miR-122 Deficiency in Mice Enhances Regeneration in Healthy Liver but Drives Pathological Repair and Functional Decline in Fibrotic Liver

microRNA-122 (miR-122) is the most abundant hepatic microRNA and a key regulator of hepatocyte proliferation, metabolism and differentiation. Although widely studied in hepatocellular carcinoma, its role in liver regeneration remains unexplored. This study investigated how miR-122 deficiency modulates liver regeneration under physiological conditions and during chronic liver injury. A miR-122-deficient mouse model (miR-122−/−) was generated using CRISPR/Cas9, and liver regeneration was assessed after two-thirds partial hepatectomy (PHx) in healthy and CCl4-induced fibrotic livers. In healthy liver, miR-122 expression was transiently downregulated within 24 h after PHx, suggesting a physiological role in cell cycle entry. After PHx in non-fibrotic livers, miR-122−/− mice showed increased basal proliferation and accelerated regeneration, associated with Cyclin D1 and RhoA overexpression, enhanced cytokinesis and a predominance of diploid hepatocytes. In contrast, miR-122 deficiency markedly exacerbated CCl4-induced fibrosis, leading to cirrhosis-like architecture, impaired hepatocyte function, and severe metabolic dysregulation. Despite increased proliferation after PHx, fibrotic miR-122−/− mice exhibited severely impaired regeneration and near-complete mortality. Proteomic analyses revealed metabolic failure, oxidative stress, and inflammatory activation, creating an unfavorable environment for tissue repair. In conclusion, miR-122 plays a dual role in liver regeneration. While its suppression enhances regeneration in healthy liver, loss of miR-122 under fibrotic conditions drives pathological repair, metabolic failure and lethality, highlighting its critical role in chronic liver disease.