*2.11. Intracellular ROS Production, Ca2+ Concentration, and Mitochondrial Mmembrane Potential*

ROS generation was determined using the fluorogenic probe 20 ,70 -dichlorodihydrofluorescein (H2DCFDA; Thermo Fisher; Waltham, MA, USA). The mitochondrial membrane potential (MMP) was determined using the cationic JC-1 dye (BD Biosciences). The Ca2+ concentration was measured using the Ca2+-sensitive fluorescent probe Fluo-4 AM (Thermo Fisher). Cells (5 <sup>×</sup> <sup>10</sup><sup>5</sup> ) were plated in six-well plates, grown to confluence, and treated with naringenin at various concentrations (100, 250, and 500 µM) for indicated times. After incubation, cells were stained with H2DCFDA (10 µM), Fluo-4 AM (3 µg/mL), and JC-1 (5 µg/mL) to determine ROS production, Ca2+ levels, and MMP, respectively. NAC and DPI were used as ROS inhibitors, and BAPTA-AM was used to control the intracellular Ca2+ level. Cells were determined using the BD Accuri C5 flow cytometer and BD Accuri C6 software (version 1.0.264.21, BD Biosciences).

### *2.12. Small Interfering RNA Transfection*

Small interfering RNAs (siRNAs) against ATG5 (sense: 50 -GUGAGAUAUGGUUUGA AUAdTdT-30 and antisense: 30 -UAUUCAAACCAUAUCUCACdTdT-50 ), Beclin 1 (sense: 50 - GUUUGGAGAUCUUAGAGCAdTdT-30 and antisense: 30 -UGCUCUAAGAUCUCCAAAC dTdT-50 ) and a nonspecific scrambled siRNA were purchased from Sigma-Aldrich (Merck KGaA). HOS and U2OS cells (5 <sup>×</sup> <sup>10</sup><sup>5</sup> ) were seeded in six-well plates and transfected with Lipofectamine 3000 (Invitrogen, Rockville, MD, USA) mixed with a serum-free medium containing ATG5 siRNA, Beclin 1 siRNA, and scrambled siRNA. Transfection was performed under 5%CO<sup>2</sup> at 37 ◦C for 24 h.

### *2.13. Autophagy Assay*

Cell autophagy was determined using DAPgreen (Dojindo Molecular Technologies, Inc., Kumamoto, Japan) and acridine orange (AO; Sigma-Aldrich; Merck KgaA) according to the manufacturers' instructions. HOS and U2OS cells (3 <sup>×</sup> <sup>10</sup><sup>5</sup> ) were seeded in sixwell plates overnight. Subsequently, cells were washed with PBS and incubated with DAPgreen and AO for autophagy detection at 37 ◦C for 30 min. After cells were washed twice with PBS, they were treated with the indicated concentrations of naringenin (100, 250, and 500 µM) for 24 h. After treatment, the image of the cells was obtained under a 200× microscope to determine the visual intensity of green fluorescence (DAPgreen) and orange/green fluorescence (AO; autophagy) using a Nikon ECLIPSE Ti and NIS-Elements AR microscope (software version 5.02.01).

### *2.14. Caspase Activity Assay*

The caspase activity assay is based on the ability of active enzymes to cleave chromophores from the enzyme substrate Ac-DEVD-pNA (caspase-3; cat. #1008) or Ac-LEHDpNA (caspase-9; cat. #1076; BioVision Inc., Milpitas, CA, USA). To examine the activity of caspase-3 and -9, HOS and U2OS cell lysates were prepared and incubated with caspase-3 and -9 substrates. Immunocomplexes were incubated with the peptide substrate in the assay buffer (100 mM NaCl, 50 mM 4-(2-hydroxyethyl)-1-piperazine ethanesulfonic acid, 10 mM dithiothreitol, 1 mM EDTA, 10% glycerol, and 0.1% CHAPS; pH 7.4) at 37 ◦C for 6 h. The release of *p*-nitroaniline was monitored using an enzyme-linked immunosorbent assay reader at 405 nm. The results are presented as the percentage change in activity compared with the untreated control.

### *2.15. Transmission Electron Microscopy*

HOS and U2OS cells (1 <sup>×</sup> <sup>10</sup><sup>5</sup> ) were treated with naringenin as previously described. Cells were fixed for 10 min in 50% Karnovsky's fixative. Cells were collected and centrifuged at 1500× *g* for 5 min. The pellet was washed and stored in 70% Karnovsky's fixative at 4 ◦C until embedding and then analyzed by means of a transmission electron

microscope. The sections were observed under a JEOL JEM-1400 electron microscope (Tokyo, Japan).

