Effect of apigenin isolated from Aster yomena against Candida albicans: apigenin-triggered apoptotic pathway regulated by mitochondrial calcium signaling

Author’s Accepted Manuscript Effect of apigenin isolated from Aster yomena a g a i ns t Candida albicans: apigenin-triggered apoptotic pathway regulated by mitochondrial calcium signaling Wonjong Lee, Eun-Rhan Woo, Dong Gun Lee www.elsevier.com/locate/jep

PII: DOI: Reference:

S0378-8741(18)33226-4 https://doi.org/10.1016/j.jep.2018.11.005 JEP11586

To appear in: Journal of Ethnopharmacology Received date: 1 September 2018 Revised date: 22 October 2018 Accepted date: 3 November 2018 Cite this article as: Wonjong Lee, Eun-Rhan Woo and Dong Gun Lee, Effect of apigenin isolated from Aster yomena against Candida albicans: apigenintriggered apoptotic pathway regulated by mitochondrial calcium signaling, Journal of Ethnopharmacology, https://doi.org/10.1016/j.jep.2018.11.005 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Effect of apigenin isolated from Aster yomena against Candida albicans: apigenin-triggered apoptotic pathway regulated by mitochondrial calcium signaling

Wonjong Leea, Eun-Rhan Woob and Dong Gun Lee a*

a

School of Life Sciences, BK 21 Plus KNU Creative BioResearch Group, College of Natural

Sciences, Kyungpook National University, 80 Daehakro, Bukgu, Daegu, 41566, Republic of Korea b

College of Pharmacy, Chosun University, 375 Seosukdong, Donggu, Gwangju 61452,

Republic of Korea Wonjong [email protected]; Eun-Rhan Woo- [email protected]; Dong Gun [email protected]

*Corresponding author: Dong Gun Lee, School of Life Sciences, BK 21 Plus KNU Creative BioResearch Group, College of Natural Sciences, Kyungpook National University, 80 Daehakro, Bukgu, Daegu, 41566, Republic of Korea. Tel.: +82 53 950 5373; Fax: +82 53 955 5522; E-mail: [email protected] ABSTRACT 1

Ethnopharmacological relevance: Aster yomena, a perennial herb that grows mainly in South Korea, has been employed in the traditional temple food for antibiotic efficacy. Recently, it was reported that apigenin isolated from A. yomena has a physical antifungal mechanism targeting membrane against Candida albicans. Aim of the study: Our study aimed to investigate the biochemical responses underlying the antifungal activity of apigenin isolated from A. yomena due to lack studies reporting the investigation of intracellular responses of apigenin in C. albicans. Materials and Methods: Apigenin was isolated from the aerial parts of A. yomena. To evaluate apigenin-induced inhibitory effects and membrane damages, the measurement of the cell viability assay and the flux of cytosolic components were performed with at various concentrations. Intracellular external potassium and calcium levels were assayed by an ion-selective electrode meter, Fura2-AM and Rhod2-AM, respectively. Mitochondrial dysfunctions were analyzed by using JC-1, Mitotracker Green FM, and MitoSOX Red dye. H2DCFDA, glutathione, and MDA assay were used to detect oxidative damage. Also, flow cytometry was carried out to detect apoptotic hallmarks using Annexin V-PI, TUNEL, and FITC-VAD-FMK staining. Tetraethylammoniumchloride (TEA), Ruthenium red (RR), and N-acetylcysteine (NAC) were used as a potassium channel blocker, mitochondrial calcium uptake inhibitor, and reactive oxygen species (ROS) scavenger, respectively. Results:

2

We confirmed that there was no decrease of cell survival percentages in crude extracts of A. yomena treatment, however, only isolated apigenin has the antifungal effect in C. albicans. Apigenin triggered a dose-dependent mitochondrial calcium uptake followed by mitochondrial dysfunction, loss of the membrane potential and an increase in the mitochondrial mass and ROS. Apigenin also induced intracellular redox imbalance as indicated by the ROS accumulation, glutathione oxidation, and lipid peroxidation. Interestingly, NAC failed the restore the mitochondrial calcium levels and thus alleviate the mitochondrial damages, however, RR reduced the apigenin-induced redox imbalance. Furthermore, apigenin induced apoptosis activation marked by the phosphatidylserine exposure, DNA fragmentation, and caspase activation. The pro-apoptotic effect of apigenin was counteracted by RR and NAC pretreatment. In particular, RR significantly reduced the pro-apoptotic responses. Conclusions: Apigenin isolated from A. yomena induced mitochondrial-mediated apoptotic pathway, and mitochondrial calcium signaling is main factor in its pathway in C. albicans. Graphical abstract

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Abbreviations: CFU, Colony-forming units; GSH, Reduced glutathione; GSSG, Glutathione disulfide; H2DCFDA, 2’,7’-dichlorodihydrofluorescein diacetate; MDA, Malondialdehyde; MIC, Minimum inhibitory concentration; MMP, Mitochondrial membrane potential; NAC, Nacetylcysteine; PS, Phosphatidylserine; ROS, Reactive oxygen species; RR, Ruthenium red; TBARS, Thiobarbituric acid reactive substances; TEA, Tetraethylammonium chloride;

Keywords: apigenin; Aster yomena; Candida albicans; calcium; mitochondria; apoptosis

1. Introduction 4

Candida albicans is the most widespread human fungal pathogen responsible for clinical conditions ranging from irritation and superficial infections of the vaginal and oral mucosa to life-threatening systemic diseases in immunocompromised hosts like the elderly, infants, or patients with AIDS (Phillips et al., 2003; Yun, D.G. and Lee, D.G., 2016). With the continuous emergence of Candida strains that are resistant to conventional antibiotics, the rates of morbidity and mortality due to candidiasis are increasing progressively (Odds et al., 2003; Yapar, 2014). Thus, there is a crucial need to discover novel and more effective antifungal agents to combat fatal pathogenic fungi. The medicinal properties of many herbal plants have been used for a long time to treat with a variety of conditions (Dorman and Deans, 2000). By producing therapeutic effects such as anti-cancer activity and anti-hepatotoxic, they can exert physiological actions on the human body (Verpoorte et al., 2000). Moreover, the antimicrobial compounds that have been isolated from medicinal herbs have minimal side effects and low toxicity (Morrissey and Osbourn, 1999). For these reasons, there has been increased interest in the medicinal properties of numerous herbal plants. Especially, many plants produce special biomolecules to protect themselves in response to an infection or stress caused by pathogenic microorganisms (Yun et al., 2015). These naturally, organic, and small occurring biomolecules, which are called phytochemicals such as flavonoids, isoflavones, and anthocyanins, participate in the defense against infection by pathogenic microorganism (Goossens et al., 2003). Aster yomena is a perennial herb that grows mostly in South Korea (Kim et al., 2014). In temple food of traditional Korean Buddhism, A. yomena utilized as mixed vegetables with spices or seasoned cooked vegetables for medicinal efficacies including antibiosis, antivirus, 5

expectoration, and asthma (Ahn, 1998; Kim et al., 2006). Among the various traditionally known effects of A. yomena, we focused on the antimicrobial action. Recently, the our study reported that apigenin isolated from A. yomena causes physical damage to the fungal cell membrane resulting in the cell shrinkage and leakage of intracellular components (Lee et al., 2018). However, the study demonstrated only physical damage, but the intracellular mechanism underlying antifungal activity of apigenin remains elusive. Here, the current study investigated the biochemical responses encompassing the cellular stress induced by apigenin isolated from A. yomena in C. albicans. 2. Materials and Methods 2.1. Isolation of apigenin Apigenin was isolated from the aerial parts of A. yomena described previously (Lee et al., 2018). Aerial parts of A. yomena (Asteraceae) were collected, air dried (yield = 1.9 kg), and subjected to 3 washes (under reflux) with methanol (MeOH), resulting in the production of 120.1 g residue. The MeOH extract was resuspended in water and partitioned sequentially using equal volumes of dichloromethane (CH2Cl2), ethyl acetate (EtOAc), and n-butanol (BuOH). Each fraction was subjected to vacuum evaporation, which yielded CH2Cl2 (23.6 g), EtOAc (15.2 g), n-BuOH (48.8 g), and water (48.2 g) extracts. The CH2Cl2 fraction (15 g) was subjected to silica gel column chromatography (CC) using a gradient solvent system of hexane:acetone (100:1→1:1), and twelve subfractions (D1–D12) were collected. Subfraction D11 (2.5 g) was subjected to YMC Sep-Pack (YMC, Kyoto, Japan) fractionation using 50, 80, and 100% MeOH as elution solvents, resulting in 3 subfractions (D111–D113). Subfraction D113 (900.9 mg) was subjected to silica gel CC using a gradient solvent system of 6

