Inhibition of Hedgehog Delays Liver Regeneration through Disrupting the Cell Cycle

Liver regeneration is a complicated biological process orchestrated by various liver resident cells. Hepatic cell proliferation and reconstruction of the hepatic architecture involve multiple signaling pathways. It has been reported that the Hh signal is involved in liver regeneration. However, the signal transduction pathways and cell types involved are ill studied. This study aimed to investigate hedgehog signal response cell types and the specific molecular mechanism involved in the process of liver regeneration. Partial hepatectomy (PH) of 70% was performed on ICR (Institute of Cancer Research) mice to study the process of liver regeneration. We found that the hedgehog signal was activated significantly after PH, including hedgehog ligands, receptors and intracellular signaling molecules. Ligand signals were mainly expressed in bile duct cells and non-parenchymal hepatic cells, while receptors were expressed in hepatocytes and some non-parenchymal cells. Inhibition of the hedgehog signal treated with vismodegib reduced the liver regeneration rate after partial hepatectomy, including inhibition of hepatic cell proliferation by decreasing Cyclin D expression and disturbing the cell cycle through the accumulation of Cyclin B. The current study reveals the important role of the hedgehog signal and its participation in the regulation of hepatic cell proliferation and the cell cycle during liver regeneration. It provides new insight into the recovery of the liver after liver resection.     

The roles of signaling pathways in liver repair and regeneration

The liver has remarkable regeneration potency that restores liver mass and sustains body hemostasis. Liver regeneration through signaling pathways following resection or moderate damages are well studied. Various cell signaling, growth factors, cytokines, receptors, and cell types implicated in liver regeneration undergo controlled hypertrophy and proliferation. Some aspects of liver regeneration have been discovered and many investigations have been carried out to identify its mechanisms. However, for optimizing liver regeneration more should be understood about mechanisms that control the growth of hepatocytes and other liver cell types in adults. The current paper deals with the possible applicability of liver regeneration signaling pathways as a target for therapeutic approaches and preventing various liver damages. Furthermore, the latest findings of spectrum-specific signaling pathway mechanisms that underlie liver regeneration are briefly described.     

The role of Kupffer cells in rat liver regeneration revealed by cell-specific microarray analysis

Liver regeneration after partial hepatectomy is a process with various types of cells involved. The role of Kupffer cells (KCs) in liver regeneration is still controversial. In this study we isolated KCs from regenerating liver and conducted cell-specific microarray analysis. The results demonstrated that the controversial role of KCs in liver regeneration could be explained with the expression patterns of TGF-α, IL-6, TNF, and possibly IL-18 during liver regeneration. IL-18 may play an important role in negative regulation of liver regeneration. The functional profiles of gene expression in KCs also indicated that KC signaling might play a negative role in cell proliferation: signaling genes were down regulated before cell division. Immune response genes in KCs were also down regulated during liver regeneration, demonstrating similar expression profiles to that of hepatocytes. The expression patterns of key genes in these functional categories were consistent with the temporal functional profiles.     

Liver tissue engineering: Recent advances in the development of a bio-artificial liver

Orthotopic liver transplantation is the most common treatment for patients with end-stage liver failure. However, liver transplantation is greatly limited by a donor shortage. Liver tissue engineering may offer a promising strategy to solve this problem by providing transplantable, bioartificial livers. Diverse types of cells, biomaterials, and growth factor delivery systems have been tested for efficient regeneration of liver tissues that possess hepatic functions comparable to native livers. This article reviews recent advances in liver tissue engineering and describes cell sources, biomaterial scaffolds, and growth factor delivery systems that are currently being used to improve the regenerative potential of tissue-engineered livers.     

Consideration on the mechanism of liver regeneration on experimental results]

Liver has enough functional capacity and regeneration ability. Liver parenchymal cells are usually stable, and it is thought being at stage of Go in cell cycle. But liver cells are easily into progressive stage and the liver recovers its functions and volume after partial hepatectomy or liver injury. Usually the studies of liver regeneration are done by studying control of growth of isolated hepatocytes in primary culture in vitro, and by pathological considerations of experimental injured models of liver in vivo. In this paper, we considered known several hepatotrophic factors including our experimental results, and regeneration mechanisms by noticing appearance of albumin positive hepatocytes in injured Nagase analbuminemic rat (NAR).     