### *2.16. Statistical Analysis*

All results were analyzed using the GraphPad Prism program (GraphPad, San Diego, CA, USA). Quantified results were expressed as the mean ± standard deviation (SD) and analyzed with one-way ANOVA followed by Fisher's least significant difference (LSD) post-hoc test. For all results, *p* < 0.05 was considered a significant difference.

### **3. Results**

### *3.1. Induction of Cell Apoptosis in Human Osteosarcoma Cells by Naringenin*

Naringenin contains the skeleton structure of flavanone and three hydroxy groups at 40 , 5 0 , and 70 carbons ((2*S*)-40 ,50 ,70 -trihydroxyflavan-4-one; Figure 1A). To determine the effect of naringenin on human osteosarcoma, HOS and U2OS cells were treated with varying concentrations of naringenin for 24 h. The results indicated that naringenin reduced the viability of osteosarcoma cells but not normal hFOB 1.19 cells in a concentration-dependent manner (Figure 1B–D), indicating that naringenin selectively inhibited the growth of osteosarcoma cells but exhibited less cytotoxicity in normal human bone cells. In addition, the antiproliferative effect of naringenin on HOS and U2OS cells was evaluated through a colony formation assay. The results demonstrated that compared with no treatment, naringenin treatment significantly reduced the number of colonies in a dose-dependent manner (Figure 1E,F). Cell apoptosis caused by naringenin was examined using with DAPI staining. The results indicated that naringenin increased DNA condensation and morphological changes in HOS and U2OS cells (Figure 1G).

To explore whether naringenin inhibited the viability and proliferation of HOS and U2OS cells by inducing apoptosis, apoptotic cells were detected by performing Annexin V/PI double-labeling and the TUNEL assay. Treatment with varying concentrations of naringenin significantly promoted the apoptosis of osteosarcoma cells in a dose-dependent manner (Figure 2A,B). Similar results were obtained in the TUNEL assay (Figure 2C,D). To investigate the inhibitory effects of naringenin on the proliferation of HOS and U2OS cells, we examined cell cycle distribution after 24 h treatment with naringenin. The percentages of naringenin-treated osteosarcoma cells in the G2/M phase were significantly higher than those of control cells (Figure 2E). To verify this change, the levels of G2/M phase regulatory proteins (CDK1 and cyclin B) were examined by western blotting. The results indicated that compared with control cells, naringenin-treated cells exhibited decreased CDK1 and cyclin B levels and upregulated p21 expression (Figure 2F). These findings indicated that naringenin induced cell apoptosis and prolonged G2/M arrest in human osteosarcoma cells.

control.

**Figure 1.** Naringenin inhibits the proliferation of human osteosarcoma cells. (**A**) Structure of naringenin. (**B**–**D**) hFOB 1.19, HOS, and U2OS cells were treated with the indicated concentration of naringenin for 24 h, and cell proliferation was determined using the CCK-8 assay. (**E**,**F**) Colony formation assay was performed in HOS and U2OS cells treated with naringenin at various concentrations (100, 250, and 500 μM) for 7 days. Quantitative results indicated the number of colonies per group. (**G**) After cells were incubated with various concentrations of naringenin for 24 h, the nucleus morphology was determined through 4′,6-diamidino-2-phenylindole staining, and cells were photographed. Magnification, ×200. Results are expressed as the mean ± SD of four independent experiments. \* *p* < 0.05 compared with **Figure 1.** Naringenin inhibits the proliferation of human osteosarcoma cells. (**A**) Structure of naringenin. (**B**–**D**) hFOB 1.19, HOS, and U2OS cells were treated with the indicated concentration of naringenin for 24 h, and cell proliferation was determined using the CCK-8 assay. (**E**,**F**) Colony formation assay was performed in HOS and U2OS cells treated with naringenin at various concentrations (100, 250, and 500 µM) for 7 days. Quantitative results indicated the number of colonies per group. (**G**) After cells were incubated with various concentrations of naringenin for 24 h, the nucleus morphology was determined through 40 ,6-diamidino-2-phenylindole staining, and cells were photographed. Magnification, ×200. Results are expressed as the mean ± SD of four independent experiments. \* *p* < 0.05 compared with control.

