Unveiling the dark side of glucose-regulated protein 78 (GRP78) in cancers and other human pathology: a systematic review

Abstract

Glucose-Regulated Protein 78 (GRP78) is a chaperone protein that is predominantly expressed in the lumen of the endoplasmic reticulum. GRP78 plays a crucial role in protein folding by assisting in the assembly of misfolded proteins. Under cellular stress conditions, GRP78 can translocate to the cell surface (csGRP78) were it interacts with different ligands to initiate various intracellular pathways. The expression of csGRP78 has been associated with tumor initiation and progression of multiple cancer types. This review provides a comprehensive analysis of the existing evidence on the roles of GRP78 in various types of cancer and other human pathology. Additionally, the review discusses the current understanding of the mechanisms underlying GRP78's involvement in tumorigenesis and cancer advancement. Furthermore, we highlight recent innovative approaches employed in downregulating GRP78 expression in cancers as a potential therapeutic target.

Introduction

Glucose-Regulated Protein 78 (GRP78) is a heat shock protein chaperone that has garnered extensive attention in various disease progression (Gopal and Pizzo 2021). GRP78 primarily resides in the lumen of the endoplasmic reticulum, a cellular organelle responsible for protein synthesis and folding (Lee 2001). It plays a vital role in maintaining protein homeostasis by assisting in the assembly and proper folding of newly synthesized proteins (Pyrko et al. 2007).Under normal physiological conditions, GRP78 ensures the efficient folding of proteins and prevents the accumulation of misfolded or unfolded proteins in the endoplasmic reticulum (Fu et al. 2008). However, in response to cellular stress, such as nutrient deprivation, hypoxia, or exposure to toxins, the demand for proper protein folding increases. In these conditions, GRP78 expression is upregulated to meet the heightened protein-folding requirements (Koumenis et al. 2002). Interestingly, in addition to its role within the endoplasmic reticulum, GRP78 has been found to undergo translocation to the cell surface. This cell surface localization gives rise to a distinct form of GRP78 known as cell surface GRP78 (csGRP78) (Misra et al. 2005). The translocation of GRP78 to the cell surface is believed to be a stress response mechanism that allows cells to adapt and survive adverse conditions (Lee 2001).

csGRP78 has been implicated in the development and progression of various types of cancer (Dong et al. 2005). It has been observed that csGRP78 expression is often upregulated in cancer cells compared to normal cells (Dong et al. 2011). This overexpression is associated with several hallmarks of cancer, including increased cell survival, enhanced proliferation, angiogenesis (formation of new blood vessels to support tumor growth), and resistance to chemotherapy and radiation therapy (Sedighzadeh et al. 2021). At the cell surface, csGRP78 interacts with specific ligands, such as hormones, growth factors, and extracellular matrix proteins. These interactions trigger signaling pathways that promote cell survival, activate pro-survival and pro-growth signaling cascades, and contribute to the acquisition of malignant traits by cancer cells (Dong et al. 2005; Fu et al. 2008). Understanding the intricate involvement of GRP78, particularly csGRP78, in cancer biology would therefore be crucial in developing effective therapeutic strategies. Targeting GRP78 or its interactions with ligands holds promise for disrupting the survival and growth pathways that drive cancer progression (Fu et al. 2008). Therefore, numerous research efforts have been directed towards unraveling the underlying mechanisms of GRP78 in cancer and exploring innovative approaches to modulate its expression or function (Ninkovic et al. 2020; Qiao et al. 2020).

In this review, we aim to provide a systematic breakdown of the correlation between GRP78 and various types of cancer. By compiling and analyzing relevant research findings, we seek to elucidate the extensive role of GRP78 in cancer pathogenesis and shed light on recent advances in downregulating GRP78 as a potential therapeutic avenue.

Discovery and types of GRPs

In the 1970s, researchers (Pouysségur et al. 1977; Hightower 1980) identified a group of proteins that were constitutively produced and activated in response to glucose deprivation, and they were given the moniker glucose-regulated proteins (GRPs). One of the most extensively studied GRPs is GRP78. The chaperone protein GRP78 was first identified and described by Stone et al. in 1974 (Stone et al. 1974). Initially, in 1976, it was believed that GRP78 was a 73 kDa membrane protein associated with viral transformation. However, subsequent studies by Shiu et al. in 1977 demonstrated that this membrane protein was primarily involved in glucose regulation within cells rather than viral transformation (Shiu et al. 1977).

