Long intergenic non-coding rna apoc1p1-3 inhibits apoptosis by decreasing α-tubulin acetylation in breast cancer

ABSTRACT Increasing evidence indicates that long non-coding RNAs (lncRNAs) act as important regulatory factors in tumor progression. However, their roles in breast cancer remain largely

unknown. In present studies, we identified aberrantly expressed long intergenic non-coding RNA _APOC1P1-3_ (_lincRNA-APOC1P1-3)_ in breast cancer by microarray, verified it by quantitative

real-time PCR, and assessed methylation status in the promoter region by pyrosequencing. We also investigated the biological functions with plasmid transfection and siRNA silencing

experiments, and further explored their mechanisms by RNA pull-down and RNA immunoprecipitation to identify binding proteins. We found that 224 lncRNAs were upregulated in breast cancer,

whereas 324 were downregulated. The _lincRNA-APOC1P1-3_ was overexpressed in breast cancer, which was related to tumor size and hypomethylation in its promoter region. We also found that

_APOC1P1-3_ could directly bind to tubulin to decrease _α_-tubulin acetylation, to inactivate caspase-3, and to inhibit apoptosis. This study demonstrates that overexpression of _APOC1P1-3_

can inhibit breast cancer apoptosis. SIMILAR CONTENT BEING VIEWED BY OTHERS ETS-1-ACTIVATED LINC01016 OVER-EXPRESSION PROMOTES TUMOR PROGRESSION VIA SUPPRESSION OF RFFL-MEDIATED DHX9

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2021 MAIN Long non-coding RNAs (lncRNAs) are a group of non-protein-coding transcripts longer than 200 nucleotides. They are found in sense or antisense orientation to protein-coding genes,

within introns of protein-coding genes or in intergenic regions of the genome. Although significant numbers of lncRNAs have been identified, most of them remain largely uncharacterized and

little is known about their functions.1 There are reports that they not only interact directly with DNA, mRNAs or proteins (such as transcription factors), but also with other regulatory

non-coding RNAs.2 By binding to regulatory components and forming lncRNA–gene complexes, they cause genetic regulations or epigenetic modifications.3 Recently, lncRNAs draw attention on

their potential contribution towards disease etiology. Accumulating findings implicate that lncRNAs are expressed aberrantly in the cancer development process, including proliferation,

metastasis, and apoptosis. For example, _lncRNA-GAS5_ expression is significantly downregulated in breast cancer cells, promoting apoptosis.4 The long intergenic non-coding RNA (lincRNA)

_p21_, which contains p53-binding sites for activation during DNA damage, is regarded as an important repressor in the p53-mediated pathway and apoptosis.5 The lncRNAs involved in breast

carcinogenesis are still in need of further exploring. Apolipoprotein C-I pseudogene 1 (_APOC1P1_) is a pseudogene located in 19q13.2 between apolipoprotein C-I and apolipoprotein C-IV. It

encodes three RNA transcript variants that belong to lincRNA family. The variant 3 (_lincRNA-APOC1P1-3_), which is shorter than the variant 1 and 2, lacks an alternate internal segment and

uses an alternate internal splice site. Its expression and function in human diseases are unknown. In present studies, we tested the hypothesis that _APOC1P1-3_ overexpression involved in

breast cancer progression. Using the microarray, we confirmed that _lincRNA-APOC1P1-3_ is highly expressed in breast cancer tissues. Microarray results were validated with quantitative

real-time PCR in breast cancer cell lines and tissues. Biological functions of _lincRNA-APOC1P1-3_ were assessed by gain _versus_ loss function studies and regulatory mechanisms were

investigated by RNA pull-down, RNA immunoprecipitation (RIP), and pyrosequencing. Our data support this hypothesis. RESULTS LINCRNA-APOC1P1-3 IS OVEREXPRESSED IN BREAST CANCERS Our

microarray results (NCBI GEO accession: GSE80266) showed that 224 lncRNAs increased and 324 decreased in breast cancer tissues (fold change ⩾1.5, Supplementary Table S4). Hierarchical

clustering showed systematic variations in expression of lncRNAs in normal _versus_ cancer tissues (Figures 1a–c). We found that _lincRNA-APOC1P1-3_ (fold change=2.02, _P_-value=0.02, and

full length=631 bp) met the selection criteria and then was taken into further validation. To investigate the role of _APOC1P1-3_, we compared its expression profiles in cultured cells

