AMPKα1 controls hepatocyte proliferation independently of energy balance by regulating Cyclin A2 expression

Hepatic Snai1 and Snai2 promote liver regeneration and suppress liver fibrosis in mice

Liver injury stimulates hepatocyte replication and hepatic stellate cell (HSC) activation, thereby driving liver regeneration. Aberrant HSC activation induces liver fibrosis. However, mechanisms underlying liver regeneration and fibrosis remain poorly understood. Here, we identify hepatic Snai1 and Snai2 as important transcriptional regulators for liver regeneration and fibrosis. Partial hepatectomy or CCl4 treatment increases occupancies of Snai1 and Snai2 on cyclin A2 and D1 promoters in the liver. Snai1 and Snai2 in turn increase promoter H3K27 acetylation and cyclin A2/D1 expressions. Hepatocyte-specific deletion of both Snai1 and Snai2, but not one alone, suppresses liver cyclin A2/D1 expression and regenerative hepatocyte proliferation after hepatectomy or CCl4 treatments but augments CCl4-stimulated HSC activation and liver fibrosis. Conversely, Snai2 overexpression in the liver enhances hepatocyte replication and suppresses liver fibrosis after CCl4 treatment. These results suggest that hepatic Snai1 and Snai2 directly promote, via histone modifications, reparative hepatocyte replication and indirectly inhibit liver fibrosis.

Inhibition of TCA cycle improves the anti-PD-1 immunotherapy efficacy in melanoma cells via ATF3-mediated PD-L1 expression and glycolysis

BACKGROUND

anti-Programmed Death-1 (anti-PD-1) immunotherapy has shown promising manifestation in improving the survival rate of patients with advanced melanoma, with its efficacy closely linked to Programmed cell death-Ligand 1 (PD-L1) expression. However, low clinical efficacy and drug resistance remain major challenges. Although the metabolic alterations from tricarboxylic acid (TCA) cycle to glycolysis is a hallmark in cancer cells, accumulating evidence demonstrating TCA cycle plays critical roles in both tumorigenesis and treatment.

METHODS

The plasma levels of metabolites in patients with melanoma were measured by nuclear magnetic resonance (NMR) spectroscopy. The effect of pyruvate dehydrogenase subunit 1 (PDHA1) and oxoglutarate dehydrogenase (OGDH) on immunotherapy was performed by B16F10 tumor-bearing mice. Flow cytometry analyzed the immune microenvironment. RNA sequencing analyzed the global transcriptome alterations in CPI613-treated melanoma cells. The regulation of PD-L1 and glycolysis by PDHA1/OGDH-ATF3 signaling were confirmed by Quantitative real-time polymerase chain reaction (qRT-PCR), western blotting, dual-luciferase reporter gene, Chromatin immunoprecipitation (ChIP)-quantitative PCR and Seahorse assay. The relationship between PDHA1/OGDH-ATF3-glycolysis and the efficacy of melanoma anti-PD-1 immunotherapy was verified in the clinical database and single-cell RNA-seq (ScRNA-Seq).

RESULTS

In our study, the results showed that significant alterations in metabolites associated with glycolysis and the TCA cycle in plasma of patients with melanoma through NMR technique, and then, PDHA1 and OGDH, key enzymes for regulation TCA cycle, were remarkable raised in melanoma and negatively related to anti-PD-1 efficacy through clinical database analysis as well as ScRNA-Seq. Inhibition of PDHA1 and OGDH by either shRNA or pharmacological inhibitor by CPI613 dramatically attenuated melanoma progression as well as improved the therapeutic efficacy of anti-PD-1 against melanoma. Most importantly, suppression of TCA cycle remarkably raises PD-L1 expression and glycolysis flux through AMPK-CREB-ATF3 signaling.

CONCLUSIONS

Taken together, our results demonstrated the role of TCA cycle in immune checkpoint blockade and provided a novel combination strategy for anti-PD-1 immunotherapy in melanoma treatment.

Hepatocyte β-catenin loss is compensated by Insulin-mTORC1 activation to promote liver regeneration

BACKGROUND AND AIMS

Liver regeneration (LR) following partial hepatectomy (PH) occurs via activation of various signaling pathways. Disruption of a single pathway can be compensated by activation of another pathway to continue LR. The Wnt-β-catenin pathway is activated early during LR and conditional hepatocyte loss of β-catenin delays LR. Here, we study mechanism of LR in the absence of hepatocyte-β-catenin.

APPROACH AND RESULTS

Eight-week-old hepatocyte-specific Ctnnb1 knockout mice (β-catenin ΔHC ) were subjected to PH. These animals exhibited decreased hepatocyte proliferation at 40-120 h and decreased cumulative 14-day BrdU labeling of <40%, but all mice survived, suggesting compensation. Insulin-mediated mechanistic target of rapamycin (mTOR) complex 1 (mTORC1) activation was uniquely identified in the β-catenin ΔHC mice at 72-96 h after PH. Deletion of hepatocyte regulatory-associated protein of mTOR (Raptor), a critical mTORC1 partner, in the β-catenin ΔHC mice led to progressive hepatic injury and mortality by 30 dys. PH on early stage nonmorbid Raptor ΔHC -β-catenin ΔHC mice led to lethality by 12 h. Raptor ΔHC mice showed progressive hepatic injury and spontaneous LR with β-catenin activation but died by 40 days. PH on early stage nonmorbid Raptor ΔHC mice was lethal by 48 h. Temporal inhibition of insulin receptor and mTORC1 in β-catenin ΔHC or controls after PH was achieved by administration of linsitinib at 48 h or rapamycin at 60 h post-PH and completely prevented LR leading to lethality by 12-14 days.

