Angiogenesis-independent endothelial protection of liver: role of VEGFR-1

Interleukin-6 mediates angiotensinogen gene expression during liver regeneration

Background

Angiotensinogen is the precursor of angiotensin II, which is associated with ischemia-reperfusion injury. Angiotensin II reduces liver regeneration after hepatectomy and causes dysfunction and failure of reduced-size liver transplants. However, the regulation of angiotensinogen during liver regeneration is still unclear.

Aims

To investigate the regulation of angiotensinogen during liver regeneration for preventing angiotensin II-related ischemia-reperfusion injury during liver regeneration.

Methods

A mouse in vitro partial hepatectomy animal model was used to evaluate the expression of interleukin-6 (IL-6) and angiotensinogen during liver regeneration. Serum IL-6 and angiotensinogen were detected by enzyme immunoassay (EIA). Angiotensinogen mRNA was detected by RT-PCR. Tissue levels of angiotensinogen protein were detected by Western blot analysis. Primary cultures of mouse hepatocytes were used to investigate IL-6-induced angiotensinogen. Chemical inhibitors were used to perturb signal transduction pathways. Synthetic double-stranded oligodeoxynucleotides (ODNs) were used as ‘decoy’ cis-elements to investigate transcription. Ki 67 staining and quantification were used to verify liver regeneration.

Results

In the in vivo model, the levels of serum IL-6 and angiotensinogen correlated. In the in vitro model, IL-6 transcriptionally regulated angiotensinogen expression. Additionally, IL-6 mediated angiotensinogen expression through the Janus kinase (JAK)/signal transducer and activator of transcription 3 (STAT3) and JAK/p38 signaling. Decoy ODN analyses revealed that STAT3 and nuclear factor-kB (NF-kB) also played critical roles in the transcriptional regulation of angiotensinogen by IL-6. IL-6-mediated signaling, JAK2, STAT3 and p38 inhibitors reduced angiotensinogen expression in the partially hepatectomized mice.

Conclusion

During liver regeneration, IL-6-enhanced angiotensinogen expression is dependent on the JAK/STAT3 and JAK/p38/NF-kB signaling pathways. Interruption of the molecular mechanisms of angiotensinogen regulation may be applied as the basis of therapeutic strategies for preventing angiotensin II-related ischemia-reperfusion injury during liver regeneration.

Liver sinusoidal endothelial cells and liver regeneration

Liver sinusoidal endothelial cells (LSECs) have long been noted to contribute to liver regeneration after liver injury. In normal liver, the major cellular source of HGF is the hepatic stellate cell, but after liver injury, HGF expression has been thought to increase markedly in proliferating LSECs. However, emerging data suggest that even after injury, LSEC expression of HGF does not increase greatly. In contrast, bone marrow progenitor cells of LSECs (BM SPCs), which are rich in HGF, are recruited to the liver after injury. This Review examines liver regeneration from the perspective that BM SPCs that have been recruited to the liver, rather than mature LSECs, drive liver regeneration.

Leukotriene B4 type-1 receptor signaling promotes liver repair after hepatic ischemia/reperfusion injury through the enhancement of macrophage recruitment

Recruited macrophages play a critical role in liver repair after acute liver injury. Leukotriene B4 (LTB4) is a potent chemoattractant for macrophages. In this study, we investigated the role of LTB4 receptor type 1 (BLT1) in liver repair during hepatic ischemia/reperfusion (I/R) injury. BLT1-knockout mice (BLT1(-/-)) or their wild-type counterparts (WT) were subjected to partial hepatic I/R. Compared with WT, BLT1(-/-) exhibited delayed liver repair and hepatocyte proliferation accompanied by a 70% reduction in the recruitment of macrophages and a 70-80% attenuation in hepatic expression of epidermal growth factor (EGF), vascular endothelial growth factor (VEGF), and VEGF receptor 1 (VEGFR1). Disruption of BLT1 signaling also reduced the expression of EGF by 67% on recruited macrophages expressing VEGFR1 in the injured liver. Treatment of WT mice with an EGF-neutralizing antibody delayed liver repair and reduced macrophage recruitment, compared with control immunoglobulin G (IgG). BLT1 signaling enhanced the expression of VEGF, VEGFR1, and EGF in isolated peritoneal macrophages in vitro. These results indicate that BLT1 signaling plays a role in liver repair after hepatic I/R through enhanced expression of EGF in recruited macrophages and that the development of a specific agonist for BLT1 could be useful for liver recovery from acute liver injury.

Phosphoproteomic analysis implicates the mTORC2-FoxO1 axis in VEGF signaling and feedback activation of receptor tyrosine kinases

The vascular endothelial growth factor (VEGF) signaling pathway plays a pivotal role in normal development and also represents a major therapeutic target for tumors and intraocular neovascular disorders. The VEGF receptor tyrosine kinases promote angiogenesis by phosphorylating downstream proteins in endothelial cells. We applied a large-scale proteomic approach to define the VEGF-regulated phosphoproteome and its temporal dynamics in human umbilical vein endothelial cells and then used siRNA (small interfering RNA) screens to investigate the function of a subset of these phosphorylated proteins in VEGF responses. The PI3K (phosphatidylinositol 3-kinase)-mTORC2 (mammalian target of rapamycin complex 2) axis emerged as central in activating VEGF-regulated phosphorylation and increasing endothelial cell viability by suppressing the activity of the transcription factor FoxO1 (forkhead box protein O1), an effect that limited cellular apoptosis and feedback activation of receptor tyrosine kinases. This FoxO1-mediated feedback loop not only reduced the effectiveness of mTOR inhibitors at decreasing protein phosphorylation and cell survival but also rendered cells more susceptible to PI3K inhibition. Collectively, our study provides a global and dynamic view of VEGF-regulated phosphorylation events and implicates the mTORC2-FoxO1 axis in VEGF receptor signaling and reprogramming of receptor tyrosine kinases in human endothelial cells.

Increased plasma levels of microparticles expressing CD39 and CD133 in acute liver injury

BACKGROUND

We have previously demonstrated that CD133 and CD39 are expressed by hematopoietic stem cells (HSC), which are mobilized after liver injury and target sites of injury, limit vascular inflammation, and boost hepatic regeneration. Plasma microparticles (MP) expressing CD39 can block endothelial activation. Here, we tested whether CD133 MP might be shed in a CD39-dependent manner in a model of liver injury and could potentially serve as biomarkers of liver failure in the clinic.

METHODS

Wild-type and Cd39-null mice were subjected to acetaminophen-induced liver injury. Mice were sacrificed and plasma MP were isolated by ultracentrifugation. HSC and CD133 MP levels were analyzed by fluorescence-activated cell sorting. Patients were enrolled with acute (n=5) and acute on chronic (n=5) liver injury with matched controls (n=7). Blood was collected at admission and plasma CD133 and CD39 MP subsets were analyzed by fluorescence-activated cell sorting.

