drug-related fatal grade 4 adverse events such as elevated levels of aspartate aminotransferase and alanine transaminase in a phase II trial during the treatment of metastatic uveal melanoma (NCT01200238) [41], and showed several grade 3/4 adverse effects including leukopenia, fatigue, diarrhea, and increased level of alkaline phosphatase according to a phase II trial for treating advanced esophagogastric cancer (NCT01167114) [28]. Actually, the latter trial (i.e., NCT01167114) was terminated due to insufficient evidence of efficacy in addition to the severe toxicity observed [28]. Other prematurally aborted investigations for the same reason include, e.g., a phase II study of another resorcinol-based inhibitor AUY922 in treating relapsed or refractory non-Hodgkin lymphoma (NCT01485536) [42]. The purine-based inhibitor Debio0932, though showed acceptable patient tolerability, exhibited minimal clinical activity with only 10% patients showing stable disease and 20% patients achieving partial response in a multi-center uncontrolled open-label non-randomized dose-escalation clinical trial in the treatment of therapeutic-resistant advanced cancers (NCT01168752) [43]. Overall, clinical trials on HSP90 inhibitors in the form of small molecules for cancer treatment as a monotherapy has declined in recent years due to their undesirable efficacies and significant toxicities. Therefore, establishment of therapeutics with improved inhibition on HSP90 and reduced side effects may provide a new avenue for HSP90 inhibitors to become clinically practical in use.

3. Targeting cancer hallmarks via inhibiting HSP90

The 10 canonical cancer hallmarks [44] can be summarized into 8 representative features, i.e. uncontrolled cell proliferation, cell death evasion, cell metabolism reprogramming, immune surveillance evasion, cancer angiogenesis, cancer metastasis, cancer-associated inflammation, and genome integrity interrogation. Inhibiting HSP90 has been shown capable of stabilizing proteins involved in sustaining cells with these major properties and assisting in their maturation [45].

3.1. HSP90 inhibition and reduced cell proliferation

The proliferation of normal cells is tightly controlled by multiple intracellular signal transductions. Mitogenic growth signals, initiated through the binding of transmembrane receptors to distinctive classes of signaling molecules, are needed to trigger the transition of cells from a quiescent to an active proliferative state; and the signals are transmitted by intracellular signal transduction pathways that are activated by the growth stimulation. In cancer cells, these growth promoting pathways are constitutively active even without external growth stimuli.

Many intracellular growth signalings are controlled by the "kinome" that is composed of various protein kinases and subjected to a precise control in normal cells. HSP90 has been identified with an essential role in maintaining the stability and activity of many kinases required for sustaining cell proliferation. Actually, many kinases have been considered oncogenic given their tumorigenic role once abnormally activated. Kinases chaperoned by HSP90 and contribute to cell proliferation include, primarily but are not limited to, "receptor tyrosine kinases", "non-receptor tyrosine kinases" (Src family kineases), "mitogen-activated protein kinase (MAPK)-related kinases", and "cell cycle kinases" (Fig. 2).

Fig. 2.

Heat shock protein 90 (HSP90) chaperons many onco-proteins driving cancer hallmarks. Endoplasmic reticulum (ER) is more oxidative, and cytosol, nucleus, and mitochondria are more reductive. HSP90 contributes in adjusting proteostasis to the redox environment in each cellular compartment, aberrant regulation of which leads to cancer hallmarks. ① HSP90 chaperons receptor tyrosine kinases (RTKs), non-receptor tyrosine kinase (Src), and primary mediators of the mitogen-activated protein kinase (MAPK) pathway such as rapidly accelerated fibrosarcoma (Raf), towards uncontrolled cell proliferation. ② Many proteins driving cells to escape various death signals such as glutathione peroxidase 4 (GPX4) in ferroptosis, mixed lineage kinase domain-like (MLKL) in necroptosis, cellular FADD-like IL-1β-converting enzyme-inhibitory protein (c-FLIP) in apoptosis,Unc-51 like autophagy activating kinase 1 (ULK1) in autophagy, and cyclin-dependent kinases (CDKs) such as CDK4 are chaperoned by HSP90. ③ HSP90 stabilizes hypoxia-inducible factor 1 alpha (HIF1α) under hypoxia that induces several key enzymes involved in glycolysis such as hexokinase 2 (HKII), phosphofructokinase 1 (PFK1), glyceraldehyde 3-phosphate dehydrogenase (GAPDH), and lactate dehydrogenase alpha (LDHA), as well as modulates the stability and activity of v-myc avian myelocytomatosis viral oncogene homologue (c-Myc) that regulates the expression of several glycolytic enzymes such as LDHA and pyruvate kinase muscle isozyme 2 (PKM2), leading to aerobic glycolysis and metabolic reprogramming. ④ HSP90 chaperons major histocompatibility complex class I (MHC-I) to enable cells with the ability of evading immune surveillance. ⑤ HSP90 chaperons vascular endothelial growth factor (VEGFR) family members and key components of the phosphoinositide-3-kinase (PI3K)/protein kinase B (AKT)/endothelial nitric oxide synthase (eNOS) pathway such as eNOS and AKT to enable tumor angiogenesis. ⑥ HSP90 promotes tumor metastasis by chaperoning extracellular matrix proteins such as matrix metallopeptidase 2 (MMP2), MMP7, MMP9, as well as epithelial mesenchymal transition (EMT)-related tanscniption factor (TF) reactive oxygen species (ROS). ⑦ HSP90 promotes cancer-associated inflammation by chaperoning several pro-inflammatory proteins such as toll-like receptor 4 (TLR4) and nuclear factor kappa B (NF-κB). ⑧ HSP90 activates DNA polymerase-η, interrogating genome integrity. APC: antigen presentation cell; ERK: extracellular regulated protein kinase; MAPKK: mitogen-activated protein kinase (MAPK) kinase; PDK1: pyruvate dehydrogenase kinase 1; Ras: rat sarcoma viral oncogene homologue.

