Expression of human choline kinase in NIH 3T3 fibroblasts increases the mitogenic potential of insulin and insulin-like growth factor I

Choline kinase-α protein and phosphatidylcholine but not phosphocholine are required for breast cancer cell survival

High levels of total choline and phosphocholine (PC) are consistently observed in aggressive cancers. Choline kinase (Chk) catalyzes choline phosphorylation to produce PC in phosphatidylcholine (PtdCho) biosynthesis. PtdCho is the most abundant phospholipid in eukaryotic cell membranes and plays a dual role as the structural component of membranes and as a substrate to produce lipid second messengers such as phosphatidic acid and diacylglycerol. Chk-α, but not Chk-β, is overexpressed in various cancers, and is closely associated with tumor progression and invasiveness. We have previously shown that downregulation of mRNA using small interfering RNA (siRNA) against Chk-α (siRNA-Chk) or Chk short hairpin RNA, and the resultant decrease of Chk-α protein levels, significantly reduced proliferation in breast cancer cells and tumors. A novel potent and selective small-molecule Chk-α inhibitor, V-11-0711, that inhibits the catalytic activity of Chk has recently been developed. Here, we used triple negative MDA-MB-231 and SUM149 breast cancer cells to further investigate the role of Chk-α in cancer, by examining Chk-α protein levels, cell viability/proliferation, choline phospholipid and lipid metabolism, lipid droplet formation, and apoptosis, following treatment with V-11-0711. Under the conditions used in this study, treatment with V-11-0711 significantly decreased PC levels but did not reduce cell viability as long as Chk-α protein and PtdCho levels were not reduced, suggesting that Chk-α protein and PtdCho, but not PC, may be crucial for breast cancer cell survival. These data also support the approach of antitumor strategies that destabilize Chk-α protein or downregulate PtdCho in breast cancer treatment.

The role of phospholipids in the biological activity and structure of the endoplasmic reticulum

The endoplasmic reticulum (ER) is an interconnected network of tubular and planar membranes that supports the synthesis and export of proteins, carbohydrates and lipids. Phospholipids, in particular phosphatidylcholine (PC), are synthesized in the ER where they have essential functions including provision of membranes required for protein synthesis and export, cholesterol homeostasis, and triacylglycerol storage and secretion. Coordination of these biological processes is essential, as highlighted by findings that link phospholipid metabolism in the ER with perturbations in lipid storage/secretion and stress responses, ultimately contributing to obesity/diabetes, atherosclerosis and neurological disorders. Phospholipid synthesis is not uniformly distributed in the ER but is localized at membrane interfaces or contact zones with other organelles, and in dynamic, proliferating ER membranes. The topology of phospholipid synthesis is an important consideration when establishing the etiology of diseases that arise from ER dysfunction. This review will highlight our current understanding of the contribution of phospholipid synthesis to proper ER function, and how alterations contribute to aberrant stress responses and disease. This article is part of a Special Issue entitled: Functional and structural diversity of endoplasmic reticulum.

MR evaluation of response to targeted treatment in cancer cells

The development of molecular technologies, together with progressive sophistication of molecular imaging methods, has allowed the further elucidation of the multiple mutations and dysregulatory effects of pathways leading to oncogenesis. Acting against these pathways by specifically targeted agents represents a major challenge for current research efforts in oncology. As conventional anatomically based pharmacological endpoints may be inadequate to monitor the tumor response to these targeted treatments, the identification and use of more appropriate, noninvasive pharmacodynamic biomarkers appear to be crucial to optimize the design, dosage and schedule of these novel therapeutic approaches. An aberrant choline phospholipid metabolism and enhanced flux of glucose derivatives through glycolysis, which sustain the redirection of mitochondrial ATP to glucose phosphorylation, are two major hallmarks of cancer cells. This review focuses on the changes detected in these pathways by MRS in response to targeted treatments. The progress and limitations of our present understanding of the mechanisms underlying MRS-detected phosphocholine accumulation in cancer cells are discussed in the light of gene and protein expression and the activation of different enzymes involved in phosphatidylcholine biosynthesis and catabolism. Examples of alterations induced in the MRS choline profile of cells exposed to different agents or to tumor environmental factors are presented. Current studies aimed at the identification in cancer cells of MRS-detected pharmacodynamic markers of therapies targeted against specific conditional or constitutive cell receptor stimulation are then reviewed. Finally, the perspectives of present efforts addressed to identify enzymes of the phosphatidylcholine cycle as possible novel targets for anticancer therapy are summarized.

