Leptin prolonged administration inhibits the growth and glucocorticoid secretion of rat adrenal cortex

Harnessing the Power of Leptin: The Biochemical Link Connecting Obesity, Diabetes, and Cognitive Decline

In this review, the current understanding of leptin’s role in energy balance, glycemic regulation, and cognitive function is examined, and its involvement in maintaining the homeostatic “harmony” of these physiologies is explored. The effects of exercise on circulating leptin levels are summarized, and the results of clinical application of leptin to metabolic disease and neurologic dysfunction are reviewed. Finally, pre-clinical evidence is presented which suggests that synthetic peptide leptin mimetics may be useful in resolving not only the leptin resistance associated with common obesity and other elements of metabolic syndrome, but also the peripheral insulin resistance characterizing type 2 diabetes mellitus, and the central insulin resistance associated with certain neurologic deficits in humans.

Postnatal leptin is necessary for maturation of numerous organs in newborn rats

The postnatal leptin surge, described particularly in rodents, has been demonstrated to be crucial for hypothalamic maturation and brain development. In the present study, the possible general effects of this hormone on maturation of numerous peripheral organs have been explored. To test this hypothesis, we used a leptin antagonist (L39A/D40A/F41A) to investigate the effects of the blockage of postnatal leptin action on neonatal growth and maturation of organs involved in metabolism regulation, reproduction and immunity. For that purpose, newborn female pups were subcutaneously injected from days 2-13 with either saline or leptin antagonist and sacrificed at weaning. Organs were submitted to histological and immunohistochemical analyses. Leptin antagonist treatment clearly impaired the maturation of pancreas, kidney, thymus and ovary. All these alterations, at the organ level, occurred without changes in the whole-body mass of the animals. Leptin antagonist treatment induced: (1) a reduction in b cell area and a concomitant increase of a cells in Langherans islets in the pancreas, (2) a reduction in the number of glomeruli and a persistence of immature glomeruli in kidney, (3) an increase in the thymic cortical layer thickness, reflecting an unmatured stage, (4) a drastic reduction of the pool of primordial follicles, in ovaries. All these results strongly argue for a crucial role of leptin for the achievement of organ maturation, opening new perspectives in the field of leptin physiology and organ development.

Adrenocortical insufficiency in Otsuka Long-Evans Tokushima Fatty rats, a type 2 diabetes mellitus model

In diabetes, dysregulation of the hypothalamic-pituitary-adrenocortical (HPA) axis causes effects such as elevation of corticotropin (ACTH) and glucocorticoids. Cholecystokinin and its receptors are involved in the HPA axis and influence the regulation of the HPA axis. We examined adrenocortical function in Otsuka Long-Evans Tokushima Fatty (OLETF) rats, a model of type 2 diabetes mellitus, that lack the cholecystokinin A receptor. We measured adrenal weight, plasma ACTH, serum and urinary corticosterone, and serum leptin in OLETF rats at 5 to 36 weeks of age. Messenger RNA (mRNA) expression of 11beta-hydroxysteroid dehydrogenase and 5alpha-reductase type 1 in adrenal glands of the rats were examined. Long-Evans Tokushima Otsuka (LETO) rats were used as controls. In OLETF rats at 32 to 36 weeks of age, plasma ACTH was significantly higher (P < .001); serum corticosterone and 24-hour urinary corticosterone were significantly lower (P < .005); and adrenal weight was significantly lower (P < .005) than those in LETO rats. At the same ages, serum leptin in OLETF rats was significantly higher (P < .001) than that in LETO rats. In the younger OLETF rats, these changes were not observed. Overall, there was an inverse correlation between serum corticosterone and serum leptin (r = -0.374, P < .0005), whereas there was a positive correlation between plasma ACTH and serum leptin (r = 0.654, P < .0001). At 5 and 36 weeks of age, mRNA expression of 5alpha-reductase type 1 in the adrenal gland of OLETF rats was significantly higher (P < .05) than that of LETO rats, whereas there was no significant difference in mRNA expressions of 11beta-hydroxysteroid dehydrogenase types 1 and 2. We showed that adrenocortical insufficiency and adrenal atrophy were acquired in OLETF rats, and the possibility of elevated serum leptin relates to this phenomenon.

