Characterization of the regulation and function of zinc-dependent histone deacetylases during rodent liver regeneration

Lysine lactylation in the regulation of tumor biology

Lysine lactylation (Kla), a newly discovered post-translational modification (PTM) of lysine residues, is progressively revealing its crucial role in tumor biology. A growing body of evidence supports its capacity of transcriptional regulation through histone modification and modulation of non-histone protein function. It intricately participates in a myriad of events in the tumor microenvironment (TME) by orchestrating the transitions of immune states and augmenting tumor malignancy. Its preferential modification of metabolic proteins underscores its specific regulatory influence on metabolism. This review focuses on the effect and the probable mechanisms of Kla-mediated regulation of tumor metabolism, the upstream factors that determine Kla intensity, and its potential implications for the clinical diagnosis and treatment of tumors.

Genetically encoded RNA-based sensors with Pepper fluorogenic aptamer

Sensors to measure the abundance and signaling of intracellular molecules are crucial for understanding their physiological functions. Although conventional fluorescent protein-based sensors have been designed, RNA-based sensors are promising imaging tools. Numerous RNA-based sensors have been developed. These sensors typically contain RNA G-quadruplex (RG4) motifs and thus may be suboptimal in living cells. Here we describe RNA-based sensors based on Pepper, a fluorogenic RNA without an RG4 motif. With Pepper, we engineered various sensors for metabolites, synthetic compounds, proteins and metal ions in vitro and in living cells. In addition, these sensors show high activation and selectivity, demonstrating their universality and robustness. In the case of sensors responding to S-adenosylmethionine (SAM), a metabolite produced by methionine adenosyltransferase (MATase), we showed that our sensors exhibited positively correlated fluorescence responding to different SAM levels. Importantly, we revealed the SAM biosynthesis pathway and monitored MATase activity and gene expression spatiotemporally in living individual human cells. Additionally, we constructed a ratiometric SAM sensor to determine the inhibition efficacy of a MATase inhibitor in living cells. Together, these sensors comprising Pepper provide a useful platform for imaging diverse cellular targets and their signaling pathway.

MSL1 Promotes Liver Regeneration by Driving Phase Separation of STAT3 and Histone H4 and Enhancing Their Acetylation

Male-specific lethal 1 (MSL1) is critical for the formation of MSL histone acetyltransferase complex which acetylates histone H4 Lys16 (H4K16ac) to activate gene expression. However, the role of MSL1 in liver regeneration is poorly understood. Here, this work identifies MSL1 as a key regulator of STAT3 and histone H4 (H4) in hepatocytes. MSL1 forms condensates with STAT3 or H4 through liquid-liquid phase separation to enrich acetyl-coenzyme A (Ac-CoA), and Ac-CoA in turn enhances MSL1 condensate formation, synergetically promoting the acetylation of STAT3 K685 and H4K16, thus stimulating liver regeneration after partial hepatectomy (PH). Additionally, increasing Ac-CoA level can enhance STAT3 and H4 acetylation, thus promoting liver regeneration in aged mice. The results demonstrate that MSL1 condensate-mediated STAT3 and H4 acetylation play an important role in liver regeneration. Thus, promoting the phase separation of MSL1 and increasing Ac-CoA level may be a novel therapeutic strategy for acute liver diseases and transplantation.

Small-Molecule-Mediated Split-Aptamer Assembly for Inducible CRISPR-dCas9 Transcription Activation

Inducible CRISPR-dCas9 transcription system has become a powerful tool for transcription regulation and sensing. Here, we develop a new concept of small-molecule-mediated split-aptamer assembly for inducible CRISPR-dCas9 transcription activation, allowing quantitative detection and imaging of S-adenosyl methionine (SAM) in live cells. This inducible transcription system is designed by integrating one fragment of a split SAM aptamer to guide RNA (gRNA) and the other to MS2 arrays. SAM-mediated reassembly of the split fragments recruits an MCP-fused transcription activator to the gRNA-dCas9 complex, activating the expression of a near-infrared fluorescent protein for imaging. We demonstrate that this inducible transcription system achieves quantitative detection of SAM with high sensitivity in live cells. Our system shows that methionine adenosyltransferase 1A (MAT1A) and MAT2A can both catalyze SAM production in live cells and the SAM levels in cancer cells can be increased via upregulation of MAT1A mRNA by epigenetic inhibitors. This split-aptamer assembly strategy could afford a new approach for controlling the CRISPR-dCas9 system, enabling conditional transcription regulation in response to endogenous metabolites in live cells.

