**6. Ribosomal proteins**

Machinery that physically executes the protein synthesis on the matrices of mRNA is based on ribosomes. This complex entities are formulated of rRNA core and ribosomal proteins (RPs) of small (RPS) or large (RPL) subunit. Recent evidence demonstrated that ribosomes can contain different RPL/RPS, thus indicating a heterogeneity among ribosomes. Additionally, different expression of some RPL and RPS in the tissues has been observed. There are RPs, such as RPL38, which regulate translation of the Homeobox genes during embryo development [84], showing involvement of RPs in directing tissue organization. This heterogeneity of the translational machinery is further amplified by proteins associating/interacting with ribosomes [85, 86] (reviewed in more detail here [87]).

Existence of ribosomopathies demonstrates that RPs can play significant role in determining the cell fate. The first ribosomopathy which has been recognized was the Diamond-Blackfan anemia (DBA) caused by defect in RPS19 gene, what leads to the bone marrow failure [88]. Since then, more similar aberrations related to pathological state have been discovered. It has been observed that ribosomopathies are often related to cancers including leukemia, bone marrow failure and anemia.

*Acute Leukemias*

transformation.

**5. Translation initiation**

*in vitro* and *in vivo*. Transcripts altered by sudemycin treatment encoded proteins involved in receptor and signal transduction activities [65]. Noteworthy, SF3B1 has distinct effect on the transcript splicing than SRSF2. Mutation in any of them led to hyperactivation of NF-κB signaling, whilst simultaneous mutation of both SF3B1

The splicing regulatory network is complex and full of various cross-talk regulations and interactions. Due to interactive nature of regulatory factors, which influence each other's activity, lowering the level of one factor activates a compensatory mechanism that is mediated by another factor. This refers to both SR [67] as well as hnRNP proteins [68]. Thus, the results should be interpreted cautiously, because the observed effect resulting from a loss of one splicing factor may in fact be the secondary effect of a change in the network. Nevertheless, targeting of the *cis*- or *trans*-regulating elements gives possibility to hit precisely the source of oncogenic

Previously we described the processes, which regulate translation of the specific RNA in a controlled manner. The initiation of translation is another step on the way of protein synthesis, which is tightly controlled. The process can be remodeled by multiple cellular intrinsic signaling pathways that can be active in malignant cells. Constitutive activation of the PI3K/Akt/mTOR signaling pathway [69, 70] has been observed in various types of leukemia, including acute lymphoblastic leukemia (ALL), Philadelphia (Ph) chromosome positive and Ph-like acute lymphoblastic leukemia (BCR-ABL1-like ALL) or AML. Continuous activity of the PI3K/Akt/ mTOR pathway contributes to unregulated proliferation and leads to resistance to therapy with tyrosine kinase inhibitors (TKI) [71]. Activation of mTOR results in phosphorylation of S6K kinase and eukaryotic translation initiation factor 4E binding protein (4E-BP1), promoting cap-dependent mRNA initiation of translation and increased protein synthesis in leukemia cells [72]. On the other hand, mTOR pathway stimulates cap-independent translation mediated by internal ribosome entry sites IRES, mainly by activation of eIF4A helicase [73]. Another signaling pathway that regulates initiation of translation is the Ras/MAPK/ERK pathway. Its activity has also been found in leukemia cells [71, 74]. Activation of that pathway resulted in phosphorylation of the translation initiation factor eIF4E by MNK1/2 kinases. This contributed to increased β-catenin mRNA translation efficiency and activation of the Wnt/β-catenin signaling pathway, which plays an important role in differentiation and proliferation of leukemia cells [71, 75]. Microenvironmental signals, such as acute hypoxia or nutrient deprivation, trigger so called Integrated Stress Response (ISR) pathway, which shapes the mRNA translation. There are four protein kinases activated dependently on the stressor type: GCN – amino acid deprivation, PKR – appearance of viral RNA, PERK – accumulation of unfolded/ misfolded proteins in the ER and HRI – oxidative stress, heme deficiency, osmotic shock and heat shock. Activation of these kinases in response to stress leads to phosphorylation of eukaryotic initiation factor 2 subunit alpha (eIF2α) and 4E-BP, which orchestrate number of downstream events regulating translation. ISR has been shown to be active in leukemia cells and displayed pro-survival properties of

Changes in cellular signaling provide a great opportunity for the anti-leukemia treatment strategy. One is based on the inhibition of PI3K/Akt/mTOR [77] and Ras/MAPK/ERK [78] signaling pathways. Rapamycin, an inhibitor of the mTOR signaling pathway, has been tested in the context of leukemia treatment [79].

and SRSF2 displayed synthetically lethal effect [66].

