Many therapies have been developed for cancer patients, but chemotherapy is still a key therapy in cancer treatment. However, some limitations, such as adverse side effects, drug resistance, and limited efficacy, remain to be overcome [1
]. Therefore, there is a need to develop more effective and safe treatment options that can minimize these limitations. In this respect, there is increasing attention being paid to the importance of natural compounds derived from herbal medicines and food constituents that have been traditionally used to prevent and treat various diseases [3
]. In particular, numerous naturally occurring agents have been reported to cause growth arrest and induce apoptosis in cancer cells, without showing toxicity to normal cells [5
]. These compounds have also emerged as an alternative to chemopreventive and chemotherapeutic agents, because they can specifically regulate various cellular signaling pathways in cancer cells due to enormous structural diversity [7
]. Moreover, many anti-cancer drugs, including etoposide, vinblastine, paclitaxel, and vincristine, are typical examples of naturally-occurring anti-cancer agents currently in clinical use [9
Licochalcone A (LCA, 4’,4-Dihydroxy-3-α,α-dimethylallyl-6-methoxychalcone) is a phenolic chalcone compound found in the root of Glycyrrhiza glabra
or G. inflata
, belonging to the plant family Fabaceae, which is widely used in herbal medicines and as a traditional food in Asia [12
]. LCA displays a number of pharmacological activities, such as anti-microbial activity [13
] and anti-inflammatory [15
] and antioxidant properties [17
], and is also considered to have potential anti-cancer agents in the results of various cancer cell models. For example, LCA has been reported to inhibit cell proliferation through cell cycle arrest at the G2/M phase in various types of cancer cells, including MCF-7 breast cancer cells [7
], A549 lung cancer cells [19
], HepG2 human hepatoma cells [20
], and U87 glioma cells [21
]. In addition, the anti-cancer effects of LCA have been reported in nasopharyngeal cancer, breast cancer, cervical cancer, oral cancer, epithelial ovarian carcinoma, bladder cancer cells, and so on [7
]. These anti-cancer effects have been shown to involve the death receptor (DR)-dependent extrinsic or mitochondria-dependent intrinsic pathways, which are representative apoptosis inducing pathways. It was also found that the anti-cancer effect of LCA was accompanied by the disturbance of various cellular signaling pathways, including mitogen activated protein kinase and phosphatidylinositol 3-kinase/AKT signaling cascades [24
]. Furthermore, LCA showed a strong cytotoxic effect through reactive oxygen species (ROS)-dependent apoptosis in various cancer cell lines [20
]. Yuan et al. suggested that LCA induced oxidative stress, and consequently caused T24 cell apoptosis via the mitochondrial-dependent pathway and endoplasmic reticulum (ER) stress [32
]. In their subsequent study, they demonstrated that LCA inhibits T24 cell proliferation by increasing intracellular ROS generation [37
]. Although they demonstrated that LCA suppresses the proliferation of T24 cells via ROS production, they did not establish an accurate role of ROS. In 2016, the same research team reported that LCA induces apoptosis of T24 bladder cancer cells via increasing intracellular Ca2+
levels, which may be associated with mitochondrial dysfunction and ER stress [26
]. However, mRNA levels of caspase showed no great change, and caspase activities were not assessed. Although the possibility has recently been proposed of the growth inhibitory activity of LCA in bladder cancer cells [26
], the underlying molecular mechanism remains unclear. Therefore, in this study, we investigated the anti-cancer efficacy of LCA in human bladder cancer cells, focusing on the mechanisms associated with the induction of cell cycle arrest and apoptosis.
Disruption of cell cycle regulation is clearly implicated in the development and progression of most tumors, and discontinuation of this process is considered to be an important strategy to inhibit the proliferation of cancer cells [5
]. In particular, it is clear that the induction of apoptosis by many anti-cancer agents is associated with cell cycle arrest at specific checkpoints [38
]. Therefore, we first investigated whether the suppression of bladder cancer cell proliferation by LCA was associated with cell cycle arrest. The results of flow cytometry analysis showed that LCA caused G2/M phase arrest in the T24 human bladder cancer cell line, similar to the results of previous studies in several human cancer cell lines [7
], suggesting that G2/M phase arrest is one of the mechanisms of the anti-cancer effect of LCA in T24 cells.