RESULTS

HSC and CD133 MP levels were significantly increased only in the plasma of wild-type mice with acetaminophen hepatotoxicity (P<0.05). No increases in CD133 MP were noted in Cd39-null mice. Plasma MP increases were observed in patients with liver injury. These MP were characterized by significantly higher levels of CD39 (P<0.05).

CONCLUSIONS

HSC and plasma CD133 MP levels increase in a CD39-dependent manner during experimental acute liver injury. Increased levels of CD39 MP are differentially noted in patients with liver injury. Further research is needed to determine whether MP fluxes are secondary to pathophysiologic insults to the liver or might reflect compensatory responses.

The RNA aptamer inhibiting human vesicular endothelial growth factor receptor 1 without affecting cytokine binding

Angiogenesis, a process of new blood vessel formation, is crucial not only for many physiological events but also for a number of diseases. The signaling pathways through members of the vesicular endothelial growth factor (VEGF) family play fundamental roles in angiogenesis. In this study, we identified inhibitory RNA aptamers against human Flt-1, a receptor of VEGF. One of the isolates, aptamer #38, showed a 50% inhibitory concentration (IC50) of 23 nM in the cell-based autophosphorylation assay, and the IC50 value was decreased to 6.3 nM upon removal of 32 dispensable nucleotides from parental #38 with a length of 72 nucleotides. Interestingly, the surface plasmon resonance-based or affinity resin-based binding study revealed that #38 and its shortened derivative, #38Jr, do not interfere with binding of VEGF or heparin, a functional cofactor, to Flt-1. Importantly, aptamer #38 does not affect the decoy activity of soluble Flt-1. These findings suggest that #38 prevents the conformational activation of Flt-1 associated with VEGF. Therefore, aptamer #38 might provide us with a unique tool for blocking the VEGF signaling specific to Flt-1, unlike most other known VEGF signaling blockers such as VEGF inhibitors, anti-Flt-1 antibodies, and decoy soluble receptors.

Conditional genetic elimination of hepatocyte growth factor in mice compromises liver regeneration after partial hepatectomy

Hepatocyte growth factor (HGF) has been shown to be indispensable for liver regeneration because it serves as a main mitogenic stimulus driving hepatocytes toward proliferation. We hypothesized that ablating HGF in adult mice would have a negative effect on the ability of hepatocytes to regenerate. Deletion of the HGF gene was achieved by inducing systemic recombination in mice lacking exon 5 of HGF and carrying the Mx1-cre or Cre-ER(T) transgene. Analysis of liver genomic DNA from animals 10 days after treatment showed that a majority (70-80%) of alleles underwent cre-induced genetic recombination. Intriguingly, however, analysis by RT-PCR showed the continued presence of both unrecombined and recombined forms of HGF mRNA after treatment. Separation of liver cell populations into hepatocytes and non-parenchymal cells showed equal recombination of genomic HGF in both cell types. The presence of the unrecombined form of HGF mRNA persisted in the liver in significant amounts even after partial hepatectomy (PH), which correlated with insignificant changes in HGF protein and hepatocyte proliferation. The amount of HGF produced by stellate cells in culture was indirectly proportional to the concentration of HGF, suggesting that a decrease in HGF may induce de novo synthesis of HGF from cells with residual unrecombined alleles. Carbon tetrachloride (CCl4)-induced regeneration resulted in a substantial decrease in preexisting HGF mRNA and protein, and subsequent PH led to a delayed regenerative response. Thus, HGF mRNA persists in the liver even after genetic recombination affecting most cells; however, PH subsequent to CCl4 treatment is associated with a decrease in both HGF mRNA and protein and results in compromised liver regeneration, validating an important role of this mitogen in hepatic growth.

Role of the microenvironment in the pathogenesis and treatment of hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is the most common primary liver tumor and the third greatest cause of cancer-related death worldwide, and its incidence is increasing. Despite the significant improvement in management of HCC over the past 30 years, there are no effective chemoprevention strategies, and only one systemic therapy has been approved for patients with advanced tumors. This drug, sorafenib, acts on tumor cells and the stroma. HCC develops from chronically damaged tissue that contains large amounts of inflammation and fibrosis, which also promote tumor progression and resistance to therapy. Increasing our understanding of how stromal components interact with cancer cells and the signaling pathways involved could help identify new therapeutic and chemopreventive targets.

New paradigms in post-hepatectomy liver failure

INTRODUCTION

Liver failure after hepatectomy remains the most feared postoperative complication. Many risk factors are already known, related to patient's comorbidities, underlying liver disease, received treatments and type of resection. Preoperative assessment of functional liver reserve must be a priority for the surgeon.

METHODS

Physiopathology of post-hepatectomy liver failure is not comparable to fulminant liver failure. Liver regeneration is an early phenomenon whose cellular mechanisms are beginning to be elucidated and allowing most of the time to quickly recover a functional organ. In some cases, microscopic and macroscopic disorganization appears. The hepatocyte hyperproliferation and the asynchronism between hepatocytes and non-hepatocyte cells mitosis probably play a major role in this pathogenesis.

RESULTS

Many peri- or intra-operative techniques try to prevent the occurrence of this potentially lethal complication, but a better understanding of involved mechanisms might help to completely avoid it, or even to extend the possibilities of resection.

CONCLUSION

Future prevention and management may include pharmacological slowing of proliferation, drug or physical modulation of portal flow to reduce shear-stress, stem cells or immortalized hepatocytes injection, and liver bioreactors. Everything must be done to avoid the need for transplantation, which remains today the most efficient treatment of liver failure.

Long-term culture and coculture of primary rat and human hepatocytes

The liver is the largest internal organ in mammals, serving a wide spectrum of vital functions. Loss of liver function due to drug toxicity or viral infection is a major cause of death in the United States. The development of Bioartificial Liver (BAL) devices and the demand for pharmaceutical and cosmetic toxicity screening require the development of long-term hepatocyte culture techniques. However, primary hepatocytes rapidly lose their cuboidal morphology and liver-specific functions over a few days in culture. Accumulation of stress fibers, loss of metabolic function, and cell death are known phenomena. In recent years, several techniques were developed that can support high levels of liver-specific gene expression, metabolic and synthetic function for several weeks in culture. These include the collagen double-gel configuration, hepatocyte spheroids, coculture with endothelial cells, and micropatterned cocultures with 3T3-J2 fibroblasts. This chapter covers the current status of hepatocyte culture techniques, including: hepatocyte isolation, media formulation, oxygen supply, heterotypic cell-cell interactions, and basic functional assays.