Over the years, the understanding of GRPs has expanded, and various types of GRPs have been identified with their distinct functions. These GRPs act as chaperones and are activated in response to cellular stress. Here are some of the key types of GRPs and their important functions (Table 1):

Table 1 Types of GRPs with their important functions

A comprehensive literature search using the terms "GRP78" or "BiP" reveals a vast number of publications on the topic, reflecting the extensive research interest in GRP78. Most references cited in this article are derived from reliable sources such as PubMed, Web of Science, and reputable scientific publishers like Elsevier and Wiley. This study provides a deeper insight into the role of GRP78 and various diseases, particularly cancer. GRP78 has been implicated in the pathogenesis and progression of various cancers, highlighting its significance as a potential therapeutic target (Luo et al. 2018). Investigating the subcellular processes and functions of GRP78 offers valuable insights into the development of novel strategies for the treatment of associated diseases.

Description of GRP78 coupled with its roles

GRP78, also known as glucose-regulated protein 78, is a chaperone protein that primarily resides in the lumen of the endoplasmic reticulum (ER) due to its carboxyl KDEL retention motif, but it can also be found on the cell surface (Lee 2014). It is encoded by the Hsp5a gene and belongs to the heat shock protein-70 (Hsp70) family. Although GRP78 is the most common member of the Hsp70 family, it lacks a heat shock element in its promoter, preventing activation by heat shock (Casas 2017). In normal cell states, GRP78 remains inactive while bound to transmembrane stress detectors of the unfolded protein response (UPR) pathway, including PRK-like endoplasmic reticulum kinase (PERK), inositol-requiring enzyme (IRE1), and activating transcription factor 6 (ATF6) (Pfaffenbach and Lee 2011) (Fig. 1). This binding keeps GRP78 in an inactive state, ready to respond to ER stress. The regulation of GRP78 involves the binding of nuclear transcription factor Y (NFY), SP1, and histone deacetylase 1 (HDAC1) to stress response elements (EREs) present in its promoter region, thereby maintaining its low basal transcription level (Fig. 1) (Yoshida et al. 1998; Roy and Lee 1999; Baumeister et al. 2005). Interestingly, GRP78 plays a dual role within the endoplasmic reticulum. Firstly, it functions as a resident chaperone, facilitating proper protein folding and preventing protein aggregation. This chaperone activity of GRP78 ensures the correct folding and assembly of newly synthesized polypeptides. Secondly, GRP78 acts as a key regulator of the UPR pathway. During ER stress, the accumulation of unfolded proteins sequesters GRP78 away from PERK, IRE1, and ATF6, thereby activating these transmembrane proteins (Fig. 2) (Hendershot 2004). The UPR is a cellular response mechanism that aims to restore ER homeostasis under stress conditions. It is divided into two phases: the early, pro-survival UPR, which activates adaptive responses to mitigate ER stress, and the late, pro-apoptotic UPR, which triggers cell death if ER stress becomes overwhelming(Roller and Maddalo 2013a). It is noteworthy that the persistent activation of the UPR pathway and the dysregulation of GRP78 have been implicated in various malignancies. Genes upstream or downstream of the UPR are frequently upregulated in cancer cells, suggesting that the sustained activation of this pathway provides tumors with a growth advantage(Roller and Maddalo 2013a).

Fig. 1

Illustration of the inactive state of GRP78 and the presence of stress response elements (ERES) positioned upstream of the TATA element. In cells that are not experiencing stress, the ERES region is bound by NFY, SP1, and HDAC1, leading to the maintenance of low transcription levels for GRP78. Additionally, GRP78, located within the lumen of the endoplasmic reticulum (ER), interacts with the ER stress sensors (IRE1, ATF6, and PERK) and acts as a signaling component for the unfolded protein response (UPR)

Fig. 2

Representation of the general mechanism by which GRP78 functions within the endoplasmic reticulum (ER). GRP78 plays a crucial role in assisting ER proteins by promoting their stability and facilitating the process of protein folding. This is achieved through its interaction with misfolded proteins and unassembled complexes, thereby initiating the ER-associated degradation (ERAD) pathway. Within this pathway, GRP78 targets the misfolded proteins for degradation through a series of events that involve ubiquitination and subsequent proteasomal degradation

