Background
Hypoxia is one of the most common features of solid tumors, which is a driving force for cancer metastasis and is generally caused by an imbalance between rapid proliferation and insufficient angiogenesis [
1‐
3]. In response to hypoxic stress, the transcription factor hypoxia-inducible factor-1α (HIF-1α) is stabilized to transcribe multiple genes involving in cancer cell proliferation, stemness, energy metabolism, metastasis, and drug resistance [
4‐
6]. Mounting studies have revealed the existence of hypoxic fractions in colorectal cancer (CRC) [
7‐
10], the second leading cause of cancer death worldwide [
11]. Indeed, hypoxia and the expression of HIFs are closely associated with increased drug resistance and distant metastasis, resulting in poor survival of CRC patients [
1]. However, although substantive evidence has supported the regulatory mechanism of HIFs, and many targeted therapeutic strategies have been developed to kill hypoxic tumor cells, lack of well-designed clinical trials has limited their application [
12‐
14]. Therefore, identifying more upstream regulatory factors or downstream effectors of HIFs holds great potential for identifying new diagnostic biomarkers or therapeutic targets which may be of particular scientific significance for treating hypoxic tumors.
Extensive data have demonstrated that hypoxia-induced activation of HIFs also modulates several aspects of epigenetic mechanisms to regulate cancer progression, especially long non-coding RNAs (lncRNAs) [
4,
15‐
17]. LncRNAs are a type of RNA transcripts longer than 200 nucleotides in length, the dysregulation of which has been reported to participate in diverse biological processes in cancer cells, including metabolism, growth and stress response [
18‐
20]. To date, numerous lncRNAs, such as
NEAT1,
MALAT1,
MIR31HG, and
RAB11B-AS1, have been reported to be activated to promote tumor progression under hypoxic condition [
21‐
23]. For example, lncRNA
RAB11B-AS1 was transcribed by HIF-2 under hypoxia, which enhanced VEGFA and ANGPTL4 expression, promoting angiogenesis and distant metastasis in breast cancer [
23]. However, studies focusing on how hypoxia-induced lncRNAs facilitate CRC progression are still limited and their regulatory mechanisms and functions need to be further elucidated.
Antisense lncRNAs are transcribed from the opposite strand of either protein or non-protein coding genes [
24]. A growing number of studies are demonstrating that antisense lncRNAs function in several aspects of gene regulation by exerting cis or trans regulation [
25,
26]. Indeed, antisense lncRNAs play important roles in many biological processes including cancer initiation and development, mainly through interacting with DNAs, RNAs and proteins [
27‐
29]. For example, a conserved antisense lncRNA,
BDNF-AS, was reported to negatively regulate its sense transcript both in vitro and in vivo by changing the chromatin structure near the BDNF locus [
30]. Another well-studied example of antisense lncRNA in cancer is
HOTAIR (HOX transcript antisense RNA), which has been demonstrated to promote proliferation, invasion, metastasis, and drug resistance in a number of cancer types, highlighting the great potential of antisense lncRNAs as diagnostic or prognostic indicators for cancer treatment [
31‐
33]. Therefore, it is scientifically important to identify more antisense lncRNA candidates to benefit the development of novel strategies for cancer therapy.
In this context, we searched the TCGA databases to define hypoxia-regulated antisense lncRNAs involved in CRC. Among several candidates, we identified that lncRNA STEAP3-AS1 was transcriptionally induced by hypoxia, which was aberrantly upregulated in clinical CRC tissues and positively correlated with poor prognosis of CRC patients. Further, we found that lncRNA STEAP3-AS1 interacted with YTHDF2, thus upregulating mRNA stability of STEAP3 and consequent STEAP3 protein expression. STEAP3 protein then activated Wnt/β-catenin signaling in an iron-dependent manner, resulting in CRC progression. These findings may unveil a novel pathway for explaining hypoxia-promoted CRC and present potential biomarkers or targets for predicting and treating CRC.
