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J. Biol. Chem., Vol. 279, Issue 30, 31139-31148, July 23, 2004
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From the Departments of Medicine and Molecular Pharmacology, Diabetes Research and Training Center, Albert Einstein College of Medicine, Bronx, New York 10461
Received for publication, January 15, 2004 , and in revised form, April 15, 2004.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONEXPERIMENTAL PROCEDURESRESULTSDISCUSSIONREFERENCES |
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONEXPERIMENTAL PROCEDURESRESULTSDISCUSSIONREFERENCES |
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Evolutionary pressures may have favored the selection of genes, which maximize energy storage when food availability is high (15–18). We and others have proposed that a rapid increase in caloric intake initiates a "tug of war" between peripheral "anabolic signals" (19) and hypothalamic "catabolic signals" (20–26). The effects of hormones, such as leptin (27–31) and insulin (24, 25, 32, 33), and perhaps nutrients, such as fatty acids (21, 23, 26), within the hypothalamus initiate a negative feedback, which includes restraint on food intake, stimulation of energy expenditure, and decreased output of nutrients from endogenous sources (mainly from the liver). Animals and humans may be susceptible to weight gain and altered metabolic regulation when this negative feedback is disrupted. The rapid onset of leptin resistance in rodent models of voluntary overfeeding provides initial support for this theory (34, 35).
Here we test the hypothesis that short term increase in caloric intake rapidly induces resistance to the central effects of the long-chain fatty acid, oleic acid (OA). Thus, we examined whether changes in nutritional status lead to alterations in the central effects of OA on feeding behavior and glucose production.
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EXPERIMENTAL PROCEDURES |
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TOPABSTRACTINTRODUCTIONEXPERIMENTAL PROCEDURESRESULTSDISCUSSIONREFERENCES |
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80 kcal/day); 2) HS ad libitum (HS; 95 kcal/day); 3) HF ad libitum (HF140; 140 kcal/day); 4) HF calorie-restricted (HF55; 55 kcal/day); and 5) HF pair-fed to SC (HF80; 80 kcal/day). In addition, we performed metabolic studies on Zucker Fatty rats (n = 10) fed an SC diet.
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-cyclodextrin (HPB, Cyclodextrin Technologies Development Inc.). The latter has been shown to provide an excellent vehicle for the central delivery of fatty acids (23, 37, 38). OA was solubilized in 45% HPB to a final concentration of 17 mM. The HPB-OA solution was diluted in artificial cerebrospinal fluid to the appropriate concentration used for each ICV injection (30 or 300 nmol/5 µl). HPB alone at a similar concentration as in the OA studies was used in all vehicle control studies. Feeding Behavior Studies—This experimental protocol was designed to examine the acute effect of ICV OA on food intake in three experimental groups fed a standard chow (SC), a high sucrose diet (HS95), and a high fat diet (HF140). SC and HF140 animals were allowed to eat their diets ad libitum. HS animals were given free access to a 20% sucrose solution in addition to the standard chow diet for 3 days. Following 3 days of ad libitum feeding in all groups, on study day 0 (Fig. 1A), rats were given an ICV bolus injection of either 5 µl of OA (30 nmol) or vehicle at a rate of 1 µl/min using a gas-tight syringe (Hamilton Corp.) 1 h before the start of the dark cycle. Food intake was measured at the same time daily for 3 days post-injection.
