Coexistence of C/EBP alpha, beta, growth-induced proteins and DNA synthesis in hepatocytes during liver regeneration. Implications for maintenance of the differentiated state during liver growth

Molecular Pathways Governing the Termination of Liver Regeneration

The liver has the unique capacity to regenerate, and up to 70% of the liver can be removed without detrimental consequences to the organism. Liver regeneration is a complex process involving multiple signaling networks and organs. Liver regeneration proceeds through three phases: the initiation phase, the growth phase, and the termination phase. Termination of liver regeneration occurs when the liver reaches a liver-to-body weight that is required for homeostasis, the so-called "hepatostat." The initiation and growth phases have been the subject of many studies. The molecular pathways that govern the termination phase, however, remain to be fully elucidated. This review summarizes the pathways and molecules that signal the cessation of liver regrowth after partial hepatectomy and answers the question, "What factors drive the hepatostat?" SIGNIFICANCE STATEMENT: Unraveling the pathways underlying the cessation of liver regeneration enables the identification of druggable targets that will allow us to gain pharmacological control over liver regeneration. For these purposes, it would be useful to understand why the regenerative capacity of the liver is hampered under certain pathological circumstances so as to artificially modulate the regenerative processes (e.g., by blocking the cessation pathways) to improve clinical outcomes and safeguard the patient's life.

Three Dimensional Bioprinting for Hepatic Tissue Engineering: From In Vitro Models to Clinical Applications

Fabrication of functional organs is the holy grail of tissue engineering and the possibilities of repairing a partial or complete liver to treat chronic liver disorders are discussed in this review. Liver is the largest gland in the human body and plays a responsible role in majority of metabolic function and processes. Chronic liver disease is one of the leading causes of death globally and the current treatment strategy of organ transplantation holds its own demerits. Hence there is a need to develop an in vitro liver model that mimics the native microenvironment. The developed model should be a reliable to understand the pathogenesis, screen drugs and assist to repair and replace the damaged liver. The three-dimensional bioprinting is a promising technology that recreates in vivo alike in vitro model for transplantation, which is the goal of tissue engineers. The technology has great potential due to its precise control and its ability to homogeneously distribute cells on all layers in a complex structure. This review gives an overview of liver tissue engineering with a special focus on 3D bioprinting and bioinks for liver disease modelling and drug screening.

Chemokine production induced by Corynebacterium pseudotuberculosis in a murine model

Corynebacterium pseudotuberculosis is the etiological agent of caseous lymphadenitis. The main clinical sign of this disease is the development of granulomas, especially in small ruminants; however, the pathways that are involved in the formation and maintenance of these granulomas are unknown. Cytokines and chemokines are responsible for the migration of immune cells to specific sites and tissues; therefore, it is possible that chemokines participate in abscess formation. This study aimed to evaluate the induction of chemokine production by two C. pseudotuberculosis strains in a murine model. A highly pathogenic (VD57) and an attenuated (T1) strain of C. pseudotuberculosis, as well as somatic and secreted antigens derived from these strains, was used to stimulate murine splenocytes. Then, the concentrations of the chemokines CCL-2, CCL-3, CCL-4, and CCL-5 and the cytokines IL-1 and TNF were measured in the culture supernatants. The VD57 strain had a higher ability to stimulate the production of chemokines when compared to T1 strain, especially in the early stages of stimulation, which can have an impact on granuloma formation. The T1 lysate antigen was able to stimulate most of the chemokines studied herein when compared to the other antigenic fractions of both strains. These results indicate that C. pseudotuberculosis is a chemokine production inducer, and the bacterial strains differ in their induction pattern, a situation that can be related to the specific behavior of each strain.

Global properties of regulatory sequences are predicted by transcription factor recognition mechanisms

Background

Mammalian genomes contain millions of putative regulatory sequences, which are delineated by binding of multiple transcription factors. The degree to which spacing and orientation constraints among transcription factor binding sites contribute to the recognition and identity of regulatory sequence is an unresolved but important question that impacts our understanding of genome function and evolution. Global mechanisms that underlie phenomena including the size of regulatory sequences, their uniqueness, and their evolutionary turnover remain poorly described.

Results

Here, we ask whether models incorporating different degrees of spacing and orientation constraints among transcription factor binding sites are broadly consistent with several global properties of regulatory sequence. These properties include length, sequence diversity, turnover rate, and dominance of specific TFs in regulatory site identity and cell type specification. Models with and without spacing and orientation constraints are generally consistent with all observed properties of regulatory sequence, and with regulatory sequences being fundamentally small (~ 1 nucleosome). Uniqueness of regulatory regions and their rapid evolutionary turnover are expected under all models examined. An intriguing issue we identify is that the complexity of eukaryotic regulatory sites must scale with the number of active transcription factors, in order to accomplish observed specificity.

Conclusions

Models of transcription factor binding with or without spacing and orientation constraints predict that regulatory sequences should be fundamentally short, unique, and turn over rapidly. We posit that the existence of master regulators may be, in part, a consequence of evolutionary pressure to limit the complexity and increase evolvability of regulatory sites.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13059-021-02503-y.

Direct conversion of porcine primary fibroblasts into hepatocyte-like cells

The pig is an important model organism for biomedical research, mainly due to its extensive genetic, physiological and anatomical similarities with humans. Until date, direct conversion of somatic cells into hepatocyte-like cells (iHeps) has only been achieved in rodents and human cells. Here, we employed lentiviral vectors to screen a panel of 12 hepatic transcription factors (TF) for their potential to convert porcine fibroblasts into hepatocyte-like cells. We demonstrate for the first time, hepatic conversion of porcine somatic cells by over-expression of CEBPα, FOXA1 and HNF4α2 (3TF-piHeps). Reprogrammed 3TF-piHeps display a hepatocyte-like morphology and show functional characteristics of hepatic cells, including albumin secretion, Dil-AcLDL uptake, storage of lipids and glycogen and activity of cytochrome P450 enzymes CYP1A2 and CYP2C33 (CYP2C9 in humans). Moreover, we show that markers of mature hepatocytes are highly expressed in 3TF-piHeps, while fibroblastic markers are reduced. We envision piHeps as useful cell sources for future studies on drug metabolism and toxicity as well as in vitro models for investigation of pig-to-human infectious diseases.

Cellular plasticity balances the metabolic and proliferation dynamics of a regenerating liver

The adult liver has an exceptional ability to regenerate, but how it maintains its specialized functions during regeneration is unclear. Here, we used partial hepatectomy (PHx) in tandem with single-cell transcriptomics to track cellular transitions and heterogeneities of ∼22,000 liver cells through the initiation, progression, and termination phases of mouse liver regeneration. Our results uncovered that, following PHx, a subset of hepatocytes transiently reactivates an early-postnatal-like gene expression program to proliferate, while a distinct population of metabolically hyperactive cells appears to compensate for any temporary deficits in liver function. Cumulative EdU labeling and immunostaining of metabolic, portal, and central vein-specific markers revealed that hepatocyte proliferation after PHx initiates in the midlobular region before proceeding toward the periportal and pericentral areas. We further demonstrate that portal and central vein proximal hepatocytes retain their metabolically active state to preserve essential liver functions while midlobular cells proliferate nearby. Through combined analysis of gene regulatory networks and cell-cell interaction maps, we found that regenerating hepatocytes redeploy key developmental regulons, which are guided by extensive ligand-receptor-mediated signaling events between hepatocytes and nonparenchymal cells. Altogether, our study offers a detailed blueprint of the intercellular crosstalk and cellular reprogramming that balances the metabolic and proliferative requirements of a regenerating liver.

