Leptin treatment reduces body fat but does not affect lean body mass or the myostatin-follistatin-activin axis in lean hypoleptinemic women

Activins, follistatins and inhibins in postmenopausal osteoporosis: A proof of concept, case-control study

BACKGROUND

Bone metabolism has been proposed to be affected by the activins-follistatins-inhibins (AFI) hormonal system. We aimed to evaluate AFI in patients with osteoporosis and osteopenia compared with postmenopausal and premenopausal controls.

METHODS

In this case-control study, circulating levels of the AFI system were evaluated, individually and jointly, between postmenopausal women with osteoporosis (BMD T-score ≤-2.5; n = 25) or osteopenia (BMD T-score >-2.5 and ≤-1; n = 25) and postmenopausal women with normal BMD (T-score >-1.0; n = 25) or premenopausal women with normal BMD (Z-score >-1.0; n = 25), with and without adjustment for potential confounders.

RESULTS

In the sum of participants, AFI molecules and their ratios followed an opposite pattern of correlations for age and BMI vs. BMD. In unadjusted models, FSTL3 concentrations were higher, whereas activin B, inhibin A and inhibin B and the ratios of activin B/follistatin and activin B/FSTL3 were lower in the three postmenopausal groups compared with the premenopausal group. Activin A/follistatin and activin AB/follistatin ratios were lower in the osteoporosis group than the other three groups. After adjustment for BMI and age, inhibin B (p = 0.005), and the ratios of activin A/follistatin (p = 0.009), activin B/follistatin (p = 0.040) and activin AB/follistatin (p = 0.003) were lower in the osteoporotic group compared with the other groups. In fully adjusted logistic regression analysis log(inhibin B) (p = 0.041), log(activinA/follistatin) (p = 0.014), log(activinB/follistatin) (p = 0.025) and log(activinAB/follistatin) (p = 0.021), but not FSTL3, remained independently associated with the presence of osteoporosis.

CONCLUSIONS

Lower inhibin B and higher ratios of activins A, B, and AB to follistatin are associated with lumbar spine BMD and the presence of osteoporosis independently from age or BMI.

Leptin mediates the regulation of muscle mass and strength by adipose tissue

Adipose tissue secretes numerous cytokines (termed 'adipokines') that have known or hypothesized actions on skeletal muscle. The majority of adipokines have been implicated in the pathological link between excess adipose and muscle insulin resistance, but approximately half also have documented in vitro effects on myogenesis and/or hypertrophy. This complexity suggests a potential dual role for adipokines in the regulation of muscle mass in homeostasis and the development of pathology. In this study, we used lipodystrophic 'fat-free' mice to demonstrate that adipose tissue is indeed necessary for the development of normal muscle mass and strength. Fat-free mice had significantly reduced mass (∼15%) and peak contractile tension (∼20%) of fast-twitch muscles, a slowing of contractile dynamics and decreased cross-sectional area of fast twitch fibres compared to wild-type littermates. These deficits in mass and contractile tension were fully rescued by reconstitution of ∼10% of normal adipose mass, indicating that this phenotype is the direct consequence of absent adipose. We then showed that the rescue is solely mediated by the adipokine leptin, as similar reconstitution of adipose from leptin-knockout mice fails to rescue mass or strength. Together, these data indicate that the development of muscle mass and strength in wild-type mice is dependent on adipose-secreted leptin. This finding extends our current understanding of the multiple roles of adipokines in physiology as well as disease pathophysiology to include a critical role for the adipokine leptin in muscle homeostasis. KEY POINTS: Adipose-derived cytokines (adipokines) have long been implicated in the pathogenesis of insulin resistance in obesity but likely have other under-appreciated roles in muscle physiology. Here we use a fat-free mouse to show that adipose tissue is necessary for the normal development of muscle mass and strength. Through add-back of genetically modified adipose tissue we show that leptin is the key adipokine mediating this regulation. This expands our understanding of leptin's role in adipose-muscle signalling to include development and homeostasis and adds the surprising finding that leptin is the sole mediator of the maintenance of muscle mass and strength by adipose tissue.

