Clinically detectable metastases represent the end-products of a complex series of cell-biological events, which are collectively termed the invasion-metastasis cascade. During metastatic progression, tumor cells exit their primary sites of growth (local invasion, intravasation), translocate systemically (survival in the circulation, arrest at a distant organ site, extravasation), and adapt to survive and thrive in the foreign microenvironments of distant tissues (initial survival in a foreign microenvironment and micrometastasis formation, metastatic colonization). Carcinoma cells are depicted in red.

Metastatic progression is not an exclusively cell-autonomous process. Indeed, carcinoma cells enlist non-neoplastic stromal cells to aid in each step of the invasion-metastasis cascade. Examples of the roles of stromal cells during metastasis are illustrated. Carcinoma cells are depicted in red. Angptl4: angiopoietin-like-4; CSF-1: colony stimulating factor-1; EGF: epidermal growth factor; IL-4: interleukin-4; MMP-9: matrix metalloproteinase- 9; OPN: osteopontin; SDF-1: stromal cell-derived factor-1.

Carcinomas originating from a particular epithelial tissue form detectable metastases in only a limited subset of theoretically possible distant organ sites. Shown here are the most common sites of metastasis for six well-studied carcinoma types. Primary tumors are depicted in red. Thickness of black lines reflects the relative frequencies with which a given primary tumor type metastasizes to the indicated distant organ site.

The number of distinct molecular programs required for metastatic colonization is incredibly high, due to considerations stemming from both the distant organ site that is being colonized and the tissue-of-origin of the primary tumor from which the metastases were initially spawned. (A) An individual primary tumor deploys distinct genetic and/or epigenetic programs in order to colonize different metastatic sites. Accordingly, a primary breast tumor (depicted in red), utilizes unique signal transduction pathways to metastasize to bone, brain, liver, or lung. (B) Carcinomas originating from two different tissues may deploy distinct molecular programs in order to colonize the same metastatic organ site. For example, primary breast tumors (upper red lesion) initiate signaling pathways that yield osteolytic bone metastases, while primary prostate tumors (lower red lesion) spawn osteoblastic bone metastases that are driven by unrelated molecular programs. IL-11: interleukin-11; SPARC: secreted protein acidic and rich in cysteine; ST6GALNAC5: ST6 (alpha-N-acetyl-neuraminyl-2,3-beta-galactosyl-1,3)-Nacetylgalactosaminide alpha-2,6-sialyltransferase-5.

Certain steps of the invasion-metastasis cascade are successfully completed with only extraordinary inefficiency. Work in experimental models has revealed that the process of metastatic colonization typically represents the rate-limiting step of the invasion-metastasis cascade, with a rate of attrition that often exceeds 99% of those cells that initially survive in a foreign microenvironment to form micrometastases. Red x-marks denote the approximate cumulative fraction of intravenously implanted tumor cells that have died after passage through the indicated steps of the invasion-metastasis cascade.

A number of models have recently been proposed in order to explain how tumor cell populations evolve to acquire molecular alterations in metastasis virulence genes. (A) The normal differentiation programs of the cells-of-origin from which certain primary tumors are derived may already dictate the altered activity of various metastasis virulence genes (depicted in gray). Upon subsequent oncogenic transformation and systemic dissemination, these cells may therefore be capable of completing the process of metastatic colonization. (B) Cells that are only partially metastasis-competent (i.e., tumor cells that have acquired a series of mutations that confer the capacity to disseminate systemically, but are initially unable to colonize foreign microenvironments) may arrive at distant organs, where they then undergo further genetic and/or epigenetic evolution within these foreign microenvironments in order to achieve full metastatic competence. Such molecular evolution would likely include alterations in metastasis virulence genes. (C) Purely by chance, mutations in metastasis virulence genes may accumulate stochastically as “passenger mutations” within tumor cell clones that bear unrelated “driver mutations” that serve to fuel the clonal expansion of these cells within primary tumors. (D) The phenomenon of tumor self-seeding indicates that already-metastasized cells are capable of re-infiltrating the primary tumor from which they originated. Hence, carcinoma cells present in metastases (which have come to acquire molecular alterations in metastasis virulence genes via either of the models proposed above – as indicated by the asterisk) may become increasingly represented within their primary-tumor-of-origin (re-infiltrating cells are depicted in blue). (E) The parallel progression model asserts that quasi-normal epithelial cells (depicted in orange) disseminate very early from pre-neoplastic lesions. Subsequently, these cells undergo molecular evolution at future sites of metastasis formation. Notably, such sites represent locations where mutations in metastasis virulence genes are now selectively advantageous. Carcinoma cells are depicted in red.

Because metastases are culpable for >90% of cancer-associated mortality, truly efficacious anti-metastatic therapies are desperately needed. (A) Various rationally designed anti-metastatic compounds trigger measurable responses in pre-clinical preventative settings where treatment is initiated prior to the formation of primary tumors or metastases. (B) Unfortunately, however, many agents that display efficacy in preventative pre-clinical models fail to impair metastasis in pre-clinical intervention settings where treatment is initiated only after the formation of small micrometastases (depicted in blue). Because carcinoma patients frequently already harbor significant numbers of disseminated tumor cells at the time of initial disease presentation, the ultimate translational utility of compounds that are unable to alter the behavior of already-formed metastases is likely be quite limited. In contrast, dasatinib, medroxyprogesterone acetate (MPA), miR-31 mimetics, bisphosphonates, denosumab, SD-208, and LY2157299 inhibit the metastatic outgrowth of already-disseminated tumor cells in intervention assays (depicted in gray). (C) In the end, agents that are capable of eliciting the regression of already-established macroscopic metastases may possess the greatest clinical utility. Compounds displaying such efficacy in pre-clinical intervention settings are quite rare, though several examples have been reported – namely, miR-31 mimetics, bisphosphonates, denosumab, SD-208, and LY2157299. In contrast, many other compounds are incapable of altering the behavior of already-established macroscopic metastases (indicated in blue) – including agents that display efficacy against small micrometastases prior to their overt metastatic colonization. Carcinoma cells are depicted in red. Therapeutic agents whose mechanism of action is believed to principally involve the targeting of non-neoplastic stromal cells are presented within the orange boxes. MMP: matrix metalloproteinase; MPA: medroxyprogesterone acetate.