Studies investigating the transcriptional activation mechanism of GRP78 have contributed to the identification of novel intracellular signaling pathways that transmit ER stress signals to the nucleus, initiating the transcription of genes involved in the unfolded protein response (UPR) (Luo and Lee 2013; Mori 2000; Chang et al. 1989). Figure 3 illustrates the cascade of downstream pathways initiated by UPR stress sensors when unfolded peptides accumulate. Upon activation, PERK forms homodimers and undergoes autophosphorylation, enabling the alpha subunit of eukaryotic translation initiation factor 2 (elf2) to be phosphorylated (Ibrahim et al. 2019; Li et al. 2008; Liu et al. 2020). Phosphorylated elf2 inhibits translation initiation, triggering the activation of the transcription factor ATF4 and its target gene C/EBP homologous protein (CHOP). CHOP, in turn, induces apoptosis by downregulating BCL-2 (an antiapoptotic outer mitochondrial membrane protein) and activating the expression of Bax and Bim genes (Roller and Maddalo 2013a). IRE1 also homodimerizes and auto phosphorylates before cleaving and splicing the mRNA encoding X-box binding protein 1. Additionally, IRE1 recruit’s tumor necrosis factor α receptor-associated factor 2 (TRAF2), which activates JUN amino-terminal kinases (JNK), ultimately leading to apoptosis. Furthermore, ATF6 translocate to the Golgi apparatus after dissociation from GRP78 and subsequently undergoes cleavage, allowing its active form, cATF6, to enter the nucleus. In the nucleus, cATF6 acts as a transcription factor, upregulating proteins essential for enhancing the ER's capacity for protein folding (Ibrahim et al. 2019). The pathways mediated by PERK, IRE1, and ATF6 intersect with each other and elicit cellular responses that can be both pro-survival and pro-apoptotic. Importantly, the induction of GRP78 transcription is not solely triggered by ER stress. Some scholarly articles have demonstrated the activation of GRP78 transcription by histone deacetylase inhibitors (HDAC1) without concurrently inducing the overall stress response (Baumeister et al. 2009). Additionally, impaired autophagy in tumor cells has been shown to upregulate ER chaperones in response to metabolic stress (Mathew et al. 2009).

Fig. 3

A proposed comprehensive pathway illustrating the involvement of GRP78 as a central regulator in the cellular response to endoplasmic reticulum (ER) stress, known as the unfolded protein response (UPR). In this pathway, GRP78 acts as a master regulator, coordinating the UPR signaling cascade

In tumor microenvironments where GRP78 is overexpressed, it localizes to the cell membrane surface (Roller and Maddalo 2013b). Referred to as cell surface GRP78 (csGRP78), this protein plays a significant role in various signaling events and acts as a co-receptor in studies involving different tumor types, influencing tumor cell survival, proliferation, and motility. The relocation of GRP78 to the cell surface has been strongly associated with drug resistance and cellular transformation (Roller and Maddalo 2013b; Chen et al. 2022). Additionally, alterations in cell surface GRP78 have been shown to impact the behavior of cancer stem cell populations in diverse tumor types (Chen et al. 2018a, b). One well-studied pathway involving cell surface GRP78 is its interaction with the tumorigenic PI3K/AKT pathway, facilitated by complex formation with PI3K (phosphoinositide-3-kinase) (Lee et al. 2022a) (Fig. 4). This interaction has been observed in prostate cancer (Zhang et al. 2013). In a study conducted by Zhang et al. in 2013 (Zhang et al. 2013), they discovered an insertion mutation in the N-terminus domain of GRP78. This mutation resulted in a decrease in complex formation, while the protein's expression and ability to move to the cell surface remained stable, similar to the normal, unmutated protein. The researchers concluded that blocking csGRP78 could present a distinct strategy to inhibit PI3K activity and potentially overcome treatment resistance in cancer cells.

