β-Elemene Promotes Apoptosis Induced by Hyperthermia via Inhibiting HSP70.

Thermotherapy has been presented as a promising strategy to be used as an effective nonsurgical technique for colorectal carcinoma. Although this strategy presents several advantages, including low toxicity and high repeatability, thermotherapy often needs to be combined with other therapies because residual tumor cells that survive hyperthermal treatment often lead to relapse. In this study, we evaluated the effects of β-elemene, which has been proven to have the potential to reverse chemotherapy drug resistance, on promoting the antitumor effects of hyperthermia. β-elemene treatment significantly promoted apoptosis after 2 hours of hyperthermia treatment and blocked cell cycle phases at G1/G0. β-elemene also significantly decreased colony formation and tumor formation abilities after hyperthermia treatment. β-elemene treatment significantly decreased HSP70, but not HSP90 or HSP27, induced by hyperthermia treatment without disturbing HSP70 mRNA. It was also found that β-elemene decreased phosphorylated ERK1/2 induced by hyperthermia. Regain of HSP70 reversed β-elemene-mediated apoptosis, indicating that β-elemene may induce apoptosis by decreasing HSP70. Moreover, β-elemene treatment significantly decreased invasion capacity by decreasing the EMT, which was induced by hyperthermia treatment. Taken together, our results offer a potential strategy for CRC therapy via the combination of hyperthermia and β-elemene.

1. Introduction

β-elemene, a sesquiterpene compound, is found in several herbs and is primarily found in Curcuma wenyujin and used in traditional Chinese medicine to treat a variety of cancers. To date, no serious adverse reactions have been reported with β-elemene. Studies have shown that β-elemene ameliorates clinical outcomes in patients with esophageal squamous cell carcinoma (ESCC) with mild side effects [1]. Additionally, patients with glioblastoma have significantly improved survival after treatment with β-elemene and temozolomide (TMZ) [2]. Bin et al. conducted a systematic literature review to evaluate the efficacy of an injectable solution of β-elemene in combination chemotherapy for lung cancer. The results showed that β-elemene improved patient survival and tumor response and reduced toxicity rates [3]. As demonstrated in current clinical studies, β-elemene is a promising adjunctive therapy touted for its synergistic effect in ameliorating outcomes in patients with malignant tumors.

Hyperthermia (HT) is a branch of external treatment of traditional Chinese medicine that involves heating a tumor to a temperature below 43°C. This technique has been applied clinically for nearly 40 years and has been shown to greatly improve the efficacy of radiotherapy and chemotherapy for primary and recurrent tumors. HT, as a radioactive and chemical sensitizer, has been proven to inhibit various DNA repair processes, clear hypoxic tumor cells, and enhance the tumor cell uptake of chemotherapeutics [4]. Peritoneal metastasis is the second leading cause of death in patients with colorectal cancer. Cytoreductive surgery (CRS) combined with hyperthermia intraperitoneal chemotherapy (HIPEC) is currently the preferred method for the treatment of peritoneal metastasis of colorectal cancer [5].

β-elemene has been shown to boost the sensitivity of cancer cells to chemoradiotherapy and to be effective against multidrug resistance in malignancies. It can reverse the multidrug resistance of breast cancer cells by downregulating the expression of BCRP and P-gp [6-8]. In KB-C2/ABCB1 cells, β-elemene remarkably enhances the antitumor effects of colchicine, vinblastine, and paclitaxel on cells by blocking the efflux of paclitaxel, as well as the sensitivity of the BCRP-high expression cell line NCI-H460/mx20 to mitoxantrone [9]. In A549/DDP or A549/ER cells, β-elemene reverses drug resistance by inhibiting the efflux of P-gp or MRP-dependent drugs [10-12]. By upregulating the expression of the E3 ubiquitin ligases c-Cbl and Cbl-β, β-elemene enhances the killing effect of adriamycin, daunorubicin, and epirubicin on K562/DNR and SGC7901/ADR cells, inhibits the PI3K/AKT pathway, and reduces P-gp [13].

