- Email: [email protected]
tRNA Modification: Is Cancer Having a Wobble?
TRECAN 153 No. of Pages 4
Forum
tRNA Modification: Is Cancer Having a Wobble? Francesca Rapino,1,3 Sylvain Delaunay,1,3 Zhaoli Zhou,1,3 Alain Chariot,2,3,4 and Pierre Close1,3,* Translational control of protein synthesis supports tumor development and progression to metastasis. Wobble tRNA modifications are required during translation elongation and sustain proteome homeostasis. Recent work has highlighted the surprising upregulation of the wobble uridine 34 (U34) tRNA cascade in cancer, which underlies the specific requirement for this pathway in tumor development. Introduction Tumorigenesis induces the rewiring of many fundamental processes and leads to increased cellular stress and upregulation of adaptation mechanisms [1]. Cancer therapy aims to target tumor cells with limited impact on normal tissue. Changes in proteome composition are observed in cancer cells and require modifications to the protein synthesis machinery to induce the reprogramming of translation, which supports tumor cell survival, invasion, and metastasis [2]. Modifications at the wobble U34 of certain tRNAs (i.e. mainly at tRNAUUU[10_TD$IF]Lys, tRNAUUCGlu, tRNAUUGGln, and tRNAUCUArg) promote decoding during translation and sustain proteome homeostasis in response to cellular stress [3]. Despite their expected generalized importance, the genetic inhibition of enzymes modifying the wobble U34 tRNAs has surprising specificity for cancer cells and does not affect normal tissue
homeostasis [4,5], pointing to new oppor- to differences in the expression of the tunities for the development of anticancer canonical translation initiation proteins [5]. drugs. Therefore, despite the requirement of Wobble U34 tRNA Modification Is U34-modifying enzymes for global mRNA translation, their crucial role may be Needed to Overcome Specific highlighted in pathophysiology, especially Stress Conditions Post-transcriptional nucleoside modifica- in cancer. tions of tRNAs at the anticodon wobble U34 are highly conserved [6]. In mam- Dynamic Regulation of tRNA mals, enzymes catalyzing chemical U34 Expression in Cancer modification include the acetyltransferase Transformed cells are characterized by Elongator (Elp1–6), the methyltransferase increased cellular proliferation and protein Alkylation repair homolog 8 (Alkbh8), synthesis. Global changes in protein synand Ubiquitin-related modifier 1 (Urm1) thesis as well as in selective translation of pathway, enclosing the thiouridylases specific mRNAs support cancer developcytosolic thiouridylase homolog 1/2 ment and progression [2]. Emerging evi(Ctu1/Ctu2) (Figure 1). In yeast, many of dence indicates that tRNA expression is the enzymes modifying U34 tRNA are not modulated during tumorigenesis [13–15]. essential, although their importance has Although still debatable, the latest data been revealed under stress conditions [3]. indicate that tRNA molecules have an The lack of U34 tRNA modification leads active role as tumor drivers [14]. Indeed, to codon-specific translational pausing the overexpression of tRNAUUCGlu, a tar[7–9], but consequences for protein out- get of U34 modification specifically upreput remain dependent on the context. On gulated in invasive breast cancer, is the one hand, it was shown that proteins sufficient to promote invasion and metascontaining clusters of cognate codons are tasis in vivo by enhancing EXOSC2 and subject to enhanced ribosome pausing in GRIPAP1 expression [14]. Conversely, its yeast lacking U34 modification and are downregulation impacts breast cancer more likely to be downregulated [7]. On metastatic potential in vivo [14]. Importhe other hand, two studies have recently tantly, differences observed in tRNA demonstrated that the loss of U34 modi- expression profiles correlate with fication causes perturbation of cellular changes in the mRNA expression of signaling and generates chronic proteo- genes exhibiting compatible codon usage toxic stress that decreases global cellular [13–15]. Together, these