Cellular and Molecular Basis of Leptin Resistance in Obese Patients

Obesity is a major risk factor for diabetes,cardiovascular disease,and certain cancers.It arises from a chronic positive energy balance.This is usually due to unrestricted access to food in the context of genetic and epigenetic vulnerabilities.And an increasingly sedentary lifestyle.Over the past few decades,our understanding of the fluid and nervous systems that mediate energy balance control has greatly improved.However,our ability to formulate effective strategies to slow the current obesity epidemic has been hampered,mainly because we have limited knowledge of the resistance mechanisms of metabolic hormones such as leptin.Resistance to leptin is a sign of obesity,leptin is a key hormone for neuroendocrine control of energy balance.In the past few years,we have made tremendous progress in our in-depth research on the cellular pathways that disrupt the action of leptin.In this review,we discuss the molecular mechanisms of leptin resistance and how to use them as targets for obesity drug therapy.

The Mechanism of Regulating Leptin Resistance in Obesity and the Influence of Adjusting Methylation of OB-R,POMC Gene Promoter of Wendan Decoction (温胆汤)

Objective:To explore the mechanism of Wendan Decoction(温胆汤)regulating leptin resistance in obesity by analyzing the level of leptin in peripheral blood,the expression of leptin receptor and signal pathway molecule POMC in hypothalamus.To analyze the methylation status of the gene promoters of leptin receptor and POMC in obese rats fed with high fat diet,and the influence of adjusting methylation of OB-R,POMC gene promoter about Wendan decoction.Methods:Model rats were randomly divided into model control group and drug observation group,additional normal rats with 8 rats in each group.The models were established by high fat diet.The observation group was treated with Wendan Decoction(温胆汤)and the other two groups were treated with sterilized drinking water once a day for 6 weeks.Body weight was recorded every week during the intervention.Peripheral blood and hypothalamus were collected at the end of the experiment.The serum leptin level was measured by ELISA method,and the expression of OB-R and POMC in hypothalamus was detected by RT-qPCR method.Methylation level of OB-R and POMC promoter detected by methylation specific PCR.Results:Our results revealed that Wendan Decoction(温胆汤)could effectively reduce body weight,the body weight in the drug observation group was significantly lower than that in the model control group,and the difference between the two groups was statistically significant.The level of leptin in peripheral blood and the expression of OB-R in hypothalamus were consistent with the trend of body weight.The heavier the weight,the higher the level of leptin and OB-R.Wendan Decoction(温胆汤)could reduce the level of leptin and OB-R in peripheral blood while reducing body weight.Compared with the model control group,the difference was statistically significant.The heavier the weight,the lower the level of POMC expression.Wendan Decoction(温胆汤)could increase the expression of POMC while reducing body weight,which had statistical significance compared with the model control group.The methylation status of OB-R and POMC promoter in hypothalamus was not significantly associated with obesity induced by high fat diet,and Wendan Decoction(温胆汤)had no effect on methylation status.Conclusions:Wendan Decoction(温胆汤)could inhibit obesity by affecting the expression of leptin receptor and POMC mRNA in hypothalamus to a large degree.There was no obvious change about the methylation status of the gene promoters of leptin receptor and POMC in obese rats fed with high fat diet.And it was not related to the change of methylation status of gene promoter about the mechanism of Wendan Decoction(温胆汤).

Leptin signaling and leptin resistance

Leptin is secreted into the bloodstream by adipocytes and is required for the maintenance of energy homeostasis and body weight.Leptin deficiency or genetic defects in the components of the leptin signaling pathways cause obesity.Leptin controls energy balance and body weight mainly through leptin receptor b(LEPRb)-expressing neurons in the brain,particularly in the hypothalamus.These LEPRb-expressing neurons function as the first-order neurons that project to the second-order neurons located within and outside the hypothalamus,forming a neural network that controls the energy homeostasis and body weight.Multiple factors,including inflammation and endoplasmic reticulum(ER)stress,contribute to leptin resistance.Leptin resistance is the key risk factor for obesity.This review is focused on recent advance about leptin action,leptin signaling,and leptin resistance.

Leptin in normal physiology and leptin resistance

Since the discovery of leptin as an adipokine in 1994, much progress has been made in the research about leptin. Circulating leptin binds to leptin receptor, activates STAT3-dependent and STAT3-independent signaling pathways, and plays an effective role in energy home- ostasis, neuroendocrine function and metabolism mainly through acting on the central nervous system, especially the hypothalamus. Leptin resistance is considered as a key risk factor for obesity. Various mechanisms have been formu- lated in order to explain leptin resistance, including impairment in leptin transport, attenuation in leptin sig- naling, ER stress, inflammation and deficiency in autop- hagy. Here, we review our current knowledge about leptin action, leptin signaling and leptin resistance, hoping to provide new ideas for the battle against obesity.

