Review Article | Published:

Pre-metastatic niches: organ-specific homes for metastases

Nature Reviews Cancer volume 17, pages 302317 (2017) | Download Citation

Abstract

It is well established that organs of future metastasis are not passive receivers of circulating tumour cells, but are instead selectively and actively modified by the primary tumour before metastatic spread has even occurred. Sowing the 'seeds' of metastasis requires the action of tumour-secreted factors and tumour-shed extracellular vesicles that enable the 'soil' at distant metastatic sites to encourage the outgrowth of incoming cancer cells. In this Review, we summarize the main processes and new mechanisms involved in the formation of the pre-metastatic niche.

Key points

  • Organs of future metastasis are selectively and actively modified by the primary tumour before metastatic spread has occurred.

  • Tumours induce the formation of microenvironments in distant organs that are conducive to the survival and outgrowth of tumour cells before their arrival at these sites. These microenvironments are termed pre-metastatic niches (PMNs).

  • PMN formation is a stepwise process resulting from the combined systemic effects of tumour-secreted factors and tumour-shed extracellular vesicles.

  • PMN formation is initiated with local changes such as the induction of vascular leakiness, remodelling of stroma and extracellular matrix, followed by systemic effects on the immune system.

  • The development of new technologies and approaches to identify PMNs in distant organ sites in patients could revolutionize cancer treatment and lead to pre-emptive treatments to hinder metastasis.

  • The PMN is a new paradigm for the initiation of metastasis. Our ability to fight metastasis would benefit greatly from understanding the pathological processes occurring before the development of macrometastases.

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References

  1. 1.

    The distribution of secondary growths in cancer of the breast. Lancet 133, 571–573 (1889). This was the first time the requirement for a suportive microenvironment, or 'fertile soil' in metastatic outgrowth was recognized.

  2. 2.

    Neoplastic Diseases: A Treatise on Tumours (W. B. Saunders Company, 1928).

  3. 3.

    & Organ selectivity for implantation survival and growth of B16 melanoma variant tumor lines. J. Natl Cancer Inst. 57, 1199–1202 (1976). This study was the first to provide experimental evidence for organotropic metastasis.

  4. 4.

    & Role of organ selectivity in the determination of metastatic patterns of B16 melanoma. Cancer Res. 40, 2281–2287 (1980).

  5. 5.

    The distribution of secondary growths in cancer of the breast. 1889. Cancer Metastasis Rev. 8, 98–101 (1989).

  6. 6.

    & The metastatic niche: adapting the foreign soil. Nat. Rev. Cancer 9, 285–293 (2009).

  7. 7.

    et al. VEGFR1-positive haematopoietic bone marrow progenitors initiate the pre-metastatic niche. Nature 438, 820–827 (2005). This was the first proof-of-principle study demonstrating the existence of and stepwise progression of the PMN.

  8. 8.

    The lymph node pre-metastatic niche. J. Mol. Med. (Berl.) 93, 1173–1184 (2015).

  9. 9.

    & Cancer tills the premetastatic field: mechanistic basis and clinical implications. Clin. Cancer Res. 22, 3725–3733 (2016).

  10. 10.

    & Complex metastatic niches: already a target for therapy? Curr. Opin. Cell Biol. 31, 29–38 (2014).

  11. 11.

    & Microenvironmental regulation of tumor progression and metastasis. Nat. Med. 19, 1423–1437 (2013).

  12. 12.

    & Microenvironmental regulation of metastasis. Nat. Rev. Cancer 9, 239–252 (2009).

  13. 13.

    & Metastatic colonization: settlement, adaptation and propagation of tumor cells in a foreign tissue environment. Semin. Cancer Biol. 21, 99–106 (2011).

  14. 14.

    , & Cancer to bone: a fatal attraction. Nat. Rev. Cancer 11, 411–425 (2011).

  15. 15.

    The metastatic niche and stromal progression. Cancer Metastasis Rev. 31, 429–440 (2012).

  16. 16.

    & Neutrophils fan cancer's flames. EMBO J. 34, 2211–2212 (2015).

  17. 17.

    , & Radiologic diagnosis of extrathoracic metastases to the lung. Oncology (Williston Park) 12, 431–438 (1998).

  18. 18.

    , , , & Mouse models of advanced spontaneous metastasis for experimental therapeutics. Nat. Rev. Cancer 11, 135–141 (2011).

  19. 19.

    et al. VEGFR1 expression is related to lymph node metastasis and serum VEGF may be a marker of progression in the follow-up of patients with differentiated thyroid carcinoma. Eur. J. Endocrinol. 164, 277–284 (2011).

  20. 20.

    et al. VEGF-C-induced lymphangiogenesis in sentinel lymph nodes promotes tumor metastasis to distant sites. Blood 109, 1010–1017 (2007).

  21. 21.

    et al. VEGF-A induces tumor and sentinel lymph node lymphangiogenesis and promotes lymphatic metastasis. J. Exp. Med. 201, 1089–1099 (2005).

  22. 22.

    et al. CD44v6 dependence of premetastatic niche preparation by exosomes. Neoplasia 11, 1093–1105 (2009).

  23. 23.

    , , & Postoperative enoximone in coronary surgery. Systemic and coronary hemodynamics and regional systolic function. Arch. Mal. Coeur Vaiss. 83, 13–17 (in French) (1990

  24. 24.

    et al. Visualization of immediate immune responses to pioneer metastatic cells in the lung. Nature 531, 513–517 (2016).

  25. 25.

    et al. Human CD133-positive hematopoietic progenitor cells initiate growth and metastasis of colorectal cancer cells. Carcinogenesis 35, 2771–2777 (2014).

  26. 26.

    , , , & Development and characterization of a reliable mouse model of colorectal cancer metastasis to the liver. Clin. Exp. Metastasis 30, 903–918 (2013).

  27. 27.

    , , & Role of vascular endothelial growth factor receptor 1-positive hematopoietic progenitor cell clusters in human colorectal carcinoma metastasis. Nan Fang Yi Ke Da Xue Xue Bao 28, 696–699 (in Chinese) (2008).

  28. 28.

    et al. Tissue inhibitor of metalloproteinases (TIMP)-1 creates a premetastatic niche in the liver through SDF-1/CXCR4-dependent neutrophil recruitment in mice. Hepatology 61, 238–248 (2015).

