MTDH-SND1 interaction is crucial for expansion and activity of tumor-initiating cells in diverse oncogene- and carcinogen-induced mammary tumors

Abstract

The Metadherin gene (MTDH) is prevalently amplified in breast cancer and associated with poor prognosis; however, its functional contribution to tumorigenesis is poorly understood. Using mouse models representing different subtypes of breast cancer, we demonstrated that MTDH plays a critical role in mammary tumorigenesis by regulating oncogene-induced expansion and activities of tumor-initiating cells (TICs), whereas it is largely dispensable for normal development. Mechanistically, MTDH supports the survival of mammary epithelial cells under oncogenic/stress conditions by interacting with and stabilizing Staphylococcal nuclease domain-containing 1 (SND1). Silencing MTDH or SND1 individually or disrupting their interaction compromises tumorigenenic potential of TICs in vivo. This functional significance of MTDH-SND1 interaction is further supported by clinical analysis of human breast cancer samples.

Figure 1. Systemic deletion of Mtdh inhibits mammary tumor formation and metastasis

(A) Schematic representation of WT and mutant Mtdh allele. Green boxes represent exons 1–12. Primers (F, forward; R, reverse) used for genotyping are indicated above the corresponding genomic sequences. (B) LacZ expression in WT and KO embryo at day 10.5, visualized by X-gal staining. (C) MTDH protein immunobloting in MECs freshly dissociated from 8-week-old female mice with indicated Mtdh genotype. (D) Kinetics of mammary tumor onset in MMTV-PyMT females of indicated Mtdh genotypes. Mtdh^+/+ (n = 13), Mtdh^+/− (n = 26) and Mtdh^−/− (n = 30). (E) Percentage of tumor-free mammary glands at indicated ages in the same cohort of mice as in (D). (F) Total tumor burden of PyMT;Mtdh^+/+, PyMT;Mtdh^+/− and PyMT;Mtdh^−/− cohorts evaluated at indicated age. Statistical comparison was done between Mtdh^+/+ and Mtdh^−/− groups. Data represent mean ± SEM (n > 20). (G) Number of lung metastatic nodules in PyMT;Mtdh^+/+ (n = 15), PyMT;Mtdh^+/− (n = 11) and PyMT;Mtdh^−/− (n = 14) animals. Error bars represent 5–95 percentiles. (H) Kinetics of mammary tumor onset in MMTV-ErbB2 mice of the indicated genotypes. Mtdh^+/+ (n = 22), Mtdh^+/− (n = 31) and Mtdh^−/− (n = 27). (I) Percentage of mice from same cohorts as in (H) bearing indicated number of tumors at 300 days of age. (J) Incidence of lung metastasis in tumor-bearing MMTV-ErbB2 mice from Mtdh^+/+ (n = 30) and Mtdh^−/− (n = 23) groups. (K) Number of metastatic lesions per lung section in the same cohorts of mice from (J). Error bars represent 5–95 percentiles. (L) Kinetics of mammary tumor onset in MMTV-Wnt mice of the indicated genotypes. Mtdh^+/+ (n = 31), Mtdh^+/− (n = 48) and Mtdh^−/− (n = 31). (M) Percentage of mice from same cohorts as in (L) bearing indicated number of tumors at 300 days of age. (N) Kinetics of mammary tumor onset in mice with indicated Mtdh genotype treated with MPA and DMBA as indicated (top). Tumor latency was recorded as days after first DMBA treatment. Mtdh^+/+ (n = 19), Mtdh^+/− (n = 13) and Mtdh^−/− (n = 10). (O) Percentage of mice from same cohorts as in (N) bearing indicated number of tumors at 4 months of age. Statistics: (D, H, L, N) log rank test. (E, I, J, M, O) Chi-square test. (G, K) Mann-Whitney test. (F) Student’s t-test. ***p < 0.001, **p < 0.01, *p < 0.05. See also Figure S1.