(MCF10A _versus_ BT549, MCF7, MDA-MB-231, MDA-MB-453, MDA-MB-468, MCF7/Adr, and T47D) and 25 pairs fresh tissues (cancer _versus_ matched normal tissues) with qPCR. Again, our data showed

_lincRNA-APOC1P1-3_ was overexpressed in both breast cancer cell lines and tissues (Figures 1d and e). HYPOMETHYLATION IN APOC1P1 PROMOTER REGION Methylation of gene promoter has been proved

to be eventful in gene epigenetic regulation. To determine whether methylation modifications exist in _APOC1P1_ gene promoter region, we quantified C/G methylation levels in the first exon

of _APOC1P1_ with its upstream 1000-bp region using pyrosequencing in 3 normal and 10 breast tissues to quantify the degree of methylation at each CG site. All 16 C/G sites of the designated

region were subjected to pyrosequencing. The pyrosequencing results showed that one of the 16 C/G methylation sites was significantly hypomethylated in breast cancer tissues when compared

with normal tissues (Figure 2), whereas the other fifteen sites showed no differences. These results indicate that the hypomethylation of the C/G site may contribute to the upregulation of

_APOC1P1_ in breast cancer. LINCRNA-APOC1P1-3 IS RELATED TO THE TUMOR SIZE To characterize the role of _APOC1P1-3_ overexpression in breast cancers, we examined the relationship between

expression of _APOC1P1-3_ and clinicopathologic parameters (age, molecular subtypes (luminal A like, luminal B like, HER2 positive, and triple negative),7 breast cancer biomarkers (estrogen

receptor (ER), progesterone receptor (PgR), and HER2), lymph node status, distant metastasis, and pTNM stage). We found that _APOC1P1-3_ expression was positively associated with tumor size

(_P_=0.0142). Tumors with a larger volume (⩾2.5 cm) tended to exhibit higher _APOC1P1-3_ expression. However, there was no significant relationship between _APOC1P1-3_ expression and other

parameters (Table 1). LINCRNA-APOC1P1-3 REGULATES EARLY APOPTOSIS IN BREAST CANCER CELLS To determine the biological function of _lincRNA-APOC1P1-3_, we performed gain/loss function studies.

We found that MCF7 and MDA-MB-231 cells can be effectively upregulated and downregulated by _pcDNA3.1_ and _siRNA_ transfection, respectively (Figure 3a). The CCK8 proliferation assay

showed that viable cells in _siRNA/Control_ group and _pcDNA3.1/APOC1P1-3_ (_APOC1P1-3_ overexpression) group were more than those in _siRNA/APOC1P1-3_ (_APOC1P1-3_ knockdown) group and

_pcDNA3.1/Control_ group, respectively (Figure 3b). Further, flow cytometry (for early apoptosis) demonstrated that the upregulation of _APOC1P1-3_ inhibited, whereas downregulation induced

cell apoptosis (Figure 3c). However, cell cycles were not affected (Figure 3d). Furthermore, we found that alterations of _APOC1P1-3_ affected caspase-3 activation, whereas did not affect

ER, PgR, HER2, and epithelial growth factor receptor (EGFR) expressions (Figure 4). LINCRNA-APOC1P1-3 CAN BIND _Α_-TUBULIN AND MODIFY ITS ACETYLATION To investigate the mechanism of

_APOC1P1-3_-induced early apoptosis, we determined whether _APOC1P1-3_ could bind and interact with apoptosis-related proteins. Therefore, we first conducted RNA pull-down assay in MCF7

cells to determine binding proteins (Figure 5a). The mass spectrometry analysis for the specific band revealed that tubulin was a potential binding protein (Supplementary Table S5). To

validate the mass spectrometry result, we performed a western blot using the captured protein from RNA pull-down assays in MDA-MB-231 and MCF7 cells (including _α_-tubulin and _β_-tubulin;

Figure 5b). Furthermore, we performed a RIP assay with antibodies for _α_-tubulin and _β_-tubulin, and detected a significant enrichment of _APOC1P1-3_ by further qPCR study (Figure 5c).