  29. 29.

    et al. Pancreatic cancer exosomes initiate pre-metastatic niche formation in the liver. Nat. Cell Biol. 17, 816–826 (2015).

  30. 30.

    et al. Glypican-1 identifies cancer exosomes and detects early pancreatic cancer. Nature 523, 177–182 (2015).

  31. 31.

    Metastasis to bone: causes, consequences and therapeutic opportunities. Nat. Rev. Cancer 2, 584–593 (2002).

  32. 32.

    et al. LOX-mediated collagen crosslinking is responsible for fibrosis-enhanced metastasis. Cancer Res. 73, 1721–1732 (2013).

  33. 33.

    et al. The hypoxic cancer secretome induces pre-metastatic bone lesions through lysyl oxidase. Nature 522, 106–110 (2015).

  34. 34.

    et al. Evidence for a causal role of parathyroid hormone-related protein in the pathogenesis of human breast cancer-mediated osteolysis. J. Clin. Invest. 98, 1544–1549 (1996).

  35. 35.

    et al. Interleukin-6 in the bone marrow microenvironment promotes the growth and survival of neuroblastoma cells. Cancer Res. 69, 329–337 (2009).

  36. 36.

    et al. Enhanced expression of interleukin-6 in bone and serum of metastatic renal cell carcinoma. Hum. Pathol. 29, 421–424 (1998).

  37. 37.

    et al. Breast cancer cells interact with osteoblasts to support osteoclast formation. Endocrinology 140, 4451–4458 (1999).

  38. 38.

    et al. A multigenic program mediating breast cancer metastasis to bone. Cancer Cell 3, 537–549 (2003). This landmark study defined the breast cancer cell-intrinsic determinants of bone metastasis.

  39. 39.

    et al. MMP-7 promotes prostate cancer-induced osteolysis via the solubilization of RANKL. Cancer Cell 7, 485–496 (2005).

  40. 40.

    et al. ADAMTS1 and MMP1 proteolytically engage EGF-like ligands in an osteolytic signaling cascade for bone metastasis. Genes Dev. 23, 1882–1894 (2009).

  41. 41.

    Examining the metastatic niche: targeting the microenvironment. Semin. Oncol. 37 (Suppl. 2), S2–S14 (2010).

  42. 42.

    et al. Bone morphogenetic protein-6 promotes osteoblastic prostate cancer bone metastases through a dual mechanism. Cancer Res. 65, 8274–8285 (2005).

  43. 43.

    & Osteoblasts in prostate cancer metastasis to bone. Nat. Rev. Cancer 5, 21–28 (2005).

  44. 44.

    et al. Canonical WNT signaling promotes osteogenesis by directly stimulating Runx2 gene expression. J. Biol. Chem. 280, 33132–33140 (2005).

  45. 45.

    et al. Regulation of osteoblastogenesis and bone mass by Wnt10b. Proc. Natl Acad. Sci. USA 102, 3324–3329 (2005).

  46. 46.

    , & Brain metastases as preventive and therapeutic targets. Nat. Rev. Cancer 11, 352–363 (2011).

  47. 47.

    et al. Alterations in pericyte subpopulations are associated with elevated blood–tumor barrier permeability in experimental brain metastasis of breast cancer. Clin. Cancer Res. 22, 5287–5299 (2016).

  48. 48.

    et al. In vivo characterization of changing blood-tumor barrier permeability in a mouse model of breast cancer metastasis: a complementary magnetic resonance imaging approach. Invest. Radiol. 46, 718–725 (2011).

  49. 49.

    Targeting metastasis. Nat. Rev. Cancer 16, 201–218 (2016).

  50. 50.

    et al. Breast-cancer-secreted miR-122 reprograms glucose metabolism in premetastatic niche to promote metastasis. Nat. Cell Biol. 17, 183–194 (2015). This study was the first to provide evidence of metabolic reprogramming of stromal cells in PMNs through miRNA cargo shuttled by tumour-derived microvesicles.

  51. 51.

    et al. Tumour exosome integrins determine organotropic metastasis. Nature 527, 329–335 (2015). This landmark study demonstrated that organotropic metastasis can be orchestrated in a tumour cell-autonomous manner through exosome-expressed integrins.

  52. 52.

    et al. MMP9 induction by vascular endothelial growth factor receptor-1 is involved in lung-specific metastasis. Cancer Cell 2, 289–300 (2002).

  53. 53.

    et al. The S100A8-serum amyloid A3–TLR4 paracrine cascade establishes a pre-metastatic phase. Nat. Cell Biol. 10, 1349–1355 (2008). This study demonstrated that chemokines induced in PMNs facilitate metastasis in a TLR4-dependent manner, through effects on local innate immune cells.

  54. 54.

    et al. G-CSF-initiated myeloid cell mobilization and angiogenesis mediate tumor refractoriness to anti-VEGF therapy in mouse models. Proc. Natl Acad. Sci. USA 106, 6742–6747 (2009).

  55. 55.

    , , & Monocyte chemoattractant protein-1: a key mediator in inflammatory processes. Int. J. Biochem. Cell Biol. 41, 998–1001 (2009).

  56. 56.

    et al. Monocyte chemotactic protein-1 (MCP-1) acts as a paracrine and autocrine factor for prostate cancer growth and invasion. Prostate 66, 1311–1318 (2006).

  57. 57.

    et al. Monocyte chemotactic protein 1 promotes lung cancer-induced bone resorptive lesions in vivo. Neoplasia 11, 228–236 (2009).

  58. 58.

    et al. Targeting CCL2 with systemic delivery of neutralizing antibodies induces prostate cancer tumor regression in vivo. Cancer Res. 67, 9417–9424 (2007).

  59. 59.

    et al. Significant correlation of monocyte chemoattractant protein-1 expression with neovascularization and progression of breast carcinoma. Cancer 92, 1085–1091 (2001).

  60. 60.

    et al. Monocyte chemoattractant protein-1 serum levels in patients with breast cancer. Tumour Biol. 25, 14–17 (2004).

  61. 61.

    et al. CCL2 recruits inflammatory monocytes to facilitate breast-tumour metastasis. Nature 475, 222–225 (2011).

