Angiogenesis and metastasis

Concept

Angiogenesis and metastasis

T-100Home BU-307Threads regulation · information

Statement

How tumours secure a blood supply and spread.

Why it matters

hallmarks-of-cancer lists the capabilities a tumour must acquire to become clinically dangerous; angiogenesis and metastasis are two of the most consequential of those capabilities. A tumour that cannot recruit its own blood supply is limited to roughly a millimetre or two in diameter by diffusion alone, and a tumour that cannot leave its tissue of origin, however large, is in principle surgically curable in a way a metastatic cancer is not. This result traces the specific cellular mechanisms behind both, building on the loss of proliferative control described in cell-cycle-checkpoints and oncogenes-tumour-suppressors.

Hypotheses

Oxygen and nutrients diffuse effectively from a blood vessel over only a limited distance (roughly 100–200 micrometres) before falling to growth-limiting levels.Without this diffusion limit, there would be no selective pressure at all driving a growing tumour to recruit new vasculature; a tumour could simply keep growing indefinitely on existing blood supply alone. Metastasis requires a tumour cell to complete every step of a multi-step cascade (Proof, below) in sequence; failure at any single step blocks the process.Because of this, only a very small fraction of disseminated tumour cells ever successfully establish a distant metastasis, even though large numbers can be shed from some tumours into the circulation over their lifetime (Discussion).

Proof

\text{Hypoxia}\ \Rightarrow\ \text{HIF-1}\alpha\text{ stabilised}\ \Rightarrow\ \text{VEGF transcription}\uparrow

As a tumour outgrows its existing blood supply, its interior becomes hypoxic (Hypotheses); the transcription factor HIF-1α, normally degraded rapidly under adequate oxygen tension, is stabilised under hypoxia and drives transcription of pro-angiogenic genes, chief among them vascular endothelial growth factor (VEGF). A

\text{VEGF}\to\text{endothelial VEGF receptor}\ \Rightarrow\ \text{endothelial proliferation \& sprouting toward the tumour}

Secreted VEGF diffuses to nearby existing blood vessels and binds receptors on endothelial cells, triggering their proliferation, migration and organisation into new capillary sprouts — the "angiogenic switch" that supplies the tumour with fresh vasculature. A

\text{New tumour vasculature is structurally leaky and disorganised}\ \Rightarrow\ \text{easier access to the circulation for tumour cells}

Unlike normal vasculature, vessels formed under strong, unregulated pro-angiogenic signalling are characteristically abnormal — irregular, leaky, poorly organised — which supplies the growing mass but, incidentally, also gives nearby tumour cells easier physical access to the bloodstream. A

E\text{-cadherin}\downarrow,\ \text{matrix metalloproteinases}\uparrow\ \Rightarrow\ \text{basement-membrane degradation \& local invasion}

A subset of tumour cells undergo a partial epithelial-to-mesenchymal-like change, downregulating cell-cell adhesion molecules such as E-cadherin and upregulating proteases such as matrix metalloproteinases, which degrade the basement membrane and allow local invasion into surrounding tissue. A

\text{Intravasation}\to\text{survival in circulation}\to\text{extravasation}\to\text{colonisation}

Invading cells enter the leaky tumour vasculature from Step 3 (intravasation), must survive transit through the circulation — a hostile environment for most cells, and a major source of the process's inefficiency — arrest in a distant capillary bed, extravasate into the new tissue, and, only rarely, proliferate there into a clinically detectable secondary tumour rather than remaining dormant or dying. B

Result

\text{Angiogenesis: hypoxia}\to\text{HIF-1}\alpha\to\text{VEGF}\to\text{new vasculature}\ \big|\ \text{Metastasis: invasion}\to\text{intravasation}\to\text{survival}\to\text{extravasation}\to\text{colonisation}

Reading. Two related capabilities, both downstream consequences of the uncontrolled proliferation and genomic instability described across this unit (oncogenes-tumour-suppressors, clonal-evolution-of-tumours), let a tumour outgrow the diffusion limit that would otherwise cap its size, and let it establish disease at sites far from where it began.

Scope. Describes the generic cascade shared by most solid tumours; the specific molecular players, preferred distant organs, and rate-limiting step differ substantially between cancer types, and colonisation efficiency in particular is very low, meaning circulating tumour cells alone do not guarantee metastatic disease will follow.

Corollaries & converses

clonal-evolution-of-tumours frames angiogenic and invasive capability as traits that arise because clones expressing them are selected for within the tumour, exactly as any other advantageous somatic mutation would be.
Anti-angiogenic therapies, targeting VEGF signalling directly, attempt to starve a tumour by blocking Step 2, a strategy motivated entirely by this mechanism.
Converse: a tumour that fails to activate angiogenesis (Steps 1–2) remains constrained to a small, often clinically insignificant size regardless of how many other hallmark capabilities (hallmarks-of-cancer) it has acquired, since diffusion alone then caps its growth.