chloroform:MeOH (30:1→100% MeOH). Of the 6 subfractions that were collected (D1131– D1136), subfraction D1135 (113.2 mg) was purified by semi-preparative high-performance liquid chromatography (75% MeOH), resulting in the isolation of apigenin (3.9 mg) which is dissolved in dimethyl sulfoxide (DMSO). Apigenin was subjected to fast atom bombardment mass spectrometry (FAB-MS), proton nuclear magnetic resonance (1H NMR), and 13C NMR, yielding the following data: FAB-MS m/z: 271 [M+]; 1H-NMR (500 MHz, CD3OD) δ: 7.89 (2H, d, J = 8.8 Hz, H-2' and H-6'), 6.92 (2H, d, J = 8.8 Hz, H-3' and H-5'), 6.72 (1H, s, H-3), 6.45 (1H, d, J = 2.1 Hz, H-6), 6.16 (1H, d, J = 2.1 Hz, H-8); and

13

C-NMR (125 MHz, CD3OD) δ: 181.7 (s, C-4), 165.2 (s, C-5),

163.8 (s, C-2), 161.5 (s, C-4'), 161.4 (s, C-7), 157.5 (s, C-9), 128.5 (d, C-2',6'), 121.2 (s, C-1'), 116.1 (d, C-3', 5'), 103.5 (s, C-10), 102.8 (d, C-3), 99.2 (d, C-6), 94.2 (d, C-8). 2.2. Cell culture preparation C. albicans (ATCC 90028) culture was obtained from the American Type Culture Collection Center (ATCC; Manassas, VA, USA). The yeast cells were grown on yeast extractpeptone-dextrose broth (YPD; Difco, Sparks, MD, USA) agar plates for 15 h at 28°C. and dissolved in dimethyl sulfoxide (DMSO). Amphotericin B and H2O2 were purchased from Sigma-Aldrich (St Louis, MO, USA). Tetraethylammoniumchloride (TEA), Ruthenium red (RR), and N-acetylcysteine (NAC) were used as a potassium channel blocker, mitochondrial calcium uptake inhibitor, and ROS scavenger, respectively and were procured from SigmaAldrich. 2.3. Cell viability with apigenin treatment

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Antifungal activity of crude extracts and apigenin was tested with the colony-forming units (CFUs) assay. C. albicans cells were treated with different concentrations of MeOH extracts and apigenin and incubated for 12 h at 28 °C. Cells were washed with phosphate buffered saline (PBS) and diluted serially. Each dilution was plated on YPD agar and incubated at 28 °C overnight. The number of CFUs/mL was used to calculate the percentage cell viability using the following formula, (CFU of cells treated with test compounds)/(CFU of untreated cells) × 100 (Yun and Lee, 2017). 2.4. Membrane disruption assay Harvested C. albicans (2 × 105 cells/mL) cells were resuspended in phosphate buffered saline (PBS), treated with various concentrations of apigenin (0.625, 1.25, 2.5, 5, and 10 µg/mL), and incubated at 28 °C for 4 h. After incubation and centrifugation, the cells were resuspended in PBS. Membrane disruption was measured by staining with 9 μM propidium iodide (PI) according to the manufactrurer’s instructions and analyzed using a FACSVerse flow cytometer (Becton Dickinson, NJ, USA) (Lee et al., 2018). 2.5. Measurement of external potassium levels To measure the external potassium levels, the C. albicans cells (2 × 105 cells/mL) were treated with apigenin for 4 h at 28 C. After centrifugation at 12,000 rpm for 5 min (Sorvall Biofuge Fresco), supernatants were transferred to 24-well plates. A potassium ionic strength adjuster was resuspended in 3 mL distilled water and incubated for 10 min for stabilization. The potassium voltage was then measured using an ion-selective electrode meter (Orion Star A214; Thermo Fisher Scientific, USA). Sonication was performed to determine the total 8

external potassium voltage. External potassium level (expressed in percentage) was calculated by the following formula: external potassium level (%) = 100

×

([K+]−[K+]0)/([K+]t−[K+]0), where [K+] represents the potassium voltage of the treated samples, and [K+]0 and [K+]t denote the potassium voltages of untreated and sonicated samples, respectively. 2.6. Analysis of cytosolic and mitochondrial calcium levels Fura-2AM and Rhod-2AM (Molecular Probes) were used to measure the calcium levels in the cytosol and mitochondria, respectively (Roe et al., 1990). To inhibit the intracellular calcium uptake, cells (2 × 105 cells/mL) were pretreated with 0.5 mM RR for 10 min before treating with apigenin for 4 h. The cells were then washed twice with Krebs buffer (pH 7.4) and treated with 1 % bovine serum albumin and 0.01 % pluronic F-127 (Molecular Probes). The cells were stained with 5 µM Fura-2AM or 10 µM Rhod-2AM according to manufacturer’s instructions and and incubated for 40 min, after which the cells were washed twice with calcium-free Krebs buffer. Fluorescence intensities were monitored with a spectrofluorophotometer (Shimadzu RF-5301PC; Shimadzu, Kyoto, Japan) and calcium levels were measured using the RFPC software for calculations using fluorescence intensities at wavelengths of 340 nm (excitation) and 510 nm (emission) for Fura-2AM and 552 nm (excitation) and 581 nm (emission) for Rhod-2AM. 2.7. Measurement of the flux of cytosolic components Harvested C. albicans cells (2 × 105 cells/mL) were resuspended in phosphate buffered saline (PBS), treated with 2.5 µg/mL apigenin, and incubated at 28 °C for 4 h. The presence 9

of reducing sugars in the supernatant of the treated cells was estimated according to the protocol of Masuko et al. that was used for measuring neutral sugars in oligosaccharides, proteoglycans, glycoproteins, and glycolipids (Masuko et al., 2005). Total proteins concentration in the supernatant was estimated using the Bradford assay as described by Lee et al. (Lee et al., 2018). 2.8. Measurement of the change in mitochondrial membrane potential 5,5ʹ,6,6ʹ-Tetrachloro-1,1ʹ,3,3ʹ-tetraethylbenzimidazolylcarbocyanine

iodide

(JC-1;

Molecular Probes) was used to evaluate the changes in mitochondrial membrane potential (MMP). In healthy cells, the lipophilic and positively charged JC-1 dye accumulates in the negatively charged mitochondria, which results in an intense orange fluorescence in addition to the green fluorescence of the monomeric JC-1 inside the cytosol. Loss of MMP in apoptotic cells prevents JC-1 accumulation in the mitochondria, and thus, the apoptotic cells only fluoresce green (Sowa-Jasilek et al., 2016). C. albicans cells (2 × 105 cells/mL) were incubated with 2.5 µg/mL apigenin and 2.5 µM H2O2 for 4 h, and stained with 2.5 µg/mL JC1 in warm phosphate-buffered saline (PBS) for 20 min. The mean fluorescence intensities of monomeric JC-1 (FL1) and the aggregated JC-1 (FL2) were measured using the FACSVerse flow cytometer (Becton Dickinson). The ratio of FL2 to FL1 fluorescence intensities was calculated. To confirm whether ROS affects MMP, cells were pretreated with 5 mM NAC for 10 min before adding apigenin. 2.9. Analysis of mitochondrial mass change

