**4.1. Evidence-based studies**

Gliomagenesis is still an unknown, incurable, and lethal process. New and effective treatment strategies are the necessity and understanding the gliomagenesis is essential in order to develop these options. Experimental evidence indicates that antioxidants are sometimes friend, and in some cases, they are the foe.

### *4.1.1. In vitro studies*

In 1995, Zhu et al. carried out a study to clarify the effects of selenium on rat and human glioblastoma multiforme cell lines. They used sodium selenite and showed that selenium had anti-proliferative effects on both A172 human glioblastoma cells and C6 rat glioblastoma cells, but it was more effective on human glioblastoma cells [39].

In 1997, Vartak et al. showed that some polyunsaturated fatty acids: gamma-linoleic acid (GLA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA) supplementations increased the radiosensitivity and also radiation response on 36B10 rat astrocytoma cells [40].

In 1998, Vartak et al. compared the effects of GLA and linoleic acid (LA) on 36B10 malignant rat astrocytoma and normal rat astrocytes. They found that GLA was cytotoxic for astrocytoma cells, but not astrocytes. LA was not effective for both cells. It suggested that GLA might be used for astrocytoma treatment [41].

In 1999, Arora-Kuruganti et al. examined roles of oxidant (H<sup>2</sup> O2 ) and antioxidant (N-acetylcysteine, NAC) on U373-MG astrocytoma cell line. They observed that tumor cell proliferation was inhibited by NAC. NAC also induced H2 O2 [25].

In the beginning 2000s, the first study came from Rooprai et al. They checked some antiinvasive and anti-proliferative agents: swainsonine, captopril, tangeretin, and nobiletin on four different glioma cell lines: ependymoma, oligoastrocytoma, anaplastic astrocytoma, and glioblastoma multiforme. Firstly, they observed that each cell line showed difference response against agents. They found that the most effective agent was nobiletin on four different cell lines and it was decreased the MMP-2 and -9 secretions [42].

In 2001, Naidu et al. studied the effects of ascorbyl stearate (Asc-S) on human glioblastoma multiforme cells. They used different doses of Asc-S on T98G human glioblastoma cell lines for 24 h. They showed that Asc-S inhibited insulin-like growth factor-dependent cell proliferation in a dose-dependent manner. Asc-S modulated IGF-R expression, in consequence of this situation programmed cell death was triggered on T98G cells by Asc-S [43].

In 2007, Rooprai et al. studied on IPSB-18 human astrocytoma cells. They treated cells with selenite and found that selenite was altered the expressions of matrix metalloproteinases and their inhibitors. It was also decreased the epidermal growth factor (EGFR) expression. This was suggested that selenite had anti-metastatic effects [44].

In 2013, Pozsgai et al. studied on quercetin effects on glioblastoma standard treatment. They found that combination treatment provided significant reduction in cell viability in U251 and DBTRG-05MG glioblastoma multiforme cell lines. They also showed that quercetin alone, or in a combination with IR triggered the apoptosis [29]. In 2016, Lou et al. found that quercetin nanoparticles stimulated the autophagy and apoptosis by activating AKT/erk/caspase 3 signaling pathway [45].

In 2017, increasing cell proliferation of glioblastoma multiforme cell lines with low doses of selenomethionine was showed by Harmanci et al. [46].

The combination of berbamine and paclitaxel were decreased the cell proliferation on U87 glioblastoma multiforme cells [47].

Higher levels of ascorbate led the DNA strand breakages by creating genotoxic and metabolic stress on glioma cells, but it also caused the development of radioresistance [48].