Global Microbiota-Dependent Histone Acetylation Patterns Are Irreversible and Independent of Short Chain Fatty Acids

BACKGROUND AND AIMS

Although germ-free mice are an indispensable tool in studying the gut microbiome and its effects on host physiology, they are phenotypically different than their conventional counterparts. While antibiotic-mediated microbiota depletion in conventional mice leads to physiologic alterations that often mimic the germ-free state, the degree to which the effects of microbial colonization on the host are reversible is unclear. The gut microbiota produce abundant short chain fatty acids (SCFAs), and previous studies have demonstrated a link between microbial-derived SCFAs and global hepatic histone acetylation in germ-free mice.

APPROACH AND RESULTS

We demonstrate that global hepatic histone acetylation states measured by mass spectrometry remained largely unchanged despite loss of luminal and portal vein SCFAs after antibiotic-mediated microbiota depletion. In contrast to stable hepatic histone acetylation states, we see robust hepatic transcriptomic alterations after microbiota depletion. Additionally, neither dietary supplementation with supraphysiologic levels of SCFA nor the induction of hepatocyte proliferation in the absence of microbiota-derived SCFAs led to alterations in global hepatic histone acetylation.

CONCLUSIONS

These results suggest that microbiota-dependent landscaping of the hepatic epigenome through global histone acetylation is static in nature, while the hepatic transcriptome is responsive to alterations in the gut microbiota.

Histone Acetyltransferases and Stem Cell Identity

Acetylation of histones is a key epigenetic modification involved in transcriptional regulation. The addition of acetyl groups to histone tails generally reduces histone-DNA interactions in the nucleosome leading to increased accessibility for transcription factors and core transcriptional machinery to bind their target sequences. There are approximately 30 histone acetyltransferases and their corresponding complexes, each of which affect the expression of a subset of genes. Because cell identity is determined by gene expression profile, it is unsurprising that the HATs responsible for inducing expression of these genes play a crucial role in determining cell fate. Here, we explore the role of HATs in the maintenance and differentiation of various stem cell types. Several HAT complexes have been characterized to play an important role in activating genes that allow stem cells to self-renew. Knockdown or loss of their activity leads to reduced expression and or differentiation while particular HATs drive differentiation towards specific cell fates. In this study we review functions of the HAT complexes active in pluripotent stem cells, hematopoietic stem cells, muscle satellite cells, mesenchymal stem cells, neural stem cells, and cancer stem cells.

GATA6 modulates the ductular reaction to bile duct ligation

BACKGROUND

GATA6, a transcription factor expressed in cholangiocytes, has been implicated in the response to liver injury. In biliary atresia, a disease characterized by extrahepatic bile duct obstruction, liver expression of GATA6 increases with pathological bile duct expansion and decreases after successful Kasai portoenterostomy. The aim of this study was to garner genetic evidence that GATA6 is involved in ductular formation/expansion.

METHODS

The murine Gata6 gene was conditionally deleted using Alb-cre, a transgene expressed in hepatoblasts (the precursors of hepatocytes and cholangiocytes) and mature hepatocytes. Bile duct ligation (BDL) was used to model biliary obstruction.

