Teresa Fernández-Agulló

Central Leptin Regulates Total Ceramide Content and Sterol Regulatory Element Binding Protein-1C Proteolytic Maturation in Rat White Adipose Tissue

Obesity and type 2 diabetes are associated with insulin and leptin resistance, and increased ceramide contents in target tissues. Because the adipose tissue has become a central focus in these diseases, and leptin-induced increases in insulin sensitivity may be related to effects of leptin on lipid metabolism, we investigated herein whether central leptin was able to regulate total ceramide levels and the expression of enzymes involved in ceramide metabolism in rat white adipose tissue (WAT). After 7 d central leptin treatment, the total content of ceramides was analyzed by quantitative shotgun lipidomics mass spectrometry. The effects of leptin on the expression of several enzymes of the sphingolipid metabolism, sterol regulatory element binding protein (SREBP)-1c, and insulin-induced gene 1 (INSIG-1) in this tissue were studied. Total ceramide levels were also determined after surgical WAT denervation. Central leptin infusion significantly decreased both total ceramide content and the long-chain fatty acid ceramide species in WAT. Concomitant with these results, leptin decreased the mRNA levels of enzymes involved in de novo ceramide synthesis (SPT-1, LASS2, LASS4) and ceramide production from sphingomyelin (SMPD-1/2). The mRNA levels of enzymes of ceramide degradation (Asah1/2) and utilization (sphingomyelin synthase, ceramide kinase, glycosyl-ceramide synthase, GM3 synthase) were also down-regulated. Ceramide-lowering effects of central leptin were prevented by local autonomic nervous system denervation of WAT. Finally, central leptin treatment markedly increased INSIG-1 mRNA expression and impaired SREBP-1c activation in epididymal WAT. These observations indicate that in vivo central leptin, acting through the autonomic nervous system, regulates total ceramide levels and SREBP-1c proteolytic maturation in WAT, probably contributing to improve the overall insulin sensitivity.

Changes in the neuroendocrine control of energy homeostasis by adiposity signals during aging

Energy balance in mammals is modulated by peripheral signals that inform the brain about the magnitude of fat stores, the amount of food in the gastrointestinal tract, and the level of nutrients such as glucose in the circulation. Among these, insulin and leptin are considered adiposity signals involved in the long-term maintenance of fat stores. Here we review the mechanisms of action of leptin and insulin in the hypothalamus and how these mechanisms are altered during aging in rat models. Aged rats are characterized by increased fat mass, central leptin and insulin resistance, and hyperleptinemia. Leptin resistance is manifested by its failure to inhibit food intake, deplete fat stores, down regulate its own expression in adipose tissue, and increase energy expenditure. Moreover, leptin and insulin signaling are decreased in hypothalamus from aged rats. Calorie restriction and fasting studies provide controversial data on the cause-effect interrelationship between increased adiposity and development of central leptin resistance. Although in the absence of obesity leptin resistance seems to be a characteristic of aged animals, adiposity could either reinforce it or cause an early onset of this resistance. More studies are necessary to clarify the role of the hypothalamus in the development of age-associated obesity and insulin resistance.

Age-associated development of inflammation in Wistar rats: Effects of caloric restriction

Context: Insulin resistance and type 2 Diabetes have been associated to a low grade of inflammation and their prevalence increase with ageing. Objective: To analyse the development of inflammation in adipose tissue, liver, muscle and hypothalamus during ageing and the effects of caloric restriction. Materials and methods: We have analysed the expression of inflammatory cytokines (TNFα, IL1-β, IL-12B and IL-6), proteins involved in macrophage recruitment (MCP-1, CCR2), TLR4 and macrophage markers (CD11c, CD11b and arginase1). Immunohistochemistry of macrophages has also been performed. Results: All studied tissues present signs of inflammation during ageing, but with different pattern and intensity. Caloric restriction decreases the expression of most of inflammatory markers. Discussion and conclusions: These data indicate a role of adiposity in the development of inflammation and insulin resistance during ageing. Dietetic intervention could be a useful tool to ameliorate the development of inflammation and insulin resistance associated with ageing.

Involvement of protein tyrosine phosphatases and inflammation in hypothalamic insulin resistance associated with ageing: Effect of caloric restriction

Aged Wistar rats present central insulin resistance associated with ageing. Several steps of the insulin signaling pathway have been described to be impaired in aged rats at hypothalamic level. In the present article we have explored possible alterations in protein tyrosine phosphatases (PTPs) involved in insulin receptor dephosphorylation, as well as pro-inflammatory pathways and serine kinases such as inhibitory kappa β kinase-nuclear factor kappa-B (IKKβ-NFκB), p38 mitogen-activated protein kinase (p38) and protein kinase C θ (PKCθ) that may also be involved in the decreased insulin signaling during ageing. We detected that ageing brings about a specific increase in insulin receptor tyrosine phosphatase activity and PTP1B serine phosphorylation. Increased association of PTP1B and leukocyte common antigen-related tyrosine protein phosphatase (LAR) with insulin receptor was also observed in hypothalamus from aged rats. Besides these mechanisms, increased activation of the IKKβ-NFκB pathway, p38 and PKCθ serine/threonine kinases were also detected. These data contribute to explain the hypothalamic insulin resistance associated with ageing. Caloric restriction ameliorates most of the effects of ageing on the above mentioned increases in PTPs and serine/threonine kinases activities and points to age-associated adiposity and inflammation as key factors in the development of age-associated insulin resistance.

