2-Mercaptoacetate, an inhibitor of fatty acid oxidation, decreases the membrane potential in rat liver in vivo

Enoyl-CoA hydratase mediates polyhydroxyalkanoate mobilization in Haloferax mediterranei

Although polyhydroxyalkanoate (PHA) accumulation and mobilization are one of the most general mechanisms for haloarchaea to adapt to the hypersaline environments with changeable carbon sources, the PHA mobilization pathways are still not clear for any haloarchaea. In this study, the functions of five putative (R)-specific enoyl-CoA hydratases (R-ECHs) in Haloferax mediterranei, named PhaJ1 to PhaJ5, respectively, were thoroughly investigated. Through gene deletion and complementation, we demonstrated that only certain of these ECHs had a slight contribution to poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) biosynthesis. But significantly, PhaJ1, the only R-ECH that is associated with PHA granules, was shown to be involved in PHA mobilization in this haloarchaeon. PhaJ1 catalyzes the dehydration of (R)-3-hydroxyacyl-CoA, the common product of PHA degradation, to enoyl-CoA, the intermediate of the β-oxidation cycle, thus could link PHA mobilization to β-oxidation pathway in H. mediterranei. This linkage was further indicated from the up-regulation of the key genes of β-oxidation under the PHA mobilization condition, as well as the obvious inhibition of PHA degradation upon inhibition of the β-oxidation pathway. Interestingly, 96% of phaJ-containing haloarchaeal species possess both phaC (encoding PHA synthase) and the full set genes of β-oxidation, implying that the mobilization of carbon storage in PHA through the β-oxidation cycle would be general in haloarchaea.

Enhancing hepatic mitochondrial fatty acid oxidation stimulates eating in food-deprived mice

Hepatic fatty acid oxidation (FAO) has long been implicated in the control of eating. Nevertheless, direct evidence for a causal relationship between changes in hepatic FAO and changes in food intake is still missing. Here we tested whether increasing hepatic FAO via adenovirus-mediated expression of a mutated form of the key regulatory enzyme of mitochondrial FAO carnitine palmitoyltransferase 1A (CPT1mt), which is active but insensitive to inhibition by malonyl-CoA, affects eating and metabolism in mice. CPT1mt expression increased hepatocellular CPT1 protein levels. This resulted in an increase in circulating ketone body levels in fasted CPT1mt-expressing mice, suggesting an increase in hepatic FAO. These mice did not show any significant changes in cumulative food intake, energy expenditure, or respiratory quotient after 4-h food deprivation. After 24-h food deprivation, however, the CPT1mt-expressing mice displayed increased food intake. Thus expression of CPT1mt in the liver increases hepatic FAO capacity, but does not inhibit eating. Rather, it may even stimulate eating after prolonged food deprivation. These data do not support the hypothesis that an increase in hepatic FAO decreases food intake.

Antiepileptic effects of endogenous beta-hydroxybutyrate in suckling infant rats

Physiological ketosis is a hallmark of metabolism in suckling infants. However, little is known on the impact of physiological ketosis on brain excitability. We addressed this question in suckling rats in vivo. 16-channel extracellular field potential recordings were performed from somatosensory barrel cortex at postnatal days 5-9 non-anaesthetized rat pups. Seizures were induced by the volatile convulsant agent flurothyl. One hour after blockade of physiological ketogenesis using combined administration of beta-oxidation inhibitors mercaptoacetate, insulin and glucose to prevent hypoglycemia, we found no significant change in the flurothyl-induced electrographic seizures. However, build-up of seizures during two repetitive flurothyl applications was strongly aggravated in the animals with blocked ketogenesis. The effect of ketogenesis inhibitors was reversed by exogenous beta-hydroxybutyrate. Diazepam exerted anticonvulsive action both under physiological ketosis and after blockade of ketogenesis, and bumetanide had no significant anticonvulsive effects in both conditions. Thus, physiological ketosis reduces excitability in the immature brain and elimination of physiological ketosis results in elimination of this anticonvulsant effect. Our study raises concern that the changes in diet, and pharmacological manipulations such as glucose infusion, and pathologies such as hyperinsulinism which break natural ketosis, may be a potential risk factor for epileptogenesis in nursing infants.