RESULTS

Alb-Cre;Gata6^flox/flox mice were viable and fertile. Cre-mediated recombination of Gata6 in hepatocytes had little impact on cellular structure or function. GATA6 immunoreactivity was retained in a majority of biliary epithelial cells in adult Alb-Cre;Gata6^flox/flox mice, implying that surviving cholangiocytes were derived from hepatoblasts that had escaped biallelic Cre-mediated recombination. Although GATA6 immunoreactivity was preserved in cholangiocytes, Alb-cre;Gata6^flox/flox mice had a demonstrable biliary phenotype. A neutrophil-rich infiltrate surrounded newly formed bile ducts in neonatal Alb-Cre;Gata6^flox/flox mice. Foci of fibrosis/necrosis, presumed to reflect patchy defects in bile duct formation, were observed in the livers of 37% of adult Alb-cre;Gata6^flox/flox mice and 0% of controls (p < 0.05). Most notably, Alb-cre;Gata6^flox/flox mice had an altered response to BDL manifest as reduced survival, impaired bile ductule proliferation, increased parenchymal necrosis, reduced fibrosis, and enhanced macrophage accumulation in the portal space.

CONCLUSIONS

GATA6 orchestrates intrahepatic biliary remodeling and mitigates liver injury following extrahepatic bile duct obstruction.

Histone deacetylase activity is required for Botrylloides leachii whole-body regeneration

The colonial tunicate Botrylloides leachii is exceptional at regenerating from a piece of vascular tunic after loss of all adults from the colony. Previous transcriptome analyses indicate a brief period of healing before regeneration of a new adult (zooid) in as little as 8-10 days. However, there is little understanding of how the resulting changes to gene expression, required to drive regeneration, are initiated and how the overall process is regulated. Rapid changes to transcription often occur in response to chromatin changes, mediated by histone modifications such as histone acetylation. Here, we investigated a group of key epigenetic modifiers, histone deacetylases (HDAC), which are known to play an important role in many biological processes such as development, healing and regeneration. Through our transcriptome data, we identified and quantified the expression levels of HDAC and histone acetyltransferase enzymes during whole-body regeneration (WBR). To determine whether HDAC activity is required for WBR, we inhibited its action using valproic acid and trichostatin A. HDAC inhibition prevented the final morphological changes normally associated with WBR and resulted in aberrant gene expression. Botrylloides leachii genes including Slit2, TGF-β, Piwi and Fzd4 all showed altered mRNA levels upon HDAC inhibition in comparison with the control samples. Additionally, atypical expression of Bl_Piwi was found in immunocytes upon HDAC inhibition. Together, these results show that HDAC function, specifically HDAC I/IIa class enzymes, are vital for B. leachii to undergo WBR successfully.

Osteopontin promotes collagen I synthesis in hepatic stellate cells by miRNA-129-5p inhibition

Activation of hepatic stellate cells (HSCs) is an essential event in the initiation and progression of liver fibrosis. HSCs are believed to be the major source of collagen-producing myofibroblasts in fibrotic livers. A key feature in the pathogenesis of liver fibrosis is fibrillar Collagen I (Col 1) deposition. Osteopontin (OPN), an extracellular matrix (ECM) cytokine expressed in HSCs, could drive fibrogenesis by modulating the HSC pro-fibrogenic phenotype and Col 1 expression. Here, we aimed to investigate the molecular mechanism of OPN regulating the activation of HSCs. Our results showed that hepatic expression of OPN was increased in patients with liver fibrosis. In addition, hepatic OPN was positively correlated with Col 1 and α-SMA. Recombinant OPN (rOPN) upregulated Col 1 and α-SMA expression in LX-2 cells. However, OPN knockdown downregulated Col 1 expression. The 3'-UTR of the collagen 1 (Col 1) was identified to bind miR-129-5p. Transfection of miR-129-5p mimic in HSC resulted in a marked reduction of Col 1 expression. Conversely, a decrease in miR-129-5p in HSCs transfected by anti-sense miR-129-5p (AS-miR-129-5p) caused Col 1 upregulation. Furthermore, luciferase reporter assay showed that miR-129-5p directly target the 3'-UTR of Col1α1 mRNA via repressing its post-transcriptional activities. Finally, miR-129-5p level was decreased in fibrotic liver of human, and reduced by rOPN treatment. In contrast, miR-129-5p was induced in HSCs transfected by OPN siRNA. These data suggested that OPN induces Col 1 expression via suppression of miR-129-5p in HSCs.