Fig. 4

A breakdown of cell surface GRP78 (csGRP78) roles in AKT/P13K/ mTOR pathway. AKT and phosphoinositide-dependent kinase-1 (PDK1) are both drawn to the membrane after PI3K produces the second messenger PIP3. Due to proximity, PDK1 can phosphorylate AKT at position T308 of the activation loop (T-loop). The rapamycin-resistant mTOR complex 2 then phosphorylates AKT at residue S473 of the hydrophobic motif (mTORC2), a key step in the complete activation of AKT's kinase activity. TSC2, a negative regulator of mTORC1 activity, is one of several additional substrates that AKT itself has the ability to phosphorylate

GRP78 role in various diseases

GRP78, a master chaperone protein of the unfolded protein response (UPR), plays a crucial role in various diseases, mainly cancer. In cancer, GRP78 has been implicated in chemotherapy resistance and the tumor virulence (Cook and Clarke 2015). Tumor cells often face a hostile metabolic environment characterized by low glucose levels, acidity, and nutrient deprivation (Ferraresi et al. 2020; Wang et al. 2009). Interestingly, the UPR, which involves the activation of GRP78, is triggered in response to oxygen or glucose starvation to promote cell survival (Wojtkowiak et al. 2012; Ferraresi et al. 2020; Wang et al. 2009; Koumenis 2006). Elevated expression levels of GRP78 have been observed in initial tumors compared to benign tissues, indicating its involvement in tumor progression and aggressiveness (Wojtkowiak et al. 2012). GRP78 acts as a multifunctional protein in cancer, contributing to diverse cellular processes such as protein folding, ER homeostasis, regulation of apoptosis, and protection against cellular stress (Ferraresi et al. 2020).

The role of GRP78 extends beyond cancer. Here, we highlighted some diseases where GRP78 has been implicated: (Fig. 5 and Table2 below):

Fig. 5

Diagrammatical Illustration showing multifactorial implications of GRP78 in various diseases

Table 2 Shows various role of GRP78 in various cancer with mechanisms

Novel approaches employed in downregulating GRP78 implicated in various diseases

Several researchers have been actively exploring the possibilities of targeting GRP78 downregulation in cancer therapeutic strategy. These efforts aim to restore normal cellular function and counteract the detrimental effects associated with GRP78 dysregulation. Recent publications have shed light on various innovative approaches employed in targeting GRP78 expression or function. Downregulation of GRP78 expression has been investigated as a potential therapeutic strategy in several diseases(Luo et al. 2018; Wang et al. 2017a, b). In prostate cancer, Lu et al. utilized RNA interference technology to downregulate GRP78 and GRP74 expression in the PC-3 cell line, and this result in reduced cell migration and induction of apoptosis(Lu et al. 2019). Another study focused on inhibiting the VEGF/GRP78 axis using the sFLT01 protein in human prostate cancer cells (DU145). The researchers observed downregulation of GRP78 and matrix metallopeptidase proteins 2&9 (MMP2&9) transcript levels, along with increased expression of tissue inhibitor of metalloproteinase proteins 1&2 (TIMP1&2), suggesting that sFLT01 inhibited cancer cell proliferation and invasiveness (Taghizadeh et al. 2021).

In breast cancer, a proteomic approach identified cell surface GRP78 and Dermcidin (DCD) as cooperative regulators of breast cancer cell migration through Wnt signaling. The interaction between GRP78 and DCD was found to be involved in the regulation of stem cell and cancer cell migration(Lager et al. 2021). Additionally, upregulating ATAD3A in colorectal cancer cells was shown to stabilize GRP78, reduce endoplasmic reticulum (ER) stress, and decrease chemotherapy-induced cancer cell death(Huang et al. 2021a, b). Cardiac glycosides (CGs), such as Lanatoside C (LanC), have been investigated for their inhibitory effect on GRP78 activation induced by ER stress in pancreatic cancer cells. The inhibition of GRP78 activation was associated with apoptosis induction in pancreatic cancer cells(Ha et al. 2021). Furthermore, Suppressor of cytokine signaling-3 (SOCS3) was found to target GRP78 ubiquitination for proteasomal degradation, thereby blocking ER stress and mitophagy pathways in cardiac hypertrophy, which could potentially prevent heart failure(Liu et al. 2021).

