RNA polymerase III (Pol III) is mainly responsible for the transcription of tRNA and 5S rRNA in eukaryotes (Wang and Gerber, 2020). Of note, the transcription of tRNAs by RNA Pol III was influenced by all kinds of oncogenes and tumor-suppressor genes (Rollins et al., 2007; Johnson et al., 2008). The transcription of Pol III in healthy cells is inhibited by tumor suppressors (White, 2008). However, this limitation is compromised during cell transformation, and oncogene products exacerbated this problem, which stimulated the output of Pol III (White, 2008). Interestingly, there is a significant positive correlation between telomerase reverse transcriptase (TERT) and tRNA levels in cancer. TERT directly binds to the Pol III subunit RPC32 and upregulates the recruitment of chromatin, resulting in an increase in the occupancy rate of Pol III on the tRNA genes (Khattar et al., 2016). TERT is significantly enriched at tRNA^Met, tRNA^Arg, and tRNA^Lys genes, regulating the expression of these tRNAs, thus controlling the rate of protein synthesis in cancer cells and promoting tumorigenesis to a certain extent (Khattar et al., 2016).

Mutations of tRNA usually occur in mitochondria due to the lack of protective histones, introns, and effective DNA repair systems in mitochondrial DNA (mtDNA) (Wallace, 2015). These mutations in certain mt-tRNAs, such as tRNA^His, tRNA^Ala, tRNA^Leu, tRNA^Ser, and tRNA^Thr, have strong pathogenicity and are highly likely to be related to the carcinogenesis of lung cancer (Wang et al., 2015; He et al., 2016). mtDNA mutation destroys the secondary structure of tRNA itself and affects the tRNA posttranscriptional modifications and aminoacylation, which causes a decrease in the mitochondrial protein synthesis and the inability to meet the respiratory phenotype and the threshold of ATP required by normal cells, thereby promoting tumorigenesis (Lu et al., 2009).

Of note, recent studies found that the newly designed β32_33 peptide could penetrate the mitochondrial membrane and improve the viability of the cells containing mt-tRNA^Leu(UUR) m.3243A>G and mt-tRNA^Lys m.8344A>G mutations by stabilizing the structure of mt-tRNA mutants (Perli et al., 2020). Moreover, mitochondrially targeted zinc finger nucleases (mtZFNs) shifted the heteroplasmy of the mt-tRNA^ALA m.5024C>T mutant, thereby rescuing mitochondrial functions (Gammage and Viscomi, 2018). These findings indicate that correcting the pathogenic mt-tRNA mutations can rescue disease phenotypes, which provides new ideas for the treatment of lung cancer. In all, both tRNA transcription and mutation are involved in the pathogenesis of lung cancer.

In addition, the dysregulation of tRNA levels is closely related to the prognosis of lung cancer. Analyzing the expression level of tRNA in lung adenocarcinoma tissues and paracarcinoma tissues by the tRNA RT-qPCR array, Kuang et al. found that there were differences in many tRNA levels, such as tRNA^Asn, tRNA^Ile, and tRNA^Leu (Kuang et al., 2019). Through further analysis of the correlation between the tRNA expression and clinicopathological characteristics, they unraveled that the expression of three tRNAs, tRNA^Ile, tRNA^Pro, and tRNA^Lys was related to tumor differentiation, and patients with a higher expression of mt-tRNA^Glu and tRNA^Tyr and lower expression of tRNA^Thr and tRNA^Asn had a higher risk of relapse (Kuang et al., 2019). Moreover, the levels of tRNA^Lys, mt-tRNA^Ser, and tRNA^Tyr were associated with cancer-specific survival (Kuang et al., 2019). These tRNAs were used as variables to construct a prognostic model of lung adenocarcinoma and could accurately predict cancer-specific survival in patients with lung adenocarcinoma (Kuang et al., 2019).

