Leptin Resistance and Endoplasmic Reticulum Stress: ER Proteostasis as a Proposed Convergence Node—A Mechanistic Synthesis of Preclinical Evidence

1. Introduction

The signal linking adipose mass to feeding behaviour was predicted by Coleman’s parabiosis experiments [1] and identified molecularly with the cloning of the obese (ob) gene [2]. That recombinant leptin reduces food intake [3,4], that the receptor was subsequently cloned [5], and that loss-of-function mutations in the leptin gene cause early-onset obesity reversible by replacement therapy [6,7] together established the hormone’s role in energy balance. Leptin acts by binding LepRb on POMC and AgRP neurons of the arcuate nucleus, thereby suppressing food intake [8,9].

What is seen in common obesity is hyperleptinaemia rather than leptin deficiency: affected individuals fail to mount the expected anorexigenic response to leptin. This state, termed leptin resistance, distinguishes rare type 1 (leptin-deficient) obesity from the far more common type 2 (leptin-resistant) form [8]. Roughly one in eight people worldwide now lives with obesity, and the prevalence is projected to keep rising through 2050 [10,11]. Leptin resistance is both the principal barrier to the therapeutic use of leptin [12,13,14] and a contributor to cardiometabolic complications [9].

This review conceptualises ER proteostasis as a candidate regulatory hub at which metabolic, inflammatory, developmental and nutritional inputs intersect and are routed towards a leptin-resistant phenotype. The framework rests on three testable hypotheses — input independence, node primacy, and the proteostasis–stress distinction — and further argues that leptin resistance may comprise several phenotypes differing in their dominant mechanism. The mechanisms are presented not as an established model but as an explicitly falsifiable framework.

2. Materials and Methods

Literature Search Strategy

This is a narrative review. The reporting principles of the PRISMA 2020 statement [15] were applied not to conduct a systematic review but to document study selection transparently. Searches of PubMed/MEDLINE and Scopus were completed on 19 July 2026.

The following search string was applied identically to both databases:

( "leptin resistance" OR "leptin signaling" ) AND ( JAK2 OR STAT3 OR PI3K OR Akt OR MAPK OR ERK OR AMPK OR mTOR OR FoxO1 OR SOCS3 OR PTP1B OR "ER stress" OR "endoplasmic reticulum stress" OR "unfolded protein response" OR UPR OR PERK OR IRE1 OR ATF6 OR CHOP OR XBP1 )

Only studies published on or after 1 January 2015 were eligible. Landmark pre-2015 work is cited for historical context in the Introduction and Section 3 but was not entered into the evidence table. Studies were included if they reported original research, addressed leptin resistance or leptin signal transduction as a primary topic, linked at least one ER stress or UPR component (PERK, IRE1α, ATF6, CHOP, XBP1, GRP78/BiP, eIF2α, ERAD) mechanistically to leptin signalling, and were available in English as full text. Studies treating leptin only superficially were excluded, as were records without abstracts, conference proceedings, case reports and retracted publications.

Of 421 records retrieved (PubMed/MEDLINE, n = 244; Scopus, n = 177), 83 duplicates were removed. Of the 338 unique records, 58 were excluded at title and abstract screening and a further 258 at full-text assessment (target mechanism not the primary focus, n = 189; signalling studies with no ER stress component, n = 54; reviews, n = 15). Twenty-two studies entered the synthesis and the evidence table, and the animal studies were classified on the basis of the SYRCLE tool [16] (Figure 1; Table 1).

During the preparation of this manuscript, a large language model (Claude, Anthropic; model claude-opus-5) was used solely for language editing and academic style refinement of the English text. No scientific content, data, interpretations, or conclusions were generated by the tool. The corresponding author reviewed and approved all language-related edits and takes full responsibility for the content of the manuscript.

3. Leptin Physiology

Leptin is a 167-amino-acid polypeptide encoded by the ob gene and produced mainly by mature adipocytes of white adipose tissue; circulating concentrations correlate strongly with total fat mass [2,8]. Of the receptor isoforms, only LepRb carries a full-length intracellular domain and can therefore transmit JAK2/STAT3 signalling; it is densely expressed in POMC and AgRP neurons. The soluble isoform sOb-R determines the size of the free leptin pool [5,17,18]. Because canonical leptin physiology has been reviewed extensively elsewhere [8,9], only three features that intersect with the ER are emphasised here.