**272**

those cells [76].

Difference in expression of ribosomal proteins between normal and malignant tissues has been found [89] (for review see [90]). It has also been proposed that aberrant expression of ribosomal proteins might support cancer progression. Overexpression of RPL23 in CD34-positive myelodysplastic cells (MDS) had impact on therapy effectiveness and was associated with poor prognosis [91]. Moreover, CD34-positive cells refractory to azacitidine treatment, displayed upregulation of RPL15, RPL28, RPL31 and RPL32 ribosomal proteins [92]. Recent study of MDS revealed that progression to AML is accompanied by elevated expression of some ribosomal proteins in CD123-positive cells [93]. Contrary to this, some ribosomal proteins play a tumor suppressor role in development of leukemia. Loss of RPL11 promoted lymphomagenesis [94], deletion in RPL5 gene has been found in multiple myeloma [95] and mutations in RPL5 and RPL10 contribute to development of T-cell acute lymphoblastic leukemia [96]. Additionally, mutation in RPL10 caused upregulation of phosphoserine phosphatase, which stimulated proliferation of cancer cells [97], deletion of a fragment of chromosome 5 led to myelodysplastic syndrome, which may progress to AML caused by RPS14 haploinsufficiency [98], deletion of RPL22 led to T-cell ALL by inducing a stemness factor [99] and mutation of RPS15 has been discovered to drive chronic lymphoblastic leukemia (CLL) development [100] and to cause cancer relapse [101]. Altogether, these examples clearly show that abnormally expressed ribosomal proteins are strong candidates for leukemia drivers.

It has been shown that ribosomal proteins associated with the ribosome define pool of mRNA transcripts that are selectively translated by this ribosome [102]. There are RPs that facilitate translation upon stress by interacting with IRES or allowing for translation with the use of alternative upstream open reading frames [uORFs]. For instance, RPS5 regulates binding of transcripts with IRES-2 [103] and RPS25 regulates binding of IRES-1 in 40S subunit [104].

Phosphorylation or other modifications of ribosomal proteins might also have impact on the spectrum of translated proteins. However, even if the phosphorylation of RPS6 is well documented, its physiological role remains not clear (for review see [105]). More recently, a phosphorylation of RPL12 has been identified to facilitate translation of AU-rich mRNAs during mitosis [86].

Selectivity for mRNA binding by the particular RPs shows that besides being a part of the translational machinery, they might actually play an important regulatory role of this process. Furthermore, identification of numerous proteins that interact with ribosome, so called ribosome associated proteins, has revealed that its activity is shaped by the microenvironment [85]. Changes of mRNA translation upon RPL12 phosphorylation during cell cycle progression [86] represent an example of how cells could use the translational machinery itself, to adapt the proteome to the current demands. This creates possibility for cellular signals to have impact on the translation machinery, allowing adjustment of the profile of synthesized proteins and by that way contributing to leukemia progression.

Targeting of ribosome activity could be a strategy of leukemia treatment. Based on cryo-EM structure, an antibiotic called cycloheximide (CHX) has been designed, that stalls ribosomes on mRNA [106]. However, due to high toxicity level, CHX is mainly used in molecular biology assays nowadays. Homoharringtonine (omacetaxine, HHT) is now the only FDA approved drug to treat CML patients refractory to TKIs [107]. Its mechanism of action is based on prevention of binding of tRNA to the ribosome [108], while at the cellular level this compound reduces the level of anti-apoptotic proteins Bcl2 and MCL-1, thus guiding leukemic cells into the apoptotic pathway [109]. Targeting of monosome translation by HHT has been recently intensively tested. This drug is also examined in terms of AML treatment. Omacetaxine occurred to be highly potent in subpopulation of myelodysplastic

