Brassica Glucosinolates Improve Metabolism Efficiency of Drosophila Melanogaster by Promoting Efficient Mitochondrial Respiration

Introduction: Obesity in the United States is the most important contributor to the development of Type II diabetes (T2DM). A 10-fold increase of diet-related childhood T2DM has been noted over the past 20 years, and no cost effective therapeutics exist to stem the epidemic of T2DM. Resveratrol (RS) and glucosinolates have been shown to reduce oxidative stress in cultured cells and we investigated whether supplementation of a high fat diet (HFD) with these compounds reduces the oxygen consumption rate (OCR) of Drosophila melanogaster larvae. We hypothesize that these compounds will reduce the oxygen consumption rate in 3 day-old fly larvae.

Methods: HFD (30 % coconut oil) has previously been shown to increase inflammation and decrease insulin signaling, mimicking the metabolic stress seen in patients with T2DM. Wild type Drosophila melanogaster larvae were fed for 1 - 3days with a HFD only or supplemented with RS, glucoraphanin (GL) (pure glucosinolate), or a mixture of glucosinolates derived from cabbage (CB), and broccoli (BR) respectively at a concentration of 50 μmol/mL per group. Larvae were also maintained on a regular diet (RD) as controls. Larvae were transferred to a Seahorse XF 24 Extracellular flux analyzer on day 3 to evaluate real time OCR. A second group of larvae (n=15) was raised to adulthood (18-28 days) under similar experimental conditions to evaluate the expression of genes associated with either insulin transcription/fat-storage/metabolism (TAG, CAPNC), or lifespan (SIRT1, SIRT2), or insulin signaling/gluconeogenesis (DILP2, DILP3, PEPCK, F16BPF, JNK, IGF2) by RT-qPCR.

Results: Larvae raised on a HFD showed a 37.2 % increase in OCR when compared to larvae raised on a RD (p<0.022). However, the OCR was significantly reduced when HFD was supplemented with either GL (133.3%, p< 0.001), or BR (59.4%, p<0.004), or CB (75.7%, p< 0.003) when compared to HFD. No statistically significant change in the OCR was seen in the presence of RS (p< 0.245). Adult flies showed a 4-fold increase (p< 0.0001) in the expression of the TAG gene when raised on a HFD in the presence of CB compared to HFD alone. None of the other genes evaluated showed a statistical difference in expression under the tested conditions.

Conclusions: Our data show that a HFD significantly increased the OCR, which was substantially revered by the addition of glucosinolates GL, BR, CB perhaps suggesting that these compound could exert bio-protective effects on mitochondrial respiration. Furthermore, CB significantly increased TAG gene expression perhaps indicative of increased synthesis of insulin and fat storage. Whether glucosinolates may also hold similar therapeutic values in mammals remains to be determined.