Insights into the regulation of tumor dormancy by angiogenesis in experimental tumors

While it is well established that an angiogenic switch marks escape from tumor dormancy in xenograft models, the molecular pathways involved in the control of tumor cell proliferation or survival by angiogenesis remain substantially uncharted. We recently demonstrated that signals stemming from angiogenic endothelial cells (EC) regulate the behavior of dormant cancer cells. Specifically, we observed that the Notch ligand Dll4, induced by angiogenic factors in EC, triggers Notch3 activation in neighboring tumor cells and promotes a tumorigenic phenotype. Evidence that Notch signaling is involved in tumor dormancy was further strengthened by the observation that MKP-1 levels-a broadly expressed phosphatase-are controlled by Notch3 by regulation of protein ubiquitination and stability. Notch3 and MKP-1 levels are consistently low in dormant tumors, and this is accompanied by relatively high levels of phosphorylated p38, a canonical MKP-1 target previously associated with maintenance of tumor dormancy. These results elucidate a novel angiogenesis-driven mechanism involving the Notch and MAPK pathways that controls tumor dormancy. More in general, angiogenic EC could form part of the vascular niche, a specialized microenvironment which appears to regulate metastatic outgrowth and future studies are needed to clarify the contribution of EC in the regulation of cancer stem cell behavior in the niche.The notion that EC could communicate signals to tumor cells raises questions about the possibility of achieving tumor dormancy by counteracting angiogenesis. In experimental tumors, anti-VEGF drugs typically prune the newly formed vasculature, thus reducing microvessel density, blood flow, and perfusion. These drugs eventually increase hypoxia and cause tumor necrosis but dormancy is rarely observed. Our group recently reported that anti-VEGF therapy causes a dramatic depletion of glucose and an exhaustion of ATP levels in tumors. Moreover, we found that the central metabolic checkpoint LKB1/AMPK-a cellular sensor of ATP levels that supports cell viability in response to energy stress-is activated by anti-VEGF therapy in experimental tumors and it has a key role in induction of sustained tumor regression. These functional links between activation of the LKB1/AMPK by anti-angiogenic therapy and tumor dormancy suggest a role for metabolism in the regulation of this phenomenon.

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.

Signals and cells involved in regulating liver regeneration

Liver regeneration is a complex phenomenon aimed at maintaining a constant liver mass in the event of injury resulting in loss of hepatic parenchyma. Partial hepatectomy is followed by a series of events involving multiple signaling pathways controlled by mitogenic growth factors (HGF, EGF) and their receptors (MET and EGFR). In addition multiple cytokines and other signaling molecules contribute to the orchestration of a signal which drives hepatocytes into DNA synthesis. The other cell types of the liver receive and transmit to hepatocytes complex signals so that, in the end of the regenerative process, complete hepatic tissue is assembled and regeneration is terminated at the proper time and at the right liver size. If hepatocytes fail to participate in this process, the biliary compartment is mobilized to generate populations of progenitor cells which transdifferentiate into hepatocytes and restore liver size.

Hepatic vascular endothelial growth factor regulates recruitment of rat liver sinusoidal endothelial cell progenitor cells

BACKGROUND & AIMS

After liver injury, bone marrow-derived liver sinusoidal endothelial cell progenitor cells (BM SPCs) repopulate the sinusoid as liver sinusoidal endothelial cells (LSECs). After partial hepatectomy, BM SPCs provide hepatocyte growth factor, promote hepatocyte proliferation, and are necessary for normal liver regeneration. We examined how hepatic vascular endothelial growth factor (VEGF) regulates recruitment of BM SPCs and their effects on liver injury.

METHODS

Rats were given injections of dimethylnitrosamine to induce liver injury, which was assessed by histology and transaminase assays. Recruitment of SPCs was analyzed by examining BM SPC proliferation, mobilization to the circulation, engraftment in liver, and development of fenestration (differentiation).

RESULTS

Dimethylnitrosamine caused extensive denudation of LSECs at 24 hours, followed by centrilobular hemorrhagic necrosis at 48 hours. Proliferation of BM SPCs, the number of SPCs in the bone marrow, and mobilization of BM SPCs to the circulation increased 2- to 4-fold by 24 hours after injection of dimethylnitrosamine; within 5 days, 40% of all LSECs came from engrafted BM SPCs. Allogeneic resident SPCs, infused 24 hours after injection of dimethylnitrosamine, repopulated the sinusoid as LSECs and reduced liver injury. Expression of hepatic VEGF messenger RNA and protein increased 5-fold by 24 hours after dimethylnitrosamine injection. Knockdown of hepatic VEGF with antisense oligonucleotides completely prevented dimethylnitrosamine-induced proliferation of BM SPCs and their mobilization to the circulation, reduced their engraftment by 46%, completely prevented formation of fenestration after engraftment as LSECs, and exacerbated dimethylnitrosamine injury.

CONCLUSIONS

BM SPC recruitment is a repair response to dimethylnitrosamine liver injury in rats. Hepatic VEGF regulates recruitment of BM SPCs to liver and reduces this form of liver injury.

Implication of hepatic stem cells in functional liver repopulation

The liver has an enormous potential to restore the parenchymal tissue loss due to injury. This is accomplished by the proliferation of either the hepatocytes or liver progenitor cells in cases where massive damage prohibits hepatocytes from entering the proliferative response. Under debate is still whether hepatic stem cells are involved in liver tissue maintenance and regeneration or even whether they exist at all. The definition of an adult tissue-resident stem cell comprises basic functional stem cell criteria like the potential of self-renewal, multipotent, i.e. at least bipotent differentiation capacity and serial transplantability featuring the ability of functional tissue repopulation. The relationship between a progenitor and its progeny should exemplify the lineage commitment from the putative stem cell to the differentiated cell. This is mainly assessed by lineage tracing and immunohistochemical identification of markers specific to progenitors and their descendants. Flow cytometry approaches revealed that the liver stem cell population in animals is likely to be heterogeneous giving rise to progeny with different molecular signatures, depending on the stimulus to activate the putative stem cell compartment. The stem cell criteria are met by a variety of cells identified in the fetal and adult liver both under normal and injury conditions. It is the purpose of this review to verify hepatic stem cell candidates in the light of the stem cell definition criteria mentioned. Also from this point of view adult stem cells from non-hepatic tissues such as bone marrow, umbilical cord blood or adipose tissue, have the potential to differentiate into cells featuring functional hepatocyte characteristics. This has great impact because it opens the possibility of generating hepatocyte-like cells from adult stem cells in a sufficient amount and quality for their therapeutical application to treat end-stage liver diseases by stem cell-based hepatocytes in place of whole organ transplantation.

