Background
Atherosclerosis (AS), an inflammatory disease, is a major cause of vascular diseases, including myocardial infarction, ischemic stroke and cardiovascular disease [
1]. AS is featured by lipid deposition and contains fat-rich macrophages (mø) [
2]. Multiple types of cells were revealed to take part in the pathogenesis of AS, such as vascular smooth muscle cells (VSMCs), endothelial cells (ECs) and mø [
3]. Recent review revealed that SMCs regulated the production of many matrix proteins and promoted the development of the inflammatory response to lipid, suggesting the importance of SMCs in AS progression [
4]. Besides, previous research explained that oxidized low-density lipoprotein (ox-LDL) was a vital risk factor for AS [
5]. Thus, profoundly revealing the mechanism underlying ox-LDL-induced abnormal transformation of VSMCs can provide us with reliable targets for AS therapy.
Long noncoding RNA (lncRNA) is a non-protein-coding RNA with at least 200 base pairs in size, featured by high conservatism [
6]. LncRNA can transfer messages and guide molecule to ribonucleoprotein complexes, thereby playing important roles in biological processes [
7]. An increasing number of researches revealed that lncRNAs were involved in the progression of various diseases, including AS [
8‐
10]. LncRNA myocardial infarction associated transcript (MIAT) was also revealed to mediate AS process. As reported, MIAT could inhibit efferocytosis via upregulating clusters of differentiation 47 (CD47) through binding to microRNA-149-5p (miR-149-5p) [
11]. MIAT also had ability to modulate diabetes mellitus-caused microvascular dysfunction [
12]. However, there were few studies on the development of AS mediated by MIAT [
13,
14].
MiRNAs are a category of noncoding RNAs with about 20 nucleotides in size, and regulate gene expression via targeting their noncoding sequences [
15]. At present, miRNAs have been unveiled to play crucial parts in modulating cardiovascular cell functions, such as protecting against cardiac dysfunction after cerebral ischemia-reperfusion, inhibiting autophagy, and regulating cardiomyocyte growth [
16‐
18]. Another miRNA, miR-641, only was reported to participate in the regulation of cancer progression. For example, Kong et al. indicated that miR-641 was under-expressed in lung cancer cells, and repressed cell proliferation [
19]. In the study of Li and his colleagues, we found the cancer-promoting role of miR-641 in pancreatic cancer by associating with LINC01963 [
20]. Stromal interaction molecule 1 (STIM1), an important factor in regulating calcium channels, can dramatically affect intracellular Ca
2+ [
21]. Researches unveiled that STIM1 was widely expressed in nonexcitable cells, including VSMCs [
22]. He et al. showed that STIM1 silencing repressed cell apoptosis and reduced intracellular Ca
2+ accumulation in cardiomyocytes [
23]. Xu and his colleagues reported that resveratrol improved cardiac functional recovery via repressing STIM1-mediated regulation of intracellular Ca
2+ [
24]. Therefore, STIM1 was important for function of multiple cell types. Based on the above data, miR-641/STIM1 were hypothesized to participate in the regulatory mechanism of MIAT in AS, and whether miR-641/STIM1 pathway was responsible for the molecular mechanism by which MIAT regulated AS development needed to be explored.
The study aimed to investigate the therapeutic target for AS and reveal the role of MIAT in ox-LDL-induced VSMCs and the inner molecular mechanism.
Methods
Cell culture and storage
Human aorta vascular smooth muscle cells (VSMCs) were purchased from Procell (Wuhan, China) and grown in F12K medium (Procell, Wuhan, China) with 10% fetal bovine serum (FBS; Procell, Wuhan, China) and 1% penicillin/streptomycin (Procell, Wuhan, China) at 37˚C in an incubator with 5% CO2.
Plasmid construction, oligonucleotide synthesis and cell transfection
The small interfering RNA against MIAT (si-MIAT), the mimic of miR-641 (miR-641 mimic), the inhibitor of miR-641 (miR-641 inhibitor) and controls (si-NC, miR-NC mimic and miR-NC inhibitor) were provided by GenePharma (Shanghai, China). The overexpression plasmids of MIAT (oe-MIAT) and STIM1 (pcDNA-STIM1) as well as their controls (Vector and pcDNA-NC) were built by Geneseed (Guangzhou, China). Lipofectamine 2000 (Thermo Fisher, Waltham, MA, USA) was employed for cell transfection based on the instructions of manufacturer. The sequences of oligonucleotides were si-MIAT 5′-GCATTTGGTTTCAGTTCTT-3′, miR-641 mimic 5′-AAAGACAUAGGAUAGAGUCACCUC-3′, miR-641 inhibitor 5′-GAGGUGACUCUAUCCUAUGUCUUU-3′, si-NC 5′-GCAGTTGTTACTGTTTCTT-3’, miR-NC mimic 5′-UUUGUACUACACAAAAGUACUG-3′ and miR-NC inhibitor 5′-CAGUACUUUUGUGUAGUACAAA-3′.
