Reversal of diabetic nephropathy by a ketogenic diet

Intensive insulin therapy and protein restriction delay the development of nephropathy in a variety of conditions, but few interventions are known to reverse nephropathy. Having recently observed that the ketone 3-beta-hydroxybutyric acid (3-OHB) reduces molecular responses to glucose, we hypothesized that a ketogenic diet, which produces prolonged elevation of 3-OHB, may reverse pathological processes caused by diabetes. To address this hypothesis, we assessed if prolonged maintenance on a ketogenic diet would reverse nephropathy produced by diabetes. In mouse models for both Type 1 (Akita) and Type 2 (db/db) diabetes, diabetic nephropathy (as indicated by albuminuria) was allowed to develop, then half the mice were switched to a ketogenic diet. After 8 weeks on the diet, mice were sacrificed to assess gene expression and histology. Diabetic nephropathy, as indicated by albumin/creatinine ratios as well as expression of stress-induced genes, was completely reversed by 2 months maintenance on a ketogenic diet. However, histological evidence of nephropathy was only partly reversed. These studies demonstrate that diabetic nephropathy can be reversed by a relatively simple dietary intervention. Whether reduced glucose metabolism mediates the protective effects of the ketogenic diet remains to be determined.

Naloxone, but not valsartan, preserves responses to hypoglycemia after antecedent hypoglycemia: role of metabolic reprogramming in counterregulatory failure

OBJECTIVE

Hypoglycemia-associated autonomic failure (HAAF) constitutes one of the main clinical obstacles to optimum treatment of type 1 diabetes. Neurons in the ventromedial hypothalamus are thought to mediate counterregulatory responses to hypoglycemia. We have previously hypothesized that hypoglycemia-induced hypothalamic angiotensin might contribute to HAAF, suggesting that the angiotensin blocker valsartan might prevent HAAF. On the other hand, clinical studies have demonstrated that the opioid receptor blocker naloxone ameliorates HAAF. The goal of this study was to generate novel hypothalamic markers of hypoglycemia and use them to assess mechanisms mediating HAAF and its reversal.

RESEARCH DESIGN AND METHODS

Quantitative PCR was used to validate a novel panel of hypothalamic genes regulated by hypoglycemia. Mice were exposed to one or five episodes of insulin-induced hypoglycemia, with or without concurrent exposure to valsartan or naloxone. Corticosterone, glucagon, epinephrine, and hypothalamic gene expression were assessed after the final episode of hypoglycemia.

RESULTS

A subset of hypothalamic genes regulated acutely by hypoglycemia failed to respond after repetitive hypoglycemia. Responsiveness of a subset of these genes was preserved by naloxone but not valsartan. Notably, hypothalamic expression of four genes, including pyruvate dehydrogenase kinase 4 and glycerol 3-phosphate dehydrogenase 1, was acutely induced by a single episode of hypoglycemia, but not after antecedent hypoglycemia; naloxone treatment prevented this failure. Similarly, carnitine palmitoyltransferase-1 was inhibited after repetitive hypoglycemia, and this inhibition was prevented by naloxone. Repetitive hypoglycemia also caused a loss of hypoglycemia-induced elevation of glucocorticoid secretion, a failure prevented by naloxone but not valsartan.

CONCLUSIONS

Based on these observations we speculate that acute hypoglycemia induces reprogramming of hypothalamic metabolism away from glycolysis toward β-oxidation, HAAF is associated with a reversal of this reprogramming, and naloxone preserves some responses to hypoglycemia by preventing this reversal.