CONCLUSIONS

Insulin-mTORC1 activation compensates for β-catenin loss to enable LR after PH. mTORC1 signaling in hepatocytes itself is critical to both homeostasis and LR and is only partially compensated by β-catenin activation. Dual inhibition of β-catenin and mTOR may have notable untoward hepatotoxic side effects.

The Pivotal Mediating Role of Adenosine Monophosphate-Activated Protein Kinase (AMPK) in Liver Tight Junctions and Liver Regeneration of a Partial-Hepatectomy Mouse Model

PURPOSE

This study aims to explore the pivotal mediating role of adenosine monophosphate-activated protein kinase (AMPK) in liver tight junctions and liver regeneration of a partial hepatectomy (PH) mouse model.

METHODS

A 70% PH mouse model was used. Firstly, mice were randomly divided into sham, 70% PH, AMPK-activated, and AMPK-inhibited groups. Then serum levels of alanine aminotransferase, aspartate transaminase, total bilirubin, direct bilirubin, albumin, and prealbumin were tested on postoperative days 1, 2 and 3. Furthermore, the expression of tight junction proteins like occludin, claudin-3, and ZO-1, together with bile salt export pump (BSEP), which reflects liver function, and AMPK were measured by Western blot and quantitative real-time polymerase chain reaction. Moreover, the expression of tight junction proteins, BSEP, and Ki-67 were examined by immunohistochemistry.

RESULTS

After 70% PH, without intervention, the changes in expression of hepatic tight junction proteins (occludin, claudin-3, and ZO-1) were consistent with that of BSEP, which could reflect liver function. After treatment with AMPK activator, the high expression status of tight junction proteins occurred in advance and was maintained stably and for a longer time. It was beneficial to liver function and liver regeneration was promoted at early periods and enhanced continuously after PH.

CONCLUSIONS

Activation of AMPK could effectively enhance the expression of hepatic tight junction proteins after PH. Therefore, it could speed up the recovery of liver function and promote liver regeneration especially early after PH.

Proteomics Indicates Lactate Dehydrogenase Is Prognostic in Acetaminophen-Induced Acute Liver Failure Patients and Reveals Altered Signaling Pathways

Better biomarkers to predict death early in acute liver failure (ALF) are needed. To that end, we obtained early (study day 1) and later (day 3) serum samples from transplant-free survivors (n = 28) and nonsurvivors (n = 30) of acetaminophen-induced ALF from the NIH-sponsored Acute Liver Failure Study Group and from control volunteers (n = 10). To identify proteins that increase early in serum during ALF, we selected individuals from this cohort for whom alanine aminotransferase was lower on day 1 than day 3, indicating a time point before peak injury (n = 10/group). We then performed untargeted proteomics on their day 1 samples. Out of 1682 quantifiable proteins, 361 were ≥ 4-fold elevated or decreased in ALF patients versus controls and 16 of those were further elevated or decreased ≥ 4-fold in nonsurvivors versus survivors, indicating potential to predict death. Interestingly, 1 of the biomarkers was lactate dehydrogenase (LDH), which is already measured in most clinical laboratories. To validate our proteomics results and to confirm the prognostic potential of LDH, we measured LDH activity in all day 1 and 3 samples from all 58 ALF patients. LDH was elevated in the nonsurvivors versus survivors on both days. In addition, it had prognostic value similar to the model for end-stage liver disease and outperformed the King's College Criteria, while a combination of model for end-stage liver disease and LDH together outperformed either alone. Finally, bioinformatics analysis of our proteomics data revealed alteration of numerous signaling pathways that may be important in liver regeneration. Overall, we conclude LDH can predict death in APAP-induced ALF.

Ferulic Acid Ameliorates Hepatic Inflammation and Fibrotic Liver Injury by Inhibiting PTP1B Activity and Subsequent Promoting AMPK Phosphorylation

Chronic inflammation in response to persistent exogenous stimuli or damage results in liver fibrosis, which subsequently progresses into malignant liver diseases with high morbidity and mortality. Ferulic acid (FA) is a phenolic acid widely isolated from abundant plants and exhibits multiple biological activities including anti-oxidant, anti-inflammation and enhancement of immune responses. Adenosine monophosphate-activated protein kinase (AMPK) functions as a critical energy sensor and is regulated through the phosphorylation of liver kinases like LKB1 or dephosphorylation by protein tyrosine phosphatases (PTPs). However, the role of FA in carbon tetrachloride (CCl₄)-induced chronic inflammation and liver fibrosis and AMPK activation has not been elucidated. Here we reported that FA ameliorated CCl₄-induced inflammation and fibrotic liver damage in mice as indicated by reduced levels of serum liver function enzyme activities and decreased expression of genes and proteins associated with fibrogenesis. Additionally, FA inhibited hepatic oxidative stress, macrophage activation and HSC activation via AMPK phosphorylation in different liver cells. Mechanically, without the participation of LKB1, FA-induced anti-inflammatory and anti-fibrotic effects were abrogated by a specific AMPK inhibitor, compound C. Combining with the results of molecular docking, surface plasmon resonance and co-immunoprecipitation assays, we further demonstrated that FA directly bound to and inhibited PTP1B, an enzyme responsible for dephosphorylating key protein kinases, and eventually leading to the phosphorylation of AMPK. In summary, our results indicated that FA alleviated oxidative stress, hepatic inflammation and fibrotic response in livers through PTP1B-AMPK signaling pathways. Taken together, we provide novel insights into the potential of FA as a natural product-derived therapeutic agent for the treatment of fibrotic liver injury.