Megakaryocytes promote hepatoepithelial liver cell development in E11.5 mouse embryos by cell-to-cell contact and by vascular endothelial growth factor A signaling

In the mouse embryo, hematopoietic progenitor cells migrate to the fetal liver (FL) between gestational days (E) 9.5 and 10.5, where they rapidly expand to form the main fetal reservoir of hematopoietic cells. The embryonic megakaryocyte progenitors (MKPs) in the E11.5 FL were identified as CD49f(H) CD41(H) (and c-Kit(D)KDR(+)CD42(+)CD9(++)CD31(+)) cells, expressing several hepato-specific proteins. Unlike adult bone marrow megakaryocytes (MKs), embryonic MKPs were CD45(-) and represent an abundant population in the FL. The CD49f(H)CD41(H) MKPs purified by cytometry differentiated in vitro to produce proplatelets, independent of thrombopoietin stimulation, and they responded to stimulation with adenosine diphosphate, thrombin, and the PAR4 thrombin receptor-activating peptide. Moreover, after removing CD49f(H)CD41(H) MKPs from purified E11.5 FL hepatoepithelial-enriched cell preparations (c-Kit(D)CD45(-)Ter119(-)), the remaining CD49f(D) cells neither differentiated nor survived in vitro. Indeed, direct cell-to-cell contact between the CD49f(H) CD41(H) and CD49f(D) populations was required to promote the hepatocyte differentiation of CD49f(D) cells. The addition of vascular endothelial growth factor A (VEGF-A) and medium conditioned by E11.5 CD49f(H)CD41(H) MKPs produced a partial effect on CD49f(D) cells, inducing the formation of hepatoepithelial layers. This effect was abolished by anti-VEGF-A antibodies. Together, these findings strongly suggest that CD49f(H)CD41(H) MKPs are fundamental to promote FL development, as proposed in adult liver regeneration.

CONCLUSION

The cells of the MK lineage present in the developing mouse embryo liver promote the growth of hepatoepithelial cells in vitro through VEGF-A signaling and may play a role in liver development in vivo.

Vascular instruction of pancreas development

Blood vessels course through organs, providing them with essential nutrient and gaseous exchange. However, the vasculature has also been shown to provide non-nutritional signals that play key roles in the control of organ growth, morphogenesis and homeostasis. Here, we examine a decade of work on the contribution of vascular paracrine signals to developing tissues, with a focus on pancreatic β-cells. During the early stages of embryonic development, blood vessels are required for pancreas specification. Later, the vasculature constrains pancreas branching, differentiation and growth. During adult life, capillaries provide a vascular niche for the maintenance of β-cell function and survival. We explore the possibility that the vasculature constitutes a dynamic and regionalized signaling system that carries out multiple and changing functions as it coordinately grows with the pancreatic epithelial tree.

Role of VEGF in organogenesis

The cardiovascular system, consisting of the heart, blood vessels and hematopoietic cells, is the first organ system to develop in vertebrates and is essential for providing oxygen and nutrients to the embryo and adult organs. Work done predominantly using the mouse and zebrafish as model systems has demonstrated that Vascular Endothelial Growth Factor (VEGF, also known as VEGFA) and its receptors KDR (FLK1/VEGFR2), FLT1 (VEGFR1), NRP1 and NRP2 play essential roles in many different aspects of cardiovascular development, including endothelial cell differentiation, migration and survival as well as heart formation and hematopoiesis. This review will summarize the approaches taken and conclusions reached in dissecting the role of VEGF signalling in vivo during the development of the early cardiovasculature and other organ systems. The VEGF-mediated assembly of a functional vasculature is also a prerequisite for the proper formation of other organs and for tissue homeostasis, because blood vessels deliver oxygen and nutrients and vascular endothelium provides inductive signals to other tissues. Particular emphasis will therefore be placed in this review on the cellular interactions between vascular endothelium and developing organ systems, in addition to a discussion of the role of VEGF in modulating the behavior of nonendothelial cell populations.

Two drugs with paradoxical effects on liver regeneration through antiangiogenesis and antifibrosis: Losartan and Spironolactone: a pharmacologic dilemma on hepatocyte proliferation

BACKGROUND

There is a strong relationship between liver regeneration and angiogenesis and fibrosis. It is known that Spironolactone, an aldosterone antagonist, acting on rennin-aldosterone axis, and Losartan, an angiotensin II type I antagonist, have both antifibrotic and antiangiogenic effects. Theoretically, the end result of these mechanisms with contradictory influences on liver regeneration is not known well. In this study, we aimed to reveal the effects on liver regeneration of administration of Spironolactone and Losartan, having contradicting effects on regeneration through antiangiogenesis and antifibrosis.

MATERIALS AND METHODS

A total of 72 Wistar albino rats were divided into control, Spironolactone, and Losartan groups and subdivided to conduct examinations on days 1, 3, 5, and 7. The specimens were treated with proliferating cell nuclear antigen to evaluate the characteristics of liver regeneration; with phosphorylated Smad2 (phospho-Smad2), serum transforming growth factor beta (TGF-B) 1, and tissue TGF-B1 to evaluate the termination of regeneration and with vascular endothelial growth factor receptor 2, Flk-1/KDR, to evaluate angiogenesis.

RESULTS

The proliferating cell nuclear antigen-labeling index was found to be significantly higher in Spironolactone and Losartan groups than in the control group on days 1, 3, and 5 (P = 0.031, 0.0023, and 0.032, respectively). Vascular endothelial growth factor receptor 2, Flk-1/KDR, expression was significantly lower in Spironolactone and Losartan groups than in the control group on days 3, 5, and 7 (P = 0.032, 0.0024, and 0.007, respectively). Phospho-Smad2 was significantly lower on days 1, 3, and 5 in Spironolactone and Losartan groups than in the control group (P = 0.011, 0.0020, and 0.05, respectively). Tissue TGF-B1 levels were significantly lower in Spironolactone and Losartan groups than in the control group only on day 3 (P = 0039). Serum TGF-B1 levels in Losartan groups were significantly different from those of control and Spironolactone groups only on day 1 (P < 0.05).

CONCLUSIONS

Liver regeneration, expected to decrease on day 3, was prolonged and increased even on day 5 despite antiangiogenic effects of Losartan and Spironolactone, which in fact inhibit fibrosis through phospho-Smad2 and increase regeneration. In addition, serum and tissue TGF-B1 levels are not sensitive enough to show active TGF-B1 for the evaluation of regeneration.

Liver regeneration after liver transplantation

BACKGROUND/PURPOSE

The liver has a remarkable capacity to regenerate after injury or resection. The aim of this review is to outline the mechanisms and factors affecting liver regeneration after liver transplantation.

METHODS

Relevant studies were reviewed using Medline, PubMed and Springer databases.

RESULTS

A variety of cytokines (such as interleukin-6 and tumor necrosis factor-α), growth factors (like hepatocyte growth factor and transforming growth factor-α) and cells are involved in liver regeneration. Several factors affect liver regeneration after transplantation such as ischemic injury, graft size, immunosuppression, steatosis, donor age and viral hepatitis.

CONCLUSION

Liver regeneration has been studied for many years. However, further research is essential to reveal the complex processes affecting liver regeneration, which may provide novel strategies in the management of liver transplantation recipients and donors.

