Background
Chronic myeloid leukemia (CML) was identified as a neoplastic disease of hematopoietic stem cells, arising from a fusion event termed
BCR-ABL [
1]. The oncogenic fusion protein BCR-ABL with aberrant tyrosine kinase function is encoded by the
BCR-ABL [
2], leading to constitutively activation of various substrates including CRKL, PI3K/AKT, and MAPK signaling pathways, resulting in malignant cell proliferation [
3‐
5]. Tyrosine kinase inhibitor (TKI) imatinib mesylate (IM), which has the benefit of significantly enhancing patient prognoses, has been used extensively in the clinical management of CML individuals [
6]. However, individuals receiving long-term IM treatment risk developing resistance since BCR-ABL (the root cause of CML) cannot be entirely eradicated. Imatinib resistant CML patients had a significantly increased risk of relapse after discontinuation [
7,
8]. The pathogenesis of CML is well understood, however, the underlying molecular mechanisms during disease development and TKI resistance are required to be clarified, thus providing novel insights for diagnosis and therapy in CML.
In the last few decades, the underlying mechanisms of CML progression and TKI resistance predominantly focused on messenger RNAs (mRNAs) that encode specific proteins [
9,
10] or long non-coding RNAs (lncRNAs) [
11,
12] or micro RNAs (miRNAs). miR-185, for example, has been uncovered to function as a repressor of PAK6, mediating Imatinib resistance in TKIs resistant leukemic cells, indicating that miR-185 might be a novel target for treating TKIs resistant CML patients [
13]. As a result of the rapid advancement of high-throughput sequencing, circular RNAs (circRNAs) have come into view. As a unique kind of regulating non-coding RNAs [
14], circRNA lacks the classic cap and poly-A tail structure retained by linear RNAs [
15]. A stable circular form renders circRNA more suitable as a molecular target [
16]. Accumulating evidence elucidated that circRNAs were extensively engaged in regulating the development of many malignancies, including liver cancer [
17], lung cancer [
18], and leukemia [
19,
20]. Diverse functions of circRNAs are rested with different subcellular localization, including sponging miRNA, acting as a scaffold to decoy RNA binding proteins (RBPs) and translated peptides in the cytoplasm, while mediating transcription of parental genes in the nucleus [
14,
21‐
23].
Amounts of evidence illustrated circRNAs derived from tumor-related parental genes can regulate the onset and progression of malignancies. For example, circMYBL2, produced from
MYBL2, which was reported to be linked to leukemia, could promote the advancement of
FLT3-ITD-mutant acute myeloid leukemia [
20]. In addition, by stimulating the Wnt signal pathway, circ β-catenin, which was generated from
CTNNB1, translated a novel β-catenin isoform and accelerated hepatocellular carcinoma cell growth [
24]. In CML, the roles of circRNA are yet obscure. It was suggested that fusion-circRNA (f-circRNA) derived from
BCR-ABL participated in regulating the progression of CML [
25,
26]. However, few pieces of evidence uncovered that the roles of circRNAs originated from other known genes in
BCR-ABL cascades, such as
CRKL.
CRKL also termed
CRK-like gene, produces an oncogenic protein CRKL that has been pinpointed as a predominant element of tyrosine phosphorylation [
27]. It has been suggested that CRKL complexes directly with BCR-ABL via the SH3 domain, triggering signaling cascades such as PI3K-AKT pathway that contribute to leukemic transformation [
28]. Besides, CRKL lacking the SH2 domain reduced c-MYC expression level in imatinib-resistant cells harboring BCR-ABL T315I mutation [
29]. Recent studies showed that hsa_circ_0001206, produced by the second exon of
CRKL pre-mRNA, was a tumor suppressor and restrained cell growth by regulating KLF5 through mediating miR-141 in prostate cancer [
30]. Besides, several lines of evidence suggested that circCRKL blocked AML progression through targeting miR-196-5p [
31].
In this work, we discovered that circCRKL, a circRNA that originated from the second exon of CRKL, is significantly highly expressed and accelerates the BCR-ABL+ cells proliferation including imatinib-resistant cell line K562/G01. Our data further confirmed that circCRKL modulates BCR-ABL via decoying miR-877-5p to hasten cell proliferation. Therefore, our findings suggested that circCRKL impels CML progression by acting an oncogenic role, and targeting circCRKL might represent a novel therapeutic option for CML patients.