Cellular plasticity balances the metabolic and proliferation dynamics of a regenerating liver

The adult liver has an exceptional ability to regenerate, but how it maintains its specialized functions during regeneration is unclear. Here, we used partial hepatectomy (PHx) in tandem with single-cell transcriptomics to track cellular transitions and heterogeneities of ∼22,000 liver cells through the initiation, progression, and termination phases of mouse liver regeneration. Our results uncovered that, following PHx, a subset of hepatocytes transiently reactivates an early-postnatal-like gene expression program to proliferate, while a distinct population of metabolically hyperactive cells appears to compensate for any temporary deficits in liver function. Cumulative EdU labeling and immunostaining of metabolic, portal, and central vein-specific markers revealed that hepatocyte proliferation after PHx initiates in the midlobular region before proceeding toward the periportal and pericentral areas. We further demonstrate that portal and central vein proximal hepatocytes retain their metabolically active state to preserve essential liver functions while midlobular cells proliferate nearby. Through combined analysis of gene regulatory networks and cell-cell interaction maps, we found that regenerating hepatocytes redeploy key developmental regulons, which are guided by extensive ligand-receptor-mediated signaling events between hepatocytes and nonparenchymal cells. Altogether, our study offers a detailed blueprint of the intercellular crosstalk and cellular reprogramming that balances the metabolic and proliferative requirements of a regenerating liver.

Liver regeneration: biological and pathological mechanisms and implications

The liver is the only solid organ that uses regenerative mechanisms to ensure that the liver-to-bodyweight ratio is always at 100% of what is required for body homeostasis. Other solid organs (such as the lungs, kidneys and pancreas) adjust to tissue loss but do not return to 100% of normal. The current state of knowledge of the regenerative pathways that underlie this 'hepatostat' will be presented in this Review. Liver regeneration from acute injury is always beneficial and has been extensively studied. Experimental models that involve partial hepatectomy or chemical injury have revealed extracellular and intracellular signalling pathways that are used to return the liver to equivalent size and weight to those prior to injury. On the other hand, chronic loss of hepatocytes, which can occur in chronic liver disease of any aetiology, often has adverse consequences, including fibrosis, cirrhosis and liver neoplasia. The regenerative activities of hepatocytes and cholangiocytes are typically characterized by phenotypic fidelity. However, when regeneration of one of the two cell types fails, hepatocytes and cholangiocytes function as facultative stem cells and transdifferentiate into each other to restore normal liver structure. Liver recolonization models have demonstrated that hepatocytes have an unlimited regenerative capacity. However, in normal liver, cell turnover is very slow. All zones of the resting liver lobules have been equally implicated in the maintenance of hepatocyte and cholangiocyte populations in normal liver.

Functional compensation precedes recovery of tissue mass following acute liver injury

The liver plays a central role in metabolism, protein synthesis and detoxification. It possesses unique regenerative capacity upon injury. While many factors regulating cellular proliferation during liver repair have been identified, the mechanisms by which the injured liver maintains vital functions prior to tissue recovery are unknown. Here, we identify a new phase of functional compensation following acute liver injury that occurs prior to cellular proliferation. By coupling single-cell RNA-seq with in situ transcriptional analyses in two independent murine liver injury models, we discover adaptive reprogramming to ensure expression of both injury response and core liver function genes dependent on macrophage-derived WNT/β-catenin signaling. Interestingly, transcriptional compensation is most prominent in non-proliferating cells, clearly delineating two temporally distinct phases of liver recovery. Overall, our work describes a mechanism by which the liver maintains essential physiological functions prior to cellular reconstitution and characterizes macrophage-derived WNT signals required for this compensation.

The liver possesses the ability to regenerate following sudden injury. Here, the authors use single-cell RNA-sequencing and in situ transcriptional analyses to identify a new phase of liver regeneration in mice aimed at maintaining essential functions throughout the regenerative process.

Graft Regeneration and Functional Recovery in Patients with Early Allograft Dysfunction After Living-Donor Liver Transplantation

Background

Successful graft regeneration is important in living-donor liver transplantation (LDLT) because partial liver grafts are used. Early allograft dysfunction (EAD) is an intermediate outcome that affects the long-term postoperative course in liver transplantation. The aim of the present study was to investigate liver graft regeneration under EAD development in LDLT.

Material/Methods

The data of 226 patients who underwent LDLT from September 2010 to July 2014 were retrospectively analyzed. The patients were classified into 2 groups: one with and one without EAD. Graft regeneration, functional recovery, and long-term patient survival were compared between the 2 groups.

Results

The grafts grew more vigorously in the EAD group than in the non-EAD group, as evidenced by the larger absolute (ALV) and relative liver volumes (RLV) of the former on postoperative days (POD) 7 and 21. The median (interquartile range) RLVs of the non-EAD group versus the EAD group were as follows: 55.2 (47.9–65.8) vs. 53.7 (46.6–64.5)% preoperatively, p>0.05; 76.1 (66.9–85.7) vs. 86.7 (73.9–96.8)% on POD 7, p<0.01; 79.6 (69.3–91.2) vs. 93.7 (79.6–101.6)%, p<0.01 on POD 21. In the early postoperative period, hepatic function, measured as total bilirubin and international normalized ratio, was higher in the EAD group; however, after EAD development, graft function recovered in these patients. In the follow-up period, overall patient survival was comparable between the 2 groups.

Conclusions

The liver grafts of EAD patients steadily regenerated, such that the development of EAD did not affect long-term patient survival after LDLT.

Pattern analysis uncovers a chronic ethanol-induced disruption of the switch-like dynamics of C/EBP-β and C/EBP-α genome-wide binding during liver regeneration

Chronic ethanol intake impairs liver regeneration through a system-wide alteration in the regulatory networks driving the response to injury. Our study focused on the initial phase of response to 2/3rd partial hepatectomy (PHx) to investigate how adaptation to chronic ethanol intake affects the genome-wide binding profiles of the transcription factors C/EBP-β and C/EBP-α. These factors participate in complementary and often opposing functions for maintaining cellular differentiation, regulating metabolism, and governing cell growth during liver regeneration. We analyzed ChIP-seq data with a comparative pattern count (COMPACT) analysis, which exhaustively enumerates temporal patterns of discretized binding profiles to identify dominant as well as subtle patterns that may not be apparent from conventional clustering analyses. We found that adaptation to chronic ethanol intake significantly alters the genome-wide binding profile of C/EBP-β and C/EBP-α before and following PHx. A subset of these ethanol-induced changes include C/EBP-β binding to promoters of genes involved in the profibrogenic transforming growth factor-β pathway, and both C/EBP-β and C/EBP-α binding to promoters of genes involved in the cell cycle, apoptosis, homeostasis, and metabolic processes. The shift in C/EBP binding loci, coupled with an ethanol-induced increase in C/EBP-β binding at 6 h post-resection, indicates that ethanol adaptation may change both the amount and nature of C/EBP binding postresection. Taken together, our results suggest that chronic ethanol consumption leads to a spatially and temporally reorganized activity at many genomic loci, resulting in a shift in the dynamic balance and coordination of cellular processes underlying regenerative response.

A cellular, molecular, and pharmacological basis for appendage regeneration in mice

Regenerative medicine aims to restore normal tissue architecture and function. However, the basis of tissue regeneration in mammalian solid organs remains undefined. Remarkably, mice lacking p21 fully regenerate injured ears without discernable scarring. Here we show that, in wild-type mice following tissue injury, stromal-derived factor-1 (Sdf1) is up-regulated in the wound epidermis and recruits Cxcr4-expressing leukocytes to the injury site. In p21-deficient mice, Sdf1 up-regulation and the subsequent recruitment of Cxcr4-expressing leukocytes are significantly diminished, thereby permitting scarless appendage regeneration. Lineage tracing demonstrates that this regeneration derives from fate-restricted progenitor cells. Pharmacological or genetic disruption of Sdf1-Cxcr4 signaling enhances tissue repair, including full reconstitution of tissue architecture and all cell types. Our findings identify signaling and cellular mechanisms underlying appendage regeneration in mice and suggest new therapeutic approaches for regenerative medicine.

