The cellular structures where mRNA translation takes place are the ribosomes, which are integrated by two subunits. Ribosomal subunits are macromolecular ribonucleoprotein complexes composed of ribosomal RNA (rRNA) and ribosomal proteins (RPs). In eukaryotes, the large (L) subunit 60S possesses three rRNA, 5S, 5.8S, 28S, and 47 proteins termed RPL. The small (S) 40S subunit contains an 18S rRNA and 33 proteins termed RPS. RPs primarily participate in ribosome assembly and protein synthesis (Weisser and Ban, 2019).

The translation process is divided into phases initiation, elongation, termination, and ribosome recycling, and the whole process is mostly regulated at the initiation phase (Hershey et al., 2019). In eukaryotes, translation initiation consists of the recruitment of an mRNA to the ribosome and placement of the mRNA translation initiation site (TIS) in the peptidyl (P) site of the 40S ribosomal subunit to establish the mRNA reading frame (Hershey et al., 2019). During the initiation phase, two checkpoints represent the major targets of regulation. One of them is the recognition of the cap structure 7-methylguanosine (m⁷GpppN) at the 5′-end of the mRNA by the cap-binding protein eIF4E (Figure 1A). For this to occur, eIF4E makes up a complex with the scaffold protein eIF4G and the RNA helicase eIF4A, the so-called eIF4F complex. Next, the poly(A)-binding protein (PABP) binds to the poly(A) tail at the 3′-end of the mRNA and eIF4G, thus promoting mRNA circularization. Simultaneously, a free 40S ribosomal subunit interacts with eIF1, eIF1A, eIF3, eIF5, and the ternary complex (TC, consisting of eIF2 bound to an initiator Met-tRNA_i ^Met and GTP) to form a 43S pre-initiation complex (PIC) (Figure 1B). The 43S PIC is then recruited at the 5′-end of the mRNA through the interaction of ribosome-bound eIF3 and eIF4G. These interactions drive the formation of a 48S PIC (Figure 1B). Afterward, eIF4A unwinds stem-loop structures of 5′-untranslated region (UTR), allowing the 43S PIC to scan the 5′-UTR to reach a TIS (usually an AUG codon) (Hinnebusch, 2014; Lind and Åqvist, 2016; Pelletier and Sonenberg, 2019).

When a TIS is reached, eIF1 and eIF1A drive the faithful selection of a TIS. In this process, the initiator Met-tRNA_i ^Met is positioned in the ribosome’s peptidyl (P) site, making Watson-Crick base-pairing between the start codon AUG and the anticodon of the Met-tRNA_i ^Met. As a result, scanning arrests and the Met-tRNA_i ^Met and eIF1A get tightly positioned within the P-site. Then, GTP-eIF5B promotes the release of eIF1 and eIF5B, facilitating the joining of a 60S subunit to the 48S PIC to form an 80S initiation complex, which is competent to start translation elongation (Figure 1C) (Hinnebusch, 2014; Lind and Åqvist, 2016; Pelletier and Sonenberg, 2019).

A second major point of regulation relies on the eIF2 activity. eIF2 is formed by α, β, and γ subunits. eIF2 activity is regulated by the reversible phosphorylation of eIF2α Ser51, being p-eIF2α (the phosphorylated form of eIF2α) an inhibitor of global protein synthesis. In addition, different kinases phosphorylate eIF2α: the heme-regulated inhibitor kinase (HRI) that is activated under heme deprivation or arsenite exposure; the double-stranded RNA protein kinase (PKR) that responds to viral infection; the general control non-derepressible 2 (GCN2) that is activated by uncharged tRNA and thus senses amino acid starvation; and the PKR-like endoplasmatic reticulum kinase (PERK) that is activated by unfolded proteins in endoplasmic reticulum (Wek, 2018).