Acidosis-induced activation of distal nephron principal cells triggers Gdf15 secretion and adaptive proliferation of intercalated cells

AIM

Type A intercalated cells of the renal collecting duct participate in the maintenance of the acid/base balance through their capacity to adapt proton secretion to homeostatic requirements. We previously showed that increased proton secretion stems in part from the enlargement of the population of proton secreting cells in the outer medullary collecting duct through division of fully differentiated cells, and that this response is triggered by growth/differentiation factor 15. This study aimed at deciphering the mechanism of acid load-induced secretion of Gdf15 and its mechanism of action.

METHODS

We developed an original method to evaluate the proliferation of intercalated cells and applied it to genetically modified or pharmacologically treated mice under basal and acid-loaded conditions.

RESULTS

Gdf15 is secreted by principal cells of the collecting duct in response to the stimulation of vasopressin receptors. Vasopressin-induced production of cAMP triggers activation of AMP-stimulated kinases and of Na,K-ATPase, and induction of p53 and Gdf15. Gdf15 action on intercalated cells is mediated by ErbB2 receptors, the activation of which triggers the expression of cyclin d1, of p53 and anti-proliferative genes, and of Egr1.

CONCLUSION

Acidosis-induced proliferation of intercalated cells results from a cross talk with principal cells which secrete Gdf15 in response to their stimulation by vasopressin. Thus, vasopressin is a major determinant of the collecting duct cellular homeostasis as it promotes proliferation of intercalated cells under acidosis conditions and of principal cells under normal acid-base status.

Altered metabolism by autophagy defection affect liver regeneration

Autophagy is the primary intracellular catabolic process for degrading and recycling long-lived proteins and damaged organelles, which maintains cellular homeostasis. Autophagy has key roles in development and differentiation. By using the mouse with liver specific knockout of autophagy related gene 5 (Atg5), a gene essential for autophagy, we investigated the possible role of autophagy in liver regeneration after 70% partial hepatectomy (PHx). Ablation of autophagy significantly impaired mouse liver regeneration, and this impairment was associated with reduced hepatocellular proliferation rate, down-regulated expression of cyclins and tumor suppressors, and increased hepatocellular apoptosis via the intrinsic apoptotic pathway. Ablation of autophagy does not affect IL-6 and TNF-α response after PHx, but the altered hepatic and systemic metabolic responses were observed in these mice, including reduced ATP and hepatic free fatty acid levels in the liver tissue, increased glucose level in the serum. Autophagy is required to promote hepatocellular proliferation by maintaining normal hepatic and systemic metabolism and suppress hepatocellular apoptosis in liver regeneration.

Hsa_circ_0003732 promotes osteosarcoma cells proliferation via miR-545/CCNA2 axis

Osteosarcoma (OS) is a primary malignant bone tumor with a high fatality rate. Circular RNAs (circRNAs) are a type of endogenous noncoding RNA that have been verified to participate in cancer pathophysiological processes. We aim to investigate the roles of circRNAs in osteosarcoma tumorigenesis. In the present study, we showed that hsa_circ_0003732 was up-regulated in OS tissues and elevated level of hsa_circ_0003732 was linked to poor prognosis of OS patients. Functional investigation indicated that hsa_circ_0003732 promoted proliferation of OS cells. Furthermore, we identified miR-545 as the hsa_circ_0003732-associated microRNA and CCNA2 was a direct target of miR-545. In addition, hsa_circ_0003732 could elevate CCNA2 expression via miR-545, resulting in the promotion of OS cells proliferation. Altogether, our findings demonstrate that hsa_circ_0003732 promotes OS cells proliferation via miR-545/CCNA2 axis and imply hsa_circ_0003732 may be a potential prognosis biomarker and therapeutic target for OS.

Mesenchymal stem cells ameliorate lipid metabolism through reducing mitochondrial damage of hepatocytes in the treatment of post-hepatectomy liver failure

Hepatectomy is an effective therapeutic strategy for many benign and malignant liver diseases, while the complexity of liver anatomy and the difficulty of operation lead to complications after hepatectomy. Among them, post-hepatectomy liver failure (PHLF) is the main factor threatening the life of patients. At present, liver transplantation is an effective approach for PHLF. However, the application of liver transplantation has been largely limited due to the shortage of donors and the high cost of such operation. Therefore, it is urgently necessary to develop a new treatment for PHLF. Mesenchymal stem cells (MSCs) have become a new treatment regimen for liver diseases because of their easy access and low immunogenicity. Our study found that there were some subtle connections between MSCs and liver lipid metabolism in the PHLF model. We used MSC transplantation to treat PHLF induced by 90% hepatectomy. MSC transplantation could restore the mitochondrial function, promote the β-oxidation of fatty acid (FA), and reduce the lipid accumulation of hepatocytes. In addition, interleukin 10 (IL-10), a cytokine with immunoregulatory function, had an important role in lipid metabolism. We also found that MSCs transplantation activated the mammalian target of rapamycin (mTOR) pathway. Therefore, we explored the relationship between mitochondrial damage and lipid metabolism abnormality or PHLF. MSCs improved mitochondrial function and corrected abnormal lipid metabolism by affecting the mTOR pathway in the treatment of PHLF. Collectively, MSC transplantation could be used as a potential treatment for PHLF.

