HIF-1α/Beclin1-Mediated Autophagy Is Involved in Neuroprotection Induced by Hypoxic Preconditioning

Hypoxic preconditioning (HPC) exerts a protective effect against hypoxic/ischemic brain injury, and one mechanism explaining this effect may involve the upregulation of hypoxia-inducible factor-1 (HIF-1). Autophagy, an endogenous protective mechanism against hypoxic/ischemic injury, is correlated with the activation of the HIF-1α/Beclin1 signaling pathway. Based on previous studies, we hypothesize that the protective role of HPC may involve autophagy occurring via activation of the HIF-1α/Beclin1 signaling pathway. To test this hypothesis, we evaluated the effects of HPC on oxygen-glucose deprivation/reperfusion (OGD/R)-induced apoptosis and autophagy in SH-SY5Y cells. HPC significantly attenuated OGD/R-induced apoptosis, and this effect was suppressed by the autophagy inhibitor 3-methyladenine and mimicked by the autophagy agonist rapamycin. In control SH-SY5Y cells, HPC upregulated the expression of HIF-1α and downstream molecules such as BNIP3 and Beclin1. Additionally, HPC increased the LC3-II/LC3-I ratio and decreased p62 levels. The increase in the LC3-II/LC3-I ratio was inhibited by the HIF-1α inhibitor YC-1 or by Beclin1-short hairpin RNA (shRNA). In OGD/R-treated SH-SY5Y cells, HPC also upregulated the expression levels of HIF-1α, BNIP3, and Beclin1, as well as the LC3-II/LC3-I ratio. Furthermore, YC-1 or Beclin1-shRNA attenuated the HPC-mediated cell viability in OGD/R-treated cells. Taken together, our results demonstrate that HPC protects SH-SY5Y cells against OGD/R via HIF-1α/Beclin1-regulated autophagy.


Introduction
Cerebrovascular diseases, such as stroke, have a high incidence of morbidity and mortality. Despite decades of extensive research and clinical practice, effective treatments for stroke are still limited; thus, current research continues to explore effective means of prevention (Jain et al. 2016). One possible therapeutic approach is the use of hypoxic preconditioning (HPC). HPC, also called hypoxia-induced tolerance, is defined as a phenomenon in which a brief period of hypoxia protects against subsequent, otherwise lethal insults (Stetler et al. 2014). HPC activates multiple endogenous protective mechanisms against severe or even fatal ischemic stroke and other acute attacks (Elena and Mikhail 2015). In vivo and in vitro studies have confirmed the protective effects of HPC against cerebrovascular stroke, but the mechanism behind this protection remains unclear (Lu et al. 2005;Yu et al. 2013). Studies have shown that HPC protection is eliminated in hypoxia-inducible factor-1α (HIF-1α) knockout mice subjected to neonatal hypoxia/ischemia (Sheldon et al. 2014). HPC induces tolerance to oxygen-glucose deprivation (OGD) in astrocytes, at least in part, via HIF-1α-linked upregulation of the P450 epoxygenase pathway (Liu and Alkayed 2005). HIF-1α is critically involved in ischemic preconditioning against stroke by modulating inflammatory responses in rats , suggesting that HIF-1α plays a key role in the regulation of HPC protection.
Autophagy is a lysosome-dependent degradation pathway that can clear away damaged organelles such as mitochondria and biological macromolecules including proteins, thus maintaining intracellular homeostasis (Denton et al. 2015). Rapamycin (Rapa) regulates autophagy through inhibition of the nutrient-sensing molecule known as mammalian target of rapamycin (mTOR) (Nair and Ren 2012). Phosphatidylinositol 3-kinase (PI3K) is a key regulator of autophagy, as it plays an important role in many biological processes, including controlling the activation of mTOR. 3-Methyladenine (3-MA) inhibits autophagy by blocking autophagosome formation via the inhibition of class III PI3K (Abdulrahman et al. 2011;Jin et al. 2015). The autophagy level is low under physiological conditions but can be increased under hypoxic conditions (Park et al. 2018); this increase can aggravate injury but can also play a protective role (Denton et al. 2015). The effective control of autophagy is important for the development of new treatment strategies for hypoxia-induced brain injury. Recent studies show that HPC plays a neuroprotective role by activating modest autophagy (Sheng et al. 2014;Wang et al. 2013b;Wang et al. 2014), but the mechanism involved is unclear.
The HIF-1α signaling pathway is closely related to hypoxia-induced autophagy (Wu et al. 2015). HIF-1 is a critical nuclear transcription factor in hypoxia adaptation and is composed of a labile HIF-1α subunit and a stable HIF-1β subunit (Loboda et al. 2012;Semenza 2012). HIF-1α is degraded by proteasomes under normoxic conditions. In response to hypoxia, however, HIF-1α is transferred into the nucleus and interacts with the HIF-1β subunit to form HIF-1 to initiate or enhance transcription of effector genes, including BNIP3/BNIP3L, insulin-like growth factor II, and vascular endothelial growth factor (Chu and Jones 2016;Xiong et al. 2015).
B cell leukemia/lymphoma 2 (Bcl-2)/adenovirus (E1B)-19KD-interacting protein 3 (BNIP3) is one of the Bcl-2 homology 3 (BH-3)-only proteins. BNIP3 is known to be involved in the regulation of both apoptosis and autophagy (Vasagiri and Kutala 2014). This protein activates autophagy by inhibiting the function of mTOR (Shi et al. 2014) and dissociates Beclin1 from Beclin1/Bcl-2 or Beclin1/Bcl-XL complexes by competing with Beclin1 during the formation of the autophagosome (Ney 2015). Accumulated evidence shows that the activation of autophagy is regulated by the HIF-1α/BNIP3 pathway (Lu et al. 2016;Wu et al. 2015;Zhao et al. 2012). Beclin1 is a well-known key regulator of autophagy; this protein controls the autophagic process by regulating class III PI3K-dependent generation of phosphatidylinositol 3-phosphate (PI3P) and the subsequent recruitment of additional autophagy-associated (ATG) proteins that promote autophagosome formation (Wirawan et al. 2012), which plays an essential role in autophagy activation (Maejima et al. 2016;Sinha and Levine 2008). Recent studies have confirmed that mimicking hypoxia with CoCl 2 induces autophagy via activation of the HIF-1/BNIP3/Beclin1 pathway in skeletal myotubes (Chen et al. 2017b). In this study, we adapted an oxygen-glucose deprivation/reperfusion (OGD/R) model in SH-SY5Y cells and determined whether the HIF-1α/Beclin1 signaling pathway modulates autophagy and whether such regulation contributes to HPC-induced protection against OGD/R.

