Over the past years, extensive research has been conducted concerning the impact of leptin on various respiratory disorders. Mounting evidence have been published, as the picture is becoming more complex. The scope of this review is to decode the existing data and provide a detailed description of the involvement of leptin in the most common disease entities associated with the respiratory system.
Obstructive sleep apnoea-hypopnoea syndrome (OSAHS) and obesity hypoventilation syndrome (OHS) (Table 4)
Table 4
The role of leptin in OSAHS and OHS
Ip et al68 (2000) | Leptin significantly correlated with AHI | Only males/Limited number of patients/Potential influence by comorbidities/No adjustment for FM |
Campo et al78 (2007) | Higher leptin is associated with reduced respiratory drive and reduced hypercapnic response | Conditions of blood sampling unknown/Potential influence by comorbidities |
Philips et al82 (2000) | Increased leptin in OSAHS | Only males/Limited number of patients/Low statistical power |
Barcelo et al86 (2005) | Decrease in leptin after nCPAP treatment in non-obese OSAHS | Only males/Limited number of patients/No adjustment for FM |
Shimizu et al90 (2002) | Significant decrease in leptin after 1 day of nCPAP The decrease of leptin correlated with cardiac sympathetic function | Only males/Limited number of patients/Potential influence by comorbidities Low statistical power |
Phipps et al96 (2002) | Leptin is a predictor for the presence of hypercapnia | Limited number of patients/Sex unknown |
OSAHS is a common disorder characterized by repeated episodes of partial or complete upper airway obstruction during sleep [
63]. Approximately 90% of patients with OHS, a condition defined as a combination of obesity (i.e. BMI ≥ 30 Kg/m
2) and sleep disordered breathing, have concurrent OSAHS (i.e. apnoea-hypopnoea index (AHI) > 5) [
64], while 10-15% of patients with OSAHS develop hypoventilation and daytime hypercapnia [
65].
Obesity is considered to be the most important risk factor of OSAHS [
66]. The impact of obesity in sleep disordered breathing was originally reported to be mechanical but recent data suggest that adipose tissue can contribute to the genesis of the syndrome through its metabolic activity. The established role of leptin as a respiratory stimulant (discussed extensively above) raised the possibility that OSAHS may represent a leptin-deficient state. Inversely, several groups have demonstrated higher circulating leptin levels in OSAHS patients, when compared to age, sex, and weight-matched controls [
67‐
72], while others have failed to document such a difference [
73,
74]. However, a collective comparison of these findings is difficult, since many of the aforementioned studies have included patients with comorbid conditions (e.g. arterial hypertension) that could serve as confounding factors [
68,
74]. The preceding data, exhibit substantial weakness originating from the relatively small number of subjects included and, additionally, the male predominance in the majority of these reports raises difficulties in extrapolating the results to the female sex.
In the light of these data, researchers have hypothesized that OSAHS is a leptin-resistant state, and that a relative deficiency in CNS leptin levels, due to an impaired transport across the blood-brain barrier, may induce hypoventilation, therefore contribute to the genesis of the syndrome [
75‐
77]. Unfortunately, literature lacks data to confirm or to decline such a hypothesis, since, to our knowledge, no study until today has investigated leptin levels in cerebrospinal fluid (CSF) in OSAHS patients. Another explanation is an impairment in leptin activity in CNS, caused by down-regulation of central leptin receptors or defects in second messenger system [
54,
76‐
78]. Recently, researchers have identified a single nucleotide polymorphism in the leptin receptor gene associated with the presence of OSAHS [
79]. This single amino acid change in the Ob-R molecule may result in altered signal transduction, generating a state of leptin resistance, in consistency with the latter hypothesis. However, others have failed to confirm an association of leptin and leptin receptor gene variations with the development of OSAHS [
80], although the results should be interpreted with caution since the number of patients enrolled have been reported to be underpowered to detect a sufficient effect [
81].