Development of Insulin Resistance During Aging: Involvement of Central Processes and Role of Adipokines

Aging in mammals associates with the development of peripheral insulin resistance. Additionally, adiposity usually increases with aging and this could play a relevant role in the gradual impairment of insulin action. In fact, fat accretion leads to changes in the expression and circulating concentrations of factors originated in adipose tissue like leptin, resistin and inflammatory cytokines which have been shown to modulate insulin signaling in insulin target tissues acting both, directly or through the central nervous system. Even insulin action on peripheral target tissues has been recently demonstrated to be partially mediated by its central action, suggesting that a decrease in central insulin action could be involved in the development of peripheral insulin resistance. In the present review we analyze the available research data on aging-associated insulin resistance making emphasis in the following aspects: 1) The time-course of development of overall insulin resistance and the evolution of changes in circulating adipokines; 2) The effect of caloric restriction and the decrease of adiposity in insulin action; 3) The influence of changes in the central action of factors like leptin or insulin in the development and maintenance of insulin resistance during aging.

Differential Development of Inflammation and Insulin Resistance in Different Adipose Tissue Depots Along Aging in Wistar Rats: Effects of Caloric Restriction

The prevalence of insulin resistance and type 2 diabetes increases with aging and these disorders are associated with inflammation. Insulin resistance and inflammation do not develop at the same time in all tissues. Adipose tissue is one of the tissues where inflammation and insulin resistance are established earlier during aging. Nevertheless, the existence of different fat depots states the possibility of differential roles for these depots in the development of age-associated inflammation and insulin resistance. To explore this, we analyzed insulin signaling and inflammation in epididymal, perirenal, subcutaneous, and brown adipose tissues during aging in Wistar rats. Although all tissues showed signs of inflammation and insulin resistance with aging, epididymal fat was the first to develop signs of inflammation and insulin resistance along aging among white fat tissues. Subcutaneous adipose tissue presented the lowest degree of inflammation and insulin resistance that developed latter with age. Brown adipose tissue also presented latter insulin resistance and inflammation but with lower signs of macrophage infiltration. Caloric restriction ameliorated insulin resistance and inflammation in all tissues, being more effective in subcutaneous and brown adipose tissues. These data demonstrate differential susceptibility of the different adipose depots to the development of age-associated insulin resistance and inflammation.

Regulation of Insulin-Stimulated Glucose Uptake in Rat White Adipose Tissue upon Chronic Central Leptin Infusion: Effects on Adiposity

Leptin enhances the glucose utilization in most insulin target tissues and paradoxically decreases it in white adipose tissue (WAT), but knowledge of the mechanisms underlying the inhibitory effect of central leptin on the insulin-dependent glucose uptake in WAT is limited. After 7 d intracerebroventricular leptin treatment (0.2 μg/d) of rats, the overall insulin sensitivity and the responsiveness of WAT after acute in vivo insulin administration were analyzed. We also performed unilateral WAT denervation to clarify the role of the autonomic nervous system in leptin effects on the insulin-stimulated [(3)H]-2-deoxyglucose transport in WAT. Central leptin improved the overall insulin sensitivity but decreased the in vivo insulin action in WAT, including insulin receptor autophosphorylation, insulin receptor substrate-1 tyrosine-phosphorylation, and Akt activation. In this tissue, insulin receptor substrate-1 and glucose transporter 4 mRNA and protein levels were down-regulated after central leptin treatment. Additionally, a remarkable up-regulation of resistin, together with an augmented expression of suppressor of cytokine signaling 3 in WAT, was also observed in leptin-treated rats. As a result, the insulin-stimulated glucose transporter 4 insertion at the plasma membrane and the glucose uptake in WAT were impaired in leptin-treated rats. Finally, denervation of WAT abolished the inhibitory effect of central leptin on glucose transport and decreased suppressor of cytokine signaling 3 and resistin levels in this tissue, suggesting that resistin, in an autocrine/paracrine manner, might be a mediator of central leptin antagonism of insulin action in WAT. We conclude that central leptin, inhibiting the insulin-stimulated glucose uptake in WAT, may regulate glucose availability for triacylglyceride formation and accumulation in this tissue, thereby contributing to the control of adiposity.

