1. Introduction
Hepatoblastoma is the most common primary liver malignancy in children, accounting for approximately 1% of all pediatric cancers and 80% of liver cancers in children under the age of five [
1,
2]. Despite being a rare disease, hepatoblastoma has garnered significant attention due to its aggressive nature and the challenges it poses in pediatric oncology. The etiology of hepatoblastoma remains largely unknown, although it is thought to involve a combination of genetic, environmental, and developmental components of undefined importance [
2]. The survival rate for patients with localized hepatoblastoma has improved with advances in surgical techniques and chemotherapy; however, the prognosis for those with metastatic or recurrent disease remains poor [
3,
4,
5,
6]. This underscores the urgent need to elucidate the molecular mechanisms underlying hepatoblastoma development and progression to identify novel therapeutic targets.
The liver is a central hub for metabolic processes, including gluconeogenesis, glycolysis, fatty acid oxidation, and the metabolite detoxification [
7]. These metabolic pathways are tightly regulated to maintain homeostasis and support the liver's diverse physiological functions. In the context of liver cancer, including hepatoblastoma, metabolic reprogramming is a well-recognized phenomenon [
8,
9,
10,
11]. Cancer cells often undergo a metabolic shift known as the Warburg effect, characterized by increased glycolysis and lactate production even in the presence of sufficient oxygen [
12,
13]. This shift supports rapid cell proliferation by providing the necessary building blocks for biomass production and by maintaining redox balance [
14,
15].
In hepatoblastoma, metabolic reprogramming is particularly relevant due to the liver's intrinsic metabolic functions. Alterations in metabolic pathways can influence tumor growth, survival, and resistance to therapy. For instance, enhanced glycolysis, glutaminolysis, and fatty acid synthesis have been implicated in the proliferation and survival of hepatoblastoma cells [
16,
17,
18]. Understanding how these metabolic pathways are regulated in hepatoblastoma can provide insights into the disease's pathogenesis and identify potential metabolic vulnerabilities that can be targeted therapeutically.
Epigenetics refers to heritable changes in gene expression that do not involve alterations in the DNA sequence. These changes are often mediated by modifications such as DNA methylation, histone modifications, and non-coding RNAs [
19,
20,
21]. DNA methylation, in particular, plays a critical role in regulating gene expression and maintaining cellular identity. It is mediated by a family of enzymes known as DNA methyltransferases (DNMTs), including DNMT1, DNMT3A, and DNMT3B, which add methyl groups to cytosine residues in DNA, typically leading to gene silencing[
22,
23,
24]. In liver cancer, aberrant DNA methylation patterns are common and contribute to tumorigenesis by silencing tumor suppressor genes or activating oncogenes [
25,
26,
27]. Hepatoblastoma, like other cancers, exhibits widespread epigenetic dysregulation, including abnormal DNA methylation [
28,
29]. The role of DNMTs in hepatoblastoma is of particular interest, as these enzymes can modulate the expression of genes involved in key oncogenic pathways and metabolic processes. Investigating the epigenetic landscape of hepatoblastoma can reveal critical insights into how DNA methylation and other epigenetic modifications contribute to tumor development and progression.
Oxidative stress, characterized by an imbalance between the production of reactive oxygen species (ROS) and the cell's antioxidant defenses, plays a pivotal role in cancer biology. Mitochondria, the cell's powerhouse, are both a major source and target of ROS [
30,
31]. In liver cancer, including hepatoblastoma, mitochondrial dysfunction and oxidative stress are common features that drive tumorigenesis. ROS can induce DNA damage, promote genetic instability, and activate signaling pathways that support cancer cell survival and proliferation [
32,
33,
34].
In hepatoblastoma, oxidative stress and mitochondrial dysfunction are intertwined with metabolic reprogramming and epigenetic regulation. For example, impaired mitochondrial oxidative phosphorylation can increase ROS production, while DNA methylation changes can alter the expression of genes involved in oxidative stress responses [
35,
36,
37]. Understanding the interplay between oxidative stress, mitochondrial function, and the metabolic and epigenetic landscape of hepatoblastoma can provide a comprehensive view of the molecular mechanisms driving this disease.