Cell Culture
Human SH-SY5Y neuroblastoma cells were purchased from the cell bank of the Chinese Academy of Sciences (Shanghai, China) and maintained in Invitrogen Dulbecco's modified Eagle's medium (DMEM; Thermo Fisher Scientific, Waltham, MA, USA) supplemented with 10% fetal bovine serum, 3.7 g/L sodium bicarbonate, 100 units/mL penicillin, and 100 g/mL streptomycin (Gibco-Thermo Fisher Scientific) at 37°C in a tissue culture incubator with 5% CO 2 and 95% relative humidity. We added 5 mM 3-MA (Sigma, St. Louis, MO, USA) or 25 mM Rapa (Cell Signaling Technology, Danvers, MA, USA) to inhibit or induce autophagy, respectively.

OGD/R and HPC
Ischemic injury was induced in vitro using OGD/R, as described previously (Darshit and Ramanathan 2016;Guo et al. 2014;Zhu et al. 2016). Briefly, 5 × 10 4 /mL cells were maintained in glucose-free and serum-free DMEM in a hypoxia incubation chamber (Thermo, Forma 3427, USA) under hypoxic conditions (1% O 2 , 5% CO 2 , and 94% N 2 ) for 10 h and then cultured under normoxic conditions (5% CO 2 , 95% air) in normal DMEM for a further 12 h. For HPC (Peterson et al. 2011;Tzeng et al. 2010;Wang et al. 2013b), cells were successively incubated (5% O 2 , 5% CO 2 , and 90% N 2 ) for 9 h and under normoxic conditions for 12 h in normal DMEM. The control culture was always maintained in normal DMEM and put in the incubator under normoxic conditions.
In Situ One-step TUNEL Cell apoptosis was determined using a terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) kit according to the manufacturer's instructions (Nanjing KGI Biological Technology Development Co. Ltd., China). Briefly, cells were fixed with 4% paraformaldehyde in PBS for 1 h followed by permeabilization with 0.1% Triton X-100 in PBS for 10 min and then incubated with TUNEL detection solution at 37°C for 1 h. Cell nuclei were counterstained with DAPI (Roche Molecular Systems, Inc., Pleasanton, CA, USA) for 20 min. TUNEL-positive cells had pyknotic nuclei with dark green fluorescent staining, indicative of apoptosis. Images were captured using a fluorescence microscope (Olympus IX71, Japan). Five random visual fields from each of five slides were observed at a magnification of × 200. TUNEL-positive cells were counted using Image-Pro Plus 5.0 (Media Cybernetics Inc., Rockville, MD, USA). The experiments were independently repeated three times.