Wu et al. showed that the expression rate and intensity of the membrane protein HSP70 in H22 cells treated with β-elemene and mitomycin C (MMC) were remarkably higher than those in untreated H22 cells (P < 0.001) [14]. When H22 cells were treated with β-elemene combined with MMC, the expression of HSP70 increased most obviously, with an expression rate of 95.1%. Heat shock treatment combined with other stress-inducing factors may further increase the expression intensity of HSP70. C57BL/6 mice immunized with β-elemene-treated H22 cells induced specific antitumor immunity, which was partially blocked by anti-HSP70 monoclonal antibodies. Three out of 11 mice immunized with β-elemene-treated H22 cells developed tumors, compared with 5 out of 10 mice immunized with anti-HSP70 monoclonal antibody. In addition, all control mice developed tumors. In summary, β-elemene is touted to enhance the immunogenicity of tumor cells to a certain extent by inducing the expression of HSP70 on the tumor cell surface.

The present study assessed the effects of β-elemene on malignancies in hyperthermia-treated HCT116 cells, including apoptosis, invasion, cell proliferation, and colony formation. Levels of HSP27, 70, and 90 were analyzed to assess the apoptosis-promoting effect of β-elemene on hyperthermia-induced apoptosis. Finally, we provided a novel strategy for curing colorectal carcinoma by using β-elemene.

2. Material and Methods

2.1. Cell Culture and Treatment

Human colorectal cancer HCT116 was purchased from the American Cultural Collection (ATCC, Manassas, VA, USA). Preservation cells (DMEM, Life Technologies, Grand Island, NY, US) were supplemented with 10% thermoinactivated fetal bovine serum (FBS, Gibco, Thermo Scientific Inc., Waltham, MA, USA) in Dulbecco's modified Eagle medium and incubated at 37°C/5% CO2. The medium was refreshed every three days and suspended with 0.25% trypsin when the cells covered 80-90% of the plate (Gibco, Thermo Scientific Inc., Waltham, MA, USA).

For the hyperthermia exposure, medium was preheated to 42°C and mixed thoroughly with indicated concentration of β-elemene and then being employed for 2-hour incubation in Forma Series II3110 incubator (Thermo Fisher Scientific, Inc., Waltham, MA, USA). Then, cells were coincubated with β-elemene to 37°C until analysis was performed.

2.2. Cell Viability

The cells (1 × 103) were seeded overnight in 96-well plate and cultured for 24 hours with a certain concentration of β-elemene (cat. No. 515-13-9, Sigma-Aldrich) at 37 or 42°C. After medium removal, the plate was washed three times with ice-cold PBS, and the plates were incubated with 10 μl of CCK-8 solution for 4-hour extra incubation at 37°C. Absorption rates of OD450 were calculated.

2.3. Apoptosis Assay

After treatment, the cells were harvested by trypsin treatment, washed, and resuspended with PBS. Annexin V-FITC labeling Apoptosis Assay Kit detects apoptotic cells (BD Biologics, USA) in accordance with manufacturer instruction. Simply put, the prepared cells were cultured at room temperature for 15 minutes in a 1×binding buffer of 100 μl, plus 5 μl of Annexin V-FITC and 5 μl of PI. The cells were then granulated, replaced with a 1×binding buffer in place of supernatant, and analyzed by a 3-laser Navios flow cytometer (Beckman Coulter, Brea, CA, USA).

2.4. Cell Cycle Analysis

To analyze the cell cycle phase by quantifying DNA content, cells were collected, washed with cold PBS, and immobilized with 70% ice-cold ethanol avoiding from light at 4°C. The fixed cells were then washed with PBS three times, with final concentrations of 100 μg/ml of RNase A and 40 μg/ml of propidium iodide (PI, Beyotime) in the dark for 15 minutes. The cells were then analyzed using three laser-channel flow cytometry (Beckman Coulter, Brea, CA).

2.5. Colony Formation

The cells were seeded on six well plates at a density of 1,000 cells per well. After overnight attachment, the cells were cultured for 10 days until a visible clone emerged. For colony staining, 500 μg/l of Giemsa solution (Kegen, Nanjing, China) was placed in each well and incubated for 30 minutes.