First, the functional competence of the receptor depends not only on its expression level but on correct folding within the ER. Newly synthesised LepRb is prone to misfolding, is a substrate of the SEL1L–HRD1 ERAD complex, and loss of SEL1L in POMC neurons attenuates leptin signalling; notably, retention of the receptor within the ER occurs independently of ER stress [19]. Second, receptor expression can fall under pathological conditions: in a model of congenital hypothyroidism, hypothalamic OB-R expression decreased by 45–63%, pSTAT3 by 51–56% and pERK by 48–50%, while SOCS3 rose [20]. Third, leptin resistance is not exclusively a central phenomenon: mice fed a high-fat diet develop local ER stress and leptin resistance within adipose tissue, and α-melanocyte-stimulating hormone (αMSH) reverses this picture by lowering GRP78 and CHOP [21].

4. Canonical Leptin Signalling

Binding of leptin to LepRb activates JAK2, which phosphorylates tyrosine residues on the cytoplasmic tail of the receptor and thereby creates docking sites for partially dissociable parallel arms (Figure 2).

The JAK2/STAT3 pathway, one of the principal mediators of leptin’s anorexigenic effects, is frequently used as a readout of leptin resistance. Yet stimuli as different as selenoprotein M deficiency, homocysteine, 4-HNE and glucose deprivation all lower pSTAT3, which shows that the marker is sensitive but not mechanism-specific. In diet-induced obese mice, moreover, STAT3 DNA-binding activity increases, and pharmacological reduction of pSTAT3 facilitates leptin-dependent POMC expression [22] — evidence that reduced pSTAT3 does not correspond to leptin resistance under all conditions.

PI3K/Akt is among the principal pathways at which leptin and insulin signalling intersect within the hypothalamus: mice carrying a kinase-domain mutation in PI3K-C2α develop leptin resistance, age-dependent insulin resistance and obesity together [23]. Perinatal nutritional interventions have also been reported to alter hypothalamic IRS1 and AMPK phosphorylation and thereby to affect leptin signalling permanently [24].

Evidence for a role of the MAPK/ERK arm in leptin resistance is limited and largely observational [20]. AMPK is suppressed by leptin; in a glucose-deprivation model, flurbiprofen reversed the suppression of pSTAT3 without altering UPR activation, which suggests that UPR activation alone may not fully account for leptin resistance [25].

mTOR is linked to ER proteostasis through its role as a negative regulator of autophagy, but the supporting studies are confined to cardiac and hepatic tissue [26,27,28]. FoxO1, for its part, competes with STAT3 at the POMC promoter and so shapes the transcriptional balance [22,29], raising the possibility that leptin resistance also reflects a shift in the balance between opposing transcriptional regulators. No primary study has examined the relationship between PI3K/Akt, mTOR or FoxO1 and ER stress directly in the context of leptin resistance; these links remain inferential (Table 2).

5. Molecular Basis of Leptin Resistance

For leptin to exert its central effects it must cross from the circulation into the brain, and the saturable nature of this transport may create an early bottleneck under hyperleptinaemic conditions [12]. Tanycytes surrounding the mediobasal hypothalamus have been shown to form an ERK-dependent gate for leptin, and this mechanism is disrupted in diet-induced obesity [30].

Upstream of this step lies the soluble receptor (sOb-R), which regulates the free leptin pool. Pharmacological and genetic modulation of hepatic CB1R alters sOb-R levels and hepatic leptin resistance; peripheral CB1R blockade is ineffective in CHOP-deficient mice; and CHOP regulates the sOb-R promoter directly [18]. ER stress can therefore interfere with leptin signalling through the circulating leptin pool, before the target cell is reached at all.

Impairment at the level of the receptor can arise in two ways. Direct evidence for reduced receptor expression comes from the congenital hypothyroidism model [20]. The more fundamental mechanism concerns receptor folding and maturation: LepRb has been identified as a substrate of the SEL1L–HRD1 ERAD complex, and loss of SEL1L in POMC neurons leads to accumulation of the receptor within the ER and to leptin resistance [19]. That this effect arises independently of classical ER stress suggests that the ER acts as a basal regulator of receptor biogenesis and quality control, and it constitutes the strongest support for the framework proposed here.

SOCS3 is among the principal negative-feedback regulators of leptin signalling: induced by STAT3, it binds Tyr985 of LepRb and suppresses JAK2 activity [31]. Sustained activation of this loop may contribute to leptin resistance, and elevated SOCS3 has been demonstrated in models of congenital hypothyroidism, high-fat feeding and cafeteria diet [20,21,32]. In the only study to address the relationship with ER stress directly, elevated CHOP and spliced XBP1 in pancreatic islets of ob/ob mice approached control levels after phenylbutyric acid (PBA), with a parallel change in SOCS3 expression [33]; the evidence is nonetheless confined to a single tissue and model.