Material and methods
Antibodies and reagents
Antibodies used are as follows: anti-STEAP3 (cat# sc-376327, 1:1000 dilution), anti-Axin (cat# sc-293190, 1:500 dilution), anti-p-GSK3β (Ser 9) (cat# sc-373800, 1:1000 dilution) and anti-GSK3β (cat# sc-9166, 1:1000 dilution) were purchased from Santa Cruz Biotechnology; anti-ZO-1 (cat# 8193, 1:1000 dilution), anti-E-cadherin (cat# 3195, 1:1000 dilution), anti-Vimentin (cat# 5741, 1:1000 dilution), anti-Claudin-1 (cat# 13995, 1:1000 dilution), anti-GAPDH (cat# 5174, 1:2000 dilution), anti-β-catenin (cat# 8480, 1:1000 dilution), anti-Histone H3 (cat# 4499, 1:1000 dilution), anti-Snail (cat# 3879, 1:1000 dilution), anti-Slug (cat# 9585, 1:1000 dilution) and anti-HIF-1α (cat# 36169, 1:1000 dilution) were purchased from Cell Signaling Technology; anti-m6A (cat# ab208577, 1:1000 dilution), anti-YTHDF1 (cat# ab252346, 1:1000 dilution), anti-YTHDF2 (cat# ab246514, 1:1000 dilution), anti-METTL3 (cat# ab195352, 1:1000 dilution), anti-METTL14 (cat# ab220030, 1:1000 dilution) were purchased from Abcam.
Regents used in this study: Dimethyloxalylglycine (DMOG, cat# S7483) and actinomycin D (Act D, cat# S8964) were obtained from Selleck; DMSO (cat# D2650), Crystal Violet (cat# C0775), MTT (cat# M2128), FeSO4 (cat# 215422) and CoCl2 (cat# 15862) were obtained from Millipore Sigma. DMOG, actinomycin D and CoCl2 were dissolved in DMSO; Crystal Violet, MTT and FeSO4 were dissolved in ddH2O.
Cell lines and cell culture
CRC cells (HCT116, RKO, DLD-1, LoVo, SW480, SW620, HT29), the nonmalignant human colon epithelial cell line NCM460 and HEK293T were obtained from ATCC and maintained in Dulbecco’s modified Eagle’s medium (Gibco) supplemented with 10% fetal bovine serum (BI), 100 U/mL penicillin, and 100 μg/mL streptomycin (Invitrogen).
Patient-derived organoid
The generation of patient-derived organoid was performed as previously described [
34]. Briefly, CRC tissue from patients was minced into small fragments (1–3 mm
3) pieces and digested by collagenase II and TrypLE Express Enzyme at 37 °C. The organoid was then embedded within Matrigel and cultured in 48-well plates supplemented with human complete feeding medium in a humidified incubator with 5% CO
2 at 37 °C.
Establishment of stable knockdown and overexpressed CRC cells
Lentiviral vectors or target plasmid were co-transfected into HEK293T cells with packaging vectors. Lentivirus particles were harvested at 24 and 48 h after transfection to infect CRC cells. shRNA plasmids were synthesized, annealed and cloned into pLKO.1 vector. Stable overexpressed gene were generated by using the pCDH expression vectors.
The sequences of shRNA and overexpression primers were as follows: STEAP3-AS1 shRNA#1, F 5′-CCGGGCACCTTTAAACTGTCCTACACTCGAGTGTAGGACAGTTTAA AGGTGCTTTTTG-3′, R 5′-AATTCAAAAAGCACCTTTAAACTGTCCTACACTCGAG TGTAGGACAGTTTAAAGGTGC-3′; STEAP3-AS1 shRNA#2, F 5′-CCGGG CTGTTCCGTGGAGCCATTATCTCGAGATAATGGCTCCACGGAACAGCTTTTTG-3′, R 5′-AATTCAAAAAGCTGTTCCGTGGAGCCATTATCTCGAGATAATGGCTCCACGGAA CAGC-3′; STEAP3-AS1 overexpression primer, F 5′-GAATTCAGACCC AAACCCCAGAGTCAT-3′, R 5′-GGATCCAGAGATGGGACCTCCCTGTGT-3′.
Western blot
CRC cells were harvest after washing twice with cold PBS. The pellet was resuspended and incubated on ice in lysis buffer for 30 min, and then the lysate was obtained by centrifugation at 12000×g for 10 min. Proteins were separated by SDS-PAGE, transferred onto PVDF membranes, blocked in 5% nonfat milk and then blotted with specific antibodies.
RT-qPCR
Total RNA was extracted from tissues and cells using Trizol (Invitrogen, USA) and reverse transcribed to cDNA by using a Reverse Transcription Kit (Takara, Dalian, China). The RNA transcripts levels were analyzed using a Bio-rad CFX96 real-time PCR system (Biorad, USA) and normalized to GAPDH. Primers used in RT-qPCR were listed in Supplementary Table S
1.