Class I histone deacetylase inhibition improves pancreatitis outcome by limiting leukocyte recruitment and acinar-to-ductal metaplasia

BACKGROUND AND PURPOSE

Pancreatitis is a common inflammation of the pancreas with rising incidence in many countries. Despite improvements in diagnostic techniques, the disease is associated with high risk of severe morbidity and mortality and there is an urgent need for new therapeutic interventions. In this study, we evaluated whether histone deacetylases (HDACs), key epigenetic regulators of gene transcription, are involved in the development of the disease.

EXPERIMENTAL APPROACH

We analysed HDAC regulation during cerulein-induced acute, chronic and autoimmune pancreatitis using different transgenic mouse models. The functional relevance of class I HDACs was tested with the selective inhibitor MS-275 in vivo upon pancreatitis induction and in vitro in activated macrophages and primary acinar cell explants.

KEY RESULTS

HDAC expression and activity were up-regulated in a time-dependent manner following induction of pancreatitis, with the highest abundance observed for class I HDACs. Class I HDAC inhibition did not prevent the initial acinar cell damage. However, it effectively reduced the infiltration of inflammatory cells, including macrophages and T cells, in both acute and chronic phases of the disease, and directly disrupted macrophage activation. In addition, MS-275 treatment reduced DNA damage in acinar cells and limited acinar de-differentiation into acinar-to-ductal metaplasia in a cell-autonomous manner by impeding the EGF receptor signalling axis.

CONCLUSIONS AND IMPLICATIONS

These results demonstrate that class I HDACs are critically involved in the development of acute and chronic forms of pancreatitis and suggest that blockade of class I HDAC isoforms is a promising target to improve the outcome of the disease.

WITHDRAWN: Epigenetics in Chronic Liver Disease

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Branched-chain amino acid supplementation in treatment of liver cirrhosis: Updated views on how to attenuate their harmful effects on cataplerosis and ammonia formation

Branched-chain amino acid (BCAA; valine, leucine, and isoleucine) supplementation is common for patients with liver cirrhosis due to decreased levels of BCAA in the blood plasma of these patients, which plays a role in pathogenesis of hepatic encephalopathy and cachexia. The unique pharmacologic properties of BCAA also are a factor for use as supplementation in this population. In the present article, BCAA is shown to provide nitrogen to alpha-ketoglutarate (α-KG) for synthesis of glutamate, which is a substrate for ammonia detoxification to glutamine (GLN) in the brain and muscles. The article also demonstrates that the favorable effects of BCAA supplementation might be associated with three adverse effects: draining of α-KG from tricarboxylic acid cycle (cataplerosis), increased GLN content and altered glutamatergic neurotransmission in the brain, and activated GLN catabolism to ammonia in the gut and kidneys. Cataplerosis of α-KG can be attenuated by dimethyl-α-ketoglutarate, l-ornithine-l-aspartate, and ornithine salt of α-KG. The pros and cons of GLN elimination from the body using phenylbutyrate (phenylacetate), which may impair liver regeneration and decrease BCAA levels, should be examined. The therapeutic potential of BCAA might be enhanced also by optimizing its supplementation protocol. It is concluded that the search for strategies attenuating adverse and increasing positive effects of the BCAA is needed to include the BCAA among standard medications for patients with cirrhosis of the liver.

Cochlear hair cell regeneration after noise-induced hearing loss: Does regeneration follow development?

