Restoring balance to liver stem cell research

Liver repair by intra- and extrahepatic progenitors

Despite its remarkable capacity for endogenous regeneration, the mammalian liver is vulnerable to a number of chronic or acute conditions that exceed or circumvent the proliferative capabilities of its mature cell complement. Bipotential hepatic progenitors, or "oval cells," have been shown to contribute to organ regeneration under such circumstances, both in human patients and in animal models. These progenitors are attractive agents for cell therapy, but have thus far proven challenging to isolate and manipulate. New reports indicating that transplanted bone marrow cells (BMCs) can also generate hepatocytes and contribute to liver repair have attracted considerable attention, because these cells are familiar and accessible to both clinicians and scientists. Recently, the issue of whether nuclear transfer (via cell fusion between donor BMC and recipient hepatocyte) or previously unrecognized differentiation potential (i.e., plasticity/transdifferentiation of BMC) is the primary origin of donor-derived hepatocytes has generated considerable controversy. In the liver, most evidence supports cell fusion as the key agent in the reversal of hepatopathology. However, regardless of their origin, the frequency of hepatocyte correction events is low. As is the case for the delivery of intrahepatic progenitors, substantial improvements in the understanding of this process will be needed before clinical application becomes practical.

Gastrointestinal stem cells. III. Emergent themes of liver stem cell biology: niche, quiescence, self-renewal, and plasticity

This essay will address areas of liver stem/progenitor cell studies in which consensus has emerged and in which controversy still prevails over consensus, but it will also highlight important themes that inevitably should be a focus of liver stem/progenitor cell investigations in coming years. Thus concepts regarding cell plasticity, the existence of a physiological/anatomic stem cell niche, and whether intrahepatic liver stem/progenitor cells comprise true stem cells or progenitor cells (or both) will be approached in some detail.

In vitro transdifferentiation of adult bone marrow Sca-1+cKit− cells cocultured with fetal liver cells into hepatic-like cells without fusion

Several research groups have recently reported that certain bone marrow cells (BMCs) differentiate into hepatocytes in vitro as well as in vivo in rodents. However, it has yet to be elucidated what factors effectively trigger and sustain transdifferentiation of BMCs. In the present study, we specifically asked whether the presence of murine fetal liver cells (FLCs) triggered and supported in vitro transdifferentiation of murine BMCs. Fractionated BMCs from green fluorescence protein (GFP)-expressing transgenic mice and FLCs from ROSA26 mice (X-gal(+) FLCs) were cocultured in the presence of hepatocyte growth factor in laminin-coated dishes. We found that Sca-1(+) BMCs gave rise to adherent hepatic-like cells, which expressed albumin as assessed with immunocytochemistry and RNA-polymerase chain reaction (PCR), and alpha-fetoprotein and cytokeratin 19 as examined with RNA-PCR. When GFP(+)Sca-1(+)cKit(-) cells were cocultured with X-gal(+) FLCs, all GFP(+) albumin-producing cells were negative for X-gal, showing that cell fusion was not associated in the observed BMCs' differentiation into hepatic-like cells. Titration analysis revealed that 1 of 5,943 Sca-1(+)cKit(-) cells had the ability to proliferate and differentiate into hepatic-like cells. These data strongly suggest that BMCs differentiate into hepatic-like cells in the presence of FLCs and that the present method may be useful for propagating BMC-derived hepatocytic progenitors and for investigating the nature of those cells.

Human cord blood stem cells generate human cytokeratin 18-negative hepatocyte-like cells in injured mouse liver

Differentiation of adult bone marrow (BM) cells into nonhematopoietic cells is a rare phenomenon. Several reports, however, suggest that human umbilical cord blood (hUCB)-derived cells give rise to hepatocytes after transplantation into nonobese diabetic-severe combined immunodeficient (NOD-SCID) mice. Therefore, we analyzed the hepatic differentiation potential of hUCB cells and compared the frequency of newly formed hepatocyte-like cells in the livers of recipient NOD-SCID mice after transplantation of hUCB versus murine BM cells. Mononuclear cell preparations of hUCB cells or murine BM from enhanced green fluorescent protein transgenic or wild-type mice were transplanted into sublethally irradiated NOD-SCID mice. Liver regeneration was induced by carbon tetrachloride injury with and without subsequent hepatocyte growth factor treatment. By immunohistochemistry and reverse transcriptase-polymerase chain reaction, we detected clusters of hepatocyte-like cells in the livers of hUCB-transplanted mice. These cells expressed human albumin and Hep Par 1 but mouse CK18, suggesting the formation of chimeric hepatocyte-like cells. Native fluorescence microscopy and double immunofluorescence failed to detect single hepatocytes derived from transplanted enhanced green fluorescent protein-transgenic mouse BM. Fluorescent in situ hybridization rarely revealed donor-derived hepatocyte-like cells after cross-gender mouse BM transplantation. Thus, hUCB cells have differentiation capabilities different from murine BM cells after transplantation into NOD-SCID mice, demonstrating the importance of further testing before hUCB cells can be used therapeutically.