Projects per year
The peroxisomal proliferator activator receptors (PPARs) are ligand-activated transcription factors that play important roles in the regulation of lipid metabolism. Three PPAR isoforms have been identified: PPARα, PPARβ/δ, and PPARγ. Each isoform has specific functions determined by their relative abundance in a cell as well as ligand specificity.
A highly sensitive PPAR target gene is represented by Angiopoietin-like 4 (ANGPTL4), which was discovered by two independent groups in 2000. ANGPTL4 is produced in a number of organs including liver and adipose tissue where its expression is governed by PPARα and PPARγ, respectively. Upon secretion, ANGPTL4 is cleaved into n- and c-terminal fragments that have divergent functions. nANGPTL4 is known to function as an inhibitor of lipoprotein lipase and hepatic lipase, whereas cANGPTL4 is involved in a number of processes including tumorigenesis and wound healing and is known to interact with integrins β1 and β5.
In this thesis we set out to expand our knowledge on the molecular function of ANGPTL4 in the regulation of lipid metabolism. We used a variety of animal models, cell culture, biochemical assays, and other functional measurements to zoom in on previously unexplored aspects of ANGPTL4.
Feeding mice deficient in ANGPLT4 a diet rich in long-chain saturated fatty acids elicited a complex phenotype and Angptl4-/- mice ultimately died from fibrinopurulent peritonitis. In contrast, the prevalence of the lethal phenotype was absent when the fat component of the high-fat diet was changed to medium-chain fatty acids, suggesting a role for increased chyle flow. Indeed, Angptl4-/- mice had dramatically enlarged mesenteric lymph nodes which contained numerous lipid laden macrophages. In vitro experiments showed that PPARβ/δ mediated induction of ANGPTL4 inhibits macrophage LPL. In the absence of ANGPTL4 there is increased lipid uptake in mesenteric lymph node macrophages, leading to ER stress and subsequent inflammatory response.
Additionally, Angptl4-/- mice gain more weight when fed a high-fat diet containing mainly unsaturated fatty acids. The increased body weight and adiposity was unrelated to food intake, activity, or energy expenditure. Remarkably, we observed increased lipid digestion in Angptl4-/- mice, which coincided with increased luminal lipase activity in the intestines of Angptl4-/- mice. Using biochemical assays we reveal that ANGPTL4 inhibits pancreatic lipase.
In the second part of this thesis we identified a novel PPAR target gene, hypoxia inducible lipid droplet associated (HILPDA). We observed HILPDA expression to be increased in liver slices exposed to a synthetic PPARα ligand. Additionally, oral dosing of similar ligand induced a marked increase in Hilpda expression in wild-type mice but not in Pparα-/- mice. PPAR mediated induction of Hilpda expression was found to be mediated by a conserved and functional PPRE located 1200 base pair or the transcription start site of HILPDA. Functional characterization of HILPDA in liver was performed via adeno-associated virus mediated overexpression. Interestingly, increased hepatic expression of HILPDA was associated with the development of a fatty liver, which could be attributes to a decrease in hepatic VLDL production.
HILPDA was also found to be highly expressed in both human and mouse adipose tissue, where its expression is under the control of PPARγ and β-adrenergic receptor. Moreover, adipose tissue HILPDA expression was increased with fasting and decreased with high-fat feeding. Despite the regulation of adipose tissue HILPDA by PPARγ, we observed no effect of HILPDA on adipogenesis. Furthermore, adipose tissue specific Hilpda knock-out mice showed no major metabolic perturbations upon fasting. However, overexpression of HILPDA in adipocytes significantly reduced the release of NEFA upon β-adrenergic receptor activation. Induction of HILPDA by β-adrenergic receptor stimulation may be part of feedback mechanism to regulate adipocyte lipolysis.
In conclusion, in thesis we have extended the current knowledge on the function of ANGPTL4. We show that ANGPTL4 serves as an important regulator in the process of lipid digestion and also in the protection of macrophages that reside in mesenteric lymph nodes that are exposed to high concentrations of lipid. HILPDA is a novel PPAR target that is involved in hepatic VLDL secretion and adipocyte lipolysis. Future research will focus on elucidating the mechanistic aspects of the regulation and function of HILPDA.
|Qualification||Doctor of Philosophy|
|Award date||15 May 2014|
|Place of Publication||Wageningen|
|Publication status||Published - 2014|
- lipid metabolism
- genotype nutrition interaction