Liver Research Center, Rhode Island Hospital and The Warren Alpert Medical School of Brown University, Providence, RI
Address reprint requests to: Jack R. Wands, M.D., Liver Research Center, Rhode Island Hospital, Alpert Medical School of Brown University, 55 Claverick Street, Providence, RI 02903. E-mail: Jack_Wands_MD@Brown.edu; fax: 401-444-2939.
Potential conflict of interest: Nothing to report.
This work was supported by grants from the Ruth L. Kirschstein National Research Service Award (NRSA) Institutional Research Training (IT-32 DK60415), the National Institutes of Health/National Cancer Institute (NIH)/NCI (R01CA123544), the National Institute for General Medical Sciences (NIGMS), an NIH RI-INBRE Faculty Development Research Project Grant (8P20GM103430-12), 2012 URI Division of Research & Economic Development and URI Council for Research Proposal Development Grants, and 2011 AACR-FNAB Fellows Grant for Translational Pancreatic Cancer Research (11-30-14-DONG).
Hepatocellular carcinoma (HCC) has a poor prognosis as a result of widespread intra- and extrahepatic metastases. There is an urgent need to understand signaling cascades that promote disease progression. Aspartyl-(asparaginyl)-β-hydroxylase (ASPH) is a cell-surface enzyme that generates enhanced cell motility, migration, invasion, and metastatic spread in HCC. We hypothesize that inhibition of its enzymatic activity could have antitumor effects. Small molecule inhibitors (SMIs) were developed based on the crystal structure of the ASPH catalytic site followed by computer-assisted drug design. Candidate compounds were tested for inhibition of β-hydroxylase activity and selected for their capability to modulate cell proliferation, migration, invasion, and colony formation in vitro and to inhibit HCC tumor growth in vivo using orthotopic and subcutaneous murine models. The biological effects of SMIs on the Notch signaling cascade were evaluated. The SMI inhibitor, MO-I-1100, was selected because it reduced ASPH enzymatic activity by 80% and suppressed HCC cell migration, invasion, and anchorage-independent growth. Furthermore, substantial inhibition of HCC tumor growth and progression was observed in both animal models. The mechanism(s) for this antitumor effect was associated with reduced activation of Notch signaling both in vitro and in vivo. Conclusions: These studies suggest that the enzymatic activity of ASPH is important for hepatic oncogenesis. Reduced β-hydroxylase activity generated by the SMI MO-I-1100 leads to antitumor effects through inhibiting Notch signaling cascade in HCC. ASPH promotes the generation of an HCC malignant phenotype and represents an attractive molecular target for therapy of this fatal disease. (Hepatology 2014;60:1302–1313)
If you can't find a tool you're looking for, please click the link at the top of the page to "Go to old article view". Alternatively, view our Knowledge Base articles for additional help. Your feedback is important to us, so please let us know if you have comments or ideas for improvement.
Hepatocellular carcinoma (HCC) is the fifth-most prevalent and third-most fatal type of cancer worldwide. HCC is currently being diagnosed in over 500,000 people each year globally, and the incidence is increasing. Despite improvements in detection and management, the 5-year survival rate of HCC is 15% (localized 28%, regional 7%, and distant 2%), and 80%-90% of patients present with advanced unresectable disease at diagnosis resulting from widespread intra- and extrahepatic metastases. It is urgent to explore the molecular mechanisms of HCC development, progression, and metastasis in an attempt to identify new molecular targets for therapy. Indeed, there is an intense search for identification and development of small molecules with antitumor effects against HCC; nevertheless, there is only one drug, sorafenib, which is an inhibitor of multiple kinases, such as Raf, vascular endothelial growth factor receptor, platelet-dervied growth factor receptor beta, c-Kit, and RET, that has been approved for clinical use in this disease.[2, 3]
Aspartate β-hydroxylase (ASPH) is an ∼86-kD type II transmembrane protein and a member of the α-ketoglutarate-dependent dioxygenase family. It catalyzes the hydroxylation of aspartyl and asparaginyl residues in epidermal growth factor (EGF)-like domains of various proteins, such as Notch and Jagged (JAG), which play critical roles in cell growth, differentiation, adhesion, and migration. The DNA and protein sequences of ASPH are highly conserved between human and murine orthologs, so that the same monoclonal antibodies (mAbs) and RNA/DNA primers can be used to identify and characterize ASPH expression in both species. ASPH was originally described to be overexpressed in HCC and cholangiocarcinoma patients. However, because the liver is derived from an early progenitor cell type and ASPH is expressed in the embryo, we hypothesize that ASPH may re-emerge during tumorigenesis and play a role in the pathogenesis of HCC. ASPH is rarely expressed in normal adult tissue, except for proliferating trophoblastic cells of the placenta, an invasive tissue. Clinical investigation has revealed that ASPH overexpression portends a poor prognosis with early disease reoccurrence of colon cancer. In HCC, there has been a significant association of ASPH expression with worse clinical outcome of patients who received surgery, and ASPH has been considered to be an independent risk factor for a reduced survival. ASPH functions to enhance the HCC malignant phenotype by increasing cell migration, invasion, and metastasis and therefore could be considered as a potent therapeutic target for treatment of this disease.
