Leptin deficiency: clinical implications and opportunities for therapeutic interventions

Gut microbiota in overweight and obesity: crosstalk with adipose tissue

Overweight and obesity are characterized by excessive fat mass accumulation produced when energy intake exceeds energy expenditure. One plausible way to control energy expenditure is to modulate thermogenic pathways in white adipose tissue (WAT) and/or brown adipose tissue (BAT). Among the different environmental factors capable of influencing host metabolism and energy balance, the gut microbiota is now considered a key player. Following pioneering studies showing that mice lacking gut microbes (that is, germ-free mice) or depleted of their gut microbiota (that is, using antibiotics) developed less adipose tissue, numerous studies have investigated the complex interactions existing between gut bacteria, some of their membrane components (that is, lipopolysaccharides), and their metabolites (that is, short-chain fatty acids, endocannabinoids, bile acids, aryl hydrocarbon receptor ligands and tryptophan derivatives) as well as their contribution to the browning and/or beiging of WAT and changes in BAT activity. In this Review, we discuss the general physiology of both WAT and BAT. Subsequently, we introduce how gut bacteria and different microbiota-derived metabolites, their receptors and signalling pathways can regulate the development of adipose tissue and its metabolic capacities. Finally, we describe the key challenges in moving from bench to bedside by presenting specific key examples.

Physiological Appetite Regulation and Bariatric Surgery

Obesity remains a common metabolic disorder and a threat to health as it is associated with numerous complications. Lifestyle modifications and caloric restriction can achieve limited weight loss. Bariatric surgery is an effective way of achieving substantial weight loss as well as glycemic control secondary to weight-related type 2 diabetes mellitus. It has been suggested that an anorexigenic gut hormone response following bariatric surgery contributes to weight loss. Understanding the changes in gut hormones and their contribution to weight loss physiology can lead to new therapeutic treatments for weight loss. Two distinct types of neurons in the arcuate hypothalamic nuclei control food intake: proopiomelanocortin neurons activated by the anorexigenic (satiety) hormones and neurons activated by the orexigenic peptides that release neuropeptide Y and agouti-related peptide (hunger centre). The arcuate nucleus of the hypothalamus integrates hormonal inputs from the gut and adipose tissue (the anorexigenic hormones cholecystokinin, polypeptide YY, glucagon-like peptide-1, oxyntomodulin, leptin, and others) and orexigeneic peptides (ghrelin). Replicating the endocrine response to bariatric surgery through pharmacological mimicry holds promise for medical treatment. Obesity has genetic and environmental factors. New advances in genetic testing have identified both monogenic and polygenic obesity-related genes. Understanding the function of genes contributing to obesity will increase insights into the biology of obesity. This review includes the physiology of appetite control, the influence of genetics on obesity, and the changes that occur following bariatric surgery. This has the potential to lead to the development of more subtle, individualised, treatments for obesity.

Sex-Specific Effects of the Genetic Variant rs10487505 Upstream of leptin in the Development of Obesity

The SNP rs10487505 in the promotor region of the leptin gene was reported to be associated with decreased circulating leptin and increased body mass index (BMI). However, the phenotypic outcomes affected by rs10487505 in the leptin regulatory pathway have not been systematically studied. Therefore, the aim of this study was to elucidate the influence of rs10487505 on leptin mRNA expression and obesity-related parameters. We genotyped rs10487505 in DNA samples from 1665 patients with obesity and lean controls and measured leptin gene expression in paired samples of adipose tissue (AT, N = 310), as well as circulating leptin levels. We confirm the leptin-lowering effect of rs10487505 in women. In contrast to the previously reported data from population-based studies, in this mainly obese cohort, we describe a lower mean BMI in women carrying the C allele of rs10487505. However, no association of rs10487505 with AT leptin mRNA expression was found. Our data suggest that reduced circulating leptin levels are not a result of the direct silencing of leptin mRNA expression. Furthermore, leptin reduction by rs10487505 does not associate with BMI in a linear manner. Instead, the decreasing effect on BMI might be dependent on the severity of obesity.

Obesity Programs Macrophages to Support Cancer Progression

Obesity induces multifactorial effects such as dyslipidemia, insulin resistance, and arterial hypertension that influence the progression of many diseases. Obesity is associated with an increased incidence of cancers, and multiple mechanisms link obesity with cancer initiation and progression. Macrophages participate in the homeostasis of adipose tissue and play an important role in cancer. Adipose tissue expansion in obesity alters the balance between pro- and anti-inflammatory macrophages, which is a primary cause of inflammation. Chronic low-grade inflammation driven by macrophages is also an important characteristic of cancer. Adipocytes secrete various adipokines, including adiponectin, leptin, IL6, and TNFα, that influence macrophage behavior and tumor progression. Furthermore, other metabolic effects of obesity, such as hyperlipidemia, hyperglycemia, and hypercholesterolemia, can also regulate macrophage functionality in cancer. This review summarizes how obesity influences macrophage-tumor cell interactions and the role of macrophages in the response to anticancer therapies under obese conditions.

