Combinative treatment of β-elemene and cetuximab is sensitive to KRAS mutant colorectal cancer cells by inducing ferroptosis and inhibiting epithelial-mesenchymal transformation

Abstract

Background and Purpose: RAS mutations limit the effectiveness of anti-epidermal growth factor receptor (EGFR) monoclonal antibodies in combination with chemotherapy for metastatic colorectal cancer (mCRC) patients. Therefore, new cell death forms have focused on identifying indirect targets to inhibit Ras-induced oncogenesis. Recently, emerging evidence has shown the potential of triggering ferroptosis for cancer therapy, particularly for eradicating aggressive malignancies that are resistant to traditional therapies. Methods: KRAS mutant CRC cell HCT116 and Lovo were treated with cetuximab and β-elemene, a bioactive compound isolated from Chinese herb Curcumae Rhizoma. Ferroptosis and epithelial-mesenchymal transformation (EMT) were detected in vitro and in vivo. Orthotopic CRC animal model were established and the tumor growth was monitored by IVIS bioluminescence imaging. Tumor tissues were collected to determine ferroptosis effect and the expression of EMT markers after the treatment. Results: CCK-8 assay showed that synergetic effect was obtained when 125 µg/ml β-elemene was combined with 25 µg/ml cetuximab in KRAS mutant CRC cells. AV/PI staining suggested a non-apoptotic mode of cell death after the treatment with β-elemene and cetuximab. In vitro, β-elemene in combination with cetuximab was shown to induce iron-dependent reactive oxygen species (ROS) accumulation, glutathione (GSH) depletion, lipid peroxidation, upregulation of HO-1 and transferrin, and downregulation of negative regulatory proteins for ferroptosis (GPX4, SLC7A11, FTH1, glutaminase, and SLC40A1) in KRAS mutant CRC cells. Meanwhile, combinative treatment of β-elemene and cetuximab inhibited cell migration and decreased the expression of mesenchymal markers (Vimentin, N-cadherin, Slug, Snail and MMP-9), but promoted the expression of epithelial marker E-cadherin. Moreover, ferroptosis inhibitors but not other cell death suppressors abrogated the effect of β-elemene in combination with cetuximab on KRAS mutant CRC cells. In vivo, co-treatment with β-elemene and cetuximab inhibited KRAS mutant tumor growth and lymph nodes metastases. Conclusions: Our data for the first time suggest that the natural product β-elemene is a new ferroptosis inducer and combinative treatment of β-elemene and cetuximab is sensitive to KRAS mutant CRC cells by inducing ferroptosis and inhibiting EMT, which will hopefully provide a prospective strategy for CRC patients with RAS mutations.

Keywords: KRAS mutation; colorectal cancer; epithelial-mesenchymal transformation; ferroptosis; β-elemene.

Figure 1

Combinative treatment of β-elemene and cetuximab was sensitive to KRAS mutant CRC cells. (A) The sensitivity of KRAS mutant and wild-type colorectal cancer cells to cetuximab treatment (25 µg/ml) for 24 h was detected by CCK-8 assay. The mean ± s.d. is shown. **P < 0.01. (B) The inhibitory effects and cytotoxicity of co-treatment with β-elemene (125 µg/ml) and cetuximab (25 µg/ml) in KRAS mutant CRC cells was determined after the treatment for 24 h. (C) Representative cell morphological changes are detected by light microscopy. Scale bar = 100 μm. (D) Representative results of annexin V-FITC/PI staining and quantitative analysis after the treatment (β-elemene 125 µg/ml, cetuximab 25 µg/ml) for 24 h. The mean ± s.d. is shown. **P < 0.01. (E) Representative results of cell cycle and quantitative analysis after the treatment (β-elemene 125 µg/ml, cetuximab 25 µg/ml) for 24 h. (F) The colony-formation assay was performed and colony numbers are shown (β-elemene 125 µg/ml, cetuximab 25 µg/ml). The mean ± s.d. is shown. **P < 0.01.