Two major pathways signal to the initiation phase of translation, namely, the mitogen-activated protein kinases (MAPK) pathway and the phosphatidylinositol 3-kinase (PI3K)/protein kinase B (Akt)/mammalian target of rapamycin complex 1 (mTORC1) pathway (Figure 2A). MAPKs are activated by mitogens and stress stimuli, and are coupled to the translation machinery via the downstream phosphorylation of Mitogen-activated protein kinase-interacting kinase 1 and 2 (MNK1/2) that phosphorylate eIF4E (Roux and Topisirovic, 2018; Proud, 2019). On the other side, the mTORC1 pathway senses, integrates, and drives responses to stress, cellular energy status, nutrient availability, hormones, and mitogens to control cellular proliferation and survival (Figure 2B). In response to these cues, the mTOR cascade controls translation initiation via eIF4E-binding proteins 1 and 2 (4E-BP1/2) which bind to eIF4E depending on their phosphorylation status: whereas dephosphorylated 4E-BPs bind eIF4E, phosphorylated proteins dissociate from eIF4E. Interaction of 4E-BPs to eIF4E inhibits the formation of the eIF4E/eIF4G complex, thereby repressing cap-dependent translation (Fonseca et al., 2016; Roux and Topisirovic, 2018).

We describe the relevance of eIFs in GB biology. First, we give an account of the eIFs integrating the 43S PIC (Figure 3), followed by the components of the eIF4F complex (Figure 4).

Early studies in Zebrafish reported that the loss of some ribosomal proteins (RPs) increases cancer risk (Amsterdam et al., 2004), suggesting a tumor suppressor ability. More recently, RPs have been reported to have both pro- and anti-tumoral activities. However, their specific role in translation (if any) remains unknown.

RPL5 is a significant candidate tumor suppressor in GB and other types of cancer, including aggressive, chronic lymphocytic leukemia, melanoma, and breast cancer. RPL5 is heterozygously mutated in about 2.6% of GB tumors (Fancello et al., 2017). RPL5 is also heterozygously deleted in about 11% of GB tumors, and patients expressing low RPL5 have poorer prognoses than patients expressing high RPL5 levels (Fancello et al., 2017). Both types of injuries could impact the RPL5/RPL11/5S rRNA complex assembly that binds to MDM2 and blocks MDM2-mediated ubiquitination of p53, leading to p53 stabilization in GB tumors with anti-tumoral effects (Orsolic et al., 2020).

RPS27, also termed metallopanstimulin-1 (MPS1), is an oncogenic protein in different cancers involved in cell invasion and migration through integrin β4 (ITGB4) activity, which is a downstream target of RPS27 (Yang et al., 2012), and MDM2-p53 signaling (Xiong et al., 2011). RPS27 is overexpressed in GB and gliomas of grade II/III, glioma stem-like cells, and macrophages associated with the tumor tissue. Moreover, RPS27 is released from tumors and can be measured in the patient’s serum. Therefore, this could be useful as a tumor marker for early detection (Feldheim et al., 2020a; Feldheim et al., 2020b).

Other RPs are commonly overexpressed in GB tumors, including RPS11, RPS20, RPL5, and RPS27 (Yong et al., 2015; Fancello et al., 2017; Feldheim et al., 2020a; Vastrad et al., 2020). The involvement of different RP in the GB biology is summarized in Table 2 and Figure 5.

Non-tumor stem cells renew themselves, giving rise to new cells, such as neurons, astrocytes and oligodendrocytes, and maintaining homeostasis throughout the organism. In contrast, GB stem cells (GSC) lack control of cell proliferation and differentiation, causing an uncontrolled response. That is why GSC have been attributed the role of the high number of recurrences of CNS tumors (Prager et al., 2020). Despite the existence of several therapeutic strategies, the prognosis for GB patients has not improved over the past decades due to chemoresistance and high relapse after surgery, a phenomenon possibly due to the involvement of GSC (Prager et al., 2020). Relapses are partially due to the heterogeneous presence of GSC and glia-like cancer stem cells that express stemness markers (Bhaduri et al., 2020). GSCs retain their potential for self-renewal, resistance to chemo and radiotherapies, high capacity of DNA repair, and increased mitochondrial reserve (Bao et al., 2006; Vlashi et al., 2011; Shirakawa et al., 2020; Tang et al., 2021). Overexpression of RPs in GSCs is associated with poor prognostic, and RPL11, and RPS20 overexpression has been proposed as a marker of poor prognostic in newly diagnosed and primary GB tumors. RPL11 high expression also is positively associated with unmethylated O6-methylguanine-DNA methyltransferase (MGMT) and wild-type IDH (Yong et al., 2015).

