A novel fusion protein TBLR1-RARα acts as an oncogene to induce murine promyelocytic leukemia: identification and treatment strategies.

Acute promyelocytic leukemia (APL) is characterized by a specific chromosome translocation involving RARα and its fusion partners. For decades, the advent of all-trans retinoic acid (ATRA) synergized with arsenic trioxide (As2O3) has turned most APL from highly fatal to highly curable. TBLR1-RARα (TR) is the tenth fusion gene of APL identified in our previous study, with its oncogenic role in the pathogenesis of APL not wholly unraveled. In this study, we found the expression of TR in mouse hematopoietic progenitors induces blockade of differentiation with enhanced proliferative capacity in vitro. A novel murine transplantable leukemia model was then established by expressing TR fusion gene in lineage-negative bone marrow mononuclear cells. Characteristics of primary TR mice revealed a rapid onset of aggressive leukemia with bleeding diathesis, which recapitulates human APL more accurately than other models. Despite the in vitro sensitivity to ATRA-induced cell differentiation, neither ATRA monotherapy nor combination with As2O3 confers survival benefit to TR mice, consistent with poor clinical outcome of APL patients with TR fusion gene. Based on histone deacetylation phenotypes implied by bioinformatic analysis, HDAC inhibitors demonstrated significant survival superiority in the survival of TR mice, yielding insights into clinical efficacy against rare types of APL.

RUNX1 inhibits proliferation and induces apoptosis of t(8;21) leukemia cells via KLF4-mediated transactivation of P57.

RUNX1 is a key transcription factor in hematopoiesis and its disruption is one of the most common aberrations in acute myeloid leukemia. RUNX1 alterations affect its DNA binding capacity and transcriptional activities, leading to the deregulation of transcriptional targets, and abnormal proliferation and differentiation of myeloid cells. Identification of RUNX1 target genes and clarification of their biological functions are of great importance in the search for new therapeutic strategies for RUNX1-altered leukemia. In this study, we identified and confirmed that KLF4, a known tumor suppressor gene, as a direct target of RUNX1, was down-regulated in RUNX1-ETO leukemia. RUNX1 bound to KLF4 promoter in chromatin to activate its transcription, while the leukemogenic RUNX1-ETO fusion protein had little effect on this transactivation. KLF4 was also identified as a novel binding partner of RUNX1. RUNX1 interacted with KLF4 through Runt domain and further co-activated its target genes. However, RUNX1-ETO competed with RUNX1 to bind KLF4 through Runt and ETO domains, and abrogated transcription of KLF4. Finally, overexpression experiments indicated that RUNX1 inhibited proliferation and induced apoptosis of t(8;21) leukemia cells via KLF4-mediated upregulation of P57. These data suggest KLF4 dysregulation mediated by RUNX1-ETO enhances proliferation and retards apoptosis, and provides a potential target for therapy of t(8;21) acute myeloid leukemia.

Identification of JL1037 as a novel, specific, reversible lysine-specific demethylase 1 inhibitor that induce apoptosis and autophagy of AML cells.

Lysine-specific demethylase 1 (LSD1) has been recognized as a potential therapeutic target for acute myeloid leukemia (AML). Herein, we identified a novel LSD1 inhibitor, JL1037, via Computer Aided Drug Design technology. JL1037 is a potent, selective and reversible LSD1 inhibitor with IC50s of 0.1 µM and >1.5 µM for LSD1 and monoamine oxidases A/B (MAO-A/B), respectively. Treatment of THP-1 and Kasumi-1 cell lines with JL1037 resulted in dose dependent accumulation of H3K4me1 and H3K4me2, the major substrates of LSD1, as well as inhibition of cell proliferation, blockade of cell cycle and induction of apoptosis. Further investigations demonstrated that JL1037 could upregulate cell cycle-related proteins P21, P57, pro-apoptotic protein Bax and downregulate anti-apoptosis proteins Bcl-2 and Bcl-XL. JL1037 appeared to activate autophage response in AML cell lines as well as primary cells from AML patients by increasing LC3-II expression and the formation of autophagosomes and autolysosomes in cytoplasm. Co-treatment with autophagy inhibitor chloroquine (CQ) enhanced JL1037-induced cell apoptosis. Moreover, daily intravenous administration of JL1037 tended to reduce tumor burden and prolong the survival of t(8;21) leukemia mice. In conclusion, JL1037 exhibited potent anti-leukemia effect and could be a potential therapeutic agent for AML treatment.