GEOFFREY HARRIS PRIZE LECTURE 2018: Novel pathways regulating neuroendocrine function, energy homeostasis and metabolism in humans

The discovery of leptin, an adipocyte-secreted hormone, set the stage for unraveling the mechanisms dictating energy homeostasis, revealing adipose tissue as an endocrine system that regulates appetite and body weight. Fluctuating leptin levels provide molecular signals to the brain regarding available energy reserves modulating energy homeostasis and neuroendocrine response in states of leptin deficiency and to a lesser extent in hyperleptinemic states. While leptin replacement therapy fails to provide substantial benefit in common obesity, it is an effective treatment for congenital leptin deficiency and states of acquired leptin deficiency such as lipodystrophy. Current evidence suggests that regulation of eating behavior in humans is not limited to homeostatic mechanisms and that the reward, attention, memory and emotion systems are involved, participating in a complex central nervous system network. It is critical to study these systems for the treatment of typical obesity. Although progress has been made, further studies are required to unravel the physiology, pathophysiology and neurobehavioral mechanisms underlying potential treatments for weight-related problems in humans.

The Effect of Chronic Exercise Training on Leptin: A Systematic Review and Meta-Analysis of Randomized Controlled Trials

BACKGROUND

Leptin is a hormone associated with satiety, lipid oxidation, energy expenditure, and energy homeostasis. To date, the current body of research examining the effect of chronic exercise training on leptin has yielded inconsistent results.

OBJECTIVE

The purpose of this meta-analysis was to provide a quantitative estimate of the magnitude of change in leptin levels following participation in exercise interventions lasting ≥ 2 weeks.

METHODS

All studies included were peer-reviewed and published in English. To be included, studies randomized human participants to an exercise training group or non-exercise comparison group for an exercise training intervention. Leptin levels were measured at baseline, during, and/or after completion of the exercise training program. Random-effects models were used to aggregate a mean effect size (ES) and 95% confidence intervals (CIs), and identify potential moderators.

RESULTS

Seventy-two randomized controlled trials met the inclusion criteria and resulted in 107 effects (n = 3826). The mean ES of 0.24 (95% CI 0.16-0.32, p < 0.0001) indicated a decrease in leptin following an exercise training program. A decrease in %Fat (β = - 0.07, p < 0.01) was associated with a decrease in leptin after accounting for the type of control group (β = - 0.38, p < 0.0001) used in each study.

CONCLUSION

These results suggest that engaging in chronic exercise training (≥ 2 weeks) is associated with a decrease in leptin levels for individuals regardless of age and sex. However, a greater decrease in leptin occurred with a decreased percentage of body fat.

Protein synthesis signaling in skeletal muscle is refractory to whey protein ingestion during a severe energy deficit evoked by prolonged exercise and caloric restriction

BACKGROUND

Exercise and protein ingestion preserve muscle mass during moderate energy deficits.

OBJECTIVE

To determine the molecular mechanisms by which exercise and protein ingestion may spare muscle mass during severe energy deficit (5500 kcal/day).

DESIGN

Fifteen overweight, but otherwise healthy men, underwent a pre-test (PRE), caloric restriction (3.2 kcals/kg body weight/day) + exercise (45 min one-arm cranking + 8 h walking) for 4 days (CRE), followed by a control diet (CD) for 3 days, with a caloric content similar to pre-intervention while exercise was reduced to less than 10,000 steps per day. During CRE, participants ingested either whey protein (PRO, n = 8) or sucrose (SU, n = 7) (0.8 g/kg body weight/day). Muscle biopsies were obtained from the trained and untrained deltoid, and vastus lateralis.

RESULTS

Following CRE and CD, serum concentrations of leptin, insulin, and testosterone were reduced, whereas cortisol and the catabolic index (cortisol/total testosterone) increased. The Akt/mTor/p70S6K pathway and total eIF2α were unchanged, while total 4E-BP1 and Thr^37/464E-BP1 were higher. After CRE, plasma BCAA and EAA were elevated, with a greater response in PRO group, and total GSK3β, pSer⁹GSK3β, pSer⁵¹eIF2α, and pSer⁵¹eIF2α/total eIF2α were reduced, with a greater response of pSer⁹GSK3β in the PRO group. The changes in signaling were associated with the changes in leptin, insulin, amino acids, cortisol, cortisol/total testosterone, and lean mass.

CONCLUSIONS

During severe energy deficit, pSer⁹GSK3β levels are reduced and human skeletal muscle becomes refractory to the anabolic effects of whey protein ingestion, regardless of contractile activity. These effects are associated with the changes in lean mass and serum insulin, testosterone, and cortisol concentrations.

