As miRNAs, tRFs can reduce mRNA stability by mediating target gene deacetylation, thereby promoting mRNA degradation and instability. tRFs prioritizes the inhibition of ribosome proteins and translational initiation or elongation factors of mRNA translation through antisense pairing in Drosophila melanogaster (Karaiskos et al., 2015; Babiarz et al., 2008; Eichhorn et al., 2014). In human cells, The GW182 protein inhibits translation and promotes the degradation of target mRNAs, and the tRF-3 target mRNA pairs in the RNA-induced silencing complex associate with GW182 proteins, which means that tRFs can affect the function of RNA-induced silencing complex by regulating the stability of mRNA (Kuscu et al., 2018; Ren et al., 2019). Furthermore, in mature B lymphocytes, tRF-3s derived from tRNA^Gly-GCC (referred as CU1276) possess miRNA-like structure and function, thereby repressing mRNA transcripts by destabilizing mRNA, and can inhibit protein translation and the cleavage of a partially complementary target site, thereby suppressing proliferation (Shao et al., 2017a). Although some tRFs have similar functions to miRNAs, the formers have been expressed preferentially bind argonaute1, argonaute3, and argonaute4 to promote RNA-induced silencing complex formation and reduce the stability of mRNA, thus inhibiting mRNA translation, rather than binding argonaute2 like miRNA (Kumar et al., 2014; Haussecker et al., 2010) (Figure 2). In addition, tRF-2s blocks the interplay of Y-box binding protein 1 (YBX-1) and YBX-1 mRNAs by competitively binding to YBX-1. This reduces the stability of these mRNAs, which could subsequently reduce the genetic stability of human breast cancer cell metastasis (Goodarzi et al., 2015) (Figure 2). Moreover, some tRF-3s chimeras have been related to histone mRNAs and can thus affect mRNA stability by competing with stem-ring binding proteins in human cells (Kumar et al., 2014) (Figure 2). Although there is an increased understanding of some of the functions of tRFs in regulating mRNA stability, the functions of tiRNAs remain elusive.

Some studies have shown that tiRNAs, mainly tiRNA-5s, could decrease translation speed by 10–15% (Yamasaki et al., 2009). For instance, tiRNA-5s from tRNA^Ala and tRNA^Cys can form a G-quadruplex-like structure that selectively binds to eIF4G/eIF4A in the translation of the starting complex, thereby inhibiting cap-dependent translation of cellular mRNAs rather than traditionally internal ribosome entry site mediated translation (Ivanov et al., 2011; Ivanov et al., 2014). tiRNAs can also selectively repress the housekeeping components’ translation under stress conditions, thereby reducing cell energy consumption without affecting the generation of pro-survival proteins (Li and Hu, 2012). These studies suggest that tiRNAs are produced to regulate the translation process under stress conditions and are not intended to reduce the level of functional maturity tRNAs affecting mRNA function (Ivanov et al., 2011; Ivanov et al., 2014).

In addition, some researches have proved that tRFs plays a positive role in reducing protein translation behavior. For example, tRF-5s is supposedly involved in new mechanisms underlying the regulation of small RNA in human cell by repressing protein translation through conserved residues in tRNAs present in tRF-5s without the need for complementary target sites in mRNA (Sobala and Hutvagner, 2013). tRF-5s derived from tRNA^Val-GAC in Haloferax volcanii has been revealed to bind the small ribosomal subunit near the mRNA channel, leading to substitution in the initiation complex and thereby attenuating global translation both in vivo and in vitro (Gebetsberger et al., 2017).

