Epithelial cell adhesion molecule fragments and signaling in primary human liver cells

Insights on epithelial cells at the single-cell level in hepatocellular carcinoma prognosis and response to chemotherapy

Background: Hepatocellular carcinoma (HCC) originates from Epithelial cells, and epithelial lineage plasticity has become a promising research direction for advancing HCC treatment. This study aims to focus on Epithelial cells to provide target insights for detecting HCC prognosis and response to drug therapy. Methods: Single-cell RNA sequencing (scRNA-seq) data from GSE149614 were clustered using Seurat, and the differentiation and evolution of epithelial cells were analyzed by Monocle 2. Scissor+ and Scissor- Epithelial cells associated with the prognostic phenotypes of bulk RNA-seq of HCC were screened using the Scissor algorithm for differential analysis to screen candidate genes. Candidate genes were overlapped with prognostic related genes screened by univariate Cox regression, and the Least Absolute Shrinkage and Selection Operator (LASSO) sparse penalty was imposed on the intersection genes to construct a risk assessment system. Results: Eight major cell subpopulations of HCC were identified, among which the proportion of epithelial cells in non-tumor liver tissues and HCC tissues was significantly different, and its proportion increased with advanced clinical stage. During the progression of HCC, the whole direction of epithelial cells differentiation trajectory was towards enhanced cell proliferation. Differential analysis between Scissor+ and Scissor- epithelial cells screened 1,265 upregulated and 191 downregulated prognostic candidate genes. Wherein, the upregulated genes were enriched in Cell processes, Genetic information processing, Metabolism and Human disease with Infection. Nevertheless, immune system related pathways took the main proportions in downregulated genes enriched pathways. There were 17 common genes between upregulated candidate genes and prognostic risk genes, of which CDC20, G6PD and PLOD2 were selected as components for constructing the risk assessment system. Risk score showed a significant correlation with tumor stage, epithelial-mesenchymal transition (EMT) related pathways and 22 therapeutic drugs, and was an independent prognostic factor for HCC. Conclusion: This study revealed the cellular composition of HCC, the differentiation evolution and functional landscape of epithelial cells in the further deterioration of HCC, and established a 3-gene risk model, which was closely related to clinical features, EMT, and drug sensitivity prediction. These findings provided insights in patient prognosis and drug therapy detection for HCC.

Characterization of CD326-positive human hepatic stem cells

Aim of the study

CD326 has been used as a single marker to enrich for hepatic stem cell populations in the liver. However, bile duct epithelium is also positive for CD326, which impedes the selection of pure hepatic stem cell populations. Some markers have been proposed to be co-expressed by hepatic stem cells but these have not been systematically compared. Therefore, we determined the percentages and compared the characteristics of human liver cells expressing potential stem cell surface markers.

Material and methods

We analyzed CD326 expression in human liver tissues from fetal, neonatal, pediatric, and adult stages using immunohistochemistry. In flow cytometry, we quantified fetal liver cells for their co-expression of CD326 with CD56, CD117, CD44, CD90, CD49f, LGR5 and SSEA4. We analyzed the various fractions for their quantitative expression of genes typically associated with progenitors and hepatic lineages.

Results

12.5% of cells were positive for CD326; of these, 63.5% co-expressed CD44. The lowest co-expression percentages were for SSEA4 (2.1%) and LGR5 (0.7%). Fractions revealed distinct gene expression patterns. Of all combinations, cells that co-expressed surface CD326 and SSEA4 demonstrated the highest gene expression for the proliferation marker MKi67 and hepatic markers DLK1, AFP and ALB, and were the only fraction negative for the biliary epithelial marker KRT19. Histology of adult and fetal liver showed cells positive for CD326 and SSEA4 but negative for CK19.

Conclusions

CD326-positive cells represent a heterogeneous population, which in combination with SSEA4 potentially distinguishes bile duct epithelium from hepatic stem cells. These findings can help to further classify human hepatic progenitor stages.

