Liver Glucose Production Research Articles

Hepato-incretin function of GLP-1: novel concept and target in type 1 diabetes.

The liver is a key player in glucose regulation, and several factors are involved in this regulation (1). Glucagon, autonomic nerves, and the adrenals stimulate hepatic glucose production to allow sufficient brain glucose delivery. Insulin inhibits hepatic glucose production, preventing excess hepatic glucose release. This is of particular importance after meals, as is evident from the negative correlation between early insulin response after meals and postprandial glucose (2). Therefore, low insulin and high glucagon levels—such as observed in type 1 and type 2 diabetes—sustain liver glucose production, thereby resulting in postprandial and fasting hyperglycemia. Thus, reducing hepatic glucose production is an important target in the treatment of diabetes (3). The incretin hormone GLP-1 inhibits hepatic glucose production through its ability to stimulate insulin secretion and inhibit glucagon secretion (4). As GLP-1 is released after meal ingestion in proportion to meal size (5), this effect allows meal-dependent inhibition of hepatic glucose production. However, GLP-1 may also inhibit endogenous glucose production through an islet-independent mechanism (6–9). This allows not only a rapid shutoff of endogenous glucose production after meal ingestion but also incretin therapy to be potent in insulin-deficient diabetes. A study in the current issue of Diabetes presents convincing evidence for such an islet-independent inhibition of endogenous glucose production by GLP-1 (10). The study by Jun et al. (10) leveraged the unique glucagon receptor …

Intranasal insulin suppresses endogenous glucose production in humans compared with placebo in the presence of similar venous insulin concentrations.

Intranasal insulin (INI) has been shown to modulate food intake and food-related activity in the central nervous system in humans. Because INI increases insulin concentration in the cerebrospinal fluid, these effects have been postulated to be mediated via insulin action in the brain, although peripheral effects of insulin cannot be excluded. INI has been shown to lower plasma glucose in some studies, but whether it regulates endogenous glucose production (EGP) is not known. To assess the role of INI in the regulation of EGP, eight healthy men were studied in a single-blind, crossover study with two randomized visits (one with 40 IU INI and the other with intranasal placebo [INP] administration) 4 weeks apart. EGP was assessed under conditions of an arterial pancreatic clamp, with a primed, constant infusion of deuterated glucose and infusion of 20% dextrose as required to maintain euglycemia. Between 180 and 360 min after administration, INI significantly suppressed EGP by 35.6% compared with INP, despite similar venous insulin concentrations. In conclusion, INI lowers EGP in humans compared with INP, despite similar venous insulin concentrations. INI may therefore be of value in treating excess liver glucose production in diabetes.

Phosphatidic acid: a new therapeutic lead to suppress hepatic glucose production.

Phosphatidic acid: a new therapeutic lead to suppress hepatic glucose production.

Pivotal role of cAMP in the activation of liver glycogen breakdown in high-fat diet fed mice

AimsLiver glycogen catabolism was evaluated in male Swiss mice fed a high-fat diet rich in saturated fatty acids (HFD) or normal fat diet (NFD) during one week. Main methodsLiver glycogenolysis (LG) and liver glucose production (LGP) were measured either under basal or stimulated conditions (infusion of glycogenolytic agents). Thus, isolated perfused livers from HFD and NFD mice were infused with glycogenolytic agents, i.e., glucagon, epinephrine, phenylephrine, isoproterenol, adenosine-3′-5′-cyclic monophosphate (cAMP), N6,2′-O-dibutyryl-cAMP (DB-cAMP), 8-bromoadenosine-cAMP (8-Br-cAMP) or N6-monobutyryl-cAMP (N6-MB-cAMP). Moreover, glycemia and liver glycogen content were measured. Key findingsGlycemia, liver glycogen content and basal rate of LGP and LG were not influenced by the HFD. However, LGP and LG were lower (p<0.05) in HFD mice during the infusions of glucagon (1nM), epinephrine (20μM) or phenylephrine (20μM). In contrast, the activation of LGP and LG during the infusion of isoproterenol (20μM) was not different (HFD vs. NFD). Because glucagon showed the most prominent response, the effect of cAMP, its intracellular mediator, on LGP and LG was investigated. cAMP (150μM) showed lower activation of LGP and LG in the HFD group. However, the activation of LGP and LG was not influenced by HFD whether DB-cAMP (3μM), 8-Br-cAMP (3μM) or N6-MB-cAMP (3μM) were used. SignificanceThe activation of LGP and LG depends on the intracellular availability of cAMP. It can be concluded that cAMP played a pivotal role on the activation of LG in high-fat diet fed mice.

