Correction of liver disease by hepatocyte transplantation in a mouse model of progressive familial intrahepatic cholestasis

Gene Therapy for Inherited Liver Disease: To Add or to Edit

Patients suffering from an inherited severe liver disorder require lifelong treatment to prevent premature death. Until recently, the only curative treatment option was liver transplantation, which requires lifelong immune suppression. Now, liver-directed gene therapy, which is a much less invasive procedure, has become a market-approved treatment for hemophilia A and B. This may pave the way for it to become the treatment of choice for many other recessive inherited liver disorders with loss-of-function mutations. Inherited liver disease with toxic-gain-of-function or intrinsic hepatocyte damage may require alternative applications, such as integrating vectors or genome editing technologies, that can provide permanent or specific modification of the genome. We present an overview of currently available gene therapy strategies, i.e., gene supplementation, gene editing, and gene repair investigated in preclinical and clinical studies to treat inherited severe liver disorders. The advantages and limitations of these gene therapy applications are discussed in relation to the underlying disease mechanism.

In vivo selection of hepatocytes

The liver is a highly regenerative organ capable of significant proliferation and remodeling during homeostasis and injury responses. Experiments of nature in rare genetic diseases have illustrated that healthy hepatocytes may have a selective advantage, outcompete diseased cells, and result in extensive liver replacement. This observation has given rise to the concept of therapeutic liver repopulation by providing an engineered selective advantage to a subpopulation of beneficial hepatocytes. In vivo selection can greatly enhance the efficiency of both gene and cell transplantation therapies for hepatic diseases. In vivo hepatocyte selection has also enabled the expansion of human hepatocytes in animals, creating novel models of human liver disease and biology. Finally, recent work has shown that somatic mutations produce clonal expansion of injury-resistant hepatocytes in most chronic liver diseases. In this review, we will address the role of hepatocyte selection in disease pathophysiology and therapeutic strategies.

Emerging drugs for the treatment of progressive familial intrahepatic cholestasis: a focus on phase II and III trials

INTRODUCTION

Progressive familial intrahepatic cholestasis (PFIC) is a group of disorders characterized by inappropriate bile formation, causing hepatic accumulation of bile acids and, subsequently, liver injury. Until recently, no approved treatments were available for these patients.

AREAS COVERED

Recent clinical trials for PFIC treatment have focused on intestine-restricted ileal bile acid transporter (IBAT) inhibitors. These compounds aim to reduce the pool size of bile acids by interrupting their enterohepatic circulation. Other emerging treatments in the pipeline include systemic IBAT inhibitors, synthetic bile acid derivatives, compounds targeting bile acid synthesis via the FXR/FGF axis, and chaperones/potentiators that aim to enhance the residual activity of the mutated transporters.

EXPERT OPINION

Substantial progress has been made in drug development for PFIC patients during the last couple of years. Although data concerning long-term efficacy are as yet only scarcely available, new therapies have demonstrated robust efficacy in a considerable fraction of patients at least on the shorter term. However, a substantial fraction of PFIC patients do not respond to these novel therapies and thus still requires surgical treatment, including liver transplantation before adulthood. Hence, there is still an unmet medical need for long-term effective medical, preferably non-surgical, treatment for all PFIC patients.

Wnt-β-catenin in hepatobiliary homeostasis, injury, and repair

Wnt-β-catenin signaling has emerged as an important regulatory pathway in the liver, playing key roles in zonation and mediating contextual hepatobiliary repair after injuries. In this review, we will address the major advances in understanding the role of Wnt signaling in hepatic zonation, regeneration, and cholestasis-induced injury. We will also touch on some important unanswered questions and discuss the relevance of modulating the pathway to provide therapies for complex liver pathologies that remain a continued unmet clinical need.

Genetic alterations and molecular mechanisms underlying hereditary intrahepatic cholestasis

Hereditary cholestatic liver disease caused by a class of autosomal gene mutations results in jaundice, which involves the abnormality of the synthesis, secretion, and other disorders of bile acids metabolism. Due to the existence of a variety of gene mutations, the clinical manifestations of children are also diverse. There is no unified standard for diagnosis and single detection method, which seriously hinders the development of clinical treatment. Therefore, the mutated genes of hereditary intrahepatic cholestasis were systematically described in this review.

Rescue of infant progressive familial intrahepatic cholestasis type 3 mice by repeated dosing of AAV gene therapy

Background & Aims

Gene therapy using recombinant adeno-associated virus (rAAV) vector carrying multidrug resistance protein 3 (MDR3) coding sequence (AAV8-MDR3) represents a potential curative treatment for progressive familial intrahepatic cholestasis type 3 (PFIC3), which presents in early childhood. However, patients with the severest form of PFIC3 should receive treatment early after detection to prevent irreversible hepatic fibrosis leading ultimately to liver transplantation or death. This represents a challenge for rAAV-based gene therapy because therapeutic efficacy is expected to wane as rAAV genomes are lost owing to hepatocyte division, and the formation of AAV-specific neutralising antibodies precludes re-administration. Here, we tested a strategy of vector re-administration in infant PFIC3 mice with careful evaluation of its oncogenicity - a particular concern surrounding rAAV treatment.

Methods

AAV8-MDR3 was re-administered to infant Abcb4 ^-/- mice 2 weeks after a first dose co-administered with tolerogenic nanoparticles carrying rapamycin (ImmTOR) given at 2 weeks of age. Eight months later, long-term therapeutic efficacy and safety were assessed with special attention paid to the potential oncogenicity of rAAV treatment.

Results

Co-administration with ImmTOR mitigated the formation of rAAV-specific neutralising antibodies and enabled an efficacious second administration of AAV8-MDR3, resulting in stable correction of the disease phenotype, including a restoration of bile phospholipid content and healthy liver function, as well as the prevention of liver fibrosis, hepatosplenomegaly, and gallstones. Furthermore, efficacious repeat rAAV administration prevented the appearance of liver malignancies in an animal model highly prone to developing hepatocellular carcinoma.

Conclusions

These outcomes provide strong evidence for rAAV redosing through co-administration with ImmTOR, as it resulted in a long-term therapeutic effect in a paediatric liver metabolic disorder, including the prevention of oncogenesis.

Impact and implications

Redosing of gene therapy for inborn hepatobiliary disorders may be essential as effect wanes during hepatocyte division and renewal, particularly in paediatric patients, but the approach may carry long-term risks of liver cancer. Viral vectors carrying a therapeutic gene exerted a durable cure of progressive familial intrahepatic cholestasis type 3 in infant mice and reduced the risk of liver cancer only following a second administration.

Cell and cell-derivative-based therapy for liver diseases: current approaches and future promises

INTRODUCTION

According to the recent updates from World Health Organization, liver diseases are the 12th most common cause of mortality. Currently, orthotopic liver transplantation (OLT) is the most effective and the only treatment for end-stage liver diseases. Owing to several shortcomings like finite numbers of healthy organ donors, lifelong immunosuppression, and complexity of the procedure, cell and cell-derivatives therapies have emerged as a potential therapeutic alternative for liver diseases. Various cell types and therapies have been proposed and their therapeutic effects evaluated in preclinical or clinical studies, including hepatocytes, hepatocyte-like cells (HLCs) derived from stem cells, human liver stem cells (HLSCs), combination therapies with various types of cells, organoids, and implantable cell-biomaterial constructs with synthetic and natural polymers or even decellularized extracellular matrix (ECM).

AREAS COVERED

In this review, we highlighted the current status of cell and cell-derivative-based therapies for liver diseases. Furthermore, we discussed future prospects of using HLCs, liver organoids, and their combination therapies.

EXPERT OPINION

Promising application of stem cell-based techniques including iPSC technology has been integrated into novel techniques such as gene editing, directed differentiation, and organoid technology. iPSCs offer promising prospects to represent novel therapeutic strategies and modeling liver diseases.