results suggest fitness [8,9]. that changes in proteome expression and, by extension, in codon usage, during In mammals, Elongator is required during tumorigenesis are supported by a embryonic and cortical development dynamic and changing repertoire of [10,11]. Conversely, Alkbh8 / [103_TD$IF] mice do tRNAs (Figure 2). not harbor any obvious phenotype under normal conditions [12]. Elp[104_TD$IF]3 activity is Implication of U34 tRNA also dispensable for normal intestine or Modification in Cancer mammary gland homeostasis in mice, but Recent data showed that U34 enzymes is critically required in tumor development are upregulated in human cases of colon in both organs [4,5]. Loss of U34 enzymes adenocarcinoma and breast cancer [4,5]. in the developing brain causes upregula- Remarkably, it was shown using condition of an unfolded protein response tional deletion of Elp3 in the mouse intes(UPR) that is responsible for the pheno- tinal epithelium, [106_TD$IF]that this enzyme is type (i.e., microcephaly) [10]. Surprisingly, critically required for Wnt-dependent in breast cancer cells, U34 loss of function intestinal tumor initiation by sustaining a does not lead to the induction of UPR or subpool of Lgr5+[105_TD$IF]/Dclk1+/Sox9+ cancer
Trends in Cancer, Month Year, Vol. xx, No. yy
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TRECAN 153 No. of Pages 4
OH 3’ 5’P
URM pathway O
O
O
O
O
U34
CH3 C OH HN
HN
O
O
O HN
O N
N
R
R
R
(ELP1–6)
U34
ALKBH8
CH3 C O CH3
HN
S
N
Elongator
O
CH3 C O CH3
N
CTU1/CTU2
R
(TRM9-like domain)
cm5 U34
mcm5 U34
mcm5s2 U34
Figure 1. Enzymatic Cascade Leading to the mcm5s2 Modification at Wobble Uridine 34 (U34) in tRNAs. The U34 mcm5s2 tRNA modification is catalyzed by successive activities of the acetyltransferase Elongator, the methyltransferase TRM9-like domain of Alkylation repair homolog 8 (ALKBH8), and the urmylation (URM) pathway, which encloses the cytosolic thiouridylase homolog 1 and 2 (CTU1/CTU2). These enzymes modify U34 of tRNAs into carboxy-methyl-uridine (cm5[9_TD$IF]-U), 5-methoxycarbonyl-methyl-uridine (mcm5-U), and 5-methoxycarbonyl-methyl-2-thiouridine (mcm5s2-U), respectively, mainly in tRNAUUULys, tRNAUUCGlu, tRNAUUGGln, tRNAUCCGly, and tRNAUCUArg. The mcm5s2 modification in tRNAs ensures cognate codon decoding and translation efficiency.
stem cells. Specifically, synthesis of Sox9 requires U34 tRNA modifications and, therefore, it is not properly expressed upon Elp3 deficiency [4]. These data indicate that the maintenance of cancer stem cells and intestinal tumor initiation both rely on U34 tRNA modification. The U34 tRNA-modifying enzymes ELP3, CTU1, and CTU2 were also found to be upregulated in biopsies from patients with invasive breast cancer. Oncogene expression in normal mammary gland epithelial cells leads to the upregulation of both the U34 tRNA enzymes and the U34-modified (i.e., thiolated) tRNAs. Strikingly, Elp3 genetic inactivation in the breast epithelium delayed PyMTinduced tumor appearance and strongly impacted metastasis formation. U34 tRNA modification enzymes were induced in cells undergoing epithelial– mesenchymal transition (EMT) and in invasive breast cancer cells, suggesting
2
Trends in Cancer, Month Year, Vol. xx, No. yy
that wobble U34-modified tRNAs are required to translate mRNA candidates encoding proinvasive proteins. Among them is the oncoprotein Dek, whose mRNA sequence contains almost 20% of codons that require wobble U34-modified tRNAs for efficient translation, but which was lacking in Elp3-deficient breast tumors. Consequently, these tumors were also unable to generate proinvasive protrusions [5]. In vitro, loss of U34 modification impacted the invasion potential, but not proliferation, of tumor cells [5]. Together, these data support a model in which the increase in the expression of specific mRNAs encoding oncoproteins and relying on U34 tRNA modifications for translation, sustains cancer stem cell maintenance and metastasis in vivo. In cancer, malignant transformation is often accompanied by alterations in intracellular proteome homeostasis. As such,