Effects of N-adamantyl-4-methylthiazol-2-amine on hyperglycemia, hyperlipidemia and oxidative stress in streptozotocin-induced diabetic rats

Thiazole derivatives are attractive candidates for drug development because they can be efficiently synthesized and are active against a number of diseases and conditions, including diabetes. In our present study, we investigated the anti-inflammatory and antioxidant properties of N-adamantyl-4-methylthiazol-2-amine (KHG26693), a new thiazole derivative, in a streptozotocin (STZ)-induced model of diabetes mellitus. STZ-induced diabetic rats were intraperitoneally administered KHG26693 (3mg/kg-body weight/day) for 4 weeks. KHG26693 administration significantly decreased blood glucose, triglycerides, and cholesterol and increased insulin. KHG26693 also suppressed several inflammatory responses in STZ-induced diabetic rats, as evidenced by decreased levels of serum tumor necrosis factor-α, interleukin-1β, and nitric oxide. Additionally, KHG26693 significantly modulated hepatic lipid peroxidation, catalase and superoxide dismutase activity, and the nonenzymatic antioxidant status (e.g., vitamins C and E), and reduced the glutathione content. These anti-inflammatory/antioxidative actions occurred as a result of the downregulation of inducible nitric oxide synthase and nuclear factor-kappa B. Taken together, our results suggest that KHG26693 successfully reduces the production of oxidative stress in STZ-induced diabetic rats by regulating the oxidation–reduction system, specifically increasing antioxidant capacity. Furthermore, KHG26693 treatment significantly reverted the key enzymes of glucose metabolism, such as glucokinase, glucose-6-phosphatase, glycogen synthase, glycogen phosphorylase, and fructose-1,6-bisphosphatase, to near-normal levels in liver tissues. These results indicate that KHG26693 normalizes disturbed glucose metabolism by enhancing glucose utilization and decreasing liver glucose production via insulin release, suggesting the possibility of future diabetes treatments.

Hypothalamic AMPK activation blocks lipopolysaccharide inhibition of glucose production in mice liver

Endotoxic hypoglycaemia has an important role in the survival rates of septic patients. Previous studies have demonstrated that hypothalamic AMP-activated protein kinase (hyp-AMPK) activity is sufficient to modulate glucose homeostasis. However, the role of hyp-AMPK in hypoglycaemia associated with endotoxemia is unknown. The aims of this study were to examine hyp-AMPK dephosphorylation in lipopolysaccharide (LPS)-treated mice and to determine whether pharmacological hyp-AMPK activation could reduce the effects of endotoxemia on blood glucose levels. LPS-treated mice showed reduced food intake, diminished basal glycemia, increased serum TNF-α and IL-1β levels and increased hypothalamic p-TAK and TLR4/MyD88 association. These effects were accompanied by hyp-AMPK/ACC dephosphorylation. LPS-treated mice also showed diminished liver expression of PEPCK/G6Pase, reduction in p-FOXO1, p-AMPK, p-STAT3 and p-JNK level and glucose production. Pharmacological hyp-AMPK activation blocked the effects of LPS on the hyp-AMPK phosphorylation, liver PEPCK expression and glucose production. Furthermore, the effects of LPS were TLR4-dependent because hyp-AMPK phosphorylation, liver PEPCK expression and fasting glycemia were not affected in TLR4-mutant mice. These results suggest that hyp-AMPK activity may be an important pharmacological target to control glucose homeostasis during endotoxemia.

Hypothalamic inhibition of acetyl-CoA carboxylase stimulates hepatic counter-regulatory response independent of AMPK activation in rats.