2.6. Tumor Formation in Soft Agar

Soft agar clonogenic assays were performed to assess tumor formation capacity in vitro. Each of the 6 well plates contained 2 ml 0.5% (w/v) low melting point agar (St. Louis Aldridge, MO) DMEM medium with 10% FBS. 5 × 103 cells were uniformly mixed in 2 ml of low melting point agar at 0.3% and 10% FBS, and 5 × 103 cells were added to the polymerized alkali solution. Plates were incubated (37°C, 5% CO2) for 14 days before colony numbers and diameter were observed under a microscope.

2.7. Lentivirus Infection

The lentivirus carrying cytomegalovirus (CMV) was provided by Shanghai Jikai Gene Chemical Co., Ltd. The coding sequence for HSP70 (NCBI Gene ID: 3308) was inserted into cloning restriction sites. The packed lentiviral particles were tittered and adjusted to 1 × 109 titer per ml. At ~80% confluency, cells seeded in 6-well plate were infected with lentivirus or empty control at a multiplicity of infection of 20 and incubated at 37°C with 5% CO2 for 72 h, without the presence of β-elemene.

2.8. Western Blot

Following treatment, cells were lysed and treated using SoniConvert® Tissue cell convertor (DocSense, Chengdu, China) by following manufacturer's instruction in chilled lysis buffer containing 50 mM Tris-HCl (pH 7.4), 150 mM NaCl, 1% Triton X-100, 5 mM EDTA, 1 mM Na3VO4, 1 mM NaF, 10 μM PMSF on ice for 10 min. By centrifugation at 12,000 g for 10 min at 4°C, supernatant was collected. The extracted protein concentrations were measured using Bicinchoninic Acid Kit for Protein Determination (Sigma-Aldrich, St. Louis, MO, USA) according to the manufacturer's protocol. For each sample, 15 μg of total protein was fractionated on a 8% SDS-PAGE gel and transferred onto a nitrocellulose membrane (Millipore, Billerica, MA, U.S.A.). Then, the membrane was incubated with HSP27 (cat. No. ab10936), HSP70 (cat. No. ab2787), HSP90 (cat. No. ab59459), β-actin (cat. No. ab8226), E-cadherin (cat. No. ab76055), vimentin (cat. No. ab8069), and N-cadherin (cat. No. ab76011). All these antibodies were bought from Abcam. p-ERK1/2 (cat. No. 4377T) and ERK1/2 (cat. No. 4695T) were bought from Cell Signaling Technology (CST).

Then, membrane was incubated with HRP-conjugated secondary antibody (goat anti-rabbit IgG H&L antibody, Cat. No. ab7090, Abcam, Cambridge, England) at dilution of 1 : 5000 for 2 h at room temperature. Then, ECL reagents (Cat. No. PRN2232; GE Healthcare Bio-Sciences) were used for immunodetection.

2.9. RT-qPCR

Total RNA was isolated using TRIzol Reagent (Life Technology), followed by integrity and purity verification using UV spectrophotometry and gel electrophoresis on formaldehyde denaturation gel. For preventing cross-contamination, separate room for RNA extraction and RT-qPCR assay was required.

For analyzing mRNA, SYBR Green Master Mix (Life Technologies, Grand Island, NY, US) was employed in a ABI7500 system (Applied Biosystems, Foster City, CA, USA) under the following conditions: 95°C 10 min, 60 cycles of 95°C 15 s, and 60°C 1 min. The specific primers used were as follows: β-actin forward: 5′-CATGTACGTTGCTATCCAGGC-3′; reverse: 5′-CTCCTTAATGTCACGCACGAT-3′; HSP70 forward: 5′-GCATCGAGACTATCGCTAATGAG-3′; and reverse: 5′- TGCAAGGTTAGATTTTTCTGCCT-3′.

2.10. Statistical Analysis

Statistical analysis was performed with SPSS version 13.0. Data are expressed as mean ± standard deviation (SD; x ± s). The statistical significance of the difference between groups was assessed by Student's t tests and one-way analysis of variance followed by Bonferroni post hoc analysis. P < 0.05 was considered to indicate a statistically significant difference.