EpCAM is essential for maintenance of the small intestinal epithelium architecture via regulation of the expression and localization of proteins that compose adherens junctions

Epithelial cell adhesion molecule (EpCAM) is highly expressed in mammalian intestines, and is essential for maintaining the homeostasis of the intestinal epithelium. EpCAM protein is localized at tight junctions and the basolateral membrane of the intestinal epithelium, where it interacts with many cell adhesion molecules. To explore the molecular functions of EpCAM in regulating adherens junctions in the intestinal epithelium, EpCAM knockout embryos and newborn pups were analyzed. Hematoxylin and eosin staining was used to assess the histology of the duodenum, jejunum, ileum and colon from wild-type and EpCAM^−/− mice at E18.5, P0 and P3. The expression and localization of adherens junction-associated genes and genes that encode the proteins that participate in the assembly of adherens junctions were measured at the mRNA and protein levels using qPCR, western blot analysis and immunofluorescence staining. The results showed that although there was no significant damage to the intestines of EpCAM^−/− mice at E18.5 and P0, they were significantly damaged at P3 in mutant mice. The expression of adherens junction-associated genes in EpCAM mutant mice was normal at the mRNA level from E18.5 to P3, but their protein levels were gradually reduced and mislocalized from E18.5 to P3. The expression of nectin 1, which can regulate the assembly and adhesion activity of E-cadherin, was also gradually reduced at both the mRNA and protein levels in the intestinal epithelium of EpCAM mutant mice from E18.5 to P3. In summary, the loss of EpCAM may cause the reduction and mislocalization of proteins that compose adherens junctions partly via the downregulation of nectin 1 in the intestines.

Expression and function of epithelial cell adhesion molecule EpCAM: where are we after 40 years?

EpCAM (epithelial cell adhesion molecule) was discovered four decades ago as a tumor antigen on colorectal carcinomas. Owing to its frequent and high expression on carcinomas and their metastases, EpCAM serves as a prognostic marker, a therapeutic target, and an anchor molecule on circulating and disseminated tumor cells (CTCs/DTCs), which are considered the major source for metastatic cancer cells. Today, EpCAM is reckoned as a multi-functional transmembrane protein involved in the regulation of cell adhesion, proliferation, migration, stemness, and epithelial-to-mesenchymal transition (EMT) of carcinoma cells. To fulfill these functions, EpCAM is instrumental in intra- and intercellular signaling as a full-length molecule and following regulated intramembrane proteolysis, generating functionally active extra- and intracellular fragments. Intact EpCAM and its proteolytic fragments interact with claudins, CD44, E-cadherin, epidermal growth factor receptor (EGFR), and intracellular signaling components of the WNT and Ras/Raf pathways, respectively. This plethora of functions contributes to shaping intratumor heterogeneity and partial EMT, which are major determinants of the clinical outcome of carcinoma patients. EpCAM represents a marker for the epithelial status of primary and systemic tumor cells and emerges as a measure for the metastatic capacity of CTCs. Consequentially, EpCAM has reclaimed potential as a prognostic marker and target on primary and systemic tumor cells.

Effects of Delta-Like Noncanonical Notch Ligand 1 Expression of Human Fetal Liver Hepatoblasts on Hematopoietic Progenitors

Although the hepatic and hematopoietic progenitors of the liver are well characterized, the interactions between these two lineages remain mostly elusive. Hepatoblasts express delta-like noncanonical Notch ligand 1 (Dlk1), whose cleaved extracellular domain can become a soluble protein. We assessed the effects of DLK1 gene expression knockdown in cultures of total fetal liver cells. Furthermore, we separated Dlk1⁺ hepatoblasts from the total liver cell fraction and investigated effects of direct cell contact. Dlk1⁻ cells were cultured either without Dlk1⁺ hepatoblasts, in direct contact with hepatoblasts, or separated from hepatoblasts by a porous membrane in inserts to inhibit cell contact but allow free exchange of molecules. Expression of the hepatic and hematopoietic genes, colony forming unit potential of various hematopoietic progenitors, and cell numbers and types were investigated. We found that DLK1 knockdown in total fetal liver cell cultures decreased total cell numbers. The expression of hepatic progenitor genes and mature hematopoietic genes was affected. Hematopoietic BFU-E and CFU-GM colony numbers were reduced significantly. The depletion of Dlk1⁺ hepatoblasts in culture decreased the potential of all hematopoietic progenitors to form colonies of all types and reduced the percentage of mature hematopoietic cells. The addition of hepatoblasts in inserts to Dlk1⁻ cells further decreased the potential to form the CFU-GM and CFU-GEMM colonies and the percentage of mature hematopoietic cells but increased total cell numbers. Conclusively, direct contact of Dlk1 supports hematopoietic progenitor expansion and functionality that cannot be reconstituted in coculture without direct cell contact.