Adipose tissue and reproductive health

The understanding of adipose tissue role has evolved from that of a depot energy storage organ to a dynamic endocrine organ. While genetics, sexual phenotype and sex steroids can impact the mass and distribution of adipose tissue, there is a counter-influence of white adipocytes on reproduction. This primarily occurs via the secretion of adipokines, the most studied of which- leptin and adiponectin- are highlighted in this article. Leptin, the "satiety hormone" primarily acts on the hypothalamus via pro-opiomelanocortin (POMC), neuropeptide Y (NPY), and agouti-related peptide (AgRP) neurons to translate acute changes in nutrition and energy expenditure, as well as chronic adipose accumulation into changes in appetite and potentially mediate insulin resistance via shared pathway and notably impacting reproductive health via influence on GnRH secreting neurons. Meanwhile, adiponectin is notable for its action in mediating insulin sensitivity, with receptors found at every level of the reproductive axis. Both have been examined in the context of physiologic and pathologic reproductive conditions. Leptin has been shown to influence puberty, pregnancy, hypothalamic amenorrhea, and lipodystrophy, and with a potential therapeutic role for both metabolic and reproductive health. Adiponectin mediates the relative state of insulin resistance in pregnancy, and has been implicated in conditions such as polycystic ovary syndrome and reproductive malignancies. There are numerous other adipokines, including resistin, visfatin, chemerin and retinol binding protein-4, which may also play roles in reproductive health and disease states. The continued examination of these and other adipokines in both normal reproduction and reproductive pathologies represents an important avenue for continued study. Here, we seek to provide a broad, yet comprehensive overview of many facets of these relationships and highlight areas of consideration for clinicians and future study.

Regulation of the activins-follistatins-inhibins axis by energy status: Impact on reproductive function

BACKGROUND

We have previously demonstrated that the adipose tissue derived hormone leptin controls reproductive function by regulating the hypothalamic-pituitary-gonadal axis in response to energy deficiency. Here, we evaluate the activins-follistatins-inhibins (AFI) axis during acute (short-term fasting in healthy people) and chronic energy deficiency (women with hypothalamic amenorrhea due to strenuous exercise [HA]) and investigate their relation to leptin and reproductive function in healthy subjects and subjects with HA.

METHODS

The AFI axis was investigated in: a) A double-blinded study in healthy subjects having three randomly assigned admissions, each time for four days: in the isocaloric fed state, complete fasting with placebo treatment, complete fasting with leptin replacement, b) A case-control study comparing women with HA vs healthy controls, c) An open-label interventional study investigating leptin treatment in women with HA over a period of up to three months, d) A randomized interventional trial investigating leptin treatment vs placebo in women with HA for nine months.

RESULTS

The circulating levels of activin A, activin B, follistatin and follistatin-like 3 change robustly in response to acute and chronic energy deficiency. Leptin replacement in acute energy deprivation does not affect the levels of these hormones suggesting an independent regulation by these two hormonal pathways. In chronic energy deficiency, leptin replacement restores only activin B levels, which are in turn associated with an increase in the number of dominant follicles.

CONCLUSIONS

We demonstrate for the first time that the AFI axis is affected both by acute and chronic energy deficiency. Partial restoration of a component of the axis, i.e. activin B only, through leptin replacement is associated with improved reproductive function in women with HA.

Bone metabolism in anorexia nervosa and hypothalamic amenorrhea

Anorexia nervosa (AN) and hypothalamic amenorrhea (HA) are states of chronic energy deprivation associated with severely compromised bone health. Poor bone accrual during adolescence followed by increased bone loss results in lifelong low bone density, degraded bone architecture, and higher risk of fractures, despite recovery from AN/HA. Amenorrhea is only one of several compensatory responses to the negative energy balance. Other hypothalamic-pituitary hormones are affected and contribute to bone deficits, including activation of hypothalamic-pituitary-adrenal axis and growth hormone resistance. Adipokines, particularly leptin, provide information on fat/energy stores, and gut hormones play a role in the regulation of appetite and food intake. Alterations in all these hormones influence bone metabolism. Restricted in scope, current pharmacologic approaches to improve bone health have had overall limited success.