Leptin in normal physiology and leptin resistance

Since the discovery of leptin as an adipokine in1994, much progress has been made in the research about leptin. Circulating leptin binds to leptin receptor, activates STAT3-dependent and STAT3-independent signaling pathways, and plays an effective role in energy homeostasis, neuroendocrine function and metabolism mainly through acting on the central nervous system, especially the hypothalamus. Leptin resistance is considered as a key risk factor for obesity. Various mechanisms have been formulated in order to explain leptin resistance, including impairment in leptin transport, attenuation in leptin signaling, ER stress, inflammation and deficiency in autophagy. Here, we review our current knowledge about leptin action, leptin signaling and leptin resistance, hoping to provide new ideas for the battle against obesity.

Leptin signaling and leptin resistance

With the prevalence of obesity and associated comorbidities,studies aimed at revealing mechanisms that regulate energy homeostasis have gained increasing interest.In 1994,the cloning of leptin was a milestone in metabolic research.As an adipocytokine,leptin governs food intake and energy homeostasis through leptin receptors(LepR)in the brain.The failure of increased leptin levels to suppress feeding and elevate energy expenditure is referred to as leptin resistance,which encompasses complex pathophysiological processes.Within the brain,LepR-expressing neurons are distributed in hypothalamus and other brain areas,and each population of the LepR-expressing neurons may mediate particular aspects of leptin effects.In LepR-expressing neurons,the binding of leptin to LepR initiates multiple signaling cascades including janus kinase(JAK)–signal transducers and activators of transcription(STAT)phosphatidylinositol 3-kinase(PI3K)-protein kinase B(AKT),extracellular regulated protein kinase(ERK),and AMP-activated protein kinase(AMPK)signaling,etc.,mediating leptin actions.These findings place leptin at the intersection of metabolic and neuroendocrine regulations,and render leptin a key target for treating obesity and associated comorbidities.This review highlights the main discoveries that shaped the field of leptin for better understanding of the mechanism governing metabolic homeostasis,and guides the development of safe and effective interventions to treat obesity and associated diseases.

An Overview of Leptin and the Th1/Th2 Balance

Adipocytes produce hormones and adipokines, among others leptin, with metabolic and inflammatory responses. The role of leptin involves different subsets of the immune system. Low levels of serum leptin concentrations are associated with bacterial infections and produce susceptibility to allergic diseases and type Th2 autoimmune diseases. High levels of leptin are associated with Th1 autoimmune diseases via inflammatory responses. Leptin resistance, that happens in obesity, is characterized by the presence of high levels of serum leptin itself accompanied by a clinical situation of a Th2 response. There is a relationship between obesity, leptin resistance, altered immunity and sex hormones. Serum leptin concentration (Th1 response) in women is higher than in men maybe to achieve the balance Th1/Th2 because of the high level of estrogenic hormones (Th2 response) in women.

Influence of catgut implantation at acupoints on leptin and insulin resistance in simple obesity rats

OBJECTIVES:To investigate the influence of catgut implantation at acupoints on leptin resistance(LR) and insulin resistance(IR) in the simple obesity rat.METHODS:Rats were made obese with high-fat diets,after which surgical catgut was implanted at Zusanli(ST 36) and Neiting(ST 44) acupoints once a week for 4 weeks(implantation group).Rats from the implantation group were compared with normal rats and unoperated obese rats(control group).Gene expression of the leptin receptor(OB-R) was evaluated using RT-PCR and northern blot.Serum and hypothalamus leptin and insulin(INS) levels were determined by radioimmunoassay.Body weight,Lee's index,body fat,serum and hypothalamus leptin and INS levels,and hypothalamic OB-R gene expression were determined before and after treatment.RESULTS:Body weights,Lee's index,body fat,and serum leptin and INS levels were significantly higher in obese than in normal rats.Hypothalamic leptin and INS levels and OB-R gene expression were significantly lower in obese rats.Catgut implantation at acupoint promoted weight loss and decreased serum leptin and INS levels.Hypothalamic leptin and INS levels and OB-R gene expression increased significantly.CONCLUSIONS:Catgut implantation at acupoint adjusts central and peripheral leptin and promotes hypothalamic OB-R gene expression.This may be an important method for regulation of LR,IR and abnormal endocrinology and metabolism.