Suppressed tubulin polymerization and increased _α_-tubulin acetylation contribute to apoptosis of cancer cells.8, 9 To further confirm effects of _APOC1P1-3_ on acetylation levels of

_α_-tubulin and attenuation of apoptosis, we examined acetylated _α_-tubulin levels in MCF7 and found that exogenous expression of _APOC1P1-3_ significantly reduced acetylated _α_-tubulin

(Figure 5d), although the total protein content of _α_/_β_-tubulin were unchanged. Furthermore, acetyltransferase inhibition with Trichostatin A demonstrated that the increasing acetylation

of _α_-tubulin induced cell apoptosis (Figure 5e). DISCUSSION Identification of lncRNA is one of the most significant discoveries in contemporary science. LncRNAs have an essential role in

epigenetics,10 transcriptional regulation, growth and development,11 and constitute part of the nucleus.12 LncRNAs also function in tumor cell proliferation, apoptosis, invasion, and

metastasis.13 In current studies, we found _lincRNA_-_APOC1P1-3_ was overexpressed in breast cancer and the promoter region was hypomethylated. _APOC1P1-3_ could bind to _α_-tubulin and

affect its acetylation, leading to cell apoptosis inhibition. On the basis of these findings, we propose a regulatory mechanism for _APOC1P1-3_ in breast cancer (Figure 6). Methylation of

gene promoter is important in gene epigenetic regulation. Hypomethylation of lncRNA has been found in cancers.14, 15 Some breast cancer related genes, such as _BRCA1_,16 are known to be

regulated by methylation modification. Recently, the methylation of lncRNA was also reported.17 As expression of _APOC1P1-3_ is high in breast cancer, using pyrosequencing to detect the CpG

methylation levels, we examined whether the promoter region was hypomethylated. Cross talk occurs between lncRNAs and methylation regulatory network. Presence of CpG island demethylation in

the lncRNA promoter leads to overexpression of lncRNA transcription.18 Our study suggests that the hypomethylation of lncRNA promoter regulates expression of lncRNA. We have predicted

binding proteins of the hypomethylation region using AliBaba 2.1, which suggests transcription factor Sp1 is a potential binding protein (Figure 7). In view of the important regulatory role

of Sp1 in gene expression, we consider overexpression of _APOC1P1-3_ may be due to the promoter hypomethylation followed by Sp1 activation. Further investigation on involvements of histone

demethylases and Sp1 is needed. LncRNAs affect tumor proliferation via cell cycle and apoptosis.13 Caspase-3 is the main executor of apoptosis. Expression levels of cleaved caspase-3 reflect

caspase-3 activities and degrees of apoptosis. Thus, we assessed early apoptosis and caspase-3 activation (CCK8 assay, flow cytometry, and western blot analysis) during _APOC1P1-3_

silencing and overexpression. We found that _APOC1P1-3_ repressed apoptosis of breast cancer to facilitate its proliferation through altering the apoptotic protein levels. These results

support that _APOC1P1-3_ regulates the breast cancer development by regulating apoptosis. In spite of the complexity and diversity of mechanisms, most studies report that lncRNAs exert

effects by directly binding to chromatin modification complexes (_HOTAIR_, _Xist_, and _Tsil_)19, 20, 21 or non-chromatin modification proteins (Dreh).22 We used an RNA pull-down assay to

identify binding proteins. Mass spectrometry, western blots, and RIP identified tubulin as a specific binding protein. Tubulin is the major constituent of microtubules and cytoskeletal

structure, and has critical role in cell mitosis and chromosome segregation, as well as cell proliferation and migration.23 Post-translational modifications of _α_- and _β_-tubulin are key

in regulation.24, 25, 26 _α_-Tubulin acetylation (transfer of the acetyl group from acetyl-coenzyme A to Lys-40) regulates the structure and function of microtubules.27 Inhibition of tubulin

polymerization and increased acetylation of _α_-tubulin contribute to cancer cell apoptosis.8, 9 We found that _APOC1P1-3_ bound to tubulin, and _APOC1P1-3_ overexpression decreased

_α_-tubulin acetylation, supporting that tubulin may be a target of _APOC1P1-3._ However, the effects of _APOC1P1-3_ on acetylation of _α_-tubulin remains unknown. Apolipoprotein C-1 (APOC1)

protein is highly expressed in pancreatic cancer. It stimulates cell proliferation and prevents cell apoptosis.28 However, APOC1 protein was found to be downregulated in breast cancer

patients.29 _APOC1P1_ is the pseudogene of _APOC1._ Generally, the antisense transcripts produced from pseudogenes can hybridize to corresponding mRNAs, forming dsRNAs cleaved by Dicer to

endogenous siRNAs.30 Our findings provide an explanation for low expression levels of APOC1 in breast cancer patients. Further study is clearly needed to investigate the interaction between

the two genes. In summary, our study demonstrates that _lincRNA-APOC1P1-3_ is overexpressed in breast cancer, and its upregulation promotes cell proliferation by suppressing cell apoptosis.