The initial transcription product of RNA Pol III is a precursor of tRNA (pre-tRNA), which must undergo a series of complex biological processes before being converted into mature tRNA (Vannini and Cramer, 2012). Under the effect of sex hormones, hypoxia, and other stress conditions, pre-tRNAs and mature tRNAs are found to be significant signaling molecules, which are broken down into tRNA derivatives (Thompson et al., 2008; Veneziano et al., 2016). Under stress, tRNA breakdown products are more frequently found in tumor tissues (Dong et al., 2020; Farina et al., 2020; Wang et al., 2021b). According to the different cleavage sites, tRNA derivatives are mainly divided into two types: tRNA-derived stress-induced RNAs (tiRNAs) generated by cleaving the anticodon loops of the mature tRNAs (Saikia and Hatzoglou, 2015; Shigematsu and Kirino, 2017) and tRNA-derived fragments (tRFs) formed by cutting the mature or precursor tRNAs in the D-loop, T-loop, and other positions (Keam and Hutvagner, 2015; Pekarsky et al., 2016). Strikingly, increasing evidence argued that tRNA derivatives were dysregulated in lung cancer (Pekarsky et al., 2016; Balatti et al., 2017). ts-3676 and ts-4521, which were derived from tRNA^Thr and tRNA^Ser, respectively, could not only interact with the Argonaute proteins Ago1 and Ago2 to act as microRNAs but also interact with the P-element–induced wimpy testis (Piwi)–like protein 2 (PiwiL2) to serve as Piwi-interacting small RNAs (piRNAs) (Pekarsky et al., 2016). It was worth noting that these two tsRNAs were significantly downregulated and mutated in lung cancer tissues than normal lung tissues (Pekarsky et al., 2016). In addition, using pathway analysis software to evaluate the roles of ts-4521 in cancer, Balatti et al. found that the downregulation of ts-4521 was related to the cell proliferation and apoptosis-related signal pathways (Balatti et al., 2017). Moreover, the overexpression of ts-46 and ts-47 had a strong inhibitory effect on cell colony formation in lung cancer cells (Balatti et al., 2017).

Another study found that tRFs could effectively regulate kinase activity. tRF-Leu-CAG, derived from tRNA^Leu(CAG), regulated the aurora kinase A (AURKA) activity in NSCLC, thereby mediating cell proliferation and cell cycle progression (Shao et al., 2017). AURKA, a serine–threonine kinase, was related to the maturation and separation of the centrosome and regulated the assembly and stability of the spindle, thus playing an important role in mitosis. The overexpression of tRF-Leu-CAG enormously increased the activity of AURKA and promoted the cell proliferation and G0/G1 cell cycle progression in NSCLC, which would be conducive to the deterioration of cancer (Shao et al., 2017). In contrast, AURKA was significantly downregulated in H1299 cells transfected with the tRF-Leu-CAG inhibitor (tRFi), indicating that this tRF might be a potential target for the treatment of lung cancer (Shao et al., 2017). In addition, Chiou et al. have demonstrated that antisense oligonucleotides can induce the silencing of tRNA fragments (Chiou et al., 2018). Therefore, it can be speculated that blocking tRNA fragments with oncogenic activity through ASO may be a potential strategy for the treatment of lung cancer.

Various RNA species are released into the extracellular space in the form of extracellular vesicles (EVs) or complexes with proteins, collectively known as extracellular RNA (exRNA) (Happel et al., 2020). Interestingly, exRNAs could not only serve as potential biomarkers for lung cancer (Wang et al., 2020) but also could act as signaling molecules to regulate tumorigenesis (Li et al., 2019). Among them, tRNAs and tRFs are considered to be one of the most abundant RNA components in the extracellular compartment (Torres and Martí, 2021). It is worth noting that the abundance of tRFs in plasma EVs from lung squamous cell carcinoma patients was higher than that from lung adenocarcinoma and healthy individuals, indicating that ex-tRFs might contribute to the development of lung squamous cell carcinoma (Li et al., 2018a). Taken together, the dysregulation of tRNAs and tRNA derivatives has a stake in the pathogenesis of lung cancer.

There are always various modifications after the transcription of tRNAs, which affect not only the stability of tRNAs and codon recognition but also the stability of tRNA transcripts (Martinez et al., 2017; Guo and Ng, 2019; Kimura et al., 2020; Tavares et al., 2021). The number and type of individual tRNA modifications are different, and mammalian cytoplasmic tRNA is estimated to carry 13-14 modifications on an average (Chan et al., 2010). The tRNA modification patterns produced under different environmental stresses are different, indicating that tRNA modification plays a regulatory role in the cellular response to stress (Chan et al., 2010). The pathogenesis of cancer changes the tRNA modification chemistry, which occurs after oxidative stress (Endres et al., 2019). Increasing evidence argues that tRNA modification and corresponding tRNA-modifying enzymes not only play an important role in translation but also are important signal molecules in the pathogenesis of cancer (Rashad et al., 2020; Rosselló-Tortella et al., 2020; Zhang et al., 2020c). Dong et al. found that the modification profiles of tRNA in rapidly proliferating cancer cells were roughly the same, indicating that there was a proliferation-related modification regulation in rapidly proliferating cancer cells, and tRNA had a positive regulatory effect in rapidly proliferating cancer cells (Dong et al., 2016). However, the roles of these tRNA modifications in rapidly proliferating cells remain to be further studied.

