Abstract
MicroRNA-122 (miR-122) is the most abundant liver-specific microRNA, comprising ~ 70% of the hepatic miRNA pool, and a central regulator of lipid metabolism, inflammation, fibrosis, viral replication, and hepatocarcinogenesis. This review synthesizes experimental, clinical, and molecular evidence on the role of miR-122 across the spectrum of liver disease, including metabolic dysfunction-associated fatty liver disease (MAFLD) and steatohepatitis (MASH), drug-induced acute liver injury, hepatitis B and C virus (HBV/HCV) infection, hepatocellular carcinoma (HCC), and colorectal cancer liver metastasis. Mechanistically, miR-122 governs hepatic lipogenesis through the Sirt1/LKB1/AMPK axis, modulates inflammation via LPS/TLR-4/FoxO3 signaling, and exerts tumor-suppressive and antiviral effects through Cyclin G1/p53, HO-1, NDRG3, GALNT10, PEG10, and NEGR1. A recurring theme is the compartment- and stage-dependent behavior of miR-122: hepatic expression declines with disease progression, whereas circulating levels rise with hepatocyte injury, reconciling apparently contradictory reports and underscoring the importance of specimen source and disease stage in biomarker interpretation. We further contrast the etiology-specific regulation of miR-122 in HBV- versus HCV-associated disease, in which epigenetic silencing and interferon-linked mechanisms drive divergent expression. Finally, we critically appraise the failed clinical translation of anti-miR-122 therapeutics (miravirsen, RG-101), highlighting viral resistance, safety liabilities, and the tumor-suppressor paradox that constrains inhibition-based strategies. Collectively, miR-122 emerges as a minimally invasive biomarker and a biologically informative, though therapeutically challenging, target in liver disease.