Hepatocellular carcinoma (HCC) is the fifth most common cancer worldwide malignancy and the third leading cause of cancer death in patients. Several studies demonstrated that hepatic cancer stem cells (HCSCs), also called tumor-initiating cells, are involved in regulation of HCC initiation, tumor progression, metastasis development, and drug resistance. Despite the extensive research, the underlying mechanisms by which HCSCs are regulated remain still unclear. MicroRNAs (miRNAs) are able to regulate a lot of biological processes such as self-renewal and pluripotency of HCSCs, representing a new promising strategy for treatment of HCC chemotherapy-resistant tumors. In this review, we synthesize the latest findings on therapeutic regulation of HCSCs by miRNAs, in order to highlight the perspective of novel miRNA-based anticancer therapies for HCC treatment.
Hepatocellular carcinoma (HCC) is the fifth most common cancer in the world and in most cases it develops in patients with chronic liver diseases such as viral infections and cirrhosis. Treatment for primary liver cancer depends on the location and stage of the cancer and liver’s functionality; however, in many cases HCC is diagnosed in patients with an advanced stage. Thus, it is difficult to treat patients at surgical and pharmacological levels [
The emerging cancer stem cell (CSC) theory based on targeting of CSCs [
Hepatic cancer stem cells (HCSCs) represent a subpopulation of cells positive for different markers including CD133, CD90, and EpCAM [
It is of note that these signaling pathways are subjected to different homeostasis system and in particular are regulated epigenetically by miRNAs. MiRNAs, by acting as oncogenes or oncosuppressors, are able to regulate many biological processes such as self-renewal and pluripotency of HCSCs [
In this review, we summarized the latest findings on the therapeutic regulation of HCSCs induced by miRNA and we try to elucidate the underlying mechanisms in order to highlight the perspective of novel miRNA-based anticancer therapies for HCC treatment.
Cancer stem cells (CSCs) or tumor-initiating cells (T-ICs) are tumor cells discovered in solid and hematological tumors. These cells share stem-like properties and are involved in the tumorigenesis, in development of metastases, and in self-renewal processes. Bonnet and Dick described CSCs for the first time, in acute myeloid leukemia [
It has been demonstrated that CSCs play important roles in the tumor initiation and maintenance but also in metastasis and cancer relapse induced by the chemoresistance to the conventional therapies [
Hepatic CSCs (HCSCs) have been isolated from heterogeneous tumor tissues, based on the specific surface markers and functional properties. It is of note that various markers have been identified for hepatic cancer stem cells, including CD133, CD90, and EpCAM [
The principal HCSCs molecular markers are summarized in Table
Cell surface marker of hepatic cancer stem cells.
Marker | Localization | Structure | Role in cancer | Ref. |
---|---|---|---|---|
CD133 |
Adult stem cells | Pentaspan transmembrane glycoprotein | Self-renewal, tumorigenicity, chemoresistance, and invasiveness | [ |
CD90 |
Adult stem cells | Glycosyl-PI-anchored glycoprotein | Tumor formation, self-renewal, and metastasis | [ |
CD44 | Mammalian cell types | Cell surface-glycoprotein | Tumor formation, chemoresistance, and metastatic ability | [ |
CD13 | Predominantly on cells in G1/G0 phase | Zn2+ dep. Membrane bound ectopeptidase | Tumorigenicity, cell proliferation, self-renewal, and chemoresistance | [ |
CD24 |
Differentiating cells | Cell adhesion glycoprotein | Self-renewal, tumor formation, metastasis, and chemoresistance | [ |
OV6 | Oval cells in fetal liver | Surface antigen | Tumor formation and chemoresistance | [ |
DLK1 | Stem/progenitor hepatic and fetal liver cells | — | Proliferation, self-renewal, tumor formation, and tumor growth | [ |
EpCAM | Progenitors and stem cells | Transmembrane glycoprotein | Invasiveness, self-renewal, and tumor formation | [ |
GEP | Fetal liver | Hepatic oncofetal protein | Tumorigenicity and chemoresistance | [ |
SP |
Cell subpopulation efflux chemotherapy drugs through ABC transporters | — | Tumorigenicity, self-renewal, and chemoresistance | [ |
HCSCs show specific features of pluripotency and self-renewal; this phenotype is strictly regulated by different types of molecular effectors involved in many pathways. Here, we review the latest findings on the most important mediators involved in HCSCs regulation.
