Abstract and Introduction
Background: Based on the results of multiple studies, multiple signaling pathways is a major cause of resistence to chemotherapy in leukemia cells. Signal transducer and activator of transcription 5 (STAT5) is among these factors; it plays an essential role in proliferation of leukemic cells.
Methods: We obtained the materials used in our study via PubMed search from 1996 through 2019. The key search terms included "STAT5," "acute leukemia," "leukemogenesis," and "mutation."
Results: On activation, STAT5 not only inhibits apoptosis of leukemic cells via activating the B-cell lymphoma 2 (BCL-2) gene but also inhibits resistance to chemotherapy by enhancing human telomerase reverse transcriptase (hTERT) expression and maintaining telomere length in cells. It has also been shown that a number of mutations in the STAT5 gene and in related genes alter the expression of STAT5.
Conclusion: The identification of STAT5 and the factors activated in its up- or downstream expression, affecting its function, contribute to better treatments such as targeted therapy rather than chemotherapy, improving the quality of life patients.
T-cell acute lymphoblastic leukemia (T-ALL) is a progressive and invasive malignant neoplasm that occurs due to impaired function of oncogenes. Although new therapeutic strategies have increased patient survival, there is still resistance to treatment that results in the death of some patients. Lack of identification of molecular pathways involved in the proliferation and survival of T-ALL cells is a main factor in lack of response to treatment in some patients. Therefore, the detection of molecular pathways is necessary for targeted therapy to increase the susceptibility of T-ALL cells to chemotherapy.
In normal conditions, interleukin (IL)–7 is a main mediator in the proliferation and differentiation of T-cell precursors, which activates the Janus kinase/signal transducer and activator of transcription (JAK/STAT) signaling pathway. The signal transducer and activator of transcription 5 (STAT5) is a downstream molecule of IL-7 that increases lymphopoiesis in the JAK/STAT signaling pathway. The results of various investigations, such as Abraham et al, have indicated that although a number of STAT5 effects in T-cell differentiation (including activation or inactivation of some oncogenes) are changed during the formation of malignant neoplasms (such as T-ALL), other functions, such as increasing differentiation and decreasing apoptosis of T-ALL cells, are not altered These factors lead to the resistance of malignant cells to chemotherapy because of increased proliferation and insensitivity to apoptosis. Therefore, in this study, we evaluated the changes in STAT5-related signaling pathways and identified pathways that are prone to targeted therapy, to increase the survival of patients with T-ALL.
Basic Function of STAT5
STAT5 is a transcription factor with 2 isoforms: a and b. Research findings[7,8] have shown that STAT5 is activated by a variety of cytokines, including IL-2 and IL-7, during the interaction of these cytokines with their receptors, which leads to the activation of JAK/STAT pathway, causing STAT5 phosphorylation and activation. STAT5 is implicated in the differentiation of specific subtypes of T-cells, causing differentiation and increased survival of TCD8+ cells by activating Runt-related transcription factor (RUNX3), which is a major factor in this process.[9,10] The results of another study indicated that STAT5 also plays a role in the differentiation of regulatory T-Cells (T-regs) through forkhead box P3 (FOXP3). STAT5 is involved in the differentiation of B lymphocytes through its effect on the differentiation of common lymphocyte progenitor (CLP) into pro-B and pro-B differentiation into pre-B, causing rearrangement of heavy chain genes in late stages of B-lymphocyte differentiation.[13,14]
STAT5-related Signaling Pathways in Survival or Apoptosis of Leukemic Cells
Approximately 5% and 20% to 30% of ALL cases involve the presence of the Philadelphia chromosome (Ph-positive ALL) in children and adults, respectively. Moreover, patients with chronic myeloid leukemia (CML) have tested Ph positive. Therefore, the molecular mechanisms and therapeutic strategies of Ph-positive ALL and CML are expected to be somewhat similar.[15,16] However, research results have shown that Ph-positive ALL has higher proliferative and invasive capacity with a poor prognosis. The expression of the Src (SRC proto-oncogene, nonreceptor tyrosine kinase) family of tyrosine kinases (including Lyn [LYN proto-oncogene], an Src-family tyrosine kinase) is a factor that increases the invasiveness of Ph-positive ALL cells. Lyn is activated by breakpoint cluster region protein–Abelson murine leukemia viral oncogene homolog 1 (BCR-ABL1), which is a fusion protein formed by the Ph chromosome. Consequently, Lyn triggers the phosphorylation of its downstream molecule STAT5, enhancing the survival of Ph-positive ALL by increasing proliferation and expression of the antiapoptotic B-cell lymphoma 2 (BCL-2) molecule.[18,19] Lyn also increases the expression of BCL-2 by activating the AKT/mTOR (mammalian target of rapamycin) and Raf (proto-oncogene serine/threonine-protein kinase)/MEK (mitogen-activated protein kinase) pathways. Contrary to expectations, the results of recent studies, such as the study published in 2018 by Guo et al, have shown that curcumin (an herbal medicine used as a clinical trial drug in animal models for leukemia treatment) increases the autophagy and apoptosis of Ph-positive ALL cells by activating the Raf/MEK pathway and that AKT/mTOR is located upstream of the Raf/MEK pathway.