PP2Acα inhibits PFKFB2-induced glycolysis to promote termination of liver regeneration

The mechanisms underlying the initiation and proliferation of liver regeneration (LR) has been extensively studied using the partial hepatectomy (PHx) model, while little is known about the termination of LR. PP2Acα (protein phosphatase 2 A catalytic subunit α isoform) is the catalytic subunit of protein phosphatase 2 A (PP2A), accounting for most of intracellular serine/threonine phosphatase activity. We have previously observed that termination of LR delayed in PP2Acα liver-specific knockout (LKO) mice after PHx. In our study, we used phospho explorer antibody array analysis to screen the potential phosphorylation targets of PP2Acα, and PP2Acα had a great influence on the hepatic phosphoproteomic signaling in the termination of LR after PHx. We then tested the phosphorylation changes and metabolic function of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase-2 (PFKFB2), an isoform of the key glycolytic enzyme PFKFB, which was significantly regulated by PP2Acα knockout. PP2Acα knockout enhanced glycolysis in vivo and in vitro, while adenoviral-mediated RNAi of PFKFB2 reversed the extension of postoperative liver regeneration in KO mice along with the downregulation of glycolysis. Therefore, we demonstrated that PP2Acα liver-specific knockout regulated the hepatocytes glycolysis via activating PFKFB2, thus enhancing liver regeneration during the termination stage.

Exercise Improves Outcomes of Surgery on Fatty Liver in Mice: A Novel Effect Mediated by the AMPK Pathway

OBJECTIVE

To investigate whether exercise improves outcomes of surgery on fatty liver, and whether pharmacological approaches can substitute exercising programs.

SUMMARY OF BACKGROUND DATA

Steatosis is the hepatic manifestation of the metabolic syndrome, and decreases the liver's ability to handle inflammatory stress or to regenerate after tissue loss. Exercise activates adenosine monophosphate-activated kinase (AMPK) and mitigates steatosis; however, its impact on ischemia-reperfusion injury and regeneration is unknown.

METHODS

We used a mouse model of simple, diet-induced steatosis and assessed the impact of exercise on metabolic parameters, ischemia-reperfusion injury and regeneration after hepatectomy. The same parameters were evaluated after treatment of mice with the AMPK activator 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR). Mice on a control diet served as age-matched controls.

RESULTS

A 4-week-exercising program reversed steatosis, lowered insulin levels, and improved glucose tolerance. Exercise markedly enhanced the ischemic tolerance and the regenerative capacity of fatty liver. Replacing exercise with AICAR was sufficient to replicate the above benefits. Both exercise and AICAR improved survival after extended hepatectomy in mice challenged with a Western diet, indicating protection from resection-induced liver failure.

CONCLUSIONS

Exercise efficiently counteracts the metabolic, ischemic, and regenerative deficits of fatty liver. AICAR acts as an exercise mimetic in settings of fatty liver disease, an important finding given the compliance issues associated with exercise. Exercising, or its substitution through AICAR, may provide a feasible strategy to negate the hepatic consequences of energy-rich diet, and has the potential to extend the application of liver surgery if confirmed in humans.

Loss of AMPKalpha1 Triggers Centrosome Amplification via PLK4 Upregulation in Mouse Embryonic Fibroblasts

Recent evidence indicates that activation of adenosine monophosphate-activated protein kinase (AMPK), a highly conserved sensor and modulator of cellular energy and redox, regulates cell mitosis. However, the underlying molecular mechanisms for AMPKα subunit regulation of chromosome segregation remain poorly understood. This study aimed to ascertain if AMPKα1 deletion contributes to chromosome missegregation by elevating Polo-like kinase 4 (PLK4) expression. Centrosome proteins and aneuploidy were monitored in cultured mouse embryonic fibroblasts (MEFs) isolated from wild type (WT, C57BL/6J) or AMPKα1 homozygous deficient (AMPKα1^−/−) mice by Western blotting and metaphase chromosome spread. Deletion of AMPKα1, the predominant AMPKα isoform in immortalized MEFs, led to centrosome amplification and chromosome missegregation, as well as the consequent aneuploidy (34–66%) and micronucleus. Furthermore, AMPKα1 null cells exhibited a significant induction of PLK4. Knockdown of nuclear factor kappa B2/p52 ameliorated the PLK4 elevation in AMPKα1-deleted MEFs. Finally, PLK4 inhibition by Centrinone reversed centrosome amplification of AMPKα1-deleted MEFs. Taken together, our results suggest that AMPKα1 plays a fundamental role in the maintenance of chromosomal integrity through the control of p52-mediated transcription of PLK4, a trigger of centriole biogenesis.

Global Phosphoproteomic Analysis Reveals Significant Metabolic Reprogramming in the Termination of Liver Regeneration in Mice

Phosphorylation is crucial in regulating various biological processes. However, comprehensive phosphoproteomic profiling in the termination of liver regeneration (LR) is still missing. Here, we used Tandem Mass Tag (TMT) labeling coupled with phosphopeptide enrichment and two-dimensional (2D) liquid chromatography-mass spectrometry (LC-MS)/MS analysis to establish a global phosphoproteomic map in the liver of mice at day 5 after partial hepatectomy (PH). Altogether, 9731 phosphosites from 3443 proteins were identified and 7802 phosphosites from 2980 proteins were quantified. Motif analysis of the identified phosphosites revealed a diverse array of consensus sequences, suggesting that multiple kinase families including ERK/MAPK, PKA/PKC, CaMK-II, CKII, and CDK may be involved in the termination of LR. Functional clustering analysis of proteins with dysregulated phosphosites showed that they mainly participate in metabolic pathways, DNA replication, and tight junction. More importantly, the deletion of PP2Acα in the liver remarkably changes the overall phosphorylation profile, indicating its critical role in regulating the termination of LR. Finally, several differentially phosphorylated sites were validated by co-immunoprecipitation and Western blot. Taken together, our data unravel the first comprehensive phosphoproteomic map in the termination of LR in mice, which greatly expands our knowledge in the complicated regulation of this process and provides new directions for the treatment of liver cancer using liver resection.

