Consumption of a high-fat diet, but not regular endurance exercise training, regulates hypothalamic lipid accumulation in mice

Abstract

Obesity is characterised by increased storage of fatty acids in an expanded adipose tissue mass and in peripheral tissues such as the skeletal muscle and liver, where it is associated with the development of insulin resistance. Insulin resistance also develops in the central nervous system with high-fat feeding. The capacity for hypothalamic cells to accumulate/store lipids, and the effects of obesity remain undefined. The aims of this study were (1) to examine hypothalamic lipid content in mice with increased dietary fat intake and in obese ob/ob mice fed a low-fat diet, and (2) to determine whether endurance exercise training could reduce hypothalamic lipid accumulation in high-fat fed mice. Male C57BL/6 mice were fed a low- (LFD) or high-fat diet (HFD) for 12 weeks; ob/ob mice were maintained on a chow diet. HFD-exercise (HFD-ex) mice underwent 12 weeks of high-fat feeding with 6 weeks of treadmill exercise training (increasing from 30 to 70 min day(-1)). Hypothalamic lipids were assessed by unbiased mass spectrometry. The HFD increased body mass and hepatic lipid accumulation, and induced glucose intolerance, while the HFD-ex mice had reduced body weight and improved glucose tolerance. A total of 335 lipid molecular species were identified and quantified. Lipids known to induce insulin resistance, including ceramide (22%↑), diacylglycerol (25%↑), lysophosphatidylcholine (17%↑), cholesterol esters (60%↑) and dihexosylceramide (33%↑), were increased in the hypothalamus of HFD vs. LFD mice. Hypothalamic lipids were unaltered with exercise training and in the ob/ob mice, suggesting that obesity per se does not alter hypothalamic lipids. Overall, hypothalamic lipid accumulation is regulated by dietary lipid content and is refractory to change with endurance exercise training.

Figure 1. Metabolic characterisation of mice in response to high-fat feeding and exercise training

A, changes in body weight presented as a percentage of starting body weight. n = 10 per group, *P < 0.05 HFD vs. HFD-ex at specific time point. B, epididymal fat pads were excised and weighed at time of killing. n = 10, per group *P < 0.05 vs. HFD. C, livers were excised from LFD, HFD and HFD-ex mice and TAG content analysed. n = 8 per group, *P < 0.05 vs. HFD. D, an endurance test was performed on all mice before and after the 6 weeks of exercise training. LFD (n = 10), HFD (n = 10) HFD-ex (n = 5), *P < 0.05 vs. HFD. E, glucose tolerance tests were performed for LFD, HFD and HFD-ex mice at the end of the exercise period. n = 5 per group. F, the incremental area under the curve was calculated from the GTT data. n = 5 per group, *P < 0.05 vs. HFD.

Figure 2. Hypothalamic lipid accumulation in response to high-fat feeding

The hypothalamus was excised from LFD and HFD mice and analysed for total lipid content. A, phospholipid content, lysophosphatidylcholine (LPC), odd chain phosphatidylcholine (odd PC), phosphatidylcholine (PC), alkylphosphatidylcholine (PC(O)), alkenylphosphatidylcholine (PC(P)), phosphatidylethanolamine (PE), alkylphosphatidylethanolamine (PE(O)), alkylphosphatidylethanolamine (PE(P)), phosphatidylglycerol (PG), phosphatidylinositol (PI) and phosphatidylserine (PS). bis(monoacylglycero)phosphate (BMP) B, sterol lipid content, cholesterol ester (CE) and cholesterol (COH). C Sphingolipids dihydroceramide (dhCer), ceramide (Cer), sphingomyelin (SM), hydroxyphingomyelin (SM(OH)), G_M3 ganglioside (GM3), monohexosylceramide (MHC), dihexosylceramide (DHC) and trihexosylceramide (THC). D Glycerolipid content, diacylglycerol (DAG) and triaclyglycerol (TAG). LFD, white bars, HFD black bars. n = 13–14 per group, *P < 0.05 vs. LFD.

Figure 3. Hypothalamic DAG and TAG fatty acid content in response to high-fat feeding

Hypothalamic DAG (A) and TAG (B) content was analysed for the total amount of saturated (Sat), monounsaturated (Mono) and polyunsaturated (Poly) fatty acids. LFD, white bars; HFD, black bars; n = 13–14 in each group, *P < 0.05 vs. LFD.

Figure 4. Individual species of hypothalamic bioactive lipids in response to high-fat feeding

A, individual ceramide species. B, the 10 most abundant diacylglycerol (DAG) species. LFD, white bars; HFD, black bars; n = 13–14 in each group, *P < 0.05 vs. LFD.

Figure 5. Hypothalamic stress signalling in response to high-fat feeding, obesity and exercise training

The hypothalamus was excised and analysed for IκBα protein expression in LFD vs. HFD (A), LFD vs. ob/ob (B), and in HFD vs. HFD-ex (C). The hypothalamus was also analysed for pJNK protein expression in LFD vs. HFD (D), LFD vs. ob/ob (E) and HFD vs. HFD-ex (F). n = 3–11 per group, *P < 0.05 vs. control.