ASPH expression is “shut off” in the adult only to re-emerge during oncogenesis, where its function may be required for a generation of malignant phenotype.[7, 10, 11] This study attempts to address several questions critical to the function of ASPH in HCC. Is ASPH a major activator of Notch signaling (which is known to promote cell migration, invasion, and metastasis)? Can ASPH be directly linked to upstream growth factor signaling pathways important in pathogenesis? Furthermore, is the β-hydroxylase enzymatic activity of ASPH essential for its biological and pathological effects? We hypothesized that discovery of small molecule inhibitors (SMIs) against β-hydroxylase activity of ASPH may produce antitumor effects and that their mechanism(s) of action involves inhibition of Notch signaling in HCC.
Materials and Methods
Synthesis and Characterization of SMIs of β-Hydroxylase Activity
We developed, characterized, and evaluated novel SMIs for ASPH β-hydroxylase activity. Formulation of these compounds was based on the crystal structure of the ASPH catalytic site, followed by computer-generated drug design that led to the synthesis of a series of parent compounds and derivatives likely to fit into the pocket of the catalytic site and inhibit enzymatic activity. The first step was a three-component reaction, including an aromatic aldehyde, glyoxal bisulfate addition product, and potassium cyanide, to yield an arylhydroxytetronimide. In the second step, arylhydroxytetronimide was sulfonylated with phenylmethansulfonyl chloride in dry tetrahydrofuran to yield compounds presented in Supporting Fig. 1. Compounds were characterized by 1H and C nuclear magnetic resonance, high-resolution mass spectroscopy, high performance liquid chromatography, infrared spectroscopy, melting point, elemental analysis, and binding to ASPH by isothermal titration calorimetry.
The FOCUS HCC cell line was derived from a hepatitis B virus (HBV)-related poorly differentiated human HCC, and the subclone, BNLT3, was derived from a murine hepatoma parenteral cell line after serial passage through immunodeficient mice to generate a clone with a highly aggressive malignant phenotype. The other human HCC cell lines included Huh7, Hep3B, and HepG2; the murine embryo fibroblast cell line, NIH3T3, was used as a negative control for ASPH expression. The above-mentioned cell lines were obtained from the American Type Culture Collection (Manassas, VA). Cell lines were maintained in Dulbecco's modified Eagle's medium supplemented with 10% fetal bovine serum and 2 mM of L-glutamine and cultivated in a humidified incubator at 37°C with 5% CO2. Cells were harvested with 0.25% Trypsin/0.03% ethylenediaminetetraacetic acid for further investigations.