PTP1B is a tyrosine phosphatase that terminates leptin signalling by dephosphorylating JAK2 and is localised to the ER membrane — a position that makes it one of the most direct links between ER proteostasis and leptin signalling. In a model of chronic sleep fragmentation, increased food intake was accompanied by hypothalamic UPR activation, unchanged ObR expression and reduced STAT3 phosphorylation; the picture was attributed to PTP1B, and both TUDCA and CHOP haploinsufficiency were protective [34]. PTP1B has also been shown to be modifiable by diet [32,35].

These four mechanisms do not rest on evidence of equal strength. The strongest causal findings come from receptor quality control and from mechanisms upstream of the UPR, where conditional genetic models link molecular changes to physiological endpoints [19,34,36]. Much of the evidence for SOCS3 and PTP1B, by contrast, is correlative: hypothalamic SOCS3 knockdown experiments are lacking [20], the ER stress–PTP1B relationship has not been tested with neuron-specific deletion, and the effect of PBA on SOCS3 suggests that the relationship may vary by tissue [33]. No mechanism, including tanycyte-mediated transport, is supported by human data [30].

6. Endoplasmic Reticulum Stress

6.1. UPR Activation

This review distinguishes ER proteostasis, ER stress and the UPR. ER proteostasis denotes the basal capacity of the ER to sustain protein folding, quality control and degradation; ER stress denotes the point at which the unfolded protein load exceeds that capacity; and the UPR denotes the cellular programme that responds through the PERK–eIF2α–ATF4, IRE1α–XBP1 and ATF6 arms — initially adaptive, but capable of impairing leptin signalling once it becomes chronic. The distinction matters, because retention of LepRb following loss of ERAD can produce leptin resistance in the absence of ER stress, showing that disrupted proteostasis can affect signalling independently of the UPR [19].

Findings that all three UPR arms are activated together in the context of leptin resistance are consistent across models: sleep fragmentation [34], diet-induced obese rats [35], activation reversible by phenethyl isothiocyanate [37], and increased hypothalamic ER stress gene expression in neonates in a maternal obesity model [38]. The foundation of this framework is the study establishing the role of the IRE1α–XBP1 pathway in POMC neurons [39].

Regulators upstream of the UPR indicate that the mechanism can be situated within a broader proteostasis network: deletion of TAK1 (MAP3K7) increases SREBP-mediated lipogenesis and ER capacity and thereby prevents ER-stress-driven leptin resistance and hyperphagic obesity under a high-fat diet [36].

6.1.1. Lipid Overload

Western-type diets rich in saturated fat are thought to trigger the UPR by altering ER membrane composition and protein-folding capacity. In apolipoprotein E-deficient mice, a Western diet raised markers of ER stress, whereas diallyl disulfide lowered these markers together with hypercholesterolaemia [40]; indole-3-carbinol has been reported to reduce plasma leptin and thereby to attenuate ER-stress-associated hyperleptinaemia [41].

These findings appear at first sight to conflict with the TAK1 study: in ApoE−/− models, reducing lipid load was associated with improvement in ER stress markers, whereas in TAK1 deletion the increase in lipogenesis was interpreted as an expansion of ER capacity [36]. The discrepancy suggests that what matters is less the absolute lipid load than the balance between that load and the processing capacity of the ER — an interpretation that has not been tested directly.

6.1.2. Oxidative ER Stress

Because the ER lumen is an oxidising environment, shifts in redox balance can affect protein-folding capacity. The lipid peroxidation product 4-HNE has been reported to reduce leptin-stimulated STAT3 phosphorylation in a dose-dependent manner through the eIF2α–CHOP pathway; global tyrosine phosphorylation was unaffected, suggesting relative selectivity for leptin signalling [42]. Dehydroascorbic acid similarly raised GRP78, CHOP and spliced XBP1 while lowering pSTAT3 [43], and redox disturbance was shown to affect leptin signalling in selenoprotein M deficiency as well [44].

6.1.3. Nutrient-Derived ER Stress

Nutrient excess is not the only relevant input; nutrient deprivation and specific metabolites also affect ER stress and leptin signalling. Glucose deprivation activates the UPR by increasing GRP78 and CHOP expression while reducing pSTAT3 and pSTAT5 [25], and elevated homocysteine selectively suppresses STAT3 activity [45]. From a developmental perspective, protein restriction combined with a high-fat diet has been associated with hypothalamic leptin resistance, a response attenuated by taurine supplementation [46].

Findings that the several UPR arms are activated together across independent models are consistent, and CNS-specific Tak1 deletion offers causal support [36]; the load-versus-capacity interpretation, by contrast, is an inference drawn from the available data.

