Putative liver progenitor cells: conditions for long-term survival in culture

The cholangiocyte marker, BD. 1, forms a stable complex with CLIP170 and shares an identity with eIF3a, a multifunctional subunit of the eIF3 initiation complex

We have previously described the generation of a monoclonal antibody recognizing a novel cholangiocyte marker, designated BD.1, that is expressed by fetal and adult rat cholangiocytes but not hepatocytes or the hepatic progenitor cells known as oval cells. In the present report, we have undertaken a comprehensive examination of BD.1 expressed by long-term cultures of bile duct epithelial cells (BDEC) and prostate epithelial cells (PEC). We show that with continued passage, the levels of BD.1 expressed by BDEC and PEC drop significantly, a decrease that is temporally associated with transition from a diploid to an aneuploid karyotype. Cell cycle analysis revealed cell cycle dependent expression of BD.1 characterized by decreased BD.1 levels within the first 10 h after release from serum starvation followed by reacquisition as cells entered S phase. MAb BD.1 recognized a 170 kDa protein in Western blots and showed strong reactivity with a 170 kDa band in blots prepared from phosphoproteins isolated by metal affinity chromatography. Analysis by mass spectrometry of tryptic peptides generated from BD.1 purified by continuous elution electrophoresis identified the plus end microtubule-binding protein, CLIP170, in the fraction reactive with MAb BD.1. Double immunofluorescence with MAb BD.1 and a MAb specific for CLIP170 showed that both were reactive with intrahepatic bile ducts. However, overexpression or siRNA knockdown of CLIP170 in 293T cells did not significantly alter BD.1 levels, indicating that CLIP170 and BD.1 were distinct, co-migrating proteins. Immunoprecipitation analysis with MAb BD.1 and anti-CLIP170 antibodies showed that under microtubule depolymerizing conditions the two proteins could be co-precipitated with both antibodies, leading us to conclude they were capable of forming stable complexes. Two different protocols were devised to enrich for the CLIP170 binding protein recognized by MAb BD.1. Analysis of tryptic peptides by LC-ESI-MS/MS identified BD.1 as eIF3a, the largest subunit of the elongation initiation factor 3 (eIF3) complex. This identity was confirmed by the simultaneous knockdown of both BD.1 and eIF3a by eIF3a-specific siRNAs and by the strong reactivity of MAb BD.1 with the 170 kDa protein immunoprecipitated with the anti-eIF3a antibody, 5H10. Based on these findings, we concluded that the BD.1 antigen was identical to eIF3a, a multifunctional subunit of the eIf3 complex shown here to associate with microtubules through its interactions with CLIP170.

Liver progenitor cell interactions with the extracellular matrix

Liver progenitor cells (LPCs) are a promising source of cells to treat liver disease by cell therapy, due to their capability for self-replication and bipotentiality. In order to establish useful culture systems of LPCs and apply them to future clinical therapies, it is necessary to understand their interactions with their microenvironment and especially with the extracellular matrix (ECM). There is considerable evidence from in vivo studies that matrix proteins affect the activation, expansion, migration and differentiation of LPCs, but the information on the role that specific ECMs play in regulating LPCs in vitro is more limited. Nevertheless, current studies suggest that laminin, collagen type III, collagen type IV and hyaluronic acid help to maintain the undifferentiated phenotype of LPCs and promote their proliferation when cultured in media supplemented with growth factors chosen for LPC expansion, whereas collagen type I and fibronectin are generally associated with a differentiated phenotype under the same conditions. Experimental evidence suggests that α6β1 and α5β1 integrins as well as CD44 on the surface of LPCs, and their related downstream signals, are important mediators of interactions between LPCs and the ECM. The interactions of LPCs with the ECM form the focus of this review and the contribution of ECM molecules to strategies for optimizing in vitro LPC cultures for therapeutic applications is discussed.