3. Results

3.1. β-Elemene Promotes Nonapoptotic and Apoptotic Cell Death Induced by Hyperthermia

To evaluate the effect of β-elemene on hyperthermia-induced cell death, we firstly detect the cytotoxicity of β-elemene on colorectal carcinoma cell HCT116 under different concentration. As it is shown by performing CCK-8 assay, 60 μg/ml of β-elemene obviously decreased cell viability of HCT116 after 24-hour treatment and calculated IC30 is 100 μg/ml, and IC50 is 120 μg/ml (Figure 1(a)). For 2-hour treatment under 42°C with β-elemene and extra 22-hour treatment under 37°C with β-elemene, inhibition of cell viability was then performed, and results presented that addition of 60-100 μg/ml of β-elemene decreased cell viability significantly (Figure 1(b)).

Figure 1

β-Elemene promotes hyperthermia-induced cell death in HCT116 cells. (a) 0-200 μg/ml of β-elemene was incubated with HCT116 cells for 24 h, and then, cell viability was measured by performing CCK-8 assay. (b) After 2-hour incubation at 42°C with β-elemene and 22-hour incubation at 37°C with β-elemene, cell viability was measured by performing CCK-8 assay and inhibitory rate was calculated. ∗P < 0.05 vs. mock group. (c) After 2-hour incubation at 42°C with β-elemene and 22-hour incubation at 37°C with β-elemene, Annexin V-FITC/PI double staining was performed followed by flow cytometry assay was performed, and nonapoptotic or apoptotic cell death was calculated, respectively. ∗P < 0.05 and ∗∗P < 0.01 vs. mock group.

After 2-hour treatment under 42°C with β-elemene and extra 22-hour treatment under 37°C with β-elemene, Annexin V-FITC/PI double staining was performed followed by flow cytometry assay to evaluate the situation of cell death. As it is shown in Figure 1(c), 60-100 μg/ml of β-elemene treatment significantly increased both nonapoptotic and apoptotic cell death. By considering intensive decrease in cell viability under 100 μg/ml of β-elemene, 60 and 80 μg/ml of β-elemene were employed for further analysis.

3.2. β-Elemene Promotes Inhibition of Malignant Behaviors in HCT116 Cells

To figure out whether β-elemene promotes inhibitory effects of hyperthermia on malignant behaviors in HCT116 cells, we analyzed cell cycle distribution after β-elemene treatment under hyperthermia condition. As shown in Figure 2(a), addition of β-elemene at 37°C failed to affect cell cycle phase distribution, which is consistent with previous finding presenting that 60 μg/ml of β-elemene failed to affect cell viability (Figure 1(a)). 42°C treatment significantly increased proportion of G1/G0 phase, which is further increased by addition of 60 μg/ml of β-elemene. This demonstrated that under 42°C condition, 60 μg/ml of β-elemene further arrested cell cycle at G1/G0 phase. To further confirm its effect in long term, colony formation and tumor formation in soft agar were performed. As it is shown in Figures 2(b) and 2(c), expectedly, 60 μg/ml of β-elemene failed to affect these behaviors. Under 42°C condition, 60 μg/ml of β-elemene further decreased colonies ≥ 50 μm in diameter. Notably, β-elemene decreased numbers of colonies ≥ 50 μm in diameter, but instead of increasing small colonies, it indicated that inhibitory effect of β-elemene on proliferation did not last in long term.

Figure 2

β-elemene promotes inhibition of hyperthermia on cell proliferation. After 2-hour incubation under 37°C or 42°C with or without 60 μg/ml of β-elemene, cells were incubated for extra 22 h under 37°C with or without 60 μg/ml of β-elemene. Then, cell cycle distribution was analyzed by performing PI staining followed by flow cytometry assay ((a) ∗P < 0.05 vs. 37°C without β-elemene; #P < 0.05 vs. 42°C without β-elemene), colony formation ((b) ∗P < 0.05 vs. 42°C without β-elemene), and tumor formation ((c) ∗P < 0.05 vs. 42°C without β-elemene) were analyzed.