Novel AAV-mediated genome editing therapy improves health and survival in a mouse model of methylmalonic acidemia

Methylmalonic acidemia (MMA) is an inborn error of metabolism mostly caused by mutations in the mitochondrial methylmalonyl-CoA mutase gene (MMUT). MMA patients suffer from frequent episodes of metabolic decompensation, which can be life threatening. To mimic both the dietary restrictions and metabolic decompensation seen in MMA patients, we developed a novel protein-controlled diet regimen in a Mmut deficient mouse model of MMA and demonstrated the therapeutic benefit of mLB-001, a nuclease-free, promoterless recombinant AAV GeneRide^TM vector designed to insert the mouse Mmut into the endogenous albumin locus via homologous recombination. A single intravenous administration of mLB-001 to neonatal or adult MMA mice prevented body weight loss and mortality when challenged with a high protein diet. The edited hepatocytes expressed functional MMUT protein and expanded over time in the Mmut deficient mice, suggesting a selective growth advantage over the diseased cells. In mice with a humanized liver, treatment with a human homolog of mLB-001 resulted in site-specific genome editing and transgene expression in the transplanted human hepatocytes. Taken together, these findings support the development of hLB-001 that is currently in clinical trials in pediatric patients with severe forms of MMA.

Novel Gene-Correction-Based Therapeutic Modalities for Monogenic Liver Disorders

The majority of monogenic liver diseases are autosomal recessive disorders, with few being sex-related or co-dominant. Although orthotopic liver transplantation (LT) is currently the sole therapeutic option for end-stage patients, such an invasive surgical approach is severely restricted by the lack of donors and post-transplant complications, mainly associated with life-long immunosuppressive regimens. Therefore, the last decade has witnessed efforts for innovative cellular or gene-based therapeutic strategies. Gene therapy is a promising approach for treatment of many hereditary disorders, such as monogenic inborn errors. The liver is an organ characterized by unique features, making it an attractive target for in vivo and ex vivo gene transfer. The current genetic approaches for hereditary liver diseases are mediated by viral or non-viral vectors, with promising results generated by gene-editing tools, such as CRISPR-Cas9 technology. Despite massive progress in experimental gene-correction technologies, limitations in validated approaches for monogenic liver disorders have encouraged researchers to refine promising gene therapy protocols. Herein, we highlighted the most common monogenetic liver disorders, followed by proposed genetic engineering approaches, offered as promising therapeutic modalities.

Case Report: A Rare Heterozygous ATP8B1 Mutation in a BRIC1 Patient: Haploinsufficiency?

Benign recurrent intrahepatic cholestasis (BRIC) is an autosomal recessive disorder characterized by recurrent cholestasis. ATPase class I, type 8B, member 1 (ATP8B1) encodes familial intrahepatic cholestasis 1 (FIC1), which acts as a phosphatidylserine reversing enzyme in the tubule membrane of hepatocytes to mediate the inward translocation of phosphatidylserine (PS). At present, dozens of ATP8B1 pathogenic mutations have been identified that mainly cause BRIC1 and progressive familial intrahepatic cholestasis 1 (PFIC1). The diagnosis of BRIC1 is based on symptoms, laboratory tests, imaging, liver histology, and genetic testing. BRIC1 treatment seeks to prevent recurrence and reduce disease severity. At present, the main treatment methods include ursodeoxycholic acid (UDCA), rifampin, cholestyramine and haemofiltration, and endoscopic nasobiliary drainage (ENBD). Here, we report a 17-year-old patient with cholestasis who has a rare heterozygous ATP8B1 gene mutation (p.T888K). The patient was treated with UDCA, glucocorticoids and haemofiltration, after which bilirubin levels gradually returned to normal. This case was thought to be caused by an ATP8B1 heterozygous mutation, which may be related to haploinsufficiency (HI).

Gene Therapy for Acquired and Genetic Cholestasis

Cholestatic diseases can be caused by the dysfunction of transporters involved in hepatobiliary circulation. Although pharmacological treatments constitute the current standard of care for these diseases, none are curative, with liver transplantation being the only long-term solution for severe cholestasis, albeit with many disadvantages. Liver-directed gene therapy has shown promising results in clinical trials for genetic diseases, and it could constitute a potential new therapeutic approach for cholestatic diseases. Many preclinical gene therapy studies have shown positive results in animal models of both acquired and genetic cholestasis. The delivery of genes that reduce apoptosis or fibrosis or improve bile flow has shown therapeutic effects in rodents in which cholestasis was induced by drugs or bile duct ligation. Most studies targeting inherited cholestasis, such as progressive familial intrahepatic cholestasis (PFIC), have focused on supplementing a correct version of a mutated gene to the liver using viral or non-viral vectors in order to achieve expression of the therapeutic protein. These strategies have generated promising results in treating PFIC3 in mouse models of the disease. However, important challenges remain in translating this therapy to the clinic, as well as in developing gene therapy strategies for other types of acquired and genetic cholestasis.

Recent updates on progressive familial intrahepatic cholestasis types 1, 2 and 3: Outcome and therapeutic strategies

Recent evidence points towards the role of genotype to understand the phenotype, predict the natural course and long term outcome of patients with progressive familial intrahepatic cholestasis (PFIC). Expanded role of the heterozygous transporter defects presenting late needs to be suspected and identified. Treatment of pruritus, nutritional rehabilitation, prevention of fibrosis progression and liver transplantation (LT) in those with end stage liver disease form the crux of the treatment. LT in PFIC has its own unique issues like high rates of intractable diarrhoea, growth failure; steatohepatitis and graft failure in PFIC1 and antibody-mediated bile salt export pump deficiency in PFIC2. Drugs inhibiting apical sodium-dependent bile transporter and adenovirus-associated vector mediated gene therapy hold promise for future.

Cell and Tissue Therapy for the Treatment of Chronic Liver Disease

Liver disease is an important clinical problem, impacting 600 million people worldwide. It is the 11th-leading cause of death in the world. Despite constant improvement in treatment and diagnostics, the aging population and accumulated risk factors led to increased morbidity due to nonalcoholic fatty liver disease and steatohepatitis. Liver transplantation, first established in the 1960s, is the second-most-common solid organ transplantation and is the gold standard for the treatment of liver failure. However, less than 10% of the global need for liver transplantation is met at the current rates of transplantation due to the paucity of available organs. Cell- and tissue-based therapies present an alternative to organ transplantation. This review surveys the approaches and tools that have been developed, discusses the distinctive challenges that exist for cell- and tissue-based therapies, and examines the future directions of regenerative therapies for the treatment of liver disease.

Correction of a urea cycle defect after ex vivo gene editing of human hepatocytes

Ornithine transcarbamylase deficiency (OTCD) is a monogenic disease of ammonia metabolism in hepatocytes. Severe disease is frequently treated by orthotopic liver transplantation. An attractive approach is the correction of a patient’s own cells to regenerate the liver with gene-repaired hepatocytes. This study investigates the efficacy and safety of ex vivo correction of primary human hepatocytes. Hepatocytes isolated from an OTCD patient were genetically corrected ex vivo, through the deletion of a mutant intronic splicing site achieving editing efficiencies >60% and the restoration of the urea cycle in vitro. The corrected hepatocytes were transplanted into the liver of FRGN mice and repopulated to high levels (>80%). Animals transplanted and liver repopulated with genetically edited patient hepatocytes displayed normal ammonia, enhanced clearance of an ammonia challenge and OTC enzyme activity, as well as lower urinary orotic acid when compared to mice repopulated with unedited patient hepatocytes. Gene expression was shown to be similar between mice transplanted with unedited or edited patient hepatocytes. Finally, a genome-wide screening by performing CIRCLE-seq and deep sequencing of >70 potential off-targets revealed no unspecific editing. Overall analysis of disease phenotype, gene expression, and possible off-target editing indicated that the gene editing of a severe genetic liver disease was safe and effective.