Many receptor tyrosine kinases have been shown to be associated with HSP90 such as HER2 and epithelial growth factor receptor (EGFR), the binding of which to HSP90 is essential for these receptors to be active and stable. Suppressing HSP90 using its inhibitor AUY922 rewired the sensitivity of HER2-positive gastric cancer developed resistance to the HER2 inhibitor lapatinib [46]. Similarly, a HSP90-phosphoinositide-3-kinase (PI3K) dual inhibitor suppressed melanoma cell proliferation by interfering with the interactions between HSP90 and EGFR as well as the associated downstream pathways [47].

Src family kinases are cytoplasmic proteins that are often translocated to the cell membrane to relay growth signals after activation. Members of Src family whose functionality require HSP90 chaperone machinery include, e.g., pp60_v-Src, Yes, Fes, Fps, and Lck [48]. Take pp60_v-Src as the example, it remains at the inactive state when forming a cytosolic complex with HSP90 and its client protein cell division cycle 37 (CDC37), and becomes activated on stimulation when being dissociated from the complex together with HSP90 [49]. Geldanamycin, a HSP90 inhibitor reverted v-Src transformed fibroblast cells by blocking the formation of the pp60_v-Src-HSP90 heteroprotein complex [50].

The MAPK cascade plays a major role in transmitting the proliferation signals from the cytoplasm to the nucleus. Specifically, adaptor proteins such as Sos accumulate to the cytoplasmic domain of the tyrosine kinase receptors that activates rat sarcoma viral oncogene homologue (Ras); activated Ras stimulates rapidly accelerated fibrosarcoma (Raf) that are serine/threonine kinase such as B-Raf and Raf1; stimulated Raf proteins phosphorylate MAPK kinases (MAPKKs) that subsequently phosphorylate MAPKs such as extracellular regulated protein kinases 1 (ERK1) and ERK2; activated ERK1/2 translocate into the nucleus to phosphorylate various proteins including transcription factors responsible for cell proliferation [51]. Several components of this cascade such as B-Raf and Raf1 are associated with HSP90 and Cdc37 [52]. Inhibiting HSP90 using 17-AAG led to increased degradation of B-Raf and arrested growth of melanoma cells, Hodgkin lymphoma cells, and colorectal cancer cells [53].

Components of the cell cycle clock, specifically those governing the G₁/S and G₂/M transits, are responsible for eliciting the anti-proliferation signals. Tumor cells typically loose the ability to enter the G₀ phase, and thus divide infinitely due to its constantly cycling cell cycle. Cyclin-dependent kinases (CDKs) play essential roles in regulating the cell cycle machinery, several of which are chaperoned by HSP90. For instance, CDK4, activated by binding to cyclin D, controls the G₁/S transition by phosphorylating Rb protein and other transcription factors [54]; HSP90 forms a complex with Cdc37 to stabilize CDK4 and assists in its binding to cyclin D [55]; disrupting the interactions between HSP90 and Cdc37 using celastrol (a HSP90 inhibitor) resulted in reduced CDK4 level, halted cell cycle and arrested proliferation of lymphocytic lymphoma cell [56].

3.2. HSP90 inhibition and controlled cell death

PCD, manifestes in varied forms such as apoptosis, necroptosis, autophagy, and ferroptosis, plays a vital role in maintaining cellular homeostasis by removing damaged and senescent cells. These diversified cell death programs share a complex yet coordinated system to mediate pathogenesis, rendering synchronous targeting of which difficult to achieve.