A subject of ongoing controversy is whether the presence of hyperleptinemia in OSAHS derives from adiposity or it reflects causality due to the effects of sleep-disordered breathing. Leptin levels are 50% higher in OSAHS patients than in controls, suggesting that other factors besides obesity contribute to the elevation of leptin [
82]. In consistency with the previous results, leptin levels are significantly correlated with several indices of OSAHS severity, i.e. AHI, percentage of sleep time with less than 90% hemoglobin saturation (%T90), oxygen desaturation index, as well as with a variety of anthropometric measurements, including BMI, waist-to-hip ratio (WHR), and skinfold thickness [
68‐
70,
72,
75,
83,
84]. However, the data derived are rather contradictive; some researchers have documented a significant positive correlation of leptin levels with AHI, even when controlled for BMI [
70], while others have reported no significant correlation between leptin values after adjustment for BMI, WHR and waist circumference, with measures of disease severity, although WHR and T%90 were found to be the most significant variables in a model predicting leptin [
69]. In keeping with the aforementioned concepts, other researchers have documented that BMI is the only parameter significantly and independently associated with leptin concentrations [
83]. Similarly, other groups have reported that adiposity measures are the only predictive factors of leptin levels, while AHI was not found to be significant [
75].
To make matters more complicated, studies have documented significantly higher leptin levels in non-obese OSAHS patients versus controls [
85,
86]. Data suggest that repeated sleep hypoxemia may promote leptin production independently of the degree of obesity. However, the authors provided evidence indicating that the location of the body fat deposition (e.g. visceral fat accumulation) may account for the increased leptin concentrations in non-obese OSAHS subjects [
85]. Clearly, the aforementioned findings are inconclusive and due to their associative nature, cannot substantiate causality.
Additional studies examining the effects of nasal continuous positive airway pressure (nCPAP) treatment were designed to elucidate the exact association of leptin with OSAHS. Leptin levels decrease significantly in OSAHS patients, treated with nCPAP for a period of 3 days to 6 months, without any significant change in BMI observed [
68,
83,
87‐
89]. The significant reduction in circulating leptin following 1 to 4 days of nCPAP therapy [
87,
90] suggests that OSAHS itself may stimulate, at least in part, leptin production independently of obesity. However, the mechanisms responsible are yet unclear, and no definite conclusions can be made since several groups have reported no significant changes in leptin levels after the application of nCPAP [
91,
92]. Interestingly, Barcelo et al [
86] documented a marginal, yet significant, decrease in leptin levels associated with nCPAP treatment in non-obese OSAHS patients, while leptin concentrations were reported unchanged in obese subjects. Similarly, others have illustrated a more pronounced reduction of leptin levels in non-obese patients versus obese OSAHS patients [
89]. The physiological explanation has not been fully elucidated, but data in the literature suggest that the decrease in leptin might be explained by the effect of treatment on sympathetic nerve activation [
90], or may be associated with changes in haemodynamics and visceral blood flow [
83]. Other possible explanations include the reduction in visceral fat accumulation and stress levels [
93], or a reverse in the Ob-R sensitivity [
94], consistent with the hypothesis of leptin resistance discussed above.
Few studies in the literature have examined the possible implication of leptin in OHS. As argued earlier, leptin deficient mice exhibit similar to OHS features, i.e. CO
2 retention and depressed HCVR [
95]. In obese patients, hyperleptinemia is associated with a reduction in respiratory drive and hypercapnic response, irrespective of anthropometric measurements [
78], while circulating leptin is a predictor for the presence of hypercapnia [
76,
96]. Leptin concentrations are statistically significantly lower in OHS patients without OSAHS, when compared to BMI matched eucapnic obese subjects without OSAHS [
97]. Additionally, the authors demonstrated a significant increase in leptin values following long-term non-invasive mechanical ventilation (NIVM), although the levels were still lower than those at the eucapnic group. Inversely, other researchers have reported a significant reduction in leptin levels in OHS patients receiving NIVM [
98]. However, a direct comparison of these results can be misleading, since Yee et al [
98] enrolled subjects with OHS associated with OSAHS. In contrast, others have reported higher circulating levels of leptin in OHS when compared to eucapnic obese subjects despite similar degree of body fat [
96]. Serum leptin served as a predictor for the presence of hypercapnia, suggesting that higher and not lower leptin levels predisposes to OHS. However, this study included patients with concurrent OSAHS that could serve as a confounding factor. In the light of these data, some have raised the possibility that OHS may be characterized by a more profound degree of leptin resistance than OSAHS, although this hypothesis requires further validation by more extensive studies [
93].