Hypothalamic responses to fasting indicate metabolic reprogramming away from glycolysis toward lipid oxidation

Nutrient-sensitive hypothalamic neurons regulate energy balance and glucose homeostasis, but the molecular mechanisms mediating hypothalamic responses to nutritional state remain incompletely characterized. To address these mechanisms, the present studies used quantitative PCR to characterize the expression of a panel of genes the hypothalamic expression by nutritional status of which had been suggested by DNA microarray studies. Although these genes regulate a variety of function, the most prominent set regulate intermediary metabolism, and the overall pattern clearly indicated that a 48-h fast produced a metabolic reprogramming away from glucose metabolism and toward the utilization of alternative fuels, particularly lipid metabolism. This general reprogramming of intermediary metabolism by fasting was observed both in cortex and hypothalamus but most prominently in hypothalamus. The effect of fasting on the expression of these genes may be mediated by reduction in plasma glucose or glucose metabolism, rather than leptin, because they were generally recapitulated by hypoglycemia even in the presence of elevated insulin and in vitro by low glucose but were not recapitulated in ob/ob mice. These studies suggest that fasting reduces glucose metabolism and thus minimizes the production of hypothalamic malonyl-coenzyme A. However, because the reprogramming of glucose metabolism by fasting was also observed in cortex, this apparent substrate competition may mediate more general responses to nutritional deprivation, including those responsible for the protective effects of dietary restriction. The present studies also provide a large panel of novel glucose-regulated genes that can be used as markers of glucose action to address mechanisms mediating hypothalamic responses to nutritional state.

Secrets of the lac operon. Glucose hysteresis as a mechanism in dietary restriction, aging and disease

Elevated blood glucose associated with diabetes produces progressive and apparently irreversible damage to many cell types. Conversely, reduction of glucose extends life span in yeast, and dietary restriction reduces blood glucose. Therefore it has been hypothesized that cumulative toxic effects of glucose drive at least some aspects of the aging process and, conversely, that protective effects of dietary restriction are mediated by a reduction in exposure to glucose. The mechanisms mediating cumulative toxic effects of glucose are suggested by two general principles of metabolic processes, illustrated by the lac operon but also observed with glucose-induced gene expression. First, metabolites induce the machinery of their own metabolism. Second, induction of gene expression by metabolites can entail a form of molecular memory called hysteresis. When applied to glucose-regulated gene expression, these two principles suggest a mechanism whereby repetitive exposure to postprandial excursions of glucose leads to an age-related increase in glycolytic capacity (and reduction in beta-oxidation of free fatty acids), which in turn leads to an increased generation of oxidative damage and a decreased capacity to respond to oxidative damage, independent of metabolic rate. According to this mechanism, dietary restriction increases life span and reduces pathology by reducing exposure to glucose and therefore delaying the development of glucose-induced glycolytic capacity.

Acute induction of gene expression in brain and liver by insulin-induced hypoglycemia

The robust neuroendocrine counterregulatory responses induced by hypoglycemia protect the brain by restoring plasma glucose, but little is known about molecular responses to hypoglycemia that may also be neuroprotective. To clarify these mechanisms, we examined gene expression in hypothalamus, cortex, and liver 3 h after induction of mild hypoglycemia by a single injection of insulin, using cDNA microarray analysis and quantitative real-time PCR. Real-time PCR corroborated the induction of six genes (angiotensinogen, GLUT-1, inhibitor of kappaB, inhibitor of DNA binding 1 [ID-1], Ubp41, and mitogen-activated protein kinase phosphatase-1 [MKP-1]) by insulin-induced hypoglycemia in the hypothalamus: five of these six genes in cortex and three (GLUT-1, angiotensinogen, and MKP-1) in liver. The induction was due to hypoglycemia and not hyperinsulinemia, since fasting (characterized by low insulin and glucose) also induced these genes. Four of these genes (angiotensinogen, GLUT-1, ID-1, and MKP-1) have been implicated in enhancement of glucose availability, which could plausibly serve a neuroprotective role during acute hypoglycemia but, if persistent, could also cause glucose-sensing mechanisms to overestimate plasma glucose levels, potentially causing hypoglycemia-induced counterregulatory failure. Although using cDNA microarrays with more genes, or microdissection, would presumably reveal further responses to hypoglycemia, these hypoglycemia-induced genes represent useful markers to assess molecular mechanisms mediating cellular responses to hypoglycemia.

