Hypoxia facilitates triple-negative breast cancer stem cells enrichment and stemness maintenance through oxidized ataxia telangiectasia mutated-induced one-carbon metabolism

Figure 1 Hypoxia promotes enrichment and DNA damage-independent oxidative activation of ataxia telangiectasia mutated. A: Flow cytometry analysis of CD44+/CD24- cell populations in Hs578T and MDA-MB-231 cells after exposure to hypoxia or normoxia; B: Quantitative reverse transcriptase polymerase chain reaction analysis of cancer stem cells-associated genes (c-Myc, octamer-binding protein 4, Kruppel-like factor 4, sex-determining region Y-box 2, NANOG) in mammosphere cultures under hypoxia; C-E: Western blot analysis of phosphorylated ataxia telangiectasia mutated, γH2AX, and 53BP1 in Hs578T and MDA-MB-231 cancer stem cells under normoxia, hypoxia, or H₂O₂ treatment. Data were presented as mean ± SD (n = 3). ^aP < 0.05; ^bP < 0.01; ns: not significant. KLF4: Kruppel-like factor 4; SOX2: Sex-determining region Y-box 2; OCT4: Octamer-binding protein 4; p-ATM: Phosphorylated ataxia telangiectasia mutated.

Figure 2 Oxidized ataxia telangiectasia mutated promotes triple-negative breast cancer-cancer stem cells enrichment and stemness maintenance. A: Representative images showing inhibition of mammosphere formation by the ataxia telangiectasia mutated inhibitor Ku60019; B: Quantification of mammosphere size; C: Quantification of mammosphere number; D-F: Effects of ataxia telangiectasia mutated knockdown on cancer stem cell enrichment (D), mammosphere size (E), and mammosphere number (F); G and H: Western blot analysis of stemness-associated proteins (c-Myc, sex-determining region Y-box 2, Kruppel-like factor 4, octamer-binding protein 4) following Ku60019 treatment (G) or ataxia telangiectasia mutated knockdown (H). Scale bar: 200 μm. Data were presented as mean ± SD (n = 3). ^aP < 0.05; ^bP < 0.01. KLF4: Kruppel-like factor 4; SOX2: Sex-determining region Y-box 2; p-ATM: Phosphorylated ataxia telangiectasia mutated; shATM: Ataxia telangiectasia mutated knockdown.

Figure 3 Hypoxia and oxidized ataxia telangiectasia mutated facilitate metabolic remodeling in triple-negative breast cancer-cancer stem cells. A: Volcano plot showing differentially regulated metabolites (115 upregulated, 129 downregulated) in MDA-MB-231 cancer stem cells under hypoxia compared with normoxia; B: Pathway enrichment analysis of differentially regulated metabolites; C: Kyoto Encyclopedia of Genes and Genomes classification of altered metabolic pathways; D: Volcano plot showing metabolic changes in hypoxic cancer stem cells after Ku60019 treatment; E: Pathway enrichment analysis after Ku60019 treatment; F: Kyoto Encyclopedia of Genes and Genomes classification of altered metabolic pathways after Ku60019 treatment.

Figure 4 Oxidized ataxia telangiectasia mutated promotes serine hydroxymethyltransferase 2 and methylenetetrahydrofolate dehydrogenase 2 expression through c-Myc. A and B: Western blot analysis of phosphorylated ataxia telangiectasia mutated, c-Myc, serine hydroxymethyltransferase 2 (SHMT2), and methylenetetrahydrofolate dehydrogenase 2 (MTHFD2) in Hs578T and MDA-MB-231 cells after treatment with Ku60019 and ataxia telangiectasia mutated knockdown; C: Consensus c-Myc binding motif; D: Schematic representation of predicted c-Myc binding sites in the human MTHFD2 and SHMT2 promoter regions; E: Luciferase assay showed SHMT2 and MTHFD2 relative luciferase activity; F: Representative chromatin immunoprecipitation (ChIP)-polymerase chain reaction (PCR) showing c-Myc occupancy at the MTHFD2 and SHMT2 promoters; input and immunoglobulin G served as controls; G and H: ChIP-quantitative PCR analysis demonstrating c-Myc enrichment at the MTHFD2 (G) and SHMT2 (H) promoters in Hs578T and MDA-MB-231 cells. ChIP-quantitative PCR enrichment expressed as % input relative to immunoglobulin G. Data were presented as mean ± SD (n = 3). ^aP < 0.05; ^bP < 0.01. p-ATM: Phosphorylated ataxia telangiectasia mutated; MTHFD2: Methylenetetrahydrofolate dehydrogenase 2; SHMT2: Serine hydroxymethyltransferase 2; IgG: Immunoglobulin G.