BackgroundHypothalamic AMPK acts as a cell energy sensor and can modulate food intake, glucose homeostasis, and fatty acid biosynthesis. Intrahypothalamic fatty acid injection is known to suppress liver glucose production, mainly by activation of hypothalamic ATP-sensitive potassium (K(ATP)) channels. Since all models employed seem to involve malonyl-CoA biosynthesis, we hypothesized that acetyl-CoA carboxylase can modulate the counter-regulatory response independent of nutrient availability.Methodology/Principal FindingsIn this study employing immunoblot, real-time PCR, ELISA, and biochemical measurements, we showed that reduction of the hypothalamic expression of acetyl-CoA carboxylase by antisense oligonucleotide after intraventricular injection increased food intake and NPY mRNA, and diminished the expression of CART, CRH, and TRH mRNA. Additionally, as in fasted rats, in antisense oligonucleotide-treated rats, serum glucagon and ketone bodies increased, while the levels of serum insulin and hepatic glycogen diminished. The reduction of hypothalamic acetyl-CoA carboxylase also increased PEPCK expression, AMPK phosphorylation, and glucose production in the liver. Interestingly, these effects were observed without modification of hypothalamic AMPK phosphorylation.Conclusion/SignificanceHypothalamic ACC inhibition can activate hepatic counter-regulatory response independent of hypothalamic AMPK activation.

Changes in liver gluconeogenesis during the development of Walker‐256 tumour in rats

Few studies have investigated liver gluconeogenesis in cancer and there is no agreement as to whether the activity of this pathway is increased or decreased in this disease. The aim of this study was to evaluate gluconeogenesis from alanine, pyruvate and glycerol, and related metabolic parameters in perfused liver from Walker-256 tumour-bearing rats on days 5 (WK5 group), 8 (WK8 group) and 12 (WK12 group) of tumour development. There was reduction (P < 0.05) of liver glucose production from alanine and pyruvate in WK5, WK8 and WK12 groups, which was accompanied by a decrease (P < 0.05) in oxygen consumption. Moreover, there was higher (P < 0.05) pyruvate and lactate production from alanine in the WK5 group and a marked reduction (P < 0.05) of pyruvate and urea production from alanine in the WK12 group. In addition, liver glucose production and oxygen consumption from glycerol were not reduced in WK5, WK8 and WK12 groups. Thus the, the results show inhibition of hepatic gluconeogenesis from alanine and pyruvate, but not from glycerol, on days 5, 8 and 12 of Walker-256 tumour development, which can be attributed to the metabolic step in which the substrate enters the gluconeogenic pathway.

Oral Glutamine Is Superior Than Oral Glucose to Promote Glycemia Recovery in Mice Submitted to Insulin-Induced Hypoglycemia

The effect of the oral administration of blood glucose precursors on glycemia recovery and liver glucose production in fasted mice subjected to insulin-induced hypoglycemia (IIH) was investigated. IIH was obtained with increasing doses (from 0.5 to 2.0 U·kg−1) of intraperitoneal regular insulin where glycemia was evaluated from 0 to 300 min after insulin injection. The dose of 1.0 U·kg−1 showed the best results, that is, a clear glycemia recovery phase without convulsions or deaths. Thus, this dose was used in all experiments. Afterwards, mice submitted to IIH received orally by gavage: saline (control group), glucose (100 mg·kg−1), glycerol (100 mg·kg−1), lactate (100 mg·kg−1), alanine (100 mg·kg−1), or glutamine (100 mg·kg−1). It was observed that glutamine was more effective in promoting glycemia recovery if compared with glucose, lactate, glycerol, or alanine. In agreement with these results, the best performance in terms of liver glucose production was obtained when glutamine was used as glucose precursors. These results open perspectives for clinical studies to investigate the impact of oral administration of gluconeogenic amino acids to promote glycemia recovery during hypoglycemia.

