RPMI-1640 and fetal bovine serum (FBS) were purchased from Gibco (Thermo Fisher Scientific, Inc., Waltham, MA, USA). MTT and monodansylcadaverine (MDC) were purchased from Sigma-Aldrich (Merck KGaA, Darmstadt, Germany). YM201636 (cat. no. sc-204193) was obtained from Santa Cruz Biotechnology, Inc. (Dallas, TX, USA). Monoclonal mouse anti-human EGFR antibody (cat. no. sc-71034; 1:200), monoclonal mouse anti-human phospho (p)-EGFR antibody (cat. no. sc-81490; 1:200), monoclonal mouse anti-human β-actin antibody (cat. no. sc-47778; 1:1,000) were all purchased from Santa Cruz Biotechnology, Inc. Polyclonal rabbit anti-human microtubule associated protein 1 light chain 3 (LC3) A/B antibody (cat. no. 14600-1-AP; 1:1,000) was purchased from ProteinTech Group, Inc. (Chicago, IL, USA). The horseradish peroxidase (HRP)-conjugated secondary antibodies including goat anti-rabbit IgG (cat. no. sc-2054; 1:1,000) and goat anti-mouse IgG (cat. no. sc-2973; 1:1,000) were also purchased from Santa Cruz Biotechnology, Inc.

All cell lines (HepG2, Huh-7 and H22) used in the present study were purchased from the Cell Bank of Type Culture Collection of Chinese Academy of Sciences (Shanghai, China). These cells were cultured in RPMI-1640 medium supplemented with 10% FBS, 100 U/ml penicillin and 100 µg/ml streptomycin, at 37°C in a humidified incubator containing 5% CO₂.

HepG2 and Huh-7 cells were transiently transfected with pcDNA3.1-epidermal growth factor receptor (EGFR) plasmid or control pcDNA3.1 plasmid (Shanghai GeneChem Co., Ltd., Shanghai, China) using Lipofectamine^® 2000 reagent (Invitrogen; Thermo Fisher Scientific, Inc.) according to the manufacturer's instructions. Briefly, the cells were seeded in 6-well plates at 5×10⁵/well and cultured at 37°C in a humidified atmosphere with 5% CO₂. The subsequent day, when the cells were ~70% confluent, they were transfected with a mixture of 3 µl Lipofectamine^® 2000 and 2 µg plasmids. At 4 h post-transfection, the cell culture medium was replaced with RPMI-1640 medium. After 24 h, these transfected cells were collected to perform MDC staining or western blot analysis.

Cells were seeded in 96-well plates at an initial density of 4×10³ cells/well in 90 µl medium and cultured overnight. As cells reached 30% confluence, various concentrations of YM201636 (0.1, 0.2, 0.5, 1, 2 and 5 µM) were added to the cells, which were then incubated for 24 h. Subsequently, 50 µl 1 mg/ml of MTT (Sigma-Aldrich; Merck KGaA) was added to each well for 4 h. Then, the supernatant was removed and 100 µl DMSO was subsequently added to solubilize the crystal products at room temperature for 10 min. The optical density (OD) was measured at a wavelength of 490 nm using a microplate reader (BioTek Instruments, Inc., Winooski, VT, USA). The growth inhibition ratio was calculated as follows: Growth inhibition ratio (%) = (OD_control - OD_drug)/OD_control × 100; where OD_control is the OD of the group treated with vehicle. The experiments were repeated at least 3 times. To verify whether the mechanism of autophagy promoted cell survival or cell death, HepG2 and Huh-7 cells were pretreated with 5 mM 3-methyladenine (3-MA) for 30 min followed by cotreatment with 2 µM YM201636 for 24 h. Then, an MTT assay was performed. The growth inhibition ratio was calculated as follows: Growth inhibition ratio (%) = (OD_control - OD_drug)/OD_control × 100; where OD_control is the OD of the group treated with vehicle. To examine the effects of EGFR overexpression on cell growth, HepG2 and Huh-7 cells (80% confluence) seeded in a 6-well plate were transfected with pcDNA3.1-EGFR or control pcDNA3.1 plasmids using Lipofectamine^® 2000. After 24 h, the cells were harvested and seeded into 96-well plates at a density of 4×10³ cells/well for another 24 h. An MTT assay was performed to determine the OD value of each group. During incubation, EGF was not added to the culture medium to avoid EGFR activation.

