Metastatic Cascade: Key Stages and Molecular Drivers

Cancer metastasis causes most cancer-related deaths, making it a critical area of study. It involves a complex process where cancer cells spread from the primary tumor to distant organs, forming secondary tumors. Understanding this progression at the cellular and molecular levels is essential for developing more effective treatments.

Key stages and molecular mechanisms drive metastasis, influenced by interactions with the surrounding microenvironment and immune system. Advancements in detection methods are improving early identification of metastatic disease.

Key Stages in the Cascade

The metastatic cascade comprises interdependent events, beginning with local invasion. Epithelial-to-mesenchymal transition (EMT) enhances cellular motility and invasiveness by downregulating E-cadherin and upregulating mesenchymal markers like N-cadherin and vimentin. This shift enables cells to detach from the primary tumor and infiltrate surrounding tissues by degrading the extracellular matrix (ECM) through matrix metalloproteinases (MMPs), particularly MMP-2 and MMP-9, which break down type IV collagen in the basement membrane.

Once cancer cells breach the basement membrane, they enter circulation through intravasation, mediated by interactions with endothelial cells and pericytes. Tumor-associated angiogenesis, driven by vascular endothelial growth factor (VEGF), creates leaky vasculature that facilitates cellular entry into the bloodstream. Circulating tumor cells (CTCs) must withstand shear forces and evade anoikis, a form of programmed cell death triggered by ECM detachment. Some CTCs survive by forming clusters or associating with platelets, which shield them from hemodynamic stress and immune surveillance. Research in Nature shows platelet-coated CTCs exhibit enhanced survival and metastatic potential.

After surviving in circulation, tumor cells exit the vasculature through extravasation, a process mirroring intravasation but occurring at distant organ sites. The organotropism of metastases—where cancer cells preferentially colonize specific organs—is influenced by chemokine signaling and adhesion molecule expression. For example, breast cancer cells often metastasize to bone due to their affinity for CXCL12-expressing osteoblasts, while colorectal cancer cells frequently target the liver, interacting with sinusoidal endothelial cells expressing adhesion molecules like E-selectin.

Following extravasation, disseminated tumor cells (DTCs) must adapt to the new microenvironment and establish micrometastases. Some DTCs enter dormancy due to unfavorable conditions such as immune surveillance or insufficient angiogenesis. Dormant cells can persist for years before reactivating and proliferating. The transition from dormancy to active growth is influenced by local stromal interactions, metabolic shifts, and epigenetic modifications. A study in Cell reported that neutrophil extracellular traps (NETs) remodel the ECM, creating a permissive niche for dormant breast cancer cells to reactivate in the lung.

Molecular Drivers

Metastasis is driven by molecular regulators that orchestrate cellular plasticity, survival, and colonization. Transcription factors like Snail, Slug, and Twist play a central role in EMT, repressing epithelial markers like E-cadherin while upregulating mesenchymal proteins such as vimentin. High Snail expression correlates with poor prognosis in breast and colorectal cancers. Non-coding RNAs, particularly microRNAs (miRNAs) like miR-200, also regulate EMT. Loss of miR-200 expression increases invasiveness by failing to suppress ZEB1 and ZEB2, transcriptional repressors that drive EMT.

Signaling pathways such as TGF-β, Wnt/β-catenin, and PI3K/AKT/mTOR regulate proliferation, survival, and adhesion. TGF-β signaling, initially a tumor suppressor, promotes metastasis in advanced cancers by sustaining EMT and fostering an immunosuppressive microenvironment. Wnt/β-catenin signaling enhances metastatic traits by promoting stem-like properties, while PI3K/AKT/mTOR activation confers resistance to apoptosis and supports metabolic adaptations. These pathways often intersect, complicating therapeutic targeting.

Genomic alterations shape metastatic potential through mutational selection and heterogeneity. TP53 mutations, among the most frequent in metastatic cancers, lead to unchecked proliferation and resistance to cell death. Loss of p53 function allows cells to bypass anoikis. Chromosomal instability generates copy number variations that amplify oncogenes like MYC, MET, and KRAS, reinforcing metastatic traits. Whole-genome sequencing of metastatic lesions reveals differences from primary tumors, highlighting the evolutionary pressures driving metastasis.

