Weight-sparing effect of insulin detemir: a consequence of central nervous system-mediated reduced energy intake?

Insulin therapy is often associated with adverse weight gain. This is attributable, at least in part, to changes in energy balance and insulin's anabolic effects. Adverse weight gain increases the risk of poor macrovascular outcomes in people with diabetes and should therefore be mitigated if possible. Clinical studies have shown that insulin detemir, a basal insulin analogue, exerts a unique weight-sparing effect compared with other basal insulins. To understand this property, several hypotheses have been proposed. These explore the interplay of efferent and afferent signals between the muscles, brain, liver, renal and adipose tissues in response to insulin detemir and comparator basal insulins. The following models have been proposed: insulin detemir may reduce food intake through direct or indirect effects on the central nervous system (CNS); it may have favourable actions on hepatic glucose metabolism through a selective effect on the liver, or it may influence fluid homeostasis through renal effects. Studies have consistently shown that insulin detemir reduces energy intake, and moreover, it is clear that this shift in energy balance is not a consequence of reduced hypoglycaemia. CNS effects may be mediated by direct action, by indirect stimulation by peripheral mediators and/or via a more physiological counter-regulatory response to insulin through restoration of the hepatic-peripheral insulin gradient. Although the precise mechanism remains unclear, it is likely that the weight-sparing effect of insulin detemir can be explained by a combination of mechanisms. The evidence for each hypothesis is considered in this review.

The von Hippel-Lindau tumor suppressor gene inhibits hepatocyte growth factor/scatter factor-induced invasion and branching morphogenesis in renal carcinoma cells.

Loss of function in the von Hippel-Lindau (VHL) tumor suppressor gene occurs in familial and most sporadic renal cell carcinomas (RCCs). VHL has been linked to the regulation of cell cycle cessation (G(0)) and to control of expression of various mRNAs such as for vascular endothelial growth factor. RCC cells express the Met receptor tyrosine kinase, and Met mediates invasion and branching morphogenesis in many cell types in response to hepatocyte growth factor/scatter factor (HGF/SF). We examined the HGF/SF responsiveness of RCC cells containing endogenous mutated (mut) forms of the VHL protein (VHL-negative RCC) with that of isogenic cells expressing exogenous wild-type (wt) VHL (VHL-positive RCC). We found that VHL-negative 786-0 and UOK-101 RCC cells were highly invasive through growth factor-reduced (GFR) Matrigel-coated filters and exhibited an extensive branching morphogenesis phenotype in response to HGF/SF in the three-dimensional (3D) GFR Matrigel cultures. In contrast, the phenotypes of A498 VHL-negative RCC cells were weaker, and isogenic RCC cells ectopically expressing wt VHL did not respond at all. We found that all VHL-negative RCC cells expressed reduced levels of tissue inhibitor of metalloproteinase 2 (TIMP-2) relative to the wt VHL-positive cells, implicating VHL in the regulation of this molecule. However, consistent with the more invasive phenotype of the 786-0 and UOK-101 VHL-negative RCC cells, the levels of TIMP-1 and TIMP-2 were reduced and levels of the matrix metalloproteinases 2 and 9 were elevated compared to the noninvasive VHL-positive RCC cells. Moreover, recombinant TIMPs completely blocked HGF/SF-mediated branching morphogenesis, while neutralizing antibodies to the TIMPs stimulated HGF/SF-mediated invasion in vitro. Thus, the loss of the VHL tumor suppressor gene is central to changes that control tissue invasiveness, and a more invasive phenotype requires additional genetic changes seen in some but not all RCC lines. These studies also demonstrate a synergy between the loss of VHL function and Met signaling.

Metabolic vs. hedonic obesity: a conceptual distinction and its clinical implications.

