About Seed and Soil

More than one century after Paget’s clairvoyant « seed and soil » hypothesis to explain the clinical logistics of metastases [1], our view on tumor development and invasion has been enriched with the interconnected concepts of “tumor cells plasticity” and “tumor cells-microenvironment cross-interaction and cross-manipulation”. The term “cell plasticity” was originally used in 1999 by Thiery JP and Chopin D [2] to describe the molecular mechanisms involved in Epithelial to Mesenchymal Transition (EMT) during morphogenesis and tumor progression. Tumor cells plasticity refers now to the protean capacity of tumor cells to shift from one differentiation state to another [3]. Interestingly, the acquired stem characteristics may include a higher level of interaction, and manipulation, of microenvironment. This tumor cells’ feature of “changing” and “be changed by” “their own world” is presumably quite common in life, not only at the human beings level, but also at the cellular level, starting from the fertilized egg which, at the highest level of stemness, in order to grow generates its own soil, the so called embryonic adnexa carrying the fetal genome. To which extent tumors behave like another body growing into our body is open to future investigations. However, it is now clear that tumor cells-stromal cells interactions are part of the « cellular social network » playing a key role in microenvironment modification, tumor development, growth, invasion and colonization, and thus are a good target for therapeutic options.

The study of the Circulating Tumor Cells (CTC)-microenvironment cross-reprogramming interaction suggests an « egg and chicken dilemma » about which one comes first in the early steps of transformation, whether the upcoming tumorigenic cell or the upcoming tumorigenic stroma. It seems plausible that, in humans, any scenario could be possible since some tumors, for instance liver cancer, may arise directly from oncogenetic mutations or from persistent chronic inflammation or from both [4].

This issue of Cancer Microenvironment updates our view about the interplay between « Circulating Tumor Cells » and « Tumor Microenvironment », the latter being intended as both the takeoff and landing ground for the invasive “army” of CTC. Far from being an exhaustive review of a so complex field, it aims to spread light on the role of CTC-microenvironment interaction on tumor cells growth, invasion and colonization.

D. Meseure and K. Drack [5] provide an extensive, brilliant and meaningful update of the literature on « Pervasive neoplastic and stromal cells reprogramming in CTC dissemination and metastatic colonization ». The chapter starts describing the molecular mechanisms involved in reciprocal stromal and epithelial reprogramming at the site of primary tumor. On one side, this reprogramming generates a tumorigenic microenvironment, including stromal biophysical and biochemical remodeling and cells (stromal, non-immune/inflammatory and immune/inflammatory) changes. On the other side, cancer cells undergo genetic and epigenetic modifications allowing their reprogramming and acquisition of adaptive behaviors. These complex bilateral modifications foster reciprocal specific interactions between cancer cells and stromal cells underlying tumor formation, development and progression.

Tumor invasion is the biological and clinical expression of the deadly tumor character and CTC are the most studied expression of tumor invasion. They are characterized by marked heterogeneity, including variable EMT/stem, migratory, homing and dormancy characteristics, as well as capacities of self-seeding and of generating micro or macro metastatic foci.

The Authors [5] stress the role of hypoxia, immune surveillance, cell dedifferentiation to stem properties, autophagy, escape to apoptosis, EMT and its reversion in homing sites to MET, allowing peripheral tumor cells proliferation and development of metastases.

A particular aspect of tumor development and progression to an invasive phenotype is the role played by metastasis suppressor genes. In the chapter “Microenvironmental influences on metastasis suppressor expression and function during a metastatic cell’s journey”, D. Welch et al. [6] provide a clear and extensive review of this rapidly growing field and discuss the molecular mechanisms involved in metastasis suppressor genes action. Interestingly, the different metastasis suppressing genes appear to regulate practically every step of the metastatic cascade, including cell growth, angiogenesis, EMT, adhesion, intravasation, cell transit, survival, transport, migration, local invasion and colonization. Furthermore, as pointed out by the Authors, metastasis suppressor genes are cell(s)-type specific and microenvironment-type specific and their action has to be considered as part of the dynamic and mutual tumor cells-microenvironment interplay and plasticity. Their therapeutic and prognostic use is thus linked to a better understanding of their regulation and function in specific tissue settings.

Animal models are key to expand our knowledge of the metastatic process and of the CTC/microenvironment interaction in vivo. In the chapter “Orthotopic mouse models of tumor invasion expressing fluorescent reporters produce imageable circulating tumor cells” R. Hoffman [7] provides an interesting review of orthotopic mouse models carrying GFP labeled and RFP labeled prostate or lung cancer cells. These models can be used for a better understanding of the mechanisms involved in tumor invasion and to test cancer cells drug responses in vivo. They also allow imunomagnetic isolation of fluorescent CTC for their subsequent study. Interestingly, these studies provided results showing a potential exchange of genetic material among tumor cells having different metastatic capacities with relative transfer of metastatic behavior. If confirmed in humans, this process would add a level of complexity to the study of tumor cells and could probably help explaining their complex genetic heterogeneity.