A novel small molecule antagonist of choline kinase-α that simultaneously suppresses MAPK and PI3K/AKT signaling

Choline kinase-α expression and activity are increased in multiple human neoplasms as a result of growth factor stimulation and activation of cancer-related signaling pathways. The product of choline kinase-α, phosphocholine, serves as an essential metabolic reservoir for the production of phosphatidylcholine, the major phospholipid constituent of membranes and substrate for the production of lipid second messengers. Using in silico screening for small molecules that may interact with the choline kinase-α substrate binding domain, we identified a novel competitive inhibitor, N-(3,5-dimethylphenyl)-2-[[5-(4-ethylphenyl)-1H-1,2,4-triazol-3-yl]sulfanyl] acetamide (termed CK37) that inhibited purified recombinant human choline kinase-α activity, reduced the steady-state concentration of phosphocholine in transformed cells, and selectively suppressed the growth of neoplastic cells relative to normal epithelial cells. Choline kinase-α activity is required for the downstream production of phosphatidic acid, a promoter of several Ras signaling pathways. CK37 suppressed MAPK and PI3K/AKT signaling, disrupted actin cytoskeletal organization, and reduced plasma membrane ruffling. Finally, administration of CK37 significantly decreased tumor growth in a lung tumor xenograft mouse model, suppressed tumor phosphocholine, and diminished activating phosphorylations of ERK and AKT in vivo. Together, these results further validate choline kinase-α as a molecular target for the development of agents that interrupt Ras signaling pathways, and indicate that receptor-based computational screening should facilitate the identification of new classes of choline kinase-α inhibitors.

Involvement of human choline kinase alpha and beta in carcinogenesis: a different role in lipid metabolism and biological functions

We have summarized here the importance of ChoKα1 in human carcinogenesis. ChoKα1 displays its oncogenic activity through activation of specific signaling pathways that influence on cell proliferation and survival. It is overexpressed in a large number of human tumors with an incidence of 40-60% of all tumors investigated. Currently, there is an active effort in the development of strategies to knockdown the activity of ChoKα through specific siRNA or small molecules inhibitors. Results from genetic silencing or from treatment with MN58b, a well characterized ChoKα inhibitor showing antiproliferative and antitumoral effect in mice xenografts, provide strong support to this concept, indicating that the design of new antitumoral drugs must be selective against this isoform. However, affecting the other two known isoforms of ChoK may have also therapeutic consequences since the physiologically active form of ChoK may be constituted by homo or heterodimers. Furthermore, alteration of the ChoKβ activity might lead to a change in the lipid content of the cells of particular tissues such as skeletal muscle as described in the ChoKβ null mice (Sher et al., 2006). Finally, the identification of the ChoKα1 isoform as an excellent novel tool for the diagnosis and prognosis of cancer patients may have clinical consequences of immediate usefulness. On one hand, the use of specific monoclonal antibodies against ChoKα1 as a tool for diagnosis in paraffin embedded samples from patient biopsies, through standard immunohistochemistry techniques, can now be achieved (Gallego-Ortega et al., 2006). On the other hand, it has been recently described the prognostic value of determination of ChoKα1 expression levels in non-small cell lung cancer using real time quantitative PCR technology (Ramírez de Molina et al., 2007). Therefore, further research should be supported on the utility of ChoK isoforms as a promising area to improve cancer diagnosis and treatment.