Splanchnic net balance of oxygen and metabolites in response to a discontinuous mesenteric vein infusion of ammonium in sheep

To simulate daily episodes of high absorption associated with the intake of diets with high N content, four wethers (42 ± 3.4 kg body weight), fitted with permanent catheters in the femoral artery and splanchnic vessels, were infused with 340 μmol into the mesenteric vein for 3 h, during the morning meal, over seven consecutive days. On the 7th day, mass transfers of , urea, glucose, lactate, ß-OH-butyrate and O2 were measured across portal-drained viscera (PDV), liver and splanchnic tissues during the last 90 min of the infusion. Measurements were repeated on the following day, at the same time, without the infusion. Plasma concentration in the portal vein (+332 μm; p = 0.006), portal absorption (+424 μmol/min; p < 0.001), liver uptake (+375 μmol/min; p = 0.003) and urea N production (+338 μmol/min; p = 0.059) were higher during infusion. Mass transfers of urea, glucose, lactate, ß-OH-butyrate and O2 across the PDV, and glucose, lactate, ß-OH-butyrate and O2 across the liver, were not altered by the infusion. Results suggest that a daily, discontinuous increase in portal flow during a meal stimulates liver removal and urea N production but does not significantly affect liver glucose production and O2 consumption in sheep.

Infliximab treatment prevents hyperglycemia and the intensification of hepatic gluconeogenesis in an animal model of high fat diet-induced liver glucose overproduction

The effect of infliximab on gluconeogenesis in an animal model of diet-induced liver glucose overproduction was investigated. The mice were treated with standard diet (SD group) or high fat diet (HFD group). HFD group were randomly divided and treated either with saline (100 µl/dose, ip, twice a day) or infliximab (10 µg in 100 µl saline per dose, ip, twice a day, i.e., 0.5 mg/kg per day). SD group also received saline. The treatment with infliximab or saline started on the first day of the introduction of the HFD and was maintained during two weeks. After this period, the mice were fasted (15 h) and anesthetized. After laparotomy, blood was collected for glucose determination followed by liver perfusion in which L-alanine (5 mM) was used as gluconeogenic substrate. HFD group treated with saline showed higher (p < 0.05) liver glucose production from L-alanine and fasting hyperglycemia. However, these metabolic changes were prevented by infliximab treatment. Therefore, this study suggested that infliximab could prevent the glucose overproduction and hyperglycemia related with glucose intolerance and type 2 diabetes.

Inhibition of hypothalamic inflammation reverses diet-induced insulin resistance in the liver.

Defective liver gluconeogenesis is the main mechanism leading to fasting hyperglycemia in type 2 diabetes, and, in concert with steatosis, it is the hallmark of hepatic insulin resistance. Experimental obesity results, at least in part, from hypothalamic inflammation, which leads to leptin resistance and defective regulation of energy homeostasis. Pharmacological or genetic disruption of hypothalamic inflammation restores leptin sensitivity and reduces adiposity. Here, we evaluate the effect of a hypothalamic anti-inflammatory approach to regulating hepatic responsiveness to insulin. Obese rodents were treated by intracerebroventricular injections, with immunoneutralizing antibodies against Toll-like receptor (TLR)4 or tumor necrosis factor (TNF)α, and insulin signal transduction, hepatic steatosis, and gluconeogenesis were evaluated. The inhibition of either TLR4 or TNFα reduced hypothalamic inflammation, which was accompanied by the reduction of hypothalamic resistance to leptin and improved insulin signal transduction in the liver. This was accompanied by reduced liver steatosis and reduced hepatic expression of markers of steatosis. Furthermore, the inhibition of hypothalamic inflammation restored defective liver glucose production. All these beneficial effects were abrogated by vagotomy. Thus, the inhibition of hypothalamic inflammation in obesity results in improved hepatic insulin signal transduction, leading to reduced steatosis and reduced gluconeogenesis. All these effects are mediated by parasympathetic signals delivered by the vagus nerve.

