Insulin signaling and glucose homeostasis in mice lacking protein tyrosine phosphatase alpha

Studies in cultured cells have implicated protein tyrosine phosphatase alpha (PTPalpha) as a potential regulator of insulin signaling. The physiological role of PTPalpha in insulin action was investigated using gene-targeted mice deficient in PTPalpha. PTPalpha-null animals had normal body weights and circulating levels of glucose and insulin in random fed and fasted states. In glucose and insulin tolerance tests, their efficiency of blood glucose clearance was comparable to wild-type mice. Kinetics and extents of insulin-stimulated insulin receptor and IRS-1 tyrosine phosphorylation were similar in wild-type and PTPalpha(-/-) liver, muscle, and adipose tissue. However, the association of IRS-1 and PI 3-K was altered in PTPalpha(-/-) liver, with increased insulin-independent and reduced insulin-stimulated association compared to wild-type samples. This did not affect activation of the downstream signaling effector Akt. Our data indicate that PTPalpha is not a negative regulator of insulin signaling and does not perform an essential role in mediating the physiological action of insulin.     

Insulin and beta receptor modulation of K homeostasis in nephrectomized dogs with hyperkalemia

In nephrectomized dogs infused with 2 mEq KCl/kg/hr a homeostatic mechanism retards the development of hyperkalemia by transferring about 70% of the K load to intracellular fluid. beta Adrenergic receptor activity is importantly involved in the transfer process; halting it with propranolol reduces the proportion transferred to less than 35%. The addition of pancreatectomy increases the involvement of beta receptor activity; propranolol treatment now reduces the proportion transferred to less than 20%. Insulin treatment, on the other hand, not only improves transfer of a K load, it also alters the response to propranolol. Nephrectomized dogs treated with 2 U insulin/kg/hr deposit some 80% of the infused K in intracellular fluid. After beta receptor blockade, nearly 90% is transferred. The results suggest that in the K homeostatic mechanism of nephrectomized dogs, insulin and beta receptors may be reciprocally related. K transfer mediated by beta receptors improves after pancreatectomy, and insulin mediated K transfer improves after beta receptors are inactivated.     

Glucose homeostasis during canine pregnancy: Insulin resistance, ketosis, and hypoglycemia

Through a variety of mechanisms, pregnancy causes insulin resistance, which suppresses the intracellular transport of glucose and increases blood glucose concentrations. In the extreme, gestational diabetes (GDM) mellitus may develop. In addition to insulin resistance, pregnant bitches have decreased ability to produce glucose via gluconeogenesis, glycogenolysis and lipolysis, because the normal multi-factorial responses to hypoglycemia are blunted late in pregnancy. Simply fasting late-pregnant bitches is sufficient to cause blood glucose and insulin concentrations to decrease and ketones to increase. The purpose of this paper is to review the clinical implications of pregnancy-associated changes in glucose homeostasis in bitches.     

Glucose homeostasis during canine pregnancy: Insulin resistance,ketosis, and hypoglycemia

Through a variety of mechanisms, pregnancy causes insulin resistance, which suppresses the intracellular transport of glucose and increases blood glucose concentrations. In the extreme, gestational diabetes (GDM) mellitus may develop. In addition to insulin resistance, pregnant bitches have decreased ability to produce glucose via gluconeogenesis, glycogenolysis and lipolysis, because the normal multi-factorial responses to hypoglycemia are blunted late in pregnancy. Simply fasting late-pregnant bitches is sufficient to cause blood glucose and insulin concentrations to decrease and ketones to increase. The purpose of this paper is to review the clinical implications of pregnancy-associated changes in glucose homeostasis in bitches.     

Insulin resistance and whole body energy homeostasis in obese adolescents with fatty liver disease

Obese adolescents are at risk of developing NAFLD and type 2 diabetes. We measured noninvasively the IHF content of obese adolescents to ascertain whether it is associated with insulin resistance and abnormal energy homeostasis. IHF content, whole body energy homeostasis, insulin sensitivity, and body composition were measured using localized hepatic (1)H-MRS, indirect calorimetry, fasting-derived and 3-h-OGTT-derived surrogate indexes (HOMA2 and WBISI), and DEXA, respectively, in 54 obese adolescents (24 female and 30 male, age 13 +/- 2 yr, BMI >99th percentile for their age and sex). NAFLD (defined as IHF content >5% wet weight) was found in 16 individuals (30%) in association with higher ALT (P < 0.006), Hb A(1c) (P = 0.021), trunk fat content (P < 0.03), and lower HDL cholesterol (P < 0.05). Individuals with NAFLD had higher fasting plasma glucose (89 +/- 8 vs. 83 +/- 9 mg/dl, P = 0.01) and impaired insulin sensitivity (HOMA2 and WBISI, P < 0.05). Meanwhile, parameters of insulin secretion were unaffected. Their reliance on fat oxidation in the fasting state was lower (RQ 0.83 +/- 0.08 vs. 0.77 +/- 0.05, P < 0.01), and their ability to suppress it during the oral glucose challenge was impaired (P < 0.05) vs. those with normal IHF content. When controlling for trunk fat content, the correlation between IHF content and insulin sensitivity was weakened, whereas the correlation with fasting lipid oxidation was maintained. In conclusion, NAFLD is common in childhood obesity, and insulin resistance is present in association with increased trunk fat content. In contrast, the rearrangement of whole body substrate oxidation in these youngsters appeared to be an independent feature.     