Chronic Leptin Deficiency Improves Tolerance of Physiological Damage and Host-Pathogen Cooperation during Yersinia pseudotuberculosis Infection

To combat infections, hosts employ a combination of antagonistic and cooperative defense strategies. The former refers to pathogen killing mediated by resistance mechanisms, while the latter refers to physiological defense mechanisms that promote host health during infection independent of pathogen killing, leading to an apparent cooperation between the host and the pathogen. Previous work has shown that Leptin, a pleiotropic hormone that plays a central role in regulating appetite and energy metabolism, is indispensable for resistance mechanisms, while a role for Leptin signaling in cooperative host-pathogen interactions remains unknown. Using a mouse model of Yersinia pseudotuberculosis (Yptb) infection, an emerging pathogen that causes fever, diarrhea, and mesenteric lymphadenitis in humans, we found that the physiological effects of chronic Leptin-signaling deficiency conferred protection from Yptb infection due to increased host-pathogen cooperation rather than greater resistance defenses. The protection against Yptb infection was independent of differences in food consumption, lipolysis, or fat mass. Instead, we found that the chronic absence of Leptin signaling protects from a shift to lipid utilization during infection that contributes to Yptb lethality. Furthermore, we found that the survival advantage conferred by Leptin deficiency was associated with increased liver and kidney damage. Our work reveals an additional level of complexity for the role of Leptin in infection defense and demonstrates that in some contexts, in addition to tolerating the pathogen, tolerating organ damage is more beneficial for survival than preventing the damage.

Early-onset severe obesity due to homozygous p.R105W (c313C> T) mutation in leptin gene in Turkish siblings: Two cases reports

Congenital leptin deficiency (CLD) is a rare cause of monogenic form obesity due to homozygous or compound heterozygous mutations in the LEP gene. To date, nine pathogenic mutations have been reported. In this study, we present are; an 18-year-old morbidly obese girl and a 14-year-old obese brother, both with homozygous mutation in the LEP gene [p.R105W (c313C> T)] and their data after three years of recombinant leptin treatment. To date, few cases of CLD have been reported in the literature. The cases reported here were siblings who were not diagnosed despite presentation at the clinic due to obesity in childhood, and diagnosis was delayed until adolescence. Clinicians need to consider CLD, a monogenic form of obesity in children with early severe obesity onset, especially if they are the child of a consanguineous marriage.

Association Between Leptin, Cognition, and Structural Brain Measures Among "Early" Middle-Aged Adults: Results from the Framingham Heart Study Third Generation Cohort

BACKGROUND

There is growing interest in the pathophysiological processes of preclinical Alzheimer's disease (AD), including the potential role of leptin. Human studies have shown that both low and high levels of leptin can be associated with worse neurocognitive outcomes, suggesting this relationship may be moderated by another risk factor.

OBJECTIVE

We examined the association between plasma leptin levels and both neuropsychological test performance and structural neuroimaging and assessed whether body mass index (BMI) is an effect modifier of these associations.

METHODS

Our study sample consisted of 2,223 adults from the Framingham Heart Study Third Generation Cohort (average age = 40 years, 53% women).

RESULTS

Among the entire sample, there was no association between leptin and any of the neuropsychological domain measures or any of the MRI brain volume measures, after adjustment for BMI, APOE4, and other clinical factors. However, we did observe that BMI category was an effect modifier for the association between leptin and verbal memory (p for interaction = 0.03), where higher levels of leptin were associated with better performance among normal weight participants (BMI 18.5-24.9) kg/m2 (beta = 0.12, p = 0.02). No association was observed between leptin level and verbal memory test performance among participants who were overweight or obese.

CONCLUSION

These findings suggest that the association between leptin and cognitive function is moderated by BMI category. Prospective examination of individuals transitioning from middle age to older adulthood will help to clarify the contribution of leptin to AD and other neurodegenerative conditions.

The association between body composition, leptin levels and glucose dysregulation in youth with cystic fibrosis

BACKGROUND

Optimization of nutritional status is recommended in patients with cystic fibrosis (CF) given the association between lower body mass index (BMI) and poor clinical outcomes. However, higher BMI and body fat correlate with glucose impairment and higher leptin levels in the general population. Differences in body composition and leptin levels between the categories of glucose tolerance were assessed in youth with CF and healthy controls.

METHODS

In a cross-sectional study, 59 adolescents and young adults with CF and 15 healthy controls matched by age and gender, underwent body composition analysis using dual energy X-ray absorptiometry (DXA) and a 2-hour oral glucose tolerance test (OGTT). Measures of insulin sensitivity, β-cell insulin secretion and fasting leptin levels were obtained.

RESULTS

Of the participants with CF, 62% were classified as abnormal glucose tolerant and 22% with cystic fibrosis related diabetes (CFRD). Patients with CFRD had a lower fat mass index (FMI) z-score, wt z-score and leptin levels compared to the control group (-1.86 vs. - 0.59, p=0.01; -1.86 vs 0.44, p=<0.001 and 7.9 vs vs. 27.7 µg/L, p=0.01). Leptin correlated positively with FMI z-score, BMI, weight z-score and indices of insulin secretion. FMI z-score correlated positively with higher insulin resistance (HOMA-IR), and lower insulin sensitivity (Matsuda index) (r=0.31; p =0.01 and r=-0.29; p=0.02, respectively) in the CF group.

CONCLUSIONS

This study shows that despite new therapeutic strategies, youth with CF have lower body fat, weight z-score and leptin levels, particularly in subjects with early onset CFRD.

Association of Leptin Levels and Disease Activity in Patients with Early Rheumatoid Arthritis

OBJECTIVE

Previous studies have reported a link between metabolic parameters and disease activity in rheumatoid arthritis (RA), although the evidence is limited in early RA. We aimed to investigate the relationship between disease activity and adipocytokine levels in subjects with early RA.

METHODS

Forty-seven patients with early RA (symptom duration ≤12 months) were enrolled. Disease activity was determined by DAS28-CRP. Patients were treated with DMARDs according to the EULAR recommendations. Subjects were tested before and five months after treatment.

