Emerging Role of Selenium Metabolic Pathways in cancer: New Therapeutic Targets for Cancer

Abstract

Selenium is incorporated into the body via the selenocysteine biosynthesis pathway, which is critical in the synthesis of selenoproteins such as glutathione peroxidases and thioredoxin reductases. Selenoproteins, which play a key role in several biological processes including ferroptosis, drug resistance, endoplasmic reticulum stress, and epigenetic processes, are guided by selenium uptake.

In this review, we critically analyze the molecular mechanisms of selenium metabolism and its potential as a therapeutic target for cancer. Selenocysteine ​​insertion sequence binding protein 2 (SECISBP2), which is a positive regulator for the expression of selenoproteins, would be a novel prognostic predictor and an alternate target for cancer. We highlight strategies that attempt to develop a novel selenium metabolism-based approach to uncover a new metabolic drug target for cancer therapy. Moreover, we expect extensive clinical use of SECISBP2 as a specific biomarker in cancer therapy in the near future. Of note, scientists face additional challenges in conducting successful research, including investigations on anti-cancer peptides to target SECISBP2 intracellular protein.

Introduction

The development and progression of cancer is a result of cellular mutations, some of which result in the use of redundant intracellular pathways that bypass the effect of anticancer drugs [Liu et al., 2020; Song et al., 2020], making treatment very challenging [Vasan et al., 2019]. Indeed, drug resistance and the resulting ineffectiveness of the drug treatment are responsible for up to 90% of cancer-related deaths [Wang et al., 2019a]. An attractive approach to overcome this hurdle is interfering with the metabolic pathways that are essential for cancer cell survival [Dong et al., 2017; Voorde et al., 2019; Weyandt et al., 2017].

Selenocysteine (Sec) is the 21^st amino acid, a rare amino acid. A recent study confirms that the selenocysteine biosynthetic pathway is a critical component in the synthesis of selenoproteins, and is essential for the survival of cancer cells, but not normal cells [Carlisle et al., 2020]. Selenocysteine is similar to cysteine, varying only by the substitution of selenium in the place of sulfide [Kalishwaralal et al., 2015; Reich and Hondal, 2016]. Selenocysteine has unique redox chemistry which in many biological systems endows selenoproteins with high redox activity. Recoding of the UGA stop codon by selenocysteine in the presence of selenocysteine-specific transfer ribonucleic acid (Sec tRNA^[Ser]Sec) leads to the specific incorporation of selenocysteine into 25 human selenoproteins [Howard and Copeland, 2019]. Selenocysteine ​​insertion sequence binding protein 2 (SECISBP2) allows the coding of UGA, rather than terminating translation, via binding to the selenocysteine insertion sequence (SECIS). Varying levels of selenoproteins have been observed in a wide variety of physiological processes and diseases[Liu et al., 2020; Song et al., 2020] and there is strong evidence of an association between the expression of selenium-containing proteins in serum and cancer risk and tumor aggressiveness [Short and Williams, 2017; Wu et al., 2021].

To date, 25 selenoproteins including glutathione peroxidase (GPX) and thioredoxin reductase (TrxR) have been identified in the human genome and 24 have been identified in the mouse genome [Regina et al., 2016]. Some studies, including clinical trials, support the role of selenium in preventing cancer [Klein et al., 2011]. Studies show that optimum levels of selenium are beneficial. However, this micronutrient is not effective when levels are higher than those needed for saturating the expression of selenoprotein [Njoroge et al., 2017; Voorde et al., 2019]. Our knowledge regarding the roles of several selenoproteins includes the finding that cancer is rapidly protected from the effects of cancer drugs [Hangauer et al., 2017; Liu et al., 2020; Song et al., 2020]. However, selenoproteins have complex roles and functions similar to those of antioxidants.