induction of a proteotoxic stress phenotype, to a certain extent, is recognized as a prosurvival mechanism to promote cancer cell adaptation (reviewed in [16]). Given the recent finding that ELP3 deficiency generates proteotoxic stress response in yeast and nematodes [8], one can also envision that additional disturbances of proteome homeostasis upon Elp3 deficiency are strongly deleterious for cancer cells and, consequently, impair tumorigenesis. Indeed, perturbation of proteome homeostasis in cancer, for instance by agents affecting protein folding within the endoplasmic reticulum (ER) or by inhibitors of the proteasome or HSP90, has been suggested as a potential therapeutic strategy [16]. Despite being attractive, this hypothesis is not yet supported by data in cancer cells because impaired wobble U34 tRNA modification [107_TD$IF]in established cell lines are not associated with apparent proteotoxic stress [5]. Consistently, Elp3 deficiency in
TRECAN 153 No. of Pages 4
Normal cell
Cancer cell proinvasive Wnt
Oncogenic pathways
U34 enzymes
Adapve proteome
U34 enzymes
RNAPIII
Tumorigenesis metastasis
RNAPIII tRNA gene
Nucleus
tRNA gene
Nucleus
mcm5s2 - modificaon
Figure 2. Increase in tRNA Abundance and Specificity Supports Tumorigenesis. Various oncogenic pathways converge on RNA Polymerase III (RNAPIII) to promote tRNA gene expression. Interestingly, not only the overall tRNA abundance is increased, but changes in the abundance of individual tRNAs are also observed in cancer [13–15]. Importantly, wobble uridine 34 (U34) tRNA-modifying enzymes (U34 enzymes) that utilize specific tRNAs as substrates are upregulated in Wnt-driven colon cancer as well as in breast cancer. Activation of the Wnt pathway, oncogene expression, or induction of the epithelial–mesenchymal transition (EMT) all lead to enhanced levels of U34 enzymes. The tRNA targets of the mcm5[10_TD$IF]s2 modification are depicted in yellow, pink, and brown; other colors depict U34 tRNA modificationindependent tRNAs. The current model predicts that the upregulation of tRNAs and their modifying enzymes is required for the establishment of an adaptive proteome that will support tumorigenesis.
established cancer cell lines does not impair cellular survival, but strongly affects the [108_TD$IF]invasion potential, migratory capacity, and ability of cancer stem cells to grow in spheroid culture [4,5,17].
What Makes the U34 tRNA Modification Pathway Attractive [109_TD$IF]As a Target? Despite the absence of genetic mutation in any of the genes implicated in this pathway, U34 tRNA modification gain of function may be required for tumor development. Indeed, the surprising specificity of Elp3 or other U34 tRNA-modifying enzymes in cancer highlights their potential as targets for drug development. U34 tRNA modification enzymes are weakly expressed in most healthy tissues (with the exception of the developing cortex [10]) and dispensable in normal stem cells. Conversely, they are specifically required in the maintenance of cancer stem cells and in metastasis formation.
Future work needs to define the specificity of U34-modifying enzymes by investigating more systematically their role in healthy tissues and in cancer. On the one hand, the specificity may come from proteome expression and the increased demand to translate mRNAs with specific codon usage. Identifying proteins whose synthesis requires wobble U34 tRNA modification is crucial to understand the importance of this pathway in cancer. On the other hand, the specificity may also rely on the activation of signaling pathways that regulate U34 tRNA-modifying enzyme expression and/or activity. Recent work highlighted the Wnt pathway and EMT as regulators of the U34 tRNA modification activity (Figure 2) [4,5]. A better understanding of the dynamics underlying U34 tRNA modification regulation as well as of the oncogenic pathways involved in the activation of the U34 tRNA modification enzymes is indispensable for further defining their role in cancer.
