hepatocellular carcinoma ( hcc ) is the sixth most common malignant disease worldwide and the third greatest cause of cancer - related death .
the etiology of hcc has been reported to be related to a variety of diseases such as viral hepatitis [ 2 , 3 ] , alcoholic hepatitis , nonalcoholic fatty liver disease ( nafld ) [ 5 , 6 ] , and metabolic syndrome including diabetes mellitus [ 7 , 8 ] .
the sequences from chronic hepatitis and liver cirrhosis cause de novo hcc   ( figure 1 ) .
hcc is considered to have increased invasiveness with malignant transformation and metastatic potential [ 10 , 11 ] .
the clinical therapy for hcc includes various modalities such as liver resection , liver transplantation , transarterial chemoembolization ( tace ) , percutaneous ethanol injection therapy ( peit ) , radiofrequency ablation ( rfa ) , and chemotherapy including molecular - targeted therapy .
the reason for the high recurrence rate of hcc could be proliferation of epithelial cells and increased arterial vascularity [ 19 , 20 ] . furthermore , hcc cells themselves express various growth factors such as vascular endothelial growth factor ( vegf ) , platelet - derived growth factor ( pdgf ) , epidermal growth factor ( egf ) , fibroblast growth factor ( fgf ) , and insulin - like growth factor ( igf ) , which induce cell proliferation in an autocrine fashion . the receptors of these growth factors activate intracellular signals such as the raf / mek / erk pathway   and the pi3k / akt / mtor pathway , which induce proliferation of both cancer and endothelial cells ( figure 2 ) . these growth factors , including their intracellular molecules , are considered to be a specific target for hcc treatment .   in clinical trials ,
sorafenib , which is an inhibitor of the vegf receptor ( vegfr ) and pdgf receptor ( pdgfr ) , has been proven to have a survival benefit for nonresectable hcc compared a placebo in the best supportive care ( bsc ) setting [ 27 , 28 ] .
phase iii trials are ongoing to determine the survival benefit in patients who receive surgery or ablation . on the other hand ,
the survival benefit of sorafenib has been limited to a few of months and other pathways need to be blocked to achieve longer survival .
side effects of sorafinib therapy are obstacles to continuation of the therapy because normal cells also express vegfr and pdgfr , and sorafenib severely damages both normal and cancer cells
. the development of anticancer drugs must focus on a specific target that is restricted to the cancer cells .
cancer stem cells have been shown to be a target for cancer - specific therapy recently .
the abilities of self - renewal and infinite proliferation are closely related to the nature of hcc development [ 30 , 31 ] .
stem cells in the liver are divided into several cell types , including oval cells , small hepatocytes , and progenitor cells   ( figure 1 ) .
liver cancer stem cells and hcc cells could derive from mutation of these stem cells .
the origin of the stem cells could be either from mature hepatocytes or from bone marrow cells   ( figure 1 ) .
thus , specific stem cell - based therapy could be another strategy to overcome the high recurrence rate of hcc .
we describe the molecular pathogenesis , molecular therapy , and stem cell - targeted therapy for hcc treatment in this review .
vegf , pdgf , egf , fgf , and igf , growth factors that facilitate high vascularity and cancer cell proliferation , are expressed not only in cancer cells but also in other surrounding cells .
the high expression of the growth factors is also associated with tumor invasion and portal thrombosis [ 19 , 20 , 22 ] . among the growth factors ,
high expression of egf is related to differentiation and invasion by the cancer cells . on the other hand ,
therefore , growth factors play an important role in proliferation of cancer cells not only in an autocrine fashion but also in a paracrine fashion through surrounding cells . in addition
based on the high vascularity of hcc , endothelial cells could be a target for hcc treatment .
this approach could be promising because endothelial cells have normal cell physiology with stable genetic regulation , which can be easily manipulated by molecular target therapy .
tyrosine kinase type receptors , such as vegfr , pdgfr , egfr , fgfr , and igfr , activate intracellular ras in the raf / mek / erk pathway [ 1923 ] .
subsequently , ap-1 family members such as c - jun and c - fos activate expression of various genes that induce cell proliferation and vasculogenesis   ( figure 2 ) .
the activation of the raf / mek / erk pathway is related to the disease progression of hcc   and hbv - related hcc development .
furthermore , hcv core protein activates raf and is considered to play a role in the development of hcc .
ras and raf play important roles in which intracellular signals activate expression of various genes   ( figure 2 ) .
erk regulates more than one hundred intracellular substrates directly and gene expression indirectly as cell kinase to activate transcription factors and cell cycle regulators [ 44 , 45 ] .
activation of erk is closely related to cancer cell proliferation and , thus , inhibition of erk could have an anticancer effect .

				  the phosphatidylinositol-3 kinase ( pi3k ) pathway plays an important role in the proliferation and survival of cancer cells in various solid tumors , including hcc   ( figure 2 ) .
inactivation of akt has been shown to improve the antitumor effect of sorafenib in an animal model and thus it could have potential use for hcc treatment .
the pi3k pathway is regulated by phosphatase and tensin homolog deleted on chromosome 10 ( pten ) negatively and the expression of pten is suppressed in half of hcc cells clinically   ( figure 2 ) .
in fact , pten expression is suppressed by hbx protein in hbv hepatitis patients , and downregulation of pten is associated with tumor grade progression , tumor stage , and poor overall prognosis .
sorafenib is an inhibitor of raf that is activated by vegf and pdgf [ 5052 ] .
it has been tested in phase iii clinical trials , such as the sharp trial   and asia - pacific trial   ( table 1 ) .
it improves overall survival in patients with advanced hcc compared to patients administered a placebo in the bsc setting .