Coexistence of insulin resistance and increased glucose tolerance in pregnant rats: A physiological mechanism for glucose maintenance

AimThe contribution of insulin resistance (IR) and glucose tolerance to the maintenance of blood glucose levels in non diabetic pregnant Wistar rats (PWR) was investigated. Main methodsPWR were submitted to conventional insulin tolerance test (ITT) and glucose tolerance test (GTT) using blood sample collected 0, 10 and 60min after intraperitoneal insulin (1U/kg) or oral (gavage) glucose (1g/kg) administration. Moreover, ITT, GTT and the kinetics of glucose concentration changes in the fed and fasted states were evaluated with a real-time continuous glucose monitoring system (RT-CGMS) technique. Furthermore, the contribution of the liver glucose production was investigated. Key findingsConventional ITT and GTT at 0, 7, 14 and 20days of pregnancy revealed increased IR and glucose tolerance after 20days of pregnancy. Thus, this period of pregnancy was used to investigate the kinetics of glucose changes with the RT-CGMS technique. PWR (day 20) exhibited a lower (p<0.05) glucose concentration in the fed state. In addition, we observed IR and increased glucose tolerance in the fed state (PWR-day 20 vs. day 0). Furthermore, our data from glycogenolysis and gluconeogenesis suggested that the liver glucose production did not contribute to these changes in insulin sensitivity and/or glucose tolerance during late pregnancy. SignificanceIn contrast to the general view that IR is a pathological process associated with gestational diabetes, a certain degree of IR may represent an important physiological mechanism for blood glucose maintenance during fasting.

Time sequence of the intensification of the liver glucose production induced by high‐fat diet in mice

It is well established that the development of insulin resistance shows a temporal sequence in different organs and tissues. Moreover, considering that the main aspect of insulin resistance in liver is a process of glucose overproduction from gluconeogenesis, we investigated if this metabolic change also shows temporal sequence. For this purpose, a well-established experimental model of insulin resistance induced by high-fat diet (HFD) was used. The mice received HFD (HFD group) or standard diet (COG group) for 1, 7, 14 or 56 days. The HFD group showed increased (P < 0.05 versus COG) epididymal, retroperitoneal and inguinal fat weight from days 1 to 56. In agreement with these results, the HFD group also showed higher body weight (P < 0.05 versus COG) from days 7 to 56. Moreover, the changes induced by HFD on liver gluconeogenesis were progressive because the increment (P < 0.05 versus COG) in glucose production from l-lactate, glycerol, l-alanine and l-glutamine occurred 7, 14, 56 and 56 days after the introduction of the HFD schedule, respectively. Furthermore, glycaemia and cholesterolemia increased (P < 0.05 versus COG) 14 days after starting the HFD schedule. Taken together, the results suggest that the intensification of liver gluconeogenesis induced by an HFD is not a synchronous 'all-or-nothing process' but is specific for each gluconeogenic substrate and is integrated in a temporal manner with the progressive augmentation of fasting glycaemia.

Venlafaxine and atenolol disrupt epinephrine-stimulated glucose production in rainbow trout hepatocytes

The beta-blocker atenolol (ATEN), and the selective serotonin and norepinephrine reuptake inhibitor, venlafaxine (VEN) are found in municipal wastewater effluents, but little is known about the effect of these pharmaceuticals on aquatic animals. We tested the hypothesis that VEN and ATEN disrupt acute stress mediated glucose production in fish liver. To this end, rainbow trout ( Oncorhynchus mykiss) hepatocytes were exposed in vitro to different concentrations (0, 0.1, 10, 1000 nM) of VEN or ATEN and glucose production in response to either cortisol or epinephrine (two key stress hormones) was ascertained. Both VEN and ATEN did not affect either the unstimulated or cortisol (100 ng/mL)-stimulated glucose release over a 24 h period. The acute (3 h) unstimulated glucose production by isolated hepatocytes in suspension was also not modified by ATEN, while VEN (100 and 1000 nM) reduced basal glucose release. However, ATEN, even at concentration as low as 0.01 nM completely abolished epinephrine (1 μM)-induced glucose production in trout hepatocytes. Interestingly, VEN also suppressed epinephrine-induced glucose production but only at higher concentrations (100 and 1000 nM). Neither VEN nor ATEN significantly impacted the glucose production in response to either 8-bromo-cAMP (cAMP analogue) or glucagon (a metabolic hormone that increases glucose production) stimulation. ATEN but not VEN attenuated the epinephrine-induced increase in glucose transporter 2 (GLUT2) mRNA abundance in trout hepatocytes. Taken together, our results suggest that the impact of ATEN and VEN on glucose production involves inhibition of β-adrenoceptor signaling in trout hepatocytes. Overall, VEN and ATEN are beta-blockers and may disrupt the adaptive acute glucose response to a secondary stressor in rainbow trout.