Cell growth and proliferation assays
Cell viability was detected by adding 5% MTT and incubation at 37 °C for 2 h at 0, 24, 48, 72 and 96 h. The absorbance of each well was measured at 570 nm. All experiments were performed in at least triplicate.
For colony formation assay, 500 cells were planted and maintained in each well of 24 well plates for 2 weeks. The medium was refreshed every 3 days. Colonies were fixed with 4% paraformaldehyde for 1 hour and then stained with 0.1% crystal violet for 30 min and washed with ddH2O. The colony numbers of each well were counted.
Transwell migration and invasion assays
Migration and invasion assays were performed using Transwell chamber system (Corning, USA). For migration assay, 5 × 104 cells were seeded in the upper chamber of an insert with 0.2 ml FBS-free starvation medium, and 0.6 ml culture media with 20% FBS were added outside the chamber in the wells of the plate. For invasion assays, the upper chamber of the insert was pre-coated with Matrigel (Millipore Sigma) before plating cells. After incubation for 48 h, cells were fixed with 4% paraformaldehyde for 1 hour and then stained with 0.1% crystal violet for 30 min. After rinsing with water and airing, migrating or invading cells were imaged and counted using a Leica DM2500 microscope.
Immunofluorescence
After seeding onto the glass cover slides (WHB scientific, cat# whb-24-cs) and leaving at 37 °C overnight, cells were fixed by 4% formaldehyde, and then permeabilized with 0.3% Triton X-100 and blocked with 5% BSA. After being incubated with indicated primary antibodies (1:100 dilution) at 4 °C overnight, slides were incubated with Alexa Fluor 488/594-conjugated secondary antibodies (1:200 dilution) for 1 hour at room temperature, followed by DAPI staining of the nuclei (Solarbio, cat# C0060, 1:4000 dilution) at room temperature for 10 minutes. Finally, images were captured using confocal laser scanning microscopy (Carl Zeiss Microimaging) in Pub-lab of West China School of Basic Medical Sciences & Forensic Medicine, Sichuan University.
Immunohistochemistry
Immunohistochemistry was performed following a standard protocol. Briefly, slides were deparaffinized with xylene and ethanol, and the endogenous peroxidase was blocked by 3% H2O2 for 10 minutes. After being incubated in retrieval buffer and boiled for 3.5 minutes, slides were washed with PBS for 3 times and blocked with 5% normal serum. Then the slides were incubated with primary antibody at 4 °C overnight followed by 60 minutes-treatment of MaxVision HRP solution (MXB Biotechnology, cat# 5020). After being stained with DAB Peroxidase Substrate (MXB Biotechnology, cat# 0031), the antigen levels were detected using EnVision Detection System (Agilent Technologies, K5007).
Co-immunoprecipitation (co-IP)
After being collected and washed 2 times with pre-cooled PBS, cells were lysed using 1 mL complete protease and phosphatase inhibitor added IP lysis buffer (100 mM NaCl, 20 mM Tris-HCl, pH 7.5, 0.5 mM EDTA, 0.1% NP-40) and incubated on ice for 30 min. After centrifugation at 12000 rpm at 4 °C for 10 min, 80 μL of the supernatant was transferred and mix with 5× lording buffer in a new tube as the control. The remaining supernatant was transferred to a new tube and incubated with 1 μg indicated antibody overnight at 4 °C. After being washed 3 times with IP lysis buffer, 30 μL of protein A/G agarose beads (GE Healthcare, cat# 17-0963-03) were added into the mixture, and rotated for another 2 h at 4 °C. The beads were washed using washing buffer (150 mM NaCl, 0.5 mM EDTA, 20 mM Tris-HCl, pH 7.4, 0.5% NP-40) for 3 times, and proteins were separated by SDS-PAGE loading buffer with 10 min incubation at 100 °C, followed by immunoblotting analysis.
In vivo orthotopic implantation and spleen injection model
Six to eight weeks old male BALB/c nu/nu mice were used. For orthotopic implantation, 1 × 107 PBS suspended cells was injected subcutaneously. Tumors were collected and sliced into 3 × 3 mm pieces for orthotopic implantation once their diameter reached 1 cm. After being anesthetized and laparotomized, CRC tissues were positioned in the wound and tied down using a suture. The intestines were then placed back followed by closing the peritoneum after sterilization. For the spleen injection model, 5 × 105 cells were injected into the spleen through an incision on the left side of abdomen. Mice were sacrificed at 6–8 weeks after implantation or injection to examine the lung and liver metastases. H&E staining was performed after tissues were fixed in 4% formaldehyde.