Notably, in recent studies, it has been found that tRNA modification was regulated by FTSJ1 in lung cancer, which had a tumor-suppressor effect (He et al., 2020). Eighteen types of tRNA modifications and up to 7 tRNA modification genes in NSCLC tumor tissues were significantly downregulated compared with normal tissues, of which the expression level of 2′-O-methyladenosine (Am) modification was the lowest (He et al., 2020). Further research showed that the amount of Am in tRNAs was significantly related to the expression of FTSJ1, which exerted significant tumor suppressor ability via interacting with DNA damage-regulated autophagy modulator 1 (DRAM1) (He et al., 2020). Similarly, another study indicated that tRNA methyltransferase 9-like (hTRM9L) attenuated the cell cycle by downregulating cyclin D1 and restricted the migration and invasion potential by changing the expression of cadherin in lung cancer (Wang et al., 2018). However, hTRM9L was significantly downregulated in lung cancer tissues, which was closely related to the poor prognosis of patients and was an independent prognostic factor for lung cancer patients (Wang et al., 2018). Collectively, the tRNA-modifying enzymes FTSJ1 and hTRM9L act as tumor suppressors, and measures that can promote their overexpression may be effective in treating lung cancer.

Additionally, some tRNA-modifying enzymes promoted the development of lung cancer (Kato et al., 2005; Coll-SanMartin and Davalos, 2021). hDUS2, a homolog of yeast and bacterial tRNA-dihydrouridine synthases (DUSs), was highly expressed in NSCLC samples, and its high levels were related to the poor prognosis of lung cancer patients (Kato et al., 2005). This enzyme facilitated the formation of dihydrouridine in tRNAs and increased the translation efficiency by interacting with glutamyl-prolyl-tRNA synthetase (EPRS), thereby contributing to tumorigenesis. Significantly, NSCLC cells transfected with si-hDUS2-#2 showed a decrease in dihydrouridine levels and growth inhibition, suggesting that the selective inhibition of hDUS2 might have the potential to treat NSCLC (Kato et al., 2005). Coll-SanMartin et al. observed that tRNA isopentenyltransferase 1 (TRIT1) catalyzed the N⁶-isopentenyladenosine (i⁶A) modification at the 37th position of tRNAs, and it showed gene amplification–related overexpression in small cell lung cancer (Coll-SanMartin and Davalos, 2021). Importantly, cancer cells with TRIT1 gene amplification were more sensitive to the drug arsenic trioxide, which provided a theoretical basis for the clinical treatment of such small cell lung cancer patients.

It is recognized that tRNA can combine with its homologous amino acids through ARS-mediated aminoacylation, thus transporting amino acids to the ribosomes to participate in protein synthesis (Kwon et al., 2019; Zhou et al., 2020b). In mammalian cells, ARSs can not only exist in their free form in the cytoplasm but also interact with three ARS-interacting multifunctional proteins (AIMPs) to form a multiple tRNA synthetase complex (MSC) (Zhou et al., 2020a). Previously, ARSs and AIMPs were regarded as housekeeping molecules without additional functions. However, growing evidence indicates that ARSs and AIMPs are involved in tumorigenesis (Yum et al., 2016; Hyeon et al., 2019; Zhang et al., 2020a).

Di et al. found that isoleucyl-tRNA synthetase 2 (IARS2) acted as an oncogene in NSCLC by activating the protein kinase B (AKT)/mammalian target of rapamycin (mTOR) pathway (Di et al., 2019). IARS2 was highly expressed in NSCLC tissues, and silencing IARS2 could inhibit the activity of lung cancer cells and reduce the tumorigenicity of cancer cells in nude mice. Not only leucyl-tRNA synthetase (LRS) was significantly upregulated in lung cancer cell A549 but also its mRNA was highly expressed in primary lung cancer tissues (Shin et al., 2008). To explore the carcinogenic potential of the overexpression of LRS in lung cancer, Shin et al. knocked down the LRS in A549 cells and found that the growth and migration of cancer cells were significantly inhibited, indicating that this molecule played an important role in the development of lung cancer (Shin et al., 2008). Of note, AIMP2 had a tumor suppressor activity on lung cancer cells by Smad ubiquitination regulatory factors 2 (Smurf2) (Kim et al., 2016). AIMP2 was phosphorylated by transforming the growth factor-β (TGF-β)–activated p38MAPK, and the phosphorylated AIMP2 was separated from the MSC (Kim et al., 2016). Subsequently, the dissociated AIMP2 translocated to the nucleus and interacted with Smurf2 to exert its nuclear function. On the one hand, this interaction promoted the degradation of ubiquitin-mediated FUSE-binding protein (FBP) and thus downregulated c-Myc. On the other hand, it inhibited the binding of Smurf2 to chromosomal region maintenance 1 (CRM1), thereby reducing the nuclear export of Smurf2 to sustain TGF-β signaling (Kim et al., 2016). TGF-β signaling is involved in the regulation of a variety of cellular functions, including cell proliferation, differentiation, migration, and apoptosis (Siegel and Massagué, 2003), and its alteration can lead to human diseases, such as cancer (Massagué, 2008; Ikushima and Miyazono, 2010).

