Leptin’s Complex Role in Metabolic dysfunction-Associated Fatty Liver Disease (MAFLD)
Table of Contents
Metabolic dysfunction-associated fatty liver disease (MAFLD) has emerged as a predominant chronic liver condition globally, intricately linked wiht obesity, type 2 diabetes (T2DM), and metabolic syndrome. Its pathogenesis is complex,following a “multiple-hit” hypothesis that involves triglyceride accumulation,insulin resistance (IR),lipotoxicity,chronic inflammation,and oxidative stress. Among the various adipokines implicated, leptin, a hormone central to energy homeostasis, has been identified as a critical player. This review synthesizes the current understanding of leptin’s structure, signaling, and its multifaceted-and frequently enough paradoxical-roles in the initiation and progression of MAFLD.
Leptin: Structure and receptor
Leptin is a 167-amino acid polypeptide hormone, primarily secreted by white adipose tissue, with levels correlating with body fat mass. It exerts its biological effects by binding to its receptor, Ob-R. Among several splice variants, the long isoform, Ob-Rb, is the primary signaling receptor, expressed both in the central nervous system (CNS) and peripheral tissues like the liver. The finding of leptin-deficient (ob/ob) and receptor-deficient (db/db) mouse models, which exhibit severe hepatic steatosis and IR, was pivotal in elucidating leptin’s metabolic functions.
Leptin Receptor Signaling and MAFLD
Upon leptin binding, Ob-Rb activates the JAK2-STAT3 signaling pathway, which regulates genes involved in metabolism and appetite. this pathway also induces SOCS3, a key negative feedback regulator that contributes to leptin resistance.Additionally, leptin signaling engages the PI3K/Akt and AMPK pathways, which are crucial for improving insulin sensitivity and promoting fatty acid oxidation in the liver. Dysregulation of these pathways is a hallmark of metabolic dysfunction in MAFLD.
Leptin Resistance
A central concept in the pathophysiology of MAFLD is leptin resistance, a state commonly observed in obesity where elevated circulating leptin fails to elicit appropriate physiological responses. Mechanisms include impaired JAK-STAT signaling,increased SOCS3 expression,and defective transport across the blood-brain barrier. This resistance disrupts leptin’s ability to regulate energy balance and metabolism, creating a vicious cycle that exacerbates IR and promotes hepatic lipid accumulation.
The dual Role of Leptin in MAFLD Pathogenesis
The role of leptin in MAFLD is complex and context-dependent, acting as both a protector and a promoter of disease.
* Glucose Metabolism and Insulin Resistance: Leptin improves glucose homeostasis through central and peripheral mechanisms. In the CNS, it enhances insulin sensitivity. In the liver, it inhibits gluconeogenesis. However, in states of hyperleptinemia, leptin can damage pancreatic β-cells and disrupt insulin signaling pathways, thereby worsening IR and creating a metabolic environment conducive to MAFLD progression.
* Lipid Metabolism: Physiologically, leptin is anti-steatotic. It promotes hepatic fatty acid β-oxidation, inhibits lipogenesis, and enhances the export of very low-density lipoprotein triglycerides (VLDL-TC). This is evidenced by the reversal of steatosis in leptin-deficient mice upon leptin treatment. However, in the prevalent condition of leptin resistance, these beneficial effects are blunted, and high leptin levels may even promote lipogenesis via upregulation of SREBP-1, contributing to hepatic fat accumulation.
* Inflammation and Fibrosis: Leptin exhibits potent pro-inflammatory properties. It promotes a pro-inflammatory M1 macrophage phenotype, stimulates the production of cytokines like TNF-α, IL-6, and IL-1β, and supports the proliferation of inflammatory T-cells.