Hypothalamic SIRT1 prevents age-associated weight gain by improving leptin sensitivity in mice

AIMS/HYPOTHESIS

Obesity is associated with ageing and increased energy intake, while restriction of energy intake improves health and longevity in multiple organisms; the NAD(+)-dependent deacetylase sirtuin 1 (SIRT1) is implicated in this process. Pro-opiomelanocortin (POMC) and agouti-related peptide (AgRP) neurons in the arcuate nucleus (ARC) of the hypothalamus are critical for energy balance regulation, and the level of SIRT1 protein decreases with age in the ARC. In the current study we tested whether conditional Sirt1 overexpression in mouse POMC or AgRP neurons prevents age-associated weight gain and diet-induced obesity.

METHODS

We targeted Sirt1 cDNA sequence into the Rosa26 locus and generated conditional Sirt1 knock-in mice. These mice were crossed with mice harbouring either Pomc-Cre or Agrp-Cre and the metabolic variables, food intake, energy expenditure and sympathetic activity in adipose tissue of the resultant mice were analysed. We also used a hypothalamic cell line to investigate the molecular mechanism by which Sirt1 overexpression modulates leptin signalling.

RESULTS

Conditional Sirt1 overexpression in mouse POMC or AgRP neurons prevented age-associated weight gain; overexpression in POMC neurons stimulated energy expenditure via increased sympathetic activity in adipose tissue, whereas overexpression in AgRP neurons suppressed food intake. SIRT1 improved leptin sensitivity in hypothalamic neurons in vitro and in vivo by downregulating protein-tyrosine phosphatase 1B, T cell protein-tyrosine phosphatase and suppressor of cytokine signalling 3. However, these phenotypes were absent in mice consuming a high-fat, high-sucrose diet due to decreases in ARC SIRT1 protein and hypothalamic NAD(+) levels.

CONCLUSIONS/INTERPRETATION

ARC SIRT1 is a negative regulator of energy balance, and decline in ARC SIRT1 function contributes to disruption of energy homeostasis by ageing and diet-induced obesity.

Miglitol prevents diet-induced obesity by stimulating brown adipose tissue and energy expenditure independent of preventing the digestion of carbohydrates

Miglitol is an alpha-glucosidase inhibitor that improves post-prandial hyperglycemia, and it is the only drug in its class that enters the bloodstream. Anecdotally, miglitol lowers patient body weight more effectively than other alpha-glucosidase inhibitors, but the precise mechanism has not been addressed. Therefore, we analyzed the anti-obesity effects of miglitol in mice and in the HB2 brown adipocyte cell line. Miglitol prevented diet-induced obesity by stimulating energy expenditure without affecting food intake in mice. Long-term miglitol treatment dose-dependently prevented diet-induced obesity and induced mitochondrial gene expression in brown adipose tissue. The anti-obesity effect was independent of preventing carbohydrate digestion in the gastrointestinal tract. Miglitol effectively stimulated energy expenditure in mice fed a high-fat high-monocarbohydrate diet, and intraperitoneal injection of miglitol was sufficient to stimulate energy expenditure in mice. Acarbose, which is a non-absorbable alpha glucosidase inhibitor, also prevented diet-induced obesity, but through a different mechanism: it did not stimulate energy expenditure, but caused indigestion, leading to less energy absorption. Miglitol promoted adrenergic signaling in brown adipocytes in vitro. These data indicate that circulating miglitol stimulates brown adipose tissue and increases energy expenditure, thereby preventing diet-induced obesity. Further optimizing miglitol's effect on brown adipose tissue could lead to a novel anti-obesity drug.

FoxO1 as a double-edged sword in the pancreas: analysis of pancreas- and β-cell-specific FoxO1 knockout mice

Diabetes is characterized by an absolute or relative deficiency of pancreatic β-cells. New strategies to accelerate β-cell neogenesis or maintain existing β-cells are desired for future therapies against diabetes. We previously reported that forkhead box O1 (FoxO1) inhibits β-cell growth through a Pdx1-mediated mechanism. However, we also reported that FoxO1 protects against β-cell failure via the induction of NeuroD and MafA. Here, we investigate the physiological roles of FoxO1 in the pancreas by generating the mice with deletion of FoxO1 in the domains of the Pdx1 promoter (P-FoxO1-KO) or the insulin 2 promoter (β-FoxO1-KO) and analyzing the metabolic parameters and pancreatic morphology under two different conditions of increased metabolic demand: high-fat high-sucrose diet (HFHSD) and db/db background. P-FoxO1-KO, but not β-FoxO1-KO, showed improved glucose tolerance with HFHSD. Immunohistochemical analysis revealed that P-FoxO1-KO had increased β-cell mass due to increased islet number rather than islet size, indicating accelerated β-cell neogenesis. Furthermore, insulin-positive pancreatic duct cells were increased in P-FoxO1-KO but not β-FoxO1-KO. In contrast, db/db mice crossed with P-FoxO1-KO or β-FoxO1-KO showed more severe glucose intolerance than control db/db mice due to decreased glucose-responsive insulin secretion. Electron microscope analysis revealed fewer insulin granules in FoxO1 knockout db/db mice. We conclude that FoxO1 functions as a double-edged sword in the pancreas; FoxO1 essentially inhibits β-cell neogenesis from pancreatic duct cells but is required for the maintenance of insulin secretion under metabolic stress.