Regulation of hepatocyte identity and quiescence

The liver is a highly differentiated organ with a central role in metabolism, detoxification and systemic homeostasis. To perform its multiple tasks, liver parenchymal cells, the hepatocytes, express a large complement of enabling genes defining their complex phenotype. This phenotype is progressively acquired during fetal development and needs to be maintained in adulthood to guarantee the individual's survival. Upon injury or loss of functional mass, the liver displays an extraordinary regenerative response, mainly based on the proliferation of hepatocytes which otherwise are long-lived quiescent cells. Increasing observations suggest that loss of hepatocellular differentiation and quiescence underlie liver malfunction in chronic liver disease and pave the way for hepatocellular carcinoma development. Here, we briefly review the essential mechanisms leading to the acquisition of liver maturity. We also identify the key molecular factors involved in the preservation of hepatocellular homeostasis and finally discuss potential strategies to preserve liver identity and function.

Molecular mechanisms controlling the phenotype and the EMT/MET dynamics of hepatocyte

The complex spatial and paracrine relationships between the various liver histotypes are essential for proper functioning of the hepatic parenchymal cells. Only within a correct tissue organization, in fact, they stably maintain their identity and differentiated phenotype. The loss of histotype identity, which invariably occurs in the primary hepatocytes in culture, or in vivo in particular pathological conditions (fibrosis and tumours), is mainly because of the phenomenon of epithelial-to-mesenchymal transition (EMT). The EMT process, that occurs in the many epithelial cells, appears to be driven by a number of general, non-tissue-specific, master transcriptional regulators. The reverse process, the mesenchymal-to-epithelial transition (MET), as yet much less characterized at a molecular level, restores specific epithelial identities, and thus must include tissue-specific master elements. In this review, we will summarize the so far unveiled events of EMT/MET occurring in liver cells. In particular, we will focus on hepatocyte and describe the pivotal role in the control of EMT/MET dynamics exerted by a tissue-specific molecular mini-circuitry. Recent evidence, indeed, highlighted as two transcriptional factors, the master gene of EMT Snail, and the master gene of hepatocyte differentiation HNF4α, exhorting a direct reciprocal repression, act as pivotal elements in determining opposite cellular outcomes. The different balances between these two master regulators, further integrated by specific microRNAs, in fact, were found responsible for the EMT/METs dynamics as well as for the preservation of both hepatocyte and stem/precursor cells identity and differentiation. Overall, these findings impact the maintenance of stem cells and differentiated cells both in in vivo EMT/MET physio-pathological processes as well as in culture.

Chemokine-mediated robust augmentation of liver engraftment: a novel approach

Effective repopulation of the liver is essential for successful clinical hepatocyte transplantation. The objective was to improve repopulation of the liver with human hepatocytes using chemokines. We used flow cytometry and immunohistochemistry assays to identify commonly expressed chemokine receptors on human fetal and adult hepatocytes. The migratory capacity of the cells to various chemokines was tested. For in vivo studies, we used a nude mouse model of partial hepatectomy followed by intraparenchymal injections of chemokine ligands at various concentrations. Human fetal liver cells transformed with human telomerase reverse transcriptase were used for intrasplenic cell transplantation. Repopulation and functionality were assessed 4 weeks after transplantation. The receptor CXCR3 was commonly expressed on both fetal and adult hepatocytes. Both cell types migrated efficiently toward corresponding CXC chemokine ligands 9, 10, and 11. In vivo, animals injected with recombinant chemokines showed the highest cell engraftment compared with controls (p<.05). The engrafted cells expressed several human hepatic markers such as cytokeratin 8 and 18 and albumin as well as transferrin, UGT1A1, hepatocyte nuclear factor (1α, 1β, and 4α), cytochrome CYP3A1, CCAAT/enhancer binding protein (α and β), and human albumin compared with controls. No inflammatory cells were detected in the livers at 4 weeks after transplantation. The improved repopulation of transplanted cells is likely a function of the chemokines to mediate cell homing and retention in the injured liver and might be an attractive strategy to augment repopulation of transplanted hepatocytes in vivo.

Adult hepatocytes are generated by self-duplication rather than stem cell differentiation

The liver is thought to utilize facultative stem cells, also known as "oval cells" or "atypical ductal cells" (ADCs), for regeneration following various types of injury. However, this notion has been based largely on in vitro studies and transplantation models; where lineage tracing has been used, results have been conflicting and effect sizes have been small. Here, we used genetic and nucleoside analog-based tools to mark and track the origin and contribution of various cell populations to liver regeneration in vivo following several ADC-inducing insults. We report that, contrary to prevailing stem-cell-based models of regeneration, virtually all new hepatocytes come from preexisting hepatocytes.

Involvement of oxidative stress-induced DNA damage, endoplasmic reticulum stress, and autophagy deficits in the decline of β-cell mass in Japanese type 2 diabetic patients

OBJECTIVE

Deficits of β-cells characterize the islet pathology in type 2 diabetes. It is yet to be clear how the β-cell loss develops in type 2 diabetes. We explored the implication of oxidative stress, endoplasmic reticulum (ER)-induced stress, and autophagy deficit in the β-cell decline in Japanese type 2 diabetic patients.

RESEARCH DESIGN AND METHODS

Pancreases from recent autopsy cases of 47 type 2 diabetic and 30 nondiabetic subjects were investigated on the islet structure with morphometric analysis. Volume densities of islet (Vi), β-cell (Vβ), and α-cell (Vα) were measured. To evaluate cell damage of endocrine cells, immunohistochemical expressions of oxidative stress-related DNA damage as expressed by γH2AX, ER stress-related cell damage as CCAAT/enhancer 1 binding protein-β (C/EBP-β), and autophagy deficit as P62 were semiquantified, and their correlations to islet changes were sought.

RESULTS

Compared with nondiabetic subjects, Vβ was reduced in diabetic subjects. Contrariwise, there was an increase in Vα. There was a significant link between reduced Vβ and increased HbA1c levels (P < 0.01) and a trend of inverse correlation between Vβ and duration of diabetes (P = 0.06). Expressions of γH2AX, P62, and C/EBP-β were all enhanced in diabetic islets, and reduced Vβ correlated with the intensity of γH2AX expression but not with C/EBP-β or P62 expressions. Combined expressions of γH2AX, P62, and C/EBP-β were associated with severe reduction of Vβ.

CONCLUSIONS

β-Cell deficit in type 2 diabetes was associated with increased oxidative stress and may further be augmented by autophagic deficits and ER stress.

Principles of liver regeneration and growth homeostasis

Liver regeneration is perhaps the most studied example of compensatory growth aimed to replace loss of tissue in an organ. Hepatocytes, the main functional cells of the liver, manage to proliferate to restore mass and to simultaneously deliver all functions hepatic functions necessary to maintain body homeostasis. They are the first cells to respond to regenerative stimuli triggered by mitogenic growth factor receptors MET (the hepatocyte growth factor receptor] and epidermal growth factor receptor and complemented by auxiliary mitogenic signals induced by other cytokines. Termination of liver regeneration is a complex process affected by integrin mediated signaling and it restores the organ to its original mass as determined by the needs of the body (hepatostat function). When hepatocytes cannot proliferate, progenitor cells derived from the biliary epithelium transdifferentiate to restore the hepatocyte compartment. In a reverse situation, hepatocytes can also transdifferentiate to restore the biliary compartment. Several hormones and xenobiotics alter the hepatostat directly and induce an increase in liver to body weight ratio (augmentative hepatomegaly). The complex challenges of the liver toward body homeostasis are thus always preserved by complex but unfailing responses involving orchestrated signaling and affecting growth and differentiation of all hepatic cell types.

Temporal mapping of CEBPA and CEBPB binding during liver regeneration reveals dynamic occupancy and specific regulatory codes for homeostatic and cell cycle gene batteries

Dynamic shifts in transcription factor binding are central to the regulation of biological processes by allowing rapid changes in gene transcription. However, very few genome-wide studies have examined how transcription factor occupancy is coordinated temporally in vivo in higher animals. Here, we quantified the genome-wide binding patterns of two key hepatocyte transcription factors, CEBPA and CEBPB (also known as C/EBPalpha and C/EBPbeta), at multiple time points during the highly dynamic process of liver regeneration elicited by partial hepatectomy in mouse. Combining these profiles with RNA polymerase II binding data, we find three temporal classes of transcription factor binding to be associated with distinct sets of regulated genes involved in the acute phase response, metabolic/homeostatic functions, or cell cycle progression. Moreover, we demonstrate a previously unrecognized early phase of homeostatic gene expression prior to S-phase entry. By analyzing the three classes of CEBP bound regions, we uncovered mutually exclusive sets of sequence motifs, suggesting temporal codes of CEBP recruitment by differential cobinding with other factors. These findings were validated by sequential ChIP experiments involving a panel of central transcription factors and/or by comparison to external ChIP-seq data. Our quantitative investigation not only provides in vivo evidence for the involvement of many new factors in liver regeneration but also points to similarities in the circuitries regulating self-renewal of differentiated cells. Taken together, our work emphasizes the power of global temporal analyses of transcription factor occupancy to elucidate mechanisms regulating dynamic biological processes in complex higher organisms.