CO2/α-Ketoglutarate-Dependent Capture Assay for Measurement of β-Hydroxylase Activity
ASPH β-hydroxylase activity was measured in protein lysates derived from FOCUS HCC cells treated with 5 µM of SMIs using an EGF-like domain-containing peptide (DGDQCETSPCQNQGKCKDGLGYETCTCLEGFEGKNC) as the substrate. In this assay, C-labeled α-ketoglutarate releases CO2, as described previously.[4, 14, 15] Incubations were carried out at 37°C for 1 hour in a final volume of 40 µL, containing 48 µg of clarified cell extract derived from FOCUS cells providing the source of ASPH β-hydroxylase protein and the 60 µM of EGF containing peptide substrate. Radioactivity captured was quantified by a phosphoimager. We modified the assay to make it specific for the β-hydroxylase activity of ASPH because the reaction was run in a 96-well plate previously coated with mAb FB-50 prepared against the N-terminus of ASPH to first allow direct immunologic capture of the enzyme followed by the subsequent enzymatic assay measurement.
All cell lines were cultured until they reached the logarithmic phase of growth and then were placed into a 96-well microtiter-plate at a density of 1 × 104 cells/well. To evaluate the effects of SMIs on cell viability, cell lines overexpressing ASPH or vector control were treated with SMIs at concentrations ranging from 0.3 to 5 µM for 24 hours, after which 10 μL of 5 mg/mL of 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl-tetrazolium bromide (MTT; Sigma-Aldrich, St. Louis, MO) was added to each well. After incubation for 3 hours and removing the supernatant, 50 μL of acidic isopropanol was added and the color reaction was quantified with a microplate reader at a test wavelength of 570 nm and a reference wavelength of 630 nm. Subsequently, solutions were discarded and 50 µL of 10 µg/mL of Hoechst 33342 solution (Molecular Probes, Eugene, OR) was added into each well. After 5 minutes of incubation at room temperature, the color reaction was quantified with a microplate reader at a test wavelength of 360 nm and a reference wavelength of 460 nm. Each experiment was carried out in triplicate.
Details on colony-formation assay, motility and invasion assays, lentiviral-infected stable cell lines with overexpression or knockdown of ASPH, and western blotting analysis can be found in the Methods section of the Supporting Information.
In Vivo Studies
A subcutaneous (SC) tumor model was used to analyze the in vivo effects of SMIs of β-hydroxylase activity, as described previously. Six-week-old female nu/nu nude mice (Charles River Laboratories, Wilmington, MA) were kept under pathogen-free conditions, fed standard chow, and given free access to sterilized water. Mice were anesthetized by isoflurane, and then SC xenografts were established by inoculating 1 × 107 FOCUS cells into the right dorsal flank. Palpable tumors were confirmed on day 3 after inoculation, and mice were randomized into treatment or control groups to receive an SMI (MO-I-1100) or saline, respectively. MO-I-1100 (50 mg/kg) was prepared in saline and administered by intraperitoneal (IP) injection. Tumor size was measured using calipers every other day, and tumor volumes were calculated as A×B2×0.5 (A, length; B, width).
An orthotopic xenograft model was also created by direct intrahepatic inoculation of FOCUS cells, as described previously. On day 3 after inoculation, MO-I-1100 (50 mg/kg) or saline was administered to mice by IP injection on 5 consecutive days per week for 2 weeks. At 4 weeks after initiation of treatment, mice were sacrificed to assess the antitumor effects of MO-I-1100. All in vivo procedures were approved by the institutional animal care and use committee of Rhode Island Hospital (Providence, RI). Details on immunohistochemistry (IHC) can be found in the Supporting Information.
ASPH Expression in Human HCC
Levels of ASPH expression were evaluated by IHC using the FB-50 mAb that reacts to the N-terminus of ASPH, in human HCC tumors (n = 27) of diverse etiologies (HBV [n = 8], hepatitis C virus [n = 12], and alcohol cirrhosis [n = 7] related), dysplastic nodules (n = 9), and adjacent non-neoplastic normal liver (n = 36; as shown in Fig. 1A,B). ASPH was expressed in 22 of 27 (81.48%) human HCC tumors derived from 86 tissue microarray (TMA) cores. No immunoreactivity was observed in normal liver parenchyma. Dysplastic regenerating nodules in cirrhotic liver were also negative for ASPH expression, indicating that ASPH up-regulation may be closely associated with malignant transformation. Most HCCs had a staining intensity of ASPH expression (bottom panels of Fig. 1A) with a mean semiquantitative value of 1.37 (Fig. 1C). The high frequency of ASPH overexpression in human HCC suggests that it may be an important factor in HCC tumor development and progression.
