The Importance of Brain Tumor-Initiating Cells in Glioblastoma
A Q&A with Anita Hjelmeland, PhD, Associate Professor of Cell, Developmental and Integrative Biology at the University of Alabama Birmingham School of Medicine
Q: The inner workings of malignant gliomas are mysterious to many of us. Why does the prognosis of patients with these tumors remain poor?
A: Glioblastoma is a primary brain tumor that is treated with surgery, radiation, and chemotherapy. While surgical removal of glioblastoma is a goal, glioblastoma cells move into the normal brain where they cannot be removed and where many chemical therapies do not reach. The latter is due to a special wall of blood vessels called the blood-brain barrier that protects the brain from toxins. To overcome these obstacles, researchers are developing ways to identify tumor cells during surgery and break down the blood brain barrier for short periods of time.
Another reason that glioblastoma is difficult to treat involves the body’s defense against invaders—the immune system. The immune system usually will not attack cells it recognizes as “self” (if it does, autoimmune diseases will develop), but special immune cells can recognize and destroy infected self cells that have different proteins on the cell surface than do normal cells. Glioblastoma cells also have different-from-normal proteins that could be targeted by drugs, but glioblastoma cells often block the activity of immune cells. To improve the treatment of glioblastoma, there are clinical trials testing the effects of drugs or viruses that are designed to activate the immune system.
A final reason that glioblastoma cures remain elusive is that tumor cells are not all the same. There are different genetic or mutational features that could lead to resistance to any one targeted therapy. Glioblastoma cells also behave differently depending on their environment: for example, lower oxygen levels can promote resistance to radiation. Furthermore, glioblastoma cells can resemble specialized cells in the brain to differing degrees, reflecting a difference in stem-cell state. To improve our ability to target all tumor cells, researchers seek to identify ways to prevent therapeutic resistance and combine therapies to try to make them more effective.
Q: Your research delves seriously into the role of brain tumor-initiating cells (BTICs). What are BTICs, what do they do, and how might an understanding of them lead to improved therapies?
A: Glioblastoma cells can look and act more or less like normal stem cells in the brain, the neural stem cells. Neural stem cells are important during development and in brain diseases because they remake themselves, a process called self-renewal, and make specialized (differentiated) brain cells like neurons. Neural stem cells and differentiated brain cells can be distinguished by levels of different proteins called markers, which can also be expressed by glioblastoma cells. Glioblastoma cells with neural stem cell markers and the ability to self-renew or differentiate are called cancer stem cells or glioblastoma stem cells. Human glioblastoma stem cells have a greater capacity to cause tumors to initiate or grow in mice without an immune system: this tumor-initiating ability has led to the alternative name of BTICs.
BTICs can comprise only a small portion of the overall number of tumor cells present but could be especially important to eradicate. BTICs better survive chemo- and radiotherapy, and can live in environments where therapies are less effective. BTICs possess a high capacity to invade and readily move into the normal brain. Therefore, BTICs are believed to be the cells that remain after surgery, radiation or chemotherapy. Any such cells that remain after treatment, as their name denotes, can stimulate glioblastoma to grow anew. Thus, it is imperative that we make strong efforts to understand how to eradicate BTICs along with the more differentiated tumor cells in the hope of extending patient survival. Our ability to ever cure glioblastoma could rest upon that success.
It is important to note that glioblastoma is not alone in harboring tumor-initiating cells. By studying ways to combat BTICs in glioblastoma, progress is likely to be made towards understanding better therapies for other malignancies with such a capacity for therapeutic resistance and recurrence.
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