Fails without

Drop the diffusion-limit assumption (imagine unlimited passive nutrient and oxygen diffusion into tissue): a tumour could grow indefinitely without ever needing to recruit its own blood supply, and angiogenesis would not be a rate-limiting, clinically targetable step in tumour progression at all.
Break the metastatic cascade at any single step (e.g. a detached cell fails to resist anoikis and dies before it can extravasate): the entire metastatic outcome fails regardless of how successfully the earlier steps proceeded — exactly why the great majority of cells shed from a primary tumour never go on to establish a clinically detectable secondary lesion.

Common errors

Assuming metastasis occurs whenever any tumour cell enters the bloodstream; Step 5's cascade has many sequential failure points, and most circulating tumour cells die in transit or fail to colonise (Hypotheses).
Treating angiogenesis and metastasis as the same process; angiogenesis (Steps 1–2) supplies a primary tumour with blood and, incidentally, provides one route for intravasation (Step 5), but local invasion (Step 4) can occur without successful metastasis, whereas metastasis cannot occur without prior local invasion.
Believing metastatic cells seed distant organs with equal probability everywhere; organ tropism is common and non-random, driven by circulation patterns and by compatibility between a given tumour type and a given distant tissue environment.
Forgetting that a tumour's own new vasculature is structurally abnormal (Step 3), part of why some solid tumours remain poorly and unevenly perfused despite active angiogenesis.

Discussion

Judah Folkman proposed in 1971 that tumour growth is angiogenesis-dependent, a hypothesis that took roughly two decades to gain wide acceptance but ultimately led directly to anti-angiogenic cancer therapy as a clinical strategy, motivated exactly by Step 2 of this cascade.

The metastatic cascade's very low overall efficiency — only a small fraction of cells that complete intravasation ever establish a growing secondary tumour — means colonisation, not intravasation or transit survival, is now understood to be the single most rate-limiting step for most cancers; some disseminated cells persist for extended periods in a distant tissue as non-dividing, clinically undetectable dormant cells before ever forming a detectable metastasis, if they do at all.

Common misconception: that metastases are made of a distinct, more "evolved" or different cell type than the primary tumour. Metastatic cells are descendants of the primary tumour's own cell population (clonal-evolution-of-tumours), simply the subset whose acquired mutations happened to also confer the specific capabilities in Steps 4–5, not a separate lineage.

Worked examples

\text{Tumour core distance from nearest vessel}>200\,\mu\text{m}\ \Rightarrow\ \text{local hypoxia}\ \Rightarrow\ \text{Step 1 triggered}

A rapidly growing tumour whose interior lies beyond the effective diffusion range of its existing vasculature (Hypotheses) develops a hypoxic, sometimes partly necrotic, core; imaging showing central necrosis in a larger tumour but not in a smaller one of the same type is consistent with the larger mass having locally outpaced its blood supply before, or without, adequately activating angiogenesis. A

\text{Detection of circulating tumour cells in blood}\neq\text{confirmed metastasis}

Because most circulating tumour cells fail to complete the full cascade of Step 5, their detection reflects successful intravasation only, not successful colonisation; clinical interpretation of such findings requires acknowledging the cascade's low overall efficiency rather than treating detection as equivalent to a confirmed secondary tumour. A

\text{Angiogenic switching is a local, hypoxia-triggered event; circulating-cell detection reflects intravasation, not colonisation}

Reading. Both examples illustrate the same underlying point: each stage of the cascade (Proof) is a distinct, separately assessable event, not a single all-or-nothing process.

Scope. Distinguishing stages matters clinically, since interventions and prognosis differ depending on which stage of the cascade a given tumour or finding reflects.

Problems

A researcher blocks VEGF signalling in a mouse tumour model using an antibody that binds VEGF directly. Predict the effect on tumour growth beyond a few millimetres in diameter, and explain the mechanism, referencing the Proof.
Solution
Blocking VEGF prevents Step 2 (endothelial receptor engagement and new vessel sprouting); the tumour cannot recruit sufficient new vasculature, so growth beyond the diffusion limit (Hypotheses) is blocked or slowed, and the tumour interior remains hypoxic and increasingly necrotic — the outcome predicted by, and consistent with, Folkman's angiogenesis-dependence hypothesis.
Two patients have primary tumours of identical size and grade, but Patient A's tumour cells show strong matrix metalloproteinase expression and low E-cadherin, while Patient B's tumour cells show the opposite. Which patient is at greater near-term risk of metastasis, and why?
Solution
Patient A's tumour cells match the Step 4 profile more closely (basement-membrane-degrading, low cell-cell adhesion), indicating greater local invasive capacity and hence a higher risk of proceeding further along the metastatic cascade; size and grade alone, without evaluating these markers, would not distinguish the two patients' risk.
Explain why a tumour could theoretically satisfy every other hallmark of cancer (hallmarks-of-cancer) yet remain clinically insignificant, referencing the Corollaries.
Solution
Per the Corollaries' Converse, a tumour that fails to activate angiogenesis remains capped by the diffusion limit regardless of its other acquired capabilities, since without new vasculature it cannot grow beyond roughly a millimetre or two in size, limiting its clinical significance even if fully malignant in every other respect.