Gene Therapy for Progressive Familial Intrahepatic Cholestasis: Current Progress and Future Prospects

Progressive Familial Intrahepatic Cholestasis (PFIC) are inherited severe liver disorders presenting early in life, with high serum bile salt and bilirubin levels. Six types have been reported, two of these are caused by deficiency of an ABC transporter; ABCB11 (bile salt export pump) in type 2; ABCB4 (phosphatidylcholine floppase) in type 3. In addition, ABCB11 function is affected in 3 other types of PFIC. A lack of effective treatment makes a liver transplantation necessary in most patients. In view of long-term adverse effects, for instance due to life-long immune suppression needed to prevent organ rejection, gene therapy could be a preferable approach, as supported by proof of concept in animal models for PFIC3. This review discusses the feasibility of gene therapy as an alternative for liver transplantation for all forms of PFIC based on their pathological mechanism. Conclusion: Using presently available gene therapy vectors, major hurdles need to be overcome to make gene therapy for all types of PFIC a reality.

Pediatric Cholestatic Liver Disease: Review of Bile Acid Metabolism and Discussion of Current and Emerging Therapies

Cholestatic liver diseases are a significant cause of morbidity and mortality and the leading indication for pediatric liver transplant. These include diseases such as biliary atresia, Alagille syndrome, progressive intrahepatic cholestasis entities, ductal plate abnormalities including Caroli syndrome and congenital hepatic fibrosis, primary sclerosing cholangitis, bile acid synthesis defects, and certain metabolic disease. Medical management of these patients typically includes supportive care for complications of chronic cholestasis including malnutrition, pruritus, and portal hypertension. However, there are limited effective interventions to prevent progressive liver damage in these diseases, leaving clinicians to ultimately rely on liver transplantation in many cases. Agents such as ursodeoxycholic acid, bile acid sequestrants, and rifampicin have been mainstays of treatment for years with the understanding that they may decrease or alter the composition of the bile acid pool, though clinical response to these medications is frequently insufficient and their effects on disease progression remain limited. Recently, animal and human studies have identified potential new therapeutic targets which may disrupt the enterohepatic circulation of bile acids, alter the expression of bile acid transporters or decrease the production of bile acids. In this article, we will review bile formation, bile acid signaling, and the relevance for current and newer therapies for pediatric cholestasis. We will also highlight further areas of potential targets for medical intervention for pediatric cholestatic liver diseases.

Wnt/β-Catenin Signaling Plays a Protective Role in the Mdr2 Knockout Murine Model of Cholestatic Liver Disease

BACKGROUND AND AIMS

The Wnt/β-catenin signaling pathway has a well-described role in liver pathobiology. Its suppression was recently shown to decrease bile acid (BA) synthesis, thus preventing the development of cholestatic liver injury and fibrosis after bile duct ligation (BDL).

APPROACH AND RESULTS

To generalize these observations, we suppressed β-catenin in Mdr2 knockout (KO) mice, which develop sclerosing cholangitis due to regurgitation of BA from leaky ducts. When β-catenin was knocked down (KD) in KO for 2 weeks, hepatic and biliary injury were exacerbated in comparison to KO given placebo, as shown by serum biochemistry, ductular reaction, inflammation, and fibrosis. Simultaneously, KO/KD livers displayed increased oxidative stress and senescence and an impaired regenerative response. Although the total liver BA levels were similar between KO/KD and KO, there was significant dysregulation of BA transporters and BA detoxification/synthesis enzymes in KO/KD compared with KO alone. Multiphoton intravital microscopy revealed a mixing of blood and bile in the sinusoids, and validated the presence of increased serum BA in KO/KD mice. Although hepatocyte junctions were intact, KO/KD livers had significant canalicular defects, which resulted from loss of hepatocyte polarity. Thus, in contrast to the protective effect of β-catenin KD in BDL model, β-catenin KD in Mdr2 KO aggravated rather than alleviated injury by interfering with expression of BA transporters, hepatocyte polarity, canalicular structure, and the regenerative response.

CONCLUSIONS

The resulting imbalance between ongoing injury and restitution led to worsening of the Mdr2 KO phenotype, suggesting caution in targeting β-catenin globally for all cholestatic conditions.

The Thyromimetic Sobetirome (GC-1) Alters Bile Acid Metabolism in a Mouse Model of Hepatic Cholestasis

Chronic cholestasis results from bile secretory defects or impaired bile flow with few effective medical therapies available. Thyroid hormone triiodothyronine and synthetic thyroid hormone receptor agonists, such as sobetirome (GC-1), are known to impact lipid and bile acid (BA) metabolism and induce hepatocyte proliferation downstream of Wnt/β-catenin signaling after surgical resection; however, these drugs have yet to be studied as potential therapeutics for cholestatic liver disease. Herein, GC-1 was administered to ATP binding cassette subfamily B member 4 (Abcb4^-/-; Mdr2^-/-) knockout (KO) mice, a sclerosing cholangitis model. KO mice fed GC-1 diet for 2 and 4 weeks had decreased serum alkaline phosphatase but increased serum transaminases compared with KO alone. KO mice on GC-1 also had higher levels of total liver BA due to alterations in expression of BA detoxification, transport, and synthesis genes, with the net result being retention of BA in the hepatocytes. Interestingly, GC-1 does not induce hepatocyte proliferation or Wnt/β-catenin signaling in KO mice, likely a result of decreased thyroid hormone receptor β expression without Mdr2. Therefore, although GC-1 treatment induces a mild protection against biliary injury in the early stages of treatment, it comes at the expense of hepatocyte injury and is suboptimal because of lower expression of thyroid hormone receptor β. Thus, thyromimetics may have limited therapeutic benefits in treating cholestatic liver disease.

Gene therapy for progressive familial intrahepatic cholestasis type 3 in a clinically relevant mouse model

Progressive familial intrahepatic cholestasis type 3 (PFIC3) is a rare monogenic disease caused by mutations in the ABCB4 gene, resulting in a reduction in biliary phosphatidylcholine. Reduced biliary phosphatidylcholine cannot counteract the detergent effects of bile salts, leading to cholestasis, cholangitis, cirrhosis and ultimately liver failure. Here, we report results from treating two- or five-week-old Abcb4-/- mice with an AAV vector expressing human ABCB4, resulting in significant decreases of PFIC3 disease biomarkers. All male mice achieved a sustained therapeutic effect up through 12 weeks, but the effect was achieved in only 50% of females. However, two-week-old females receiving a second inoculation three weeks later maintained the therapeutic effect. Upon sacrifice, markers of PFIC3 disease such as, hepatosplenomegaly, biliary phosphatidylcholine and liver histology were significantly improved. Thus, AAV-mediated gene therapy successfully prevented PFIC3 symptoms in a clinically relevant mouse model, representing a step forward in improving potential therapy options for PFIC3 patients.

Liver-directed gene therapy results in long-term correction of progressive familial intrahepatic cholestasis type 3 in mice

BACKGROUND & AIMS

Progressive familial intrahepatic cholestasis type 3 (PFIC3), for which there are limited therapeutic options, often leads to end-stage liver disease before adulthood due to impaired ABCB4-dependent phospholipid transport to bile. Using adeno-associated virus serotype 8 (AAV8)-mediated gene therapy, we aimed to restore the phospholipid content in bile to levels that prevent liver damage, thereby enabling stable hepatic ABCB4 expression and long-term correction of the phenotype in a murine model of PFIC3.

METHODS

Ten-week-old Abcb4-/- mice received a single dose of AAV8-hABCB4 (n = 10) or AAV8-GFP (n = 7) under control of a liver specific promoter via tail vein injection. Animals were sacrificed either 10 or 26 weeks after vector administration to assess transgene persistence, after being challenged with a 0.1% cholate diet for 2 weeks. Periodic evaluation of plasma cholestatic markers was performed and bile duct cannulation enabled analysis of biliary phospholipids. Liver fibrosis and the Ki67 proliferation index were assessed by immunohistochemistry.