Genomewide microRNA down-regulation as a negative feedback mechanism in the early phases of liver regeneration

The liver is one of the few organs that have the capacity to regenerate in response to injury. We carried out genomewide microRNA (miRNA) microarray studies during liver regeneration in rats after 70% partial hepatectomy (PH) at early and mid time points to more thoroughly understand their role. At 3, 12, and 18 hours post-PH ∼40% of the miRNAs tested were up-regulated. Conversely, at 24 hours post-PH, ∼70% of miRNAs were down-regulated. Furthermore, we established that the genomewide down-regulation of miRNA expression at 24 hours was also correlated with decreased expression of genes, such as Rnasen, Dgcr8, Dicer, Tarbp2, and Prkra, associated with miRNA biogenesis. To determine whether a potential negative feedback loop between miRNAs and their regulatory genes exists, 11 candidate miRNAs predicted to target the above-mentioned genes were examined and found to be up-regulated at 3 hours post-PH. Using reporter and functional assays, we determined that expression of these miRNA-processing genes could be regulated by a subset of miRNAs and that some miRNAs could target multiple miRNA biogenesis genes simultaneously. We also demonstrated that overexpression of these miRNAs inhibited cell proliferation and modulated cell cycle in both Huh-7 human hepatoma cells and primary rat hepatocytes. From these observations, we postulated that selective up-regulation of miRNAs in the early phase after PH was involved in the priming and commitment to liver regeneration, whereas the subsequent genomewide down-regulation of miRNAs was required for efficient recovery of liver cell mass.

CONCLUSION

Our data suggest that miRNA changes are regulated by negative feedback loops between miRNAs and their regulatory genes that may play an important role in the steady-state regulation of liver regeneration.

Hierarchies of transcriptional regulation during liver regeneration

The remarkable capacity of the liver to regenerate after severe injury or disease has excited interest for centuries. The goal of harnessing this process in treatment of liver disease, and the appreciation of the parallels between regeneration and tumor development in the liver, remain a major driver for research in this area. Studies of liver regeneration as a model system offer a view of intricate and precisely timed regulatory pathways that drive the process toward completion. Successful regeneration of the liver mass demands a hierarchal and well-controlled balance between proliferative and metabolic functions, which is orchestrated by signaling and regulation of transcription factors. Control and regulation of these cascades of transcriptional activities, necessary for induction, renewal, and cessation of liver growth, are the focus of this chapter.

Role of AMP-activated protein kinase in the control of hepatocyte priming and proliferation during liver regeneration

The enzyme AMP-activated protein kinase (AMPK) is the main energy sensor in cells and is responsible for controlling the balance of anabolic/catabolic processes under metabolic stress conditions. This metabolic control exerted by AMPK is critical for energy-demanding situations, such as liver regeneration. Immediately after partial hepatectomy (PH), the liver undergoes the priming phase, mediated by the proinflammatory cytokines tumor necrosis factor (TNF) and interleukin-6, which promote responsiveness of hepatocytes to growth factors, such as hepatocyte growth factor (HGF) and epidermal growth factor, which lead to proliferation. In addition to its metabolic function, AMPK is likely to be a key mediator in both hepatocyte priming and the proliferative phases, induced by TNF-α and HGF, respectively. TNF-α-induced AMPK activation has been shown to be necessary for nuclear factor κappa B (NF-κB)-induced inducible nitric oxide synthase expression and for blocking TNF-α-induced apoptosis. On the other hand, HGF-induced LKB1/AMPK activation has been found to play a critical role in controlling Hu antigen R cytosolic localization and endothelial nitric oxide synthase activation, and consequently Cyclin D1 and Cyclin A expressions, and nitric oxide generation, respectively. During PH, levels of S-adenosylmethionine (SAMe), the principal methyl donor in the liver, have to decrease to allow liver proliferation. Our studies also show that SAMe inhibits hepatocyte proliferation by controlling the hepatocyte's responsiveness to mitogenic signals such as HGF through the inhibition of AMPK activity. In summary, these data highlight the essential role of AMPK in controlling the balance between hepatocyte metabolic adaptations, cell cycle progression and apoptosis during liver regeneration.

Suppression of liver regeneration and hepatocyte proliferation in hepatocyte-targeted glypican 3 transgenic mice

Glypican 3 (GPC3) belongs to a family of glycosylphosphatidylinositol-anchored, cell-surface heparan sulfate proteoglycans. GPC3 is overexpressed in hepatocellular carcinoma. Loss-of-function mutations of GPC3 result in Simpson-Golabi-Behmel syndrome, an X-linked disorder characterized by overgrowth of multiple organs, including the liver. Our previous study showed that GPC3 plays a negative regulatory role in hepatocyte proliferation, and this effect may involve CD81, a cell membrane tetraspanin. To further investigate GPC3 in vivo, we engineered transgenic (TG) mice overexpressing GPC3 in the liver under the control of the albumin promoter. GPC3 TG mice with hepatocyte-targeted, overexpressed GPC3 developed normally in comparison with their nontransgenic littermates but had a suppressed rate of hepatocyte proliferation and liver regeneration after partial hepatectomy. Moreover, gene array analysis revealed a series of changes in the gene expression profiles in TG mice (both in normal mice and during liver regeneration). In unoperated GPC3 TG mice, there was overexpression of runt related transcription factor 3 (7.6-fold), CCAAT/enhancer binding protein alpha (2.5-fold), GABA A receptor (2.9-fold), and wingless-related MMTV integration site 7B (2.8-fold). There was down-regulation of insulin-like growth factor binding protein 1 (8.4-fold), Rab2 (5.6-fold), beta-catenin (1.7-fold), transforming growth factor beta type I (3.1-fold), nodal (1.8-fold), and yes-associated protein (1.4-fold). Changes after hepatectomy included decreased expression in several cell cycle-related genes.

CONCLUSION

Our results indicate that in GPC3 TG mice, hepatocyte overexpression of GPC3 suppresses hepatocyte proliferation and liver regeneration and alters gene expression profiles, and potential cell cycle-related proteins and multiple other pathways are involved and affected.

Embryonic and adult stem cell systems in mammals: ontology and regulation

Stem cells are defined as having the ability to self-renew and to generate differentiated cells. During embryogenesis, cells are initially proliferative and pluripotent and then they gradually become restricted to different cell fates. In the adult, tissue stem cells are normally quiescent, but become proliferative upon injury. Knowledge from developmental biology and insights into the properties of stem cells are keys to further understanding and successful manipulation. Here, we first focus on ES cells, then on embryonic development, and then on tissue stem cells of endodermally derived tissues, particularly the liver and pancreas.

RNA interference against hepatic epidermal growth factor receptor has suppressive effects on liver regeneration in rats

Liver regeneration after a two-thirds partial hepatectomy (PHx) is a complex process requiring interaction and cooperation of many growth factors and cytokines and cross talk between multiple pathways. Along with hepatocyte growth factor and its receptor MET (HGF-MET), the epidermal growth factor receptor (EGFR) signaling pathway is activated within 60 minutes after PHx. To investigate the role of EGFR in liver regeneration, we used two EGFR-specific short hairpin silencing RNAs to inhibit EGFR expression in regenerating normal rat liver. Suppression of EGFR mRNA and protein was evident in treated rats. There was also a demonstrable decrease but not complete elimination of bromo-deoxyuridine incorporation and mitoses at 24 hours after PHx. In addition, we observed up-regulation of MET and Src as well as activation of the ErbB-3-ErbB-2-PI3K-Akt pathway and down-regulation of STAT 3, cyclin D1, cyclin E1, p21, and C/EBP beta. The decrease in the ratio of C/EBP alpha to C/EBP beta known to occur after PHx was offset in shEGFR-treated rats. Despite suppression of hepatocyte proliferation lasting into day 3 after PHx, liver weight restoration occurred. Interestingly, hepatocytes in shEGFR-treated rats were considerably larger when compared with ScrRNA-treated controls. The data indicate that although the MET and EGFR pathways are similar, the contributions made by MET and EGFR are unique and are not compensated by each other or other cytokines.