_APOC1P1-3_ can bind to tubulin, and then increase _α_-tubulin acetylation and inhibit apoptosis. In addition, the promoter region of _APOC1P1_ is hypomethylated, which contributes to the

transcription activation and _APOC1P1-3_ overexpression. We conclude that _lincRNA-APOC1P1-3_ is involved in the breast cancer development. MATERIALS AND METHODS The information for tumor

tissues, cell lines, PCR, western blot, immunohistochemistry, proliferation assay, and cell cycle assay were provided in the Supplementary Materials. LNCRNA EXPRESSION MICROARRAY ANALYSIS

Five matched breast cancer and normal tissues were used for microarray (Table S1). Total RNA was extracted using TRIzol (Ambion, Carlsbad, CA, USA), and transcribed into fluorescent cDNA

using Quick Amp Labeling kit (Agilent, Palo Alto, CA, USA). After hybridization, using Human LncRNA Microarray v2.0 (Arraystar, Rockville, MD, USA), slides were scanned with the Agilent DNA

Microarray Scanner (Agilent p/n G2565BA) and analyzed with Agilent Feature Extraction software v. 11.5.1.1. Quantile normalization and subsequent data processing were performed using the

Agilent GeneSpring GX v11.5.1. Differentially expressed lncRNAs with statistical significance were identified through volcano plot filtering (threshold: _P_-value ⩽0.05, fold change ⩾1.5,

and false discovery rate ⩽0.05). Microarray array data analysis was completed by Shanghai KangChen bio-tech (Shanghai, China). The candidate lncRNAs should meet the following criteria: (1)

RNA length <3 kb; (2) negative X-hybrization (cross-hybridization) result: the probe can not be hybridized with other lncRNAs or mRNAs; (3) sequences do not overlap with nearby mRNAs; (4)

_P_-value, as small as possible; fold change, as big as possible; and raw intensity, as high as possible; and (5) comparable with the latest version of the relative database (NCBI Reference

Sequence, UCSC Knowngenes, and Ensembl Genome). PYROSEQUENCING The pyrosequencing work was accomplished by the cpgbiotech company (Shanghai, China). Three normal breast tissues and 10

breast cancer tissues were obtained from Huashan Hospital, Fudan University. Primers were designed by PyroMark Assay Desigen Software 2.0 (Qiagen, Hilden, Germany). Amplification primers

sequences (in 5′–3′ orientitation) and the sequencing primers are listed in the Supplementary Table S2. One of the primers must be biotinylated, which enables conversion of the PCR product

to a single-stranded DNA template for pyrosequencing. The technological processes were: (1) bisulfite treatment and elution of genomic DNA (C→U and mC→mC; Qiagen); (2) PCR amplification (U→T

and mC→C; PyroMark PCR kit, Qiagen, Hilden, Germany). Both methylated and unmethylated DNA sequences of the designated regions were amplified with its specific primers; (3)

streptavidin-coated beads separated specific PCR products into single strand; (4) sequencing primer was added, which annealed to a fixed single-stranded DNA template; and (5) quantitative

methylation detection by pyrosequencing was completed with Biotage PyroMark Q24 system (Qiagen, Hilden, Germany) according to manufacturer’s instructions, and data were analyzed with

PyroMark software (Qiagen). Calculation of C:T peaks represent the methylation.6 PLASMID AND SIRNA TRANSFECTION The cDNA encoded full-length _lincRNA-APOC1P1-3_ was PCR-amplified using

primers (5′-CAACCAAGCCCTCCAGCAAG-3′ and 5′-GCCTCAGCCTCCCGAATAG-3′), amplification was performed for 35 cycles at 95 °C for 45 s, at 60 °C for 45 s, and at 72 °C for 1 min, and subcoloned

into _Bam HI_ and _Xho I_ sites of a pcDNA3.1 vector (Invitrogen, Carlsbad, CA, USA), named _pcDNA3.1/APOC1P1-3_. Transfections for _pcDNA3.1/APOC1P1-3_ and _siRNA/APOC1P1-3_ (Supplementary

