Autophagy is important for liver homeostasis, and the deficiency leads to injury, inflammation, ductular reaction (DR), fibrosis, and tumorigenesis. It is not clear how these events are mechanistically linked to autophagy deficiency. Here, we reveal the role of high-mobility group box 1 (HMGB1) in two of these processes. First, HMGB1 was required for DR, which represents the expansion of hepatic progenitor cells (HPCs) implicated in liver repair and regeneration. DR caused by hepatotoxic diets (3,5-diethoxycarbonyl-1,4-dihydrocollidine [DDC] or choline-deficient, ethionine-supplemented [CDE]) also depended on HMGB1, indicating that HMGB1 may be generally required for DR in various injury scenarios. Second, HMGB1 promoted tumor progression in autophagy-deficient livers. Receptor for advanced glycation end product (RAGE), a receptor for HMGB1, was required in the same two processes and could mediate the proliferative effects of HMBG1 in isolated HPCs. HMGB1 was released from autophagy-deficient hepatocytes independently of cellular injury but depended on NRF2 and the inflammasome, which was activated by NRF2. Pharmacological or genetic activation of NRF2 alone, without disabling autophagy or causing injury, was sufficient to cause inflammasome-dependent HMGB1 release. In conclusion, HMGB1 release is a critical mechanism in hepatic pathogenesis under autophagy-deficient conditions and leads to HPC expansion as well as tumor progression.

Partial hepatectomy (PH) triggers hepatocyte proliferation–mediated liver repair and is widely used to study the mechanisms governing liver regeneration in mice. However, the dynamics of the hepatocyte proliferative response to PH remain unclear. We found that PH-induced mouse liver regrowth was driven by four consecutive waves of hepatocyte replication. The first wave exhibited the highest magnitude followed by two moderate waves and one minor wave. Underlying this continuous hepatocyte replication was persistent activation of cell cycle components throughout the period of liver regeneration. Hepatocyte mitotic activity in the first three proliferative cycles showed a circadian rhythm manifested by three corresponding mitosis peaks, which were always observed at Zeitgeber time 0. The Bmal1-Clock/Wee1/Cdc2 pathway has been proposed by others to govern the circadian rhythm of hepatocyte mitosis during liver regeneration. However, we did not observe the correlations in the expression or phosphorylation of these proteins in regenerating livers. Notably, Bmal1 protein displayed frequent changes in hepatic distribution and cellular localization as the liver regrowth progressed. Further, three waves of hepatic fat accumulation occurred during hepatic regeneration. The first started before and lasted through the first round of hepatocyte proliferation, whereas the second and third occurred concomitantly with the second and third mitotic peaks, respectively. PH-induced liver regeneration consists of four continuous waves of hepatocyte proliferation coupled with three waves of hepatic fat accumulation. Bmal1, Wee1, and Cdc2 may not form a pathway regulating the circadian rhythm of hepatocyte mitosis during liver regeneration.

Pregnancy is characterized by physiological adjustments in the maternal compartment. In this investigation, the influence of pregnancy on maternal liver was examined in CD-1 mice. Dramatic changes were observed in the size of the maternal liver during pregnancy. Livers doubled in weight from the non-pregnant state to day 18 of pregnancy. The pregnancy-induced hepatomegaly was a physiological event of liver growth confirmed by DNA content increase and detection of hepatocyte hyperplasia and hypertrophy. Growth of the liver was initiated following implantation and peaked at parturition. The expression and/or activities of key genes known to regulate liver regeneration, a phenomenon of liver growth compensatory to liver mass loss, were investigated. The results showed that pregnancy-dependent liver growth was associated with interleukin (IL)-6, tumor necrosis factor α, c-Jun and IL-1 β, but independent of hepatocyte growth factor, fibroblast growth factor 1, tumor necrosis factor receptor 1, constitutive androstane receptor and pregnane X receptor. Furthermore, maternal liver growth was associated with the activation of hepatic signal transducer and activator of transcription 3, β-catenin and epidermal growth factor receptor, but pregnancy did not activate hepatic c-Met. The findings suggest that the molecular mechanisms regulating pregnancy-induced liver growth and injury-induced liver regeneration exhibit overlapping features but are not identical. In summary, the liver of the mouse adapts to the demands of pregnancy via a dramatic growth response driven by hepatocyte proliferation and size increase.

The transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2) regulates various cellular activities, including redox balance, detoxification, metabolism, autophagy, proliferation, and apoptosis. Several studies have demonstrated that Nrf2 regulates hepatocyte proliferation during liver regeneration. The aim of this study was to investigate how Nrf2 modulates the cell cycle of replicating hepatocytes in regenerating livers. Wild-type and Nrf2 null mice were subjected to 2/3 partial hepatectomy (PH) and killed at multiple time points for various analyses. Nrf2 null mice exhibited delayed liver regrowth, although the lost liver mass was eventually restored 7 days after PH. Nrf2 deficiency did not affect the number of hepatocytes entering the cell cycle but did delay hepatocyte mitosis. Mechanistically, the lack of Nrf2 resulted in increased mRNA and protein levels of hepatic cyclin A2 when the remaining hepatocytes were replicating in response to PH. Moreover, Nrf2 deficiency in regenerating livers caused dysregulation of Wee1, Cdc2, and cyclin B1 mRNA and protein expression, leading to decreased Cdc2 activity. Thus, Nrf2 is required for timely M phase entry of replicating hepatocytes by ensuring proper regulation of cyclin A2 and the Wee1/Cdc2/cyclin B1 pathway during liver regeneration.