Characterization of the peroxisome proliferator activated receptor coactivator 1 alpha (PGC 1alpha) expression in the murine brain

The peroxisome proliferator activated receptor coactivator 1 alpha (PGC-1alpha) is a nuclear transcriptional coactivator that is expressed in brown adipose tissue, brain, heart and kidney as well as cold-exposed skeletal muscle. In liver, white and brown adipose tissue, PGC-1alpha expression is regulated in a manner suggesting a role in energy homeostasis. To characterize PGC-1alpha expression in the rodent brain and to determine brain PGC-1alpha regulation, we used in situ hybridization histochemistry in C57Bl/6J mice and Sprague-Dawley rats. We found that PGC-1alpha is widely expressed in brain areas, including in the olfactory bulb, cerebral cortex, the diagonal band of Broca, the medial septal nucleus, reticular thalamic nucleus, the striatum and globus pallidus, the hippocampus, the substantia nigra, the mesencephalic nucleus of the trigeminal nerve, the cochlear nucleus and the superior olivary complex. In contrast, PGC-1alpha expression was absent in the hypothalamus. To evaluate PGC-1alpha expression under different physiologic states in these various brain areas, we examined expression with fasting, leptin treatment and cold exposure (4 h at 4 degrees C) and found no change, nor was expression changed in the brain of the leptin-deficient ob/ob mice and the hyperleptinemic UCP-DTA mice. Hence, PGC-1alpha is widely expressed in the rodent brain, but is not regulated by states of caloric deficiency, leptin, obesity or cold exposure. Its functional role in the brain requires further study.

Reducing hypothalamic AGRP by RNA interference increases metabolic rate and decreases body weight without influencing food intake

BACKGROUND

Several lines of evidence strongly suggest that agouti-related peptide (AGRP) plays a key role in the regulation of metabolic function but ablation of the AGRP gene has no apparent effect on metabolic function. Since specific pharmacological antagonists of AGRP do not presently exist, we assessed if reduction of hypothalamic AGRP mRNA by RNA interference (RNAI) would influence metabolic function, an outcome suggesting that pharmacological antagonists might constitute useful reagents to treat obesity.

RESULTS

The RNAI protocol specifically reduced hypothalamic expression of AGRP mRNA by 50% and resulted in reduction of AGRP peptide immunoreactivity. Physiologically, the reduction in AGRP levels was associated with increased metabolic rate and reduced body weight without changes in food intake.

CONCLUSION

AGRP can function to increase body weight and reduce metabolic rate without influencing food intake. The present study demonstrates that RNAI protocols can be used to assess physiological function of neuronal genes in vivo.

Validated genomic approach to study differentially expressed genes in complex tissues

Microarray-based genomic techniques allow the simultaneous determination of relative levels of expression of a large number of genes. Studies of the transcriptome in complex neurobiological systems are uniquely demanding due to the heterogeneous nature of these cells. Most brain regions contain a large variety of cell populations that are closely intermingled. The expression of any specific gene may be restricted to a subpopulation of cells, and changes in gene expression may occur in only a small fraction of the cells expressing that transcript. Due to this dilution effect, many genes of interest are expected to have relatively low levels of expression in tissue homogenates. Furthermore, biologically significant differences in expression may result in only small fold-changes. Therefore genomic approaches using brain dissections must be optimized to identify potentially regulated transcripts and differential expression should be confirmed using quantitative assays. We evaluated the effects of increasing tissue complexity on detection of regulated transcripts in focused microarray studies using a mouse cell line, mouse hypothalamus and mouse cortex. Regulated transcripts were confirmed by quantitative real-time PCR. As tissue complexity increased, distinguishing significantly regulated genes from background variation became increasingly more difficult. However, we found that cDNA microarray studies using regional brain dissections and appropriate numbers of replicates could identify genes showing less than 2-fold regulation and that most regulated genes identified fell within this range.