Choline kinase overexpression increases invasiveness and drug resistance of human breast cancer cells

A direct correlation exists between increased choline kinase (Chk) expression, and the resulting increase of phosphocholine levels, and histological tumor grade. To better understand the function of Chk and choline phospholipid metabolism in breast cancer we have stably overexpressed one of the two isoforms of Chk-alpha known to be upregulated in malignant cells, in non-invasive MCF-7 human breast cancer cells. Dynamic tracking of cell invasion and cell metabolism were studied with a magnetic resonance (MR) compatible cell perfusion assay. The MR based invasion assay demonstrated that MCF-7 cells overexpressing Chk-alpha (MCF-7-Chk) exhibited an increase of invasion relative to control MCF-7 cells (0.84 vs 0.3). Proton MR spectroscopy studies showed significantly higher phosphocholine and elevated triglyceride signals in Chk overexpressing clones compared to control cells. A test of drug resistance in MCF-7-Chk cells revealed that these cells had an increased resistance to 5-fluorouracil and higher expression of thymidylate synthase compared to control MCF-7 cells. To further characterize increased drug resistance in these cells, we performed rhodamine-123 efflux studies to evaluate drug efflux pumps. MCF-7-Chk cells effluxed twice as much rhodamine-123 compared to MCF-7 cells. Chk-alpha overexpression resulted in MCF-7 human breast cancer cells acquiring an increasingly aggressive phenotype, supporting the role of Chk-alpha in mediating invasion and drug resistance, and the use of phosphocholine as a biomarker of aggressive breast cancers.

Photoactivable sphingosine as a tool to study membrane microenvironments in cultured cells

Human fibroblasts from normal subjects and Niemann-Pick A (NPA) disease patients were fed with two labeled metabolic precursors of sphingomyelin (SM), [(3)H]choline and photoactivable sphingosine, that entered into the biosynthetic pathway allowing the synthesis of radioactive phosphatidylcholine and SM, and of radioactive and photoactivable SM ([(3)H]SM-N(3)). Detergent resistant membrane (DRM) fractions prepared from normal and NPA fibroblasts resulted as highly enriched in [(3)H]SM-N(3). However, lipid and protein analysis showed strong differences between the two cell types. After cross-linking, different patterns of SM-protein complexes were found, mainly associated with the detergent soluble fraction of the gradient containing most cell proteins. After cell surface biotinylation, DRMs were immunoprecipitated using streptavidin. In conditions that maintain the integrity of domain, SM-protein complexes were detectable only in normal fibroblasts, whereas disrupting the membrane organization, these complexes were not recovered in the immunoprecipitate, suggesting that they involve proteins belonging to the inner membrane layer. These data suggest that differences in lipid and protein compositions of these cell lines determine specific lipid-protein interactions and different clustering within plasma membrane. In addition, our experiments show that photoactivable sphingolipids metabolically synthesized in cells can be used to study sphingolipid protein environments and sphingolipid-protein interactions.

Activation of choline kinase by extracellular Ca2+ is Ca(2+)-sensing receptor, Galpha12 and Rho-dependent in breast cancer cells

Breast cancer cell metastases to bone result in osteolysis and release of large quantities of Ca2+ into the bone microenviroment. Extracellular Ca2+ (Ca(o)2+) acting through the Ca(2+)-sensing receptor (CaR), a member of G protein-coupled receptor superfamily, plays an important role in the regulation of multiple signaling pathways. Here, we find that expression of the CaR and Galpha(12) is significantly up-regulated in breast cancer cells (MDA-MB-231 and MCF-7) compared with nonmalignant breast cells (Hs 578Bst and MCF-10A). Ca(o)2+ induces a significant increase in extracellular [(3)H]phosphocholine (P-cho) production in breast cancer cells. Using an anti-CaR antibody to block Ca(o)2+ binding to the CaR and small interfering RNA (siRNA) to silence CaR gene expression, our data demonstrate that [(3)H]P-cho production in response to Ca(o)(2+)-stimulation is CaR-dependent. By analyzing cellular lipid profiles and using siRNA to silence choline kinase (ChoK) expression, we determine that the production of [3H]P-cho is primarily related to CaR-induced ChoK activation, and not degradation of choline phospholipids. Finally, by pretreatment of the cells with either pertussis toxin or C3 exoenzyme, co-immunoprecipiation of Galpha(i), Galpha(q) or Galpha12 with the CaR, and RhoA translocation, we found that the enhancement of ChoK activation and P-cho production in breast cancer cells occurs via a CaR-Galpha12-Rho signaling pathway.