An activating mutation of AKT2 and human hypoglycemia.

Pathological fasting hypoglycemia in humans is usually explained by excessive circulating insulin or insulin-like molecules or by inborn errors of metabolism impairing liver glucose production. We studied three unrelated children with unexplained, recurrent, and severe fasting hypoglycemia and asymmetrical growth. All were found to carry the same de novo mutation, p.Glu17Lys, in the serine/threonine kinase AKT2, in two cases as heterozygotes and in one case in mosaic form. In heterologous cells, the mutant AKT2 was constitutively recruited to the plasma membrane, leading to insulin-independent activation of downstream signaling. Thus, systemic metabolic disease can result from constitutive, cell-autonomous activation of signaling pathways normally controlled by insulin.

Comparative effects of short-term and long-term insulin-induced hypoglycemia on glucose production in the perfused livers of weaned rats

The liver glucose production (LGP) levels of 15-h overnight fasted weaned rats submitted to short-term insulin-induced hypoglycemia (ST-IIH) and long-term IIH (LT-IIH) were compared. Experiments to characterize ST-IIH or LT-IIH that followed an intraperitoneal (ip) injection (1.0U/kg) of regular (ST-IIH) or insulin detemir (LT-IIH) were performed and glycemia were measured 0 (normoglycemic control), 0.5h (ST-IIH), 4h and 6h (LT-IIH) later. The values of glycemia (mg/dl) were 77.8±7.2 (normoglycemic control), 26.2±6.1 (ST IIH 0.5h), 21.2±7.6 (LT-IIH 4h) and 35.3±14.5 (LT-IIH 6.0). The LGP levels were measured in the rats submitted to ST-IIH (0.5h) and LT-IIH (4h or 6h). The rats that received ip saline were used as the normoglycemic control group (COG). The livers from the COG and IIH groups (ST-IIH or LT-IIH) were perfused in situ with infusion of L-alanine (5mM), L-glutamine (10mM), glutamine dipeptide (5mM), L-lactate (2mM) or glycerol (2mM). The ST-IIH rats showed a higher LGP level than COG group following the L-glutamine infusion (p<0.05), but the LGP levels that were measured following the L-lactate, L-alanine, glutamine dipep-tide (5mM), L-lactate (2mM) or glycerol infusion remained unchanged. Moreover, if the period of IIH was expanded to 4h following insulin injection, the LGP levels induced by L-alanine, glutamine dipeptide or glycerol infusion also increased (p<0.05, LT-IIH vs. COG). However, the LGP from the L-lactate infusion remained unchanged until 6h after insulin injection. In conclusion, these results suggest that the intensification of liver gluconeogenesis during ST-IIH and LT-IIH in weaned rats is not a synchronous “all or nothing” process; instead, this process integrated in a temporal manner and is specific for each gluconeogenic substrate.

Lipids in the Wrong Place: Visceral Fat and Nonalcoholic Steatohepatitis

There is substantial evidence in both animal (1,2) and human models of lipodystrophy (3) that the accumulation of excessive fat in ectopic sites that normally lack adipose tissue, such as the skeletal muscle and the liver, may trigger whole-body insulin resistance. This article will review the evidence showing the condition of fatty liver as an independent and perhaps better indicator of disease than excessive accumulation of visceral adipose tissue. On the basis of the hypothesis that a fatty liver may increase the risk of cardiovascular disease (CVD), this article will not only focus on the metabolic-related features, but also on the cardiovascular risk factors affecting individuals with fatty liver. The biomarkers of CVD will therefore be analyzed in patients with fatty livers. Finally, epidemiological evidence linking fatty liver to the development of type 2 diabetes and CVD will be given. ### Metabolic features of individuals with insulin impairment: what contributes more, fatty liver or visceral adipose tissue? Nonalcoholic fatty liver disease (NAFLD) is often found together with insulin resistance syndrome (4,5) and is commonly found in those suffering from visceral adiposity (3,4). Multiple insulin-related metabolic abnormalities in both organs and tissues have been reported, including impaired insulin-mediated inhibition of hepatic glucose production in livers (4–8), impaired insulin-stimulated glucose metabolism in skeletal muscle tissue (4–9), and impaired insulin-dependent control of lipolysis in adipose tissue (4–10). These metabolic abnormalities are often present along with the increased visceral adipose tissue and a fatty liver. It is very difficult to determine the biological relevance of each anthropometric feature because they are so commonly found together in humans. Recent studies have shown that of these two indicators, i.e., intrahepatic fat content (IHF) and excess visceral adipose tissue, IHF is statistically a more relevant indicator than excess visceral adipose tissue. In insulin-treated type 2 diabetic patients, the IHF content was more …

