Epithelial-mesenchymal transition in cancer metastasis through the lymphatic system

Abstract

It was already in the 18th century when the French surgeon LeDran first noted that breast cancer patients with spread of tumor cells to their axillary lymph nodes had a drastically worse prognosis than patients without spread (LeDran et al., ). Since then, metastatic spread of cancer cells to regional lymph nodes has been established as the most important prognostic factor in many types of cancer (Carter et al., ; Elston and Ellis, ). However, despite its clinical importance, lymph metastasis remains an underexplored area of tumor biology. Fundamental questions, such as when, how, and perhaps most importantly, why tumor cells disseminate through the lymphatic system, remain largely unanswered. Accordingly, no treatment strategies exist that specifically target lymph metastasis. The identification of epithelial-mesenchymal transition (EMT) as a mechanism, which allows cancer cells to dedifferentiate and acquire enhanced migratory and invasive properties, has been a game changer in cancer research. Conceptually, EMT provides an explanation for why epithelial cancers with poor differentiation status are generally more aggressive and prone to metastasize than more differentiated cancers. Inflammatory cytokines, such as TGF-β, which are produced and secreted by tumor-infiltrating immune cells, are potent inducers of EMT. Thus, reactivation of EMT also links cancer-related inflammation to invasive and metastatic disease. Recently, we found that breast cancer cells undergoing TGF-β-induced EMT acquire properties of immune cells allowing them to disseminate in a targeted fashion through the lymphatic system similar to activated dendritic cells during inflammation. Here, we review our current understanding of the mechanisms by which cancer cells spread through the lymphatic system and the links to inflammation and the immune system. We also emphasize how imaging techniques have the potential to further expand our knowledge of the mechanisms of lymph metastasis, and how lymph nodes serve as an interface between cancer and the immune system.

Keywords: EMT; TGF-β; chemokines; immune cell properties; lymph metastasis; targeted migration.

Figure 1

Cancer cells undergoing TGF‐β‐induced EMT acquire properties of immune cells allowing them to disseminate through the lymphatic system similar to DCs during inflammation. Upon activation, DCs acquire cell surface expression of the chemokine receptor CCR7, which allows them to sense and migrate in a targeted fashion toward lymphatic capillaries secreting the CCR7 ligand, CCL21. Endothelial cells of lymphatic capillaries are oak leaf‐shaped and are connected through button‐like junctions, which allows cells to intravasate without junctional disorganization. Subsequently, DCs migrate to lymph nodes where they interact with other cells of the immune system and perform antigen presentation. Recent studies show that similar to activated DCs, cancer cells undergoing TGF‐β‐induced EMT gain the expression of CCR7 and migrate in a targeted fashion through the lymphatic system (Pang et al., 2016). TGF‐β may also enhance the migration of EMT cells toward lymphatic capillaries by inducing the expression of CCL21 in lymphatic endothelial cells. It remains to be determined how EMT cells migrate and interact with the immune system within draining lymph nodes.

Figure 2

Intravital imaging techniques to study lymphatic dissemination of cancer cells. (A) Photomerge composition of lymphatic and blood circulatory systems of the lower hind limb of a Prox1‐GFP mouse. In the upper box, white arrows indicate a lymphatic valve and the saphena artery. In the main figure, white arrows indicate the afferent vessels, the saphena artery, and the popliteal lymph node (LN). (B) Schematic representation of the surgical intravital two‐photon model in which two different areas of the limb are imaged. On the one hand, the injection site, located in the footpad (1), and on the other hand, the popliteal area, where the arrival of the cancer cells is monitored (2). After administration of anesthesia, the animal is immobilized, the popliteal LN is surgically exposed, and interactions between the cancer and immune cells are recorded using intravital two‐photon microscopy. (C) Schematic representation of the most representative areas and immune cell populations in the popliteal LN. SSM, subcapsular sinus macrophages; MM, medullary macrophages; DC, dendritic cells; B, B‐cell follicle.