Knockdown of CREB3 activates endoplasmic reticulum stress and induces apoptosis in glioblastoma

Glioblastoma is a highly malignant type of central nervous system tumor. In the present study, the results of RNA sequencing indicated that cAMP responsive element binding protein 3 (CREB3) was upregulated in tumor tissues from patients with GBM. The cAMP responsive element binding protein 3 (CREB3) pathway is a major contributor to the malignant progression of glioblastoma. In this study, we explored the mechanisms by which CREB3 regulates the proliferation, invasion and apoptosis of glioblastoma. Pairs of glioblastoma and normal tissues were subjected to RNA sequencing. Then, qRT-PCR and Western blotting were used to detect CREB3 levels in glioblastoma tissues and cell lines, respectively. CREB3 was upregulated in glioblastoma tissues and cell lines. Overexpression of CREB3 promoted the proliferation and invasion of SHG-44 cells, while downregulation of CREB3 inhibited the invasion of U251MG cells. Knockdown of CREB3 also induced apoptosis in U251MG cells and increased the protein levels of BAX, active caspase 3, p-PERK, p-eIF2α and ATF4. An in vivo study in nude mice bearing U251MG cell xenografts confirmed these results. Our findings indicate that CREB3 functions as a tumor promoter in glioblastoma, and thus could serve as a treatment target in glioblastoma patients.


INTRODUCTION
Glioblastoma is a highly complicated and malignant type of central nervous system tumor, accounting for >70% of all brain tumors [1,2]. Glioblastoma cases can be categorized into grades I to IV, based on their histological characteristics and the World Health Organization criteria [3]. Grade IV glioblastoma, also known as glioblastoma multiforme, is characterized by strong invasiveness, microvascular proliferation and tissue ischemic necrosis [4]. Although advanced methods of surgery, chemotherapy and radiotherapy are available for glioblastoma treatment, the median survival of patients is only 1 (IRE1) and activating transcription factor 6 (ATF6), respectively [9,10]. The UPR alleviates stress and restores ER function by inhibiting translation and increasing the degradation of misfolded proteins [11]. Recent studies have indicated that ERS helps to maintain the homeostasis of glioblastoma cells, and thus is an important contributor to glioblastoma genesis and tumor progression [12][13][14].
The cAMP responsive element binding protein 3 (CREB3) is an ERS-related protein that is considered to be a transcriptional coregulator [15]. The CREB3 subfamily includes five basic leucine zipper transcription factors: CREB3, CREB3L1, CREB3L2, CREB3L3 and CREB3L4 [16]. All the CREB3 subfamily members appear to be involved in the UPR [17]. The CREB3 pathway has been reported to promote the malignant progression of glioblastoma [2]; however, the mechanism remains elusive.
In the current study, we performed gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses to determine the differentially expressed genes (DEGs) in glioblastoma and adjacent normal tissues. We also investigated the effects of overexpressing or knocking down CREB3 in glio-blastoma cells to explore a new therapeutic strategy for this disease.

DEG screening in glioblastoma and normal tissues
RNA sequencing analysis was used to identify DEGs between glioblastoma and adjacent normal tissues. The downregulated and upregulated mRNAs in glioblastoma tissues compared with normal tissues are shown in Figure  1A. In addition, MA and volcano plots were generated to demonstrate the DEGs between glioblastoma and adjacent tissues, based on the thresholds of an adjusted P-value < 0.05 and a log2 fold-change ≥ 2 ( Figure 1B). GO and KEGG pathway enrichment analyses were also performed, which revealed that the DEGs were mainly involved in molecular transduction and translation regulation ( Figure 1C and 1D).

The expression of DEGs in glioblastoma and normal tissues
Among the DEGs, CREB3 was obviously upregulated in glioblastoma tissues compared with adjacent normal tissues ( Figure 2A). Receiver operating characteristic (ROC) curve analysis was used to discriminate glioblastoma tissue from adjacent normal tissue. The area under the curve for CREB3 was 0.7845 in discriminating between glioblastoma and adjacent normal tissues, indicating that the experimental results were reliable ( Figure 2B). As shown in Table 1, CREB3 expression correlated with clinicopathological parameters such as the tumor volume, KI67 expression, distant metastasis and World Health Organization stage. In addition, the threeyear relapse-free survival rate was 45.0% in patients with low levels of CREB3, while it was 5.0% in patients with high levels of CREB3 ( Figure 2C). The overall survival was also better in the low-CREB3 group than in the high-CREB3 group ( Figure 2D). These data suggest that CREB3 is upregulated in glioblastoma and is associated with a poor prognosis.