This review addresses the role of several selenoproteins such as thioredoxin reductase 1 (TrxR1) and glutathione peroxidase 4 (GPX4) and explores the potential manipulation of molecular pathways of selenium metabolism[Ganther, 1999] in cancer therapy. Since the function of selenocysteine ​​insertion sequence binding protein 2 (SECISBP2) is a necessary factor for the synthesis of selenoprotein in mammals, we propose that SECISBP2 can interact with key selenoamino acid metabolic proteins in a variety of cancer pathways and affects the synthesis of selenoproteins such as TrxR1 and GPX4 [Labunskyy et al., 2014]. This strategy would further expand the scope of current efforts to develop TrxR1 and GPX4 drug inhibitors as an essential goal in the targeting of selenocysteine metabolism [Moosmayer et al., 2021; Xu and Fang, 2021].

Selenium

Selenium is one of the essential micronutrients for mammals. Previously, scientists have shown interest in selenium-containing proteins (selenoproteins) or enzymes, since they are major component of antioxidant enzymes such as GPX and TrxR (selenoenzymes). Selenoenzyme has strong antioxidant activity and offers a protective function in cell defense, safeguarding cells from oxidizing damage caused by reactive oxygen species (ROS) including superoxide, peroxide, hydroxyl radical, nitrogen oxide, and peroxynitrite.

Figure 1 illustrates the key role of selenium in the production of selenoenzymes which in turn prevents tumor formation by decreasing ROS-mediated mutation and tumor initiation. However, several studies contradict this notion and speculate the protective role of selenium in human cancers [Voorde et al., 2019]. It is noteworthy that when triple-negative breast cancer cell lines were cultured in the new physiological medium, Plasmax (selenium medium), the colony-forming ability of cancer cells were enhanced when compared to those cells grown in the conventional DMEM-F12 medium [Voorde et al., 2019]. These results suggest that the selenium present in Plasmax protects cells from impairment by lipid peroxidation and ferroptosis. The concentration of selenium in tumor tissue is almost four-fold higher compared to that in normal cells [Charalabopoulos et al., 2006; Lossow et al., 2021], in particular drug-resistant cancer cells, suggesting that cancer cells utilize selenoenzymes to protect themselves from ROS-producing anticancer agents [Liu et al., 2020; Song et al., 2020; Viswanathan et al., 2017].

Figure 1:

Graphical illustration of the role of selenium in augmenting drug resistance in cancer cells under A. in vitro and B. in vivo (via ferroptosis) conditions. (Created by Biorender.com).

Ferroptosis

Although the scientific community has known about ferroptosis, a type of programmed cell death, for quite a long time, Stockwell’s group coined the term ferroptosis and described its key features [WS et al., 2014]. Cell death induced by cysteine withdrawal was initially discovered in 1955 by Eagle H [H, 1955], who also conceived the fundamental concepts of what is known today as the minimum cell growth condition in culture. Excess extracellular glutamate inhibits cysteine uptake and imparts remarkable oxidative stress to nerve cells during the development of central nervous system diseases, trauma, and other neurodegenerative diseases, resulting in cell death through a pathway called oxytosis [Yao and Asayama, 2017].

Ferroptosis refers to a novel iron-dependent mechanism of regulated cell death. It possesses unique biochemical and cytological characteristics which demarcate it from other forms of cell death such as autophagy, apoptosis and necrosis [Lee et al., 2020]. The hallmarks of ferroptosis include lethal accumulation of reactive oxygen species (ROS) and over production of lipid peroxidation. Lipid peroxidation stress and cell membrane damage caused due to the excessive oxidation of phospholipids containing polyunsaturated fatty acids (PUFAs) in the cell membrane is the key driver of ferroptosis induction since PUFAs have the tendency to form lipid peroxide and consequently induce ferroptosis. The oxidation and anti-oxidation of esterified fatty acids is mainly regulated by GPX4. There is a concomitant increase in the levels of ROS with a decrease in the expression levels of GPX4. Subsequently, the long-chain PUFAs present on the membrane are oxidized, which leads to the induction of ferroptosis. The occurrence of ferroptosis is thus, majorly influenced by lipid peroxidation, Glutathione metabolism and iron metabolism. Some of the key molecules involved in this process are lipoxygenase (LOX), ferritin, transferrin receptor-1 (TFR1), cystine/glutamic acid reverse transporter (system Xc-), and GPX4 [Xie et a., 2021]. At the molecular level, ferroptosis is regulated by the interplay of different pathways such as mevalonate, transsulfuration, glutaminolysis and p62-Keap1-Nuclear factor (erythroid-derived 2)-like 2 (Nrf2) pathway [Viswanathan et al., 2017; Jiang et al., 2021; Chen et al., 2021; Jiang et al., 2020].