Cascades of transcription regulation during liver regeneration

An increasing demand for new strategies in cancer prevention and regenerative medicine requires a better understanding of molecular mechanisms that control cell proliferation in tissue-specific manner. Regenerating liver is a unique model allowing use of biochemical, genetic, and engineering tools to uncover molecular mechanisms and improve treatment of hepatic cancers, liver failure, and fibrotic disease. Molecular mechanisms of liver regeneration involve extra- and intracellular factors to activate transcription of genes normally silenced in quiescent liver. While many upstream signaling pathways of the regenerating liver have been extensively studied, our knowledge of the downstream effectors, transcription factors (TFs), remains limited. This review describes consecutive engagement of pre-existing and de novo synthesized TFs, as cascades that regulate expression of growth-related and metabolic genes during liver regeneration after partial hepatectomy in mice. Several previously recognized regulators of regenerating liver are described in the light of recently identified co-activator and co-repressor complexes that interact with primary DNA-binding TFs. Published results of gene expression and chromatin immunoprecipitation analyses, as well as studies of transgenic mouse models, are used to emphasize new potential regulators of transcription during liver regeneration. Finally, a more detailed description of newly identified transcriptional regulators of liver regeneration illustrates the tightly regulated balance of proliferative and metabolic responses to partial hepatectomy.

The Met protooncogene is a transcriptional target of NF kappaB: implications for cell survival

NF kappaB transcription factor regulates gene expression in response to extracellular stimuli such as TNF alpha. The genes regulated by NF kappaB encode for proteins which control cell growth and survival. Met is the tyrosine kinase receptor for hepatocyte growth factor, and it too promotes cell mitogenesis and survival. Previously, we showed that Met gene expression is regulated by TNF alpha. In this report, we identify and characterize a TNF alpha response element in the Met promoter. This element contains tandem C/EBP sites adjacent to an NF kappaB site. Binding of the NF kappaB p65 subunit and C/EBP beta to this element is induced by TNF alpha. To examine the interplay of NF kappaB and Met in vivo, we determined that Met mRNA and protein levels are reduced in the livers of p65-/- mice as compared to controls. In p65-/- mouse embryonic fibroblasts (MEFs), Met induction by TNF alpha is abrogated while Met's basal gene expression is reduced by half as compared to controls. When overexpressed in p65-/- MEFs, Met confers resistance to TNF-alpha-mediated cell death. Conversely, expression of dominant negative Met in wild-type MEFs renders them sensitive to cell death induced by TNF alpha. A similar response following TNF alpha challenge was observed in hepatocytic cells treated with siRNA to knockdown endogenous Met. Together, these results indicate that the Met gene is a direct target of NF kappaB and that Met participates in NF kappaB-mediated cell survival.

p21 is required for dextrose-mediated inhibition of mouse liver regeneration

The inhibitory effect of dextrose supplementation on liver regeneration was first described more than 4 decades ago. Nevertheless, the molecular mechanisms responsible for this observation have not been elucidated. We investigated these mechanisms using the partial hepatectomy model in mice given standard or 10% dextrose (D10)-supplemented drinking water. The results showed that D10-treated mice exhibited significantly reduced hepatic regeneration compared with controls, as assessed by hepatocellular bromodeoxyuridine (BrdU) incorporation and mitotic frequency. D10 supplementation did not suppress activation of hepatocyte growth factor (HGF), induction of transforming growth factor alpha (TGF-alpha) expression, or tumor necrosis factor alpha-interleukin-6 cytokine signaling, p42/44 extracellular signal-regulated kinase (ERK) activation, immediate early gene expression, or expression of CCAAT/enhancer binding protein beta (C/EBPbeta), but did augment expression of the mito-inhibitory factors C/EBPalpha, p21(Waf1/Cip1), and p27(Kip1). In addition, forkhead box M1 (FoxM1) expression, which is required for normal liver regeneration, was suppressed by D10 treatment. Finally, D10 did not suppress either FoxM1 expression or hepatocellular proliferation in p21 null mice subjected to partial hepatectomy, establishing the functional significance of these events in mediating the effects of D10 on liver regeneration.

CONCLUSION

These data show that the inhibitory effect of dextrose supplementation on liver regeneration is associated with increased expression of C/EBPalpha, p21, and p27, and decreased expression of FoxM1, and that D10-mediated inhibition of liver regeneration is abrogated in p21-deficient animals. Our observations are consistent with a model in which hepatic sufficiency is defined by homeostasis between the energy-generating capacity of the liver and the energy demands of the body mass, with liver regeneration initiated when the functional liver mass is no longer sufficient to meet such demand.

Oltipraz promotion of liver regeneration after partial hepatectomy: The role of PI3-kinase-dependent C/EBPbeta and cyclin E regulation

Oltipraz, a representative cancer chemopreventive agent, regenerates cirrhotic liver via CCAAT/enhancer binding protein beta (C/EBPbeta). This study examined the effect of oltipraz on liver regeneration after partial hepatectomy (PH) and explored the role of phosphatidylinositol 3-kinase (PI3K) pathway responsible in liver regeneration. Oltipraz treatment (30 mg/kg/day, for 3 days) promoted liver regeneration in PH rats, but did not increase hepatocyte growth factor production. Subcellular fractionation and electrophoretic mobility shift assays showed that oltipraz treatment increased C/EBPbeta-DNA binding activity in the liver of sham control rats and further enhanced PH-mediated nuclear translocation of C/EBPbeta. The expression of cyclin E and the activity of cyclin E-dependent kinase were both enhanced by oltipraz treatment of PH rats. The signaling pathway that controls C/EBPbeta and cyclin E were studied in H4IIE cells, a rat-derived hepatocyte cell line. Oltipraz potentiated the nuclear accumulation of C/ EBPbeta and C/EBPbeta-DNA binding activity in cells incubated in a medium containing serum. PI3K and its downstream kinase, p70S6 kinase, were both required for C/EBPbeta-dependent induction of cyclin E by oltipraz, as shown by chemical inhibition and plasmid transfection experiments. The results of this study demonstrate that oltipraz treatment enhances liver regeneration after PH, which involves activation of C/EBPbeta and C/EBPbeta-dependent cyclin E expression via the PI3K-p70S6 kinase pathway.

Identification of neuronal target genes for CCAAT/enhancer binding proteins

CCAAT/Enhancer Binding Proteins (C/EBPs) play pivotal roles in the development and plasticity of the nervous system. Identification of the physiological targets of C/EBPs (C/EBP target genes) should therefore provide insight into the underlying biology of these processes. We used unbiased genome-wide mapping to identify 115 C/EBPbeta target genes in PC12 cells that include transcription factors, neurotransmitter receptors, ion channels, protein kinases and synaptic vesicle proteins. C/EBPbeta binding sites were located primarily within introns, suggesting novel regulatory functions, and were associated with binding sites for other developmentally important transcription factors. Experiments using dominant negatives showed C/EBPbeta to repress transcription of a subset of target genes. Target genes in rat brain were subsequently found to preferentially bind C/EBPalpha, beta and delta. Analysis of the hippocampal transcriptome of C/EBPbeta knockout mice revealed dysregulation of a high percentage of transcripts identified as C/EBP target genes. These results support the hypothesis that C/EBPs play non-redundant roles in the brain.