In the chapter “Angiotropism, pericytic mimicry and extravascular migratory metastasis in melanoma: an alternative to Intravascular Cancer Dissemination” C Lugassy et al. [8] provide an exciting review of Lugassy & Barnhill’s long lasting contribution to the field of tumor invasion through angiotropism, also called Extravascular Migratory Metastasis (EVMM). They first described angiotropism by histological observations in patients with melanoma, a finding which has then been found to be a prognostic factor in these patients. The Authors have studied this phenomenon not only in patients, but also in vitro, in animals and have explored the molecular characteristics of this route of invasion. Tumor cells may thus invade through a different microenvironment from that of primary tumor and of distant metastases, located at the abluminal vascular surface, which is remarkably described by the Authors. As a matter of fact, UV exposure of a genetically engineered animal model of melanoma has been shown to promote metastatic progression via angiotropism and tumor cells migration along the abluminal vascular surface. Furthermore, molecular data suggest that tumor cells involved in EVMM harbor stem cells profiles allowing them to behave like embryonic migrating cells. Finally, the Authors report and discuss angiotropism, pericytic mimicry and extravascular migratory metastasis in pancreatic cancers and in other solid tumors providing an important window for studies exploring this alternative route of tumor invasion and its clinical impact. Since during embryonic life cells migration occurs at the earliest steps of development, it is sensible to think that tumors of any type displaying angiotropism, pericytic mimicry and extravascular migration could have superior capabilities to create metastases. This of course is ground for future investigations and related search of therapeutic options.

In the chapter “Crosstalk between CTC, immune system and hypoxic tumor microenvironment” S. Chouaib et al. [9] provide a remarkable review linking microenvironmental hypoxia, EMT, and CTC/CSC (cancer stem cells) escape from the immune system. The Authors report the molecular pathways involved in the hypoxia-induced EMT at the tumor microenvironment level. They analyze the immune-escape mechanisms related to immune cells and to tumor cells. EMT-related suppressive conditions allowing tumor cells immune evasion and generation of CTC may be induced by increased expression of TLRs on dendritic cells. Furthermore, the increased expression of cell-surface CD95 reported in peripheral T helper cells is expected to induce helper T cell apoptosis. At the tumor cells level, the complete loss of MHC molecules and the loss of expression of tumor antigens has been reported in many types of cancers and is an important mechanism to escape T-cell mediated immune response. Furthermore, the expression by tumor cells of molecules like PD-L1 and HLA-G or HLA-E may induce resistance of tumor cells to immune-induced death.

CTC are known to be heterogeneous and some of them have been reported to express EMT and CSC markers which, in turn, are involved in decreased sensitivity to cytotoxic immune effectors. The Authors thus remark that those CTC with EMT and CSC markers could be resistant to cell-mediated cell death. A decreased immune response has also been reported in patients with CTC. Finally, platelets and lymphocytes may aggregate around CTCs in the blood microenvironment and this is expected to protect CTC from NK cell-mediated lysis.

Studies on the mechanisms involved in CTC escape from the immune system are at their very beginning and whether CTC express or up-regulate surface molecules such as PD-L1 and HLA-G is not clear at present but is an interesting field for future investigations.

In this setting, the Authors suggest that the EMT generated by the hypoxic tumor microenvironment could favor the emergence of CTC and CSC with increased metastatic and immune resistance potential, a view which could lead to an interesting path for therapeutic action.

Finally, in the Chapter “Circulating Tumor Cells: who is the killer?” by P. Paterlini Bréchot [10], the field of CTC is discussed from the scientist and the clinician different perspectives. The Author raises the issue of the technical challenges, different methods and related clinical interest of the current tests for CTC detection, a domain which has been the focus of intensive scientific activity over the last 15 years. The Chapter also underlines the critical issue of diagnostic identification of CTC and the burning  questions which have remained unanswered, the achievements and limitations, and future directions for patients and scientists.

This Issue of Cancer Microenvironment updates our knowledge and understanding of the extensive cross talk and interaction leading to tumor development, tumor cells invasion, tumor cells circulation and colonization. Reciprocal stromal and epithelial reprogramming and tumor cells plasticity are new exciting areas for intensive investigation. Tumor invasion may proceed through blood and also outside blood vessels through angiotropic invasion but the possible connection and reciprocal impact of these two “arms” are completely unknown at present. The study of interactions involving CTC in the blood microenvironment, including their cross talk/reprogramming, their “protection from/exposure to” immune system, their stimulation to undergo EMT and to evolve toward CSC and their different ways to die may open new avenues of research leading to unravel the process leading to metastases. Finally, clinical studies performed using a highly sensitive and diagnostic approach to identify tumor cells in blood, referred to as Circulating Cancer Cells (CCC), could change the clinical practice in oncology and reduce mortality through a very early diagnosis of invasive cancers.

Overall, this Issue shows new routes of investigation and stimulates more fundamental and clinical work on invading tumor cells and their changing microenvironment: a perspective which is also a step forward in our battle to beat cancer.