Overexpression of FoxO1 in the hypothalamus and pancreas causes obesity and glucose intolerance

Recent studies have revealed that insulin signaling in pancreatic β-cells and the hypothalamus is critical for maintaining nutrient and energy homeostasis, the failure of which are hallmarks of metabolic syndrome. We previously reported that forkhead transcription factor forkhead box-containing protein of the O subfamily (FoxO)1, a downstream effector of insulin signaling, plays important roles in β-cells and the hypothalamus when we investigated the roles of FoxO1 independently in the pancreas and hypothalamus. However, because metabolic syndrome is caused by the combined disorders in hypothalamus and pancreas, to elucidate the combined implications of FoxO1 in these organs, we generated constitutively active FoxO1 knockin (KI) mice with specific activation in both the hypothalamus and pancreas. The KI mice developed obesity, insulin resistance, glucose intolerance, and hypertriglyceridemia due to increased food intake, decreased energy expenditure, and impaired insulin secretion, which characterize metabolic syndrome. The KI mice also had increased hypothalamic Agouti-related protein and neuropeptide Y levels and decreased uncoupling protein 1 and peroxisome proliferator-activated receptor γ coactivator 1α levels in adipose tissue and skeletal muscle. Impaired insulin secretion was associated with decreased expression of pancreatic and duodenum homeobox 1 (Pdx1), muscyloaponeurotic fibrosarcoma oncogene homolog A (MafA), and neurogenic differentiation 1 (NeuroD) in islets, although β-cell mass was paradoxically increased in KI mice. Based on these results, we propose that uncontrolled FoxO1 activation in the hypothalamus and pancreas accounts for the development of obesity and glucose intolerance, hallmarks of metabolic syndrome.

Genetic defect in phospholipase Cδ1 protects mice from obesity by regulating thermogenesis and adipogenesis

OBJECTIVE

Regulation of obesity development is an important issue to prevent metabolic syndromes. Gene-disrupted mice of phospholipase Cδ1 (PLCδ1), a key enzyme of phosphoinositide turnover, seemed to show leanness. Here we examined whether and how PLCδ1 is involved in obesity.

RESEARCH DESIGN AND METHODS

Weight gain, insulin sensitivity, and metabolic rate in PLCδ1(-/-) mice were compared with PLCδ1(+/-) littermate mice on a high-fat diet. Thermogenic and adipogenetic potentials of PLCδ1(-/-) immortalized brown adipocytes and adipogenesis of PLCδ1-knockdown (KD) 3T3L1 cells, or PLCδ1(-/-) white adipose tissue (WAT) stromal-vascular fraction (SVF) cells, were also investigated.

RESULTS

PLCδ1(-/-) mice showed marked decreases in weight gain and mass of epididymal WAT and preserved insulin sensitivity compared with PLCδ1(+/-) mice on a high-fat diet. In addition, PLCδ1(-/-) mice have a higher metabolic rate such as higher oxygen consumption and heat production. When control immortalized brown adipocytes were treated with thermogenic inducers, expression of PLCδ1 was decreased and thermogenic gene uncoupling protein 1 (UCP1) was upregulated to a greater extent in PLCδ1(-/-) immortalized brown adipocytes. In contrast, ectopic expression of PLCδ1 in PLCδ1(-/-) brown adipocytes induced a decrease in UCP expression, indicating that PLCδ1 negatively regulates thermogenesis. Importantly, accumulation of lipid droplets was severely decreased when PLCδ1-KD 3T3L1 cells, or PLCδ1(-/-) WAT SVF cells, were differentiated, whereas differentiation of PLCδ1(-/-) brown preadipocytes was promoted.

CONCLUSIONS

PLCδ1 has essential roles in thermogenesis and adipogenesis and thereby contributes to the development of obesity.

[Normal and abnormal aging of cardiovascular system]

Studies of normal aging in the cardiovascular system in humans are affected by the study population. Besides intrinsic biological aging, extrinsic factors including overt or latent cardiovascular diseases as well as life style variables such as physical activity, diet, alcohol and smoking may influence the age-related changes of cardiovascular function. We have recruited "normal" elderly subjects from community-dwelling volunteers by extensive health screening procedures including treadmill maximum exercise tests. Some of their cardiovascular functions, such as various cardiovascular regulatory functions, were altered compared to normal young subjects, while others such as resting hemodynamics were not. Interrelationships among various autonomic functions in the elderly were not recognized. Although general effects of life styles on circulatory regulatory functions were not clearly indicated, variables such as sodium intake or body mass index appeared to affect some of the sympathetic nervous functions. Furthermore, hypertension in the elderly had much less impact on cardiovascular functions than is generally expected, based on the results from young or middle-aged subjects. To identify factors which either modify (accelerate) or do not affect the aging of the cardiovascular functions is important not only to achieve a good aging process but also to establish therapeutic goals in elderly subjects.