Liver-specific ablation of integrin-linked kinase in mice results in abnormal histology, enhanced cell proliferation, and hepatomegaly

Hepatocyte differentiation and proliferation are greatly affected by extracellular matrix (ECM). Primary hepatocytes cultured without matrix dedifferentiate over time, but matrix overlay quickly restores differentiation. ECM also is critical in liver regeneration where ECM degradation and reconstitution are steps in the regenerative process. Integrin-linked kinase (ILK) is a cell-ECM-adhesion component implicated in cell-ECM signaling by means of integrins. We investigated the role of ILK in whole liver by using the LoxP/Cre model system. ILK was eliminated from the liver by mating homozygous ILK-floxed animals with mice expressing Cre-recombinase under control of the alpha fetoprotein enhancer and albumin promoter. After ablation of ILK, animals are born normal. Soon after birth, however, they develop histologic abnormalities characterized by disorderly hepatic plates, increased proliferation of hepatocytes and biliary cells, and increased deposition of extracellular matrix. Cell proliferation is accompanied by increased cytoplasmic and nuclear stabilization of beta-catenin. After this transient proliferation of all epithelial components, proliferation subsides and final liver to body weight ratio in livers with ILK deficient hepatocytes is two times that of wild type. Microarray analysis of gene expression during the stage of cell proliferation shows up-regulation of integrin and matrix-related genes and a concurrent down-regulation of differentiation-related genes. After the proliferative stage, however, the previous trends are reversed resulting in a super-differentiated phenotype in the ILK-deficient livers.

CONCLUSION

Our results show for the first time in vivo the significance of ILK and hepatic ECM-signaling for regulation of hepatocyte proliferation and differentiation.

Tissue-specific transcription factors in progression of epithelial tumors

Dedifferentiation and epithelial-mesenchymal transition are important steps in epithelial tumor progression. A central role in the control of functional and morphological properties of different cell types is attributed to tissue-specific transcription factors which form regulatory cascades that define specification and differentiation of epithelial cells during embryonic development. The main principles of the action of such regulatory systems are reviewed on an example of a network of hepatocyte nuclear factors (HNFs) which play a key role in establishment and maintenance of hepatocytes--the major functional type of liver cells. HNFs, described as proteins binding to promoters of most hepatospecific genes, not only control expression of functional liver genes, but are also involved in regulation of proliferation, morphogenesis, and detoxification processes. One of the central components of the hepatospecific regulatory network is nuclear receptor HNF4alpha. Derangement of the expression of this gene is associated with progression of rodent and human hepatocellular carcinomas (HCCs) and contributes to increase of proliferation, loss of epithelial morphology, and dedifferentiation. Dysfunction of HNF4alpha during HCC progression can be either caused by structural changes of this gene or occurs due to modification of up-stream regulatory signaling pathways. Investigations preformed on a model system of the mouse one-step HCC progression have shown that the restoration of HNF4alpha function in dedifferentiated cells causes partial reversion of malignant phenotype both in vitro and in vivo. Derangement of HNFs function was also described in other tumors of epithelial origin. We suppose that tissue-specific factors that underlie the key steps in differentiation programs of certain tissues and are able to receive or modulate signals from the cell environment might be considered as promising candidates for the role of tumor suppressors in the tissue types where they normally play the most significant role.

CCAAT/enhancer binding protein-beta is a transcriptional regulator of peroxisome-proliferator-activated receptor-gamma coactivator-1alpha in the regenerating liver

The transcriptional coactivator peroxisome-proliferator-activated receptor-gamma coactivator-1alpha (PGC-1alpha) is induced in the liver in response to fasting and coordinates the activation of targets necessary for increasing energy production for gluconeogenesis and ketogenesis. After partial hepatectomy, the liver must restore its mass while maintaining metabolic homeostasis to ensure survival. Here we report that PGC-1alpha is rapidly and dramatically induced after hepatectomy, with an amplitude of induction that exceeds the fasting response. Maximal activation of PGC-1alpha after hepatectomy is dependent on the basic leucine zipper transcription factor, CCAAT/enhancer binding protein-beta (C/EBPbeta), a critical factor in hepatocyte proliferation. We demonstrate in vivo C/EBPbeta binding to C/EBP and cAMP response element sites in the PGC-1alpha promoter and show that the C/EBP site is essential for PGC-1alpha activation. Expression of the PGC-1alpha target, carnitine palmitoyl transferase 1a, the rate-limiting enzyme in fatty acid beta-oxidation, and of long-chain acyl-coenzyme A dehydrogenase, an enzyme involved in beta-oxidation of long chain fatty acids, was significantly reduced in C/EBPbeta(-/-) livers after hepatectomy. These findings identify C/EBPbeta as a direct activator of PGC-1alpha in the regenerating liver. The demonstration of a functional link between C/EBPbeta and PGC-1alpha activation provides a likely mechanism for how upstream signaling pathways in the regenerating liver can enable the adaptation to the changed metabolic status.

Serial changes in expression of functionally clustered genes in progression of liver fibrosis in hepatitis C patients

AIM: To investigate the relationship of changes in expression of marker genes in functional categories or molecular networks comprising one functional category or multiple categories in progression of hepatic fibrosis in hepatitis C (HCV) patients.

METHODS: Marker genes were initially identified using DNA microarray data from a rat liver fibrosis model. The expression level of each fibrosis associated marker gene was analyzed using reverse transcription-polymerase chain reaction (RT-PCR) in clinical biopsy specimens from HCV-positive patients (n = 61). Analysis of changes in expression patterns and interactions of marker genes in functional categories was used to assess the biological mechanism of fibrosis.

RESULTS: The profile data showed several biological changes associated with progression of hepatic fibrosis. Clustered genes in functional categories showed sequential changes in expression. Several sets of clustered genes, including those related to the extracellular matrix (ECM), inflammation, lipid metabolism, steroid metabolism, and some transcription factors important for hepatic biology showed expression changes in the immediate early phase (F1/F2) of fibrosis. Genes associated with aromatic amino acid (AA) metabolism, sulfur-containing AA metabolism and insulin/Wnt signaling showed expression changes in the middle phase (F2/F3), and some genes related to glucose metabolism showed altered expression in the late phase of fibrosis (F3/F4). Therefore, molecular networks showing serial changes in gene expression are present in liver fibrosis progression in hepatitis C patients.

CONCLUSION: Analysis of gene expression profiles from a perspective of functional categories or molecular networks provides an understanding of disease and suggests new diagnostic methods. Selected marker genes have potential utility for biological identification of advanced fibrosis.

CCAAT/enhancer binding protein-beta promotes the survival of intravascular rat pancreatic tumor cells via antiapoptotic effects

A transcriptional factor, CCAAT/enhancer binding protein-beta (C/EBP-beta), regulates a variety of cell functions in normal and neoplastic cells. Although the involvement of C/EBP-beta in metastasis has been demonstrated clinicopathologically in several types of human cancer, the mechanism by which it functions during the multistep process of metastasis remains largely unknown. We investigated the role of C/EBP-beta in the intravascular step of hematogenous metastasis in a rat pancreatic tumor cell line, AR42J-B13, as this step profoundly affects metastatic efficiency. C/EBP-beta-transfected AR42J-B13 (betaB13) cells acquired considerable resistance against serum toxicity, which was primarily mediated by apoptosis in vitro. Upregulated expression of Bcl-2 and Bcl-xL was seen in betaB13 cells. Enhanced early survival of intraportally injected betaB13 cells in the BALB/c nu/nu male mice liver, detected by the mRNA of a vector-specific gene, was observed. Nick-end labeling analysis of the tumor-injected liver revealed significantly lower rates of apoptosis among intravascular betaB13 tumor cells than among empty vector-transfected AR42J-B13 (mB13) cells. Finally, intrasplenically injected betaB13 cells established a larger number of colonies in the liver than did the mB13 cells. These findings suggest that C/EBP-beta may enhance hematogenous metastasis and its antiapoptotic effects may promote the survival of intravascular tumor cells.