Materials and methods
Cell culture and management
Human chronic myeloid leukemia cell lines (K562, K562/G01, and KCL22), acute monocytic leukemia cell line (THP-1), human lymphoblastic cell line (TK6), and 293 T were all maintained in Roswell Park Memorial Institut-1640 (RPMI-1640, Gibco, USA) or Dulbecco's modified eagle medium (DMEM, Gibco, USA) with 10% fetal bovine serum (FBS, Gibco, USA) respectively. All cells were cultivated at 37 °C in a suitable atmosphere containing 5% CO2. The Chinese Academy of Science's Cell Bank provided all of the cell lines utilized in this work.
The bone marrow samples of newly diagnosed CML patients and normal donors were obtained from the Department of Hematology, the Second Affiliated Hospital of Chongqing Medical University. The information was illustrated in Additional file
3: Table S1. The Human bone marrow mononuclear cells (BMMCs) separation Kit (TBD, China) was used to isolate BMMCs from the samples. Before the trial began, all patients signed informed permission forms. The ethics committee of Chongqing Medical University gave its approval to this investigation.
RNA extraction and RT-qPCR
TRizol reagent (Accurate, China) was used to extract total RNA from patient samples and cultured cells. To reverse transcribe circRNA and messenger RNA (mRNA) into complementary DNA (cDNA), a reverse transcription kit (Accurate, China) was employed. The stem-loop primer was used to reverse transcribe miR-877-5p. The quantitative PCR was carried out through SYBR Green Premix Pro Taq HS qPCR Kit (Accurate, China) following the product’s instructions.
Actin or
U6 was used as RT-qPCR internal parameters. The 2
−ΔΔCT method was utilized to examine the relative expression of RNA. All specific primers sequences used in this investigation were listed in Additional file
3: Table S2.
RNase R resistance and Actinomycin D analysis of circCRKL
The 1 μg of total RNA extraction of K562, and K562/G01 cells were digested at 37 °C for 10 min after adding 2U/mg of RNase R (Geneseed, China), then the mixture was hatched at 70 °C for another 10 min to inactive the RNase R. Subsequently, the relative RNA enrichment was detected through RT-qPCR.
The 5 μg/mL actinomycin D was mixed with K562 and K562/G01 cells and reacted for 4, 8, and 12 h respectively, the total RNA was then extracted. The relative RNA expression was then identified using RT-qPCR.
Fluorescence in situ hybridization (FISH)
The specific circCRKL probe was labeled with cy3 and synthesized by RiboBio (Guangzhou, China). The assay was conducted in accordance with the manufacturer’s orders. In brief, K562 and K562/G01 cells were washed and fixed with 4% paraformaldehyde at room temperature (RT) for 10 min before being permeabilized with 0.5% Triton X-100 at 4 °C for 5 min. Then circCRKL probe was hybridized with cells in a dark moist chamber at 37 °C overnight. To stain the nuclei, 4′,6-diamidino-2-phenylindole (DAPI) was used. A confocal microscope was utilized to acquire photos.
Infection of lentivirus
For circCRKL down-regulation, small interfering oligos targeting circCRKL (the sequences are illustrated in Additional file
3: Table S3) were designed and constructed into lentivirus (GeneChem, China), which were subsequently infected into BCR-ABL
+ and THP-1 cells. Stable transfection was established through culturing cells in RPMI-1640 medium supplemented with 2 μg/ml puromycin (Solarbio, China) for 7 days, and maintained with 1 µg/ml of that for another 7 days.
CCK-8 assay
The cell viability was determined using a CCK-8 kit (TargetMol, China). Cells were seeded at the number of 3000 per well in a 96-well plate and cultured in an incubator for 24 h before receiving treatment of 10 μl CCK-8 reagent for 3 h. Subsequently, the absorbance was assessed at 450 nm. Three duplicates were set in each group.
Cells were seeded at a density of 100 per well into a 96-well plate. After being stilling cultured using RMPI-1640 with 10% FBS for 7 days, photos were taken and the number of colonies was counted. At least three duplicates were set in each group.
Flow cytometry assay
For cell cycle assay, at least 1 × 106 infected K562 and K562/G01 cells were immobilized through pre-cooled 70% ethanol at 4 °C overnight. After that, each sample was left in a lucifugal chamber at RT for 30 min after receiving 500 μl of propidium iodide (PI) staining reagent. Then cell cycle was detected and analyzed through a flow cytometer (CytoFlex, Beckman coulter).