3-(4,5-Dimethylthazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay
VSMCs were diluted in F12K medium (Procell, Wuhan, China) and grown in 96-well plates for 16 h. Then, cells were incubated with various concentrations (0, 25, 50, 75 and 100 µg/mL) of ox-LDL (Yeasen, Shanghai, China) for 24 h or 50 µg/mL ox-LDL for different time (0, 12, 24, 36 and 48 h). After that, MTT reagent (Solarbio, Beijing, China) was incubated with cells for 4 h. Cell supernatant was discarded and dimethyl sulfoxide (Sigma, St. Louis, MO, USA) was used to dissolve formazan. The cell viability was determined by assessing the output of wavelength at 490 nm with a Varioskan LUX Multimode microplate reader (Thermo Fisher, Waltham, MA, USA).
Quantitative real-time polymerase chain reaction (qRT-PCR)
Cultured VSMCs were collected and lysed with TransZol (TransGen, Beijing, China). RNA was isolated using an RNAsimple kit (Tiangen, Beijing, China). Then, cDNA was synthesized with a FastKing RT Kit (Tiangen, Beijing, China) or MicroRNA Reverse Transcription Kit (Thermo Fisher, Waltham, MA, USA). For determining the expression levels of MIAT, miR-641 and STIM1, SuperReal PreMix Color (Tiangen, Beijing, China) was mixed with synthesized cDNA and primers, and added into a 96-well IQ5 thermocycler (Bio-Rad, Hercules, CA, USA). After that, Data were assessed by the 2−∆∆Ct method. Glyceraldehyde 3-phosphate dehydrogenase (GAPDH) and U6 acted as controls. The sequences of forward and reverse primers were MIAT 5′-GTGGCTCAGGAGTGCTTC-3′ and 5′-ACTTGCCCAGGGTTGTAG-3′; miR-641 5′-ACACTCCAGCTGGGGAGGTGACTCTATCCTAT-3′ and 5′-TGGTGTCGTGGAGTCG-3′; STIM1 5′-TTGGATTCTTCCCGTTCT-3′ and 5′-CTGGGCTGGAGTCTGTTT-3′; GAPDH 5′-GGTCACCAGGGCTGCTTT-3′ and 5′-GGAAGATGGTGATGGGATT-3′; U6 5′-CTCGCTTCGGCAGCACA-3′ and 5′-AACGCTTCACGAATTTGCGT-3′.
VSMCs were seeded in 6-well plates for 16 h and treated with 50 µg/mL ox-LDL (Yeasen, Shanghai, China). Twenty-four hours later, si-MIAT, miR-641 inhibitor, miR-641 mimic or pcDNA-STIM1 was transfected into the cells at ~70% confluence with controls according to the defined purposes. Then, cells were cultured for 2 weeks. F12K medium (Procell, Wuhan, China) was renewed every 3 days during culture. The forming colonies were immobilized with paraformaldehyde (Sigma, St. Louis, MO, USA) and then dyed with crystal violet (Sigma, St. Louis, MO, USA). Cell colony-forming ability was determined by assessing the number of colonies. A colony was deemed when cell numbers over 50.
DNA content quantitation assay
Cell cycle was detected by DNA content quantitation assay. In short, cultured VSMCs were collected and fixed with cold ethanol (Millipore, Bradford, MA, USA). Cells were precipitated by centrifugation at 300 g for 5 min, and incubated with RNase A (Solarbio, Beijing, China) at 37 °C for 30 min. After that, propidium iodide (PI; Solarbio, Beijing, China) was employed to stain cells at 4 °C for 30 min. Samples were assessed by a flow cytometry (Thermo Fisher, Waltham, MA, USA).
Wound-healing assay
VSMCs were grown in 6-well plates and treated with different purposes. Cells were cultured until its confluence reached about 100 %. Cell wounds were created with 10-µL pipette tips and cells were then cultivated in FBS-free F12K medium (Procell, Wuhan, China). At 24 h after culture, the width of the wounds was measured under an inverted microscope (Nikon, Tokyo, Japan) with a 40(×) magnification, and cell migratory ability was determined by analyzing wound width.