Previous studies report eight tissue- and substrate-specific isoforms of GPX in mammals [Brigelius-Flohé and Maiorino, 2013]. Among these GPX4, a phospholipid-hydroperoxidatic enzyme, is remarkable for its activity in reducing complex lipid peroxides and in regulating ferroptosis. Proteomic analysis using mass spectrometry has allowed researchers to identify the role of GPX4 as a regulator of ferroptosis [WS et al., 2014]. Recently, TrxR1 has been proposed to partner with GPX4 to protect cells from the lethal aggregation of lipid peroxides. These results suggest the involvement of TrxR1 selenoenzyme in the regulation of ferroptosis sensitivity [Cai et al., 2020]. The link between the micronutrient selenium and ferroptosis is established via two negative regulators of ferroptosis and the selenium-dependent enzymes GPX4 and TrxR1. Selenoenzymes, the byproducts of the selenium metabolite pathway, have sparked a growing interest among cancer biologists as a potential new target for cancer therapy. However, researchers need to understand the role of selenium metabolism in the progression of cancer and its vulnerability to cancer cells before using it as an alternative therapeutic approach.

GPX4

GPX, which belongs to an antioxidant enzyme family, negatively regulates the formation of peroxidized phospholipids. One of the significant antioxidant enzymes in humans is GPX4. The active site of GPX4 is a tetrad composed of Cys(Sec), Asn, Gln, and Trp catalytic sites located near the protein surface [Ingold et al., 2018]. Any mutation on this site, in particular selenocysteine, significantly impairs GPX4’s normal function [Cheff et al., 2021].

GPX4 plays an important role in inhibiting the non-apoptotic cell death process called ferroptosis by preventing the accumulation of lipid ROS and the production of lipid peroxidation under oxidative stress [WS et al., 2014]. GPX4 is known to reduce lipid hydroperoxides in biological membranes [Brigelius-Flohé and Maiorino, 2013], which explains why cells treated with erastin show high ROS lipid levels during ferroptosis. Furthermore, a decrease in the expression of GPX4 in cancer cells generates lipid ROS and induces ferroptosis, thereby validating the hypothesis of erastin-induced cell death via the inhibition of GPX4 [WS et al., 2014]. Conversely, overexpression of GPX4 prevents cell death caused by ferroptosis inducers such as erastin, RAS-selective lethal (RSL3), suggesting that GPX4 is a negative regulator of cell death by ferroptosis [WS et al., 2014].

Studies of modified GPX4 (selenium replaced with sulfur) in mouse models reveal that GPX4 protects neurons from oxidative stress-induced neurological complications [Ingold et al., 2018]. In addition to the neuroprotective function of GPX4, overexpression or depression of GPX4 plays a significant role in tumor progression [Kinowaki et al., 2018]. Although GPX4 is uncommon in tumor tissues, its higher abundance is noted in both colon carcinoma and hepatocellular carcinoma [Guerriero et al., 2015; Yagublu et al., 2011]. These studies implicate targeting GPX4 as a useful clinical therapeutic opportunity in cancer [Taguchi et al., 2021].

TrxR1

Thioredoxin reductase 1 (TrxR1) is another Sec-containing selenoprotein. It is an NADPH-based oxidoreductase and selenocysteine is an active site in the TrxR1 enzyme. The crystal structure of TrxR1 presents that a selenenylsulfide motif with the selenium of selenocysteine is involved in the reduction of TrxR1 substrate [Cheng et al., 2009].