3.3. β-Elemene Decreased HSP70 Protein Level, but Not HSP90 or HSP27, Induced by Hyperthermia

Hyperthermia is well-recognized to induce heat shock protein family expression, including HSP27, 70, and 90. To confirm whether β-elemene affects these protein levels, we performed western blot and found that addition of β-elemene significantly decreased HSP70, but not HSP90 or 27 (Figure 3(a)). Moreover, mRNA level of HSP70 was not detectably affected by β-elemene treatment, indicating that β-elemene affects HSP70 protein level by decreasing its stability (Figure 3(b)). We then compared the relative levels of the downstream ERK1/2 signaling after 24-hour hyperthermia. Similarly, the exposure of HCT116 cells to 42°C resulted in increased phosphorylation of ERK1 and ERK2, which was reversed by addition of β-elemene significantly. We then significantly overexpressed HSP70 by introducing lentivirus containing HSP70-coding sequence, to maintain HSP70 protein level after β-elemene treatment and detect expression of p-ERK1/2 and ERK1/2 total protein. Interestingly, overexpression of HSP70 failed to decrease phosphorylation of ERK1/2 after β-elemene treatment, indicating that β-elemene regulates ERK1/2 signaling in a HSP70-independent manner (Figure 3(c)).

Figure 3

β-Elemene decreased HSP70 and phosphorylation of ERK1/2 induced by hyperthermia. (a) After hyperthermia treatment, western blot was performed to detect HSP27, HSP70 and HSP90 protein levels. ∗P < 0.05 vs. 37°C/mock group; #P < 0.05 vs. 42°C/mock group. (b) After hyperthermia treatment with or without β-elemene, RT-qPCR assay was performed to detect HSP70 mRNA. ∗P < 0.05 vs. 37°C/mock group. (c) After hyperthermia treatment with or without β-elemene, western blot was performed to detect total ERK1/2 and phosphorylated ERK1/2 protein levels. ∗P < 0.05 vs. 37°C/mock group; #P < 0.05 vs. 42°C/mock group.

3.4. β-Elemene Induces Cell Death Partially via Deceasing HSP70

HSP70 exerts protective effect against hyperthermia. To detect whether β-elemene promotes hyperthermia-induced cell death by inhibiting HSP70, we evaluate the cell death after HSP70 knockdown. As it is shown in Figure 4(a), after hyperthermia treatment, HSP70 was increased which was reversed by addition of β-elemene. Infection of lentivirus containing HSP70 coding sequence also upregulated HSP70 protein level. By detecting apoptosis by performing Annexin V-FITC/PI double staining, it was observed that presence of HSP70 significantly decreased apoptosis induced by hyperthermia exposure (Figure 4(b)).

Figure 4

β-Elemene promotes apoptosis induced by hyperthermia. After hyperthermia treatment with or without β-elemene, the efficiency of overexpressed HSP70 was measured by western blot ((a) ∗P < 0.05 vs. 42°C/mock group; #P < 0.05 vs. 42°C/β-elemene/vector group), and its effect on apoptosis is detected by performing Annexin V-FITC/PI double staining ((b) ∗P < 0.05 vs. 42°C/mock group; #P < 0.05 vs. 42°C/β-elemene/vector group).

3.5. β-Elemene Inhibited EMT Induced by Hyperthermia Treatment

According to previous report, hyperthermia promotes epithelial-to-mesenchymal-like transition of breast cancer cells [15]. This promotes us to confirm whether hyperthermia promotes EMT in CRCs. As it is shown in Figure 5(a) by performing Transwell assay, 2-hour treatment of 42°C significantly promotes cell invasive capacity. Expectedly, addition of β-elemene significantly decreased transferred cells (Figure 5(a)). Interestingly, HSP70 inhibition by adding PFT-μ also decreased transferred cells (Figure 5(a)). We further measured the expression of EMT-related markers to further evaluate the effects of β-elemene on hyperthermia induced EMT promotion. The results of western blot analysis are shown in Figure 5(b). The expression level of an epithelial marker, E-cadherin, was significantly downregulated under hyperthermal stress and is reversed by addition of β-elemene (Figure 5(b)). In contrast, a mesenchymal marker, vimentin, was upregulated by hyperthermia and, expectedly, be reversed by addition of β-elemene. Taken together, β-elemene inhibited EMT, which is induced by hyperthermia, at least partially by targeting to HSP70.

Figure 5

β-Elemene inhibited EMT induced by hyperthermia. (a) After addition of β-elemene or HSP70 inhibitor, PFT-μ, invasion capacity was measured by performing Transwell assay. ∗P < 0.05, 42°C/mock. (b) The hallmarkers of EMT, including E-cadherin (E-cad), vimentin, and N-cadherin (N-cad), were measured by performing western blot. ∗P < 0.05, 42°C/mock.

