Parry, Phillip V.; Engh, Johnathan A.
It is well known that human glioblastoma multiforme (GBM) contains rests of tumor initiating cells known as cancer stem cells (CSCs).1 Both non-cancerous neural stem cells (NSCs) and CSCs share cell signaling pathways and surface markers that permit cell growth; however, the links between the NSCs and CSCs have not been clearly defined.2 Identifying these signaling pathways and surface markers has been a major focus in the molecular characterization of GBMs because of their potential to be therapeutic targets. Previous work identified the transcription factor Hes3 as a marker of established NSCs in the adult rodent and primate brain.3,4 Recent work from Park et al demonstrate the critical role of Hes3 as an identifying marker for the CSC population in GBM cell cultures and a potential target for treatment of GBM.5
In order to determine the value of Hes3 as a potential therapeutic target, the group began by studying the role of Hes3 in NSCs. These experiments showed that Hes3 is upregulated by signals that promote STAT3-serine phosphorylation in the absence of STAT3-tyrosine phosphorylation. In NSCs, bFGF, insulin, Notch ligands and Angiopoietin 2 induce such STAT3-serine phosphorylation. Upregulation of Hes3 by these mechanisms led to an increase in the number of NSCs. Conversely, they showed that signals such as Ciliary Neurotrophic Factor (CNTF), EGF and serum components activated the JAK/STAT pathway, leading to STAT3-tyrosine phosphorylation. As a result, these signals did not induce Hes3 expression.
Therefore, they hypothesized that GBM cells cultured in the presence of bFGF and JAK I would produce similar Hes3 expression as seen in NSCs. GBM cells tested in the presence of FGF and JAK I demonstrated the highest STAT3-Ser and lowest STAT3-Tyr phosphorylation. Under these conditions, the cells were able to proliferate rapidly. Additionally, Hes3 expression was the highest in GBM cells grown under these conditions.
The researchers then investigated the role of Hes3 in the expansion of GBM in order to determine its potential as an anti-cancer target. The GBM cell lines were grown in bFGF and JAK I conditions, then challenged with 2 separate short interference RNAs (siRNA) to reduce Hes3 expression (Figure 1). All cultures transfected with either of the Hes3 siRNAs had reduction of Hes3 expression and a reduction in cell number. Additionally, Hes3 siRNA-transfected GBM cell survival assays demonstrated a reduction in cell viability.
In conclusion, the team should be heralded for their cumulative efforts spent over the past eight years in elucidating the elaborate cellular signaling pathways of CSC in GBM. The researchers have defined two essential new aspects about the molecular signaling pathways of gliomas: 1) Primary GBM cell cultures express Hes3 under conditions that support STAT3-Ser phosphorylation while suppressing STAT3-Tyr phosphorylation; 2) Hes3 interference reduces cell viability and the numbers of cells in culture. Delineating these important intercellular signaling pathways of GBM proliferation will hopefully assist in the development of GBM specific molecular therapies, thereby providing new treatment options for patients harboring this ominous disease.
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