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Insulin Action Studies—The experimental protocol herein was designed to examine the effect of nutritional status on the ability of long-chain fatty acids in the hypothalamus to modulate carbohydrate metabolism. Sprague-Dawley rats (n = 43) and Zucker Fatty rats (n = 10) were implanted with chronic catheters as described previously (36). After full recovery from the catheterization, Sprague-Dawley rats were randomized into three groups HF55 (n = 16), HF80 (n = 9), or HF140 (n = 18) and were allowed to consume their allocated diet for 3 days. On the night prior to the in vivo study, all rats received 55 kcal to ensure a similar nutritional state at the start of the metabolic studies. In a separate experiment, Zucker Fatty rats (n = 10) were fed a standard chow diet and allowed to fast for 5 h prior to the metabolic studies. All studies were performed in awake, unstressed, chronically catheterized rats. At t = 0 (Fig. 3A), a primed continuous infusion of ICV OA (total dose 30 or 300 nmol) or vehicle (HBP 10% in artificial cerebrospinal fluid) was initiated and maintained throughout the duration of the study. Plasma glucose was measured periodically initiating at the onset (t = 0) of the ICV infusion and lasting throughout the duration of the study. Plasma samples for determination of insulin, leptin, and nonesterified fatty acid concentration were obtained at the onset (t = 0) and at 30-min intervals during the study. At t = 120, a primed continuous infusion of high pressure liquid chromatography-purified [3-3H]glucose (PerkinElmer Life Sciences; 40 µCi bolus, 0.4 µCi/min for duration of the study) was initiated and maintained for the last 4 h of the study. Samples for determination of [3H]glucose-specific activity were obtained at 10-min intervals throughout infusions. Finally, at t = 240, a pancreatic insulin clamp study was initiated and maintained for 2 h. During this procedure, a primed continuous infusion of regular insulin (1 milliunit/kg/min) was administered, and a variable infusion of 25% glucose solution was started at t = 240 and periodically adjusted to clamp the plasma glucose concentration at 7 mM. The rate of insulin infusion was designed to replace the plasma insulin concentration at approximately the average basal levels in post-absorptive rats. In order to control for possible effects of the ICV injections on endocrine pancreas, somatostatin (3 µg/kg/min) was co-infused with insulin to inhibit endogenous insulin secretion. At the end of the in vivo studies, rats were anesthetized (pentobarbital 55 mg/kg body weight, intravenously), and tissue samples were freeze-clamped in situ with aluminum tongs pre-cooled in liquid nitrogen. All tissue samples were stored at –80 °C for subsequent analysis.
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Gene Expression Analysis—Total RNA was isolated from hypothalami and liver with Trizol (Invitrogen). NPY, Glc-6-Pase, or PEPCK expression was measured by Northern blot analysis. Hypothalamic RNA was analyzed using probes for prepro-NPY and -actin. To assess the effect of ICV OA on the expression of hepatic enzymes, total RNA was isolated from freeze-clamped liver tissues from rats subjected to insulin clamp studies. Glc-6-Pase and PEPCK cDNA were obtained as described previously (44–46). Probes were labeled with [
-32P]dCTP by using a random primer kit (Stratagene). Quantification was performed by scanning densitometry, normalizing for -actin signal and 18 S ribosomal RNA to correct for loading variabilities. Agouti-related protein (AGRP) and pro-opiomelanocortin (POMC) gene expressions were measured by quantitative PCR. Single-stranded cDNA synthesis and real time PCRs with hypothalamic total RNA were performed as described before (25). The following primers were used for the quantitative PCR: agouti-related protein, forward, 5'-GCCATGCTGACTGCAATGTT-3', and reverse, 5'-TGGCTAGGTGCGACTACAGA-3'; proopiomelanocortin, forward, 5'-CCAGGCAACGGAGATGAAC-3', and reverse, 5'-TCACTGGCCCTTCTTGTGC-3'; and -actin, forward, 5'-TGAGACCTTCAACACCCCAGCC-3', and reverse, 5'-GAGTACTTGCGCTCAGGAGGAG-3'. The copy number of each transcript was measured against a copy number standard curve of cloned target templates. Expression of each transcript was normalized to the copy number for -actin. Normalization with the glyceraldehyde phosphodehydrogenase copy number yielded similar results (data not shown).
Comparisons between groups were made by analysis of variance, and all values are presented as the mean ± S.E. Specifically, for the feeding data presented in Fig. 1, the two curves for vehicle and oleic acid were first compared with each other by using analysis of variance for repeated measures. If statistical differences were revealed, the differences between each time point were estimated using Student's t test. The study protocol was reviewed and approved by the Institutional Animal Care and Use Committee of the Albert Einstein College of Medicine.
