Joan C. Rodríguez

Thyroid Hormone Regulates the Hypotriglyceridemic Gene APOA5

The apolipoprotein AV gene (APOA5) is a key determinant of plasma triglyceride levels, a major risk factor for coronary artery disease and a biomarker for the metabolic syndrome. Since thyroid hormones influence very low density lipoprotein triglyceride metabolism and clinical studies have demonstrated an inverse correlation between thyroid status and plasma triglyceride levels, we examined whether APOA5 is regulated by thyroid hormone. Here we report that 3,5,3′-triiodo-l-thyronine (T3) and a synthetic thyroid receptor β (TRβ) ligand increase APOA5 mRNA and protein levels in hepatocytes. Our data revealed that T3-activated TR directly regulates APOA5 promoter through a functional direct repeat separated by four nucleotides (DR4). Interestingly, we show that upstream stimulatory factor 1, a transcription factor associated with familial combined hyperlipidemia and elevated triglyceride levels in humans, and upstream stimulatory factor 2 cooperate with TR, resulting in a synergistic activation of APOA5 promoter in a ligand-dependent manner via an adjacent E-box motif. In rats, we observed that apoAV levels declines with thyroid hormone depletion but returned to normal levels upon T3 administration. In addition, treatments with a TRβ-selective agonist increased apoAV and diminished triglyceride levels. The identification of APOA5 as a T3 target gene provides a new potential mechanism whereby thyroid hormones can influence triglyceride homeostasis. Additionally, these data suggest that TRβ may be a potential pharmacological target for the treatment of hypertriglyceridemia.

Differential regulation of the human versus the mouse apolipoprotein AV gene by PPARalpha. Implications for the study of pharmaceutical modifiers of hypertriglyceridemia in mice.

Mice have been used widely to define the mechanism of action of fibric acid derivatives. The fibrates are pharmacological agonists of the peroxisome proliferator-activated receptor alpha (PPARalpha), whose activation in human subjects promotes potent reduction in plasma levels of triglycerides (TG) with concomitant increase in those of HDL-cholesterol. The impact of PPARalpha agonists on gene expression in humans and rodents is however distinct; such distinctions include differential regulation of key genes of lipid metabolism. We evaluated the question as to whether the human and murine genes encoding apolipoprotein apoAV, a regulator of plasma concentrations of TG-rich lipoproteins, might be differentially regulated in response to fibrates. Fenofibrate, a classic PPARalpha agonist, repressed expression of mouse Apoa5 in vivo in a mouse model transgenic for the human APOA5 gene; by contrast, expression of the human ortholog was up-regulated. Our findings are consistent with the presence of a functional PPAR-binding element in the promoter of the human APOA5 gene; this element is however degenerate and non-functional in the corresponding mouse Apoa5 sequence, as demonstrated by reporter assays and gel shift analyses. These data further highlights the distinct mechanisms which are implicated in the metabolism of TG-rich lipoproteins in mice as compared to man. They equally emphasize the importance of the choice of a mouse model for investigation of the impact of pharmaceutical modifiers on hypertriglyceridemia.

Regulation of human class I alcohol dehydrogenases by bile acids

Class I alcohol dehydrogenases (ADH1s) are the rate-limiting enzymes for ethanol and vitamin A (retinol) metabolism in the liver. Because previous studies have shown that human ADH1 enzymes may participate in bile acid metabolism, we investigated whether the bile acid-activated nuclear receptor farnesoid X receptor (FXR) regulates ADH1 genes. In human hepatocytes, both the endogenous FXR ligand chenodeoxycholic acid and synthetic FXR-specific agonist GW4064 increased ADH1 mRNA, protein, and activity. Moreover, overexpression of a constitutively active form of FXR induced ADH1A and ADH1B expression, whereas silencing of FXR abolished the effects of FXR agonists on ADH1 expression and activity. Transient transfection studies and electrophoretic mobility shift assays revealed functional FXR response elements in the ADH1A and ADH1B proximal promoters, thus indicating that both genes are direct targets of FXR. These findings provide the first evidence for direct connection of bile acid signaling and alcohol metabolism.

Apolipoprotein AV: gene expression, physiological role in lipid metabolism and clinical relevance

The apolipoprotein APOA5 gene, a member of the gene cluster on chromosome 11q23 that includes APOA1, APOC3 and APOA4, has gained considerable interest as it encodes ApoAV, a key determinant of circulating levels of potentially atherogenic triglyceride-rich lipoproteins (TRL). Indeed, strong associations between genetic variants of the APOA5 gene sequence and elevated triglyceride (TG) levels have been established. This apolipoprotein may potentiate lipolysis of TRL through facilitation of lipoprotein interaction with lipoprotein lipase. In addition, ApoAV may enhance clearance of remnant lipoproteins by mediating their interaction with the LDL receptor-related protein (LRP)1. The implication of ApoAV in intravascular TRL metabolism is further supported by studies that have demonstrated upregulation of APOA5 gene expression by nuclear receptors (PPARα, FXR and HNF4α) and hormones (thyroxine) involved in hypotriglyceridemic pathways. APOA4 expression may equally be modulated by nutritional status and, more ...

Regulation of N-Myc downstream regulated gene 2 by bile acids.

Here we report that bile acid chenodeoxycholic acid (CDCA) and synthetic farnesoid X receptor (FXR) agonist GW4064 robustly induced tumor suppressor N-Myc downstream regulated gene 2 (NDRG2) expression in human hepatoma cells and primary hepatocytes. Knockdown of FXR abolished the induction by CDCA, whereas overexpression of a constitutively active form of FXR increased NDRG2 expression. A FXR-response element was identified within intronic regions of human and murine genes. Moreover, mice given GW4064 exhibit an increase of Ndrg2 expression in liver and kidney, where both NDRG2 and FXR are enriched. The identification of NDRG2 as a bile acid regulated gene may provide novel knowledge toward the understanding of NDRG2 physiological function and the link between metabolism and cancer.