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Mitotracker Green FM is a fluorescent dye that passively diffuses across the plasma membrane and accumulates in active mitochondria and was thus used to measure mitochondrial mass and number (Lee and Lee, 2018). C. albicans cells (2 × 105 cells/mL) were treated with 2.5 µg/mL apigenin and 2.5 µM H2O2 for 4 h followed by incubation with 0.1 µM Mitotracker Green FM for 30 min at 28 °C. The stained cells were investigated using the FACSVerse flow cytometer. To confirm whether ROS affects the mitochondrial mass, cells were pretreated with 5 mM NAC for 10 min before adding apigenin. 2.10. Detection of mitochondrial superoxide levels Mitochondrial superoxide levels were measured using the MitoSOX Red Mitochondrial Superoxide Indicator (Molecular Probes) according to the manufacturer’s instructions. C. albicans cells were cultured in YPD medium at 28 °C and then resuspended in PBS. Yeast cells (2 × 105 cells/mL) were treated with 2.5 µg/mL apigenin and 2.5 µM H2O2 at 28 °C for 4 h. The cells were harvested by centrifugation at 12,000 rpm for 5 min and incubated in PSB with 5 mM MitoSOX Red for 30 min at 28 °C. The stained cells were washed three times with PBS and the fluorescent cells were analyzed using the FACSVerse flow cytometer (Lee et al., 2017). To confirm whether ROS affects mitochondrial superoxide level, cells were pretreated with 5 mM NAC for 10 min before adding apigenin. 2.11. Intracellular ROS detection Intracellular ROS accumulation was assessed using 2’,7’-dichlorodihydrofluorescein diacetate (H2DCFDA; Molecular Probes) (Lee and Lee, 2018). C. albicans cells (2 × 105 cells/mL) were treated with 2.5 µg/mL apigenin and 2.5 µM H2O2 at 28 °C for 4 h. After 11

washing with PBS, the cells were stained with H2DCFDA following the manufacturer’s instructions and analyzed using the FACSVerse flow cytometer. To confirm whether calcium movement affects ROS generation, cells were pretreated with 0.5 mM RR for 10 min before adding apigenin. 2.12. Glutathione analysis Reduced glutathione (GSH) and glutathione disulfide (GSSG) were measured by glutathione reductase enzymatic recycling method as described by Yun et al. (Yun, J. and Lee, D.G., 2016). C. albicans cells (2 × 105 cells/mL) were treated with 2.5 µg/mL apigenin and 2.5 µM H2O2 at 28 °C for 4 h. To determine the total glutathione levels, the cells were pelleted and resuspended in 5% (w/v) 5-sulfosalicylic acid solution to prevent protein precipitation. Subsequently, the cells were exposed to three freeze-thaw cycles for cell lysis. The cell lysate was collected to measure total glutathione levels. To measure GSSG, the lysates were treated with 2-vinylpyridine at room temperature for 1 h. Glutathione levels were measured in the cell lysate at 415 nm using a spectrophotometer (DU530; Beckman Coulter, Fullerton, CA) and normalized to the total cellular protein using Bradford assay. To confirm whether calcium movement affects ROS generation and the resultant glutathione levels, cells were pretreated with 0.5 mM RR for 10 min before adding apigenin. 2.13. Lipid peroxidation assay Lipid peroxidation was quantified based on a thiobarbituric acid reactive substances (TBARS) assay (Lee and Lee, 2017). C. albicans cells (2 × 105 cells/mL) were treated with 2.5 µg/mL apigenin and 2.5 µM H2O2 for 4 h, harvested by centrifugation at 12,000 rpm for 5 12

min (Sorvall Biofuge Fresco) and sonicated twice on ice in the lysis buffer (10 mM Tris–HCl [pH 8.0], 1 mM EDTA, 100 mM NaCl, 2% Triton X-100, and 1% SDS). The unlysed cells and debris were centrifuged at 12,000 rpm for 5 min (Sorvall Biofuge Fresco) and the supernatant was added to 0.5% (w/v) thiobarbituric acid solution. The mixture was heated for 30 min at 95 °C and cooled on ice. Absorbance of the reaction mixture was measured at 532 nm using a spectrophotometer. To confirm whether calcium movement affects ROS generation and thus the lipid peroxidation, the cells were pretreated with 0.5 mM RR for 10 min before apigenin treatment and TBARS assay. 2.14. Measurement of phosphatidylserine exposure Externalization of phosphatidylserine (PS) in yeast can be monitored by double staining with annexin V-FITC and propidium iodide (Madeo et al., 2002). To prepare protoplasts, C. albicans cells (2 × 105 cells/mL) were incubated with 0.1 M potassium phosphate buffer (pH 6.0) containing 20 mg/mL lysis enzyme and 1 M sorbitol for 4 h at 28 °C. To confirm whether calcium and ROS affects the apoptotic mechanism of apigenin, RR and NAC was employed. The protoplasts were preincubated with 0.5 mM RR and 5 mM NAC for 10 min before treatment with apigenin. Cells incubated with 2.5 µg/mL apigenin or 2.5 µM H2O2 for 4 h. The FITC Annexin V Apoptosis Detection Kit (BD Pharmingen) was used to measure the PS exposed on the cell surface as a result of apoptosis using the FACSVerse flow cytometer. 2.15. Analysis of DNA fragmentation DNA fragmentation can be analyzed using the terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL) assay, which is used to detect apoptotic DNA cleavage in 13

the individual nuclei by labeling 3’-OH ends of DNA with fluorescent dUTP (Tian, J. et al., 2017). To confirm whether calcium and ROS affects the apoptotic mechanism of apigenin, RR and NAC was employed. C. albicans cells (2 × 105 cells/mL) were preincubated with 0.5 mM RR and 5 mM NAC for 10 min before treatment with apigenin. Cells incubated with 2.5 µg/mL apigenin or 2.5 µM H2O2 for 4 h. The cells were then washed with PBS (pH 7.4), and incubation in a permeabilization solution (0.1 % sodium citrate and 0.1 % Triton X-100) for 2 min on ice. The cells were washed with PBS again, and an in situ cell death detection kit (Roche) was used for the TUNEL assay at 37 °C for 1 h using the spectrofluorophotometer at wavelengths of 495 nm (excitation) and 519 nm (emission). 2.16. Measurement of metacaspase activation Caspases can be detected by a FITC-VAD-FMK staining (Tian, J. et al., 2017). VADFMK, which is a pancaspase inhibitor, was used to determine caspase activity. To confirm whether calcium and ROS affects the apoptotic mechanism of apigenin, RR and NAC was employed. C. albicans cells (2 × 105 cells/mL) were preincubated with 0.5 mM RR and 5 mM NAC for 10 min before treatment with apigenin. Cells incubated with 2.5 µg/mL apigenin or 2.5 µM H2O2 for 4 h. The cell samples were washed twice and then incubated with FITC-VAD-FMK for 20 min. Fluorescence intensity was measured using the FACSVerse flow cytometer. 2.17. Statistical analysis All the experiments were performed in triplicates and the values were expressed as the means ± standard deviation (SD). After confirming the normality of distribution using the 14

Shapiro-Wilk test, statistical comparisons between various groups were carried out by analysis of variance (ANOVA) followed by Tukey’s post-hoc test for three-group comparisons using SPSS software (version 23; SPSS/IBM). Intergroup differences were considered statistically significant at p-values < 0.05. 3. Results 3.1. Apigenin induced disruption of intracellular ion homeostasis at sub-MIC The antifungal activity was analyzed by CFU assay, and crude extracts (MeOH extracts) and apigenin were tested. There was no decrease of cell viability in various concentration treatments of crude extracts (0.625, 1.25, 2.5, 5, and 10 µg/mL) (Fig. 1). Like as crude extracts, apigenin showed no effect in low concentrations (0.625 and 1.25 µg/mL). However, cell viabilities were decreased in high concentrations (2.5, 5, and 10 µg/mL). Although minimum inhibitory concentration (MIC) of apigenin was determined by a two-fold serial dilution via the Clinical and Laboratory Standards Institute (CLSI) method in our previous study (Lee et al., 2018), interestingly, the significant decreased cell viability was observed from 1/2 MIC (2.5 µg/mL) lower than the previously reported MIC (5 µg/mL). These results demonstrate that apigenin treatment decreases the cell survival even at 1/2 MIC. On the basis of these results, we confirmed that only isolated apigenin at 1/2 MIC has the antifungal effect in C. albicans. In our previous study, apigenin at the MIC was reported to induce cell membrane perturbations (Lee et al., 2018). Membrane damage induced by apigenin contributed to the leakage of intracellular potassium, calcium, and sugars as well as the disruption of the osmotic balance, eventually leading to cell death. However, apigenin treatment at sub-MIC 15

