Discuss ways in which the Smad/TGF-? signalling can be dysregulated in human disease, and using specific examples from recent research papers
Transforming growth factor beta (TGF-?) are ligands discovered in the early 1980s. These ligands work alongside Smad proteins as a signalling pathway. To initiate signalling TGF-? dimer binds to type II receptors at the cell surface. Both type I and II receptors form serine/ threonine kinases. (Zhike, 2014) When type II receptor is activated it phosphorylates type I receptor. This then phosphorylates R-Smad (Smad 2/3) to form a complex with CoSmad. The complex formed act as transcription factor which functions in the nucleus of the cell. These transcription factors bind to promoter regions of DNA at TGF-? response elements. (Clarke, 2006) The signalling pathway has been recognised as a key regulator for fibrosis immune responses and angiogenesis. TGF-? plays a major role in tumorigenesis and other biological processes, including tumour proliferation, metastasis, inflammation, angiogenesis, and immune response.
Cells are often regulated in the body, where some cells will undergo apoptosis or proliferation. Cancer is the result of mutations in DNA, where Proto-Oncogenes and or Tumour suppressor genes malfunctions. Unlike normal cells they will constantly divide and form tumour(s). Benign tumours are isolated to one area and are generally harmless. Malignant tumours however will spread via the blood stream or lymphatic system around the body and are harmful. TGF-? can be both tumour suppressing and tumour promoting.
For normal cells to divide, in G1 phase DNA is replicated. DNA polymerase is set off from portions of the DNA known as origins of replication. In order for DNA polymerase to bind to origins of replication, pre-replication complexes have to be prepared. To move from G1 phase into S phase of the cell cycle, the origins of replication fires so the levels of Cyclin D/cdk4 complex rises. The transition of stage only occurs when Cyclin D/cdk4 complex phosphorylates the retinoblastoma protein to inactivate it. p15 and p21 are examples of tumour suppressor genes which are regulated by the R-Smad/coSmad complex. p15 binds to Cyclin dependant kinase 4 (CDK4) which are required for pre-replication complexes. This therefore stops cells dividing rapidly into tumours. p21 on the other hand also binds with Cyclin D but can also bind with Cyclin A which is crucial for the S phase going into G2 phase. These two genes can down regulate the division of cells.
It is found that people who have tumours have either mutation or deletion for Smad4, T?RII, or Smad2. Colon and gastric cancers for example have inactive T?R-II with microsatellite instability. In which suggests the significantly role that TGF-? signalling plays in tumours suppression. (Kretzschmar, 2000) Without the functioning signalling pathway there is no control of the tumour allowing proliferation of cancer cells and fibroblasts. (Yang, 2009) However with Smad/TGF-? signalling still in tack it can arrest proliferation and induce cell death and hence limits the chance of tumours.
On the other hand, TGF-? can also be tumour promoters. They act upon the surrounding immune cells, endothelial and smooth-muscle cells. Thus causing immunosuppression, angiogenesis allowing the cancer cells to be more invasive. As the tumour starts to establish itself in the tissue, TGF-? is expressed in large quantities. This supresses the inflammatory reaction where it stops the immune cells, by inducing T cells and natural killer cells to cell apoptosis. (LI, 1999) Thus allowing itself to grow into larger size. When more TGF-? are expressed it uses matrix metalloproteinases (MMPs) which causes stromal cells to produce more matrix. When the tumour mass is too great the blood supply is cut off and will stop the cells from dividing. TGF? type I receptors (ALK1 and ALK5) are both expressed. This leads to Smad2/3 pathway activation resulting in angiogenic resolution. In addition, TGF?/ALK5 induces Smad1/5 and generates transcriptional responses for angiogenesis. (Lebrun, 2012) This is where Vascular endothelial growth factor (VEGF) promotes growth of new blood vessels. Providing the tumour with the needed nutrients and oxygen.
Furthermore, Metastasis is the spread of tumour from one organ to another. Epithelial mesenchymal cell transition (EMT) are when epithelial cells downregulate E-cadherin, Zonula Occludens (ZO) cell-cell junction dissolution and lose apical-basolateral cell polarity to acquire a mesenchymal cell phenotype. For this to happen it relies on surrounding cells to produce TGF-? which activates R-Smad/coSmad complex. (Wang, 2014) The R-Smad/coSmad complex then activates Snail, Zeb and Twist transcription factors which supresses the production of E-cadherin. (Heerboth, 2015) This is one of the problems which cancers can have, where they become invasive and metastasis mesenchymal cells. The tumour becomes more migratable when it’s in this state and travels by intravasation in the lymphatic and or circulatory systems into other organs. When reaching the second organ site they under mesenchymal epithelial transition (MET) which is the reverse process to EMT. MET occurs because they need to be stationary to proliferate, whereas mesenchymal cannot.
When testing with chicken hearts, having TGF-? blocked with antibodies and or antisense oligonucleotides, inhibits EMT in chicken atrioventricular explants, whereas when the TGF-?2 was recovered EMT was induced. (Xu, 2016)