2.1. Metabolic modeling of HCC
Here, we employed TRFBA-CORE, a benchmark-driven algorithm developed by an extensive evaluation of other cancer genome-scale metabolic modeling methods, to find potential drug targets for HCC 27. We also reconstructed two cell-specific GEMs using mCADRE and CORDA algorithms to compare their predictive capabilities with TRFBA-CORE in simulating a set of defined metabolic tasks for the liver 12,19,26. CORDA was able to predict a wider range of liver metabolic tasks (Figure 1). However, the enrichment of cell-specific models with essential genes for HepG2 showed that only GEM generated by TRFBA-CORE was able to significantly predict essential genes (Enrichment p values = 0.005, 0.098, and 0.086, for TRFBA-CORE, CORDA, and mCADRE, respectively).
The use of growth-correlated key reactions and the FASTCORE minimal modeling approach may explain the ability of TRFBA-CORE to significantly predict HepG2 essential genes. The size of generated models further shows that the TRFBA-CORE model has the minimum number of reactions required to render the network flux consistent (Figure 2).
2.2. Identification of potential drug targets for HCC
Of 139 essential genes predicted by TRFBA-CORE, 31 genes (22%) disrupted defined metabolic functions in energy metabolism and redox equilibrium, or biomass production, predominantly in the OXPHOS pathway (Supplementary table S1). Therefore, we excluded these genes due to their potential adverse effects on healthy cells, and performed a gene-set enrichment analysis using the remaining 108 predicted essential genes to better understand their functional impacts. Interestingly gene ontology (GO) biological processes showed an enrichment of processes involved in cholesterol, sterols and steroids biosynthesis (Supplementary table S2). Furthermore, KEGG (Supplementary table S3) and Reactome (Supplementary table S4) biochemical pathways involved in lipid and lipoprotein metabolism, biosynthesis, cholesterol and steroids regulation, and carbohydrate and amino acids metabolisms had a significant overlap with the predicted essential genes.
Cholesterol is a key molecule in cell membrane structure, and a prerequisite for essential hormones and bile acids. The liver is one of the main organs for the synthesis and metabolism of endogenous cholesterol in the body 28,29. The growth of HCC tumors is dependent on cholesterol biosynthesis, and the use of statins (inhibitors of HMG-CoA reductase) has been suggested as a candidate therapy for HCC. Although this enzyme is a key regulatory enzyme in cholesterol synthesis, several other enzymes in the cholesterol biosynthesis pathway have been targeted to prevent or reduce cell proliferation in various cancers 28. Furthermore, previous findings suggest that the mevalonate pathway plays a role in the development of HCC 30–32. Of the 18 essential genes predicted for cholesterol metabolism, 17 metabolic genes (excluding sterol O-acyltransferase 1 (SOAT1)) has been previously identified to be involved in the production of potential antimetabolites reducing the growth of HCC tumors 28.
Moreover, glucocorticoids, such as dexamethasone, are a class of steroids widely used as anti-inflammatory agents. Ma et al. showed that the expression of two essential enzymes that regulate endogenous glucocorticoids and glucogenesis was altered in malignant hepatocytes, so that the expression of 11-beta dehydrogenase hydroxysteroid type 1 (HSD11B1) and 2 (HSD11B2) in mouse and human tumor cells were up and down-regulated, respectively. Moreover, the expression ratio of HSD11B1:HSD11B2 was related to the prognosis and survival of patients with HCC, so that patients with a high ratio of HSD11B1: HSD11B2 had a longer survival time than others 33,34. Also, it has been shown that HSD11B1 expression usually leads to a decrease in cell proliferation, while HSD11B2 expression is involved in an increase in cell proliferation. Several studies showed that the expression of these two genes varies in different tumors and can provide a suitable microenvironment for tumor growth 35,36. Therefore, HSD11B2, which was the only predicted essential gene in the steroid biosynthesis pathway, was selected for further investigation.