Figure 5 Serine hydroxymethyltransferase 2 and methylenetetrahydrofolate dehydrogenase 2 promote cancer stem cells enrichment and stemness maintenance in triple-negative breast cancer. A: Effects of serine hydroxymethyltransferase 2 knockdown on mammosphere formation, size, and number in Hs578T and MDA-MB-231 cancer stem cells; B: Effects of methylenetetrahydrofolate dehydrogenase 2 knockdown on mammosphere formation, size, and number; C and D: Western blot analysis of stemness-associated proteins Kruppel-like factor 4 and sex-determining region Y-box 2 after serine hydroxymethyltransferase 2 or methylenetetrahydrofolate dehydrogenase 2 knockdown. Scale bar: 200 μm. Data were presented as mean ± SD (n = 3). ^aP < 0.05. MTHFD2: Methylenetetrahydrofolate dehydrogenase 2; SHMT2: Serine hydroxymethyltransferase 2; KLF4: Kruppel-like factor 4; SOX2: Sex-determining region Y-box 2.

Figure 6 c-Myc regulates serine hydroxymethyltransferase 2 and methylenetetrahydrofolate dehydrogenase 2 expression and cancer stem cell stemness. A: Representative images of mammospheres formed by MDA-MB-231 cells with control, c-Myc knockdown (shc-Myc), or c-Myc overexpression (ov-c-Myc). Scale bar = 200 μm; B and C: Quantification of mammosphere size (B) and mammosphere number (C); D: Western blot analysis of c-Myc, serine hydroxymethyltransferase 2, methylenetetrahydrofolate dehydrogenase 2, Kruppel-like factor 4, and sex-determining region Y-box 2 in MDA-MB-231 cells after shc-Myc or ov-c-Myc treatment; E: Volcano plot showing global metabolic changes following c-Myc knockdown; F: Pathway enrichment analysis of differential metabolites; G: Kyoto Encyclopedia of Genes and Genomes classification of altered metabolic pathways after c-Myc knockdown. MTHFD2: Methylenetetrahydrofolate dehydrogenase 2; SHMT2: Serine hydroxymethyltransferase 2; KLF4: Kruppel-like factor 4; SOX2: Sex-determining region Y-box 2.

Figure 7 Oxidized ataxia telangiectasia mutated sustains triple-negative breast cancer-cancer stem cells stemness via serine hydroxymethyltransferase 2- and methylenetetrahydrofolate dehydrogenase 2-mediated one-carbon metabolism. A: NADPH/NADP+ ratio in cancer stem cells (CSCs) under hypoxia compared with normoxia; B and C: NADPH/NADP+ ratio decreased in CSCs after Ku60019 treatment (B) or shc-Myc knockdown (C); D-F: Effects of one-carbon metabolite supplementation on mammosphere size (D) and number (E and F) in CSCs with serine hydroxymethyltransferase 2 (SHMT2) knockdown or methylenetetrahydrofolate dehydrogenase 2 (MTHFD2) knockdown; G: Western blot analysis of MTHFD2, SHMT2, Kruppel-like factor 4, and sex-determining region Y-box 2 expression following SHMT2 or MTHFD2 knockdown under hypoxia. Data were presented as mean ± SD (n = 3). ^aP < 0.05. MTHFD2: Methylenetetrahydrofolate dehydrogenase 2; SHMT2: Serine hydroxymethyltransferase 2; KLF4: Kruppel-like factor 4; SOX2: Sex-determining region Y-box 2.