6.2. Molecular Convergence of Distinct Stressors on Leptin Resistance Effectors

One of the patterns that stands out in this review is that stressors as varied as sleep fragmentation, homocysteine, 4-HNE, glucose deprivation, Western-type diet and maternal obesity all appear to act through a limited set of shared effectors — SOCS3, PTP1B and CHOP. Of these, CHOP is the strongest candidate integrating mechanism: CHOP haploinsufficiency protects against hypothalamic leptin resistance [34], CHOP participates directly in regulation of the sOb-R promoter [18], and the effects of 4-HNE are mediated through the eIF2α–CHOP axis [42].

The localisation of PTP1B to the ER membrane provides a structural basis for considering SOCS3, PTP1B and ER-stress-dependent effectors within a common proteostatic context. That α-MSH lowers GRP78 and CHOP while suppressing SOCS3 and raising pSTAT3 further suggests that these mechanisms interact within a connected regulatory network rather than operating as independent processes [21]. The therapeutic hypothesis that follows is that modulating the mechanisms at which stressors converge might achieve broader efficacy than targeting each stressor separately; this approach has not yet been tested directly.

This shared pattern suggests that particular regulatory mechanisms may be common to distinct pathophysiological inputs. For CHOP, genetic findings support a causal role; for SOCS3 and PTP1B the evidence is weaker. Because the stressors have not been assessed within the same model under comparable conditions, it remains difficult to determine whether the observed convergence is a general biological property or an apparent pattern arising from similarities between models (Table 3). Molecular convergence is therefore a hypothesis requiring direct comparative testing.

6.3. A Possible ER Stress-Mediated Link Between Leptin and Insulin Resistance

The metabolic consequences of ER stress may not be confined to leptin signalling; similar mechanisms may also contribute to insulin resistance, placing the two disorders on a shared cellular footing. That loss of SEL1L in POMC neurons is associated with hepatic steatosis, glucose intolerance and insulin resistance as well as leptin resistance [19], and that TUDCA improved findings related to both forms of resistance in a maternal obesity model [38], lend support to this possibility. A plausible molecular basis is that PTP1B dephosphorylates both JAK2 and the insulin receptor, and that PI3K/Akt is a signalling component shared by the two hormones. No direct human evidence shows that correcting ER proteostasis improves both forms of resistance together. Table 4 compares the mechanisms discussed in Section 5 and Section 6 by the UPR arm affected, the point at which each acts, and the strength of the evidence supporting it.

6.4. The Causality Problem: Separating Direct Effects from Weight Loss

Most of the intervention studies summarised here share a common interpretive difficulty: nearly all report reductions in body weight and/or adiposity alongside the intervention (Table 5). Because weight loss by itself reduces hypothalamic ER stress and leptin resistance, it cannot be determined whether the observed improvement reflects a direct effect of the intervention or the accompanying weight loss.

The only study whose design overcomes this difficulty is the Roux-en-Y gastric bypass (RYGB) study of Chen et al. [47], which used a weight-matched caloric restriction control and showed that improvements in hypothalamic ER stress and leptin sensitivity were specific to RYGB. It is thus the sole direct basis for the claim that reducing ER stress can restore leptin sensitivity independently of weight loss, and it awaits independent replication; wider adoption of weight-matched controls in this field is needed.

A second difficulty concerns the target specificity of chemical chaperones: beyond reducing ER stress, PBA and TUDCA act on numerous targets, including histone deacetylase inhibition and bile acid signalling. Findings obtained with these agents therefore provide supportive but limited evidence, whereas findings based on CHOP genetics constitute a firmer basis.

Third, findings obtained solely in vitro warrant additional caution: four of these studies used SH-SY5Y neuroblastoma cells transfected to overexpress Ob-Rb, a tumour line that does not carry hypothalamic neuronal identity, and both receptor density and stressor concentrations lie outside the physiological range. Such studies are valuable for generating hypotheses but are not sufficient for inference about leptin resistance in vivo.

7. Intercellular Interactions

Leptin resistance is less a cell-autonomous event within an isolated neuron than the product of continuous interaction among hypothalamic neurons, microglia, astrocytes and peripheral tissues. The low-grade inflammatory response that overnutrition induces in the hypothalamus is accompanied by ER stress induction and defective autophagy, and the process is mediated through interaction with non-neuronal cells [35,47]. A comparable relationship has been described at the level of sympathetic output: silencing Sirt1 or FoxO1 in the arcuate nucleus reduces leptin-mediated inflammation and sympathetic nerve activity [29].