TrxR1 is one of the key enzymes controlling cell function and the antioxidant system that plays a major role in sustaining low levels of ROS, as well as stimulating many cell survival pathways such as the NF-κB and the hypoxia-inducible factor 1 pathways. TrxR1 is regulated by the Nrf-2 transcription factor and is highly expressed in various cancer types, including blood cancers such as lymphoma and multiple myeloma [AA et al., 2018]. TrxR1 is a major enzyme that controls cell function and plays an important role in regulating growth factor-stimulated protein phosphorylation through activating protein tyrosine phosphatases such as protein tyrosine phosphatase 1B[Arnér, 2021]. The role of the TrxR system in cancer development and progression may vary depending on the cancer stage. In the initial stage, the TrxR1 system’s direct antioxidant activity tends to prevent normal cell malignancy by preventing oxidative stress induced by xenobiotics or carcinogenetic agents. However, the amount of TrxR system components increases considerably as the normal cells become malignant. For example, the TrxR1 in most solid tumor cells is overexpressed and associated with aggressive tumor growth in the lung, breast, colorectum, pancreas, liver, and gastric cells [Ghareeb and Metanis, 2020]. A high level of TrxR1 expression in non-small lung cell cancer leads to aggressive growth in tumors and a decreased patient survival rate. These studies reveal that an increase in TrxR enhances carcinogenic processes in numerous ways, including enhanced tumor formation, angiogenesis induction, and an increase in drug resistance to cancer therapies [Jia et al., 2019]. Recent studies suggest that a ferroptosis drug induces rapid ferroptosis-like cell death by inhibiting thioredoxin enzyme activity in different cancer cells [Tang et al., 2021]. Although there is no evidence to support the idea that TrxR1 inhibitor induces lipid peroxidation [Cai et al., 2020], researchers show interest in the biological properties of TrxR1 in ferroptosis and its potential as a novel target in cancer therapy.

GPX2

Glutathione peroxidase 2 (GPX2), also known as gastrointestinal GPX, is a member of the GPX family of antioxidant enzymes found in mammals. Human glutathione peroxidases (GPX2) are selenoproteins, and their expression is regulated by the availability of selenium (Se) [Brigelius-Flohé and Kipp, 2009]. So far, several research works have contributed to our knowledge about the role of GPX2 and TrxR1 modulation in cancer progression [Arnér, 2020; Brigelius-Flohé and Kipp, 2009]. In most cases of cancers such as lung adenocarcinomas, human breast cancer, human adenocarcinomas, and human colorectal cancer, up-regulation of GPX2 and TrxR1 has been reported. Multi-drug-resistant cancer cells express both enzymes (GPX2 and TrxR1). As a target of the Wnt pathway, GPX2 appears to play an important role in the proliferation and self-renewal of the intestinal epithelium. However, it also supports the proliferation of cancer cells. It is still unclear whether or not TrxR1 also regulates the Wnt pathway. Previous studies have suggested that β-catenin can activate the promoter of GPX2, and the upregulation of β-catenin in the nucleus is associated with poor prognosis [Kipp et al., 2012; Peters et al., 2018; Wang et al., 2019b].

SEPW1

Selenoprotein W1 (SEPW1) is a 9.4 kDa Sec-containing thioredoxin-like protein with a suspected antioxidant function and plays a significant role in cell cycle control and progression [WC and Z, 2011]. SEPW1 is one of the most highly expressed selenoproteins in humans and is also found in mice, rats, sheep, monkeys, rabbits, guinea pigs, and cattle. It is expressed in all twenty two human organs that have been tested, with the highest levels seen in the brain, testes, and muscles [Z et al., 2015]. SEPW1 protein is controlled at the messenger RNA (mRNA) level by Se intake [WC et al., 2009]. A recent study showed that SEPW1 siRNA blocked EGF-induced downstream pathway activation (MAPK, PI3K/Akt, JAK/STAT, and p53) and EGF-induced cell cycle entry. Understanding the mechanism of EGFR regulation by SEPW1 would reveal the link between selenium food supplements and cancer prevention [Z et al., 2015].