HSPs share similar roles with PCD in regulating cellular homeostasis, making these proteins possible to modulate PCD in a unified fashion under diverse physiological states. Indeed, HSP90, among the diversified classes of HSPs, has been proposed as a common regulatory nodal of PCD. For example, HSP90 suppressed apoptosis [57], inhibited necroptosis [58], and prevented ferroptosis [59]. It is worth to mention the dual roles played by autophagy during cancer initiation and progression. That is, while autophagy is tumor-suppressive via eliminating abnormal cells and organelles towards preserved genome [60], it is tumor-promotive by aiding transformed cells in surviving stress for promoted metastasis and gained drug-resistance [61]. HSP90 was reported to suppress cytoprotective autophagy in, e.g., colorectal cancer cells [62] and breast cancer cells [63].

Inhibiting HSP90 using 17-AAG induced apoptosis in lung cancer cells by decreasing the expression of cellular FADD-like IL-1β-converting enzyme-inhibitory protein (c-FLIP) (a master anti-apoptotic regulation) [64], mediated necroptosis by inhibiting mixed lineage kinase domain-like (MLKL) phosphorylation at Ser227 and Ser358 in HEK293T cells [65], and prevented autophagy by destabilizing Unc-51 like autophagy activating kinase 1 (ULK1) that is a mammalian homologue of ATG1 involved in the nucleation and extension of autophagosome in human chronic myeloid leukemia cells [66]. The HSP90 inhibitor gambogic acid triggered ferroptosis in colorectal cancer cells by reducing the expression of glutathione peroxidase 4 (GPX4) as a result of concomitant glutathione (GSH) depletion and reactive oxygen species (ROS) elevation [59].

3.3. HSP90 inhibition and rewired cell metabolism

Metabolic rewiring and plasticity are distinctive features of cancer cells characterized by a shift from oxidative phosphorylation (OXPHOS) to glycolysis, namely the Warburg effect.

HSP90 plays an essential role in regulating the equilibrium between glycolytic metabolism and OXPHOS in response to external perturbations towards a favorable environment for tumor initiation and progression [17]. HSP90 expression is up-regulated in many human malignancies [67], inhibition of which has been associated with a metabolic shift from glycolysis to OXPHOS. Specifically, HSP90 can rewire cell metabolism via either directly controlling the stability and functionality of enzymes involved in cancer cell metabolism, or indirectly regulating metabolic pathways through oncogenic signaling (Fig. 2).

Many glycolytic enzymes such as pyruvate kinase muscle isozyme 2 (PKM2) and glyceraldehyde 3-phosphate dehydrogenase (GAPDH) are clients of HSP90 [17]. Take PKM2 as the example, HSP90 brings glycogen synthase kinase-3β (GSK3β) to PKM2 to form a complex and phosphorylate PKM2 at Thr328, which sustains the high expression and activity of PKM2 in hepatocellular carcinoma tissues [17]. Alternatively, HSP90 interferes with several oncogenic pathways involved in metabolic plasticity such as those mediated by hypoxia-inducible factor 1 alpha (HIF1α) [68], v-myc avian myelocytomatosis viral oncogene homologue (c-Myc) [69], PI3K [70], HER2 [46], and Src family kinases [71]. All these mediators promote glycolysis, enhanced stability or activity of which leads to carcinogenesis. For instance, HIF1α shifts cell metabolism from OXPHOS to glycolysis in response to hypoxia by inducing the expression of genes encoding glucose transporters and glycolytic enzymes, as well as genes decreasing OXPHOS [72]. Altogether, these studies support the role of HSP90 in remodeling the balance between glycolysis and OXPHOS in response to extracellular stimuli that is relevant to drug resistance and carcinogenesis; and inhibiting HSP90 may represent an effective anti-cancer strategy by targeting cancer cell metabolism and reverting drug resistance.

Though relatively little is known on the role of HSP90 played in lipid metabolism, we may find clues from its protective effect on oncogenes through its interactions with lipid associated proteins. For example, HSP90 promoted HER2-positive breast cancer progression via interacting with pStAR-related lipid transfer domain protein 3 (STARD3) to protect HER2 from degradation, where STARD3 participates in the transportation of cholesterols between the ER and endosomes [73].

3.4. HSP90 inhibition and restored immune surveillance

While the immune system of a healthy individual can recognize, monitor, and eventually eliminate most malignant cells, a small number of transformed cells may escape immune clearance, leading to rapid disease progression. Specifically, tumor peptides are degraded in the cytosol, brought to the ER for MHC class I (MHC-I) loading, and presented to the surface of antigen presentation cells such as macrophages and DCs, the whole process of which is referred to as "antigen presentation". Antigens presented are then recognized by CD8⁺ T cells to initiate adaptive immune response, namely cross-priming.

