Inhibition of DNA-dependent protein kinase reduced palmitate and oleate-induced lipid accumulation in HepG2 cells

Abstract

The aim of this study is to investigate the involvement of DNA-dependent protein kinase (DNA-PK) in palmitate and oleate-induced lipid accumulation in hepatocytes. We treated HepG2 with free fatty acids (FFA) (0.33 mM palmitate and 0.66 mM oleate) mixture to induce lipid accumulation. Cellular lipid was determined by Nile Red staining followed by flow cytometry detection as well as phase contrast and fluorescence microscope examination. Cell viability was detected by MTT assay. Apoptosis was detected by DAPI staining. Lipogenic gene expression was examined by real-time PCR at mRNA level and Western blotting at protein level. Promoter transcriptional activity was measured by dual luciferase assay. FFA treatment neither affected HepG2 cells viability nor induced DNA fragmentation, while induced cellular lipid accumulation was associated by the upregulation of sterol regulatory element-binding protein-1 (SREBP1) and fatty acid synthase (FAS) at both mRNA and protein levels. Interestingly, we also found that both the protein phosphatase 2A (PP2A) protein expression and DNA-PK activity were increased in these cells. Inhibition of PP2A by okadaic acid, knockdown of DNA-PK by siRNA or inhibition of DNA-PK by specific DNA-PK inhibitors curtailed the FFA-induced upregulations of the SREBP1 mRNA expression and the nuclear active SREBP1 protein expression, and reduced FFA-induced upregulation of FAS promoter transcriptional activity and lipid accumulation. This is the first time suggesting that inhibition of DNA-PK reduced FFA-induced lipid accumulation in hepatocytes. This finding might help us better understand non-alcoholic steatohepatitis pathogenesis.