AFP, alpha-fetoprotein; ATP, alkaline triphosphate; CD, cluster of differentiation; CYP, cytochrome P450; DDC, 3,5-diethoxycarbonyl-1,4-dihydrocollidine; ESCs, embryonic stem cells; EpCAM, epithelial cell adhesion molecule; EZH2, enhancer of zeste homolog 2; FAH, fumarylacetoacetate hydrolase; HCV, hepatitis C virus; HCC, hepatocellular carcinoma; HDAC, histone deacetylase; iPSCs, induced pluripotent stem cells; LPCs, liver progenitor cells; MAPK, mitogen-activated protein kinase; miRNA, microRNA; PARP, poly(ADP-ribose) polymerase; TGF-β, transforming growth factor beta; TISCs, tumor-initiating stem-like cells; TLR-4, Toll-like receptor-4; YAP1, yes-associated protein 1.
Stem cells have potential for therapy of liver diseases, but may also be involved in the formation of liver cancer. Recently, the American Association for the Study of Liver Diseases Henry M. and Lillian Stratton Basic Research Single Topic Conference “Stem Cells in Liver Diseases and Cancer: Discovery and Promise” brought together a diverse group of investigators to define the status of research on stem cells and cancer stem cells in the liver and identify problems and solutions on the path to clinical translation. This report summarizes the outcomes of the conference and provides an update on recent research advances. Progress in liver stem cell research includes isolation of primary liver progenitor cells (LPCs), directed hepatocyte differentiation of primary LPCs and pluripotent stem cells, findings of transdifferentiation, disease-specific considerations for establishing a therapeutically effective cell mass, and disease modeling in cell culture. Tumor-initiating stem-like cells (TISCs) that emerge during chronic liver injury share the expression of signaling pathways, including those organized around transforming growth factor beta and β-catenin, and surface markers with normal LPCs. Recent investigations of the role of TISCs in hepatocellular carcinoma have provided insight into the transcriptional and post-transcriptional regulation of hepatocarcinogenesis. Targeted chemotherapies for TISC are in development as a means to overcome cellular resistance and mechanisms driving disease progression in liver cancer. (HEPATOLOGY 2012;55:298–306)
Recent Advances in Liver Stem Cell Research
Liver stem cell research promises to improve the outcomes of patients with liver diseases. Advances in liver stem cell research may lead to new cell therapies and may facilitate the development of new drugs by providing faithful liver disease models.
John Gearhart, who codirected the conference, introduced unanswered questions and technical hurdles that remain to be overcome in stem cell research. In many tissues, stem cells have yet to be specifically identified and isolated. As a consequence, the current understanding of the mechanisms that facilitate proliferation and differentiation of tissue-specific stem cells is limited, which has also hampered the generation of therapeutically effective surrogate cells from alternative cell sources, such as pluripotent stem cells. To collaboratively solve these problems, stem cell researchers need to agree on a standard of phenotypical markers and functional read-outs and establish therapeutic efficacy and safety in rigorous animal models.
Research findings presented at the conference and additional advances reported in the last 2 years have moved the liver stem cell research field closer to realizing its potential by answering some long-standing questions, overcoming persistent technical hurdles, and making unexpected discoveries.
Isolation of Adult Clonogenic and Bipotential Liver Progenitor Cells
Adult liver progenitor cells (LPCs) are believed to provide a back-up system for replenishing hepatocytes and biliary epithelial cells when the regenerative capabilities of these cells are impaired, such as in chronic injury states. LPCs emerge and expand in periportal areas of the injured mouse, rat, and human liver. Recently, the long-standing hypothesis that adult LPCs reside within or derive from the epithelial lining of bile ducts has been confirmed in mice by lineage tracing of cells expressing the transcription factor Sox9 (Fig. 1).1
LPCs can be delineated from mature biliary epithelial cells, which also express Sox9, based on expression of the transcription factor Foxl1, or a combination of cell-surface epitopes, including the duct cell marker MIC1-1C3, and the general stem cell marker prominin-1 (cluster of differentiation [CD]133) (Fig. 1).2, 3 Cell isolation using these markers produces liver cell populations in which approximately 4% of the cells form colonies in culture that consist of both hepatocytes and biliary epithelial cells. Interestingly, clonogenic and bipotential adult LPCs cannot only be isolated from mice with ductular reactions induced by the drug 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC), but also from healthy livers.3 Accumulating evidence suggests that similarly potent cells also exist in the adult human liver, where they express the marker epithelial cell adhesion molecule (EpCAM) (Fig. 1).4
Improving Hepatocyte Differentiation of Pluripotent Stem Cells
A detailed understanding of the signals guiding hepatic specification in development has facilitated the production of cells equivalent to LPCs from mouse and human embryonic stem cells (ESCs) (Fig. 1).5-9 Though some hepatocyte functions are lacking or underdeveloped in these ESC derivatives in culture, including the expression of certain cytochrome P450 (CYP) enzymes, the cells can undergo further maturation after being transplanted into livers of mice or rats. Moreover, like primary hepatocytes, mouse ESC-derived hepatocytes can proliferate extensively after transplantation and repopulate the failing livers of fumarylacetoacetate hydrolase (FAH)-deficient mice.10 Much effort is currently focused on devising protocols that robustly produce human ESC-derived hepatocytes with similar proliferative abilities.