  62. 62.

    et al. Primary tumor hypoxia recruits CD11b+/Ly6Cmed/Ly6G+ immune suppressor cells and compromises NK cell cytotoxicity in the premetastatic niche. Cancer Res. 72, 3906–3911 (2012).

  63. 63.

    & Chemokine (C-C motif) ligand 2 engages CCR2+ stromal cells of monocytic origin to promote breast cancer metastasis to lung and bone. J. Biol. Chem. 284, 29087–29096 (2009).

  64. 64.

    & Organotropism of breast cancer metastasis. J. Mammary Gland Biol. Neoplasia 12, 153–162 (2007).

  65. 65.

    et al. Tumor entrained neutrophils inhibit seeding in the premetastatic lung. Cancer Cell 20, 300–314 (2011).

  66. 66.

    , & S100 proteins in cancer. Nat. Rev. Cancer 15, 96–109 (2015).

  67. 67.

    & Building the niche: the role of the S100 proteins in metastatic growth. Semin. Cancer Biol. 22, 216–225 (2012).

  68. 68.

    et al. Functions of S100 proteins. Curr. Mol. Med. 13, 24–57 (2013).

  69. 69.

    , , & Tumour-mediated upregulation of chemoattractants and recruitment of myeloid cells predetermines lung metastasis. Nat. Cell Biol. 8, 1369–1375 (2006).

  70. 70.

    et al. Hypoxia-inducible factor 1 is a master regulator of breast cancer metastatic niche formation. Proc. Natl Acad. Sci. USA 108, 16369–16374 (2011).

  71. 71.

    et al. Inhibitors of hypoxia-inducible factor 1 block breast cancer metastatic niche formation and lung metastasis. J. Mol. Med. (Berl.) 90, 803–815 (2012).

  72. 72.

    et al. Hypoxia-induced lysyl oxidase is a critical mediator of bone marrow cell recruitment to form the premetastatic niche. Cancer Cell 15, 35–44 (2009).

  73. 73.

    et al. Lysyl oxidase is essential for hypoxia-induced metastasis. Nature 440, 1222–1226 (2006).

  74. 74.

    , & Hypoxic enhancement of exosome release by breast cancer cells. BMC Cancer 12, 421 (2012).

  75. 75.

    et al. Hypoxia-inducible factors and RAB22A mediate formation of microvesicles that stimulate breast cancer invasion and metastasis. Proc. Natl Acad. Sci. USA 111, E3234–E3242 (2014).

  76. 76.

    & As we wait: coping with an imperfect nomenclature for extracellular vesicles. J. Extracell. Vesicles (2013).

  77. 77.

    , & Biogenesis, secretion, and intercellular interactions of exosomes and other extracellular vesicles. Annu. Rev. Cell Dev. Biol. 30, 255–289 (2014).

  78. 78.

    & Extracellular vesicles: exosomes, microvesicles, and friends. J. Cell Biol. 200, 373–383 (2013).

  79. 79.

    et al. Melanoma exosomes educate bone marrow progenitor cells toward a pro-metastatic phenotype through MET. Nat. Med. 18, 883–891 (2012). This was the first study to demonstrate that exosomes secreted by highly metastatic tumours promote metastasis by permanently educating bone marrow progenitors and recruiting them to PMNs.

  80. 80.

    et al. Cellular disposal of miR23b by RAB27-dependent exosome release is linked to acquisition of metastatic properties. Cancer Res. 74, 5758–5771 (2014).

  81. 81.

    , & The secreted factors responsible for pre-metastatic niche formation: old sayings and new thoughts. Semin. Cancer Biol. 21, 139–146 (2011).

  82. 82.

    et al. Exosome-mediated transfer of mRNAs and microRNAs is a novel mechanism of genetic exchange between cells. Nat. Cell Biol. 9, 654–659 (2007).

  83. 83.

    et al. Tumour microvesicles contain retrotransposon elements and amplified oncogene sequences. Nat. Commun. 2, 180 (2011).

  84. 84.

    et al. Glioblastoma microvesicles transport RNA and proteins that promote tumour growth and provide diagnostic biomarkers. Nat. Cell Biol. 10, 1470–1476 (2008).

  85. 85.

    et al. Intercellular transfer of the oncogenic receptor EGFRvIII by microvesicles derived from tumour cells. Nat. Cell Biol. 10, 619–624 (2008).

  86. 86.

    et al. Embryonic stem cell-derived microvesicles reprogram hematopoietic progenitors: evidence for horizontal transfer of mRNA and protein delivery. Leukemia 20, 847–856 (2006).

  87. 87.

    et al. Microvesicles derived from activated platelets induce metastasis and angiogenesis in lung cancer. Int. J. Cancer 113, 752–760 (2005).

  88. 88.

    , , & Enhancing effect of platelet-derived microvesicles on the invasive potential of breast cancer cells. Transfusion 46, 1199–1209 (2006).

  89. 89.

    , & Shedding microvesicles: artefacts no more. Trends Cell Biol. 19, 43–51 (2009).

  90. 90.

    et al. Tumour-released exosomes and their implications in cancer immunity. Cell Death Differ. 15, 80–88 (2008).

  91. 91.

    , , , & Membrane-derived microvesicles: important and underappreciated mediators of cell-to-cell communication. Leukemia 20, 1487–1495 (2006).

  92. 92.

    et al. Microvesicles released from human renal cancer stem cells stimulate angiogenesis and formation of lung premetastatic niche. Cancer Res. 71, 5346–5356 (2011).

  93. 93.

    , & Exosomes released by melanoma cells prepare sentinel lymph nodes for tumor metastasis. Cancer Res. 71, 3792–3801 (2011).

  94. 94.

    , , , & Sorting it out: regulation of exosome loading. Semin. Cancer Biol. 28, 3–13 (2014).

  95. 95.

    et al. Tumor exosomal RNAs promote lung pre-metastatic niche formation by activating alveolar epithelial TLR3 to recruit neutrophils. Cancer Cell 30, 243–256 (2016).

  96. 96.

    et al. Activation of hematopoietic stem/progenitor cells promotes immunosuppression within the pre-metastatic niche. Cancer Res. 76, 1335–1347 (2016).

  97. 97.

    et al. Platelet factor 4 is produced by subsets of myeloid cells in premetastatic lung and inhibits tumor metastasis. Oncotarget (2016).