**Figure 2.** Naringenin induces apoptosis and cell cycle arrest at the G2/M phase in human osteosarcoma cells. (**A**–**D**) Cells were treated with naringenin at various concentrations (100, 250, and 500 μM) for 24 h. Apoptotic cells were determined using with Annexin V/PI and TUNEL staining. (**E**) Flow cytometry was performed to determine the cell cycle distribution of HOS and U2OS cells treated with naringenin at various concentrations (100, 250, and 500 μM) for 8 h. (**F**) Levels of p21, CDK1, cyclin B, and p53 proteins in naringenin-treated cells for 8 h were detected through western blotting. Results are expressed as the mean ± SD of four independent experiments. \* *p* < 0.05 compared with control. **Figure 2.** Naringenin induced apoptosis and cell cycle arrest at the G2/M phase in human osteosarcoma cells. (**A**–**D**) Cells were treated with naringenin at various concentrations (100, 250, and 500 µM) for 24 h. Apoptotic cells were determined using with Annexin V/PI and TUNEL staining. (**E**) Flow cytometry was performed to determine the cell cycle distribution of HOS and U2OS cells treated with naringenin at various concentrations (100, 250, and 500 µM) for 8 h. (**F**) Levels of p21, CDK1, cyclin B, and p53 proteins in naringenin-treated cells for 8 h were detected through western blotting. Results are expressed as the mean ± SD of four independent experiments. \* *p* < 0.05 compared with control.

### *3.2. Induction of ROS-Mediated ER Stress by Naringenin in Osteosarcoma Cells* cells pretreated with BAPTA-AM, a Ca2+ cell-permeable chelator, markedly reduced

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in osteosarcoma cells (Figure 3C,D).

*3.2. Induction of ROS-Mediated ER Stress by Naringenin in Osteosarcoma Cells* 

ROS plays a crucial role in the apoptotic process in many cell types [24]. ER stress is

To determine whether naringenin induced apoptosis by triggering ER stress, we first

examined its effect on the mobilization of Ca2+. The Ca2+ level was significantly increased in naringenin-treated cells compared with control cells (Figure 4A,B). The results revealed that aberrant Ca2+ imbalance increased in naringenin-treated cells in a dose-dependent manner. We next determined whether ER stress-associated proteins, glucose-regulated proteins (GRP78 and GRP 94), and calpain proteins (calpain I and calpain II) are induced by naringenin in osteosarcoma cells. As shown in Figure 4C, naringenin increased the expression of GRP78, GRP94, and calpain I and II in a dose-dependent manner. Moreover,

induced by the accumulation of ROS, leading to mitochondrial dysfunction and apoptosis [25]. As shown in Figure 3A,B, naringenin induced ROS production and accumulation. Pretreating cells with the antioxidant NAC and the inhibitor of the flavoproteindependent oxidase DPI reduced ROS production and naringenin-induced cell apoptosis

ROS plays a crucial role in the apoptotic process in many cell types [24]. ER stress is induced by the accumulation of ROS, leading to mitochondrial dysfunction and apoptosis [25]. As shown in Figure 3A,B, naringenin induced ROS production and accumulation. Pretreating cells with the antioxidant NAC and the inhibitor of the flavoprotein-dependent oxidase DPI reduced ROS production and naringenin-induced cell apoptosis in osteosarcoma cells (Figure 3C,D). naringenin-mediated cell apoptosis (Figure 4D). In addition, naringenin-induced Ca2+ release was inhibited by ROS inhibitors (DPI or NAC; Figure 4E). These findings indicated that naringenin induced cell apoptosis in osteosarcoma cells through ROS production and caused ER stress.

**Figure 3.** Naringenin induces reactive oxygen species (ROS) generation in human osteosarcoma cells. (**A**, **B**) Cells were treated with naringenin (100, 250, and 500 μM) for 1 h and stained with H2DCFDA. The percentage of ROS production was determined by flow cytometry. (**C**) Cells were treated as described in A-B. ROS inhibitors inhibited ROS production. (**D**) After cells were pretreated with a ROS inhibitor for 1 h, followed by stimulation with naringenin for 24 h, cell viability was examined by using the CCK-8 assay. Results are expressed as the mean ± SD of three independent experiments. \* *p* < 0.05 compared with the control group; # *p* < 0.05 compared with the naringenin-treated group. **Figure 3.** Naringenin induces reactive oxygen species (ROS) generation in human osteosarcoma cells. (**A**,**B**) Cells were treated with naringenin (100, 250, and 500 µM) for 1 h and stained with H2DCFDA. The percentage of ROS production was determined by flow cytometry. (**C**) Cells were treated as described in A-B. ROS inhibitors inhibited ROS production. (**D**) After cells were pretreated with a ROS inhibitor for 1 h, followed by stimulation with naringenin for 24 h, cell viability was examined by using the CCK-8 assay. Results are expressed as the mean ± SD of three independent experiments. \* *p* < 0.05 compared with the control group; # *p* < 0.05 compared with the naringenin-treated group.