The progression of the cell cycle in eukaryotic cells is strictly controlled by the interaction of cyclins and Cdks with their inhibitory factors. For example, D-type cyclins bind to and activate Cdk4 and Cdk6, leading to sequential progression from the G1 to S phase. On the other hand, cyclin A/Cdk2 and Cdc2 complexes control S and G2 phases, while cyclin B/Cdc2 complex regulates induction of G2/M transition and achievement of mitosis [38
]. In addition, Wee1, a tyrosine kinase, induces phosphorylation of Cdc2, resulting in inhibition of cyclin B-Cdc2 activity and preventing entry into mitosis [41
]. In the current study, when T24 cells were exposed to LCA, expression of cyclin A, cyclin B1, and Wee1 was markedly reduced without significant changes in expression of Cdk2 and Cdc2. Consistent with the results of previous studies reported in human hepatoma and oral squamous cell carcinoma cells [20
], we also found that expression of p21 in LCA-treated T24 cells was significantly induced. The typical Cdk inhibitor p21, belonging to the KIP/CIP family, was first reported as a major inducer of tumor suppressor p53-dependent cell cycle arrest induced by DNA damage, but it can act as a mediator of p53-independent cell arrest in various types of cancer cells [43
]. When p21 expression increases, it forms complexes with Cdks to reduce kinase activity and inhibit cell cycle progression [43
]. Our data showed that complexes immunoprecipitated with Cdk2 and Cdc2 antibodies in LCA-treated cells contained a greater amount of immunologically detectable p21 protein compared to untreated control cells, which may have contributed the reduction of Cdk2 and Cdc2 activity, ultimately leading to G2/M arrest. Because T24 cells are mutant p53 gene-bearing cells [46
], increased p21 expression by LCA seems to contribute to G2/M inhibition regardless of p53 gene status. Collectively, our data suggest that LCA-triggered G2/M arrest was due to decreased expression of cyclin A, cyclin B1, and Wee1, and an increase in p21 levels through a p53-independent mechanism.
Since LCA-mediated cell cycle arrest at the G2/M phase has been reported to be related to apoptosis in various cancer cell lines [7
], we also evaluated whether G2/M arrest by LCA was associated with apoptosis induction in T24 cells. Based on the results of morphological changes and flow cytometry analysis, we found that the anti-proliferative effect of LCA was achieved through the induction of apoptosis associated with G2/M arrest. Apoptosis is a highly conserved physiological mechanism for eliminating malignant cells without causing damage to normal cells or surrounding tissues, and can be broadly divided into extrinsic and intrinsic pathways in mammalian cells [47
]. The extrinsic pathway is characterized by the activation of caspase-8 by the formation of the death-inducing signal complex through binding of death ligands to the cell surface DRs [48
]. On the other hand, activation of the intrinsic pathway is caused by the activation of caspase-9 through the release of pro-apoptotic factors, such as cytochrome c
, from the mitochondrial intermembrane space into the cytoplasm. This pathway is tightly regulated by the Bcl-2 protein family that includes pro- and anti-apoptotic proteins, which guard mitochondrial integrity, and control the release of cytochrome c
through the mitochondrial transition pore [47
]. Caspases-8 and -9, which correspond to the initiator caspases of each pathway, ultimately activate apoptosis through the cleavage of various cellular substrates, such as PARP, by activating down stream executioner caspases, including caspase-3 and -7 [47
]. As shown in previous studies using human nasopharyngeal and ovarian carcinoma cells [23
], our results demonstrate that LCA activated caspase-3 as well as caspase-8 and -9, and induced the cleavage of PARP. Therefore, we speculated that the pro-apoptotic effect of LCA in T24 cells could occur by simultaneously activating extrinsic and intrinsic pathways. In addition, consistent with previous studies [24
], we confirmed that mitochondrial dysfunction was induced in LCA-treated cells, which was accompanied by the promotion of cytosolic release of cytochrome c
, suggesting that cytochrome c
release was due to an increase in mitochondrial membrane permeability. Additionally, we examined the effects of LCA on the expression of Bax and Bcl-2, both of which regulate mitochondrial membrane permeabilization [47
], and found that LCA down-regulated Bcl-2/Bax ratio. These results imply that LCA treatment shifted the equilibrium between Bax and Bcl-2 in mitochondria, resulting in permeability of the mitochondrial membrane.
