Introduction
Obesity is recognized by the World Health Organization as the pandemic of the 21st century.1 It is characterized by the excessive accumulation and dysfunction of adipose tissue, as well as systemic-grade inflammation with potentially negative health outcomes. This dysfunction involves imbalanced adipokine secretion and disrupted cellular communication mediated by extracellular vesicles like exosomes.2 Continuous deviation of biological parameters (infra- or supraphysiological responses) due to prolonged stressors leads to wear and tear on the regulatory systems. This burden, or cost of adaptation, reflects the prevalence of the allostatic state, commonly known as allostatic load. If the over-activation of allostatic responses persists over time, it can result in illness and become life-threatening, a condition referred to as allostatic overload.3 This leads to the onset and development of chronic diseases and increases mortality risk. In the context of this article, these disruptions detrimentally affect metabolic equilibrium and are closely tied to metabolic diseases, including chronic inflammation, insulin resistance (IR), lipid overload, organelle stress, and subsequent modulation of gene expression. Consequently, obesity increases susceptibility to cardiovascular diseases, type 2 diabetes mellitus (T2DM), chronic kidney disease, and various cancers.4–7 Adipose tissue releases adipokines such as tumor necrosis factor-alpha, which drives cellular and systemic inflammation, and leptin, which regulates appetite and sympathetic nervous activity. Paradoxically, obese individuals exhibit leptin resistance, possibly due to blood-brain barrier (BBB) saturation, rendering exogenous leptin administration ineffective in treating obesity.8
Preliminary data show that reduced sensitivity to leptin leads to diet-induced obesity.9 Hyperleptinemia serves as a marker of leptin resistance (LR); however, the precise diagnosis or explanation of LR remains elusive.10 Despite this, LR is closely associated with obesity and is directly correlated with the amount of adipose tissue an individual possesses.11 This, in turn, influences the development of metabolic diseases, whose pathophysiology converges in obesity and IR.12,13 Thus, there is a correlation between genetics, dietary habits, and leptin deficiency in metabolic syndrome (MS).14 This review aimed to discuss the intricate molecular relationships between leptin, obesity, and IR.
Procedure of the review
Protocol and registration
This bioinformatics-assisted review was conducted and reported according to previously published procedures.15,16 The study is part of “The Project ATA”, a multicenter study developed by the DBSS Research Division (ClinicalTrials.gov ID NCT05758311) in cooperation with several universities and research centers across America and Europe. Procedures were published and made freely available to avoid unnecessary duplication.17
Search strategy and information sources
The PubMed/MEDLINE, ScienceDirect, and Google Scholar databases were searched using the keywords “leptin”, “insulin resistance”, and “obesity”.
Manual curation and bioinformatics-assisted review
The bioinformatics-assisted review addresses the lack of systematization in reviews that aimed at updating and/or analyzing phenomena at the molecular level. To overcome the subjective and time-consuming manual extraction of information, this approach utilizes experimentally validated, high-level, manually curated, and reproducible data (open-source bioinformatics tools). The prioritization of biological elements was based on pathways and the regulation of leptin metabolism and signaling. In this study, several public databases and repositories were accessed to retrieve manually curated biological information, including UniProtKB (https://www.uniprot.org/ ),18 PDB (https://www.rcsb.org/ ),19 Ensembl (https://www.ensembl.org/index.html ),20 Gene Ontology Resource (http://geneontology.org/ ),21 The Human Protein Atlas (https://www.proteinatlas.org/ ),22 and BioGPS–Gene Portal System (http://biogps.org/ ).23 Readers may refer to our recent article for a selection of widely used online bioinformatics databases and an accompanying website that includes additional information (https://sites.google.com/view/compgenomtools/ ).24
Findings presentation
The functional annotations were discussed within the literature review and expert clinical interpretation as follows: i) Obesity, leptin resistance, and metabolic syndrome; ii) Relevant molecular mechanisms of leptin resistance; iii) Convergence of molecular pathways between leptin and insulin resistance; and iv) Conclusions.
