Skip to main content
  • Viewpoint
  • Published:

New insights into signalling networks regulating breast cancer stem cells

Abstract

In a recent paper, Aceto and colleagues report that Src homology 2 domain-containing protein tyrosine phosphatase 2 (Shp2) plays a critical role in maintenance of breast tumour-initiating cells, and they define novel effectors downstream of Shp2 that regulate cellular invasion and self-renewal, including the transcription factors c-Myc and zinc finger E-box binding homeobox 1 and the suppressor of miRNA biogenesis lin-28 homolog B. These findings provide important mechanistic insights into breast tumourigenesis and highlight Shp2 as a potential therapeutic target.

The cancer stem cell (CSC) hypothesis proposes that certain malignancies, including breast cancer, are driven by a population of CSCs that are tumorigenic and capable of self-renewal and differentiation into nontumorigenic cells. Tumour-initiating cells (TICs) are operationally defined as cells that seed tumours when transplanted in mice [1]. The highlighted study addresses the role of Src homology 2 domain-containing protein tyrosine phosphatase 2 (Shp2) in maintenance of the CSC/TIC sub-population [2]. In breast cancer, signalling via this phosphatase can be amplified via upregulation of particular receptor tyrosine kinases, such as erbB2 (also known as HER2) or c-Met [3, 4], or via overexpression of the docking protein growth factor receptor bound 2-associated binder 2 [5–8]. From a signalling perspective, Shp2 is required for maximal and/or sustained activation of the Ras/Erk pathway by a variety of growth factor and cytokine receptors, but it also regulates other pathways, including phosphatidylinositol 3-kinase/Akt activation [3, 4].

Previously, the Agazie group demonstrated that Shp2 knockdown reduces anchorage-independent growth of breast cancer cells and promotes acquisition of an epithelial phenotype [9], but whether Shp2 regulated breast TICs was unclear. Aceto and colleagues address this question using conditional, miRNA-adapted shRNA-mediated knockdown of Shp2 in a variety of HER2 or triple-negative breast cancer cell lines, as well as MCF-10A mammary epithelial cells engineered to overexpress HER2 and HER3. Shp2 knockdown attenuated invasion in vitro and in vivo, blocked growth of orthotopic tumour xenografts, and reduced lung metastasis [2]. Moreover, Shp2 knockdown reduced the self-renewal capacity of tumoursphere-forming cells as well as the number of cells exhibiting a CSC phenotype. Single cells isolated from Shp2-deficient tumours exhibited a reduced ability to re-seed new tumours, providing strong evidence that Shp2 is required for the maintenance and propagation of TICs.

Given the potent biological effects of Shp2 ablation, a critical question was the nature of the signalling and transcriptional networks regulated by this phosphatase. The data presented indicate that Erk plays a key role downstream of Shp2 [2], but other pathways probably also contribute. In addition, transcript profiling identified an Shp2 signature of 180 genes that exhibited reduced expression upon Shp2 knockdown. This included the genes encoding zinc finger E-box binding homeobox 1, a transcription factor that induces epithelial-to-mesen-chyme transition, and the c-Myc target lin-28 homolog B, a repressor of Let-7 miRNA biogenesis. Consistent with these data, Shp2 knockdown resulted in increased expression of let-7a and let-7b, and downregulation of the let-7 targets Ras and c-Myc. Since let-7 targets include proteins upstream of Shp2, these studies identify a novel positive-acting feedback loop that regulates maintenance of TICs.

Interestingly, a previous paper from the Struhl laboratory demonstrated that transient Src signalling sets up a self-reinforcing feedback loop for CSC maintenance in which NF-κB activates lin-28 homolog B to repress Let-7 processing, which then leads to IL-6 expression [10], highlighting the importance of positive feedback loops in maintaining this cellular phenotype. It would be interesting to determine the role of the miR-200 family of miRNAs in this system, given that miR-200 forms a double-negative regulatory loop with zinc finger E-box binding homeobox 1 and thereby represses epithelial-to-mesenchyme transition and the CSC phenotype [11].

While Shp2 was not consistently overexpressed in breast cancer compared with normal breast tissue, the Shp2 signature was enriched in the triple-negative subtype versus the luminal or HER2 breast cancer subtypes, in invasive cancers versus ductal carcinoma in situ, and in poor-prognosis breast cancers. While this enrichment might be indicative of Shp2 activation, it should be noted that other pathways are likely to converge on this gene set, and the expression pattern might reflect intrinsic properties of particular breast cancer phenotypes rather than a role for Shp2 per se. However, the authors present a strong case that Shp2 represents a potential therapeutic target in at least a subset of HER2 and triple-negative breast cancers, two breast cancer subtypes often associated with a poor prognosis [12].

A major issue in the field of CSC biology has been the role that phenotypic plasticity plays in acquisition of a stem-like phenotype, with some data suggesting that all cancer cells have the capacity to acquire this state in response to environmental cues [1]. If this is true, then therapeutics designed to target CSCs may not lead to durable clinical responses in the adjuvant setting. A highlight of this work is the rigorous investigation of the TIC phenotype following Shp2 knockdown, in which the authors show not only quantitative inhibition of tumour growth and tumour-initiating capacity, but also the failure to reacquire the stem-like state after release from Shp2 knockdown - suggesting that in these models depletion of Shp2 leads to irreversible loss of stem-like capacity. A key implication of these findings is that components of the identified positive feedback loop represent potential therapeutic targets, and in this context it is worth noting that small-molecule inhibitors of Shp2 are currently in preclinical development [13]. In addition, characterisation of the upstream regulator(s) of Shp2 in the TICs may identify additional targets, such as specific receptor tyrosine kinases, which are amenable to inhibitor-based or antibody-based therapies.

