The biphasic nature of hypoxia-induced directional migration of activated human hepatic stellate cells.

Liver fibrogenesis is sustained by pro-fibrogenic myofibroblast-like cells (MFs), mainly originating from activated hepatic stellate cells (HSC/MFs) or portal (myo)fibroblasts, and is favoured by hypoxia-dependent angiogenesis. Human HSC/MFs were reported to express vascular-endothelial growth factor (VEGF) and VEGF-receptor type 2 and to migrate under hypoxic conditions. This study was designed to investigate early and delayed signalling mechanisms involved in hypoxia-induced migration of human HSC/MFs. Signal transduction pathways and intracellular generation of reactive oxygen species (ROS) were evaluated by integrating morphological, cell, and molecular biology techniques. Non-oriented and oriented migration were evaluated by using wound healing assay and the modified Boyden's chamber assay, respectively. The data indicate that hypoxia-induced migration of HSC/MFs is a biphasic process characterized by the following sequence of events: (a) an early (15 min) and mitochondria-related increased generation of intracellular ROS which (b) was sufficient to switch on activation of ERK1/2 and JNK1/2 that were responsible for the early phase of oriented migration; (c) a delayed and HIF-1α-dependent increase in VEGF expression (facilitated by ROS) and its progressive, time-dependent release in the extracellular medium that (d) was mainly responsible for sustained migration of HSC/MFs. Finally, immunohistochemistry performed on HCV-related fibrotic/cirrhotic livers revealed HIF-2α and haem-oxygenase-1 positivity in hepatocytes and α-SMA-positive MFs, indicating that MFs were likely to be exposed in vivo to both hypoxia and oxidative stress. In conclusion, hypoxia-induced migration of HSC/MFs involves an early, mitochondrial-dependent ROS-mediated activation of ERK and JNK, followed by a delayed- and HIF-1α-dependent up-regulation and release of VEGF.

Liver fibrosis: a dynamic and potentially reversible process.

In any chronic liver disease (CLDs), whatever the aetiology, reiteration of liver injury results in persisting inflammation and progressive fibrogenesis, with chronic activation of the wound healing response in CLDs, representing a major driving force for progressive accumulation of ECM components, eventually leading to liver cirrhosis. Cirrhosis is characterized by fibrous septa dividing the hepatic parenchyma into regenerative pseudo-lobules, as well as by extensive changes in vascular architecture, the development of portal hypertension and related complications. Liver fibrogenesis (i.e., the dynamic process leading to increased deposition of ECM and much more) can lead to different patterns of fibrosis and is sustained by myofibroblast-like cells (MFs) of different origin, with activated hepatic stellate cells (HSC/MFs) being the major cell type involved. Major pro-fibrogenic mechanisms also include oxidative stress, as well as derangement of epithelial-mesenchymal interactions and, as recently suggested, the process of epithelial to mesenchymal transition (EMT). Liver fibrosis has been considered traditionally as an irreversible process but experimental and clinical literature data published in the last decade have suggested that both the removal of the aetiological agent or condition, as well as an effective therapy, can result in significant regression of liver fibrosis. This is usually associated, particularly in animal models, with induction of apoptosis in MFs but, unfortunately, human HSC/MFs are much more resistant to apoptosis than murine MFs. However, clinical studies provided no unequivocal evidence for a complete reversal of cirrhosis or a significant reversal of vascular changes in conditions of established cirrhosis.

Epithelial-mesenchymal transition: from molecular mechanisms, redox regulation to implications in human health and disease.

Epithelial to mesenchymal transition (EMT) is a fundamental process, paradigmatic of the concept of cell plasticity, that leads epithelial cells to lose their polarization and specialized junctional structures, to undergo cytoskeleton reorganization, and to acquire morphological and functional features of mesenchymal-like cells. Although EMT has been originally described in embryonic development, where cell migration and tissue remodeling have a primary role in regulating morphogenesis in multicellular organisms, recent literature has provided evidence suggesting that the EMT process is a more general biological process that is also involved in several pathophysiological conditions, including cancer progression and organ fibrosis. This review offers first a comprehensive introduction to describe major relevant features of EMT, followed by sections dedicated on those signaling mechanisms that are known to regulate or affect the process, including the recently proposed role for oxidative stress and reactive oxygen species (ROS). Current literature data involving EMT in both physiological conditions (i.e., embryogenesis) and major human diseases are then critically analyzed, with a special final focus on the emerging role of hypoxia as a relevant independent condition able to trigger EMT.

Hepatic myofibroblasts: a heterogeneous population of multifunctional cells in liver fibrogenesis.

Hepatic myofibroblasts constitute a heterogeneous population of highly proliferative, pro-fibrogenic, pro-inflammatory, pro-angiogenic and contractile cells that sustain liver fibrogenesis and then fibrotic progression of chronic liver diseases of different aetiology to the common advanced-stage of cirrhosis. These alpha-smooth muscle actin-positive myofibroblast-like cells, according to current literature, mainly originate by a process of activation and trans-differentiation that involves either hepatic stellate cells or fibroblasts of portal areas. Hepatic myofibroblasts can also originate from bone marrow-derived cells, including mesenchymal stem cells or circulating fibrocytes able to engraft chronically injured liver, as well as, in certain conditions, by a process of epithelial to mesenchymal transition involving hepatocytes and cholangiocytes. Hepatic myofibroblasts may have also additional crucial roles in modulating immune response and in the cross talk with hepatic progenitor (stem) cells as well as with malignant cells of either primary hepatocellular carcinomas or of metastatic cancers.