tsRNAs have recently emerged as important regulators of ribosome biogenesis. Particularly, in the lower organism Tetrahymena thermophila, tsRNAs are composition of the precursor ribosomal RNA splicing complex (Couvillion et al., 2012). In Drosophila, tsRNAs restrain global translation by impeding ribosome biogenesis. Mechanistically, Drosophila argonaute2-bound tsRNAs preferentially inhibit the mRNA translation of ribosome proteins or translational initiation or elongation factors via an RNA-like pathway, thus attenuating overall translation (Couvillion et al., 2012; Dou et al., 2019). In addition, tRF-3s can recruit exonuclease Xrn2 and Tan1 protein to form compounds by specifically binding to the Twi12 protein, which cleaves and processes precursor ribosomal RNA to enhance ribosomal RNA synthesis in physiological conditions in Tetrahymena (Couvillion et al., 2012). In mammalian cells, tRF-3s from tRNA^Leu-CAG bind at least two ribosomal protein mRNAs to itself, such as ribosome proteins 28 and ribosome proteins 15, to promote translation (Kim et al., 2017) (Figure 2). However, the mechanism by which tsRNAs regulate ribosomal biogenesis in human remains to be explored.

Recent studies have unveiled that tsRNAs may function as epigenetic factors to regulate gene expression. Obese rat model under the control of a high-fat diet, increased levels of tRF^Glu-TTC directly targeted the transcription factors from the Kruppel-like factor (KLF) family, such as KLF9, KLF11, and KLF12, which are injected themselves into multiplication, apoptosis, differentiation and progress, and significantly suppressed their target mRNA expression, thus preventing the differentiation of preadipocytes (Shen et al., 2019) (Figure 2). Moreover, tRF^Glu-TTC suppressed adipogenesis by inhibiting lipids transcription factors’ expression (Shen et al., 2019). tRF-3s of different lengths can block reverse transcription and post-transcription silence by 18 and 22 nts long respectively, thus silence the long terminal repeat reverse transcription transposer (Schorn et al., 2017). Furthermore, Dicer-like 1 processes tRF-5s and then integrates into argonaute1, which, like miRNA, regulates genomic stability by targeting the transcriptional elements in plant Arabidopsis thaliana (Martinez et al., 2017). Intriguingly, tsRNAs from high-fat diet male sperm injected into normal fertilized eggs of mature mouse sperm caused gene expression changes in the early embryo and triggered islet metabolic pathways independent of DNA methylation in CpG-enriched regions. This proves that tsRNAs may be a paternal epigenetic factor in the intergenerational inheritance of metabolic diseases affected by a mediated diet (Chen et al., 2016).

tsRNAs can also act as an agonist for viral reverse transcription and promote the viral reverse transcription process in various ways. For example, tRF-3s can combine with the primer-binding site of human T cell leukemia virus type 1 RNA to initiate reverse transcription and promote viral synthesis of HIV-infected host cells (Ruggero et al., 2014). Meanwhile, the respiratory syncytial virus infection induces the angiotensincutting2 tRNAs to produce tiRNAs, thus triggering stress response in host cells. respiratory syncytial virus uses host tiRNAs as primers to promote its replication and improve the infection efficiency (Wang et al., 2013; Deng et al., 2015; Zhou et al., 2017).

Moreover, host cellular proteins can regulate retroviral replication by binding to tRNAs, thereby affecting all steps in the viral life cycle. In certain circumstances, aminoacyl-tRNA synthetases bind tRNAs and link them with their corresponding amino acids and cognate aminoacyl-tRNA synthetases to facilitate tRNA primer selection, thus promoting viral reverse transcription (Jin and Musier-Forsyth, 2019).

Studies have also revealed the potential role of tsRNAs as novel immune factors. tsRNAs are highly and stably expressed in hematopoietic and lymphatic organs and blood compared with other tissues (Dhahbi, 2015). This suggests that tsRNAs may participate in the immune process. In addition, when the body is in an acute inflammatory state, tsRNAs levels in the blood increase rapidly, particularly in the sera of mice and monkeys with acute and chronic hepatitis B and active hepatitis B virus infection, as well as chimpanzees with chronic viral hepatitis (Zhang et al., 2014; Selitsky et al., 2015). Likewise, tRF-5s derived from tRNA^Glu can lead to the inhibition of CD1A expression by compounding with PIWIL4 and PIWIL1, which was able to promote the maturation of monocytes into dendritic cells (Zhang et al., 2016) (Figure 3). Moreover, tsRNAs can also activate the immune response of Th1 and cytotoxic T lymphocyte by interacting directly with toll-like receptors (Wang et al., 2006) (Figure 3).