The Inhibition of Aldose Reductase Accelerates Liver Regeneration through Regulating Energy Metabolism

Objectives

Our previous study showed that aldose reductase (AR) played key roles in fatty liver ischemia-reperfusion (IR) injury by regulating inflammatory response and energy metabolism. Here, we aim to investigate the role and mechanism of AR in the regeneration of normal and fatty livers after liver surgery.

Methods

The association of AR expression with liver regeneration was studied in the rat small-for-size liver transplantation model and the mice major hepatectomy and hepatic IR injury model with or without fatty change. The direct role and mechanism of AR in liver regeneration was explored in the AR knockout mouse model.

Results

Delayed regeneration was detected in fatty liver after liver surgery in both rat and mouse models. Furthermore, the expression of AR was increased in liver after liver surgery, especially in fatty liver. In a functional study, the knockout of AR promoted liver regeneration at day 2 after major hepatectomy and IR injury. Compared to wild-type groups, the expressions of cyclins were increased in normal and fatty livers of AR knockout mice. AR inhibition increased the expressions of PPAR-α and PPAR-γ in both normal liver and fatty liver groups after major hepatectomy and IR injury. In addition, the knockout of AR promoted the expressions of SDHB, AMPK, SIRT1, and PGC1-α and PPAR-

Conclusions

The knockout of AR promoted the regeneration of normal and fatty livers through regulating energy metabolism. AR may be a new potential therapeutic target to accelerate liver regeneration after surgery.

Therapeutic targets for liver regeneration after acute severe injury: a preclinical overview

Introduction: Liver transplantation is the only viable treatment with a proven survival benefit for acute liver failure (ALF). Donor organ shortage is, however, a major hurdle; hence, alternative approaches that enable liver regeneration and target acute severe hepatocellular damage are necessary.Areas covered: This article sheds light on therapeutic targets for liver regeneration and considers their therapeutic potential. ALF following extensive hepatocyte damage and small-for-size syndrome (SFSS) are illuminated for the reader while the molecular mechanisms of liver regeneration are assessed in accordance with relevant therapeutic strategies. Furthermore, liver background parameters and predictive biomarkers that might associate with liver regeneration are reviewed.Expert opinion: There are established and novel experimental strategies for liver regeneration to prevent ALF resulting from SFSS. Granulocyte-colony stimulating factor (G-CSF) is a promising agent targeting liver regeneration after acute severe injury. Autophagy and hepatocyte senescence represent attractive new targets for liver regeneration in acute severe hepatic injury. Liver support strategies, including tissue engineering, constitute novel regenerative means; the success of this is dependent on stem cell research advances. However, there is no firm clinical evidence that these supportive strategies may alleviate hepatocellular damage until liver transplantation becomes available or successful self-liver regeneration occurs.

LKB1 as a Gatekeeper of Hepatocyte Proliferation and Genomic Integrity during Liver Regeneration

Liver kinase B1 (LKB1) is involved in several biological processes and is a key regulator of hepatic metabolism and polarity. Here, we demonstrate that the master kinase LKB1 plays a dual role in liver regeneration, independently of its major target, AMP-activated protein kinase (AMPK). We found that the loss of hepatic Lkb1 expression promoted hepatocyte proliferation acceleration independently of metabolic/energetic balance. LKB1 regulates G₀/G₁ progression, specifically by controlling epidermal growth factor receptor (EGFR) signaling. Furthermore, later in regeneration, LKB1 controls mitotic fidelity. The deletion of Lkb1 results in major alterations to mitotic spindle formation along the polarity axis. Thus, LKB1 deficiency alters ploidy profile at late stages of regeneration. Our findings highlight the dual role of LKB1 in liver regeneration, as a guardian of hepatocyte proliferation and genomic integrity.

Role of the α2 subunit of AMP-activated protein kinase and its nuclear localization in mitochondria and energy metabolism-related gene expressions in C2C12 cells

BACKGROUND

AMP-activated protein kinase (AMPK), a heterotrimer with α1 or α2 catalytic subunits, acts as an energy sensor and regulates cellular homeostasis. Whereas AMPKα1 is necessary for myogenesis in skeletal muscle, the role of AMPKα2 in myogenic differentiation and energy metabolism-related gene expressions has remained unclear. We here examined the specific roles of AMPKα1 and AMPKα2 in the myogenic differentiation and mitochondria and energy metabolism-related gene expressions in C2C12 cells.

MATERIALS AND METHODS

Stable C2C12 cell lines expressing a scramble short hairpin RNA (shRNA) or shRNAs specific for AMPKα1 (shAMPKα1), AMPKα2 (shAMPKα2), or both AMPKα1 and AMPKα2 (shPanAMPK) were generated by lentivirus infection. Lentiviruses encoding wild-type AMPKα2 (WT-AMPKα2) or AMPKα2 with a mutated nuclear localization signal (ΔNLS-AMPKα2) were also constructed for introduction into myoblasts. Myogenesis was induced by culture of C2C12 myoblasts for 6 days in differentiation medium.