FHL2 interacts with iASPP and impacts the biological functions of leukemia cells.

iASPP is an inhibitory member of apoptosis-stimulating proteins of p53 (ASPP) family, which inhibits p53-dependent apoptosis. iASPP was highly expressed in acute leukemia, inhibited leukemia cells apoptosis and promoted leukemogenesis. In order to clarify its mechanism, a yeast two-hybrid screen was performed and FHL2 was identified for the first time as one of the binding proteins of iASPP. FHL2 was highly expressed in K562 and Kasumi-1 cells. FHL2 and iASPP interacted with each other and co-localized in both nucleus and cytoplasm. Either FHL2 or iASPP silenced could reduce cell proliferation, induce cell cycle arrest at G0/G1 phase, and increase cell apoptosis. Western blot analysis showed that the level of p21 and p27 increased, CDK4, E2F1, Cyclin E and anti-apoptotic proteins Bcl-2 and Bcl-xL reduced. Interestingly, when FHL2 was knocked down, the protein expression level of iASPP also decreased. Similarly, the expression of FHL2 would reduce when iASPP was silenced. These results indicated that FHL2 might be a novel potential target for acute myelocytic leukemia treatment.

Rac1 GTPase Promotes Interaction of Hematopoietic Stem/Progenitor Cell with Niche and Participates in Leukemia Initiation and Maintenance in Mouse.

Interaction between hematopoietic stem/progenitor cells (HSPCs) with their niche is critical for HSPC function. The interaction also plays an important role in the multistep process of leukemogenesis. Rac1 GTPase has been found to be highly expressed and activated in leukemia patients. Here, by forced expression of constitutively active form of Rac1 (Rac1-V12) in HSPCs, we demonstrate that active Rac1 promotes interaction of HSPC with niche. We then established an active Rac1 associated acute myeloid leukemia (AML) model by expression of Rac1-V12 cooperated with AML1-ETO9a (AE9a) in mouse HSPCs. Compared with AE9a alone, Rac1-V12 cooperated with AE9a (AER) drives an AML with a short latency, demonstrating that activation of Rac1 GTPase in mice promotes AML development. The mechanism of this AML promotion is by a better homing and lodging of leukemia cells in niche, which further enhancing their colony formation, quiescence and preventing leukemia cells from apoptosis. Further study showed that an inhibitor targeting activated Rac1 can increase the efficacy of chemotherapeutic agents to leukemia cells. This study provides evidence that activation of Rac1 promotes leukemia development through enhancing leukemia cells' homing and retention in niche, and suggests that inhibition of Rac1 GTPase could be an effective way of eliminating AML cells. Stem Cells 2016;34:1730-1741.

[Effect of TBLR1-RARα Fusion Gene on Erythroid Differentiation of K562 Cells].

OBJECTIVE: To explore the effects of TBLR1-RARα on the differentiation induction of leukemia cell line K562 cells into erythroid lineage and to investigate its related mechanisms. METHODS: Tet-Off inducible system was used to construct the conditional expression vector of TBLR1-RARα fusion gene by cloning the TBLR1-RARα fragment into lentivirus vector pLVX-Tight-Puro, the expression of TBLR1-RARα fusion gene was induced by doxycycline (Dox). Then, K562 cells were transfected with lentivirus pLVX-Tight-Puro-TBLR1-RARα-flag, and the expression of fusion proteins was verified by Western blot. After treatment of K562 with all-trans retinoid acid (ATRA), real time RT-PCR was performed to test the expression of erythroid differentiation-related CD71 and α, ε, γ-globins gene. Flow cytometry was used also to analyze the expression of erythroid differentiation markers CD71 and CD235a. Benzidine staining was used to detect the production of hemoglobin in K562 cells. RESULTS: qRT-RCR showed that ATRA could increase the expression level of CD71 and α, ε, γ-globin genes when TBLR1-RARα was expressed. After treatment of ATRA, the proportion of CD71(+) cells detected by the flow cytometry also increased. Benzidine staining showed that ATRA could induce hemoglobin production in K562 cells with TBLR1-RARα fusion gene expression. CONCLUSION: The expression of TBLR1-RARα fusion gene contribute to ATRA-inducing differentiation of K562 cells into erythroid lineage.

Up-regulated A20 promotes proliferation, regulates cell cycle progression and induces chemotherapy resistance of acute lymphoblastic leukemia cells.

A20, also known as tumor necrosis factor-α (TNFα)-induced protein 3 (TNFAIP3), has been identified as a key regulator of cell survival in many solid tumors. However, little is known about the protein expression level and function of A20 in acute lymphoblastic leukemia (ALL). In this study, we found that A20 is up-regulated in ALL patients and several cell lines. Knockdown of A20 in Jurkat, Nalm-6, and Reh cells resulted in reduced cell proliferation, which was associated with cell cycle arrest. Phospho-ERK (p-ERK) was also down-regulated, while p53 and p21 were up-regulated in A20 knockdown cells. In addition, A20 knockdown induced apoptosis in Jurkat and Reh cells and enhanced the sensitivity of these cell lines to chemotherapeutic drugs. These results indicate that A20 may stimulate cell proliferation by regulating cell cycle progression. A20 inhibited apoptosis in some types of ALL cells, thereby enhancing their resistance to chemotherapy. This effect was abolished through A20 silencing. These findings suggest that A20 may contribute to the pathogenesis of ALL and that it may be used as a new therapeutic target for ALL treatment.