Figure 2. Mammary glands from Mtdh^−/− mice exhibit defects in oncogene-induced expansion and tumorigenic potential.<

br>(A) Representative whole mounts (top panels, scale bar 1 mm) and H&E stained sections (bottom panels, scale bar 200 μm) of preneoplastic mammary glands from MMTV-PyMT (4 weeks), MMTV-Wnt (6 weeks), MMTV-ErbB2 (6 months) mice of the indicated genotypes. (B) Flow cytometry of CD45⁻CD31⁻TER119⁻ (Lin⁻) MECs from mammary glands of 6-week-old females of the indicated genotypes. (C, D) Quantification of luminal (C) and basal (D) cells analyzed in (B) (n = 4). (E) Mammosphere formation assays with WT or KO MECs dissociated from preneoplastic glands of MMTV-PyMT (n = 6), MMTV-Wnt (n = 4), MMTV-ErbB2 (n = 6) mice. Assays performed in triplicates for each mammary gland. (F) Mammary tumor incidence (left) and size (right) 3 months after orthotopic transplantations of unsorted MECs dissociated from preneoplastic glands of PyMT;Mtdh^+/+ and PyMT;Mtdh^−/− mice. (G) Mammary tumor incidence (left) and size (right) 8 weeks after orthotopic transplantations of indicated sorted CD24⁺CD29^low luminal or CD24⁺CD29^high basal MECs from preneoplastic glands of PyMT;Mtdh^+/+ mice. (H, I) Mammary tumor incidence (H) and volumes (I) 8 weeks after orthotopic transplantations of Lin⁻CD24⁺CD29^low luminal cells from preneoplastic glands of PyMT;Mtdh^+/+ and PyMT;Mtdh^−/− mice. Statistics: (C–E) Student’s t-test. (F–I), tumor incidence based on limiting dilution analysis and tumor volume based on Mann-Whitney test. ***p < 0.001, **p < 0.01, *p < 0.05. Data represent mean ± SEM. See also Figure S2.

Figure 3. MTDH is intrinsically required for oncogene-induced TICs functionality

(A) Schematic diagram of MMTV-Mtdh transgene construct and breeding scheme used to generate PyMT;Mtdh^−/− mice with (Mtdh^−/− +Tg) or without the MMTV-Mtdh transgene. (B) MTDH protein levels in PyMT-induced tumors from Mtdh^+/+, Mtdh^−/−, or Mtdh^−/− +Tg mice. (C) Quantification of CD24⁺CD29^low luminal population in Lin⁻ MECs (n = 4) from preneoplastic mammary glands of 6-week-old females of the indicated genotypes. (D) Kinetics of mammary tumor onset in MMTV-PyMT females of the indicated genotypes. Mtdh^−/− (n = 21), Mtdh^−/− +Tg (n = 20). (E) Average number of tumor-free mammary glands at indicated ages in the same cohort of mice as in (D). (F) Tumor burden of same cohorts of mice as in (D). (G, H) MTDH was knocked down by two independent shRNA (KD1 and KD2) in freshly dissociated PyMT;Mtdh^+/+ pMECs and in vitro mammosphere (G, n = 5, each in triplicates) and in vivo tumor formation assays were performed (H, incidence at 3 months). FC, fold changes. (I, J) Mouse MTDH was expressed in freshly dissociated PyMT;Mtdh^−/− pMECs via lentivirus transduction and in vitro mammosphere (I, n = 4, each in triplicates) and in vivo tumor formation (J) assays were performed. (K) Schematic diagram of experiments in L–O. (L) Mammosphere formation of ALDH-positive or ALDH-negative tumor cells from PyMT;Mtdh^+/+ tumors. (M) MTDH was knocked down in sorted ALDH⁺ cells from PyMT;Mtdh^+/+ tumors and mammosphere assays were performed. (N) Mammosphere formation of Lin⁻CD24⁺CD61⁺ or Lin⁻CD24⁺CD61⁻ tumor cells from Wnt;Mtdh^+/+ tumors. (O) MTDH was knocked down in sorted Lin⁻CD24⁺CD61⁺ cells from Wnt;Mtdh^+/+ tumors and mammosphere assays were performed. Statistics: (C, G, I, L–O) Student’s t-test. (D) Log rank test. (E) Chi-square test. (F) Mann-Whitney test. (H, J) Limiting dilution analysis. ***p < 0.001, **p < 0.01, *p < 0.05. Data represent mean ± SEM. See also Figure S3.