In addition, the specific nucleoside motifs of tRNAs may be a structural determinant of innate immune recognition. For example, the interaction between the human tRNA^Ala stem loop and the D and T rings of tRNA^His may be epitopes of autoantibodies in the sera of patients with idiopathic inflammatory myopathy (Bunn and Mathews, 1987), and the anti-adenovirus infection-induced tRNAs fungal protective cell therapy (Alvarado-Vásquez et al., 2005). Finally, the major histocompatibility complex contains the largest tRNA gene cluster in human, which also coexist with immune-related functions are co-located. This may imply the role of tRNA in the immune system (Horton et al., 2004). These findings overall support the proposition that tRNAs may act as immune signaling molecules.

Both mitochondrial and cytosolic tRNAs have been shown to bind to cytochrome c. This binding inhibits the interaction between cytochrome c and apoptotic protease activating factor-1, thus blocking activating factor-1 oligomerization and caspase activation, which eventually preventing apoptosis (Mei et al., 2010). Another study revealed that under high osmotic pressure or stress, angiogenin-induced tiRNAs can inhibit apoptotic formation and activity by binding cytochrome c to form a ribonucleoprotein complex (Saikia et al., 2014) (Figure 3). Moreover, tsRNAs can also regulate micro-organisms found in human. In the oral cavity, the presence of Fusobacterium nucleatum triggers the release of tsRNAs, which may inhibit the growth of the former by interfering with the biosynthesis of bacterial proteins (He et al., 2018).

At present, studies exploring the function of tRNAs are only emerging. Future researches are expected to reveal the further regulatory role of tRNAs and tsRNAs in biological functions.

Different factors can induce the abnormal expression of tRNAs in breast cancer, thereby promoting tumor progression. For example, the stimulation of ethanol activates c-Jun N-terminal kinase 1, which promotes the proliferation of Brf1 and ERα. Subsequently, the interaction between Brf1 and ERα upregulates Pol III gene transcription to enhance the production of tRNA, ultimately leading to the development of breast cancer. However, tamoxifen can hold in the incident of breast cancer by containing the effects of Brf1 and ERα, also indirectly inhibits the generation of tRNA (Fang et al., 2017) (Figure 5). Similarly, the TATA box-binding protein human Maf1 and the oncogene Ras can promote the transcription of tRNAs by targeting RNA pol Ⅲ, particularly the Brf1 subunit of TFⅢ B factor, thereby promoting tumor progression (Wang et al., 1997; Shen et al., 1998; Rollins et al., 2007; Johnson et al., 2008).

Enzymes catalyzing tRNA modifications play significant roles in the biological processes in breast cancer (Towns and Begley, 2012; Frye and Watt, 2006; Delaunay et al., 2016). In human, the overexpression of the U34-modifying enzymes Elp3 and Ctu1/2 directly promoted the translation of the oncoprotein DEK by catalyzing the mcm⁵s²-U34 tRNA modification. Increased DEK can then bind the LEF1 internal ribosome entry site sequence, thereby increasing the translation of the oncogenic LEF-1 mRNA and promoting the invasion and metastasis of breast cancer cells (Delaunay et al., 2016) (Figure 5). Furthermore, the high expression of tRNA^Arg-CCG and tRNA^GIu-UUC in breast cancer can promote the invasion and metastasis of cancer cells by directly upregulating the expression of EXOSC2 and enhancing that of GRIPAP1 (Goodarzi et al., 2016).

In a study cohort of Polish women with breast cancer, the tertiary structure of mt-tRNAs was affected by genetic mutation, resulting in the severe impairment of the mitochondrial protein synthesis, and thus affecting cell proliferation (Grzybowska-Szatkowska and Slaska, 2012). Further, mutations in mt-tRNAs are participated in the carcinogenesis of breast cancer, such as mt-tRNA^Asp (Meng et al., 2016), but the specific mechanism of mtRNA mutation in cancer is unknown.