The strongest evidence that this interaction can be reversed comes from bariatric surgery: RYGB improved hypothalamic gliosis, inflammatory signalling and ER stress in diet-induced obese rats, the improvement was associated with increased central leptin responsiveness, and it was not observed in the weight-matched control [47] (Section 6.4).

Autophagy represents a further node in this network: a berberine-containing herbal preparation attenuated hypothalamic ER stress and leptin resistance through modulation of autophagy, with the findings confirmed in both microglial and neuronal cell models [35]. In the periphery, αMSH attenuates ER-stress-induced leptin resistance in adipose tissue, whereas ER stress induced by tunicamycin or thapsigargin reduces leptin sensitivity [21] — pointing to a link between central melanocortin signalling and peripheral adipose expansion that operates through ER stress.

8. Developmental Programming and Epigenetic Regulation

Evidence that leptin resistance can be permanently programmed by early-life conditions continues to accumulate.

8.1. Developmental Programming

Maternal obesity has been reported to disrupt the development of melanocortin circuits in offspring, producing neonatal hyperleptinaemia and leptin resistance, to upregulate ER-stress-related genes in the hypothalamus, and to be corrected — in both its metabolic and its neurodevelopmental components — by neonatal TUDCA [38]. That an intervention delivered within a critical developmental window can alter an otherwise persistent phenotype points to a causal role for early-life ER stress.

Programming has been shown to begin at the level of the placenta: maternal obesity activates placental insulin/IGF-I/mTOR and leptin pathways [49], and placental dysfunction is associated with increased SOCS3 and PTPN2 expression [50]. The same axis operates postnatally, where perinatal diet composition and gestational thyroid hormone insufficiency produce persistent and partly sex-specific hypothalamic leptin signalling defects in adulthood [20,24,46].

The most direct evidence that this programming is reversible comes from silencing maternal SOCS3: a Socs3-shRNA lentivirus administered in early gestation to obese dams attenuated postnatal obesity in the offspring [48]. This suggests that increased hypothalamic SOCS3 is not merely a marker of early-life programming but also a mediator of it.

8.2. Epigenetic and Post-Translational Mediators

The first layer of epigenetic regulation comprises histone modification and the acetylation balance. Hypothalamic leptin action has been shown to be mediated by histone deacetylase 5 (HDAC5), which shapes the transcriptional output of leptin signalling at the chromatin level [51]. In parallel, hypothalamic expression of the nuclear receptor Nur77 falls in obese mice, leptin fails to reduce body weight in Nur77 knockout animals, and Nur77 recruits p300 and dissociates HDAC1, thereby increasing STAT3 acetylation and transcriptional activity [52].

The second layer has been defined at the level of DNA methylation and non-coding RNAs, and it includes human data: adiposity-associated DNA methylation signatures in adolescents have been related to leptin concentrations and perinatal factors [53], and circulating microRNAs in children with obesity have been associated with leptin resistance [54]. Experimentally, a maternal high-fat diet impairs leptin signalling in the neonatal hypothalamus and upregulates the type-1 cannabinoid receptor through sex-specific epigenetic changes [55].

The two layers do not rest on evidence of equal strength. Findings on developmental programming carry relatively strong causal support because they are reversible by intervention within a critical window [38,48], whereas the DNA methylation and microRNA findings are cross-sectional and correlational. More importantly, no direct mechanistic link between ER stress and epigenetic marks has been demonstrated; the fact that these two literatures remain unintegrated is the most conspicuous gap in the framework proposed here.

9. A Unified Mechanistic Model and the Proposed Conceptual Framework

9.1. Integrating the Findings into a Single Mechanistic Model

The findings summarised in this review can be read not as a series of independent observations but as a single mechanistic route (Figure 3). In the proposed model, stressors that appear unrelated — lipid overload, oxidative products, nutrient-derived stressors, maternal obesity and sleep fragmentation — converge on hypothalamic ER stress; ER stress activates all three arms of the UPR; and that activation converges on a narrow set of effectors (CHOP, SOCS3, PTP1B). These effectors can be argued to suppress LepRb–JAK2–STAT3 signalling and so to produce leptin resistance, which in turn drives hyperphagia and increased adiposity, feeds back onto ER stress, and closes a self-reinforcing loop. No single study has demonstrated the route in its entirety.

A second arm runs parallel to this route: LepRb quality control mediated by the SEL1L–HRD1 ERAD system and occurring independently of ER stress [19]. On this view, the ER can shape leptin signalling through both its stress response and its basal proteostatic function.