Metabolic regulation by protein tyrosine phosphatases

Obesity and the metabolic syndrome and their associated morbidities are major public health issues, whose prevalence will continue to increase in the foreseeable future. Aberrant signaling by the receptors for leptin and insulin plays a pivotal role in development of the metabolic syndrome. More complete molecular-level understanding of how both of these key signaling pathways are regulated is essential for full characterization of obesity, the metabolic syndrome, and type lI diabetes, and for developing novel treatments for these diseases. Phosphorylation of proteins on tyrosine residues plays a key role in mediating the effects of leptin and insulin on their target cells. Here, we discuss the molecular methods by which protein tyrosine phosphatases, which are key physiological regulators of protein phosphorylation in vivo, affect signaling by the leptin and insulin receptors in their major target tissues.

Protective Effects of Chaihu Shugan San(柴胡疏肝散) on Nonalcoholic Fatty Liver Disease in Rats with Insulin Resistance

Objective： To investigate the protective effects of Chinese medicine formulation Chaihu Shugan San（柴胡疏肝散, CHSGS） on nonalcoholic fatty liver disease（NAFLD） in rats with insulin resistance（IR） and its molecular mechanisms. Methods： Male Sprague-Dawley rats were randomly divided into six groups： the control group, the model group, Dongbao Gantai group（东宝肝泰, DBGT, 0.09 g methionine/kg）, CHSGS high-dose group（CHSG-H, 12.6 g crude drug/kg）, CHSGS medium-dose group（CHSG-M, 6.3 g crude drug/kg）, and CHSGS low-dose group（CHSG-L, 3.15 g crude drug/kg）. After establishing the NAFLD rat model and treatment for 8 weeks, total cholesterol（TC）, triglyceride（TG）, high-density lipoprotein cholesterol（HDL-C）, free fatty acid（FFA）, fasting blood glucose（FBG）, fasting insulin（FINS） contents in blood serum, and TC, TG contents in the hepatic homogenate were measured by an automatic biochemical analyzer, and a homeostasis model assessment was applied to assess the status of IR, insulin sensitivity index（ISI）, and homeostasis model assessment for insulin secretion（HOMA-IS）. The expression levels of adiponectin and leptin mRNA in liver tissue were analyzed by reverse transcription polymerase chain reaction. Pathological changes of livers were observed by hematoxylineosin staining of paraffin section. Results： Compared with the model group, the serum levels of TC, TG, FFA, FBG, FINS, IRI, ISI, and the liver levels of TC and TG in CHSG-H, CHSG-M, CHSG-L groups showed significant declines（P〈0.01 or P〈0.05）; the serum levels of HDL-C, HOMA-IS were significantly increased（P〈0.01 or P〈0.05）; the expression of leptin mRNA was dramatically decreased and the expression of adiponectin mRNA was increased in the hepatic tissue（P〈0.01 or P〈0.05）. The fatty deposition of liver cells could also be alleviated. Conclusion： CHSGS could up-regulate the expression of adiponectin mRNA and down-regulate the expression of leptin mRNA on the liver, suggesting the CHSGS had positive therapeutic effect on NAFLD in rats with IR.

Obesity:an evolutionary context

People completely lacking body fat(lipodystrophy/lipoatrophy)and those with severe obesity both show profound metabolic and other health issues.Regulating levels of body fat somewhere between these limits would,therefore,appear to be adaptive.Two different models might be contemplated.More traditional is a set point(SP)where the levels are regulated around a fixed level.Alternatively,dual-intervention point(DIP)is a system that tolerates fairly wide variation but is activated when critically high or low levels are breached.The DIP system seems to fit our experience much better than an SP,and models suggest that it is more likely to have evolved.A DIP system may have evolved because of two contrasting selection pressures.At the lower end,we may have been selected to avoid low levels of fat as a buffer against starvation,to avoid disease-induced anorexia,and to support reproduction.At the upper end,we may have been selected to avoid excess storage because of the elevated risks of predation.This upper limit of control seems to have malfunctioned because some of us deposit large fat stores,with important negative health effects.Why has evolution not protected us against this problem?One possibility is that the protective system slowly fell apart due to random mutations after we dramatically reduced the risk of being predated during our evolutionary history.By chance,it fell apart more in some people than others,and these people are now unable to effectively manage their weight in the face of the modern food glut.To understand the evolutionary context of obesity,it is important to separate the adaptive reason for storing some fat(i.e.the lower intervention point),from the nonadaptive reason for storing lots of fat(a broken upper intervention point).The DIP model has several consequences,showing how we understand the obesity problem and what happens when we attempt to treat it.