The upregulation of CREB3 promoted the proliferation of SHG-44 cells
Next, qRT-PCR and Western blotting were used to detect the expression of CREB3 in human astrocytes (HA1800 cells) and three glioblastoma cell lines (SHG-44, U251MG and U87MG). CREB3 was upregulated the most in U251MG cells compared with HA1800 cells; in fact, there was no difference in CREB3 levels between SHG-44 cells and HA1800 cells ( Figure 3A-3C).
To determine the function of CREB3 in glioblastoma, we overexpressed CREB3 in SHG-44 cells. CREB3 was markedly upregulated in SHG-44 cells transfected with lenti-CREB3 plasmids ( Figure 3D-3F). A CCK-8 assay demonstrated that CREB3 overexpression markedly enhanced cell proliferation compared with NC treatment ( Figure 3G). Similarly, an immunofluorescence assay revealed that KI67-positivity was significantly greater among CREB3-overexpressing cells than among NC cells ( Figure 3H and 3I). These results indicate that the upregulation of CREB3 promoted the proliferation of SHG-44 cells.

The downregulation of CREB3 induced apoptosis in U251MG cells
We next used two different shRNAs (CREB3-shRNA1 and CREB3-shRNA2) to downregulate CREB3 in  U251MG cells. QRT-PCR and Western blotting illustrated that CREB3-shRNA2 downregulated CREB3 more significantly than CREB3-shRNA1 in U251MG cells ( Figure 4A-4C). The apoptosis rate increased markedly in U251MG cells following their transfection with CREB3-shRNA2 ( Figure 4D and 4E). Moreover, the protein levels of BAX and active caspase 3 were greater in U251MG cells transfected with CREB3-shRNA2 than in those transfected with the NC ( Figure 4F and 4G). These data illustrate that the downregulation of CREB3 induced apoptosis in U251MG cells.

The upregulation of CREB3 increased the invasion of SHG-44 cells
Since glioblastoma is an aggressive type of brain tumor [18], we investigated whether overexpressing or downregulating CREB3 would alter the invasive abilities of glioblastoma cells. As shown in Figure 5A and 5B, the invasive abilities of SHG-44 cells increased significantly after the cells were transfected with lenti-CREB3 plasmids. On the other hand, the downregulation of CREB3 markedly inhibited the invasion of U251MG cells compared with NC treatment ( Figure 5C and 5D).

The downregulation of CREB3 activated ERS pathway proteins in U251MG cells
We next explored whether the inhibition of CREB3 induced apoptosis in glioblastoma cells by regulating the ERS pathway. As indicated in Figure 6A-6E, the downregulation of CREB3 significantly increased the protein levels of p-PERK, p-eIF2α and ATF4 compared with NC treatment. In addition, as shown in Figure 7A, the level of PERK was markedly decreased following transfection with PERK-siRNA2. Moreover, downregulation of CREB3 significantly increased the protein levels of p-PERK, p-eIF2α and ATF4 in U251MG cells, which were obviously reversed following transfection with PERK-siRNA2 ( Figure 7B-7E). Furthermore, downregulation of CREB3 obviously inhibited the proliferation of U251MG cells, while downregulated PERK in CREB3-shRNA2 treated cells, significantly reversed the growth-inhibitory role of CREB3-shRNA2 in U251MG cells ( Figure 7F). Meanwhile, downregulation of CREB3 obviously induced apoptosis of U251MG cells, which was markedly reversed following transfection with PERK-siRNA2 ( Figure 7G and 7H). These results suggest that the downregulation of CREB3 induced apoptosis in U251MG cells by activating ERS pathway proteins. AGING