RESULTS

The amount of AMPKα2 increased progressively, whereas that of AMPKα1 remained constant, during the differentiation of myoblasts into myotubes. Expression of shPanAMPK or shAMPKα1, but not that of shAMPKα2, attenuated the proliferation of myoblasts as well as the phosphorylation of both acetyl-CoA carboxylase and the autophagy-initiating kinase ULK1 in myotubes. Up-regulation of myogenin mRNA, a marker for the middle stage of myogenesis, was attenuated in differentiating myotubes expressing shPanAMPK or shAMPKα1. In contrast, up-regulation of gene expression for muscle creatine kinase (MCK), a late-stage differentiation marker, as well as for genes related to mitochondrial biogenesis including the transcriptional coactivator peroxisome proliferator-activated receptor-γ coactivator-1α1 and α4 (PGC-1α1 and PGC-1α4) and mitochondria-specific genes such as cytochrome c were attenuated in myotubes expressing shAMPKα2 or shPanAMPK. The diameter of myotubes expressing shPanAMPK or shAMPKα2 was reduced, whereas that of those expressing shAMPKα1 was increased, compared with myotubes expressing scramble shRNA. A portion of AMPKα2 became localized to the nucleus during myogenic differentiation. The AMPK activator AICAR (5-aminoimidazole-4-carboxamide ribonucleotide) and 2-deoxyglucose (2DG) each induced the nuclear translocation of WT-AMPKα2, but not that of ΔNLS-AMPKα2. Finally, expression of WT-AMPKα2 increased the mRNA abundance of PGC-1α1 and MCK mRNAs as well as cell diameter and tended to increase that of PGC-1α4, whereas that of ΔNLS-AMPKα2 increased only the abundance of MCK mRNA, in myotubes depleted of endogenous AMPKα2.

CONCLUSION

TAMPKα1 and AMPKα2 have distinct roles in myogenic differentiation of C2C12 cells, with AMPKα1 contributing to the middle stage of myogenesis and AMPKα2 to the late stage. AMPKα2 regulates gene expressions including MCK, PGC-1α1 and PGC-1α4 and mitochondria-specific genes such as cytochrome c during the late stage of differentiation. Furthermore, the nuclear translocation of AMPKα2 is necessary for maintenance of PGC-1α1 mRNA during myogenesis.

AMP-Activated Protein Kinase (AMPK)-Dependent Regulation of Renal Transport

AMP-activated kinase (AMPK) is a serine/threonine kinase that is expressed in most cells and activated by a high cellular AMP/ATP ratio (indicating energy deficiency) or by Ca²⁺. In general, AMPK turns on energy-generating pathways (e.g., glucose uptake, glycolysis, fatty acid oxidation) and stops energy-consuming processes (e.g., lipogenesis, glycogenesis), thereby helping cells survive low energy states. The functional element of the kidney, the nephron, consists of the glomerulus, where the primary urine is filtered, and the proximal tubule, Henle's loop, the distal tubule, and the collecting duct. In the tubular system of the kidney, the composition of primary urine is modified by the reabsorption and secretion of ions and molecules to yield final excreted urine. The underlying membrane transport processes are mainly energy-consuming (active transport) and in some cases passive. Since active transport accounts for a large part of the cell's ATP demands, it is an important target for AMPK. Here, we review the AMPK-dependent regulation of membrane transport along nephron segments and discuss physiological and pathophysiological implications.

Insight on ALPPS - Associating Liver Partition and Portal Vein Ligation for Staged Hepatectomy - mechanisms: activation of mTOR pathway

BACKGROUND

ALPPS procedure has been introduced to increase the volume of future liver remnant. The mechanisms underlying the accelerated regeneration observed with ALPPS are unknown. It was hypothesized that AMPK/mTOR is activated as an integrating pathway for metabolic signals leading to proliferation and cell growth. Our aim was to analyze increase in liver volume, proliferation parameters and expression of AMPK/mTOR pathway-related molecules in patients undergoing ALPPS.

METHODS

A single center prospective study of patients undergoing ALPPS was performed from 2013 to 2015. Liver and serum samples, clinical laboratory results and CT-scan data were obtained. ELISA, Ki-67 immunostaining and qRT-PCR were performed in deportalized and remnant liver tissue in both stages of the procedure.

RESULTS

11 patients were enrolled. Remnant liver volume increased 112 ± 63% (p < 0.05) in 9.1 ± 1.6 days. Proliferation-related cytokines IL-6, TNF-α, HGF and EGF significantly increased, while higher Ki-67 immunostaining and cyclin D expression were observed in remnant livers after ALPPS. mTOR, S6K1, 4E-BP1, TSC1 and TSC2 expression were significantly increased in remnant livers at second stage, while AMPK and Akt increased only in deportalized liver samples.

CONCLUSION

Rapid liver regeneration with ALPPS might be associated with hepatocyte proliferation induced by mTOR pathway activation.

Mesenchymal Stem Cells Enhance Liver Regeneration via Improving Lipid Accumulation and Hippo Signaling

The liver has the potential to regenerate after injury. It is a challenge to improve liver regeneration (LR) after liver resection in clinical practice. Bone morrow-derived mesenchymal stem cells (MSCs) have shown to have a role in various liver diseases. To explore the effects of MSCs on LR, we established a model of 70% partial hepatectomy (PHx). Results revealed that infusion of MSCs could improve LR through enhancing cell proliferation and cell growth during the first 2 days after PHx, and MSCs could also restore liver synthesis function. Infusion of MSCs also improved liver lipid accumulation partly via mechanistic target of rapamycin (mTOR) signaling and enhanced lipid β-oxidation support energy for LR. Rapamycin-induced inhibition of mTOR decreased liver lipid accumulation at 24 h after PHx, leading to impaired LR. And after infusion of MSCs, a proinflammatory environment formed in the liver, evidenced by increased expression of IL-6 and IL-1β, and thus the STAT3 and Hippo-YAP pathways were activated to improve cell proliferation. Our results demonstrated the function of MSCs on LR after PHx and provided new evidence for stem cell therapy of liver diseases.