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RESULTS |
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TOPABSTRACTINTRODUCTIONEXPERIMENTAL PROCEDURESRESULTSDISCUSSIONREFERENCES |
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70–140 kcal/day. Because control rats undergoing the same experimental procedures consumed 80 kcal/day when exposed to standard chow, we also compared the ad libitum fed rats to a group of pair-fed rats receiving 80 kcal/day and to a group of calorie-restricted rats receiving 55 kcal/day of the same highly palatable diet. This approach allowed us to investigate the central effects of OA at three levels of daily caloric intake reflecting moderate caloric restriction, pair-feeding, and overfeeding. It should be noted that data reported below for the pair-fed group are quite similar to those obtained in rats fed standard chow (23). Thus, this experimental approach allowed us to examine whether the central effects of OA on food intake and insulin action are modulated by short term changes in caloric intake.
Voluntary Overfeeding Rapidly Curtails the Effects of Central OA on Feeding Behavior and Hypothalamic NPY and AGRP Expression—We examined whether ICV OA modulates feeding behavior in animals following 3 days of voluntary overfeeding. Food intake was monitored daily in animals fed ad libitum either a high palatable diet (HF140) to induce hyperphagia or a standard chow (SC; caloric intake 78 ± 3 kcal/day) diet for 3 days prior to ICV injections. To examine the effects of macronutrient composition of the diet (e.g. high fat versus high carbohydrate) on OA sensitivity, we also studied an additional group of animals that were allowed free access to a 20% sucrose solution in addition to their standard chow (HS; caloric intake of 93 ± 4 kcal/day, reflecting an increased total caloric intake of 20% compared with SC animals). After 3 days on their assigned regimen, all animals received a bolus of either OA (30 nmol) or vehicle (HPB) via an indwelling ICV catheter 1 h prior to the onset of the dark cycle. Food intake was monitored for a total of 3 days following ICV injections (Fig. 1A), and the effects of OA were compared with those elicited by vehicle alone within each experimental group. ICV OA resulted in a 48 and 52% decrease in food intake compared with pre-injection basal levels when administered to SC animals (Fig. 1, B and E) on the 1st and 2nd day following the ICV injection, respectively. However, after 3 days of moderate overfeeding (HS95) ICV OA decreased daily food intake by 21% on the 1st day and by 17% on the 2nd day (Fig. 1, D and E). Finally, following 3 days of marked voluntary hyperphagia (HF140), ICV OA decreased daily food intake by only 11% on the 1st day and by –2% on the 2nd day (Fig. 1, C and E). The modest decreases observed in the HS95 and HF140 groups were not statistically different from those observed after ICV injection of vehicle alone (Fig. 1, C and D). All groups returned to base-line food intake 3 days after the OA injection (Fig. 1, B–E).
We next examined a potential mechanism by which resistance to hypothalamic OA may develop. We reported previously that hypothalamic NPY mRNA was decreased (by 50%) after ICV OA compared with ICV vehicle in rats fed standard chow following prolonged (16 h) fasting (23). Here we asked whether changes in the nutritional status modulate the ability of ICV OA to restrain the hypothalamic expression of NPY. To this end, we assessed the abundance of NPY mRNA in the hypothalamus by Northern blot analysis following 3 days of ad libitum feeding (140 kcal/day) on a highly palatable diet or restricted to 55 kcal/day on the same diet. Both experimental groups were given a single bolus injection of 30 nmol of OA or vehicle (HPB) followed by an overnight fast. Hypothalamic tissue was harvested the following morning, and Northern blot analysis was performed (Fig. 2A). Quantification by densitometry, utilizing -actin as a reference transcript, demonstrated that the average NPY mRNA levels were similar in HF55 and HF140 (Fig. 2C). ICV OA suppressed NPY mRNA expression in the 55-kcal/day group by 60% (Fig. 2D). This decrease is similar to that reported previously (23) in rats fed standard chow at 70 kcal/day following prolonged fasting. However, ICV OA decreased hypothalamic NPY mRNA by only 28% in fasted animals following 3 days of voluntary overfeeding (Fig. 2, C and D). These data provide evidence that the hyperphagia displayed in animals given free access to a highly palatable diet is partly due to defective regulation of NPY expression in the hypothalamus. To investigate further the potential mechanisms responsible for the OA resistance, we next analyzed the effect of OA on the gene expression of the hypothalamic neuropeptides, agouti-related protein (AGRP) and pro-opiomelanocortin (POMC). Total hypothalami were obtained from ICV vehicle (n = 9) or ICV OA (n = 8) administered animals that were fed either 55 or 140 kcal/day for 3 days. Quantitative analysis of hypothalamic RNA demonstrated that OA suppressed AGRP expression by 75% in animals fed 55 kcal/day as compared with vehicle (Fig. 2D). Conversely, animals fed 140 kcal/day failed to suppress AGRP expression when treated with ICV OA. In fact, the expression of AGRP was 120% of its level in ICV vehicle-infused animals (Fig. 2D). On the other hand, hypothalamic POMC expression was not altered following ICV OA treatment (Fig. 2E). Thus, 140 kcal/day animals displayed marked resistance to the ability of OA to alter AGRP expression in the hypothalamus, and this may partially account for the hyperphagia displayed by these animals.