Leptin and the regulation of the hypothalamic-pituitary-adrenal axis

Leptin, the product of the obesity gene (ob) predominantly secreted from adipocytes, plays a major role in the negative control of feeding and acts via a specific receptor (Ob-R), six isoforms of which are known at present. Evidence has been accumulated that leptin, like other peptides involved in the central regulation of food intake, controls the function of the hypothalamic-pituitary-adrenal (HPA) axis, acting on both its central and peripheral branches. Leptin, along with Ob-R, is expressed in the hypothalamus and pituitary gland, where it modulates corticotropin-releasing hormone and ACTH secretion, probably acting in an autocrine-paracrine manner. Only Ob-R is expressed in the adrenal gland, thereby making it likely that leptin affects it by acting as a circulating hormone. Although in vitro and in vivo findings could suggest a glucocorticoid secretagogue action in the rat, the bulk of evidence indicates that leptin inhibits steroid-hormone secretion from the adrenal cortex. In keeping with this, leptin was found to dampen the HPA axis response to many kinds of stress. In contrast, leptin enhances catecolamine release from the adrenal medulla. This observation suggests that leptin activates the sympathoadrenal axis and does not appear to agree with its above-mentioned antistress action. Leptin and/or Ob-R are also expressed in pituitary and adrenal tumors, but little is known about the role of this cytokine in the pathophysiology.

Different forms of obesity as a function of diet composition

OBJECTIVE

To characterize the phenotype of obesity on a high-carbohydrate diet (HCD) as compared to a high-fat diet (HFD) or moderate-fat diet (MFD).

METHODS AND PROCEDURES

In four experiments, adult Sprague-Dawley rats (275-300 g) were maintained for several weeks on a: (1) HFD with 50% fat; (2) balanced MFD with 25% fat; or (3) HCD with 10% fat/65% carbohydrate. Then, based on the amount of body fat accumulated in four dissected fat pads, the animals were subgrouped as lean (lowest tertile) or obese (highest tertile) and characterized with multiple measures.

RESULTS

The obese rats of these diet groups, with 70-80% greater body fat than the lean animals, exhibited elevated levels of leptin and insulin and increased activity of lipoprotein lipase in adipose tissue (aLPL), with no change in muscle LPL. Characteristics common to the obese rats on the HFD or MFD, but not seen on the HCD, were hyperphagia, elevated circulating levels of triglycerides (TG), nonesterified fatty acids (NEFA) and glucose, and a significant increase in beta-hydroxyacyl-CoA dehydrogenase (HADH) activity in muscle, reflecting its greater capacity to metabolize fat. This was accompanied by a significant increase in expression of the peptide, galanin (GAL), in the paraventricular nucleus (PVN), as measured by in situ hybridization and real-time quantitative PCR, and also in GAL peptide immunoreactivity. These measures of GAL were consistently, positively correlated with circulating TG levels and also with HADH activity in muscle. In contrast to these fat-associated changes, rats that became obese on an HCD maintained normal caloric intake and levels of TG, NEFA, and glucose. They also showed no change in PVN GAL mRNA or peptide. Instead, they exhibited a significant reduction in HADH activity compared to the lean animals, along with increased activity of phosphofructokinase in muscle, a key enzyme in glycolysis.

CONCLUSION

Specific characteristics of obesity, including expression of hypothalamic peptides, are dependent upon diet composition. Whereas obesity on an HFD is associated with hyperphagia and elevated lipids, fat metabolism in muscle, and fat-stimulated peptides such as GAL, obesity on an HCD with a similar increase in body fat shows none of these characteristics and instead exhibits a metabolic pattern in muscle that favors carbohydrate over fat oxidation. These results suggest the existence of multiple forms of obesity with different underlying mechanisms that are diet dependent.

Leptin and the treatment of obesity: its current status

Leptin, the protein product of the ob gene, is primarily an adipocyte-secreted hormone, whose functional significance is rapidly expanding. Although early research efforts were focused on defining leptin's role in reversing obesity in rodents, there is now substantial evidence indicating that its influence extends to several hypothalamic-pituitary-endocrine axes, including gonadal, adrenal, thyroid, growth hormone, and pancreatic islets. A role for leptin in hematopoiesis, angiogenesis, immune function, osteogenesis, and wound healing has also been documented. The results of recent clinical trials with recombinant human leptin indicated that its effectiveness in restoring energy balance and correcting obesity-related endocrinopathies in genetically obese rodent models extended only partially to the management of human obesity. New efforts in drug development have focused on leptin-related synthetic peptide agonists as potential anti-obesity pharmacophores.