Body weight is determined via both metabolic and hedonic mechanisms. Metabolic regulation of body weight centres around the 'body weight set point', which is programmed by energy balance circuitry in the hypothalamus and other specific brain regions. The metabolic body weight set point has a genetic basis, but exposure to an obesogenic environment may elicit allostatic responses and upward drift of the set point, leading to a higher maintained body weight. However, an elevated steady-state body weight may also be achieved without an alteration of the metabolic set point, via sustained hedonic over-eating, which is governed by the reward system of the brain and can override homeostatic metabolic signals. While hedonic signals are potent influences in determining food intake, metabolic regulation involves the active control of both food intake and energy expenditure. When overweight is due to elevation of the metabolic set point ('metabolic obesity'), energy expenditure theoretically falls onto the standard energy-mass regression line. In contrast, when a steady-state weight is above the metabolic set point due to hedonic over-eating ('hedonic obesity'), a persistent compensatory increase in energy expenditure per unit metabolic mass may be demonstrable. Recognition of the two types of obesity may lead to more effective treatment and prevention of obesity.

Beneficial effects of dietary acarbose in the streptozotocin-induced diabetic rat.

Diabetes is characterized by hyperphagia, polydipsia, polyuria, and elevations in blood and urinary glucose. It has also been documented that beta-adrenergic responsiveness is reduced in diabetes. The intestinal glucosidase inhibitor, acarbose (BAY G 5421), decreases postprandial glycemia by delaying carbohydrate absorption. The purpose of this study was to evaluate the effects of chronic acarbose treatment (20 and 40 mg/100 g of diet) on the metabolic and adrenergic parameters altered in streptozotocin (STZ) (50 mg/kg, intravenously [IV] )-induced diabetes. Metabolic parameters were measured daily for 8 weeks. Diabetic rats were hyperphagic, polydipsic, and polyuric within 1 week of STZ treatment. Acarbose treatment did not consistently effect the food intake but did reduce water intake, urinary output, blood glucose, and the urinary loss of glucose associated with STZ-induced diabetes. Adrenergic responses were assessed by monitoring the increase in tail skin temperature (TST) associated with administration of isoproterenol. Diabetic rats were less responsive than controls and acarbose treatment restored responses toward that of the controls. Additionally, 3H-NE release from the tail artery was elevated in the diabetic rat and restored to normal in the acarbose-treated animals. Collectively these data suggest that acarbose treatment is effective in reducing the severity of metabolic and autonomic complications associated with STZ-induced diabetes.

Patterns of hyperphagia in the Zucker obese rat: a role for fat cell size and number?

The hypothesis that adipocyte size and number influence feeding behavior, via as yet unidentified signals to the CNS, is reviewed. The proposal is made that, due to several metabolic alterations which favor lipid deposition, the genetically obese Zucker rat (fafa) may be an appropriate model in which to study feeding-adipose tissue relationships. Data from several studies are presented demonstrating that the developing male Zucker fatty rat displays hyperphagia during the growth period which reaches a peak, or "break point," and then declines such that intake of fatty and lean rats becomes comparable at approximately 20 weeks of age. Beyond week 20, cycles of hyperphagia of several weeks' duration can be detected in fatty rats. The above feeding changes are related to data showing that on a laboratory chow-type diet, adipocytes approach maximal size at 15-16 weeks in the fatty rat, while accelerated proliferation of adipocytes takes place following week 20. During growth, responding for food in an operant task by fatty rats varies in accord with the pattern of hyperphagia. Further studies in the fatty rat show that the duration and magnitude of developmental hyperphagia can be altered by manipulating the caloric density and macronutrient content of the diet, with fat containing diets leading to the earliest break point of developmental hyperphagia. Some theoretical problems with the notion of adipose tissue feedback control of feeding behavior are discussed.

Naltrexone and human eating behavior: a dose-ranging inpatient trial in moderately obese men.