Liver sinusoidal endothelial cell progenitor cells promote liver regeneration in rats

The ability of the liver to regenerate is crucial to protect liver function after injury and during chronic disease. Increases in hepatocyte growth factor (HGF) in liver sinusoidal endothelial cells (LSECs) are thought to drive liver regeneration. However, in contrast to endothelial progenitor cells, mature LSECs express little HGF. Therefore, we sought to establish in rats whether liver injury causes BM LSEC progenitor cells to engraft in the liver and provide increased levels of HGF and to examine the relative contribution of resident and BM LSEC progenitors. LSEC label-retaining cells and progenitors were identified in liver and LSEC progenitors in BM. BM LSEC progenitors did not contribute to normal LSEC turnover in the liver. However, after partial hepatectomy, BM LSEC progenitor proliferation and mobilization to the circulation doubled. In the liver, one-quarter of the LSECs were BM derived, and BM LSEC progenitors differentiated into fenestrated LSECs. When irradiated rats underwent partial hepatectomy, liver regeneration was compromised, but infusion of LSEC progenitors rescued the defect. Further analysis revealed that BM LSEC progenitors expressed substantially more HGF and were more proliferative than resident LSEC progenitors after partial hepatectomy. Resident LSEC progenitors within their niche may play a smaller role in recovery from partial hepatectomy than BM LSEC progenitors, but, when infused after injury, these progenitors engrafted and expanded markedly over a 2-month period. In conclusion, LSEC progenitor cells are present in liver and BM, and recruitment of BM LSEC progenitors is necessary for normal liver regeneration.

Implanted adipose-derived stem cells attenuate small-for-size liver graft injury by secretion of VEGF in rats

Graft injury after small-for-size liver transplantation impairs graft function and threatens the survival of the recipients. The use of adipose-derived stem cells (ADSCs) for liver injury protection and repair is promising. Our aim was to investigate the role of vascular endothelial growth factor (VEGF) secreted by ADSCs in the treatment of small-for-size liver graft injury. Studies were performed using ADSCs with VEGF secretion blocked by RNA interference. In vitro, ADSCs prevented apoptosis of freshly isolated liver sinusoidal endothelial cells (LSECs) by secretion of VEGF. Syngeneic 35% orthotopic liver transplantation followed by implantation of syngeneic ADSCs through the portal vein system was performed using Wistar rats. We found VEGF secreted by implanted ADSCs improved graft microcirculatory disturbances, serum liver function parameters and survival. The improved microcirculatory status was also reflected by reduced hepatocellular damage, especially LSEC apoptosis and improved liver regeneration. These effects were accompanied by decreased expression of endothelin receptor type A, increased Bcl-2/Bax ratio, decreased expression of Bad and elevated proportion of phosphorylated Bad. In conclusion, implanted syngeneic ADSCs attenuated small-for-size liver graft injuries and subsequently enhanced liver regeneration in a rat 35% liver transplantation model. The VEGF secreted by implanted ADSCs played a crucial role in this process.

Notch signaling in ocular vasculature development and diseases

Ocular angiogenesis, characterized by the formation of new blood vessels in the avascular area in eyes, is a highly coordinated process involved in retinal vasculature formation and several ocular diseases such as age-related macular degeneration, proliferative diabetic retinopathy and retinopathy of prematurity. This process is orchestrated by complicated cellular interactions and vascular growth factors, during which endothelial cells acquire heterogeneous phenotypes and distinct cellular destinations. To date, while the vascular endothelial growth factor has been identified as the most critical angiogenic agent with a remarkable therapeutic value, the Notch signaling pathway appears to be a similarly important regulator in several angiogenic steps. Recent progress has highlighted the involvement, mechanisms and therapeutic potential of Notch signaling in retinal vasculature development and pathological angiogenesis-related eye disorders, which may cause irreversible blindness.

Echinomycin decreases induction of vascular endothelial growth factor and hepatocyte regeneration in acetaminophen toxicity in mice

Up-regulation of vascular endothelial growth factor (VEGF) is important to hepatocyte regeneration in the late stages of acetaminophen (APAP) toxicity in the mouse. This study was conducted to examine the relationship of hypoxia-inducible factor 1α (HIF-1α) to VEGF and hepatocyte regeneration in APAP toxicity using an inhibitor of HIF-1α DNA-binding activity, echinomycin (EC). B6C3F1 male mice were treated with APAP (200 mg/kg IP), followed by EC (0.15 mg IP) and killed at 4 hr. Serum alanine aminotransferase (ALT), necrosis, hepatic glutathione (GSH) and APAP protein adducts were comparable in the APAP/EC and the APAP/veh mice at 4 hr. Additional studies showed that high dose EC (0.3 mg) reduced hepatic VEGF but also lowered hepatic GSH. Subsequent studies were performed using the 0.15-mg dose of EC. Although EC 0.15 mg had no effect on hepatic VEGF levels at 8 hr, by 24 hr VEGF levels were decreased by 40%. Toxicity (ALT and histopathology) was comparable in the APAP and APAP/EC groups at 24 and 48 hr. Proliferating cell nuclear antigen expression was reduced by both Western blot analysis and immunohistochemical staining in the APAP/EC mice at 48 hr. The data support the hypothesis that induction of HIF-1α, its binding to DNA and subsequent expression of VEGF are important factors in hepatocyte regeneration in APAP toxicity in the mouse.

Expression of vascular endothelial growth factor (VEGF) transcript and protein in the testis of several vertebrates, including endangered species

Vascular endothelial growth factor (VEGF) is known to influence the testis function. To establish the role of VEGF in the testis of a variety of species, we analyzed the expression of VEGF transcript using human gene-specific primers by semiquantitative reverse transcription-polymerase chain reaction (RT-PCR) analysis in the testes of 18 vertebrates, including a few endangered species. An amplicon of 566 bp representing VEGF(165) was identified in testis of all species in this study. Sequence analysis of these amplicons revealed 84 to 96% homology to available human VEGF sequence and to the VEGF sequences of other species in GenBank. Immunohistochemical analysis revealed expression of VEGF protein, primarily in Sertoli and Leydig cells and occasionally in the germ cells of the testis sections. It can be concluded from this study that expression of VEGF transcript is conserved in the testis of several vertebrates and may have a role in the process of spermatogenesis.

Endothelial nitric oxide synthase is a key mediator of hepatocyte proliferation in response to partial hepatectomy in mice

Endothelial nitric oxide synthase (eNOS) is a critical modulator of vascular tone and blood flow and plays major roles in liver physiology and pathophysiology. Nitric oxide (NO) is widely recognized as one of the key humoral factors important for the initiation of liver regeneration in response to partial hepatectomy. Liver regeneration in response to partial hepatectomy is dependent on the efficiency of growth factor-mediated cell-cycle progression. Epidermal growth factor receptor (EGFR) is a critical mediator of multiple hepatic mitogens, such as epidermal growth factor (EGF), transforming growth factor alpha, amphiregulin, and heparin-binding EGF in regenerating livers. However, the functional significance of endothelial nitric oxide synthase (eNOS) expressed in hepatocytes, and its potential role in EGFR-mediated hepatocyte proliferation, remains unexplored. We sought to determine whether eNOS is essential for hepatocyte proliferation in response to partial hepatectomy (PH). Our studies with eNOS knockout (eNOS(-/-) ) mice suggest that eNOS activation is essential for the efficient induction of early events and elicitation of a robust hepatocyte proliferative response to PH. Moreover, eNOS expression is essential for the efficient early induction of matrix metalloprotease-9, a known mediator of extracellular matrix remodeling and growth factor activation in regenerating livers. Our in vitro studies suggest that eNOS is a critical mediator of EGF-induced hepatocyte proliferation, potentially via its influence on the induction of early growth response-1 (Egr-1) and phosphorylation of c-Jun--known mediators of cell-cycle progression. EGF-induced eNOS phosphorylation at Ser 1177 is dependent on the phosphorylation and activation of EGFR/PI3 kinase/AKT signaling in hepatocytes.