A novel SAHA-bendamustine hybrid induces apoptosis of leukemia cells.

Hybrid anticancer drugs are of great therapeutic interests as they can potentially overcome the deficiencies of conventional chemotherapy drugs and improve the efficacy. Many studies have revealed that the combination of histone deacetylase inhibitors (HDACi) and alkylating agents have synergistic effects. We reported a novel hybrid NL-101, in which the side chain of bendamustine was replaced with the hydroxamic acid of HDACi vorinostat (SAHA). NL-101 exhibited efficient anti-proliferative activity on myeloid leukemia cells especially Kasumi-1 and NB4 cells, accompanied by S phase arrest and caspase-3 dependent apoptosis. Importantly, it presented both the properties of HDAC inhibition and DNA damaging, as assessed by the acetylation of histone H3 and DNA double-strand breaks marker γ-H2AX. NL-101 also down-regulated the expression of anti-apoptotic protein Bcl-xL which was involved in the mitochondrial death pathway. Meanwhile, NL-101 induced apoptosis and DNA damage in primary cells from acute myeloid leukemia (AML) patients. NL-101 treatment could significantly prolong the survival time of t(8;21) leukemia mice with enhanced efficacy than bendamustine. These data demonstrate that NL-101 could be a potent and selective agent for leukemia treatment.

[Silence potentiates chemosensitivity of K562 cells to SAHA].

Ribosomal protein S27a (RPS27a) can perform extra-ribosomal functions besides imparting a role in ribosome biogenesis and post-translational modifications of proteins. The RPS27a gene has been reported to be over-expressed in breast fibroadenomas, colorectal and renal cancers, advanced-phase chronic myeloid leukemia (CML) and acute leukemia (AL) patients. This study was purposed to explore the function of RPS27a in CML-erythroleukemia cell line K562 cells. RPS27a was silenced by short hairpin RNA (shRNA) in K562 cells. Furthermore, the proliferation changes of K562 cells was detected by MTT method after silencing the RPS27a with suberoylanilide hydroxamic acid (SAHA), then the IC50 of K562-sh1/sh2 and K562-scr cells to SAHA was measured. The results indicated that compared with K562-scr cells, the IC50 of K562-sh1/sh2 to SAHA at 24 h and 48 h decreased (P < 0.01); RPS27a silence significantly increased the percentage of apoptotic K562-sh1/sh2 cells after incubation with 1 µmol/L, 2 µmol/L and 5 µmol/L SAHA for 24 h and 48 h as compared with that of K562-scr cells (P < 0.01). K562-sh1, K562-sh2 and K562-scr cells after incubation with or without 2 µmol/L SAHA for 48 h presented apoptosis features: i. e. chromatin condensation, nucleic fragmentation and apoptotic body formation. It is concluded that RPS27a can inhibit the apoptosis of K562 cells and RPS27a silence can potentiate sensitivity of K562 cells to SAHA.

TBLR1 fuses to retinoid acid receptor α in a variant t(3;17)(q26;q21) translocation of acute promyelocytic leukemia.

The majority of acute promyelocytic leukemia (APL) cases are characterized by the PML-RARα fusion gene. Although the PML-RARα fusion gene can be detected in >98% of APL cases, RARα is also found to be fused with other partner genes, which are also related to all-trans retinoic acid (ATRA)-dependent transcriptional activity and cell differentiation. In this study, we identified a novel RARα fusion gene, TBLR1-RARα (GenBank KF589333), in a rare case of APL with a t(3;17)(q26;q21),t(7;17)(q11.2;q21) complex chromosomal rearrangement. To our knowledge, TBLR1-RARα is the 10th RARα chimeric gene that has been reported up to now. TBLR1-RARα contained the B-F domains of RARα and exhibited a distinct subcellular localization. It could form homodimers and also heterodimers with retinoid X receptor α. As a result, TBLR1-RARα exhibited diminished transcriptional activity by recruitment of more transcriptional corepressors compared with RARα. In the presence of pharmacologic doses of ATRA, TBLR1-RARα could be degraded, and its homodimerization was abrogated. Moreover, when treated with ATRA, TBLR1-RARα could mediate the dissociation and degradation of transcriptional corepressors, consequent transactivation of RARα target genes, and cell differentiation induction in a dose- and time-dependent manner.