**2. Hypoxia-inducible factor**

Oxygen supply and consumption are tightly regulated and dynamically balanced in most normal tissues. However, supply and consumption of oxygen in tumor cells are usually decoupled due to the loss of physiological control and aberrant molecular signaling that provide malignant growth and survival advantages. Hypoxia appears in tumor cells when the metabolic demand for oxygen exceeds its extracellular availability. One of the main early cellular events responded upon hypoxia is activation of hypoxia-inducible factor 1 and 2 (HIF-1 and HIF-2), a critical heterodimeric transcription factor. HIF then in turn binds to hypoxiaresponsive elements (HREs), with the minimal core sequence 5'-CGTG-3', and influences the expression of various genes involved in angiogenesis, metabolism, pH regulation, proliferation, metastasis, and a wide range of other signaling processes.

#### **2.1. Structure of hypoxia-inducible factor and domain structure of α subunits**

The structure of HIF was identified as a dimer protein composed of HIF-1β and HIF-1α subunits. HIF-1β, the aryl hydrocarbon receptor nuclear translocator (ARNT), and its highly homologous protein ARNT2 and ARNT3, is constitutively expressed. All the three HIF-β subunits have the potential in forming dimers with various HIF-α subunits [5]. In normoxic conditions, HIF-1α is expressed ubiquitously at low closely balanced levels in all organs and has six different splice variants [6]. HIF-2α is most abundantly expressed in the lung, followed by the heart, brain, liver, and various other organs. Despite their similarities in mediating transcriptional responses to hypoxia, HIF-1α and HIF-2α have distinct, nonredundant functions (reviewed in Semenza [2004] [7]). HIF-3α is the least-studied member of the family and has multiple splice variants [5]. The functional domains of HIF include DNA-binding region basic helix-loop-helix (bHLH), HIF dimerization-binding region PER-ARNT-SIM (PAS), oxygen tension modulated N-terminal transactivation domain (N-TAD), and C-terminal transactivation domain (C-TAD). HIF-α subunit also contains oxygen-dependent degradation domain (ODDD).

#### **2.2. Oxygen-dependent hypoxia-inducible factor regulation**

further differentiate into mature myeloid cell to play its role in the hematopoietic system. In comparison, in CML, hematopoietic stem cell is preferred to differentiate myeloid cells (leukemia cells). CML is much milder due to these leukemia cells still partial functions to maintain homeostasis. However, the leukemia cells uncontrollably divide, build up in the bone marrow, and spill over into the blood. Over time, the cells settle in other parts of the body, especially in the spleen (causing splenomegaly), and it can also promote into a fast-growing AML. The American Cancer Society reports that incidence rates have increased over the past few decades, estimating that in 2015 about 20,830 new cases of AML and 14,620 new cases of CML were diagnosed, and 10,460 deaths from AML and 4650 deaths from CML would occur in the USA [1]. Currently, the majority of AML patients still have a poor prognosis, making the development of novel therapies a priority. Prognosis is influenced by a combination of cytogenetic and genetic characteristics of the disease, together with clinical features and the patient's age. In the albeit heterogeneous genetic landscape of myeloid leukemia, many myeloid leukemia patients exhibit recurrent mutations in genes encoding epigenetic regulators [2]. It is thus becoming increasingly clear that epigenetic dysfunction plays a key role in leukemogenesis of AML and CML [3]. More importantly, the epigenetic regulators CREB-binding protein (CBP) and p300 histone acetyltransferases (HATs), as important HIF co-transcriptional factors, facilitate leukemogenesis and represent therapeutic targets in AML [4]. Here, we have focused on the effect of dysregulated epigenetic programs in the development and maintenance of myeloid leukemia. In addition, we have discussed recent advances

Oxygen supply and consumption are tightly regulated and dynamically balanced in most normal tissues. However, supply and consumption of oxygen in tumor cells are usually decoupled due to the loss of physiological control and aberrant molecular signaling that provide malignant growth and survival advantages. Hypoxia appears in tumor cells when the metabolic demand for oxygen exceeds its extracellular availability. One of the main early cellular events responded upon hypoxia is activation of hypoxia-inducible factor 1 and 2 (HIF-1 and HIF-2), a critical heterodimeric transcription factor. HIF then in turn binds to hypoxiaresponsive elements (HREs), with the minimal core sequence 5'-CGTG-3', and influences the expression of various genes involved in angiogenesis, metabolism, pH regulation, prolifera-

The structure of HIF was identified as a dimer protein composed of HIF-1β and HIF-1α subunits. HIF-1β, the aryl hydrocarbon receptor nuclear translocator (ARNT), and its highly homologous protein ARNT2 and ARNT3, is constitutively expressed. All the three HIF-β subunits have the potential in forming dimers with various HIF-α subunits [5]. In normoxic conditions, HIF-1α is expressed ubiquitously at low closely balanced levels in all organs and has six different

in therapies specifically targeting these key epigenetic mechanisms.

tion, metastasis, and a wide range of other signaling processes.