CONCLUSION

Collectively, these results highlight a hitherto unrecognized role for eNOS activation in hepatocyte proliferation with implications for targeted therapies to enhance liver regenerative response in chronic disorders.

Insulin resistance and liver microcirculation in a rat model of early NAFLD

BACKGROUND & AIMS

Insulin contributes to vascular homeostasis in peripheral circulation, but the effects of insulin in liver microvasculature have never been explored. The aim of this study was to assess the vascular effects of insulin in the healthy and fatty liver.

METHODS

Wistar rats were fed a control or a high fat diet (HFD) for 3days, while treated with a placebo, the insulin-sensitizer metformin, or the iNOS inhibitor 1400W. Vascular responses to insulin were evaluated in the isolated liver perfusion model. Insulin sensitivity at the sinusoidal endothelium was tested by endothelium-dependent vasodilation in response to acetylcholine in the presence or absence of insulin and by the level of liver P-eNOS after an insulin injection.

RESULTS

Rats from the HFD groups developed liver steatosis. Livers from the control group showed a dose-dependent hepatic vasodilation in response to insulin, which was blunted in livers from HFD groups. Metformin restored liver vascular insulin-sensitivity. Pre-treatment with insulin enhanced endothelium-dependent vasodilation of the hepatic vasculature and induced hepatic eNOS phosphorylation in control rats but not in HFD rats. Treatment with metformin or 1400W restored the capacity of insulin to enhance endothelium dependent vasodilation and insulin induced eNOS phosphorylation in HFD rats.

CONCLUSIONS

The administration of a HFD induces insulin resistance in the liver sinusoidal endothelium, which is mediated, at least in part, through iNOS upregulation and can be prevented by the administration of metformin. Insulin resistance at the hepatic vasculature can be detected earlier than inflammation or any other sign of advanced NALFD.

Blood vessels restrain pancreas branching, differentiation and growth

How organ size and form are controlled during development is a major question in biology. Blood vessels have been shown to be essential for early development of the liver and pancreas, and are fundamental to normal and pathological tissue growth. Here, we report that, surprisingly, non-nutritional signals from blood vessels act to restrain pancreas growth. Elimination of endothelial cells increases the size of embryonic pancreatic buds. Conversely, VEGF-induced hypervascularization decreases pancreas size. The growth phenotype results from vascular restriction of pancreatic tip cell formation, lateral branching and differentiation of the pancreatic epithelium into endocrine and acinar cells. The effects are seen both in vivo and ex vivo, indicating a perfusion-independent mechanism. Thus, the vasculature controls pancreas morphogenesis and growth by reducing branching and differentiation of primitive epithelial cells.

A Src family kinase inhibitor improves survival in experimental acute liver failure associated with elevated cerebral and circulating vascular endothelial growth factor levels

BACKGROUND AND AIMS

Acute liver failure (ALF) is frequently complicated by cerebral oedema, systemic inflammation and multiorgan dysfunction. Vascular endothelial growth factor (VEGF) may stimulate liver regeneration but it can also be pro-inflammatory, activating endothelial cells and increasing permeability, actions mediated through Src kinase signalling. We therefore examined whether a Src inhibitor could have therapeutic potential in ALF.

METHODS

Murine ALF was induced with azoxymethane. Liver pathology was graded by a blinded examiner and apoptosis quantified by immunohistochemistry. Cerebral VEGF expression was imaged using VEGF-green fluorescent protein transgenic mice. Circulating and macrophage-secreted VEGF levels were measured. Experimental animals received a Src inhibitor or vehicle controls.

RESULTS

VEGF was undetectable in normal plasma but reached a mean of 835 pg/ml at grade III encephalopathy (P<0.001). Ammonia, lipopolysaccharide and interferon-gamma acted synergistically to enhance VEGF secretion by macrophages. Production of VEGF by cerebral cortical astrocytes increased with disease progression. Late treatment with inhibitors of Src or VEGF did not improve liver histology, encephalopathy or survival. However, early use of a Src kinase inhibitor significantly reduced hepatic injury, delayed encephalopathy and allowed 25% of mice to survive an otherwise lethal insult.

CONCLUSION

Systemic and cerebral VEGF levels are significantly elevated during experimental ALF and may be exacerbated by hyperammonemia and macrophage activation. Early use of a Src inhibitor reduced hepatocellular injury and enabled survival, indicating such agents may have some promise in the treatment of ALF.

Aquaporin-1 promotes angiogenesis, fibrosis, and portal hypertension through mechanisms dependent on osmotically sensitive microRNAs

Changes in hepatic vasculature accompany fibrogenesis, and targeting angiogenic molecules often attenuates fibrosis in animals. Aquaporin-1 (AQP1) is a water channel, overexpressed in cirrhosis, that promotes angiogenesis by enhancing endothelial invasion. The effect of AQP1 on fibrogenesis in vivo and the mechanisms driving AQP1 expression during cirrhosis remain unclear. The purpose of this study was to test the effect of AQP1 deletion in cirrhosis and explore mechanisms regulating AQP1. After bile duct ligation, wild-type mice overexpress AQP1 that colocalizes with vascular markers and sites of robust angiogenesis. AQP1 knockout mice demonstrated reduced angiogenesis compared with wild-type mice, as evidenced by immunostaining and endothelial invasion/proliferation in vitro. Fibrosis and portal hypertension were attenuated based on immunostaining, portal pressure, and spleen/body weight ratio. AQP1 protein, but not mRNA, was induced by hyperosmolality in vitro, suggesting post-transcriptional regulation. Endothelial cells from normal or cirrhotic mice were screened for microRNA (miR) expression using an array and a quantitative PCR. miR-666 and miR-708 targeted AQP1 mRNA and were decreased in cirrhosis and in cells exposed to hyperosmolality, suggesting that these miRs mediate osmolar changes via AQP1. Binding of the miRs to the untranslated region of AQP1 was assessed using luciferase assays. In conclusion, AQP1 promotes angiogenesis, fibrosis, and portal hypertension after bile duct ligation and is regulated by osmotically sensitive miRs.