Characterization of melanin concentrating hormone and preproorexin expression in the murine hypothalamus

Melanin concentrating hormone (MCH) and the orexins (A and B) have been identified as neuropeptides localized to the lateral hypothalamic area (LHA) and are potential regulators of energy homeostasis. Potential factors regulating expression of both MCH and the orexins include fasting and leptin. Previous studies have generated conflicting data and, as there is little leptin receptor expressed in the lateral hypothalamus, it is likely that any observed leptin effects on these peptides are indirect. In this study, we examined MCH and preproorexin expression in mice in physiological states of starvation, with or without leptin administration, in addition to characterizing MCH and preproorexin expression in well-known obesity models, including ob/ob and UCP-DTA mice. Neuropeptide Y (NPY) expression in the arcuate nucleus was used as a positive control. After a 60-h fast, expression of both NPY and MCH mRNA was increased (by 148 and 33%, respectively) while preproorexin expression in the murine LHA did not change. Leptin administration to fasted mice blunted the rise in MCH and NPY expression towards control levels. In contrast, there was a 78% increase in preproorexin expression in fasted mice in response to peripheral leptin administration. MCH expression was increased (by 116%) in ob/ob mice at baseline, as we have previously reported. In addition, leptin treatment of ob/ob mice blunted the increase in MCH expression. In contrast, preproorexin expression did not differ in the leptin-deficient ob/ob mice or in the obese hyperleptinemic brown adipose tissue deficient (UCP-DTA) mice in comparison with controls. In summary, MCH expression is increased in two states of decreased leptin, fasting and ob/ob mice, and leptin replacement blunts MCH expression in both paradigms. Thus, MCH expression appears to be regulated by leptin. In contrast, preproorexin expression does not respond acutely to fasting, although it is acutely increased by leptin treatment during fasting. These preproorexin responses are in contrast to those seen with well-characterized orexigenic neuropeptides, such as NPY and AgRP, suggesting that appetite regulation may not be a significant physiological role of orexins. This conclusion is further supported by the observation that orexin ablated mice have arousal and not feeding deficits.

Melanin-concentrating hormone receptor is a target of leptin action in the mouse brain

Melanin-concentrating hormone (MCH) is a hypothalamic neuropeptide that is important in the regulation of energy homeostasis. MCH signals via a seven-transmembrane G protein-coupled receptor, which is coupled to Galpha(i). This receptor was initially cloned in rat and human and designated SLC-1 because of its homology to the somatostatin receptor. In rat brain, it is expressed in a pattern that mirrors the previously described pattern of projections of MCH-immunoreactive fibers. In the present study we cloned the mouse MCH receptor (MCH-R) ortholog by a rapid amplification of 5'- and 3'-cDNA ends approach and have found it to be 98% homologous with the rat sequence. We have characterized MCH-R messenger RNA distribution in the mouse brain by in situ hybridization and have shown MCH-R to be expressed in diverse brain areas implicated in the regulation of feeding, body adiposity, and sensory integration of smell and gustatory inputs, including the hypothalamus [paraventricular nucleus (magnocellular part) and dorsomedial, ventromedial, and arcuate nucleus], areas of the olfactory pathway, and the nucleus of the solitary tract. We also studied MCH-R regulation and found that MCH-R expression is increased 7-fold by 48-h fasting or genetic leptin deficiency (ob/ob mice) and is completely blunted by leptin administration. In contrast, MCH-R messenger RNA expression remains unaltered in genetic MCH deficiency. Our findings suggest that MCH-R constitutes a central target of leptin action in the mammalian brain.

Melanin-concentrating hormone overexpression in transgenic mice leads to obesity and insulin resistance

