Cysteine Dioxygenase 1 Mediates Erastin-Induced Ferroptosis in Human Gastric Cancer Cells

Abstract

Background: Ferroptosis is a recently discovered form of iron-dependent nonapoptotic cell death. It is characterized by loss of the activity of the lipid repair enzyme, glutathione peroxidase 4 (GPX4), and accumulation of lethal reactive lipid oxygen species. However, we still know relatively little about ferroptosis and its molecular mechanism in gastric cancer (GC) cells. Here, we demonstrate that erastin, a classic inducer of ferroptosis, induces this form of cell death in GC cells and that cysteine dioxygenase 1 (CDO1) plays an important role in this process.

Methods: We performed quantitative real-time polymerase chain reaction, Western blotting, cell viability assay, reactive oxygen species (ROS) assay, glutathione assay, lipid peroxidation assay, RNAi and gene transfection, immunofluorescent staining, dual-luciferase reporter assay, transmission electron microscopy, and chromatin immunoprecipitation assay to study the regulation of ferroptosis in GC cells. Mouse xenograft assay was used to figure out the mechanism in vivo.

Results: Silencing CDO1 inhibited erastin-induced ferroptosis in GC cells both in vitro and in vivo. Suppression of CDO1 restored cellular GSH levels, prevented ROS generation, and reduced malondialdehyde, one of the end products of lipid peroxidation. In addition, silencing COO1 maintained mitochondrial morphologic stability in erastin-treated cells. Mechanistically, c-Myb transcriptionally regulated CDO1, and inhibition of CDO1 expression upregulated GPX4 expression.

Conclusions: Our findings give a better understanding of ferroptosis and its molecular mechanism in GC cells, gaining insight into ferroptosis-mediated cancer treatment.

Figure 1

CDO1 suppression contributes to ferroptosis resistance. (A) GC cells were treated with 0, 2.5, 5, 10, 15, 20, or 40 μg/ml erastin for 24 hours. Cell viabilities were assayed by the MTT assay. (B) GPX4, solute carrier family 7 member 11(Slc7a11), and CDO1 expression were assessed by the qRT-PCR in GC cells. (C) GC cells were treated with erastin (E, 10 μg/ml) and with or without Z-VAD-FMK (apoptosis inhibitor), necrostatin (necroptosis inhibitor), 3-methyladenine (3-MA, autophagy inhibitor), and ferrostatin-1 or liproxstatin-1 (ferroptosis inhibitors) for 24 hours. Cell viability was assayed by the MTT assay. (D) GC cells were treated with or without erastin (10 μg/ml). LDH release assay was examined. (E) The siRNA sequences [CDO1 siRNA (2) and (3)] were designed for silencing CDO1. The mRNA levels of CDO1 in GC cells were examined by qRT-PCR. (F) The siRNA sequences [CDO1 siRNA (2) and (3)] were designed for silencing CDO1. The protein levels of CDO1 in GC cells were examined by Western blotting. (G) CDO1 siRNA (2)– and CDO1 siRNA (3)–silenced GC cells were treated with erastin (0, 2.5, 5, 10, 15, and 20 μg/ml) for 24 hours, and cell viabilities were assayed by the MTT assay. Quantitative data are presented as means ± SD from three independent experiments. ^⁎P < .05, ^⁎⁎P < .01, ^⁎⁎⁎P < .001.

Figure 2

CDO1 regulates ferroptosis biomarkers. CDO1-silenced GC cells were treated with 10 μg/ml erastin for 24 hours. (A) GSH, (B) ROS, and (C) MDA levels were assayed. CDO1 siRNA-silenced GC cells were treated with 10 μg/ml erastin for 24 hours. GPX4 expression was assessed by the qRT-PCR (D), Western blotting (E), and immunofluorescence (F). (G) Transmission electron microscopic images of CDO1 siRNA (2)– and CDO1 siRNA(3)–silenced AGS cells treated with 10 μg/ml erastin for 24 hours. Left, 1 × 10⁴ magnification, scale bar = 2 μm; right, 5 × 10⁴ magnification, scale bar = 500 nm. Quantitative data are presented as means ± SD from three independent experiments. ^⁎P < .05, ^⁎⁎P < .01, ^⁎⁎⁎P < .001.