Potential roles of AMP-activated protein kinase in liver regeneration in mice with acute liver injury

Liver regeneration post severe liver injury is crucial for the recovery of hepatic structure and function. The energy sensor AMP‑activated protein kinase (AMPK) has a crucial role in the regulation of nutrition metabolism in addition to other energy‑intensive physiological and pathophysiological processes. Cellular proliferation requires intensive energy and nutrition support, therefore the present study investigated whether AMPK is involved in liver regeneration post carbon tetrachloride (CCl4)‑induced acute hepatic injury. The experimental data indicated that phosphorylation level of AMPK increased 48 h post‑CCl4 exposure, which was accompanied with upregulation of proliferating cell nuclear antigen (PCNA) and recovery of alanine aminotransferase (ALT) level. Pretreatment with the AMPK inhibitor compound C had no obvious effects on ALT elevation in plasma and histological abnormalities in liver 24 h post CCl4 exposure. However, treatment with compound C 24 h post CCl4 exposure significantly suppressed CCl4‑induced AMPK phosphorylation, PCNA expression and ALT recovery. These data suggest that endogenous AMPK was primarily activated at the regeneration stage in mice with CCl4‑induced acute liver injury and may function as a positive regulator in liver regeneration.

Perioperative oral supplementation with fish oil promotes liver regeneration following partial hepatectomy in mice via AMPK activation

The present study aimed to observe the effects of perioperative oral supplementation with fish oil (FO) on liver regeneration in mice and examine the potential mechanism. A total of 120 male ICR mice were randomly divided into 5 groups: Sham, Control, fish oil (FO), Compound C [the AMP‑activated protein kinase (AMPK) inhibitor dorsomorphin], and Compound C + FO. Changes in liver function, alterations in hepatocyte proliferation and in the expression of polarization markers, and activation of AMPK signaling were examined following partial hepatectomy (PH). The results demonstrated that restoration of serum alanine aminotransferase (ALT) and total bilirubin (TBIL) levels were significantly faster in FO‑treated mice compared with Control mice, and this effect was suppressed by treatment with Compound C. FO‑treated mice exhibited increased numbers of Ki‑67 positive hepatocytes and their postoperative liver‑to‑body weight ratio was significantly increased compared with the Control mice, which was also suppressed by co‑treatment with the AMPK inhibitor. Furthermore, protein expression of Occludin, Claudin‑3, tight junction protein 1 and bile salt export pump was gradually increased in FO‑treated mice compared with Control, whereas Compound C treatment reversed this effect. Therefore, the present study revealed that perioperative oral supplementation with FO may promote liver regeneration and improved liver function in mice following PH through AMPK activation.

Functional characterization of AMP-activated protein kinase signaling in tumorigenesis

AMP-activated protein kinase (AMPK) is a ubiquitously expressed metabolic sensor among various species. Specifically, cellular AMPK is phosphorylated and activated under certain stressful conditions, such as energy deprivation, in turn to activate diversified downstream substrates to modulate the adaptive changes and maintain metabolic homeostasis. Recently, emerging evidences have implicated the potential roles of AMPK signaling in tumor initiation and progression. Nevertheless, a comprehensive description on such topic is still in scarcity, especially in combination of its biochemical features with mouse modeling results to elucidate the physiological role of AMPK signaling in tumorigenesis. Hence, we performed this thorough review by summarizing the tumorigenic role of each component along the AMPK signaling, comprising of both its upstream and downstream effectors. Moreover, their functional interplay with the AMPK heterotrimer and exclusive efficacies in carcinogenesis were chiefly explained among genetically altered mice models. Importantly, the pharmaceutical investigations of AMPK relevant medications have also been highlighted. In summary, in this review, we not only elucidate the potential functions of AMPK signaling pathway in governing tumorigenesis, but also potentiate the future targeted strategy aiming for better treatment of aberrant metabolism-associated diseases, including cancer.

Identification of a mammalian glycerol-3-phosphate phosphatase: Role in metabolism and signaling in pancreatic β-cells and hepatocytes

Obesity, and the associated disturbed glycerolipid/fatty acid (GL/FA) cycle, contribute to insulin resistance, islet β-cell failure, and type 2 diabetes. Flux through the GL/FA cycle is regulated by the availability of glycerol-3-phosphate (Gro3P) and fatty acyl-CoA. We describe here a mammalian Gro3P phosphatase (G3PP), which was not known to exist in mammalian cells, that can directly hydrolyze Gro3P to glycerol. We identified that mammalian phosphoglycolate phosphatase, with an uncertain function, acts in fact as a G3PP. We found that G3PP, by controlling Gro3P levels, regulates glycolysis and glucose oxidation, cellular redox and ATP production, gluconeogenesis, glycerolipid synthesis, and fatty acid oxidation in pancreatic islet β-cells and hepatocytes, and that glucose stimulated insulin secretion and the response to metabolic stress, e.g., glucolipotoxicity, in β-cells. In vivo overexpression of G3PP in rat liver lowers body weight gain and hepatic glucose production from glycerol and elevates plasma HDL levels. G3PP is expressed at various levels in different tissues, and its expression varies according to the nutritional state in some tissues. As Gro3P lies at the crossroads of glucose, lipid, and energy metabolism, control of its availability by G3PP adds a key level of metabolic regulation in mammalian cells, and G3PP offers a potential target for type 2 diabetes and cardiometabolic disorders.

Evolving Lessons on the Complex Role of AMPK in Normal Physiology and Cancer

AMP kinase (AMPK) is an evolutionarily conserved enzyme required for adaptive responses to various physiological and pathological conditions. AMPK executes numerous cellular functions, some of which are often perceived at odds with each other. While AMPK is essential for embryonic growth and development, its full impact in adult tissues is revealed under stressful situations that organisms face in the real world. Conflicting reports about its cellular functions, particularly in cancer, are intriguing and a growing number of AMPK activators are being developed to treat human diseases such as cancer and diabetes. Whether these drugs will have only context-specific benefits or detrimental effects in the treatment of human cancer will be a subject of intense research. Here we review the current state of AMPK research with an emphasis on cancer and discuss the yet unresolved context-dependent functions of AMPK in human cancer.