Despite concerns about epigenetic differences between induced pluripotent stem cells (iPSCs) and ESCs, evidence suggests that these pluripotent stem cell types are equally effective in giving rise to hepatocytes in culture.11-13 Moreover, liver function has been shown to be normal in chimeric mice generated by injection of wild-type mouse iPSCs into FAH-deficient blastocysts.14 Because all hepatocytes are iPSC derived in these mice, the finding establishes that mouse iPSCs are, in principle, capable of full hepatocyte differentiation (Fig. 1). In addition, the cellular origin of human iPSCs (i.e., whether they are derived from hepatocytes, fibroblasts, bone marrow mesenchymal stem cells, or keratinocytes) has been reported to not affect their ability for hepatic specification.15
However, advancing the differentiation of ESCs or iPSCs from an LPC to a mature hepatocyte stage in culture appears to require improved culture systems. Along these lines, coculture of primary human LPCs or hepatocytes with mesenchymal cells promotes or stabilizes hepatocyte differentiation, respectively.16, 17 Alternatively, direct and sequential application of growth factors and matrices provided by mesenchymal liver cells can be used to more closely replicate normal liver development.16, 18 Other findings presented at the conference show that prevention of epithelial-mesenchymal transition is also needed to achieve and maintain hepatocyte differentiation of human fetal liver cells in culture. These refined cell-culture conditions likely have a similar effect on LPCs derived from ESCs or iPSCs. In fact, hepatocyte differentiation and function of ESCs has been shown to significantly improve on polymer matrices.19 Importantly, advanced differentiation of ESC-derived hepatocytes does not only improve their function, but may also reduce the risk of tumor formation after transplantation.20 As an alternative approach to promoting hepatocyte differentiation, forced overexpression of transcription factors, such as Hex or a combination of Foxa2, Hnf4α, and C/ebpα, has been reported (Fig. 1).21, 22
Forced and Spontaneous Transdifferentiation
Lineage conversion of somatic cells by forced overexpression of cell-type–specific transcription factors is emerging as an alternative to reprogramming that bypasses the pluripotent state and its potential hazards. Along these lines, overexpression of the chromatin-modifying factors Foxa3 and Gata4, together with the transcription factor Hnf1α, in adult mouse fibroblasts lacking the tumor suppressor p19ARF, has been shown to induce a conversion into cells that resemble hepatocytes.23 Similar results have been obtained by coexpressing any of the 3 Foxa genes and Hnf4α in otherwise unmodified embryonic or adult mouse fibroblasts (Fig. 1).24 Induced hepatocyte-like cells generated with these few essential factors lack certain hepatocyte functions in culture, but can repopulate livers of FAH-deficient mice and prolong their survival. If similar cells could be generated from human cells, they would have great potential for both liver research and liver cell therapy.25 An example of spontaneous lineage conversion is provided by the finding that, in states of severe biliary injury, periportal hepatocytes can activate the biliary transcription factor Hnf1β, and transdifferentiate into biliary epithelial cells in rats (Fig. 1).26
Successful liver repopulation of FAH-deficient mice with hepatocytes derived from mouse ESCs, iPSCs, or by fibroblast transdifferentiation shows that the cells replicate the proliferative abilities of primary hepatocytes. However, taking advantage of post-transplant expansion to increase the efficacy of liver cell therapy has been limited to a few liver diseases that provide a growth advantage for normal hepatocytes, such as FAH deficiency, Wilson's disease, and progressive familial intrahepatic cholestasis. The recent finding, that transplanted hepatocytes spontaneously expand and repopulate the liver in a mouse model of α1-antitrypsin deficiency, extends this list to include the most common genetic liver disease.27 A strategy to achieve efficient engraftment and selective expansion of transplanted hepatocytes in other liver diseases is to combine localized liver irradiation with stimulation of hepatocyte proliferation.28, 29 As reported at the conference, a clinical trial employing this strategy for therapy of metabolic liver diseases with primary human hepatocytes is currently ongoing at the University of Pittsburgh.
