Background and Aims Non-dividing hepatocytes in end-stage liver disease indicates permanent growth arrest similar to senescence. Identifying senescence in vivo is often challenging and mechanisms inhibiting senescence are poorly understood. In lower organisms mitochondrial unfolded protein response (UPR MT ) helps in increasing longevity; however, its role in senescence and liver disease is poorly understood. Aim of this study was to identify hepatocyte senescence and the role of UPR MT in cryptogenic cirrhosis. Methods Doxorubicin was used to induce senescence in non-neoplastic hepatocytes (PH5CH8) and hepatoma cells (HepG2 and Huh7). Senescence-associated markers and unfolded protein response was evaluated by fluorescence microscopy, immunoblotting and gene expression. Explants/biopsies from normal, fibrosis, compensated and decompensated cirrhosis without any known etiology were examined for presence of senescence and UPR MT by immunohistochemistry and gene expression. Results Accumulation of senescent hepatocytes in cryptogenic cirrhosis was associated with reduced proliferation, increased expression of γH2AX and p21, together with loss of LaminB1. Dysfunctional mitochondria and compromised UPR MT were key features of senescent hepatocytes both in vitro and also in decompensated cirrhosis. Intriguingly, compensated cirrhotic liver mounted strong UPR MT , with high levels of mitochondrial protease, CLPP. Overexpression of CLPP inhibited senescence in vitro , by reducing mitochondrial ROS and altering oxygen consumption. Conclusions Our results implicate a role of hepatocyte senescence in cryptogenic cirrhosis together with a crucial role of UPR MT in preventing hepatocyte senescence. A compromised UPR MT may shift the fate of cirrhotic liver toward decompensation by exaggerating hepatocyte senescence. Restoring CLPP levels at least in cell culture appears as a promising strategy in mitohormesis, thereby, preventing senescence and possibly improving hepatocyte function.

In multicellular organisms majority of the cells remain in a non-dividing states of either quiescence (reversible) or senescence (irreversible). In the present study, gene expression signatures unique to quiescence and senescence were identified using microarray in osteosarcoma cell line, U2OS. It was noted that certain genes and pathways like NOD pathway was shared by both the growth arrest conditions. A major highlight of the present study was increased expression of number of chemokines and cytokines in both quiescence and senescence. While senescence-associated secretory phenotype (SASP) is well known, the quiescence-associated secretory phenotype (QASP) is relatively unknown and appeared novel in this study. ARID5A, a subunit of SWI/SNF complex was identified as a quiescence associated gene. The endogenous expression of ARID5A increased during serum starved condition of quiescence. Overexpression of ARID5A resulted in more number of cells in G0/G1 phase of cell cycle. Further ARID5A overexpressing cells when subjected to serum starvation showed a pronounced secretory phenotype. Overall, the present work has identified gene expression signatures which can distinguish quiescence from senescence.

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Presence of dysfunctional senescent hepatocyte is a hallmark feature of cirrhosis. We now report the presence of senescent hepatocytes (p21 and p53 positive) in vicinity of infiltrated immune cells in hepatocellular carcinoma. Hence, we checked if senescent cells can alter fate of macrophage polarization and neutrophil extracellular trap (NETs) formation. Using an in vitro assay, senescence was induced in hepatoma cells (HepG2 and Huh7 cells) by doxorubicin treatment and senescent cell showed secretory phenotype with strong expression of cytokines (IL1β, IL6, IL8 and IL13) as evaluated by Flow cytometry. The senescent secretome from hepatoma cells induced macrophage differentiation predominantly with M2 markers (CD80, CD86) while that of non-senescent cell induced M1 phenotype (CD163, CD206) as analysed by flow cytometry. Human hepatocellular carcinoma harbouring senescent hepatocytes showed presence of M2 macrophages, while M1 macrophages were predominant in non-tumorous region. Additionally, the senescent secretome from Huh7 cells (p53mut) enhanced the NETs formation, while HepG2 (p53+/+) secretome had an inhibitory affect In conclusion, the “pro-inflammatory” senescent secretome drives non-inflammatory type M2 macrophage polarization and modulate neutrophil traps thereby modulating the microenvironment towards tumor promotion. Targeting senescent hepatocyte secretome appears a promising therapeutic target in liver cancer in future.Abstract FigureWork Highlight (Diagrammatic Representation).