Flow Cytometric Evaluation of Apoptosis
Quantification of apoptotic cells was also performed by flow cytometry using Annexin V-fluorescein isothiocyanate (FITC) and propidium iodide (PI). In brief, cells were seeded in 24-well plates at a density of 1 × 10 5 cells per well. Cells were harvested by trypsinization and gently washed twice with PBS after the various treatments, as indicated in the figures. The cells were stained with Annexin V-FITC using an Apoptosis Detection Kit (Beijing Biosea Biotechnology Co. Ltd., China) and PI for 15 min in the dark. Finally, apoptosis was quantified using a flow cytometer (FACSCalibur, BD Biosciences, San Jose, CA, USA). The data were collected from at least 10,000 cells derived from three independent experiments.

Knockdown of Beclin1 Expression in SH-SY5Y Cells
A short hairpin RNA (shRNA) construct was designed targeting a 21-nt sequence (5′-CCCGTGGAATGGAA TGAGATT-3′) within the human Beclin1 gene (transcript variant: NM_003766.4). The shRNA hairpin sequence is shown in Table 1, in which TTCAAGAGA was used as the stem-loop junction and TTTTTT was used as the transcription terminator. GATCC was added to the 5′ end of the positive-sense primer to complement the cut of the BamHI enzyme (sense sequence). A KpnI site was added just behind the transcription terminator for identification of positive clones. CTTAA was added to the 5′ end of the negative-sense primer to complement the cut of the EcoRI enzyme (antisense sequence). After linearization with both BamHI and EcoRI, Lenti-U6-GFPpuro vector was incubated at 16°C overnight with T4 DNA ligase, the annealed double-stranded oligo, followed by transformation of Escherichia coli Stbl3 competent cells. Positive colonies were selected for KpnI digestion and sequencing. A total 10 μg of Lenti-U6-shRNA-GFP-puro recombinant

Quantitative PCR
Total RNA was extracted from cells with TRIzol Reagent (Thermo Fisher Scientific) and transcribed into cDNA using a reverse transcription kit (Takara Biotechnology Co. Ltd., Dalian, China). The relative mRNA level of the target gene was detected using the Roche LightCycler® 480 Real-Time PCR system (Roche, Basel, Switzerland) according to the SYBR® Premix Ex Taq instructions (Takara Biotechnology Co. Ltd.). The primer sequences were as follows: forward 5′-CAGGAACTCACAGCTCCATT-3′ and reverse 5′-CATC AGATGCCTCCCCAATC-3′ for Beclin1 and forward 5′-ACCACACCTTCTACAATGA-3′ and reverse 5′-ATAG CACAGCCTGGATAG-3′ for β-actin. The results were quantified by the comparative CT (threshold cycle) method using β-actin as an internal control (Chen et al. 2017a).

Statistical Analysis
Data were analyzed using SPSS 18.0 software (SPSS Inc., Chicago, IL, USA). All values are presented as the mean ± standard deviation. Statistical analyses for the comparison of two groups were performed using Student's t test, and oneway analysis of variance followed by a post hoc least significant difference multiple comparison test was used for the comparison of more than two groups. P < 0.05 was considered statistically significant.

HPC Protects SH-SY5Y Cells Against OGD/R Injury
To investigate the effects of HPC on OGD/R-induced SH-SY5Y cells, we conducted experiments as illustrated in Fig. 1a. Cells were treated with hypoxia for 9 h followed by conditions. d Summary of the mean ± SD from three independent experiments. *P < 0.05, **P < 0.01, and ***P < 0.001 vs. control group; ## P < 0.01 vs. OGD/R group normoxia for 12 h, and thereafter, they were treated with OGD for 10 h followed by reperfusion for 12 h. Next, cell viability, apoptosis, and cleaved caspase-3 level were assessed. As shown in Fig. 1c, OGD/R treatment significantly decreased cell viability compared with that of control cells. The OGD/Rinduced reduction in cell viability was significantly alleviated by HPC treatment (Fig. 1c). Additionally, OGD/R-induced cell apoptosis was significantly attenuated by HPC treatment ( Fig. 1d-g). The cleaved caspase-3 level, which was robustly induced by OGD/R treatment, was significantly suppressed by HPC treatment (Fig. 1h, i). Our results demonstrate that HPC protected SH-SY5Y cells against OGD/R-induced injury.