Role of the Cytokine-like Hormone Leptin in Muscle-bone Crosstalk with Aging

The cytokine-like hormone leptin is a classic adipokine that is secreted by adipocytes, increases with weight gain, and decreases with weight loss. Additional studies have, however, shown that leptin is also produced by skeletal muscle, and leptin receptors are abundant in both skeletal muscle and bone-derived mesenchymal (stromal) stem cells. These findings suggest that leptin may play an important role in muscle-bone crosstalk. Leptin treatment in vitro increases the expression of myogenic genes in primary myoblasts, and leptin treatment in vivo increases the expression of microRNAs involved in myogenesis. Bone marrow adipogenesis is associated with low bone mass in humans and rodents, and leptin can reduce marrow adipogenesis centrally through its receptors in the hypothalamus as well as directly via its receptors in bone marrow stem cells. Yet, central leptin resistance can increase with age, and low circulating levels of leptin have been observed among the frail elderly. Thus, aging appears to significantly alter leptin-mediated crosstalk among various organs and tissues. Aging is associated with bone loss and muscle atrophy, contributing to frailty, postural instability, and the incidence of falls. Therapeutic interventions such as protein and amino acid supplementation that can increase muscle mass and muscle-derived leptin may have multiple benefits for the elderly that can potentially reduce the incidence of falls and fractures.

Leptin in congenital and HIV-associated lipodystrophy

Leptin is a hormone secreted by adipocytes that regulates energy metabolism via peripheral action on glucose synthesis and utilization as well as through central regulation of food intake. Patients with decreased amounts of fat in their adipose tissue (lipoatrophy) will have low leptin levels, and hypoleptinemic states have been associated with a variety of metabolic dysfunctions. Pronounced complications of insulin resistance, dyslipidemia and fatty liver are observed in patients suffering from congenital or acquired generalized lipodystrophy while somewhat less pronounced abnormalities are associated with human immunodeficiency virus (HIV) and the use of highly active antiretroviral therapy, the so-called HIV-associated lipodystrophy. Previous uncontrolled open-label studies have demonstrated that physiological doses of leptin repletion have corrected many of the metabolic derangements observed in subjects with rare fat maldistribution syndromes such as generalized lipodystrophy. In the much more commonly encountered HIV-associated lipodystrophy, leptin replacement has been shown to decrease central fat mass and to improve insulin sensitivity, dyslipidemia, and glucose levels. The United States Food and Drug Administration has recently granted approval for recombinant leptin therapy for congenital and acquired generalized lipodystrophy, however large, well-designed, placebo-controlled studies are needed to assess long-term efficacy, safety and adverse effects of leptin replacement. In this review, we present the role of leptin in the metabolic complications of congenital and acquired lipodystrophy and discuss current and emerging clinical therapeutic uses of leptin in humans with lipodystrophy.

20 years of leptin: role of leptin in energy homeostasis in humans

The hyperphagia, low sympathetic nervous system tone, and decreased circulating concentrations of bioactive thyroid hormones that are common to states of congenital leptin deficiency and hypoleptinemia following and during weight loss suggest that the major physiological function of leptin is to signal states of negative energy balance and decreased energy stores. In weight-reduced humans, these phenotypes together with pronounced hypometabolism and increased parasympathetic nervous system tone create the optimal circumstance for weight regain. Based on the weight loss induced by leptin administration in states of leptin deficiency (obese) and observed similarity of phenotypes in states of congenital and dietary-induced states of hypoleptinemia (reduced obese), it has been suggested that exogenous leptin could potentially be useful in initiating, promoting, and sustaining weight reduction. However, the responses of human beings to exogenous leptin administration are dependent not only on extant energy stores but also on energy balance. Leptin administration to humans at usual weight has little, if any, effect on body weight while leptin administration during weight loss mitigates hunger, especially if given in supraphysiological doses during severe caloric restriction. Leptin repletion is most effective following weight loss by dietary restriction. In this state of weight stability but reduced energy stores, leptin at least partially reverses many of the metabolic, autonomic, neuroendocrine, and behavioral adaptations that favor weight regain. The major physiological function of leptin is to signal states of negative energy balance and decreased energy stores. Leptin, and pharmacotherapies affecting leptin signaling pathways, is likely to be most useful in sustaining weight loss.