Lack of MHC-I molecule presentation is one major mechanism leading to tumor immune escape. It has been proposed that the whole antigen presentation process is accompanied by HSPs, and antigenic peptides chaperoned by HSPs are more efficient presented by MHC-I than free peptides by orders of magnitude [74]. In addition, HSP90 is involved in both antigen presentation and cross-priming, inhibition of which using radicicol may result in the generation of "empty" MHC-I on antigen donor cells for compromised ability in cross-priming (Fig. 2).

Tumor associated macrophages (TAMs), originated from monocytes, are crucial during carcinogenesis. By screening 172 compounds involved in the differentiation of monocytes into TAMs, HSP90 was found being secreted by Triple-Negative Breast Cancer (TNBC) cells and capable of promoting TAM differentiation [75]. In another study, HSP90 was characterized to promote colorectal cancer progression via stabilizing macrophage migration inhibitory factor (MIF), where MIF was initially identified as a T lymphocyte-elicited inhibitor of macrophage motility [76].

3.5. HSP90 inhibition and attenuated cancer angiogenesis

Angiogenesis refers to a process where new blood vessels develop from existing capillaries to create a complete, regular, and mature vascular network. In tumor cells, abundant growth factors and cytokines are typically produced in response to various stimuli such as hypoxia, ischemia, acidosis and high interstitial pressure to stimulate abnormally regulated angiogenesis and meet the high demand of tumor cells on cell proliferation and metabolism.

Lots of evidence has implicated the promotive role of HSP90 in tumor angiogenesis that mediates a pro-angiogenic response on receptor binding in target cells such as endothelial cells, and induces the secretion of pro-angiogenic factors such as growth factors or cytokines in tumor cells [77], with the most important factors involved being VEGF and endothelial nitric oxide synthase (eNOS) (Fig. 2).

VEGF is a pro-angiogenic factor exerting an essential role in enhancing vascular permeability, stimulating migration, and promoting endothelial cell survival. HSP90 mediates a pro-angiogenic response on receptor binding in target cells such as endothelial cells and pericytes, and induces the secretion of pro-angiogenic stimuli such as growth factors and cytokines from tumor cells by stabilizing VEGF [78]. HSP90 inhibitors appear to directly target several components of the VEGF signaling axis such as integrins, focal adhesion kinase or neuropilin co-receptors [79]. For instance, 17-AAG showed potent anti-neoplastic activity both in vitro and in vivo by suppressing the expression of VEGFR1 in human vascular endothelial cells, reducing VEGFR2 in human endothelial cells, and decreasing the level of VEGFR3 in lymphatic endothelial cells [80].

Another pathway playing a vital role in angiogenesis is PI3K/protein kinase B (AKT)/eNOS signaling as eNOS-knockout mice were featured by impaired angiogenesis in response to either VEGF or hypoxia [81]. Later, HSP90 was identified with a scaffold role in forming the AKT/pyruvate dehydrogenase kinase 1 (PDK1)/eNOS complex, and found to help release nitric oxide (that was angiogenesis-promotive) from endothelial cells through interacting with eNOS. Suppressing HSP90 using 17-AAG attenuated the PI3K/AKT/eNOS axis with substantially lowered AKT and eNOS levels in endothelial cells that, ultimately, led to decreased angiogenesis [82].

3.6. HSP90 inhibition and arrested cancer metastasis

Metastasis refers to the development of secondary tumors distant from the primary cancer. The metastatic cascade is comprised of five key steps, i.e., invasion, intravasation, circulation, extravasation, and colonization. The EMT process is required for metastasis initiation that permits epithelial cells to attain the mesenchymal phenotype and movement plasticity [83]. Typically, EMT-derived tumor cells acquire stem cell properties and exhibit marked therapeutic resistance [84]. For instance, over-expression of Twist, Snail or FOXC2 not only characterized the CD24⁻CD44⁺ cell cohort but also identified cells carrying the mesenchymal phenotype in breast cancers [85]. EMT can be viewed as a selection process of cells capable of breaking the epithelial cell-cell adherens junction and gaining the ability of movement subjected to a series of genetic and epigenetic alterations. Cancer cells capable of completing the EMT process must be able to devote more energy to break the force imposed by cell-cell connections, and thus need to have a high metabolic plasticity regarding how the energy could be produced and utilized. On the other hand, cancer stem cells (CSCs) are featured with metabolic plasticity that enables them to shift between glycolysis and OXPHOS and survive diverse stress conditions. Thus, it is no wonder that cells having completed the EMT process, in general, manifest the CSC-like features or, in other words, only cells with high cancer stemness can survive the EMT selection process. This can also be used to explain the phenomenon that EMT-derived cancer cells are more likely to develop metastasis and gain drug resistance.