Increasing evidence demonstrates that many anti-cancer agents promote growth inhibition and apoptosis for removal of cancer cells through pro-oxidant properties, such as increasing ROS accumulation, or destroying cellular antioxidant systems [51
]. Several previous studies have reported that many phytochemicals generate ROS to activate cell cycle arrest and apoptosis signaling in cancer cells [53
]. In particular, mitochondria are a major target of ROS and a major subcellular organelle responsible for the production of ROS in the cells [56
]. These observations suggest that inducing the production of ROS in cancer cells can be used in therapeutic strategies. Therefore, we further assessed whether LCA-induced G2/M arrest and apoptosis in T24 cells was correlated with the production of ROS. Consistent with previous studies [20
], our results showed that LCA-treatment markedly increased the levels of ROS production. However, the quenching of ROS generation using NAC significantly diminished LCA-induced cytosolic release of cytochrome c
to control levels, indicating that ROS act as upstream signaling molecules to enhance LCA-mediated mitochondrial dysfunction in T24 cells. Our results also demonstrated that the presence of NAC markedly attenuated LCA-induced down-regulation of cyclin A and cyclin B1, and up-regulation of p21. Subsequently, NAC pre-treatment also significantly protected against LCA-mediated G2/M arrest and cell death, confirming that increasing ROS may serve a key contributor to the anti-cancer effects of LCA.
Taken together, these results suggest that the production of ROS by LCA plays a critical role in the induction of G2/M arrest and apoptosis through simultaneous initiation of both extrinsic and intrinsic pathways in human bladder cancer cells. However, future studies are warranted to explore the precise molecular mechanisms of intracellular signaling pathways associated with the production of ROS and to evaluate the anti-cancer properties of LCA in in vivo bladder cancer models. In addition, further studies are required for bioavailability and PK analysis for clinical application of LCA. These further studies are expected to suggest the scientific evidence of LCA for treatment or prevention of cancers, including bladder cancer.
4. Materials and Methods
4.1. Cell Culture and LCA Treatment
Human bladder cancer T24 and 5637, Chang liver, and HaCat keratinocyte cells were purchased from the American Type Culture Collection (Manassas, MD, USA). T24 and 5637 cells were cultured in RPMI 1640 medium containing 10% heat-inactivated fetal bovine serum (FBS). Chang and HaCaT cells were cultured in Eagle’s minimum essential medium and Dulbecco’s modified Eagle’s medium, respectively, containing 10% FBS. All media were also supplemented with 100 U/mL of penicillin and 100 μg/mL of streptomycin (all from WelGENE Inc., Daegu, Korea) at 37 °C in an atmosphere containing 5% CO2 in a humidified incubator. LCA obtained from Sigma-Aldrich Chemical Co. (St. Louis, MO, USA) was dissolved in dimethyl sulfoxide (DMSO, Sigma-Aldrich Chemical Co., St. Louis, MO, USA) as a 100 mM stock solution.
4.2. Cell Viability
The cells were seeded at a density of 1 × 104 cells/well in 96-well plates and allowed to attach to the well. After 24 h, cells were treated with the desired concentrations of LCA in triplicate for 48 h and the cells were then incubated with 50 μg/mL MTT solution (Invitrogen, Waltham, MA, USA) for 2 h. The formazan crystals were dissolved in DMSO, and the optical density of the solution was determined using a microplate reader (Molecular Device Co., Sunnyvale, CA, USA) at 540 nm wavelength to determine the number of viable cells. The morphological changes of cells were observed and photographed under an inverted phase-contrast microscope (Carl Zeiss, Oberkochen, Germany).
4.3. Determination of Cell Cycle Distribution by Flow Cytometric Analysis
PI staining was applied to analyze the DNA content and cell cycle distribution. After treatment with different concentrations of LCA for 48 h, the detached cells and adherent cells were collected, and cell suspensions of approximately 1 × 106 cells were generated. The cells were fixed in ice-cold 70% ethanol (in phosphate-buffered saline, PBS) at 4 °C for 30 min. The cells were re-suspended in PBS containing 40 μg/mL PI, 100 μg/mL RNase A, 0.1% (w/v) sodium citrate, and 0.1% (v/v) Nonidet-P40 (all from Sigma-Aldrich Chemical Co.), then incubated on ice for 30 min in the dark at room temperature. Finally, the cells were analyzed for cell cycle perturbation using a flow cytometer (BD Biosciences, San Jose, CA, USA) and Cell Quest software was used to determine the relative DNA content. The frequency of sub-G1 cells was calculated to estimate apoptotic cells.
4.4. Detection of Apoptotic Morphological Changes
Changes of nuclear morphology were assessed by DAPI staining. After 48 h treatment with LCA, the cells were washed with PBS, and fixed with 3.7% paraformaldehyde (Sigma-Aldrich Chemical Co.) in PBS for 10 min at room temperature. The fixed cells were washed with PBS, and stained with 1 μg/mL DAPI solution (Sigma-Aldrich Chemical Co.) for 10 min in the dark. The cells were washed with PBS, and the fluorescence intensity was observed and photographed under a fluorescence microscope (Carl Zeiss, Oberkochen, Germany).
