Regulation of the survival and differentiation of hepatic stem/progenitor cells by acyclic retinoid
The liver is the largest organ in the adult human body and is important for maintaining
homeostasis. Adult hepatocytes and liver parenchymal cells express genes and enzymes
for several metabolic functions. In addition, the liver includes several non-parenchymal
cells such as cholangiocytes, sinusoidal endothelial cells, stellate cells, and Kupffer
cells. Hepatocytes and cholangiocytes are differentiated from the same source: the
stem/progenitor cells in embryonic livers. Guan and colleagues, in a recent article
in Stem Cell Research Therapy, investigated a new mechanism regulating the survival and differentiation of these
stem/progenitor cells by retinoic acid signals 1].
There are two types of liver progenitor cells: foetal hepatic progenitor cells and
postnatal/adult hepatic progenitor cells. Liver formation begins with the specification
of the foetal hepatic progenitor cells derived from the foregut endoderm. During mid-
to late-foetal liver development, hepatic progenitor cells differentiate into hepatocytes
and cholangiocytes. It has been reported that the interaction between hepatic cells
and hematopoietic cells is important for foetal liver maturation 2]. Moreover, in cases of serious injury to the adult liver, such as that induced by
retrorsine or 2-acetylaminofluorine treatment in combination with partial hepatectomy,
the number of characteristic progenitor-like cells expressing both cholangiocellular
and hepatocellular markers increases in periportal regions 3].
These foetal and adult hepatic progenitor cells are well characterized by specific
cell surface antibodies and cell purification methods such as flow cytometry. Suzuki
and colleagues 4] found CD29 and CD49f to be specific markers and established in vitro colony formation assays of hepatic progenitor cells. In addition, markers such as
Dlk, Liv2, E-cadherin, CD13, and CD133 are reported to be specific markers of foetal
hepatic progenitor cells. CD13 and CD133 are also expressed in progenitor cells derived
from adult livers. Recent studies revealed that long-term proliferative hepatic progenitor
cells exist in mouse and human adult livers. LGR5+ and EpCAM+ progenitor cells can form cystic structures and expand in vitro for several months 5], 6]. An in vitro culture system of purified cells using antibodies against these cell surface antigens
is useful for analyses of molecular mechanisms regulating the proliferation and differentiation
of hepatic progenitor cells.
It is known that, in addition to stem/progenitor cells in the normal liver, cancer
stem-like cells are present in hepatocellular carcinomas (HCCs) or other types of
cancers. Cancer stem-like cells have characteristics similar to those exhibited by
somatic stem cells and are important for tumour initiation, metastasis, and recurrence.
Retinoic acid is a natural derivative of vitamin A and regulates its target genes
by binding to nuclear receptors such as retinoic acid receptors (RARs) and retinoid
X receptors (RXRs). Retinoic acid is important for the differentiation and proliferation
of stem/progenitor cells in both somatic and cancer tissues. For example, retinoic
acid is used for the differentiation of pluripotent stem cells into mature functional
cells. In addition, acyclic retinoid (ACR) exhibited anti-HCC effects in several hepatoma
models 7], 8]. In contrast, liver regeneration in vivo is induced by the addition of all-trans retinoic acid (ATRA) 9]. Thus, the exact effects of retinoic acid signals on normal hepatic stem/progenitor
cells (HpSCs) in somatic tissues remain unknown.
Guan and colleagues 1] stated that the addition of ACR regulated the survival and differentiation of HpSCs
derived from mouse embryonic liver. Peretinoin is a synthetic ACR that has been known
to reduce the risk of HCC recurrence or death. In addition, peretinoin inhibits the
replication and release of hepatitis C virus 10]. Guan and colleagues isolated HpSCs by using specific cell surface antigens (CD29
and CD49f) and flow cytometry and cultured them in a low-density colony assay system.
These cells have a high colony-forming ability and express several retinoic acid-binding
receptors – namely, Rar?, Rxr?, and Rxr? – at higher levels than those expressed by
adult hepatocytes. The addition of peretinoin significantly reduced the total number
of cells and colony formation by HpSCs in a dose-dependent manner. After peretinoin
treatment, the clonal expansion of HpSCs was suppressed and the colony size was significantly
reduced. In addition, the expression of the cell cycle gene cyclin D1 was decreased
whereas the expression of cdk inhibitor p21Cip1 was upregulated by the addition of peretinoin. These results suggested that ACR inhibits
the expansion of stem/progenitor cells derived from embryonic livers. This cell cycle
arrest is involved in the differentiation of HpSCs. The authors found that ACR induced
maturation of HpSCs into hepatocytic cells. The addition of peretinoin induced the
expression of cells positive for the hepatocyte marker albumin in this culture system.
In contrast, the HpSC marker genes such as ?-feto protein, Cd44, and Dlk were significantly
downregulated by peretinoin. The expression of cytokeratin 19, a cholangiocyte marker,
was also decreased, suggesting that the addition of ACR promoted hepatic differentiation,
but not cholangiocytic differentiation, of HpSCs. In addition, ACR regulated HpSC
survival via apoptosis.
Guan and colleagues have provided evidence that retinoic acids regulate proliferation
and differentiation of HpSCs during liver development. Whether retinoic acids ATRA
and ACR positively or negatively affect cell proliferation in normal liver and hepatic
carcinoma tissues is controversial. The present study clearly showed that ACR suppressed
the proliferation and survival of stem/progenitor cells in normal embryonic liver
tissues. A synthetic retinoid ACR might have a cell signal cascade that is different
from that of natural retinoic acids. Hepatic cancer stem cells also exhibit stem/progenitor
characteristics similar to those exhibited by normal HpSCs. Therefore, this study
suggested that ACR not only plays the role of an anti-tumour drug in HCC but also
inhibits tumour-initiating cells such as hepatic cancer stem cells.