To investigate the effects of the long-acting opiate antagonist naltrexone on spontaneous human eating behavior, eight moderately obese male paid volunteers were housed in a hospital metabolic unit for 28 days and offered palatable foods ad lib by a platter service method. Under double-blind conditions, equally divided doses of 100, 200 and 300 mg naltrexone, or an acetaminophen placebo, were administered twice daily in tablet form for 3-day periods each, according to a Latin Square design. The doses of naltrexone resulted in decreases of daily caloric intake from placebo level, but these reductions were neither statistically significant nor dose-related. When the averaged effects of the doses were compared to placebo, five subjects showed intake reductions but the overall intake reduction of 301.5 +/- 198.1 kcal/day (mean +/- SEM) was not statistically significant. Naltrexone administration failed to selectively alter intakes of individual meals and snacks or macronutrient consumption patterns. During active drug periods, subjects lost 0.62 +/- 0.22 lb over 3 days, while during the placebo period, subjects gained 0.46 +/- 0.68 lb. However, there was no reliable change of basal metabolic rate as a function of naltrexone administration. The present results, which indicate that naltrexone administration is relatively ineffective in reducing food intake and inducing body weight loss in obese humans, are thus in contrast with reports that administration of opiate antagonist agents promote significant reductions of food intake and attenuations of body weight gain in experimental animals.

Satiety in the obese Zucker rat: effects of carbohydrate type and acarbose (Bay g 5421).

Despite the obese Zucker rat's hyperphagia on carbohydrate diets such as laboratory chow, this laboratory has found that its satiety response to glucose and other simple sugars is comparable to that of its lean control rat. To further investigate carbohydrate satiety in the Zucker rat, the short-term feeding behavior of obese and lean rats was observed following intragastric infusions (7.2 kcal in 10 ml) of corn starch and the starch hydrolysates Polycose and dextrin. There were no reliable between-genotype differences in the feeding inhibitory effects of Polycose and dextrin. However, in obese rats, the satiety effect of corn starch was delayed and reduced compared to that observed in lean rats (p less than 0.04). To modify the effect of corn starch, rats were administered 0.2 or 0.6 mg/infusion of the carbohydrate digestive inhibitor acarbose (Bay g 5421). Acarbose significantly reduced the satiety effect of corn starch in lean rats (p less than 0.001), and further attenuated satiety in obese rats (p less than 0.02). Since secretion of pancreatic amylase, the enzyme that initiates starch digestion, is decreased in obese rats, this result suggests that alterations of digestive and/or absorptive processes may underlie the obese rat's impaired satiety response to complex carbohydrate.

Diet composition determines course of hyperphagia in developing Zucker obese rats.

Previous observations from this laboratory indicate that, during growth, the hyperphagia of the male genetically obese Zucker rat reaches a peak or "breakpoint" and then declines. To examine the effect of dietary macronutrient content on the course of hyperphagia, groups of male lean and obese rats were maintained from 5-28 weeks of age on powdered chow, or isocaloric diets (3.6 kcal/g) containing 72% of calories as corn oil, dextrose, or soy isolate protein (n = 5 lean and obese rats/diet). On chow, hyperphagia was maintained at a level of 7-8 g above lean control intake until a "breakpoint" was reached at 17 weeks, and obese intake declined to lean control level. On the fat diet, hyperphagia was increased to 10 g/day when a breakpoint was reached at 8 weeks. On the dextrose and protein diets, hyperphagia at a level of 3-4 g/day reached breakpoints at weeks 18 and 16, respectively. On all diets, the intakes of obese rats were precisely equal to the intakes of lean control rats by weeks 19-20. These data show that the magnitude and duration of hyperphagia in the developing obese rat are influenced by diet composition. Previously, we have proposed that the obese rat's hyperphagia arises from rapid adipocyte filling. Since high-fat diets facilitate adipocyte enlargement, the early "breakpoint" of hyperphagia seen with the high-fat diet may indicate that this feeding stimulation decreases as the fat cells of the obese rat approach maximal size.

Tolbutamide affects food ingestion in a manner consistent with its glycemic effects in the rat.

The sulphonylurea tolbutamide possesses the ability to stimulate insulin release, produce hypoglycemia and increase food intake; however, no study has investigated the effects of moderate doses which do not produce frank hypoglycemia. Forty male rats received injections of tolbutamide at 0, 5, 15, 25 or 50 mg/kg body weight. The injections terminated a 2-hr fast and occurred at light offset, insuring a meal. Food intakes were then recorded for two hr following injection. Tolbutamide at 5 and 15 mg doses decreased food intake during the first half-hour or hour, respectively. In parallel experiments, 10 rats were sampled for blood prior to injection of tolbutamide or saline at doses cited above, and again at 10 and 40 min following injection in the absence of food. Plasma was then analyzed for insulin and glucose. Both 5 and 15 mg tolbutamide produced a mild, reliable increase in insulin accompanied by a decrease of 5 to 15 mg/dl in plasma glucose. On the other hand, the 50 mg dose produced a marked increase in insulin and a decrease of approximately 25% in plasma glucose. Thus, the present studies suggest that when endogenous insulin levels are modestly raised by tolbutamide, such that only moderate reductions of circulating glucose were observed, decreases in food intake occur.