Defined as an adaptive mechanism in response to stress in yeast, wobble U34 tRNA modification may represent an attractive convergent mechanism to which cancer cells become addicted during the course of tumorigenesis (Figure 2). This new family of enzymes that regulate U34 tRNA modifications may represent targets of a new class of inhibitors to be used alone or in combination with existing drugs to better treat cancer. Acknowledgments Our work is supported by grants from the Belgian National Funds for Scientific Research (FNRS: MIS F:4532.13), grants from the University of Liege, the Belgian Foundation against Cancer (FAC: FAF-F/ 2016/840), and TELEVIE, as well as by the Walloon Excellence in Life Sciences and Biotechnology (WELBIO). We are also grateful to the Fonds Leon Fredericq and the Centre Anticancéreux of the CHU Liege for their financial support. 1 Laboratory of Cancer Signaling, University of Liège, 4000 Liège, Belgium
Trends in Cancer, Month Year, Vol. xx, No. yy
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TRECAN 153 No. of Pages 4
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Laboratory of Medical Chemistry, University of Liège, 4000 Liège, Belgium 3 GIGA-Molecular Biology of Diseases, GIGA-Research,
5. Delaunay, S. et al. (2016) Elp3 links tRNA modification to IRES-dependent translation of LEF1 to sustain metastasis in breast cancer. J. Exp. Med. 213, 2503–2523
University of Liège, 4000 Liège, Belgium WELBIO[102_TD$IF], Avenue Pasteur 6, 1300 Wavre, Belgium
6. El Yacoubi, B. et al. (2012) Biosynthesis and function of posttranscriptional modifications of transfer RNAs. Ann. Rev. Genet. 46, 69–95
4
*Correspondence: [email protected] (P. Close). http://dx.doi.org/10.1016/j.trecan.2017.02.004 References 1. Nagel, R. et al. (2016) Drugging the addict: non-oncogene addiction as a target for cancer therapy. EMBO Rep. 1, 505–527 2. Truitt, M.L. and Ruggero, D. (2016) New frontiers in translational control of the cancer genome. Nat. Rev. Cancer 16, 288–304 3. Ranjan, N. and Rodnina, M.V. (2016) tRNA wobble modifications and protein homeostasis. Translation 4, e1143076 4. Ladang, A. et al. (2015) Elp3 drives Wnt-dependent tumor initiation and regeneration in the intestine. J. Exp. Med. 212, 2057–2075
4
Trends in Cancer, Month Year, Vol. xx, No. yy
7. Deng, W. et al. (2015) Trm9-catalyzed tRNA modifications regulate global protein expression by codon-biased translation. PLoS Genet. 11, e1005706 8. Nedialkova, D.D. and Leidel, S.A. (2015) Optimization of codon translation rates via tRNA modifications maintains proteome integrity. Cell 161, 1–13 9. Zinshteyn, B. and Gilbert, W.V. (2013) Loss of a conserved tRNA anticodon modification perturbs cellular signaling. PLoS Genet. 9, e1003675 10. Laguesse, S. et al. (2015) A dynamic unfolded protein response contributes to the control of cortical neurogenesis. Dev. Cell 35, 553–567 11. Chen, Y.-T. et al. (2009) Loss of mouse Ikbkap, a subunit of elongator, leads to transcriptional deficits and embryonic lethality that can be rescued by human IKBKAP. Mol. Cell. Biol. 29, 736–744
12. Songe-Møller, L. et al. (2010) Mammalian ALKBH8 possesses tRNA methyltransferase activity required for the biogenesis of multiple wobble uridine modifications implicated in translational decoding. Mol. Cell. Biol. 30, 1814–1827 13. Pavon-Eternod, M. (2009) tRNA over-expression in breast cancer and functional consequences. Nucleic Acids Research 37, 7268–7280 14. Goodarzi, H. et al. (2016) Modulated expression of specific tRNAs drives gene expression and cancer progression. Cell 165, 1416–1427 15. Gingold, H. et al. (2014) A dual program for translation regulation in cellular proliferation and differentiation. Cell 158, 1281–1292 16. Dai, C. et al. (2012) Proteotoxic stress of cancer: implication of the heat-shock response in oncogenesis. J. Cell. Physiol. 227, 2982–2987 17. Close, P. et al. (2012) DERP6 (ELP5) and C3ORF75 (ELP6) regulate tumorigenicity and migration of melanoma cells as subunits of elongator. J. Biol. Chem. 287, 32535– 32545