Chronic Obstructive Pulmonary Disease (COPD) (Table 5)
Table 5
The role of leptin in COPD
Cachexia-stable COPD | Takabatake et al102
(1999) | Leptin production regulated physiologically and not correlated with TNF-α or sTNF-R | Only males/Limited number of patients/No adjustment for FM |
| Takabatake et al104 (2001) | Absence of circadian rhythm of leptin | Only males/Limited number of patients |
| Schols et al106 (1999) | Leptin related to sTNF-R55 in emphysema | Only males/Limited number of patients/Patients received CS |
Exacerbation | Creutzberg et al108 (2000) | Increased leptin (serial measurements) Leptin positively correlated with sTNFR-55 | Limited number of patients/Patients with hospital stay < 7 days excluded/Patients received CS/Only severe COPD |
| Kythreotis et al109 (2009) | Leptin positively correlated with TNF-α | Patients received CS |
COPD is a disease state characterized by airflow limitation that is not fully reversible, usually progressive, and associated with an abnormal inflammatory response of the lung to noxious particles or gases [
99]. Researchers have speculated that a potential link between obesity and COPD subsists since low BMI and weight loss is associated with increased mortality in patients suffering from COPD [
100]. However, the mechanisms underlying this association are not yet fully elucidated.
Studies in the literature have examined the hypothesis that underlying abnormalities in the leptin feedback mechanism might be involved in the impaired energy balance responsible for the cachexic status and muscle wasting commonly seen in COPD [
101]. However, researchers have failed to demonstrate the presence of inappropriately increased leptin levels in cachexic stable COPD patients [
102,
103], while there is no statistically significant relationship detected between circulating leptin and the activated TNF-α system [
102‐
105]. In contrast, others have reported a significant partial correlation coefficient between leptin and soluble tumour necrosis factor receptor 55 (sTNF-R55), when adjusted for fat mass (FM) and oral corticosteroid use in the emphysematous subtype of COPD, but not in chronic bronchitis patients, while leptin levels were associated with FM in line with the reported feedback mechanism involved in the regulation of body weight [
106]. Although leptin seems to be regulated physiologically, low leptin levels may contribute to sexual disturbances, impaired glucose tolerance, and higher frequency of pulmonary infection, observed in COPD patients [
102], while leptin has been associated with the presence of osteoporosis in COPD subjects [
62]. To gain a more comprehensive understanding, Takabatake et al [
104] examined the circadian rhythm of circulating leptin in COPD and documented its absence in cachexic COPD patients, while it was preserved in normal weight COPD subjects. Interestingly, the very low frequency component of heart rate variability, which has been considered to reflect neuroendocrine and thermoregulatory influences to the heart, showed similar diurnal rhythm with circulating leptin in all study groups [
104]. These data suggest that the loss of the physiologic pattern of leptin release may have clinical importance in the pathophysiologic features in cachexic patients with COPD, such as abnormalities of the autonomous nervous system and the hypothalamic-pituitary axes, or may represent a compensatory mechanism to maintain body fat content [
104].
Researchers have investigated the possible involvement of leptin during the acute exacerbations of COPD. Malnourished patients experiencing exacerbation, exhibit significantly higher leptin levels, compared to normal-weight stable COPD patients, an observation not replicated when compared to malnourished stable COPD patients [
107]. Similar results have been reported by other groups [
103]. Importantly, leptin values, corrected for FM, are significantly elevated in COPD patients during acute exacerbation versus controls [
108,
109]. Leptin concentrations gradually decrease throughout the exacerbation, but when corrected for FM, remain significantly elevated during hospitalization [
108,
109]. The normal feedback regulation of leptin by FM is preserved on Day 7 of the exacerbation, although dissociation has been reported on Day 1, possibly due to a temporary dysfunction related to the event [
108]. The natural logarithm (LN) of leptin is inversely correlated with the dietary intake/resting energy expenditure index (indicating the role of leptin in energy balance) and positively correlated with sTNF-R55 (after correction for FM) [
108]. Other researchers have reported a positive correlation between TNF-α and leptin on Day 1 of admission [
109]. sTNF-R55 significantly explains 66% of the variation in energy balance in Day 7 of the exacerbation, while leptin is excluded, suggesting that the influence of leptin is under the control of the systemic inflammatory response [
108].