  98. 98.

    et al. Pulmonary vascular destabilization in the premetastatic phase facilitates lung metastasis. Cancer Res. 69, 7529–7537 (2009). This landmark study demonstrated that ANGPT2-, MMP3- and MMP10-dependent pulmonary vascular destabilization is an early event occurring during the pre-metastatic phase, which promotes the extravasation of tumour cells and facilitates lung metastasis.

  99. 99.

    et al. Primary tumours modulate innate immune signalling to create pre-metastatic vascular hyperpermeability foci. Nat. Commun. 4, 1853 (2013).

  100. 100.

    et al. Endothelial focal adhesion kinase mediates cancer cell homing to discrete regions of the lungs via E-selectin up-regulation. Proc. Natl Acad. Sci. USA 108, 3725–3730 (2011).

  101. 101.

    et al. Gr-1+CD11b+ myeloid cells tip the balance of immune protection to tumor promotion in the premetastatic lung. Cancer Res. 70, 6139–6149 (2010).

  102. 102.

    et al. Mediators of vascular remodelling co-opted for sequential steps in lung metastasis. Nature 446, 765–770 (2007).

  103. 103.

    et al. TGFβ primes breast tumors for lung metastasis seeding through angiopoietin-like 4. Cell 133, 66–77 (2008).

  104. 104.

    et al. Inhibition of endothelial FAK activity prevents tumor metastasis by enhancing barrier function. J. Cell Biol. 204, 247–263 (2014).

  105. 105.

    et al. Carcinoma-produced factors activate myeloid cells through TLR2 to stimulate metastasis. Nature 457, 102–106 (2009).

  106. 106.

    et al. Genes that mediate breast cancer metastasis to the brain. Nature 459, 1005–1009 (2009).

  107. 107.

    et al. Analysis of tumour- and stroma-supplied proteolytic networks reveals a brain-metastasis-promoting role for cathepsin S. Nat. Cell Biol. 16, 876–888 (2014).

  108. 108.

    & Contribution of platelets to tumour metastasis. Nat. Rev. Cancer 11, 123–134 (2011).

  109. 109.

    , & Impact of venous thromboembolism and anticoagulation on cancer and cancer survival. J. Clin. Oncol. 27, 4902–4911 (2009).

  110. 110.

    et al. Coagulation facilitates tumor cell spreading in the pulmonary vasculature during early metastatic colony formation. Cancer Res. 64, 8613–8619 (2004).

  111. 111.

    et al. Recruitment of monocytes/macrophages by tissue factor-mediated coagulation is essential for metastatic cell survival and premetastatic niche establishment in mice. Blood 119, 3164–3175 (2012).

  112. 112.

    , & Platelets guide the formation of early metastatic niches. Proc. Natl Acad. Sci. USA 111, E3053–E3061 (2014).

  113. 113.

    et al. A link between inflammation and metastasis: serum amyloid A1 and A3 induce metastasis, and are targets of metastasis-inducing S100A4. Oncogene 34, 424–435 (2015).

  114. 114.

    et al. Myeloid-derived suppressor cells are implicated in regulating permissiveness for tumor metastasis during mouse gestation. J. Clin. Invest. 121, 2794–2807 (2011).

  115. 115.

    et al. Pulmonary alveolar macrophages contribute to the premetastatic niche by suppressing antitumor T cell responses in the lungs. J. Immunol. 194, 5529–5538 (2015).

  116. 116.

    et al. Interactions between cancer stem cells and their niche govern metastatic colonization. Nature 481, 85–89 (2012). This landmark study demonstrated that a subpopulation of cancer stem cells is responsible for metastatic colonization and that this process depends on signals provided by the stromal niche.

  117. 117.

    Periostin in fibrillogenesis for tissue regeneration: periostin actions inside and outside the cell. Cell. Mol. Life Sci. 68, 3201–3207 (2011).

  118. 118.

    et al. Periostin is a key niche component for wound metastasis of melanoma. PLoS ONE 10, e0129704 (2015).

  119. 119.

    et al. Periostin promotes immunosuppressive premetastatic niche formation to facilitate breast tumour metastasis. J. Pathol. 239, 484–495 (2016).

  120. 120.

    et al. Myeloid progenitor cells in the premetastatic lung promote metastases by inducing mesenchymal to epithelial transition. Cancer Res. 72, 1384–1394 (2012).

  121. 121.

    & New functions for the matrix metalloproteinases in cancer progression. Nat. Rev. Cancer 2, 161–174 (2002).

  122. 122.

    , & Type IV collagen and corneal epithelial adhesion and migration. Effects of type IV collagen fragments and synthetic peptides on rabbit corneal epithelial cell adhesion and migration in vitro. Invest. Ophthalmol. Vis. Sci. 32, 2766–2773 (1991).

  123. 123.

    , , , & Regulation of tumor cell chemotaxis by type IV collagen is mediated by a Ca2+-dependent mechanism requiring CD47 and the integrin αVβ3. J. Biol. Chem. 275, 4796–4802 (2000).

  124. 124.

    , & Matrix metalloproteinases: regulators of the tumor microenvironment. Cell 141, 52–67 (2010).

  125. 125.

    et al. C-C chemokine receptor 5 on stromal cells promotes pulmonary metastasis. Cancer Res. 65, 3374–3379 (2005).

  126. 126.

    et al. C-C chemokine receptor 5 on pulmonary fibrocytes facilitates migration and promotes metastasis via matrix metalloproteinase 9. Am. J. Pathol. 173, 253–264 (2008).

  127. 127.

    et al. Lysyl oxidase-like 2 (LOXL2) and E47 EMT factor: novel partners in E-cadherin repression and early metastasis colonization. Oncogene 34, 951–964 (2015).

  128. 128.

    , , & Multiscale modeling of form and function. Science 324, 208–212 (2009).

  129. 129.

    et al. Tensile forces govern germ-layer organization in zebrafish. Nat. Cell Biol. 10, 429–436 (2008).

  130. 130.

    & Breast cancer by proxy: can the microenvironment be both the cause and consequence? Trends Mol. Med. 15, 5–13 (2009).

  131. 131.

    et al. Matrix crosslinking forces tumor progression by enhancing integrin signaling. Cell 139, 891–906 (2009).