Noise-induced hearing loss (NIHL) affects a large number of military personnel and civilians. Regenerating inner-ear cochlear hair cells (HCs) is a promising strategy to restore hearing after NIHL. In this review, we first summarize recent transcriptome profile analysis of zebrafish lateral lines and chick utricles where spontaneous HC regeneration occurs after HC damage. We then discuss recent studies in other mammalian regenerative systems such as pancreas, heart and central nervous system. Both spontaneous and forced HC regeneration occurs in mammalian cochleae in vivo involving proliferation and direct lineage conversion. However, both processes are inefficient and incomplete, and decline with age. For direct lineage conversion in vivo in cochleae and in other systems, further improvement requires multiple factors, including transcription, epigenetic and trophic factors, with appropriate stoichiometry in appropriate architectural niche. Increasing evidence from other systems indicates that the molecular paths of direct lineage conversion may be different from those of normal developmental lineages. We therefore hypothesize that HC regeneration does not have to follow HC development and that epigenetic memory of supporting cells influences the HC regeneration, which may be a key to successful cochlear HC regeneration. Finally, we discuss recent efforts in viral gene therapy and drug discovery for HC regeneration. We hope that combination therapy targeting multiple factors and epigenetic signaling pathways will provide promising avenues for HC regeneration in humans with NIHL and other types of hearing loss.

HDACi Valproic Acid (VPA) and Suberoylanilide Hydroxamic Acid (SAHA) Delay but Fail to Protect against Warm Hepatic Ischemia-Reperfusion Injury

BACKGROUND

Histone deacetylases (HDAC) catalyze N-terminal deacetylation of lysine-residues on histones and multiple nuclear and cytoplasmic proteins. In various animal models, such as trauma/hemorrhagic shock, ischemic stroke or myocardial infarction, HDAC inhibitor (HDACi) application is cyto- and organoprotective and promotes survival. HDACi reduce stress signaling, cell death and inflammation. Hepatic ischemia-reperfusion (I/R) injury during major liver resection or transplantation increases morbidity and mortality. Assuming protective properties, the aim of this study was to investigate the effect of the HDACi VPA and SAHA on warm hepatic I/R.

MATERIAL AND METHODS

Male Wistar-Kyoto rats (age: 6-8 weeks) were randomized to VPA, SAHA, vehicle control (pre-) treatment or sham-groups and underwent partial no-flow liver ischemia for 90 minutes with subsequent reperfusion for 6, 12, 24 and 60 hours. Injury and regeneration was quantified by serum AST and ALT levels, by macroscopic aspect and (immuno-) histology. HDACi treatment efficiency, impact on MAPK/SAPK-activation and Hippo-YAP signaling was determined by Western blot.

RESULTS

Treatment with HDACi significantly enhanced hyperacetylation of Histone H3-K9 during I/R, indicative of adequate treatment efficiency. Liver injury, as measured by macroscopic aspect, serum transaminases and histology, was delayed, but not alleviated in VPA and SAHA treated animals. Importantly, tissue destruction was significantly more pronounced with VPA. SAPK-activation (p38 and JNK) was reduced by VPA and SAHA in the early (6h) reperfusion phase, but augmented later on (JNK, 24h). Regeneration appeared enhanced in SAHA and VPA treated animals and was dependent on Hippo-YAP signaling.

CONCLUSIONS

VPA and SAHA delay warm hepatic I/R injury at least in part through modulation of SAPK-activation. However, these HDACi fail to exert organoprotective effects, in this setting. For VPA, belated damage is even aggravated.

miRNA profiles in livers with different mass deficits after partial hepatectomy and miR-106b~25 cluster accelerating hepatocyte proliferation in rats