Selenoenzyme Inhibitor

Selenoenzymes are the main regulators of antioxidant-mediated defense and redox signaling and can be inhibited by species of methyl mercury and by the gold compound medication auranofin [Pickering et al., 2020; Yang et al., 2020]. RSL3 and ML162 are selenoenzyme (GPX4) inhibitors, both possessing a highly reactive chloroacetamide group that covalently inactivates GPX4 and several other selenoproteins at the active site, resulting in its decreased in vivo usage [Conrad et al., 2021; Hangauer et al., 2017]. Interestingly, a research group from Japan first identified the GPX4 peptide inhibitor and its binding site in selenocysteine [Sakamoto et al., 2017]. GXpep-3 peptide (VPCPYLPLWNCAGK) may contribute to the development of GPX4-targeted drugs [Sakamoto et al., 2017]. Unfortunately, studies on GPX4 inhibitors over the past decade have not been able to successfully produce any drug that treats cancer.

So far, no anti-carcinogenic medicines that directly target the selenoenzyme mechanism are available clinically, even though several compounds have been developed as some selenoenzyme (TrxR1 or GPX4) inhibitors with promising potential under in vitro conditions [Hangauer et al., 2017]. Some inhibitors also demonstrate cross-reactivity with glutathione and other components in the TrxR1 or GPX4 system but cause severe side effects [Jovanović et al., 2020; Ren et al., 2017]. One of the earliest studies showed that treatment with selenium nanoparticles (SeNPs) resulted in significant cytotoxicity in two ovarian cancer cell lines, SKOV-3 and OVCAR-3. However, SeNPs dramatically increased histone methylation at three histone marks, namely H3K4, H3K27, and H3K9. SeNPs also impact the expression of genes linked to hallmarks of cancer such as DNA repair activation, which leads to drug resistance [Toubhans et al., 2020]. Epigenetic modification is involved in their different immune checkpoints/ligands in the tumor microenvironment [Sasidharan Nair et al., 2018]. SeNPs downregulate PD-1 expression in T cells. In addition, another study investigating immune checkpoints like PDL-1, CTLA-4, LAG-3, and TIM-3 showed that TIGIT receptors were not significantly changed in both the selenium (+) and selenium (−) groups after three months [M et al., 2021]. This difficulty makes it hard to target the selenoenzyme mechanism without major side effects since the critical selenoenzyme (TrxR1 or GPX4) system functions for normal cell survival. Consequently, it is necessary to develop potent and non-toxic cytosolic selenoenzyme (TrxR1 or GPX4) inhibitors [Ghareeb and Metanis, 2020]. Nevertheless, we are urgently and expectantly exploring research to target the molecular mechanism of selenoenzyme synthesis and the therapeutic effectiveness of potential ferroptosis target drug candidates. Since SECISBP2 is essential for the expression of selenoproteins including GPX4 and TrxR1, here, we hypothesize that SECISBP2 is a potential new therapeutic target for cancer therapy that controls the abundance of selenoenzyme.

Selenoenzyme (TrxR1 or GPX4)-Specific Target Biomarkers

The tumor microenvironment (TME) is made up of stromal cells, endothelial cells, immune cells, and regulatory T cells [Chung et al., 2021]. The TME’s dynamics and interactions are critical for preserving cancer cell properties and regulating cell transformation, proliferation, and metastasis [Wu et al., 2019]. Diverse miRNA and mRNA transfer signals between tumor cells and the TME via direct contact with different types of extracellular vesicles. Exosomes have been discovered as the key factors in facilitating cell-to-cell communication between tumor cells and the microenvironment, which takes part in signaling pathways in the TME (Figure 2), and have the potential capabilities to modulate the TME and contribute in tumor metastasis, immune response [CF et al., 2017; H et al., 2021], and multidrug resistance [Skalska et al., 2019]. Exosomal selenoprotein mRNAs enter cancer cells and may be involved in regulating selenoprotein synthesis [H et al., 2021].

Figure 2:

Diagrammatic representation of the molecular mechanisms which manifest SECISBP2 as a potential therapeutic target for cancer treatment. (Created by Biorender.com).

Proteins in which selenium is incorporated non-specifically, selenocysteine and selenomethionine, can replace their respective sulfur counterparts, cysteine, and methionine. The physiological consequences of this non-specific incorporation remain unclear. Proteins that specifically bind selenium have been studied very little, which explains why their molecular mechanism has not yet been elucidated [Guignardi and Schiavon, 2017].