Similarly, tiRNAs are abnormally expressed in breast cancer and are involved in tumorigenesis (Goodarzi et al., 2015; Honda et al., 2015; Balatti et al., 2017). Decreased abundances of 26 specific circulatory tiRNAs from the tRNA^Gly, tRNA^Glu, and tRNA^Lys heterogenous receptors was observed in ER-positive breast cancer (Dhahbi et al., 2014). The same study have suggested that inflammatory breast cancer is associated with an increase in tiRNA^Ala (Dhahbi et al., 2014). Some tiRNAs, such as tiRNA^Asp-5 and tiRNA^His-5, are significantly overexpressed in breast cancer, and the knockout of tiRNA-5s can inhibit tumor proliferation (Honda et al., 2015). These findings provide sufficient evidence of the involvement of abnormal tiRNAs expression in the course of breast cancer.

The connection of tsRNAs in breast cancer progression has also been confirmed by several studies. First, the anomalous demonstration of tiRNAs has been sighted at some stages of the carcinogenesis process [19]. The downregulation of tiRNA^Val-5 in the serum is positively associated with lymph node metastasis and cancer stage progression, whereas its overexpression inhibits malignant cell activity (Mo et al., 2019). Meanwhile, the expression of tRF-3s are strongly downregulated in invasive advanced breast cancer, whereas that of tRF-1s were raised in an advanced cancer cell, thus suggesting that tRF-3s and tRF-1s may be related to advanced pathological changes in cancer (Balatti et al., 2017). Second, tsRNAs can regulate breast cancer progression by affecting gene transcription. For example, tRFs from tRNA^Glu, tRNA^Asp, tRNA^Gly, and tRNA^Tyr contend with YB-1 for the transcription of endogenous cancer genes, thereby undermining the stability of transcriptions of proto-oncogene and reducing their expression. This subsequently inhibits breast cancer progression (Goodarzi et al., 2015). Similarly, in breast cancer cells, tRFs derived from tRNA^Tyr, tRNA^Asp, tRNA^Gly, and tRNA^Glu can inhibit tumor progression by displacing the 3′-UTRs of multiple oncogenic transcripts from the RNA-binding protein YBX-1, thus reducing their stability (Goodarzi et al., 2015). Another study showed that tiRNA^Val-5 leads to the inhibition of c-myc and cyclinD1 by downregulating the FZD3-Wnt/β-catenin axis, which inhibits the progression of breast cancer (Mo et al., 2019) (Figure 5). Third, the impairment of the tiRNAs production also affect cancer progression. estrogen and its receptors promote the angiogenin cutting mature tRNA anticodon ring, thus producing large amounts of tiRNAs in ER-positive breast cancer. This accumulation makes for cells proliferating, which may promote tumor occurrence and tumor growth (Honda et al., 2015). Meanwhile, in a hypoxic environment, angiogenin induces the production of tsRNAs, and tsRNAs then interact with interleukin-6 to promote the phosphorylation of signal transducers and activators of transcription proteins. This promotes the transcription of the hypoxia inducible factor-1α, as well as those of multidrug-resistant genes and glycolytic proteins, ultimately leading to cytochemical resistance (Cui et al., 2019) (Figure 5). These studies suggest the varied functions of tsRNAs in cancer pathogenesis.

mt-tRNA gene mutations have been found to promote the development of lung cancer (Lu et al., 2009). These mutations damage the secondary structure of tRNAs, thereby affecting post-transcriptional modifications and aminoacylation, which can change the specificity, stability, or affinity of tRNAs (Brulé et al., 1998). Further, these mutations caused a decrease in mitochondrial protein synthesis and the cellular inability to reach the respiratory phenotypes and ATP thresholds required by normal cells to promote lung cancer (Lu et al., 2009) (Figure 6). The drug BC-Li-0186 when combined with leucyl-tRNA synthetase inhibited its activity, reduced the abundance of tRNA-carrying leucine, and prevented leucyl-tRNA synthetase mediated the non-classical mammalian target of rapamycin complex 1. This ultimately restrains the development of non-small cell lung cancer (Kim et al., 2019) (Figure 6).