Partial hepatectomy (PH) promotes the reentry of quiescent hepatocytes into cell cycle for regrowth. miRNA profiles in livers with different mass deficits after PH have not been investigated and miRNAs implicated in liver regeneration remain unclear. We generated miRNA profiles from normal and remnant livers at 6, 12, 24, and 36 hours after 1/3 or 2/3PH using microarrays. Compared with normal livers, the proportion of altered miRNAs decreased with time after 1/3PH, but increased after 2/3PH. Most of altered miRNAs between 1/3 and 2/3PH exhibited similar up- or down-regulation, but lower expression magnitude for 1/3PH. Among differentially expressed miRNAs between 2/3PH with robust DNA replication and 1/3PH with a minimal replicative response, we identified miR-101a, miR-92a, miR-25, miR-93 and miR-106b as key regulators of cell cycle. In 2/3PH model, overexpression of miR-106b~25 cluster tended to accelerate liver regeneration, while inhibition of miR-106b~25 cluster markedly repressed regenerative response and delayed recovery of liver function. Mechanistically, RB1 and KAT2B with cell cycle arrest activity were identified as novel targets of miR-106b/93 and miR-25, respectively. Overall, we featured miRNA profiles and dynamics after 1/3 and 2/3PH, and identified miR-106b~25 cluster as being involved in timely cell cycle entry of hepatocytes after PH.

Phenylbutyrate exerts adverse effects on liver regeneration and amino acid concentrations in partially hepatectomized rats

Phenylbutyrate is recommended in urea cycle disorders and liver injury to enhance nitrogen disposal by the urine. However, hypothetically there may be adverse responses to the use of phenylbutyrate in the treatment of liver disease because of its role as a histone deacetylase inhibitor and its stimulatory effect on branched-chain alpha-keto acid dehydrogenase, the rate-limiting enzyme in the catabolism of branched-chain amino acids (BCAA; valine, leucine and isoleucine). We report the effects of phenylbutyrate on liver regeneration and amino acid levels in plasma of partially hepatectomized (PH) rats. Phenylbutyrate or saline was administered at 12-h intervals to PH or laparotomized rats. Phenylbutyrate delayed the onset of liver regeneration compared to the saline-treated controls, as indicated by lower hepatic DNA specific activities 18 and 24( ) h post-PH, decreased hepatic fractional protein synthesis rates 24 h post-PH and lowered the increases in liver weights and hepatic protein and DNA contents 48 h after PH. Hepatic DNA fragmentation (a hallmark of apoptosis) was higher in the phenylbutyrate-treated animals than in controls. Phenylbutyrate decreased the glutamine and BCAA concentrations and the ratio of the BCAA to aromatic amino acids (phenylalanine and tyrosine) in the blood plasma in both hepatectomized and laparotomized animals. In conclusion, the delayed onset of liver regeneration and the decrease in BCAA/AAA ratio in blood suggest that phenylbutyrate administration may be disastrous in subjects with acute hepatic injury and BCAA supplementation is needed when phenylbutyrate is used therapeutically.

Postponing the Hypoglycemic Response to Partial Hepatectomy Delays Mouse Liver Regeneration

All serious liver injuries alter metabolism and initiate hepatic regeneration. Recent studies using partial hepatectomy (PH) and other experimental models of liver regeneration implicate the metabolic response to hepatic insufficiency as an important source of signals that promote regeneration. Based on these considerations, the analyses reported here were undertaken to assess the impact of interrupting the hypoglycemic response to PH on liver regeneration in mice. A regimen of parenteral dextrose infusion that delays PH-induced hypoglycemia for 14 hours after surgery was identified, and the hepatic regenerative response to PH was compared between dextrose-treated and control mice. The results showed that regenerative recovery of the liver was postponed in dextrose-infused mice (versus vehicle control) by an interval of time comparable to the delay in onset of PH-induced hypoglycemia. The regulation of specific liver regeneration-promoting signals, including hepatic induction of cyclin D1 and S-phase kinase-associated protein 2 expression and suppression of peroxisome proliferator-activated receptor γ and p27 expression, was also disrupted by dextrose infusion. These data support the hypothesis that alterations in metabolism that occur in response to hepatic insufficiency promote liver regeneration, and they define specific pro- and antiregenerative molecular targets whose regenerative regulation is postponed when PH-induced hypoglycemia is delayed.