Low-carbohydrate diets affect energy balance and fuel homeostasis differentially in lean and obese rats

In parallel with increased prevalence of overweight people in affluent societies are individuals trying to lose weight, often using low-carbohydrate diets. Nevertheless, long-term metabolic consequences of those diets, usually high in (saturated) fat, remain unclear. Therefore, we investigated long-term effects of high-fat diets with different carbohydrate/protein ratios on energy balance and fuel homeostasis in obese (fa/fa) Zucker and lean Wistar rats. Animals were fed high-carbohydrate (HC), high-fat (HsF), or low-carbohydrate, high-fat, high-protein (LC-HsF-HP) diets for 60 days. Both lines fed the LC-HsF-HP diet displayed reduced energy intake compared with those fed the HsF diet (Zucker, -3.7%) or the HC diet (Wistar rats, -12.4%). This was not associated with lower weight gain relative to HC fed rats, because of increased food efficiencies in each line fed HsF and particularly LC-HsF-HP food. Zucker rats were less glucose tolerant than Wistar rats. Lowest glucose tolerances were found in HsF and particularly in LC-HsF-HP-fed animals irrespective of line, but this paralleled reduced plasma adiponectin levels, elevated plasma resistin levels, higher retroperitoneal fat masses, and reduced insulin sensitivity (indexed by insulin-induced hypoglycemia) only in Wistar rats. In Zucker rats, however, improved insulin responses during glucose tolerance testing and tendency toward increased insulin sensitivities were observed with HsF or LC-HsF-HP feeding relative to HC feeding. Thus, despite adverse consequences of LC-HsF diets on blood glucose homeostasis, principal differences exist in the underlying hormonal regulatory mechanisms, which could have benefits for B-cell functioning and insulin action in the obese state but not in the lean state.     

Insulin/TOR signaling in growth and homeostasis: a view from the fly world

The insulin/TOR pathway is a conserved regulator of cell and organism growth in metazoans. Over the last several years, an array of signaling inputs to this pathway has been defined. However the growth-regulatory outputs are less clear. Drosophila has proven to be a powerful genetic model system in which to study insulin/TOR signaling. This review highlights recent studies in Drosophila that have identified essential outputs and key effectors of the pathway. These include the regulation of ribosome synthesis, mRNA translation, autophagy and endocytosis, through downstream effectors such as Myc, FOXO, HIF1-alpha, TIF-IA, 4EBP and Atg1. This network of outputs and effectors can regulate cell and organismal metabolism, and is essential for the control of tissue growth, responses to starvation and stress, and aging. The mechanisms identified in Drosophila likely operate in most metazoans, and are relevent to our understanding of diseases caused by aberrent insulin/TOR signaling such as cancer, diabetes and obesity.     

Insulin receptor substrate 3 is not essential for growth or glucose homeostasis.

The insulin receptor substrates (IRS) 1 and 2 are required for normal growth and glucose homeostasis in mice. To determine whether IRS-3, a recently cloned member of the IRS family, is also involved in the regulation of these, we have generated mice with a targeted disruption of the IRS-3 gene and characterized them. Compared with wild-type mice, the IRS-3-null mice showed normal body weight throughout development, normal blood glucose levels in the fed and fasted state and following an oral glucose bolus, and normal fed and fasted plasma insulin levels. IRS-3 is most abundant in adipocytes and is tyrosine-phosphorylated in response to insulin in these cells. Therefore, isolated adipocytes were analyzed for changes in insulin effects. Insulin-stimulated glucose transport in the adipocytes from the IRS-3-null mice was the same as in wild-type cells. The extent of tyrosine phosphorylation of IRS-1/2 following insulin stimulation was similar in adipocytes from IRS-3-null and wild-type mice, and the insulin-induced association of tyrosine-phosphorylated IRS-1/2 with phosphatidylinositol 3-kinase and SHP-2 was not detectably increased by IRS-3 deficiency. Thus, IRS-3 was not essential for normal growth, glucose homeostasis, and glucose transport in adipocytes, and in its absence no significant compensatory augmentation of insulin signaling through IRS-1/2 was evident.     

