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Liver regeneration and degeneration are the ying and yang forces operating in liver following any injury, which ultimately determines the fate towards survival or failure leading to death. The various liver assaults such as attack by viruses, alcohol, fat deposition, autoimmune disorders all lead to degenerative changes in the hepatocytes leading to their exhaustion. Hence, it is extremely crucial to understand the mechanisms behind hepatocyte degeneration so that therapeutic interventions can be designed to prevent the degenerative changes. Broadly, the degenerative changes include cell death such as apotosis, necrosis and permanent growth arrest viz., cellular senescence.
Apoptosis: A prevalent mechanism to study liver injury
Apoptosis is the major mode of cell death in chronic viral hepatitis, though it is combined with necrosis (necroptosis) in cholestatic livers. Multiple signalling pathways can activate an intermixed regulated apoptotic network by involving host, pathogen, and microenvironment; including ER stress, death receptor-mediated cascade, ROS generation, and mitochondrial dysfunction, whereas necrosis is an acute, sudden and uncontrolled. The pathophysiologic role of apoptosis is complicated by contradictory evidences where lack of apoptosis or increased liver apoptosis may both result in hepatocellular cancer depending on the liver microenvironment. So the understanding of liver regeneration with both anti-apoptosis and pro-apoptosis is vital and beneficial for studying the liver repair responses. Even during acute liver insult the liver microenvironments are exposed to instant high levels of proinflammatory cytokines such as TNF-α, IL-1β, INF-ï§, caused by host-derived (‘DAMPs’) or pathogen-derived (‘PAMPs’) danger signals (e.g. LPS). Pro- and anti-apoptotic activities within a particular cell decide whether the balance will tip to one side or the other during liver injury. Inflection of cell specific apoptotic pathways may offer an effective way to protect against hepatic injury and HCC development by reducing the pressure for clonal expansion. Last year we uncovered the long term controversy on the ambivalent role of the pro-apoptotic BH3-only protein Bid in tumorigenesis (Biswas S et al. Cell Death Differ 2013; 20: 869–877, Editorial Choice) and PUMA in carcinogen (i.e. DEN; diethylnitrosamine)-driven liver cancer model of mice. It is also well established that in mammals, the liver serves as the main hematopoietic organ during a considerable period of prenatal ontogeny. In context of liver environment, the mechanisms of hepatic degeneration also comprised of a combination of cell-intrinsic and -extrinsic causes that ultimately alter the function of monocytes, macrophages and other mature blood lineages. Understanding the basis of on cell cycle and apoptotic pathways that modulate the hematopoietic stem and progenitor cells (HSPCs) accompanied with a progressive alteration of vasculature in bone marrow during development of fibrosis/ cirrhosis is important for new therapeutic resolution in the liver disease progression.
Non-apoptotic cell death pathways
Besides understanding the apoptotic mechanisms, another area of interest is with regard to non-apopotic cell death mechanisms. In this regard a recent work from our group ( Babul et al, 2014) has shown cyclophilin B inhibition mediates a non-apoptotic form of alternate cell death viz., paraptosis. An attempt is being made to exploit the caspase independent cell death mechanisms for eliminating the cancer cells.
In context of liver, a degenerative feature such as cellular senescence not only acts as a barrier for regeneration but also to cancer progression (Ramakrishna et al, 2012, 2013). Hence cellular senescence has both beneficial and detrimental effects. Currently, studies are underway to examine the cytopathological alterations associated with senescence to develop new cellular markers to identify hepatocyte senescence especially in vivo. Additionally senescent cell appears to be metabolically active and we are trying to understand its cell autonomous and non-cell-autonomous functions in context of liver injury.