Complementary vascular and matrix regulatory pathways underlie the beneficial mechanism of action of sorafenib in liver fibrosis

Paracrine signaling between hepatic stellate cells (HSCs) and liver endothelial cells (LECs) modulates fibrogenesis, angiogenesis, and portal hypertension. However, mechanisms regulating these processes are not fully defined. Sorafenib is a receptor tyrosine kinase inhibitor that blocks growth factor signaling in tumor cells but also displays important and not yet fully characterized effects on liver nonparenchymal cells including HSCs and LECs. The aim of this study was to test the hypothesis that sorafenib influences paracrine signaling between HSCs and LECs and thereby regulates matrix and vascular changes associated with chronic liver injury. Complementary magnetic resonance elastography, micro-computed tomography, and histochemical analyses indicate that sorafenib attenuates the changes in both matrix and vascular compartments that occur in response to bile duct ligation-induced liver injury in rats. Cell biology studies demonstrate that sorafenib markedly reduces cell-cell apposition and junctional complexes, thus reducing the proximity typically observed between these sinusoidal barrier cells. At the molecular level, sorafenib down-regulates angiopoietin-1 and fibronectin, both released by HSCs in a manner dependent on the transcription factor Kruppel-like factor 6 , suggesting that this pathway underlies both matrix and vascular changes associated with chronic liver disease.

CONCLUSION

Collectively, the results of this study demonstrate that sorafenib inhibits both matrix restructuring and vascular remodeling that accompany chronic liver diseases and characterize cell and molecular mechanisms underlying this effect. These data may help to refine future therapies for advanced gastrointestinal and liver diseases characterized by abundant fibrosis and neovascularization.

The VEGF family in cancer and antibody-based strategies for their inhibition

Angiogenesis is required in normal physiological processes, but is also involved in tumor growth, progression and metastasis. Vascular endothelial growth factor (VEGF), a primary mediator of angiogenesis in normal physiology and in disease, and other VEGF family members and their receptors provide targets that have been explored extensively for cancer therapy. Small molecule inhibitors and antibody/protein-based strategies that target the VEGF pathway have been studied in multiple types of cancer. This review will focus on VEGF pathway targeting antibodies that are currently being evaluated in pre-clinical and clinical studies.

Developmental and pathological angiogenesis

The formation of the vascular network is an intricate and complex process that is an obligate requirement during vertebrate development. The cardiovascular system is the first organ to develop and reach a functional state, which underscores the crucial role of the vasculature in the developing embryo. The development of the vasculature into highly branched conduits needs to occur in numerous sites and in precise patterns to supply oxygen and nutrients to the rapidly expanding tissue of the embryo. This process is mediated by the coordinated response of vascular endothelial and mural cells to the heterogeneous angiogenic cues provided by tissues and organs, whereas aberrant regulation and coordination of angiogenic signals during development result in lethality, impaired organ development, or disease states. This article reviews the essential signaling pathways required for establishment of the vertebrate vasculature with a major focus on a key regulatory factor, vascular endothelial growth factor (VEGF). We also discuss current knowledge of physiological angiogenic processes as well as their disruptions in pathological processes, particularly tumorigenesis.

Thrombospondin-1: an islet endothelial cell signal of importance for β-cell function

OBJECTIVE

Loss of thrombospondin (TSP)-1 in pancreatic islets has been shown to cause islet hyperplasia. This study tested the hypothesis that endothelial-derived TSP-1 is important for β-cell function.

RESEARCH DESIGN AND METHODS

Islet function was evaluated both in vivo and in vitro. Messenger RNA and protein expression were measured by real-time PCR and Western blot, respectively. The role of endothelial-derived TSP-1 for β-cell function was determined using a transplantation design in which recipient blood vessels either were allowed to grow or not into the transplanted islets.

RESULTS

TSP-1-deficient mice were glucose intolerant, despite having an increased β-cell mass. Moreover, their islets had decreased glucose-stimulated insulin release, (pro)insulin biosynthesis, and glucose oxidation rate, as well as increased expression of uncoupling protein-2 and lactate dehydrogenase-A when compared with control islets. Almost all TSP-1 in normal islets were found to be derived from the endothelium. Transplantation of free and encapsulated neonatal wild-type and TSP-1-deficient islets was performed in order to selectively reconstitute with TSP-1-positive or -negative blood vessels in the islets and supported that the β-cell defects occurring in TSP-1-deficient islets reflected postnatal loss of the glycoprotein in the islet endothelial cells. Treatment of neonatal TSP-1-deficient mice with the transforming growth factor (TGF)β-1-activating sequence of TSP-1 showed that reconstitution of TGFβ-1 activation prevented the development of decreased glucose tolerance in these mice. Thus, endothelial-derived TSP-1 activates islet TGFβ-1 of importance for β-cells.

CONCLUSIONS

Our study indicates a novel role for endothelial cells as functional paracrine support for pancreatic β-cells.

Expression of a functional VEGFR-1 in tumor cells is a major determinant of anti-PlGF antibodies efficacy

PlGF, one of the ligands for VEGFR-1, has been implicated in tumor angiogenesis. However, more recent studies indicate that genetic or pharmacological inhibition of PlGF signaling does not result in reduction of microvascular density in a variety of tumor models. Here we screened 12 human tumor cell lines and identified 3 that are growth inhibited by anti-PlGF antibodies in vivo. We found that efficacy of anti-PlGF treatment strongly correlates with VEGFR-1 expression in tumor cells, but not with antiangiogenesis. In addition, PlGF induced VEGFR-1 signaling and biological responses in tumor cell lines sensitive to anti-PlGF, but not in refractory tumor cell lines or in endothelial cells. Also, genetic ablation of VEGFR-1 signaling in the host did not affect the efficacy of PlGF blockade. Collectively, these findings suggest that the role of PlGF in tumorigenesis largely consists of promoting autocrine/paracrine growth of tumor cells expressing a functional VEGFR-1 rather than stimulation of angiogenesis.

Vascular endothelial growth factor receptor-1 activation mediates epithelial to mesenchymal transition in hepatocellular carcinoma cells

PURPOSE

To explore the molecular mechanism of Vascular endothelial growth factor receptor-1 (VEGFR-1) in invasion and metastasis of hepatocellular carcinoma.

METHODS

Reverse transcription polymerase chain reaction was performed to test expression of VEGFR-1 and its ligand VEGF-B19 in four hepatoma carcinoma cell. Fluorescent immunohistochemistry and western blotting were used to test the change of expression of E-cadherin or α-catenin.

RESULTS

VEGF-B-treated cells exhibited a change in E-cadherin from an organized, membrane-bound structure to a disorganized state that was dispersed throughout the cytoplasm. The maximal changes in E-cadherin were observed 24 hr after treatment of cells with VEGF-B. α-catenin was observed to translocate to the nucleus from its usual membrane-bound location 24 hr after treatment with either VEGF-B. Expression of the epithelial adhesion molecules E-cadherin was observed to decrease 48 hours after VEGF-B treatment. The nuclear expression of α-catenin was observed to increase 24 hr after treatment with VEGF-B.

