The TGFbeta system

Question 1

What processes are regulated by TGFbeta?

Answer 1

Embryogenesis
Repair, induction of cell adhesion molecules, modulation of ECM
Immune regulation
Suppression of proliferation with late G1 arrest
Apoptosis

Question 2

What are the receptors for TGFbeta?

Answer 2

These are heterotetrameric receptors with intracellular ser/thr kinase domains where RII binds TGFbeta this then activates RI by phosphorylating it at the GS box (a stretch of G and S residues preceding the kinase domain)

Question 3

How does TGFbeta generate a diversity of responses?

Answer 3

Through Smad cofactors as well as some multiple alternative pathways including the Ras pathway

Question 4

How does the smad family transduce a signal from the TGFbeta?

Answer 4

Smad2 and 3 contain two mad-homologous domains MH1 which has a DN binding function and MH2 which has transcriptional activation function
They are phosphorylated by TbetaRI at the C terminal site allowing them to hetero-oligomerise through their MH2 domains to Smad4
This has an MH1 autoinhibitory function which typically prevents it from interacting with non-phosphorylated Smad2/3 however when it is complexed it will be translocated to the nucleus where it activates transcription by binding to things such as AP-1, HAT

Question 5

What members of the Smad family regulate the activating Smads (2/3,4)

Answer 5

Smad 6 and 7 which inhibit Smad-receptor and Smad-Smad interactions

Question 6

What is AP-1?

Answer 6

This is found in the promoters of the collagnenase I, collagen IV and plasminogen activator inhibitor-1 genes

Question 7

What is HAT?

Answer 7

Histone acetyltransferase co activators which facilitate transcription through acetylating histone lysine residues opening up DNA to transcription and recruiting RNA polymerase II

Question 8

What was the effect of over expressing TGFbeta in keratin promoters to generate skin tumours in transgenic mice?

Answer 8

When there was increased TGFbeta there were fewer papillomas produced but a higher likelihood of the formed papillomas to progress to a carcinoma
This helped to illustrate how TBGFbeta can act as tumour suppressor early in development but as an oncogene later in tumour development

Question 9

Why does TGFbeta act as tumour suppressor?

Answer 9

It can suppress the proliferation of epithelial cells as it induces p21CIP1, however many cancers can become resistant to the anti-proliferative action of TGFbeta

Question 10

How does TGFbeta promote cancer development?

Answer 10

It induces the the expression of Snail 1/2 that repress the E-cadherin gene (driving EMT)
Supresses apoptosis
Enhances angiogenesis
Suppresses immunity through preventing IL-2 production which induces the apoptosis of peripheral T cells, blocking T cell differentiation and the function of CD8 T cells
Favours the development of cancer stem cells

Question 11

What are the RII mutations seen in cancer?

Answer 11

RER+ CRC and stomach cancers have homozygous 1-bp deletion mutations in an A10 BAT-RII sequence of TbetaRII which prematurely terminate the receptor, removing transmembrane and intracellular domains
There are also heterozygous mutations also occur which indicates a 2-hit inactivation with one allele having a BAT-RII mutation other mutations may occur on the other allele

Question 12

What is the RER+ cancer phenotype?

Answer 12

These are a group of cancers resulting from the loss of mismatch repair and typically contain microsatellite instability
Tumours containing this phenotype include hereditary non-polyposis colorectal cancer
More than 50% of sporadic tumours on the RHS of the colon

Question 13

What are the RI mutations seen in cancer?

Answer 13

This occur in some primary breast carcinomas and (more frequently) in LN metastases which results in decreased responsiveness of TGFbeta-repressible promoters and TGFbeta inducible promoters
Some of these cancers become TGFbeta resistant following loss of TbetaRI expression

Question 14

What is IGFIIR?

Answer 14

The gene for the insulin-like growth factor-II receptor has a coding region microsatellite,
This protein binds, internalises and degrades IGFII as well as binding and activating the latent complex of TGFbeta

Question 15

What are the IGFIIR mutations seen in cancer?

Answer 15

Many RER+ tumours have IGFIIR mutations typically frameshifts in a G8 tract causing premature termination
In a given tumour either TGFbetaII or IGFIIR genes undergo mutations with IGFIIR mutation being associated with higher expression of latent TGFbeta1 and of IGFII

Question 16

What are the Smad mutations seen in cancer?

Answer 16

The deleted in pancreatic cancer gene 4 is inactivated in 40% of pancreatic cancers, and undergoes loss of heterozygosity in 60% of CRCs this gene was later identified as the gene for Smad4
There are also mutations in the MH1 domain interfere with DNA binding and reduce the stability of Smad2 and Smad4 causing them to be ubiquitinated by ubiquituitin ligases and degraded
As well as mutations in MH2 domain interfere with Smad hetero-oligomerisation and transcriptional activation

Question 17

What are the effects of TGFbeta on pRb?

Answer 17

TGFbeta arrests the cell cycle late in G1, it arrests the phosphorylation of pRb through inhibition of the cyclin D and E dependent protein kinases by inducing an inhibitor of Cdk4

Question 18

What can occur between c-Myc and TGFbeta in cancer?

Answer 18

Overexpression of c-Myc can lead to resistance to the antiproliferative effects of TGFbeta with at least part of this effect may be the formation of a repressive Sp1-Smad-cMyc complex at a TGFbeta reponse element that controls p15 expression
However TGFbeta may repress the expression c-Myc, Cdk4, Id proteins and Cyclin A

Question 19

What were the divergent effects of TGFbeta signalling in mammary cancer shown in Mice expressing MMTV-ERBB2 and MMTV-TbetaRI?

Answer 19

The TbetaRI gene delayed the appearance of primary tumours and reduced 4 fold the frequency of cells containing phosphorylated histone 3 (a proliferation marker) but increased the number of lung metastases growing in the parenchyma which may be a result of increased expression of integrins and MMPs

Question 20

What occurs to TbetaRII in cancers without MIN?

Answer 20

There are mutations typically in the kinase domain inactivate TbetaRII with recurrent tamoxifen resistant breast cancers show frequent mutations this may be due to the therapy increasing TGFbeta expression which selects for resistant cells