Hypoxia regulates choline kinase expression through hypoxia-inducible factor-1 alpha signaling in a human prostate cancer model

The intensity of the total choline (tCho) signal in spectroscopic images of tumors is spatially heterogeneous. The likewise heterogeneous physiologic tumor microenvironment may contribute to this heterogeneity. We therefore investigated the relationship between hypoxia, choline metabolites, and choline kinase (Chk) in a human prostate cancer model. Human PC-3 prostate cancer cells were engineered to express enhanced green fluorescent protein (EGFP) under hypoxic conditions. These PC-3-5HRE-EGFP cells were characterized in culture and as tumors transplanted in mice using (1)H magnetic resonance spectroscopy (MRS) and MRS imaging (MRSI) combined with EGFP fluorescence microscopy and imaging. Hypoxic EGFP-fluorescing tumor regions colocalized with regions of high tCho in combined MRSI and optical imaging studies. Cellular phosphocholine (PC) and tCho concentrations as well as Chk expression levels significantly increased following exposure of PC-3 cells to hypoxia. A putative promoter region located 5' of the translation start site of the human chk-alpha gene was cloned and luciferase (Luc)-based reporter vector constructs were generated. Luc reporter assays provided evidence that some of the putative hypoxia response elements (HRE) within this putative chk-alpha promoter region functioned in vitro. Chromatin immunoprecipitation assays using an antibody against hypoxia-inducible factor (HIF)-1 alpha showed that HIF-1 can directly bind this region of the endogenous chk-alpha promoter in hypoxic PC-3-5HRE-EGFP cells. These data suggest that HIF-1 activation of HREs within the putative chk-alpha promoter region can increase Chk-alpha expression within hypoxic environments, consequently increasing cellular PC and tCho levels within these environments.

Choline metabolism in cancer: implications for diagnosis and therapy

Magnetic resonance studies from the last 10 years have conclusively demonstrated that choline metabolism is altered in a wide variety of cancers. In cancer, the choline metabolite profile is characterized by an elevation of phosphocholine and total choline-containing compounds. This elevation is increasingly being used as an endogenous biomarker of cancer. Importantly, the enzymes and pathways resulting in these distinct alterations in phosphocholine and total choline may provide novel molecular targets for specific, targeted anticancer therapies. In this article, we have summarized some of the magnetic resonance spectroscopy and positron emission tomography techniques that are currently available, or will be in the near future, for choline metabolism-based diagnosis, staging and therapy assessment in cancer patients. This review also outlines currently known molecular alterations that cause the aberrant choline metabolite profile in cancers and concludes with a summary of recent research findings that may, in the future, lead to novel anticancer therapies targeting choline metabolism.

Therapeutic targets and biomarkers identified in cancer choline phospholipid metabolism

Choline phospholipid metabolism is altered in a wide variety of cancers. The choline metabolite profile of tumors and cancer cells is characterized by an elevation of phosphocholine and total choline-containing compounds. Noninvasive magnetic resonance spectroscopy can be used to detect this elevation as an endogenous biomarker of cancer, or as a predictive biomarker for monitoring tumor response to novel targeted therapies. The enzymes directly causing this elevation, such as choline kinase, phospholipase C and phospholipase D may provide molecular targets for anticancer therapies. Signal transduction pathways that are activated in cancers, such as those mediated by the receptor tyrosine kinases breakpoint cluster region-abelson (Bcr-Abl), c-KIT or epidermal growth factor receptor (EGFR), correlate with the alterations in choline phospholipid metabolism of cancers, and also offer molecular targets for specific anticancer therapies. This review summarizes recently discovered molecular targets in choline phospholipid metabolism and signal transduction pathways, which may lead to novel anticancer therapies potentially being monitored by magnetic resonance spectroscopy techniques.