Forum
tRNA Modification: Is Cancer Having a Wobble? Francesca Rapino,1,3 Sylvain Delaunay,1,3 Zhaoli Zhou,1,3 Alain Chariot,2,3,4 and Pierre Close1,3,* Translational control of protein synthesis supports tumor development and progression to metastasis. Wobble tRNA modifications are required during translation elongation and sustain proteome homeostasis. Recent work has highlighted the surprising upregulation of the wobble uridine 34 (U34) tRNA cascade in cancer, which underlies the specific requirement for this pathway in tumor development. Introduction Tumorigenesis induces the rewiring of many fundamental processes and leads to increased cellular stress and upregulation of adaptation mechanisms [1]. Cancer therapy aims to target tumor cells with limited impact on normal tissue. Changes in proteome composition are observed in cancer cells and require modifications to the protein synthesis machinery to induce the reprogramming of translation, which supports tumor cell survival, invasion, and metastasis [2]. Modifications at the wobble U34 of certain tRNAs (i.e. mainly at tRNAUUU[10_TD$IF]Lys, tRNAUUCGlu, tRNAUUGGln, and tRNAUCUArg) promote decoding during translation and sustain proteome homeostasis in response to cellular stress [3]. Despite their expected generalized importance, the genetic inhibition of enzymes modifying the wobble U34 tRNAs has surprising specificity for cancer cells and does not affect normal tissue
homeostasis [4,5], pointing to new oppor- to differences in the expression of the tunities for the development of anticancer canonical translation initiation proteins [5]. drugs. Therefore, despite the requirement of Wobble U34 tRNA Modification Is U34-modifying enzymes for global mRNA translation, their crucial role may be Needed to Overcome Specific highlighted in pathophysiology, especially Stress Conditions Post-transcriptional nucleoside modifica- in cancer. tions of tRNAs at the anticodon wobble U34 are highly conserved [6]. In mam- Dynamic Regulation of tRNA mals, enzymes catalyzing chemical U34 Expression in Cancer modification include the acetyltransferase Transformed cells are characterized by Elongator (Elp1–6), the methyltransferase increased cellular proliferation and protein Alkylation repair homolog 8 (Alkbh8), synthesis. Global changes in protein synand Ubiquitin-related modifier 1 (Urm1) thesis as well as in selective translation of pathway, enclosing the thiouridylases specific mRNAs support cancer developcytosolic thiouridylase homolog 1/2 ment and progression [2]. Emerging evi(Ctu1/Ctu2) (Figure 1). In yeast, many of dence indicates that tRNA expression is the enzymes modifying U34 tRNA are not modulated during tumorigenesis [13–15]. essential, although their importance has Although still debatable, the latest data been revealed under stress conditions [3]. indicate that tRNA molecules have an The lack of U34 tRNA modification leads active role as tumor drivers [14]. Indeed, to codon-specific translational pausing the overexpression of tRNAUUCGlu, a tar[7–9], but consequences for protein out- get of U34 modification specifically upreput remain dependent on the context. On gulated in invasive breast cancer, is the one hand, it was shown that proteins sufficient to promote invasion and metascontaining clusters of cognate codons are tasis in vivo by enhancing EXOSC2 and subject to enhanced ribosome pausing in GRIPAP1 expression [14]. Conversely, its yeast lacking U34 modification and are downregulation impacts breast cancer more likely to be downregulated [7]. On metastatic potential in vivo [14]. Importhe other hand, two studies have recently tantly, differences observed in tRNA demonstrated that the loss of U34 modi- expression profiles correlate with fication causes perturbation of cellular changes in the mRNA expression of signaling and generates chronic proteo- genes exhibiting compatible codon usage toxic stress that decreases global cellular [13–15]. Together, these results suggest fitness [8,9]. that changes in