Glioblastomas are highly aggressive brain tumors with a very poor prognosis. Even with aggressive treatment, recurrence is nearly systematic. In an article published in Scientific Reports, researchers from IRCM (CEA-Jacob) have decrypted molecular mechanisms that enable treatment resistance and escape for a subpopulation of cells, and in so doing, identified a potential target to avoid recurrence. A patent application has been filed, giving hope for new therapeutic possibilities.
Glioblastomas are the most common primary¹ brain tumor in humans. They show high aggressiveness and have very poor prognoses. Despite standardized treatment associating surgery, chemotherapy and repeated radiation therapy, recurrence is, for all practical reasons, systematic. A particular sub-population of cells, called glioma stem-like cells (GSCs), appears to play an important role in recurrence and treatment resistance. GSCs have regenerative capacities comparable to those of normal neural stem cells².
Researchers from the
Genetic Stability, Stem Cells and Radiation mixed research unit (IRCM / CEA-Jacob) decided to take a look at the behavior of GSCs under exposure to ionizing radiation. In an article published in Scientific Reports, they showed that such radiation exposure specifically stimulates the migratory capacities and invasiveness of human GSCs, and furthermore shed light on the underlying molecular mechanisms.
The researchers performed in vitro experiments involving GSCs isolated from several patients and different experimental methods such as time-lapse videomicroscopy and cell invasion assays. They also performed in vivo tests by injecting irradiated CSGs into mouse brains. They reported that ionizing radiation that was not lethal for the GSCs stimulated these latter's migratory capacities, and that stimulation remained observable for at least several days after radiation exposure.
To determine why that was, the team studied the underlying molecular mechanisms and illustrated the implication of two molecules: hypoxia-inducible factor 1α (HIF1α, a transcription factor³) and the junction-mediating and regulatory protein (JMY). Experiments involving gene knockdown and pharmacological inhibition enabled the demonstration of GSC migration in response to ionizing radiation. That aspect was accompanied by a transitory accumulation of HIF1α in the cell nucleus, which drove in turn an accumulation of JMY (the expression of which is driven by HIF1α) in the cell cytoplasm. JMY is known primarily as a partner protein for p53⁴ but it is also involved in the assembly of the actin filaments of the cytoskeleton. And, indeed, the CEA researchers observed the densification of the actin network, which is strictly dependent on HIF1α and JMY, in GSCs as a response to ionizing radiation. Furthermore, in their work, the inactivation of JMY inhibited radiation-induced GSC migration, confirming the role of the HIF1α/JMY pathway in the phenomenon.
Ionizing radiation provokes an accumulation of the transcription factor HIF1α (white arrows, top image) in the nuclei of glioma stem-like cells (GSCs) (white arrows top-right image). The accumulation of the transcription factor leads to an increase in JMY expression, and that in turn to an increase in the polymerization of actin filaments (middle images). These molecular mechanisms enable the migration of GSCs when exposed to ionizing radiation, as observed in vitro (bottom-left image) and in vivo (bottom-right image). © L. Gauthier / F. Boussin
GSCs, with their "stem-cell like" characteristics, are rare within the tumor. To study the effects of sublethal ionizing radiation on the cells without their stem cell attributes, the researchers cultured the GSCs so as to provoke their differentiation and thus the loss of those attributes. Remarkably, the ionizing radiation was unable to stimulate migration in the resulting differentiated cells, demonstrating that the identified migratory mechanism was specific to the stemness of the GSCs. Furthermore, the HIF1α/JMY pathway was not stimulated in the differentiated cells when they were exposed to the ionizing radiation.
Standard treatment for glioblastomas comprises daily radiation therapy sessions continued over several weeks. The phenomenon detected by the IRCM team could cause certain cancerous cells to migrate away from the irradiation site and thus favor later recurrence.
HIF1α makes for a difficult therapeutic target because of its role in the regulation of the expression of numerous genes. Acting upon it would thus result in a plethora of adverse effects. JMY, on the other hand, may prove very useful as a therapeutic target for the prevention of recurrence after radiation therapy.
1 The term "primary tumor" describes the original tumor from which cancerous cells may escape and provoke metastases elsewhere in the body.
2 Stem cells are cells with some level of pluripotency, that is, an ability to differentiate into a range of cell types. They can be identified by their specific markers. Neural stem cell, for example, can differentiate into a range of cells present in the brain, such as nerve or glial cells.
3 A transcription factor is a protein that, within the cell nucleus, attaches to a specific DNA sequence to initiate or regulate gene expression, that is, the cellular process via which a genetic code is converted into an active molecule, usually a protein, in the cytoplasm of the cell.
4 p53 is a transcription factor involved in the regulation of a great number of cellular functions such as proliferation or apoptosis (programmed cell death). It is commonly considered to be a tumor suppressor gene. Indeed, p53 is found to be mutated in more than half of human cancers.