The role of hepatocytes and oval cells in liver regeneration and repopulation

The liver has the unique capacity to regulate its growth and mass. In rodents and humans, it grows rapidly after resection of more than 50% of its mass. This growth process, as well as that following acute chemical injury is known as liver regeneration, although growth takes place by compensatory hyperplasia rather than true regeneration. In addition to hepatocytes and non-parenchymal cells, the liver contains intra-hepatic "stem" cells which can generate a transit compartment of precursors named oval cells. Liver regeneration after partial hepatectomy does not involve intra or extra-hepatic (hemopoietic) stem cells but depends on the proliferation of hepatocytes. Transplantation and repopulation experiments have demonstrated that hepatocytes, which are highly differentiated and long-lived cells, have a remarkable capacity for multiple rounds of replication. In this article, we review some aspects of the regulation of hepatocyte proliferation as well as the interrelationships between hepatocytes and oval cells in different liver growth processes. We conclude that in the liver, normally quiescent differentiated cells replicate rapidly after tissue resection, while intra-hepatic precursor cells (oval cells) proliferate and generate lineage only in situations in which hepatocyte proliferation is blocked or delayed. Although bone marrow stem cells can generate oval cells and hepatocytes, transdifferentiation is very rare and inefficient.     

Fas engagement accelerates liver regeneration after partial hepatectomy

Fas (CD95) is a receptor involved in induction of apoptotic cell death of Fas-bearing cells, including hepatocytes and T cells. Injection of Fas-specific antibodies into mice leads to fulminant hepatic failure and death. Fas also transduces growth-promoting signals in proliferating T cells, fibroblasts and some tumor cells. Here we show that partial hepatectomy, which triggers the immediate onset of liver regeneration, protected mice against the lethal effects of Fas-specific antibodies and prevented hepatocyte apoptosis in response to Fas engagement in vivo. Furthermore, Fas engagement accelerated liver regeneration after partial hepatectomy. Liver regeneration kinetics were delayed in mutant mice with decreased cell surface Fas expression (lpr mice). In contrast, regeneration was not delayed in lpr-cg mutant mice, which have a Fas mutation that prevents Fas-induced death but not Fas-dependent proliferative stimulation. Our results indicate that Fas engagement on cells in regenerating or healing tissues may promote cell growth.     

What fires prometheus? The link between inflammation and regeneration following chronic liver injury

Liver progenitor cells (LPCs) play a major role in the regeneration process after chronic liver damage, giving rise to hepatocytes and cholangiocytes. Thus, they provide a cell-based therapeutic alternative to organ transplant, the current treatment of choice for end-stage liver disease. In recent years, much attention has focused on unravelling the cytokines and growth factors that underlie this response. Liver regeneration following acute damage is achieved by proliferation of mature hepatocytes; yet similar cytokines, most related to the inflammatory process, are implicated in both acute and chronic liver regeneration. Thus, many recent studies represent attempts to identify LPC-specific factors. This review summarises our current understanding of LPC biology with a particular focus on the liver inflammatory response being associated with the induction of LPCs in the liver. We will describe: (i) the pathways of liver regeneration following acute and chronic damage; (ii) the similarities and differences between the two pathways; (iii) the liver inflammatory environment; (iv) the unique features of liver immunology as well as (v) the interactions between liver immune cells and LPCs. Combining data from studies on the LPC-driven regeneration process with the knowledge in the field of liver immunology will improve our understanding of the LPC response and allow us to regulate these cells in vivo and in vitro for future therapeutic strategies to treat chronic liver disease.     

Liver stem/progenitor cells: their characteristics and regulatory mechanisms

Liver stem cells give rise to both hepatocytes and bile duct epithelial cells also known as cholangiocytes. During liver development hepatoblasts emerge from the foregut endoderm and give rise to both cell types. Colony-forming cells are present in the liver primordium and clonally expanded cells differentiate into either hepatocytes or cholangiocytes depending on culture conditions, showing stem cell characteristics. The growth and differentiation of hepatoblasts are regulated by various extrinsic signals. For example, periportal mesenchymal cells provide a cue for bipotential hepatoblasts to become cholangiocytes, and mesothelial cells covering the parenchyma support the expansion of foetal hepatocytes by producing growth factors. The adult liver has an extraordinary capacity to regenerate, and after 70% hepatectomy the liver recovers its original mass by replication of the remaining hepatocytes without the activation of liver stem cells. However, in certain types of liver injury models, liver stem/progenitor-like cells, known as oval cells in rodents, proliferate around the portal vein, while the roles of such cells in liver regeneration remain a matter of debate. Clonogenic and bipotential cells are also present in the normal adult liver. In this minireview we describe recent studies on liver stem/progenitor cells by focusing on extracellular signals.     