Figure 4. SND1 is necessary for MTDH-mediated tumor initiation

(A) Combination of MTDH re-expression and SND1 knockdown in PyMT;Mtdh^−/− tumor cells. The efficiency of SND1 KD and MTDH re-expression was assessed by western blotting. (B, C) In vitro mammosphere (B) and in vivo tumor formation (C, 6 weeks) assays were performed with cells generated in (A). +/− indicate whether the denoted protein is present (+) or absent (−) based on western blotting in (A). (D) SND1 was knocked down in PyMT;Mtdh^+/+ or Wnt;Mtdh^+/+ pMECs cells and mammosphere assays were performed in triplicates. (E, F) Tumor incidence (E) and volume (F) after orthotopic transplantations of control or SND1-KD PyMT;Mtdh^+/+ pMECs. Statistics: (B, D) Student’s t-test. (C, E) Limiting dilution analysis. (F) Mann-Whitney test. ***p < 0.001, **p < 0.01, *p < 0.05. Data represent mean ± SEM. See also Figure S4.

Figure 5. Determination of key regions and residues mediating the MTDH-SND1 interaction

(A) Schematics of MTDH fragments and mutants with indicated SND1-binding capability. + indicates binding and – indicates no binding based on results shown below. Two putative nuclear localization signals (432–451 for NLS2 and 561–580 for NLS3) are denoted by green boxes. In the enlarged view of the minimal binding region 386–407, 9 residues were targeted for mutagenesis in the current study. Mutations highlighted in red or purple either completely or strongly reduced the binding, respectively. (B) Pulldown of His6-SND1ΔC by GST-tagged MTDH fragments with indicated boundaries. The bound proteins were examined by SDS-PAGE and visualized by Coomassie blue staining. (C) Pulldown of His6-SND1ΔC by GST-tagged WT or triple mutant MTDH fragments (364–582). For (B) and (C), 1/10 of the His6-SND1ΔC input was shown, and GST alone was used as a negative control. Representative results of 3 independent experiments are shown. (D, E) Lysates from HEK293T cells expressing the indicated ectopic human SND1, AGO2 or MTDH were immunoprecipitated with anti-Myc and immunoblotted with the indicated antibodies.

Figure 6. SND1-binding deficient MTDH fails to promote tumor-initiating potential of MECs

(A, E) Lysates from PyMT;Mtdh^−/− MECs reconstituted with vector control, WT or mutant murine MTDH were immunoprecipitated with anti-MTDH antibody and immunoblotted for indicated proteins. (B, F) Mammosphere assays were performed with PyMT;Mtdh^−/− pMECs reconstituted with indicated Mtdh constructs. (C, D, G, H) In vivo tumor formation (C, G for tumor incidence; D, H for tumor volumes) were performed at limiting numbers using PyMT;Mtdh^−/− pMECs reconstituted with indicated WT or mutant MTDH. *Note: mouse W391D MTDH corresponds to human W394D MTDH; and mouse W398D MTDH corresponds to human W401D MTDH. Statistics: (B, F) Student’s t-test. (C, G) Limiting dilution analysis. (D, H) Mann-Whitney test. Data represent mean ± SEM. ***p < 0.001, **p < 0.01, *p < 0.05.