An alternative mechanism that could be harnessed for effective liver cell replacement therapy is suggested by the ability of LPCs isolated from fetal rats to spontaneously repopulate the livers of wild-type rats, in particular, if the animals are aged.30 Similarly, as reported at the conference, adult LPCs emerge and expand in a rat model of end-stage liver cirrhosis. However, the finding that hepatocyte function is impaired in these animals suggests that the cirrhotic liver environment may not only cause loss of hepatocyte differentiation, but may also prevent LPC maturation. Thus, liver cirrhosis may prohibit effective cell therapy with both hepatocytes and LPCs.
Because most chronic liver diseases are associated with cirrhosis, alternative methods of hepatocyte delivery are being developed: The colonization of lymph nodes with transplanted hepatocytes creates therapeutically effective liver tissue outside of the recipient's liver.31 Decellularized liver matrix from cadaveric livers may provide the three-dimensional structure needed for creating transplantable liver tissue in culture.32, 33
Liver Disease Modeling
Patient-derived iPSCs have been differentiated into hepatocytes to generate cell-culture models of the liver diseases α1-antitrypsin deficiency, familial hypercholesterolemia, glycogen storage disease type 1a, and Wilson's disease.13, 34 In addition, a model of maturity-onset diabetes of the young type I was presented at the conference. Although iPSC-derived hepatocytes lack certain hepatocyte functions in culture, the disease phenotypes of these models are sufficiently distinctive and respond to established drugs, suggesting that they can be used to screen for new therapeutic agents for these liver diseases.
Stem Cell Conclusions
Recent advances in liver stem cell research can be expected to move the field forward in various ways: Even if primary LPCs will not be used for transplantation directly, the newfound ability to identify and isolate adult LPCs provides a system to investigate the mechanisms that guide LPC lineage choice and maturation. These analyses may reveal new regulators that, together with advanced cell-culture systems, may be effective in improving hepatocyte differentiation of pluripotent stem cells or fibroblasts. Improved hepatocyte differentiation protocols would not only benefit cell therapy strategies, but would also facilitate using patient-derived iPSCs to screen for CYP-dependent drug hepatotoxicity or to model liver diseases that require full hepatocyte differentiation for disease manifestation.
Successful stem-cell–based liver cell therapies will not only require that fully functional surrogate hepatocytes are used, but also that a therapeutically effective mass of these cells can be established in the recipient. Depending on the recipient's liver disease, a growth advantage for transplanted hepatocytes generated from primary or pluripotent stem cells in culture can be harnessed or created to replace a sufficient number of hepatocytes. In addition, identification of the signals and pathways that afford spontaneous proliferation of adult or fetal LPCs may facilitate targeted pre- or post-transplant expansion of like cells derived from pluripotent stem cells. However, in liver diseases where cirrhosis acts as a barrier to conventional hepatocyte replacement therapy, alternative sites or modes for cell delivery may be required.
The conference concluded with an expert panel discussion that reviewed the challenges of translating advances in liver stem cell research into therapies for patients. As evident from recent advances, methods of efficient cell delivery and directed differentiation of pluripotent stem cells into hepatocytes have become available or appear within reach. Other tasks, such as establishing therapeutic efficacy, protection from rejection, and safety of stem-cell–based liver cell therapies, may require major additional efforts, including the development of large animal models of liver diseases.
Emerging Concepts in the Field of Liver Tumor-Initiating Stem-Like Cells
Because LPCs are activated during chronic injury and cirrhosis, from which 90% of hepatocellular carcinoma (HCC) develops, LPC-derived HCC was originally proposed by Sell in 1989, 5 years before the sentinel work in leukemia.35, 36 In the last 2 years, an explosion of genomic profiling and molecular pathogenesis discovery has led to a more fundamental understanding of the role of tumor-initiating stem-like cells (TISCs) in HCC progression. TISCs form the fulcrum of the hierarchic cancer model and are generally defined with four criteria: high efficiency self-renewal, differentiation along at least two independent lineages (i.e., hepatocyte and cholangiocyte), resistance to traditional genotoxic therapy, and capacity to establish and recapitulate the original tumor.37 Here, we review emerging concepts within the controversial field of HCC TISC, with a focus on new discoveries that continue to reiterate the redundant, overlapping, and integrated signaling pathways that are deregulated in TISC populations and HCC.