Western blot analysis
Cells were harvested and lysed with RIPA buffer (Sigma, St. Louis, MO, USA) possessing proteinase K (Millipore, Bradford, MA, USA). Then, proteins were denaturalized at 95 °C, and lysates were loaded onto 12% bis-tris-acrylamide gel (Thermo Fisher, Waltham, MA, USA) to separate proteins. The protein bands were transferred onto polyvinylidene fluoride membranes (Millipore, Bradford, MA, USA), which were then immersed in 5% non-fat milk (Solarbio, Beijing, China). Subsequently, the membranes were incubated with anti-proliferating cell nuclear antigen (anti-PCNA) (1:1500; Affinity, Nanjing, China), anti-nuclear proliferation marker (anti-Ki-67) (1:1500; Affinity, Nanjing, China), anti-phospho-focal adhesion kinase (anti-p-FAK) (1:1000; Affinity, Nanjing, China), anti-Ago2 (1:1000; Affinity, Nanjing, China), anti-IgG (1:2000; Abcam, Cambridge, UK), anti-STIM1 (1:1500; Affinity, Nanjing, China) and anti-GAPDH (1:15,000, Affinity, Nanjing, China). The membranes were incubated with horseradish peroxidase-marked secondary antibody (1:5000; Affinity, Nanjing, China). The protein bands were presented with RapidStep ECL Reagent (Millipore, Bradford, MA, USA), and protein expression was determined by Image J software (NIH, Bethesda, MD, USA).
Transwell migration and invasion assays
The migrated and invaded cells were determined by transwell chambers without or with Matrigel (Corning, Madison, New York, USA). In brief, cells were seeded in the upper chambers containing FBS-free F12K medium (Procell, Wuhan, China) after treated with ox-LDL, si-MIAT, si-NC, miR-641 inhibitor, miR-NC inhibitor, miR-641 mimic, miR-NC mimics, pcDNA-STIM1 or pcDNA-NC. In the lower chambers, F12K medium containing 15% FBS (Procell, Wuhan, China) was added. Twenty-four hours later, cell supernatant was discarded, and cells were incubated with paraformaldehyde (Sigma, St. Louis, MO, USA) and crystal violet (Sigma, St. Louis, MO, USA), respectively. Results were determined via counting the number of cells in the lower chambers under a microscope (Nikon, Tokyo, Japan) at a 100(×) magnification.
Dual-luciferase reporter assay
The binding sites between miR-641 and MIAT or STIM1 were firstly assessed through starbase online database (
http://starbase.sysu.edu.cn/agoClipRNA.php?source=mRNA). And the wild-type (WT) and mutant (MUT) plasmids of MIAT and the 3’-untranslated region (3’UTR) of STIM1 were built by Geneseed Co., Ltd. (Guangzhou, China), and named as WT-MIAT, WT-STIM1 3’UTR, MUT-MIAT and MUT-STIM1 3’UTR. Constructed plasmids and synthesized oligonucleotides were transfected into the VSMCs at ~70% confluence using Lipofectamine 2000 (Thermo Fisher, Waltham, MA, USA). Post-culture of 48 h, the cells were collected and lysed using lysis buffer (Promega, Madison, WI, USA). Luciferase activities were detected with a dual luciferase reporter assay kit (Promega; Madison, WI, USA).
Renilla luciferase activity served as a control.
RNA immunoprecipitation (RIP) assay
MiR-641 mimic was transfected into VSMCs with miR-NC mimics as a control. Post-transfection of 48 h, the cells were collected and lysed with RIP lysis buffer (Millipore, Bradford, MA, USA) containing protease inhibitor (Millipore, Bradford, MA, USA). After that, lysates were incubated with the magnetic beads bound with anti-Ago2 (RIP-Ago2; Abcam, Cambridge, UK) or anti-IgG (RIP-IgG; Abcam, Cambridge, UK). Twenty-four hours later, the magnetic beads were washed, and MIAT and STIM1 expression were determined by qRT-PCR.
Statistical analysis
Data derived from three independent duplicate tests were assessed by SPSS 21.0 software (IBM, Somers, NY, USA). Results were expressed as means ± standard deviations (SD). Significant differences were compared with two-tailed Student’s t-tests between the two groups or one-way analysis of variance (ANOVA) with Tukey’s test among three or more groups. Statistical significance was defined when P value < 0.05.