Peripheral effects of FAAH deficiency on fuel and energy homeostasis: role of dysregulated lysine acetylation

Background

FAAH (fatty acid amide hydrolase), primarily expressed in the liver, hydrolyzes the endocannabinoids fatty acid ethanolamides (FAA). Human FAAH gene mutations are associated with increased body weight and obesity. In our present study, using targeted metabolite and lipid profiling, and new global acetylome profiling methodologies, we examined the role of the liver on fuel and energy homeostasis in whole body FAAH^−/− mice.

Methodology/Principal Findings

FAAH^−/− mice exhibit altered energy homeostasis demonstrated by decreased oxygen consumption (Indirect calorimetry). FAAH^−/− mice are hyperinsulinemic and have adipose, skeletal and hepatic insulin resistance as indicated by stable isotope phenotyping (SIPHEN). Fed state skeletal muscle and liver triglyceride levels was increased 2–3 fold, while glycogen was decreased 42% and 57% respectively. Hepatic cholesterol synthesis was decreased 22% in FAAH^−/− mice. Dysregulated hepatic FAAH^−/− lysine acetylation was consistent with their metabolite profiling. Fasted to fed increases in hepatic FAAH^−/− acetyl-CoA (85%, p<0.01) corresponded to similar increases in citrate levels (45%). Altered FAAH^−/− mitochondrial malate dehydrogenase (MDH2) acetylation, which can affect the malate aspartate shuttle, was consistent with our observation of a 25% decrease in fed malate and aspartate levels. Decreased fasted but not fed dihydroxyacetone-P and glycerol-3-P levels in FAAH^−/− mice was consistent with a compensating contribution from decreased acetylation of fed FAAH^−/− aldolase B. Fed FAAH^−/− alcohol dehydrogenase (ADH) acetylation was also decreased.

Conclusions/Significance

Whole body FAAH deletion contributes to a pre-diabetic phenotype by mechanisms resulting in impairment of hepatic glucose and lipid metabolism. FAAH^−/− mice had altered hepatic lysine acetylation, the pattern sharing similarities with acetylation changes reported with chronic alcohol treatment. Dysregulated hepatic lysine acetylation seen with impaired FAA hydrolysis could support the liver''s role in fostering the pre-diabetic state, and may reflect part of the mechanism underlying the hepatic effects of endocannabinoids in alcoholic liver disease mouse models.     

Insulin release from the pancreas and fuel metabolism during late gestation in chemically diabetic rats

The effects of chemical diabetes and fasting on fuel metabolism and insulin secretory activity in late pregnancy were investigated. Female Wistar rats were made chemically diabetic (CD) by intravenous injection of streptozotocine (30 mg/kg) 2 weeks before conception. When CD pregnant rats were fed, plasma glucose and insulin levels were not significantly different from those of normal pregnant rats. Ketone body levels, however, were higher in CD pregnant rats than in normal pregnant rats, indicating insulin resistance in CD rats. Insulin secretion from the perfused pancreas caused by arginine or glucose was markedly decreased in CD pregnant rats. The pregnant rats were fasted for 2 days, from day 19 to 21 of gestation. Plasma glucose and insulin concentrations decreased similarly in the two groups, whereas ketone body concentrations in CD pregnant rats were significantly higher than those in normal pregnant rats. Glucose-induced insulin secretion by the perfused pancreas was markedly attenuated by fasting and was not significantly different in normal and CD pregnant rats. These observations suggest that diabetes mellitus accelerates starvation in late gestation, due to increased insulin resistance and poor insulin secretion, and that fasting in diabetic pregnancy amplifies ketogenesis.     