Molecular characterization and localization of Plasmodium falciparum choline kinase

Generation of phosphocholine by choline kinase is important for phosphatidylcholine biosynthesis via Kennedy pathway and phosphatidylcholine biosynthesis is essential for intraerythrocytic growth of malaria parasite. A putative gene (Gene ID PF14_0020) in chromosome 14, having highest sequence homology with choline kinase, has been identified by BLAST searches from P. falciparum genome sequence database. This gene has been PCR amplified, cloned, over-expressed and characterized. Choline kinase activity of the recombinant protein (PfCK) was validated as it catalyzed the formation of phosphocholine from choline in presence of ATP. The K(m) values for choline and ATP are found to be 145+/-20 microM and 2.5+/-0.3 mM, respectively. PfCK can phosphorylate choline efficiently but not ethanolamine. Southern blotting indicates that PfCK is a single copy gene and it is a cytosolic protein as evidenced by Western immunoblotting and confocal microscopy. A model structure of PfCK was constructed based on the crystal structure of choline kinase of C. elegans to search the structural homology. Consistent with the homology modeling predictions, CD analysis indicates that the alpha and beta content of PfCK are 33% and 14%, respectively. Since choline kinase plays a vital role for growth and multiplication of P. falciparum during intraerythrocytic stages, we can suggest that this well characterized PfCK may be exploited in the screening of new choline kinase inhibitors to evaluate their antimalarial activity.

RNA interference-mediated choline kinase suppression in breast cancer cells induces differentiation and reduces proliferation

Choline kinase is overexpressed in breast cancer cells and activated by oncogenes and mitogenic signals, making it a potential target for cancer therapy. Here, we have examined, for the first time, the effects of RNA interference (RNAi)-mediated down-regulation of choline kinase in nonmalignant and malignant human breast epithelial cell lines using magnetic resonance spectroscopy (MRS) as well as molecular analyses of proliferation and differentiation markers. RNAi knockdown of choline kinase reduced proliferation, as detected by proliferating cell nuclear antigen and Ki-67 expression, and promoted differentiation, as detected by cytosolic lipid droplet formation and expression of galectin-3. The functional importance of RNAi-mediated choline kinase down-regulation on choline phospholipid metabolism was confirmed by the significant reduction of phosphocholine detected by MRS. These results strongly support the targeting of choline kinase in breast cancer cells with RNAi and show the potential ability of noninvasive MRS to detect and evaluate future treatments incorporating such strategies.

Noninvasive Magnetic Resonance Spectroscopic Pharmacodynamic Markers of the Choline Kinase Inhibitor MN58b in Human Carcinoma Models

MN58b is a novel anticancer drug that inhibits choline kinase, resulting in inhibition of phosphocholine synthesis. The aim of this work was to develop a noninvasive and robust pharmacodynamic biomarker for target inhibition and, potentially, tumor response following MN58b treatment. Human HT29 (colon) and MDA-MB-231 (breast) carcinoma cells were examined by proton (1H) and phosphorus (31P) magnetic resonance spectroscopy (MRS) before and after treatment with MN58b both in culture and in xenografts. An in vitro time course study of MN58b treatment was also carried out in MDA-MB-231 cells. In addition, enzymatic assays of choline kinase activity in cells were done. A decrease in phosphocholine and total choline levels (P < 0.05) was observed in vitro in both cell lines after MN58b treatment, whereas the inactive analogue ACG20b had no effect. In MDA-MB-231 cells, phosphocholine fell significantly as early as 4 hours following MN58b treatment, whereas a drop in cell number was observed at 48 hours. Significant correlation was also found between phosphocholine levels (measured by MRS) and choline kinase activities (r2 = 0.95, P = 0.0008) following MN58b treatment. Phosphomonoesters also decreased significantly (P < 0.05) in both HT29 and MDA-MB-231 xenografts with no significant changes in controls. 31P-MRS and 1H-MRS of tumor extracts showed a significant decrease in phosphocholine (P < or = 0.05). Inhibition of choline kinase by MN58b resulted in altered phospholipid metabolism both in cultured tumor cells and in vivo. Phosphocholine levels were found to correlate with choline kinase activities. The decrease in phosphocholine, total choline, and phosphomonoesters may have potential as noninvasive pharmacodynamic biomarkers for determining tumor response following treatment with choline kinase inhibitors.