9.2. ER Proteostasis as a Convergence Node

On this basis, the following testable framework is proposed:

Endoplasmic reticulum proteostasis may function as a central convergence node that translates metabolic, inflammatory, developmental and nutrient-derived signals into leptin resistance.

What distinguishes the framework is that it positions the ER not as one step in a chain leading to leptin resistance but as a regulatory hub converting different classes of input into a common output. Three hypotheses follow, each corresponding to a directly testable experimental design.

The first is that the effector signature is independent of the input: whatever the stressor, a similar effector pattern encompassing CHOP, SOCS3 and PTP1B would be expected. The available data appear consistent with this expectation, but no study has tested different stressors within the same model under comparable conditions.

The second is that intervening at the shared effector node will prove more effective than targeting upstream stressors individually. Work on chemical chaperones and CHOP genetics offers partial support, but no study has compared node-level intervention with upstream intervention within a single model.

The third is that disrupted ER proteostasis can produce leptin resistance even when the ER stress response is not activated. The SEL1L–HRD1 ERAD finding of Mao et al. [19] is the only direct evidence supporting this hypothesis and represents the most original aspect of the framework; were it confirmed, it would follow that studies measuring only UPR markers are missing part of the leptin resistance phenotype.

The framework requires the ER stress and ER proteostasis literatures to be brought together, and it carries an explicit falsification criterion: a demonstration that different stressors produce markedly different effector signatures would invalidate it.

Table 3 lists these three hypotheses, together with two further hypotheses concerning the load-to-capacity ratio and translatability to humans, in terms of the expected outcome, the proposed experimental design, the falsifying finding and the current state of testing.

10. Clinical Implications

Detail about molecular mechanism acquires clinical meaning only to the extent that it can be translated into patient care. Figure 4 sets out the therapeutic approaches considered in this section by level of intervention, together with the nature of the evidence available for each.

10.1. Clinical Outcomes of Leptin Replacement Therapy

The therapeutic potential of leptin has been tested in a single clinical trial of the hormone itself, with largely negative results: in a randomised, double-blind, placebo-controlled dose-escalation study, 73 obese and 54 lean participants received daily subcutaneous recombinant leptin, and placebo-adjusted weight loss in the obese group remained around 3% even at the highest doses [58].

Within the proposed framework this is an expected result: if leptin resistance is generated simultaneously at the receptor, post-receptor and bioavailability levels, raising circulating ligand will correct none of these steps. In common obesity the aim is therefore not to administer leptin but to repair the response to it. States of genuine deficiency, such as congenital leptin deficiency and lipodystrophy, fall outside this assessment.

10.2. Human Data on Chemical Chaperones

Chemical chaperones have been tested in humans, but leptin endpoints were not assessed. Kars et al. [56] gave TUDCA (1,750 mg/day) or placebo to 20 participants with obesity for four weeks and reported an approximately 30% increase in hepatic and muscle insulin sensitivity in the TUDCA arm. Xiao et al. [57] showed that two weeks of oral sodium phenylbutyrate (7.5 g/day) partly attenuated lipid infusion-induced insulin resistance.

These two trials point in two directions at once. Interventions targeting ER stress are feasible in humans, are tolerated, and can produce measurable metabolic benefit, and the interest is extending to other metabolic diseases [59,60]. Against this, both trials measured only insulin endpoints, the samples were small (n = 20 and n = 8) and the exposures brief; the assumption that chemical chaperones correct leptin resistance in humans therefore remains untested and represents the most concrete gap in the field.

10.3. PTP1B as a Therapeutic Target

PTP1B is the member of the proposed effector node that appears most amenable to drug development [61]. The aminosterol trodusquemine was reported to suppress appetite and produce fat-specific weight loss in diet-induced obese mice [62] and was well tolerated in phase 1 trials, yet the development programme for the obesity indication was not pursued.

The reason is not inefficacy alone but the structural nature of the target: the catalytic site of PTP1B is highly polar and charged, molecules that bind it remain too polar to cross the cell membrane, and structural similarity to other members of the phosphatase family creates a selectivity problem. Current work has therefore turned to allosteric and natural product-derived inhibitors [63]. Correctly identifying an effector node, in other words, does not make that node druggable.

10.4. Bariatric Surgery and Incretin-Based Therapies

The mechanism of bariatric surgery bears directly on the proposed framework (Section 6.4 and Section 7): that RYGB improves hypothalamic ER stress and inflammation, that this is associated with increased central leptin sensitivity, and that it is not seen in a weight-matched control [47] offers a mechanistic explanation for why surgically induced weight loss is more durable than equivalent loss achieved by diet.

