| Abstract: | Glucocorticoids are important regulators of lipid homeostasis, and chronically elevated glucocorticoid levels induce hypertriglyceridemia, hepatic steatosis, and visceral obesity. The occupied glucocorticoid receptor (GR) is a transcription factor. However, those genes regulating lipid metabolism under GR control are not fully known. Angiopoietin-like 4 (ANGPTL4, fasting-induced adipose factor), a protein inhibitor of lipoprotein lipase, is synthesized and secreted during fasting, when circulating glucocorticoid levels are physiologically increased. We therefore tested whether the ANGPTL4 gene (Angptl4) is transcriptionally controlled by GR. We show that treatment with the synthetic glucocorticoid dexamethasone increased Angptl4 mRNA levels in primary hepatocytes and adipocytes (2–3-fold) and in the livers and white adipose tissue of mice (∼4-fold). We tested the mechanism of this increase in H4IIE hepatoma cells and found that dexamethasone treatment increased the transcriptional rate of Angptl4. Using bioinformatics and chromatin immunoprecipitation, we identified a GR binding site within the rat Angptl4 sequence. A reporter plasmid containing this site was markedly activated by dexamethasone, indicative of a functional glucocorticoid response element. Dexamethasone treatment also increased histone H4 acetylation and DNase I accessibility in genomic regions near this site, further supporting that it is a glucocorticoid response element. Glucocorticoids promote the flux of triglycerides from white adipose tissue to liver. We found that mice lacking ANGPTL4 (Angptl4−/−) had reductions in dexamethasone-induced hypertriglyceridemia and hepatic steatosis, suggesting that ANGPTL4 is required for this flux. Overall, we establish that ANGPTL4 is a direct GR target that participates in glucocorticoid-regulated triglyceride metabolism.Glucocorticoids are steroid hormones that act as key transcriptional regulators of human metabolism during the fasted state, when their levels are physiologically increased. In particular, glucocorticoids facilitate the mobilization of triglycerides (TG)2 from the white adipose tissue (WAT) for use by the liver in processes such as gluconeogenesis, TG synthesis, and very low density lipoporotein synthesis and secretion (1, 2). However, the full set of genes that mediate this effect via transcriptional control by glucocorticoids is not known.Fasting also increases circulating levels of angiopoietin-like 4 (ANGPTL4, a fasting-induced adipose factor). ANGPTL4 is a protein secreted by the liver and WAT that can inhibit lipoprotein lipase (LPL) activity and stimulate WAT lipolysis (3–5). LPL hydrolyzes lipoprotein TG, promoting fatty acid storage in the WAT. This activity is counterbalanced by that of lipases, which hydrolyze stored TG, promoting fatty acid release by adipocytes. Therefore, one could predict that reducing ANGPTL4 activity would promote WAT TG storage, whereas increasing it would favor lipolysis. This prediction is consistent with the results of genetic and physiological studies. First, mice lacking ANGPTL4 (Angptl4−/−) have decreased plasma TG levels and an increased capacity for weight gain (6). By contrast, mice overexpressing Angptl4 in the WAT have a dramatically limited capacity for TG storage and increased levels of plasma TG, fatty acids, and glycerol (7). Plasma TG and fatty acid levels are similarly increased in mice by adenoviral overexpression of Angptl4 in the liver (8) and by systemic injection of recombinant ANGPTL4 (9, 10). Finally, a recent large population-based study uncovered sequence variations in Angptl4 that are associated with loss of function and reduced plasma TG levels in humans (11). In summary, these data suggest that transcriptional modulation of Angptl4 expression could serve as an important regulatory mechanism in TG homeostasis.We previously showed that Angptl4 mRNA levels are increased by glucocorticoids in A549 lung epithelial cells, suggesting that glucocorticoids may exert transcriptional control over Angptl4 expression (12). Several lines of evidence support this hypothesis in metabolic tissues. First, excess glucocorticoids promote hypertriglyceridemia, as also seen in models where ANGPTL4 levels are increased (1, 13, 14). Second, reducing the ratio of active to inactive glucocorticoids by pharmacologically inhibiting 11β-hydroxysteroid dehydrogenase type I increased plasma TG clearance and decreased liver TG synthesis, two components of the phenotype seen in Angptl4−/− mice (6, 15). Therefore, it is possible that both physiologic and pathophysiologic responses to glucocorticoids may involve the regulation of Angptl4 expression in metabolic tissues. However, whether Angptl4 expression is indeed directly regulated by glucocorticoids in the liver and WAT remains unexplored.Given the prominent role of ANGPTL4 in systemic TG metabolism, we explored the regulation of Angptl4 by glucocorticoids. We first examined whether glucocorticoids regulate Angptl4 expression in primary hepatocytes and adipocytes, mouse livers and WAT, and established cell lines. We then dissected the mechanism of glucocorticoid-regulated Angptl4 expression using a rat hepatoma cell line, H4IIE. Finally, we used Angptl4−/− mice to investigate the potential role of ANGPTL4 in glucocorticoid-regulated TG homeostasis. |