TGF-
Emerging evidences indicate that Smad7 also regulates Wnt/
One of the most recurrent pathways involved in HCSCs regulation is the Wnt/
Key signaling pathways that regulate the function of hepatic cancer stem cells. The cartoon recapitulates the principle signaling pathways that regulate the function of hepatic CSCs such as Wnt/
It has been showed that cytoplastmatic and nuclear accumulation of
Nuclear
These data suggest that Wnt signaling is involved in HCSCs maintenance.
The EpCAM signaling starts with a sequential cleavage of the surface protein, operated by TNF-
The PI3K/AKT/mTOR signaling that has been found to be deregulated in 40–50% of HCC cases, with less differentiated tumors and with reduction of free disease survival [
Specifically, the activation of IRS1, an intracellular mediator of insulin signaling, induces the activation of PI3K (Phosphatidylinositol 3-Kinase). This leads to the phosphorylation of PKB (protein kinase B)/AKT mediated by PDK1 (Pyruvate Dehydrogenase Kinase Isozyme 1), a positive regulator of the tuberous sclerosis (TSC1-TSC2) complex; the latter promotes the activation of mTORC1, a mammalian target of rapamycin complex 1, through the small GTPase Rheb (Ras homolog enriched in brain). mTORC1 can target and activate S6K1 (ribosomal protein S6 kinase) and 4E-BP1 (eukaryotic initiation factor 4E binding protein 1), major regulators of protein translation. The Phosphatase PTEN (Phosphatase and tensin homolog) physiologically inhibits the downstream activity of PI3K/AKT axis and is frequently deregulated in HCC (66% of tumor incidence in PTEN-deficient mice) [
Hedgehog pathway plays an important role during embryonic development and in cell fate maintenance. It is activated by binding of ligands (Desert, Indian, and Sonic Hedgehog) to the membrane based patched (Ptc) receptors [
Figure
Recent studies showed the role of miRNA in many biological processes, including the regulation of carcinogenesis, sharing both oncogenes and oncosuppressor functions [
Emerging evidences suggest that miRNAs play a key role also in the maintenance, progression, chemoresistance, and disease relapse of HCSCs [
Here, we summarized the latest findings on the therapeutic targets of miRNA in HCSCs.
Recently it has been demonstrated that miR-10b represents a switch factor between liver normal stem cells (LNSCs) and liver cancer stem cells (LCSCs). This malignant transformation is mediated by the enhanced expression of the axis miR-10b/HOX transcript antisense RNA (HOTAIR) that induces the degradation of E-cadherin pattern in LNSCs, thus facilitating the epithelial-to-mesenchymal transition (EMT) [
Several studies demonstrated that miR-155 acts as oncogenic miRNA, through the interaction with the axis TGF-
It is of note that miRNAs are able to regulate several biological mechanisms. For example, miR-122 has a key role in glycolytic metabolism. It induces a reversion of malignancy phenotype of HCSCs, by regulation of glycolysis, which is more active in HCSC CD133+, via inhibition of PDK4 and LDHA [
Several studies showed that the oncosuppressor miR-125b reduces EMT, through SMAD2/4 protein association [
A clinical study reported the dualistic effect of miR-150, also as an oncogene, together with miR-155 and miR-223. Their suppression is due to a decrement of EpCAM+ cell population [
Moreover, studies performed on miR-200a demonstrated that it regulates stemness of HCSCs with a dual activity. Overexpression of this miRNA switches on the transition from LCSC to HCSC that it is observed through the expression analysis of N-cadherin, ZEB2, and vimentin [
MiR-612 regulates the EMT through a direct interaction with AKT2 [
Recent studies have highlighted the key role of miR-181 in HCSCs stemness maintenance through the interaction with let-7 family members. It has been demonstrated that let-7/miR-181 axis is upregulated in HCSCs, and this condition leads to chemoresistance to doxorubicin or sorafenib treatment [
Finally, recent studies reported the involvement of several miRNAs in HCSCs regulation/maintenance, through interaction with molecular target poorly studied. MiR-152, for example, shows an oncosuppressor role by targeting KIT receptor [
The regulatory functions of miRNAs targeted in HCSCs are summarized in Table
The regulatory functions of miRNAs in HCSCs biology.