RESULTS

Stable transgene expression was achieved in all animals that received AAV8-hABCB4 up to 26 weeks after administration. AAV8-hABCB4 expression restored biliary phospholipid excretion, increasing the phospholipid and cholesterol content in bile to levels that ameliorate liver damage. This resulted in normalization of the plasma cholestatic markers, alkaline phosphatase and bilirubin. In addition, AAV8-hABCB4 prevented progressive liver fibrosis and reduced hepatocyte proliferation for the duration of the study.

CONCLUSION

Liver-directed gene therapy provides stable hepatic ABCB4 expression and long-term correction of the phenotype in a murine model of PFIC3. Translational studies that verify the clinical feasibility of this approach are warranted.

LAY SUMMARY

Progressive familial intrahepatic cholestasis type 3 (PFIC3) is a severe genetic liver disease that results from impaired transport of lipids to bile, which makes the bile toxic to liver cells. Because therapeutic options are currently limited, this study aims to evaluate gene therapy to correct the underlying genetic defect in a mouse model of this disease. By introducing a functional copy of the missing gene in liver cells of mice, we were able to restore lipid transport to bile and strongly reduce damage to the liver. The proliferation of liver cells was also reduced, which contributes to long-term correction of the phenotype. Further studies are required to evaluate whether this approach can be applied to patients with PFIC3.

Chemical cocktails enable hepatic reprogramming of human urine-derived cells with a single transcription factor

Human liver or hepatocyte transplantation is limited by a severe shortage of donor organs. Direct reprogramming of other adult cells into hepatic cells may offer a solution to this problem. In a previous study, we have generated hepatocyte-like cells from mouse fibroblasts using only one transcription factor (TF) plus a chemical cocktail. Here, we show that human urine-derived epithelial-like cells (hUCs) can also be transdifferentiated into human hepatocyte-like cells (hiHeps) using one TF (Foxa3, Hnf1α, or Hnf4α) plus the same chemical cocktail CRVPTD (C, CHIR99021; R, RepSox; V, VPA; P, Parnate; T, TTNPB; and D, Dznep). These hiHeps express multiple hepatocyte-specific genes and display functions characteristic of mature hepatocytes. With the introduction of the large T antigen, these hiHeps can be expanded in vitro and can restore liver function in mice with concanavalin-A-induced acute liver failure. Our study provides a strategy to generate functional hepatocyte-like cells from hUCs by using a single TF plus a chemical cocktail.

Cell-based liver therapies: past, present and future

Liver transplantation represents the standard treatment for people with an end-stage liver disease and some liver-based metabolic disorders; however, shortage of liver donor tissues limits its availability. Furthermore, whole liver replacement eliminates the possibility of using native liver as a possible target for future gene therapy in case of liver-based metabolic defects. Cell therapy has emerged as a potential alternative, as cells can provide the hepatic functions and engraft in the liver parenchyma. Various options have been proposed, including human or other species hepatocytes, hepatocyte-like cells derived from stem cells or more futuristic alternatives, such as combination therapies with different cell types, organoids and cell-biomaterial combinations. In this review, we aim to give an overview of the cell therapies developed so far, highlighting preclinical and/or clinical achievements as well as the limitations that need to be overcome to make them fully effective and safe for clinical applications.This article is part of the theme issue 'Designer human tissue: coming to a lab near you'.

Suppression of Hepatic Bile Acid Synthesis by a non-tumorigenic FGF19 analogue Protects Mice from Fibrosis and Hepatocarcinogenesis

Critical regulation of bile acid (BA) pool size and composition occurs via an intensive molecular crosstalk between the liver and gut, orchestrated by the combined actions of the nuclear Farnesoid X receptor (FXR) and the enterokine fibroblast growth factor 19 (FGF19) with the final aim of reducing hepatic BA synthesis in a negative feedback fashion. Disruption of BA homeostasis with increased hepatic BA toxic levels leads to higher incidence of hepatocellular carcinoma (HCC). While native FGF19 has anti-cholestatic and anti-fibrotic activity in the liver, it retains peculiar pro-tumorigenic actions. Thus, novel analogues have been generated to avoid tumorigenic capacity and maintain BA metabolic action. Here, using BA related Abcb4^-/- and Fxr^-/- mouse models of spontaneous hepatic fibrosis and HCC, we explored the role of a novel engineered variant of FGF19 protein, called FGF19-M52, which fully retains BA regulatory activity but is devoid of the pro-tumoral activity. Expression of the BA synthesis rate-limiting enzyme Cyp7a1 is reduced in FGF19-M52-treated mice compared to the GFP-treated control group with consequent reduction of BA pool and hepatic concentration. Treatment with the non-tumorigenic FGF19-M52 strongly protects Abcb4^-/- and Fxr^-/- mice from spontaneous hepatic fibrosis, cellular proliferation and HCC formation in terms of tumor number and size, with significant reduction of biochemical parameters of liver damage and reduced expression of several genes driving the proliferative and inflammatory hepatic scenario. Our data bona fide suggest the therapeutic potential of targeting the FXR-FGF19 axis to reduce hepatic BA synthesis in the control of BA-associated risk of fibrosis and hepatocarcinoma development.

Human hepatocyte transplantation for liver disease: current status and future perspectives

Liver transplantation is the accepted treatment for patients with acute liver failure and liver-based metabolic disorders. However, donor organ shortage and lifelong need for immunosuppression are the main limitations to liver transplantation. In addition, loss of the native liver as a target organ for future gene therapy for metabolic disorders limits the futuristic treatment options, resulting in the need for alternative therapeutic strategies. A potential alternative to liver transplantation is allogeneic hepatocyte transplantation. Over the last two decades, hepatocyte transplantation has made the transition from bench to bedside. Standardized techniques have been established for isolation, culture, and cryopreservation of human hepatocytes. Clinical hepatocyte transplantation safety and short-term efficacy have been proven; however, some major hurdles-mainly concerning shortage of donor organs, low cell engraftment, and lack of a long-lasting effect-need to be overcome to widen its clinical applications. Current research is aimed at addressing these problems, with the ultimate goal of increasing hepatocyte transplantation efficacy in clinical applications.

Hepatocyte transplantation: Consider infusion before incision

Human hepatocyte transplantation is undergoing study as a bridge, or even alternative, to orthotopic liver transplantation (OLT). This technique has undergone multiple developments over the past thirty years in terms of mode of delivery, source and preparation of cell cultures, monitoring of graft function, and use of immunosuppression. Further refinements and improvements in these techniques will likely allow improved graft survival and function, granting patients higher yield from this technique and potentially significantly delaying need for OLT.