GLP-1 receptor agonists raise a different question: liraglutide has been reported to attenuate the fall in free plasma leptin during maintenance of weight loss [64]. Since weight loss normally lowers free leptin and thereby facilitates weight regain, this effect may contribute to the drug’s efficacy. The study did not, however, assess sensitivity, and the effect of GLP-1 receptor agonists on hypothalamic ER stress has not been examined in humans. Table 6 summarises the interventions that target this axis, together with the current state of human data for each.

10.5. Molecular Heterogeneity of Leptin Resistance and a Phenotyping Framework

One of the most direct clinical implications of this review is that leptin resistance is not a single entity: the findings in Section 5 and Section 6 show that it can arise at different molecular levels and that a set of phenotypes distinguished by dominant mechanism exists.

Table 7 proposes a phenotyping framework on this basis, linking five questions: which patient, which dominant mechanism, which biomarker, which intervention and which primary endpoint. Because an intervention effective in only one subgroup will appear ineffective overall in an unstratified trial, establishing this chain is a precondition for making the axis clinically testable. The history of the leptin field illustrates the risk: recombinant leptin proved ineffective in an unselected obese population [58] but markedly effective in the leptin-deficient phenotype [7].

The status of this framework should be stated plainly: none of the proposed phenotypes has been confirmed in humans. The classification is a hypothesis-generating structure derived from animal and laboratory findings, and the biomarkers listed are candidates. Its purpose is not to offer a clinical algorithm but to define a testable stratification hypothesis.

The weakest link in the framework is measurement: hypothalamic ER stress cannot be measured directly in humans, so distinguishing an ER-dominant phenotype depends on unvalidated indirect markers. The second weak link is the endpoint, since no standard method exists for measuring leptin sensitivity in a clinical setting. Until both gaps are addressed, the pairings in Table 7 cannot be tested (Table 2).

10.6. Determinants of Individual Response Variability

The framework assumes that individuals do not respond identically to the same metabolic load — the founding premise of the personalised nutrition literature [66], which has empirical support [67]. A plausible molecular counterpart is that, where ER folding capacity differs between individuals, the same dietary lipid load will generate different degrees of ER stress [36] (Section 6.1.1); this inference remains conceptual.

The first source of variability is genotype. The most direct human evidence comes from the LEPR Q223R (rs1137101) variant, which reduces leptin binding and predicts annual weight gain in childhood obesity [68]. Rare variants illustrate the same principle at the level of proteostasis: the Cys604Ser variant of LepRb is prone to misfolding and is processed as an ERAD substrate [19]. Because no study has shown that a genotype-tailored intervention alters outcome, the evidence remains at the level of hypothesis [69].

The second source is developmental windows; the determinant of response variability with the strongest evidence base is early-life exposure (Section 8). Its clinical implication is twofold: the timing of an intervention may determine its efficacy, and sex should be treated as an independent response modifier [24,38,48,55].

The third source is diet-derived modulators, and it is notable that most of the compounds found effective along this axis are dietary in origin: phenethyl isothiocyanate, indole-3-carbinol, diallyl disulfide, proanthocyanidins, biochanin A, berberine, taurine and a Rubisco-derived plant peptide [32,35,37,40,41,46,70,71] (Table 5). Three caveats apply: the studies were conducted almost entirely in animal models and at doses above the range attainable through diet; most reported concurrent weight loss, so direct effects cannot be separated (Section 6.4); and differences in bioavailability and microbiota-mediated metabolism may alter efficacy [72].

10.7. Implications for Clinical Practice

As far as the current evidence allows, the clinical implications can be grouped under four points.

The first concerns the interpretation of hyperleptinaemia: a high leptin concentration in a patient with obesity reflects a defective response rather than an excess of hormone, and leptin measurement does not currently function as a decision tool in routine practice.

The second concerns diet composition: although most of the compounds of interest are dietary in origin, the findings remain at the level of animal models and do not support supplementation. They provide mechanistic support only for existing advice to reduce saturated fat and excess caloric load.

The third concerns the nature of weight loss: surgical data suggest that the same amount of weight loss may have different effects on the central leptin response depending on how it is achieved, so that the method as well as the magnitude of loss deserves attention.

The fourth concerns the prospect of phenotype-based stratification: adiposity-associated DNA methylation signatures and circulating microRNAs [53,54] may represent measurable molecular correlates of individual differences in response. These findings are correlational and unvalidated, however, and provide no basis for any clinical decision at present.

The clinical value of this review lies not in offering a treatment recommendation but in explaining mechanistically why current treatments do and do not work, and in defining a concrete research question ready for testing: can agents targeting ER proteostasis restore leptin sensitivity in humans?