Comparison of the effects of pre-exercise feeding of glucose, glycerol and placebo on endurance and fuel homeostasis in man

Six men were studied during exercise to exhaustion on a cycle ergometer at 73% of VO2max following ingestion of glycerol, glucose or placebo. Five of the subjects exercised for longer on the glucose trial compared to the placebo trial (p less than 0.1; 108.8 vs 95.9 min). Exercise time to exhaustion on the glucose trial was longer (p less than 0.01) than on the glycerol trial (86.0 min). No difference in performance was found between the glycerol and placebo trials. The ingestion of glucose (lg X kg-1 body weight) 45 min before exercise produced a 50% rise in blood glucose and a 3-fold rise in plasma insulin at zero min of exercise. Total carbohydrate oxidation was increased by 26% compared to placebo and none of the subjects exhibited a fall in blood glucose below 4 mmol X 1-1 during the exercise. The ingestion of glycerol (lg X kg-1 body weight) 45 min before exercise produced a 340-fold increase in blood glycerol concentration at zero min of exercise, but did not affect resting blood glucose or plasma insulin levels; blood glucose levels were up to 14% higher (p less than 0.05) in the later stages of exercise and at exhaustion compared to the placebo or glucose trials. Both glycerol and glucose feedings lowered the magnitude of the rise in plasma FFA during exercise compared to placebo. Levels of blood lactate and alanine during exercise were not different on the 3 dietary treatments.(ABSTRACT TRUNCATED AT 250 WORDS)     

Genomics and homeostasis

The Cannon lecture this year illustrates how knowledge of DNA sequences of complex living organisms is beginning to shape the landscape of physiology in the 21st century. Enormous challenges and opportunities now exist for physiologists to relate the galaxy of genes to normal and pathological functions. The first extensive genomic systems biology map for cardiovascular and renal function was completed last year as well as a new hypothesis-generating tool ("physiological profiling") that enables us to hypothesize relationships between specific genes responsible for the regulation of regulatory pathways. Techniques of chromosomal substitution (consomic and congenic rats) are beginning to confirm statistical results from linkage analysis studies, narrow the regions of genetic interest for positional cloning, and provide genetically well-defined control strains for physiological studies. Patterns of gene expression identified by microarray and mapping of expressed genes to chromosomal sites are adding to the understanding of systems physiology. The previously unimaginable goal of connecting approximately 36,000 genes to the complex functions of mammalian systems is indeed well underway.     

Phosphatidylcholine and choline homeostasis

Phosphatidylcholine (PC) is made in mammalian cells from choline via the CDP-choline pathway. Animals obtain choline primarily from the diet or from the conversion of phosphatidylethanolamine (PE) to PC followed by catabolism to choline. The main fate of choline is the synthesis of PC. In addition, choline is oxidized to betaine in kidney and liver and converted to acetylcholine in the nervous system. Mice that lack choline kinase (CK) alpha die during embryogenesis, whereas mice that lack CKbeta unexpectedly develop muscular dystrophy. Mice that lack CTP:phosphocholine cytidylyltransferase (CT) alpha also die during early embryogenesis, whereas mice that lack CTbeta exhibit gonadal dysfunction. The cytidylyltransferase beta isoform also plays a role in the branching of axons of neurons. An alternative PC biosynthetic pathway in the liver uses phosphatidylethanolamine N-methyltransferase to catalyze the formation of PC from PE. Mice that lack the methyltransferase survive but die from steatohepatitis and liver failure when placed on a choline-deficient diet. Hence, choline is an essential nutrient. PC biosynthesis is required for normal very low density lipoprotein secretion from hepatocytes. Recent studies indicate that choline is recycled in the liver and redistributed from kidney, lung, and intestine to liver and brain when choline supply is attenuated.     

Ghrelin and glucose homeostasis

Ghrelin plays an important physiological role in modulating GH secretion, insulin secretion and glucose metabolism. Ghrelin has direct effects on pancreatic islet function. Also, ghrelin is part of a mechanism that integrates the physiological response to fasting. However, pharmacologic studies indicate the important obesogenic/diabetogenic properties of ghrelin. This is very likely of physiological relevance, deriving from a requirement to protect against seasonal periods of food scarcity by building energy reserves, predominantly in the form of fat. Available data indicate the potential of ghrelin blockade as a means to prevent its diabetogenic effects. Several studies indicate a negative correlation between ghrelin levels and the incidence of type 2 diabetes and insulin resistance. However, it is unclear if low ghrelin levels are a risk factor or a compensatory response. Direct antagonism of the receptor does not always have the desired effects, however, since it can cause increased body weight gain. Pharmacological suppression of the ghrelin/des-acyl ghrelin ratio by treatment with des-acyl ghrelin may also be a viable alternative approach which appears to improve insulin sensitivity. A promising recently developed approach appears to be through the blockade of GOAT activity, although the longer term effects of this treatment remain to be investigated.     

Calcium regulation and homeostasis

The use of techniques to visualize the stimulus-induced changes in [Ca2+]i that occur at the single cell level has revealed that intracellular Ca2+ signals can be remarkably organized in space (waves), as well as in time (oscillations). New insights are beginning to emerge into how these complex Ca2+ signals may be generated, and into how Ca2+ signals may be transmitted from cell to cell.