RESULTS

Early RA patients with high disease activity (DAS28-CRP > 4.9) had greater BMI (31.2 ± 6.8 kg/m² vs. 26.7 ± 4.1 kg/m²; p = 0.006) and higher leptin levels (14.62 ± 15.60 ng/mL vs. 7.82 ± 8.00 ng/mL; p = 0.048). Levels of other adipocytokines were not significantly different. Leptin levels were similar in subjects with mild/moderate disease activity and controls. DAS28-CRP was correlated with leptin (r = 0.303, p = 0.039). Leptin levels decreased significantly after treatment (from 10.86 ± 12.34 ng/mL to 9.22 ± 9.29 ng/mL; p = 0.047) along with insulin levels (from 13.68 ± 21.90 mU/L to 7.09 ± 4.72 mU/L; p = 0.010) and HOMA-IR (from 4.39 ± 9.53 to 1.70 ± 1.38; p = 0.012). HDL cholesterol levels increased (from 41 ± 10 mg/dL48 ± 10 mg/dL; p <0.001).

CONCLUSION

Leptin levels were associated with disease activity in patients with early RA and these levels decreased after treatment with DMARDs. Further research is needed to elicit leptin's role to regulate disease activity in early RA.

Impact of Genetic Variations and Epigenetic Mechanisms on the Risk of Obesity

Rare genetic obesity disorders are characterized by mutations of genes strongly involved in the central or peripheral regulation of energy balance. These mutations are effective in causing the early onset of severe obesity and insatiable hunger (hyperphagia), suggesting that the genetic component can contribute to 40–70% of obesity. However, genes’ roles in the processes leading to obesity are still unclear. This review is aimed to summarize the current knowledge of the genetic causes of obesity, especially monogenic obesity, describing the role of epigenetic mechanisms in obesity and metabolic diseases. A comprehensive understanding of the underlying genetic and epigenetic mechanisms, with the metabolic processes they control, will permit adequate management and prevention of obesity.

The effect of underweight on female and male reproduction

Chronic energy deficiency can impair the hypothalamic-pituitary-gonadal (HPG) axis and lead to hypothalamic anovulation in underweight women. This review presents the syndromes related to underweight status that are associated with infertility, summarizes the underlying mechanisms, and reviews the available treatment options. Eating disorders, such as anorexia nervosa (AN), constitute the most common cause of infertility in underweight women, who, in addition, experience miscarriages, and sexual dysfunction. The relative energy deficiency in sports (RED-S; former terminology: athlete's triad) involves menstrual dysfunction due to low energy availability, which results in anovulation. Moreover, lipodystrophies, malnutrition, starvation, systematic illnesses (malignancies, endocrinopathies, infectious diseases, advanced chronic diseases, neurologic illnesses), and the utilization of drugs can cause excessive weight loss. They may result in fertility problems due to the loss of adipose tissue and the subsequent hormonal disturbances. Each of these conditions requires multidisciplinary management. Nutritional counseling should target the restoration of energy balance by increasing intake and reducing output. Medical treatment, recommended only for patients who did not respond to standard treatment, may include antipsychotics, antidepressants, or leptin administration. Finally, psychiatric treatment is considered an integral part of the standard treatment.

Adipokines in anorexia nervosa: A systematic review and meta-analysis

OBJECTIVE

The association between adipokine dysregulation and weight loss of patients with anorexia nervosa (AN) has been long investigated, in search of a causal relationship. We sought to: a) synthesize the available evidence on potential differences between AN patients and controls with regards to adipokine measurements (namely, leptin, adiponectin, resistin, soluble leptin receptor, visfatin, vaspin and omentin), b) estimate the potential differences between constitutionally thin (CT) subjects and AN patients, and c) present the available evidence with regards to biomarker efficacy of adipokines in AN.

METHODS

A structured literature search, last updated in 2/2019, was conducted in the following databases: MEDLINE, clinicaltrials.gov, PsycINFO, PSYNDEX and WHO Registry Network. The primary outcome was the standardized mean difference of each adipokine between AN patients and controls of normal BMI. Secondary outcomes included the correlation of leptin with BMI and bone mineral density among AN patients. The study protocol is published in PROSPERO (CRD42018116767).

RESULTS

In a total of 622 screened studies, after exclusion of non-relevant articles and duplicates, 84 reports on leptin, 31 reports on adiponectin, 12 on resistin, 10 on soluble leptin receptor, 5 on visfatin, 3 on vaspin and omentin were finally included in the meta-analysis. Publication bias assessment underlined the possibility of non-significant studies being underrepresented; still, significant heterogeneity renders this statement inconclusive. Leptin [ELISA: SMD (95% CI): -3.03 (-4, -2.06)], radioimmunoassay [RIA: -3.84 (-4.71, -2.98)] and resistin [-1.67 (-2.85, -0.48)] were significantly lower in patients with AN compared with controls, whereas visfatin decrease did not reach significance (-2.03 (-4.38, 0.3). Mean adiponectin, vaspin and soluble leptin receptor levels were significantly higher. In subgroup analysis, a significantly attenuated SMD was reported in ELISA studies compared with RIA studies. Leptin was significantly lower in AN patients compared to CT subjects and BMI marginally did not appear to confound the result. In all analyses, except for the correlation of leptin with BMI in AN patients, high heterogeneity was present. Meta-regression analysis indicated a potential confounding action of controls' BMI and age on leptin SMD and between-assay differences. Publication bias assessment underlined the possibility of nonsignificant studies being underrepresented; still, further investigation did not corroborate this and significant heterogeneity renders this statement inconclusive.

CONCLUSION

A distinct profile of adipokine dysregulation is apparent in AN patients, following the anticipated pattern of low BMI. A precise estimation of the magnitude is hindered by heterogeneity, partly caused by varying assays and methodologies. Interestingly, while mean leptin levels are lower in AN subjects compared with constitutionally thin women, there is an overlap in individual levels between the two groups and therefore, they cannot be used to differentiate between these states.