HSP90 promotes tumor metastasis by interacting with and promoting the activities of extracellular matrix proteins such as matrix metalloproteinase 2 (MMP2), MMP7, and MMP9, tissue plasminogen activator (PLAT) and lysyl oxidase-like protein 2 (LOXL2) (Fig. 2) [86,87]. Applying 17-AAG decreased ovarian tumor metastasis, where both the expression and activity of MMP7 were reduced [88].

Alternatively, HSP90 activates the EMT process by increasing the expression of EMT-related transcription factors such as Snail, Slug, Zeb1, Twist1, and down-regulating E-cadherin (Fig. 2) [89]. Indeed, suppressing EMT using 17-AAG abrogated Twist1 transcription in ovarian, renal, and nasopharyngeal cancer cells [90].

Several cell-surface receptors such as low-density lipoprotein receptor-related protein 1 (LRP1) [91], toll-like receptor 4 (TLR4), [92] and HER2 [70] have been reported to mediate HSP90 signaling towards promoted cell migration (Fig. 2). For instance, LRP1 mediated HSP90-induced cell migration by forming a complex with HSP90 with the help of clusterin to activate AKT, ERK, and nuclear factor kappa B (NF-κB) pathways [91]; and inhibiting HSP90 using SL-145 was proposed as an effective strategy to arrest the migration of triple negative breast cancer cells via attenuating AKT and ERK signalings [93].

3.7. HSP90 inhibition and reduced cancer-associated inflammation

HSPs including HSP90 are key players during inflammation, and thus were also named chaperonkine. Focusing on HSP90, it functions as a stabilizer of many oncogenes such as JAK2 and tightly regulates several key regulators of pathways with pivotal roles in mediating inflammatory responses such as NF-κB, JAK2/STAT, and TLR4 signalings (Fig. 2) [94].

HSP90 inhibitors can attenuate inflammation by suppressing certain pro-inflammatory mediators in many types of cells that are not limited to cancers. For instance, treating mice carrying atherosclerotic plaque with either 17-AAG or 17-DMAG reduced the secretion of pro-inflammatory cytokines by suppressing the activities of transcription factors signal transducer and activator of transcription (STATs) and nuclear factor kappa B (NF-κB) [95]. 17-AAG also attenuated inflammation in autoimmune encephalomyelitis and severe sepsis [96]. Geldanamycin inhibited tumor necrotic factor alpha (TNF-α)-mediated NF-κB activation by dissociating the inhibitor of kappa B kinase (IKK) complex in several in vitro models [97], and suppressed TNF-α-mediated IL-8 gene expression in cultured human respiratory epithelium [98]. The synthetic HSP90 inhibitor EC144 decreased the secretion of pro-inflammatory cytokines such as TNF-α and interleukin 6 (IL-6) by deactivating TLR4 signaling in a mouse model of endotoxic shock, a pathological phenomenon resulted from systemic release of pro-inflammatory factors (Fig. 2) [99]. SNX-7081, another synthetic HSP90 inhibitor, prevented NF-κB nuclear translocation and nitric oxide production following the stimulation of Jurkat cells by pro-inflammatory cytokines TNF-α and IL-1β [96].

3.8. HSP90 inhibition and improved genome integrity

Genome integrity has been considered as another enabling cancer hallmark, the loss of which may lead to accelerated genome mutation and, consequently, cancer initiation and progression.

HSP90 inhibitors can enhance the sensitivity of cancer cells to DNA-damaging agents, given the facilitating role of HSP90 in activating DNA polymerase-η (a DNA polymerase enabling replication via ultraviolet-induced pyrimidine dimers) through proper folding (Fig. 2). For instance, 17-AAG sensitized HeLa and HEK293 cells to the cytotoxic effects of ultraviolet radiation by inhibiting the activity of DNA polymerase-η [100].

Besides the suppressive role of HSP90 inhibition on DNA polymerase-η-generated mutations, HSP90 inhibitors may also increase the mutation frequency by enhancing the activity of transposons under certain context. For instance, transposons were suppressed by a mechanism mediated by Piwi-interacting RNAs (piRNAs) [101], the biogenesis of which required HSP90 in germ cells of D. melanogaster [102]. HSP90 inactivation led to de novo mutations by enhancing transposon mobility as a result of reduced piRNA expression [102]. In addition, HSP90 inhibitors caused the loss of the activity of Protein Arginine Methyltransferase 5 (PRMT5) and thus reduced the methylation of several piRNA-interacting proteins, leading to abrogated piRNA-mediated suppression on transposon mobility via impairing interactions between Piwi proteins and piRNAs [103].