Diet composition alters the satiety effect of cholecystokinin in lean and obese Zucker rats.

Although exogenous administration of the peptide cholecystokinin (CCK) has been shown to reduce food intake in a variety of experimental situations, few studies have examined the influence of dietary content upon CCK's effectiveness, particularly in obese states. To evaluate the effectiveness of CCK administration in animals consuming high fat diets, groups of obese and lean Zucker rats were maintained on laboratory chow (CH), a high fat diet isocaloric to chow (IF), or a hypercaloric fat diet (HF). After a 17 hr fast, rats were given intraperitoneal injections of saline or ascending doses of 0.06 to 2.0 micrograms/kg of the synthetic octapeptide of CCK. On all diets, obese rats required higher doses of CCK to significantly reduce feeding and showed smaller intake reductions than lean rats (p less than 0.001). Despite higher baseline caloric intakes (p less than 0.001), rats of both genotypes maintained on HF displayed larger reductions of intake than those fed IF or CH (p less than 0.001). Intake reductions by either genotype maintained on IF or CH were not reliably different. The manner in which the satiety effect of CCK was enhanced in rats consuming the calorically dense, palatable HF diet is unclear but may be related to orosensory and/or postingestive attributes of the diet.

Iron appetite and latent learning in rats.

Two experiments are reported which show that rats are capable of forming an association between the presence of iron in a solution when it is not specifically needed and a subsequent state of iron deficiency. Specifically, rats were trained to lever press for water while thirsty. One group received ferrous ions in addition to the water. When these rats were subsequently rendered iron deficient, they lever pressed more under extinction conditions as a graded function of lower hemoglobin levels. Controls that either did not receive ferrous ions during training or received solutions other than ferrous solutions during training did not respond this way under extinction conditions. This is therefore a type of latent learning previously demonstrated only for sodium appetite.

The satiety effects of intragastric macronutrient infusions in fatty and lean Zucker rats.

To evaluate satiety in the hyperphagic, genetically obese Zucker "fatty" (fafa) rat, food-deprived fatty and lean (FaFa) control rats were given equicaloric intragastric infusions consisting largely of fat, carbohydrate, or protein. Relative to distilled water infusion, these infusions resulted in immediate reductions of food intake in both fatty and lean rats allowed to feed 20 min post-infusion. Cumulative food intakes remained reduced throughout the 2 hr period of observation. Thus, despite its hyperphagia, the fatty rat is responsive to the satiating effect of infused nutrients. However, the relative satiating effectiveness of the macronutrient infusions differed for the two genotypes. In lean rats, the different macronutrient infusions resulted in equivalent reductions of feeding. In contrast, in fatty rats, fat was the least satiating and protein was the most satiating macronutrient. Moreover, compared to lean rats, fatty rats displayed less initial suppression of feeding after fat infusion and greater overall suppression after protein infusion. These effects are consistent with the long-term feeding behavior of the fatty rat for the different macronutrients and may be related to pre- and postabsorptive metabolic alterations that have been documented in this animal.

Positive effects of acarbose in the diabetic rat are not altered by feeding schedule.