The Role of Adipose Tissue in the Pathogenesis and Therapeutic Outcomes of Inflammatory Bowel Disease

Though historically regarded as an inert energy store, adipose tissue is a complex endocrine organ, which is increasingly implicated in the pathogenesis of inflammatory bowel disease (IBD). Accumulating evidence points to visceral adipose tissue and specifically to its mesenteric component, or “creeping fat” as impacting on the disease course through its immunomodulatory properties. On the one hand, mesenteric fat acts as a physical barrier to inflammation and is involved in controlling host immune response to translocation of gut bacteria. On the other hand, however, there exists a strong link between visceral fat and complicated course of the disease with unfavorable therapeutic outcomes. Furthermore, “creeping fat” appears to play different roles in different IBD phenotypes, with the greatest pathogenetic contribution probably to an ileal form of Crohn’s disease. In this review, we summarize and discuss the existing literature on the subject and identify high-priority areas for future research. It may be that a better understanding of the role of mesenteric fat in IBD will determine new therapeutic targets and translate into improved clinical outcomes.

Lower serum adiponectin level is associated with lipodystrophy among HIV-infected men in the Study to Understand the Natural History of HIV/AIDS in the Era of Effective Therapy (SUN) study

OBJECTIVES

Adiponectin levels are inversely related to cardiovascular risk and are low in diabetics and obese persons. We examined the association between adiponectin concentration and HIV-associated lipodystrophy, which remains unclear.

METHODS

The Study to Understand the Natural History of HIV/AIDS in the Era of Effective Therapy (SUN) was a prospective cohort study of HIV-infected adults conducted in four US cities. Lean body and fat masses were assessed using dual-energy X-ray absorptiometry scans. Using baseline data from 2004 to 2006, we defined lipodystrophy using a sex-specific fat mass ratio and performed cross-sectional analyses of associated risks using multivariable logistic regression.

RESULTS

Among 440 male participants (median age 42 years; 68% non-Hispanic white; 88% prescribed combination antiretroviral therapy; median CD4 lymphocyte count 468 cells/μL; 76% with viral load < 400 HIV-1 RNA copies/mL; 5% diabetic; median body mass index 25 kg/m² ), median concentrations of leptin and adiponectin were 3.04 ng/L [interquartile range (IQR) 1.77-5.43 ng/L] and 8005 μg/mL (IQR 4950-11 935 μg/mL), respectively. The prevalence of lipodystrophy was 14%. Lipodystrophy was significantly associated with increasing age [prevalence ratio (PR) 1.50; 95% confidence interval (CI) 1.10-2.06, per 10 years], adiponectin < 8005 μg/mL (PR 5.02; 95% CI 2.53-9.95), ever stavudine use (PR 2.26; 95% CI 1.36-3.75), CD4 cell count > 500 cells/μL (PR 2.59; 95% CI 1.46-4.61), viral load < 400 copies/mL (PR 3.98; 95% CI 1.25-12.6), highly sensitive C-reactive protein < 1.61 mg/L (PR 1.91; 95% CI 1.11-3.28) and smoking (PR 0.42; 95% CI 0.22-0.78).

CONCLUSIONS

Among men in this HIV-infected cohort, the prevalence of lipodystrophy was similar to previous estimates for persons living with HIV, and was associated with lower adiponectin levels, potentially indicating increased cardiovascular disease risk.

Clinical Trials Required to Assess Potential Benefits and Side Effects of Treatment of Patients With Anorexia Nervosa With Recombinant Human Leptin

The core phenotype of anorexia nervosa (AN) comprises the age and stage dependent intertwining of both its primary and secondary (i.e., starvation induced) somatic and mental symptoms. Hypoleptinemia acts as a key trigger for the adaptation to starvation by affecting diverse brain regions including the reward system and by induction of alterations of the hypothalamus-pituitary-“target-organ” axes, e.g., resulting in amenorrhea as a characteristic symptom of AN. Particularly, the rat model activity-based anorexia (ABA) convincingly demonstrates the pivotal role of hypoleptinemia in the development of starvation-induced hyperactivity. STAT3 signaling in dopaminergic neurons in the ventral tegmental area (VTA) plays a crucial role in the transmission of the leptin signal in ABA. In patients with AN, an inverted U-shaped relationship has been observed between their serum leptin levels and physical activity. Albeit obese and therewith of a very different phenotype, humans diagnosed with rare congenital leptin deficiency have starvation like symptoms including hypothalamic amenorrhea in females. Over the past 20 years, such patients have been successfully treated with recombinant human (rh) leptin (metreleptin) within a compassionate use program. The extreme hunger of these patients subsides within hours upon initiation of treatment; substantial weight loss and menarche in females ensue after medium term treatment. In contrast, metreleptin had little effect in patients with multifactorial obesity. Small clinical trials have been conducted for hypothalamic amenorrhea and to increase bone mineral density, in which metreleptin proved beneficial. Up to now, metreleptin has not yet been used to treat patients with AN. Metreleptin has been approved by the FDA under strict regulations solely for the treatment of generalized lipodystrophy. The recent approval by the EMA may offer, for the first time, the possibility to treat extremely hyperactive patients with AN off-label. Furthermore, a potential dissection of hypoleptinemia-induced AN symptoms from the primary cognitions and behaviors of these patients could ensue. Accordingly, the aim of this article is to review the current state of the art of leptin in relation to AN to provide the theoretical basis for the initiation of clinical trials for treatment of this eating disorder.