did not significantly increase PI fluorescence (data not shown) and the resultant leakage of sugar and protein (Table 1), thus indicating that it does not affect membrane integrity. Interestingly, the apigenin treatment at sub-MIC resulted in a leakage of the intracellular potassium and an accumulation of intracellular calcium in the cytosol (Table 1). Regulation of intracellular ion concentrations is essential for the maintenance of cell volume and membrane potential, and the ion movement occurs through ion channels as well as membrane disruption (Kondratskyi et al., 2015). Therefore, the potassium ion channel blocker TEA was used to verify the cause of apigenin-induced potassium ion leakage. When apigenin was used at sub-MIC, high external potassium levels were observed (53.26 ± 2.91 %) (Fig. 2A). There was no significant increase in external potassium levels of TEA-pretreated cells (23.33 ± 3.69 %) and untreated cells (19.61 ± 6.35 %). Taken together, these results indicate that apigenin induced ion channel-mediated potassium leakage and not the cell membrane damage in C. albicans. As shown Table 1, apigenin induced cytosolic calcium ion accumulation. Cytosolic free calcium levels in the yeast are tightly regulated and maintained at low levels through calcium storage including endoplasmic reticulum, mitochondria and vacuole (Carraro and Bernardi, 2016). When calcium levels increase in the cytosol, the ions are rapidly moved to the mitochondria (Orrenius et al., 2003); therefore, we measured dose-dependent cytosolic and mitochondrial calcium levels following treatment with mitochondrial calcium uptake inhibitor RR. It reported that RR disrupt mitochondrial calcium uptake (Lupetti et al., 2004). RR also is endomembrane calcium channel blocker (Calvert and Sanders, 1995). At the MIC of apigenin, we observed decreased cytosolic calcium levels in concordance with the results of our previous study. Besides, apigenin treatment at sub-MIC increased the calcium ion 16

levels in the mitochondria (Fig. 2B, C). Pretreatment of the cells with RR resulted in a higher cytosolic calcium levels at the sub-MIC of apigenin, thus indicating that the apigenin treatment induced movement of calcium ion from the cytosol to mitochondrial in a dosedependent manner. In addition, the significant increase in the cytosolic calcium level with the research about endomembrane calcium channel blocker of RR (Calvert and Sanders, 1995) indicates that RR inhibits mitochondrial calcium uptake. 3.2. Apigenin induced mitochondrial dysfunction in C. albicans Mitochondrial calcium overload induces mitochondrial damage (Arrington et al., 2006). To investigate the apigenin-induced changes in mitochondria, JC-1, Mitotracker Green, and MitoSOX Red dyes were used. MMP is an important indicator of the mitochondrial function; therefore, JC-1 staining was used to detect MMP (Hwang et al., 2012). The cationic dye, JC-1 produces orange fluorescence owing to the formation of dye aggregates in the mitochondrial matrix of normal cells. In contrast, JC-1 is released into the cytoplasm and produces a green fluorescence in apoptotic cells due to changes in the MMP (Lee and Lee, 2018). A decrease in the mean fluorescence intensity of J-aggregates points to the release of JC-1 dye from the mitochondria due to its depolarization which is indicated by the ratio of FL2 to FL1 fluorescence. This ratio was higher in the untreated cells (50.80 ± 2.67) than in apigenintreated cells (30.76 ± 4.58) (Fig. 3A). Mitochondrial mass change was measured via Mitotracker Green staining. The fluorescence intensity in case of the apigenin-treated cells was higher than that in untreated cells (Fig. 3B), indicating increased mitochondrial mass. A increase in mitochondrial mass occurred when mitochondrial proliferation and fusion, and it was a cellular response to compensate for reduced mitochondrial function (Ampawong et al., 17

2017; Arakaki et al., 2006; Mahyar-Roemer et al., 2001; Nugent et al., 2007; Peng et al., 2016). Accordingly, the increased mitochondrial mass indicates mitochondrial dysfunction. We also used the MitoSOX Red, which is a dihydroethidium derivative designed to assay mitochondrial superoxide, to determine apigenin-induced mitochondrial disruption. MitoSOX Red fluorescence intensity after apigenin treatment increased by 66.47 ± 0.69 % compared to that in untreated cells (21.07 ± 0.38 %) (Fig. 3C), indicating that apigenin treatment increased ROS accumulation in the mitochondria. These results indicated that apigenin induced disruption of mitochondrial membrane potential and a resultant mitochondrial dysfunction. 3.3. Association between calcium and reactive oxygen species in apigenin-induced cell stress In apigenin-treated cells, intracellular ROS levels were measured by the H2DCFDA fluorescent dye. H2DCFDA reacts with accumulated intracellular ROS and is oxidized to DCF that exhibits fluorescence. Apigenin increased H2DCFDA fluorescence intensity by 45.09 ± 1.83 % compared to the untreated cells (21.34 ± 2.86 %) (Fig. 4A). The increase in the apigenin-induced fluorescence was reduced when cells were pretreated with RR (23.53 ± 2.82 %). These results suggest that apigenin-induced mitochondrial calcium overload resulted in the ROS accumulation inside mitochondria. Glutathione is considered as one of the most abundant intracellular thiol because it reaches millimolar concentration in most cell types. It plays an important role in maintaining redox homeostasis, which is critical for the proper functioning of cell (Yun, J. and Lee, D.G., 2016). When C. albicans was treated with apigenin, the GSH/GSSG ratio was decreased compared to the untreated and RR-pretreated cells (Fig. 4B). Thus, apigenin treatment led to calcium signaling dependent increase in 18

GSSG relative to GSH, indicating an oxidative stress to the cell. Lipid peroxidation is an important marker for oxidative stress because the attack of ROS forms lipid peroxides on the cellular lipids. TBARS assay was used to measure lipid peroxidation through malondialdehyde (MDA) level, which is considered as by-product of lipid peroxidation (Lee and Lee, 2017). High levels of MDA were observed after treatment with apigenin compared to the untreated and RR-pretreated cells (Fig. 4C). This confirmed that apigenin triggers oxidative damage to the intracellular lipids, and the inhibition of mitochondrial calcium overload led to reduce apigenin-induced lipid peroxidation. Taken together, these results indicate that the apigenin-induced ROS accumulation disturbs intracellular redox states and it is mediated by mitochondrial calcium signaling modulated by ROS generation. Additionally, we used NAC to investigate relationship between calcium, mitochondrial homeostasis, and ROS generation by apigenin. NAC-pretreated cells reversed the apigenininduced increase in both the cytosolic and mitochondrial calcium levels (Fig. 5A). The FL1/FL2 ratio of NAC pretreated cells was different from that observed in apigenin alone treatment (Fig. 5B). Mitochondrial mass also did not alter significantly in NAC pretreated cells compared to apigenin treatment alone (Fig. 5C). Although there were no significant changes in the mitochondrial dysfunction after NAC pretreatment, mitochondrial superoxide levels were reduced in NAC pretreated cells compared to apigenin treatment alone (Fig. 5D). In summary, these results indicate that apigenin-induced ROS did not affect disruption of calcium and mitochondria. 3.4. Apigenin triggered calcium-associated apoptosis

19

The disruption of calcium homeostasis and excessive ROS generation activated apoptotic responses (Simon et al., 2000). Thus, we confirmed induction of apoptosis using Annexin V, TUNEL assay, and FITC-VAD-FMK assay. Annexin V binds to PS and exhibits minimal binding to other phospholipids including phosphatidylcholine and sphingomyelin that present constitutively in the outer leaflet of the plasma membrane (Segawa et al., 2014). When apigenin treated, apoptotic cells with externalized PS showed increased exposure of PS on the surface compared to the untreated cells (Fig. 6A). Cells pretreated with NAC and RR exhibited reduced Annexin V associated fluorescence compared to only apigenin-treated cells. Although NAC pretreatment also resulted in the reduced fluorescence intensity compared to apigenin alone treatment, the reduction was lesser in comparison to that with RR pretreatment. This suggests that RR pretreatment reduced apigenin-induced PS externalization to a higher extent than the NAC pretreatment. As DNA fragmentation is observed during the late stage of apoptosis (Tian, J. et al., 2017), TUNEL assay was used to confirm the DNA fragmentation. Apigenin-treated cells showed increased TUNEL fluorescence intensity compared to the untreated cells (Fig. 6B). These results indicate that treatment of C. albicans with apigenin induced DNA margination and fragmentation. However, DNA fragmentation was reduced by RR or NAC pretreatment with apigenin, in particular by RR pretreatment. In yeast, metacaspases are caspase-like cysteine proteases that are implicated in mitochondrial dysfunction (Tian, J. et al., 2017). We observed that the cells treated with apigenin exhibited intracellular metacaspase activation as indicated by the increased FITC-VAD-FMK fluorescence compared to the untreated cells. Metacaspase activity increased significantly to 37.78 ± 2.81 % after treatment with apigenin compared to that in untreated cells (16.47 ± 0.28 %). However, in RR and NAC-pretreated cells with apigenin, metacaspase activity was 20