Research results have indicated that when a cell is exposed to stress conditions, such as deficiency of the metabolic substrates, activation of AKT/mTOR pathway leads to the induction of autophagy and the death of cells. However, studies of the AKT/mTOR pathway in leukemia cells have revealed that cell proliferation can be prevented by targeting this pathway. Thus, it can be concluded that the identification of up- and downstream factors of AKT/mTOR pathway in cells experiencing autophagy due to the lack of metabolic substrates could present a proper strategy to induce AKT/mTOR mediated autophagy in leukemia cells.
In addition to the activation of Lyn and the JAK/STAT pathway, BCR-ABL1 activates the PI3K/AKT/mTOR pathway through a critical member of the aforementioned pathway named GAB2 (Grb-associated binder-2), a signal transducer of BCR-ABL1. AKT is phosphorylated when PI3K (phosphatidylinositol-4,5-bisphosphate 3-kinase) is stimulated, which leads to the activation of mTORC1 (mammalian target of rapamycin complex 1), after which mTORC1 phosphorylates its substrates S6 kinase (S6K, a ribosomal protein) and 4E–binding protein–1 (4E-BP-1, a eukaryotic initiation factor) that increase the expression of mouse double minute 2 homolog (MDM2). Finally, MDM2 enhances the cell cycle and prevents apoptosis by suppressing P53 expression (Figure 1).
The proliferation of ALL cells becomes uncontrollable when STAT5 forms a complex with JAK3. BLNK (B-cell linker), a signal transducer molecule that is present during differentiation and activation of B-cells, prevents the formation of STAT/JAK3 complex and suppresses ALL proliferation, causing cell-cycle arrest through P21 expression. Nevertheless, because BLNK level is reduced in ALL, the induction of its expression could decrease the survival of malignant cells. Research[25,26] has revealed that BLNK promotes the expression of other genes, including C-myc (MYC proto-oncogene), in addition to P21. The Notch signaling pathway in T-ALL leads to the proliferation of malignant cells due to the expression of a series of genes such as hairy/enhancer of split (HES1). However, HES-1, together with poly adenosine 5′-diphosphate (ADP) ribose polymerase 1 (PARP1) in B-ALL, leads to the apoptosis of cancer cells. Also, the expression of cluster of differentiation (CD)59 on the surface of T-ALL cells activates the Notch/STAT5 pathway and can prevent the apoptosis of leukemia cells. In addition, because Notch signaling in T-ALL increases the C-myc level, it can be said that the investigation of pathways and molecules that induce apoptosis through C-myc and PARP1 mediation in B-ALL might induce apoptosis of T-ALL cells by targeted therapy.
STAT5 Effect on Acute Myeloid Leukemia (AML) Prognosis
AML is an invasive hematologic malignant neoplasm characterized by the accumulation of blasts in blood and bone marrow, which is treated in many cases by chemotherapy and bone-marrow transplantation. Mutation in FMS-like tyrosine kinase 3 (FLT3) is a common cause of AML. The activation of FLT-3 has been shown to activate its downstream factors, including STAT5, AKT, and MAPK/ERK (extracellular regulated mitogen-activated protein kinase), which eventually increases the expression of MCL-1 (MCL1 apoptosis regulator) antiapoptotic factor.
Although the application of FLT-3 inhibitors can improve patient conditions to some extent, it is known that the secretion of cytokines such as IL-3 and GM-CSF (granulocyte-macrophage colony-stimulating factor) activates the JAK2/STAT pathway, which increases the survival of leukemia cells. Also, the findings of another study were that IL-3 and GM-CSF cytokines boost the survival of leukemia cells by increasing the expression of AXL (AXL receptor tyrosine kinase). Finally, the use of FLT-3 inhibitors, along with inhibition of STAT5, can increase the ability of patients to recover by reducing the survival of leukemia cells.
Investigation results have also shown that increasing activation of HOXA9 leads to resistance to antibody treatment against STAT. The findings of other studies, such as Savino et al, have also indicated that the transduction of IL-7 signals leads to resistance of patients to treatment by targeting STAT.