MDC, a specific marker for autophagic vacuoles, was used to examine whether YM201636 induced autophagy. HepG2 and Huh-7 cells were grown on coverslips in a 6-well plate overnight at 3×10^5/well, and then treated with 2 or 5 µM of YM201636 for 24 h. The cells were collected and washed with ice-cold PBS 3 times, then incubated with 50 µM MDC at 37°C for 30 min. The stained cells were washed, fixed with 4% paraformaldehyde for 10 min at room temperature and analyzed with the imaging flow cytometer FlowSight^® (Amnis^®; Merck KGaA). A laser set at 405 nm was used for excitation. Bright field and MDC images (green fluorescence) were collected in channels 1 and 8, respectively. The stained cells (1×10^4/sample) were analyzed using IDEAS version 6.0 software (Merck KGaA).

The total proteins were isolated from cancer cells or allograft tumors using radio immunoprecipitation assay buffer (Beyotime Institute of Biotechnology, Haimen, China) following the manufacturer's instructions. The protein concentration was determined using a bicinchoninic acid assay kit (Pierce; Thermo Fisher Scientific, Inc.). Samples were denatured in 5X SDS-sample buffer at 95°C for 5 min. Total proteins (40 µg/well) were separated using SDS-PAGE on 10% gels for EGFR detection or 12% gels for LC3 detection. Following separation, the protein was transferred onto polyvinylidene difluoride membranes. Subsequently, the membranes were blocked using 5% non-fat milk in TBS-Tween (TBS-T) at room temperature for 1 h. Following blocking, membranes were incubated with corresponding primary antibodies overnight at 4°C, then washed 3 times with TBST and incubated with the appropriate HRP-conjugated secondary antibody, and then washed 3 times with TBST. Proteins were detected using the enhanced chemiluminescence plus reagents (Beyotime Institute of Biotechnology). The western blots were visualized using a FluroChem E Imager (Protein Simple, San Jose, CA, USA). Quantity One Software (Quantity One 462; Bio-Rad Laboratories, Inc., Hercules, CA, USA) was used to calculate the alteration of corresponding protein expression.

Male BALB/c mice (weighing 20–25 g; aged 5 weeks) were purchased from the Laboratory Animal Center of Henan (Zhengzhou, China). All animal procedures were performed with the approval of the Institutional Animal Care and Use Committee of Henan University (Kaifeng, China). For the development of solid tumors, mice were subcutaneously injected with 2×10⁶ H22 cells. At 1 day after inoculation, the mice were randomly divided into the control and YM201636 groups, which were treated with 5% DMSO or 2 mg/kg YM201636, respectively, via oral administration once daily for 7 consecutive days. The mice were anesthetized using ether for ~30 sec via the respiratory route, and the heart rate and respiratory rate were monitored to ensure the animals were simply anesthetized. Then the mice were euthanized by cervical dislocation, and solid tumors were isolated and weighed. Meanwhile, the heart, liver, kidney, lung, and spleen of the mice were collected and weighed on the last day. The organ weight index was investigated for systemic toxicity evaluation as follows: Organ index (%) = (organ weight/bodyweight) × 100.

Statistical analyses were performed using GraphPad Prism 5 software for Windows (GraphPad Software, Inc., La Jolla, CA, USA). All data are expressed as the mean ± standard error. A two-tailed unpaired t-test was used for the comparison of the mean values between two groups. One-way analysis of variance (ANOVA) followed by a Dunnett's test or two-way ANOVA followed by a Bonferroni post hoc test was used for multiple comparisons. P<0.05 was considered to indicate a statistically significant difference.

In order to determine whether the PIKfyve inhibitor YM201636 affects the cell viability of hepatoma, HepG2 and Huh-7 cells were cultured and incubated in the presence of increasing concentrations of the drug for 24 h. Cell viability rates were then detected using an MTT assay. Following incubation of HepG2 and Huh-7 cells with 0.1,0.2, 0.5, 1, 2 and 5 µM YM201636 for 24 h, YM201636 reduced the HepG2 and Huh-7 cell viability in a dose-dependent manner (Fig. 1). The data indicated that YM201636 may inhibit the cell growth of liver cancer cell lines.