**2.1. Structure of hypoxia-inducible factor and domain structure of α subunits**

**2. Hypoxia-inducible factor**

106 Myeloid Leukemia

The regulation of HIF by the extent of oxygen pressure is dependent on the intermediator that affects HIF-α protein stability and/or its ability to bind cofactors essential for transcriptional activity. In normoxia, HIF-α is strictly controlled by two types of oxygen sensors. First, 2-oxoglutarate (2-OG)-dependent prolyl hydroxylase domain (PHD) proteins could hydroxylate two prolyl residues (Pro402 and/or Pro<sup>564</sup>) in the N-TAD of HIF-1α ODDD regions (**Figure 1**) [8, 9]. Currently, three functional 2-OG-dependent PHD have been identified—PHD1, PHD2, and PHD3,—and all three require oxygen, Fe2+, and 2-OG as cofactors. This modification of HIF-α promotes its destruction by the proteasomal system through interaction with von Hippel-Lindau (VHL) protein, a component of an E3 ubiquitin ligase complex [10]. A second

**Figure 1.** HIF-1α regulation and HIF-1α dependent gene expression under hypoxia.

oxygen sensor called factor-inhibiting HIF-1 (FIH-1) involves hydroxylation of an asparagine residue (Asn803) in the C-TAD of HIF-1α, which also utilizes oxygen as a substrate [11]. The hydroxyl modification of Asn803 blocks the binding of the cofactor proteins) CREB-binding protein (CBP) and p300 thus inhibit HIF transcriptional activity. In hypoxia, HIF-α is regulated through "bicephalous" transcriptional nature in an FIH-dependent or FIH-independent manner [12]. In short, PHD has a lower affinity for oxygen than HIF and therefore is more rapidly inhibited. Consequently, genes require only the N-TAD to be induced. As oxygen decreases further, the inhibition of C-TAD is released and HIF-1α retains full transcription activity.

conversion of pyruvate to acetyl-CoA, TCA cycle, and mitochondrial biogenesis are inhibited through downregulation of pyruvate dehydrogenase kinase (PDK) 1 and 3 [17, 18]. Even though the glycolysis produces far less energy than TCA cycle per glucose molecule, it has a significant higher throughout. In addition, the accumulated by-products could be used as sources of carbon to produce nucleotides and lipids for proliferating cells [19]. The classic view of metabolism is that of a self-correction of homeostasis responding to microenvironment. In this model, for cancer to arise, tumor hypoxia selects cells depending on anaerobic metabolism [20]. Secondary mutations are needed to give cells the ability to transform the capability to alter existing cell metabolism in a way that supports cell growth. One example is that of mouse embryonic fibroblasts that reduce

The direct consequence of glycolysis is the production of lactic acid by hypoxic tumor cells leading to tumor acidosis. Intracellular acidosis poses a threat to cell survival. Readjusting intracellular pH (pHi) is a critical strategy to protect against apoptosis and cell death. HIF

by HIF. This reaction facilitates proton generation in the extracellular space, which contributes to acidification in tumor microenvironment, while preventing acidification of intercel-

Wadding first proposed the concept of epigenetics in 1915 and believed that the phenotypes generated from certain genotype within the scope of epigenetics. Meanwhile, he explained the expression of the genetic materials in the entire life process for the first time by using the concept of "Whole View" [25]. Holiday summarized with a more comprehensive explanation that "epigenetics refers to the changes of the gene expression manner with no difference and/ or change of heritage DNA sequence" [26]. Epigenetics is involved in individual development and the gene expression regulation in the biological process, however, it could also lead to

Histone acetylation, a process closely related to transcriptional activation is one of major epigenetic modifications. Histone acetylation occurs in the lysine residue located at the end of the histone N-terminal. Histone acetylation induces relaxed and/or higher-order structure of chromatins through transcriptional regulation [28]. In addition, the acetylated histone produces a signal that binds to the protein, producing intrinsic activities or capped-chromatin remodeling complexes, thereby promoting the transcriptional induction. Histone acetylation is mainly controlled by the inhibitory activities of histone acetyltransferases (HATs) and histone

to 1% O2

HIF-Regulated Leukemogenesis Through the Advances on Epigenetic Mechanism

[22, 23]. Moreover, two transmembrane carbonic anhydrases (CAs) catalyze

, suggesting HIF stable modified metabolic repro-

/H<sup>+</sup>

, CA IX, and XII overexpressed in tumors also regulated

, and continued low oxygen

http://dx.doi.org/10.5772/intechopen.75769

109

exchanger (NHE1) facilitate

oxygen consumption when switching from 20% O2

upregulated monocarboxylate transporter 4 (MCT4) and Na<sup>+</sup>

− and H+

**4. HIF regulation in epigenetic modification**

human diseases when it is interfered [27].

**4.1. Histone acetylation**

consumption when returning to 20% O2

gramming [21].

exportation of H+

to be hydrated to HCO3

lular milieu of cancer cell [24].

CO2

#### **2.3. Metabolic-dependent HIF regulation**

The metabolic intermediates are also the key regulators disrupting the hemostasis of HIF activates. As mentioned earlier, PHDs are 2-OG-dependent dioxygenases, catalyzing the conversion of a prolyl residue, molecular oxygen, 2-OG to hydroxyprolyl, carbon dioxide, and succinate using ferrous iron as cofactor. In addition, succinate also intermediates in the tricarboxylic acid (TCA) cycle catalyzed by succinate dehydrogenase (SDH) to fumarate in mitochondria. SDH dysfunction in cells raises the levels of succinate, which accumulates and leaks out to cytosol [13]. The increased level of succinate also inhibits PHDs activity due to accumulation succinate feedback, leading to the stabilization of HIF-α and activation of HIF complex. Similarly, deficiency of fumarate hydratase (FH) leads to accumulation of fumarate in the cytosol. Due to chemical similarity of fumarate to succinate, FH-deficient cells could also inhibit PHDs [14]. Other metabolic changes, such as diseases related to iron homeostasis, also crosstalk with HIF regulation. Hepcidin, a small polypeptide, plays a central role in regulating iron uptake. Iron demand in bone marrow increases when erythropoiesis is stimulated by hypoxia via increased erythropoietin (EPO) synthesis. Iron overload disease like hemochromatosis and iron decrease in anemia, feedback hepcidin production through VHL-HIF regulation [15].