Traditional Chinese Medicine (TCM) treatments have also been explored for their ability to downregulate GRP78 expression. TCMSini's San (SNS), used for depression-related symptoms, was shown to attenuate GRP78 expression, preventing its interaction with LRP5 on the cell surface and inhibiting breast cancer stem cell signaling through the Wnt and β-catenin pathway(Zheng et al. 2021; Liu et al. 2022; Sadeghipour et al. 2022). Another TCM remedy, Ai Du Qing (ADQ), downregulated GRP78 expression, leading to the degradation of β-catenin and attenuating chemoresistance in breast cancer cells (Liao et al. 2021). Palmatine (Fig. 6), a plant-derived compound with various protective properties, was found to downregulate GRP78 expression in a streptozotocin-induced diabetic rat model. Palmatine treatment inhibited the upregulation of GRP78 in the pancreas, suggesting its potential as a therapeutic agent for diabetes(Okechukwu et al. 2021). In the field of immunotherapy, chimeric antigen receptor (CAR) T cells targeting cell surface GRP78 (GRP78.1x, GRP78.2x, and GRP78.3 × CAR T cells) demonstrated potent anti-tumor activity against acute myeloid leukemia (AML) cells. The CAR T cells induced the production of anti-tumor cytokines and effectively suppressed tumor progression in preclinical models (Yu et al. 2022) (Fig. 6).Recently, the activation of GRP78 ATPase by ZBM-H has been shown to suppress A549 lung cancer cell migration by promoting the degradation of integrin β4 (ITGB4). ZBM-H treatment resulted in decreased ITGB4 protein levels and inhibited the epithelial-mesenchymal transition (EMT) process in A549 cells. The activation of GRP78 ATPase by ZBM-H also facilitated the interaction between annexin A7 (ANXA7) and heat shock cognate 70 kDa protein (Hsc70), which contributed to the regulation of selective autophagy and degradation of ITGB4(Ning et al. 2022a, b).

Fig. 6

Overview of Synthetic and Natural Products that has been employed in Downregulating GRP78 in various cancers. Natural and synthetic compounds with anticancer properties that suppress GRP78 induction has been reported below; however, they exert pleiotropic effects

These studies highlight various novel approaches employed to downregulate GRP78 expression and investigate its implications in different diseases. The use of RNA interference technology, protein inhibitors, traditional Chinese medicine, cardiac glycosides, and immunotherapy strategies targeting cell surface GRP78 have shown promising results in inhibiting tumor cell proliferation, migration, and inducing apoptosis. Additionally, the modulation of GRP78 expression has been explored in the context of diabetes, cardiac hypertrophy, and psychological stress-related diseases. Targeting GRP78 and exploring its regulatory mechanisms offer potential avenues for therapeutic interventions in cancer, diabetes, cardiovascular diseases, and other conditions associated with GRP78 dysregulation. Further research is needed to elucidate the underlying mechanisms and optimize the therapeutic strategies targeting GRP78 to improve patient outcomes.

Agents that target cell surface GRP78

Agents that target cell surface GRP78 include specific peptides, both conjugated and unconjugated, as well as the human plasminogen factor kringle 5. These agents have demonstrated efficient targeting of cell surface GRP78. Additionally, antibodies against cell surface GRP78 have been developed with diverse mechanisms to induce cancer cell death and suppress GRP78-mediated oncogenic signaling(Lee 2014; Hernandez and Cohen 2022). Numerous strategies have been employed to reduce cell surface GRP78 levels, and various inhibitory agents have been investigated. The table below provides a brief overview of phytochemicals, peptides, and antibodies that have shown inhibitory effects against cell surface GRP78 (Table 3) (Elfiky et al. 2020; Shimizu et al. 2022).

Table 3 Shows various Agents that target cell surface GRP78

Despite, various strategies employed, there are some compounds that are site specific when binding. Several compounds like Honokiol, Salicylate and epigallocatechin gallate; directly bind to ATP binding domain to inactivate GRP78 in cancer(Ermakova et al. 2006; Martin et al. 2013). More so, alpha 2 macroglobulin and Mouse C38 and C107 binds with cell surface GRP78 at the N and C terminal respectively (Misra et al. 2009)(Fig. 7 and Table 4).

Fig. 7

Depicts some significant agents that specifically target GRP78 on cell surface

Table 4 Shows site specific binding compounds

Clinical significance of GRP78

The clinical significance of glucose-regulated protein 78 (GRP78) in cancer is profound, as it participates in several tumor biology-related processes, suggesting its potential as a therapeutic target. GRP78 holds significance in three key aspects: tumor therapy, enhancement of therapeutic benefits, and as a prognostic biomarker.

