Roles of Sp1 and protein kinase C in regulation of human serum paraoxonase 1 (PON1) gene transcription in HepG2 cells

Human serum paraoxonase 1 (PON1) is associated with high-density lipoprotein, and inhibits oxidative modification of low-density lipoprotein in vitro. Therefore, PON1 is supposed to protect against atherosclerosis in vivo. In this study, we investigated the direct effect of Sp1 on PON1 transcription in HepG2 cells using a reporter gene assay. A deletion analysis of the PON1 upstream region revealed that dominant promoter elements were present within a sequence between −269 and −97 bp, which contained a consensus binding site for Sp1, and an electrophoretic mobility shift analysis (EMSA) indicated the Sp1 binding to the upstream sequence. In accordance with this, overexpression of Sp1 dramatically enhanced PON1 promoter activity, and the Sp1 inhibitor mithramycin inhibited Sp1-induced promoter activation in a dose-dependent manner. The basal promoter activity was also enhanced by phorbol 12-myristate 13-acetate (PMA), and synergistic promoter activation was observed when Sp1-transfected cells were treated with PMA. The PMA-induced promoter activation was inhibited by mithramycin. In addition, overexpression of the dominant negative version of PKCα or ζ, significantly reduced PON1 promoter activity. These data suggest that Sp1 acts as a positive regulator of PON1 transcription, and that an interaction between Sp1 and PKC is a key mechanism for the effect of Sp1 on PON1 transcription.