lecture 23: epithelial to mesenchymal transition (EMT) and stem cells

Question 1

Can epithelial cells transition to mesenchymal cells?

Answer 1

• yes, and vice versa

Question 2

What happens when epithelial cells undergo EMT?

Answer 2

• disassemble cell junctions
• lose their apico-basal polarity
• become more loosely associated
• become more motile
• these characteristics result from changes in gene expression
• approximately 4000 genes change

Question 3

What is a characteristic of EMT?

Answer 3

• down regulation of E-cadherin
• intermediate EMT states allow migration of cell sheets, epithelial remodelling and individual cells with more motile characteristics within the epithelium

Question 4

What do adherens junctions do? What is the role of cadherins in these?

Answer 4

• form a continuous belt below the tight junctions, the zonula adherens, in epithelial cells that bring actin filaments into alignment
• cadherins form the transmembrane linkages

Question 5

What regulates EMT?

Answer 5

• multiple signalling pathways
• note that many are associated with regulation of stem cell maintenance and differentiation
• the transcription facotrs listed are mainly repressors of E-cadherin
  
  • transcription factors:
    
    • Snail (Snai1)
    • Slug (Snai2)
    • Zeb1
    • Zeb2 (Sip1)
    • Twist1
  • Signalling pathways
    
    • tgf-beta
    • wnt
    • notch
    • tna-alpha
    • hypoxia
    • receptor tyrosine kinase
  • microRNAs
    
    • miR-10b
    • miR-9
    • miR-200 family

Question 6

For what is EMT important?

Answer 6

• in the development of tissues and organs
• e.g.
  
  • cutaneous structures (hair, feathers, sweat glands, mammary glands)
  • limb
  • gut organs
  • foregut and respiratory associated organs
  • kidney
  • tooth

Question 7

What is the relationship between EMT and cancer?

Answer 7

• EMT has long been associated with tumour metastasis but recent evidence suggests that EMT is associated with tumour progression and development of cancer stem cells

Question 8

What are snail proteins?

Answer 8

• snail proteins are transcriptional repressors
• snail family in drosophila: snail, escargot, worniu
• snail family in mouse/human: snai1, snai2, snai3

Question 9

With what are snail proteins associated?

Answer 9

• radio-resistance
• quiescent haematopoietic stem cells (HSCs) are resistant to gamma-irradiation
• snai2 mutant HSCs are radiosensitive
• irradiation → stabilisation of p53 → Snai2 inhibits puma (normally → apoptosis)
• snai2 represses Puma and prevents apoptosis of HSCs

Question 10

What genes regulate EMT?

Answer 10

• Snail genes
  
  • inhibit epithelial markers
  • inhibit proliferation
  • lead to mesenchymal markers
  • changes in cell shape, cell movements, invasion
  • survival

Question 11

In what way does EMT generate cells with properties of stem cells?

Answer 11

• immortalised human mammary epithelial cells (HMECs) expressing Snail or Twist or treated with TGFbeta1 undergo EMT
• cells downregulate E-cadherin and upregulate mesenchymal genes
• cells express high CD44 and low CD24 (mammary stem cell profile)
• cells can efficiently form mammospheres
• naturally arising CD44^high/CD24^low cells exhibit the properties of stem cells and appear to have undergone EMT
• normal mouse mammary stem cells express markers of EMT

Question 12

What is seen in intestinal epithelial stem cells?

Answer 12

• express mesenchymal markers
• snai1 is prominently expressed in the nuclei of crypts

Question 13

Where are the most primitive mammary stem cells found?

Answer 13

• in basal cells of the epithelial ducts in the mammary gland
• snai2 together with sox9 can induce stem cell characteristics in differentiated luminal cells

Question 14

What is the role of microRNAs in EMT?

Answer 14

• play key roles
• epithelial cells express high levels of miR-200 family miRNAs that repress stem cell markers
• induction of EMT induces expression of Zeb1 which represses miR-200
• miR-200c suppresses normal mammary outgrowth in vivo

Question 15

What do paracrine and autocrine signals do in the breast?

Answer 15

• induce and maintain mesenchymal and stem cell states
• inducers/maintainers of migration/self-renewal
  
  • TGF-beta
  • wnt/beta-catenin
  • wnt5a
• inhibitors of migration/self-renewal
  
  • BMP4
  • DKK1
  • SFRP1

Question 16

What is the role of poised chromatin at the ZEB1 promoter?

Answer 16

• enables breast cancer cell plasticity and enhances tumourigenecity

Question 17

What could the link between EMT and stemness indicate?

Answer 17

• could indicate that upregulation of some EMT inducers could lead to BOTH metastasis and tumour growth
• metastatic cells may have an inherent ability to seed new tumours via stem cell properties

Question 18

What are the review points?

Answer 18

• what is epithelial to mesenchymal transition (EMT)?
• what are characteristics of EMT?
• EMT causes downregulation of E-cadherin
• multiple pathways and transcription factors can induce EMT
• EMT associated with tumour progression /development of cancer stem cells
• snail genes are associated with acquisition of radio-resistance
• evidence that EMT generates cells with properties of stem cells (Mani)
• downregulation of specific miRNAs by ZEB1 is observed in stem cells
• cell signals maintain mesenchymal and stem cell state
• poised chromatin allows rapid transition to a cancer stem cell state