Background

Over the past two decades, nonalcoholic fatty liver disease (NAFLD) has progressed from a relatively unknown disease to the leading cause of chronic liver disease worldwide [1]. Its global frequency is quickly increasing, reaching up to 25% in developed countries like the United States [2]. NAFLD is a degenerative disease caused by the buildup of intracellular lipid droplets in liver cells, which can induce inflammation, cell death, and even more advanced stages such as nonalcoholic steatohepatitis (NASH) (with or without fibrosis), cirrhosis, and liver cancer [3, 4]. Currently, pharmacological options for NAFLD are limited. Treatment cornerstones are a healthy lifestyle and weight loss. There is still an unmet therapeutic need [5].

NAFLD is bidirectionally associated with components of the metabolic syndrome [6], a cluster of alterations that includes centripetal obesity, decreased HDL cholesterol concentrations, increased triglyceride concentrations, arterial hypertension, and hyperglycemia [7,8,9]. This syndrome has become one of the epidemics of the twenty-first century. Causative factors include insulin resistance, leptin, lipocalins, microbiota alterations, and epigenetics [10, 56]. In this regard, leptin should have an anti-steatosis impact on hepatocytes [15]. However, no therapeutics for NAFLD are directed at this target. As a result, the finding that leptin and NAFLD are correlated may be useful for assessing disease risk, preventing NAFLD, combining existing therapy regimens for potentiation, and identifying prospective targets for novel drug development. Our study, based on the literature, did find a significant association between increased leptin levels and reduced incidence of NAFLD, and our findings not only coincide with previous literature but also validate the hypothesis in the literature through database analysis of real-world case sources.

Leptin signals through binding to its receptors, mainly Lep Rb, which is a long stretch of extracellular structure, a transmembrane region and an elongated intracellular extension. As Lep Rb does not possess intrinsic kinase activity, the conformational change of Lep Rb upon leptin binding to Lep Rb induces the activation of Janus kinase (JAK2) phosphorylation, which phosphorylates three tyrosine residues (Y985, Y1077 and Y1138) in the intracellular extension of Lep Rb. These phosphorylated tyrosine residues then recruit proteins containing the SH2 phosphorylation recognition domain for downstream signaling. Currently, the most studied leptin signaling is the JAK/signal transducer and activator of transcription (STAT) pathway. Leptin and NAFLD exert their effects mainly through the JAK2/STAT3 pathway [57, 58]. An important role of leptin is to direct the storage of triglycerides in adipocytes and prevent their deposition in non-adipose tissues such as the liver, thus preventing hepatocyte lipotoxicity and apoptosis. Leptin also inhibits the production of hepatic glucose and the formation of new hepatic fat, acting as an insulin-like agent to prevent the development of NAFLD. Studies have shown that chronic central leptin infusion can reduce hepatic lipid synthesis gene expression and triglyceride levels by stimulating hepatic sympathetic activity and that this effect of leptin is associated with the PI3K signaling pathway, blocking which can specifically induce hepatic steatosis without causing obesity. In addition, leptin promotes fatty acid oxidation in the liver and increases fatty acid consumption in the liver [59]. In addition to its direct effects on the liver, leptin also affects hepatic glucose metabolism indirectly through its central regulation. Leptin infusion into the ventricles of type 1 diabetes mice inhibited the expression of glucagon, consistent with the phenotype of peripheral hyperleptinemia [60]. Specific expression of Lep Rb in the arcuate nucleus of the rat hypothalamus by adenoviral transfection improves peripheral insulin sensitivity and reduces hepatic gluconeogenesis in leptin receptor-deficient Koletsky rats [61]. The regulation of hepatic glucose by leptin may be related to the effect of its phosphatidylinositol 3-kinase (PI3K), which increases insulin signaling and decreases the expression of glucose synthesis genes such as glucose-6-phosphatase (G-6-P) and phosphoenolpyruvate carboxykinase (PEPCK) [61]. In addition, the effects of leptin can also be mediated by central neural regulation, e.g., selective severance of the hepatic vagus nerve can prevent hypothalamic leptin from regulating hepatic insulin sensitivity.

