The DHR96 nuclear receptor controls triacylglycerol homeostasis in Drosophila

Abstract

Triacylglycerol (TAG) homeostasis is an integral part of normal physiology and essential for proper energy metabolism. Here we show that the single Drosophila ortholog of the PXR and CAR nuclear receptors, DHR96, plays an essential role in TAG homeostasis. DHR96 mutants are sensitive to starvation, have reduced levels of TAG in the fat body and midgut, and are resistant to diet-induced obesity, while DHR96 overexpression leads to starvation resistance and increased TAG levels. We show that DHR96 function is required in the midgut for the breakdown of dietary fat and that it exerts this effect through the CG5932 gastric lipase, which is essential for TAG homeostasis. This study provides insights into the regulation of dietary fat metabolism in Drosophila and demonstrates that the regulation of lipid metabolism is an ancestral function of the PXR/CAR/DHR96 nuclear receptor subfamily.

Figure 1. DHR96 mutants are sensitive to starvation and display decreased levels of TAG

(A) Mature adult male Canton-S (CanS) control flies, DHR96¹ mutants, and the DHR96^2X overexpression strain were subjected to complete starvation and the number of surviving animals was determined at four hour intervals. Results are presented as a percent of the total population, which consisted of 15 groups of 20 animals for each genotype. Similar results were obtained in a second independent experiment as well as from adult flies staged at 1–2 days after eclosion. (B and C) Total glycogen (B) and TAG (C) levels were measured in CanS control flies, DHR96¹ mutants, and the DHR96^2X overexpression strain under both fed conditions and after a 20 hour starvation. Glycogen and TAG levels were normalized for total protein and are presented as normalized to a wild-type level of 100%. (D–F) Nile red staining of dissected fat body cells from 1–2 day old adult male flies shows reduced cell size and lipid droplet size in DHR96¹ mutants (E) and increased lipid droplet size in DHR96^2X animals (F), relative to CanS controls (D). (G–I) Oil Red O staining of dissected adult midguts shows reduced lipid accumulation in the DHR96¹ mutant (H) and enlarged lipid droplets in the DHR96^2X overexpression strain (I), relative to CanS controls (G). The white brackets mark comparable regions of the midgut epithelium. The lumen of the midgut can be seen in the DHR96¹ mutant (arrow). Scale bars are 75 μm (D–F) or 400 μm (G–I). Error bars represent +/− S.E. * p<0.05 and **p<0.001.

Figure 2. DHR96 mutants are resistant to diet-induced obesity

(A) Mature adult male Canton-S (CanS) control flies and DHR96¹ mutants were subjected to either a low calorie 0.5 SY or high calorie 2.0 SY diet for seven days, after which TAG levels were determined. TAG levels were normalized for total protein and are presented as normalized to a wild-type level of 100%. (B) Food uptake is slightly reduced in DHR96 mutants, as determined by scintillation counting of Canton-S (CanS) control flies and DHR96¹ mutants that were maintained on food supplemented with ³²P-dCTP. (C) DHR96 mutants can efficiently absorb dietary fatty acids, as determined by scintillation counting of Canton-S (CanS) control flies and DHR96¹ mutants that were maintained on food supplemented with ³H-oleic acid (p=0.18). Error bars represent +/− S.E. * p<0.05 and ***p<0.0001.

Figure 3. DHR96 mutants are resistant to treatment with Orlistat

(A) Mature adult male Canton-S (CanS) control flies and DHR96¹ mutants were transferred to a diet either with or without 2.0 µM Orlistat for 5–7 days, after which TAG levels were determined. TAG levels were normalized for total protein and are presented as normalized to a wild-type level of 100%. Error bars represent +/− S.E. ***p<0.0001. (B–E) Oil Red O staining of dissected adult midguts shows that lipid accumulation is significantly reduced in CanS flies fed Orlistat (C), relative to controls (B) and undetectable in DHR96¹ mutants maintained on either normal food (D) or food supplemented with Orlistat (E). Low levels of lipids can be detected in the midgut lumen of DHR96 mutants and CanS flies fed Orlistat (arrows). Scale bars are 400 μm.

Figure 4. DHR96 functions in the midgut to control dietary TAG breakdown

(A) Midguts dissected from mature adult male Canton-S (CanS) control flies and DHR96¹ mutants were homogenized and increasing amounts of lysate were tested for TAG lipolytic activity by assaying for the release of glycerol from a glycerol tributyrate emulsion. (B) Mature adult male Canton-S (CanS) control flies and DHR96¹ mutants were transferred to lipid extracted 1.0 SY medium alone (−lipid), medium supplemented with TAG (+TAG), or medium supplemented with free fatty acids (+FFA) for 7 days, after which TAG levels were determined. TAG levels were normalized for total protein and are presented as normalized to a level of 100% in CanS flies on the unsupplemented lipid extracted medium. (C) Tissue specific expression of a wild-type UAS-DHR96 transgene in the midgut of DHR96 mutants, driven by Mex-GAL4 (Mex>DHR96), is sufficient to rescue the lean phenotype. In contrast, little effect is seen when Cg-GAL4 is used to drive fat body-specific expression of the UAS-DHR96 transgene (CG>DHR96). The presence of the GAL4 drivers alone or the UAS-DHR96 transgene in the DHR96 mutant background has no effect on TAG levels (data not shown). Error bars represent +/− S.E. ***p<0.0001.

Figure 5. DHR96 regulates the CG5932 gastric lipase gene to control the breakdown of dietary TAG

(A) RNA was isolated from either fed or starved CanS control flies (+ DHR96) and DHR96¹ mutants (− DHR96) and analyzed by northern blot hybridization for CG5932 transcription. Hybridization to detect rp49 mRNA was used as a control for loading and transfer. (B) An Act-GAL4 driver or the UAS-CG5932 RNAi transgene alone have no effect on whole animal TAG levels (above) or CG5932 mRNA levels (below), as determined by northern blot hybridization. Combining these two transgenes, however, effectively reduces CG5932 expression and leads to a significant reduction in total TAG levels. Hybridization to detect rp49 mRNA was used as a control for loading and transfer. Error bars represent +/− S.E. ***p<0.0001.

Figure 6. Midgut-specific expression of CG5932 rescues the lean phenotype of DHR96 mutants

TAG levels were determined in mature adult male Canton-S (CanS) control flies or DHR96¹ mutants that carried different transgenes. Either the midgut-specific Mex-GAL4 driver alone or two different insertions of the UAS-CG5932 rescue transgene alone (UAS-CG5932¹ or UAS-CG5932²) had little or no effect on whole animal TAG levels. The increased level of TAG seen in the presence of UAS-CG5932² alone could be due to background expression from a flanking regulatory element, as is often seen with P-element insertions. In contrast, combining the Mex-GAL4 driver with each UAS-CG5932 rescue transgene restored normal TAG levels. TAG levels were normalized for total protein and are presented as normalized to a wild-type level of 100%. Error bars represent +/− S.E. **p<0.001 and ***p<0.0001.