A researcher finds that blocking EMT transcription factors in a tumor cell line reduces invasiveness in vitro but does not reduce metastatic colony formation in vivo. Which concept best explains this discrepancy?
AEMT is irreversible, so cells that already underwent EMT before treatment was applied remain metastatic
BBlocking transcription factors only affects gene expression, not protein function, leaving metastasis unchanged
CMetastasis can occur through collective migration and dissemination without full EMT, so blocking EMT alone is insufficient
DIn vivo conditions reactivate EMT through signals in the tumor microenvironment that bypass the transcription factors
EMT is not the only route to metastasis. Collective migration — groups of cells moving together while retaining some epithelial features — can also drive dissemination. Moreover, partial EMT (neither fully epithelial nor fully mesenchymal) may actually be more metastasis-competent than complete EMT in some cancers. The simple narrative that 'more EMT equals more metastasis' is incorrect. In vitro invasion assays test individual cell motility, but metastasis in vivo involves intravasation, circulation survival, extravasation, and colonization — steps not fully captured by EMT alone.
Question 2 Multiple Choice
What is the key initiating molecular event in EMT that allows an epithelial cell to detach from the epithelial sheet?
AUpregulation of vimentin, which replaces the cortical actin network and enables cell motility
BSecretion of matrix metalloproteinases that digest the basement membrane beneath the epithelial layer
CRepression of E-cadherin by transcription factors Snail, Slug, or Twist, dissolving adherens junctions
DActivation of β-catenin signaling, which drives proliferation and loosens cell-cell contacts
The initiating step is E-cadherin repression. E-cadherin is the adhesion molecule holding epithelial cells together at adherens junctions; without it, the sheet dissolves and cells become individual. Snail, Slug, and Twist directly repress the E-cadherin gene. Cytoskeletal reorganization (vimentin upregulation) and MMP secretion are important downstream events, but they follow from — rather than cause — the loss of E-cadherin-mediated cohesion.
Question 3 True / False
EMT occurs in normal embryonic development as well as in cancer.
TTrue
FFalse
Answer: True
EMT is an indispensable developmental program. During gastrulation, epiblast cells undergo EMT to form the mesoderm and endoderm. Neural crest cells use EMT to migrate and give rise to peripheral neurons, melanocytes, and craniofacial structures. In wound healing, keratinocytes at wound edges partially undergo EMT to migrate across the wound bed. Cancer repurposes this normal program — EMT is not intrinsically pathological.
Question 4 True / False
A cancer cell that has undergone complete EMT is permanently committed to the mesenchymal state and cannot revert.
TTrue
FFalse
Answer: False
EMT is reversible. Many disseminated tumor cells undergo mesenchymal-epithelial transition (MET) at metastatic sites, re-establishing an epithelial phenotype to colonize new tissue. This reversibility is clinically important: it means EMT is a dynamic state, not a permanent fate switch. It also means that cells may oscillate between states, and that partial EMT — maintaining characteristics of both — may persist as the most metastasis-competent state in some cancers.
Question 5 Short Answer
Why might partial EMT — rather than complete EMT — be the most metastasis-competent state in some cancers?
Think about your answer, then reveal below.
Model answer: Partial EMT cells retain aspects of both epithelial and mesenchymal identity. The residual epithelial properties allow cohesive collective migration (increasing survival in circulation and seeding efficiency at distant sites), while the mesenchymal properties provide enough invasiveness to breach the basement membrane and intravasate. Completely EMT-derived cells may be too dispersed and poorly coordinated to seed metastases efficiently. Full reversal to epithelial state (MET) is also easier from a partial EMT state, facilitating colonization of the metastatic site.
This is a counterintuitive finding that challenges the simple 'EMT drives metastasis' narrative. The most dangerous cancer cells may not be the most mesenchymal — they may be in an intermediate, plastic state. This has therapeutic implications: targeting full EMT may miss the most metastasis-competent cell population.