Molecular mechanisms of early growth response protein-1 (EGR-1) expression by quercetin in INS-1 beta-cells

Early growth response-1 (EGR-1), one of immediate early response genes, is involved in diverse cellular response. We recently reported that quercetin increased catalytic subunit of γ-glutamylcysteine ligase (GCLC) via the interaction of EGR-1 to GCLC promoter in INS-1 beta-cells. Therefore, this study investigated molecular mechanisms of quercetin-induced EGR-1 expression in INS-1 cells. Quercetin significantly induced EGR-1 protein and its mRNA expressions. This induction of EGR-1 was completely blocked by pretreatment with a PKA inhibitor, H89 and partially blocked by a p38 inhibitor, SB203580. Additionally, the siRNA-mediated inhibition of PKAα and p38 resulted in significant reduction of quercetin-induced EGR-1 promoter activity. Also, quercetin-induced EGR-1 protein expression was significantly decreased in the cells transfected with PKAα siRNA. Study using truncated EGR-1 promoter constructs showed that serum response element (SRE) sites, not cAMP response element site, were essential for EGR-1 transcription. However, electrophoretic mobility shift assay showed that quercetin did not affect the band intensity of DNA-protein complex on SRE site of EGR-1 promoter. Also, immune-shift assay using serum response factor (SRF) and phospho-SRF antibodies showed no difference between control and quercetin-treated groups. Collectively, quercetin-induced EGR-1 expression is largely dependent on PKA and partly on p38 MAPK pathway, and SRE sites of EGR-1 promoter are involved in quercetin-induced EGR-1 transcriptional activity.