HNF4α is required for Tkfc promoter activation by ChREBP.

Triokinase/FMN cyclase (Tkfc) is involved in fructose metabolism and is responsible for the phosphorylation of glyceraldehyde to glyceraldehyde-3-phosphate. In this study, we showed that refeeding induced hepatic expression of Tkfc in mice. Luciferase reporter gene assays using the Tkfc promoter revealed the existence of 2 hepatocyte nuclear factor 4α (HNF4α)-responsive elements (HNF4RE1 and HNF4RE2) and 1 carbohydrate-responsive element-binding protein (ChREBP)-responsive element (ChoRE1). Deletion and mutation of HNF4RE1 and HNF4RE2 or ChoRE1 abolished HNF4α and ChREBP responsiveness, respectively. HNF4α and ChREBP synergistically stimulated Tkfc promoter activity. ChoRE1 mutation attenuated but maintained HNF4α responsiveness, whereas HNF4RE1 and HNF4RE2 mutations abolished ChREBP responsiveness. Moreover, Tkfc promoter activity stimulation by ChREBP was attenuated upon HNF4α knockdown. Furthermore, Tkfc expression was decreased in the livers of ChREBP-/- and liver-specific HNF4-/- (Hnf4αΔHep) mice. Altogether, our data indicate that Tkfc is a target gene of ChREBP and HNF4α, and Tkfc promoter activity stimulation by ChREBP requires HNF4α.

Sulforaphane suppresses the activity of sterol regulatory element-binding proteins (SREBPs) by promoting SREBP precursor degradation.

Sterol regulatory element-binding proteins (SREBPs) are transcription factors that regulate various genes involved in cholesterol and fatty acid synthesis. In this study, we describe that naturally occurring isothiocyanate sulforaphane (SFaN) impairs fatty acid synthase promoter activity and reduces SREBP target gene (e.g., fatty acid synthase and acetyl-CoA carboxylase 1) expression in human hepatoma Huh-7 cells. SFaN reduced SREBP proteins by promoting the degradation of the SREBP precursor. Amino acids 595-784 of SREBP-1a were essential for SFaN-mediated SREBP-1a degradation. We also found that such SREBP-1 degradation occurs independently of the SREBP cleavage-activating protein and the Keap1-Nrf2 pathway. This study identifies SFaN as an SREBP inhibitor and provides evidence that SFaN could have major potential as a pharmaceutical preparation against hepatic steatosis and obesity.

Chrysin reduces the activity and protein level of mature forms of sterol regulatory element-binding proteins.

Sterol regulatory element-binding proteins (SREBPs) are transcription factors that regulate the expression of genes involved in fatty acid and cholesterol biosynthetic pathways. The present study showed that the flavonoid chrysin impairs the fatty acid synthase promoter. Chrysin reduces the expression of SREBP target genes, such as fatty acid synthase, in human hepatoma Huh-7 cells and impairs de novo synthesis of fatty acids and cholesterol. Moreover, it reduces the endogenous mature, transcriptionally active forms of SREBPs, which are generated by the proteolytic processing of precursor forms. In addition, chrysin reduces the enforced expressing mature forms of SREBPs and their transcriptional activity. The ubiquitin-proteasome system is not involved in the chrysin-mediated reduction of SREBPs mature forms. These results suggest that chrysin suppresses SREBP activity, at least partially, via the degradation of SREBPs mature forms. Abbreviations: ACC1: acetyl-CoA carboxylase 1; DMEM: Dulbecco's modified Eagle's medium; FAS: fatty acid synthase; GAPDH: glyceraldehyde-3-phosphate dehydrogenase; 25-HC: 25-hydroxycholesterol; HMGCS: HMG-CoA synthase; LDH: lactate dehydrogenase; LPDS: lipoprotein-deficient serum; PI3K: phosphatidylinositol 3-kinase; SCD1: stearoyl-CoA desaturase; SREBPs: sterol regulatory element-binding proteins.

Kaempferol stimulates gene expression of low-density lipoprotein receptor through activation of Sp1 in cultured hepatocytes.

A high level of plasma low-density lipoprotein (LDL) cholesterol is considered a risk factor for atherosclerosis. Because the hepatic LDL receptor (LDLR) is essential for clearing plasma LDL cholesterol, activation of LDLR is a promising therapeutic target for patients with atherosclerotic disease. Here we demonstrated how the flavonoid kaempferol stimulated the gene expression and activity of LDLR in HepG2 cells. The kaempferol-mediated stimulation of LDLR gene expression was completely inhibited by knockdown of Sp1 gene expression. Treatment of HepG2 cells with kaempferol stimulated the recruitment of Sp1 to the promoter region of the LDLR gene, as well as the phosphorylation of Sp1 on Thr-453 and Thr-739. Moreover, these kaempferol-mediated processes were inhibited in the presence of U0126, an ERK pathway inhibitor. These results suggest that kaempferol may increase the activity of Sp1 through stimulation of Sp1 phosphorylation by ERK1/2 and subsequent induction of LDLR expression and activity.