Development and Characterizations of β-Hydroxylase SMIs
Figure 2C depicted representatives of parent compounds synthesized and examined for inhibition of β-hydroxylase activity using a high-throughput screening approach. In brief, Fig. 2A describes the enzymatic reaction catalyzed by ASPH with the liberation of CO2 as the readout. Figure 2B represents a “strong hit” with MO-I-1100, which inhibits the β-hydroxylase activity of ASPH by approximately 80%. Figure 2C describes the structure of representative parent compounds synthesized and examined for inhibition of β-hydroxylase activity. When inhibitors of ASPH enzymatic activity were identified, we performed a functional MTT assay to test their effects on cell viability over a range of drug concentrations. Examples of positive and negative results among several candidate compounds are depicted in Fig. 2C (bottom). Figure 2D suggests that the expression level of ASPH determined cellular response to the SMI, because MO-I-1100 had no effect on viability of NIH3T3 cells, which lacked ASPH on the cell surface, whereas FOCUS cells, which have a high level ASPH expression (Fig. 3A), were inhibited by MO-I-1100 at 1 µM (Fig. 2D). MO-I-1100 had no effect on the proliferation rate of FOCUS or NIH3T3 cells over 24 hours (Fig. 2E), which was important to interpret its independent effects on cell migration and invasion presented in Fig. 4. In addition, MO-I-1100 was not cytotoxic to FOCUS cells. However, this SMI of ASPH β-hydroxylase activated proapoptosis mechanisms through Notch signaling (Supporting Fig. 2).
Additional human HCC cell lines, including a murine BNLT3 hepatoma cell subclone, were used to evaluate the inhibitory effects of SMIs on cell viability over 24 hours. ASPH expression levels were measured by western blotting analysis using both FB-50 mAb and polyclonal rabbit antibodies prepared against recombinant ASPH. Over a range of MO-I-1100 concentrations (0.3-5.0 µM), there was no effect on cell viability of NIH3T3 cells that lacked ASPH expression (Fig. 3A, bottom). In contrast, the murine BNLT3 hepatoma cells with very low levels of ASPH by western blotting, but easily detectable levels using a more sensitive fluorescence-activated cell sorting (FACS) analysis or IHC with polyclonal rabbit anti-ASPH (displayed in Supporting Fig. 3), revealed reduced viability only at a 5-µM concentration. Likewise, Hep3B, with low ASPH levels only detectable with polyclonal anti-ASPH, exhibited reduced viability at 5 µM, whereas FOCUS, HepG2, and Huh7, with higher ASPH levels, demonstrated reduced viability at a concentration as low as 1 or 1.25 µM of MO-I-1100. Thus, the level of ASPH expression correlated with response to MO-I-1100 with respect to cell viability, which depended, in part, on the concentration of SMIs employed. However, when FOCUS cells were incubated with MO-I-1100 (5 µM) for 3 weeks, there was a substantial reduction of cell growth (Fig. 3B).
The ability to form colonies in soft agar is considered to be a reliable test for tumorigenic potential. In order to evaluate effects of MO-I-1100 on anchorage-independent colony formation, cultivation of FOCUS cells in soft agar with the β-hydroxylase inhibitor was performed. After 3 weeks of incubation, MO-I-1100 reduced colony formation of FOCUS cells (Fig. 3C). Treatment with MO-I-1100 revealed a dose-dependent reduction of colony number and size (Fig. 3D,E).
Overexpression of ASPH in Huh7 Cells Promotes a Malignant Phenotype
Huh7 cells have low-level ASPH expression, which was increased with lentiviral transfection of an ASPH expression plasmid and promoted cell migration and invasion (as shown in Supporting Fig. 4). Previous studies have demonstrated small interfering RNA (siRNA) knockdown reduced this phenotype. More important, knockdown of ASPH expression in FOCUS cells by specific short hairpin RNAs reduced the malignant phenotype, as characterized by decreased colony formation in soft agar as an index of reduced cellular transformation (Supporting Fig. 5).