proteome expression and, by extension, in codon usage, during In mammals, Elongator is required during tumorigenesis are supported by a embryonic and cortical development dynamic and changing repertoire of [10,11]. Conversely, Alkbh8 / [103_TD$IF] mice do tRNAs (Figure 2). not harbor any obvious phenotype under normal conditions [12]. Elp[104_TD$IF]3 activity is Implication of U34 tRNA also dispensable for normal intestine or Modification in Cancer mammary gland homeostasis in mice, but Recent data showed that U34 enzymes is critically required in tumor development are upregulated in human cases of colon in both organs [4,5]. Loss of U34 enzymes adenocarcinoma and breast cancer [4,5]. in the developing brain causes upregula- Remarkably, it was shown using condition of an unfolded protein response tional deletion of Elp3 in the mouse intes(UPR) that is responsible for the pheno- tinal epithelium, [106_TD$IF]that this enzyme is type (i.e., microcephaly) [10]. Surprisingly, critically required for Wnt-dependent in breast cancer cells, U34 loss of function intestinal tumor initiation by sustaining a does not lead to the induction of UPR or subpool of Lgr5+[105_TD$IF]/Dclk1+/Sox9+ cancer
Trends in Cancer, Month Year, Vol. xx, No. yy
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TRECAN 153 No. of Pages 4
OH 3’ 5’P
URM pathway O
O
O
O
O
U34
CH3 C OH HN
HN
O
O
O HN
O N
N
R
R
R
(ELP1–6)
U34
ALKBH8
CH3 C O CH3
HN
S
N
Elongator
O
CH3 C O CH3
N
CTU1/CTU2
R
(TRM9-like domain)
cm5 U34
mcm5 U34
mcm5s2 U34
Figure 1. Enzymatic Cascade Leading to the mcm5s2 Modification at Wobble Uridine 34 (U34) in tRNAs. The U34 mcm5s2 tRNA modification is catalyzed by successive activities of the acetyltransferase Elongator, the methyltransferase TRM9-like domain of Alkylation repair homolog 8 (ALKBH8), and the urmylation (URM) pathway, which encloses the cytosolic thiouridylase homolog 1 and 2 (CTU1/CTU2). These enzymes modify U34 of tRNAs into carboxy-methyl-uridine (cm5[9_TD$IF]-U), 5-methoxycarbonyl-methyl-uridine (mcm5-U), and 5-methoxycarbonyl-methyl-2-thiouridine (mcm5s2-U), respectively, mainly in tRNAUUULys, tRNAUUCGlu, tRNAUUGGln, tRNAUCCGly, and tRNAUCUArg. The mcm5s2 modification in tRNAs ensures cognate codon decoding and translation efficiency.
stem cells. Specifically, synthesis of Sox9 requires U34 tRNA modifications and, therefore, it is not properly expressed upon Elp3 deficiency [4]. These data indicate that the maintenance of cancer stem cells and intestinal tumor initiation both rely on U34 tRNA modification. The U34 tRNA-modifying enzymes ELP3, CTU1, and CTU2 were also found to be upregulated in biopsies from patients with invasive breast cancer. Oncogene expression in normal mammary gland epithelial cells leads to the upregulation of both the U34 tRNA enzymes and the U34-modified (i.e., thiolated) tRNAs. Strikingly, Elp3 genetic inactivation in the breast epithelium delayed PyMTinduced tumor appearance and strongly impacted metastasis formation. U34 tRNA modification enzymes were induced in cells undergoing epithelial– mesenchymal transition (EMT) and in invasive breast cancer cells, suggesting
2
Trends in Cancer, Month Year, Vol. xx, No. yy
that wobble U34-modified tRNAs are required to translate mRNA candidates encoding proinvasive proteins. Among them is the oncoprotein Dek, whose mRNA sequence contains almost 20% of codons that require wobble U34-modified tRNAs for efficient translation, but which was lacking in Elp3-deficient breast tumors. Consequently, these tumors were also unable to generate proinvasive protrusions [5]. In vitro, loss of U34 modification impacted the invasion potential, but not proliferation, of tumor cells [5]. Together, these data support a model in which the increase in the expression of specific mRNAs encoding oncoproteins and relying on U34 tRNA modifications for translation, sustains cancer stem cell maintenance and metastasis in vivo. In cancer, malignant transformation is often accompanied by alterations in intracellular proteome homeostasis. As such,