A universal system to select gene-modified hepatocytes in vivo

Many genetic and acquired liver disorders are amenable to gene and/or cell therapy. However, the efficiencies of cell engraftment and stable genetic modification are low and often subtherapeutic. In particular, targeted gene modifications from homologous recombination are rare events. These obstacles could be overcome if hepatocytes that have undergone genetic modification were to be selectively amplified or expanded. We describe a universally applicable system for in vivo selection and expansion of gene-modified hepatocytes in any genetic background. In this system, the therapeutic transgene is coexpressed with a short hairpin RNA (shRNA) that confers modified hepatocytes with resistance to drug-induced toxicity. An shRNA against the tyrosine catabolic enzyme 4-OH-phenylpyruvate dioxygenase protected hepatocytes from 4-[(2-carboxyethyl)-hydroxyphosphinyl]-3-oxobutyrate, a small-molecule inhibitor of fumarylacetoacetate hydrolase. To select for specific gene targeting events, the protective shRNA was embedded in a microRNA and inserted into a recombinant adeno-associated viral vector designed to integrate site-specifically into the highly active albumin locus. After selection of the gene-targeted cells, transgene expression increased 10- to 1000-fold, reaching supraphysiological levels of human factor 9 protein (50,000 ng/ml) in mice. This drug resistance system can be used to achieve therapeutically relevant transgene levels in hepatocytes in any setting.

p27Kip1 Inactivation Provides a Proliferative Advantage to Transplanted Hepatocytes in DPPIV/Rag2 Double Knockout Mice after Repeated Host Liver Injury

Studies were conducted to develop a new DPPIV–/–/Rag2–/– mouse model for hepatocyte transplantation by allogeneic and xenogeneic cells and to compare the proliferative capacity of p27 null hepatocytes versus normal hepatocytes in this system. Dipeptidyl peptidase IV (DPPIV) gene knockout mice, in which wild-type (wt) DPPIV+ donor hepatocytes can be readily identified by enzyme histochemistry, were bred with Rag2 null mice to prepare immunotolerant DPPIV–/–/Rag2–/– double knockout mice. DPPIV–/–/Rag–/– mice were transplanted with wt hepatocytes or p27 null mouse hepatocytes, which show enhanced cell cycle activity due to disruption of the Kip1 cyclin kinase inhibitor gene, and liver repopulation was assessed under nonproliferative versus proliferative experimental conditions. After their initial engraftment, transplanted wt hepatocytes did not proliferate in untreated livers or increase significantly in response to an acute liver regenerative stimulus. p27 null hepatocytes engrafted with the same efficiency as wt hepatocytes, but showed a noticeable, although not statistically significant, increase in proliferation in response to partial hepatectomy or acute CCl4 administration. Repeated treatments with CCl4 substantially increased proliferation and liver repopulation by p27 null hepatocytes but not by wt hepatocytes. These results suggest that p27 gene inactivation does not overcome proliferative restrictions imposed on hepatocytes by the normal liver, but that after repeated episodes of toxic liver injury, the augmented proliferative capacity of p27 null hepatocytes leads to significant liver repopulation compared with wt hepatocytes. These properties of p27-deficient hepatocytes could prove useful as a target for liver repopulation in patients with intermittent or a low level of chronic liver injury.

Efficient liver repopulation of transplanted hepatocyte prevents cirrhosis in a rat model of hereditary tyrosinemia type I

Hereditary tyrosinemia type I (HT1) is caused by a deficiency in the enzyme fumarylacetoacetate hydrolase (Fah). Fah-deficient mice and pigs are phenotypically analogous to human HT1, but do not recapitulate all the chronic features of the human disorder, especially liver fibrosis and cirrhosis. Rats as an important model organism for biomedical research have many advantages over other animal models. Genome engineering in rats is limited till the availability of new gene editing technologies. Using the recently developed CRISPR/Cas9 technique, we generated Fah(-/-) rats. The Fah(-/-) rats faithfully represented major phenotypic and biochemical manifestations of human HT1, including hypertyrosinemia, liver failure, and renal tubular damage. More importantly, the Fah(-/-) rats developed remarkable liver fibrosis and cirrhosis, which have not been observed in Fah mutant mice or pigs. Transplantation of wild-type hepatocytes rescued the Fah(-/-) rats from impending death. Moreover, the highly efficient repopulation of hepatocytes in Fah(-/-) livers prevented the progression of liver fibrosis to cirrhosis and in turn restored liver architecture. These results indicate that Fah(-/-) rats may be used as an animal model of HT1 with liver cirrhosis. Furthermore, Fah(-/-) rats may be used as a tool in studying hepatocyte transplantation and a bioreactor for the expansion of hepatocytes.

The potential of induced pluripotent stem cell derived hepatocytes

Orthotopic liver transplantation remains the only curative treatment for liver disease. However, the number of patients who die while on the waiting list (15%) has increased in recent years as a result of severe organ shortages; furthermore the incidence of liver disease is increasing worldwide. Clinical trials involving hepatocyte transplantation have provided encouraging results. However, transplanted cell function appears to often decline after several months, necessitating liver transplantation. The precise aetiology of the loss of cell function is not clear, but poor engraftment and immune-mediated loss appear to be important factors. Also, primary human hepatocytes (PHH) are not readily available, de-differentiate, and die rapidly in culture. Hepatocytes are available from other sources, such as tumour-derived human hepatocyte cell lines and immortalised human hepatocyte cell lines or porcine hepatocytes. However, all these cells suffer from various limitations such as reduced or differences in functions or risk of zoonotic infections. Due to their significant potential, one possible inexhaustible source of hepatocytes is through the directed differentiation of human induced pluripotent stem cells (hiPSCs). This review will discuss the potential applications and existing limitations of hiPSC-derived hepatocytes in regenerative medicine, drug screening, in vitro disease modelling and bioartificial livers.

Hepatocyte transplantation program: Lessons learned and future strategies

This review aims to share the lessons we learned over time during the setting of the hepatocyte transplantation (HT) program at the Hepatic Cell Therapy Unit at Hospital La Fe in Valencia. New sources of liver tissue for hepatocyte isolation have been explored. The hepatocyte isolation and cryopreservation procedures have been optimized and quality criteria for assessment of functionality of hepatocyte preparations and suitability for HT have been established. The results indicate that: (1) Only highly viable and functional hepatocytes allow to recover those functions lacking in the native liver; (2) Organs with steatosis (≥ 40%) and from elderly donors are declined since low hepatocyte yields, viability and cell survival after cryopreservation, are obtained; (3) Neonatal hepatocytes are cryopreserved without significant loss of viability or function representing high-quality cells to improve human HT; (4) Cryopreservation has the advantage of providing hepatocytes constantly available and of allowing the quality evaluation and suitability for transplantation; and (5) Our results from 5 adults with acute liver failure and 4 from children with inborn metabolic diseases, indicate that HT could be a very useful and safe cell therapy, as long as viable and metabolically functional human hepatocytes are used.

In Situ Transplantation of Alginate Bioencapsulated Adipose Tissues Derived Stem Cells (ADSCs) via Hepatic Injection in a Mouse Model

Objective

Adipose tissue derived stem cells (ADSCs) transplantation has recently gained widespread enthusiasm, particularly in the perspective to use them as potential alternative cell sources for hepatocytes in cell based therapy, mainly because of their capability of hepatogenic differentiation in vitro and in vivo. But some challenges remain to be addressed, including whether ADSCs can be provided effectively to the target organ and whether subsequent proliferation of transplanted cells can be achieved. To date, intrasplenic injection is the conventional method to deliver ADSCs into the liver; however, a number of donor cells retained in the spleen has been reported. In this study, our objective is to evaluate a novel route to transplant ADSCs specifically to the liver. We aimed to test the feasibility of in situ transplantation of ADSCs by injecting bioencapsulated ADSCs into the liver in mouse model.

Methods

The ADSCs isolated from human alpha 1 antitrypsin (M-hAAT) transgenic mice were used to allow delivered ADSCs be readily identified in the liver of recipient mice, and alginate was selected as a cell carrier. We first evaluated whether alginate microspheres are implantable into the liver tissue by injection and whether ADSCs could migrate from alginate microspheres (study one). Once proven, we then examined the in vivo fate of ADSCs loaded microspheres in the liver. Specifically, we evaluated whether transplanted, undifferentiated ASDCs could be induced by the local microenvironment toward hepatogenic differentiation and the distribution of surviving ADSCs in major tissue organs (study two).

Results

Our results indicated ADSCs loaded alginate microspheres were implantable into the liver. Both degraded and residual alginate microspheres were observed in the liver up to three weeks. The viable ADSCs were detectable surrounding degraded and residual alginate microspheres in the liver and other major organs such as bone marrow and the lungs. Importantly, transplanted ADSCs underwent hepatogenic differentiation to become cells expressing albumin in the liver. These findings improve our understanding of the interplay between ADSCs (donor cells), alginate (biomaterial), and local microenvironment in a hepatectomized mouse model, and might improve the strategy of in situ transplantation of ADSCs in treating liver diseases.