the clinical phase ii trial of sunitinib for hcc treatment showed severe grade 3 to 4 side effects .
brivanib , erotinib , and tsu-68 , which are inhibitors of growth factor receptors , have been clinically tested for advanced hcc patients as well .
the response rates to single doses of sorafenib [ 27 , 28 ] , sunitinib [ 53 , 54 ] , brivanib , erlotinib , and tsu-68   were 2.33.3% , 2.72.9% , 5.0% , 9.0% , and 8.6% respectively ( table 1 ) .
phase ii clinical trials using bevacizumab , a vegfr inhibitor , and cetuximab , an egfr inhibitor , had 13% and 0% rrs , respectively ( table 1 ) .
although the clinical results of single doses of these molecular - targeted agents were not totally satisfactory , bevacizumab and tsu-68 achieved 2 - 3% complete responses [ 57 , 58 ] ( table 1 ) .
in addition , a phase ii clinical trial of combination therapy using erlotinib and bevacizumab had a 25% response rate   ( table 1 ) .
therefore , combination of these agents with appropriate management of the side effects could improve survival of patients with advanced hcc in the future .
ongoing clinical trials using molecular - targeted agents for hcc are shown in table 2 .
the storm trial is a phase iii clinical trial using a single dose of sorafenib alone for adjuvant therapy after liver resection and ablation .
the silius trial is a phase iii clinical trial using combination therapy with transarterial chemoinfusion ( taci ) and sorafenib for advanced hcc patients .
the space trial and tactics trial are a phase ii clinical trial using tace and sorafenib for advanced hcc patients .
the brisk - ps trial is designed for second therapy using brivanib for advanced hcc patients resistant to sorafenib . the brisk - ta employs adjuvant therapy using brivanib after tace , and the brisk - fl trial is a comparative clinical trial using sorafenib alone and brivanib alone .
the clinical results of these molecular - targeted therapies have not all been published yet and we will need to interpret the results carefully in the future .
molecular markers of cancer stem cells are shared with either normal stem cells or progenitor cells .
cd133 , cd90 , cd44 , and epcam [ 63 , 64 ] have been shown to be markers for cancer stem cells in hcc patients ( table 3 ) .
these biomarkers can be useful to estimate the prognosis of hcc and they could be useful for specific targeted therapy for cancer cells .  
nsc74859 is a specific inhibitor of signal transducer and activator of transcription 3 ( stat3 ) activation and it decreases cd133-positive cells with suppression of cancer development .
in addition , cd133 cells increase in pten deleted mice , which indicates that pten can play an important role to regulate cd133-positive cancer stem cells .
these basic studies suggest that cd133 can be a molecular target for hcc treatment .  
the atp - binding cassette ( abc ) transporters are a family of membrane transporters such as mdr1 , abcg2 , and abcc2 .
a combination of chemotherapy and inhibitors of abc transporters could decrease not only the number of hcc cells but also that of cancer stem cells   ( table 3 ) .  
cd90 is expressed in oval cells and progenitor cells and its expression is related to tumor development .
cd44 is a receptor of hyaluronate expressed on the cell surface and is often coexpressed with cd90 [ 62 , 74 ] .
anti - cd44 treatment induces apoptosis in cd90-positive cells and thus cd44 plays an important role in the survival of cancer cells [ 75 , 76 ] .
epcam is another cell marker for progenitor cells and the direct target of the wnt / beta catenin pathway [ 63 , 77 ] .
the knockdown of the wnt / beta catenin by small interfering rna ( sirna ) of wnt / beta catenin decreases the number of epcam - positive cells with suppression of tumor development and induces apoptosis .
cd13-positive cells are also potential cancer stem cells   ( table 3 ) . these cells can be found in the peripheral areas of hcc after tace treatment , which is considered to be related to tumor recurrence .
furthermore , inhibitors of cd13 such as 24f can suppress the invasion and angiogenesis of hcc .
the molecular pathogenesis of hcc is important to understand the mechanism of tumor development as well as the high - recurrence behavior of hcc .
furthermore , each step of the molecular signals could be a target to control tumor progression .
further clinical studies using single agents and combination therapies need to be conducted for hcc treatment .
the clinical benefits of cell - targeted therapy for cancer stem cells are eagerly awaited .
 the prognosis of hepatocellular carcinoma ( hcc ) is affected by tumoral factors and liver functions ; therefore it is often difficult to select the appropriate therapeutic methods for hcc . 
 recently , two global phase iii trials showed that sorafenib , which is a tyrosine kinase inhibitor , improved the prognosis of patients with advanced hcc . 
 as a new therapeutic strategy for hcc , sorafenib is expected to expand the indication for hcc in the future . 
 however , it alone is insufficient for the molecular - targeted treatment of hcc because the signaling pathway exists not only in cancer cells but also in normal cells . 
 recently , cancer stem cells ( cscs ) have attracted attention as a novel therapeutic target for hcc . 
 there is now much evidence that stem cell properties such as self - renewal , unlimited proliferation , and differentiation are highly relevant to cancer recurrence and the drug resistance of hcc . in this review , 
 we describe the molecular pathogenesis and the current state and future development of molecular- and csc - therapeutic targeted agents for hcc , citing various reports . 
1. Introduction
2. Role of Growth Factors and Angiogenesis in HCC
3. Role of RAF/MEK/ERK Signaling Pathway in Developing HCC
4. Role of PI3K/AKT/mTOR Signaling Pathway in Developing HCC
5. Molecular-Targeting Clinical Therapies and Trials for HCC
6. Molecular Markers of Cancer Stem Cells in HCC
7. Summary