Previous studies have demonstrated that knockdown or inhibition of PIKfyve induces secretory autophagy in multiple cell lines (12,13). Therefore, the present study investigated whether decreased cell viability was associated with autophagy in liver cancer cell lines using an MDC staining assay. Following treatment with 2 or 5 µM YM201636 for 24 h, HepG2 and Huh-7 cells exhibited strong MDC staining (Fig. 2), with enhanced fluorescence in cells treated with 5 µM YM201636. Additionally, the percentage of autophagic cells in the population was increased in HepG2 and Huh-7 cells treated with YM201636 for 24 h (Fig. 2). To further confirm whether YM201636 induces autophagy in liver cancer cell lines, the ratio of LC3I to LC3II was determined, an established autophagosome-related marker (14,15). The results demonstrated that YM201636 significantly promoted the conversion of LC3I to LC3II in HepG2 and Huh-7 cells (Fig. 3). To verify whether the mechanism of autophagy promoted cell survival or cell death, the autophagy inhibitor 3-MA was applied to inhibit YM201636 induced-autophagy in HepG2 and Huh-7 cells; 3-MA inhibits autophagy by blocking autophagosome formation via the inhibition of type III phosphatidylinositol 3-kinases (PI3Ks) (16,17). Co-treatment with 5 mM 3-MA for 24 h attenuated the inhibitory effects of YM201636 on liver cancer cell lines (Fig. 4). Collectively, these results suggested that YM201636 inhibits the growth of HepG2 and Huh-7 cells via the induction of autophagy.

It has been previously reported that PIKfyve inhibition blocks the lysosomal degradation of EGFR, resulting in increased expression levels of EGFR in MCF-10A cells (18), and overexpression of inactive EGFR has been demonstrated to be associated with autophagy (19). Therefore, the present study investigated whether YM201636-induced autophagy is dependent upon EGFR overexpression. To test this hypothesis, the total protein expression of EGFR was determined in HepG2 and Huh-7 cells treated with YM201636. Western blotting demonstrated that the total protein expression levels of EGFR were significantly upregulated in HepG2 and Huh-7 cells treated with 2 µM YM201636 for 24 h (Fig. 5). To investigate whether the increased EGFR was activated in HepG2 and Huh-7 cells treated with YM201636, the phosphorylation levels of EGFR at Tyr¹⁰⁶⁸, an indicator of EGFR activation, were examined. As shown in Fig. 5, the phosphorylation levels of EGFR at Tyr¹⁰⁶⁸ were notably unaffected in HepG2 and Huh-7 cells treated with YM201636. The results suggested that the increased total protein constituted inactive EGFR.

To further investigate the hypothesis that inactive EGFR mediates autophagy in HepG2 and Huh-7 cells, HepG2 and Huh-7 cells were transfected with the pcDNA3.1-EGFR plasmid with no EGF treatment. At 24 h after transfection, the expression of EGFR was increased significantly (Fig. 6). Subsequently, whether overexpression of EGFR was able to affect autophagy in liver cancer cells was examined. As demonstrated in Fig. 7, HepG2 and Huh-7 cells transfected with EGFR exhibited strong MDC staining and an increase in the proportion of autophagic cells. Additionally, overexpression of EGFR also promoted the transformation of LC3I into LC3II in HepG2 and Huh-7 cells (Fig. 8). Furthermore, the effects of EGFR overexpression on liver cancer cell growth were determined. Overexpression of EGFR in HepG2 and Huh-7 cells inhibited cell proliferation (Fig. 9). The results indicated that YM201636 induced-autophagy was dependent on EGFR overexpression in HepG2 and Huh-7 cells.

To further evaluate the antitumor activity of the PIKfyve inhibitor, YM201636, in vivo, an allograft model of mouse liver cancer was established by subcutaneously injecting H22 cells (mouse hepatoma cell line) into BALB/c mice. Transplantation of H22 cells into BALB/c mice induced the in situ formation of tumors. Following treatment with 2 mg/kg YM201636 for 7 consecutive days, the tumor weight of the YM201636 group was lower than that of the negative control group. For example, the mean weight of tumors in the YM201636-treated group was 0.31±0.05 g, compared with 0.67±0.07 g in the control group (Fig. 10A and B). Additionally, YM201636 promoted the expression of EGFR total protein in allograft tumors; however, the phosphorylation levels of EGFR at Tyr¹⁰⁶⁸ were not notably altered (Fig. 10C and D). In addition, visceral organ indices were also examined to evaluate adverse effects of YM201636. No significant differences in the visceral organ indexes (heart, liver, spleen, lung and kidney) were observed in the YM201636 group compared with the control group (Fig. 10E). These results suggested that YM201636 may inhibit tumor growth without notable systemic toxicity at the dose applied in the present study.