Figure 1

SOSTDC1 is highly expressed in TNBC and positively correlated with malignancy and Olaparib resistance. A) Overlap analysis of the upregulated genes in TNBC tumors (TNBC versus non‐TNBC >two‐fold, PRJNA739366), BTICs from two TNBC PDXs (BTICs B) versus the rest non‐BTICs (R), PRJNA376644) and an Olaparib‐resistant gene cluster of TNBC cells (GSE165914). (B) SOSTDC1 mRNA expression level of patients in the TCGA database with different molecular subtypes of breast cancer. Statistical significance was determined using Mann–Whitney U‐tests. C) Immunohistochemistry analysis of SOSTDC1 expression in para‐tumor tissues and tumor tissues from BC patients. Representative images of different molecular subtypes were shown and H‐Scores were calculated. Data represent the mean ± SEM, following unpaired Student's two‐sided t‐tests. ^* p < 0.05; ^** p < 0.01; ^*** p < 0.001. Scale bar, 100 µm. D) mRNA and protein expression of SOSTDC1 in different subtypes of BC cell lines. E) Overall survival and relapse‐free survival of TNBC patients in Kaplan‐Meier Plotter with high or low SOSTDC1 expression. Statistical significance was assessed using a log‐rank test. F) mRNA and protein expression of SOSTDC1 in sorted BTICs (B) and the rest non‐BTICs (R) of SUM149 and SUM159. ^* p < 0.05 (unpaired Student's two‐sided t‐tests). G,H) Mammosphere formation assays of SOSTDC1‐knockdown SUM149 cells G) or SOSTDC1‐overexpressing SUM159 cells H). Representative images were shown and data were presented as mean ± SEM. Statistical significance was determined using the one‐way ANOVA or unpaired Student's two‐sided t‐tests. ^* p < 0.05; ^** p < 0.01. Scale bar, 100 µm. I–J) The percentage of BTIC population in SOSTDC1‐knockdown SUM149 cells I) or SOSTDC1‐overexpressing SUM159 cells J). Data were presented as mean ± SEM, following the one‐way ANOVA or unpaired Student's two‐sided t‐tests. ^*** p < 0.001; ^**** p < 0.0001. K,L) Survival analysis of SOSTDC1‐knockdown SUM149 cells K) or SOSTDC1‐overexpressing SUM159 cells L) in response to Olaparib; n = 3 independent biological samples. M,N) The percentage M) and absolute number N) of BTIC population in SOSTDC1‐knockdown SUM149 cells treated with 10 nmol L⁻¹ Olaparib for 5d. Data were presented as mean ± SEM, following the two‐way ANOVA. ^* p < 0.05; ^*** p < 0.001; ns, not significant. O) Tumor volumes of SOSTDC1‐knockdown SUM149 xenografts (four mice per group, two sites each mouse, 5000 cells per site). Data were presented as mean ± SEM. ^*** p < 0.001 (the one‐way ANOVA). P) Tumor volumes of SOSTDC1‐overexpressing MDA‐MB‐231 xenografts (three mice per group, two sites each mouse, 1 million cells per site). Data were presented as mean ± SEM. ^*** p < 0.001 (unpaired Student's two‐sided t‐tests.). Q,R) The percentage of BTIC population of SOSTDC1‐knockdown SUM149 xenografts Q) or SOSTDC1‐overexpressing MDA‐MB‐231 xenografts R). Data were presented as mean ± SEM, following the one‐way ANOVA or unpaired Student's two‐sided t‐tests. ^** p < 0.01; ^*** p < 0.001. S–U) The effect of Olaparib treatment (50 mg kg⁻¹, intraperitoneally, once a day) on SUM149 xenograft growth (SOSTDC1‐knockdown and control) in nude mice (three mice per group, two sites each mouse, 0.8 million cells per site). Tumor volumes and a representative image of tumors were shown in S). The percentage of the BTIC population in each group was shown in (T), and absolute BTIC numbers of each group are shown in (U). Data were presented as mean ± SEM, following the two‐way ANOVA. ^* p < 0.05; ^** p < 0.01; ^**** p < 0.0001; ns, not significant.