Deregulation of Transforming Growth Factor Beta Signaling as a Molecular Pathway to TISC-Based Malignancy
Transforming growth factor beta (TGF-β) signals through intermediary SMAD proteins, which activate differentiation programs and inhibit cell-cycle progression during early carcinogenesis.38 In TISC-driven hepatocarcinogenesis, the loss of intermediary regulators, such as β2-Spectrin, results in the malignant transformation of liver stem and progenitor cells to TISCs via loss of differentiation and growth-arrest signals (Fig. 1).39 Within liver TISC populations, increased expression of ESC transcription factors Oct4 and Nanog, driven by loss of TGF-β differentiation signals, propagates self-renewal characteristics. Small molecule promoters of TGF-β signaling, which may restore growth-arrest and differentiation signals in TISCs, have been proposed as a TISC-targeting strategy. In related work, targeting of the fifth subunit of the COP9 signalosome (CSN5), which is functionally interconnected with TGF-β signaling, resulted in decreased tumor growth of human HCC cell lines in vivo.40
Because chronic hepatitis C virus (HCV) infection is the primary cause of HCC in the United States, murine models of HCV-induced HCC are highly relevant. In HCV core+ or NS5A+ transgenic mice, up-regulation of Toll-like receptor-4 (TLR4) expression during HCV-induced chronic injury was associated with impaired TGF-β signaling, up-regulated Nanog expression, and increased malignant potential, a process that is exacerbated by a high-fat diet.41, 42 TLR4 activation occurred predominantly in Nanog-dependent CD133+CD49f+ TISCs. Targeting Nanog directly in these TISCs results in decreased tumor initiation, by down-regulating cellular growth regulators. TLR4-initiated and Nanog-dependent activation of yes-associated protein 1 (YAP1), a regulator of Hippo signaling, results in inhibition of TGF-β through suppression of nuclear translocation of SMAD3. Silencing Yap1 results in suppression of Nanog transcription, restoration of TGF-β/SMAD3 signaling, and sensitization of TISCs to rapamycin and sorafenib.
Deregulation of β-Catenin Signaling in Liver TISCs
Canonical β-catenin signaling through activation of TCF/LEF promoters is a general mechanism of stem cell function, resulting in stem cell proliferation, survival, and inhibition of differentiation. Mutations in β-catenin and related complex proteins often result in β-catenin activation without Wnt initiation.43 Although β-catenin mutations are well characterized in HCC, mutations that protect β-catenin from degradation are not, by themselves, sufficient to induce HCC in murine models.43, 44 β-catenin activation is also found in normal LPCs, proliferating in response to chronic liver injury. Conversely, in liver-specific β-catenin knockout mice, LPC proliferation is reduced in response to the DDC diet.45 Further illustrating the importance of β-catenin for LPC proliferation, rare LPCs with normal β-catenin expression eventually emerge in this model and repopulate the liver.46
Genomic Profiling in Support of TISC-based HCC
Genomic profiling has emerged as a powerful tool for the understanding of comprehensive regulatory pathways in cancer biology, and recent work proposes that HCC can be subdivided within established differentiation stages, based on profiling analysis.47 Based on transcriptome profiles, HCC with a progenitor (e.g., EpCAM+) phenotype demonstrates TISC traits, such as self-renewal, bipotency, tumor-sphere formation, and increased tumor initiation, compared to EpCAM− HCC.48 Recent work also demonstrates that HCC expressing a cytokeratin-19 signature is TISC derived and carries a poor prognosis.49 In addition, integrative profiling provides insight into molecular mechanisms favoring tumor metastasis.48, 50, 51 Within TISC-based tumors, genomic profiling confirms the activation of key oncogenic signals from mitogen-activated protein kinase (MAPK), phosphatidyl inositol phosphate kinase, and β-catenin pathways, compared to mature hepatocyte-based HCC. These findings are supported by work demonstrating that TISCs, identified by “side-population” analysis, exhibit strong tumor-initiation ability, chemotherapy resistance, and express high levels of the pluripotency-associated transcription factors, Nanog, Oct4, c-Myc, and Sox2. This TISC signature is enriched using a 3-day treatment with the DNA methyltransferase inhibitor, zebularine, followed by isolation of the side population. During this enrichment process, methyltransferase inhibitors induce differentiation in all but the most resistant TISCs.37
Chromatin Accessibility, Epigenetic Regulation, and Transcriptional Regulation in TISC