The airflow limitation in COPD is linked to structural changes, including the presence of an abnormal inflammatory pattern detected in each lung compartment [
110]. AKR/J mice (i.e. a strain that presents similar to COPD anatomic abnormalities following cigarette smoke exposure for 4 months) exhibit reduced Ob-R expression in the airway wall, upon smoke exposure [
111]. Inversely, stimulation of bronchial epithelial cells and alveolar type II pneumocytes, isolated from human lung tissue, with increasing doses of cigarette smoke condensate results in a significant induction of leptin and Ob-R
b m-RNA, suggesting that smoking itself may increase the expression of the leptin/leptin receptor system in lung tissue [
8]. However, others have demonstrated down-regulation of leptin/leptin receptor system in bronchial epithelial cells of proximal airways of mild-to-severe COPD patients, when compared to tissues obtained from non-smoking subjects [
48], while immunohistochemical studies show that leptin expression is increased in bronchial epithelial cells and alveolar macrophages in the peripheral lung of COPD patients (GOLD stage 4) [
8]. Additionally, leptin is over-expressed in the submucosa of proximal airways of COPD patients [
48]. The diversities observed in pulmonary leptin/leptin-receptor system expression among COPD patients, symptomatic smokers and never-smokers despite similar anthropometric measurements, lend further support to the concept of local production of leptin in the lung [
8].
Accumulated evidence suggest that leptin may be involved in the local inflammatory response seen in the airways of COPD patients, hypothetically regulating the infiltration and the survival of inflammatory cells in the submucosa of COPD patients [
48]. Interestingly, leptin's up-regulation in the proximal airways correlates to the expression of activated T lymphocytes (mainly CD8
+) and to the absence of apoptotic T cells [
48]. In addition, leptin is detected in induced sputum of patients with COPD, whereas it is significantly positively correlated with inflammatory markers measured in induced sputum, such as CRP and TNF-α [
112]. Importantly, plasma and sputum leptin levels are inversely correlated. In harmony with the previous results, the presence of Ob-R
b in lung epithelium and inflammatory cells combined with the fact that the lung is a source of leptin, suggests the existence of a paracrine cross-talk between resident pulmonary epithelial cells and immune cells in response to noxious particles [
8]. This hypothesis needs further validation by subsequent studies, enrolling a larger number of patients and including experiments that will shed further light to the pathophysiological role of leptin in the pathogenesis of COPD.
Recently, researchers have reported that COPD patients carrying minor alleles of polymorphisms in the Ob-R gene are less susceptible to loss of lung function, as indicated by %FEV
1 decline [
111]. Although the functional significance is not known, these data have led to the hypothesis that the Ob-R gene may serve as a novel candidate gene for COPD.
Asthma (Table 6)
Table 6
The role of leptin in asthma
Structural changes | Bruno et al9 (2009) | Leptin/leptin receptor expression in bronchial epithelial cells is reduced in mild uncontrolled and severe asthma | Limited number of patients/Patients treated with corticosteroids |
Animal studies | Shore et al117 (2003) | Increased response to ozone in ob/ob mice |
ob/ob mice exhibit low lung size (potential mechanical bias) |
| Luet et al119 (2006) | Increased responses to ozone in db/db mice |
db/db mice exhibit low lung size (potential mechanical bias)/Only female mice |
| Johnston et al121 (2008) | Mice with diet-induced obesity exhibit innate AHR | Control mice were overweight |
| Shore et al127 (2005) | Enhanced metacholine responsiveness in leptin-treated mice | Clinical relevance unknown |
Clinical studies | Guler et al124 (2004) | Leptin is a predictive factor for childhood asthma | No adjustment for FM/Lack of correlation of leptin with PFT |
| Sood et al126 (2006) | Higher leptin in asthmatics | Asthma diagnosis based on self-questionnaire/No adjustment for FM |
Asthma represents a chronic inflammatory disorder of the airways associated with airway hyper-responsiveness that leads to recurrent episodes of widespread, and often reversible, airflow obstruction within the lung [
113]. Obesity is a risk factor for asthma, while studies indicate that adiposity may increase disease severity in asthmatic subjects and possibly alter the efficacy of standard asthma medications [
114‐
116]. The mechanisms underlying the relationship between obesity and asthma have not been fully established yet, however, experimental evidence suggests that changes in adipose-tissue derived hormones, including leptin, as well as other factors, are possibly implicated.