> To determine whether naringenin induced apoptosis by triggering ER stress, we first examined its effect on the mobilization of Ca2+. The Ca2+ level was significantly increased in naringenin-treated cells compared with control cells (Figure 4A,B). The results revealed that aberrant Ca2+ imbalance increased in naringenin-treated cells in a dosedependent manner. We next determined whether ER stress-associated proteins, glucoseregulated proteins (GRP78 and GRP 94), and calpain proteins (calpain I and calpain II) are induced by naringenin in osteosarcoma cells. As shown in Figure 4C, naringenin increased the expression of GRP78, GRP94, and calpain I and II in a dose-dependent manner. Moreover, cells pretreated with BAPTA-AM, a Ca2+ cell-permeable chelator, markedly reduced naringenin-mediated cell apoptosis (Figure 4D). In addition, naringenininduced Ca2+ release was inhibited by ROS inhibitors (DPI or NAC; Figure 4E). These findings indicated that naringenin induced cell apoptosis in osteosarcoma cells through ROS production and caused ER stress.

**Figure 4.** Naringenin triggers Ca2+ release and causes endoplasmic reticulum (ER) stress in human osteosarcoma cells. (**A**,**B**) Cells were treated with naringenin (100, 250 and 500 μM) for 5 h and stained with Fluo-4 AM. Ca2+ expression was examined by using flow cytometry. (**C**) After cells were treated with varying concentrations of naringenin for 8 h, the protein expression of GRP78, GRP94, calpain I, and calpain II was examined using with western blotting, and β-actin was used as the internal control. (**D**) After cells were pretreated with BAPTA-AM for 1 h, followed by stimulation with naringenin for 24 h, cell viability was examined using the CCK-8 assay. (**E**) After cells were pretreated with a ROS inhibitor for 1 h, followed by stimulation with naringenin for 5 h, Ca2+ expression was examined by using flow cytometry. Results are expressed as the mean ± SD of four independent experiments. \* *p* < 0.05 compared with the control group. # *p* < 0.05 compared with the naringenin-treated group. *3.3. Involement of Cell Apoptosis in Naringenin-Induced Mitochondrial Dysfunction in Human Osteosarcoma*  ROS production and accumulation in mitochondria reduce MMP, thereby triggering **Figure 4.** Naringenin triggers Ca2+ release and causes endoplasmic reticulum (ER) stress in human osteosarcoma cells. (**A**,**B**) Cells were treated with naringenin (100, 250 and 500 µM) for 5 h and stained with Fluo-4 AM. Ca2+ expression was examined by using flow cytometry. (**C**) After cells were treated with varying concentrations of naringenin for 8 h, the protein expression of GRP78, GRP94, calpain I, and calpain II was examined using with western blotting, and β-actin was used as the internal control. (**D**) After cells were pretreated with BAPTA-AM for 1 h, followed by stimulation with naringenin for 24 h, cell viability was examined using the CCK-8 assay. (**E**) After cells were pretreated with a ROS inhibitor for 1 h, followed by stimulation with naringenin for 5 h, Ca2+ expression was examined by using flow cytometry. Results are expressed as the mean ± SD of four independent experiments. \* *p* < 0.05 compared with the control group. # *p* < 0.05 compared with the naringenin-treated group.

### the mitochondrial apoptotic pathway [26]. To examine the effect of naringenin on HOS and U2OS cells, JC-1 staining was performed to determine the fluorescence ratio of orange and green fluorescence between normal and unhealthy mitochondria. Orange *3.3. Involement of Cell Apoptosis in Naringenin-Induced Mitochondrial Dysfunction in Human Osteosarcoma*

