Invasion is the process by which cancer cells directly extend and penetrate into neighboring tissues in cancer. It is generally distinguished from metastasis, which is the spread of cancer cells through the circulatory system or the lymphatic system to more distant locations. The two are nonetheless closely related, and lymphovascular invasion is generally the first step of metastasis.
The two main patterns of cancer cell invasion by cell migration are collective cell migration and individual cell migration, by which tumor cells overcome barriers of the extracellular matrix and spread into surrounding tissues. Either pattern of cell migration exhibits distinct morphological features and is governed by specific biochemical and molecular genetic mechanisms.
Two types of migrating tumor cells, mesenchymal (fibroblast-like) and amoeboid, can be observed in various patterns of cancer cell invasion. This article describes the key differences between the variants of cancer cell migration, the role of epithelial-mesenchymal and related transitions, as well as the significance of different tumor factors and stromal molecules in tumor invasion. Morphological manifestations of the invasion patterns are characterized by a variety of tissue (tumor) structures. (Creative Commons Attribution License)
Invasive growth and metastasis The results of numerous experimental and clinical studies of malignant neoplasms have indicated that invasive growth and metastasis are the main manifestations of tumor progression, which constitute two closely related processes.
A malignant tumor is defined by its capacity to initiate a biological phenomenon known as the metastatic cascade, a complex multi-stage process in which cell invasion precedes further cancer progression and the formation of metastases in distant organs and tissues. Massive metastatic lesions lead to the development of organ failure. The range between the "end" points of a complex invasive metastatic process–an invasion of the primary tumor into surrounding tissues and the formation of metastatic foci–comprises several stages, the passage of which is strictly necessary for the successful development and subsequent progression of tumor growth: intravasation, survival and presence in the systemic circulation, extravasation with subsequent colonization of organs by tumor cells, and the formation of clinically detectable metastases. Tumor growth is accompanied by increasing pressure on surrounding extracellular matrix structures, whereas the tissue microenvironment works to retain its functional-anatomic integrity by increasing pressure on the tumor cells. The factors limiting the growth of malignant neoplasms include the basement membrane and various components of the surrounding stroma, increased interstitial pressure, limited oxygen supply to tumor cells and the production of reactive oxygen species, and persistent contact with immune system cells. Due to intratumoral heterogeneity, some tumor cells may undergo regression and death, while others, resilient against opposing microenvironmental factors, acquire an aggressive phenotype and the capacity to metastasize.
Invasive tumor growth is enabled by the detachment of malignant cells from the tumor mass due to a reduction in or complete loss of intercellular adhesion molecules. This allows the cells to gain anomalously high motility enabling penetration through the stiff structural elements of the surrounding stroma.
The process by which epithelial cells lose their cell polarity and cell–cell adhesion, and gain migratory and invasive properties to become mesenchymal stem cells, is referred to as an epithelial-mesenchymal transition (EMT). EMTs are a normal feature of diverse biological processes such as embryogenesis and wound healing. However, in the context of metastasis, they facilitate the invasion of tumor cells into other areas of the body.
Physiological prototypes of invasive growth Tumor cells have the ability to replicate the mechanisms and migration patterns typically seen in normal, non-tumor cells during various physiological processes. Like normal cells, tumor cells can activate these mechanisms to alter their shape, create favorable conditions for movement, and reshape nearby tissues to form pathways for migration. However, tumor cells, in contrast to normal cells, do not have physiological "stop signals" to terminate these processes. This leads to the establishment of the migration mechanisms and promotes the progression and spread of the tumor.
Malignant cells were found to use built-in genetic programs to implement the processes that determine invasive growth and metastasis. For example, the movement of individual cells observed during embryonic development and inflammation (e.g., leukocyte migration) is similar to the dissemination of cancer cells during tumor progression and metastasis.
Along with single cell migration, collective cell migration can occur when groups of firmly interconnected tumor cells migrate together. This type of migration indicates tissue rearrangement, underlies the processes of embryonic morphogenesis, and is also an essential component of the healing of wound surfaces.
In this way malignant tumor cells use the mechanisms of both collective and single cell migration as physiological prototypes in the process of invasive growth and metastasis.
Patterns of invasive growth Two distinct patterns of invasive growth are distinguished based on specific morphological and molecular genetic parameters: single-cell migration and collective cell migration. The migration type is predominantly influenced by characteristics of the tissue microenvironment, and is dependent on molecular changes within the tumor cells.
Determination of the invasion mechanism used by single migrating cells during migration is a complex task. Before 2015, studies examining this at the molecular and morphological levels were few in numbers and mostly were carried out in vitro using specific cell lines.