20 years of leptin: role of leptin in human reproductive disorders

Leptin, as a key hormone in energy homeostasis, regulates neuroendocrine function, including reproduction. It has a permissive role in the initiation of puberty and maintenance of the hypothalamic-pituitary-gonadal axis. This is notable in patients with either congenital or acquired leptin deficiency from a state of chronic energy insufficiency. Hypothalamic amenorrhea is the best-studied, with clinical trials confirming a causative role of leptin in hypogonadotropic hypogonadism. Implications of leptin deficiency have also emerged in the pathophysiology of hypogonadism in type 1 diabetes. At the other end of the spectrum, hyperleptinemia may play a role in hypogonadism associated with obesity, polycystic ovarian syndrome, and type 2 diabetes. In these conditions of energy excess, mechanisms of reproductive dysfunction include central leptin resistance as well as direct effects at the gonadal level. Thus, reproductive dysfunction due to energy imbalance at both ends can be linked to leptin.

Metreleptin: first global approval

Metreleptin is an analogue of the human hormone leptin being developed by Amylin Pharmaceuticals (a subsidiary of Bristol-Myers Squibb) for the subcutaneous treatment of metabolic disorders including lipodystrophy. The compound is expected to improve insulin sensitivity, hypertriglyceridaemia and hyperglycaemia in patients with lipodystrophy who are unresponsive to conventional treatment. Metreleptin has been approved in Japan as a leptin therapy for the treatment of lipodystrophy. Amylin has also completed a submission for regulatory approval to the US FDA for metreleptin in the treatment of diabetes mellitus and/or hypertriglyceridaemia in patients with rare forms of lipodystrophy. Clinical development of the drug is also underway in the USA for the treatment of type 1 diabetes. Amgen was previously assessing the use of metreleptin as a treatment for amenorrhoea; however, it appears that development in this indication has been discontinued. This article summarizes the milestones in the development of metreleptin leading to this first approval for lipodystrophy.

Irisin mRNA and circulating levels in relation to other myokines in healthy and morbidly obese humans

OBJECTIVE

Skeletal muscle is considered to be an endocrine organ that secretes a number of myokines including follistatin (FST), myostatin (MSTN), activin A, and the newly identified irisin. Irisin's biology and function exhibit similarities with the functions of the FST-MSTN-activin A axis. It remains unknown whether there is any interplay among these molecules. The aim of this study is to examine potential associations of irisin with the FST, MSTN, and activin A axis.

METHODS

Two observational studies were performed to evaluate the associations of irisin with the other three peptides. Study A included 150 healthy males aged 18.48±0.16 years with BMI 23.18±3.75 kg/m(2). Fasting serum samples were used to measure the levels of the molecules of interest. Study B included 14 morbidly obese individuals, candidates for bariatric surgery, aged 53.14±8.93 years with BMI 50.18±10.63 kg/m(2). Blood samples were obtained after an overnight fast. Eight out of the 14 participants consented to an optional thigh biopsy during their bariatric surgery. Using the above blood and tissue samples, we measured circulating levels and muscle mRNA of irisin, FST, MSTN, and activin A.

RESULTS

We report that FNDC5 mRNA in muscle is positively correlated with FST mRNA expression in morbidly obese subjects (ρ=0.93, P<0.001). We also found that circulating irisin is positively correlated with FST circulating levels among lean subjects (ρ=0.17, P=0.05) while this association was suggestive among the obese (ρ=0.56, P=0.07).

CONCLUSION

The newly identified myokine irisin may be positively associated with FST at both the mRNA and circulating protein level.

Leptin's role in lipodystrophic and nonlipodystrophic insulin-resistant and diabetic individuals

Leptin is an adipocyte-secreted hormone that has been proposed to regulate energy homeostasis as well as metabolic, reproductive, neuroendocrine, and immune functions. In the context of open-label uncontrolled studies, leptin administration has demonstrated insulin-sensitizing effects in patients with congenital lipodystrophy associated with relative leptin deficiency. Leptin administration has also been shown to decrease central fat mass and improve insulin sensitivity and fasting insulin and glucose levels in HIV-infected patients with highly active antiretroviral therapy (HAART)-induced lipodystrophy, insulin resistance, and leptin deficiency. On the contrary, the effects of leptin treatment in leptin-replete or hyperleptinemic obese individuals with glucose intolerance and diabetes mellitus have been minimal or null, presumably due to leptin tolerance or resistance that impairs leptin action. Similarly, experimental evidence suggests a null or a possibly adverse role of leptin treatment in nonlipodystrophic patients with nonalcoholic fatty liver disease. In this review, we present a description of leptin biology and signaling; we summarize leptin's contribution to glucose metabolism in animals and humans in vitro, ex vivo, and in vivo; and we provide insights into the emerging clinical applications and therapeutic uses of leptin in humans with lipodystrophy and/or diabetes.