HPC Activates Autophagy in SH-SY5Y Cells
LC3-II is a hallmark protein of autophagy. Under normal conditions, LC3 protein exists in the cytosol as type I (LC3-I).
When autophagy is activated, LC3-I is recruited to autophagosomes and subsequently converted to LC3-II (Mizushima 2004). As shown in Fig. 2a, b, the ratio of LC3-II to LC3-I was increased in cells treated with HPC or OGD/R compared with that in cells under normal conditions; the LC3-II/LC3-I ratio in cells subjected to HPC followed by OGD/R was also increased, exceeding the value for cells treated with OGD/R alone. Protein p62, one of the selective substrates of autophagy, is localized at autophagosomes and constantly degraded by the autophagy-lysosomal pathway, which increases in response to a decrease in autophagic activity or a total loss of autophagic function (He et al. 2015). As shown in Fig. 2c, d, the p62 level was decreased in cells treated with HPC or OGD/R compared with that in cells kept under normal conditions. The p62 level in cells subjected to HPC followed by OGD/R was also decreased, reaching a value significantly lower than that in cells treated with OGD/R alone.
The results demonstrated that both HPC and OGD/R upregulated the autophagy level in SH-SY5Y cells. Moreover, HPC further increased the autophagy level, followed by OGD/R.

Autophagy Is Involved in the Neuroprotection of HPC Against OGD/R-Induced Injury
To examine whether autophagy was involved in neuroprotection by HPC against OGD/R-induced injury, we applied Rapa, an agonist of autophagy (Nair and Ren 2012). Treating the cells with Rapa increased OGD/Rcell viability (Fig. 3a, b). Additionally, OGD/R-induced apoptosis was significantly suppressed by Rapa (Fig.  3c-f). These results show that Rapa protects SH-SY5Y cells against OGD/R-induced injury through activating autophagy (Fig. 3g, h).
The role of autophagy in HPC-induced neuroprotection was further studied by using 3-methyladenine (3-MA), an autophagy inhibitor. As shown in Fig. 4, administration of 5 mM (McFarland et al. 2013) 3-MA to cells treated with HPC + OGD/R significantly decreased cell viability and promoted cell apoptosis (Fig. 4b-d). Moreover, 3-MA significantly increased the cleaved caspase 3 level in cells treated with HPC + OGD/R (Fig. 4e, f). Our results indicate that inhibition of autophagy with 3-MA attenuated HPC-induced neuroprotection, providing evidence to suggest that autophagy is involved in the mechanism by which HPC protects SH-SY5Y cells against OGD/R-induced injury.

