| dc.description.abstract | Liver cirrhosis places an increasing burden on healthcare worldwide. Currently the
only treatment is liver transplantation. Whilst liver transplant has a relatively good
five-year survival, donor organ shortage costs many lives every year and results in
lifelong immunosuppression. Alternative treatments are thus urgently needed. It is
with this background that there is understandable interest for the development of stem
cell therapies for liver regeneration. The identification of putative liver stem cells has
brought closer the previously separate fields of liver ontology, regeneration, and
carcinogenesis. Significant overlaps in the regulation of these processes are now being
described. For example, studies in embryonic liver development have already provided
the basis for directed differentiation of human embryonic stem cells and induced
pluripotent stem cells into hepatocyte-like cells. As a result, the understanding of the
cell biology of proliferation and differentiation in the liver has been improved. This
knowledge can be used to improve the function of hepatocyte-like cells for drug
testing, bio-artificial livers, and transplantation. In parallel, the mechanisms regulating
cancer cell biology are now clearer, providing fertile soil for novel therapeutic
approaches. Recognition of the relationships between development, regeneration, and
carcinogenesis, and the increasing evidence for the role of stem cells in all of these
areas, has sparked fresh enthusiasm in understanding the underlying molecular
mechanisms and has led to new targeted therapies for liver cirrhosis and primary liver
cancers.
Human liver progenitor cells (LPCs) have therapeutic potential but their in vitro
culture results in inadequate differentiation, function, and phenotypic instability
reflecting an incomplete understanding of in vivo processes. LPCs can be robustly
isolated from second trimester human foetal livers by immunoselection for
EpCAM+/CD29+/CD49d+/CD49e–/CD235a–/CD45– cells. Expression profiling of
mRNA and microRNA in human foetal LPCs was performed and compared with
mature human hepatocytes and human embryonic stem cells undergoing hepatocytic
differentiation. Foetal LPCs exhibit a distinct transcriptome profile consistent with a
stem cell signature, cell division, and some liver-specific functions. Bioinformatic integration of microRNA and mRNA datasets revealed that microRNAs up-regulated
in LPCs targeted genes involved in metabolic processes implying repression of the
mature hepatocyte phenotype. Control of LPC gene expression therefore occurs at both
transcriptional and, via microRNAs, post-transcriptional levels. Furthermore,
transcription factor binding site analyses revealed enriched E2F1 motif in gene and
microRNA promoters suggesting feedback control in determining LPC fate. Foetal
LPCs were capable of differentiation to a hepatocytic phenotype in the presence of
appropriate paracrine signals provided by EpCAM– non-parenchymal cells (NPCs),
which consist mainly of endothelial cells and hepatic stellate cells. Fibronectin, despite
being produced in abundance by EpCAM– NPCs, had no effect on LPC synthetic
function in vitro. The expression of fibronectin in the perisinusoidal space suggests its
potential role of modulating cross-talk between hepatoblasts/hepatocytes, liver
sinusoidal endothelial cells, and hepatic stellate cells. Fibronectin expression in the
portal vein mesenchyme and laminin α5 expression along the ductal plate suggest that
both matrix molecules, located in close proximity to LPCs, may be important in
supporting the LPC niche. Findings in this work provide insight into the regulation of
the human foetal LPC functional phenotype, bringing stem cell-based therapies for
liver disease one step closer. | en |