Zebrafish Mutants Carrying Leptin a (lepa) Gene Deficiency Display Obesity, Anxiety, Less Aggression and Fear, and Circadian Rhythm and Color Preference Dysregulation

Leptin, a hormone secreted by peripheral adipose tissues, regulates the appetite in animals. Recently, evidence has shown that leptin also plays roles in behavioral response in addition to controlling appetite. In this study, we examined the potential function of leptin on non-appetite behaviors in zebrafish model. By using genome editing tool of Transcription activator-like effector nuclease (TALEN), we successfully knocked out leptin a (lepa) gene by deleting 4 bp within coding region to create a premature-translation stop. Morphological and appetite analysis showed the lepa KO fish display a phenotype with obese, good appetite and elevation of Agouti-related peptide (AgRP) and Ghrelin hormones, consistent with the canonical function of leptin in controlling food intake. By multiple behavior endpoint analyses, including novel tank, mirror biting, predator avoidance, social interaction, shoaling, circadian rhythm, and color preference assay, we found the lepa KO fish display an anxiogenic phenotype showing hyperactivity with rapid swimming, less freezing time, less fear to predator, loose shoaling area forming, and circadian rhythm and color preference dysregulations. Using biochemical assays, melatonin, norepinephrine, acetylcholine and serotonin levels in the brain were found to be significantly reduced in lepa KO fish, while the levels of dopamine, glycine and cortisol in the brain were significantly elevated. In addition, the brain ROS level was elevated, and the anti-oxidative enzyme catalase level was reduced. Taken together, by performing loss-of-function multiple behavior endpoint testing and biochemical analysis, we provide strong evidence for a critical role of lepa gene in modulating anxiety, aggression, fear, and circadian rhythm behaviors in zebrafish for the first time.

In vitro fertilization/intracytoplasmic insemination and thyroid function: reviewing the evidence

Recent findings, that specific G protein-coupled TSH receptors (TSHR) and the nuclear thyroid hormone receptors (THRs) are widely expressed in reproductive tissues, reveal the close links between hypothalamus-pituitary-thyroid and hypothalamus-pituitary-gonadal axes. It has been suggested that thyroid function as well as thyroid autoimmunity (TAI) have an impact on Assisted Reproduction Technology (ART) reproductive outcome. Lately, it became evident that ovarian stimulation (OS) may also have an impact on thyroid function. This narrative review describes the mutual interaction between thyroid function and OS, by reviewing the current evidence, assessing the pathophysiological links and arriving at practical recommendations.

Addressing the Perfect Storm: Biomarkers in Obesity and Pathophysiology of Cardiometabolic Risk

BACKGROUND

The worldwide rise of obesity has provoked intensified research to better understand its pathophysiology as a means for disease prevention. Several biomarkers that may reflect various pathophysiological pathways that link obesity and cardiometabolic diseases have been identified over the past decades.

CONTENT

We summarize research evidence regarding the role of established and novel obesity-related biomarkers, focusing on recent epidemiological evidence for detrimental associations with cardiometabolic diseases including obesity-related cancer. The reviewed biomarkers include biomarkers of glucose–insulin homeostasis (insulin, insulin-like growth factors, and C-peptide), adipose tissue biomarkers (adiponectin, omentin, apelin, leptin, resistin, and fatty-acid-binding protein-4), inflammatory biomarkers (C-reactive protein, interleukin 6, tumor necrosis factor α), and omics-based biomarkers (metabolites and microRNAs).

SUMMARY

Although the evidence for many classical obesity biomarkers, including adiponectin and C-reactive protein (CRP), in disease etiology has been initially promising, the evidence for a causal role in humans remains limited. Further, there has been little demonstrated ability to improve disease prediction beyond classical risk factors. In the era of “precision medicine,” there is an increasing interest in novel biomarkers, and the extended list of potentially promising biomarkers, such as adipokines, cytokines, metabolites, and microRNAs, implicated in obesity may bring new promise for improved, personalized prevention. To further evaluate the role of obesity-related biomarkers as etiological and early-disease-prediction targets, well-designed studies are needed to evaluate temporal associations, replicate findings, and test clinical utility of novel biomarkers. In particular, studies to determine the therapeutic implications of novel biomarkers beyond established metabolic risk factors are highly warranted.

Leptin applications in 2015: what have we learned about leptin and obesity?

PURPOSE OF REVIEW

To summarize previous and current advancements for leptin therapeutics, we described how leptin may be useful in leptin deficient states such as lipodystrophy, for which leptin was recently approved, and how it may be useful in the future for typical obesity.

RECENT FINDINGS

The discovery of leptin in 1994 built the foundation for understanding the pathophysiology and treatment of obesity. Leptin therapy reverses morbid obesity related to congenital leptin deficiency and appears to possibly treat lipodystrophy, a finding which has led to the approval of leptin for the treatment of lipodystrophy in the USA and Japan. Typical obesity, on the other hand, is characterized by hyperleptinemia and leptin tolerance. Thus, leptin administration has proven ineffective for inducing weight loss on its own but could possibly be useful in combination with other therapies or for weight loss maintenance.

SUMMARY

Leptin is not able to treat typical obesity; however, it is effective for reversing leptin deficiency-induced obesity and is possibly useful in lipodystrophy. New mechanisms and pathways involved in leptin resistance are continuously discovered, whereas the development of new techniques and drug combinations which may improve leptin's efficacy and safety regenerate the hope for its use as an effective treatment for typical obesity.