  132. 132.

    et al. Biomechanical remodeling of the microenvironment by stromal caveolin-1 favors tumor invasion and metastasis. Cell 146, 148–163 (2011).

  133. 133.

    et al. Extracellular matrix mediators of metastatic cell colonization characterized using scaffold mimics of the pre-metastatic niche. Acta Biomater. 33, 13–24 (2016).

  134. 134.

    & Fibronectin splice variants: understanding their multiple roles in health and disease using engineered mouse models. IUBMB Life 63, 538–546 (2011).

  135. 135.

    et al. Id1 suppresses anti-tumour immune responses and promotes tumour progression by impairing myeloid cell maturation. Nat. Commun. 6, 6840 (2015).

  136. 136.

    et al. Granulocyte-colony stimulating factor promotes lung metastasis through mobilization of Ly6G+Ly6C+ granulocytes. Proc. Natl Acad. Sci. USA 107, 21248–21255 (2010).

  137. 137.

    et al. Invasive breast cancer reprograms early myeloid differentiation in the bone marrow to generate immunosuppressive neutrophils. Proc. Natl Acad. Sci. USA 112, E566–E575 (2015).

  138. 138.

    et al. Matrix metalloproteinase-9 triggers the angiogenic switch during carcinogenesis. Nat. Cell Biol. 2, 737–744 (2000).

  139. 139.

    & Matrix metalloproteinase-9 is required for tumor vasculogenesis but not for angiogenesis: role of bone marrow-derived myelomonocytic cells. Cancer Cell 13, 193–205 (2008).

  140. 140.

    et al. The lack of type I interferon induces neutrophil-mediated pre-metastatic niche formation in the mouse lung. Int. J. Cancer 137, 837–847 (2015).

  141. 141.

    & The two faces of interferon-γ in cancer. Clin. Cancer Res. 17, 6118–6124 (2011).

  142. 142.

    & Neutrophils support lung colonization of metastasis-initiating breast cancer cells. Nature 528, 413–417 (2015).

  143. 143.

    , , & The new deal: a potential role for secreted vesicles in innate immunity and tumor progression. Front. Immunol. 6, 66 (2015).

  144. 144.

    Fundamental limits of spatial resolution in PET. Nucl. Instrum. Methods Phys. Res. A 648 (Suppl. 1), S236–S240 (2011).

  145. 145.

    , , II & T1 lung cancers: sensitivity of diagnosis with fluorodeoxyglucose PET. Radiology 223, 453–459 (2002).

  146. 146.

    et al. Measuring response in solid tumors: unidimensional versus bidimensional measurement. J. Natl Cancer Inst. 91, 523–528 (1999).

  147. 147.

    Tumor heterogeneity: mechanisms and bases for a reliable application of molecular marker design. Int. J. Mol. Sci. 13, 1951–2011 (2012).

  148. 148.

    , , & Prognostic impact of CD133 immunoexpression in node-negative invasive breast carcinomas. Anticancer Res. 31, 1315–1320 (2011).

  149. 149.

    et al. Incremental increase in VEGFR1+ hematopoietic progenitor cells and VEGFR2+ endothelial progenitor cells predicts relapse and lack of tumor response in breast cancer patients. Breast Cancer Res. Treat. 132, 235–242 (2012).

  150. 150.

    et al. Identification of the high-risk group for metastasis of gastric cancer cases by vascular endothelial growth factor receptor-1 overexpression in peripheral blood. Br. J. Cancer 96, 1723–1728 (2007).

  151. 151.

    et al. Targeting VEGFR1- and VEGFR2-expressing non-tumor cells is essential for esophageal cancer therapy. Oncotarget 6, 1790–1805 (2015).

  152. 152.

    et al. Myeloid clusters are associated with a pro-metastatic environment and poor prognosis in smoking-related early stage non-small cell lung cancer. PLoS ONE 8, e65121 (2013).

  153. 153.

    et al. CD8+ T-cell immunosurveillance constrains lymphoid premetastatic myeloid cell accumulation. Eur. J. Immunol. 45, 71–81 (2015).

  154. 154.

    et al. S1PR1-STAT3 signaling is crucial for myeloid cell colonization at future metastatic sites. Cancer Cell 21, 642–654 (2012). This study demonstrated that myeloid cells depend on S1PR1–STAT3 signalling to participate in PMN formation.

  155. 155.

    et al. Prognostic significance of neutrophilic infiltration in benign lymph nodes in patients with muscle-invasive bladder cancer. Eur. Urol. (2016).

  156. 156.

    et al. Powerful anti-tumor and anti-angiogenic activity of a new anti-vascular endothelial growth factor receptor 1 peptide in colorectal cancer models. Oncotarget 6, 10563–10576 (2015).

  157. 157.

    et al. A high HIF-1α expression genotype is associated with poor prognosis of upper aerodigestive tract carcinoma patients. Oral Oncol. 48, 130–135 (2012).

  158. 158.

    et al. Molecular changes in pre-metastatic lymph nodes of esophageal cancer patients. PLoS ONE 9, e102552 (2014).

  159. 159.

    et al. Primary tumor-secreted lymphangiogenic factors induce pre-metastatic lymphvascular niche formation at sentinel lymph nodes in oral squamous cell carcinoma. PLoS ONE 10, e0144056 (2015).

  160. 160.

    et al. Factors associated with collagen metabolism in the lymph node pre-metastatic niche in oral cancer. Oral Surg. Oral Med. Oral Pathol. Oral Radiol. 120, e155–e156 (2015).

  161. 161.

    & Targeted therapies: pazopanib: carving a niche in a crowded therapeutic landscape. Nat. Rev. Clin. Oncol. 7, 362–363 (2010).

  162. 162.

    US National Library of Medicine. ClinicalTrials.gov (2016).

  163. 163.

    et al. Vesicle-MaNiA: extracellular vesicles in liquid biopsy and cancer. Curr. Opin. Pharmacol. 29, 47–53 (2016).

  164. 164.

    , , , & Do circulating tumor cells, exosomes, and circulating tumor nucleic acids have clinical utility? A report of the association for molecular pathology. J. Mol. Diagn. 17, 209–224 (2015).