Table S3) were performed using the lipofectamine 2000 (Invitrogen) with Opti-MEM (Gibco, Grand Island, NY, USA) according to the manufacturer’s instructions. Total RNA and protein were

collected after 24 and 48 h, respectively. APOPTOSIS DETECTION Flow cytometry was used to detect the apoptotic cells. After transfection with lipofectamine 2000 and Opti-MEM for 6 h, cells

were maintained in fresh medium supplemented with 1% FBS for 24 h. Thereafter, cells were collected and washed with phosphate-buffered saline. Finally, cell apoptosis was detected by flow

cytometry (BD Bioscience, Franklin Lakes, NJ, USA) after incubation with annexin V-FITC and propidium iodide for 15 min. Data were acquired with a BD FACSVerse system and BD FACSuite

software. (San Jose, CA, USA) RNA PULL-DOWN Biotin-labeled, full-length _APOC1P1-3_ RNA and antisense _APOC1P1-3_ were prepared with Biotin RNA Labeling Mix (Roche, Indianapolis, IN, USA)

and T7 RNA polymerase (Roche). Biotinylated RNAs were treated with RNase-free DNase I (Roche) and purified with the RNeasy Mini kit (Qiagen, Valencia, CA, USA). Cell proteins were extracted

with the ProteoJETTM Cytoplasmic and Nuclear Protein Extraction kit (Fermentas, St. Leon-Rot, Germany), and then mixed with biotin-labeled RNAs. Washed streptavidin agarose beads

(Invitrogen) were added to each binding reaction, incubated at room temperature for 1 h, washed five times and boiled in SDS buffer. Retrieved protein was detected by SDS gel

electrophoresis. RNA IMMUNOPRECIPITATION The RIP test was performed with the Magna RIP RNA-Binding Protein Immunoprecipitation kit (Millipore, Bedford, MA, USA) and _α_-tubulin (cat.# 2144,

Cell Signaling Technology, Beverly, MA, USA) according to manufacturer’s instructions. In brief, beads were mixed with tubulin antibody or IgG and cell lysate, and rotated at room

temperature for 4 h. The co-precipitated RNAs were detected by RT-PCR. Total RNAs (input controls) and isotype controls were assayed simultaneously to demonstrate that detected signals were

from RNAs, specifically bound to _α_-tubulin. STATISTICAL ANALYSIS Data were analyzed using SPSS 17.0 (Chicago, IL, USA). For comparisons, one-way analyses of variance, Fisher’s exact tests,

_χ_2-tests, and two-tailed student’s _t_-tests were performed. _P_<0.05 was considered to be statistically significant. The diagrams were completed with Prism 5.0 (GraphPad Software, La

Jolla, CA, USA). ABBREVIATIONS * lncRNA: long non-coding RNA * lincRNA: long intergenic non-coding RNA * APOC1P1: Apolipoprotein C-I pseudogene 1 * APOC1: Apolipoprotein C-1 * RIP: RNA

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PubMed  Google Scholar  Download references ACKNOWLEDGEMENTS This research was supported by grants from the Natural Science Foundation of China (81372849, 81172507, 81272387, and 81470857),

Shanghai Municipal Commission of Health (2012-433) and Natural Science Foundation of Science and Technology Commission of Shanghai Municipality (16ZR1403300). AUTHOR INFORMATION Author notes

* X-H Liao and J-G Wang: The two authors contributed equally to this work and are co-first authors. AUTHORS AND AFFILIATIONS * Department of Physiology and Pathophysiology, School of Basic

Medical Sciences, Fudan University, No. 138 Yixueyuan Road, Shanghai, China X-H Liao, L-Y Li, L Lv, J-G Yu, J-Y Yang, Q Lu & P Zhou * Department of Pathology, School of Basic Medical

Sciences, Fudan University, No. 138 Yixueyuan Road, Shanghai, China J-G Wang, D-M Zhou, K-H Ren & X-P Liu * Department of Breast Surgery, Huashan Hospital, Fudan University, No. 12

Urumqi Middle Road, Shanghai, China Y-T Jin & Q Zou * Department of Medicine, University of Louisville, Louisville, 40292, KY, USA J Yu * Department of Pathology, The Fifth People's

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Wang, JG., Li, LY. _et al._ Long intergenic non-coding RNA _APOC1P1-3_ inhibits apoptosis by decreasing _α_-tubulin acetylation in breast cancer. _Cell Death Dis_ 7, e2236 (2016).

https://doi.org/10.1038/cddis.2016.142 Download citation * Received: 03 December 2015 * Revised: 16 April 2016 * Accepted: 27 April 2016 * Published: 26 May 2016 * Issue Date: May 2016 *

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