Early leptin intervention reverses perturbed energy balance regulating hypothalamic neuropeptides in the pre- and postnatal calorie-restricted female rat offspring

Pre- and postnatal calorie restriction is associated with postnatal growth restriction, reduced circulating leptin concentrations, and perturbed energy balance. Hypothalamic regulation of energy balance demonstrates enhanced orexigenic (NPY, AgRP) and diminished anorexigenic (POMC, CART) neuropeptide expression (PN21), setting the stage for subsequent development of obesity in female Sprague-Dawley rats. Leptin replenishment during the early postnatal period (PN2-PN8) led to reversal of the hypothalamic orexigenic:anorexigenic neuropeptide ratio at PN21 by reducing only the orexigenic (NPY, AgRP), without affecting the anorexigenic (POMC, CART) neuropeptide expression. This hypothalamic effect was mediated via enhanced leptin receptor (ObRb) signaling that involved increased pSTAT3/STAT3 but reduced PTP1B. This was further confirmed by an increase in body weight at PN21 in response to intracerebroventricular administration of antisense ObRb oligonucleotides (PN2-PN8). The change in the hypothalamic neuropeptide balance in response to leptin administration was associated with increased oxygen consumption, carbon dioxide production, and physical activity, which resulted in increased milk intake (PN14) with no change in body weight. This is in contrast to the reduction in milk intake with no effect on energy expenditure and physical activity observed in controls. We conclude that pre- and postnatal calorie restriction perturbs hypothalamic neuropeptide regulation of energy balance, setting the stage for hyperphagia and reduced energy expenditure, hallmarks of obesity. Leptin in turn reverses this phenotype by increasing hypothalamic ObRb signaling (sensitivity) and affecting only the orexigenic arm of the neuropeptide balance.

Leptin: A hormone linking activation of neuroendocrine axes with neuropathology

Leptin, a peptide hormone secreted by adipocytes, plays a central role in controlling appetite and weight in both rodents and humans. Basic science and clinical research suggest that this hormone not only affects the regulation of the neuroendocrine axes, but also exerts effects on the central nervous system with subsequent alterations in psychological functions. For instance, leptin suppresses cortisol secretion during stress-related activation of the adrenal axis. As psychiatric disorders like depression are associated with hypercortisolism, leptin is proposed to exert anti-depressant-like effects due to its inhibition of chronically overactive hypothalamo-pituitary-adrenal axis function. Moreover, leptin status of depressed patients could serve as a prognostic marker for therapy response. Besides its influence on neuroendocrine pathways leptin seems to have direct central effects on brain development and neuroplasticity. Low leptin levels have been shown to be associated with increased risk of developing dementia, supporting the idea of a pro-cognitive effect of leptin. These areas may have direct clinical implications and deserve to be studied further in the future.

Leptin and the brain: influences on brain development, cognitive functioning and psychiatric disorders

Receptors of leptin, the prototypical adipokine, are expressed throughout the cortex and several other areas of the brain. Although typically studied for its role in energy intake and expenditure, leptin plays a critical role in many other neurocognitive processes and interacts with various other hormones and neurotransmitters to perform these functions. Here, we review the literature on how leptin influences brain development, neural degradation, Alzheimer's disease, psychiatric disorders, and more complicated cognitive functioning and feeding behaviors. We also discuss modulators of leptin and the leptin receptor as they relate to normal cognitive functioning and may mediate some of the actions of leptin in the brain. Although we are beginning to better understand the critical role leptin plays in normal cognitive functioning, there is much to be discovered.

New pharmacological perspectives for the leptin receptor in the treatment of obesity

After its discovery in 1994, leptin became the great hope as an anti-obesity treatment based on its ability to reduce food intake and increase energy expenditure. However, treating obese people with exogenous leptin was unsuccessful in most cases since most of them present already high circulating leptin levels to which they do not respond anymore defining the so-called state of "leptin resistance." Indeed, leptin therapy is unsuccessful to lower body weight in commonly obese people but effective in people with rare single gene mutations of the leptin gene. Consequently, treatment of obese people with leptin was given less attention and the focus of obesity research shifted toward the prevention and reversal of the state of leptin resistance. Many of these new promising approaches aim to restore or sensitize the impaired function of the leptin receptor by pharmacological means. The current review will focus on the different emerging therapeutic strategies in obesity research that are related to leptin and its receptor.

Association of serum adiponectin, leptin, and resistin concentrations with the severity of liver dysfunction and the disease complications in alcoholic liver disease

Background and aims. There is growing evidence that white adipose tissue is an important contributor in the pathogenesis of alcoholic liver disease (ALD). We investigated serum concentrations of total adiponectin (Acrp30), leptin, and resistin in patients with chronic alcohol abuse and different grades of liver dysfunction, as well as ALD complications. Materials and Methods. One hundred forty-seven consecutive inpatients with ALD were prospectively recruited. The evaluation of plasma adipokine levels was performed using immunoenzymatic ELISA tests. Multivariable logistic regression was applied in order to select independent predictors of advanced liver dysfunction and the disease complications. Results. Acrp30 and resistin levels were significantly higher in patients with ALD than in controls. Lower leptin levels in females with ALD compared to controls, but no significant differences in leptin concentrations in males, were found. High serum Acrp30 level revealed an independent association with advanced liver dysfunction, as well as the development of ALD complications, that is, ascites and hepatic encephalopathy. Conclusion. Gender-related differences in serum leptin concentrations may influence the ALD course, different in females compared with males. Serum Acrp30 level may serve as a potential prognostic indicator for patients with ALD.

Design and validation of a homogeneous time-resolved fluorescence-based leptin receptor binding assay

The pleiotropic cytokine hormone leptin, by activating its receptor OB-R, plays a major role in many biological processes, including energy homeostasis, immune function, and cell survival and proliferation. Abnormal leptin action is associated with obesity, autoimmune diseases, and cancer. The pharmacological characterization of OB-R and the development of synthetic OB-R ligands are still in their infancy because currently available binding assays are not compatible with ligand saturation binding experiments and high-throughput screening (HTS) approaches. We have developed here a novel homogeneous time-resolved fluorescence-based binding assay that overcomes these limitations. In this assay, fluorescently labeled leptin or leptin antagonist binds to the SNAP-tagged OB-R covalently labeled with terbium cryptate (Tb). Successful binding is monitored by measuring the energy transfer between the Tb energy donor and the fluorescently labeled leptin energy acceptor. Ligand binding saturation experiments revealed high-affinity dissociation constants in the subnanomolar range with an excellent signal-to-noise ratio. The assay performed in a 384-well format shows high specificity and reproducibility, making it perfectly compatible with HTS applications to identify new OB-R agonists or antagonists. In addition, fluorescently labeled leptin and SNAP-tagged OB-R will be valuable tools for monitoring leptin and OB-R trafficking in cells and tissues.