Dietary fat sensing via fatty acid oxidation in enterocytes: possible role in the control of eating

Various mechanisms detect the presence of dietary triacylglycerols (TAG) in the digestive tract and link TAG ingestion to the regulation of energy homeostasis. We here propose a novel sensing mechanism with the potential to encode dietary TAG-derived energy by translating enterocyte fatty acid oxidation (FAO) into vagal afferent signals controlling eating. Peripheral FAO has long been implicated in the control of eating ( 141 ). The prevailing view was that mercaptoacetate (MA) and other FAO inhibitors stimulate eating by modulating vagal afferent signaling from the liver. This concept has been challenged because hepatic parenchymal vagal afferent innervation is scarce and because experimentally induced changes in hepatic FAO often fail to affect eating. Nevertheless, intraperitoneally administered MA acts in the abdomen to stimulate eating because this effect was blocked by subdiaphragmatic vagal deafferentation ( 21 ), a surgical technique that eliminates all vagal afferents from the upper gut. These and other data support a role of the small intestine rather than the liver as a FAO sensor that can influence eating. After intrajejunal infusions, MA also stimulated eating in rats through vagal afferent signaling, and after infusion into the superior mesenteric artery, MA increased the activity of celiac vagal afferent fibers originating in the proximal small intestine. Also, pharmacological interference with TAG synthesis targeting the small intestine induced a metabolic profile indicative of increased FAO and inhibited eating in rats on a high-fat diet but not on chow. Finally, cell culture studies indicate that enterocytes oxidize fatty acids, which can be modified pharmacologically. Thus enterocytes may sense dietary TAG-derived fatty acids via FAO and influence eating through changes in intestinal vagal afferent activity. Further studies are necessary to identify the link between enterocyte FAO and vagal afferents and to examine the specificity and potential physiological relevance of such a mechanism.

Differential influences of peroxisome proliferator-activated receptors gamma and -alpha on food intake and energy homeostasis

Chronic treatment with compounds activating peroxisome proliferator-activated receptor (PPAR)gamma and -alpha influences body energy stores, but the underlying mechanisms are only partially known. In a chronic-dosing study, equiefficacious antihyperglycemic doses of the PPAR gamma agonist pioglitazone and PPAR alpha/gamma dual activator ragaglitazar were administered to obesity-prone male rats. The PPAR alpha agonist fenofibrate had no effect on insulin sensitivity. Pioglitazone transiently increased and fenofibrate transiently decreased food intake, whereas ragaglitazar had no impact on feeding. As a result, body adiposity increased in pioglitazone-treated rats and decreased in fenofibrate-treated rats. PPAR gamma compounds markedly increased feed efficiency, whereas PPAR alpha agonist treatment decreased feed efficiency. In fenofibrate-treated rats, plasma acetoacetate was significantly elevated. Plasma levels of this potentially anorectic ketone body were unaffected in pioglitazone- and ragaglitazar-treated rats. High-fat feeding markedly increased visceral fat pads, and this was prevented by pioglitazone and ragaglitazar treatment. Pioglitazone treatment enlarged subcutaneous adiposity in high-fat-fed rats. In conclusion, PPAR gamma activation increases both food intake and feed efficiency, resulting in net accumulation of subcutaneous body fat. The impact of PPAR gamma activation on feeding and feed efficiency appears to be partially independent because the PPAR alpha component of ragaglitazar completely counteracts the orexigenic actions of PPAR gamma activation without marked impact on feed efficiency.

Physiological effect of circulating glucagon on the hepatic membrane potential

The pancreatic hormone glucagon hyperpolarizes the liver cell membrane under various conditions. Here we investigated the physiological relevance of this effect by testing the influence of infusions of glucagon antiserum on the liver cell membrane potential in vivo. Intracellular microelectrode recordings of liver cells (up to 60/rat over 2 h) were done in anesthetized male rats. Livers were fixed in place, and recordings were done 10-30 min after intraperitoneal injections of glucagon or hepatic portal vein infusions of glucagon or specific polyclonal glucagon antibodies raised in rabbits. The isotonic lactose vehicle was used as a control for glucagon, and equal amounts of nonimmunized rabbit IgG were used as a control for glucagon antibodies. Intraperitoneal glucagon (400 microg/kg) hyperpolarized the liver cell membrane up to 12 mV, and intraportal glucagon (10 or 60 microg/kg) dose dependently hyperpolarized the liver cell membrane by 3-7 mV. Intraportal infusion of glucagon antiserum (in vitro binding capacity of 4 ng glucagon/rat) significantly depolarized the liver cell membrane by approximately 2.5 mV. The effects of both glucagon and glucagon antiserum reversed after 60-90 min. We conclude that glucagon is a physiologically important modulator of the liver cell membrane potential.