P0028 Isoliquiritigenin causes DNA damage and inhibits ATM expression in oral squamous cell carcinoma

Abstract

Background Isoliquiritigenin (ISL) is a natural compound in licorice and has chemopreventive and anti-tumour activities. ISL induces tumour cell apoptosis and DNA damage in cervical cancer cells, and induces ATM-associated DNA repair signalling. However, ISL down-regulates ATM and phosphorylated ATM (p-ATM) in human oral squamous cell carcinoma (OSCC) cell lines. The aim of this study was to investigate the ATM inhibition mechanism of ISL in OSCC cell lines. Methods mRNA and protein expression were detected by reverse transcription-PCR (RT-PCR) and western blotting, respectively. The promoter activity was detected by firefly and renilla luciferase reporter system. DNA damage was analysed by terminal deoxynucleotidyl transferase dUTP nick-end labelling (TUNEL) assay and gamma-H2A.X protein expression. microRNA expression was detected by quantitative RT-PCR. Findings ISL induced OSCC cell cycle G2/M phase arrest, apoptosis, and DNA damage. However, the DNA repair-associated ATM and p-ATM proteins were down-regulated, ATM mRNA levels were unchanged, and the p-ATM downstream signals were inhibited. The down-regulated ATM protein expression might be caused by promoter activity inhibition, microRNA over-expression, and protein degradation. The ATM promoter in OSCC cell line was unchanged after ISL treatment. The microRNAs miR203a and miR421, which target ATM, were both down-regulated. When blocking caspase activity with Z-DVED-FMK, the down-regulation of ATM was reversed. Interpretation ISL-inhibited ATM expression was not regulated by miR203a and miR421 at a transcriptional level. The down-regulation of ATM was caused by ISL activating caspase. These data indicate that ISL induced apoptosis and inhibited DNA repair in OSCC. ISL might be a promising chemopreventive agent against oral cancer.