Core Framework & Diagram Why Does Cancer Metastasize?
July 7, 20261 Min Read How Does Inflammation Promote Cancer?
July 7, 2026Cancer's real lethality isn't growing at its origin — it's that it goes elsewhere
—— The biology of metastasis: why ninety percent of cancer deaths come from metastasis, not the primary tumor.
I. Epithelial-mesenchymal transition (EMT): how cancer cells 'get wings''
Normal epithelial cells are 'social' — tightly connected to each other, with clear polarity, firmly attached to the basement membrane, not actively migrating. Epithelial-mesenchymal transition (EMT) is the key strategy cancer cells use to overcome this obstacle. During EMT, cancer cells lose epithelial characteristics (E-cadherin expression downregulated, cell-cell junctions loosen); acquire mesenchymal characteristics (N-cadherin, Vimentin upregulated, migratory capacity, can degrade extracellular matrix); transform from 'bricks' into 'guerrilla fighters,' dissociating from fixed tissue structures, crossing through basement membrane and stroma toward blood vessels. Core transcription factors driving EMT are SNAIL, TWIST, and ZEB1/2 — activated by TGF-β, Wnt, Notch signals, continuously induced by hypoxia, inflammatory factors, and fibroblasts in the tumor microenvironment.
EMT's profound connection to immune evasion: A 2019 Cancer Cell study revealed that the higher a cancer cell's degree of EMT completion, the lower MHC-I expression and higher PD-L1 expression — meaning EMT simultaneously grants cancer cells migratory capacity and enhances their ability to evade the immune system. This isn't coincidental — it's because common transcription factors (like SNAIL) simultaneously regulate both EMT and immune evasion-related genes. The cancer cells with greatest metastatic potential are simultaneously those best at evading immune pursuit — metastasis and immune evasion are two sides of the same coin.
2. 'Lone rangers' in the blood: the lethal challenge CTCs face
Cancer cells entering the blood immediately face a perilous battlefield. Anoikis: normal epithelial cells leaving the basement membrane trigger a special form of programmed death — anoikis — an important anti-cancer mechanism. Metastatic cancer cells must evolve resistance to anoikis — primarily through activating PI3K/AKT survival pathways, upregulating BCL-2 and other anti-apoptotic proteins, and using integrins and platelets to provide 'alternative anchoring' signals. NK cell 'blood patrol': NK cells are the primary anti-tumor effector cells in blood circulation and the main force eliminating CTCs. Research estimates a CTC's average 'lifespan' in blood circulation is only 2.5 hours — the rest are cleared by NK cells and other immune cells.
CTCs able to survive long enough to complete homing often evolved several anti-NK cell protection mechanisms. The most important is binding with platelets — CTCs express tissue factor and other molecules on their surfaces that recruit platelets to wrap around themselves, forming a 'platelet coat.' This platelet coat protects CTCs in two dimensions: physically concealing surface ligands recognized by NK cells (NKG2D ligands are obscured); simultaneously platelets secrete TGF-β, directly suppressing NK cell cytotoxic activity.
This 'CTC-platelet' complex discovery raised an intriguing intervention thought: can low-dose aspirin (antiplatelet aggregation) by preventing platelet wrapping of CTCs, enhance NK cell clearance of CTCs, thereby reducing metastasis risk? Multiple prospective studies (including a 2016 large Oxford University meta-analysis) show daily low-dose aspirin correlates with reduced metastasis risk in colorectal cancer, stomach cancer, and other cancer types.
3. 'Pre-metastatic niche': cancer sends scouts first
In 2005, David Lyden's team published a shocking finding in Nature: before cancer cells reach the target organ, the target organ has already begun, under influence of signals from the primary tumor, reshaping its own microenvironment in advance, 'preparing soil' for arriving cancer cells — this pre-remodeled microenvironment is called the 'Pre-metastatic Niche' (PMN). PMN formation requires several key steps: the primary tumor secretes VEGF, PlGF and other factors that induce bone marrow-derived hematopoietic progenitors to pre-migrate and establish at distant organs, forming 'pioneer cell groups'; these pioneer cells remodel the target organ's local microenvironment, upregulating fibronectin to provide molecular anchor points for subsequent CTC 'homing'; exosomes from the primary tumor carry specific integrin 'molecular address tags,' guiding metastatic cancer cells toward specific organs.
PMN theory's clinical implications are extremely important: if we can intervene before PMN 'matures' — blocking the primary tumor's distant signals or directly targeting key molecules in PMN — we could potentially eradicate metastasis before metastatic lesions form.
4. Organ-specific metastasis: why breast cancer 'prefers' bone
Breast cancer → bone: breast cancer cells highly express CXCR4; bone marrow stromal cells highly secrete CXCL12 (CXCR4's ligand) — a precise 'chemical guidance system' pulling breast cancer cells toward bone marrow. The bone microenvironment is also rich in TGF-β and IGF-1, supporting breast cancer cell growth; simultaneously TGF-β released during bone destruction creates a positive feedback loop driving 'bone lysis → TGF-β → promote cancer cell growth → more bone lysis,' making breast cancer bone metastasis particularly hard to control.
Colorectal cancer → liver: colorectal venous blood directly flows into the liver via the portal vein — this is a physical 'highway'; after colorectal cancer cells enter blood, their first stop is the liver. Lung cancer → brain: lung cancer cells highly express ST6GalNAc5 and similar molecules helping them cross the blood-brain barrier and establish in the brain. Understanding the molecular basis of organ-specific metastasis directly points to anti-metastasis treatment targets: targeting the CXCR4-CXCL12 axis can interfere with breast cancer cell homing to bone; bisphosphonates (by inhibiting osteoclasts) break the 'bone lysis positive feedback cycle' and have been clinically proven to reduce the progression rate of breast cancer bone metastases.
5. NK cells' special role in anti-metastasis
Throughout the entire metastasis cascade, NK cells' role is severely underestimated — they aren't only the primary force eliminating CTCs in blood circulation, but are the most important 'checkpoint guards' in the entire metastasis process, setting up barriers at multiple critical junctures.
In the circulation phase, NK cells are the primary effector cells eliminating CTCs. Research found individuals with high NK cell activity have significantly lower CTC counts than those with low NK activity; in animal models with impaired NK function (chronic stress models, sleep deprivation models), CTC survival time significantly extends and metastasis lesion formation rates significantly rise.
In the metastasis establishment phase, tissue-resident NK cells in target organs (liver, lung, bone marrow) are the most important force clearing freshly arrived CTCs. Liver's tissue-resident NK cells are the primary natural barrier against colorectal cancer liver metastasis; lung's tissue-resident NK cells are important obstacles to lung metastasis formation. Research found colorectal cancer patients with higher pre-surgery NK cell activity had significantly lower post-surgery liver metastasis rates. This is direct clinical evidence of NK cell activity as an anti-metastasis prognostic indicator.
This has direct implications for 'anti-metastasis NK cell strategies': any intervention that can maintain or enhance NK cell activity during the peri-operative period (the most vulnerable period for NK function) theoretically could reduce post-surgery metastasis risk. Pre-surgery regular exercise, adequate sleep, and reducing pre-surgery anxiety (anxiety elevates cortisol, directly suppressing NK cells) — these seemingly 'soft' recommendations have specific mechanistic support and clinical significance in the NK cell anti-metastasis framework.
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