Long-term correction of type 1 and 2 diabetes by central leptin gene therapy independent of effects on appetite and energy expenditure

Adipocyte-derived leptin is a hormone associated with the regulation of energy homeostasis, including glucose metabolism. Hyperleptinemia, induced by the consumption of energy-enriched diets, inhibits leptin transport across the blood-brain barrier, and thereby produces leptin insufficiency in the hypothalamus. As a result of sustained leptin insufficiency, the hypothalamic restraint on pancreatic insulin secretion is lost. Additionally, both glucose metabolism and energy expenditure are also diminished, and both type 1 and type 2 diabetes are induced. A replication-deficient recombinant adeno-associated virus vector engineered to encode the leptin gene (rAVV-LEP) has been used in models of diabetes as a novel therapeutic approach. After rAVV-LEP injection in ob/ob mice, hypothalamic leptin expression was increased, body weight was suppressed, and hyperinsulinemia was ameliorated. Additionally injection of rAVV-LEP into the hypothalamus suppressed the expression of orexigenic neuropeptide Y (NPY) and enhanced anorexigenic pro-opiomelanocortin (POMC) in the arcuate nucleus (ARC) in rats. It is proposed that central leptin gene therapy should be tested clinically to reduce the worldwide epidemic of obesity, diabetes, and shortened life span. In this article, the information has been assembled from published review articles on this topic.

Leptin and mucosal immunity

Enhanced susceptibility to infection has long been recognized in children with congenital deficiency of leptin or its receptor. Studies in mice have demonstrated that leptin deficiency affects both the innate and acquired immune systems. Here, we review recent studies that demonstrate the impact on immunity of a common non-synonomous polymorphism of the leptin receptor. In a Bangladesh cohort of children, the presence of two copies of the ancestral Q223 allele was significantly associated with resistance to amebiasis. Children and mice with at least one copy of the leptin receptor 223R mutation were more susceptible to amebic colitis. Leptin signaling in the intestinal epithelium and downstream STAT3 (signal transducer and activator of transcription 3) and SHP2 (Src homology phosphatase 2) signaling were required for protection in the murine model of amebic colitis. Murine models have also implicated leptin in protection from other infections, including Mycobacterium tuberculosis, Klebsiella pneumoniae, and Streptococcus pneumoniae. Thus, the role of leptin signaling in infectious disease and specifically leptin-mediated protection of the intestinal epithelium will be the focus of this review.

Leptin as a modulator of neuroendocrine function in humans

Leptin, a peptide hormone secreted by adipocytes in proportion of the amount of energy stored in fat, plays a central role in regulating human energy homeostasis. In addition, leptin plays a significant permissive role in the physiological regulation of several neuroendocrine axes, including the hypothalamic-pituitary-gonadal, -thyroid, -growth hormone, and -adrenal axes. Decreased levels of leptin, also known as hypoleptinemia, signal to the brain a state of energy deprivation. Hypoleptinemia can be a congenital or acquired condition, and is associated with alterations of the aforementioned axes aimed at promoting survival. More specifically, gonadotropin levels decrease and become less pulsatile under conditions of energy deprivation, and these changes can be at least partially reversed through leptin administration in physiological replacement doses. Similarly, leptin deficiency is associated with thyroid axis abnormalities including abnormal levels of thyrotropin-releasing hormone, and leptin administration may at least partially attenuate this effect. Leptin deficiency results in decreased insulin-like growth factor 1 levels which can be partially ameliorated through leptin administration, and leptin appears to have a much more pronounced effect on the growth of rodents than that of humans. Similarly, adrenal axis function is regulated more tightly by low leptin in rodents than in humans. In addition to congenital leptin deficiency, conditions that may be associated with decreased leptin levels include hypothalamic amenorrhea, anorexia nervosa, and congenital or acquired lipodystrophy syndromes. Accumulating evidence from proof of concept studies suggests that leptin administration, in replacement doses, may ameliorate neuroendocrine abnormalities in individuals who suffer from these conditions.

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.

Pivotal role of leptin-hypothalamus signaling in the etiology of diabetes uncovered by gene therapy: a new therapeutic intervention?

The incidence of diabetes mellitus has soared to epidemic proportion worldwide. The debilitating chronic hyperglycemia is caused by either lack of insulin as in diabetes type 1 or its ineffectiveness as in diabetes type 2. Frequent replacement of insulin with or without insulin analogs for optimum glycemic control are the conventional cumbersome therapies. Recent application of leptin gene transfer technology has uncovered the participation of adipocytes-derived leptin-dependent hypothalamic neural signaling in glucose homeostasis and demonstrated that a breakdown in this communication due to leptin insufficiency in the hypothalamus underlies the etiology of chronic hyperglycemia. Reinstatement of central leptin sufficiency by hyperleptinemia produced either by intravenous leptin infusion or a single systemic injection of recombinant adenovirus vector encoding leptin gene suppressed hyperglycemia and evoked euglycemia only transiently in rodent models of diabetes type 1. In contrast, stable restoration of leptin sufficiency, solely in the hypothalamus, with biologically active leptin transduced by an intracerebroventicular injection of recombinant adeno-associated virus vector encoding leptin gene (rAAV-lep) abolished hyperglycemia and imposed euglycemia through the extended duration of experiment by stimulating glucose disposal in the periphery in models of diabetes type 1. Further, similar hypothalamic leptin transgene expression abrogated chronic hyperglycemia and hyperinsulinemia, the predisposing risk factors of the age and environmentally acquired diabetes type 2, and instituted euglycemia by independently activating relays that stimulate glucose metabolism and repress hyperinsulinemia and improve insulin sensitivity in the periphery. Consequently, this durable antidiabetic efficacy of one time rAAV-lep neurotherapy offers a potential novel substitute for insulin therapy following preclinical trials in subhuman primates and humans.