induction of a proteotoxic stress phenotype, to a certain extent, is recognized as a prosurvival mechanism to promote cancer cell adaptation (reviewed in [16]). Given the recent finding that ELP3 deficiency generates proteotoxic stress response in yeast and nematodes [8], one can also envision that additional disturbances of proteome homeostasis upon Elp3 deficiency are strongly deleterious for cancer cells and, consequently, impair tumorigenesis. Indeed, perturbation of proteome homeostasis in cancer, for instance by agents affecting protein folding within the endoplasmic reticulum (ER) or by inhibitors of the proteasome or HSP90, has been suggested as a potential therapeutic strategy [16]. Despite being attractive, this hypothesis is not yet supported by data in cancer cells because impaired wobble U34 tRNA modification [107_TD$IF]in established cell lines are not associated with apparent proteotoxic stress [5]. Consistently, Elp3 deficiency in
TRECAN 153 No. of Pages 4
Normal cell
Cancer cell proinvasive Wnt
Oncogenic pathways
U34 enzymes
Adapve proteome
U34 enzymes
RNAPIII
Tumorigenesis metastasis
RNAPIII tRNA gene
Nucleus
tRNA gene
Nucleus
mcm5s2 - modificaon
Figure 2. Increase in tRNA Abundance and Specificity Supports Tumorigenesis. Various oncogenic pathways converge on RNA Polymerase III (RNAPIII) to promote tRNA gene expression. Interestingly, not only the overall tRNA abundance is increased, but changes in the abundance of individual tRNAs are also observed in cancer [13–15]. Importantly, wobble uridine 34 (U34) tRNA-modifying enzymes (U34 enzymes) that utilize specific tRNAs as substrates are upregulated in Wnt-driven colon cancer as well as in breast cancer. Activation of the Wnt pathway, oncogene expression, or induction of the epithelial–mesenchymal transition (EMT) all lead to enhanced levels of U34 enzymes. The tRNA targets of the mcm5[10_TD$IF]s2 modification are depicted in yellow, pink, and brown; other colors depict U34 tRNA modificationindependent tRNAs. The current model predicts that the upregulation of tRNAs and their modifying enzymes is required for the establishment of an adaptive proteome that will support tumorigenesis.
established cancer cell lines does not impair cellular survival, but strongly affects the [108_TD$IF]invasion potential, migratory capacity, and ability of cancer stem cells to grow in spheroid culture [4,5,17].
What Makes the U34 tRNA Modification Pathway Attractive [109_TD$IF]As a Target? Despite the absence of genetic mutation in any of the genes implicated in this pathway, U34 tRNA modification gain of function may be required for tumor development. Indeed, the surprising specificity of Elp3 or other U34 tRNA-modifying enzymes in cancer highlights their potential as targets for drug development. U34 tRNA modification enzymes are weakly expressed in most healthy tissues (with the exception of the developing cortex [10]) and dispensable in normal stem cells. Conversely, they are specifically required in the maintenance of cancer stem cells and in metastasis formation.
Future work needs to define the specificity of U34-modifying enzymes by investigating more systematically their role in healthy tissues and in cancer. On the one hand, the specificity may come from proteome expression and the increased demand to translate mRNAs with specific codon usage. Identifying proteins whose synthesis requires wobble U34 tRNA modification is crucial to understand the importance of this pathway in cancer. On the other hand, the specificity may also rely on the activation of signaling pathways that regulate U34 tRNA-modifying enzyme expression and/or activity. Recent work highlighted the Wnt pathway and EMT as regulators of the U34 tRNA modification activity (Figure 2) [4,5]. A better understanding of the dynamics underlying U34 tRNA modification regulation as well as of the oncogenic pathways involved in the activation of the U34 tRNA modification enzymes is indispensable for further defining their role in cancer.
