Interplay of Dietary Fatty Acids and Cholesterol Impacts Brain Mitochondria and Insulin Action

Abstract

Overconsumption of high-fat and cholesterol-containing diets is detrimental for metabolism and mitochondrial function, causes inflammatory responses and impairs insulin action in peripheral tissues. Dietary fatty acids can enter the brain to mediate the nutritional status, but also to influence neuronal homeostasis. Yet, it is unclear whether cholesterol-containing high-fat diets (HFDs) with different combinations of fatty acids exert metabolic stress and impact mitochondrial function in the brain. To investigate whether cholesterol in combination with different fatty acids impacts neuronal metabolism and mitochondrial function, C57BL/6J mice received different cholesterol-containing diets with either high concentrations of long-chain saturated fatty acids or soybean oil-derived poly-unsaturated fatty acids. In addition, CLU183 neurons were stimulated with combinations of palmitate, linoleic acid and cholesterol to assess their effects on metabolic stress, mitochondrial function and insulin action. The dietary interventions resulted in a molecular signature of metabolic stress in the hypothalamus with decreased expression of occludin and subunits of mitochondrial electron chain complexes, elevated protein carbonylation, as well as c-Jun N-terminal kinase (JNK) activation. Palmitate caused mitochondrial dysfunction, oxidative stress, insulin and insulin-like growth factor-1 (IGF-1) resistance, while cholesterol and linoleic acid did not cause functional alterations. Finally, we defined insulin receptor as a novel negative regulator of metabolically stress-induced JNK activation.

Central Acting Hsp10 Regulates Mitochondrial Function, Fatty Acid Metabolism, and Insulin Sensitivity in the Hypothalamus

“…Especially, palmitate and stearate were elevated in Hsp10 KD cells, compared to control. It has been shown that high palmitate levels are present in the CSF of obese patients and patients with metabolic syndrome [ 49 , 50 ], and palmitate is able to induce a neuroinflammatory response with hypothalamic insulin and leptin resistance [ 22 , 23 , 51 ]. Therefore, the Hsp10 KD-induced change in lipid composition might be causative for insulin or leptin resistance.…”

“…Yet, mitochondrial dysfunction impacts cellular energy metabolism and proper clearance of ROS, as well as fatty acid metabolism [ 21 ]. Although the brain does not depend on fatty acid metabolism as its energy source, fatty acid sensing in the hypothalamus seems to control metabolism [ 22 , 23 ]. Increases in saturated long-chain fatty acids, diverse sphingolipids, or ceramides are linked to insulin resistance, indicating that an increase in overall fatty acid and lipid metabolism is detrimental for insulin action.…”

Enhanced Palmitate-Induced Interleukin-8 Formation in Human Macrophages by Insulin or Prostaglandin E2

“…The human monocytic cell line THP-1 was cultivated in medium RPMI1640 with 10% heat-inactivated FCS and 1% antibiotics (all from Biochrom AG, Berlin, Germany) and seeded in 35 mm diameter culture plates with 1 × 10 6 cells per plate. Monocytes were differentiated into macrophages by the addition of 100 ng/mL phorbol-12-myristate-13-acetate (PMA) (Sigma-Aldrich, Taufkirchen, Germany) for 24 h. After removing the medium, macrophages were washed with RPMI1640 and incubated in RPMI1640 without PMA, supplemented with 0.5% serum and 1% antibiotics for 24 h. For cell experiments and the preparation of supernatants, macrophages were stimulated for another 24 h with 100 nM of insulin (Sigma-Aldrich), 100 µM of palmitate (dissolved under alkaline conditions and coupled to bovine serum albumin (BSA), as described previously [ 16 ], or a respective control), 10 µM of PGE 2 (Enzo Life Sciences, Lörrach, Germany), or 1 µM of agonists (17-phenyl trinor prostaglandin E 2 for EP1/3, 19-(R)-hydroxyprostaglandin E 2 for EP2 and CAY10598 for EP4, all Cayman Chemical, Ann Arbor, Michigan, USA) or EP4-antagonist (ONO AE3-208, Cayman Chemical, Ann Arbor, Michigan, USA). The used concentrations were consistent through all experiments.…”

Untangling the effect of insulin action on brain mitochondria and metabolism

“…33 The importance of IR signalling for mitochondrial function has been further demonstrated by showing that IR inhibition in hypothalamic neurones decreases mitochondrial respiration. 34 Conversely, insulin increases basal and maximum respiration in immortalised hypothalamic neurones. This is accompanied by ERK-dependent induction of MSR, which is also achieved by IGF-1 stimulation.…”

“…120 On the other hand, stimulation of immortalised hypothalamic neurones with the most abundant saturated fatty acid palmitate, as an in vitro model of lipotoxicity, induces mitochondrial dysfunction, neuroinflammation and insulin/IGF-1 resistance. 34,121 Furthermore, HFD-fed rodents show clear signs of hypothalamic inflammation with insulin resistance, mitochondrial dysfunction and elevated JNK activity. 21,122 The importance of JNK activation for the establishment of inflammation, oxidative stress 123 and insulin resistance has been demonstrated recently by neuronespecific loss-of-function of DUSP8, a phosphatase deactivating JNK, in male mice fed a HFD.…”