Adipogenic potential can be activated during muscle regeneration

Fatty degeneration is observed in various neuromuscular diseases, but the mechanism(s) of its initiation remains unclear. To gain insight into the regulation of fatty degeneration, we employed a freeze-induced model of muscle degeneration/regeneration. Using this model, we examined the distribution of adipocyte-like cells with Oil Red-O staining and the expression pattern of adipogenic transcriptional factors, an adipocyte-terminal differentiation marker, and Wnt10b signaling molecules during muscle regeneration. Mice were subjected to freeze injury, and the gastrocnemius muscles were isolated 1, 3, 5, 7, 10, 14 and 28 days after surgery. Adipocyte-like cells with nuclei were readily observed, but not in normal muscle. Large amount of lipid accumulation was also observed in regenerating muscle. The area of Oil Red-O staining was significantly increased from 3 to 5 days after muscle injury and then rapidly decreased to almost control levels by day 10. Adipogenic transcriptional factors, sterol regulatory element binding protein-1c, CCAAT/enhancer-binding proteins alpha, beta and delta, peroxisome-proliferator activated receptors gamma1 and gamma2, and the terminal differentiation marker, leptin were significantly up-regulated in the early stage of muscle regeneration, suggesting activation of the adipogenic potential. Secreted Frizzled-related protein-2, a Wnt pathway inhibitory protein, was strongly up-regulated 3 days after muscle injury, suggesting active repression of the Wnt10b pathway. In regenerating muscle, expression of CCAAT/enhancer-binding protein alpha and peroxisome-proliferator activated receptor gamma2 proteins were increased 3 days after muscle injury. Taken together, our results suggest that adipogenic potential can be activated during muscle regeneration through increased adipogenic signaling in conjunction with decreased Wnt10b signaling.

Portal blood flow regulates volume recovery of the rat liver after partial hepatectomy: molecular evaluation

BACKGROUND/AIM

Liver regeneration is a finely tuned process that is closely regulated by multiple cell cycle steps. Although the portal blood flow affects liver regeneration, the molecular mechanism by which the blood flow regulates gene expression and liver function is largely unknown. The aim of this study was to investigate the molecular effect of portal blood flow on hepatocyte proliferation and gene regulation during liver regeneration.

MATERIALS AND METHODS

We developed a simple surgical rat model to investigate the relation between portal blood flow and liver regeneration by partially ligating the portal trunk with 8-0 Proline sutures under microscopy to reduce the blood flow by 40%. We investigated recovery of liver volume, DNA synthesis, and gene expression associated with cell cycle regulators, comparing partially hepatectomized (PH) rats without (PH group; n = 30) and with partial portal ligation (PHPL group; n = 30) for 7 days after the operation.

RESULTS

The hepatic tissue blood flow and the recovery ratio between liver weight and body weight in the PHPL group were significantly lower than in the PH group after hepatectomy. The peak 5-bromo-2'-deoxyuridine labeling index in the PHPL group was delayed and weak compared with the PH group. The expression of CT-1 and cyclin D, E, and B mRNAs indicated that the liver regeneration in the PHPL group was delayed and weak. In addition, there was reciprocal expression of C/EBPalpha and C/EBPbeta mRNAs, an observation supported by their nuclear protein levels. Furthermore, the cytochrome P-450 protein level in the PHPL group was higher than that in the PH group 1 day after hepatectomy.

CONCLUSION

The portal blood flow regulates the activity of liver regeneration and the gene expression associated with cell cycle regulators, while the functions are maintained.

C/EBPalpha: a tumour suppressor in multiple tissues?

The CCATT/enhancer binding protein alpha, C/EBPalpha, is a key transcription factor involved in late differentiation events of several cell types. Besides acting as a classical transcription factor, C/EBPalpha is also a well-characterized inhibitor of mitotic growth in most cell lines tested. In line with its anti-mitotic properties, C/EBPalpha has been shown to interact with, and alter the activities of, several cell cycle related proteins and a number of models as to the mechanistics of C/EBPalpha-mediated growth repression have been proposed. More recently, several reports have indicated that C/EBPalpha acts as a tumour suppressor in the hematopoietic system and that mutation within C/EBPalpha is sufficient to induce tumourigenesis. Here, we will review these data and probe the possibility that C/EBPalpha also act as a tumour suppressor in other C/EBPalpha-expressing tissues.

C/EBPbeta contributes to hepatocyte growth factor-induced replication of rodent hepatocytes

BACKGROUND/AIMS

Hepatocyte replication can be induced in vivo by hepatocyte growth factor (HGF), which might be used for gene therapy or to promote liver regeneration. However, the biochemical steps critical for this process are not clear. C/EBPbeta and C/EBPalpha are liver-enriched transcription factors that induce and inhibit hepatocyte replication, respectively. Because of their role in hepatocyte replication, this study examined the effect of HGF upon C/EBP proteins in vivo.

METHODS

Rats were treated with HGF, and the effect upon C/EBPs was evaluated in liver extracts. Normal or C/EBPbeta-deficient mice were treated with HGF, and the effect upon hepatocyte replication was determined.

RESULTS

HGF had no effect in rat liver upon C/EBPalpha or C/EBPbeta mRNA, nuclear protein, or nuclear DNA binding activity. However, HGF increased phosphorylated p90-RSK and ERK to 18- and 3-fold normal, respectively. These kinases phosphorylate C/EBPbeta and increase its transcriptional activity. The percentage of hepatocytes that replicated in C/EBPbeta-deficient mice after HGF administration was only 1.1%, which was lower than the value of 6.6% for hepatocytes from HGF-treated normal mice (P=0.005).

CONCLUSIONS

C/EBPbeta contributes to the induction of hepatocyte replication in response to HGF in rodents, which is likely due to post-translational modifications.

Dephosphorylated C/EBPalpha accelerates cell proliferation through sequestering retinoblastoma protein

CCAAT/enhancer-binding protein alpha (C/EBPalpha) has been previously considered a strong inhibitor of cell proliferation which uses multiple pathways to cause growth arrest. In this paper, we describe a new function of C/EBPalpha, which is an acceleration of cell proliferation. This new function of C/EBPalpha is created in proliferating livers by protein phosphatase 2A-mediated dephosphorylation of C/EBPalpha at Ser193. The Ser193-dephosphorylated C/EBPalpha interacts with retinoblastoma protein (Rb) independently on E2Fs and sequesters Rb, leading to a reduction of E2F-Rb repressors and to acceleration of proliferation. This new function of C/EBPalpha requires Rb, since the dephosphorylated C/EBPalpha does not promote proliferation in Rb-negative cells. We also show that a balance of Rb and Ser193-dephosphorylated C/EBPalpha determines if the cells are growth arrested or have an increased rate of proliferation. Consistently with these findings, a significant portion of Rb is sequestered into Rb-C/EBPalpha complexes in proliferating livers, and E2F-Rb complexes are not detectable in these livers. Our data demonstrate a new pathway by which the phosphorylation-dependent switch of biological functions of C/EBPalpha promotes liver proliferation.

Liver-enriched transcription factors in liver function and development. Part II: the C/EBPs and D site-binding protein in cell cycle control, carcinogenesis, circadian gene regulation, liver regeneration, apoptosis, and liver-specific gene regulation

In the first part of our review (see Pharmacol Rev 2002;54:129-158), we discussed the basic principles of gene transcription and the complex interactions within the network of hepatocyte nuclear factors, coactivators, ligands, and corepressors in targeted liver-specific gene expression. Now we summarize the role of basic region/leucine zipper protein family members and particularly the albumin D site-binding protein (DBP) and the CAAT/enhancer-binding proteins (C/EBPs) for their importance in liver-specific gene expression and their role in liver function and development. Specifically, regulatory networks and molecular interactions were examined in detail, and the experimental findings summarized in this review point to pivotal roles of DBP and C/EBPs in cell cycle control, carcinogenesis, circadian gene regulation, liver regeneration, apoptosis, and liver-specific gene regulation. These regulatory proteins are therefore of great importance in liver physiology, liver disease, and liver development. Furthermore, interpretation of the vast data generated by novel genomic platform technologies requires a thorough understanding of regulatory networks and particularly the hierarchies that govern transcription and translation of proteins as well as intracellular protein modifications. Thus, this review aims to stimulate discussions on directions of future research and particularly the identification of molecular targets for pharmacological intervention of liver disease.