Identification of an epigenetic signature of early mouse liver regeneration that is disrupted by Zn-HDAC inhibition

Liver regeneration has been well studied with hope of discovering strategies to improve liver disease outcomes. Nevertheless, the signals that initiate such regeneration remain incompletely defined, and translation of mechanism-based pro-regenerative interventions into new treatments for hepatic diseases has not yet been achieved. We previously reported the isoform-specific regulation and essential function of zinc-dependent histone deacetylases (Zn-HDACs) during mouse liver regeneration. Those data suggest that epigenetically regulated anti-proliferative genes are deacetylated and transcriptionally suppressed by Zn-HDAC activity or that pro-regenerative factors are acetylated and induced by such activity in response to partial hepatectomy (PH). To investigate these possibilities, we conducted genome-wide interrogation of the liver histone acetylome during early PH-induced liver regeneration in mice using acetyL-histone chromatin immunoprecipitation and next generation DNA sequencing. We also compared the findings of that study to those seen during the impaired regenerative response that occurs with Zn-HDAC inhibition. The results reveal an epigenetic signature of early liver regeneration that includes both hyperacetylation of pro-regenerative factors and deacetylation of anti-proliferative and pro-apoptotic genes. Our data also show that administration of an anti-regenerative regimen of the Zn-HDAC inhibitor suberoylanilide hydroxamic acid (SAHA) not only disrupts gene-specific pro-regenerative changes in liver histone deacetylation but also reverses PH-induced effects on histone hyperacetylation. Taken together, these studies offer new insight into and suggest novel hypotheses about the epigenetic mechanisms that regulate liver regeneration.

Dietary aflatoxin-induced stunting in a novel rat model: evidence for toxin-induced liver injury and hepatic growth hormone resistance

BACKGROUND

Despite a strong statistical correlation between dietary aflatoxin B1 (AFB1)-exposure and childhood stunting, the causal mechanism remains speculative. This issue is important because of emerging interest in reduction of human aflatoxin exposure to diminish the prevalence and complications of stunting. Pediatric liver diseases cause growth impairment, and AFB1 is hepatotoxic. Thus, liver injury might mediate AFB1-associated growth impairment. We have developed a rat model of dietary AFB1-induced stunting to investigate these questions.

METHODS

Newly-weaned rats were given AFB1-supplemented- or control-diets from age 3-9 wk, and then euthanized for serum- and tissue-collection. Food intake and weight were serially assessed, with tibial-length determined at the experimental endpoint. Serum AFB1-adducts, hepatic gene and protein expression, and liver injury markers were quantified using established methodologies.

RESULTS

AFB1-albumin adducts correlated with dietary toxin contamination, but such contamination did not affect food consumption. AFB1-exposed animals exhibited dose-dependent wasting and stunting, liver pathology, and suppression of hepatic targets of growth hormone (GH) signaling, but did not display increased mortality.

CONCLUSION

These data establish toxin-dependent liver injury and hepatic GH-resistance as candidate mechanisms by which AFB1-exposure causes growth impairment in this mammalian model. Interrogation of modifiers of stunting using this model could guide interventions in at-risk and affected children.

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.

Fibroblast growth factor 15 deficiency impairs liver regeneration in mice

Fibroblast growth factor (FGF) 15 (human homolog, FGF19) is an endocrine FGF highly expressed in the small intestine of mice. Emerging evidence suggests that FGF15 is critical for regulating hepatic functions; however, the role of FGF15 in liver regeneration is unclear. This study assessed whether liver regeneration is altered in FGF15 knockout (KO) mice following 2/3 partial hepatectomy (PHx). The results showed that FGF15 KO mice had marked mortality, with the survival rate influenced by genetic background. Compared with wild-type mice, the KO mice displayed extensive liver necrosis and marked elevation of serum bile acids and bilirubin. Furthermore, hepatocyte proliferation was reduced in the KO mice because of impaired cell cycle progression. After PHx, the KO mice had weaker activation of signaling pathways that are important for liver regeneration, including signal transducer and activator of transcription 3, nuclear factor-κB, and mitogen-activated protein kinase. Examination of the KO mice at early time points after PHx revealed a reduced and/or delayed induction of immediate-early response genes, including growth-control transcription factors that are critical for liver regeneration. In conclusion, the results suggest that FGF15 deficiency severely impairs liver regeneration in mice after PHx. The underlying mechanism is likely the result of disrupted bile acid homeostasis and impaired priming of hepatocyte proliferation.