The downregulation of CREB3 inhibited the tumor growth of U251MG xenografts in vivo
We then established a xenograft model to further assess the effects of CREB3 on glioblastoma tumor growth in vivo. As indicated in Figure 8A and 8B, the downregulation of CREB3 significantly repressed xenograft tumor growth. The tumor weights were markedly lower in the CREB3-shRNA2 group than in the NC group ( Figure  8C). The relative mRNA levels of CREB3 were notably lower in CREB3-shRNA2-transfected tumor tissues than in NC-transfected tissues ( Figure 8D). Additionally, a TUNEL assay demonstrated that apoptosis was induced in the CREB3-shRNA2 group compared with the NC group ( Figure 8E and 8F). Meanwhile, we tested the expressions of Bax and active caspase 3 in tumor tissues in vivo. The data indicated that knockdown of CREB3 significantly upregulated the protein levels of Bax and active caspase 3 in tumor tissues ( Figure 8G and 8H). As shown in Figure 9A-9E, the downregulation of CREB3 significantly increased the protein levels of p-PERK, p-eIF2α and ATF4 in vivo. These data suggest that the downregulation of CREB3 inhibited U251MG xenograft tumor growth in vivo by activating ERS pathway proteins.

DISCUSSION
Relatively few biomarkers have been applied to cancer diagnosis, therapy and prognostic evaluation, so it is important to explore new biomarkers to enhance the treatment and prognostic assessment of cancer patients [19]. In this study, GO and KEGG analyses were used to identify DEGs in patients with glioblastoma. One of these DEGs was CREB3, which is known to promote the malignant progression of glioblastoma [2].   CREB3 is most highly expressed in the hypothalamus, hippocampus and anterior pituitary [20]. Xue et al. found that the downregulation of CREB3 inhibited the growth of glioblastoma [2]. In the present study, CREB3 mRNA levels were found to be significantly greater in glioblastoma tumor tissues than in adjacent normal tissues. Additionally, lower levels of CREB3 were associated with higher relapse-free and overall survival rates in patients with glioblastoma. Thus, CREB3 may function as a tumor promoter in glioblastoma patients.
To assess the effects of CREB3 in glioblastoma, we employed a human astrocyte cell line (HA1800) and three glioblastoma cell lines (SHG-44, U251MG and U87MG). Our qRT-PCR and Western blot data indicated that CREB3 levels were greater in U251MG cells than in HA1800 cells, but were similar in SHG-44 and HA1800 cells. Therefore, we transfected SHG-44 cells with a CREB3 overexpression lentivirus, and transfected U251MG cells with CREB3-shRNA2. The overexpression of CREB3 promoted the proliferation and invasion of SHG-44 cells, while the downregulation of CREB3 inhibited the tumor growth and invasion of U251MG cells. In addition, the silencing of CREB3 induced apoptosis in U251MG cells and increased the levels of BAX and active caspase 3. A previous report indicated that the knockdown of CREB3 regulatory factor (CREBRF) inhibited cell proliferation by suppressing AKT signaling [21]. This evidence supports the notion that CREB3 functions as a tumor promoter.
CREB3 is an ERS-related protein [22,23] that downregulates the UPR as a membrane-bound basic leucine zipper transcription factor [24]. The UPR is primarily regulated by three ER-membrane-bound proteins -PERK, IRE1 and ATF6 -the latter of which also contains a basic leucine zipper domain [24]. We found that knocking down CREB3 increased p-PERK, p-eIF2α and ATF4 levels and induced apoptosis in vitro and in vivo. A previous study suggested that the inhibition of IL-6 could induce apoptosis by regulating the CREBRF-CREB3-ATG5 pathway [25]. The upregulation of CREB3 has been reported to protect cells against ERSinduced apoptosis [23]. Ma et al. indicated that the activation of PERK-eIF2α-ATF4 could induce apoptosis in glioma cells [26]. Therefore, the knockdown of CREB3 may have induced apoptosis in U251MG cells in vitro and in vivo by activating ERS pathway proteins. In conclusion, we demonstrated that CREB3 was upregulated in patients with glioblastoma. In glioblastoma cells, the overexpression of CREB3 increased proliferation and invasion, while the downregulation of CREB3 induced apoptosis and inhibited invasion. In addition, the knockdown of CREB3 activated ERS pathway proteins and induced apoptosis in U251MG cells in vitro and in vivo. These data suggest a novel mechanism whereby CREB3 may function as a tumor promoter by suppressing the ERS pathway in patients with glioblastoma. However, the complete mechanism by which knocking down CREB3 induces apoptosis in glioblastoma requires further investigation.

RNA sequencing
For RNA sequencing, total RNA was extracted from glioblastoma tissues and adjacent normal tissues with Trizol (Invitrogen, Carlsbad, CA, USA) according to the manufacturer's instructions. A Nanodrop 2000 spectrophotometer (Thermo Fisher Scientific) was used to determine the purity and quality of the total RNA samples. Small RNA libraries were generated with a Small RNA Library Preparation Kit (Illumina, CA, USA) [27]. quantitative real-time PCR (qRT-PCR) were used to analyze the RNA sequences.