For cell apoptosis assay, at least 5 × 105 infected K562 and K562/G01 cells were collected and washed, then cells were suspended with 100 μl pre-cooled PBS and treated with 10 μl Annexin V APC and 5 μl DAPI in the dark at RT for 15 min before being determined with a flow cytometer (CytoFlex, Beckman coulter).
RNA Immunoprecipitation (RIP)
K562/G01 cells were used to conducted RIP assay with the BersinBio™ RNA-Binding Protein Immunoprecipitation Kit (BersinBio, China) in accordance with the manufacturer’s protocols. In a nutshell, at least 2 × 107 cells were harvested, washed, and lysed using 900 μl RIP lysis buffer, then cell lysates were added with 4 μg of anti-AGO2 (Abcam, USA) or the equal amounts of anti-IgG (BersinBio) and incubated at 4 °C overnight. The mixture was subsequently added with 40 μl protein A/G magnetic beads and incubated at 4 °C for 2 h. Finally, the RNAs were extracted and purified using phenol–chloroform and the relative abundance of miR-877-5p and circCRKL was examined through RT-qPCR.
Western blot analysis
To lyse cells, a radio immunoprecipitation assay (RIPA) lysing solution including phosphatase and protease inhibitor was employed, and total protein was then extracted and quantified through a bicinchoninic acid (BCA) protein extraction assay kit (Beyotime, China). For western blot assay, 40 μg of protein samples were loaded and isolated with 8% sodium dodecyl sulfate–polyacrylamide gel electrophoresis (SDS-PAGE). Subsequently, blocking the polyvinylidene fluoride (PVDF) membranes containing protein blots with 5% nonfat milk powder at RT for 2 h before incubating the primary antibody (1:1000) at 4 °C overnight. After that, the membranes were incubated with relevant secondary antibodies (1:5000) at RT for 90 min. Finally, the Bio-Rad Gel Imaging System was used to visualize and analyze the signals.
The wild-type (WT) reporters of circCRKL and ABL containing binding sites with miR-877-5p or mutant (MUT) ones were cloned into the pmiR-GLO luciferase vector (Genecreate, China). The vectors along with miR-NC or miR-877-5p mimic (RiboBio, China) were then transfected into 293 T cells. The Dual-luciferase Reporter Assay System (Promega, USA) was employed to evaluate the luciferase activities after culturing for 2 days.
Biotinylated RNA pulldown assay
The specific probe targeting the back-splicing site of circCRKL and NC oligo probe was coupled by biotin and purchased from RiboBio (Guangzhou, China). The probe sequences were demonstrated in Additional file
3: Table S4. Briefly, approximately 4 × 10
7 K562/G01 cells were harvested and fixed using 1% formaldehyde, then the cells were lysed, sonicated, and centrifuged at 16,000
g at 4 °C for 15 min. The biotin-coupled circCRKL probe or NC oligo probe was incubated with the supernatant obtained from lysates at RT for 4 h. After that, streptavidin-coated magnetic beads (MCE, USA) were utilized to pull down the biotin-labeled RNA mixture. Finally, with the elution of the beads, the RNA was obtained using TRizol, and the relative abundance of circCRKL and miR-877-5p was evaluated with RT-qPCR.
Xenograft leukemogenesis model
Female NOD/SCID mice aged 5 to 6 weeks were irradiated with an X-ray at a dose of 250 cGy before injection. K562/G01 cells were collected and washed with sterile PBS at a concentration of 2.5 × 107/ml after being infected with sh-NC or sh-circCRKL lentivirus. The mice were then split into two groups at random and intravenously injected with 200 μl cells, respectively. The body weights of each mouse were monitored weekly. The number of white blood cells in the peripheral blood was measured three weeks after injection. The bone marrow cells, livers, and spleens were harvested while mice were closely culled at an ethical endpoint. The animal experiments were given approval by the Ethics Committee of Chongqing Medical University.
Statistical analysis
GraphPad Prism 8 (GraphPad, USA) was used for analyzing data in this research. The student’s t-test was applied for calculating the main effect in two groups and One-Way ANOVA was used for that in three and more. p-values < 0.05 (*), p < 0.01 (**), p < 0.001 (***) and p < 0.0001 (****) were regarded as statistically significant.
