The Anti-Leukemia Effect of Ascorbic Acid: From the Pro-Oxidant Potential to the Epigenetic Role in Acute Myeloid Leukemia

Abstract

Data derived from high-throughput sequencing technologies have allowed a deeper understanding of the molecular landscape of Acute Myeloid Leukemia (AML), paving the way for the development of novel therapeutic options, with a higher efficacy and a lower toxicity than conventional chemotherapy. In the antileukemia drug development scenario, ascorbic acid, a natural compound also known as Vitamin C, has emerged for its potential anti-proliferative and pro-apoptotic activities on leukemic cells. However, the role of ascorbic acid (vitamin C) in the treatment of AML has been debated for decades. Mechanistic insight into its role in many biological processes and, especially, in epigenetic regulation has provided the rationale for the use of this agent as a novel anti-leukemia therapy in AML. Acting as a co-factor for 2-oxoglutarate-dependent dioxygenases (2-OGDDs), ascorbic acid is involved in the epigenetic regulations through the control of TET (ten-eleven translocation) enzymes, epigenetic master regulators with a critical role in aberrant hematopoiesis and leukemogenesis. In line with this discovery, great interest has been emerging for the clinical testing of this drug targeting leukemia epigenome. Besides its role in epigenetics, ascorbic acid is also a pivotal regulator of many physiological processes in human, particularly in the antioxidant cellular response, being able to scavenge reactive oxygen species (ROS) to prevent DNA damage and other effects involved in cancer transformation. Thus, for this wide spectrum of biological activities, ascorbic acid possesses some pharmacologic properties attractive for anti-leukemia therapy. The present review outlines the evidence and mechanism of ascorbic acid in leukemogenesis and its therapeutic potential in AML. With the growing evidence derived from the literature on situations in which the use of ascorbate may be beneficial in vitro and in vivo, we will finally discuss how these insights could be included into the rational design of future clinical trials.

Keywords: acute myeloid leukemia; ascorbic acid; epigenetic regulation; oxidative stress; vitamin C.

FIGURE 1

Ascorbic acid uptake. Ascorbic acid, also known as Vitamin C, enters the cell either in its reduced form (ascorbate, ASC) by sodium-dependent vitamin C transporters (SVCTs) or in its oxidized form (dehydroascorbate, DHA) via facilitative glucose transporters (GLUTs). In the cytosol, DHA is rapidly reduced back to ASC in the presence of glutathione (GSH). Created with Biorender.com.

FIGURE 2

Biological functions and mechanisms of action of ascorbic acid. (A) Ascorbic acid plays an important role in several biological processes by acting as a cofactor for 2-oxoglutarate dependent dioxygenases (2-OGDDs) that have a wide range of biological functions, including collagen synthesis, HIF-1α degradation and biosyntesis of catecholamines. (B) Ascorbic acid can act as an epigenetic regulator by enanching the activity of TET2 enzyme, that catalyzes the conversion of 5-methylcytosine (5-mC), into 5-hydroxymethylcytosine (5-hmC), inducing DNA demethylation. (C) Ascorbic acid has important roles in scavenging free radicals, having the ability to donate an electron to reactive oxygen species (ROS) to form a relatively stable ascorbyl-free radical (AFR). Created with Biorender.com.

FIGURE 3

The anti-cancer effects of ascorbic acid. (A) A high concentration of ascorbic acid (ASC) increases the labile iron pool (LIP) of tumor cells, induces the production of increased level of ROS directly damaging mitochondria and DNA, and ultimately stimulates apoptotic pathways. (B) Tumor cells can uptake DHA at higher rates and then internally reduce it to ASC. This reduction triggers scavenging of glutathione (GSH), inducing oxidative stress, the inactivation of glyceraldehyde 3-phosphate dehydrogenase (GAPDH), the inhibition of glycolytic flux, an energy crisis and cell death. (C) In the hypoxic conditions of tumor microenvironment, there is a repression of HIF-1α hydroxilation. As a result, HIF-1α accumulates in the cytoplasm and translocates into the nucleus, promoting the transcription of its targets involved in prcesses such as angiogenesis, glycolysis and anaerobic metabolism, metastasis and resistance to therapy. Under ascorbic acid treatment, HIF-1α is hydroxylated by the prolyl hydroxylases, ultimately leading to polyubiquitination and proteasomal degratadion of HIF-1α. (D) Ascorbic acid binds to the catalytic domain of TET and facilitates DNA demethylation and re-expression of important tumor suppressor genes. Created with Biorender.com.

FIGURE 4

Activity of pharmacologic doses of ascorbic acid in AML with TET2, IDH1/2 or WT1 altered pathways. (A) TET2 mutations result in a nonfunctional enzyme with hypermethylation of gene promoters, and prevention of oxidation of 5-methylcytosine (5mC) to 5-hydroxymethylcytosine (5hmc). (B) The presence of gain-of-function mutations in IDH1/2 genes results in the overproduction of the oncometabolite 2-hydroxyglutarate (2-HG) with the inhibition of TET2 activity. (C) WT1 mutations hamper the ability of TET2 to bind and activate WT1, inhibiting the expression of WT1-target genes. (D) Ascorbic acid treatment mimics TET2 restoration, inducing a reversal of defective DNA methylation and cell differentiation, ultimately inhibiting tumor progression. Created with Biorender.com.