Inhibition of lung tumorigenesis and cancer cell growth by EGCG and other chemopreventive agents

Lung cancer is the leading cause of cancer death in the United States and the most commonly diagnosed cancer worldwide and smoking is a key risk factor of lung cancer. The purpose of this thesis study is to characterize the mechanisms of 4-(methylnitrosamino)-1-(3-pyridyl)- 1-butanone (NNK)-induced lung tumorigenesis in A/J mice and the inhibitory activities of (−)-epigallocatechin-3-gallate (EGCG) and other chemopreventive agents including tocopherols, myo-inositol and atorvastatin. DNA methyltransferase 1 (DNMT1), a key enzyme mediating DNA methylation, is known to be activated in cancers, including the mouse lung tumors induced by NNK. However, it is not known how NNK treatment affects the DNMT1 expression. We found that administration of NNK to A/J mice caused elevation of DNMT1 in bronchial epithelial cells at days 1-14. DNMT1 elevation at days 1 and 3 was accompanied by an increase in -H2AX and phospho-AKT (p-AKT), which has been shown to stabilize DNMT1 at the protein level by inhibiting the ubiquitination of DNMT1. Induction of O6-methylguanine, cyclooxygenase-2 (COX-2), superoxide dismutase (SOD) and catalase by NNK treatment was also observed at days 1-14. DNMT1 expression decreased to lower levels at weeks 5-20 in lung tissues, but was highly elevated in lung tumors at week 20. In concordance with DNMT1 elevation, promoter hypermethylation of tumor suppressor genes Cdh13, Prdm2 and Runx3 was observed in lung tissues at day 3 and in lung tumors at week 20. EGCG (0.4% in diet) treatment attenuated the induction of DNMT1, p-AKT and γ-H2AX at days 1 and 3 and inhibited lung tumorigenesis. EGCG, γ-tocopherol-rich mixture of tocopherols (γ-TmT) and myo-inositol significantly reduced tumor multiplicity and tumor size in the NNK model. The EGCG-generated reactive oxygen species (ROS) caused cell growth inhibition and induction of apoptosis and oxidative DNA damage in vitro. Apoptosis and oxidative DNA damage were also induced in lung tumors by EGCG treatment at the tumor stage. Short-term γ-TmT treatment at the tumor stage induced antioxidant enzymes SOD and glutathione peroxidase. Myo-inositol treatment significantly reduced the level of p-c-Jun in lung tumors.Ph. D.Includes bibliographical referencesby Huanyu Ji