Yuhong Zou

Four waves of hepatocyte proliferation linked with three waves of hepatic fat accumulation during partial hepatectomy-induced liver regeneration.

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. Conclusion 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.

Nrf2 is involved in maintaining hepatocyte identity during liver regeneration.

Nrf2, a central regulator of the cellular defense against oxidative stress and inflammation, participates in modulating hepatocyte proliferation during liver regeneration. It is not clear, however, whether Nrf2 regulates hepatocyte growth, an important cellular mechanism to regain the lost liver mass after partial hepatectomy (PH). To determine this, various analyses were performed in wild-type and Nrf2-null mice following PH. We found that, at 60 h post-PH, the vast majority of hepatocytes lacking Nrf2 reduced their sizes, activated hepatic progenitor markers (CD133, TWEAK receptor, and trefoil factor family 3), depleted HNF4α protein, and downregulated the expression of a group of genes critical for their functions. Thus, the identity of hepatocytes deficient in Nrf2 was transiently but massively impaired in response to liver mass loss. This event was associated with the coupling of protein depletion of hepatic HNF4α, a master regulator of hepatocyte differentiation, and concomitant inactivation of hepatic Akt1 and p70S6K, critical hepatocyte growth signaling molecules. We conclude that Nrf2 participates in maintaining newly regenerated hepatocytes in a fully differentiated state by ensuring proper regulation of HNF4α, Akt1, and p70S6K during liver regeneration.

Nrf2 participates in regulating maternal hepatic adaptations to pregnancy.

Summary Pregnancy induces widespread adaptive responses in maternal organ systems including the liver. The maternal liver exhibits significant growth by increasing the number and size of hepatocytes, by largely unknown mechanisms. Nrf2 mediates cellular defense against oxidative stress and inflammation and also regulates liver regeneration. To determine whether Nrf2 is involved in the regulation of maternal hepatic adaptations to pregnancy, we assessed the proliferation and size of maternal hepatocytes and the associated molecular events in wild-type and Nrf2-null mice at various stages of gestation. We found that wild-type maternal hepatocytes underwent proliferation and size reduction during the first half, and size increase without overt replication during the second half, of pregnancy. Although pregnancy decreased Nrf2 activity in the maternal liver, Nrf2 deficiency caused a delay in maternal hepatocyte proliferation, concomitant with dysregulation of the activation of Cyclin D1, E1, and, more significantly, A2. Remarkably, as a result of Nrf2 absence, the maternal hepatocytes were largely prevented from reducing their sizes during the first half of pregnancy, which was associated with an increase in mTOR activation. During the second half of pregnancy, maternal hepatocytes of both genotypes showed continuous volume increase accompanied by persistent activation of mTOR. However, the lack of Nrf2 resulted in dysregulation of the activation of the mTOR upstream regulator AKT1 and the mTOR target p70SK6 and thus disruption of the AKT1/mTOR/p70S6K pathway, which is known to control cell size. This suggests an mTOR-dependent and AKT1- and p70S6K-independent compensatory mechanism when Nrf2 is deficient. In summary, our study demonstrates that Nrf2 is required for normal maternal hepatic adjustments to pregnancy by ensuring proper regulation of the number and size of maternal hepatocytes.

Proteomic analysis of immediate-early response plasma proteins after 70% and 90% partial hepatectomy.

Partial hepatectomy (PH) induces robust hepatic regenerative and metabolic responses that are considered to be triggered by humoral factors. The aim of the study was to identify plasma protein factors that potentially trigger or reflect the bodys immediate‐early responses to liver mass reduction.

Nuclear Factor Erythroid 2–Related Factor 2 Deficiency Results in Amplification of the Liver Fat-Lowering Effect of Estrogen

Transcription factor nuclear factor erythroid 2–related factor 2 (Nrf2) regulates multiple biologic processes, including hepatic lipid metabolism. Estrogen exerts actions affecting energy homeostasis, including a liver fat-lowering effect. Increasing evidence indicates the crosstalk between these two molecules. The aim of this study was to evaluate whether Nrf2 modulates estrogen signaling in hepatic lipid metabolism. Nonalcoholic fatty liver disease (NAFLD) was induced in wild-type and Nrf2-null mice fed a high-fat diet and the liver fat-lowering effect of exogenous estrogen was subsequently assessed. We found that exogenous estrogen eliminated 49% and 90% of hepatic triglycerides in wild-type and Nrf2-null mice with NAFLD, respectively. This observation demonstrates that Nrf2 signaling is antagonistic to estrogen signaling in hepatic fat metabolism; thus, Nrf2 absence results in striking amplification of the liver fat-lowering effect of estrogen. In addition, we found the association of trefoil factor 3 and fatty acid binding protein 5 with the liver fat-lowering effect of estrogen. In summary, we identified Nrf2 as a novel and potent inhibitor of estrogen signaling in hepatic lipid metabolism. Our finding may provide a potential strategy to treat NAFLD by dually targeting Nrf2 and estrogen signaling.