Polycomb factors, such as enhancer of zeste homolog 2 (EZH2), act as epigenetic chromatin modifiers and transcriptional repressors and are important in stem cell self-renewal programs.52 In HCC, EZH2 suppresses Wnt antagonists, resulting in functional β-catenin activation.53
MicroRNAs (miRNAs) are noncoding regulators of gene expression, and miRNA-mediated control of proliferation in liver stem cells and hepatocytes during liver regeneration and control of differentiation in TISCs during carcinogenesis have been proposed.54-56 Specifically within HCC, molecular alterations manifesting as small changes across multiple genes, can be explained by changes in miRNA expression.56 MiRNA expression profiling of HCC identified miRNA-181 as up-regulated in EpCAM+ TISCs.57 β-catenin drives miRNA-181, which targets the hepatocyte differentiation-promoting genes, CDX2 and GATA6. In addition, miRNA-122, the most abundant miRNA in hepatocytes, has been identified as an inhibitor of alpha-fetoprotein (AFP) expression and aggressive features in HCC,58 providing another link between TISC-based HCC and poor prognosis.
TISC Surface Markers and Alternative Strategies of Identification
According to the hierarchical model of tumor formation and maintenance, tumor eradication requires TISC-targeted therapy, which requires target identification. Several surface markers, many of which are used as liver stem- and progenitor-cell markers, have been utilized to identify liver TISCs in human and murine models.
CD133 is an epithelial membrane protein expressed on the apical cell surface. CD133 expression has been identified within several human HCC cell lines, and recent work demonstrates a mechanistic link between TGF-β signaling and epigenetic modification as a regulator of CD133 expression within Huh7 cells, with increasing CD133 expression correlating with tumor initiation (Fig. 1).59 A prospective isolation of single CD133+ TISCs from liver-specific Pten-deficient animals demonstrates robust tumor initiation in immune-deficient and syngenic immune-competent hosts.60 Follow-up work demonstrates that the chronic inflammatory state of the model, not the oncogenic signals resulting from Pten loss, is the primary driver of TISC formation.61
EpCAM is present in the developing liver and biliary system, and is absent in mature hepatocytes.4, 62 EpCAM thus serves as a potential link between LPC and TISC populations (Fig. 1). In HCC, EpCAM expression correlates with in vivo tumor initiation as well as patient survival.48, 57
CD44 expression is well characterized within breast TISC populations (CD44high/+CD24low/−).63 In HCC, CD44 expression correlates with tumor initiation, metastatic potential, and chemotherapy resistance (Fig. 1).64, 65In vitro, the inhibition of CD44 expression results in reduction of TISC characteristics.66
Because TISCs are proposed to be rare populations, one concern is the variability of CD133 and other marker expression, which ranges from less than 1% in MHCC97-H cells to 60% in Huh7 cells.37, 65 This discrepancy suggests that isolating TISCs, based on the coexpression of multiple markers, would be more effective than use of a single marker. A mechanism for deregulated signaling, resulting in specific β-catenin activation results in up-regulation of TISC surface marker EpCAM, effectively linking TISC signaling mechanisms to surface marker expression.48 External factors, such as matrix stiffness, have also been implicated in promoting TISCs; although increasing matrix stiffness was associated with chemotherapy resistance, decreasing matrix stiffness was associated with other TISC characteristics, such as CD133 and CD44 expression.67
Alternative methods for TISC isolation include functional assays, such as side population, in which the exclusion of Hoechst dye identifies TISCs,68, 69 and aldehyde dehydrogenase activity.70 A functional assay may be superior to cell-surface markers for TISC isolation, because functional assays isolate cells based on the ability to detoxify—a key TISC characteristic.37
In terms of novel TISC-based therapies for HCC, there is a synergistic action between the histone deacetylase (HDAC) inhibitor, vorinostat, and the poly(ADP-ribose) polymerase (PARP) inhibitor, ABT1888, in HCC cell lines.71 The use of HDAC inhibitors is supported by the epigenetic modifications that enable the maintenance of the dedifferentiated state within TISC populations.37 This combination provided a significant increase in DNA damage and apoptosis in TISC populations, which are resistant to standard genotoxic therapies. In terms of targeting signaling pathways in TISCs, ON123300, an alkaline triphosphate (ATP) mimetic kinase inhibitor,72, 73 inhibits tumor cell proliferation and induces apoptosis, primarily through inhibition of CDK4 and AMPK-related protein kinase 5, without causing broader hepatotoxicity. This ATP kinase inhibitor may provide alternative TISC targeting.
