2nd March - Role of Integrins

Question 1

What are integrins?

Answer 1

Receptors for laminin, collagen and fibronectin

Question 2

Outline the structure of an integrin

Answer 2

Made up of two non covalently associated subunits - alpha and beta
Both are membrane glycoproteins
Have a single TM region
Short cytoplasmic tail
the N-term of beta forms a loop stabilized by a disulphide bond between the 1st cysteine rich region and amino terminal cysteines
The alpha subunit binds divalent metal ions, necessary for ligand binding

Question 3

What is the main function of integrins?

Answer 3

To form a transmembrane bridge between the ECM and actin filaments of the cytoskeleton

Question 4

How does integrin mediated adhesion regulate the cell cycle?

Answer 4

Cell-anchorage is required for hyperphosphorylation of Rb and release of E2f in G1 phase of the cell cycle

Question 5

How does integrin mediated adhesion protect normal cells from apoptosis?

Answer 5

Induces PI3K

Question 6

What is the focal complex?

Answer 6

A complex that forms around the integrin in a cell mediating the polymerisation of actin –> motility

Question 7

What happens to focal contacts as the cell moves?

Answer 7

They recruit more protein becoming focal adhesions

Question 8

What are the main kinases associated with integrin signalling?

Answer 8

Focal adhesion kinases (FAK)

Src family kinases

Question 9

How does an inert integrin activate FAK?

Answer 9

Binds adaptors pacillin and talin
FAK binds to paxillin and taxin in its phosphorylated form
When there is a high density of ECM –> integrin clustering

Question 10

How does FAK-Src signallign promote an invasive phenotype?

Answer 10

JNK –> Fos/Jun TFs –> Increase in transcription of MMPs –> Invasive phenotype

Question 11

How do integrins perform inside-out signalling?

Answer 11

Integrins can bind ECM ligands modulated by Rho and Ras

Question 12

What does integrin dependent migration depend on?

Answer 12

Substration ligand density
Integrin expression level
Integrin-ligand affinity

Question 13

Why do integrins mediate cell survival?

Answer 13

The cell needs to be in a suitable environment

Question 14

How do integrins mediate cell survival?

Answer 14

Integrin bound to the ECM –> Raf and Pi3k-Akt, NFkB and decreases p53 activity –> Survival

Question 15

How does an unligated integrin mediate cell survival?

Answer 15

It induces caspases –> apoptosis

Question 16

What is anoikis?

Answer 16

Programmed cell death due to loss of ECM contact - non-adherent cell

Question 17

What is integrin mediated death?

Answer 17

An adherent cell undergoes cell death when the integrin has no ligand

Question 18

Which integrin promotes anchorage independence?

Answer 18

alphavbeta3

Question 19

How does alphavbeta3 promote anchorage independence?

Answer 19

Triggers pathways with Src and p130 CAS to allow cells to survive independently of the ECM

Question 20

What is alpha 6 beta 4?

Answer 20

An integrin which is a component of hemidesmosomes, connecting the ECM to intermediate filaments

Question 21

Which cancers often lack alpha 6 beta 4?

Answer 21

Breast and prostate

Question 22

Which cancers often upregulate alpha 6 beta 4?

Answer 22

Squamous cell carcinoma
Skin
Cervical
Head and Neck

Question 23

How is the function of alpha 6 beta 4 regulated?

Answer 23

By Laminin 5
With laminin 5 it forms hemidesmosomes for stable adhesion
With laminin 5 cleaved by MMP2 it forms focal contacts for migration

Question 24

What is the evidence that adhesion is required for metastasis?

Answer 24

The RGD peptide –| Lung metastasis when co-injected with B16-F10 mouse melanoma cells into mice

mAb agonist alpha v subunit to neutralise its effects –| tumour growth and metastasis of human M21 melanoma cells in nude mice

mAb against alpha 5 beta 1 fibronectin receptor –| breast cancer metastasis

Question 25

Outline the role of integrins in angiogenesis

Answer 25

alphaV beta3 and alpha5beta 1 are key in the formation of the vascular sprout.
To form a mature vessel the endothelial cells need to bind to the smooth muscle cells or pericytes via alpha 4 beta 1 and VCAM to provide a supporting layer

Question 26

How do fibroblasts promote collagen production?

Answer 26

Through alpha 11 beta 1

Question 27

How do integrins protect tumour cells within the circulation?

Answer 27

Platelets express alpha IIb beta 3 which binds to fibrinogen within the circulation which binds tumour cells, protecting the tumour embolus
Tumour cells secrete molecules which activate platelets leading to upregulation of alpha2bbeta 3 and the coagulation cascade
platelet adhesion molecules may assist binding to endothelial cells facilitating arrest in the capillary bed
Tumour cells may also interact with monocytes in fibrin containing aggregates. Monocytes may supply a housing system for the capillary bed through expression of L-selectin, alpha4 beta1