Abbreviations

CSC:

cancer stem cell

IL:

interleukin

miRNA:

microRNA

NF:

nuclear factor

Shp2:

Src homology 2 domain-containing protein tyrosine phosphatase 2

shRNA:

short hairpin RNA

TIC:

tumour-initiating cell.

References

  1. Visvader JE, Lindeman GJ: Cancer stem cells in solid tumours: accumulating evidence and unresolved questions. Nat Rev Cancer. 2008, 8: 755-768. 10.1038/nrc2499.

    Article  CAS  PubMed  Google Scholar 

  2. Aceto N, Sausgruber N, Brinkhaus H, Gaidatzis D, Martiny-Baron G, Mazzarol G, Confalonieri S, Quarto M, Hu G, Balwierz PJ, Pachkov M, Elledge SJ, van Nimwegen E, Stadler MB, Bentires-Alj M: Tyrosine phosphatase SHP2 promotes breast cancer progression and maintains tumor-initiating cells via activation of key transcription factors and a positive feedback signaling loop. Nat Med. 2012, 18: 529-537. 10.1038/nm.2645.

    Article  CAS  PubMed  Google Scholar 

  3. Wohrle FU, Daly RJ, Brummer T: Function, regulation and pathological roles of the Gab/DOS docking proteins. Cell Commun Signal. 2009, 7: 22-10.1186/1478-811X-7-22.

    Article  PubMed  PubMed Central  Google Scholar 

  4. Grossmann KS, Rosario M, Birchmeier C, Birchmeier W: The tyrosine phosphatase Shp2 in development and cancer. Adv Cancer Res. 2010, 106: 53-89.

    Article  CAS  PubMed  Google Scholar 

  5. Daly RJ, Gu H, Parmar J, Malaney S, Lyons RJ, Kairouz R, Head DR, Henshall SM, Neel BG, Sutherland RL: The docking protein Gab2 is overexpressed and estrogen regulated in human breast cancer. Oncogene. 2002, 21: 5175-5181. 10.1038/sj.onc.1205522.

    Article  CAS  PubMed  Google Scholar 

  6. Bentires-Alj M, Gil SG, Chan R, Wang ZC, Wang Y, Imanaka N, Harris LN, Richardson A, Neel BG, Gu H: A role for the scaffolding adapter GAB2 in breast cancer. Nat Med. 2006, 12: 114-121. 10.1038/nm1341.

    Article  CAS  PubMed  Google Scholar 

  7. Bocanegra M, Bergamaschi A, Kim YH, Miller MA, Rajput AB, Kao J, Langerod A, Han W, Noh DY, Jeffrey SS, Huntsman DG, Borresen-Dale AL, Pollack JR: Focal amplification and oncogene dependency of GAB2 in breast cancer. Oncogene. 2010, 29: 774-779. 10.1038/onc.2009.364.

    Article  CAS  PubMed  Google Scholar 

  8. Fleuren ED, O'Toole S, Millar EK, McNeil C, Lopez-Knowles E, Boulghourjian A, Croucher DR, Schramek D, Brummer T, Penninger JM, Sutherland RL, Daly RJ: Overexpression of the oncogenic signal transducer Gab2 occurs early in breast cancer development. Int J Cancer. 2010, 127: 1486-1492. 10.1002/ijc.25172.

    Article  CAS  PubMed  Google Scholar 

  9. Zhou XD, Agazie YM: Inhibition of SHP2 leads to mesenchymal to epithelial transition in breast cancer cells. Cell Death Differ. 2008, 15: 988-996. 10.1038/cdd.2008.54.

    Article  CAS  PubMed  Google Scholar 

  10. Iliopoulos D, Hirsch HA, Struhl K: An epigenetic switch involving NF-κB, Lin28, Let-7 MicroRNA, and IL6 links inflammation to cell transformation. Cell. 2009, 139: 693-706. 10.1016/j.cell.2009.10.014.

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  11. Bracken CP, Gregory PA, Kolesnikoff N, Bert AG, Wang J, Shannon MF, Goodall GJ: A double-negative feedback loop between ZEB1-SIP1 and the microRNA-200 family regulates epithelial-mesenchymal transition. Cancer Res. 2008, 68: 7846-7854. 10.1158/0008-5472.CAN-08-1942.

    Article  CAS  PubMed  Google Scholar 

  12. Brenton JD, Carey LA, Ahmed AA, Caldas C: Molecular classification and molecular forecasting of breast cancer: ready for clinical application?. J Clin Oncol. 2005, 23: 7350-7360. 10.1200/JCO.2005.03.3845.

    Article  CAS  PubMed  Google Scholar 

  13. Hellmuth K, Grosskopf S, Lum CT, Wurtele M, Roder N, von Kries JP, Rosario M, Rademann J, Birchmeier W: Specific inhibitors of the protein tyrosine phosphatase Shp2 identified by high-throughput docking. Proc Natl Acad Sci USA. 2008, 105: 7275-7280. 10.1073/pnas.0710468105.

    Article  CAS  PubMed  PubMed Central  Google Scholar 

Download references

Author information

Authors and Affiliations

Authors

Corresponding author

Correspondence to Roger J Daly.

Additional information

Competing interests

The authors declare that they have no competing interests.

Rights and permissions

Reprints and permissions

About this article

Cite this article

Swarbrick, A., Daly, R.J. New insights into signalling networks regulating breast cancer stem cells. Breast Cancer Res 14, 321 (2012). https://doi.org/10.1186/bcr3209

Download citation

  • Published:

  • DOI: https://doi.org/10.1186/bcr3209

Keywords