4. CAP as a possible HSP90 inhibitor

4.1. CAP and its onco-therapeutic potential

CAP is partially ionized plasma generated at a relatively low temperature (i.e., approximately the room temperature) while being enriched with reactive oxygen and nitrogen substances (RONS) [104]. It is typically produced by exciting the gas to a plasma state via imposing an electric field. Given its redox-modulatory role and unique multi-modality nature, CAP has been applied in diverse branches of medicine such as sterilization and wound healing [105]. The selectivity of CAP against cancer cells, i.e., selectively killing transformed cells without harming their healthy peers, has been recognized since 2007 and demonstrated in various types of cancers such as triple-negative breast cancer [106], prostate cancer [107], bladder cancer [108], colorectal cancer [109], pancreatic cancer [110], lung cancer [111], liver cancer [112], and melanoma [113]. Similar with HSP90 inhibitors, CAP was shown capable of fostering uncontrolled cell growth [114], inducing PCD (such as apoptosis [115] and ferroptosis [116]), halting cancer cell migration [108], targeting cancer angiogenesis [117], reprogramming cancer cell metabolism [118], assisting cancer cells in immune surveillance evasion [119], attenuating cancer-associated inflammation [120], and accelerating the death of cells with impaired genome integrity [121]. Clinically, CAP has been successfully used to secure the life of a 75-year-old pancreatic cancer patient in 2016 and the life of a 33-year-old patient carrying a rare recurrent incurable peritoneal sarcoma in 2019. The first FDA-approved clinical trial examining the safety and efficacy of CAP as an onco-therapy (NCT4267575) was initiated in 2019 and completed in 2021, during which 17 of 20 enrolled patients with varied types of advanced solid cancers were still alive by the end of this study.

One prevailing theory explaining the selectivity of CAP in targeting cancer cells without harming their healthy peers believes that cells once transformed to the malignant state have higher redox levels easily reaching the death threshold on external stimuli, and CAP takes on its action via imposing cells with redox stress taking advantages of such differences [122]. With incremental understandings on the mechanisms-of-action enabling the medical use of CAP, it has been proposed that CAP targets the system controlling cellular redox homeostasis without being restricted to any particular molecular target [123]. Thus, molecular hits under stress conditions may not be targeted by CAP once cells regained redox homeostasis. Such unique dynamic and multi-modal features make, on one hand, cells vulnerable to CAP less likely develop therapeutic resistance and, on the other hand, the choice of CAP for disease treatment a safe option.

Despite these pre-clinical and clinical advances, no consensus has so far been reached regarding the molecular mechanism of CAP as an onco-therapy as well as its other medical miracles. One relatively prevailing explanation attributes the selectivity of CAP against cancer cells to the relatively higher redox level of cancer cells than that of normal cells. This renders cancer cells more easily to reach the oxidative damage threshold in response to external stimuli and thus be more vulnerable to CAP treatment than their healthy peers [124]. However, this theory can not explain the higher sensitivity of CSCs to CAP treatment than the bulk tumor cells and normal cells, which have the lowest basal redox level among the three cell types [106]. Another theory states that the higher susceptibility of cancer cells to CAP-induced cell death is caused by the abnormally faster division of cancer cells than normal cells [125]. Yet, this can not explain the efficacy of CAP in tumor microenvironment (TME) editing [114] and drug resistance rewiring [104]. Thus, more relevant explanations are needed to identify the key features of CAP that enable its multifaceted medical applications.

4.2. CAP as a possible HSP90 inhibitor and proteostasis regulator

Eukaryotic cells are featured by different subcellular redox environments. Thus, proteostasis of each compartment needs to be adapted to the subcellular redox properties. Specifically, ER provides a more oxidizing environment, and conditions in the cytosol, nucleus and mitochondria are more reducing. Since the redox level controls the formation of intra- and inter-molecular disulfide bonds, cellular redox homeostasis tightly affects protein folding and, consequently, proteostasis (Fig. 3) [126].

Fig. 3.

Intrinsic connections between redox homeostasis and proteostasis, and the role of cold atmospheric plasma (CAP) as a possible proteostasis regulator. Heat shock protein 90 (HSP90) is distributed in the cytoplasm, endoplasmic reticulum (ER), mitochondria, cell membrane, and extracellular space. Take cytoplasm as the example, disturbed redox level homeostasis induces abnormal formation of intra- and inter-molecular disulfide bonds of proteins, leading to abnormal protein folding. An increased level of misfolded proteins results in protein degradation burden. On the other hand, intact HSP90 keeps the stabilization of proteins including some abnormally folded ones, leading to disturbed proteostasis. Failed proteostasis acts as a harmful stimulus to trigger inflammation as manifested as the release of pro-inflammatory cytokines. These cytokines in turn further aggregate redox imbalance, and drive the pathogenesis of diseases such as cancers and autoimmune diseases. CAP can possibly regulate proteostasis by inducing reactive oxygen species (ROS)-dependent HSP90 cleavage, leading to the degradation of abnormally folded proteins and returned proteostasis.