We compared the relative expression level of HSD11B2 in HepG2 and three non-cancerous cell lines, namely HH, HHSEC and LX-2 (Figure 3a) and observed a relatively higher expression of HSD11B2 in HepG2 cells. Therefore, we silenced HSD11B2 in HepG2 cells (Figure 3b) to examine its knockdown effect on the cell viability. Interestingly, we were able to observe a modest but significant reduction in cell viability after 48 (p-value = 0.021) and 72 (p-value = 6.5E-4) hr (Figure 4)
Glucocorticoids are essential hormones in the body, regulating the nuclear expression of approximately 5% of human genes. One of their function is to increase glucose synthesis by increasing gluconeogenesis. Ma et al. showed that glycogenesis is impaired in HCC, and that the abnormal expression of two genes, HSD11B1 and HSD11B2, affects some key genes in the glucose pathway. Although the mechanism by which malignant hepatocytes target these two genes for malignant progression is not clear, targeting HSD11B2 in a mouse model reduced tumor mass 34. Furthermore, by comparing the RNA-Seq expression data in GEPIA web server, we observed that HSD11B1 and HSD11B2 were respectively down- and up-regulated in HCC samples (P <0.05). The expression of these two genes showed a negative correlation in healthy and cancerous samples from Atlas of Cancer Genome (TCGA, Spearman R=-0.18, p=3×10-4), while, the correlation in healthy samples was positive (Spearman R = 0.36, p = 0.011). Therefore, given the lower expression of HSD11B1 in HCC cells, HSD11B2 may be a potential target gene in HCC to regulate the HSD11B1: HSD11B2 expression ratio 37,38.
2.3. Drug repositioning for HCC
Drug repurposing is an attractive approach providing cancer patients with faster and cost-effective potential therapeutics with well-characterized safety profiles 39,40. Here, we used the DrugBank database to evaluate potential compounds targeting the predicted metabolic essential genes 41. Thus, a set of metabolic drugs targeting one or more of the predicted genes was obtained, among which we selected 9 compounds by reviewing the literature and applying the following criteria; first, metabolic drug has not been used in the treatment of HCC, and secondly, has been evaluated in at least one of the other cancers, or the target gene was suggested as a therapeutic target for HCC (Table 1).
Table 1 Drugs reviewed in this study. Only intersection of predicted essential genes and targets of each therapeutic compound has been shown in metabolic targets column.
Subsystems
|
Metabolic targets
|
DrugBank accession number
|
Drug
|
Cholesterol Metabolism
|
CYP51A1
|
DB01007
|
Tioconazole
|
Nucleotides
|
RRM1
|
DB00631
|
Clofarabine
|
Nucleotides
|
TYMS
|
DB00432
|
Trifluridine
|
Pyrimidine Biosynthesis
|
DHODH
|
DB01117
|
Atovaquone
|
Nucleotides
|
ADSSL1; ADSS
|
DB05540
|
Alanosine
|
Folate Metabolism; Nucleotides
|
DHFR; TYMS
|
DB06813
|
Pralatrexate
|
Cholesterol Metabolism
|
FDPS
|
DB00710
|
Ibandronate
|
Cholesterol Metabolism
|
SQLE
|
DB00857
|
Terbinafine
|
Cholesterol Metabolism
|
FDPS
|
DB00630
|
Alendronic Acid
|
We next examined the treatment effect of these compounds on HepG2 cell viability in 5 different concentrations. Interestingly, most of the drugs, except Terbinafine, were able to significantly reduce the cell viability (Figure 5). In addition, some of these drugs showed a comparatively higher inhibitory effect compared to Sorafenib, the well-known approved drug to treat advanced HCC. For instance, Clofarabine, Alanosine, and Pralatrexate were more effective than Sorafenib at lower concentrations (100 and 1 nm). Alanosine targeted two genes adenylosuccinate synthetase like 1 (ADSSL1) and adenylosuccinate synthetase (ADSS), which TRFBA-CORE predicted as synthetic lethal. These two genes encode two isozymes, adenylosuccinate synthase 1 and 2, respectively, catalyzing a key reaction in purine nucleotide biosynthesis. The effect of Alanosine on inhibiting the growth of other cancerous cells has also been demonstrated 42. Targeting AMP, another critical precursor by Alanosine, has also been shown to be effective at the nucleotide levels 43.