Figure 3

C-Myb regulates CDO1 expression during ferroptosis. (A) GC cells were treated with erastin (10 or 30 μg/ml) for 24 hours. Expression levels of CDO1, c-Myb, and GPX4 proteins were detected by Western blotting. (B) The siRNA sequences [c-Myb siRNA (1), (2), and (3)] were designed for silencing c-Myb. The mRNA levels of c-Myb in GC cells were examined by qRT-PCR. (C) The siRNA sequences [c-Myb siRNA (1), (2), and (3)] were designed for silencing c-Myb. The protein levels of c-Myb in GC cells were examined by Western blotting. (D) Cell viabilities were determined by the MTT assay after transfection with three different c-Myb siRNA sequences to silence c-Myb expression. (E) c-Myb–silenced GC cells were treated with erastin (5, 10, or 20 μg/ml) for 24 hours, and cell viabilities were detected by the MTT assay. C-Myb was overexpressed, and (F) mRNA and (G) protein expression levels of CDO1 and GPX4 were detected by the qRT-PCR and Western blotting, respectively (V, vector; M, overexpression). (H) Immunofluorescence staining of GPX4 was determined when c-Myb was overexpressed. (I) Cell viabilities were determined by the MTT assay after overexpression of c-Myb or inhibited expression of CDO1 in GC cells treated with erastin (10 μg/ml). (J) The interaction between CDO1 and c-Myb was verified by a luciferase reporter assay. (K) The chromatin immunoprecipitation assay shows that c-Myb has three positive binding sites with the CDO1 promoter. Quantitative data are presented as means ± SD from three independent experiments. ^⁎P < .05, ^⁎⁎P < .01, ^⁎⁎⁎P < .001.

Figure 4

Suppression of CDO1 inhibits ferroptosis in vivo. (A) Nude mice were injected subcutaneously with BGC823 cells (1 × 10⁶ cells/mouse) and treated with erastin (20 mg/kg, intraperitoneally, twice every other day) starting at day 10 for 4 weeks. The tumor volume was calculated every 5 days (n = 5). (B, C) Lung, liver, spleen, kidney, heart, and grafted subcutaneous tumors of nude mice were removed and stained with hematoxylin-eosin. Magnification: 100× (left panel), scale bar = 100 μm; 400× (right panel), scale bar = 25 μm. Expression of CDO1, GPX4, and PTGS2 (E) in subcutaneous tumors of nude mice using immunohistochemistry. Magnification: 100× (left panel); scale bar = 100 μm; 200× (right panel), scale bar = 50 μm; 400 × (right panel), scale bar = 25 μm.

Figure 5

Diagram summarizing the role of CDO1 during erastin-induced ferroptosis. Erastin inhibits cellular cysteine uptake by suppressing system X_C⁻ and depleting GSH. This leads to inactivation of GPX4, increased ROS, and subsequently ferroptosis. CDO1, transcriptionally regulated by c-Myb, can transform cysteine to taurine and thereby competitively deprive cells of the cysteine used to synthesize GSH. When CDO1 expression is inhibited, the conversion of cysteine to taurine is decreased, enhancing GSH generation. Therefore, GPX4 activity is promoted, inhibiting ROS accumulation and lipid peroxidation and ultimately reducing the risk of ferroptosis.

Supplementary Figure 1

Expression of CDO1 in GC and normal gastric tissues. (A) CDO1 mRNA expression levels of 16 paired patient specimens (C, cancer; N, normal). (B) CDO1 protein expression levels of four paired patient specimens (C, cancer; N, normal). (C) Viabilities of GC cells overexpressing CDO1 following treatment with 5, 10, or 20 μg/ml erastin for 24 hours (V, vector; M, overexpression). Quantitative data are presented as means ± SD from three independent experiments. ^⁎P < .05, ^⁎⁎P < .01, ^⁎⁎⁎P < .001.

Supplementary Figure 2

GPX4 mRNA levels by alteration of CDO1. Overexpressing CDO1-GC cells were treated with 10 μg/ml erastin for 24 hours (V, vector; M, overexpression). Quantitative data are presented as means ± SD from three independent experiments. ^⁎P < .05, ^⁎⁎P < .01, ^⁎⁎⁎P < .001.

Supplementary Figure 3

Immunohistochemistry of Ki-67 in tumors. Expression of Ki-67 in subcutaneous tumors of nude mice using immunohistochemistry. Magnification: 100× (left panel); scale bar = 100 μm; 200× (right panel), scale bar = 50 μm; 400 × (right panel), scale bar = 25 μm.