4.5. Determination of Apoptotic Cell Death by Flow Cytometric Analysis
Apoptotic cell death was quantified by a flow cytometer using the Annexin V Apoptosis Detection Kit from BD Biosciences (San Diego, CA, USA), according to the manufacturer’s instruction. In brief, following treatment of the cells with LCA for 48 h, the detached cells and adherent cells were collected. The collected cells were suspended in the supplied binding buffer and then stained with FITC-conjugated annexin V and PI at room temperature in the dark for 20 min. The cells were resuspended in binding buffer and analyzed using a flow cytometer, and the annexin V+/PI− and annexin V+/PI+ cell populations were considered indicators of apoptotic cells.
4.6. Protein Extraction, Co-Iimmunoprecipitation, and Western Blot Analysis
Whole cell protein extracts were prepared and protein concentrations measured using Bradford protein assay kit (Bio-Rad Laboratories, Hercules, CA, USA), according to the manufacturer’s instructions. For co-immunoprecipitation assays, 500 μg of cell lysates from each sample was precleaned with normal rabbit IgG and protein-A-sepharose bead slurry (Amersham, Arlington Heights, IL, USA), and immunoprecipitation was conducted using 1 μg of anti-Cdk2 or Cdc2 antibody (Santa Cruz Biotechnology, Inc., Santa Cruz, CA, USA) and protein-A-sepharose (Sigma-Aldrich Chemical Co.). Protein complexes were then prepared according to previously described methods [58
]. Equal amounts of protein samples or immunoprecipitated proteins were separated by denaturing SDS-polyacrylamide gel electrophoresis, and then transferred onto PVDF membranes (Millipore, Bedford, MA, USA). The membranes were blocked with 5% skim milk in Tris-buffered saline containing 0.1% Triton X-100 (TBST) for 1 h at room temperatue, and probed with specific primary antibodies (Santa Cruz Biotechnology, Inc. and Cell Signaling Technology) at 4 °C overnight. After washing with TBST, the membranes were incubated with the appropriate horseradish peroxidase-conjugated secondary antibodies (Santa Cruz Biotechnology, Inc.) for 2 h. The expression of protein was detected by ECL kit (GE Healthcare Life Sciences, Little Chalfont, UK), and visualized by Fusion FX Image system (Vilber Lourmat, Torcy, France). All results were confirmed in three independent experiments.
4.7. Caspase Activity Assay
The activity of caspases was determined using Caspase Activity Assay Kits (R&D Systems, Minneapolis, MN, USA), according to the protocol of the manufacturer. Briefly, the detached cells and adherent cells were harvested, washed with PBS, and cell pellets resuspended in the lysis buffer provided in the kit. The supernatants were collected and incubated with the supplied reaction buffer containing dithiothreitol, with or without tetrapeptides labeled with p-nitroaniline (pNA) at 37 °C for 2 h under light-shielded conditions. The caspase activities were determined by measuring changes in absorbance at 405 nm using a microplate reader.
4.8. Measurement of MMP (ΔΨm) and ROS Production
To measure MMP, JC-1 staining was performed. After LCA treatment for 48 h, 10 μM JC-1 (Sigma-Aldrich Chemical Co.) was added to the cells for 30 min at 37 °C. Subsequently, the cells were washed with PBS to remove unbound dye, and at least 10,000 cells were collected for each sample. The amounts of MMP were detected at 488/575 nm using a flow cytometer by following the manufacturer’s protocol [59
]. Briefly, the gated region of the y-axis (JC-1 aggregate) included cells with intact mitochondrial membranes and the gated region of the x-axis (JC-1 monomer) depicted cells with loss of MMP. The production of ROS was measured using DCF-DA. At the end of the treatment with LCA, 10 μM DCF-DA (Invitrogen) was added to the incubated cells in a dark environment for 20 min at 37 °C. Subsequently, cells were analyzed for DCF fluorescence by a flow cytometer at 480 nm/520 nm.
4.9. Statistical Analysis
The results of quantitative studies are reported as mean ± SD using GraphPad Prism software (version 5.03; GraphPad Software, Inc., La Jolla, CA, USA). All experiments were repeated at least three times. To compare data, one-way analysis of variance (ANOVA) with Dunnett’s post hoc test was used, and p < 0.05 was considered to indicate a statistically significant difference.