Zebrafish xenograft model
Tg (flk1:eGFP) zebrafishes were used to establish zebrafish xenograft model of human CRC. After being anesthetized with 0.04 mg/mL tricaine (Millipore Sigma), zebrafishes received a microinjection of 200 mCherry stably expressed CRC cells. The tumor cells-bearing zebrafishes were randomly divided into two groups after examination of mCherry fluorescent signal in the next day. Zebrafishes were maintained under normal oxygen or hypoxic conditions for a period of 3 days. Finally, the mCherry fluorescent signal was read to examine the distribution and metastasis of CRC cancer cells using a stereo microscope.
In situ hybridization (ISH)
In situ hybridization assays were performed to evaluate the STEAP3-AS1 levels in the CRC xenograft model. Sections were deparaffinized with xylene and ethanol, and the endogenous peroxidase was blocked by 3% H2O2 for 10 minutes at room temperature. After being incubated with 3% citric acid and freshly diluted pepsin for about 60 s at 37 °C, slides were washed 3 times with PBS and fixed with 1% formaldehyde with addition of 0.1% DEPC for 10 min at room temperature. The sections were then pre-hybridized at 40 °C for 2 hours in a hybrid box with 20 mL 20% glycerinum placed in the bottom. Twenty microlitre hybridization liquid was then added and left at 40 °C overnight. After being washed successively with 2 × SSC, 0.5 × SSC, 0.2 × SSC (15 minutes for each), the sections were blocked with blocking reagent for 30 min at 37 °C. Next, sections were incubated with biotin-digoxigenin for 1 hour at 37 °C followed by Strept Avidin-Biotin Complex (SABC) for 20 min at 37 °C. After being incubated with biotin peroxidase, sections were subjected to DAB. This was followed by hematoxylin redye, dehydration using graded ethanol and vitrification with dimethylbenzene. Sections were analysed using an EnVision Detection System (Agilent Technologies, K5007).
Fluorescence in situ hybridization (FISH)
After being fixed with 4% formaldehyde, cells were permeabilized with 0.3% Triton X-100 and blocked with 5% BSA. Cells were then pre-hybridized at 37 °C for 30 min followed by incubation with lncRNA FISH Probe Mix for hybridization at 37 °C overnight. After being washed three times, DAPI was added to stain the nucleus. Images were captured at 555 nm using confocal laser scanning microscopy (Carl Zeiss Microimaging).
RNA immunoprecipitation (RIP)
Cells were harvested for nuclear isolation before incubating with m6A antibody for 4 h at 4 °C in 1× immunoprecipitation buffer supplemented with RNase inhibitors. Prewashed protein A/G magnetic beads (30 μL) were added and incubated overnight at 4 °C. After washing 3 times and incubating with proteinase K digestion buffer, RNA was finally extracted using phenol-chloroform and analyzed by qPCR.
RNA pulldown assay
Briefly, the in vitro biotin-labelled RNAs were transcribed with 10 × Biotin RNA labeling mix (Roche, cat# 1165597910) and T7 enzyme mix (New England Biolabs, cat# M0251S), and heated at 65 °C for 5 min. Samples were cooled to room temperature to form the proper secondary structure in the presence of 10 mM HEPES, 10 mM MgCl2 and 0.1 M NaCl. The RNAs were then incubated with Streptavidin Magnetic Beads (Beyotime Biotechnology, cat# P2151) for 15–30 minutes at room temperature with agitation. Protein lysates were then mixed with the RNA-beads complex for 30–60 minutes at 4 °C with agitation or rotation. The pulldown complexes were then washed and boiled at 95–100 °C for 5–10 minutes, followed by immunoblotting.
Chromatin immunoprecipitation (ChIP) assays
Chromatin immunoprecipitation assay was performed using a ChIP kit (Millipore Corp.) following the manufacturer’s protocol. Firstly, after being cross-linked with 1% formaldehyde, DLD-1 or SW480 cells (1 × 10
7) were sonicated at 30% maximum power for 8 min (5 s pulse after every 10 s). Supernatants were transferred into a new tube for immunoprecipitation with 1 μg of specific antibodies or IgG antibody after centrifugation at 15000×g for 10 min. The target protein and their binding DNA complex was sedimented using prewashed agarose beads (GE Healthcare, cat# 17–0963-03). After elution and purification, DNA was analyzed by RT-qPCR. Primers used in ChIP-qPCR are listed in Supplementary Table S
2.