Tissue culture: the unlimited potential

Lack of tissue-specific differentiated functions of cells in tissue culture, once thought to be due to "dedifferentiation", was shown to be due to selective overgrowth of fibroblasts by a series of simple experiments that challenged the prevailing dogma. Following this insight, enrichment culture techniques (alternate animal and culture passage) were designed to give functionally differentiated tumor cells selective advantage over the fibroblasts. These experiments resulted in the derivation of a large number of functionally differentiated clonal strains of a range of cell types, providing the final point of destruction of the dogma of "dedifferentiation." Instead, the hypothesis was proposed that cells in culture accurately represent cells in vivo, but without the complex in vivo environment. With the development of hormonally defined media and its combination with functionally differentiated clonal cell lines, this concept has been strengthened and the potential of tissue culture studies has been greatly augmented. Hormonally defined media allow the culture of cell types that cannot be grown in conventional, serum-supplemented media. These approaches demonstrate that hormonal responses and dependencies can be discovered in culture. Following this thinking and the discovery of hormonal dependencies of cancer cells has led to a new rationale for therapy. Tissue culture and cell technology continue to play an important role in solving human health problems.

Functional analysis of encapsulated hepatic progenitor cells

A major challenge in developing therapies based on progenitor or stem cell populations (from sources other than bone marrow) involves developing a mode to deliver these cells in a manner that optimizes their viability, engraftment, proliferation, and differentiation. We have previously isolated a hepatic progenitor cell (HPC) population from adult liver tissue that differentiates into hepatic and biliary cell subtypes. We postulated that, using electrostatic encapsulation, we could reproducibly generate an ex vivo environment for the HPCs. We also theorized that this approach would foster cellular viability and function of the progenitor cell population. Using this encapsulation process, we consistently produced beads with uniform diameters between 200 and 700 microm. In vitro analysis of the encapsulated beads demonstrated extended periods of viability and function based on albumin production, urea metabolism, and glycogen storage. In conclusion, HPC encapsulation fosters the subsequent differentiation of HPCs into functional cells while maintaining their viability in long-term culture. These results demonstrate the efficacy of this method using somatic-derived progenitor cell populations and pave the way for clinical therapies.

Hepatic stem cells: from inside and outside the liver?

The liver is normally proliferatively quiescent, but hepatocyte loss through partial hepatectomy, uncomplicated by virus infection or inflammation, invokes a rapid regenerative response from all cell types in the liver to perfectly restore liver mass. Moreover, hepatocyte transplants in animals have shown that a certain proportion of hepatocytes in foetal and adult liver can clonally expand, suggesting that hepatoblasts/hepatocytes are themselves the functional stem cells of the liver. More severe liver injury can activate a potential stem cell compartment located within the intrahepatic biliary tree, giving rise to cords of bipotential transit amplifying cells (oval cells), that can ultimately differentiate into hepatocytes and biliary epithelial cells. A third population of stem cells with hepatic potential resides in the bone marrow; these haematopoietic stem cells may contribute to the albeit low renewal rate of hepatocytes, but can make a more significant contribution to regeneration under a very strong positive selection pressure. In such instances, cell fusion rather than transdifferentiation appears to be the underlying mechanism by which the haematopoietic genome becomes reprogrammed.

Pancreatic-hepatic switches in vivo

The existence of a common endodermal progenitor cell to both pancreas and liver is suggested by anatomy of the development of pancreatic and liver buds in embryogenesis. Here we review the large body of evidence that pancreas to liver and liver to pancreas cell differentiation can also occur in adult life. The published data are consistent with the hypothesis that endodermal progenitor cells, capable of giving rise to multiple hepatic and pancreatic cell types continue to persist. These cells may represent a stem cell reservoir with potential in cell therapy applications in the future.