ob/ob mice exhibit significantly elevated pulmonary resistance (R
L) and responsiveness to metacholine in baseline conditions, while ozone (O
3) exposure results in greater increase in these two parameters, associated with an enhanced expression of bronchoalveolar alveolar lavage fluid (BALF) protein, eotaxin, and IL-6 when compared to lean controls [
117]. Acute leptin replacement in chronically leptin-deficient mice cannot reverse the enhanced inflammatory response. However, mice fasted overnight exhibit reduced leptin levels, associated with a significant increase in R
L and airway responsiveness following O
3 exposure, as compared to fed mice [
118]. The restoration of leptin to fed levels prevented the fasting induced changes in response to O
3. Exogenous leptin administration in wild-type mice results in increased O
3-induced cytokine and protein release into BALF [
117]. Similarly to the
ob murine model,
db/db mice (i.e. mice that lack functional Ob-R
b isoform due to a mutation in the cytoplasmic domain of the receptor) and carboxypeptidase E-deficient (CPE
fat) mice (i.e. a strain characterized by obesity, resulting from a functional mutation in the gene encoding carboxypeptidase, and increased leptin levels) present increased baseline airway responsiveness, as well as augmented responses to O
3 exposure, when compared to their lean controls [
119,
120]. In harmony with the latter results, mice with diet-induced obesity exhibit innate AHR and enhanced O
3-induced pulmonary inflammation, similar to that observed in genetically obese mice [
121]. Collectively, the aforementioned findings suggest that leptin may have the potential to augment the pulmonary response to acute O
3 exposure, but other effects of obesity may also play an important role [
122]. Since innate AHR is a common feature of leptin and leptin receptor deficient mice, as well as CPE
fat mice and mice with diet induced obesity (i.e. mice with reduced and mice with increased leptin concentrations) it seems unlikely that the adipokine can act as an intermediary in the causal pathway [
122].
Clinical studies provide confounding evidence to the mouse-model observation regarding the role of leptin in asthma. Overweight asthmatic children present twice as high leptin levels as those without asthma, despite no differences in BMI [
123]. Similar results are documented by other researchers; asthmatic children, especially asthmatic boys, exhibit higher leptin levels compared to controls [
124]. Leptin concentrations are significantly associated with bronchodilator response in overweight/obese men, but not in overweight/obese women [
125]. Furthermore, leptin levels, even when adjusted for BMI, are predictive of asthma in male subjects [
124]. Additionally, increased BMI and leptin concentrations are associated with asthma in adults, but when adjusted for leptin, no effect is observed in the association among BMI and asthma, indicating that the association is not mediated by the leptin pathway alone [
126]. In contrast, others have failed to document any direct association between leptin and the presence of asthma [
60].
Increasing evidence suggest that the pro-inflammatory effects of leptin may contribute to the higher incidence of asthma in the obese population. As discussed previously, administration of leptin to wild-type mice enhances O
3-induced airway inflammation [
117], while ovalbumin sensitization and challenge increases serum leptin levels in mice [
127]. Additionally, in animal models, exogenous leptin enhances the phagocytosis by macrophages and the production of TNF-a, IL-6 and IL-12 [
124]. Administration of pro-inflammatory cytokines, such as TNF-α and IL-1, in mice results in a dose-dependent increase in leptin concentrations [
126]. However, since these cytokines have been implicated in the pathophysiology of asthma [
124] it is conceivable that the disease-related inflammation induces the release of leptin from the adipose tissue or the lung itself, which may in turn increase airway inflammation and hyper-responsiveness through a continuous interaction [
122,
126,
128].
Over the past few years, researchers have hypothesized that decreased immunological tolerance, as a consequence of immunological changes induced by adipokines, may be implicated in the pathogenesis of allergic asthma [
129]. As argued above, leptin-treated animals exhibit augmented responses to metacholine and increased levels of IgE, following ovalbumin challenge, when compared to saline-infused mice [
127]. No difference on the inflammatory response in the airways was observed between the two study groups. In keeping with the aforementioned results, leptin and IgE levels are significantly correlated in asthmatic children [
124]. Interestingly, atopic asthmatic boys have significantly higher leptin levels than non-atopic asthmatic subjects. Additionally, in vitro studies have documented that leptin can significantly up-regulate the cell surface expression of intracellular adhesion molecule (ICAM)-1 and CD18 and suppress those of ICAM-3 and L-selectin in eosinophils [
130], while it augments alveolar macrophage leukotriene synthesis [
131]. The latter results suggest that leptin may induce accumulation of eosinophils and may enhance inflammatory processes at sites such as the lung or the airways, and thereby augment allergic airway responses, at least in part [
130,
131].