  165. 165.

    et al. Cancer-secreted miR-105 destroys vascular endothelial barriers to promote metastasis. Cancer Cell 25, 501–515 (2014).

  166. 166.

    , & Magnetic resonance imaging of melanoma exosomes in lymph nodes. Magn. Reson. Med. (2014).

  167. 167.

    et al. Molecular imaging of very late antigen-4 (alpha4beta1 integrin) in the premetastatic niche. J. Nucl. Med. 53, 779–786 (2012).

  168. 168.

    et al. Very late antigen-4 (α4β1 Integrin) targeted PET imaging of multiple myeloma. PLoS ONE 8, e55841 (2013).

  169. 169.

    et al. Label-free quantitative detection of tumor-derived exosomes through surface plasmon resonance imaging. Anal. Chem. 86, 8857–8864 (2014).

  170. 170.

    et al. P2Y2R activation by nucleotides released from the highly metastatic breast cancer cell MDA-MB-231 contributes to pre-metastatic niche formation by mediating lysyl oxidase secretion, collagen crosslinking, and monocyte recruitment. Oncotarget 5, 9322–9334 (2014).

  171. 171.

    et al. Protein tyrosine phosphatase receptor type O expression in the tumor niche correlates with reduced tumor growth, angiogenesis, circulating tumor cells and metastasis of breast cancer. Oncol. Rep. 33, 1908–1914 (2015).

  172. 172.

    et al. The CXCR4 antagonist AMD3465 regulates oncogenic signaling and invasiveness in vitro and prevents breast cancer growth and metastasis in vivo. PLoS ONE 8, e58426 (2013).

  173. 173.

    , & Angiogenesis, metastasis, and endogenous inhibition. J. Neurooncol. 50, 173–180 (2000).

  174. 174.

    & Why don't we get more cancer? A proposed role of the microenvironment in restraining cancer progression. Nat. Med. 17, 320–329 (2011).

  175. 175.

    Metastasis prevention by targeting the dormant niche. Nat. Rev. Cancer 15, 238–247 (2015).

  176. 176.

    & Obesity, adipokines, and prostate cancer (review). Int. J. Oncol. 28, 737–745 (2006).

  177. 177.

    , , & Obesity and prostate cancer: a role for adipokines. Eur. Urol. 52, 46–53 (2007).

  178. 178.

    et al. Tumor cell-educated periprostatic adipose tissue acquires an aggressive cancer-promoting secretory profile. Cell. Physiol. Biochem. 29, 233–240 (2012).

  179. 179.

    et al. Adipocytes promote ovarian cancer metastasis and provide energy for rapid tumor growth. Nat. Med. 17, 1498–1503 (2011).

  180. 180.

    et al. Chronic stress promotes tumor growth and angiogenesis in a mouse model of ovarian carcinoma. Nat. Med. 12, 939–944 (2006).

  181. 181.

    & Remodeling and homeostasis of the extracellular matrix: implications for fibrotic diseases and cancer. Dis. Model. Mech. 4, 165–178 (2011).

  182. 182.

    et al. Hepatic ischemia-reperfusion increases circulating bone marrow-derived progenitor cells and tumor growth in a mouse model of colorectal liver metastases. J. Surg. Res. 184, 888–897 (2013).

  183. 183.

    et al. Hypoxia after liver surgery imposes an aggressive cancer stem cell phenotype on residual tumor cells. Ann. Surg. 259, 750–759 (2014).

  184. 184.

    et al. Inhibition of tumor necrosis factor alpha reduces the outgrowth of hepatic micrometastasis of colorectal tumors in a mouse model of liver ischemia-reperfusion injury. J. Biomed. Sci. 21, 1 (2014).

  185. 185.

    et al. Ischemia/reperfusion accelerates the outgrowth of hepatic micrometastases in a highly standardized murine model. Hepatology 42, 165–175 (2005).

  186. 186.

    et al. Reduction of breast cancer relapses with perioperative non-steroidal anti-inflammatory drugs: new findings and a review. Curr. Med. Chem. 20, 4163–4176 (2013).

  187. 187.

    Normalization of tumor vasculature: an emerging concept in antiangiogenic therapy. Science 307, 58–62 (2005).

  188. 188.

    Nice neighborhood: emerging concepts of the stem cell niche. Cell 157, 41–50 (2014).

  189. 189.

    & Modes of resistance to anti-angiogenic therapy. Nat. Rev. Cancer 8, 592–603 (2008).

  190. 190.

    et al. Persistence of disseminated tumor cells in the bone marrow of breast cancer patients predicts increased risk for relapse — a European pooled analysis. Clin. Cancer Res. 17, 2967–2976 (2011).

  191. 191.

    et al. Persistence of solitary mammary carcinoma cells in a secondary site: a possible contributor to dormancy. Cancer Res. 62, 2162–2168 (2002).

  192. 192.

    , , & Dormant cancer cells retrieved from metastasis-free organs regain tumorigenic and metastatic potency. Am. J. Pathol. 169, 673–681 (2006).

  193. 193.

    et al. The BMP inhibitor Coco reactivates breast cancer cells at lung metastatic sites. Cell 150, 764–779 (2012). This landmark study screening for modifiers of metastatic dormancy identified organ-specific BMP signalling as a microenvironmental suppressor of metastasis.

  194. 194.

    et al. TGF-beta2 dictates disseminated tumour cell fate in target organs through TGF-beta-RIII and p38alpha/beta signalling. Nat. Cell Biol. 15, 1351–1361 (2013).

  195. 195.

    et al. Bone morphogenetic protein 7 in dormancy and metastasis of prostate cancer stem-like cells in bone. J. Exp. Med. 208, 2641–2655 (2011).

  196. 196.

    et al. The perivascular niche regulates breast tumour dormancy. Nat. Cell Biol. 15, 807–817 (2013). This is an elegant dissection of the role of vascular niches in tumour dormancy, demonstrating that stable microvasculature is required to maintain dormancy.

  197. 197.

    et al. Angiocrine factors from Akt-activated endothelial cells balance self-renewal and differentiation of haematopoietic stem cells. Nat. Cell Biol. 12, 1046–1056 (2010).

  198. 198.

    , , , & Dysfunctional endothelial cells directly stimulate cancer inflammation and metastasis. Int. J. Cancer 133, 1334–1344 (2013).

  199. 199.