Purification of fetal mouse hepatoblasts by magnetic beads coated with monoclonal anti-e-cadherin antibodies and their in vitro culture

A simple, rapid, and reproducible method of fetal hepatoblast purification was established to investigate mechanisms controlling interactions between hepatoblasts and nonparenchymal cells during liver development. Because E-cadherin is exclusively expressed on the cell membrane of hepatoblasts, magnetic beads coated with monoclonal antibodies to an extracellular epitope of its molecule were used to purify hepatoblasts from a cell suspension prepared from 12.5-day fetal mouse livers. The purity and yield in the hepatoblast fraction prepared in our protocol were more than 90% and approximately 30%, respectively. The nonparenchymal fraction rarely contained hepatoblasts; the rate of hepatoblast contamination in this fraction was less than 1%. Separate cultures of these two fractions were compared with cocultures of both fractions. In culture of the hepatoblast fraction, hepatoblasts formed aggregates similar to a bunch of grapes via their loose adhesion, floating in the medium after 24 h, and dissociated into single cells from the aggregates after 120 h of culture. By contrast, in the mixed culture, the majority of hepatoblasts formed multicellular spheroids after 24 h, and these spheroids changed into monolayer cell sheets after 120 h of culture. The cells comprising these monolayer sheets abundantly expressed albumin and carbamoylphosphate synthase I. In the mixed culture, fibroblastic cells also proliferated extensively with spreading on glass slides and surrounded the hepatoblast or hepatocyte colonies. On the other hand, fibroblastic cells spreading on glass slides decreased gradually in cultures of the nonparenchymal cell fraction alone. These findings indicated that the coexistence of hepatoblasts and nonparenchymal cells may be essential for their mutual survival, proliferation, differentiation, and morphogenesis. The conditioned medium of fetal liver cell cultures could partially replace the effects of the nonparenchymal cells on hepatoblasts in vitro. Our isolation protocol for fetal mouse hepatoblasts using immunobeads can greatly facilitate studies on mechanisms of cell-cell interactions during liver development.

Engineering liver therapies for the future

Treatment of liver disease has been greatly improved by the advent and evolution of liver transplantation. However, as demand for donor organs continues to increase beyond their availability, the need for alternative liver therapies is clear. Several approaches including extracorporeal devices, cell transplantation, and tissue-engineered constructs have been proposed as potential adjuncts or even replacements for transplantation. Simultaneously, experience from the liver biology community have provided valuable insight into tissue morphogenesis and in vitro stabilization of the hepatocyte phenotype. The next generation of cellular therapies must therefore consider incorporating cell sources and cellular microenvironments that provide both a large population of cells and strategies to maintain liver-specific functions over extended time frames. As cell-based therapies evolve, their success will require contribution from many diverse disciplines including regenerative medicine, developmental biology, and transplant medicine.

Applications of developmental biology to medicine and animal agriculture

With the complete sequence of the human genome expected by winter 2001, genomic-based drug discovery efforts of the pharmaceutical industry are focusing on finding the relatively few therapeutically useful genes from among the total gene set. Methods to rapidly elucidate gene function will have increasing value in these investigations. The use of model organisms in functional genomics has begun to be recognized and exploited and is one example of the emerging use of the tools of developmental biology in recent drug discovery efforts. The use of protein products expressed during embryo-genesis and the use of certain pluripotent cell populations (stem cells) as candidate therapeutics are other applications of developmental biology to the treatment of human diseases. These agents may be used to repair damaged or diseased tissues by inducing or directing developmental programs that recapitulate embryonic processes to replace specialized cells. The activation or silencing of embryonic genes in the disease state, particularly those encoding transcription factors, is another avenue of exploitation. Finally, the direct drug-induced manipulation of embryonic development is a unique application of developmental biology in animal agriculture.

Liver stem cells: a two compartment system

Hepatocytes and biliary epithelia are phenotypically very dissimilar, but share a common ancestry. Hepatocytes regenerate very efficiently, and their division potential indicates that many of them are functional stem cells. When hepatocyte-damaging agents also impair the regenerative ability of surviving hepatocytes, a potential stem cell system of biliary origin is activated to generate new hepatocytes - a reversal of ontogeny. Now both bile duct derived cells and hepatocytes can be isolated from the liver, genetically modified in vitro and returned to their in vivo origins where, after considerable population expansion, they can function as hepatocytes - paving the way for ex vivo gene therapy.