Given such an intrinsic interplay between protein and redox homeostasis, CAP can easily alter the outcomes of cells by regulating proteostasis. This enables CAP with a role similar to HSP90 inhibitors. Indeed, CAP was found capable of cleaving HSP90 chaperone that consequently leads to reduced viability of breast, prostate and colorectal cancer cells [127]. Specifically, CAP provoked the oxidative cleavage of HSP90 upon ROS generation by a Fenton-like reaction in the proximity to the N-terminal nucleotide binding site of HSP90 [128], and the cleavage led to the loss-of-function of HSP90 in stabilizing its client proteins [127]. It is also reported that vorinostat (a histone deacetylase inhibitor) synergized with gefitinib (an EGFR tyrosine kinase inhibitor) in triggering the death of non-small cell lung cancer cells through ROS-dependent HSP90 cleavage [129]. Actually, scientific evidence supporting the existence of ROS-mediated HSP90 cleavage and associated cytotoxicity can be traced back to 2008 when it was reported to participate in cell apoptosis through inhibiting telomerase activity [130]. Thus, being a promising onco-therapeutic tool relying on redox perturbation, it is no wonder that CAP can cleave HSP90 chaperone. It is noteworthy that CAP may be advantageous over currently available HSP90 inhibitors that are mostly in the form of small molecules in being accompanied with little adverse event if on appropriate dosage. Given that CAP targets the disturbed redox homeostasis instead of HSP90, its cutting role on HSP90 may get lost once cells returned back to proteostasis provided with the intrinsic connection between redox homeostasis and proteostasis. That is, once cells lost the homeostatic stress, the level of ROS would be pushed back to the level at the physiological condition, which can not support ROS-dependent cleavage on HSP90. Indeed, a pre-clinical study suggested CAP as a safe option for medical use without the potential of causing genotoxicity and mutagenicity [131]. The efficacy and safety of using CAP for treating diabetic wound has been critically evaluated, with desirable results achieved without evidence of causing damages to the liver and kidney both in vitro and in vivo [132]. Clinically, CAP has been shown effective and safe in treating keloid in a randomized controlled trial involving 18 patients [133].

The inhibitory role of CAP on HSP90 is determined by the tumor-promotive functions of HSP90, abnormal activity of which disturbs proteostasis. Perturbed proteostasis may lead to the burden of abnormal proteins accumulated in cells and, in particular, ER. Almost all membrane proteins and most secreted proteins formed disulfide bridge to acquire functional three-dimensional structures in ER, the process of which is mediated by protein disulfide isomerase family members and other oxidoreductases. Processes catalyzed by these enzymes contribute to the ROS turnover of ER, rendering ER highly vulnerable to proteostasis perturbation. Thus, deregulated ER proteostasis as a result of, e.g., aberrant HSP90 functionality, triggers ER stress to provoke unfolded protein response (UPR) via activating genes participating in protein folding and the anti-oxidative machinery [134]. This will ultimately lead to altered redox homeostasis and provoke of the suppression of redox modulatory tools such as CAP on HSP90.

5. Maintaining proteostasis for treating inflammation-driven diseases

5.1. Inflammation as a consequence of failed proteostasis

Proteostasis is vital in maintaining the integrity of cellular protein functionalities, as keeping balanced protein synthesis and degrading misfolded proteins are essential to preserve all cellular functions. Failed proteostasis may act as a harmful stimulus to trigger inflammation and inform the immune system to remove endangered cells that may cause possible deadly consequences. Subsequent release of inflammatory mediators such as cytokines may further disturb proteostasis by inducing oxidative stress that exacerbates protein misfolding and causes an eventual overload of the degradation system, forming a vicious cycle (Fig. 3).

5.2. Maintaining proteostasis as a possible solution of inflammation-driven disorders

Given that inflammation is a manifestation of disturbed proteostasis, restoring cellular proteostasis by inhibiting HSP90 or applying CAP may be a possible solution of diseases driven by inflammation.

Tumor-associated inflammation is an enabling hallmark of cancer. That is, pathological inflammation can be considered as a driving force of carcinogenesis as it fosters the other cancer characteristics. Accordingly, HSP90 and CAP have both been demonstrated capable of arresting the aggressiveness of transformed cells. We thus wonder whether approaches maintaining or restoring cellular proteostasis such as HSP90 inhibitors or CAP can help resolve other inflammation-driven pathologies (Fig. 3).