    Parallel progression of primary tumours and metastases. Nat. Rev. Cancer 9, 302–312 (2009).

  200. 200.

    & Tumour-cell invasion and migration: diversity and escape mechanisms. Nat. Rev. Cancer 3, 362–374 (2003).

  201. 201.

    & Lymphatic or hematogenous dissemination: how does a metastatic tumor cell decide? Cell Cycle 5, 812–817 (2006).

  202. 202.

    et al. Type I collagen receptor (α2β1) signaling promotes prostate cancer invasion through RhoC GTPase. Neoplasia 10, 797–803 (2008).

  203. 203.

    et al. Integrin α3β1 can function to promote spontaneous metastasis and lung colonization of invasive breast carcinoma. Mol. Cancer Res. 12, 143–154 (2014).

  204. 204.

    et al. Anti-α4 integrin antibody suppresses the development of multiple myeloma and associated osteoclastic osteolysis. Blood 104, 2149–2154 (2004).

  205. 205.

    Integrins in bone metastasis formation and potential therapeutic implications. Curr. Cancer Drug Targets 9, 801–806 (2009).

  206. 206.

    , & Integrins and bone metastasis: integrating tumor cell and stromal cell interactions. Bone 48, 54–65 (2011).

  207. 207.

    et al. High lung-metastatic variant of human osteosarcoma cells, selected by passage of lung metastasis in nude mice, is associated with increased expression of αvβ3 integrin. Anticancer Res. 33, 3623–3627 (2013).

  208. 208.

    et al. Cadherin-11 regulates the metastasis of Ewing sarcoma cells to bone. Clin. Exp. Metastasis 32, 579–591 (2015).

  209. 209.

    et al. Involvement of chemokine receptors in breast cancer metastasis. Nature 410, 50–56 (2001).

  210. 210.

    , , & Lung endothelial dipeptidyl peptidase IV promotes adhesion and metastasis of rat breast cancer cells via tumor cell surface-associated fibronectin. J. Biol. Chem. 273, 24207–24215 (1998).

  211. 211.

    , , & Altered mRNA expression of glycosyltransferases in human colorectal carcinomas and liver metastases. Gut 46, 359–366 (2000).

  212. 212.

    , , & E-Selectin ligand-1 controls circulating prostate cancer cell rolling/adhesion and metastasis. Oncotarget 5, 12097–12110 (2014).

  213. 213.

    et al. Identification of leukocyte E-selectin ligands, P-selectin glycoprotein ligand-1 and E-selectin ligand-1, on human metastatic prostate tumor cells. Cancer Res. 65, 5750–5760 (2005).

  214. 214.

    et al. Breast cancer bone metastasis mediated by the Smad tumor suppressor pathway. Proc. Natl Acad. Sci. USA 102, 13909–13914 (2005).

  215. 215.

    et al. Genes that mediate breast cancer metastasis to lung. Nature 436, 518–524 (2005).

  216. 216.

    et al. Induction of E-selectin expression on vascular endothelium by digestive system cancer cells. J. Gastroenterol. 31, 299–301 (1996).

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Acknowledgements

The authors gratefully acknowledge support from the following funding sources: the US National Cancer Institute (CA169538 to D.L., M.J.B. and H.P. and CA169416 to D.L. and H.P.), the US Department of Defense (W81XWH-13-1-0427 to Y.K., D.L. and J.B., W81XWH-13-1-0249 and W81XWH-14-1-0199 to D.L.), the Hartwell Foundation, the Manning Foundation, the Sohn Foundation, the STARR Consortium, the POETIC Consortium, the Paduano Foundation, Alex's Lemonade Stand Foundation, the Champalimaud Foundation, the 5th District AHEPA Cancer Research Foundation (all to D.L.) and the Daedalus Fund (Weill Cornell Medicine, to D.L and H.Z). H.P. is supported by grants from MINECO (SAF2014-54541-R), ATRES-MEDIA – AXA, Asociación Española Contra el Cáncer, WHRI Academy and Worldwide Cancer Research. A.H. is supported by a Susan Komen Foundation For the Cure Fellowship. J.T.E. is supported by a Novo Nordisk Foundation Hallas Møller stipend. G.R. is supported by a Peter Oppenheimer Fellowship, awarded by the American Portuguese Biomedical Research Fund. C.M.G is supported by a US Department of Defense Breast Cancer Research Program Era of Hope Scholar Award (W81XWH-15-1-0201), the US National Cancer Institute (CA193461-01), the National Breast Cancer Coalition's Artemis Project and the Pink Gene Foundation.

Author information

Author notes

    • Héctor Peinado
    • , Haiying Zhang
    •  & Irina R. Matei

    These authors contributed equally to this work seeds

Affiliations

  1. Children's Cancer and Blood Foundation Laboratories, Departments of Pediatrics, and Cell and Developmental Biology, Drukier Institute for Children's Health, Meyer Cancer Center, Weill Cornell Medicine, New York, New York 10021, USA.

    • Héctor Peinado
    • , Haiying Zhang
    • , Irina R. Matei
    • , Bruno Costa-Silva
    • , Ayuko Hoshino
    • , Goncalo Rodrigues
    •  & David Lyden
  2. Microenvironment and Metastasis Group, Department of Molecular Oncology, Spanish National Cancer Research Center (CNIO), Madrid 28029, Spain.

    • Héctor Peinado
  3. Systems Oncology Group, Champalimaud Research, Champalimaud Centre for the Unknown, Avenida Brasília, Doca de Pedrouços, 1400-038 Lisbon, Portugal.

    • Bruno Costa-Silva
  4. Graduate Program in Areas of Basic and Applied Biology, Abel Salazar Biomedical Sciences Institute, University of Porto, 4099-003 Porto, Portugal.

    • Goncalo Rodrigues
  5. Centre for Haematology, Department of Medicine, Hammersmith Hospital, Imperial College London, London W12 0HS, UK.

    • Bethan Psaila
  6. Center for Cancer Research, Pediatric Oncology Branch, National Cancer Institute, National Institutes of Health, Building 10-Hatfield CRC, Room 1-3940, Bethesda, Maryland 20892, USA.

    • Rosandra N. Kaplan
  7. Department of Medicine, Memorial Sloan Kettering Cancer Center, New York, New York 10065, USA.

