Fat metabolism is central to the process of energy homeostasis. When nutrients are abundant, dietary fat in the form of triacylglycerol (TAG) is broken down by gastric TAG lipases to release fatty acids. These fatty acids are absorbed by the intestine and used to resynthesize TAG, which is trafficked to the adipose tissue for storage. These TAG reserves can be accessed upon nutrient deprivation through the action of specific lipid-droplet associated lipases that release the fatty acids for energy production through mitochondrial fatty acid β-oxidation. Defects in these processes can lead to dramatic changes in TAG levels and a range of physiological disorders, including obesity, diabetes, and cardiovascular disease. The alarming rise in the prevalence of these disorders in human populations has focused attention on understanding the molecular mechanisms that coordinate dietary nutrient uptake with TAG homeostasis. As a result, many regulators of TAG metabolism have been identified, including SREBP, PPAR, and adiponectin. In spite of these advances, however, the molecular mechanisms that coordinate dietary fat uptake, synthesis, storage, and utilization, remain poorly understood.
Nuclear receptors (NRs) are ligand-regulated transcription factors that play a central role in metabolic control. They are defined by a conserved zinc-finger DNA binding domain (DBD) and a C-terminal ligand binding domain (LBD) that can impart multiple functions, including hormone binding, receptor dimerization, and transactivation. Many NRs are regulated by small lipophilic compounds that include dietary signals and metabolic intermediates, and exert their effects by directing global changes in gene expression that act to maintain metabolic homeostasis. This is exemplified by members of the mammalian PPAR, LXR, and FXR subfamilies, which play critical roles in adipogenesis, lipid metabolism, and cholesterol and bile acid homeostasis, respectively (Chawla et al., 2001
; Sonoda et al., 2008
In this study we use the fruit fly, Drosophila melanogaster
, as a simple model system to characterize the NR subfamily represented by the Pregnane X Receptor (PXR, NR1I2), Constitutive Androstane Receptor (CAR, NR1I3), and Vitamin D Receptor (VDR, NR1I1) in mammals. Previous studies have defined central roles for these receptors in sensing xenobiotic compounds and directly regulating genes involved in detoxification (Timsit and Negishi, 2007
; Willson and Kliewer, 2002
). Initial studies showed that the single ancestral Drosophila
ortholog of this NR subclass, DHR96, has similar functions (King-Jones et al., 2006
). A DHR96
null mutant displays increased sensitivity to the sedative effects of phenobarbital and the pesticide DDT as well as defects in the expression of phenobarbital-regulated genes. These studies, however, revealed other potential roles for the receptor – in particular, unexpected effects on the expression of genes that are predicted to regulate lipid and carbohydrate metabolism (King-Jones et al., 2006
). This observation is in line with recent studies that have implicated roles for the mammalian PXR and CAR NRs in metabolic control (Moreau et al., 2008
). The molecular mechanism by which they exert this effect, however, remains undefined.
Here we show that DHR96 null mutants are sensitive to starvation and have reduced levels of TAG, while DHR96 overexpression leads to starvation resistance and elevated TAG levels. A series of studies using metabolic assays, diets, and the drug Orlistat, revealed that DHR96 mutants are defective in their ability to break down dietary lipid. This model was supported by microarray studies, which showed that many genes expressed in the midgut are misregulated in DHR96 mutants, including highly reduced expression of the gastric lipase gene CG5932. We show that CG5932 is required for proper whole animal TAG levels, and that selective expression of CG5932 in the midgut of DHR96 mutants is sufficient to rescue their lean phenotype. Taken together, these data support a role for the PXR/CAR/DHR96 NR subclass in lipid metabolism and define DHR96 as a key regulator of dietary TAG breakdown in the Drosophila midgut.