fluorescence disappeared in cells when observed under the 200× microscope, indicating mitochondrial dysfunction (Figure 5A). To verify the loss of MMP, HOS and U2OS cells were analyzed using with flow cytometry, and the results revealed a shift in MMP in naringenin-treated cells (Figure 5B,C). The findings indicated the loss of MMP in cells treated with various naringenin concentrations. Mitochondrial and cytosolic proteins, cytochrome c, and proapoptotic/antiapoptotic proteins were examined in cells treated with naringenin. The release of cytochrome c due to mitochondrial dysfunction was upregulated by protein–protein interactions between Bcl-2 proteins (Figure 5D,E). ROS production and accumulation in mitochondria reduce MMP, thereby triggering the mitochondrial apoptotic pathway [26]. To examine the effect of naringenin on HOS and U2OS cells, JC-1 staining was performed to determine the fluorescence ratio of orange and green fluorescence between normal and unhealthy mitochondria. Orange fluorescence disappeared in cells when observed under the 200× microscope, indicating mitochondrial dysfunction (Figure 5A). To verify the loss of MMP, HOS and U2OS cells were analyzed using with flow cytometry, and the results revealed a shift in MMP in naringenin-treated cells (Figure 5B,C). The findings indicated the loss of MMP in cells treated with various naringenin concentrations. Mitochondrial and cytosolic proteins, cytochrome c, and proapoptotic/antiapoptotic proteins were examined in cells treated with naringenin. The release of cytochrome c due to mitochondrial dysfunction was upregulated by protein–protein interactions between Bcl-2 proteins (Figure 5D,E). Proapoptotic proteins, Bak and Bax, and cytochrome c released in the cytosol were upregulated with the increasing concentration of naringenin (Figure 5D,E). In the mitochondrial pathway of apoptosis, the downstream signaling of caspases was activated after treatment with naringenin. To determine the primary mediators of apoptosis, caspase-3 and caspase-9 were examined in

naringenin-treated HOS and U2OS cells. The results revealed the upregulation of caspases, including that of the PARP cleavage (Figure 5F). To determine the intrinsic pathway of naringenin-induced apoptosis, caspase-3 and caspase-9 activities were examined separately (Figure 5G,H). The activities of both caspases were associated with naringenin-induced cell apoptosis. Pretreatment of cells with a caspase-3 inhibitor (z-DEVD-FMK) or caspase-9 inhibitor (z-LEHD-FMK) inhibited naringenin-induced cell apoptosis (Figure 5I). Thus, these data demonstrated that naringenin induced mitochondrial dysfunction and the subsequent release of cytochrome c and activation of caspases-9 and caspases-3 in human osteosarcoma cells. mitochondrial pathway of apoptosis, the downstream signaling of caspases was activated after treatment with naringenin. To determine the primary mediators of apoptosis, caspase-3 and caspase-9 were examined in naringenin-treated HOS and U2OS cells. The results revealed the upregulation of caspases, including that of the PARP cleavage (Figure 5F). To determine the intrinsic pathway of naringenin-induced apoptosis, caspase-3 and caspase-9 activities were examined separately (Figure 5G,H). The activities of both caspases were associated with naringenin-induced cell apoptosis. Pretreatment of cells with a caspase-3 inhibitor (z-DEVD-FMK) or caspase-9 inhibitor (z-LEHD-FMK) inhibited naringenin-induced cell apoptosis (Figure 5I). Thus, these data demonstrated that naringenin induced mitochondrial dysfunction and the subsequent release of cytochrome c and activation of caspases-9 and caspases-3 in human osteosarcoma cells.

Proapoptotic proteins, Bak and Bax, and cytochrome c released in the cytosol were upregulated with the increasing concentration of naringenin (Figure 5D,E). In the