Liver regeneration: from myth to mechanism

The unusual regenerative properties of the liver are a logical adaptation by organisms, as the liver is the main detoxifying organ of the body and is likely to be injured by ingested toxins. The numerous cytokine- and growth-factor-mediated pathways that are involved in regulating liver regeneration are being successfully dissected using molecular and genetic approaches. So what is known about this process at present and which questions remain?

C/EBP beta isoforms LIP and LAP modulate progression of the cell cycle in the regenerating mouse liver

The CCAAT enhancer-binding protein (C/EBP) beta gene can produce several N-terminally truncated isoforms. Liver-enriched activator protein (LAP) is a transcriptional activator in many systems, whereas liver-enriched inhibitory protein (LIP) is regarded as a functional LAP antagonist. In this study, we examined the impact of these two proteins on cell cycle progression in the regenerating liver. Adenoviral overexpression of LAP, in addition to its role as a transactivator of liver-specific genes, led to a delayed S-phase entry of hepatocytes after partial hepatectomy (PH) in vivo. This delay was accompanied by decreased expression of cyclin A and E as well as proliferating cell nuclear antigen and decreased cyclin-dependent kinase 2 activity at the G1/S boundary. This observation is not explained by increased p21(CIP1/Waf1) expression or lack of phosphorylation of external LAP, but LAP overexpression triggered a decreased C/EBP-alpha/C/EBP-alpha-30 ratio and a reduced basal c-jun level in the liver. In contrast, adenoviral overexpression of LIP resulted in a stronger and earlier induction of cyclin A and E after PH, but did not change the timing and extent of cyclin-dependent kinase 2 activity or the amount of hepatocytes that entered S phase in this model. In the LIP expressing group, both C/EBP-alpha isoforms and c-jun were more strongly induced after PH. In conclusion, the LAP/LIP ratio is an important modulator of cell cycle progression during liver regeneration. In the context of previous studies, our results demonstrate that LAP, through a dose-dependent effect, withholds a dual activating and inhibiting role on hepatocyte proliferation in vivo.

Adult-adult living donor liver transplantation

After the first report from Denver in 1998 of a successful liver transplant in an adult using the right lobe from a living donor, the procedure was rapidly adopted by many transplant centers as a potential solution to the critical shortage of donor livers. By the end of 2000, when the National Institutes of Health held a Consensus Conference on Adult-Adult Living Donor Transplantation (AALDT), a substantial body of literature had already developed and many of the associated technical and medical pitfalls had been defined. The exponential expansion of the procedure came to a dramatic halt in January 2002 when the death of a donor occurred at Mount Sinai Hospital--the busiest AALDT center in the United States. This led to a widespread reassessment of the risks inherent in right lobe donation. Yet, the problem that drove the development of this controversial technique--the dire shortage of organs for transplantation--still persists. After a 50% drop in the number of AALDT procedures performed in the United States in 2002 compared with 2001, centers are regrouping and approaching AALDT with renewed interest, albeit with heightened awareness of the attendant risks. On November 2, 2002, a state-of-the-art symposium on AALDT was held in Boston, MA, under the combined auspices of the American Hepatico-Pancreato-Biliary Association and the American Association for the Study of Liver Diseases. This article comprises the presentations at the symposium on three subjects of critical importance concerning AALDT. These include advances in surgical technique, candidate selection, and hepatic regeneration; each subject is acknowledged by an expert in the field.

Epimorphin is involved in differentiation of rat hepatic stem-like cells through cell–cell contact

Epimorphin, a mesenchymal cell surface-associated molecule, is detected on hepatic stellate cells (HSCs) in the liver. Here, we show the involvement of epimorphin in differentiation of rat hepatic stem-like cells (HSLCs) through contact with HSCs. HSLCs, isolated from adult rats, cultured in stellate cell-conditioned medium had no phenotypic and morphological changes, whereas HSLCs co-cultured with HSCs expressed albumin, transferrin, and tyrosine aminotransferase. An anti-epimorphin antibody inhibited hepatocytic differentiation of HSLCs in co-culture. Furthermore, epimorphin induced mRNA expression of albumin, transferrin, tyrosine aminotransferase, and gamma-glutamyl transpeptidase with decrease of c-kit and musashi-1. Morphologically, HSLCs piled up when co-cultured with HSCs, which was dramatically inhibited by an anti-epimorphin antibody. HSLCs contact with epimorphin started piling up, changed their shape from flat to cuboidal, and subsequently developed bile-canaliculi-like structures. In conclusion, epimorphin is a factor that induces differentiation of hepatic stem-like cells through epithelial-mesenchymal cell contact.

Massive hepatic apoptosis associated with TGF-beta1 activation after Fas ligand treatment of IGF binding protein-1-deficient mice

Acute liver failure caused by viral hepatitis or toxic damage involves both apoptotic and necrotic pathways. IGF binding protein-1 (IGFBP-1), a hepatocyte-derived secreted protein, is required for normal liver regeneration. To determine whether IGFBP-1 could prevent liver injury that entails direct stimulation of hepatocyte apoptosis, IGFBP-1(-/-) mice, IGFBP-1(+/+) mice, and mice pretreated with Ab's against IGFBP-1 were treated with a normally sublethal dose of Fas agonist. IGFBP-1 deficiency was associated with massive hepatocyte apoptosis and caspase activation within 3 hours of Fas agonist treatment, which could be corrected by pretreatment with IGFBP-1. IGFBP-1-deficient livers had enhanced signaling via the integrin receptor at early times (0.5 to 1 hour) after Fas agonist treatment accompanied by elevated activated matrix metalloproteinase-9 (MMP-9), a known target of fibronectin signaling and activator of TGF-beta. Within 3 hours of Fas agonist treatment, elevated expression of active TGF-beta1, a hepatocyte apoptogen, was observed in IGFBP-1-deficient livers that correlated with the appearance of the apoptotic process. Both MMP-9 and TGF-beta1 expression were suppressed by IGFBP-1 treatment, supporting their role in the apoptotic process. IGFBP-1(-/-) mice also displayed increased injury in a toxic hepatic injury model caused by CCl(4). These findings indicate that IGFBP-1 functions as a critical hepatic survival factor in the liver by reducing the level of proapoptotic signals.

Hepatocyte growth factor activates CCAAT enhancer binding protein and cell replication via PI3-kinase pathway

Hepatocyte growth factor (HGF), a ligand of c-Met receptor, stimulates activation of cellular kinases via phosphatidylinositol 3-kinase (PI3-kinase). CCAAT/enhancer binding protein (C/EBP) controls cell cycle progression. The present study was designed to determine whether HGF activates C/EBP in association with the S-phase entrance for cell replication and whether PI3-kinase contributes to the activation of C/EBP. Treatment of H4IIE cells, a hepatocyte-derived cell line, with HGF increased protein binding to the C/EBP binding site at an early time. Immunodepletion, subcellular fractionation, and confocal microscopic analyses showed that the HGF-induced C/EBP DNA binding activity depended on nuclear translocation of C/EBP beta. Whereas stable transfection of the p110 catalytic subunit of PI3-kinase enhanced HGF-mediated nuclear translocation of C/EBP beta and DNA binding, stable transfection of p85 subunit or chemical inhibition of PI3-kinase completely blocked C/EBP activation. HGF increased luciferase reporter activity in cells transfected with a mammalian cell expression vector containing -1.65 kilobase rGSTA2 promoter comprising C/EBP response element (pGL-1651). Whereas transfection with pCMV500, a control vector, allowed pGL-1651 to respond to HGF, expression of dominant negative mutant C/EBP completely inhibited the ability of HGF to stimulate the reporter gene expression. Flow cytometric analysis showed that HGF caused an increase in the area of S phase with a reciprocal decrease in that of G(1) phase, suggesting that HGF promoted cell cycle progression to S phase. In conclusion, HGF induces nuclear translocation of C/EBP beta via the PI3-kinase pathway and stimulates C/EBP DNA binding and gene transcription and that the PI3-kinase-mediated C/EBP activation by HGF may contribute to cell replication.