[methyl-3H]Choline incorporation into MCF7 tumour cells: correlation with proliferation

PURPOSE

The aim of the study was to investigate the intracellular location of [methyl-(3)H]choline in MCF7 tumour cells and to determine the relationship between [methyl-(3)H]choline incorporation and proliferation.

METHODS

Tumour cells were incubated with [methyl-(3)H]choline for 10 min, and then in cold medium to simulate the rapid blood clearance of [methyl-(11)C]choline. Labelled metabolites were then extracted from cells by treating them with organic and aqueous solvents to determine the distribution of tracer between phospholipid and water-soluble metabolite pools. Aqueous extracts were subjected to thin-layer chromatography, ion exchange chromatography and a choline extraction procedure to identify (3)H-containing metabolites. Procedures were carried out on fast- and slow-growing populations of MCF7 cells to determine the relationship between choline incorporation and proliferation.

RESULTS

Only about 5% of [methyl-(3)H]choline was present as phospholipid. [methyl-(3)H]choline incorporation was found to be related to S-phase fraction. In another experiment, [methyl-(14)C]choline incorporation was found to be correlated with [methyl-(3)H]thymidine incorporation. The V(max) of choline uptake was found to be increased whilst K(m) was decreased in populations of MCF7 cells with higher proliferative fractions, compared with populations having lower proliferative fractions.

CONCLUSION

Choline incorporation into tumour cells under conditions that simulate rapid blood clearance of [methyl-(11)C]choline is correlated with proliferation. Most of the activity (about 95%) was in the non-lipid fraction of the cell.

Biochemical characterization of metastatic lymph nodes of breast cancer patients by in vitro 1H magnetic resonance spectroscopy: a pilot study

Using one-dimensional (1D) and two-dimensional (2D) proton nuclear magnetic resonance (NMR) methods, the perchloric acid extract of involved (n = 11) and noninvolved (n = 12) axillary lymph nodes (ALN) of breast cancer patients was investigated. Resonances from 40 metabolites such as lactate (Lac), glucose, several amino acids (alanine, lysine, glutamic acid, glutamine, etc.), nucleotides (adenosine triphosphate, guanosine triphosphate, uridine triphosphate, uridine monophosphate, etc.), membrane metabolites [glycerophosphocholine (GPC), phosphocoline (PC), phosphoethanolamine (PE), choline] were unambiguously assigned in both the involved and noninvolved ALN. The concentration of PC/GPC (p = 0.002) was significantly higher in the involved compared to noninvolved nodes. In addition, the concentration of glycolytic product Lac (p = 0.0001) was also found to be significantly higher in involved nodes. Increased concentration of membrane metabolites PC/GPC may be attributed to increased membrane synthesis in malignant cells and, therefore, suggests the presence of metastatic cells in lymph nodes. The higher concentration of Lac is indicative of the presence of malignant cells that derive energy via anaerobic glycolytic pathway. Present results demonstrate the potentials of in vitro proton NMR in detecting malignant cells in ALN and such studies may have an important bearing in determining the prognosis of breast cancer patients.

Loss of P53 Function in Colon Cancer Cells Results in Increased Phosphocholine and Total Choline

Mutations in the p53 gene are the most frequently observed genetic lesions in human cancers. Human cancers that contain a p53 mutation are more aggressive, more apt to metastasize, and more often fatal. p53 controls numerous downstream targets that can influence various outcomes such as apoptosis, growth arrest, and DNA repair. Based on previous observations using (1)H magnetic resonance spectroscopy (MRS), we have identified choline phospholipid metabolite intensities typical of increased malignancy. Here we have used (1)H MRS to characterize the choline phospholipid metabolite levels of p53(+/ +) and p53(-/-) cells, and demonstrated that loss of p53 function results in increased phosphocholine and total choline. These data suggest that the increased malignancy of cancer cells resulting from loss of p53 may be mediated, in part, through the choline phospholipid pathway.

