1. Angiogenesis

Angiogenesis is a physiological process that involves the development of new blood vessels from pre-existing ones. This intricate and multistep process entails the activation of endothelial cells, proliferation of cells, invasion, chemotactic migration, and differentiation into newly formed blood vessels ^[1]. Tumors require the development of new blood vessels to ensure an adequate supply of oxygen and nutrients for the tumor cells to grow and thrive. Furthermore, endothelial cells have the capability to produce growth factors through both autocrine and paracrine signaling pathways, which can effectively promote the proliferation of tumors ^[2]. The onset of angiogenesis coincides with an increase in the number of tumor cells entering the circulation, thereby promoting metastasis. Studies have consistently shown that for solid tumors, the transition from carcinoma in situ to invasive carcinoma must be accompanied by neovascularization ^[3][4][5]. Hence, the strategy of inhibiting angiogenesis could be an effective approach for treating cancer metastasis.

2. Epithelial–Mesenchymal Transition

Epithelial–mesenchymal transition (EMT) refers to a biological phenomenon in which epithelial cells undergo trans-differentiation and acquire the characteristics of mesenchymal cells. This process occurs under specific physiological and pathological conditions. During this process, epithelial cells lose cell polarity and cell adhesion, acquire migration and invasion characteristics, and undergo EMT, which is considered as the initial step of tumor metastasis ^[6][7]. EMT is characterized by a series of cellular events, including the breakdown of tight junctions, loss of apical-basal polarity, and rearrangement of the cytoskeletal structure. These changes enable tumor cells to acquire an aggressive phenotype. In the context of tumor development, the regulation of EMT is often dysregulated and influenced by various extracellular factors within the tumor microenvironment, including growth factors, inflammatory cytokines, and physical stressors such as hypoxia ^[8]. The process of EMT is intricately designed to facilitate the ability of tumor cells to acclimatize and adapt to the dynamic microenvironment of various tumors, ultimately promoting effective metastasis. One of the important features of the EMT process in solid tumors is the loss of function of E-cadherin expression, basic helix-loop-helix proteins (Twists), and forkhead box proteins (FOXCs) in tumor cells ^[9]. In addition, other transcripts involved in the EMT process include small non-coding RNAs, epigenetic regulators, and exogenous inducers. A variety of growth factors, the number of signaling pathways, and matrix metalloproteinases are also involved in the regulation of EMT ^[10]. In addition, cancer cells that undergo EMT show significant changes in morphology and molecular characteristics. These alterations include a reduction in the expression of epithelial markers, such as ZO-1 and occludin, as well as mesenchymal markers, such as N-cadherin, vimentin, and fibroblast-specific protein. These changes are further evidenced by the increased expression of fibronectin-1 ^[11].

3. Cancer Stem Cells

Tumor metastasis is not a capability shared by all cells within a tumor. Rather, cancer stem cells (CSCs) constitute a specific subset of cells that are capable of self-renewal, generating the diverse range of cells present within the tumor. CSCs play a role in tumor metastasis through direct or indirect involvement in the processes of angiogenesis and lymphangiogenesis, both of which are significant pathological changes associated with metastatic tumors ^[12]. Metastasis is driven by the evolution and selection of CSCs subsets and is considered the engine of cancer metastasis. CSCs were first identified in hematopoietic malignancies and later in a wide range of solid tumors, including breast, colon, and brain cancers ^[13]. Surface markers that facilitate cell interactions and provide them with unique properties have been used to identify CSCs ^[14]. Therefore, selectively targeting CSCs may be a promising therapeutic strategy against cancer metastasis.

4. Tumor Microenvironment

Tumor microenvironment (TME) is used to describe the specific cellular surroundings in which tumor or cancer stem cells are present ^[15]. In the 1990s, the “soil and seeds” theory of tumor metastasis proposed by Steven Paget was a milestone event in the history of TME research. He believed that tumor cells could colonize target organs only in a favorable microenvironment ^[16]. Subsequently, it was proposed that cancer cell seeds are intrinsically compatible with the microenvironmental soil of specific tissues, which helps determine metastatic organotropism ^[17]. The term “pre-metastatic niche” (PMN) was introduced in 2005 by Dr. Lyden and colleagues to describe the mechanism by which primary tumors attract bone marrow-derived cells to remote organs and create microenvironments that facilitate metastasis ^[18]. TME encompasses various components, including immune cells, blood vessels, extracellular matrix (ECM), fibroblasts, lymphocytes, myeloid-derived inflammatory cells, and signaling molecules. Through intricate interactions between the cellular and structural constituents of the TME, cancer cells gain the ability to invade surrounding tissue and propagate through a multistep metastatic process to distant sites. Thus, the TME plays a pivotal role in facilitating cancer cell metastasis. Tumor-associated macrophages (TAMs) are the main component of tumor leukocyte infiltration and play an important role in tumor metastasis ^[19]. The TME determines the interconversion between M1-type (“classically activated”) and M2-type (“alternately activated”) macrophages ^[20]. M1 macrophages elicit a response to cytokines, such as interferon-γ (IFN-γ), that impedes tumor progression by producing pro-inflammatory and immunostimulatory cytokines, including IL-12 and TNF-α. However, in various types of tumors, M2-type macrophages comprise the predominant TAMs. M2 macrophages possess immunosuppressive and growth-promoting properties, while mesenchymal cells produce copious exosomes to enhance the migratory capacity of cancer cells. Additionally, cancer-associated fibroblasts (CAFs) restructure the neighboring matrix and establish migration-promoting pathways for cancer cells ^[21].

5. Inflammation

Inflammation is a biological response that the body utilizes to counteract infection, tissue injury, or other forms of cellular stress and to promote tissue repair through restorative mechanisms ^[22]. A growing body of clinical and preclinical evidence accumulated over the past two decades has demonstrated that inflammation is a critical immune and reparative response that is indispensable for metastasis ^[23]. Inflammation is a hallmark of nearly all types of cancer and involves reciprocal communication between malignant and non-malignant cells through mediators such as cytokines, chemokines, and genetic changes. Ultimately, the inflammatory tumor microenvironment fosters tumor progression and metastasis. Interestingly, inflammation and EMT interact and maintain each other by forming a positive feedback loop. The most prominent inflammatory mediators that favor EMT and thus drive tumor cell migration, invasion, and metastatic potential include IL-1β, IL-6, IL-8, TNF-α, and some chemokines, such as CC chemokine ligand (CCL)2, CCL5 and CCL18 ^[24].

6. Genetic and Epigenetic Factors

Genetic mutations are thought to be one of the primary factors that facilitate metastatic events. Nevertheless, comprehensive sequencing studies have indicated that genetic mutations alone may not be sufficient to account for metastasis. Increasingly, it is recognized that epigenetic alterations play a crucial role in conferring additional characteristics on primary cancer cells that significantly contribute to the metastatic process ^[25]. Epigenetic modifications that promote metastatic progression occur by modifying the output of already activated transcriptional programs. The drivers of these programs are either specifically activated in cancer cells via oncogenic mutations or represent endogenous genetic and other factors that are not induced by cancer-specific oncogenic alterations. For example, hypermethylation at the loci of tumor suppressor genes, such as TP53, APC, and VHL, is frequently observed and associated with transcriptional silencing ^[26]. In addition, miRNAs can act as oncogenes with specific functions in angiogenesis, invasion, and migration, leading to cancer metastasis ^[27].

7. Extracellular Vesicles