27.33 ± 0.49 % and 34.49 ± 2.35 %, respectively (Fig. 6C). These results demonstrated that apigenin induced activation of metacaspases in C. albicans, and the RR pretreatment significantly attenuated the apigenin-induced effects on metacaspase activity. Taken together, these results indicates that apigenin-induced apoptosis is more directly related to mitochondrial calcium overload rather than ROS generation. 4. Discussion A. yomena, which belongs to chrysanthemum species, has been used d as temple food in traditional Korean Buddhism with medicinal efficacies of the plant in South Korea (Kim et al., 2014; Kim et al., 2006). From the aerial parts of A. yomena, various phenolic compounds are isolated, and apigenin is one of them (Kim et al., 2014). In cell viability assay, decreased cell survival percentages in apigenin treatment demonstrated that apigenin exerts its antifungal effect in C. albicans. However, there was no decrease of cell survival percentages in crude extracts treatment, and these results suggested that MeOH extracts have no antifungal effect in C. albicans. There were a lot of compounds, including apigenin in crude extracts and they comprised a small proportion. Due to these reasons, the antifungal effect of crude extracts seemed to be influenced unless apigenin was in crude extracts. Therefore, pure and concentrated apigenin exerts the better antifungal effect. In our previous study, apigenin was reported to induce significant cell membrane perturbations at the MIC, resulting in cell shrinkage and disruption of membrane osmotic homeostasis. Membrane disruption of C. albicans led to the leakage of intracellular components such as potassium, calcium, and sugar (Lee et al., 2018). In the present study, apigenin at the MIC induced potassium efflux although the potassium ion channel blockade. It suggests that membrane disruption caused by 21

apigenin led to potassium efflux at the MIC. However, we show that only potassium efflux was generated through the potassium ion channels whereas sugar did not leak outside the cells when the cells were treated with apigenin at a concentration below MIC. It has been described that antimicrobial compounds at concentrations below the MIC, called sub-MICs, cause various effects such as altered growth kinetics, modifications to cell wall structure, loss of adhesive properties, inhibition of toxin production or enzyme, and morphological changes (Gerber et al., 2008; Khan et al., 2014; Reeks et al., 2005). These findings suggest that apigenin at sub-MIC has a potential mechanism of action against C. albicans beyond cell membrane disruption. Therefore, we investigated the underlying intracellular mechanism of action of apigenin at sub-MIC against C. albicans. The apigenin treatment at the MIC decreased cytosolic calcium levels and did not change mitochondrial calcium levels. It supports our previous study that apigenin induced calcium efflux via membrane disruption and did not affect mitochondrial calcium level at the MIC. However, the apigenin treatment at sub-MIC induced calcium overload in the mitochondria and cytosol. Intracellular calcium acts as a secondary messenger to regulate various cellular processes (Liu et al., 2015). Potassium ion moves via ion channel also might affect intracellular calcium levels change by apigenin treatment. Flavonoid can activate the mitochondrial calcium uniporter and inhibit calcium pump leading to elevation of cytosolic calcium, initiating calcium-dependent mitochondrial-mediated cell death (Horáková, 2011; Montero et al., 2004). Excessive elevation of cytosolic calcium stimulates an increase in the mitochondrial calcium levels, which in turn induce opening of mitochondrial permeability transition pore resulting in the activation of intrinsic apoptotic pathway (Giorgi et al., 2012; Kroemer et al., 2007). Hence, intracellular calcium ions did not leak but instead migrated 22

from the cytosol to mitochondria, indicating the disruption of intracellular calcium homeostasis (Yun, D.G. and Lee, D.G., 2016). Additionally, calcium plays a direct role in regulating the expression of its signaling cascades important for cell survival (Liu et al., 2015). For example, calcium-dependent cysteine proteases mediate cleavage of several members of BCL-2 family including anti-apoptotic BCL-2 and BCL-2-like protein 1 (BCLXL) as well as pro-apoptotic BID. Also, it promotes MMP and cytochrome c release in mammalian cell (Kondratskyi et al., 2015). For this reason, disruption of calcium homeostasis results in the activation of multiple processes that lead to the cell damage (Ribeiro et al., 2006). However, although the human BCL-2 family proteins can be studied in yeast using ectopic expression (Verbandt et al., 2017), other yeast homologues for BCL-2 family proteins have not yet been found, and further studies are needed for profound understanding of the Bcl2-family proteins-associated with yeast apoptosis. In yeast, increased cytosolic calcium triggered an increase in the mitochondrial calcium level resulting in the stimulation of ROS generation (Carraro and Bernardi, 2016; Hajnoczky et al., 2006). This study confirmed that the apigenin treatment induced mitochondrial damages such as significantly loss of MMP, mitochondrial superoxide generation, and increased mass, indicating mitochondrial dysfunction. Mitochondria are essential organelles that function in energy metabolism and are also linked to cellular physiology and integrity (Kang et al., 2010). When functioning of mitochondrial is impaired, which are the major sites of ROS generation, the ROS production is promoted (Hajnoczky et al., 2006; Tian, H. et al., 2017). On the contrary, mitochondrial dysfunction can also be caused by severe ROS accumulation, as ROS target mitochondria itself (Lin and Beal, 2006). However, apigenininduced calcium movement and mitochondrial dysfunction shows similar aspects even after 23

ROS scavenging. These finding suggests that the apigenin-induced disruption of calcium homeostasis and mitochondrial damage does not affect the ROS accumulation. This is probably related to the fact that mitochondrial calcium overload could induce mitochondrial dysfunction directly (Kazak et al., 2017). Therefore, it suggests that apigenin triggered intracellular calcium movement from the cytosol to mitochondria leads to the mitochondrial dysfunction, and supports that apigenin induced ROS generation through mitochondrial calcium signaling. Apigenin induced intracellular ROS accumulation, imbalance of redox system, and lipid peroxidation are reduced by the inhibition of mitochondrial calcium overload. Excessive ROS generation results in imbalance between intracellular ROS and antioxidant system followed by oxidative stress (Tian, H. et al., 2017). Oxidative stress leads to reduced enzyme activity, severe DNA damage, lipid peroxidation, and protein oxidation (Costa and Moradas-Ferreira, 2001). Glutathione is an intracellular tripeptide antioxidant that possesses two free carboxyl and one sulfhydryl groups; it is a significant antioxidant that protects intracellular biomolecules such as enzyme, proteins, lipid, DNA from oxidative stress (Li et al., 2004). When ROS are generated, GSH is oxidized to GSSG rapidly, resulting in a decrease in GSH and an increase in GSSG content through electron donation (Gurer-Orhan et al., 2004). For these reasons, alterations in glutathione levels are an indicator of the oxidative stress (Li et al., 2004). Chan et al. reported that the phenolic B-ring of apigenin generates phenoxyl radicals that can co-oxidize other molecules such as glutathione (Chan et al., 2003). This previous research suggests that the decrease in GSH/GSSG ratio by apigenin may be due to direct interaction between apigenin and glutathione. However, the current study confirmed that the oxidation of glutathione is due to apigenin-induced ROS and not because of a direct 24