Obesity, leptin resistance, and metabolic syndrome
MS comprises a group of conditions that collectively increase the risk of cardiovascular diseases and T2DM.25 In 2009, the International Diabetes Federation, in partnership with the American Heart Association and the National Heart, Lung, and Blood Institute, established the criteria for diagnosing MS. These criteria include abdominal girth (based on population/regional cut-off points), low levels of high-density lipoprotein cholesterol, elevated fasting glucose, elevated triglycerides, and increased blood pressure. A diagnosis requires the presence of at least three out of these five components, providing a comprehensive framework for identifying individuals at risk.26
Obesity emerges as a significant precursor to MS, resulting from complex interactions among genetic, behavioral, and environmental factors.27 Unhealthy lifestyle habits, such as inadequate sleep, physical inactivity, and excessive food intake, contribute to weight gain,28 leading to increased blood leptin levels. This elevation prompts IR, which aligns with oxidative stress and mitochondrial dysfunction, both central to the pathophysiology of T2DM.29
It is now clear that obesity contributes to MS, further exacerbated by LR.30 Hyperleptinemia, which serves as a biomarker, is associated with MS due to the abundance of leptin receptors (LEPR, also known as OB-R) in pancreatic tissue. Dysfunction of these receptors can lead to diabetes.31 Moreover, hyperleptinemia has been associated with various cardiac diseases, hypertension, and stroke, thereby exacerbating MS.32 Although limited evidence initially reported promising results,33,34 the treatment with exogenous leptin for weight management has proven ineffective.35,36 Currently, the use of glucagon-like peptide-1 receptor agonists, such as Wegovy®, is a clinical option approved by the U.S. Food and Drug Administration, but further research is warranted to confirm its utility and identify side effects.37 Available data suggest that increased visceral fat, rather than obesity per se, induces MS by triggering cytokine release from macrophages, activating inflammatory signaling cascades, and causing the accumulation of reactive oxygen species (ROS) resulting from endoplasmic reticulum (ER) dysfunction.4,38,39 Interestingly, several studies have demonstrated positive results for MS management through physical activity, emphasizing exercise as a form of medicine.40–42
Relevant molecular mechanisms of leptin resistance
Table 1 summarizes the characteristics and expression patterns of all proteins discussed in this bioinformatics-assisted review for functional annotation and cross-referencing.
Table 1Functional annotation of the prioritized proteins
| Protein name | Gene name | Ensembl ID† | Gene location | UniProtKB | Subunit structure | PDB entry or AlphaFold ID | Cellular location | Expression* | ID BioGPS |
|---|
| Leptin | LEP | ENSG00000174697 | Chromosome 7: 128,241,278-128,257,629 forward strand | P41159 | Monomer | 1AX8 | Cytosol, extracellular region, and extracellular space | Adipose tissue | LEP-3952 |
| Tumor necrosis factor (TNF-α) | TNF | ENSG00000232810 | Chromosome 6: 31,575,565-31,578,336 forward strand | P01375 | Dimer of identical or non-identical chains | 1TNF | Cell surface, external side of the plasma membrane, extracellular region, extracellular space, membrane raft, neuronal cell body, phagocytic cup, plasma membrane, recycling, and endosome | Mainly immune system and bone barrow | TNF-7124 |
| Cell division control protein 42 homolog (rho GTPase Cdc42) | CDC42 | ENSG00000070831 | Chromosome 1: 22,052,627-22,101,360 forward strand | P60953 | Dimer of identical or non-identical chains | 1AJE | Centrosome, cytoplasm, cytoplasmic ribonucleoprotein granule, cytosol, dendritic spine, endoplasmic reticulum membrane, filopodium, focal adhesion, Golgi membrane, Golgi transport complex, mitotic spindle neuron projection, neuronal cell body, phagocytic vesicle, plasma membrane, and spindle midzone | Bone marrow, brain, and adipose tissue, among several others | CDC42-998 |
| Tyrosine-protein kinase JAK2 (janus kinase 2) | JAK2 | ENSG00000096968 | Chromosome 9: 4,984,390-5,129,948 forward strand | O60674 | Dimer of identical or non-identical chains | 4BBE | Caveola, cytoplasm, cytoskeleton, cytosol, endosome lumen, extrinsic component of the plasma membrane, focal adhesion, nucleus, nucleoplasm, and plasma membrane | Mainly adipose tissue, but also in, bone marrow, immune system, brain, and hematopoietic tissues, among several others | JAK2-3717 |
| Signal transducer and activator of transcription 3 (STAT3) | STAT3 | ENSG00000168610 | Chromosome 17: 42,313,324-42,388,568 reverse strand | P40763 | Dimer of identical or non-identical chains | 6TLC | Cytoplasm, cytosol, nucleus, nucleoplasm, and plasma membrane | Mainly liver and lung, but also in the immune system, and hematopoietic tissues, among several others | STAT3-6774 |