    • Jacqueline F. Bromberg
  8. Department of Molecular Biology, Princeton University, Princeton, New Jersey 08544, USA.

    • Yibin Kang
  9. Rutgers Cancer Institute of New Jersey, New Brunswick, New Jersey 08903, USA.

    • Yibin Kang
  10. Biological Systems and Engineering Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.

    • Mina J. Bissell
  11. The Garvan Institute of Medical Research and The Kinghorn Cancer Centre, Cancer Division, St Vincent's Clinical School, Faculty of Medicine, University of New South Wales, Sydney, NSW 2010, Australia.

    • Thomas R. Cox
  12. Department of Radiation Oncology, Stanford University, Stanford, California 94305, USA.

    • Amato J. Giaccia
  13. Biotech Research and Innovation Centre (BRIC), University of Copenhagen (UCPH), Copenhagen 2200, Denmark.

    • Janine T. Erler
  14. Department of Pharmacology, Tokyo Women's Medical University School of Medicine, 8-1 Kawada-cho, Tokyo 162-8666, Japan.

    • Sachie Hiratsuka
  15. Public Health Sciences Division/Translational Research Program, Fred Hutchinson Cancer Research Center, Seattle, Washington 98109, USA.

    • Cyrus M. Ghajar
  16. Department of Pediatrics, Memorial Sloan Kettering Cancer Center, New York, New York 10065, USA.

    • David Lyden

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Glossary

Disseminated tumour cells

(DTCs). Thought to originate from CTCs that reach distant organs and survive in these new distant microenvironments.

Tumour-secreted factors

Also known as the tumour secretome. The totality of factors released by tumour cells into their immediate environment or into the systemic circulation. They include growth factors, hormones, cytokines, chemokines and extracellular matrix components, as well as extracellular vesicles.

Extracellular vesicles

(EVs). A heterogeneous population of membrane- surrounded structures released by cells into the intercellular space and the circulation. Their sizes range from 30 nm to 5 μm in diameter and they include exosomes (typically 30–150 nm), microvesicles (150–1,000 nm) and apoptotic bodies (1–5 μm).

Vascular leakiness

Loss of vascular integrity resulting in increased permeability of vessels to macromolecules and cells that normally face resistance or do not cross endothelial barriers.

Circulating tumour cells

(CTCs). Rare cells shed by solid tumours into the systemic circulation at an estimated frequency of 1:500,000–1:1,000,000 circulating cells.

Metastatic niche

Microenvironment in distant organs that supports the survival and outgrowth of tumour cells.

Extracellular matrix

(ECM). Comprising molecules, specifically proteoglycans and fibrous proteins (fibronectin, collagen, elastin and laminin) secreted by stromal cells into the microenvironment, that generate an intricate network of macromolecules that fill the intercellular space.

Orthotopic

Derived from the Greek orthos, meaning right and topos, meaning place, this terminology refers to grafting a tumour into the place in the body where it would normally arise and grow.

Transgenic

Relating to or denoting an organism that contains genetic material into which DNA from an unrelated organism has been artificially introduced.

Omental tissues

A double fold of peritoneum attached to the stomach and connecting it with certain organs of the abdominal viscera, composed of the greater and the lesser omentum, which are the membranes of the bowels.

Neutrophils

Also known as polymorphonuclear leukocytes. Mature granular white blood cells with a multilobular nucleus and cytoplasm containing very fine granules. They are typically the first responders to acute inflammation, such as bacterial infection, injury or certain cancers.

Exosomes

Extracellular vesicles (typically 30–150 nm in diameter) of endocytic origin, released into the extracellular space by all cell types through the fusion of multivesicular bodies with the plasma membrane.

Kupffer cells

Specialized liver-resident phagocytic macrophages that line the walls of the liver sinusoid blood vessels.

Stellate cells

Pericytes that reside in the area between liver sinusoid blood vessels and hepatocytes. They play a prominent role in liver fibrosis and may function as liver-resident antigen-presenting cells.

Blood–brain barrier

(BBB). A complex structure formed by the tight interactions between the brain endothelium, surrounded by the basal lamina and stabilized by pericytes, glial cells and neurons.

Micrometastasis

The formation of a microscopic metastasis, usually defined as a cluster of 10–12 cells in mouse models of metastasis.

Cancer stem cells

A subset of cancer cells that share features of normal stem cells, such as self-renewal and differentiation and that can regenerate the tumour.

Fenestrated vasculature

A permeable type of vasculature that contains ultramicroscopic pores of variable sizes, usually found in kidneys and glands as well as in the circumventricular organs of the brain.

Venous thromboembolism

(VTE). Refers to either of two blood clot-related conditions: deep vein thrombosis (DVT) or pulmonary embolism (PE). DVT occurs when a blood clot forms in a deep vein whereas a PE occurs when a blood clot breaks off and circulates to the lung.

Disseminated intravascular coagulation

Systemic activation of blood coagulation, leading to fibrin accumulation, which in turn results in the formation of microvascular thrombi in vital organs.

T helper 1 (TH1) cell

Member of a subset of CD4+ T cells that can activate macrophages and mediate cellular immunity through secretion of interferon-γ (IFNγ), interleukin-2 (IL-2) and tumour necrosis factor-α (TNFα).

Metastasis-initiating cells

Rare tumour cells that have the capacity to survive and proliferate in distant metastatic sites.

Myeloid-derived suppressor cells

(MDSCs). A heterogeneous population of myeloid-derived immunosuppresive and pro-tumorigenic cells that suppress T cell function. They expand in number in pathological conditions and interact with other innate and adaptive immune cells to modulate their function. They universally express CD11b, but can be further categorized in both mice and humans on the basis of expression of additional markers into granulocytic and monocytic lineages.

Macrometastases

The outgrowth of micrometastases that are histologically or radiologically detectable.

Type I interferon

A class II α-helical cytokine essential for protection against viral infections that also plays important roles in bacterial infections, shock, autoimmunity and cancer.

Leukotrienes

Products of the eicosanoid metabolism of leukocytes that mediate inflammation and allergic reactions.

Dormancy

A latent state in which individual tumour cells are quiescent and reversibly arrested in G0 phase of the cell cycle.

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DOI

https://doi.org/10.1038/nrc.2017.6

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