Uncoupling hypoxia signaling from oxygen sensing in the liver results in hypoketotic hypoglycemic death

As the ultimate electron acceptor in oxidative phosphorylation, oxygen plays a critical role in metabolism. When oxygen levels drop, heterodimeric hypoxia-inducible factor (Hif) transcription factors become active and facilitate adaptation to hypoxia. Hif regulation by oxygen requires the protein von Hippel-Lindau (pVhl) and pVhl disruption results in constitutive Hif activation. The liver is a critical organ for metabolic homeostasis, and Vhl inactivation in hepatocytes results in a Hif-dependent shortening in life span. While albumin-Cre;VhlF/F mice develop hepatic steatosis and impaired fatty acid oxidation, the variable penetrance and unpredictable life expectancy has made the cause of death elusive. Using a system in which Vhl is acutely disrupted and a combination of ex vivo liver perfusion studies and in vivo oxygen measurements, we demonstrate that Vhl is essential for mitochondrial respiration in vivo. Adenovirus-Cre mediated acute Vhl disruption in the liver caused death within days. Deprived of pVhl, livers accumulated tryglicerides and circulating ketone and glucose levels dropped. The phenotype was reminiscent of inborn defects in fatty acid oxidation and of fasted PPARα-deficient mice and while death was unaffected by pharmacologic PPARα activation, it was delayed by glucose administration. Ex vivo liver perfusion analyses and acylcarnitine profiles showed mitochondrial impairment and a profound inhibition of liver ketone and glucose production. By contrast, other mitochondrial functions, such as ureagenesis, were unaffected. Oxygen consumption studies revealed a marked suppression of mitochondrial respiration, which, as determined by magnetic resonance oximetry in live mice, was accompanied by a corresponding increase in liver pO2. Importantly, simultaneous inactivation of Hif-1β suppressed liver steatosis and rescued the mice from death. These data demonstrate that constitutive Hif activation in mice is sufficient to suppress mitochondrial respiration in vivo and that no other pathway exists in the liver that can allow oxygen utilization when Hif is active precluding thereby metabolic collapse.

Insulin sensitivity and liver glucose production in the rat are influenced by lifetime food restriction

In both humans and rats, food restriction leads to increased insulin sensitivity and predisposition to hypoglycemia. We hypothesized that metabolic responses to hypoglycemic episodes could be altered in food-restricted rats. To test our hypothesis, plasma glucose levels and liver glucose production during insulin-induced hypoglycemia were assessed. Rats either had free access to food (FF group) or were food restricted from birth (FR group). As adults, they were subjected to insulin-induced hypoglycemia after an overnight fast. Plasma glucose was measured before (time 0) the intraperitoneal injection of insulin (1 U/kg) and at regular intervals for 300 minutes. Some FF and FR rats received oral glucose (100 mg/kg) 15 minutes after insulin injection, and the same time intervals were investigated. The FR rats showed a larger decrease and slower recovery of plasma glucose than the FF group, and this was not influenced by oral glucose. Liver glucose production from glycogenolysis and gluconeogenesis (ie, before and during the infusion of L-alanine) was higher and lower, respectively, in the FR rats than in the FF rats, either with or without oral glucose before liver perfusion. Preference for glycogenolysis could be a metabolic adaptation for the maintenance of plasma glucose levels during fasting despite lower food availability in the FR rats. However, long-term FR increased the severity of hypoglycemia and impaired plasma glucose recovery. In addition, hypoglycemia could not be prevented by glucose administration. Therefore, food restriction in individuals with intensive insulin therapy should be more rigorously examined.