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J. Biol. Chem., Vol. 280, Issue 48, 39681-39683, December 2, 2005
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1
1
2
From the
Department of Biological Chemistry, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205, the ¶Department of Bioresource Science, College of Agriculture, Ibaraki University, 3-21-1 Chu-ou, Ami, Ibaraki 300-0393, Japan, and the
Montreal Diabetes Research Center, Montreal, Quebec H2L 4M1, Canada
Received for publication, September 29, 2005 , and in revised form, October 6, 2005.
| ABSTRACT |
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| INTRODUCTION |
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C75 appears to exert its anorectic effect by disrupting the signaling system that regulates expression of the hypothalamic neuropeptides that control feeding behavior. Thus, C75 blocks the fasting-induced up-regulation of orexigenic (AgRP and NPY) and down-regulation of anorexigenic (CART and POMC) neuropeptides in the hypothalamus (3, 6). These and other findings suggested (7) that the accumulation of malonyl-CoA, a substrate of FAS and MCD, may mediate the changes in the expression of these neuropeptides and, therefore, food intake. Consistent with this hypothesis the administration of C75 leads to an increase of hypothalamic malonyl-CoA (7).
Recent studies suggest that the changes in hypothalamic malonyl-CoA may depend upon the activity of hypothalamic AMP kinase (8, 9). It has long been recognized that AMP kinase in peripheral tissues (e.g. liver, adipose, and muscle) acts as a "sensor" of cellular energy charge (10, 11). Only recently, however, has evidence been obtained for the involvement of AMP kinase sensing in the hypothalamus (8, 12, 13) where global energy status is monitored and energy intake regulated (1). Conditions under which AMP kinase would be expected to be active and ACC inactive, e.g. in the fasted state, hypothalamic malonyl-CoA is extremely low (7). Consistent with this finding, the anorexigenic hormone leptin lowers AMP kinase activity in the hypothalamus (13). Likewise, the anorexigenic FAS inhibitor, C75, appears to reduce hypothalamic AMP kinase activity (8). These findings suggest that lowering hypothalamic AMP kinase activity is required to elicit the anorexigenic effects of leptin or C75. Lowering hypothalamic AMP kinase activity might be expected to activate ACC and elevate hypothalamic malonyl-CoA. While a direct linkage of the AMP kinase signaling system to changes in malonyl-CoA in the hypothalamus is suspected, it has not been demonstrated.
Moreover, the role of malonyl-CoA as a regulator of feeding behavior in studies with C75 remains controversial because the specificity of the FAS inhibitor has been questioned (1416) raising the possibility of indirect effects. In the present paper, we describe two independent approaches that provide compelling evidence for the direct involvement of malonyl-CoA in the hypothalamic regulation of feeding behavior. It was found that overexpression of cytosolic MCD (cMCD) or treatment with a 5'-AMP kinase agonist, AICAR, decreases malonyl-CoA in hypothalamic GT1-7 neurons. We also show that intracerebroventricular (i.c.v.) administration of AICAR to mice activates the phosphorylation of ACC, lowers hypothalamic [malonyl-CoA], and increases food intake. Likewise, bilateral stereotaxic injection of adenoviral cMCD into the ventral hypothalamus increases food intake.
| EXPERIMENTAL PROCEDURES |
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Culture and Treatment of Hypothalamic GT1-7 Cells, Malonyl-CoA Concentration, and ImmunoblottingGT1-7 cells were cultured in Dulbecco's modified Eagle's medium containing 25 mM glucose and 10% fetal bovine serum under 90% air/10% CO2. As the cells approached confluence they were treated or not with 2 mM AICAR for 4 h or were transduced with the appropriate Adenoviral vector (3.5 x 109 pfu/6-cm dish). Malonyl-CoA was quantified using an ultrasensitive substrate recycling assay (7). Immunoblotting was performed as described (17) with antibodies to MCD (18) obtained from Dr. Steven Gould (Johns Hopkins University School of Medicine) and to ACC2 and phospho(Ser79)-ACC2 (which also recognizes the AMP-activated protein kinase phosphorylation sites in ACC1) from Cell Signaling, Beverly, MA.
Construction of Adenoviral Cytomegalovirus Cytosolic Malonyl-CoA Decarboxylase (Ad-cMCD) Expression VectorThe construction of a recombinant adenovirus allowing constitutive expression of MCD in the cytosol with the use of a rat MCD cDNA devoid of its mitochondrial and peroxisomal targeting sequences has been detailed before (19).
Stereotaxic Microinjection of Adenoviral Expression VectorsMale BALB/c mice (2025 g) were placed in a stereotaxic frame (David Kopf Instruments) under ketamine/xylazine anesthesia (80/12 mg/kg body weight). The arcuate nucleus and dorsomedial and ventromedial hypothalamus were targeted bilaterally using a 30-gauge needle (Hamilton) connected to a Hamilton 5-µl syringe. The injection was directed to stereotaxic coordinates 1.5 mm posterior to the bregma, ±0.5 mm lateral to midline, and 5.8 mm below the surface of the skull. Adenovirus vectors, either Ad-LacZ (6.3 x 1012 pfu/ml) or Ad-cMCD (7.8 x 1012 pfu/ml), were delivered with a syringe injector pump (World Precision Instruments Inc., Sarasota, FL) at a rate of 100 nl/min for 2.5 min (250 nl/injection site), and the entire injector system was left in place for an additional 10 min after the injections were completed. After the procedure was complete, mice were placed in a heated cage until they recovered from anesthesia after which they were returned to their cages. Food intake and body weight were measured daily for 2 weeks starting from day 3 following surgery.