In vitro experiments revealed the role of RPS6 in inducing stem cell-like features in GB. pRPS6 co-localized with the expression of Nestin and CD34-positive in the perinecrotic and perivascular GSCs niches (Shirakawa et al., 2021). Transfection of U251MG and U-87 MG cells with RPS6-specific siRNA (siRPS6) significantly decreased neurosphere formation and the average size of spheres, along with a substantial reduction of Nestin and SOX2 expression (Shirakawa et al., 2020). Furthermore, RPS6 phosphorylation on Ser235-Ser236 in glioma tissues was observed in high-grade gliomas. Moreover, RPS6 phosphorylation on residues 240–244 correlates with mTOR phosphorylation, which is associated with poor prognosis in patients with GB (Machado et al., 2018; Shirakawa et al., 2021). Again, a direct role of RPs in translation is not clear.

In response to mitogens and stress stimuli, extracellular-signal-regulated kinase (ERK) or p38 mitogen-activated protein (MAP) kinase activate downstream MNK1/2, which in turn phosphorylate eIF4E on Ser209 (Fukunaga and Hunter, 1997; Waskiewicz et al., 1997; Scheper et al., 2001). MNK1/2 interact with the carboxy-terminal of eIF4G in mammalian cells to directly phosphorylate eIF4E (Pyronnet et al., 1999). Due to the relevant role of eIF4E and p-eIF4E in cancer progression, dysregulation of the MNK1/2-eIF4E axis is linked to cancer in a wide spectrum of tissues (Yang et al., 2020; Xu et al., 2022). In GB, the MNK1-eIF4E axis regulates the translation of cancer-related mRNAs such as SMAD2, a key component of the TGF-β signaling pathway (Grzmil et al., 2011), and MNK1/2 are overexpressed in primary GB and glioma cells (Bredel et al., 2005; Grzmil et al., 2011; Xu et al., 2021). In addition, in GB U87MG cells, MNK1 knockdown resulted in a significant reduction in tumor formation when injected in a GB mice model (Ueda and al., 2010).

Several drugs target eIF4E and or the MAPK cascade to treat cancer (Figure 7A). Ribavirin is an antiviral drug that displays potent anti-cancer activity in breast, leukemia, and lung cancers (Pettersson et al., 2011; Urtishak et al., 2019; Tan et al., 2020). Previously, it was thought that these anti-cancer features were due to the ability to inhibit eIF4E activity through a structural similarity of ribavirin with the mRNA cap structure (Kentsis et al., 2004). Then, Westman et al. (2005) demonstrated that ribavirin does not compete with eIF4E for binding to the mRNA cap and impairs translation in vitro. Other experiments showed that ribavirin decreased eIF4E activity in glioma and GSCs by decreasing ERK and eIF4E Ser209 phosphorylation (Figure 7A) (Volpin et al., 2017). Although the mechanism underlying the antitumor effect of ribavirin on malignant gliomas cells remains unclear, this evidence suggests that ribavirin is involved in regulating the phosphorylation status of the ERK/MNK1/eIF4E signaling pathway (Shi et al., 2015; Volpin et al., 2017). More recent agents to treat GB include BAY1143269, a novel angiogenesis inhibitor with a potent anti-cancer activity (Wan et al., 2022). BAY1143269 inhibits eIF4E-mediated expression of oncogenic proteins, including those involved in the cell cycle and the epithelial-mesenchymal transition. In addition, BAY1143269 suppresses eIF4E phosphorylation, inhibits cell proliferation, and induces apoptosis of glioblastoma cells. This drug also promoted a significant tumor growth reduction in a xenograft mice model (Wan et al., 2022).

Another drugs act on the MNK1/2-eIF4E axis with the outcome of eIF4E Ser209 dephosphorylation (Figure 7A). Some of them are currently being evaluated in clinical trials for their capacity to reduce tumor size or cell proliferation (Abdelaziz et al., 2021; Xu et al., 2022). Chen et al. (2021) studied the antitumor activity of Osimertinib, an FDA-approved EGFR inhibitor against primary GB cells isolated from patients. They found that this drug inactivated MNK1/2 and also led to eIF4E dephosphorylation. Moreover, in GB patient-derived xenotransplants in mice, oral administration of Osimertinib suppressed tumor growth and inhibited eIF4E phosphorylation in tumor cells. CGP57380 and Cercosporamide are drugs found to be able to inhibit eIF4E and GB growth in mice (Grzmil et al., 2014). In another study, Bell et al. (2016) demonstrated that pharmacological MNK inhibition with LY2801653 (Merestinib) targeted mesenchymal glioma stem cells and was able to prolong survival in a mouse model of GB.