tsRNAs have also been associated with lung cancer development through its regulation of the biological behavior of cells. For instance, miR-4521, acts as an inhibitor in CLL, has been revealed to be a tsRNA (ts-4521), which is lowered and mutated genes in lung cancer. Meanwhile, a reduction in ts-4521 has been shown to support tumor movement by the cell proliferation-related pathways’ activation and inhibiting apoptosis-related pathways in cancer cells (Phizicky and Hopper, 2010; Pekarsky et al., 2016). In addition, tsRNAs can regulate the demonstration of oncogenes. tRF‐Leu‐CAG is highly expressed in non-small cell lung cancer tissues, promoting tumor cell proliferation and cell cycle progression by upregulating the oncogene AURKA (Shao et al., 2017b) (Figure 6). However, how to mediate other signaling pathways through AURKA remains unclear.

Melanoma is highly malignant and accounts for the majority of skin tumor deaths. The roles of tRNAs and tsRNAs in its melanoma have also been investigated. The overexpression of the promoter methionine tRNA gene promoted tumor growth and angiogenesis in mouse melanoma cells, as well as an increase in cancer cell migration, invasion, and lung colonization, thereby resulting in increased metastasis potential (Phizicky and Hopper, 2010; Clarke et al., 2016). Its upregulation in cancer-associated fibroblasts accelerated the secretion of stromal cells, especially type-II collagen, thus facilitating tumor growth and metastasis (Phizicky and Hopper, 2010) (Figure 7).

In a recent study, the accurate translation of hypoxia inducible factor 1 α mRNA in melanoma requires the participation of 34 uridine tRNA-modifying enzymes to adapt to a metabolic environment that is not conducive to growth conditions. The particular translation reprogramming of which relies partly on mTORC2-mediated enzymatic phosphorylation to modify the anti-codon of tRNA. Further, enhanced codon dependence on hypoxia inducible factor 1 α translation can promote glycolytic metabolism and the proliferation of melanoma cells (Mcmahon and Ruggero, 2018) (Figure 7). However, the specific regulatory mechanism of tRNAs in melanoma needs further validation, whereas the role of tsRNAs in melanoma has yet to be studied.

tRNAs and tsRNAs have also been implicated in the biological processes of liver (Selitsky et al., 2015) and prostate cancers (Olvedy et al., 2016b). In liver cancer, tsRNAs tRNA^Val-TAC-3, tRNA^Gly-TCC-5, tRNA^Val-AAC-5, and tRNA^Glu-CTC-5 were significantly increased in plasma exosomes (Selitsky et al., 2015) (Figure 8). Meanwhile the expression of tiRNA^Arg-CCT-5, tiRNA^Glu-CTC-5, tiRNA^Leu-CAG-5, and tiRNA^Lys-TTT-5 was downregulated in clear-cell renal-cell carcinoma, suggesting a potential role as a tumor suppressor (Zhao et al., 2018). Moreover, the relative abundance of tiRNA^Gly is 50–60% lower in hepatitis B virus and Hepatitis C virus-related cancers than normal liver tissue (Selitsky et al., 2015). These findings suggest that tRNAs and tsRNAs may have opposite effects in liver cancer and clear-cell renal-cell carcinoma and may thus be used as new diagnostic biomarkers.

tsRNA (CU1276) modulated DNA damage response and suppressed cell proliferation via the inhibition of RPA1, which is an endogenous single-stranded DNA binding protein, in B cell lymphoma cells (Maute et al., 2013) (Figure 8). Meanwhile, tRNA^Leu and pre-miRNA derived from tRF/miR-1280, can suppress the growth and transfer of colorectal cancer by inhibiting Notch signaling pathways. Particularly, tRF/miR-1280 can target the Notch ligand jagged 2 (JAG2) to repress Notch signaling pathways, which in turn inhibits the cancer stem cell phenotypes by inhibiting direct transcription of Gata1/3 and miR200b genes, thus inhibiting tumorigenesis and metastasis (Huang et al., 2017) (Figure 8).