**275**

*Targeting of Post-Transcriptional Regulation as Treatment Strategy in Acute Leukemia*

synergistic effect in combinatory therapy (as reviewed in [110]).

cells progressing towards AML [93]. Apart from that, usage of HHT has shown the

The fact that the presence of ribosomal protein or its modification might be different in cancer versus healthy cells creates opportunity to target cancer cells in more precise way, limiting the damage of healthy tissue. Design of specific small molecule inhibitors or other drugs targeting precisely the deregulated ribosomes would allow effective elimination of leukemia cells 'hiding' quiescently in the niche.

Translation regulation is a key process, which enables cancer cells to adapt the proteome according to the cellular demands and therefore survive the therapeutic treatment. Moreover, it can contribute to oncogenic transformation, because deranged translation can be a source of enhanced expression of such oncogenes as Myc. This includes modification of activity of certain RNA binding proteins or stimulation of signaling, what leads to increase of global protein synthesis rate. Thus, targeting the translation regulatory mechanisms can be an effective way to eliminate the oncogene-driven malignant cells or just limit cancer cells potential for survival. In other words, therapeutic targeting of post-transcriptional regulation of gene expression gives possibility for both, precise medicine approach as well as blockade of cancer cells proliferation, irrespective of the evolving cancer clones or

This work was supported by research grants from the National Science Centre UMO-2018/30/M/NZ3/00274 (PP-B) and UMO-2016/23/N/NZ3/02232 (MW).

Additionally, there are strategies to target ribosomal proteins biosynthesis at the step of transcription by using DNA intercalating agents such as: oxaliplatin, cisplatin or carboplatin [111] or by specific inhibition of ribosomal genes transcription by Polimerase I inhibitor CX-5461 [112]. This inhibitor showed the clinical potential in Myc expressing multiple myeloma [113, 114]. The Phase I clinical study in hematological malignancies has reported the increased patients survival/enhanced elimina-

*DOI: http://dx.doi.org/10.5772/intechopen.94421*

tion of cancer cells [115].

**7. Conclusions**

oncogene expression.

**Acknowledgements**

**Conflict of interest**

The authors declare no competing interests.

#### *Targeting of Post-Transcriptional Regulation as Treatment Strategy in Acute Leukemia DOI: http://dx.doi.org/10.5772/intechopen.94421*

cells progressing towards AML [93]. Apart from that, usage of HHT has shown the synergistic effect in combinatory therapy (as reviewed in [110]).

Additionally, there are strategies to target ribosomal proteins biosynthesis at the step of transcription by using DNA intercalating agents such as: oxaliplatin, cisplatin or carboplatin [111] or by specific inhibition of ribosomal genes transcription by Polimerase I inhibitor CX-5461 [112]. This inhibitor showed the clinical potential in Myc expressing multiple myeloma [113, 114]. The Phase I clinical study in hematological malignancies has reported the increased patients survival/enhanced elimination of cancer cells [115].

The fact that the presence of ribosomal protein or its modification might be different in cancer versus healthy cells creates opportunity to target cancer cells in more precise way, limiting the damage of healthy tissue. Design of specific small molecule inhibitors or other drugs targeting precisely the deregulated ribosomes would allow effective elimination of leukemia cells 'hiding' quiescently in the niche.

### **7. Conclusions**

*Acute Leukemias*

for leukemia drivers.

