Hepassocin (FGL-1) as a Hepatokine in Liver Physiology and Metabolic Dysfunction: A Narrative Review.

Hepassocin, also known as fibrinogen-like protein 1 (FGL-1), is a liver-derived secretory protein initially identified as a mitogenic factor involved in hepatocyte proliferation and liver regeneration. Increasing evidence has subsequently suggested that FGL-1 functions as a hepatokine linking hepatic metabolic stress to systemic metabolic regulation. Experimental and clinical studies have demonstrated that circulating FGL-1 levels are associated with obesity, insulin resistance, metabolic dysfunction-associated steatotic liver disease (MASLD), and type 2 diabetes mellitus (T2DM). Mechanistically, FGL-1 appears to contribute to metabolic dysfunction by impairing insulin signaling and promoting hepatic lipid accumulation, although its precise molecular targets remain incompletely defined. In addition to its metabolic roles, FGL-1 has been identified as a major ligand of lymphocyte activation gene-3 (LAG-3), implicating it in immune modulation and tumor progression, particularly in hepatocellular carcinoma (HCC). However, most available human data are observational, and conflicting findings from experimental models suggest that FGL-1 may function as a context-dependent mediator rather than a purely pathogenic factor. Given the expanding but sometimes conflicting evidence, a comprehensive understanding of FGL-1 biology may provide important insights into the complex interactions among hepatic stress responses, metabolic dysfunction, and immune regulation. This review therefore examines the current evidence regarding the physiological and pathological roles of FGL-1 and highlights key unresolved questions that may influence future translational research and therapeutic development.