Firstly, GRP78 can serve as a target for tumor therapy. Its overexpression in cancer cells has been consistently observed and is associated with various aspects of tumor progression, including cell survival, resistance to apoptosis, angiogenesis promotion, and facilitation of epithelial-mesenchymal transition (EMT). Targeting GRP78 with selective inhibitors or antibodies offers the potential to disrupt its pro-survival functions, sensitize cancer cells to conventional therapies, and overcome treatment resistance. By inhibiting GRP78, it becomes possible to interfere with critical cellular processes essential for cancer cell survival and growth, thereby enhancing the effectiveness of cancer treatment. Secondly, the creation of unfolded protein response (UPR)-related proteins, particularly GRP78, can enhance the therapeutic benefits of drugs commonly used in clinical practice. The UPR is a cellular stress response mechanism activated in cancer cells under conditions of increased protein folding demand and nutrient deprivation. GRP78 is a key regulator of the UPR and assists in maintaining cellular homeostasis during stress. Modulating the UPR through GRP78 can improve the efficacy of anticancer drugs by increasing their cytotoxic effects on cancer cells. This approach has the potential to enhance treatment outcomes and overcome drug resistance. Lastly, GRP78 can serve as a biomarker for evaluating cancer patients and predicting prognosis. The expression levels of GRP78 in tumor tissues or body fluids can be indicative of disease progression and patient outcomes. Assessing GRP78 as a diagnostic or prognostic biomarker holds promise for early cancer detection and risk stratification. By evaluating GRP78 expression, it becomes possible to identify high-risk patients who may benefit from aggressive treatment strategies or personalized therapies. Additionally, monitoring changes in GRP78 levels during the course of treatment can provide valuable information on treatment response and disease status.

In preclinical studies, selective inhibitors or antibodies against GRP78 have shown promising results in disrupting its pro-survival functions and sensitizing cancer cells to conventional therapies (Lee 2007). For instance, a study by Chen et al. (2022) demonstrated that an anti-GRP78 antibody, PAT-SM6, effectively inhibited tumor growth in melanoma mouse models(Chen et al. 2022). Furthermore, combination therapies involving GRP78 targeting have been investigated to enhance treatment response. Combining GRP78 inhibition with conventional therapies or emerging treatment modalities, such as immunotherapy, has shown synergistic effects, leading to improved outcomes (Lev et al. 2017). These approaches hold great potential for overcoming treatment resistance and improving patient survival. One approach to improving therapeutic outcomes is by utilizing UPR-related proteins, including GRP78, to enhance drug delivery and sensitize cancer cells to treatment. For instance, Zhao et al. (2015) developed a nanoparticle-based drug delivery system that specifically targets GRP78-expressing cancer cells, resulting in improved drug efficacy and reduced systemic toxicity (Zhao et al. 2015). By exploiting the UPR pathway, such strategies have the potential to optimize the effectiveness of conventional chemotherapeutic agents and overcome drug resistance mechanisms.

Despite the challenges in comprehending the precise role of csGRP78 in cancer, the targeting of csGRP78 has shown promise in preclinical and early clinical studies. PAT-SM6, an anti-GRP78 antibody, has demonstrated potent anti-tumoral activity in preclinical models and has progressed to phase I clinical trials in patients with malignant melanoma (Hensel et al. 2013). Further investigations are warranted to explore the therapeutic potential of csGRP78-targeted strategies and to gain a deeper understanding of its functional implications in different cancer types.

Conclusion and a Glimpse into the future

In conclusion, this study has provided a systematic breakdown and pictorial evidence showcasing the correlation between glucose-regulated protein 78 (GRP78) and various types of cancer. The consistent upregulation of GRP78 in cancer cells has been associated with tumor initiation, growth, metastasis, and resistance to therapy. The visual representation of GRP78 expression patterns in different cancer types strengthens its clinical relevance. GRP78 plays a crucial role in cancer development and progression by promoting cell survival, inhibiting apoptosis, facilitating angiogenesis, inducing epithelial-mesenchymal transition (EMT), and conferring therapy resistance. The extensive research supporting these observations highlights the potential of GRP78 as a therapeutic target in cancer treatment. The findings discussed here pave the way for several exciting future perspectives that could contribute to the development of effective cancer therapies. Firstly, there is a need to explore the therapeutic targeting of GRP78. The development of selective inhibitors or antibodies against GRP78 could disrupt its pro-survival functions and sensitize cancer cells to conventional therapies, potentially overcoming treatment resistance and improving patient outcomes. Secondly, investigating combination therapies involving GRP78 targeting is crucial. Assessing the synergistic effects of GRP78 inhibition with conventional therapies or emerging treatment modalities like immunotherapy may enhance treatment response and minimize the likelihood of relapse. Thirdly, exploring the utility of GRP78 as a diagnostic or prognostic biomarker holds promise. Further research is required to evaluate the clinical applicability of GRP78 in early cancer detection and risk stratification, enabling timely intervention and improved patient outcomes. Additionally, unraveling the intricate molecular mechanisms underlying GRP78's role in cancer progression is imperative. This knowledge will provide valuable insights into potential downstream targets and signaling pathways that can be exploited for therapeutic intervention. Finally, preclinical studies using in vitro and in vivo models should be conducted to validate the efficacy and safety of GRP78-targeted therapies before clinical translation. These studies will help optimize treatment strategies and assess potential side effects. In summary, the correlation between GRP78 and various cancers presents a promising avenue for future research and therapeutic interventions. By focusing on GRP78 as a target, we have the potential to overcome treatment resistance and improve outcomes for cancer patients. However, further investigation and validation are required to fully harness the therapeutic potential of GRP78 inhibition in clinical practice.