interaction, as indicated by the reversal of the reduction when mitochondrial calcium channel is inhibited. Besides the changes in the glutathione levels, lipid peroxidation is also considered as an oxidative stress marker. ROS can initiate lipid peroxidation easily, resulting in the MDA formation that can cause DNA damage via reacting with DNA to form deoxyguanosine and deoxyadenosine adducts (Marnett, 1999). Taken together, these findings support that the apigenin induced mitochondrial calcium signaling, mitochondrial dysfunction and redox imbalance leads to impaired cell survival of C. albicans. Apoptosis has been defined as cellular suicide, because it involves cell death resulting from the activation of processes within the cell itself (Orrenius et al., 2015). There are two major apoptotic pathways that operate through activation of caspases: the intrinsic or mitochondrial-mediated pathway; and the extrinsic or death receptor-mediated (tumor necrosis factor receptor and Fas) pathway (Madeo et al., 2009; Mao et al., 2007; Mazzoni and Falcone, 2008). Most of the membrane, PS exists in the inner leaflet of the cytoplasmic membrane (Tian, J. et al., 2017). PS is translocated to the outer layer of the lipid bilayer while the membrane remains intact during apoptosis. Accordingly, PS translocation is considered a marker of apoptosis in yeast cells (Bulteau et al., 2012). DNA fragmentation and caspase activation are also evaluated as apoptosis hallmarks (Sowa-Jasilek et al., 2016; Tian, H. et al., 2017). Caspase cleave cellular DNA and activate a proteolytic signaling cascade that induces apoptosis leading to cell death (Higuchi, 2003; Pereira et al., 2007). Caspase-like proteases and other unidentified caspases called metacaspases, which play an important role in yeast apoptosis, have been reported in C. albicans (Hao et al., 2013). Here, we observed that the apigenin-triggered cell death is attributed to the disruption of mitochondrial calcium signaling and intracellular ROS generation. Intracellular calcium and extreme ROS production leads to 25

oxidative stress and have been implicated in apoptosis (Tian, H. et al., 2017). As calcium is one of the major signaling molecules, it has been implicated in the regulation of numerous cellular functions including cell cycle regulation, caspase activation, and apoptosis (Tiwari et al., 2017). ROS are considered as the upstream modulators of caspases and Bax protein, in turn, act downstream of the processes including gene transcription, mRNA translation and activation of caspase (Maulik et al., 1998; Schulz et al., 1997; Schulz et al., 1996). Apigenin stimulated caspase-dependent apoptosis, and it was effectuated by the intracellular calcium and ROS levels. In addition, mitochondrial calcium signaling rather than redox imbalance played a major role in the apigenin-induced activation of apoptosis pathway. Since ROS was generated through calcium accumulation in apigenin-treated cells, calcium signaling is apparently more influential on apigenin-induced apoptosis. In summary, the present study shows that apigenin isolated from A. yomena at sub-MIC induces the disruption of calcium homeostasis followed by mitochondrial dysfunction. This dysfunction causes redox imbalance and the consequent activation of caspase-dependent apoptotic responses. Although apigenin-induced oxidative damage was reduced by mitochondrial calcium channel inhibitor RR, ROS scavenger NAC did not inhibit the disruption of calcium homeostasis and mitochondrial dysfunction stimulated by apigenin. Apigenin-induced apoptotic features were reduced by both NAC and RR treatment, nevertheless RR showed a greater reduction than NAC. In a sum, apigenin isolated from A. yomena triggers to the activation of mitochondrial-mediated apoptotic pathway in C. albicans, and mitochondrial calcium signaling is main initiator in its pathway. Conflicts of interest

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The authors declare that they have no conflicts of interest. Acknowledgements This work was supported by a grant from the Next-Generation BioGreen 21 Program (Project No. PJ01325603), Rural Development Administration, Republic of Korea. Author contributions W. Lee and D.G. Lee conceived the study and designed the experiment. E. Woo prepared and isolated the curde extracts and compounds. W. Lee and D.G. Lee conducted the experiment and analyzed the data. W. Lee wrote the manuscript. References Ampawong, S., Isarangkul, D., Aramwit, P., 2017. Sericin improves heart and liver mitochondrial architecture in hypercholesterolaemic rats and maintains pancreatic and adrenal cell biosynthesis. Experimental cell research 358(2), 301-314. Ahn, D.K., 1998. Illustrated Book of Korean Medicinal Herbs. Kyohak Publishing Co., Ltd., Seoul. Arakaki, N., Nishihama, T., Owaki, H., Kuramoto, Y., Suenaga, M., Miyoshi, E., Emoto, Y., Shibata, H., Shono, M., Higuti, T., 2006. Dynamics of mitochondria during the cell cycle. Biological & pharmaceutical bulletin 29(9), 1962-1965. Arrington, D.D., Van Vleet, T.R., Schnellmann, R.G., 2006. Calpain 10: a mitochondrial calpain and its role in calcium-induced mitochondrial dysfunction. American journal of physiology. Cell physiology 291(6), C1159-1171. 27

Bulteau, A.L., Planamente, S., Jornea, L., Dur, A., Lesuisse, E., Camadro, J.M., Auchere, F., 2012. Changes in mitochondrial glutathione levels and protein thiol oxidation in yfh1 yeast cells and the lymphoblasts of patients with Friedreich's ataxia. Biochim Biophys Acta 1822(2), 212-225. Calvert, C.M., Sanders, D., 1995. Inositol trisphosphate-dependent and-independent Ca2+ mobilization pathways at the vacuolar membrane of Candida albicans. Journal of Biological Chemistry 270(13), 7272-7280. Carraro, M., Bernardi, P., 2016. Calcium and reactive oxygen species in regulation of the mitochondrial permeability transition and of programmed cell death in yeast. Cell Calcium 60(2), 102-107. Chan, T.S., Galati, G., Pannala, A.S., Rice-Evans, C., O'Brien, P.J., 2003. Simultaneous detection of the antioxidant and pro-oxidant activity of dietary polyphenolics in a peroxidase system. Free radical research 37(7), 787-794. Costa, V., Moradas-Ferreira, P., 2001. Oxidative stress and signal transduction in Saccharomyces cerevisiae: insights into ageing, apoptosis and diseases. Molecular aspects of medicine 22(4-5), 217-246. Dorman, H., Deans, S.G., 2000. Antimicrobial agents from plants: antibacterial activity of plant volatile oils. Journal of applied microbiology 88(2), 308-316. Gerber, M., Walch, C., Loffler, B., Tischendorf, K., Reischl, U., Ackermann, G., 2008. Effect of sub-MIC concentrations of metronidazole, vancomycin, clindamycin and linezolid on toxin gene transcription and production in Clostridium difficile. J Med Microbiol 57(Pt 6), 776-783.

28

Giorgi, C., Baldassari, F., Bononi, A., Bonora, M., De Marchi, E., Marchi, S., Missiroli, S., Patergnani, S., Rimessi, A., Suski, J.M., 2012. Mitochondrial Ca2+ and apoptosis. Cell calcium 52(1), 36-43. Goossens, A., Häkkinen, S.T., Laakso, I., Seppänen-Laakso, T., Biondi, S., De Sutter, V., Lammertyn, F., Nuutila, A.M., Söderlund, H., Zabeau, M., 2003. A functional genomics approach toward the understanding of secondary metabolism in plant cells. Proceedings of the National Academy of Sciences 100(14), 8595-8600. Gurer-Orhan, H., Sabir, H.U., Ozgunes, H., 2004. Correlation between clinical indicators of lead poisoning and oxidative stress parameters in controls and lead-exposed workers. Toxicology 195(2-3), 147-154. Hajnoczky, G., Csordas, G., Das, S., Garcia-Perez, C., Saotome, M., Sinha Roy, S., Yi, M., 2006. Mitochondrial calcium signalling and cell death: approaches for assessing the role of mitochondrial Ca2+ uptake in apoptosis. Cell Calcium 40(5-6), 553-560. Hao, B., Cheng, S., Clancy, C.J., Nguyen, M.H., 2013. Caspofungin kills Candida albicans by causing both cellular apoptosis and necrosis. Antimicrob Agents Chemother 57(1), 326-332. Higuchi, Y., 2003. Chromosomal DNA fragmentation in apoptosis and necrosis induced by oxidative stress. Biochemical pharmacology 66(8), 1527-1535. Horáková, Ľ., 2011. Flavonoids in prevention of diseases with respect to modulation of Capump function. Interdisciplinary toxicology 4(3), 114-124. Hwang, J.H., Hwang, I.S., Liu, Q.H., Woo, E.R., Lee, D.G., 2012. (+)-Medioresinol leads to intracellular ROS accumulation and mitochondria-mediated apoptotic cell death in Candida albicans. Biochimie 94(8), 1784-1793.