Evaluation of 11C-acetate and 18F-FDG PET/CT in mouse multidrug resistance gene-2 deficient mouse model of hepatocellular carcinoma

BACKGROUND

Hepatocellular carcinoma (HCC) remains a global health problem with unique diagnostic and therapeutic challenges, including difficulties in identifying the highest risk patients. Previous work from our lab has established the murine multidrug resistance-2 mouse (MDR2) model of HCC as a reasonable preclinical model that parallels the changes seen in human inflammatory associated HCC. The purpose of this study is to evaluate modalities of PET/CT in MDR2(-/-) mice in order to facilitate therapeutic translational studies from bench to bedside.

METHODS

18F-FDG and 11C-acetate PET/CT was performed on 12 m MDR2(-/-) mice (n = 3/tracer) with HCC and 12 m MDR2(-/+) control mice (n = 3/tracer) without HCC. To compare PET/CT to biological markers of HCC and cellular function, serum alpha-fetoprotein (AFP), lysophosphatidic acid (LPA), cAMP and hepatic tumor necrosis factor α (TNFα) were quantified in 3-12 m MDR2(-/-) (n = 10) mice using commercially available ELISA analysis. To translate results in mice to patients 11C-acetate PET/CT was also performed in 8 patents suspected of HCC recurrence following treatment and currently on the liver transplant wait list.

RESULTS

Hepatic18F-FDG metabolism was not significantly increased in MDR2(-/-) mice. In contrast, hepatic 11C-acetate metabolism was significantly elevated in MDR2(-/-) mice when compared to MDR2(-/+) controls. Serum AFP and LPA levels increased in MDR2(-/-) mice contemporaneous with the emergence of HCC. This was accompanied by a significant decrease in serum cAMP levels and an increase in hepatic TNFα. In patients suspected of HCC recurrence there were 5 true positives, 2 true negatives and 1 suspected false 11C-acetate negative.

CONCLUSIONS

Hepatic 11C-acetate PET/CT tracks well with HCC in MDR2(-/-) mice and patients with underlying liver disease. Consequently 11C-acetate PET/CT is well suited to study (1) HCC emergence/progression in patients and (2) reduce animal numbers required to study new chemotherapeutics in murine models of HCC.

Use of mesenchymal stem cells to treat liver fibrosis: Current situation and future prospects

Progressive liver fibrosis is a major health issue for which no effective treatment is available, leading to cirrhosis and orthotopic liver transplantation. However, organ shortage is a reality. Hence, there is an urgent need to find alternative therapeutic strategies. Cell-based therapy using mesenchymal stem cells (MSCs) may represent an attractive therapeutic option, based on their immunomodulatory properties, their potential to differentiate into hepatocytes, allowing the replacement of damaged hepatocytes, their potential to promote residual hepatocytes regeneration and their capacity to inhibit hepatic stellate cell activation or induce their apoptosis, particularly via paracrine mechanisms. The current review will highlight recent findings regarding the input of MSC-based therapy for the treatment of liver fibrosis, from in vitro studies to pre-clinical and clinical trials. Several studies have shown the ability of MSCs to reduce liver fibrosis and improve liver function. However, despite these promising results, some limitations need to be considered. Future prospects will also be discussed in this review.

Improvement of hepatocyte transplantation efficiency in the mdr2-/- mouse model by glyceryl trinitrate

INTRODUCTION

Hepatocyte transplantation could be an alternative to liver transplantation for the treatment of metabolic diseases. However, rodent models have shown that engraftment of transplanted cells in the liver is low and requires deposition of cells in hepatic sinusoids. Splanchnic vasodilatators improved hepatocyte engraftment in a rat model. Therefore, we investigated the effect of glyceryl trinitrate (GTN) on the efficacy of cell engraftment and on liver repopulation in the mdr2-knockout mouse, a model for progressive familial intrahepatic cholestasis type 3.

METHODS

Congenic normal mdr2 (+/+) hepatocytes were isolated by two-step collagenase perfusion and transplanted into mdr2(-/-) mice livers through the portal vein in the presence or absence of GTN. Liver repopulation was assessed by immunohistochemistry, and transplanted hepatocyte function was assessed at different times after transplantation by measurement of biliary lipid secretion and quantification of fibrosis.

RESULTS

The number of engrafted cells in GTN-treated mice was significantly higher than that in control mice, and transplanted hepatocytes were found in a greater number of distal sinusoids. Levels of phospholipid secretion were significantly higher than those in the control group 3 months after hepatocyte transplantation (18.3 ± 2.3 vs. 5.2 ± 3.9 nmol/min/100 g, P < 0.0001), and the ratio of phospholipids to bile salt was greater (6.8 ± 1.3 vs. 3.2 ± 1.6, P = 0.03). The percentage area of liver fibrosis was also significantly reduced in GTN-treated mice (5.7% ± 2.3% vs. 12.4% ± 2.9%, P = 0.016).

CONCLUSION

The use of GTN improves hepatocyte engraftment and correction of metabolic disease in mdr2 (-/-) mice. This approach might be beneficial in hepatocyte transplantation for the treatment of patients with liver diseases.

The history and use of human hepatocytes for the treatment of liver diseases: the first 100 patients

Orthotopic liver transplantation remains the only curative treatment for many end-stage liver diseases, yet the number of patients receiving liver transplants remains limited by the number of organs available for transplant. There is a need for alternative therapies for liver diseases. The transplantation of isolated hepatocytes (liver cells) has been used as an experimental therapy for liver disease in a limited number of cases. Recently, the 100th case of hepatocyte transplantation was reported. This review discusses the history of the hepatocyte transplant field, the major discoveries that supported and enabled the first hepatocyte transplants, and reviews the cases and outcomes of the first 100 clinical transplants. Some of the problems that limit the application or efficacy of hepatocyte transplantation are discussed, as are possible solutions to these problems. In conclusion, hepatocyte transplants have proven effective particularly in cases of metabolic liver disease where reversal or amelioration of the characteristic symptoms of the disease is easily quantified. However, no patients have been completely corrected of a metabolic liver disease for a significant amount of time by hepatocyte transplantation alone. It is likely that future developments in new sources of cells for transplantation will be required before this cellular therapy can be fully implemented and available for large numbers of patients.

Exploiting the unique regenerative capacity of the liver to underpin cell and gene therapy strategies for genetic and acquired liver disease

The number of genetic or acquired diseases of the liver treatable by organ transplantation is ever-increasing as transplantation techniques improve placing additional demands on an already limited organ supply. While cell and gene therapies are distinctly different modalities, they offer a synergistic alternative to organ transplant due to distinct architectural and physiological properties of the liver. The hepatic blood supply and fenestrated endothelial system affords relatively facile accessibility for cell and/or gene delivery. More importantly, however, the remarkable capacity of hepatocytes to proliferate and repopulate the liver creates opportunities for new treatments based on emerging technologies. This review will summarise current understanding of liver regeneration, describe clinical and experimental cell and gene therapeutic modalities and discuss critical challenges to translate these new technologies to wider clinical utility. This article is part of a Directed Issue entitled: "Regenerative Medicine: the challenge of translation".

ABCB4: Insights from pathobiology into therapy

Adenosine triphosphate (ATP)-binding cassette, sub-family B, member 4 (ABCB4), also called multidrug resistance 3 (MDR3), is a member of the ATP-binding cassette transporter superfamily, which is localized at the canalicular membrane of hepatocytes, and mediates the translocation of phosphatidylcholine into bile. Phosphatidylcholine secretion is crucial to ensure solubilization of cholesterol into mixed micelles and to prevent bile acid toxicity towards hepatobiliary epithelia. Genetic defects of ABCB4 may cause progressive familial intrahepatic cholestasis type 3 (PFIC3), a rare autosomic recessive disease occurring early in childhood that may be lethal in the absence of liver transplantation, and other cholestatic or cholelithiasic diseases in heterozygous adults. Development of therapies for these conditions requires understanding of the biology of this transporter and how gene variations may cause disease. This review focuses on our current knowledge on the regulation of ABCB4 expression, trafficking and function, and presents recent advances in fundamental research with promising therapeutic perspectives.

