New Insights into the Control of Stem Cells: Keeping the Right Balance

“Proteins of this pathway are active on a variety of stem cells, including neural, mammary, and embryonic stem cells”, Professor Dr. Roel Nusse from Stanford University, California, USA, pointed out in his key note lecture at the opening of the international conference on “Wnt signaling in Development and Disease” at the Max Delbrück Center for Molecular Medicine (MDC) Berlin-Buch, Germany. He is one of the pioneers in this field.

There are a few signaling pathways active in embryogenesis. All are vital for the development of the embryo into a healthy organism and for maintaining life functions in the adult. These pathways operate from the cell surface to the cell nucleus with its DNA. When disrupted through mutations, severe diseases can result, such as cancer, (e.g., colon cancer), brain and heart disorders, as well as other diseases.

One of the best studied pathways today is the Wnt signaling pathway. It plays a crucial role in embryonic development, cell growth (proliferation), and maturation of cells into specialized cells (differentiation). This pathway is also an important regulator of stem cells.

Dr. Nusse laid the foundation for this field of research, when in 1982, he discovered a gene in mice he named int1 for the integration of a tumor virus into the cellular DNA. It turned out to be identical to a gene Professor Dr. Christiane Nüsslein-Volhard, Nobel Laureate from 1995, had discovered earlier in the fruitfly Drosophila melanogaster.

That gene was termed wingless (wg), because fruit flies which carry this mutated gene lack wings. Dr. Nusse suggested fusing these two names to form “wnt” (wingless plus int). Since that time, roughly 20 genes of the Wnt like genes have been discovered. But the entire pathway encompasses many dozens of genes.

Moreover, it was known that the different signaling pathways do not simply operate independently but also cross-talk with each other. Hence, dysfunctional cross-talk between pathways also plays a significant role in the development of disease.

Stem cells have the potential to develop into various cell types. They are the source for renewal of short-lived cells such as blood cells or epithelial cells, which both form the skin and line inner organs such as the mammary glands or intestine. Stem cells are also the basis for the body to regenerate after injury.

“Wnt proteins maintain the undifferentiated state of stem cells”, Dr. Nusse pointed out. In other words, Wnt proteins ensure that the reservoir of stem cells does not “dry out”.

Once the body makes use of the stem cells, they have to grow. “However, other growth factors instruct the cells to proliferate”, Dr. Nusse said. They include factors such as FGF (fibroblast growth factor) and EGF (epidermal growth factor). They regulate a different pathway, which involves tyrosine kinases, enzymes which activate or inactive proteins. A few years ago the cancer drugs Trastuzumab (brand name Herceptin) and Imatinib (brand name Gleevec), which inhibit the mutated tyrosine kinase pathways, revolutionized treatment of breast cancer, leukemias and gastrointestinal stroma tumors.

There is also an interaction between Wnt signaling and tyrosine kinases that enables the proliferating cells to mature into specialized (differentiated) cells. “This dynamic interaction ensures the right balance between proliferation and differentiation”, the Dutch cell biologist pointed out.

If this system gets out of balance, normal cells can turn into tumor cells as is seen in breast and colon cancer. In 90 percent of all human colon cancer, one of the core components of the Wnt pathway is deregulated, namely the tumor suppressor APC (adenomatous polypolis coli).

In normal cells, the signaling protein beta-catenin is chained to APC and other proteins, which target it for degradation in the cells waste basket, the proteasome. If APC is mutated, beta-catenin gets rid of its chains, accumulates in the cell’s cytoplasm and moves into the cell’s nucleus. There, it binds to the T cell factor (TCF) and turns on genes. This process is considered the trigger for the onset of breast and colon cancer.

Restauration of normal cell program in intestinal adenomas

Professor Dr. Hans Clevers from the Hubrecht Laboratory and Centre for Biomedical Genetics in Utrecht, The Netherlands, investigates how the translocation of beta-catenin from the cytoplasm into the cell nucleus turns normal colon cells into colon cancer cells. Some years ago, he discovered the interaction between beta-catenin and TCF, a discovery also made at the MDC in the research group of Professor Walter Birchmeier.

As Dr. Clevers pointed out in Berlin, this translocation process of beta-catenin and its binding to TCF is believed to represent the primary transforming event in colon cancer. “Yet, the consequence of the presence of mutationally activated beta-catenin/TCF in colon cancer cells is still unknown”, he said.

Dr. Clevers and his team constructed colon cancer cell lines in which they could induce or inhibit TCF constructs. They could show that beta-catenin/TCF inhibits differentiation of colon cells. Cells which do not differentiate run out of control and grow indefinitely, characteristic for cancer cells.

Interestingly, when the researchers inhibited beta-catenin/TCF, they could restore the differentiation program, despite the presence of many other mutations in the colon cancer cells. In addition, they were able to demonstrate that not only the Wnt cascade controls the fate of a cell, but also another signaling pathway, the Notch pathway.

When Dr. Clevers and his colleagues inhibited this Notch pathway with gamma-secretase inhibitors, they could induce cell differentiation in intestinal adenomas, benign tumors which can become malignant. “Our data imply that gamma-secretase inhibitors, developed for Alzheimer`s disease, may be of therapeutic benefit in colorectal neoplastic disease”, he reported.