Development of novel BCL-2 inhibitors for glial tumors by using in vitro and in vivo systems

Abstract

Glioblastoma Multiforme (GBM) is the most malign form of glial tumors, which accounts for the majority of brain tumor cases worldwide. There have been different approaches to treat GBM effectively, and with the advancements made for the last decade molecular pathology, target driven therapy, and personalized medicine gained attraction. One of such promising targets for GBM is Bcl-2 induced intrinsic apoptosis pathway. Anti-apoptotic members of Bcl-2 induced intrinsic apoptosis pathway have an important role in the regulation of GBM cell death. In this thesis study, we screened seven potential Bcl-2 inhibitor compounds and evaluated their effects on proliferation of GBM cells as well as their inhibitory capacity of Bcl-2 protein. Of those, I further analyzed three of them namely 58, 243, and ind-199. 58 and ind-199 compounds did not show any significant anti-proliferation effect on GBM cells. Eventually, we decided to elucidate the mechanism of action of 243 compound, a thiazolidine derivative BH3 mimetic, which was the most promising one according to the in vitro proliferation experiments. I performed colony formation assay to assess proliferation of YKG1 GBM cells, additionally to the proliferation assay with A172 GBM cells. While 243 inihibited cell growth significantly compared to control group, Bcl-2 inhibitor ABT-199 did not inhibit cell proliferation. Moreover, I tested 243 on YKG1 tumorspheres to determine its effectivity on tumor initiating cancer stem cells (CSC). Both ABT-199 and 243 had inhibitory effect on CSC proliferation, however 243 was significantly more effective than ABT-199 when compared to control group. Since 243 is a Bcl-2 inhibitor, I analyzed key players of Bcl-2 family and intrinsic apoptosis pathway. I have analyzed gene expression levels of BCL2, BCLXL, BAX, CASP3, CASP7, and CASP9. Furthermore, I also analyzed genes related with cell death which are CASP8 and TP53. Time dependent quantitative RT-PCR results suggested that, GBM cells that are treated with Bcl-2 inhibitors ABT-263 and 243 acts differently in case of gene expressions related to apoptosis. Next, we wanted to show apoptotic cell death with Annexin V-PI assay. Interestingly, we did not detect significantly elevated apoptosis in A172 cells when they are treated with either ABT-199 or 243. Similarly, cell cycle analysis showed that 243 did not have any effect on cell cycle, altough ABT-199 induced G1 phase arrest. Moreover, I determined expression levels of apoptosis related proteins PARP, Caspase-3, and Caspase-9. I used staurosporine treatment as a positive control to induce apoptosis. None of the treatment groups apart from staurosporine increased cleaved-PARP expression. Similarly, I checked if there is a difference in expression of Pro-caspase-3 and Pro-caspase-9, and observed that only stauroporine treated group expressed lower levels of Pro-caspases, indicating that cleaved forms of both Caspase-3 and 9 were produced upon staurosporine treatment only. At this point, we hypothesized that both ABT-199 and 243 could only induce limited apoptotic cell death because BCL2 expression was relatively low in A172 cell line. Expectedly, when I compared gene expression levels among different cell lines, I observed that BCL2 expression was very low in A172 cells, and it was abundant in SH-SY5Y neuroblastoma cells. Therefore, I decided to analyze apoptosis of SH-SY5Y cells after a treatment with ABT-199 and 243. Within only 48 hours of treatment with both inhibitors, I observed apoptotic cell death of SH-SY5Y cells. Hence, we had a new hypothesis that when BCL2 expression is low, upon Bcl-2 inhibitor treatment, cells may die through autophagy since Bcl-2 forms a complex with autophagy related protein Beclin 1. I showed that 243 treatment significantly upregulated autophagy related genes such as BECN1, ATG5, and MAP1LC3B, whereas ABT-199 induced autophagy on limited level. Moreover, autophagy indicative LC3B-II expression was significantly upregulated on a protein level with the 243 treatment, when compared to control as well as ABT-199 treatment. Additionally, I determined protein expression level of p53, which has a role in the interplay between apoptosis, cell cycle, and autophagy. I observed that p53 protein expression was increased upon both ABT-199 and 243 treatment, when compared to control group. Expectedly, when we performed in silico computational analysis, Beclin 1:Bcl-2 interaction and binding of 243 to their BH3 binding domains, we observed that 243 binds to Bcl-2 through important interactions. Since 243 and Beclin 1 binds to Bcl-2 from the same domain, when cells are treated with 243, Beclin 1 cannot bind to Bcl-2 and therefore it is released to initiate autophagy. In addition, we demonstrated that 243 significantly reduced in vivo tumor growth and prolonged survival in orthotropic brain tumor models, compared to vehicle group as well as ABT-263 treated animals. Furthermore, I assessed the anti-proliferative effects of 243 on primary glial cell lines as well. 243 exerted anti-proliferative effect on all patient derived glioma cell lines that have different grades and histopathology, except OLG3 cell line which is a grade 2 oligodendroglioma. According to quantitative RT- PCR results of OLG3, OLG7, and GBM9 cell lines I observed that OLG3 has a lower expression level of BCL2. These results suggest that patients with high BCL2 expression might benefit from 243 treatment. Taken together, our results indicate that 243 disrupts Beclin 1:Bcl-2 complex, hence activates autophagic cell death, and may serve as a potential therapeutic for the treatment of GBM.