However, there has been a subsequent increase in the number of studies that demonstrate interest in the molecular genetic features of tumor cells that determine the main differences between the mesenchymal and amoeboid types of cell movement during individual migration, as well as collective migration.
Collective migration
Collective migration is characterized by the migration of whole groups of cells interconnected by adhesion molecules and other communication junctions. This is the main feature of this type of invasion, since the underlying cellular mechanisms are the same processes that largely determine single cell migration.
Collective cell migration has been observed in the development and progression of breast and endometrial cancer, prostate cancer, colorectal cancer, large-cell lung carcinoma, rhabdomyosarcoma, melanoma, as well as most squamous cell carcinomas.
In the case of collective migration, cancer cells, being a part of the tumor mass or detaching from it in the form of multicellular groups, penetrate into the surrounding tissues and form thin, short chords, clusters, stripes and wide fields, as well as structures with lumen, that indicate a wide variety of structural elements involved in tumor invasion.
Collective migration is characterized by the migration of whole cell groups interconnected by cadherins and intercellular gap junctions. A moving cell group has a "leading edge" or "leading front" that uses integrins and proteases. Observable differences exist in the expression of the genes and morphology between the "leader" cells forming the leading edge and the "follower" cells that are located behind them, at the "trailing edge". The cell shape of the "leaders" often resembles mesenchymal cells and is characterized by a less pronounced ordering and structural organization, while the "followers" tend to form more tightly packed, rosette-like tubular structures with tight intercellular contacts.
In the case of collective migration, tumor cells form protrusions (pseudopodia) at the leading edge, use integrins to form focal contacts with the actin cytoskeleton, and perform proteolytic degradation of the extracellular matrix, creating a space for invasion of the tumor tissue and extensively involving the actin-myosin contractile apparatus in the process to ensure successful migration.
The differences in the polarity of collectively migrating cell groups are due to the expression of surface receptors, such as CXCR4 and CXCR7 chemokine receptors, in the "leader" cells. The growth factors and chemokines produced by stromal cells form a diffusion gradient and induce cell polarization. Involvement of chemokines, such as SDF1 (CXCL12), the fibroblast growth factor (FGF), and the transforming growth factor β (TGF-β), in these processes has been discussed in the literature.
The involvement of TGF-β in carcinogenesis is twofold. TGF-β, which acts in the epithelial cells of the mammary gland as a potent tumor suppressor at the early stages of cancer, can affect tumor development via interaction with oncogenic cytokines. Increased expression of TGF-β has been associated with the progression of tumors, which has often been observed, e.g., at the later stages of breast cancer. TGF-β is a regulator of the interactions between the tumor and stroma, which promotes collective cell migration in breast cancer.
It has been established that leader cells express podoplanin, a transmembrane glycoprotein that is expressed under normal conditions in kidney podocytes, type 1 alveolar cells, skeletal muscle cells, placenta, etc. Podoplanin expression in breast cancer cells induces cell migration and invasion with the formation of filopodia and simultaneous retention of E-cadherin expression.
Collectively migrating cancer cells can potentially use the ability of adjacent mesenchymal cells to modify the structure of the matrix and rebuild it, and then follow in their "footsteps". In in vitro experiments, the introduction of fibroblasts in the culture induces collective tumor cell migration to the underlying matrix in the form of chains. In this way, fibroblasts guide invading tumor cells, remodeling the surrounding extracellular matrix into pathways with thick collagen bundles on the sides and a lack of a matrix in the center.
LIM-kinase, an enzyme whose substrate stabilizes actin, plays a role in the collective migration of tumor cells. This protein is known to be involved in the regulation of developing invadopodias, which are structures typical of malignant tumor cells and responsible for the destruction of the surrounding extracellular matrix. Excessive activation of LIM-kinase is displayed in breast cancer. Breast tumor cells with suppressed expression of the LIM-kinase gene lose their ability to invade due to the loss of their ability to disrupt the extracellular matrix.
Single cell invasion
Single cell invasion is distinguished by individual tumor cells invading the surrounding tissues independently of each other. In this type of tumor invasion, single cell migration can occur via two different movement types: mesenchymal and amoeboid. These movement types are highly plastic and can shift from one type of migration to the other (from mesenchymal to amoeboid and vice versa). These transitions usually occur upon changes in the activity of certain cell molecules when tumor cells have to adapt to the peculiarities of the microenvironment.
Mesenchymal (fibroblast-like) cell migration
The mesenchymal mechanisms of invasive cell growth, in contrast to the amoeboid type of migration, are characterized by the occurrence of more complex processes and the involvement of a larger number of cellular molecules.