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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 reduces adiposity and exerts antisteatotic effects on nonadipose tissues. However, the mechanisms underlying leptin effects on lipid metabolism in liver and white adipose tissue have not been fully clarified. Here, we have studied the effects of central leptin administration on key enzymes and transcription factors involved in lipid metabolism in liver and epididymal adipose tissue. Intracerebroventricular leptin infusion for 7 d did not change leptin plasma levels but decreased triacylglyceride content in liver, epididymal adipose tissue, and plasma. In both tissues this treatment markedly decreased the expression of key enzymes of the de novo fatty acid (FA) synthesis such as acetyl-coenzyme A-carboxylase, FA synthase, and stearoyl-coenzyme A desaturase-1, in parallel with a reduction in mRNA expression of sterol regulatory element binding protein-1c in liver and carbohydrate regulatory element binding protein in adipose tissue. In addition, leptin also decreased phosphoenol-pyruvate carboxykinase-C expression in adipose tissue, an enzyme involved in glyceroneogenesis in this tissue. Central leptin administration down-regulates delta-6-desaturase expression in liver and adipose tissue, in parallel with the decrease of the expression of sterol regulatory element binding protein-1c in liver and peroxisome proliferator activated receptor alpha in adipose tissue. Finally, leptin treatment, by regulating adipose triglyceride lipase/hormone sensitive lipase/diacylglycerol transferase 1 expression, also established a new partitioning in the FA-triacylglyceride cycling in adipose tissue, increasing lipolysis and probably the FA efflux from this tissue, and favoring in parallel the FA uptake and oxidation in the liver. These results suggest that leptin, acting at central level, exerts tissue-specific effects in limiting fat tissue mass and lipid accumulation in nonadipose tissues, preventing the development of obesity and type 2 diabetes.

Insulin, like leptin, is considered as a lipostatic signal acting at a central level. Aging and age-associated adiposity have been related to the development of leptin resistance in Wistar rats. In the present article, hypothalamic insulin response during aging has been studied in Wistar rats. Thus, the effects of intracerebroventricular infusion of insulin during a week on food intake and body weight as well as insulin signal transduction after acute intracerebroventricular insulin administration have been studied in 3-, 8-, and 24-month-old rats. To explore the possible role of age-associated adiposity, these experiments were also performed in 8- and 24-month-old rats after 3 months of food restriction to reduce visceral adiposity index to values below those of young animals. Intracerebroventricular administration of insulin during a week was more efficient at reducing food intake and body weight in 3-month-old rats than in 8- and 24-month-old rats. Hypothalamic insulin-stimulated insulin receptor, GSK3, AKT, and p70S6K phosphorylation decreased with aging. Insulin receptor and IRS-2 phosphoserine was increased in 24-month-old rats. Food restriction improved both insulin responsiveness and insulin signaling. These data suggest that Wistar rats develop hypothalamic insulin resistance with aging. This can be explained by alterations of the signal transduction pathway. The fact that food restriction improves central insulin response and signal transduction points to the age-associated adiposity as a key player in the development of central insulin resistance.

In rodents, soluble leptin receptor (SLR) may be generated by alternative splicing of ObR mRNA and/or as a cleavage product of ObR membrane-anchored receptors. In this study, we investigated the contribution of both processes on the generation of SLR in 3-, 8-, and 24-month-old Wistar rats fed ad libitum (AL) or under food restriction (FR). To this end, we determined serum SLR levels and analyzed ObRa and ObRe mRNA expression under these physiological conditions. Additionally, we studied the cellular distribution of ObRa and the generation of SLR by N-ethyl-maleimide-induced shedding from ObRa membrane receptors in isolated adipocytes. Serum SLR levels were significantly increased in 8- and 24-month-old rats under FR, whereas similar amounts were found in rats of different ages fed AL. ObRa and ObRe mRNA expression in epididymal adipose tissue increased with aging. In contrast, after FR, ObRe mRNA expression decreased, whereas ObRa mRNA expression further increased compared with 8- and 24-month-old rats fed AL. Additionally, FR promoted a change in the distribution of ObRa between internal and plasma membranes in isolated adipocytes, increasing its presence at the cell surface. Finally, the generation of SLR by N-ethyl-maleimide-induced shedding from ObRa was also increased under FR. These data suggest that shedding of ObRa membrane-anchored receptors, rather than ObRe expression, might preferentially contribute to the generation of the increased levels of SLR in serum under conditions of FR.

Mammary gland is an organ that undergoes cycles of growth, differentiation, and function during pregnancy and lactation. Although it is known that the gland enhances its sensitivity to insulin during lactation, it remains to be investigated whether this increased sensitivity develops during pregnancy and which are the molecular mechanisms underlying such a change. To address this issue, virgin and late-pregnant rats were subjected to a continuous infusion with 50% glucose for 72 h to produce a prolonged hyperinsulinemic-euglycemic condition. Insulin sensitivity in mammary gland was determined as the glucose utilization index by using 2-[3H]-deoxyglucose. Furthermore, binding characteristics and kinase activity were studied by means of both [125I]insulin binding and in vitro phosphorylation studies with insulin receptors partially purified from mammary gland. Whereas the glucose utilization index in mammary gland from nonpregnant rats remained unaffected by hyperinsulinemia, glands from pregnant rats displayed a high insulin-dependent glucose uptake. This effect was not paralleled by changes in the binding characteristics of insulin to the high-affinity receptor, suggesting that the high insulin sensitivity of mammary gland in pregnancy is not accounted for by changes at the level of hormone-receptor interaction. Autophosphorylation studies showed that insulin-stimulated kinase activity of insulin receptors from mammary gland was 6- and 20-fold higher in pregnant than in virgin animals under normo- and hyperinsulinemic conditions, respectively. Moreover, insulin dose-response curves revealed that the efficacy of insulin to stimulate kinase activity of the insulin receptor was markedly higher in pregnant than in virgin rats, whereas its potency (ED50 approximately 15 nM) was not changed. These data, therefore, show that mammary glands develop increased insulin sensitivity during late pregnancy, caused by an augmented kinase activity of the insulin receptor.

Aging has been associated with peripheral insulin resistance in both humans and rats. However, the specific tissues that become insensitive to insulin before glucose homeostasis is altered remain to be elucidated. In the present work we studied the glucose metabolic index of a number of tissues known to be insulin sensitive in 3- and 24-month-old Wistar rats by measuring 2-deoxy-D-[1-3H]glucose uptake both under euglycemic-hyperinsulinemic conditions and in the basal state. Analysis of the glucose infusion rate to maintain normoglycemia during the clamp confirmed that the old rats show overall insulin resistance at both saturating and subsaturating insulin concentrations. The maximal response of glucose uptake to insulin as well as insulin sensitivity in red and white quadriceps were unaltered in old rats. In contrast, glucose uptake by soleus and diaphragm was poorly stimulated in old animals, and a marked decrease in insulin sensitivity was observed in both tissues. In heart, only the sensitivity to the hormone, not the maximal response, was impaired in old rats. In white adipose tissue, no significant stimulation was detected. We conclude that during aging in Wistar rats and before fasting plasma insulin and glucose levels become altered, specific tissues develop insulin resistance, whereas other remain insulin sensitive. We postulate that fat tissue plays a qualitative important role in eliciting the insulin resistance in old animals. Due to the metabolic characteristics of the aged Wistar rat, the changes reported might reflect what occurs in nonobese elderly humans, nongenetically committed to develop type 2 diabetes.