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2-fold) higher in the HF55 than in the HF80 group, this increase appears to be highly dependent on the preceding nutritional status of the animal even within the normal to low range of caloric intake. Plasma FFA concentrations did not change during the clamp period (Table I) indicating that the metabolic effects induced by ICV OA were initiated by its action within the central nervous system. The increased requirement for exogenous glucose elicited by the ICV administration of OA in the presence of basal insulin levels could be due to stimulation of glucose uptake and/or to suppression of endogenous glucose production (GP). However, the rates of glucose disappearance (Fig. 3D, Rd) were similar in the three experimental groups, and most important, ICV OA did not significantly modify them. Thus, the increase in whole body insulin action induced by ICV OA in the HF55 and HF80 groups did not reflect an increase in peripheral glucose uptake (Fig. 3D). On the other hand, ICV OA (at 30 nmol) markedly inhibited GP in the HF55 and HF80 groups but not in the ad libitum fed rats (HF140). Indeed, a 10-fold higher central infusion of OA also failed to significantly decrease GP in the ad libitum fed group (Fig. 4A). Consistent with the effect observed on GIR, the inhibition of GP by ICV OA was more pronounced (71% decrease from basal) in the HF55 group than in the HF80 group (–45% decrease from basal) (Fig. 4B). Finally, the inhibition of GP induced by ICV OA entirely accounted for its stimulation of GIR. To examine further the impact of the nutritional status in modulating the effect of ICV OA on metabolic fluxes, we plotted the changes in GP induced by ICV OA or vehicle as a function of percent decreases in body weight (Fig. 4C) and daily food intake (Fig. 4D). The inhibitory effect of ICV OA was directly proportional to the decrease in daily food intake and weight gain. Most important, there was no significant correlation between changes in GP and GIR and changes in food intake/body weight in the groups receiving ICV vehicle (data not shown). A similar correlation with OA-induced inhibition of GP was also found by plotting only rats fed ad libitum the standard chow at various levels of caloric intake (data not shown). Thus, the degree of inhibition of GP in response to central administration of OA appears to be highly dependent on short term changes in caloric intake and/or body weight.
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ICV OA Markedly Decreases the in Vivo Flux through Glc-6-Pase—GP represents the net contribution of glucosyl units derived from gluconeogenesis and glycogenolysis. However, a portion of glucose entering the liver via phosphorylation by glucokinase is also a substrate for de-phosphorylation via Glc-6-Pase. This futile cycle between glucokinase and Glc-6-Pase is commonly named glucose cycling and accounts for the difference between the total glucose output (flux through Glc-6-Pase) and GP (Fig. 5A).
