|Description|| Choline Kinase alpha (CHKA, though we have proposed to name it as ChoKα in order to distinguish it from check point kinase CHK) encodes two different isoforms. Choline Kinase alpha isoform a (ChoKαa) has 457 amino acid residues with a molecular mass of approximatively 52 kDa. Choline Kinase alpha isoform b (ChoKαb) has the same N- and C-termini but is shorter compared to isoform a, resulting in a variant of 439 amino acids and a molecular mass of approximatively 50 kDa. |
Both isoforms are active only in an oligomeric form (di- or tetrameric) and require ATP and Mg2+ for their catalytic activity (Wittenberg and Kornberg 1953).
Choline Kinase alpha isoform a (NM_001277) has been crystallized in complex with ADP and phosphocholine (referred in the paper as Choline Kinase alpha2). ATP binds in a cavity where residues from both de N and C-terminal lobes contribute to form a cleft, while the choline-binding site constitutes a deep hydrophobic groove in the C-terminal domain with a rim composed of negative charged residues. Upon binding of choline, the enzyme undergoes conformational changes independently affecting the N-terminal domain and the ATP binding loop (Malito et al. 2006).
Although much work has been made in other organisms (Paddon et al. 1982; Warden and Friedkin 1985; Kim and Carman 1999; Ramirez de Molina et al. 2002; Yu et al. 2002; Choi et al. 2005; Soto 2008), little is known about human ChoKα regulation.
It has been described that in HeLa cells, both EGF and insulin increase ChoK activity promoting the conversion of Cho to PCho, accompanied by an expansion of the PCho pool in treated cells (Uchida 1996). On the other hand, it has been suggested that Hypoxia-Inducible Factor-1α (HIF-1α) regulates ChoKα expression in a human prostate cancer model. An increase in cellular PCho and total Cho, as well as ChoK expression, has been observed following exposure of PC-3 cells to hypoxia. Furthermore, HIF-1α can directly bind to some putative hypoxia response elements (HRE) within ChoKα promoter, suggesting that HIF-1α activation of HREs within the putative ChoKα promoter region can increase ChoKα expression in hypoxic environments (Glunde et al. 2008).
|Expression|| Choline Kinase is expressed ubiquitously and concurrently (Aoyama et al. 2002). It is a vital enzyme, as homozygous ChoKa knock-out mice are lethal, indicating the indispensable role of ChoKα in early embryogenesis (Wu et al. 2008).|
|Localisation|| ChoKα is found in the cytoplasm.|
|Function|| Choline Kinase activation is necessary for membranes maintenance, cell growth and cell proliferation. It is also necessary for restoring phospholipids degraded during signal transduction. Consequently, ChoKα has an essential role in growth control and signal transduction and it has been implicated in the carcinogenic process.|
Role in metabolic process:
Choline Kinase is the first enzyme in the Kennedy pathway, responsible for de novo synthesis of phosphatidylcholine (PC), one of the major lipid components of plasma membranes in mammal cells, that is also essential for structural stability and cell proliferation. The Kennedy pathway consists of four steps. First Choline Kinase catalyzes choline phosphorylation, then phosphocholine (PCho) cytidylyl-transferase (CCT) catalyzes the formation of CDP-choline from PCho and CTP, and cholinephosphotransferase (CPT) catalyzes the final condensation reaction of CDP-choline with diacylglycerol (DAG) to generate PC. Finally, Phospholipase D (PLD) catalyses the hydrolysis of PC to generate phosphatidic acid (PA) and free choline.
ChoKα can also function as an ethanolamine kinase (EtnK) as it is able to phosphorylate ethanolamine. For a long time choline kinase and ethanolamine kinase have been considered as the same enzyme, because ChoK preparations of highly purified or recombinant enzymes from mammalian sources has been shown to have also a significant EtnK activity. Subsequently, separate genes that would encode EtnK-specific enzymes were identified (Aoyama et al. 2004).
Role in signal transduction, precursor of second messengers:
PC hydrolysis has been implicated in cell signalling. Due to the relative abundance of PC, its hydrolysis can sustain a prolonged liberation of catabolites without drastic changes in membrane phospholipids content. These long-lasting signals are thought to be important in the acquisition of the transformed phenotype. Under mitogenic stimulation by growth factors or oncogenic transformation, PLD-driven PC hydrolysis gives choline and phosphatidic acid (PA). PA can be hydrolyzed or deacylated to form DAG or lysophosphatidic acid (LPA) respectively, both with mitogenic activity. On the other hand, PCho generated from Cho by ChoK is an essential event for growth factors such as platelet-derived growth factor (PDGF) or fibroblast growth factor (FGF). Furthermore, it has been suggested a mitogenic role for PCho (Lacal 2001; Janardhan et al. 2006).
Role in the regulation of cell proliferation:
The accumulation of PC is necessary for the entrance of S phase of the cycle and cell division. It has been recently proposed that ChoKα participates in the regulation of G1-->S transition of the cell cycle at different levels (Ramirez de Molina et al. 2004). ChoKα over-expression induces the transcriptional regulation of genes involved in cell cycle such as p21, p27, and Cyclin D1 and Cyclin D3, whereas ChoKα specific inhibition reverses this effect on the regulation of cell cycle promoting genes. These results suggest the existence of ChoKα-driven co-regulated mechanism to maintain cell growth through the activation of G1-->S transition of the cell cycle (Ramirez de Molina et al. 2008).