Transforming growth factor-β signaling confers hepatic stellate cells progenitor features after partial hepatectomy

Liver regeneration involves not only hepatocyte replication but progenitor aggregation and scarring. Partial hepatectomy (PH), an established model for liver regeneration, reactivates transforming growth factor-β (TGF-β) signaling. Hepatic stellate cells (HSCs) are primarily responding cells for TGF-β and resident in stem cell niche. In the current study, PH mice were treated with SB-431542, an inhibitor of TGF-β Type I receptor, aiming to address the role of TGF-β signaling on the fate determination of HSCs during liver regeneration. After PH, control mice exhibited HSCs activation, progenitor cells accumulation, and a fraction of HSCs acquired the phenotype of hepatocyte or cholangiocyte. Blocking TGF-β signaling delayed proliferation, impaired progenitor response, and scarring repair. In SB-431542 group, merely no HSCs were found coexpressed progenitor makers, such as SOX9 and AFP. Inhibition of TGF-β pathway disturbed the epithelial-mesenchymal transitions and diminished the nuclear accumulation of β-catenin as well as the expression of cytochrome P450 2E1 in HSC during liver regeneration. We identify a key role of TGF-β signaling on promoting HSC transition, which subsequently becomes progenitor for generating liver epithelial cells after PH. This process might interact with an acknowledged stem cell function signaling, Wnt/β-catenin.     

Laminin expression during bone marrow mononuclear cell transplantation in hepatectomized rats

The adult bone marrow retains two populations of stem cells with emerging importance for the treatment of diverse liver diseases: hematopoietic stem cells (HSCs) and mesenchymal stem cells (MSCs). However, the mechanisms that control liver regeneration after bone marrow cell transplantation are still controversial. Liver regeneration after partial hepatectomy is a complex process that requires the proliferation of all hepatic cells. Growth factors, cytokines and extracellular matrix molecules are key elements in this process. Laminins are a family of extracellular matrix proteins with adhesive and chemotactic functions, expressed in the portal and centrolobular veins of the normal liver. The aim of this study was to investigate laminin expression during liver regeneration induced by partial hepatectomy followed by bone marrow mononuclear cell (BMMNC) transplantation. Rat BMMNCs were isolated by Ficoll-gradient centrifugation, stained with DAPI and injected into recently hepatectomyzed rats via the portal vein. Liver sections obtained 15 min, 1 day and 3 days after the surgery were immunolabeled with anti-rat CD34 and/or laminin primary antibodies and observed under a laser scanning confocal microscope. Results showed that 15 min after partial hepatectomy, a transplanted CD34+ HSC was found in contact with laminin, which was localized in the portal and centrolobular veins of rat livers. Furthermore, 1 and 3 days after hepatectomy, transplanted BMMNCs were found in the hepatic sinusoids expressing laminin. These results strongly suggest that laminin might be an important extracellular matrix component for bone marrow cell attachment and migration in the injured liver.     

The role of stem cells in cutaneous wound healing: what do we really know?

Wound repair is a complex process involving the orchestrated interaction of multiple growth factors, cytokines, chemokines, and cell types. Dysregulation of this process leads to problems such as excessive healing in the form of keloids and hypertrophic scars and chronic, nonhealing wounds. These issues have broad global implications. Stem cells offer enormous potential for enhancing tissue repair and regeneration following injury. The rapidly developing fields of stem cell biology and skin tissue engineering create translational opportunities for the development of novel stem cell-based wound-healing therapies.     

异质性干细胞增殖过程中的熵变化

干细胞在多细胞生物体内广泛存在,其增殖过程在生命体的生长、发育、衰老、组织修复过程中起着重要作用。正常组织中的细胞增殖过程受到严格的控制,干细胞的异常增殖与恶性肿瘤、肥胖症、再生障碍性贫血等疾病有密切关系。生命体内异质性细胞的增殖过程是复杂的动力系统行为,干细胞异常增殖过程伴随细胞的可塑性变化和细胞间相互作用的再平衡过程,如何对这一过程进行定量描述是重要的研究课题。本文构建包含细胞的增殖分化指标和异常增殖性指标异质性的干细胞增殖模型,通过所建立的模型研究由于微环境变化引起的细胞异常增殖过程的熵变化,建立不同增殖条件下的系统熵变化与宏观动力学和系统参数之间的关系。结果表明,在细胞微环境变化引起异常增殖和恢复的过程中,系统的熵与细胞数量之间存在对应关系,而与微环境变化的路径无关。 此外,熵对细胞数量的依赖关系在异常增殖和恢复阶段表现出不同的行为,显示了生物过程的微观不可逆性。本文从物理学的角度对细胞异常增殖过程中熵变化与细胞数量变化的动力学给出定量刻画,为定量描述异质性干细胞增殖过程给出新的研究思路。     

The immediate-early growth response in regenerating liver and insulin-stimulated H-35 cells: comparison with serum-stimulated 3T3 cells and identification of 41 novel immediate-early genes.