In Vitro Effects of β-Hydroxylase on Motility and Invasiveness of HCC Cell Lines
We determined whether β-hydroxylase activity was required for ASPH to mediate its effects on cell motility. Directional motility and invasiveness were measured using Boyden chamber-type culture inserts equipped with a porous membrane. FOCUS cells were pretreated with 5 µM of MO-I-1100 for 24 hours and then placed into the upper chamber. Migration and invasion were evaluated after 24 hours. Migration and invasion were reduced after MO-I-1100 treatment of FOCUS cells (Fig. 4A,C). SMIs exerted a direct inhibitory effect on this malignant phenotype, which was not secondary to cell proliferation (as shown in Fig. 2E). Migration and invasion assays were also inhibited in BNLT3 hepatoma cells expressing a low level of ASPH (a highly conserved murine ortholog) (Fig. 4B,D). The catalytic site is identical in protein sequence between human and murine orthologs of ASPH. These observations suggest that β-hydroxylase of both species is susceptible to the inhibitory effect of MO-I-1100.
Effects of the β-Hydroxylase Inhibitor, MO-I-1100, on Notch Signaling
We determined the expression of Notch receptors and ligands as well as the activated Notch intracellular domains (NICDs), which, when cleaved from the full-length receptors and transported to the nucleus, serve as transcription factors to up-regulate Notch-responsive genes in several HCC cell lines. Notch receptors 1, 2, 3, and 4 were variably expressed in these cell lines (as shown in Fig. 5A). In contrast, JAG1 was expressed in all cell lines and was particularly prominent in HepG2, Huh7, and Hep3B, whereas JAG2 and delta-like (DLL)1 and 4 had little expression. There was generation of NICDs in many of the cell lines and activated Notch 2 intracellular domain (ICD) was particularly prominent in human and murine HCC cell lines tested, suggesting that Notch signaling were present in HCC cell lines and could contribute to generation of a highly migrated, invasive, proliferative, and metastatic phenotype.
To test this hypothesis, we explored the effects of MO-I-1100 on Notch signaling in FOCUS cells. Expression levels of hairy and enhancer of split 1 (HES1) and hairy/enhancer-of-split related with YRPW motif 1 (HEY1), known to be downstream genes regulated by Notch signaling, were evaluated by western blotting analysis. Cell lysates were prepared from FOCUS cells treated with MO-I-1100 for 24 hours at various concentrations. Results showed a dose-inhibitory effect of MO-I-1100 on HES1 and HEY1 protein expression (Fig. 5B,C). These results suggest that β-hydroxylase activity may be important in modulating Notch signaling because inhibition of enzymatic activity by 80% (MO-I-1100) reduced expression of Notch-responsive genes.
Antitumor Effects of MO-I-1100 on HCC Growth and Progression in Immunodeficient Mice
The FOCUS cell line generates a highly aggressive tumor when injected SC into nude mice. Nude mice bearing FOCUS SC established tumors were treated with MO-I-1100 (20 mg/kg per day) on 5 consecutive days for 2 weeks, and then MO-I-1100 was administered every other day. Treatment of tumor-bearing mice significantly reduced HCC growth (Fig. 6A). Tumor growth and progression were substantially reduced by day 21 of treatment; mean tumor volume in treated mice was 31.7% of tumors observed in control mice (Fig. 6B). However, the size of HCC tumors at the initiation of treatment was 50 mm3, and we cannot exclude that MO-I-1100 may have other off-target effects on angiogenesis, for example. None of the MO-I-1100-treated mice showed signs of wasting or other toxicity, relative to control animals.
β-Hydroxylase Inhibitor MO-I-100 Reduces Tumor Growth in an Intrahepatic Orthotopic Xenograft Model of HCC
In order to further establish the efficacy of MO-I-1100 on HCC growth and progression, an orthotopic xenograft model of HCC was developed. MO-I-1100 at 50 mg/kg was administered to mice bearing FOCUS-generated tumors in the liver for 5 consecutive days per week for 2 weeks and then administered every other day. As a result of large tumor size in controls, all mice were sacrificed at 3 weeks and HCC intrahepatic growth was significantly inhibited by MO-I-1100 (Fig. 6C). Mean liver tumor weight in the treated group was 42.6% of control animals (Fig. 6D).