Given the importance of metabolic and epigenetic regulation in liver physiology and cancer, our study aims to explore these aspects in hepatoblastoma using single-cell RNA sequencing (scRNAseq). By profiling the transcriptomes of individual cells within hepatoblastoma tumors, we can uncover the cellular heterogeneity and identify distinct metabolic and epigenetic states. Specifically, we aim to elucidate the relationship between the metabolic status of hepatoblastoma cells and their developmental trajectory, as well as the role of DNMT-mediated DNA methylation in regulating these processes.
Our results reveal a complex interplay between metabolic reprogramming and epigenetic dysregulation in hepatoblastoma through overexpression of DNMT3B and implication of one carbon metabolism in tumor cells, highlighting potential therapeutic targets and biomarkers for this aggressive pediatric cancer. This study provides novel insights into the molecular underpinnings of hepatoblastoma and underscores the value of integrated single-cell analyses in understanding cancer biology.
3. Discussion
In hepatoblastoma, metabolic reprogramming is particularly relevant due to the liver's intrinsic metabolic functions. For instance, enhanced glycolysis, glutaminolysis, and fatty acid synthesis have been implicated in the proliferation and survival of hepatoblastoma cells [
16,
17,
18]. In the present work, up regulation of metabolic enzyme program was investigated in hepatoblastoma tumor at tissue and single cell level. We confirmed main alteration of carbohydrate metabolism which share some enzymes with amino acid biosynthesis metabolism such as: PKM, PFKM and ENO2. PKM2 is an alternative-splice isoform of the PKM gene. The M2 pyruvate kinase (PKM2) isoform is upregulated in most cancers and plays a crucial role in regulation of the Warburg effect, which is characterized by the preference for aerobic glycolysis over oxidative phosphorylation for energy metabolism. Antisense oligonucleotide-based PKM splice switching has been proposed as a targeted therapy for liver cancer [
42]. During hepatocellular carcinoma, ZEB1 has been shown to enhance Warburg effect, facilitating tumorigenesis and metastasis of liver cancer by transcriptionally activating PFKM [
12]. In amino acid biosynthesis GLUL and ASNS have been shown to be correlated to overall survival during hepatoblastoma. Hepatoblastoma samples showed strong GLUL expression and glutamine synthesis, generally as a result of CTNNB1 mutations. Glutamine depletion resulted in the inhibition of proliferation and of cell viability in embryonal hepatoblastoma cell lines [
43]. In metabolic enrichment network, steroid metabolism was also highlighted as isolated subnetwork implicating: SQLE, SOAT2, FDFT1, and CEL. Carboxyl ester lipase is implicated in reverse cholesterol transport [
44] , and confers susceptibility to alcoholic liver cirrhosis[
45]. Squalene epoxidase (SQLE) is known to promote the growth and migration of the hepatocellular carcinoma cells [
46]. During Zebrafish embryogenesis, Sterol O-Acyltransferase 2 contributes to the yolk cholesterol trafficking [
47].
During our work interconnection between metabolism and epigenetics were observed and especially with potential regulations of DNA methylation. DNMT3B belongs to epigenetic machinery implicated in DNA methylation. During hepatoblastomas a general disrupted expression of genes from the epigenetic machinery was observed, mainly UHRF1, TET1, and TET2 upregulation, in association with an enrichment of 5hmC content. These alterations support a model of active demethylation by TETs in hepatoblastoma, probably during early stages of liver development, which in combination with UHRF1 overexpression would lead to DNA hypomethylation and an increase in overall 5hmC content [
48]. During development of metabolic disfunction associated to liver cancer, it was observed some associations between the rare and common germline variants in one-carbon metabolism and DNA methylation genes [
49]. KEGG enrichment network of hepatoblastoma tumor cells has shown enrichment in one carbon metabolism enzymes, regulating pool of folates. Depletion of folates is connected to the aggressiveness of cancer phenotype [
50], and folate pool can be dependent of purine nucleotide biosynthesis. Hepatoblastoma tumor cells were displaying in up regulation of purine metabolism through enhance expression of ENTPD1, PDE4C, PDE5A, and PAPSS1. Disruption of ENTPD1 (Cd39) perturbates metabolism (purinergic signaling) during liver development [
51]. Use phosphodiesterase inhibitor during chronic liver injury and metabolic diseases potentially impact on Cyclic AMP (cAMP) signaling, particularly in the regulation of fatty acid (FA) β-oxidation and pro-inflammatory polarization of tissue-resident lymphocytes [
52]. in HEpG2 cell line, PAPSS1/2 knockdown significantly activated farnesoid X receptor (FXR), retinoid-related orphan receptor, and pregnane X receptor responsive reporters, and treatment with the FXR agonist GW4064[
53].