Hepatic progenitors of the fetal liver: Interactions with hematopoietic stem cells

The aim of this review is to summarize and give an overview on the findings of signaling between hepatic and hematopoietic progenitors of the liver. To date, there are not many findings published in the field, and the aim of this review is to cover all current publications in this area. The liver is the main site of hematopoiesis during fetal development. However, little is known about how hepatic and other non-hematopoietic progenitors potentially influence hematopoiesis and vice versa. The concurrent peaks of hepatic and hematopoietic progenitor proliferation during development indicate interactions that could possibly be mediated through cell-cell contact, extracellular matrices, cytokines and growth factors, or other signaling molecules. For example, hepatic progenitors, such as hepatic stem cells and hepatoblasts, possess characteristic surface markers that can be cleaved, giving rise to fragments of various lengths. A surface molecule of hepatoblasts has been demonstrated to play an essential role in hematopoiesis. Particularly, these effects on hematopoiesis were distinct, depending on whether it was membrane-bound or cleaved. In this review, the various hepatic and hematopoietic progenitor cell types are concisely described, and the current findings of their potential interactions are summarized.

Membrane-associated epithelial cell adhesion molecule is slowly cleaved by γ-secretase prior to efficient proteasomal degradation of its intracellular domain

Regulated intramembrane proteolysis (RIP) is a key mechanism for activating transmembrane proteins such as epithelial cell adhesion molecule (EpCAM) for cellular signaling and degradation. EpCAM is highly expressed in carcinomas and progenitor and embryonic stem cells and is involved in the regulation of cell adhesion, proliferation, and differentiation. Strictly sequential cleavage of EpCAM through RIP involves initial shedding of the extracellular domain by α-secretase (ADAM) and β-secretase (BACE) sheddases, generating a membrane-tethered C-terminal fragment EpCTF. Subsequently, the rate-limiting γ-secretase complex catalyzes intramembrane cleavage of EpCTF, generating an extracellular EpCAM-Aβ-like fragment and an intracellular EpICD fragment involved in nuclear signaling. Here, we have combined biochemical approaches with live-cell imaging of fluorescent protein tags to investigate the kinetics of γ-secretase-mediated intramembrane cleavage of EpCTF. We demonstrate that γ-secretase-mediated proteolysis of exogenously and endogenously expressed EpCTF is a slow process with a 50% protein turnover in cells ranging from 45 min to 5.5 h. The slow cleavage was dictated by γ-secretase activity and not by EpCTF species, as indicated by cross-species swapping experiments. Furthermore, both human and murine EpICDs generated from EpCTF by γ-secretase were degraded efficiently (94-99%) by the proteasome. Hence, proteolytic cleavage of EpCTF is a comparably slow process, and EpICD generation does not appear to be suited for rapidly transducing extracellular cues into nuclear signaling, but appears to provide steady signals that can be further controlled through efficient proteasomal degradation. Our approach provides an unbiased bioassay to investigate proteolytic processing of EpCTF in single living cells.

EpCAM ectodomain EpEX is a ligand of EGFR that counteracts EGF-mediated epithelial-mesenchymal transition through modulation of phospho-ERK1/2 in head and neck cancers

Head and neck squamous cell carcinomas (HNSCCs) are characterized by outstanding molecular heterogeneity that results in severe therapy resistance and poor clinical outcome. Inter- and intratumoral heterogeneity in epithelial-mesenchymal transition (EMT) was recently revealed as a major parameter of poor clinical outcome. Here, we addressed the expression and function of the therapeutic target epidermal growth factor receptor (EGFR) and of the major determinant of epithelial differentiation epithelial cell adhesion molecule (EpCAM) in clinical samples and in vitro models of HNSCCs. We describe improved survival of EGFR^low/EpCAM^high HNSCC patients (n = 180) and provide a molecular basis for the observed disparities in clinical outcome. EGF/EGFR have concentration-dependent dual capacities as inducers of proliferation and EMT through differential activation of the central molecular switch phosphorylated extracellular signal–regulated kinase 1/2 (pERK1/2) and EMT transcription factors (EMT-TFs) Snail, zinc finger E-box-binding homeobox 1 (Zeb1), and Slug. Furthermore, soluble ectodomain of EpCAM (EpEX) was identified as a ligand of EGFR that activates pERK1/2 and phosphorylated AKT (pAKT) and induces EGFR-dependent proliferation but represses EGF-mediated EMT, Snail, Zeb1, and Slug activation and cell migration. EMT repression by EpEX is realized through competitive modulation of pERK1/2 activation strength and inhibition of EMT-TFs, which is reflected in levels of pERK1/2 and its target Slug in clinical samples. Accordingly, high expression of pERK1/2 and/or Slug predicted poor outcome of HNSCCs. Hence, EpEX is a ligand of EGFR that induces proliferation but counteracts EMT mediated by the EGF/EGFR/pERK1/2 axis. Therefore, the emerging EGFR/EpCAM molecular cross talk represents a promising target to improve patient-tailored adjuvant treatment of HNSCCs.