[D-LEU-4]-OB3, a synthetic leptin agonist, improves hyperglycemic control in C57BL/6J ob/ob mice

We have recently shown that the activity of a synthetic peptide corresponding to amino acid residues 116-130 of secreted mouse leptin is contained in a restricted sequence at the amino terminus of the peptide, between residues 116-122 (Ser-Cys-Ser-Leu-Pro-Gln-Thr, OB3). Substitution of the Leu residue at position 4 of OB3 with its D-isomer ([D-Leu-4]-OB3) enhanced the ability of OB3 (1 mg/day, ip, 7 days) to reduce body weight gain, food and water intake, and serum glucose in female C57BL/6J ob/ob mice. In the present study, we have utilized a pair-feeding approach to demonstrate that the antihyperglycemic action of [D-Leu-4]-OB3 is not solely due to its effects on caloric intake. One group of female C57BL/6J ob/ob mice (n=6) was fed ad libitum, and two additional groups (n=6 per group) were allowed 3.0 g food/mouse daily, an amount previously determined to satisfy [D-Leu-4]-OB3-treated mice. At the end of the 7-day test period, vehicle-injected mice fed ad libitum were approximately 10% heavier than their initial body weights, while pair-fed mice injected with vehicle and [D-Leu-4]-OB3-treated mice lost 5% of their initial body weights. After 1 day of treatment, blood glucose was reduced by 20% in pair-fed vehicle-injected mice, and by 40% in mice given [D-Leu-4]-OB3. Food restriction reduced blood glucose throughout the 7-day study, but not to levels seen in wild-type nonobese C57BL/6J mice of the same sex and age. After 2 days of treatment with [D-Leu-4]-OB3, however, blood glucose was reduced to levels comparable to those seen in wild-type nonobese mice. [D-Leu-4]-OB3 also lowered serum insulin levels by 53% when compared to mice fed ad libitum. Neither pair-feeding nor [D-Leu-4]-OB3 treatment had any apparent effect on thermogenesis. These results suggest that [D-Leu-4]-OB3 exerts its effects on serum glucose not only by suppressing caloric intake, but also through a separate effect on glucose metabolism which may involve increased tissue sensitivity to insulin.

Effects of losartan treatment on T-cell activities and plasma leptin concentrations in primary hypertension

Recent evidence shows that leptin may contribute to elevated blood pressure (BP) and interact with the renin-angiotensin-aldosterone and cellular immune systems. Altered T-cell activities and changes in T-cell subset ratios have also been reported in hypertension. However, little is known about the effects of AT1-receptor antagonism on T-cell activities and plasma leptin concentrations in primary hypertension. We have, therefore, investigated the relationship between leptin and T-cell activities and the effect of an AT1-receptor antagonist, losartan, in primary hypertension. Twenty recently-diagnosed and untreated young adults (11 males and 9 females, age; 39.9+/-7.6 years, range 23-49 years, BMI; 27.6+/-3.7kg/m2) and 20 normotensive healthy, age-, sex- and BMI-matched controls were studied. The [3H]-thymidine uptakes of cultured lymphocytes were determined, both spontaneously and after stimulation with phytohaemagglutinin. The tests were performed before and after three months of treatment with losartan. The results indicate that the blastogenic responses of T-cells to phytohaemagglutinin are significantly higher in the patient group compared with controls (p=0.02). After normalisation of BP, T-cell responses were significantly reduced and were lower than in the controls (p=0.01). Pretreatment plasma leptin levels were significantly higher in hypertensives than in controls (p=0.01). However, losartan treatment had no significant effect on leptin concentrations; moreover, no correlation between leptin levels and T-cell activity was found. Our data show that plasma leptin levels and T-cell activity are markedly enhanced in untreated essential hypertension and that the alteration of T-cell activity is not related to plasma leptin levels. Antihypertensive treatment with losartan decreases T-cell activities but does not influence plasma leptin levels. We conclude that leptin levels are not affected by AT1-receptor blockade and are not related to T-cell activity.