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-actin mRNA (not shown) in tissue samples obtained after ICV infusion of either OA or vehicle in calorie-restricted and overfed rats. ICV OA markedly decreased glucose-6-phosphatase gene expression in calorie-restricted rats. Quantification of multiple blots by densitometry (Fig. 5, E and F), utilizing -actin as a reference transcript, demonstrated that ICV OA suppressed Glc-6-Pase mRNA expression in the HF55 group by 73% with no changes detected in the HF140 group. Conversely, ICV OA failed to significantly alter the liver expression of PEPCK in either group. The marked inhibitory effect of ICV OA on liver Glc-6-Pase expression is likely to contribute to its potent inhibition of the in vivo flux through glucose-6-phosphatase in calorie-restricted rats. |
DISCUSSION |
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TOPABSTRACTINTRODUCTIONEXPERIMENTAL PROCEDURESRESULTSDISCUSSIONREFERENCES |
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Here we report that the ability of a central administration of the long-chain fatty acid oleic acid to inhibit food intake and hypothalamic NPY expression is blunted following 3 days of voluntary overfeeding in rats. Similarly, exposure to a highly palatable diet leading to hyperphagia induces resistance to the anorectic and metabolic effects of leptin and insulin in the hypothalamus in susceptible animal models (6, 16–18). NPY, a potent orexogenic neuropeptide, is a downstream target of both leptin and insulin in the hypothalamus and their failure to restrain NPY may partially explain why common forms of obesity are characterized by normal or elevated food intake despite elevated plasma leptin and insulin levels. Of interest, NPY is also a target of LCFA-CoAs as central administration of either oleic acid, fatty-acid synthase, or CPT-1 inhibitors prevents the rise in hypothalamic NPY mRNA induced by fasting (21, 23, 26). The impaired suppression of hypothalamic NPY expression by fatty acids observed in overfed rats may play a particularly important role in determining the degree of hyperphagia following a period of fasting when circulating fatty acids are elevated and plasma leptin and insulin levels are low. However, it should also be kept in mind that the cellular metabolism of fatty acids within the hypothalamus is also likely to play a key role in modulating this nutrient signal (26). In this regard, the well established biochemical link between cellular carbohydrate and lipid metabolism may play a particularly important role. In fact, we hypothesize that the cellular levels of LCFA-CoAs are likely to represent the key signal generated in response to increased availability of fatty acids. Similarly, an increase in the availability of simple carbohydrates (such as sucrose) can markedly increase the cellular levels of LCFA-CoAs via increased levels of (glycolytically derived) malonyl-CoA leading to inhibition of fatty acid oxidation. Thus, it is likely that this central nutrient-sensing mechanism may be able to respond to increased availability of either lipids or carbohydrates. It is therefore possible that the sustained increase in cellular LCFA-CoAs leads to adaptive changes in cellular lipid metabolism (e.g. inhibition of ACC leading to decreased formation of malonyl-CoA). In keeping with this postulate, we found that increasing the daily caloric intake via either a high fat or a high sucrose diet similarly blunted the effect of ICV OA on food intake, suggesting that changes in daily caloric intake rather than in macronutrient composition of the diet are likely to account for this effect. In this regard, it is intriguing that the anorectic effects of the fatty-acid synthase inhibitor C75 are preserved in diet-induced and genetic obesity in mice (54). By contrast, the poor response to ICV OA in overfed rats may be secondary to decreased esterification of LCFA due to decreased activity of acyl-CoA synthase or increased activity of acyl-thiosterases and/or accelerated metabolism of LCFA-CoAs due to increased activity of CPT-1 or depletion of malonyl-CoA. As in other models of diet-induced obesity (6, 16–18), the lack of response to an anorectic agent may indicate impaired action (of oleic acid) on feeding behavior and/or the inability to counteract the high palatability of the high fat and high sucrose diets.
Why does a short term increase in food intake lead to impaired hypothalamic response? Perhaps this is an attempt to promote efficient energy storage as an "adaptive" response to the increased availability of food. This mechanism may have developed as a result of evolutionary pressure in keeping with the thrifty genotype hypothesis by Neel (17). It is of interest that a similar paradoxical adaptation to overfeeding has also been demonstrated for a peripheral nutrient-sensing pathway whose stimulation appears to decrease mitochondrial function and energy expenditure in response to increased nutrient availability (22).