Structure and function of choline kinase isoforms in mammalian cells

Choline kinase (CK) catalyzes the first phosphorylation reaction in the CDP-choline pathway for the biosynthesis of phosphatidylcholine (PC), yielding phosphocholine (P-Cho) from choline and ATP in the presence of Mg(2+). This enzyme exists in mammalian cells as at least three isoforms that are encoded by two separate genes termed ck-alpha and ck-beta. Each isoform is not active in its monomeric form. The active enzyme consists of either their homo- or hetero-dimeric (or oligomeric) forms. In recent years, the roles of CK in cell growth and cell stress/defense mechanisms have been intensely investigated. These functions of CK do not seem to be directly related to the net PC biosynthesis but predict another important role of this enzyme in certain cell physiology. This review summarizes briefly the recent progress of mammalian CK study which will include the gene structure of each isoform and its possible transcriptional regulation, the active configuration of the enzyme, induction of the particular isoform in chemically induced cell stress, and the possible role of this enzyme as well as of its reaction product, P-Cho, in cell growth and other cellular physiology.

Inhibition of choline kinase as a specific cytotoxic strategy in oncogene-transformed cells

Cancer treatment is in the need of selective drugs that can interfere specifically with signalling pathways affected during the carcinogenic process. Identification of new potential molecular targets is the key event in the design of new anticancer strategies. Once identified, attempts for the generation of specific molecules to regulate their function can be achieved. The relevance of deregulation of choline kinase (ChoK, E.C. 2.7.1.32) in oncogene-driven cell transformation has been previously demonstrated. Here we provide strong evidence that MN58b, a selective inhibitor of ChoK, is rather specific to this enzyme, with no effect on a variety of oncogene-activated signalling pathways involved in the regulation of cell proliferation. MN58b does not affect MAPKs, PI3K, and other enzymes involved in the regulation of phospholipid metabolism such as phospholipases C, D, and A2, CTP:phosphocholine cytidylyltransferase, or diacylglycerol choline-phosphotransferase. Consistent with this specificity, ectopic expression of ChoK resulted in resistance to its inhibitor. Finally, nontransformed cells were able to resume cell proliferation after removal of the drug, while transformed cells were irreversibly affected. These results indicate that inhibition of ChoK is a rather specific strategy for the cytotoxic treatment of transformed cells.

Choline Phospholipid Metabolites of Human Vascular Endothelial Cells Altered by Cyclooxygenase Inhibition, Growth Factor Depletion, and Paracrine Factors Secreted by Cancer Cells

Magnetic resonance studies have previously shown that solid tumors and cancer cells in culture typically exhibit high phosphocholine and total choline. Treatment of cancer cells with the anti-inflammatory agent, indomethacin (INDO), reverted the phenotype of choline phospholipid metabolites in cancer cells towards a less malignant phenotype. Since endothelial cells form a key component of tumor vasculature, in this study, we used MR spectroscopy to characterize the phenotype of choline phospholipid metabolites in human umbilical vein endothelial cells (HUVECs). We determined the effect of growth factors, the anti-inflammatory agent INDO, and conditioned media obtained from a malignant cell line, on choline phospholipid metabolites. Growth factor depletion or treatment with INDO induced similar changes in the choline phospholipid metabolites of HUVECs. Treatment with conditioned medium obtained from MDA-MB-231 cancer cells induced changes similar to the presence of growth factor supplements. These results suggest that cancer cells secrete growth factors and/or other molecules that influence the choline phospholipid metabolism of HUVECs. The ability of INDO to alter choline phospholipid metabolism in the presence of growth factor supplements suggests that the inflammatory response pathways of HUVECs may play a role in cancer cell-HUVEC interaction and in the response of HUVECs to growth factors.