In selenoproteins, selenocysteine ​​is specifically incorporated and can be substituted by cysteine ​​during protein synthesis [Lu et al., 2009]. Selenoproteins choose a specific strategy to incorporate selenocysteine in place of the UGA termination codon during translation. The mRNA encoding the selenocysteine is the UGA codon, which is normally a stop codon. The differentiation allowing the incorporation of selenocysteine by its transfer RNA instead of stopping the protein synthesis is characterized by a stem-loop form of mRNA at 3’ of the UGA codon called SECIS (selenocysteine ​​insertion sequence). This 3′ untranslated region of selenoprotein mRNAs is the location of selenocysteine when the UGA codon is recognized as SECISBP2[Shetty et al., 2014] (Figure 2). In addition, the interaction between SECISBP2’s sequence and Sec-tRNAs plays a crucial role in linking SECISBP2 to the eukaryotic Sec-spec elongation factor and 60S ribosomal subunits. A previous report proposed that SECISBP2 is a “master regulator” of selenoprotein synthesis [Bubenik et al., 2015].

Previous research indicates a poor prognosis for patients with GPX4 overexpression in solid tumors. However, GPX4 mRNA is not altered. GPX4 expression can be controlled through the regulation of protein translation or post-translational mechanisms. For example, using the ubiquitin-proteasome system [Fletcher et al., 2001; Taguchi et al., 2021] SECISBP2 protein is also significantly upregulated in several ovarian cancers, along with several other malignancies, including melanomas and colorectal and breast cancers. Thus, selenoprotein expression is highly dependent on SECISBP2. The deletion of SECISBP2 has reduced selenoprotein levels in HEK293T cells [Latrec̀he et al., 2012]. SECISBP2 has recently been shown to positively control the expression of selenoproteins such as GPX4 and TrxR1; the expression of SECISBP2 is related to poor survival across diffuse large B-cell lymphoma and renal cancer patients [Jia et al., 2020; Taguchi et al., 2021]. Thus, SECISBP2 may be useful as a prognostic predictor and a therapeutic target. Recent studies have reported that non-coding RNA (coiled-coil domain-containing protein 152) inhibits SECISBP2 selenoprotein P mRNA levels, but not SECISBP2 bound GPx4 mRNA levels [Mita et al., 2021].

It is possible that targeting SECISBP2 with inhibitors could be an effective approach to attenuate disease progression, even though there may be many challenges to this approach [Taguchi et al., 2021]. To treat cancers, antibody-based inhibitors (i.e., immunotherapeutics) could be more effective. SECISBP2 is an intracellular protein, which makes antibody-based therapy impractical since conventional extracellular proteins are the targets for these drugs. As intracellular molecules constitute almost half of the human proteome and constitute an untapped pool of potential therapeutic targets, reassessment of intracellular proteins as immunotherapeutic targets such as SECISBP2 needs to be prioritized. While there may be antibodies that recognize an intracellular protein of interest discovered through mechanistic studies, their difficulty in delivering broad antibodies (Ab ~ 150 kDa) or antibody fragments (Fab ~ 50 kDa, scFv ~ 25 kDa) into cells doesn’t make them a viable choice [Marciel and Hoffmann, 2019]. The use of phage display techniques is a new strategy that incorporates emerging technologies and is one potential way to resolve these barriers. Many peptide therapies have been developed for cancer drug development due to their strong biological properties, including high strength, low toxicity, and high specificity. Recently, the artificial cyclic peptide KS-58 was reported as the first selective inhibitor of K-RAS (G12D) cancer cells under in vivo [Buyanova et al., 2021; Sakamoto et al., 2020].