Stem cell therapy. Use of differentiated pluripotent stem cells as replacement therapy for treating disease

Pluripotent stem cells (PSCs) directed to various cell fates holds promise as source material for treating numerous disorders. The availability of precisely differentiated PSC-derived cells will dramatically affect blood component and hematopoietic stem cell therapies and should facilitate treatment of diabetes, some forms of liver disease and neurologic disorders, retinal diseases, and possibly heart disease. Although an unlimited supply of specific cell types is needed, other barriers must be overcome. This review of the state of cell therapies highlights important challenges. Successful cell transplantation will require optimizing the best cell type and site for engraftment, overcoming limitations to cell migration and tissue integration, and occasionally needing to control immunologic reactivity, as well as a number of other challenges. Collaboration among scientists, clinicians, and industry is critical for generating new stem cell-based therapies.

Hepatocyte senescence induced by radiation and partial hepatectomy in rat liver

PURPOSE

Exposure to radiation primes the liver for extensive replacement of the resident parenchymal cells by transplanted hepatocytes. The mechanisms underlying this repopulation remain to be clarified. In these studies, we examined the possible occurrence of cell senescence in vivo following radiation-associated preconditioning of the host liver.

MATERIALS AND METHODS

Fischer 344 rats underwent external-beam, computed-tomography-based partial liver irradiation. A single dose of 25 Gy was delivered to the right liver lobes (40% of liver mass). An additional group of animals received a 1/3 partial hepatectomy (removal of the left anterior lobe) four days after irradiation. Non-irradiated groups served as controls. All rats were sacrificed four weeks after the initial treatment.

RESULTS

The irradiated livers displayed several markers of cell senescence, including expression of senescence-associated-β-galactosidase (SA-β-gal), increase in cell size, and up-regulation of cyclin-dependent kinase inhibitors (CDK-I) p16 and p21. Furthermore, quantitative reverse-transcriptase polymerase chain reaction (qRT-PCR) analysis revealed activation of the senescence-associated secretory phenotype (SASP), including the cytokines interleukin 6 (IL6) and 1α (IL1α). The senescence-related changes were more prominent in rats undergoing partial hepatectomy (PH) following irradiation (IR).

CONCLUSIONS

We conclude that priming with radiation for liver repopulation results in the induction of cell senescence and the up-regulation of a senescence-associated secretory phenotype. The latter can contribute to the extensive growth of transplanted cells in this system.

Fumarylacetoacetate hydrolase deficient pigs are a novel large animal model of metabolic liver disease

Hereditary tyrosinemia type I (HT1) is caused by deficiency in fumarylacetoacetate hydrolase (FAH), an enzyme that catalyzes the last step of tyrosine metabolism. The most severe form of the disease presents acutely during infancy, and is characterized by severe liver involvement, most commonly resulting in death if untreated. Generation of FAH(+/-) pigs was previously accomplished by adeno-associated virus-mediated gene knockout in fibroblasts and somatic cell nuclear transfer. Subsequently, these animals were outbred and crossed to produce the first FAH(-/-) pigs. FAH-deficiency produced a lethal defect in utero that was corrected by administration of 2-(2-nitro-4-trifluoromethylbenzoyl)-1,3 cyclohexanedione (NTBC) throughout pregnancy. Animals on NTBC were phenotypically normal at birth; however, the animals were euthanized approximately four weeks after withdrawal of NTBC due to clinical decline and physical examination findings of severe liver injury and encephalopathy consistent with acute liver failure. Biochemical and histological analyses, characterized by diffuse and severe hepatocellular damage, confirmed the diagnosis of severe liver injury. FAH(-/-) pigs provide the first genetically engineered large animal model of a metabolic liver disorder. Future applications of FAH(-/-) pigs include discovery research as a large animal model of HT1 and spontaneous acute liver failure, and preclinical testing of the efficacy of liver cell therapies, including transplantation of hepatocytes, liver stem cells, and pluripotent stem cell-derived hepatocytes.

Cell therapy to remove excess copper in Wilson's disease

To achieve permanent correction of Wilson's disease by a cell therapy approach, replacement of diseased hepatocytes with healthy hepatocytes is desirable. There is a physiological requirement for hepatic ATP7B-dependent copper (Cu) transport in bile, which is deficient in Wilson's disease, producing progressive Cu accumulation in the liver or brain with organ damage. The ability to repopulate the liver with healthy hepatocytes raises the possibility of cell therapy in Wilson's disease. Therapeutic principles included reconstitution of bile canalicular network as well as proliferation in transplanted hepatocytes, despite toxic amounts of Cu in the liver. Nonetheless, cell therapy studies in animal models elicited major differences in the mechanisms driving liver repopulation with transplanted hepatocytes in Wilson's disease versus nondiseased settings. Recently, noninvasive imaging was developed to demonstrate Cu removal from the liver, including after cell therapy in Wilson's disease. Such developments will help advance cell/gene therapy approaches, particularly by offering roadmaps for clinical trials in people with Wilson's disease.

Improved liver function and relieved pruritus after 4-phenylbutyrate therapy in a patient with progressive familial intrahepatic cholestasis type 2

To examine the effects of 4-phenylbutyrate (4PB) therapy in a patient with progressive familial intrahepatic cholestasis type 2. A homozygous c.3692G>A (p.R1231Q) mutation was identified in ABCB11. In vitro studies showed that this mutation decreased the cell-surface expression of bile salt export pump (BSEP), but not its transport activity, and that 4PB treatment partially restored the decreased expression of BSEP. Therapy with 4PB had no beneficial effect for 1 month at 200 mg/kg/day and the next month at 350 mg/kg/day but partially restored BSEP expression at the canalicular membrane and significantly improved liver tests and pruritus at a dosage of 500 mg/kg/day. We conclude that 4PB therapy would have a therapeutic effect in patients with progressive familial intrahepatic cholestasis type 2 who retain transport activity of BSEP per se.

Therapeutic hepatocyte transplant for inherited metabolic disorders: functional considerations, recent outcomes and future prospects

The applications, outcomes and future strategies of hepatocyte transplantation (HTx) as a corrective intervention for inherited metabolic disease (IMD) are described. An overview of HTx in IMDs, as well as preclinical evaluations in rodent and other mammalian models, is summarized. Current treatments for IMDs are highlighted, along with short- and long-term outcomes and the potential for HTx to supplement or supplant these treatments. Finally, the advantages and disadvantages of HTx are presented, highlighted by long-term challenges with interorgan engraftment and expansion of transplanted cells, in addition to the future prospects of stem cell transplants. At present, the utility of HTx is represented by the potential to bridge patients with life-threatening liver disease to organ transplantation, especially as an adjuvant intervention where severe organ shortages continue to pose challenges.

Apoptosis and necrosis in the liver

Because of its unique function and anatomical location, the liver is exposed to a multitude of toxins and xenobiotics, including medications and alcohol, as well as to infection by hepatotropic viruses, and therefore, is highly susceptible to tissue injury. Cell death in the liver occurs mainly by apoptosis or necrosis, with apoptosis also being the physiologic route to eliminate damaged or infected cells and to maintain tissue homeostasis. Liver cells, especially hepatocytes and cholangiocytes, are particularly susceptible to death receptor-mediated apoptosis, given the ubiquitous expression of the death receptors in the organ. In a quite unique way, death receptor-induced apoptosis in these cells is mediated by both mitochondrial and lysosomal permeabilization. Signaling between the endoplasmic reticulum and the mitochondria promotes hepatocyte apoptosis in response to excessive free fatty acid generation during the metabolic syndrome. These cell death pathways are partially regulated by microRNAs. Necrosis in the liver is generally associated with acute injury (i.e., ischemia/reperfusion injury) and has been long considered an unregulated process. Recently, a new form of "programmed" necrosis (named necroptosis) has been described: the role of necroptosis in the liver has yet to be explored. However, the minimal expression of a key player in this process in the liver suggests this form of cell death may be uncommon in liver diseases. Because apoptosis is a key feature of so many diseases of the liver, therapeutic modulation of liver cell death holds promise. An updated overview of these concepts is given in this article.