Effects of MO-I-1100 on Notch Signaling in HCC Tumors
Nude mice with established SC tumor (approximately 200 mm3) xenografts of FOCUS cells were treated IP with MO-I-1100 (50 mg/kg) or saline for 5 consecutive days and sacrificed on day 5. Expression levels of Notch-related proteins were analyzed by western blotting analysis. Results demonstrated that activated Notch 1 (cleaved fragment) as well as downstream HES1 and HEY1 gene expression were reduced by MO-I-1100 administration (Fig. 7A). Densitometric analysis revealed significantly reduced levels of activated Notch 1, HES1, and HEY1 by 69.8%, 42%, and 39.8%, respectively, suggesting that inhibition of β-hydroxylase activity reduces Notch signaling activation in HCC tumors (Fig. 7B). Furthermore, IHC analysis of murine xenograft tumor tissue revealed that Notch 1 ICD was observed prominently in approximately 50% of the nuclei of tumor cells in control mice, and there was a significant reduction in nuclear localization of NICD after treatment of established HCC tumors for 5 days with MO-I-1100 (Fig. 7C). Indeed, percent of NICD-positive nuclei was substantially decreased after MO-I-1100 administration by 60.6% (Fig. 7C). Enzymatic activity of ASPH appeared important in regulation of Notch proteins by MO-I-1100 (as shown in Fig. 3D).
ASPH is highly expressed in HCC tumors, as suggested by several previous studies,[7, 10, 18] and increased ASPH expression predicts a worse prognosis as a result of disease reoccurrence after surgical resection. Interestingly, ASPH was not expressed in dysplastic regenerating nodules, suggesting an association with a fully transformed hepatic phenotype (as shown in Fig. 1). The biological function of ASPH is associated with increased cell motility, migration, invasion, and anchorage-independent cell growth, which are characteristics of malignant cells.[11, 19] Thus, its role in hepatic oncogenesis is proposed to promote tumor growth, progression, and metastases. Blocking its biological activities in this context may have therapeutic potential.
The Notch signaling cascades consist of four large transmembrane receptors (Notch 1, 2, 3, and 4) coupled with ligands, such as JAG1 and 2 as well as DLL1, 3, and 4, and requires cell-cell interaction for activation. A key molecular event is the cleavage of full-length receptors and the release of the Notch ICD (NICD), where it translocates to the nucleus to form a transcriptional complex that up-regulates Notch-responsive genes, such as HES and HEY, which are important not only in cell-fate determination principally during embryogenesis, but also in maintaining organ and tissue homeostasis.[20-22] Notch signaling may play a critical role in HCC development, as outlined in a concise review. Constitutive activation of Notch by conditional overexpression at the NICD produced HCC in 88%-100% of animals.[23, 24] Notch activation has been described in 30%-90% of human HCC tumors,[18, 22, 24] although its exact role and mechanisms of activation that contribute to HCC development and progression are controversial.[12, 23-26] However, it is a major regulator of cell proliferation, migration, and invasion and plays a prominent role in regulation of apoptosis.
Previous studies have established that ASPH overexpression leads to a direct interaction with Notch 1 and JAG, followed by activation of downstream Notch-responsive genes. We have recently demonstrated that reduction in enzymatic activity of ASPH by the H675Q mutation does not prevent this interaction, but blocks up-regulation of Notch-activated genes (unpublished observations). Furthermore, transient inhibition of ASPH expression by siRNA reduces cell motility, migration, and invasion. It has not been previously clarified whether the enzymatic activity was necessary for ASPH biological function. Development of an SMI MO-I-1100 addressed this issue, because an 80% reduction in β-hydroxylase activity had an effect on HCC viability and there was a general correlation between the expression level and sensitivity to this drug (as shown in Figs. 2 and 3). This finding may have clinical relevance for selecting high-ASPH-expressing tumors for targeted therapy with specific β-hydroxylase inhibitors. ASPH could serve as a biomarker to predict likelihood of antitumor responses because there was variable expression of ASPH protein levels in HCC tumors (as shown in Fig. 1). Specificity of MO-I-1100 for ASPH β-hydroxylase, but no other hydroxylases, was established through a capture step employing a mAb (FB-50) directed against the N-terminus of the protein preceding measurement of β-hydroxylase activity. We have found that ASPH overexpression is associated with Notch activation both in vitro and in vivo, and the enzymatic activity of the β-hydroxylase is important because treatment with a SMI MO-I-1100 (Fig. 2) down-regulates this signaling cascade (as shown in Figs. 6 and 7).