We previously demonstrated that chronic dietary treatment with acarbose, an alpha-glucosidase inhibitor, improves glucose homeostasis in the streptozotocin (STZ)-induced diabetic rat. In this study we evaluated the effects of 4 weeks of acarbose treatment on glucose homeostasis in STZ-diabetic rats for both meal-fed (three times daily) and ad libitum feeding conditions. Sprague Dawley male rats (n = 58) were started on a daily meal-feeding paradigm consisting of three 2-h feeding periods: 0700 to 0900 hours, 1300 to 1500 hours, and 1900 to 2100 hours. Following 2 weeks of adaptation, half of the animals were switched to ad libitum feeding. The feeding paradigm itself (meal fed versus ad lib.) affected neither body weight nor daily food intake. Twenty animals from each feeding group then received STZ (60 mg/kg i.v.), whereas control animals received vehicle injections only. Two days later, the diet of 10 STZ-treated animals from each paradigm was supplemented with acarbose (40 mg of BAY G 5421/100-g diet), and the groups were treated for 4 weeks. Untreated diabetic rats had lower body weight than vehicle-injected control rats at all time points after STZ treatment. Acarbose treatment delayed this effect on body weight. STZ treatment induced hyperphagia regardless of feeding paradigm, which was significantly attenuated by acarbose only for the first week of treatment. Untreated diabetic rats had fasting blood glucose values 4 times those of vehicle-injected controls in both the meal-fed and ad libitum-fed conditions. Acarbose significantly lowered fasting blood glucose in the treated STZ groups. Blood glucose was also assessed 0, 90, and 180 min following the start of a meal. The postprandial rise in blood glucose was significantly reduced in acarbose-treated meal-fed diabetic rats, to values not significantly different from those of vehicle-injected control rats. During the fourth week of treatment glycated hemoglobin levels were significantly higher in untreated diabetic groups compared to vehicle-injected control groups. Acarbose treatment significantly reduced this rise, regardless of the feeding paradigm. Collectively, the results demonstrate that acarbose reduces diabetes-induced increases of blood glucose and glycated hemoglobin and that the glycemic effects of acarbose are most apparent during the absorptive period. Feeding paradigm (ad lib. versus meal fed) has little or no influence on acarbose's metabolic effects, indicating that large meals are not required to realize the beneficial effects of the drug. The meal-fed STZ-diabetic rat may be a good model with which to test meal-based diabetes treatments.

Effects of a glucosidase inhibitor (acarbose, BAY g 5421) on the development of obesity and food motivated behavior in Zucker (fafa) rats.

BAY g 5421 (acarbose) inhibits carbohydrate digestion in the gut, thereby reducing the rate of glucose absorption. This experiment tested whether long term administration of acarbose to developing Zucker "fatty" (fafa) rats would, by reducing several lipogenic factors, attenuate lipid deposition and reduce the hyperphagia and increased food motivated behavior of these animals. From 7 to 20 weeks of life groups of fatty and lean (FaFa) control rats were fed 0, 20 or 40 mg acarbose/100 g maintenance diet (45% carbohydrate, 35% fat, 20% protein calories), while an additional fatty and lean group were pair-fed to respective 40 mg acarbose groups. Lean groups fed acarbose exhibited dose dependent reductions of body weight, insulin, triglycerides, retroperitoneal and epididymal pad weight, adipocyte size, LPL activity/cell (retroperitoneal pad only), and lipid deposition both in total grams of fat and as a percentage of carcass weight. Fatty groups fed acarbose exhibited dose dependent reductions of insulin, blood glucose, retroperitoneal pad weight, and, at one of the two doses used, significantly lowered body weight, (40 mg), triglycerides (20 mg) and cholesterol (20 mg). However, acarbose-fed fatty groups failed to show significant reductions of adipocyte size, number or LPL activity/cell in retroperitoneal and epididymal fat pads, and maintained their obese body composition, on a percentage basis, at levels not significantly different from that of the 0 mg fatty control group. Acarbose administration led to an initial dose dependent reduction of food intake in both genotypes, which persisted for the lean groups. Fatties fed the 20 mg dose showed a gradual tendency (ns) towards increased daily intake, lever pressed at elevated rates for food pellets, and refed at faster rates following fasting. Fatties fed the 40 mg dose maintained their daily intake at fatty control levels, did not lever press at elevated rates, and showed significantly reduced refeeding following fasting. The 40 mg fatty and both lean acarbose treated groups had decreased sucrose solution preference. Possible bases for these differing effects of the drug on feeding behavior by the groups are considered.