Adenosine Kinase: An Epigenetic Modulator and Drug Target

Abstract

Adenosine kinase (ADK, EC: 2.7.1.20) is an evolutionarily ancient ribokinase, which acts as a metabolic regulator by transferring a phosphoryl group to adenosine to form AMP. The enzyme is of interest as a therapeutic target because its inhibition is one of the most effective means to raise the levels of adenosine and hence adenosine receptor activation. For these reasons, ADK has received significant attention in drug discovery efforts in the early 2000s for indications such as epilepsy, chronic pain, and inflammation; however, the report of adverse events regarding cardiovascular and hepatic function as well as instances of microhemorrhage in the brain of preclinical models prevented further development efforts. Recent findings emphasize the importance of compartmentalization of the adenosine system reflected by two distinct isoforms of the enzyme, ADK-S and ADK-L, expressed in the cytoplasm and the cell nucleus, respectively. Newly identified adenosine receptor independent functions of adenosine as a regulator of biochemical transmethylation reactions, which include DNA and histone methylation, identify ADK-L as a distinct therapeutic target for the regulation of the nuclear methylome. This newly recognized role of ADK-L as an epigenetic regulator points toward the potential disease-modifying properties of the next generation of ADK inhibitors. Continued efforts to develop therapeutic strategies to separate nuclear from extracellular functions of adenosine would enable the development of targeted therapeutics with reduced adverse event potential. This review will summarize recent advances in the discovery of novel ADK inhibitors and discuss their potential therapeutic use in conditions ranging from epilepsy to cancer.

Keywords: adenosine kinase; cancer; drug discovery; epigenetics; epilepsy; metabolism.

FIGURE 1

Main functions of adenosine receptors. A₁ and A₃ receptors mediate the inhibitory roles of adenosine, whereas A_2A and A_2B receptors mediate the excitatory/stimulatory roles of adenosine. A selection of clinically relevant effects achieved by receptor activation or inhibition is shown.

FIGURE 2

Compartmentalization of the adenosine system and major adenosine generating and consuming pathways. Extracellular purine turnover affects adenosine receptor (AR) activation, which plays important roles in immune suppression and angiogenesis, and includes adenosine‐producing (ecto‐5′‐nucleotidase/CD73) and ‐removing pathways (adenosine deaminase, ADA). Extra‐ and intracellular levels of adenosine are exchanged via equilibrative (ENT) and concentrative (CNT) nucleoside transporters. Intracellular adenosine metabolism depends on the cytoplasmic form of adenosine kinase (ADK‐S), and the enzymes ADA, cytosolic nucleotidase (cN‐I), and adenine phosphoribosyl transferase (APRTase). Hydrolysis of S‐adenosylhomocysteine (SAH) can be a major source for adenosine and homocysteine (HCy) by SAH hydrolase (SAHH), while S‐adenosylmethionine (SAM) serves as the donor of a methyl group in the transmethylation reactions catalyzed by methyltransferases (MT). One of the methyltransferases potentially affected by this mechanism is nicotinamide N‐methyltransferase (NNMT) whose inhibition has been linked to cancer. In the cell nucleus, adenosine is part of the transmethylation pathway, which adds methyl groups to DNA (DNA‐CH3) with DNA methyltransferase (DNMT). The nuclear form of adenosine kinase (ADK‐L) drives the flux of methyl groups through the pathway leading to increased DNA and histone methylation. For the sake of clarity, only the most important enzymes are mentioned.

FIGURE 3

DNA methylation catalyzed by DNA methyltransferases (DNMT) is biochemically linked to the methionine and adenosine cycles. Methyl groups are supplied by S‐adenosylmethionine (SAM), which after the donation of a methyl group to DNA, is converted into S‐adenosylhomocysteine (SAH). The SAH hydrolase (SAHH) reaction which can cleave SAH into adenosine (ADO) and homocysteine (HCY) is bidirectional, with the thermodynamic equilibrium on the site of SAH formation. SAH, in turn, is an inhibitor of DNMTs. The SAH reaction can proceed only toward cleavage and permit DNA methylation if the products ADO and HCY are effectively removed. SAH feeds into the methionine cycle and is converted by methionine synthase (MS) converts HCY into methionine (MET), which upon reaction with ATP, replenishes the pool of SAM. ADO is removed by adenosine kinase (ADK), which feeds adenosine back into the adenosine cycle, which replenishes ATP via AMP and ADP.

FIGURE 4

Simplified diagram showing the role of adenosine and ADK inhibitors in various disease states (ADK = adenosine kinase).