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Immunohistochemical AnalysesOn day 17 after adenovirus injection, mice were placed under deep ketamine/xylazine anesthesia and perfused with 0.1 M PBS (pH 7.4) followed by 4% paraformaldehyde. Perfused brains were removed, placed in the same fixative for an additional 4 h at room temperature, and then cryoprotected in 30% sucrose, 0.1 M PBS overnight. Brains were snap-frozen on dry ice and stored at 80 °C for further analysis. Coronal brain sections (16 mm) were washed in PBS and incubated in X-gal staining solution (1 mg/ml X-gal, 5 mM K3Fe(CN)6, 5mM K4Fe(CN)6) overnight to visualize
-galactosidase activity.
| RESULTS AND DISCUSSION |
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70% (Fig. 1B). To assess the effect of overexpressing cytosolic cMCD on malonyl-CoA, hypothalamic GT1-7 neurons were transduced with an adenoviral cMCD expression vector (Ad-cMCD). Ad-cMCD harbors the mouse cDNA encoding a cMCD in which both the peroxisome and mitochondrial targeting signals have been deleted to promote expression in the cytoplasm (19). As verified by immunoblotting the expression vector markedly increased the cellular level of cMCD relative to that of endogenous MCD (Fig. 1C and inset). Moreover, overexpression of cMCD leads to a marked decrease in malonyl-CoA. This observation, and that described above (Fig. 1, A and B), validate two approaches by which malonyl-CoA can be lowered in hypothalamic neurons ex vivo, either by preventing its formation or by accelerating its removal.
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Effect of Stereotaxic Delivery of Ad-cMCD into the Ventral HypothalamusTo determine the effect of overexpressing cMCD in the ventral hypothalamus on food intake and weight gain, an Ad-cMCD expression vector was delivered directly into the ventral hypothalamus of mice by bilateral stereotaxic injection. To verify the delivery of the viral construct to the ventral hypothalamus, a control adenoviral
-galactosidase vector (Ad-LacZ) was tested using the same stereotaxic injection procedure. As illustrated in Fig. 3A, 3 days after bilateral stereotaxic injection
-galactosidase expression was detected in the ventral hypothalamus adjacent to and below the third ventricle. Expression was particularly pronounced in the region encompassing the arcurate nucleus, a region that contains sets of neurons known to function in the regulation of feeding behavior (20). This staining pattern is typical of many other stereotaxic injection experiments, thus verifying the appropriate placement of the adenoviral expression vectors in the ventral hypothalamus.
Three days after delivery of the Ad-cMCD vector into the ventral hypothalamus the mice began to exhibit modest, but consistent, increases in food intake and body weight that continued over the next 12 days (Fig. 3, B and C). Since the region into which the cMCD expression vector was delivered comprises only a small fraction of the entire hypothalamus, reliable quantification of the malonyl-CoA concentration within this limited region was not possible. Nevertheless, the same Ad-cMCD vector was shown to lower [malonyl-CoA] ex vivo when transduced into hypothalamic GT1-7 neurons (Fig. 1C).
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Why the introduction of Ad-cMCD into the ventral hypothalamus does not produce a greater effect on food intake is unclear. Conceivably, the chronic hypothalamic exposure to cMCD may prompt compensatory changes in the expression of regulatory factors in attempt to normalize food intake. Such compensatory effects occur in gene knock-out models, e.g. NPY and AgRP knock-out mice do not exhibit the expected decreases in food intake (21, 22). However, when placed in the context of leptin deficiency, as in NPY/-ob/ob mice, the expected phenotype of NPY deficiency occurs (23).
Taken together with previous findings (7, 17), these results provide compelling evidence that malonyl-CoA plays a major role in the hypothalamic control of food intake. Thus, malonyl-CoA appears to be a key cellular indicator of fuel abundance that controls multiple functions related to body weight and fuel including insulin secretion (24), insulin action (25), and food intake (1).
| FOOTNOTES |
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1 Both authors contributed equally to this work. ![]()
2 To whom correspondence should be addressed: Dept. of Biological Chemistry, Johns Hopkins University School of Medicine, 512 WBSB, 725 N. Wolfe St., Baltimore, MD 21205. Tel.: 410-955-3554; Fax: 410-955-0903; E-mail: dlane{at}jhmi.edu.
3 The abbreviations used are: ACC, acetyl-CoA carboxylase; FAS, fatty acid synthase; MCD, malonyl-CoA decarboxylase; cMCD, cytosolic MCD; Ad-cMCD, adenoviral cMCD; i.c.v., intracerebroventricular; pfu, plaque-forming unit; PBS, phosphate-buffered saline; X-gal, 5-bromo-4-chloro-3-indolyl-
-D-galactopyranoside. ![]()
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