Oestrogen receptor α is required for biochanin A-induced apolipoprotein A-1 mRNA expression in HepG2 cells

Neuropeptide Y promotes hepatic apolipoprotein A1 synthesis and secretion through neuropeptide Y Y5 receptor

OBJECTIVES

Apolipoprotein A1 (ApoA1), a major component of high-density lipoprotein (HDL), is a protective factor against cardiovascular disease (CVD). A recent epidemiological study found an association between neuropeptide Y (NPY) gene polymorphism and serum HDL levels. However, the direct effect of NPY on ApoA1 expression remains unknown. This study was designed to investigate the molecular mechanisms underlying the NPY-mediated regulation of hepatic ApoA1.

METHODS

Serum ApoA1, total cholesterol, and HDL-c and hepatic ApoA1 levels were measured after intraperitoneal administration of NPY or an NPY Y5 receptor (NPY5R) agonist in vivo. HepG2 and BRL-3A hepatocytes were treated in vitro with NPY in the presence or absence of NPY receptor antagonists, agonists, or signal transduction pathway inhibitors. Subsequently, the protein and mRNA expression of cellular and secreted ApoA1 were determined.

RESULTS

NPY considerably upregulated hepatic ApoA1 expression and stimulated ApoA1 secretion, both in vivo and in vitro. NPY5R inhibition blocked NPY-induced upregulation of ApoA1 expression, and NPY5R activation stimulated ApoA1 expression and secretion in hepatocytes. Moreover, extracellular signal-regulated protein kinases 1 and 2 (ERK1/2) and protein kinase A (PKA) inhibition almost completely blocked the upregulation of ApoA1 expression and secretion induced by NPY5R.

CONCLUSIONS

For the first time, we demonstrated that NPY5R activation promotes hepatic ApoA1 synthesis and secretion through the ERK1/2 and PKA signal transduction pathways. Thus, NPY5R may be a potential therapeutic target for treating CVD by promoting cholesterol reverse transport.

Estrogenic endocrine disruptors: Molecular mechanisms of action

A comprehensive summary of more than 450 estrogenic chemicals including estrogenic endocrine disruptors is provided here to understand the complex and profound impact of estrogen action. First, estrogenic chemicals are categorized by structure as well as their applications, usage and effects. Second, estrogenic signaling is examined by the molecular mechanism based on the receptors, signaling pathways, crosstalk/bypassing and autocrine/paracrine/homeostatic networks involved in the signaling. Third, evaluation of estrogen action is discussed by focusing on the technologies and protocols of the assays for assessing estrogenicity. Understanding the molecular mechanisms of estrogen action is important to assess the action of endocrine disruptors and will be used for risk management based on pathway-based toxicity testing.

A comparative analysis of genistein and daidzein in affecting lipid metabolism in rat liver

Effects of soy isoflavones, genistein and daidzein, on the hepatic gene expression profile and indices for lipid metabolism were compared in rats. In the first experiment (Expt. 1), animals were fed diets containing 2 g/kg of either genistein or daidzein, or a control diet free of isoflavone for 14 days. In the second experiment (Expt. 2), rats were fed diets containing 1 or 2 g/kg of genistein, or an isoflavone-free diet for 16 days. Genistein at a dietary level of 2 g/kg reduced serum triacylglycerol concentrations in both experiments, and serum concentrations of cholesterol in Expt. 2. However, daidzein at 2 g/kg did not decrease serum lipid concentrations in Expt. 1. A DNA microarray analysis in Expt. 1 showed that genistein was stronger than daidzein in affecting gene expression in liver, targeting many genes involved in lipid and carbohydrate metabolism. Detailed analyses indicated that alterations in the expression of genes related to lipogenesis are primarily responsible for the serum lipid-lowering effect of genistein. This notion was supported by analyses of the activity of enzymes involved in lipogenesis in Expt. 2.

Changes in gene expression and assessment of DNA methylation in primary human hepatocytes and HepG2 cells exposed to the environmental contaminants-Hexabromocyclododecane and 17-beta oestradiol

We evaluated the effects of two putative non-genotoxic hepatic carcinogens, hexabromocyclododecane (HBCD) and 17-beta oestradiol (E(2)) on global and CpG promoter DNA methylation in both primary human hepatocytes and hepatocellular carcinoma (HepG2) cells. The mRNA gene expression levels of genes involved particularly in cell cycle were also evaluated and potential correlation with DNA methylation status examined. HBCD at 0.03 and 0.3 ng/mL did not produce statistically significant differences in global genomic methylation. However, E(2) (0.1 ng/mL) significantly lowered global DNA methylation levels in HepG2 cells by approximately 65% (P<0.01). In primary hepatocytes, the promoter regions of N-cym and ERalpha were methylated in both control and treated groups, signifying lack of promoter demethylation by both HBCD and E(2). Furthermore, CpG promoter methylation of RB1 was observed in HepG2 cells but this was unaffected by treatments. The remaining genes (p16, C-myc, H-ras, THRalpha, histone H3, TBK1 and TNFRalpha) were unmethylated in their CpG promoter regions in both test systems. Quantitative RT-PCR showed that HBCD at 0.03 ng/mL up-regulated the expression of N-cym whereas E(2) up-regulated the expression of ERalpha and THRalpha genes in primary hepatocytes. In HepG2 cells, the mRNA gene expression levels of p16, RB1 and N-cym were significantly down regulated by HBCD (0.03 ng/mL) and E(2) (0.1 ng/mL) while HBCD at 0.3 ng/mL, significantly down regulated the expression levels of N-cym, ERalpha and ERbeta genes. Thus, while both HBCD and E(2) may alter the expression of certain genes involved in proliferation, the mechanisms appear unrelated to DNA methylation.