miRNA | Oncogene (OG) |
Molecular target/pathways | Effects | Ref. |
---|---|---|---|---|
miR-10b | OG | HOX transcript antisense RNA (HOTAIR) | E-cadherin degradation |
[ |
miR-21 | OG | PTEN, RECK, and PDCD4 | Migration/invasion | [ |
miR-122 | OS | PDK4, LDHA, and |
Glycolysis inhibition |
[ |
miR-125b | OS | SMAD2, SMAD4 | EMT | [ |
miR-142-3p | OG | CD-133 | HCSC features | [ |
miR-145 | OS | OCT4 | Tumorigenesis enhancing | [ |
miR-148a | OS | TGF- |
HCSC-features | [ |
miR-148b | OS | NRP1 | HCSC-features | [ |
miR-150 | OG/OS | 3′UTR of mRNA c-myb | Cyclin D1/Bcl-2 | [ |
miR-152 | OS | KIT | HCSC-features | [ |
miR-155 | OG | TGF- |
EMT |
[ |
miR-181 | OG | let-7 |
Doxorubicin sorafenib resistance | [ |
miR-200a | OG/OS | VASH2 in ZEB1/2 signaling |
Transition LCSC/HCSC |
[ |
miR-205 | OS | PLC |
HCSC-features | [ |
miR-214 | OS | EZH2 in Wnt/ |
EpCAM+ | [ |
miR-216a | OG | PTEN and SMAD7 |
EMT |
[ |
miR-217 | OG | PTEN and SMAD7 |
EMT |
[ |
miR-223 | OG | — | HCSC-features | [ |
miR-491 | OS | GIT-1/NF- |
HCSC-features | [ |
miR-612 | OS | AKT2 | EMT | [ |
In order to eradicate the HCSCs, several therapeutic approaches have been developed. Here we summarized the recent progress in HCSCs research related to HCC, trying to provide a possible perspective for treatment of chemotherapy-resistant HCC tumors.
Epigenetic mechanisms, such as histone modification and DNA methylation, play several roles in cancer development and progression [
Altogether, these data suggest that epigenetic therapy may represent a promising approach for the eradication of CSC in HCC.
Several studies suggested that targeting CSCs with monoclonal antibody could represent a strategy to improve the outcome of cancer therapy [
Molecular-target therapy is considered a promising therapeutic approach for HCC treatment. It has been demonstrated that self-renewal of colorectal CSC function is dependent on the BMI1 [
Another type of therapy developed for the eradication of HCSCs is based on targeting the HCSCs niches. Niches are identified, as specific microenvironments in which HCSCs and normal tissue stem cells are present. It has been demonstrated that sorafenib, the unique molecular-target drug approved to treat HCC at clinical level, may contribute to the eradication of HCSCs by targeting Raf/MEK/ERK pathway and receptor tyrosine kinases [
Recent evidences have suggested the potential application of miRNAs as novel strategy in cancer therapy for HCC. It has been previously described that the therapeutic application of miRNAs involves two different strategies [
In this review, we synthesize the latest findings on therapeutic regulation of miRNA by modulation of tumor-suppressive and oncogenic signaling pathways. Data emerging from these studies suggest that deregulation of miRNA expression controls liver cancer progression and is responsible for the chemoresistance and disease relapse of HCSCs, although the underlying mechanisms are not completely elucidated. In order to highlight the perspective of novel miRNA-based anticancer therapies for HCC treatment, more studies will be needed in the future.
The authors declare that there is no conflict of interests regarding the publication of this paper.
Sabrina Bimonte and Maddalena Leongito contributed equally to this paper.
The authors thank Massimiliano Spinelli, for kindly helping in providing informatics assistance. This work was supported by current research programs of Institute National of Tumors, IRCCS “Foundation G. Pascale,” Naples (Italy).