Novel chemoprevention strategies aimed at targeting TISC through inhibition of the MAPK pathway and/or vitamin D supplementation have been proposed.74-76 Key insights include the marked vitamin D deficiency observed in cirrhotic patients who develop HCC. In the setting of cirrhosis, vitamin D supplementation may act as a chemoprevention strategy by restoring TGF-β signaling, because vitamin D up-regulates β2-spectrin in cirrhotic patients. Within alcoholic liver disease, a sensitization of liver macrophages to portal endotoxin, which activates TLR4 on macrophages, results in the production of inflammatory cytokines and activation of p38 MAPK, both contributing to the activation of the nuclear factor kappa light-chain enhancer of activated B cell signaling cascade.42, 77 Liver cirrhosis results from increased sensitivity of hepatic stellate cells to TGF-β, leading to increased proliferation and production of extracellular matrix via activation of p38 MAPK signaling.
Unanswered Questions in the Field of Liver TISCs
New work reinforces that the TGF-β and β-catenin pathways are central to the process of TISC transformation and maintenance. Transcriptome profiling confirms poor prognosis of TISC-based HCC. At the conference, several issues were identified as areas of focus for future work. One unresolved issue is whether liver TISCs have reduced rates of proliferation, compared to the bulk tumor population. A quiescent state is proposed for TISCs, but strong evidence is lacking. A second outstanding issue is the origin of TISCs. Although the hierarchic cancer model proposes that TISCs are derived from stem cells, they may also originate from hepatocyte dedifferentiation through loss of β2-Spectrin or up-regulation of β-catenin and resultant up-regulation of Oct-4 and Nanog.
A third issue is the need for agreement on the phenotype of TISCs. For example, in the field of hematopoietic malignancy, CD34+CD38− is a standard immune phenotype for TISCs, whereas CD44high/+CD24low/− is used to identify TISCs in breast cancer. As reviewed above, surface markers, such as EpCAM, CD133, CD49f, CD44, and others, have all been proposed for identifying TISCs, as have functional traits, such as efflux of Hoechst 33342, associated with the side population.
A final unanswered question is the effect of TISC-targeted therapy on the LPC population of the regenerating liver. As LPCs and TISCs share many common pathways for proliferation and maintenance of stemness, targeting TGF-β or β-catenin may reduce the effectiveness of LPCs to regenerate the liver during cirrhosis.
Although liver researchers proposed the concept of progenitor cell-derived cancer several years before leukemia researchers, the existence of liver TISCs is still debated. Recent advances provide additional support for the liver TISC concept and, importantly, may aid therapeutic decision making by allowing risk assessment. Targeted anti-TISC therapies hold significant promise, but a better understanding of the origin and phenotype of these cells appears necessary.
Significant progress has been made in liver stem cell and liver cancer stem cell research in the last 2 years. Although normal and cancerous liver stem cells or LPCs naturally share many characteristics and markers, on most accounts, the two fields have been moving forward independently. The dual focus of the conference provided an opportunity to gather insight or update one's knowledge of the other field. In addition, the conference identified commonalities that may serve as a basis for future advances. For example, new markers of normal LPCs may be useful for further fractionation of heterogeneous TISC populations. Conversely, signaling pathways and transcription factors regulating TISC characteristics may also play a role in noncancerous liver regeneration. In addition, new cell-culture or in vivo cell-delivery systems will likely benefit research on both normal and cancerous liver stem cells or LPCs. The hope is that increased exchange and collaboration between the two fields will accelerate the development of therapies for patients with liver disease and liver cancer.
The American Association for the Study of Liver Disease Henry M. and Lillian Stratton Basic Research Single Topic Conference “Stem Cells in Liver Diseases and Cancer: Discovery and Promise” was held in Atlanta, GA, March 19-20, 2011. The meeting provided an overview and update on ongoing research efforts seeking to obtain a detailed understanding of stem cell biology in liver disease and cancer for the development of new therapies. This review is dedicated to Nelson Fausto, a leader in the field.