HPC-Induced Autophagy Is Associated with the HIF-1α/BNIP3/Beclin1 Signaling Pathway in Control Cells
We next studied the effect of YC-1, an HIF-1α inhibitor (Rajoria et al. 2014), on the expression of HIF-1α/BNIP3/ Beclin1 pathway components and LC-3 protein. As expected, the basal level of HIF-1α was relatively low, and treatment with YC-1 (10 μM) did not alter the levels of HIF-1α, BNIP3, or Beclin1 or the ratio of LC3-II to LC3-I. Compared with the control, the levels of HIF-1α, BNIP3, and Beclin1 (defined as 100%) and the ratio of LC3-II to 408.4 ± 26.7,525.0 ± 43.3%, and 1.1 ± 0.1, respectively, demonstrating that HPC robustly increased the levels of HIF-1α, BNIP3, and Beclin1 and the ratio of LC3-II/LC3-I. Treating the cells with YC-1 reduced the expression of the above proteins (Fig. 5). These results suggest that HPC activated autophagy, likely through the activation of the HIF-1α/BNIP3/Beclin1 signaling pathway.
To further confirm the role of the HIF-1α/Beclin1 signaling pathway in HPC-induced autophagy, we stably knocked down Beclin1 by using a Beclin1-shRNA lentiviral vector (Fig. 6). The expression levels of HIF-1α and BNIP3 remained unchanged in Beclin1knockdown SH-SY5Y cells after treatment with HPC, but the LC3-II/LC3-I ratio (0.18 ± 0.01) was significantly lower than that of negative control cells (0.66 ± 0.04), indicating that suppression of Beclin1 expression counteracts HPC-induced autophagy (Fig. 7). These results together with the results described above (Fig. 5) support the notion that HPC triggers autophagy through activating the HIF-1α/Beclin1 signaling pathway. Fig. 3 Effect of the autophagy agonist rapamycin on OGD/R-induced injury to SH-SY5Y cells. a Representative images showing the cell morphology of control cells, cells treated with 3 mM rapamycin (Rapa), and cells subjected to OGD/R with or without treatment using 3 mM Rapa 2 h before and during OGD/R. Scale bar is 100 μm. b Summary of the mean cell viability determined by MTT from three independent experiments using six wells of cells in each experiment. c Representative fluorescent images showing TUNEL staining (green), DAPI nuclear staining (blue), and merged images under the conditions shown in a. Scale bar is 100 μm. d Summary of the percentage of TUNEL-positive dead cells as shown in c, from three independent experiments with five different images analyzed in each experiment. e, f Analysis of cell death using flow cytometry under the conditions indicated above. g, h Western blotting showing the expression of LC-3 protein in SH-SY5Y cells. The mean results are from three independent experiments. *P < 0.05; **P < 0.01, and ***P < 0.001 vs. control group. ## P < 0.01 and ### P < 0.001 vs. OGD/R group HPC Enhanced Autophagy and the HIF-1α/BNIP3/Beclin1 Signaling Pathway in OGD/R-Injured Cells As shown in Fig. 8, the expression levels of HIF-1α, BNIP3, and Beclin1 were increased in cells treated with OGD/R (HIF-1α: 211.1 ± 12.9%; BNIP3: 172.9 ± 12.7%; Beclin1: 191.8 ± 7.3%) or HPC + OGD/R (HIF-1α: 415.5 ± 22.3%; BNIP3: 302.8 ± 10.3%; Beclin1: 407.5 ± 24.3%) compared with the levels in cells under control culture conditions. The ratio of LC3-II to LC3-I in cells subjected to OGD/R (0.65 ± 0.04) or HPC + OGD/ R (1.05 ± 0.03) was also increased. Moreover, cells treated with HPC + OGD/R had higher expression levels of the proteins mentioned above than did cells in the OGD/R group, suggesting that HPC enhanced OGD/R-induced autophagy and HIF-1α/BNIP3/Beclin1 pathway activity.
Treatment with YC-1 reduced cell viability in HPC + OGD/R cells, implying that the HIF-1α inhibitor counteracted the protective effects of HPC. Moreover, the cell viability of Beclin1knockdown cells was 68.0 ± 4.3% in the HPC + OGD/R group, which was significantly lower than the viability of the negative control cells (78.5 ± 4.8%) (Fig. 9b); this finding implies that, relative optical density of protein bands on the blots, estimated using ImageJ software. Data shown as the mean ± SD from three independent experiments. *P < 0.05, **P < 0.01 vs. control group; # P < 0.05 vs. HPC group compared with control cells, Beclin1-knockdown SH-SY5Y cells had reduced sensitivity to HPC, suggesting that Beclin1 knockdown attenuated the protective effects of HPC.