It is worth noting that GRP78 and other components of the unfolded protein response (UPR) also play important roles in autoimmune disorders and neurological disorders. Understanding the processes through which cell surface GRP78 functions will provide crucial insights into not only cancer but also these other disease areas. The overexpression of cell surface GRP78 in cancer makes it a potential target for tumor therapy(Farshbaf et al. 2020). Exploring new approaches that combine drugs promoting GRP78 translocation to the cell surface with peptides or small compounds targeting GRP78 could significantly enhance the efficacy of cancer treatments. Currently, two GRP78 antibodies, PAT-SM6 and Bold-100, have undergone testing in human clinical trials (Hensel et al. 2013; Rasche et al. 2015; Burris et al. 2016). Initial studies on PAT-SM6 in melanoma tumor-bearing mice showed potent anti-tumoral activity, and a phase I clinical trial was conducted in patients with malignant melanoma to assess the safety and anti-tumor effectiveness of the antibody (Hensel et al. 2013).

Furthermore, Bold-100 also known as IT-139, NKP1339) is a small ruthenium-based compound that specifically targets GRP78. By inhibiting GRP78, Bold-100 aims to disrupt the protective mechanisms of cancer cells, making them more vulnerable to standard cancer treatments such as chemotherapy and radiation therapy (Burris et al. 2016). The rationale behind targeting GRP78 is that it is overexpressed on the surface of cancer cells, providing them with enhanced survival advantages, including resistance to apoptosis (programmed cell death) and increased tolerance to stress conditions. By blocking GRP78, Bold-100 intends to sensitize cancer cells to treatment-induced cell death. Bold Therapeutics has conducted preclinical and early clinical studies to evaluate the safety and efficacy of Bold-100 in various types of cancers (Yoo et al. 2012).

Bold-100 is currently undergoing a global Phase 2 clinical trial for the treatment of advanced gastrointestinal cancers, including bile duct, colon, gastric, and pancreatic cancers. The trial, registered as NCT04421820, involves six sites in Canada, two in the United States, and five in South Korea. Promising results from the Phase 2 study in advanced colorectal cancer were presented at the American Association for Cancer Research (AACR) conference in April 2023. The data demonstrated a clinically significant improvement when compared to standard of care treatments. Specifically, among patients in the 3rd line and beyond with advanced colorectal cancer (n = 17), the study revealed a median Progression-Free Survival (PFS) of 4.7 months, Overall Survival (OS) of 9.8 months, and an Overall Response Rate (ORR) of 13%. The potential of Bold-100 has been recognized by the U.S. Food and Drug Administration (FDA), which has granted Orphan Drug Designations (ODDs) for gastric and pancreatic cancers. It is anticipated that in 2023, Bold-100 will also receive breakthrough therapy designations (BTDs) for colorectal cancer and potentially other indications. As demonstrated in this review, various solid tumor cell types express GRP78 on their surface, making them potential candidates for future clinical studies. Utilizing the GRP78 antibody in tumors where other antibody-based therapies are already available could offer clinical benefits through the development of mechanisms that enable targeted drug delivery.

Conclusively, further research and exploration are needed to fully exploit the therapeutic potential of GRP78 inhibition in cancer treatment, as well as to understand its implications in autoimmune and neurological disorders.