Additionally, studies have raised the issue whether leptin may play an important role on asthma pathophysiology through its ability to activate SNS. Leptin increases the activity of the adrenal medulla and sympathetic nerves in various organs, although its impact on the sympathetic nerves of the lung is unknown [
132,
133]. On the basis of this conception, researchers have examined the effects of leptin on human airway smooth muscle cells and airway remodeling associated with asthma; leptin itself cannot promote muscle proliferation, migration or cytokine synthesis, suggesting that the effects of obesity on asthma may not be attributed to a direct effect of leptin on airway smooth muscle [
47]. Leptin has no proliferative effect when administered in a human airway smooth muscle cell line culture, although it stimulates the release of VEGF by these cells [
134]. However, the expression of leptin/leptin receptor in bronchial epithelial cells is significantly reduced in patients with mild uncontrolled asthma and severe treated asthma versus patients with mild controlled treated asthma and healthy volunteers, while leptin and leptin receptor expression are inversely correlated with reticular basement membrane thickness suggesting that leptin/leptin receptor expression may be associated with the airway remodeling observed in asthma, implicating the adipokine in the homeostasis of lung tissue [
9].
Lung Cancer (Table 7)
Table 7
The role of leptin in lung cancer
Ribeiro et al138 (2006) | Polymorphism in the promoter of leptin gene associated with increased risk for NSCLC | Controls younger than patient group/Smoking status of controls unknown |
Aleman et al142 (2002) | Lower leptin in NSCLC vs controls | No adjustment for FM/Only advanced stage disease |
Karapanagiotou et al146 (2008) | No association of leptin to histological type, differentiation grade, disease stage, survival or time to disease progression | Controls and patients not age and sex matched/ Only advanced stage disease |
Carpagnano et al147 (2007) | Higher leptin in NSCLC vs controls | No adjustment for FM/Limited number o f patients/Non-advanced disease stage |
Increased BMI is significantly associated with higher death rates due to cancer [
135], and it is well established that obesity increases the risk of cancer developing in numerous sites [
136,
137]. Can leptin be the mediator linking obesity with cancer?
A functional polymorphism in the promoter region of leptin gene is associated with a threefold increased risk of developing non-small cell lung cancer (NSCLC) [
138]. The over-expressing variant is associated with earlier onset of lung cancer, but not with advanced metastatic disease, suggesting that continuous exposure to higher leptin concentrations due to the polymorphism in the leptin gene may accelerate cancer initiation [
138]. This hypothesis is further strengthened by other groups who observed increased leptin levels in NSCLC patients and recognized leptin as a risk factor for cancer, even after controlling for BMI and recent weight loss [
139].
In accordance with the previous studies, primary cultures of tracheal epithelial cells of
db/db mice demonstrate significantly lower cell proliferation versus those of their lean litternates, while administration of leptin significantly increased cell proliferative ability in lean mice, but not in
db/db mice [
49]. Leptin has a stimulatory action on a clonal cell line derived from human lung squamous cell cancer (SQ5 cells), an effect mediated through mitogen activated protein (MAP) kinase activity, indicating that leptin may act as a growth factor. On the contrary, in an experimental pulmonary metastasis model,
ob/ob and
db/db mice present a remarkably increased number of metastatic colonies when compared to wild-type mice [
140]. Administration of leptin in
ob/ob mice abolished the increase in metastasis, indicating a rather prophylactic role of leptin. However, when cancer cells were inoculated orthotopically, through a chest incision, tumor growth at the implanted site was comparable among the groups.
Studies have led to the hypothesis that leptin contributes in cancer development, at least in part, through its up-regulatory role in the inflammatory system [
141]. Leptin affects both innate and adaptive immunity by stimulating and activating neutrophils, macrophages, blood mononuclear cells, dendritic cells and T cells, and consecutively their products, which may induce chronic inflammation and lung carcinogenesis [
141]. However, until today, this complex interplay between leptin, immune system, and cancer has received only some experimental support and further investigations are required.
A number of studies have examined the possible role of leptin in the pathogenesis of cancer-related weight loss. In consistency with earlier studies [
142‐
145], Karapanagiotou et al [
146], reported no differences in serum leptin levels, adjusted for sex and BMI, among advanced NSCLC patients and healthy controls. Leptin levels did not correlate with the histological type, differentiation grade, disease stage, overall survival, or time to disease progression, and there were no differences presented between patients with and without weight loss. Therefore, leptin cannot serve as a diagnostic or prognostic factor in advanced NSCLC. Moreover, these results suggest that cancer anorexia and cachexia are not due to a dysregulation of leptin production. The aforementioned observations are in contrast with those reported by other researchers, who observed higher concentrations of leptin in NSCLC patients
vs. controls [
147]. Patients recruited in the latter study had mainly non-advanced disease and there was no adjustment of leptin levels for FM, factors that can attribute to the discrepancies among studies.