Difference in expression of ribosomal proteins between normal and malignant tissues has been found [89] (for review see [90]). It has also been proposed that aberrant expression of ribosomal proteins might support cancer progression. Overexpression of RPL23 in CD34-positive myelodysplastic cells (MDS) had impact on therapy effectiveness and was associated with poor prognosis [91]. Moreover, CD34-positive cells refractory to azacitidine treatment, displayed upregulation of RPL15, RPL28, RPL31 and RPL32 ribosomal proteins [92]. Recent study of MDS revealed that progression to AML is accompanied by elevated expression of some ribosomal proteins in CD123-positive cells [93]. Contrary to this, some ribosomal proteins play a tumor suppressor role in development of leukemia. Loss of RPL11 promoted lymphomagenesis [94], deletion in RPL5 gene has been found in multiple myeloma [95] and mutations in RPL5 and RPL10 contribute to development of T-cell acute lymphoblastic leukemia [96]. Additionally, mutation in RPL10 caused upregulation of phosphoserine phosphatase, which stimulated proliferation of cancer cells [97], deletion of a fragment of chromosome 5 led to myelodysplastic syndrome, which may progress to AML caused by RPS14 haploinsufficiency [98], deletion of RPL22 led to T-cell ALL by inducing a stemness factor [99] and mutation of RPS15 has been discovered to drive chronic lymphoblastic leukemia (CLL) development [100] and to cause cancer relapse [101]. Altogether, these examples clearly show that abnormally expressed ribosomal proteins are strong candidates

It has been shown that ribosomal proteins associated with the ribosome define pool of mRNA transcripts that are selectively translated by this ribosome [102]. There are RPs that facilitate translation upon stress by interacting with IRES or allowing for translation with the use of alternative upstream open reading frames [uORFs]. For instance, RPS5 regulates binding of transcripts with IRES-2 [103] and

Phosphorylation or other modifications of ribosomal proteins might also have impact on the spectrum of translated proteins. However, even if the phosphorylation of RPS6 is well documented, its physiological role remains not clear (for review see [105]). More recently, a phosphorylation of RPL12 has been identified to

Selectivity for mRNA binding by the particular RPs shows that besides being a part of the translational machinery, they might actually play an important regulatory role of this process. Furthermore, identification of numerous proteins that interact with ribosome, so called ribosome associated proteins, has revealed that its activity is shaped by the microenvironment [85]. Changes of mRNA translation upon RPL12 phosphorylation during cell cycle progression [86] represent an example of how cells could use the translational machinery itself, to adapt the proteome to the current demands. This creates possibility for cellular signals to have impact on the translation machinery, allowing adjustment of the profile of synthe-

RPS25 regulates binding of IRES-1 in 40S subunit [104].

facilitate translation of AU-rich mRNAs during mitosis [86].

sized proteins and by that way contributing to leukemia progression.

Targeting of ribosome activity could be a strategy of leukemia treatment. Based on cryo-EM structure, an antibiotic called cycloheximide (CHX) has been designed, that stalls ribosomes on mRNA [106]. However, due to high toxicity level, CHX is mainly used in molecular biology assays nowadays. Homoharringtonine (omacetaxine, HHT) is now the only FDA approved drug to treat CML patients refractory to TKIs [107]. Its mechanism of action is based on prevention of binding of tRNA to the ribosome [108], while at the cellular level this compound reduces the level of anti-apoptotic proteins Bcl2 and MCL-1, thus guiding leukemic cells into the apoptotic pathway [109]. Targeting of monosome translation by HHT has been recently intensively tested. This drug is also examined in terms of AML treatment. Omacetaxine occurred to be highly potent in subpopulation of myelodysplastic

**274**

Translation regulation is a key process, which enables cancer cells to adapt the proteome according to the cellular demands and therefore survive the therapeutic treatment. Moreover, it can contribute to oncogenic transformation, because deranged translation can be a source of enhanced expression of such oncogenes as Myc. This includes modification of activity of certain RNA binding proteins or stimulation of signaling, what leads to increase of global protein synthesis rate. Thus, targeting the translation regulatory mechanisms can be an effective way to eliminate the oncogene-driven malignant cells or just limit cancer cells potential for survival. In other words, therapeutic targeting of post-transcriptional regulation of gene expression gives possibility for both, precise medicine approach as well as blockade of cancer cells proliferation, irrespective of the evolving cancer clones or oncogene expression.

## **Acknowledgements**

This work was supported by research grants from the National Science Centre UMO-2018/30/M/NZ3/00274 (PP-B) and UMO-2016/23/N/NZ3/02232 (MW).

### **Conflict of interest**

The authors declare no competing interests.

*Acute Leukemias*