| Forkhead box protein O1 (FoxO1) | FOXO1 | ENSG00000150907 | Chromosome 13: 40,555,667-40,666,641 reverse strand | Q12778 | Monomer | 6QVW | Cytoplasm, cytosol, mitochondrion, nucleoplasm, and nucleus | Mainly immune system and adipose tissue, but also in the ovary, thyroid, adipocyte, among several others | FoxO1-2308 |
| Leptin receptor (LEPR) | LEPR | ENSG00000116678 | Chromosome 1: 65,420,652-65,641,559 forward strand | P48357 | 3V6O | Dimer of identical or non-identical chains | The basolateral plasma membrane, external side of the plasma membrane, and extracellular region | Mainly liver and brain, but also in adipose tissue, among several others | LEPR-3953 |
| Suppressor of cytokine signaling (SOCS3) | SOCS3 | ENSG00000184557 | Chromosome 17: 78,356,778-78,360,077 reverse strand | O14543 | AF-O14543-F1 | Monomer | Cytoplasmic side of plasma membrane and cytosol | Mainly adipose tissue and skeletal muscle but also in the liver, among several others. | SOCS3-9021 |
LR is depicted as a self-perpetuating cycle in which elevated leptin levels in the bloodstream contribute to heightened resistance, while an adequate amount of leptin is essential for its role as an anorexigenic hormone. In certain instances, diet-induced obesity is initiated by preexisting reductions in leptin sensitivity.9 The secretion of adipokines, such as leptin, induces hyperleptinemia in the central nervous system, leading to insensitivity to LEPR, which favors LR and consequently promotes further obesity.9,43
Various underlying mechanisms of central and peripheral LR are known. Dysregulation of signaling in the hypothalamus can result from different causes, such as genetic mutations affecting not only the receptors but also protein complexes and secondary messengers, disrupting proper leptin signaling. For example, the SEL1L-HRD1 complex specific to proopiomelanocortin (POMC) neurons can trap LEPR in the ER by affecting its folding and maturation.44 Additionally, increases in blood leptin concentration may trigger downregulation and desensitization through the degradation of LEPR.8 Indeed, the primary determinant of LR is impaired responsiveness or reduced sensitivity of the brain to leptin, which is attributed to altered function in BBB transport mediated by short isoforms of LEPR (i.e., LEPR-a, LEPR-b, LEPR-c). These isoforms are involved in the cerebral uptake of leptin, mainly in brain areas related to energy and adipose tissue metabolism, thereby contributing to the development of obesity.45 Elevated blood triglyceride levels from the diet may also influence the low permeability of the BBB, hindering proper leptin transport to the brain and thus impeding its anorexigenic effect.46,47 However, leptin transport to the brain can be modulated. Evidence shows that high dietary salt intake and fasting can induce obesity by promoting hyperphagia and endogenous fructose production, which in turn trigger LR.48 Although LR via the BBB is widely acknowledged, Harrison et al.49 demonstrated through fluorescence labeling of leptin in obese rats that leptin passage through the BBB remains intact.
Upon binding to LEPR, leptin initiates a signaling cascade by activating the Janus kinase (JAK) 2 tyrosine kinase family, leading to physical interaction with Rho-kinase 1. This interaction promotes the phosphorylation of tyrosine residues Y985, Y1077, and Y1138.50 Subsequently, tyrosine Y1138 binds to the Src homology 2 domain of signal transducer and activator of transcription (STAT) 3, initiating STAT3 phosphorylation. This phosphorylation induces the dimerization of STAT3 and STAT5, facilitating their translocation to the nucleus, where they act as gene regulators for neuropeptides such as POMC, agouti-related protein, and neuropeptide Y. Furthermore, to regulate LEPR signaling, STAT3 phosphorylation establishes a negative feedback mechanism by promoting the transcription of suppressor of cytokine signaling 3.50
The presence of LR in obese individuals can also be attributed to other factors, including the direct inhibition of leptin binding to its receptors by C-reactive protein. This phenomenon is directly correlated with increased leptin bioavailability in obese individuals, leading to heightened C-reactive protein binding to LEPR at multiple sites, thereby modifying the pleiotropic effects of leptin.6–16,18–29 Additionally, overexpression of cytosolic suppressor of cytokine signaling 3 may block the downstream signaling pathway of LEPR, potentially induced by obesity-induced hyperleptinemia.51,52 In LR, ER stress also affects leptin signaling, blocking the JAK2/STAT3 pathway triggered by leptin present in POMC neurons, potentially contributing to metabolic disturbances in the peripheral tissues of obese phenotypes (Fig. 1a).53,54
Convergence of molecular pathways between leptin and insulin resistance