Peripheral signalling involved in energy homeostasis control

The alarming prevalence of obesity has led to a better understanding of the molecular mechanisms controlling energy homeostasis. Regulation of energy intake and expenditure is more complex than previously thought, being influenced by signals from many peripheral tissues. In this sense, a wide variety of peripheral signals derived from different organs contributes to the regulation of body weight and energy expenditure. Besides the well-known role of insulin and adipokines, such as leptin and adiponectin, in the regulation of energy homeostasis, signals from other tissues not previously thought to play a role in body weight regulation have emerged in recent years. The role of fibroblast growth factor 21 (FGF21), insulin-like growth factor 1 (IGF-I), and sex hormone-binding globulin (SHBG) produced by the liver in the regulation of body weight and insulin sensitivity has been recently described. Moreover, molecules expressed by skeletal muscle such as myostatin have also been involved in adipose tissue regulation. Better known is the involvement of ghrelin, cholecystokinin, glucagon-like peptide 1 (GLP-1) and PYY(3-36), produced by the gut, in energy homeostasis. Even the kidney, through the production of renin, appears to regulate body weight, with mice lacking this hormone exhibiting resistance to diet-induced obesity. In addition, the skeleton has recently emerged as an endocrine organ, with effects on body weight control and glucose homeostasis through the actions of bone-derived factors such as osteocalcin and osteopontin. The comprehension of these signals will help in a better understanding of the aetiopathology of obesity, contributing to the potential development of new therapeutic targets aimed at tackling excess body fat accumulation.

The complex interaction between obesity, metabolic syndrome and reproductive axis: a narrative review

The aim of this narrative review is to provide current evidence for the interaction between obesity, metabolic syndrome (MS) and reproductive axis. Gonadotropin-releasing hormone (GnRH) pulses and, consequently, normal function of reproductive (hypothalamus-pituitary-gonadal) axis depend on normal energy balance, which presupposes sufficient food intake, reasonable energy consumption and average thermoregulatory costs. In case of an energy imbalance, reproductive dysfunction may occur. In young women, excessive leanness is accompanied by puberty delay, whereas premature puberty might be a manifestation of obesity. In a similar way, obesity in men affects fertility. Excess adipose tissue results in increased conversion of testosterone to estradiol, which may lead to secondary hypogonadism through reproductive axis suppression. Moreover, oxidative stress at the level of the testicular micro-environment may result in decreased spermatogenesis and sperm damage. Products of the adipocyte, such as leptin, adiponectin and resistin, and gut peptides, such as ghrelin, are considered to be crucial in the interaction between energy balance and reproduction. Finally, an indirect evidence for the interplay between MS and reproductive axis is the fact that when treating components of one, parameters of the other can be improved as well. These therapeutic interventions include lifestyle modifications, pharmacological agents, such as sex hormone replacement therapy, and surgical procedures. Although many issues remain unclear, the elucidation of the complex interaction between MS and reproductive axis will have obvious clinical implications in the therapeutic approach of both entities.

Leptin in relation to the lipodystrophy-associated metabolic syndrome

Leptin, an adipocyte-secreted hormone, regulates energy homeostasis as well as reproductive, neuroendocrine, immune and metabolic functions. Subjects with decreased amounts of fat in their adipose tissue, i.e., lipoatrophy, have low leptin levels. In the context of open-label, uncontrolled studies leptin administration, in physiological replacement doses, has been shown to have metabolically salutary effects in the rare patients with the syndrome of congenital lipodystrophy accompanied by leptin deficiency. Much more patients with lipodystrophy suffer from lipodystrophy and the metabolic syndrome associated with the use of highly active antiretroviral therapy. In this so called highly active antiretroviral therapy (HAART)-associated lipodystrophy and metabolic syndrome, patients demonstrate fat maldistribution with dyslipidemia, insulin resistance, and other metabolic complications. Leptin administration has been shown to decrease central fat mass and to improve fasting insulin/glucose levels and insulin sensitivity in human immunodeficiency virus-infected hypoleptinemic patients with HAART induced lipodystrophy and the metabolic syndrome. By contrast, the results of leptin treatment in leptin replete or hyperleptinemic obese individuals with glucose intolerance and diabetes mellitus have been minimal or null, presumably due to leptin tolerance or resistance that impairs leptin action. In this review, we present the emerging clinical applications and potential therapeutic uses of leptin in humans with lipodystrophy and the metabolic syndrome.

