Therapeutic Stress Influences the Symmetric Cell Division in Glioblastoma

Glioblastoma (GBM) is the most common and aggressive primary brain malignancy. Currently, GBM remains incurable because of its profound ability to recur and resist conventional therapies. Our lab and others have shown that GBM contains a subpopulation of tumor progenitor cells deemed glioma stem cells (GSCs) that drive the therapeutic resistance capabilities of recurrent GBM. We have shown that the GSC population expands following administration of temozolomide (TMZ), the standard of care chemotherapeutic agent for GBM patients. While it has been suggested that this expansion relies solely on selection, we hypothesize this process depends on the ability of TMZ-induced therapeutic stress to modulate cellular division in GSCs. GSCs can divide in three ways: (1) asymmetric division, which produces one GSC and one differentiated cancer cell, (2) symmetric division that promotes self-renewal by yielding two daughter GSCs or (3) symmetric division that support differentiation by resulting in two mature cancer cells. Preliminary studies from our laboratory found that asymmetric division occurs in 64% of divisions when cells are treated with vehicle, yet in only 30% of divisions when cells are treated with TMZ. This study also revealed that symmetric self-renewing cell division—where one GSC gives rise to two daughter GSCs—occurs at a rate of 24% in GBM cells treated with vehicle, yet at a rate of 54% following TMZ therapy (p<0.0001). Gene expression analysis during therapeutic stress revealed that the LNX1 pathway, a Notch co-regulator known to control symmetric divisions, increases 8.7-fold in patient-derived xenograft (PDX) cells (p<0.0001). Furthermore, a comparative examination of PDX cells under therapeutic stress showed that Notch antagonist Numb is significantly downregulated. Based on these observations, we propose that therapeutic stress mediates alterations in the Notch pathway that allow symmetric self-renewal to become more prevalent in GBM. We believe that the shift from asymmetric to symmetric cell division of GSCs is a key mechanism by which GSCs expand under therapeutic stress. Given GSCs’ ability to resistance therapy, it is critical that this shift is halted.