AMP is produced from the IMP by ADSS and ADSSL1, and an essential precursor in the production of DNA and RNA 44. Synthetic lethality occurs in a cell when perturbations of two genes combined are lethal, while a genetic defect in either gene alone is harmless. In this case, a genetic change, such as a defect in one tumor suppressor gene causes another gene to become essential for cell viability, and therefore, offering a therapeutic approach by selective targeting of cancer cells 45. TRFBA-CORE predicted a total of 25 synthetic lethal pairs, nine of which had previously been identified in the study of Folger et al. 46. Interestingly, none of these predicted gene pairs disrupted the defined metabolic functions in Recon 1.
Among the novel predicted synthetic lethal pairs, hydroxyl methylglutaryl-CoA synthase (HMGCS2), which is involved in cholesterol metabolism, contained eight gene pairs all of which were involved in amino acid metabolism (Supplementary table S5). Moreover, carnitine O-acyl transferase (CRAT), which facilitates the release of acetyl-CoA mitochondria into the cytosol 47 , had three gene pairs that were involved in TCA and citrate transport from mitochondria to the cytosol. The role of HMGCS2 and CRAT in several cancers has been already shown 47,48.
In addition to Alanosine, Clofarabine, one of the most successful drugs against leukemia 49, targets the large subunit of ribonucleoside-diphosphate reductase (RRM1). Notably, few other drugs targeting RRM1 have previously been suggested to inhibit HCC cell growth and other liver diseases 50,51. RRM1 and RRM2 are responsible for ribonucleotide reductase expression, which acts as a rate-limiting enzyme in dNTP production. Also, Pralatrexate targets dihydrofolate reductase (DHFR) and thymidylate synthetase (TYMS), both of which were predicted to be essential genes. Pralatrexate is an approved drug in the treatment of peripheral T cell lymphoma, and its promising outcome has been shown in some other cancer cells 52,53. However, the effect of Pralatrexate on growth was relatively constant at different concentrations (Figure 5). TYMS plays a vital role in dTMP synthesis, which is significantly upregulated in tumor cells 54. Tetrahydrofolate (THF), the biologically active form of folic acid, is an essential metabolite synthetized by DHFR and utilized by TYMS to produce dTMP, a critical compound in DNA replication 55,56.
Interestingly, Trifluridine had a larger inhibitory effect on cell growth than Sorafenib at a low concentration of 1 µm. Trifluridine/ tipiracil combination therapy has been shown to have a beneficial effect on survival in those with colon cancer 57. Furthermore, several other drugs targeting TYMS have been suggested for the treatment of HCC 58,59.
Notably, while Sorafenib showed a relatively stronger inhibitory effect on cell viability at 10 µm, thioconazole, an imidazole antifungal drug, showed a larger effect compared to the other compounds at higher concentrations (25 µm and 50 µm, Figure 5). One of the drug targets of thioconazole is lenosterol 14-alpha-demethylase (CYP51A1), which has been linked to the size of HCC tumors in the carriers of the hepatitis C virus 60. Recently, the effect of thioconazole on inhibiting the growth of cancer cells has been shown 61. Moreover, at these concentrations, the inhibitory effect of Alendronic acid, Alanosine, Ibandronate and Atovaquone, on growth was similar to that of Sorafenib. Alendronic acid and Ibandronate both inhibit farnesyl pyrophosphate synthase (FDPS). Pamidronic acid and zoldronic acid, both of which are FDPS inhibitors, have also shown antiproliferative effects in HCC 62,63. Moreover, the efficacy of Atovaquone, an antimalarial drug, has recently been demonstrated in various cancers 64,65. Atovaquone inhibits the activity of dihydroorotate dehydrogenase (DHODH), which is involved in de novo pyrimidine synthesis, resulting in a forced pause of DNA and RNA synthesis in cancer cells 66–68.