Energy deprivation alters in a leptin- and cortisol-independent manner circulating levels of activin A and follistatin but not myostatin in healthy males

CONTEXT

Activin A, myostatin, and follistatin have recently emerged as important regulatory molecules of reproduction and the musculoskeletal system. Little is known, however, about their day/night patterns of secretion and their physiological regulation by energy availability.

OBJECTIVE

The objective of the study was to explore day/night patterns of secretion and assess whether energy deprivation alters circulating levels of activin A, myostatin, follistatin, and cortisol and to examine whether leptin may mediate this effect. DESIGN, SETTING AND PATIENTS, AND INTERVENTIONS: Seven healthy lean men (aged 23.2 ± 3.7 yr, body mass index 23.6 ± 1.7 kg/m(2)) were studied for 72 h under three different conditions: on their baseline/isocaloric diet and in a complete fasting state with administration of either placebo or metreleptin. The two fasting studies were randomized and double blinded. Blood samples were obtained every 15 min from 0800 h on d 3 until 0800 h on d 4 and pooled hourly.

MAIN OUTCOME MEASURES

Serum concentrations of activin A, myostatin, follistatin, cortisol, and leptin were measured.

RESULTS

In contrast to cortisol, we demonstrated no day/night pattern of activin A, myostatin, and follistatin secretion. Activin A concentrations decreased significantly in response to energy deprivation (P < 0.01). Follistatin and cortisol concentrations increased significantly (P < 0.01 and P < 0.01, respectively). Myostatin remained unaffected (P = 0.40). Leptin administration reversed cortisol response (P < 0.01) but failed to alter activin A, follistatin, or myostatin concentrations.

CONCLUSIONS

Unlike cortisol, there is no day/night variation in the concentrations of activin A, myostatin, and follistatin in healthy young males. Although energy deprivation-induced cortisol changes are leptin mediated, the changes in follistatin and activin A concentrations occur through a leptin-independent pathway.

Leptin in human physiology and pathophysiology

Leptin, discovered through positional cloning 15 years ago, is an adipocyte-secreted hormone with pleiotropic effects in the physiology and pathophysiology of energy homeostasis, endocrinology, and metabolism. Studies in vitro and in animal models highlight the potential for leptin to regulate a number of physiological functions. Available evidence from human studies indicates that leptin has a mainly permissive role, with leptin administration being effective in states of leptin deficiency, less effective in states of leptin adequacy, and largely ineffective in states of leptin excess. Results from interventional studies in humans demonstrate that leptin administration in subjects with congenital complete leptin deficiency or subjects with partial leptin deficiency (subjects with lipoatrophy, congenital or related to HIV infection, and women with hypothalamic amenorrhea) reverses the energy homeostasis and neuroendocrine and metabolic abnormalities associated with these conditions. More specifically, in women with hypothalamic amenorrhea, leptin helps restore abnormalities in hypothalamic-pituitary-peripheral axes including the gonadal, thyroid, growth hormone, and to a lesser extent adrenal axes. Furthermore, leptin results in resumption of menses in the majority of these subjects and, in the long term, may increase bone mineral content and density, especially at the lumbar spine. In patients with congenital or HIV-related lipoatrophy, leptin treatment is also associated with improvements in insulin sensitivity and lipid profile, concomitant with reduced visceral and ectopic fat deposition. In contrast, leptin's effects are largely absent in the obese hyperleptinemic state, probably due to leptin resistance or tolerance. Hence, another emerging area of research pertains to the discovery and/or usefulness of leptin sensitizers. Results from ongoing studies are expected to further increase our understanding of the role of leptin and the potential clinical applications of leptin or its analogs in human therapeutics.