Figure 2

The effect of co-treatment with β-elemene and cetuximab on several ferroptotic events in KRAS mutant CRC cells. (A) The effect of cetuximab and β-elemene in combination with other cell death inhibitors on the cell viability of KRAS mutant HCT116 and Lovo cells after the treatment for 24 h. The mean ± s.d. is shown. (B) The cellular ROS level after the treatment (β-elemene 125 µg/ml, cetuximab 25 µg/ml) for 24 h was analyzed by a flow cytometer, **P < 0.01. (C) Intracellular GSH level in KRAS mutant HCT116 and Lovo cells after the treatment (β-elemene 125 µg/ml, cetuximab 25 µg/ml) for 24 h was detected, **P < 0.01. (D) Intracellular MDA levels in KRAS mutant HCT116 and Lovo cells after the treatment (β-elemene 125 µg/ml, cetuximab 25 µg/ml) for 24 h was detected, **P < 0.01.

Figure 3

The iron ion level and mitochondria staining were detected. (A) The chelatable iron was determined using the fluorescent indicator Phen Green SK (green) after the treatment (β-elemene 125 µg/ml, cetuximab 25 µg/ml) for 24 h. Scale bar = 100 µm. (B) The Mitochondria morphology was assessed with Mito-Tracker Green after the treatment (β-elemene 125 µg/ml, cetuximab 25 µg/ml) for 24 h. Scale bar = 50 µm.

Figure 4

The effect of co-treatment with β-elemene and cetuximab on ferroptosis-related proteins in KRAS mutant CRC cells. (A) The expression of positive regulatory proteins for ferroptosis (HO-1 and transferrin) and the negative regulatory proteins for ferroptosis (GPX4, SLC7A11, FTH1, glutaminase, and SLC40A1) were detected by western blotting after the treatment (β-elemene 125 µg/ml, cetuximab 25 µg/ml) for 24 h. (B) HCT116 and Lovo cells were treated with cetuximab (25 µg/ml) and β-elemene (125 µg/ml) with or without ferroptosis inhibitors for 24 h and cell viability was assayed. The mean ± s.d. is shown. **P < 0.01.

Figure 5

Combinative treatment of β-elemene and cetuximab suppressed the migration of KRAS mutant CRC cells by inhibiting EMT. (A) Representative results of wound healing after the treatment (β-elemene 125 µg/ml, cetuximab 25 µg/ml, DFO 20 nM) for 24 h. The mean ± s.d. is shown. **P < 0.01. (B) Transwell invasion assay was performed by the 24-transwell system and quantitative analysis. The pictures were taken 24 h after seeding (original magnification: × 100). The mean ± s.d. is shown. **P < 0.01. (C) The expression of several key EMT markers Vimentin, E-Cadherin, N-Cadherin, Slug, Snail and MMP-9 were detected after the treatment (β-elemene 125 µg/ml, cetuximab 25 µg/ml) for 24 h by western blotting.

Figure 6

The antitumor efficacy of co-treatment with β-elemene and cetuximab in vivo. (A) The scheme of tumor inoculation and systemic injection. (B) Bioluminescent imaging for HCT116-luc orthotopic xenograft colon tumors at different time points post treatment (β-elemene 50 mg/kg, cetuximab 50 mg/kg) and representative image of metastatic lymph nodes. (C) Fold change in average radiance per mouse at experimental endpoint (day 18) was analyzed for each treatment group. Data are expressed as the mean ± s.d. (D) The survival curves of mice in each group were assessed.

Figure 7

Representative immunohistochemical staining for orthotopic xenograft tumor sections. Several regulatory proteins for ferroptosis (GPX4 and Transferrin) and EMT (E-cadherin and Vimentin) were detected by immunohistochemical staining (original magnification: × 100).

Figure 8

A schematic diagram about the central role of co-treatment with β-elemene and cetuximab in ferroptosis induction and migration inhibition. Combinative treatment of β-elemene and cetuximab is sensitive to KRAS mutant CRC cells by inducing ferroptosis in both GPX4-dependent and GPX4-independent pathway and suppressing cancer migration by regulating EMT.