One heterogeneous group of disorders closely related to chronic inflammation are autoimmune diseases including, e.g., psoriasis, rheumatoid arthritis, Alzheimer's disease, type I diabetes, systemic lupus erythematosus, autoimmune encephalomyelitis, and acquired epithelial bullosa. At present, its treatment mainly relies on hormone drugs such as corticosteroids, and immunity suppressants such as azathioprine, phenylbutyrate, cyclophosphamide, cyclosporin, mycophenolate, and methotrexate [135]. These therapeutic modalities, although alleviating symptoms, are often accompanied with severe side effects that may increase the risk of getting infections and developing cancers. HSP90 inhibitors have been shown capable of regulating cytokine storms in excessive delayed and acute inflammation using acute lung injury and delayed-type hypersensitivity models [136]. Inhibiting HSP90 using ganetespib ameliorated inflammation in a mouse model of atopic dermatitis [137]. The HSP90 inhibitor C-316-1 attenuated acute kidney injury by suppressing receptor-interacting protein kinase 1 (RIPK1)-mediated inflammation and necroptosis [138]. 17-AAG suppressed nucleus pulposus inflammation and catabolism triggered by M1 macrophages [139]. Oral intake of SNX-5422 attenuated the replication of SARS-CoV-2 via dampening inflammation in airway cells [140]. AT-533 attenuated HS1-induced inflammation [141]. In addition, targeting HSP90 has been shown promising, using preclinical rodent studies, in treating encephalomyelitis, systemic lupus erythematosus, and acquired epidermal bullosa [[142], [143], [144]]. On the other hand, CAP ameliorated imiquimod-triggered psoriasis-like skin inflammation in mice [145], alleviated radiation-induced skin injury by suppressing inflammation [146], removed diabetes-induced inflammation together with the associated enzyme glycation [147], inhibited inflammation and tumor-like features of fibroblast-like synoviocytes from rheumatoid arthritis patients [148], and suppressed tunicamycin-induced inflammation with a therapeutic potential for treating respiratory diseases [149]. Additionally, CAP showed a great promise in curing neurodegenerative syndromes including Alzheimer's diseases via enhancing neural cell differentiation into neurons [150]. Enlightened by these encouraging results, more intensive efforts are required to explore the possible synergy between canonical HSP90 inhibitors and CAP in treating inflammation-driven pathologies, and examine the potency of HSP90 inhibitors, CAP or their combined use in treating diseases beyond those aforementioned.

6. Conclusion

HSP90, being the most abundant HSP family members unanimously present in cells, plays a paramount role in maintaining proteostasis, the association of which with cancers has been extensively investigated. By modulating the stability and functionality of its large spectrum of client proteins including kinases, signaling molecules, and transcription factors that orchestrate the signaling network driving carcinogenesis, HSP90 plays multifaceted roles in fostering varied hallmarks of cancers. Accordingly, several natural or synthetic HSP90 inhibitors have been established, with efficacies in arresting cancer aggressiveness being consecutively reported.

Importantly, we identify an intrinsic interplay between proteostasis and redox homeostasis, as protein equilibrium is subjected to subcellular redox modulation. This leads to the possible role of redox modulatory tools such as CAP in regulating proteostasis and arresting the malignant properties of transformed cells. Thus, we propose CAP as a proteostasis controller that conveys a similar treatment efficacy with HSP90 inhibitors but has little adverse effect. While advocating the role of CAP in serving as an innovative type of HSP90 inhibitors for cancer control, we do not exclude the possibility of other redox regulators in suppressing HSP90. In other words, CAP is a naturally existing tool that exemplifies how editing cellular redox homeostasis can be used to intervene proteostasis and disease progression, where HSP90 or similar acts as the nexus. Also, CAP is featured with multi-modality, i.e., it functions on the system orchestrating the cellular redox homeostasis that does not rely on any particular targets. Thus, its suppressive functionality on HSP90 may be extended to other molecules having close functions and not applicable to proteins with distinct roles in proteostasis.

In addition, we courageously propose HSP90 inhibitors, CAP or redox editing tools with similar mechanisms-of action as possible cure of most, if not all, inflammation-driven pathologies, since inflammation is a manifestation of failed proteostasis. This may lead to a plethora of investigations examining the efficacy of HSP90 inhibitors, CAP or their synergies in treating diseases in addition to or beyond cancers such as the heterogeneous group of autoimmune disorders. On the other hand, these insights may spark a broad interest of examining the potency of putative onco-therapeutic tools in suppressing HSP90. From the clinical perspective, currently available HSP90 inhibitor are all small molecules disturbing the activity or stability of HSP90 with unavoidable and even severe adverse effects. Therapeutic strategies dynamically targeting the system regulating redox homeostasis or proteostasis instead of restricting the targets to any particular molecular targets and exempting the targets once the imbalanced system returned homeostasis may offer clues for establishing effective regimes with desirable safety.

CRediT authorship contribution statement

Xiaofeng Dai: Writing – review & editing, Writing – original draft, Visualization, Supervision, Resources, Project administration, Funding acquisition, Conceptualization. Ruohan Lyu: Writing – review & editing. Guanqun Ge: Funding acquisition.

Consent for publication

All authors have read and agreed the content of the manuscript as well as its submission.

Declaration of competing interest

The authors declare that there are no conflicts of interest.