Combinations of chemical agents are also being tested to treat GB. For example, a combination of compounds targeting both MNK and mTOR was reported to significantly sensitize GB cells to the mTOR inhibitor rapamycin resulting in a reduction of cell proliferation (Hemmings et al., 2010). In GB cells, the MNK1 inhibitor CGP-57380 inhibited eIF4E phosphorylation, and siRNA-mediated knockdown of MNK1 expression reduced the proliferation of cells incubated with rapamycin and CGP-57380 (Figure 7A). Concomitant with eIF4E dephosphorylation, this compound downregulated cell proliferation and colony formation (Grzmil et al., 2011). Cercosporamide, in combination with rapamycin analogs (rapalogs), significantly blocked GB tumor growth in mice (Grzmil et al., 2014). Other studies have shown that TMZ promotes eIF4E phosphorylation by MNK1/2 in glioma cells. Accordingly, inhibition of MNK1/2 activity using drugs or protein knockdown sensitized GB cells to TMZ treatments, which prevented cell growth (Bell et al., 2016; Grzmil et al., 2016; Xu et al., 2021). Finally, ribavirin alone or in combination with TMZ and interferon-β (IFN-β) inhibited growth, impaired glioma cell adhesion, and migration of GSCs glioma cells by inducing cell cycle arrest in the G0/G1 phase (Ogino et al., 2014; Volpin et al., 2017; Ochiai et al., 2020). In vivo experiments showed that ribavirin significantly extended the median survival and maximized the cytotoxicity effects of TMZ and radiation, increasing cell death in glioma and GSC (Volpin et al., 2017) (Figure 7A).

In response to environmental stimuli and cellular cues, the PI3K/AKT/mTOR cascade controls cell growth, survival, and proliferation via the regulation of lipids, nucleotide, and protein synthesis. As described, dephosphorylated 4E-BPs trigger eIF4E/4E-BP complex formation and inhibit eIF4E/eIF4G interaction, thereby repressing cap-dependent translation. The PI3K/AKT/mTOR pathway is frequently dysregulated in different cancer types with consequences in cell proliferation, survival, migration and invasion (Fonseca et al., 2016; Roux and Topisirovic, 2018). The involvement of this pathway in GB is well established. mTOR signaling is hyperactivated in GB, which promotes protein synthesis and tumor size (Rich et al., 2005; Holand et al., 2011; Gini et al., 2013; Langhans et al., 2017; Mecca et al., 2018). Indeed, phosphorylated 4E-BP (p-4E-BP) levels significantly correlated with tumor grade and patient survival (Korkolopoulou et al., 2012).

Inhibiting the PI3K/AKT/mTOR signaling has emerged as a strategy to treat GB, and different small molecule inhibitors, alone or in combination, have shown promising results (Figure 7B) (Behrooz et al., 2022; Liu et al., 2022). Chlorpromazine inhibited the PI3K/AKT/mTOR cascade, arrested cell cycle in the G2/M phase, and promoted autophagy-induced cell death in GB cells (Shin et al., 2010; Tanaka et al., 2015; Oliva et al., 2017). The anti-depressant Fluvoxamine acts on this signaling cascade too, and decreases cell invasion, migration, and lamellipodia formation in GB cells. Fluvoxamine also prolonged a mice model’s survival (Hayashi et al., 2016). On the other side, Imipramine strongly reduced colony formation and mitochondrial activity in U87MG and C6 GM cells (Jeon et al., 2011; Shchors et al., 2015).