Discussion
Ischemia reperfusion causes neuronal injury (Lin et al. 2016). SH-SY5Y human neuroblastoma cells are derived from the neural crest during neurodevelopment, exhibiting similar biochemical and functional properties to neurons (Shipley et al. 2016). In this study, a model of OGD/R-induced SH-SY5Y cell injury was established by using a hypoxic incubator, as in previous reports (Darshit and Ramanathan 2016;Guo et al. 2014;Zhu et al. 2016).
HPC refers to a nonfatal hypoxia exposure that confers a high degree of tolerance to subsequent severe hypoxia or other normally fatal stress, thereby offering a protective mechanism Fig. 7 Effect of Beclin1-shRNA on HPC-induced HIF-1α, BNIP3, Beclin1, and LC-3 protein expression in SH-SY5Y cells. a Western blots showing the expression of HIF-1α, BNIP3, Beclin1, and LC-3 proteins after Beclin1-shRNA transfection of SH-SY5Y cells with HPC treatment. b-e Graphs showing the mean results for relative optical density of protein bands on the blots, estimated using ImageJ software. Data are shown as the mean ± SD from three independent experiments. *P < 0.05, **P < 0.01 vs. control group; ## P < 0.01 vs. HPC group of negative control (NC) cells Fig. 8 Effect of HPC on LC-3 and the HIF-1α/BNIP3/Beclin1 signaling pathway in OGD/R-treated SH-SY5Y cells. a Western blots showing the expression of HIF-1α, BNIP3, Beclin1, and LC-3 proteins. b, c Graphs showing the mean results for relative optical density of protein bands on the blots, estimated using ImageJ software. Data are shown as the mean ± SD from three independent experiments. *P < 0.05, **P < 0.01 vs. control group; ## P < 0.01 vs. OGD/R group (Nikiforou et al. 2015). HPC has been confirmed to play a protective role not only in cardiovascular and renal systems but also in the central nervous system. However, the underlying mechanisms remain poorly understood. Recent study found that autophagy contributes to the neuroprotection of HPC against global cerebral ischemia and ethanol neurotoxicity in rats (Wang et al. 2013b;Zhan et al. 2017). Our study showed that HPC protects SH-SY5Y cells against OGD/R injury. A previous study reported that HPC did not cause neurotoxicity but induced autophagy (Shao and Lu 2012), and our data showed that HPC not only activates autophagy in normal SH-SY5Y cells but also further enhances autophagy in OGD/R-treated SH-SY5Y cells.
Autophagy, a mechanism of defense and stress regulation, plays an important role in maintaining cellular homeostasis and cell survival (Galluzzi et al. 2014). Autophagy has both pro-survival and pro-death functions, and recent studies have confirmed that enhancing autophagy has a protective effect against brain damage Dai et al. 2017;Shen et al. 2016;Xie et al. 2016). Our results demonstrated that the autophagy agonist Rapa provided protection against OGD/R-induced injury. Moreover, 3-MA attenuated HPC-induced protection. Taken together, these results suggest that autophagy is critically involved in the protective effect of HPC, consistent with previous studies (Cui et al. 2015;Tzeng et al. 2010).
Our results also showed that HPC activated autophagy while upregulating the HIF-1α/BNIP3/Beclin1 signaling pathway in SH-SY5Y cells, an effect that was counteracted by YC-1 treatment. Moreover, knockdown of Beclin1 expression suppressed HPC-induced autophagy without any effect on the expression of HIF-1α or BNIP3. Based on these findings and on previous studies (Bellot et al. 2009;Zhao et al. 2012), we conclude that HPC activates autophagy through the HIF-1α/BNIP3/Beclin1 signaling pathway in SH-SY5Y cells. In addition, HPC further activates autophagy and enhances HIF-1α/BNIP3/Beclin1 pathway in OGD/R-treated cells. Either treatment with YC-1 or knockdown of Beclin1 expression attenuates HPCinduced neuroprotection, suggesting that the HIF-1α/ Beclin1 signaling pathway is involved in HPC-induced autophagy and neuroprotection.
In conclusion, our study reveals that HPC attenuates OGD/R-induced injury in SH-SY5Y cells by activating autophagy, and the HIF-1α/Beclin1 signaling pathway is involved in HPC-activated autophagy. Furthermore, HIF-1α and Beclin1 are also involved in HPC-induced neuroprotection.
Funding Information This work was supported in part by two grants from the National Natural Science Foundation of China (NSFC; grant numbers: 81771517, 81571205) and by a Science and Technology Key Research Project Grant from the Department of Education of Henan Province (No. 14A310009).

Compliance with Ethical Standards
Conflict of Interest The authors declare that they have no conflicts of interest. Fig. 9 The HIF-1a/Beclin1 signaling pathway is involved in regulating the protective effect of HPC against OGD/R-induced injury. a Summary of the mean cell viability determined by MTT under the indicated conditions. b Effect of Beclin1 knockdown on the protective effect of HPC. Summary of the mean cell viability determined by MTT, from three independent experiments using six wells of cells in each experiment. Data are shown as the mean ± SD. *P < 0.05, and **P < 0.01 vs. control group; # P < 0.05, and ## P < 0.01 vs. OGD/R group; △ P < 0.05, and △△ P < 0.01 vs. HPC + OGD/R group of negative control (NC) cells Open Access This article is distributed under the terms of the Creative Comm ons Attribution 4.0 International License (http:// creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.