In this study we have demonstrated the suitability of two liver-specific biomarkers (ASGR1 and miR-122-5p) for the characterization of circulating epithelial cells (CECs) from liver cirrhosis (LC) and hepatocellular carcinoma (HCC) patients as prognostic biomarkers. This proof-of-concept study demonstrates that phenotypic characterization of CECs with either ASGR1 or miR-122-5p may allow risk-stratification in patients at earlier disease stages, which is the principal basis of precision medicine.
Currently there is a lack of diagnostic and risk-stratification tools for HCC. Evaluation of circulating tumor cells (CTCs) in advanced stages is a prognostic marker for high risk of metastasis [25] and presence of CTCs after surgery or resection is a prognostic marker for PFS in HCC patients [22]; however, there are some limitations of CTC evaluation, particularly in early and pre-tumoral disease stages. Lu-Nan, Qi et al 2018 identified CTCs in more than half of the early-stage (BCLC 0-A) HCC patients [26], suggesting it may be a remarkably useful tool for cancer interception [29]. However, there are currently only very few studies addressing identification and characterization of CECs in pre-tumoral diseases such as LC [30].
Expression of ASGR1 preferentially in the sinusoidal and basolateral hepatocellular membranes makes this protein an important biomarker for HCC. In fact, it has been explored in the context of targeted therapies for HCC [46], [47], [48]. Other roles of ASGR1 such as hepatitis C virus binding allowing viral infection [49] or metastasis promotion by interaction with lectins in the tumor microenvironment through the EGFR-ERK pathway [50] are also described in relation with HCC induction. Interestingly, hepatocytes expressing low levels of ASGR1 were characterized as progenitor-like cells with higher levels of EGFR, β1 and α6 integrins expression [51]. Furthermore, variable ASGR1 expression was observed between tumor stages. For instance, Shi, B., et al (2013) demonstrated that expression of ASGR1 (based on H-scores) in normal adjacent tissue was comparable to that on hepatic cirrhosis and early HCC (grade I or well differentiated) using tissue microarrays, although its expression decreased significantly with increasing tumor stage [39]. This suggested that ASGR1 could potentially be used as an early indicator of the disease status. Likewise, Witzigmann, et al (2016) showed lower levels of ASGR1 mRNA in HCC compared to its adjacent normal tissue, as well as a reduction of mRNA according to increasing HCC stages [40]. Furthermore, a decreased ASGR1 mRNA expression was observed in metastatic or highly proliferative tumors (defined by Ki69 positivity) [40] and overexpression of ASGR1 was shown to inhibit cell migration and invasivity potential both in vitro and in vivo [37], suppressing metastasis and serving a prognostic biomarker.
Together with ASGR, miR-122-5p is another liver-specific biomarker which tumor suppressor role has also been suggested in HCC [42, 52]. In fact, miR-122-5p is one of the best diagnostic markers for HCC being usually elevated in circulation of HCC patients [44]; however, its overexpression in circulation was not correlated with that in tissue, which levels were found to be downregulated in HCC [42]. Intra-tumor heterogeneity following microenvironment stress cannot be characterized by cytokeratin positive CEC counts alone. Thus, phenotypic characterization with tissue-specific biomarkers, such as ASGR1 or miR-122-5p, may be of great importance. In fact, we identified distinct CEC subpopulations based on ASGR1/miR-122-5p staining within and between patients. Interestingly, a perfect positive correlation was found between the two markers suggesting that both could become excellent prognostic biomarkers for HCC.
Our results show that both the presence of CECs and the absence of ASGR1 expression in CECs are risk factors of developing HCC. Furthermore, ASGR1 staining has been shown to be positively correlated with miR-122-5p expression, suggesting that CECs expressing both markers originated from hepatocytes; contrarily, absence of the two biomarkers suggests a more dedifferentiated state, potentially identifying more aggressive phenotypes. These data agree with those presented by Shi., et al (2013) and Witzigmann., et al (2016), who have shown decreased ASGR1 expression levels in HCC tumor tissues, both at the mRNA and protein level [39], [40] as well as works from Hu., et al (2012) who showed a decreased miR-122-5p expression in HCC compared to controls [41].
Despite its main expression in liver, ASGR1 was also shown to be expressed in other cell types such as colon [53] or peripheral blood mononuclear cells (PBMCs) [54], albeit at lower levels compared with hepatocytes. We have also observed some level of ASGR1 expression in PBMCs of some individuals (data not shown); however, any significance of such differences among patients needs to be further investigated. One of the described roles of ASGR1 is the clearance of platelets and other prothrombotic blood components [55]. Therefore, its presence in PBMCs might arise because of this blood hemostasis process. In fact, we showed significant correlations between ASGR1 expression in CECs and INR (p = 0.025) as well as prothrombin activity (p = 0.006), validating the role of ASGR1 maintaining blood homeostasis. Furthermore, the percentage of ASGR1 expression in CECs decreased with Child Pugh-Turcotte stage suggesting it may be used as diagnostic and prognostic biomarker in earlier disease stages as an outstanding cancer interception tool [29]. Thus, our data highlights the clinical utility of characterizing CECs using ASGR1/miR-122-5p isolated from patients with chronic liver cirrhosis or HCC to identify potentially more aggressive phenotypes (loss of ASGR1/miR-122-5p) serving as prognostic tool.