29

Hwang, K.-A., Hwang, Y.-J., Song, J., 2018. Aster yomena extract ameliorates proinflammatory immune response by suppressing NF-κB activation in RAW 264.7 cells. Journal of the Chinese Medical Association 81(2), 102-110. Kang, K., Fong, W.P., Tsang, P.W., 2010. Novel antifungal activity of purpurin against Candida species in vitro. Med Mycol 48(7), 904-911. Kazak, L., Chouchani, E.T., Stavrovskaya, I.G., Lu, G.Z., Jedrychowski, M.P., Egan, D.F., Kumari, M., Kong, X., Erickson, B.K., Szpyt, J., 2017. UCP1 deficiency causes brown fat respiratory chain depletion and sensitizes mitochondria to calcium overload-induced dysfunction. Proceedings of the National Academy of Sciences, 201705406. Khan, A., Ahmad, A., Khan, L.A., Manzoor, N., 2014. Ocimum sanctum (L.) essential oil and its lead molecules induce apoptosis in Candida albicans. Res Microbiol 165(6), 411-419. Kim, A.R., Jin, Q., Jin, H.-G., Ko, H.J., Woo, E.-R., 2014. Phenolic compounds with IL-6 inhibitory activity from Aster yomena. Archives of pharmacal research 37(7), 845-851. Kim, H., Song, M. J., Potter, D., 2006. Medicinal efficacy of plants utilized as temple food in traditional Korean Buddhism. Journal of ethnopharmacology, 104(1-2), 32-46. Kondratskyi, A., Kondratska, K., Skryma, R., Prevarskaya, N., 2015. Ion channels in the regulation of apoptosis. Biochim Biophys Acta 1848(10 Pt B), 2532-2546. Kroemer, G., Galluzzi, L., Brenner, C., 2007. Mitochondrial membrane permeabilization in cell death. Physiological reviews 87(1), 99-163. Lee, H., Hwang, J.S., Lee, D.G., 2017. Scolopendin, an antimicrobial peptide from centipede, attenuates mitochondrial functions and triggers apoptosis in Candida albicans. Biochem J 474(5), 635-645.

30

Lee, H., Lee, D.G., 2017. Fungicide Bac8c triggers attenuation of mitochondrial homeostasis and caspase-dependent apoptotic death. Biochimie 133, 80-86. Lee, H., Woo, E.R., Lee, D.G., 2018. Apigenin induces cell shrinkage in Candida albicans by membrane perturbation. FEMS yeast research. Lee, W., Lee, D.G., 2018. Reactive oxygen species modulate itraconazole-induced apoptosis via mitochondrial disruption in Candida albicans. Free Radic Res 52(1), 39-50. Li, Y., Wei, G., Chen, J., 2004. Glutathione: a review on biotechnological production. Appl Microbiol Biotechnol 66(3), 233-242. Lin, M.T., Beal, M.F., 2006. Mitochondrial dysfunction and oxidative stress in neurodegenerative diseases. Nature 443(7113), 787-795. Liu, S., Hou, Y., Liu, W., Lu, C., Wang, W., Sun, S., 2015. Components of the calciumcalcineurin signaling pathway in fungal cells and their potential as antifungal targets. Eukaryot Cell 14(4), 324-334. Lupetti, A., Brouwer, C.P., Dogterom-Ballering, H.E., Senesi, S., Campa, M., van Dissel, J.T., Nibbering, P.H., 2004. Release of calcium from intracellular stores and subsequent uptake by mitochondria are essential for the candidacidal activity of an N-terminal peptide of human lactoferrin. Journal of Antimicrobial Chemotherapy 54(3), 603-608. Madeo, F., Carmona-Gutierrez, D., Ring, J., Büttner, S., Eisenberg, T., Kroemer, G., 2009. Caspase-dependent and caspase-independent cell death pathways in yeast. Biochemical and biophysical research communications 382(2), 227-231. Madeo, F., Herker, E., Maldener, C., Wissing, S., Lachelt, S., Herlan, M., Fehr, M., Lauber, K., Sigrist, S.J., Wesselborg, S., Frohlich, K.U., 2002. A caspase-related protease regulates apoptosis in yeast. Mol Cell 9(4), 911-917. 31

Mahyar-Roemer, M., Katsen, A., Mestres, P., Roemer, K., 2001. Resveratrol induces colon tumor cell apoptosis independently of p53 and precede by epithelial differentiation, mitochondrial proliferation and membrane potential collapse. International journal of cancer 94(5), 615-622. Mao, S., Park, Y., Hasegawa, Y., Tribble, G.D., James, C.E., Handfield, M., Stavropoulos, M.F., Yilmaz, Ö., Lamont, R.J., 2007. Intrinsic apoptotic pathways of gingival epithelial cells modulated by Porphyromonas gingivalis. Cellular microbiology 9(8), 1997-2007. Marnett, L.J., 1999. Lipid peroxidation-DNA damage by malondialdehyde. Mutation research 424(1-2), 83-95. Masuko, T., Minami, A., Iwasaki, N., Majima, T., Nishimura, S.-I., Lee, Y.C., 2005. Carbohydrate analysis by a phenol–sulfuric acid method in microplate format. Analytical biochemistry 339(1), 69-72. Maulik, N., Yoshida, T., Das, D.K., 1998. Oxidative stress developed during the reperfusion of ischemic myocardium induces apoptosis. Free Radic Biol Med 24(5), 869-875. Mazzoni, C., Falcone, C., 2008. Caspase-dependent apoptosis in yeast. Biochim Biophys Acta 1783(7), 1320-1327. Montero, M., Lobaton, C.D., Hernández-Sanmiguel, E., Santodomingo, J., Laura, V., Moreno, A., Alvarez, J., 2004. Direct activation of the mitochondrial calcium uniporter by natural plant flavonoids. Biochemical Journal 384(1), 19-24. Morrissey, J.P., Osbourn, A.E., 1999. Fungal resistance to plant antibiotics as a mechanism of pathogenesis. Microbiology and Molecular Biology Reviews 63(3), 708-724.

32

Nugent, S.M., Mothersill, C.E., Seymour, C., McClean, B., Lyng, F.M., Murphy, J.E., 2007. Increased mitochondrial mass in cells with functionally compromised mitochondria after exposure to both direct γ radiation and bystander factors. Radiation research 168(1), 134-142. Odds, F.C., Brown, A.J., Gow, N.A., 2003. Antifungal agents: mechanisms of action. Trends in microbiology 11(6), 272-279. Orrenius, S., Gogvadze, V., Zhivotovsky, B., 2015. Calcium and mitochondria in the regulation of cell death. Biochem Biophys Res Commun 460(1), 72-81. Orrenius, S., Zhivotovsky, B., Nicotera, P., 2003. Regulation of cell death: the calciumapoptosis link. Nat Rev Mol Cell Biol 4(7), 552-565. Peng, K., Tao, Y., Zhang, J., Wang, J., Ye, F., Dan, G., Zhao, Y., Cai, Y., Zhao, J., Wu, Q., Zou, Z., Cao, J., Sai, Y., 2016. Resveratrol Regulates Mitochondrial Biogenesis and Fission/Fusion to Attenuate Rotenone-Induced Neurotoxicity. Oxid Med Cell Longev 2016, 6705621. Pereira, C., Camougrand, N., Manon, S., Sousa, M.J., Corte-Real, M., 2007. ADP/ATP carrier is required for mitochondrial outer membrane permeabilization and cytochrome c release in yeast apoptosis. Mol Microbiol 66(3), 571-582. Phillips, A.J., Sudbery, I., Ramsdale, M., 2003. Apoptosis induced by environmental stresses and amphotericin B in Candida albicans. Proceedings of the National Academy of Sciences of the United States of America 100(24), 14327-14332. Reeks, B.Y., Champlin, F.R., Paulsen, D.B., Scruggs, D.W., Lawrence, M.L., 2005. Effects of sub-minimum inhibitory concentration antibiotic levels and temperature on growth kinetics and outer membrane protein expression in Mannheimia haemolytica and Haemophilus somnus. Canadian journal of veterinary research = Revue canadienne de recherche veterinaire 69(1), 1-10. 33