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**Figure 5.** Naringenin triggers endoplasmic reticulum (ER) stress-related mitochondrial dysfunction in human osteosarcoma cells. (**A**–**C**) Cells were treated with naringenin at various concentrations (100, 250, and 500 μM) for 24 h and stained with JC-1 to determine the reduction in mitochondrial membrane potential. The percentage of cells was analyzed by using flow cytometry, and images were obtained using a fluorescence microscope. Magnification, ×200. (**D**– **F**) Cells were treated as described in A–B. The expression of mitochondrial dysfunction-related proteins, mitochondrial and cytosolic cytochrome c, Bax, Bak, Bcl-2, Bcl-xl, caspase-3, caspase-9, and PARP was determined by western blotting. (**G**,**H**) Cells were treated as described in A–B. The upregulation of caspase-3 and caspase-9 activities was determined and analyzed using the caspase activity assay. (**I**) Cells were pretreated with caspase inhibitors separately, followed by naringenin treatment (250 μM), and cell proliferation was determined using the CCK-8 assay. Results are expressed as the mean ± SD of four independent experiments. \* *p* < 0.05 compared with the control group. # *p* < 0.05 compared with the naringenin-treated group. *3.4. Triggering of Autophagy in Human Osteosarcoma Cells by Naringenin*  **Figure 5.** Naringenin triggers endoplasmic reticulum (ER) stress-related mitochondrial dysfunction in human osteosarcoma cells. (**A**–**C**) Cells were treated with naringenin at various concentrations (100, 250, and 500 µM) for 24 h and stained with JC-1 to determine the reduction in mitochondrial membrane potential. The percentage of cells was analyzed by using flow cytometry, and images were obtained using a fluorescence microscope. Magnification, ×200. (**D**– **F**) Cells were treated as described in A–B. The expression of mitochondrial dysfunction-related proteins, mitochondrial and cytosolic cytochrome c, Bax, Bak, Bcl-2, Bcl-xl, caspase-3, caspase-9, and PARP was determined by western blotting. (**G**,**H**) Cells were treated as described in A–B. The upregulation of caspase-3 and caspase-9 activities was determined and analyzed using the caspase activity assay. (**I**) Cells were pretreated with caspase inhibitors separately, followed by naringenin treatment (250 µM), and cell proliferation was determined using the CCK-8 assay. Results are expressed as the mean ± SD of four independent experiments. \* *p* < 0.05 compared with the control group. # *p* < 0.05 compared with the naringenin-treated group.

### *3.4. Triggering of Autophagy in Human Osteosarcoma Cells by Naringenin*

Because cell autophagy regulates cell death, we examined whether naringenin can induce autophagy. The autophagy phenomenon was observed in HOS and U2OS cells treated with naringenin. Using a transmission electronic microscope, it was observed that naringenin induced the formation of numerous intracytoplasmic vacuoles in HOS cells (Figure 6A). In addition, to observe naringenin-induced autophagy, HOS and U2OS cells were stained with DAPgreen, and the green fluorescence intensity within cells was de-

group.

tected. The intensity increased with the concentration of naringenin (Figure 6B). Moreover, AO staining revealed the accumulation of acidic vesicles in HOS and U2OS cells (Figure 6C). Furthermore, we examined the expression of several proteins that serve as markers of autophagy. Naringenin increased the level of microtubule-associated protein light chain 3 (LC3)-II and the expression of ATG5, Beclin 1, and p62 in a dose-dependent manner (Figure 6D). To determine the role of autophagy in naringenin-induced cell death, siRNAs of Beclin 1 and ATG5 were transfected in HOS and U2OS cells. The results of the CCK-8 assay revealed that Beclin 1 and ATG5 siRNAs suppressed the naringenin-induced loss of cell viability (Figure 6E), indicating that the autophagic mechanism of ATG5 and Beclin 1 involved in naringenin-induced cell death promotes either survival or death and is associated with signaling transduction in programmed cell apoptosis. Furthermore, we examined naringenin-induced cell autophagy by using ROS inhibitors. We performed DAPgreen staining to observe the green fluorescence intensity (Figure 6F). Cell images were obtained using a fluorescence microscope, and it was observed that after adding the inhibitors, the autophagosome (white arrow) significantly decreased in naringenin-treated cells compared with control cells. The results indicate that naringenin-induced intracellular ROS production triggered autophagic signaling in human osteosarcoma cells. *Molecules* **2021**, *26*, x FOR PEER REVIEW 13 of 17

**Figure 6.** Naringenin initiates autophagy signaling responses in human osteosarcoma cells. (**A**) Electron microscope images of HOS cells were obtained with and without naringenin treatment for 8 h. (**B**,**C**) Cells were treated with naringenin for 8 h and stained with DAPgreen and acridine orange to determine the induction of autophagosome. (**D**) The expression of ATG5, Beclin 1, p62, and LC3-II proteins was determined by western blotting after naringenin treatment for 8 h. (**E**) siRNAs of ATG5 and Beclin 1 were transfected in cells for 24 h, followed by treatment with naringenin for 24 h. Cell proliferation was determined using the CCK-8 assay. (**F**) Cells were pretreated with ROS inhibitors, followed by **Figure 6.** Naringenin initiates autophagy signaling responses in human osteosarcoma cells. (**A**) Electron microscope images of HOS cells were obtained with and without naringenin treatment for 8 h. (**B**,**C**) Cells were treated with naringenin for 8 h and stained with DAPgreen and acridine orange to determine the induction of autophagosome. (**D**) The expression of ATG5, Beclin 1, p62, and LC3-II proteins was determined by western blotting after naringenin treatment for 8 h. (**E**) siRNAs of ATG5 and Beclin 1 were transfected in cells for 24 h, followed by treatment with naringenin for 24 h. Cell