Future Directions: Identification of specific peptide inhibitor for SECISBP2 using Phage Display Technology

In 1985, Smith reported the engineering of filamentous bacteriophage clones to display combinatorial peptide sequences at the N-terminus of discreet surface proteins [Smith, 1985a]. Researchers used a “library” of these clones to screen for and select peptides with target selectivity [Petrenko and Smith, 2015; Smith, 1985b] Since that time, phage display peptide library (PDPL) technology has been refined and expanded to meet the needs of different applications. These include the engineering of several strains of phages and peptides of varying lengths and configurations and different phage clone selection protocols tailored to specific needs [Bashari et al., 2017; Kalimuthu et al., 2018]. Interestingly, few PDPLs are available commercially and many researchers prefer to develop their custom libraries. As the libraries usually contain billions of phage clones, each displaying a unique peptide sequence, the technology has become a relatively low-cost, small volume, high throughput system for selecting binding motifs with high target affinity. PDPs are used as screening tools in immunology, cell biology, molecular imaging, drug discovery, and pharmacology [Chen et al., 2015; Pande et al., 2010; Tan et al., 2016]. Another important advantage of the use of such libraries is that the process of clone selection for a defined function, such as the ability to bind a certain cell type, does not require prior knowledge of the identity or structure of the peptide-binding moiety, affording the discovery of novel peptide targets [Kalimuthu et al., 2018]. Nevertheless, the use of PDPLs does not come without some technical and inherent difficulties. For instance, studies have found that large libraries can contain clones that display “target-unrelated peptides” (TUPs), whose biophysical properties afford them preferentially high binding to polystyrene microplate wells, are often used as solid supports in biopanning procedures [Bakhshinejad and Sadeghizadeh, 2016; Qiang et al., 2017]. Indeed, this problem has led to the establishment of TUP databases such as SAROTUP and software to ensure the discovery of true target-specific sequences [Brinton et al., 2016].

An additional related issue is the size of most libraries, which often contain 10⁸–10¹⁰ clones. It is therefore likely that this large phage pool contains many peptides with low binding affinity for the target. Together, these clones could result in significant cross-reactive or non-specific binding competition with the far less abundant, but higher affinity clones. We have sought to significantly diminish this problem by the prior removal of as many irrelevant clones as possible by pre-adsorption, either in vitro or in vivo, on a series of control targets. Figure 3 shows that such procedures reduce the phage pool by five to seven orders of magnitude, thereby increasing the chances of isolating high-affinity clones when the “absorbed phage pool”” is exposed to the target. However, two problems that limit their effectiveness as therapeutic agents are their correlation with peptides and protein medicines and their potential to contribute to unexpected immunological reactions. The D-enantiomers of peptides have many advantages over the L-enantiomers, are more resistant to hydrolysis proteases that increase half-life serum, and have fewer immune responses, making them attractive candidates for drug production [J and J, 2021; Zhou et al., 2020].

Figure 3:

Phage display peptide library (PDPL) - Biopanning techniques for particular target molecules (created by Biorender.com).

Conclusion

Since SECISBP2 is widely expressed in normal tissue and can cause accidental and adverse effects such as kidney failure and brain injury, nanocarriers for medicinal use (i.e., nanoparticles, nano-liposomes, exosomes) may be considered for the prevention of side effects by binding to particular recognition components such as aptamers and peptides. Nevertheless, no specific drug inhibitor is available worldwide. Therefore, it is important to enhance the pharmacokinetic properties of the current cancer drug inhibitors and produce new bioavailable inhibitors for future study.

Abbreviations used in this paper:

(GPX)

glutathione peroxidase

(GPX4)

glutathione peroxidase-4

(mRNA)

messenger RNA

(PDPL)

phage display peptide library

(RSL3)

RAS-selective lethal

(ROS)

reactive oxygen species

(Sec)

selenocysteine

(SECIS)

selenocysteine ​​insertion sequence

(SECISBP2)

SECIS binding protein 2

(Sec-tRNA)

selenocysteine-specific transfer ribonucleic acid

(SEPW1)

selenoprotein W1

(TUPs)

target-unrelated peptides

(CTLA-4)

cytotoxic T-lymphocyte-associated protein 4

(TrxR)

thioredoxin reductase

(TrxR1)

thioredoxin reductase 1

(TME)

T cell immunoglobulin domain and mucin domain (Tim)-3,tumor microenvironment

(PD-L1)

Programmed death-ligand 1

(LAG-3)

Lymphocyte-activation gene 3

TIGIT

(also called T cell immunoreceptor with Ig and ITIM domains)

Data availability statement:

Data sources will be made freely available to the scientific research community from the corresponding author upon reasonable request as soon as they have been documented in a publication.