Within a tumor exists a subpopulation of cells that present less abnormal characteristics; however, there exists another type that hold the potential to recreate and expand tumor mass. The latter share similar characteristics to normal stem cells, which are the unlimited ability to self-renew and the ability to maintain undifferentiated. Over the course of time, these cells will differentiate into tumor stem cells (TSC) or tumor-initiating cells with stem-like properties (TICs). Researchers previously identified TICs in glioblastomas and confirmed that TICs share functional similarities to neural stem cells (NSCs).
A similarity is that, in both cell types, BMP is a key inhibitor regulatory of TICs from glioblastomas. On the other hand, a difference is that TICs undergo genetic aberrations that are similar to tumors, which in turn give rise to the typical tumor phenotype [1]. What still remains unknown is during which point in the developmental process in tissues do TICs form and also what are the intrinsic cell signaling pathways that are disrupted in TICs. To answer this, they looked into the potential aberrant methylation in the BMPR1B promoter in 0308-TIC cell line.
Bisulfite treating the DNA in 0308 cells showed that the CpG islands are normally heavily methylated. However, this can be reversed by treating cells with 5- Azacytidine (5’azaC). First studied in 1979 by fellow USC researchers, 5’azaC is an FDA- approved chemotherapeutic agent that is also a DNA methylation inhibitor [4]. By forcing demethylation in 0308 cells, there was a significant increase in BMPR1B mRNA expression, suggesting that the control or untreated 0308-TICs do not typically express BMPR1B [1]. Furthermore, the enhancer of zeste homolog 2 (EZH2) is a key component of the polycomb repressive complex 2 (PCR2).
Together, they facilitate methylation and are highly expression in tumor cells and healthy stem cells [1]. In Lee et. al. ’s study, they used the siRNA technique to knockdown EZH2 expression in 0308 cells, which caused significant reductions in Notch1 mRNA levels, reduced cellular proliferation, and lost tumorigenicity. Notably, they discovered that the Notch1 pathway is required for EZH2-mediated breast stem cell expansion. To assess methylation patterns in the EZH2-siRNA treated cells, they found that there was an increase in BMPR1B expression and the normally significantly methylated methylated CpG islands became demethylated.
Altogether, these suggest that finding a way to block EZH2 expression in breast cancer (perhaps using or incorporating the mechanism behind 5’azaC) could possibly be a treatment for women who suffer from breast cancer. While knocking down EZH2 has the potential to demethylate the BMPR1B promoter and lose tumorigenicity in TICs and induce glial differentiation in glioblastomas, it also plays an important role in other organ systems. More specifically, it has been studied in breast stem cells.
Gonzales et. al. ’s primary goal was to better understand how EZH2 promotes breast cancer initiation and the underlying mechanism. One of the techniques they used was the MMTV (mouse mammary tumor virus)-neu mouse model to study the effect of EZH2-overexpression. The EZH2+:neu mice showed a myriad of negative side effects, including increased cell proliferation, atypical epithelial hyperplasia, and accelerated tumor initiation. Because EZH2 is known to promote Notch1 expression, these side effects are proposed to be associated with an increase in Notch1 signaling pathway [2].
While the above papers extrapolated EZH2 and Notch signaling, Xing et. al. shared some of the topics from those papers and studied the effect of Notch signaling pathway in human glioblastomas. This paper sought to specifically understand the association between Notch1 and the epidermal growth factor receptor (EGFR) gene and their overlying cell survival rate in human glioblastoma samples. When compared to healthy brain samples, the tumors exhibit more elevated Notch and EGFR mRNA and protein levels.
Further, FACS analysis showed that apoptosis in tumor tissues was almost a three-fold increase compared to normal tissue. Altogether, Xing et. al. found a positive correlation for Notch, EGFR, and apoptosis in glioblastomas [3]. In summary, the diseases covered in these papers are devastating and are in a dire need for an effective treatment.
Glioblastoma multiforme is one of the most aggressive brain tumors with a very low life expectancy of less than one year and breast cancer is one of the leading causes of cancer-related death in women [2,3]. Rather persuasively and well-executed, Lee et. al. showed that the knock-out of EZH2 has a high potential to ameliorate glioblastomas because this knock-out results in loss of tumorigenicity and increased glial differentiation.
Moreover, Gonzales et. al. built upon the existing knowledge of EZH2 and showed that, in the breast tissue, EZH2 up-regulation results in malignant breast cancer stem cell characteristics. Although the last paper is slightly weaker, its conclusions have the potential to further narrow down glioblastoma prognosis. In conclusion, these three papers accelerate the field of stem cell biology and regenerative medicine, and together, they have great potential to treat diseases.
References [1] Lee, Jeongwu, et al. “Epigenetic-Mediated Dysfunction of the Bone Morphogenetic Protein Pathway Inhibits Differentiation of Glioblastoma-Initiating Cells.” Cancer Cell 13. 1 (2008): 69-80. Web. [2] Gonzalez, Maria E. et al. “EZH2 Expands Breast Stem Cells through Activation of NOTCH1 Signaling. ” Proceedings of the National Academy of Sciences of the United States of America 111. 8 (2014): 3098–3103. PMC. Web. [3]. Xing, Zhen-yi, Lai-guang Sun, and Wu-jun Guo.
“Elevated Expression of Notch-1 and EGFR Induced Apoptosis in Glioblastoma Multiforme Patients. ” Clinical neurology and neurosurgery 131. 0 (2015): 54-8. Web. [4]! Taylor, Shirley M. , and Peter A. Jones. “Multiple New Phenotypes Induced in 10T12 and 3T3 Cells Treated with 5-Azacytidine. ” Cell 17. 4 (1979): 771-9.