TCGA analysis and RNAseq analysis
Gene expression data and the corresponding clinical information were obtained from TCGA repository using the *GDCquery* function of the TCGAbiolinks R package as well as a recent updated clinical data resource [
35]. LncRNAs upregulated in CRC were analyzed using DESeq2 R package (log2FoldChange > 0.5 & padj< 0.05). The hypoxia signature score was calculated based on a gene set as previously described [
36] using ‘ssgsea’ method of the GSVA R package, and correlations with each lncRNAs were analyzed by Pearson Correlation (coefficient > 0.15 & padj < 0.05). For the survival and kaplan-meier analysis, patients were stratified into two groups (high & low expression) using surv_cutpoint and surv_categorize function of the survminer R package and the progression-free survival was analyzed.
Total RNA was extracted using TRIzol (Invitrogen, Carlsbad, CA, USA). RNA-sequencing analysis was performed at Novogene (Tianjin, China). TCGA data was downloaded and extracted using TCGAbiolinks v.2.22. The rlog transformation and differential expression analysis were performed using DESeq2 v.1.34. The differentially expressed genes (Padj ≤0.05 and log2 (fold change) ≥ 0.5) were subjected to enriched biological pathways analysis using DAVID bioinformatic Resources (2021 Update). Enriched pathways were visualized using clusterProfiler v.4.22 and ComplexHeatmap v.2.10.
Statistics
All data were from at least three independent experiments and presented as the mean ± SD. The P value was calculated using GraphPad (version 9). P < 0.05 was considered statistically significant. Comparisons between two groups and repeated measurements over a period of time were performed by two-tailed Student t test, one-way analysis of variance (ANOVA) or two-way ANOVA. Correlation between two independent groups were performed using Pearson’s Chi-square test. Kaplan-Meier method were used to generate survival curves.
Discussion
Numerous studies support the important functions of hypoxia in facilitating tumor progression by regulating abnormal expression of proteins. The role of lncRNAs in hypoxia-mediated tumor progression remains largely elusive. In the present study, we identified that STEAP3-AS1 is a hypoxia-responsive lncRNA, which was transcribed by HIF-1α through binding to the HREs located near the STEAP3-AS1 locus. Upregulation of STEAP3-AS1 promoted proliferation and metastasis of CRC cells both in vitro and in vivo and was positively correlated with poor prognosis of CRC patients. Further studies found that STEAP3-AS1 conferred the upregulation of STEAP3 protein by interacting with YTHDF2 to prevent m6A-mediated degradation of STEAP3 mRNA, thus preserving Fe2+ concentration to activate Wnt/β-catenin and favor CRC progression. To the best of our knowledge, this is the first report identifying STEAP3-AS1 as a hypoxia-responsive lncRNA in CRC and elucidating the mechanisms underlying STEAP3-AS1-mediated CRC progression. These findings indicate that lncRNA STEAP3-AS1 may serve as a potential biomarker for clinical CRC management.
STEAP3-AS1 is an antisense lncRNA of six-transmembrane epithelial antigen of the prostate 3 (STEAP3, also known as TSAP6 or dudulin-2), which was initially identified as a potential prognostic biomarker in tongue squamous cell carcinoma (TSCC) [
45]. In addition,
STEAP3-AS1 also displayed an oncogenic role in human hepatocellular carcinoma (HCC) by acting as a competing endogenous RNA (ceRNA) and served as a risk scoring system together with three other lncRNAs (SNHG1, RUSC1-AS1, and SNHG3) to predict the outcomes of HCC patients [
46]. Recently,
STEAP3-AS1 was also reported to regulate cell cycle by modulating CDKN1C expression in colon cancer, but the detailed mechanisms were not fully understood [
47]. In this study, we found that hypoxia-induced upregulation of
STEAP3-AS1 accelerated the proliferation and metastasis of CRC cells both in vitro and in vivo and was positively correlated with the poor outcomes of CRC patients, suggesting its potential application in clinical management of CRC. Moreover,
STEAP3-AS1 caused the expression of its neighboring STEAP3 in a m
6A modification-dependent manner, which activated Wnt/β-catenin signaling to support CRC progression. Importantly, recent in silico analysis of a serum exosome-derived ceRNA network has identified
STEAP3-AS1 as an independent prognostic predictor of glioblastoma and gallbladder cancer [
48,
49], further proving its clinic value as a promising biomarker for cancers. Therefore, further studies elucidating the functions of secreted
STEAP3-AS1 in CRC progression may expand its applications for early detection or prediction of drug response in CRC patients.
