Speaker: Geert Berx
Department: JNI Oncology
Location: Erasmus MC Rotterdam
Author: Katja Slangewal
Dr. Geert Berx was one of the people how helped discovering E-cadherin as aberrantly regulated in cancers. E-cadherin is for instance lost or partially lost in invasive lobular breast cancers. However, this type of breast cancer only represents 15% of all the breast cancer types. Dr. Berx went on in studying the transcriptional regulation of the protein. He found that mutations in specific E-boxes right before the transcriptional start site can lead to the loss of E-cadherin expression in epithelial cells. This brought him to a protein called ZEB2, an important transcription factor.
ZEB2 is, together with ZEB1, part of a small but evolutionary conserved family. Both ZEB1 and ZEB2 have multiple interaction partners. One of which is E-cadherin. ZEB2 represses E-cadherin expression. This has been shown by inducing a knock-out of ZEB2. This resulted in an upregulation of E-cadherin, leading to mis-expression. On the other hand, a conditional up regulation of ZEB2 leads to the loss of E-cadherin expression. The same is true for another transcription factor called Snail. Dr. Berx and his group have focused on ZEB1, ZEB2 and Snail as transcription factors regulating E-cadherin expression. The loss of E-cadherin has a cancerous effect. The hallmarks of invasive cancer cells are occurring. This means that cells are undergoing EMT (epithelial mesenchymal transition, figure 1). This transforms the cells in invasive cells.
Figure 1: Epithelial to Mesenchymal transition. The main processes and related proteins are indicated in the picture. http://www.ijournalhs.org/article.asp?issn=2349-5006;year=2015;volume=8;issue=2;spage=77;epage=84;aulast=Angadi
EMT is a process controlled by four major interconnected regulatory networks:
- Post-translational control
- Transcriptional control
- Differential splicing
- Non-coding regulation
So far it was mainly thought that cells are either in a stable epithelial state, an unstable transition state or a stable mesenchymal state. However, dr. Berx has shown eight metastable transition states.
EMT is not only associated with invasive cancers. Dr. Berx also shows a connection to stemness of cells. This can be explained by taking a look at the reprogramming of iPSCs (induced pluripotent stem cells). When firoblasts are being reprogrammed towards iPSCs, they undergo an epithelial intermediate. This state is reach by a decrease of for instance Snail. Snail has also been shown to regulate stem cell maintenance in the intestines. ZEB2 on the other hand has been shown to be important for stem cell maintenance in embryonic hematopoiesis. When ZEB2 is knocked- out, stem cells accumulate and leukocytes, erythrocytes ad platelets are reduced. This means that ZEB2 is not necessary for stem cell maintenance, but it is necessary for faith determination.
In the recent months, dr. Berx has mainly focused on ZEB2 in melanoma cancer types. Melanomas are a product of EMT, so this makes it interesting for ZEB2 expression. A ZEB2 knock-out mouse has no pigment. This indicates that ZEB2 functions in melanocyte differentiation. A very interesting observation was made during this experiment. When ZEB2 levels decrease, ZEB1 levels automatically increase. An immunostaining revealed that ZEB1 mainly localizes in the stem cell compartments, whereas ZEB2 is mainly found in the differentiated states of melanocytes. Also, an overexpression of ZEB1 leads to a dedifferentiated gene signature in primary melanocytes. A double knock-out of both ZEB1 and ZEB2 lead to growth arrest, indicating that at least one of them is necessary for a cell to function.
In short, an increase of ZEB1 leads to a highly invasive signature and almost no proliferation. An increase of ZEB2 leads to a low invasive signature and high proliferation. This observation led to the following model:
Figure 2: The oscillating model proposed by dr. Berx and his group.
In the primary tumor ZEB2 is high. When invasion starts and EMT is happening ZEB1 takes over. Then in the metastasis ZEB2 expression increases again. So ZEB2 drives melanoma proliferation and differentiation. Enhancing the ZEB2 expression does not lead to an increased melanoma frequency. However, it does lead to an increased metastasis formation. For this to happen an oscillation between ZEB2 and ZEB1 expression is necessary. This is regulated on protein level by TNF. TNF probably marks one of the ZEB proteins for degradation. However, how this regulation exactly takes place is not known.
After reading this you will probably have noticed this: increase of ZEB2 leads to increase of metastasis. So, shouldn’t there just be a drug that inhibits ZEB2 expression? Unfortunately for drug design, biology is not that simple. ZEB2 does not only have an important role in E-cadherin expression. It is also required for natural killer cell maturation. This means that an inhibition of ZEB2 will cause an immature immune system, leading to many more problems. Luckily, there are many more options to investigate. Recently, it has been shown in a different cell line, that an increased expression of ZEB2 marks the cells sensitive to a specific drug. This observation is useful to prevent giving unnecessary medicine to patients. Dr. Berx and his group will focus on this feature in melanomas in the near future.
My final seminar, was an interesting one. Dr. Berx had a clear story. Just like my very first seminar, there were a lot of technical terms embedded in the talk. I am very happy to see the difference in how far I could understand this talk compared to two years ago. It makes the talks a lot more interesting if you can understand them properly.