Role in carcinogenesis:
PCho is an important lipid metabolite that is involved in cell proliferation as well as in tumorogenesis (Glunde et al. 2006). A role for ChoK in generation of human tumours has been reported. Studies with nuclear magnetic resonance (NMR) reveals elevated levels of PCho in human tumoral tissues in comparison with normal ones (Ruiz-Cabello and Cohen 1992; Smith et al. 1993). The generation of PCho through Kennedy pathway is considered to be one of the crucial steps in regulating growth factor stimulated cell proliferation, malignant transformation, invasion and metastasis (Lacal 2001; Rodriguez-Gonzalez et al. 2003; Glunde et al. 2006). Confirming the role of ChoK in the generation of PCho in the carcinogenic process, this enzyme has been recently described as a novel oncogene that potentiates the tumorogenic ability of other oncogenes such as RhoA (Ramirez de Molina et al. 2005).
ChoKα is over-expressed in different tumour-derived cell lines as well as in different human tumours including breast, lung, prostate and colorectal colon cancers (Ramirez de Molina et al. 2002; Ramirez de Molina et al. 2002). In addition to ChoKα over-expression, an increased enzymatic activity has been observed in human tumours such as breast (Ramirez de Molina et al. 2002) and colon cancer (Nakagami et al. 1999). Furthermore, ChoKα has been recently described as a new prognostic factor to predict patient outcome in early-stage non-small-cell lung cancer patients (Ramirez de Molina et al. 2007).
Consequently, ChoKα inhibition constitutes an efficient antitumour strategy with demonstrated antiproliferative activity in vitro and antitumoral activity in vivo (Hernandez-Alcoceba et al. 1997; Hernandez-Alcoceba et al. 1999). A dramatic difference in the response to MN58b, a specific ChoK inhibitor, has been observed between normal and tumour cells. Whereas blockage of de novo PCho synthesis by MN58b in primary cells induces pRb dephosphorylation and results in reversible cell cycle arrest in G0/G1 phase, tumour cells suffer a drastic wobble in the metabolism of main membrane lipids PC and sphingomyelin, resulting in a significant increase in the intracellular levels of ceramides that promotes cells to apoptosis (Rodriguez-Gonzalez et al. 2003; Rodriguez-Gonzalez et al. 2004; Rodriguez-Gonzalez et al. 2005).
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| Aoyama C, Liao H, Ishidate K.|
| Prog Lipid Res. 2004 May;43(3):266-81.|
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| Aoyama C, Ohtani A, Ishidate K.|
| Biochem J. 2002 May 1;363(Pt 3):777-84.|
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| Mol Pharm. 2006 Sep-Oct;3(5):496-506.|
| Hypoxia regulates choline kinase expression through hypoxia-inducible factor-1 alpha signaling in a human prostate cancer model.|
| Glunde K, Shah T, Winnard PT Jr, Raman V, Takagi T, Vesuna F, Artemov D, Bhujwalla ZM.|
| Cancer Res. 2008 Jan 1;68(1):172-80.|
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| Kim KH, Carman GM.|
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| Choline kinase: a novel target for antitumor drugs.|
| Lacal JC.|
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| Malito E, Sekulic N, Too WC, Konrad M, Lavie A.|
| J Mol Biol. 2006 Nov 24;364(2):136-51. Epub 2006 Sep 3.|
| Increased choline kinase activity and elevated phosphocholine levels in human colon cancer.|
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| Paddon HB, Vigo C, Vance DE.|
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| Choline kinase as a link connecting phospholipid metabolism and cell cycle regulation: Implications in cancer therapy.|
| Ramirez de Molina A, Gallego-Ortega D, Sarmentero-Estrada J, Lagares D, Gomez Del Pulgar T, Bandres E, Garcia-Foncillas J, Lacal JC.|
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| Inhibition of choline kinase renders a highly selective cytotoxic effect in tumour cells through a mitochondrial independent mechanism.|
| Rodriguez-Gonzalez A, Ramirez de Molina A, Banez-Coronel M, Megias D, Lacal JC.|
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| Phospholipid metabolites, prognosis and proliferation in human breast carcinoma.|
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| Regulation of the Saccharomyces cerevisiae CKI1-encoded Choline Kinase by Zinc Depletion.|
| Soto A, Carman GM.|
| J Biol Chem. 2008 Apr 11;283(15):10079-88. Epub 2008 Feb 14.|
| Stimulation of phospholipid synthesis in HeLa cells by epidermal growth factor and insulin: activation of choline kinase and glycerophosphate acyltransferase.|
| Uchida T.|
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| Regulation of choline kinase activity and phosphatidylcholine biosynthesis by mitogenic growth factors in 3T3 fibroblasts.|
| Warden CH, Friedkin M.|
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| Choline phosphokinase.|
| Wittenberg J, Kornberg A.|
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| Early embryonic lethality caused by disruption of the gene for choline kinase alpha, the first enzyme in phosphatidylcholine biosynthesis.|
| Wu G, Aoyama C, Young SG, Vance DE.|
| J Biol Chem. 2008 Jan 18;283(3):1456-62. Epub 2007 Nov 19.|
| Phosphorylation of Saccharomyces cerevisiae choline kinase on Ser30 and Ser85 by protein kinase A regulates phosphatidylcholine synthesis by the CDP-choline pathway.|
| Yu Y, Sreenivas A, Ostrander DB, Carman GM.|
| J Biol Chem. 2002 Sep 20;277(38):34978-86. Epub 2002 Jul 8.|