11. Conclusions

This review has examined leptin resistance at the intersection of intracellular signalling networks and endoplasmic reticulum stress, synthesising 22 core primary studies together with supporting literature. The aim was not to restate known mechanisms but to unify them under a single testable framework and to set out explicitly the points at which that framework could be falsified.

11.1. Principal Conclusions of the Synthesis

Three principal conclusions emerge. First, the ER appears to shape leptin signalling at the levels of receptor biogenesis, receptor bioavailability and post-receptor inhibition; that the first of these layers operates independently of ER stress, and rests on conditional genetic designs, makes it the best-supported claim in this review. Second, distinct stressors appear to converge on a narrow set of effectors, although the causal role of CHOP is supported by genetic evidence while the evidence for SOCS3 and PTP1B remains correlative. Third, this node appears pharmacologically accessible in experimental models, yet direct effects have been separated from weight-loss-dependent effects in only one weight-matched study [47].

11.2. Limitations of the Evidence Base

All of these conclusions must be read against a substantial constraint: 17 of the core evidence base rests on rodent in vivo models and 5 on cell culture alone, and no study has been conducted in human participants. Studies using the human-derived SH-SY5Y line provide data from a human cell line, not human data.

This structure determines which claims the evidence supports. Animal models strongly support the proposition that ER stress can impair hypothalamic leptin signalling and that the impairment is reversible. The same evidence base does not support the proposition that targeting ER proteostasis should be used as an obesity treatment in humans; such a proposal is at best a hypothesis worth testing.

The distinction deserves emphasis, because the history of the leptin field is itself instructive: recombinant leptin was markedly effective in rodent models yet failed in common human obesity, and there is no basis for assuming that strategies targeting ER stress will avoid a comparable discontinuity.

Further limitations are as follows: (i) the great majority of studies used male animals, and sex differences have not been examined systematically, with El-Sehrawy et al. [65] an exception; (ii) phytochemical doses exceed the range attainable through diet in humans; (iii) effect sizes vary widely across models; (iv) study durations are generally limited to 6–12 weeks; and (v) although leptin resistance manifests with different molecular signatures in different tissues, most studies treat it as a single entity.

11.3. Research Gaps

The principal research gaps and the methodological approaches required to address them are summarised in Table 2; the most important is the absence of a randomised controlled trial testing whether an intervention targeting ER stress restores leptin sensitivity in humans.

11.4. Methodological Requirements and Future Directions

Closing these gaps will require several methodological advances: single-cell and spatial transcriptomics could resolve the cell-type-specific UPR activation that tissue homogenates obscure; human iPSC-derived hypothalamic organoids could overcome the limitations of the SH-SY5Y line; multi-omic integration could be used to identify circulating biomarkers; and computational drug discovery could contribute to the design of selective PTP1B and UPR modulators [63].

How the response varies between individuals, meanwhile, is unknown. Studies stratified by genotype, epigenetic profile, microbiota composition and baseline metabolic state offer the most realistic route to testing these compounds in humans, but this is a research programme that first requires phenotyping tools to be developed.

The field should also adopt two design standards: the use of weight-matched control groups in intervention studies, and the inclusion of both sexes.

11.5. Concluding Assessment

In rodent models the endoplasmic reticulum appears not merely as a waypoint in leptin resistance but as its regulatory centre, and its multi-layered role — extending from receptor quality control to induction of negative regulators — is supported by conditional genetic designs. Recasting that observation as a convergence-node framework, however, remains at the level of hypothesis.

The most important consequence of this multi-layered organisation is that leptin resistance is not a uniform disorder; the phenotypes arising at different levels (Table 7) offer a molecular basis for explaining why the same intervention produces different outcomes in different individuals: Resolving the molecular heterogeneity of leptin resistance may be necessary before interventions aimed at restoring leptin sensitivity can be tested in the appropriate patient subgroups.

The conditional phrasing is deliberate: the evidence base consists largely of animal and cell culture studies, and in humans neither can leptin resistance phenotypes be distinguished nor can hypothalamic ER stress be measured. The stratification proposed here is therefore not a clinical strategy but a mechanistically justified research programme.

The priority for the field is not to generate further rodent studies but to take three concrete steps: (i) developing and validating circulating biomarkers capable of distinguishing leptin resistance phenotypes in humans; (ii) testing agents already administered in humans, such as TUDCA and 4-PBA, in weight-matched trials with leptin sensitivity as the primary endpoint; and (iii) designing studies in which interventions are stratified by genotype, epigenetic profile and microbiota composition.