Timing of menarche in Chinese girls with and without adolescent idiopathic scoliosis: current results and review of the literature

Age at menarche is closely related to scoliosis progression during adolescence. Current data concerning the timing of menarche between scoliotic and non-scoliotic girls in the literature are conflicting, with inconclusive results. The aim of this study was to investigate the distribution difference of age at menarche for adolescent idiopathic scoliosis (AIS) girls and normal control population and to subsequently elucidate the menarche age difference through literature reviewing. Moreover, menarche age of AIS girls with Cobb angle <40°, 40-60°, >60° were compared to estimate its association with curve severity. Menstrual status data were available for 6,376 healthy female adolescents and 2,196 AIS girls. We notice that less than 10% of healthy Chinese girls experienced onset of menses before 11.38 years, and approximately 90% of healthy Chinese girls were menstruating by 13.88 years, with a median age of 12.63 years. As for AIS girls, less than 10% started to menstruate before 11.27 years, and approximately 90% were menstruating by 14.38 years, with a median age of 12.83 years. Average menarche age in AIS (12.83 ± 1.22 years) was significantly later than that of normal control girls (12.63 ± 0.98 years) (p < 0.001). Age at menarche for AIS affected girls was significantly greater than that of normal control girls at 75%, 90% of whom had attained menarche (p = 0.001, p < 0.001). Proportion of girls starting to menstruate after 14 years was significantly higher in AIS population compared with normal controls (16.3 vs. 8.1%, p < 0.001). In addition, AIS girls with Cobb angle >60° experienced onset of menses at an average age of 13.25 years, which was significantly later than AIS girls with Cobb angle <40° (12.81 years, p < 0.05) and marginally significantly later than AIS girls with Cobb angle between 40 and 60° (12.86 years, p = 0.053). In conclusion, a tendency of delayed onset of menarche was observed in Chinese idiopathic scoliotic girls in this large sample study, especially for girls with Cobb angle >60°, which is supported by multiple previously established positive linkages on AIS etiology studies. Accordingly it is believed that late menarche may contribute importantly to abnormal pubertal growth and subsequently modulate curve behavior in AIS.

A Japanese child with asymptomatic elevation of serum creatine kinase shows PTRF-CAVIN mutation matching with congenital generalized lipodystrophy type 4

Congenital generalized lipodystrophy (CGL), characterized by generalized absence of adipose tissue, has heterogeneous causes. Recently, a novel type of CGL complicated by muscular dystrophy was categorized as CGL4 caused by PTRF-CAVIN deficiency. However, it is unknown whether CGL4 exhibits clinical abnormalities during the infantile period. Here, we describe the youngest Japanese case of CGL4-a Japanese girl with asymptomatic high serum creatine kinase (CK) levels at 3months old. She was referred to our hospital at 5months of age because of her elevated serum CK (2528IU/L). Generalized absence of adipose tissue was first recognized at 2years of age. Mutation analysis of genes known to be responsible for CGL1-3 failed to disclose any abnormalities. Instead, analysis of the PTRF-CAVIN gene encoding PTRF-CAVIN revealed compound heterozygous mutations, one allele contained an insertion (c.696_697insC) and the other allele harbored a novel nonsense mutation (c.512C>A). Our patient had low serum leptin and adiponectin levels and insulin resistance. Pathological studies on biopsied muscle disclosed mild dystrophic change and highly reduced expression of PTRF-CAVIN. It was concluded that our PTRF-CAVIN deficient patient showed not only CGL but also asymptomatic elevation of serum CK because of her mild muscle dystrophic change.

The obese phenotype-inducing N82K mutation in human leptin disrupts receptor-binding and biological activity

A novel homozygous mutation of the leptin gene was recently reported in an Egyptian child and his sister with severe early onset obesity. This mutation results from the substitution of asparagine (AAC) by lysine (AAA) at codon 103 of a non-mature (signal peptide-containing) leptin and corresponds to the N82K mutation in the mature protein. The patient had very low serum leptin levels, raising the question of whether the obese phenotype resulted from low leptin levels or from its lower intrinsic activity. To answer this question, we characterized the functional consequences of the N82K mutation. Wild-type (WT) human leptin was mutated accordingly, expressed in Escherichia coli at high yield, purified to homogeneity as a monomer and compared to WT human leptin prepared by the same methodology. Circular dichroism analysis of the mutated leptin indicated proper refolding and a secondary structure identical to that of the WT human leptin. In contrast to WT human leptin, the N82K mutant did not form a detectable complex with human leptin-binding domain (hLBD) and its binding capacity to hLBD assessed in a nonradioactive receptor-binding assay was at least 500-fold lower than that of WT human leptin. The biological activity of the N82K mutant, tested in two cell bioassays, was reduced by more than three orders of magnitude relative to WT human leptin. Therefore, though the present report does not explain the reason for the low circulating leptin levels it definitely documents that the reported obese phenotype originates not only from low serum leptin levels but also from the N82K mutant's almost total lack of intrinsic leptin activity.