Grapefruit is important not only as a fruit but also in traditional medicine [27]. It is a rich source of bioactive compounds that may serve as chemopreventive agents in cancer therapy [28]. Naringenin is the active component in grapefruit extract [29]. Naringenin exhibits various bioactivities including antioxidative, anti-inflammatory, and anticancer effects. Additionally, a review paper published in 2019 and showed the effect of naringenin has high concentration practices in breast cancer (SKBR3 and MDA-MD-231; 250 μM) [30] and liver cancer (HepG2; 100–200 μM) [31]. In this study, we found that the IC50 values for HOS and U2OS osteosarcoma cells after 24 h treatment with naringenin were 276 and 389 μM, respectively. Although a high naringenin concentration was used during the study, naringenin selectively inhibited the growth of osteosarcoma cells and exhibited less cytotoxicity in normal human bone cells. This is the first study to report the cytotoxic and antiproliferative effects of naringenin on human osteosarcoma HOS and U2OS cells. In our study, the results of CCK-8 and colony formation assays and DAPI

**4. Discussion** 

naringenin treatment (250 μM) for 8 h, and were then stained with DAPgreen to determine autophagosome formation. Images were obtained using a fluorescence microscope. Magnification, ×200. Results are expressed as the mean ± SD of four independent experiments. \* *p* < 0.05 compared with the control group. # *p* < 0.05 compared with the naringenin-treated proliferation was determined using the CCK-8 assay. (**F**) Cells were pretreated with ROS inhibitors, followed by naringenin treatment (250 µM) for 8 h, and were then stained with DAPgreen to determine autophagosome formation. Images were obtained using a fluorescence microscope. Magnification, ×200. Results are expressed as the mean ± SD of four independent experiments. \* *p* < 0.05 compared with the control group. # *p* < 0.05 compared with the naringenin-treated group.

### **4. Discussion**

Grapefruit is important not only as a fruit but also in traditional medicine [27]. It is a rich source of bioactive compounds that may serve as chemopreventive agents in cancer therapy [28]. Naringenin is the active component in grapefruit extract [29]. Naringenin exhibits various bioactivities including antioxidative, anti-inflammatory, and anticancer effects. Naringenin exerts in vitro effects on various cancer cell lines at high concentration; eg., on breast cancer (SKBR3 and MDA-MD-231; 250 µM) [16–18,30,31] and liver cancer cell lines (HepG2; 100–200 µM) [31]. In this study, we found that the IC<sup>50</sup> values for HOS and U2OS osteosarcoma cells after 24 h treatment with naringenin were 276 and 389 µM, respectively. Although a high naringenin concentration was used during the study, naringenin selectively inhibited the growth of osteosarcoma cells and exhibited less cytotoxicity in normal human bone cells. This is the first study to report the cytotoxic and antiproliferative effects of naringenin on human osteosarcoma HOS and U2OS cells. In our study, the results of CCK-8 and colony formation assays and DAPI staining revealed that naringenin significantly inhibited the proliferation of HOS and U2OS cells. Moreover, naringenin induced caspase-dependent apoptosis in HOS and U2OS cells, as determined through flow cytometry, TUNEL staining, and the expression of apoptosis-related proteins. In this study, we observed that naringenin exhibited potent antitumor activity against osteosarcoma in vitro. Naringenin-induced ROS overproduction resulted in ER stress activation and autophagy, which caused cell apoptosis. The findings indicated that naringenin plays multiple roles in signaling pathways in human osteosarcoma cells.

Cell apoptosis and autophagy, which are involved in the regulation of cancer cell death, have been widely studied. Autophagy plays a critical role in maintaining homeostasis and the pathogenesis of many human diseases, including cancer [32]. Autophagy, a dynamic and conserved catabolic process, plays dual roles in cell survival and death [20]. In some circumstances, autophagy can lead to cell death either in collaboration with apoptosis or as a backup mechanism when apoptosis is defective [33]. Autophagy can be induced through acute exposure to resveratrol while prolonging the activation of the caspase-mediated cell death pathway [34]. Because Bcl-2 family proteins control the release of cytochrome c during mitochondrial dysfunction, Beclin 1 can be upregulated to activate the autophagy pathway with autophagy-related genes (ATG) and their protein products [35]. Cleavage of LC3 is an essential step in autophagosome formation [36]. Thus, in this study, we examined whether naringenin induces autophagy in osteosarcoma cells. We evaluated the expression levels of the autophagy markers LC3-I/LC3-II, Beclin 1, and p62. Transmission electron microscopy was mainly used for the detection of autophagosomes and the autophagy level. Our results indicated that naringenin induced autophagy in osteosarcoma cells.