Several lines of investigation suggest that the hypothalamic neuropeptide melanin-concentrating hormone (MCH) regulates body weight in mammals. Obese mice lacking functional leptin overexpress the MCH message in the fed or fasted state. Acute intracerebroventricular injection of MCH increases energy intake in rats. Mice lacking the MCH gene are lean. To test the hypothesis that chronic overexpression of MCH in mice causes obesity, we produced transgenic mice that overexpress MCH (MCH-OE) in the lateral hypothalamus at approximately twofold higher levels than normal mice. On the FVB genetic background, homozygous transgenic animals fed a high-fat diet ate 10% more and were 12% heavier at 13 weeks of age than wild-type animals, and they had higher systemic leptin levels. Blood glucose levels were higher both preprandially and after an intraperitoneal glucose injection. MCH-OE animals were insulin-resistant, as demonstrated by markedly higher plasma insulin levels and a blunted response to insulin; MCH-OE animals had only a 5% decrease in blood glucose after insulin administration, compared with a 31% decrease in wild-type animals. MCH-OE animals also exhibited a twofold increase in islet size. To evaluate the contribution of genetic background to the predisposition to obesity seen in MCH-OE mice, the transgene was bred onto the C57BL/6J background. Heterozygote C57BL/6J mice expressing the transgene showed increased body weight on a standard diet, confirming that MCH overexpression can lead to obesity.

Characterization of [Phe(13), Tyr(19)]-MCH analog binding activity to the MCH receptor

Melanin concentrating hormone (MCH), a hypothalamic neuropeptide, is an important regulator of energy homeostasis in mammals. Characterization of an MCH specific receptor has been hampered by the lack of a suitable radioligand. The [Phe(13), Tyr(19)]-MCH analog has been shown by different investigators to bind specifically to cell lines of epithelial or pigment cell origin. Recently, using functional assays, the MCH receptor has been characterized as a seven transmembrane G-coupled protein initially identified as SLC-1. In the present study, we used tyrosine iodinated [Phe(13), Tyr(19)]-MCH analog, which stimulates food intake in a manner similar to that of MCH, as well as native MCH to conduct binding studies. Specific binding could not be demonstrated in intact cells of several cell lines, including A431 and B16. Specific binding associated with membranes localized to the microsomal, not the plasma membrane, fraction. Message for SLC-1 was absent in these cell lines, as assessed by Northern blot analysis. We conclude that cells previously reported to express the MCH receptor do not express SLC-1 and that both iodinated MCH and the [Phe(13), Tyr(19)]-MCH have a large component of non-specific binding. These ligands may be useful for binding studies in transfected cells with high levels of SLC-1 expression. However they do not appear to be suitable for screening for the MCH receptor as most cells demonstrate significant low affinity non-specific binding.

Characterization of expression of hypothalamic appetite-regulating peptides in obese hyperleptinemic brown adipose tissue-deficient (uncoupling protein-promoter-driven diphtheria toxin A) mice

Brown adipose tissue-deficient [uncoupling protein (UCP)-promoter-driven diphtheria toxin A (DTA)] mice develop obesity as a result of both decreased energy expenditure and hyperphagia. The hyperphagia occurs despite high serum leptin levels. Hence, this is a model of leptin-resistant obesity in which the mechanism driving hyperphagia is unknown. Leptin is a regulator of a number of hypothalamic neuropeptides involved in energy homeostasis. In ob/ob mice, leptin deficiency results in increased expression of neuropeptide Y (NPY), agouti-related protein (AGRP), and melanin-concentrating hormone (MCH), and decreased expression of POMC. We have previously shown that NPY is reduced in the UCP-DTA mouse, suggesting a normal NPY response to leptin. To define other potential sites of leptin resistance, we used in situ hybridization to evaluate the expression of messenger RNAs (mRNAs) encoding a number of peptides, including NPY, AGRP, MCH, and POMC. We confirmed that the decrease in NPY expression previously detected by Northern blots reflects a decrease in NPY expression in the arcuate nucleus. AGRP mRNA was also decreased, whereas POMC mRNA levels in the arcuate nucleus were the same as control. MCH mRNA levels in the lateral hypothalamic area were also decreased. In contrast, there was induction of NPY expression in the dorsomedial hypothalamic nucleus in the UCP-DTA animals but not in the controls. The results indicate that these neuropeptides generally respond to leptin and that the hyperphagia seen in the UCP-DTA mice is likely the result of dysregulated expression of other, as yet unexamined, hypothalamic peptides, or lies at sites distal to the hypothalamus.