Effects of age and blood pressure on the cardiovascular responses to the Valsalva maneuver

The present study was performed to obtain basic information as to the influences of two fundamental variables, age and blood pressure, on the circulatory responses during the Valsalva maneuver. Although a positive linear relationship between age and resting systolic blood pressure was present in the study population of 90 normal and uncomplicated hypertensive subjects, the pressure overshoot and the change in R-R intervals during phase 4 in the Valsalva maneuver were significantly inversely related to age or resting systolic blood pressure independently of each other variable. Furthermore, age was related negatively to the maximum R-R interval in phase 4 and positively to the decline of blood pressure post-Valsalva release in phase 3 even after adjusting for the effect of blood pressure, but age-independent relationships between these two indexes and resting systolic blood pressure were not statistically significant. The data indicate that changes in circulatory responses to the Valsalva maneuver occur with aging and high blood pressure, suggesting that the sympathetic vasoregulation and the baroreflex control of heart rate in the maneuver are impaired by these two variables. Such alterations appear to be more strongly related to age than to blood pressure level.

Differences in age-independent effects of blood pressure on baroreflex sensitivity between normal and hypertensive subjects

Inter-relationship between age, systolic blood pressure and baroreflex sensitivity index derived from the Valsalva manoeuvre was investigated in either combined or separated groups of normal and hypertensive subjects. Both in the total population as a whole and in each blood pressure subgroup, the baroreflex sensitivity index was significantly inversely related to age and to systolic blood pressure. Furthermore, age was significantly related to systolic blood pressure except in the hypertensive group. Partial correlation analysis showed that, in the total and hypertensive population, the baroreflex sensitivity index was significantly related to age and systolic blood pressure independently of each other variable. In the normal group, however, the baroreflex sensitivity index was not related to systolic blood pressure after adjusting for the effect of age, but remained significantly related to age independently of systolic blood pressure. The estimates of relative effects of the two variables on baroreflex sensitivity by multiple regression analysis were consistent with these results. Thus a prevailing concept of the inhibitory effect of blood pressure on baroreflex function may be accurate exclusively in hypertensive patients, and baroreflex function appears to be more sensitive to age-related changes in this system than to those related to blood pressure level, particularly in non-hypertensive normal subjects.

Use of a computer in analysis of heart rate and blood pressure response to the Valsalva maneuver

The relationship between systolic blood pressure (SBP) and R-R interval (RR) in phase II of the Valsalva maneuver was analyzed with the use of a nonlinear relation. The nonlinear relation introduced here includes time derivatives of SBP and RR, and is a natural extension from the linear one which defines the linear baroreflex sensitivity index (BRSI). The model equations of SBP and RR derived from the nonlinear relation predicted well the observed changes of SBP and RR in 11 normotensive subjects and 16 hypertensive patients. Two of the parameters of the relation were related to the activity of the compensatory mechanism, and discriminated well the hypertensives from the normotensives. For this analysis an interactive system of regression analysis has been developed to reduce the difficulties in the case of nonlinear regression curve fitting. The regression curves can be monitored on a graphic terminal during every regression step. The values of nonlinear parameters can be obtained easily and systematically with use of the system.

Age-related changes of baroreflex function, plasma norepinephrine, and blood pressure

Interrelationships between age, baroreflex sensitivity, plasma norepinephrine levels, and systolic blood pressure were assessed in a group of 54 normal subjects ranging in age from 14 to 77 years. Baroreflex sensitivity was measured by the change in R-R intervals per unit change in systolic blood pressure during phase 4 of the Valsalva maneuver. All correlations among these four variables were statistically significant (p less than 0.05 or 0.01). To investigate possible causal relationships between age-related changes of baroreflex sensitivity, plasma norepinephrine levels, and blood pressure, partial correlative analysis was then performed. After the effect of age was eliminated, plasma norepinephrine levels were found to be related positively to blood pressure (r = 0.29, p less than 0.05) and negatively to baroreflex sensitivity (r = -0.34, p less than 0.05). The increase in plasma norepinephrine levels could be causally related to the elevation of blood pressure, as plasma norepinephrine levels could provide an index of sympathetic activity. Furthermore, baroreflex sensitivity was found to be negatively related to age (r = -0.44, p less than 0.01) independent of plasma norepinephrine levels, whereas plasma norepinephrine levels were no longer related to age (r = 0.10) after adjusting for the effect of baroreflex sensitivity. This finding suggests that an increase in plasma norepinephrine levels with age could be mediated by the age-related change of baroreflex sensitivity.(ABSTRACT TRUNCATED AT 250 WORDS)