Neuroendocrine effects of leptin

Leptin, the product of the obesity gene, is a cytokine-like circulating protein acting as a peripheral satiety signal to the hypothalamus. It was initially described as a secreted product of white adipose cells, but more recent data have demonstrated its expression by endocrine and neuroendocrine tissues like the ovary and the hypothalamus, as well as several anterior pituitary cell types. The effects of leptin on body weight homeostasis are mediated via different hypothalamic neurotransmitters regulating appetite and energy expenditure. In addition, leptin participates to the modulation of the activity of the neuroendocrine thyrotrope, somatotrope, corticotrope and gonadotrope axes. These endocrine effects of leptin have progressively emerged as important physiological functions of this molecule. Its role as a permissive factor for puberty and normal reproductive function in adulthood is becoming widely recognized. In addition, leptin participates in the fine tuning of the corticotrope axis. Thus, by signalling body fat stores to the hypothalamus and other endocrine organs, leptin serves as a metabolic integrator of several neuroendocrine functions. The precise site of action and mode of regulation of the gonadotrope and somatotrope axes by leptin are reviewed.

Anterior pituitary corticotropes of adrenalectomized, leptin-administered rats

Leptin (LEP), the product of the ob gene is an adipose-tissue secreted hormone that acts to decrease caloric intake and increase energy expenditure. Some observations suggest the mutual relationship between leptin and blood ACTH levels. In the rat acute LEP administration enhances blood ACTH levels while prolonged treatment lowers blood corticotropin concentrations. Since the pituitary-derived ACTH is an important element in functioning of that loop, studies were undertaken to investigate the effect of prolonged LEP administration on anterior pituitary ACTH cells of adrenalectomized rats. Studies were performed on bilaterally adrenalectomized adult female rats. They were administered for 3 or 6 days with 2.7 nmol/rat/day LEP (recombinant human leptin) or with the vehicle (0.9% NaCl). LEP administration did not affect the body weight of bilaterally adrenalectomized rats. During the whole experiment the average increase in body weight was 3.9-4.3 g/day. LEP administration into adrenalectomized rats had no effect on anterior pituitary weight. This treatment resulted in a significant increase in pituitary ACTH concentration and content, and these changes were accompanied by a potent decrease in blood corticotropin level. LEP administration into adrenalectomized rats only insignificantly lowered the quantity of anterior pituitary ACTH-immunoreactive cells and their average area. On the opposite, the average volume of pituitary corticotropes of LEP-treated rats was notably lower than that in adrenalectomized-vehicle administered ones. Results of performed experiments clearly demonstrate that prolonged LEP administration results in a notable inhibition of the growth and secretory activity of anterior pituitary corticotropes of the adrenalectomized rats.

Leptin inhibits cortisol and corticosterone secretion in pathologic human adrenocortical cells

Regulation of adrenal corticosteroid secretion by leptin may involve interactions at multiple levels of the hypothalamic-pituitary-adrenal axis. To investigate the possible direct effects of leptin on corticosteroid secretion of human adrenocortical adenomas, cells from adrenocortical adenomas causing primary aldosteronism (n = 1) and Cushing's syndrome (n = 1), as well as cells from nonhyperfunctioning adrenocortical adenomas (n = 5) were isolated and incubated for 2 h with human recombinant leptin (1-1000 ng/ml) in the presence and absence of adrenocorticotrop hormone (ACTH), then cortisol, corticosterone and aldosterone concentrations in incubating media were determined using radioimmunoassays. It was found that leptin effectively and dose-dependently inhibited basal and ACTH-stimulated cortisol and corticosterone secretion in the three types of human adrenocortical adenoma cells. The inhibiting effect of basal corticosterone secretion was detectable in the presence of leptin concentration as low as 1 ng/ml, with decreases of corticosterone secretion to 34+/-4%, 57+/-11% and 79+/-9% in Cushing's syndrome, primary aldosteronism, and nonhyperfunctioning adrenocortical adenoma cells, respectively. The inhibition of basal cortisol secretion in the presence of low concentration of leptin was less prominent, but 10 ng/ml leptin significantly diminished basal cortisol secretion to 81+/-9% in adrenocortical adenoma cells from Cushing's syndrome, to 68+/-6% in adenoma cells from primary aldosteronism, and to 83+/-8% in cells from nonhyperfunctioning adenomas. The inhibition of ACTH-stimulated cortisol and corticosterone secretion by leptin was similar to those found in cells without ACTH stimulation. By contrast, leptin even at 1000 ng/ml concentration exerted no clear effect on basal and ACTH-stimulated aldosterone secretion in cells from primary aldosteronism and in those nonhyperfunctioning adenoma cells in which aldosterone secretion was detectable. These results indicate that leptin is a potent inhibitor of cortisol and corticosterone secretion in human adenomatous adrenocortical cells. The inhibition of these corticosteroids by leptin may represent a potentially important interaction that exists between leptin and the hypothalamic-pituitary-adrenal axis.