One carbon metabolism deregulation in hepatoblastoma tumor implicated: 5-methyltetrahydrofolate-homocysteine methyltransferase (MTR), aldehyde dehydrogenase 1 family member L2 (ALDH1L2), and methylenetetrahydrofolate dehydrogenase (NADP+ dependent) 1 like (MTHFD1L). Mitochondrial folate-dependent one-carbon (1-C) metabolism converts 1-C donors such as serine and glycine to formate, which is exported and incorporated into the cytoplasmic tetrahydrofolate (THF) 1-C pool [
54]. The folate cycle, through transfer of a carbon unit between tetrahydrofolate and its derivatives in the cytoplasmic and mitochondrial compartments, produces other metabolites that are essential for cell growth, including nucleotides, methionine, and the antioxidant NADPH Folate cycle enzyme MTHFD1L is known to confer metabolic advantages in hepatocellular carcinoma [
55]. During embryonic development, the methylation of DNA and histones drives cell division and regulation of gene expression through epigenesis and imprinting. Folate cycle and methyltransferase enzymes are important actors of methyl transfer processes [
56].
At single cell level, machine learning model ranked top ten mRNAs to predict tumor cell status during hepatoblastoma: FKBP10, ATP1A2, NT5DC2, UGT3A2, PYCR1, CKB, GPX7, DNMT3B, GSTP1, and OXCT1. FKBP10, is a member of the FK506-binding protein (FKBP) family, and has been implicated in cancer development [
57]. During lung cancer, gain- and loss-of-function assays show that FKBP10 boosts cancer growth and stemness via its peptidyl-prolyl-cis-trans-isomerase (PPIase) activity. Also, FKBP10 interacts with ribosomes, and its downregulation leads to reduction of translation elongation at the beginning of open reading frames (ORFs), particularly upon insertion of proline residues [
58]. ATP1A2 has been shown highly expressed during platinum-resistant ovarian cancer [
59]. During hepatocellular carcinoma, NT5DC2 promotes tumor cell proliferation by stabilizing EGFR [
60]. UGT3A2 an UDP-glycosyltransferase (UGT) is implicated in exogenous polycyclic aromatic hydrocarbons (PAHs) detoxification [
61]. pyrroline-5-carboxylate reductase (PYCR1) participates to mitochondrial proline metabolism reprogramming and promotes liver tumorigenesis [
62]. Creatine kinase B (CKB) suppresses ferroptosis by phosphorylating GPX4[
63]. GPX7 with GPX4 are known to be overexpressed in hepatocellular carcinoma tissues [
64]. GSTP1 gene is preceded by a large CpG-rich region that is frequently affected by methylation during cancer[
65]. 3-oxoacid CoA-transferase 1 (OXCT1), a rate-limiting ketolytic enzyme whose expression is suppressed in normal adult liver tissues, is re-induced by serum starvation-triggered mTORC2-AKT-SP1 signaling in HCC cells [
66]. Top ten enzymes of the hepatoblastoma metabolic score confirmed potential reprogramming of these tumoral cells.
During this work, a complex interplay between metabolic reprogramming and epigenetic dysregulation in hepatoblastoma through overexpression of DNMT3B and implication of one carbon metabolism in tumor cells, highlighting potential therapeutic targets and biomarkers for this aggressive pediatric cancer. This study provides novel insights into the molecular underpinnings of hepatoblastoma and underscores the value of integrated single-cell analyses in understanding cancer biology.