The mechanism of leptin in NAFLD has been supported by a large body of experimental data, and clinical studies on leptin and NAFLD have focused on the association of leptin or leptin receptor levels with NAFLD. The findings on circulating leptin levels in NAFLD patients are not very consistent, with some studies reporting high leptin expression in NAFLD patients [62, 63] and others finding no difference in leptin levels in NAFLD patients compared to non-NAFLD populations [64, 65]. Clinical studies of leptin and Lep R gene expression and SNPs in the NAFLD population have also been reported sporadically. Two small clinical studies showed no expression of the Lep R gene in liver tissue, while in peripheral leukocytes and abdominal adipose tissue Lep gene expression did not differ significantly between NAFLD patients and healthy populations [66, 67]. In another study, immunohistochemical staining of liver tissue for leptin showed that leptin expression was higher in patients with NAFLD than in the healthy population, consistent with altered circulating leptin levels [63]. Some of the Lep R gene SNP studies have also shown a positive association with the development of NAFLD, even if this association is not dependent on the presence of obesity. Given the complexity of clinical studies and the multilevel nature of clinical data, it is difficult to obtain direct evidence that leptin resistance causes NAFLD from the available clinical research data, which need to be interpreted with caution.

Our study has several advantages. First, the TSMR analysis method is based on the principle of Mendelian randomization-free segregation and combination, which excludes the influence of acquired factors (social environment and natural environment) on the study results at the genetic level. In order to successfully compensate for the vulnerability to confounding factors and reverse causality interference in traditional observational studies for inferring the etiology of complex disorders, the genes must arise prior to the disease with a precise causal time sequence [68, 69]. Second, this study uses publicly available GWAS summary statistics with a large sample size to obtain more precise estimates and greater statistical power, saving research costs and improving the utilization of biological information while limiting the study population mainly to individuals of European ancestry, reducing some of the bias that may arise due to population stratification. Finally, the value of this study lies in establishing an association between leptin levels and the incidence of NAFLD using a database of real-world sources. Based on our findings, it is reasonable to believe that leptin levels may be used for the assessment of NAFLD, including the screening of people who are traditionally at high risk of develo** NAFLD (e.g., those with comorbid diabetes and those who are overweight), and, more importantly, for the assessment of the risk of develo** lean NAFLD in people with normal BMI. In addition, it can be used as an indicator to evaluate the improvement potential of NAFLD. Since there are no drugs that target leptin, we believe that, based on the current state of research, leptin can reduce the incidence of NAFLD without duplicating the mechanism of action of other existing drugs for NAFLD and can be used as a complement to existing treatment regimens. Additionally, there is evidence that leptin regulation may have favorable effects on a variety of other factors, including weight loss, reducing blood sugar levels, and controlling intestinal functions [70]. Therefore, modulation of leptin levels may be used in multiple aspects of metabolic disorders and may have a wider range of potential applications. However, there are some limitations to this study. First and foremost, the majority of these GWAS data are from European populations. It needs to be determined if the findings we described would hold in other people. Second, this study lacks a multidimensional stratification of the heterogeneity of patients with NAFLD. In the future, a multicenter prospective cohort study is needed to fully consider the heterogeneity of NAFLD, integrate demographic characteristics, lifestyle, genetics and other factors to accurately identify high-risk groups for NAFLD, and develop targeted and individualized body mass control strategies, with a view to achieving accurate prevention and control of NAFLD.

Conclusion

In conclusion, our study reveals a causal relationship between leptin and NAFLD. It thus provides further insight into the factors that may be associated with a reduced risk of NAFLD development. Additionally, from a systems biology standpoint, it aids researchers in better understanding the connections between diverse diseases.