Discussion
BCR-ABL is a fusion gene that leads to the occurrence and progression of CML [
2,
36]. Despite that tyrosine kinase inhibitors such as imatinib targeting BCR-ABL oncoprotein have ameliorated survival effectively [
6], the TKI-resistance and uncontrolled progression prevent part of CML patients from curative benefit [
8,
37]. Therefore, novel treatment strategies are supposed to be provided for optimizing the outcomes of CML patients.
During the past decade, the evolution of high-throughput sequencing has brought circRNAs to the forefront. circRNAs have been widely developed for therapeutic targets in diverse diseases due to their stable circular structure. An artificial circRNA scRNA21 was applied to sponge oncogenic miR-21 to restrain gastric cancer cell proliferation [
38]. More recently, circRNA has been developed into vaccines for the treatment of SARS-CoV-2 [
39]. Accumulating evidence has proved that circRNAs generated by cancer-associated parental genes play a vital role in those cancers [
40]. For example, it has conclusively been shown that circMYBL2, derived from AML-related gene
MYBL2, exerts a cancerogenic role in leukemia harboring
FLT3-ITD mutation [
20]. However, few species of investigations on the function of such circRNAs in CML. In this current research, we firstly reported that circCRKL, back spliced from the second exon of pre-
CRKL, was highly expressed in BMMCs of CML patients and BCR-ABL
+ cell lines. Previous studies revealed that circCRKL was downregulated and mitigated carcinogenesis in prostate cancer and acute myeloid leukemia [
29,
30]. The reason circCRKL differential expressed in CML and other cancers could be explained is that circRNAs are cancer-specific and cell type-specific [
41]. We hypothesized that circCRKL can function in CML in concert with its parental gene
CRKL. As expected, further loss of function assays showed that knockdown of circCRKL dampened CML cells proliferation in cell lines and mouse models, suggesting circCRKL exerts an oncogenic role in CML.
circRNAs located in the cytoplasm can absorb specific miRNAs to control the related target genes, and to mediated tumorigenesis and progression. For example, circCDYL sponges miR-145-5p to augment tumor-suppressor TJP1 in Wilm’s tumor [
42]. circRNF220 was identified as a key upstream regulator of miR-30a, leading to elevated expression of MYSM1 and IER2 in AML cells [
43]. In this study, we noted that circCRKL is predominantly expressed in the cytoplasm of CML cell lines. Intriguingly, we found that knockdown of circCRKL markedly diminished BCR-ABL in both mRNA and protein levels. Given that circCRKL could function as a miRNAs sponge [
21], the hypothesis was established that circCRKL regulates BCR-ABL expression level via specific miRNA. Hence, several bioinformatics databases were utilized to predict the target miRNAs of circCRKL and BCR-ABL simultaneously. Then we found a miRNA, miR-877-5p, containing binding sites of both circCRKL and ABL.
Several studies uncovered that miR-877-5p serves as a suppressor in a diversity of cancers. Yu et al. discovered that circ_0061395 promoted HCC advancement by sponging miR-877-5p and increasing PIK3R3 expression [
44]. Luo et al. uncovered that miR-877-5p restrains PDAC progression via negatively regulating lncRNA NR2F1-AS1 [
45]. Additionally, researchers have confirmed miR-877-5p as a positive element in other malignancies [
46,
47]. Herein, we firstly reported the involvement of miR-877-5p in CML. The RNA pull-down and dual-luciferase reporter assays illustrate that circCRKL exerts a sponge effect on miR-877-5p. Furthermore, we discovered that in CML, miR-877-5p was downregulated, and miR-877-5p silencing could offset circCRKL knockdown-mediated the proliferation restrain in CML cells. More than that, deep investigations demonstrated that miR-877-5p silencing elevated BCR-ABL expression level, and reverse results were induced by ectopic expressing miR-877-5p. Additionally, rescue experiments demonstrated that inhibition of miR-877-5p ameliorates the reduced BCR-ABL caused by knockdown of circCRKL, which validated the hypothesis we raised. Collectively, we revealed that circCRKL could sponge miR-877-5p to regulate BCR-ABL expression level to maintain the malignancy of CML cells. However, the present research is predominantly at the stage of cell and animal trials, additional studies were required to determine whether the target can be applied to CML individuals.
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