Recent studies on the metabolic effects of the central administration of insulin (24), leptin (36), melanocortins (55), and free fatty acids support the notion that these central hypothalamic pathways are also involved in the regulation of hepatic glucose output and insulin action. Because glucose production by the liver is the major source of endogenous fuel, we have postulated that central neural circuitries concomitantly modulate exogenous and endogenous sources of energy in keeping with a negative feedback system designed to monitor and regulate the input of nutrients in the circulation (23). Circulating fatty acids are mostly bound to albumin and cross the blood-brain barrier mainly by simple diffusion in the unbound form. Unbound fatty acids can also be derived via hydrolysis of lipoproteins by lipoprotein lipase within blood or at the cerebral capillary bed. Thus, chylomicrons are likely to be a major circulating source of brain fatty acids in the post-meal state, although a combination of unbound fatty acids and locally hydrolyzed lipoproteins contribute to the brain fatty acid pool in the fasting state. A small portion of fatty acid entry into the brain may also occur via direct uptake of lipoprotein particles mediated by lipoprotein receptors in the luminal surface of the cerebrovascular endothelium (56, 57). Overall, the access of circulating free fatty acids to the central nervous system is generally proportional to their plasma concentration (58, 59), and their concentration in cerebral spinal fluid is 6% of plasma concentration in fasted anesthetized dog (60). Thus, whereas one cannot simply extrapolate the effects of ICV oleic acid to physiological conditions, our findings raise the possibility that nutritionally induced changes in the potent behavioral and metabolic effects of fatty acids within the hypothalamus can contribute to the regulation of both energy balance and insulin action. On the other hand, it has long been recognized that changes in caloric intake also have a dramatic impact on the actions of insulin on glucose metabolism (61). In this regard, hepatic insulin resistance develops within days and/or a few weeks following overfeeding in animals and humans (62).
Here we report a strong correlation between the inhibition of GP induced by ICV OA and the nutritional status of the animal (i.e. body weight/caloric intake). In the presence of basal insulin levels, stimulation of hypothalamic insulin signaling (24) or central administration of OA (23) leads to inhibition of endogenous glucose production. This "insulin-like" central effect of LCFAs appears to be at odds with their well established actions in the peripheral tissues (63–65). For instance, elevated circulating and hepatic FFA levels reduce insulin suppression of endogenous glucose production (i.e. induces hepatic insulin resistance), whereas elevated levels of central fatty acids enhance the suppression of endogenous glucose production, even in the presence of basal insulin. Similarly, increased availability of fatty acids induces the expression of Glc-6-Pase in the liver (66), whereas the central administration of OA decreases the hepatic expression of the same enzyme. It is conceivable that central effects of fatty acids provide an important restraint on their peripheral action on hepatic glucose fluxes. Because the central administration of OA failed to inhibit GP in overfed rats, we postulate that the inability of OA to inhibit hepatic glucose production and Glc-6-Pase expression leaves the peripheral effects of fatty acids unopposed by their central effects. It is therefore conceivable that a lack of response to LCFA in the hypothalamus leads to increased rate of glucose output and may contribute to the hepatic insulin resistance observed in this model.
This hypothalamic nutrient sensing may also play a role in fuel partitioning. Under normal conditions, increased availability of fatty acids in the hypothalamus results in decreased output of glucose from the liver (23, 26) in order to promote the preferential utilization of lipid in muscle and other peripheral tissues. However, when the increased availability of fatty acids is sustained, it triggers the activation of "thrifty" metabolic mechanisms designed to promote the efficient flux of lipid into energy storage (triglyceride) sites. The increased production of glucose from the liver in overfed rats may serve this goal by providing alternative fuel for oxidation.
