İnsan Mide Adenokarsinoma Hücrelerinde Helicobacter Pylori'nin Neden Olduğu DNA Çift Sarmal Kırıkları ve Oksidatif Stresin Değerlendirilmesi

Abstract

Didem, O. Evaluation of DNA Double-Strand Breaks and Oxidative Stress caused by Helicobacter pylori in Human Adenogastric Carcinoma Cells. Hacettepe University Graduate School of Health Sciences, Pharmaceutical Toxicology Program, Doctorate Thesis, Ankara,2019. It is well-known that infections caused by various microorganisms lead to the formation of reactive oxygen species (ROS) and reactive nitrogen species (RNS). Several studies have shown that these species can change the intracellular oxidant/antioxidant balance and cause oxidative stress. Oxidative stress can cause DNA damage by many different mechanisms. DNA damage can be observed in a wide range, from simple and repairable base lesions to DNA double-strand breaks (DSBs), which are the most dangerous DNA damage types. Radiation and some bacterial toxins can cause carcinogenesis by inducing DSBs. Helicobacter pylori is a gram-negative bacterium that causes chronic infection in gastric mucosa. Approximately 50% of the world’s population has been reported to be infected with chronic Helicobacter pylori infection which causes gastritis, peptic ulcer, stomach adenocarcinoma and gastric mucosal lymphoma. Helicobacter pylori infection is categorized as “Group I carcinogen” by the International Agency for Research on Cancer. The mechanisms underlying the toxicity of Helicobacter pylori infection and the mechanism of gastric carcinogenesis caused by this certain bacterium are not fully understood yet. Studies show that Helicobacter pylori infection can cause genetic instability both by direct DNA damage and by epigenetic mechanisms. Helicobacter pylori infection causes oxidative damage nuclear and mitochondrial DNA of the host. In addition, it can lead to chromosomal instability, microsatellite instability, and mutations by affecting DNA methylation. There are studies in the literature that show Helicobacter pylori infection can cause DSBs, oxidative stress and may also affect DSB repair mechanisms. However, there are not many studies that investigate the association between Helicobacter pylori infection and oxidative stress. Studies to date have generally been conducted on clinical specimens and focused on the direct treatment of infection rather than focusing on the toxicity mechanisms of the bacterium. In the light of these data, the possible cytotoxicity, intracellular ROS elevations, and oxidative stress, DNA base lesions, changes in DNA repair proteins, apoptotic and autophagic changes caused by Helicobacter pylori in gastric adenocarcinoma (AGS) cell line were investigated in detail in this thesis. In addition, AGS cells which were infected with Helicobacter pylori were investigated for the presence of histone 2A family X (H2AX) phosphorylation (-H2AX), a biomarker of DSBs. On the other hand, by evaluating that DSBs caused by Helicobacter pylori can be repaired, a DNA-protein kinase (DNA-PK, an important DSB repair protein) inhibitor was also included to the study and the related parameters were measured. Thus, we planned to foresee the outcomes of Helicobacter pylori infection in people which disrupted DSB repairs. According to the results, Helicobacter pylori infection caused significant increases in oxidative stress, changes in DNA repair proteins and increases in -H2AX levels, particularly in the presence of DNA-protein kinase inhibitor. Moreover, Helicobacter pylori infection triggered apoptosis and autophagy and cell death was significantly higher in both DNA-protein kinase inhibitor and Helicobacter pylori -exposed group.