Epigenetic modifications such as DNA methylation and histone modifications regulate metastatic plasticity. Hypomethylation of prometastatic genes and hypermethylation of tumor suppressors create an environment favoring dissemination. For instance, hypermethylation of CDH1 silences E-cadherin, promoting EMT and invasion. Histone modifications like H3K27 acetylation activate genes involved in migration and survival. These reversible changes offer therapeutic targets, with drugs like DNA methyltransferase inhibitors and histone deacetylase inhibitors being explored to reprogram metastatic cells toward a less aggressive phenotype.

Interplay With the Microenvironment

The metastatic cascade is profoundly shaped by interactions between tumor cells and their microenvironment. The extracellular matrix (ECM), composed of proteins like collagen, fibronectin, and laminins, serves as both a barrier and a conduit for tumor progression. Cancer cells manipulate the ECM by secreting enzymes like lysyl oxidase (LOX), which crosslinks collagen fibers, creating stiffened regions that facilitate migration. High ECM stiffness correlates with increased metastatic potential in breast cancer, as observed through atomic force microscopy studies.

Biochemical signaling within the tumor microenvironment (TME) also influences metastasis. Hypoxia, induced by rapid tumor expansion and insufficient vascularization, triggers adaptive responses that promote invasion. Hypoxia-inducible factor 1-alpha (HIF-1α) upregulates genes involved in angiogenesis, metabolic reprogramming, and ECM degradation, priming tumor cells for dissemination. Intravital imaging studies show hypoxic tumor regions are hotspots for cell egress.

Tumor-associated blood vessels exhibit irregular structure and high permeability, allowing cancer cells to breach endothelial barriers. Endothelial-to-mesenchymal transition (EndoMT), in which endothelial cells acquire mesenchymal traits, further destabilizes vascular integrity, promoting tumor cell escape. Additionally, pericytes, which normally support vascular stability, are often impaired in aggressive tumors, leading to disorganized vessel networks that facilitate intravasation. This disruption is particularly pronounced in glioblastomas, where compromised blood-brain barrier integrity contributes to widespread dissemination.

Immune Responses

The immune system acts as both a barrier to metastasis and an enabler of disease progression. Cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells patrol the body, targeting malignant cells for destruction through perforin and granzyme-mediated apoptosis. However, metastatic cells evade immune detection by downregulating major histocompatibility complex (MHC) molecules and upregulating immune checkpoint proteins like PD-L1, which suppress T cell activity. Elevated PD-L1 expression is linked to immune evasion in metastatic melanoma and non-small cell lung cancer.

Tumor-associated macrophages (TAMs) further facilitate immune escape by shifting toward an M2-like phenotype, which promotes tissue remodeling and immunosuppression. These macrophages secrete interleukin-10 (IL-10) and TGF-β, both of which inhibit CTL responses. Regulatory T cells (Tregs) also accumulate in metastatic lesions, suppressing effector T cell function and promoting tolerance to tumor antigens. High Treg density in metastatic sites correlates with worse prognosis in colorectal and breast cancer patients.

Detection Strategies

Detecting metastatic disease early remains a challenge. Traditional imaging techniques like computed tomography (CT), magnetic resonance imaging (MRI), and positron emission tomography (PET) are widely used, but their sensitivity varies based on tumor size and location. PET scans, often combined with fluorodeoxyglucose (FDG), effectively identify metabolically active metastases, as cancer cells exhibit increased glucose uptake. However, micrometastases often evade detection due to their small size and low metabolic activity. Advanced imaging technologies such as high-resolution diffusion-weighted MRI and molecular imaging probes targeting tumor-specific markers are improving detection.

Liquid biopsy offers a promising non-invasive alternative for metastatic surveillance. By analyzing circulating tumor DNA (ctDNA), circulating tumor cells (CTCs), and tumor-derived extracellular vesicles in blood samples, liquid biopsy provides real-time insights into tumor evolution. CtDNA levels correlate with disease burden, with higher concentrations predicting poorer prognosis in colorectal and lung cancer. Additionally, genomic profiling of ctDNA enables identification of actionable mutations, guiding personalized treatment. As technology advances, combining liquid biopsy with artificial intelligence-driven imaging analysis may enhance early detection and improve patient outcomes.