Liver regeneration provides a unique system for analysis of mitogenesis in intact, fully developed animals. Cellular immediate-early genes likely play an important role in cell cycle regulation and have been extensively studied in mitogen-stimulated fibroblasts lymphocytes but not in liver. We have begun to characterize the immediate-early growth response genes of mitogen-stimulated liver cells, specifically, regenerating liver and insulin-stimulated Reuber H-35 hepatoma cells, and to address differences in growth response between different cell types. Through subtraction and differential screening of cDNA libraries from regenerating liver and insulin-treated H-35 cells, we have extensively characterized 341 differentially expressed clones and identified 52 immediate-early genes. These genes have been partially sequenced and subjected to Northern (RNA) blot analysis, and 41 appear to be novel. Surprisingly, two-thirds of these genes are also expressed in BALB/c 3T3 cells, but only 10 were identified in previous studies of 3T3 cells, and of these, 6 include well-known genes like jun and fos, and only 4 are novel. Approximately one-third of the immediate-early genes identified in mitogen-stimulated liver cells or serum-stimulated NIH 3T3 cells are expressed in a tissue-specific fashion, indicating that cell type-specific regulation of the proliferative response occurs during the immediate-early period. Our findings indicate that the immediate-early response is unusually complex for the first step in a regulatory cascade, suggesting that multiple pathways must be activated. The abundance of immediate-early genes and the highly varied pattern of their expression in different cell types suggest that the tissue specificity of the proliferative response arises from the particular set of these genes expressed in a given tissue.     

Regeneration therapy for oral disease

The aim of this paper is to provide a review of the current understanding of the mechanisms, cell and factors required for regeneration and restoration of periodontal tissue around natural teeth. Periodontal regeneration is a complex multifactorial process involving cell populations: periodontal ligament cells, bone cells, gingival fibroblasts and epithelial cells. This paper describes bone graft, guided tissue regeneration and enamel matrix derivative with the application of growth factors.     

Exogenous growth factors stimulate the regeneration of ganglion cells in the chicken retina

Recent reports have found that the posthatch chicken retina has the capacity for neuronal regeneration. The purpose of this study was to test whether the types of cells destroyed by neurotoxic lesions influence the types of cells that are regenerated, and whether exogenous growth factors stimulate neural regeneration in the chicken retina. N-methyl-D-aspartate (NMDA) was used to destroy amacrine and bipolar cells; kainate was used to destroy bipolar, amacrine, and ganglion cells; colchicine was used to selectively destroy ganglion cells. Following toxin-induced damage, bromo-deoxyuridine was used to label proliferating cells. In some animals, growth factors were injected into the vitreous chamber of the eye. We found that the proliferation of cells within the retina was stimulated by toxin-induced cell loss, and by insulin and FGF2. After either kainate- or colchicine-induced retinal damage, some of the newly generated cells expressed markers and had the morphology of ganglion cells. The combination of insulin and FGF2 stimulated the regeneration of ganglion cells in kainate- and colchicine-treated retinas. We conclude that exogenous growth factors can be used to stimulate neural regeneration in the retina. We propose that the type of neuron destroyed in the retina may allow or promote the regeneration of that neuronal type.     

Regeneration of roots, shoots and embryos: physiological, biochemical and molecular aspects

When the proper stimuli are given, somatic plant cells may form adventitious embryos, roots or shoots. The three pathways of regeneration show apparent similarities. They consist of three analogous phases: 1) dedifferentiation (during which the tissue becomes competent to respond to the organogenic/embryogenic stimulus), 2) induction (during which cells become determined to form either a root, a shoot or an embryo), and 3) realization (outgrowth to an organ or an embryo). The first phase may involve a period of callus growth (indirect regeneration), but often cells present in the explant become competent without cell division or without cell division at a large scale (direct regeneration). In an explant, only very few cells show the organogenic/embryogenic response. In direct regeneration, the three regenerative pathways start from cells in different tissues. This is most obvious when the different types of regeneration occur in the same explant. The hormonal trigger for the dedifferentiation phase is a general one, probably auxin. During the induction phase, each pathway requires specific hormonal triggers. During the realization phase, hormones should be absent or at low concentration. The successive steps in the regeneration process coincide with events on the molecular and biochemical levels, but so far no coherent picture has emerged. In particular during the early stages of regeneration, research on these levels is hampered by a technical problem, viz., the very low proportion of cells that participate in the process of regeneration. New methods may overcome this problem. This revised version was published online in July 2006 with corrections to the Cover Date.