Currently, more than 50 PI3K or dual PI3K/mTOR inhibitors are being developed and tested in cancer therapies (Behrooz et al., 2022; Liu et al., 2022). However, only a few have shown promising results and have entered clinical trials. The more relevant include Buparlisib, which can reduce GB cell growth both in vitro and in tumors (Burger et al., 2011; Koul et al., 2012; Wen et al., 2019); BKM120 showed anti-tumor activity in GB cells and in mice BALB/c that were inoculated with U-87 MG cells (Fan Q. et al., 2017); Pilaralisib and Voxtalisib alone or in combination with conventional therapies containing TMZ have shown tumor suppression activities (Figure 7B) (Prasad et al., 2011; Wen et al., 2015; Gravina et al., 2016; Wheler et al., 2017); and Metformin, a drug used to treat type 2 diabetes, sensitized GB and GB stem cells to TMZ, a strategy that has entered early clinical trials (Yu et al., 2015; Byron et al., 2018).

Silvestrol is an organic heterocyclic compound isolated from the tropical plant Aglaia silvestris that exhibits anticancer activity. Silvestrol is an eIF4A inhibitor able to kill tumor cells in colorectal carcinoma, acute lymphoblastic leukemia, breast, and prostate cancer in mice and human cells (Figure 7C) (Cencic et al., 2009; Alachkar et al., 2013; Kogure et al., 2013). Interestingly, Silvestrol exerts antitumor effects in U251 and U-87 MG GB cells by inhibiting the expression of Cyclin D1, the PI3K/AKT/mTOR, and ERK1/2 signaling cascades (Zhang et al., 2022) (Figure 7C). Currently, the molecular mechanism underlying these observations are not well understood.

Internal ribosome entry site (IRES)-dependent mRNA translation is a cap-and eIF4E-independent mechanism that places the 43S PIC at or very near the AUG TIS without the involvement of eIF4E. IRES-dependent translation is involved in tumor growth, survival, and chemoresistance in GB cell lines (Holmes et al., 2016). The c-Myc oncogene and Cyclin D1 mRNA translation is driven by IRESs and the IRES trans-acting factors (ITAFs) hnRNP A1 (heterogeneous nuclear ribonucleoprotein A1) (Shi et al., 2005; Martin et al., 2011). Holmes identified a class of inhibitors called C11 and IRES-J007, which block the ability of the ITAF hnRNP A1 from associating with Cyclin D1 and c-Myc IRES, leading to the downregulation of mRNA translation (Figure 7D) (Holmes et al., 2016). The same research group subsequently identified Riluzole, an FDA-approved drug for the treatment of amyotrophic lateral sclerosis, that markedly blocked both Cyclin D1 and c-Myc IRES activity. Riluzole interacts with identical residues as C11 and IRES-J007 and His120 on hnRNP A1 via hydrogen bonding (Benavides-Serrato et al., 2020). In vivo co-therapy with Riluzole and the mTORC1/2 inhibitor PP242 in xenografted mice demonstrated significant tumor growth inhibition, increased apoptotic cell death, and the overall survival of mice significantly extended (Benavides-Serrato et al., 2020). Moreover, a Riluzole/TMZ co-therapy demonstrated the strongest synergism and inhibitory effect on cell proliferation in GB MGMT-positive cell lines and suppressed tumor growth in vivo. Interestingly, Riluzole in combination with TMZ therapy, avoided the TMZ-induced upregulation of MGMT through the inhibition of MGMT mRNA expression in GB (Yamada et al., 2020).

Increasing evidence shows that targeting the mitochondrial oxidative phosphorylation (OXPHOS), electron transport chain activity, and mitochondrial translation could be clinically exploitable in GB therapies for TMZ-resistant glioma cells and GSCs, which are responsible for TMZ therapy failure and disease recurrence (Oliva et al., 2010; Kuramoto et al., 2020). FDA-approved combination of the antibiotics Quinupristin and Dalfopristin (Q/D), used to treat bacterial infections, could be used to treat GB (Figure 7E) (Manzella et al., 2001). Sighel et al. proved that Q/D promote significant growth inhibition of GSCs. These molecules bind to the large mitoribosome subunit (dalfopristin at the peptidyl-transferase center and quinupristin at the begining to the exit tunnel for nascent polypeptides), stalling polypeptide synthesis, which dysregulates the activity of OXPHOS complexes I, IV, and V (Sighel et al., 2021). This dysregulation led to loss of clonogenic potential, cell cycle arrest, and death by apoptosis in GSCs in normoxia and hypoxia (Sighel et al., 2021).