Leptin accumulation in hypothalamic and dorsal raphe neurons is inversely correlated with brain serotonin content

Food intake and energy balance are among the functions regulated by serotonin in the brain. Recent studies have shown an interaction of serotonergic system with leptin, a protein released from adipose tissue that inhibits feeding behavior and increases fuel expenditure. In this study, leptin labeled with digoxigenin was injected in the lateral cerebral ventricle of 5 young adult rats (4months old) and 5 aged rats (24months old) to assess the effect of aging on the accumulation of exogenous leptin by raphe and hypothalamic neurons. Four aged rats showed an intense leptin accumulation in neuronal cell bodies, mainly at the level of the dorsal raphe nucleus. In contrast, only one young rat showed neuronal accumulation of leptin in dorsal raphe and hypothalamus. Low brain serotonin immunoreactivity was found in all animals with high neuronal leptin accumulation at the raphe nucleus, independently of their age. In contrast, high brain serotonin immunoreactivity was accompanied by a low neuronal accumulation of leptin. To determine whether differences in serotonin content were the cause of the differences in leptin accumulation an inhibitor of serotonin synthesis, p-chlorophenylalanine, was administered to young rats. Serotonin depletion resulted in an enhanced accumulation of leptin in raphe as well as in hypothalamic neurons. These findings indicate that serotonin regulates leptin uptake by neuronal cell bodies of the dorsal raphe and hypothalamus, this suggests that at least part of the effects of serotonin may be mediated by the regulation of neuronal trafficking in the brain.

Impairment of skeletal muscle insulin action with aging in Wistar rats: role of leptin and caloric restriction.

Insulin resistance develops with aging in rats in parallel to fat mass accretion, central leptin resistance and hyperleptinemia. Previous studies demonstrated that insulin resistance appears earlier in adipose tissue than in muscle during aging and pointed to a role of hyperleptinemia in the impairment of insulin action. Here we explored the evolution along aging of insulin sensitivity in soleus and EDL muscles by analyzing insulin signaling in vivo and insulin-dependent glucose transport ex vivo. A decrease in insulin action was observed in both muscles. Caloric restriction improves insulin sensitivity at early aging but not in older animals. We also tested the role of leptin on insulin action in skeletal muscle. Short-term pretreatment with leptin inhibits in vivo muscle insulin signaling and insulin-dependent glucose transport in isolated muscle strips. This effect is mediated by its action on early insulin signaling as well as by the inhibition of p38. In contrast, chronic central administration of leptin elicits an insulin sensitizing effect on soleus. These data suggest that leptin can act as muscle insulin sensitizer, when acting at central level, and as insulin antagonistic when interacting directly with soleus muscle. This effect may be relevant in situations of hyperleptinemia such as aging.

EFFECTS OF CHRONIC ACARBOSE TREATMENT ON ADIPOCYTE INSULIN RESPONSIVENESS, SERUM LEVELS OF LEPTIN AND ADIPONECTIN AND HYPOTHALAMIC NPY EXPRESSION IN OBESE DIABETIC WISTAR RATS

1 Inhibitors of intestinal glucosidases have been shown to improve glycaemic control in diabetic and obese humans and animals. In the present study, we have investigated the effect of 3 months treatment with acarbose on adiposity, food intake and the modulation of hypothalamic neuropeptide Y (NPY) in obese diabetic Wistar (WDF) rats and the possible correlation between changes in overall insulin sensitivity and the level of circulating adipokines, leptin and adiponectin. In addition, we investigated the effect of acarbose on adipocyte insulin signalling. 2 Mature male WDF rats were randomly distributed to one of three treatment groups (no acarbose or 20 or 40 mg of acarbose/100 g diet). After 3 months, blood glucose, cholesterol, triglyceride, insulin, leptin and adiponectin were analysed. Insulin signalling was determined in isolated adipocytes as the stimulation of mitogen‐activated protein kinase (MAPK) and Akt phosphorylation; the level of hypothalamic NPY was assessed by immunohistochemistry. 3 Acarbose‐treated rats had lower levels of blood glucose, cholesterol, triglyceride, insulin and leptin and an increase in adiponectin compared with untreated animals. There were no changes in bodyweight and adiposity. Stimulation of adipocyte MAPK activity by insulin was higher in rats treated with both doses of acarbose, whereas higher stimulation of Akt phosphorylation was observed with the highest dose of acarbose. Although food intake was not significantly reduced in rats treated with acarbose, the acarbose‐treated rats had lower NPY expression in the arcuate nucleus. 4 We conclude that the improvement in overall insulin sensitivity in WDF rats after prolonged acarbose treatment is paralleled by increases in circulating adiponectin and adipocyte insulin responsiveness. Acarbose neither decreases food intake nor reverts obesity, but decreases leptin levels and the expression of the orexigenic NPY in the hypothalamus.