Association Between STAT5 and Telomerase Complex
Prevention of telomere-length reduction is one of the most important factors in cell survival. Telomerase is a complex of 2 subunits: hTERT and human telomerase RNA template (hTR). The hTERT subunit has a catalytic role, and its expression leads to the maintenance of telomere length and increasing cell survival.[36,37] The results of a recent investigation have indicated that the expression of hTERT in leukemia cells enhances the resistance of those cells to chemotherapy, increasing the relapse rate of patients. However, it has been shown that different signaling pathways play a role in hTERT expression. The JAK2/STAT pathway, along with PI3K/AKT/mTOR, increases the expression of hTERT via augmentation of the expression of STAT5 (Table 1). HSP90 (heat shock protein 90), which prevents false folding of proteins, has been shown to increase hTERT expression along with the JAK2/STAT and PI3K/AKT/mTOR pathways. Hence, because of increasing expression of STAT5 in Ph-positive ALL cells and because recent investigations have revealed that the expression of HSP90 (a protein involved in the function of certain proto-oncogenes, such as BCR-ABL1 and AKT) in these patients is associated with increasing resistance to chemotherapy and survival of ALL cells (Table 1), STAT5 targeting could prevent hTERT expression and thereby render Ph-positive ALL cells susceptible to chemotherapy due to the reduction of length in telomere of Ph-positive ALL cells.
The results of certain studies, such as a study in 2011 by Yamada et al, have stated that in addition to hTERT expression, the expression of the multiple drug resistance 1 (MDR1) gene increases resistance to chemotherapy. MDR1 inhibits the toxicity of chemotherapy drugs on leukemia cells due to the expression of P-glycoprotein (P-gp), which pumps drugs outside the cell by adenosine triphosphate (ATP) consumption. In contrast, activation of the Notch pathway leads to the expression of c-myc in Ph-positive ALL, which in turn activates cellular pathways such as STAT5 and increases hTERT and P-gp expressions.[39,43] It has also been shown[44,45] that anticancer drugs activate MAPK (mitogen-activated protein kinase) in Ph-positive ALL via increasing generation of reactive oxygen species (ROS). Considering that this pathway activates c-myc, it can be stated that the use of anticancer drugs, in combination with hTERT and P-gp inhibitors, could sensitize cells to chemotherapy and prevent disease relapse.
Effect of Gene Mutations on STAT5
Several genetic changes, such as chromosomal translocations and mutations, play a role in ALL development. A large number of genes undergoing these genetic changes are involved in the development of lymphocytes, cell cycle, and apoptosis, and are proto-oncogenes in cellular processes (Table 2).[52,53] However, disruption and mutation in some genes that encode tyrosine kinases (eg, LNK) and controlling of the cell cycle in a normal state cause uncontrolled proliferation of lymphocytic precursors in ALL and lead to certain clinical symptoms.
In normal conditions, LNK (SH2B3) is an inhibitor of cell proliferation by downregulating cytokine signaling, which also acts in the development of B cells and is a self-renewal inhibitor of hematopoietic stem cells (HSCs).[54,55] Nevertheless, a loss-of-function mutation in LNK leads to an increase in the proliferation of Ph-positive ALL cells and their resistance to chemotherapy. Impaired LNK function also activates the JAK2/STAT pathway and STAT5 downstream of the IL-7 receptor, which inhibits apoptosis of Ph-positive ALL cells. The results of another study have shown that activation of the Notch pathway leads to an increase of inflammatory responses due to the differentiation of immune cells and inhibition of the JAK2/STAT pathway. It has also been noted that mutations occurring in the Notch pathway factors that increase the activity of the Notch pathway are associated with good prognosis in ALL. However, it can be stated that Notch induces differentiation of immune cells and inhibits the JAK2/STAT pathway, increasing the apoptosis of Ph-positive ALL cells.
We were surprised to read contradictory results in another study, namely, that STAT5 is located downstream of the Notch pathway, so that any increase in Notch can activate STAT5 and also enhances the activation of the Notch pathway as positive feedback. Finally, according to the aforementioned contradictory Notch results, it might not be an appropriate target for targeted therapy. Although Notch mutation and enhanced function lead to apoptosis in ALL cells, it increases proliferation and survival of ALL cells by activating STAT5. Hence, we hypothesize that in patients having mutations in Notch pathway factors, we must first evaluate the expression of STAT5, to augment the apoptosis of ALL cells by targeting STAT5 in a therapeutic approach as needed.
Lab Med. 2020;51(4):345-351. © 2020 American Society for Clinical Pathology