The downstream mechanism(s) by which OA modulates hepatic glucose fluxes has yet to be delineated. However, the marked decrease in both in vivo flux through glucose-6-phosphatase and in the hepatic expression of the glucose-6-phosphatase catalytic subunit induced by ICV OA is likely to play a key role. How does lipid sensing within the hypothalamus modulate hepatic glucose fluxes and gene expression? It is likely that rapid changes in autonomic nervous system outflow play a leading role. ICV leptin administration increases autonomic outflow in various regional sites (36). It is well known that both sympathetic and parasympathetic systems provide direct innervation of the liver, pancreas, and adipose tissue (via the splanchnic nerve and vagus nerve, respectively). Indeed, ventromedial hypothalamic lesions lead to acute and chronic hyperinsulinemia, and this can be reversed by subdiaphragmatic vagotomy (68, 69). Electrical stimulation of the lateral hypothalamus, on the other hand, fails to increase insulin secretion or change plasma glucagon concentration (68–70). However, it should be pointed out that all present studies were performed in the presence of pancreatic clamp conditions. Thus, it is not likely that changes in the levels of these pancreatic hormones can account for the effects of ICV OA on hepatic glucose fluxes. Of note, electrical stimulation of the ventromedial hypothalamus causes an increase in the activity of PEPCK and Glc-6-Pase, key gluconeogenic enzymes, and a marked suppression of pyruvate kinase, a key glycolytic enzyme in rat liver (69, 71). Stimulation of the lateral hypothalamus, on the other hand, leads to a decrease in PEPCK activity. Finally, various adipose depots receive input from autonomic nervous system. The latter has been shown in turn to regulate the gene expression and secretion of fat-derived hormones and cytokines (reviewed in Ref. 1), which have potent effects on insulin action (44, 67). To this end, the effect of ICV OA on liver glucose-6-phosphatase expression and on hepatic glucose fluxes is reminiscent of those induced by infusion of recombinant adiponectin in mice (44). However, we did not detect changes in plasma leptin and plasma adiponectin (Table I) levels during the ICV injections in any of the experimental groups.
In conclusion, we have shown that the central effects of long-chain fatty acids on food intake and GP are nutritionally regulated. Under normal circumstances biological responses (i.e. adjustments in food intake and glucose output) keep body fat and glucose homeostasis within a tight range. However, a sustained increase in the availability of nutrients may induce a rapid paralysis of this hypothalamic nutrient-sensing system (whereas short term caloric restriction enhanced this hypothalamic nutrient-sensing system). We postulate that the rapid onset of hypothalamic resistance to multiple nutritional signals such as leptin, insulin, and perhaps fatty acids contributes to the susceptibility to obesity and insulin resistance in predisposed individuals and animals.
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FOOTNOTES |
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Recipient of a Junior Faculty Award from the American Diabetes Association.
To whom correspondence should be addressed: Albert Einstein College of Medicine, 1300 Morris Park Ave., Bronx, NY 10461. Tel.: 718-430-4118; Fax: 718-430-8557; E-mail: rossetti{at}aecom.yu.edu.
1 The abbreviations used are: DM2, type 2 diabetes mellitus ICV, in the third cerebral ventricle; OA, oleic acid; NPY, neuropeptide Y; PEPCK, phosphoenolpyruvate carboxykinase; Glc-6-Pase, glucose-6-phosphatase; HPB, hydroxypropyl--cyclodextrin; SC, standard chow; HF, high fat chow; LCFA, long-chain fatty acids; GP, glucose production; GIR, glucose infusion rate; AGRP, agouti-related protein; POMC, pro-opiomelanocortin; FFA, free fatty acid; PEPCK, phosphoenolpyruvate carboxykinase.
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ACKNOWLEDGMENTS |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONEXPERIMENTAL PROCEDURESRESULTSDISCUSSIONREFERENCES |
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) resulted in rapid onset of anorexia, which lasted for 48 h. ICV vehicle (
) did not significantly modify eating behavior. C, in animals fed high fat chow, neither ICV OA (
), high sucrose (
), and high-fat chow (
). ICV OA markedly decreased food intake (by 
0.05 versus standard chow group; $, 
) OA infusion. D, the rate of glucose disappearance (Rd) during the pancreatic insulin clamp period. The rate of glucose disappearance was not significantly affected by ICV OA in any of the experimental groups. **, p = 0.0001 versus vehicle infusion; *, p < 0.03 versus vehicle infusion.