Figure 7. MTDH confers survival advantage by interacting and stabilizing pro-survival protein SND1 under stress

(A) Quantification of cleaved caspase 3-positive MECs from normal or MMTV-PyMT preneoplastic glands (n > 3) of WT or KO females. (B) The effect of CPT on the apoptosis of PyMT;Mtdh^−/− pMECs reconstituted with indicated Mtdh constructs was determined by PI and Hoechst staining. (C) The effect of CPT on the apoptosis of control or SND1-KD PyMT;Mtdh^+/+ pMECs. (D) Protein levels of SND1 and β-actin (loading control) in control or MTDH-KD PyMT;Mtdh^+/+ pMECs treated with CPT at indicated concentrations for 36 hours. Degradation curve (right) represents the average of 3 independent experiments. (E) Western blotting of SND1, MTDH and β-actin (loading control) in PyMT;Mtdh^−/− MECs reconstituted with indicated constructs after CPT treatment for 48 hours. Degradation curves (right) represent average of 3 independent experiments. (F) Heat map representation of microarray data displaying the expression of SND1-upregualted genes (n = 504, fold change > 2, p < 0.05) in control versus SND1-KD PyMT;Mtdh^+/+ pMECs under CPT (50 μM) treatment for 36 h. Color key indicates log2 values. (G) Ingenuity Pathway Analysis shows the top 5 molecular and cellular functions of SND1-upregulated genes shown in (F) and the number of molecules/genes implicated in each category. (H) Effects of SND1-upregulated genes in cell survival and cell death functions. Z scores were calculated based on gene expression changes and gene functions as specified by the ingenuity knowledge base. A given function is predicted to be significantly increased when z > 2 or decreased when z < −2. (I) GSEA plot showing the enrichment of SND1-upregulated gene signature in PyMT;Mtdh^−/− MECs rescued with mouse WT MTDH as compared to those rescued with W391D mutant MTDH. All cells were treated with CPT (50 μM). NES: normalized enrichment score. Statistics: (A–C) Student’s t-test. Data represent mean ± SEM. ***p < 0.001, **p < 0.01, *p < 0.05. See also Figure S5.

Figure 8. MTDH and SND1 are important for in vitro sphere-forming and in vivo tumor-initiating activities of human breast cancer cells

(A, B) MTDH (A) or SND1 (B) was knocked down in HMLE-Neu cells and tumorsphere assays were performed in triplicates. (C, D) MTDH (C) or SND1 (D) was knocked down in the BCM-4013 patient-derived xenografted (PDX) tumor cells and tumorsphere assays were performed in triplicates. (E) MTDH or SND1 was knocked down in MDA-MB-231 cells, and the KD efficiency was measured by immunoblotting. (F) Tumorsphere assays of MDA-MB-231 cells were peformed in triplicates. (G, H) Tumor incidence (G) and volumes (H) 5 weeks after injection of limiting numbers of MDA-MB-231 cells. (I) The protein levels of MTDH and SND1 in human invasive mammary carcinomas (n = 154) were determined by IHC staining of a breast cancer tissue microarray (BR1921a, US Biomax). The staining intensity in tumor cells was scored as 0 (negative), 1 (weak), 2 (moderate), 3 (strong). (J) Bar graph presentation of (I). (K) Representative tumor specimens with strong (tumor 1), weak (tumor 2) and negative (tumor 3) staining of MTDH and SND1. Scale bar: 200 μm (top) and 20 μm (bottom) for each tumor. (L) Schematic illustration depicting the essential role of MTDH in tumor initiation but not normal gland development. Under stress conditions during tumorigenesis, the MTDH-SND1 interaction protects SND1 from stress-induced degradation and supports the survival and activities of both basal and luminal TICs. Statistics: (A–D, F) Student’s t test. (G) Limiting dilution analysis. (H) Mann-Whitney test. (I, J) Chi-square test. Data represent mean ± SEM. ***p < 0.001, **p < 0.01, *p < 0.05. See also Figure S6.