Notch receptors, ligands, and cleaved NICDs are expressed in HCC cell lines, suggesting that constitutive activation of this signaling cascade may be important in promoting a highly invasive, metastatic phenotype. Treatment with MO-I-1100 was particularly revealing because it reduced generation of NICD and subsequent nuclear localization as well as inhibited Notch-responsive HES1 and HEY1 gene expression both in vitro and in established HCC tumors (as shown in Figs. 5 and 7). More important, MO-I-1100 treatment of established FOCUS cell-generated tumors with aggressive characteristics was substantial for antitumor effects using two animal models of SC and intrahepatic growth, which suggests that enzymatic activity of ASPH was important for tumor progression.
Several key signaling pathways are commonly activated in HCC tumors irrespective of viral or other etiologies, including insulin (IN)/insulin-like growth factor 1 (IGF1)/insulin-receptor substrate type 1 (IRS1)/mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK), IN/IGF1/IRS1/phosphatidylinositol 3-kinase (PI3K)/protein kinase B (AKT)/glycogen synthase kinase 3 beta (GSK3β), and WNT/β-catenin,[18, 20, 21, 28, 29] among many others. Indeed, activation of these pathways in a double transgenic murine model was necessary and sufficient to transform the normal liver to HCC without the participation of known genetic mutations. ASPH is a downstream target of these pathways and is up-regulated at both messenger RNA and protein levels. Because up-regulation of ASPH promotes cell proliferation, migration, invasion, malignant transformation, and growth of tumors in immunodeficient mice, it is essential to establish what the downstream effector-signaling pathway is that produces such an ASPH-associated malignant phenotype. Previous studies suggest that Notch activation may play a role in this regard.
Transcriptional regulation of ASPH is provided by tripartite signaling pathways: IN/IGF1/IRS1/MAPK/ERK; IN/IGF1/IRS1/PI3K/AKT; and WNT/β-catenin.[18, 32] Posttranscriptional regulation of ASPH is mediated by phosphorylation of GSK3β-related motifs located in the N-terminal region of the molecule. IN/IGF1/IRS1/MAPK/ERK, PI3K/AKT, and WNT/β-catenin signaling up-regulated ASPH expression has been implicated to increase motility and invasion of HCC cells.[17, 18, 32, 33] Accordingly, siRNA inhibition of ASPH expression reduces cell motility and growth.[18, 33-35] Collectively, ASPH has been hypothesized to exert its effector function by activating Notch signaling and its downstream target genes to promote cell migration and invasion.[17, 18, 32]
Our working hypothesis on the role of ASPH in HCC pathogenesis, as well as formulating a potential therapeutic strategy, is based on this signaling pathway (as shown in Fig. 8). In this scheme, overexpression of ASPH through activation of upstream growth factor signaling cascade[18, 20, 21, 28-31] facilitates an interaction with Notch receptors and ligands,[27, 36] followed by two cleavage events to generate NICDs, which serve as transcription factors after translocation into the nucleus (Fig. 8A). Level of β-hydroxylase activity is essential for its biological function because it may link upstream growth factor signaling pathways to reveal how Notch signaling cascade may be subsequently activated in HCC through ASPH overexpression. Inhibition of enzymatic activity with SMIs such as MO-I-1100 prevents activation of Notch signaling (Fig. 8B) and downstream gene activation involved in cell migration, invasion, and metastasis to produce antitumor effects on HCC growth, progression, and distant spread.
Tissue samples were provided by Dr. Murray Resnick from the Department of Pathology and Laboratory Medicine, Rhode Island and Miriam Hospitals, Alpert Medical School of Brown University. The authors also gratefully acknowledge the assistance of Drs. Tong and de le Monte in the construction of the β-hydroxylase assay.