GO and KEGG pathway analyses
GO (http://www.geneontology.org/) and KEGG (http://www.genome.jp/kegg/) enrichment analyses are widely used to determine potential gene annotations. These online bioinformatics tools allow the functional analysis of extensive lists of genes and proteins. In the present study, DEGs (http://mathworld.wolfram.com) were obtained by hypergeometric tests. GO analysis was used to allocate protein biomarkers into their corresponding pathways at three levels: biological process, cellular component and molecular function. The criteria used for GO and KEGG analyses were Pvalues ≤ 0.05. Then, a volcano map was generated.

Cell culture
The human astrocyte cell line HA1800 and glioblastoma cell lines SHG-44, U251MG and U87MG were purchased from the American Type Culture Collection (ATCC, Rockville, MD, USA). All these cell lines were cultured in Roswell Park Memorial Institute (RPMI)-1640 medium (Gibco, USA) supplemented with 10% fetal bovine serum (Gibco), 100 U/mL penicillin and 100 U/mL streptomycin (Thermo Fisher Scientific) in a humidified incubator with 5% CO2 at 37°C.

qRT-PCR
Trizol reagent (Invitrogen) was used to extract total RNA from cells in accordance with the manufacturer's procedure. Next, a cDNA synthesis kit (Invitrogen) was used to synthesize cDNA. Then, qRT-PCR was performed with a SYBR Premix Ex Taq II kit (TaKaRa, Dalian, China) on an ABI 7900HT instrument (ABI, NY, USA).

Flow cytometric analysis of cell apoptosis
U251MG cells were resuspended in a binding buffer at a concentration of 5x10 5 cells/mL. Then, the cells were treated with 10 μL of Annexin V-fluorescein isothiocyanate (FITC) and propidium iodide (PI). The cells were vortexed and incubated for 15 min at room temperature. Apoptotic cells were quantified with a BD Accuri™ C6 flow cytometer (BD Biosciences). BD Accuri™ C6 software (BD Biosciences) was used to analyze the results.

Matrigel invasion assay
Cell invasion was assessed in Transwell chambers (24well type, 8 µm pore size; Corning Incorporated, Corning, NY, USA). Briefly, the upper chamber was pre-treated with 100 μL of Matrigel. Then, SHG-44 or U251MG cells were seeded into the upper chamber with serum-free medium, while RPMI-1640 medium containing 10% fetal bovine serum was added to the lower chamber. The plates were incubated for 24 h at 37°C to allow the cells on the upper surface of the filter to migrate to the lower chamber. Then, the cells that had migrated were fixed in 100% methanol and stained with 0.05% crystal violet at room temperature. The invading cells were visualized and counted in three randomly selected fields. AGING

Animal study
Five-week-old male nude mice (n=24) were obtained from the Laboratory Animal Center of China (Shanghai, China). U251MG cells (5x10 6 cells in 100 μL of phosphate-buffered saline) with one of three different treatments (blank, NC or CREB3-shRNA2) were subcutaneously injected into the right armpit area of each mouse. The tumor volume was measured with Vernier calipers every week according to the formula: volume = (length × (width 2 )) / 2. After five weeks, the mice were sacrificed. The entire tumor from each mouse was excised, weighed and used for TUNEL staining experiments. All animal experiments were performed in accordance with our institutional guidelines, following a protocol approved by the Ethics Committees of the Affiliated Hospital of Guizhou Medical University.

TUNEL staining
Tumor tissue sections were stained with an APO-BrdU™ TUNEL Assay Kit (A23210, Thermo Fisher Scientific) in accordance with the manufacturer's instructions.

Statistical analysis
For each analysis, at least three independent experiments were performed, and all data were expressed as the mean ± standard deviation (SD). Statistical analyses were conducted with SPSS v21.0 (IBM Corp., Armonk, NY, USA) and GraphPad Prism v5.0 (GraphPad Software, Inc., La Jolla, CA, USA). Comparisons between two groups were performed with Student's t-test, while comparisons among multiple groups were performed with one-way analysis of variance followed by Dunnett's test. P<0.05 or P<0.01 was considered to indicate a statistically significant difference.