In high-grade serous ovarian cancer, upregulated tRFs can promote protein phosphorylation, transcription, cell migration, cancer pathways, MAPK, and Wnt signaling pathways, as well as regulating HMBOX1 to attack human ovarian cancer cells (Zhang et al., 2019a). This proves that tRFs play a role as a regulatory factor for cancer development in serous ovarian cancer (Figure 8). In prostate cancer, 589 differentially expressed tRFs have been detected, suggesting its potential as a biomarker (Olvedy et al., 2016b). Similarly, tRF-1001 from tRNA^Ser was eminently expressed in prostate cancer, whereas its knockdown suppressed DNA biosynthesis and cell proliferation (Anderson and Ivanov, 2014).

Cancer is the most dangerous disease known. In many cases, patients with cancer are often unaware of their early stages, and when clinical symptoms appear, the cancer is in its late stages. Finding reliable and sensitive diagnostic molecular markers is researchers have been struggling to find. Recently, the function of tRNAs and tsRNAs in the human body has gradually received clinical attention and shown great molecular marker potential.

Previous studies have shown that tsRNAs can be detected in serum and urine stabilized (Romaine et al., 2015; Santos et al., 2019). Now, with the continuous development of high-throughput sequencing technology, different types of tsRNAs are constantly detected and isolated in body fluids, and their functions are gradually reflected. For example, there are differences in abundance between prostate cancer and renal transparent cell carcinoma and tsRNAs in normal prostate tissue (Hayes et al., 2014; Zhao et al., 2018). Subsequently, many studies have found tRNAs and tsRNAs disorders may be related to the regulation of tumor genes and tumor suppressor genes, while the abnormal expression of tRNAs and tsRNAs can promote cell proliferation and inhibit cell apoptosis, further promoting cancer progression (Pavon-Eternod et al., 2013; Kwon et al., 2018). In breast cancer, ethanol stimulation can promote the proliferation of Brf1 and ER+, thereby enhancing their interaction, and enhance the production of tRNA by Pol III gene transcription, further promoting tumor progression (Fang et al., 2017). Meanwhile many studies have shown that cancer genes can regulate the expression of tsRNAs and tsRNAs may also be a key effector molecule of cancer gene regulation (Zhu et al., 2019). On the other hand, in hepatocellular carcinoma, tsRNAs tRNA^Val-TAC-3, tRNA^Gly-TCC-5, tRNA^Val-AAC-5, and tRNA^Glu-CTC-5 are significantly elevated in plasma exosomes, which may play a transcriptional role (Selitsky et al., 2015). These characteristics are sufficient to demonstrate the great potential of tsRNAs to become a biomarker of tumors. But there is still less research on tsRNAs, and hopefully more people will look to it in the future so that tsRNAs can really play its clinical role and contribute to the cause of human medicine.

tRNAs and tsRNAs, as small molecular markers related to tumor, may also play a role in molecular targeted therapy to suggest local therapeutic targets. For example, mt-tRNA mutations in lung cancer inhibit the development of NSCLC by leucyl-tRNA synthase mediated rapamycin complex 1 as a non-classical mammalian target (Kim et al., 2019). In high-grade serous ovarian cancer, tRF-03357 promotes cell proliferation, migration and invasion, partly by modulating HMBOX1. And this phenomenon can be reversed by targeting tRF-03357 (Zhang et al., 2019a). These functions indicate that tRNAs and tsRNAs can be used as therapeutic targets for clinical interventions. However, as a therapeutic target, more precise mechanisms of action and higher sensitivity specificity are required, which still leaves some unexplored areas.