Mounting evidence has indicated that antisense transcript lncRNA can positively or negatively regulate the expression of its nearby protein-coding genes [
50]. For example, lncRNA
FOXC2-AS1 directly bound to FOXC2 mRNA and increased its expression to confer doxorubicin resistance in osteosarcoma [
51], while lncRNA
HOXD-AS1 mediated the recruitment of PRC2 to the
HOXD3 promoter to significantly repress the transcription of the
HOXD3 gene [
52]. Indeed, the negative correlation between
STEAP3-AS1 and its neighboring STEAP3 has been previously reported, but the regulatory mechanism was not clear [
47]. However, in the present study, we demonstrated that
STEAP3-AS1 interacted with the m
6A reader YTHDF2 to inhibit m
6A-mediated degradation of
STEAP3 mRNA and upregulate STEAP3 protein expression to promote CRC progression. Moreover, exogenous overexpression of STEAP3 could partially rescue
STEAP3-AS1 knockdown-mediated inhibition of CRC progression, implying a positive correlation between
STEAP3-AS1 and STEAP3. As STEAP3 is a metalloreductase responsible for reducing cellular Fe
3+ to Fe
2+ [
53], increased expression of STEAP3 preserved cellular Fe
2+ concentrations to phosphorylate and inactivate GSK3β, thus activating Wnt/β-catenin signaling to favor CRC progression. In fact, the oncogenic roles of STEAP3 have already been found in HCC and glioblastoma [
40,
54], which could partially strengthen our conclusion that
STEAP3-AS1 positively correlated with STEAP3 to promote CRC progression.
m
6A is one of the most prevalent RNA modifications, which regulates RNA splicing, translation, export, and stability, especially within lncRNAs and mRNAs [
55]. Increasing studies have reported that aberrant regulations of m
6A modification on certain RNAs executed important functions to modulate tumor initiation and progression [
56,
57]. Moreover, proteins responsible for m
6A modification, including writers (such as METTL3/14), erasers (such as FTO and ALKBH5), and readers (such as YTHDF1/2/3), have been found to be overexpressed and promote the initiation and development of many cancer types [
58‐
60]. Accumulating data suggest that YTHDF2, a reader protein responsible for m
6A-mediated mRNA decay, is closely related to many aspects of human cancers by regulating multiple biological processes, such as metastasis, proliferation, differentiation and inflammation [
61‐
63]. In our study, the m
6A modification was found to occur both on lncRNA
STEAP3-AS1 and
STEAP3 mRNA. Further studies demonstrated that
STEAP3-AS1 bound to YTHDF2 through its 3′-fragment, which prevented m
6A-mediated degradation of
STEAP3 mRNA, resulting in upregulation of STEAP3 protein expression. Further investigations are required to clarify the binding motif of YTHDF2 with lncRNA
STEAP3-AS1.
The reprogramming of iron metabolism is an indispensable biological process for cancer cells, which contributes to the initiation, growth, and metastasis of tumors [
64]. The STEAP3-mediated reduction of ferric iron (Fe
3+) to ferrous iron (Fe
2+) is the major aspect of iron utilization in cancer cells [
65,
66]. A growing body of evidence has revealed that sufficient cellular Fe
2+ is essential for the activation of Wnt signaling, particularly in CRC cells, during which the regulation of β-catenin is characterized as a key event. However, to date, the mechanisms are not clearly defined [
67,
68]. In this study, by performing RNA-seq analysis and assessing Wnt activation, we confirmed the important functions of Wnt/β-catenin signaling in
STEAP3-AS1-mediated CRC progression. We also found that exogenous supplementation of Fe
2+ could partially reverse the inhibitory effect on Wnt/β-catenin signaling and CRC progression caused by
STEAP3-AS1 knockdown, indicating the pivotal role of
STEAP3-AS1/STEAP3-mediated Fe
2+ generation in activating Wnt and promoting CRC progression. Moreover, an abundant supply of Fe
2+ increased the Ser 9 phosphorylation of GSK3β and inhibited its kinase activity, thus releasing β-catenin for nuclear translocation to activate Wnt signaling. This observation is consistent with the previous findings that Fe
2+ could promote the phosphorylation of GSK3β on Ser 9 in hippocampal neurons and this phosphorylation of GSK3β inhibited its activity and activated Wnt/β-catenin signaling in cancer cells [
42,
44]. However, further studies are required to elucidate the mechanisms underlying Fe
2+-mediated Ser 9 phosphorylation and inactivation of GSK3β in cancer cells.
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