DNA methyltransferase 3B plays a protective role against hepatocarcinogenesis caused by chronic inflammation via maintaining mitochondrial homeostasis

Liver-specific deletion of de novo DNA methyltransferases protects against glucose intolerance in high-fat diet-fed male mice

Alterations to gene transcription and DNA methylation are a feature of many liver diseases including fatty liver disease and liver cancer. However, it is unclear whether the DNA methylation changes are a cause or a consequence of the transcriptional changes. It is even possible that the methylation changes are not required for the transcriptional changes. If DNA methylation is just a minor player in, or a consequence of liver transcriptional change, then future studies in this area should focus on other systems such as histone tail modifications. To interrogate the importance of de novo DNA methylation, we generated mice that are homozygous mutants for both Dnmt3a and Dnmt3b in post-natal liver. These mice are viable and fertile with normal sized livers. Males, but not females, showed increased adipose depots, yet paradoxically, improved glucose tolerance on both control diet and high-fat diets (HFD). Comparison of the transcriptome and methylome with RNA sequencing and whole-genome bisulfite sequencing in adult hepatocytes revealed that widespread loss of methylation in CpG-rich regions in the mutant did not induce loss of homeostatic transcriptional regulation. Similarly, extensive transcriptional changes induced by HFD did not require de novo DNA methylation. The improved metabolic phenotype of the Dnmt3a/3b mutant mice may be mediated through the dysregulation of a subset of glucose and fat metabolism genes which increase both glucose uptake and lipid export by the liver. However, further work is needed to confirm this.

Pivotal role for S-nitrosylation of DNA methyltransferase 3B in epigenetic regulation of tumorigenesis

DNA methyltransferases (DNMTs) catalyze methylation at the C5 position of cytosine with S-adenosyl-L-methionine. Methylation regulates gene expression, serving a variety of physiological and pathophysiological roles. The chemical mechanisms regulating DNMT enzymatic activity, however, are not fully elucidated. Here, we show that protein S-nitrosylation of a cysteine residue in DNMT3B attenuates DNMT3B enzymatic activity and consequent aberrant upregulation of gene expression. These genes include Cyclin D2 (Ccnd2), which is required for neoplastic cell proliferation in some tumor types. In cell-based and in vivo cancer models, only DNMT3B enzymatic activity, and not DNMT1 or DNMT3A, affects Ccnd2 expression. Using structure-based virtual screening, we discovered chemical compounds that specifically inhibit S-nitrosylation without directly affecting DNMT3B enzymatic activity. The lead compound, designated DBIC, inhibits S-nitrosylation of DNMT3B at low concentrations (IC₅₀ ≤ 100 nM). Treatment with DBIC prevents nitric oxide (NO)-induced conversion of human colonic adenoma to adenocarcinoma in vitro. Additionally, in vivo treatment with DBIC strongly attenuates tumor development in a mouse model of carcinogenesis triggered by inflammation-induced generation of NO. Our results demonstrate that de novo DNA methylation mediated by DNMT3B is regulated by NO, and DBIC protects against tumor formation by preventing aberrant S-nitrosylation of DNMT3B.

Multifaceted roles of aerobic glycolysis and oxidative phosphorylation in hepatocellular carcinoma

Liver cancer is a common malignancy with high morbidity and mortality rates. Changes in liver metabolism are key factors in the development of primary hepatic carcinoma, and mitochondrial dysfunction is closely related to the occurrence and development of tumours. Accordingly, the study of the metabolic mechanism of mitochondria in primary hepatic carcinomas has gained increasing attention. A growing body of research suggests that defects in mitochondrial respiration are not generally responsible for aerobic glycolysis, nor are they typically selected during tumour evolution. Conversely, the dysfunction of mitochondrial oxidative phosphorylation (OXPHOS) may promote the proliferation, metastasis, and invasion of primary hepatic carcinoma. This review presents the current paradigm of the roles of aerobic glycolysis and OXPHOS in the occurrence and development of hepatocellular carcinoma (HCC). Mitochondrial OXPHOS and cytoplasmic glycolysis cooperate to maintain the energy balance in HCC cells. Our study provides evidence for the targeting of mitochondrial metabolism as a potential therapy for HCC.

Misregulation of the expression and activity of DNA methyltransferases in cancer

In mammals, DNA methyltransferases DNMT1 and DNMT3's (A, B and L) deposit and maintain DNA methylation in dividing and nondividing cells. Although these enzymes have an unremarkable DNA sequence specificity (CpG), their regional specificity is regulated by interactions with various protein factors, chromatin modifiers, and post-translational modifications of histones. Changes in the DNMT expression or interacting partners affect DNA methylation patterns. Consequently, the acquired gene expression may increase the proliferative potential of cells, often concomitant with loss of cell identity as found in cancer. Aberrant DNA methylation, including hypermethylation and hypomethylation at various genomic regions, therefore, is a hallmark of most cancers. Additionally, somatic mutations in DNMTs that affect catalytic activity were mapped in Acute Myeloid Leukemia cancer cells. Despite being very effective in some cancers, the clinically approved DNMT inhibitors lack specificity, which could result in a wide range of deleterious effects. Elucidating distinct molecular mechanisms of DNMTs will facilitate the discovery of alternative cancer therapeutic targets. This review is focused on: (i) the structure and characteristics of DNMTs, (ii) the prevalence of mutations and abnormal expression of DNMTs in cancer, (iii) factors that mediate their abnormal expression and (iv) the effect of anomalous DNMT-complexes in cancer.