4. Discussion

In our study, the effects of β-elemene with hyperthermia on colorectal cancer (CRC) treatment were evaluated, including changes in cell proliferation, apoptosis, and invasion capacity. The results indicated that β-elemene inhibits the protein level of HSP70, which in turn promotes hyperthermia-induced apoptosis and cell cycle arrest. As a result, β-elemene could be a promising small-molecule drug for promoting hyperthermia against cancer.

β-elemene boasts a range of effects, such as inducing apoptosis, increasing p53 protein expression, targeting the Bcl-2 protein family, activating caspase-dependent and mitochondria-dependent pathways, and cell cycle arrest. Edris, Nan et al., and Yu et al. demonstrated that β-elemene has an antiproliferation effect on multiderived cancer cells mainly by inducing apoptosis and blocking the cell cycle [16-18]. In previous results, we reported that, berberine, a compound extracted from the traditional Chinese medicine Coptis chinensis, suppresses malignancies in colorectal cancer cells, indicating that natural compound extracted from herbal medicine could exert antitumor effects [19]. We found that hyperthermia (42°C) treatment for 2 hours had little effect on the cell cycle distribution of colorectal cancer cells, while the addition of β-elemene could significantly arrest the cell cycle in the G1/G0 phase. In contrast, β-elemene alone did not significantly affect cell cycle distribution. This may be attributed to the lack of effect of our chosen β-elemene concentration on cell viability.

Furthermore, β-elemene regulates the expression of several key molecules involved in tumor angiogenesis and metastasis, including vascular endothelial growth factor (VEGF), matrix metalloproteinases (MMPs), E-cadherin, N-cadherin, and vimentin, playing a regulatory role in the invasion and metastasis of a variety of tumor cells [20-25]. Consistent with these results, we found that β-elemene significantly inhibited hyperthermia-induced EMT after treatment of colorectal cancer cells. We also showed that hyperthermia treatment significantly increased invasion capacity, which was reversed by the addition of β-elemene. This finding indicates that β-elemene inhibited EMT progression induced by hyperthermia. Notably, the addition of PFT-μ, an HSP70 inhibitor, after hyperthermia treatment also inhibited the EMT, demonstrating that β-elemene inhibited the EMT potentially by decreasing HSP70.

The immune function of the body plays a vital role in the occurrence and development of cancer. Existing studies have shown that β-elemene can not only inhibit the proliferation of tumor cells but also enhance the cellular immune function associated with malignancies. According to relevant studies, β-elemene can enhance the body's immune response to tumors by changing the immune characteristics of tumor cells or in combination with other immune drugs [25-30]. To avoid the effect of β-elemene on colorectal carcinoma cells, we evaluated the cytotoxicity of β-elemene on HCT116 cells, and the concentration of β-elemene presenting no effect on cell viability was shown. Thus, the effect of β-elemene in our study mainly exerted roles after hyperthermia treatment.

Among several cellular mechanisms that destabilize the intracellular environment of cancer cells, HT has shown promising antitumor effects. Cancer cells, however, also exhibit a certain heat tolerance. In fact, HT-induced protein denaturation and aggregation may lead to upregulation of heat shock proteins, a group of molecular chaperones with cell-protective and antiapoptotic properties. HSP70s, as a class of evolutionarily conserved heat shock proteins, play a critical role in cellular homeostasis and survival. HSP70s are overexpressed in a variety of malignancies, including colorectal cancer, and are often associated with poor prognosis. Consequently, inhibition of HSP70 may be a novel approach for the treatment of colorectal cancer. We discovered that β-elemene has an inhibitory effect on HSP70 and, thus, may promote apoptosis and inhibit the EMT.

5. Conclusion

β-Elemene promotes hyperthermia-induced apoptosis by inhibiting HSP70 in colorectal carcinoma cells. The addition of β-elemene posttranscriptionally decreased HSP70 levels without disturbing HSP27 or HSP90 protein levels and inhibited cell invasion, colony formation, and EMT progression. The present study highlighted the potential role of β-elemene in promoting the effects of hyperthermia on treating colorectal carcinoma. Therefore, our results suggest β-elemene as a promising strategy to combine with hyperthermia to cure colorectal carcinoma.