Programmed cell apoptosis can be triggered by certain endogenous or exogenous signals through various ER stress-induced signals. Condensation of chromatin, fragmentation of DNA, and shedding of small fragments from cells are the innate cellular responses of apoptosis [37]. We examined mitochondrial dysfunction-induced apoptosis and observed that caspases play critical roles in the apoptotic cascade [38]. Our results revealed that naringenin increased the protein expression of caspase-3 and caspase-9. Moreover, Bcl-2 family proteins are mitochondrial apoptotic regulators that can be either proapoptotic proteins, such as Bax, or antiapoptotic proteins, such as Bcl-2. Naringenin upregulated Bax and Bak protein expression and downregulated Bcl-2 protein expression. Our results indicated that naringenin induced apoptosis through caspase-dependent apoptotic pathways by activating caspase-3/-9 and altering Bax/Bcl-2 in HOS and U2OS cells.

ROS acts as a second messenger in cell signaling by regulating various biological processes in normal and cancer cells [39]. The ROS production is shared by most chemotherapeutics due to its role in triggering cell death [40,41]. Excessive ROS generation can result in oxidative stress, DNA damage, and cell death through apoptosis [42]. For example, Zhang et al. [43] reported that libertellenone H treatment significantly increased intracellular ROS generation and induced apoptosis in human pancreatic cancer cells in a ROS-dependent manner. Similarly, curcumin derivatives markedly increased ROS production in colon cancer cells [44]. Furthermore, naringenin-inhibited cell growth was effectively increased through intercellular ROS production [45]. ROS is crucial in cellular apoptosis and autophagy pathways [46]. For example, Zhang et al. [47] demonstrated that the induction of autophagy increased ROS-mediated apoptosis in human bladder cancer cells. They reported that apoptosis and autophagy were dependent on ROS production. The obtained results suggest that ROS plays a vital role in cancer cell apoptosis and autophagy.

Accumulating evidence has indicated that apoptosis is regulated by ER stress [48,49]. The ER is a major organelle in eukaryotic cells that is involved in protein processing, intracellular calcium storage, and lipid synthesis. In the present study, naringenin exerted potent cytotoxic and apoptotic effects on human osteosarcoma cells. We found that the intracellular ROS level was increased after naringenin treatment and that the antioxidant NAC rescued the apoptosis level. In addition, naringenin stimulated the expression of ER stress-related proteins including GRP78, GRP94, calpain I, and calpain II. Naringenininduced apoptosis was suppressed by the ER stress inhibitor BAPTA-AM in HOS and U2OS cells. These results suggest that apoptosis is regulated by ER stress signaling. Moreover, ROS inhibitors (DPI and NAC) suppressed calcium release and autophagosome formation. Therefore, naringenin induced ER stress and autophagy by promoting ROS generation, thus resulting in cellular apoptosis.

In conclusion, we demonstrated that naringenin induced apoptosis through the ROSmediated ER stress signaling pathway and autophagy. Although a high concentration of naringenin was required in vitro to cause cancer cell death, naringenin is a potential agent against human osteosarcoma cells. Future studies examining novel naringenin-based derivatives are warranted.

**Author Contributions:** C.-W.L., Y.-C.C. and J.-F.L. conceived and designed the experiments, which were performed by C.-W.L., C.C.-Y.H., K.-T.P. and M.-C.C., K.-H.L., M.-L.F. and K.-H.L. analyzed the data. K.-H.L., Y.-C.C. and J.-F.L. contributed reagents/materials/analysis tools. C.-W.L., Y.-C.C., K.-H.L. and J.-F.L. wrote the paper. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by grants from The Ministry of Science and Technology (MOST-106-2314-B-038-099-MY3, MOST-109-2320-B-255-004-MY3), Taipei Medical University (TMU108- AE1-B47). Chang Gung University of Science and Technology Chang Gung University of Science Foundation (grants ZRRPF6L0011, ZRRPF3L0091, ZORPF6L0011) and Chang Gung Medical Research Program Foundation (grants CMRPF6K0041).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study.

**Data Availability Statement:** All data and materials are available for verification as needed.

**Conflicts of Interest:** The authors declare no conflict of interest.

**Sample Availability:** Samples of the compounds are not available from the authors.