Meaningfully, the abundance of tyrosine-tRNA synthetase (TyrRS) and microtubule agglutinin cross-linking factor 1 (MACF-1) in lung adenocarcinoma tissue was higher than that in adjacent normal tissues (Zhou et al., 2013). Cox regression analysis found that patients with high TyrRS or MACF-1 expression had a significantly increased risk of death. Asparaginyl-tRNA synthetase (NARS), a class II ARS, had higher levels in lung adenocarcinoma than adjacent normal tissues, and it was positively correlated with lymph node metastasis (Hsu et al., 2016). Importantly, the downregulation of NARS could inhibit the growth and migration of adenocarcinoma cells (Hsu et al., 2016). Furthermore, methionyl-tRNA synthetase (MRS) had excessive mTORC1-related activities in NSCLC tissues, which played a vital role in tumor growth and spread (Kim et al., 2017a). Importantly, its overexpression was associated with poor clinical outcomes in patients with NSCLC. Interestingly, AIMP2-lacking exon 2 (AIMP2-DX2), a tumorigenic factor, is often upregulated in many cancers (Choi et al., 2011; Lim et al., 2020). AIMP2-DX2 is produced by alternative splicing, and it is highly expressed in human lung cancer cells and patient tissues. The ratio of AIMP2-DX2 to AIMP2 was positively correlated with the cancer stage, while it was negatively correlated with patient survival (Choi et al., 2011). This was because AIMP2-DX2 reduced the proapoptotic activity of AIMP2 by competing with p53 (Choi et al., 2011). The cells with higher levels of AIMP2-DX2 had a higher tendency to form anchorage-independent colonies, were more resistant to cell death, and increased the sensitivity of lung tumors (Choi et al., 2011). In all, certain ARSs not only contribute to the development of lung cancer but also serve as potential biomarkers for its diagnosis and prognosis (Figure 2).

As ARSs and AIMPs are closely related to the development of tumors, they may become potential targets for tumor treatment. LRS played an important role in activating the mTORC1 pathway and cell growth (Kim et al., 2019). Interestingly, the new LRS inhibitor BC-LI-0186 inhibited the mTORC1 signaling pathway by interacting with the RagD site of LRS, leading to the cytotoxicity of NSCLC cells and anticancer effects in the K-ras mouse lung cancer model (Kim et al., 2017b; Kim et al., 2019). These results provide a new therapeutic strategy for NSCLC. Moreover, Lim et al. found that the heat shock protein HSP70 (HSP70) was a critical factor which affected the level of AIMP2-DX2, and it was positively correlated with the level of AIMP2-DX2 in lung cancer cells (Lim et al., 2020). HSP70 could block the seven in absentia homolog 1 (Siah1)– mediated ubiquitination of AIMP2-DX2 by interacting with AIMP2-DX2, thereby maintaining the stability of AIMP2-DX2 and further enhancing AIMP2-DX2–induced cell transformation and cancer progression (Lim et al., 2020). Notably, BC-DXI-495 specifically inhibited the interaction of AIMP2-DX2 with HSP70, thereby inhibiting tumorigenesis (Lim et al., 2020). Another research discovered that AIMP2-DX2 also could prevent oncogene-induced apoptosis and senescence by directly binding to and inhibiting p14/ARF (Oh et al., 2016). However, the inhibition of DX2-p14/ARF interaction played an antitumor effect in lung cancer and delayed tumor progression (Oh et al., 2016). Taken together, although AIMP2-DX2 is a tumorigenic factor in the lung cancer progression, it can also be used as a treatment strategy for patients suffering from lung cancer.