Dual-specificity phosphatase 6 interferes with the repressive activity of forkhead box O1 towards CYP4A11 that mediates lipid accumulation in the liver

Dual-specificity phosphatase 6 (DUSP6) is a phosphatase specific for extracellular signal-regulated kinase (ERK). Dusp6-knockout mice are resistant to diet-induced hepatic steatosis, which appears to be linked to the downregulation of cytochrome P450 4 A (CYP4A); however, its mechanism remains unclear. This study aimed to elucidate how DUSP6 regulates CYP4A11 in human hepatocyte-lineage cells by focusing on forkhead box O1 (FOXO1). HepG2 and HuH-7 cells were challenged with palmitic acid and oleic acid to induce lipid accumulation while manipulating the expression of DUSP6, FOXO1, CYP4A11, ERK, and/or AKT. Lipid accumulation was reduced by DUSP6 knockdown, resulting in decreased CYP4A11 expression despite elevated phosphorylated ERK, AKT, and FOXO1. Inhibition of ERK increased lipid accumulation, while simultaneous inhibition of ERK and AKT decreased it. Knockdown of FOXO1 or induced expression of DUSP6 increased CYP4A11 expression and lipid accumulation, whereas induced expression of FOXO1 decreased them. Chromatin-immunoprecipitation showed that FOXO1 bound to CYP4A11 promoter. Immunoprecipitations revealed that DUSP6 bound to and anchored FOXO1 in the cytoplasm. These results indicate that DUSP6 interferes with FOXO1's repressive activity towards CYP4A11 by sequestering it in the cytoplasm and preventing its nuclear translocation, which ultimately unleashes CYP4A11 and promotes lipid accumulation.