Epidermal growth factor receptor mediated proliferation depends on increased lipid droplet density regulated via a negative regulatory loop with FOXO3/Sirtuin6

The proliferation of colon cancer cells is mediated in part by epidermal growth factor receptor (EGFR) signaling and requires sustained levels of cellular energy to meet its high metabolic needs. Intracellular lipid droplets (LDs) are a source of energy used for various cellular functions and they are elevated in density in human cancer, yet their regulation and function are not well understood. Here, in human colon cancer cells, EGF stimulates increases in LD density, which depends on EGFR expression and activation as well as the individual cellular capacity for lipid synthesis. Increases in LDs are blockaded by inhibition of PI3K/mTOR and PGE2 synthesis, supporting their dependency on select upstream pathways. In colon cancer cells, silencing of the FOXO3 transcription factor leads to down regulation of SIRT6, a negative regulator of lipid synthesis, and consequent increases in the LD coat protein PLIN2, revealing that increases in LDs depend on loss of FOXO3/SIRT6. Moreover, EGF stimulates loss of FOXO3/SIRT6, which is blockaded by the inhibition of upstream pathways as well as lipid synthesis, revealing existence of a negative regulatory loop between LDs and FOXO3/SIRT6. Elevated LDs are utilized by EGF treatment and their depletion through the inhibition of lipid synthesis or silencing of PLIN2 significantly attenuates proliferation. This novel mechanism of proliferative EGFR signaling leading to elevated LD density in colon cancer cells could potentially be therapeutically targeted for the treatment of tumor progression.

Dissecting the Dual Role of AMPK in Cancer: From Experimental to Human Studies

The precise role of 5'AMP-activated kinase (AMPK) in cancer and its potential as a therapeutic target is controversial. Although it is well established that activation of this energy sensor inhibits the main anabolic processes that sustain cancer cell proliferation and growth, AMPK activation can confer on cancer cells the plasticity to survive under metabolic stress such as hypoxia and glucose deprivation, which are commonly observed in fast growing tumors. Thus, AMPK is referred to as both a "conditional" tumor suppressor and "contextual" oncogene. To add a further layer of complexity, AMPK activation in human cancer tissues and its correlation with tumor aggressiveness and progression appears to vary in different contexts. The current review discusses the different faces of this metabolic regulator, the therapeutic implications of its modulation, and provides an overview of the most relevant data available on AMPK activation and AMPK-activating drugs in human studies.

Suppression of autophagy during liver regeneration impairs energy charge and hepatocyte senescence in mice

Autophagy is a homeostatic mechanism that regulates protein and organelle turnover and uses the amino acids from degraded proteins to produce adenosine 5'-triphosphate (ATP). We investigated the activity of autophagy-associated pathways in liver regeneration after partial hepatectomy (PHx) in liver-specific autophagy-related gene 5 (Atg5) knockout (KO) mice. Liver regeneration was severely impaired by 70% PHx, with a reduction in postoperative mitosis, but a compensating increase in hepatocyte size. PHx induced intracellular adenosine triphosphate and β-oxidation reduction as well as injured cellular mitochondria. Furthermore, PHx in Atg5 KO mice enhanced hepatic accumulation of p62 and ubiquitinated proteins. These results indicated that reorganization of intracellular proteins and organelles during autophagy was impaired in the regenerating liver of these mice. Up-regulation of p21 was associated with hepatocyte senescence, senescence-associated β-galactosidase expression, irreversible growth arrest, and secretion of senescence-associated molecules, including interleukin (IL)-6 and IL-8.

CONCLUSION

These findings indicate that autophagy plays a critical role in liver regeneration and in the preservation of cellular quality, preventing hepatocytes from becoming fully senescent and hypertrophic.

Liver damage, inflammation, and enhanced tumorigenesis after persistent mTORC1 inhibition

Obesity can result in insulin resistance, hepatosteatosis, and nonalcoholic steatohepatitis (NASH) and increases liver cancer risk. Obesity-induced insulin resistance depends, in part, on chronic activation of mammalian target of rapamycin complex 1 (mTORC1), which also occurs in human and mouse hepatocellular carcinoma (HCC), a frequently fatal liver cancer. Correspondingly, mTORC1 inhibitors have been considered as potential NASH and HCC treatments. Using a mouse model in which high-fat diet enhances HCC induction by the hepatic carcinogen DEN, we examined whether mTORC1 inhibition attenuates liver inflammation and tumorigenesis. Notably, rapamycin treatment or hepatocyte-specific ablation of the specific mTORC1 subunit Raptor resulted in elevated interleukin-6 (IL-6) production, activation of signal transducer and activator of transcription 3 (STAT3), and enhanced HCC development, despite a transient reduction in hepatosteatosis. These results suggest that long-term rapamycin treatment, which also increases IL-6 production in humans, is unsuitable for prevention or treatment of obesity-promoted liver cancer.

Mechanisms of liver repair following injury

Liver injury caused by a variety of physical or chemical factors is a common disease, and severe or persistent liver injury can ultimately lead to acute liver failure. Its treatment is still a formidable challenge to clinicians. Elucidation of mechanisms underlying liver repair following injury is the cornerstone of treatment of hepatic diseases. Despite many research efforts over the past decades, the mechanisms behind liver repair following injury are still not clear. Recent studies have demonstrated that oval cells and bone marrow stem cells are involved in this complex process. A variety of cells and factors may play a role in different stages of this process. In this paper, we will review mechanisms of liver repair following injury.