Hepatocyte transplantation in bile salt export pump-deficient mice: selective growth advantage of donor hepatocytes under bile acid stress

The bile salt export pump (Bsep) mediates the hepatic excretion of bile acids, and its deficiency causes progressive familial intrahepatic cholestasis. The current study aimed to induce bile acid stress in Bsep(-/-) mice and to test the efficacy of hepatocyte transplantation in this disease model. We fed Bsep(-/-) and wild-type mice cholic acid (CA) or ursodeoxycholic acid (UDCA). Both CA and UDCA caused cholestasis and apoptosis in the Bsep(-/-) mouse liver. Wild-type mice had minimal liver injury and apoptosis when fed CA or UDCA, yet had increased proliferative activity. On the basis of the differential cytotoxicity of bile acids on the livers of wild-type and Bsep(-/-) mice, we transplanted wild-type hepatocytes into the liver of Bsep(-/-) mice fed CA or CA + UDCA. After 1-6 weeks, the donor cell repopulation and canalicular Bsep distribution were documented. An improved repopulation efficiency in the CA + UDCA-supplemented group was found at 2 weeks (4.76 ± 5.93% vs. 1.32 ± 1.48%, P = 0.0026) and at 4-6 weeks (12.09 ± 14.67% vs. 1.55 ± 1.28%, P < 0.001) compared with the CA-supplemented group. Normal-appearing hepatocytes with prominent nuclear staining for FXR were noted in the repopulated donor nodules. After hepatocyte transplantation, biliary total bile acids increased from 24% to 82% of the wild-type levels, among which trihydroxylated bile acids increased from 41% to 79% in the Bsep(-/-) mice. We conclude that bile acid stress triggers differential injury responses in the Bsep(-/-) and wild-type hepatocytes. This strategy changed the balance of the donor-recipient growth capacities and was critical for successful donor repopulation.

Hepatocyte transplantation for inherited metabolic diseases of the liver

Inherited metabolic diseases of the liver are characterized by deficiency of a hepatic enzyme or protein often resulting in life-threatening disease. The remaining liver function is usually normal. For most patients, treatment consists of supportive therapy, and the only curative option is liver transplantation. Hepatocyte transplantation is a promising therapy for patients with inherited metabolic liver diseases, which offers a less invasive and fully reversible approach. Procedure-related complications are rare. Here, we review the experience of hepatocyte transplantation for metabolic liver diseases and discuss the major obstacles that need to be overcome to establish hepatocyte transplantation as a reliable treatment option in the clinic.

Progressive familial intrahepatic cholestasis

Progressive familial intrahepatic cholestasis (PFIC) refers to a heterogeneous group of autosomal-recessive disorders of childhood that disrupt bile formation and present with cholestasis of hepatocellular origin. The exact prevalence remains unknown, but the estimated incidence varies between 1/50,000 and 1/100,000 births. Three types of PFIC have been identified and associated with mutations in hepatocellular transport-system genes involved in bile formation. PFIC1 and PFIC2 usually appear in the first months of life, whereas onset of PFIC3 may arise later in infancy, in childhood or even during young adulthood. The main clinical manifestations include cholestasis, pruritus and jaundice. PFIC patients usually develop fibrosis and end-stage liver disease before adulthood. Serum gamma-glutamyltransferase (GGT) activity is normal in PFIC1 and PFIC2 patients, but is elevated in PFIC3 patients. Both PFIC1 and PFIC2 are caused by impaired bile salt secretion due to defects in ATP8B1 encoding the FIC1 protein and in ABCB11 encoding bile salt export pump (BSEP) protein, respectively. Defects in ABCB4, encoding multidrug resistance 3 protein (MDR3), impair biliary phospholipid secretion, resulting in PFIC3. Diagnosis is based on clinical manifestations, liver ultrasonography, cholangiography and liver histology, as well as on specific tests to exclude other causes of childhood cholestasis. MDR3 and BSEP liver immunostaining, and analysis of biliary lipid composition should help to select PFIC candidates for whom genotyping could be proposed to confirm the diagnosis. Antenatal diagnosis may be proposed for affected families in which a mutation has been identified. Ursodeoxycholic acid (UDCA) therapy should be initiated in all patients to prevent liver damage. In some PFIC1 and PFIC2 patients, biliary diversion may also relieve pruritus and slow disease progression. However, most PFIC patients are ultimately candidates for liver transplantation. Monitoring of liver tumors, especially in PFIC2 patients, should be offered from the first year of life. Hepatocyte transplantation, gene therapy and specific targeted pharmacotherapy may represent alternative treatments in the future.

In vivo selection of transplanted hepatocytes by pharmacological inhibition of fumarylacetoacetate hydrolase in wild-type mice

Genetic fumarylacetoacetate hydrolase (Fah) deficiency is unique in that healthy gene-corrected hepatocytes have a strong growth advantage and can repopulate the diseased liver. Unfortunately, similar positive selection of gene-corrected cells is absent in most inborn errors of liver metabolism and it is difficult to reach the cell replacement index required for therapeutic benefit. Therefore, methods to transiently create a growth advantage for genetically modified hepatocytes in any genetic background would be advantageous. To mimic the selective pressure of Fah deficiency in normal animals, an efficient in vivo small molecule inhibitor of FAH, 4-[(2-carboxyethyl)-hydroxyphosphinyl]-3-oxobutyrate (CEHPOBA) was developed. Microarray analysis demonstrated that pharmacological inhibition of FAH produced highly similar gene expression changes to genetic deficiency. As proof of principle, hepatocytes lacking homogentisic acid dioxygenase (Hgd) and hence resistant to FAH inhibition were transplanted into sex-mismatched wild-type recipients. Time course analyses of 4-6 weeks of CEHPOBA administration after transplantation showed a linear relationship between treatment length and replacement index. Compared to controls, recipients treated with the FAH-inhibitor had 20-100-fold increases in liver repopulation. We conclude that pharmacological inhibition of FAH is a promising approach to in vivo selection of hepatocytes.

Hepatocyte senescence in vivo following preconditioning for liver repopulation

In the retrorsine (RS)-based model of massive liver repopulation, preexposure to this naturally occurring alkaloid is sufficient to prime normal host parenchymal cells to be slowly replaced by transplanted normal hepatocytes. The basis for this striking effect is yet to be fully elucidated. In the present studies the possible involvement of cell senescence was investigated. Fischer 344 rats were treated according to the RS-based protocol for hepatocyte transplantation, i.e., two doses of RS, 2 weeks apart, and were killed at 4 or 8 weeks after treatment. Control groups were given saline. Expression of senescence-associated beta-galactosidase was greatly induced in hepatocytes exposed to RS. In addition, several other changes that have been related to cell senescence were observed: these included markers of persistent activation of a DNA damage response, an increased expression of mammalian target of rapamycin, and positive regulators of the cell cycle, together with the induction of p21 and p27 cyclin-dependent kinase inhibitors. Furthermore, RS treatment increased levels of interleukin-6 in the liver, consistent with the activation of a senescence-associated secretory phenotype.

CONCLUSION

These findings indicate that RS induces hepatocyte senescence in vivo. We propose that cell senescence and the associated secretory phenotype can contribute to the selective growth of transplanted hepatocytes in this system.