CONCLUSIONS

VEGFR-1 on tumor cells may contribute to the aggressive behavior of hepatocellular carcinoma cells by inducing epithelial to mesenchymal transition (EMT). Targeting VEGFR-1 and downstream mediators of EMT may provide the foundation for the development of novel therapeutic approaches for this morbid and lethal disease.

Reconstitution of hepatic tissue architectures from fetal liver cells obtained from a three-dimensional culture with a rotating wall vessel bioreactor

Reconstitution of tissue architecture in vitro is important because it enables researchers to investigate the interactions and mutual relationships between cells and cellular signals involved in the three-dimensional (3D) construction of tissues. To date, in vitro methods for producing tissues with highly ordered structure and high levels of function have met with limited success although a variety of 3D culture systems have been investigated. In this study, we reconstituted functional hepatic tissue including mature hepatocyte and blood vessel-like structures accompanied with bile duct-like structures from E15.5 fetal liver cells, which contained more hepatic stem/progenitor cells comparing with neonatal liver cells. The culture was performed in a simulated microgravity environment produced by a rotating wall vessel (RWV) bioreactor. The hepatocytes in the reconstituted 3D tissue were found to be capable of producing albumin and storing glycogen. Additionally, bile canaliculi between hepatocytes, characteristics of adult hepatocyte in vivo were also formed. Apart from this, bile duct structure secreting mucin was shown to form complicated tubular branches. Furthermore, gene expression analysis by semi-quantitative RT-PCR revealed the elevated levels of mature hepatocyte markers as well as genes with the hepatic function. With RWV culture system, we could produce functionally reconstituted liver tissue and this might be useful in pharmaceutical industry including drug screening and testing and other applications such as an alternative approach to experimental animals.

Acetaminophen hepatotoxicity and HIF-1α induction in acetaminophen toxicity in mice occurs without hypoxia

HIF-1α is a nuclear factor important in the transcription of genes controlling angiogenesis including vascular endothelial growth factor (VEGF). Both hypoxia and oxidative stress are known mechanisms for the induction of HIF-1α. Oxidative stress and mitochondrial permeability transition (MPT) are mechanistically important in acetaminophen (APAP) toxicity in the mouse. MPT may occur as a result of oxidative stress and leads to a large increase in oxidative stress. We previously reported the induction of HIF-1α in mice with APAP toxicity and have shown that VEGF is important in hepatocyte regeneration following APAP toxicity. The following study was performed to examine the relative contribution of hypoxia versus oxidative stress to the induction of HIF-1α in APAP toxicity in the mouse. Time course studies using the hypoxia marker pimonidazole showed no staining for pimonidazole at 1 or 2h in B6C3F1 mice treated with APAP. Staining for pimonidazole was present in the midzonal to periportal regions at 4, 8, 24 and 48h and no staining was observed in centrilobular hepatocytes, the sites of the toxicity. Subsequent studies with the MPT inhibitor cyclosporine A showed that cyclosporine A (CYC; 10mg/kg) reduced HIF-1α induction in APAP treated mice at 1 and 4h and did not inhibit the metabolism of APAP (depletion of hepatic non-protein sulfhydryls and hepatic protein adduct levels). The data suggest that HIF-1α induction in the early stages of APAP toxicity is secondary to oxidative stress via a mechanism involving MPT. In addition, APAP toxicity is not mediated by a hypoxia mechanism.

HCC and angiogenesis: possible targets and future directions

Hepatocellular carcinoma (HCC), the most common primary liver tumor, is notoriously resistant to systemic therapies, and often recurs even after aggressive local therapies. HCCs rely on the formation of new blood vessels for growth, and VEGF is critical in this process. A hallmark of new vessel formation in tumors is their structural and functional abnormality. This leads to an abnormal tumor microenvironment characterized by low oxygen tension. The liver is perfused by both arterial and venous blood and the resulting abnormal microenvironment selects for more-aggressive malignancies. Anti-VEGF therapy with sorafenib was the first systemic therapy to demonstrate improved survival in patients with advanced-stage HCC. This important development in the treatment of HCC raises hope as well as critical questions on the future development of targeted agents including other antiangiogenic agents, which hold promise to further increase survival in this aggressive disease.

Vascular CD39/ENTPD1 directly promotes tumor cell growth by scavenging extracellular adenosine triphosphate

Extracellular adenosine triphosphate (ATP) is known to boost immune responses in the tumor microenvironment but might also contribute directly to cancer cell death. CD39/ENTPD1 is the dominant ectonucleotidase expressed by endothelial cells and regulatory T cells and catalyzes the sequential hydrolysis of ATP to AMP that is further degraded to adenosine by CD73/ecto-5'-nucleotidase. We have previously shown that deletion of Cd39 results in decreased growth of transplanted tumors in mice, as a result of both defective angiogenesis and heightened innate immune responses (secondary to loss of adenosinergic immune suppression). Whether alterations in local extracellular ATP and adenosine levels as a result of CD39 bioactivity directly affect tumor growth and cytotoxicity has not been investigated to date. We show here that extracellular ATP exerts antitumor activity by directly inhibiting cell proliferation and promoting cancer cell death. ATP-induced antiproliferative effects and cell death are, in large part, mediated through P2X(7) receptor signaling. Tumors in Cd39 null mice exhibit increased necrosis in association with P2X(7) expression. We further demonstrate that exogenous soluble NTPDase, or CD39 expression by cocultured liver sinusoidal endothelial cells, stimulates tumor cell proliferation and limits cell death triggered by extracellular ATP. Collectively, our findings indicate that local expression of CD39 directly promotes tumor cell growth by scavenging extracellular ATP. Pharmacological or targeted inhibition of CD39 enzymatic activity may find utility as an adjunct therapy in cancer management.

Advances in the regulation of liver regeneration

Liver regeneration is known to be a process involving highly organized and ordered tissue growth triggered by the loss of liver tissue, and remains a fascinating topic. A large number of genes are involved in this process, and there exists a sequence of stages that results in liver regeneration, while at the same time inhibitors control the size of the regenerated liver. The initiation step is characterized by priming of quiescent hepatocytes by factors such as TNF-α, IL-6 and nitric oxide. The proliferation step is the step during which hepatocytes enter into the cell cycle's G1 phase and are stimulated by complete mitogens including HGF, TGF-α and EGF. Hepatic stimulator substance, glucagon, insulin, TNF-α, IL-1 and IL-6 have also been implicated in regulating the regeneration process. Inhibitors and stop signals of hepatic regeneration are not well known and only limited information is available. Furthermore, the effects of other factors such as VEGF, PDGF, hypothyroidism, proliferating cell nuclear antigen, heat shock proteins, ischemic-reperfusion injury, steatosis and granulocyte colony-stimulating factor on liver regeneration are also systematically reviewed in this article. A tissue engineering approach using isolated hepatocytes for in vitro tissue generation and heterotopic transplantation of liver cells has been established. The use of stem cells might also be very attractive to overcome the limitation of donor liver tissue. Liver-specific differentiation of embryonic, fetal or adult stem cells is currently under investigation.