Head and neck squamous cell carcinomas (HNSCCs) display poor survival, with death rates above 55%. Major factors affecting survival are metastases’ formation and therapy resistance. Phenotypic changes during partial epithelial-mesenchymal transition (EMT) provide tumor cells with increased migration, invasion, and therapy resistance. Understanding molecular mechanisms of EMT, as a central process of the metastatic cascade and the development of therapy resistance, is therefore important. In the present work, we identified molecular cross talk between epidermal growth factor receptor (EGFR) and epithelial cell adhesion molecule (EpCAM) as a novel determinant of clinical outcome in HNSCCs. Low levels of EGFR but high levels of EpCAM (EGFR^low/EpCAM^high) were associated with favorable prognosis, with survival rates above 90%, whereas EGFR^high/EpCAM^low correlated with poor survival, below 10%. EGFR was shown to have a concentration-dependent capacity to induce proliferation and EMT. Proteolytic cleavage of the extracellular domain of EpCAM (EpEX) produces a ligand of EGFR that induces EGFR-dependent proliferation but counteracts EGF-induced EMT. We delineate an EGFR/extracellular signal–regulated kinase 1/2 (ERK1/2)/EpCAM signaling axis that may be a promising therapeutic target for HNSCCs.

EpCAM homo-oligomerization is not the basis for its role in cell-cell adhesion

Cell-surface tumor marker EpCAM plays a key role in proliferation, differentiation and adhesion processes in stem and epithelial cells. It is established as a cell-cell adhesion molecule, forming intercellular interactions through homophilic association. However, the mechanism by which such interactions arise has not yet been fully elucidated. Here, we first show that EpCAM monomers do not associate into oligomers that would resemble an inter-cellular homo-oligomer, capable of mediating cell-cell adhesion, by using SAXS, XL-MS and bead aggregation assays. Second, we also show that EpCAM forms stable dimers on the surface of a cell with pre-formed cell-cell contacts using FLIM-FRET; however, no inter-cellular homo-oligomers were detectable. Thus, our study provides clear evidence that EpCAM indeed does not function as a homophilic cell adhesion molecule and therefore calls for a significant revision of its role in both normal and cancerous tissues. In the light of this, we strongly support the previously suggested name Epithelial Cell Activating Molecule instead of the Epithelial Cell Adhesion Molecule.

A high-content screen for small-molecule regulators of epithelial cell-adhesion molecule (EpCAM) cleavage yields a robust inhibitor

Epithelial cell-adhesion molecule (EpCAM) is a transmembrane protein that regulates cell cycle progression and differentiation and is overexpressed in many carcinomas. The EpCAM-induced mitogenic cascade is activated via regulated intramembrane proteolysis (RIP) of EpCAM by ADAM and γ-secretases, generating the signaling-active intracellular domain EpICD. Because of its expression pattern and molecular function, EpCAM is a valuable target in prognostic and therapeutic approaches for various carcinomas. So far, several immunotherapeutic strategies have targeted the extracellular domain of EpCAM. However, targeting the intracellular signaling cascade of EpCAM holds promise for specifically interfering with EpCAM's proliferation-stimulating signaling cascade. Here, using a yellow fluorescence protein-tagged version of the C-terminal fragment of EpCAM, we established a high-content screening (HCS) of a small-molecule compound library (n = 27,280) and characterized validated hits that target EpCAM signaling. In total, 128 potential inhibitors were initially identified, of which one compound with robust inhibitory effects on RIP of EpCAM was analyzed in greater detail. In summary, our study demonstrates that the development of an HCS for small-molecule inhibitors of the EpCAM signaling pathway is feasible. We propose that this approach may also be useful for identifying chemical compounds targeting other disorders involving membrane cleavage-dependent signaling pathways.