Ribeiro, G.F., Corte-Real, M., Johansson, B., 2006. Characterization of DNA damage in yeast apoptosis induced by hydrogen peroxide, acetic acid, and hyperosmotic shock. Mol Biol Cell 17(10), 4584-4591. Roe, M.W., Lemasters, J.J., Herman, B., 1990. Assessment of Fura-2 for measurements of cytosolic free calcium. Cell Calcium 11(2-3), 63-73. Schulz, J.B., Bremen, D., Reed, J.C., Lommatzsch, J., Takayama, S., Wullner, U., Loschmann, P.A., Klockgether, T., Weller, M., 1997. Cooperative interception of neuronal apoptosis by BCL-2 and BAG-1 expression: prevention of caspase activation and reduced production of reactive oxygen species. Journal of neurochemistry 69(5), 2075-2086. Schulz, J.B., Weller, M., Klockgether, T., 1996. Potassium deprivation-induced apoptosis of cerebellar granule neurons: a sequential requirement for new mRNA and protein synthesis, ICE-like protease activity, and reactive oxygen species. The Journal of neuroscience : the official journal of the Society for Neuroscience 16(15), 4696-4706. Segawa, K., Kurata, S., Yanagihashi, Y., Brummelkamp, T.R., Matsuda, F., Nagata, S., 2014. Caspase-mediated cleavage of phospholipid flippase for apoptotic phosphatidylserine exposure. Science 344(6188), 1164-1168. Simon, H.-U., Haj-Yehia, A., Levi-Schaffer, F., 2000. Role of reactive oxygen species (ROS) in apoptosis induction. Apoptosis 5(5), 415-418. Sowa-Jasilek, A., Zdybicka-Barabas, A., Staczek, S., Wydrych, J., Skrzypiec, K., Mak, P., Derylo, K., Tchorzewski, M., Cytrynska, M., 2016. Galleria mellonella lysozyme induces apoptotic changes in Candida albicans cells. Microbiological research 193, 121-131.

34

Tian, H., Qu, S., Wang, Y., Lu, Z., Zhang, M., Gan, Y., Zhang, P., Tian, J., 2017. Calcium and oxidative stress mediate perillaldehyde-induced apoptosis in Candida albicans. Appl Microbiol Biotechnol 101(8), 3335-3345. Tian, J., Lu, Z., Wang, Y., Zhang, M., Wang, X., Tang, X., Peng, X., Zeng, H., 2017. Nerol triggers mitochondrial dysfunction and disruption via elevation of Ca2+ and ROS in Candida albicans. Int J Biochem Cell Biol 85, 114-122. Tiwari, M., Prasad, S., Shrivastav, T.G., Chaube, S.K., 2017. Calcium signaling during meiotic cell cycle regulation and apoptosis in mammalian oocytes. Journal of cellular physiology 232(5), 976-981. Verbandt, S., Cammue, B.P., Thevissen, K., 2017. Yeast as a model for the identification of novel survival-promoting compounds applicable to treat degenerative diseases. Mechanisms of ageing and development 161, 306-316. Verpoorte, R., van der Heijden, R., Memelink, J., 2000. Engineering the plant cell factory for secondary metabolite production. Transgenic research 9(4-5), 323-343. Yapar, N., 2014. Epidemiology and risk factors for invasive candidiasis. Therapeutics and clinical risk management 10, 95. Yun, D.G., Lee, D.G., 2016. Silibinin triggers yeast apoptosis related to mitochondrial Ca2+ influx in Candida albicans. Int J Biochem Cell Biol 80, 1-9. Yun, J., Lee, D.G., 2016. Cecropin A-induced apoptosis is regulated by ion balance and glutathione antioxidant system in Candida albicans. IUBMB Life 68(8), 652-662. Yun, J., Lee, D.G., 2017. Role of potassium channels in chlorogenic acid-induced apoptotic volume decrease and cell cycle arrest in Candida albicans. Biochim Biophys Acta 1861(3), 585-592. 35

Yun, J., Lee, H., Ko, H.J., Woo, E.R., Lee, D.G., 2015. Fungicidal effect of isoquercitrin via inducing membrane disturbance. Biochim Biophys Acta 1848(2), 695-701. Figure captions Fig. 1. Inhibition of the antifungal effects of crude extracts and apigenin at various concentrations. C. albicans cells were treated with a,0.625 µg/mL; b, 1.25 µg/mL; c, 2.5 µg/mL; d, 5 µg/mL; e, 10 µg/mL methanol extracts and apigenin. Cell viability is represented as a percentage relative to the untreated samples (*p < 0.05, **p < 0.01 versus untreated cells; one-way ANOVA followed by Tukey’s post-hoc test). Fig. 2. Intracellular ion movement in the presence of ion movement inhibitor after treatment with apigenin. Candida albicans cells were preincubated with 5 mM TEA or 0.5 mM RR for 10 min before incubating with apigenin (2.5 µg/mL apigenin for sub-MIC, 5 µg/mL apigenin for MIC) for 4 h at 28 °C. (A) Increase in external potassium levels after treatment with apigenin in TEA pretreated cells. The fluorescence intensity of (B) Fura-2AM and (C) Rhod-2AM was determined to measure calcium levels in the cytosol and mitochondria, respectively (*p < 0.05, **p < 0.01 versus untreated cells; #p < 0.05, ##p < 0.01 versus apigenin for sub-MIC treatment alone; +p < 0.05,

++

p < 0.01 versus apigenin for MIC

treatment alone; one-way ANOVA followed by Tukey’s post-hoc test). Fig. 3. Detection of mitochondrial dysfunction in apigenin-treated C. albicans. C. albicans cells were incubated with 2.5 µg/mL apigenin (Api) and 2.5 µM H2O2 for 4 h at 28 °C (A) Mitochondrial membrane depolarization was detected using the fluorescent dye, JC-1. (B) Mitochondrial mass, and (C) Mitochondrial superoxide was measured using 36

Mitotracker Green and MitoSOX Red staining, respectively (*p < 0.05, **p < 0.01 versus untreated cells; one-way ANOVA followed by Tukey’s post-hoc test). Fig. 4. Oxidative damage in apigenin-treated C. albicans. C. albicans cells were preincubated with 0.5 mM RR for 10 min before incubating with 2.5 µg/mL apigenin (Api) and 2.5 µM H2O2 for 4 h at 28 °C. (A) Intracellular ROS were measured using H2DCFDA. (B) GSH/GSSG ratio was measured after Api treatment. (C) MDA levels indicate the lipid peroxidation after treatment with Api. MDA concentration in the cells was determined using the TBARS assay (*p < 0.05, **p < 0.01 versus untreated cells; #p < 0.05, ##p < 0.01 versus Api treatment alone; one-way ANOVA followed by Tukey’s post-hoc test). Fig. 5. Detection of calcium homeostasis and mitochondrial disruption by apigenin along with the NAC pretreatment. C. albicans cells were preincubated with 5 mM NAC for 10 min before incubating with 2.5 µg/mL apigenin (Api) and 2.5 µM H2O2 for 4 h at 28°C. (A) Fura2-AM and Rhod2-AM fluorescences were determined to measure calcium levels in the cytosol and mitochondria, respectively, after either apigenin treatment alone or apigenin treatment with NAC pretreatment. (B) Mitochondrial membrane depolarization, (C) mitochondrial superoxide, and (C) mitochondrial mass were determined using JC-1, Mitotracker Green, and MitoSOX Red staining, respectively (*p < 0.05, **p < 0.01 versus untreated cells; #p < 0.05, ##p < 0.01 versus Api treatment alone; one-way ANOVA followed by Tukey’s post-hoc test). Fig. 6. Detection of apoptotic features with RR and NAC pretreatment. C. albicans cells were preincubated with 0.5 mM RR or 5 mM NAC for 10 min before incubating with 2.5 µg/mL apigenin (Api) and 2.5 µM H2O2 for 4 h at 28°C. (A) Plasma membrane PS 37

externalization was detected by annexin V-FITC fluorescence. (B) DNA fragmentation was assessed by TUNEL staining. (C) Metacaspase activation was investigated using 10 mM CaspACE FITC-VAD-FMK in situ marker. (*p < 0.05, **p < 0.01 versus untreated cells; #p < 0.05, ##p < 0.01 versus Api treatment alone; one-way ANOVA followed by Tukey’s post-hoc test).

Table 1. The flux of intracellular components caused by apigenin at sub-MIC C. albicans strains Assay Untreated

Apigenin

Amphotericin B

External potassium (%)

15.41±6.00

53.59±2.23

89.56±4.94

Intracellular calcium (relative intensity)

228.21±1.29

252.17±2.43

151.80±1.73

Sugar (soluble; fold)

1.00

1.35±0.12

2.02±0.14

Protein (soluble; fold)

1.00

1.05±0.07

0.99±0.03

38

39

40

41

42