LEPR proteins are localized in pancreatic β cells, where leptin directly influences insulin gene expression, leading to a reduction in insulin secretion. This inhibitory effect operates through various pathways, including the blockade of glucose transport, reorganization of the actin cytoskeleton, and activation of phosphodiesterase 3B via the phosphatidylinositol 3-kinase pathway. The activation of phosphodiesterase 3B results in decreased cAMP levels, which affects the protein kinase A pathway, a crucial regulator of calcium channels and exocytosis. These intricate mechanisms collectively limit insulin release.31 However, recent studies by Zhang et al.55 have revealed a novel aspect of this regulatory network. In vitro experiments conducted on pancreatic β-cells (INS-1E) demonstrated that tumor necrosis factor-alpha, in conjunction with leptin, reduces the phosphorylation level of forkhead box protein O1 (FoxO1). FoxO1 is a pivotal player in insulin regulation and may function as a transcriptional regulator of LEPR expression. This regulatory action affects insulin stimulation by glucose and promotes hyperinsulinemia (Fig. 2).56
In the brain, elevated levels of circulating leptin can lead to dysregulation by overactivation of the LEPR receptor, inducing ER stress and generating high levels of ROS.53 This surge in ROS may, in turn, trigger overactivation of the GTPase cell division control protein 42 homolog (Cdc42). Recent studies have linked dysregulation of the Rho GTPase Cdc42 to various diseases, including cancer and IR. Under physiological conditions, Cdc42 toggles between its inactive GDP-bound form and its active GTP-bound form, regulating signaling pathways involved in cellular functions such as cell morphology, cell cycle control, actin cytoskeleton dynamics, vesicle trafficking, and cell polarity.57 Additionally, Cdc42 participates in the second phase of glucose-stimulated insulin secretion in the pancreas via the serine/threonine-protein kinase PAK 1/RAF proto-oncogene serine/threonine-protein kinase, Mitogen-activated protein kinase kinase 1/extracellular signal-regulated kinase (MEK/ERK) axis,58,59 potentially enhancing insulin expression through ERK1/2-mediated promotion of NeuroD1 nuclear translocation.60 In this bioinformatics-assisted review, a plausible molecular pathway is suggested whereby ROS-induced activation of Cdc42, in conjunction with Rac1 protein, initiates inflammatory signaling pathways, particularly RAC-alpha serine/threonine-protein kinase (Akt), and serine/threonine-protein kinase PAK 1/mitogen-activated protein kinases (MAPKs) such as ERK1/2, MAPK 8 (JNK), and p38 MAPK. Upon activation, LEP/LEPR signaling upregulates 72 kDa type IV collagenase (MMP2) expression through the activation of JNK within the MAPK pathway mediated by Cdc42. This dysregulation inhibits LEPR’s signal transduction ability by cleaving its extracellular domain via MMP2.61 This potential signaling pathway provides insights into the impact of adipokines originating from adipose tissue on pancreatic β cells, offering a possible explanation for the development of T2DM. However, experimental validation of this proposed pathway is required. Xie et al.62 observed activation of pMEK, pERK, pSTAT3, Rac1/Cdc42, pFAK, and vinculin in Sca-1+ progenitor cells. Additional research in neuronal cells is warranted to elucidate other molecular pathways (Fig. 1b).
Conclusions
Obesity, a widespread condition affecting individuals across diverse demographics, primarily results from an imbalance between calorie intake and energy expenditure. However, its development is multifactorial, involving complex interactions between genetic predisposition, environmental factors, and lifestyle habits. In this bioinformatics-assisted review, we summarize the close relationship between obesity and metabolic disorders, with IR serving as an early indicator of potential MS. This link operates through complex molecular pathways, prominently featuring FoxO1/LEPR and JAK/STAT3 signaling, and possibly involving Cdc42. Nevertheless, a comprehensive understanding of LR requires further experimental validation to uncover the intricately associated molecular pathways. Such insights are crucial for devising novel therapeutic strategies for obesity management, such as glucagon-like peptide-1 receptor agonists, beyond conventional approaches like leptin administration, which has proven challenging and ineffective. This review provides researchers and practitioners with an up-to-date perspective and outlines future directions that will aid in refining our understanding and exploring innovative interventions for combating obesity.
Declarations
Acknowledgement
This bioinformatics-assisted review is part of “Project ATA” (NCT05758311), developed and supported by the Research Division of the Dynamical Business & Science Society—DBSS International SAS.
Data sharing statement
The curated data supporting the statements discussed in this article are available in the bioinformatics repositories. All identifier numbers are listed in Table 1.
Funding
This research received no external funding.
Conflict of interest
The authors have no conflict of interest related to this publication.
Authors’ contributions
Conceptualization (WAG, GH, DAB), original draft preparation (WAG, DAB), visualization (WAG), critical review and editing (CAO-C, RC, AMM-C, GH, LMG-M, JLP, DAB), and supervision (DAB). All authors have read and agreed to the published version of the manuscript.