Mechanisms of hepatic stellate cell activation

Activation of hepatic stellate cells (HSCs) in liver injury is the primary driver of hepatic fibrosis. In this Review, Tsuchida and Friedman detail the varied intracellular and extracellular signalling pathways leading to HSC activation, as well as the role of HSCs in liver fibrosis resolution and as therapeutic targets. Hepatic fibrosis is a dynamic process characterized by the net accumulation of extracellular matrix resulting from chronic liver injury of any aetiology, including viral infection, alcoholic liver disease and NASH. Activation of hepatic stellate cells (HSCs) — transdifferentiation of quiescent, vitamin-A-storing cells into proliferative, fibrogenic myofibroblasts — is now well established as a central driver of fibrosis in experimental and human liver injury. Yet, the continued discovery of novel pathways and mediators, including autophagy, endoplasmic reticulum stress, oxidative stress, retinol and cholesterol metabolism, epigenetics and receptor-mediated signals, reveals the complexity of HSC activation. Extracellular signals from resident and inflammatory cells including macrophages, hepatocytes, liver sinusoidal endothelial cells, natural killer cells, natural killer T cells, platelets and B cells further modulate HSC activation. Finally, pathways of HSC clearance have been greatly clarified, and include apoptosis, senescence and reversion to an inactivated state. Collectively, these findings reinforce the remarkable complexity and plasticity of HSC activation, and underscore the value of clarifying its regulation in hopes of advancing the development of novel diagnostics and therapies for liver disease.