Class I HDAC activity is required for renal protection and regeneration after acute kidney injury

Activation of histone deacetylases (HDACs) is required for renal epithelial cell proliferation and kidney development. However, their role in renal tubular cell survival and regeneration after acute kidney injury (AKI) remains unclear. In this study, we demonstrated that all class I HDAC isoforms (1, 2, 3, and 8) were expressed in the renal epithelial cells of the mouse kidney. Inhibition of class I HDACs with MS-275, a highly selective inhibitor, resulted in more severe tubular injury in the mouse model of AKI induced by folic acid or rhabdomyolysis, as indicated by worsening renal dysfunction, increased neutrophil gelatinase-associated lipocalin expression, and enhanced apoptosis and caspase-3 activation. Blocking class I HDAC activity also impaired renal regeneration as evidenced by decreased expression of renal Pax-2, vimentin, and proliferating cell nuclear antigen. Injury to the kidney is accompanied by increased phosphorylation of epidermal growth factor receptor (EGFR), signal transducers and activators of transcription 3 (STAT3), and Akt. Inhibition of class I HDACs suppressed EGFR phosphorylation as well as reduced its expression. MS-275 was also effective in inhibiting STAT3 and Akt phosphorylation, but this treatment did not affect their expression levels. Taken together, these data suggest that the class I HDAC activity contributes to renal protection and functional recovery and is required for renal regeneration after AKI. Furthermore, renal EGFR signaling is subject to regulation by this class of HDACs.

Elucidating the metabolic regulation of liver regeneration

The regenerative capability of liver is well known, and the mechanisms that regulate liver regeneration are extensively studied. Such analyses have defined general principles that govern the hepatic regenerative response and implicated specific extracellular and intracellular signals as regulated during and essential for normal liver regeneration. Nevertheless, the most proximal events that stimulate liver regeneration and the distal signals that terminate this process remain incompletely understood. Recent data suggest that the metabolic response to hepatic insufficiency might be the proximal signal that initiates regenerative hepatocellular proliferation. This review provides an overview of the data in support of a metabolic model of liver regeneration and reflects on the clinical implications and areas for further study suggested by these findings.

Class I histone deacetylase activity is required for proliferation of renal epithelial cells

The process of renal regeneration after acute kidney injury is thought to recapitulate renal development, and proliferation of renal proximal tubular cells (RPTCs) is a critical step in the regenerative response. Recent studies indicate that class I histone deacetylases (HDACs) are required for embryonic kidney gene expression, growth, and differentiation. The role and underlying mechanisms of class I HDAC activation in RPTC proliferation, however, remain unclear. In this study, we used cultured RPTCs to examine this issue since four class I HDAC isoforms (1, 2, 3, and 8) are abundantly expressed in this cell type. Blocking class I HDAC activity with a highly selective inhibitor, MS-275, induced global histone H3 hyperacetylation, reduced RPTC proliferation, and diminished expression of cyclin D1 and proliferating cell nuclear antigen. Silencing HDAC1, 3, or 8 with small interfering RNA resulted in similar biological effects. Activation of epidermal growth factor receptor (EGFR) and signal transducers and activators of transcription 3 (STAT3) was required for RPTC proliferation, and STAT3 functioned downstream of EGFR. Treatment with MS-275 or knockdown of HDAC1, 3, or 8 suppressed EGFR expression and phosphorylation, and silencing HDAC1 and 3 also reduced STAT3 phosphorylation. However, HDAC2 downregulation did not affect RPTC proliferation and phosphorylation of EGFR and STAT3. Collectively, these data reveal a critical role of class I HDACs in mediating proliferation of renal epithelial cells through activation of the EGFR/STAT3 signaling pathway.