Discussion
Although many achievements have been created in revealing the pathogenesis of AS, AS still poses a heavy threat to human health [
25]. Previous researches showed that lots of lncRNAs participated in regulating the process of cardiovascular diseases, including AS [
26,
27]. As reported, lnc00113 could accelerate cell proliferation and migration via mediating phosphoinositide 3-kinase (PI3K)/protein kinase B (Akt)/mTOR pathway in AS [
28]. Xia et al. indicated lncRNA gm4418 accelerated cell apoptosis and deteriorated hypertensive cerebral arteriosclerosis [
29]. In another example, retinal non-coding RNA3 (RNCR3) knockdown aggravated hypercholesterolemia and repressed the proliferative and migratory abilities of VSMCs [
30]. Herein, we found MIAT silencing repressed cell proliferation, migration and invasion by regulating miR-641/STIM1 axis in ox-LDL-induced VSMCs.
Previous studies presented that MIAT participated in the regulation of AS progression. For example, MIAT promoted atherosclerotic plaques formation, angiogenesis and inflammation, and upregulated lipid content [
11,
13]. Zhong et al. explained MIAT accelerated cell proliferation, but restrained cell apoptosis via sponging miR-181b [
31]. Additionally, it was found that MIAT was augmented and enhanced cell proliferative and metastatic capacities in ox-LDL-stimulated VSMCs [
14,
32]. In this paper, we also found MIAT was elevated in ox-LDL-induced VSMCs. MIAT absence restored ox-LDL-induced proliferation, migration and invasion in VSMCs. As reported, PCNA takes part in many aspects of DNA replication and is regarded as an important regulator of vital events at the replication fork [
33]; Ki-67 has a conserved leucine/arginine rich C-terminusand that can bind to DNA, thereby promoting chromatin compaction [
34]; FAK is a tyrosine kinase and can be recruited to sites of integrin clustering or focal adhesions, and its phosphorylation status is revealed to be correlated with cell metastasis [
35]. In the study, we found MIAT absence also attenuated the promoting effects of ox-LDL on the protein expression of PCNA, Ki-67 and p-FAK. All these evidences suggested the repressing role of MIAT in ox-LDL-induced VSMC disorders. Considering that lncRNA-miRNA-mRNA network played a vital part in unveiling the pathogenesis of cardiovascular diseases [
36], the miRNA and mRNA associated with MIAT were sought. Results exhibited that MIAT interacted with miR-641, which was further revealed to target STIM1.
Current researches revealed that miR-641 was closely correlated with disease progression. Researches indicated that miR-641 repressed cell proliferation in lung cancer [
37] and gastric cancer [
38]. Zhang et al. explained that miR-641 participated in osteoarthritis process by regulating extracellular matrix metabolism and inflammation [
39]. Additionally, miR-641 was reported to repress cisplatin resistance and erlotinib sensitivity in lung cancer [
40,
41]. Here, the paper was the first one to report the role of miR-641 in AS process. We found miR-641 expression was decreased in ox-LDL-treated VSMCs, and miR-641 inhibitor restrained MIAT knockdown-mediated impacts, which suggested that miR-641 served as a suppressor in AS evolution. Meanwhile, the evidences from our study suggested that MIAT regulated ox-LDL-induced development of VSMCs via interacting with miR-641.
Ca
2+ influx into cells is commonly mediated by capacitative Ca
2+ entry pathway, which is reported to be modulated by STIM (STIM1 and STIM2) as well as Orai proteins [
42]. This finding implied that STIM1 played a crucial part in biological behaviors of various cells. Coincidently, we found STIM1 was a target gene of miR-641. Herein, STIM1 was overexpressed in ox-LDL-treated VSMCs. And ectopic STIM1 expression impaired miR-641-mdiated effects on cell proliferation and metastasis in ox-LDL-stimulated VSMCs. The evidences from our research implicated STIM1 served as a promoter in the progression of ox-LDL-stimulated VSMCs, which was proved by the existed evidences [
43,
44]. In the meantime, the above data implied that miR-641 repressed the progression of VSMCs stimulated by ox-LDL via binding to STIM1. Given the associated relationships between MIAT and miR-641 as well as between miR-641 and STIM1, whether MIAT modulated STIM1 by binding to miR-641 was continued to be illustrated. Rescue experiments showed MIAT silencing significantly decreased STIM1 protein expression, whereas miR-641 inhibitor attenuated this impact. Additionally, miR-641 mimic also restored the upregulating impact of MIAT overexpression on STIM1 protein expression. These evidences suggested MIAT could control STIM1 expression through associating with miR-641.
However, there are some limitations that should be considered when interpreting our findings. First, the study only focuses on the roles of MIAT in regulating ox-LDL-induced cell injury in vitro, and the in vivo data are lacking in the present research, which is expected to be explored in future. To address the question, we would study the biological role of MIAT using Apoe−/− mouse. Additionally, ox-LDL-induced VSMC injury might not only attribute to MIAT/miR-641/STIM1 pathway, and there might were other singling pathways, which needed to be explored by more assays and bioinformatics methods.
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