Defined as an adaptive mechanism in response to stress in yeast, wobble U34 tRNA modification may represent an attractive convergent mechanism to which cancer cells become addicted during the course of tumorigenesis (Figure 2). This new family of enzymes that regulate U34 tRNA modifications may represent targets of a new class of inhibitors to be used alone or in combination with existing drugs to better treat cancer. Acknowledgments Our work is supported by grants from the Belgian National Funds for Scientific Research (FNRS: MIS F:4532.13), grants from the University of Liege, the Belgian Foundation against Cancer (FAC: FAF-F/ 2016/840), and TELEVIE, as well as by the Walloon Excellence in Life Sciences and Biotechnology (WELBIO). We are also grateful to the Fonds Leon Fredericq and the Centre Anticancéreux of the CHU Liege for their financial support. 1 Laboratory of Cancer Signaling, University of Liège, 4000 Liège, Belgium
Trends in Cancer, Month Year, Vol. xx, No. yy
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TRECAN 153 No. of Pages 4
2
Laboratory of Medical Chemistry, University of Liège, 4000 Liège, Belgium 3 GIGA-Molecular Biology of Diseases, GIGA-Research,
5. Delaunay, S. et al. (2016) Elp3 links tRNA modification to IRES-dependent translation of LEF1 to sustain metastasis in breast cancer. J. Exp. Med. 213, 2503–2523
University of Liège, 4000 Liège, Belgium WELBIO[102_TD$IF], Avenue Pasteur 6, 1300 Wavre, Belgium
6. El Yacoubi, B. et al. (2012) Biosynthesis and function of posttranscriptional modifications of transfer RNAs. Ann. Rev. Genet. 46, 69–95
4
*Correspondence: [email protected] (P. Close). http://dx.doi.org/10.1016/j.trecan.2017.02.004 References 1. Nagel, R. et al. (2016) Drugging the addict: non-oncogene addiction as a target for cancer therapy. EMBO Rep. 1, 505–527 2. Truitt, M.L. and Ruggero, D. (2016) New frontiers in translational control of the cancer genome. Nat. Rev. Cancer 16, 288–304 3. Ranjan, N. and Rodnina, M.V. (2016) tRNA wobble modifications and protein homeostasis. Translation 4, e1143076 4. Ladang, A. et al. (2015) Elp3 drives Wnt-dependent tumor initiation and regeneration in the intestine. J. Exp. Med. 212, 2057–2075
4
Trends in Cancer, Month Year, Vol. xx, No. yy
7. Deng, W. et al. (2015) Trm9-catalyzed tRNA modifications regulate global protein expression by codon-biased translation. PLoS Genet. 11, e1005706 8. Nedialkova, D.D. and Leidel, S.A. (2015) Optimization of codon translation rates via tRNA modifications maintains proteome integrity. Cell 161, 1–13 9. Zinshteyn, B. and Gilbert, W.V. (2013) Loss of a conserved tRNA anticodon modification perturbs cellular signaling. PLoS Genet. 9, e1003675 10. Laguesse, S. et al. (2015) A dynamic unfolded protein response contributes to the control of cortical neurogenesis. Dev. Cell 35, 553–567 11. Chen, Y.-T. et al. (2009) Loss of mouse Ikbkap, a subunit of elongator, leads to transcriptional deficits and embryonic lethality that can be rescued by human IKBKAP. Mol. Cell. Biol. 29, 736–744
12. Songe-Møller, L. et al. (2010) Mammalian ALKBH8 possesses tRNA methyltransferase activity required for the biogenesis of multiple wobble uridine modifications implicated in translational decoding. Mol. Cell. Biol. 30, 1814–1827 13. Pavon-Eternod, M. (2009) tRNA over-expression in breast cancer and functional consequences. Nucleic Acids Research 37, 7268–7280 14. Goodarzi, H. et al. (2016) Modulated expression of specific tRNAs drives gene expression and cancer progression. Cell 165, 1416–1427 15. Gingold, H. et al. (2014) A dual program for translation regulation in cellular proliferation and differentiation. Cell 158, 1281–1292 16. Dai, C. et al. (2012) Proteotoxic stress of cancer: implication of the heat-shock response in oncogenesis. J. Cell. Physiol. 227, 2982–2987 17. Close, P. et al. (2012) DERP6 (ELP5) and C3ORF75 (ELP6) regulate tumorigenicity and migration of melanoma cells as subunits of elongator. J. Biol. Chem. 287, 32535– 32545