Brian Clem

Small-molecule inhibition of 6-phosphofructo-2-kinase activity suppresses glycolytic flux and tumor growth

6-Phosphofructo-1-kinase, a rate-limiting enzyme of glycolysis, is activated in neoplastic cells by fructose-2,6-bisphosphate (Fru-2,6-BP), a product of four 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase isozymes (PFKFB1-4). The inducible PFKFB3 isozyme is constitutively expressed by neoplastic cells and required for the high glycolytic rate and anchorage-independent growth of ras-transformed cells. We report herein the computational identification of a small-molecule inhibitor of PFKFB3, 3-(3-pyridinyl)-1-(4-pyridinyl)-2-propen-1-one (3PO), which suppresses glycolytic flux and is cytostatic to neoplastic cells. 3PO inhibits recombinant PFKFB3 activity, suppresses glucose uptake, and decreases the intracellular concentration of Fru-2,6-BP, lactate, ATP, NAD+, and NADH. 3PO markedly attenuates the proliferation of several human malignant hematopoietic and adenocarcinoma cell lines (IC50, 1.4-24 μmol/L) and is selectively cytostatic to ras-transformed human bronchial epithelial cells relative to normal human bronchial epithelial cells. The PFKFB3 enzyme is an essential molecular target of 3PO because transformed cells are rendered resistant to 3PO by ectopic expression of PFKFB3 and sensitive to 3PO by heterozygotic genomic deletion of PFKFB3. Importantly, i.p. administration of 3PO (0.07 mg/g) to tumor-bearing mice markedly reduces the intracellular concentration of Fru-2,6-BP, glucose uptake, and growth of established tumors in vivo. Taken together, these data support the clinical development of 3PO and other PFKFB3 inhibitors as chemotherapeutic agents. [Mol Cancer Ther 2008;7(1):110–20]

Regulation of glucose metabolism by 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatases in cancer.

A high rate of glycolytic flux, even in the presence of oxygen, is a central metabolic hallmark of neoplastic tumors. Cancer cells preferentially utilize glycolysis in order to satisfy their increased energetic and biosynthetic requirements. This metabolic phenotype has been confirmed in human studies using positron emission tomography (PET) with (18)F-2-fluoro-deoxy-glucose which have demonstrated that tumors take up 10-fold more glucose than adjacent normal tissues in vivo. The high glucose metabolism of cancer cells is caused by a combination of hypoxia-responsive transcription factors, activation of oncogenic proteins and the loss of tumor suppressor function. Over-expression of HIF-1alpha and myc, activation of ras and loss of p53 function each have been found to stimulate glycolysis in part by activating a family of regulatory bifunctional 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatases (PFKFB). The PFKFB enzymes synthesize fructose-2,6-bisphosphate (F2,6BP) which allosterically activates 6-phosphofructo-1-kinase (PFK-1), a rate-limiting enzyme and essential control point in the glycolytic pathway. PFK-1 is inhibited by ATP when energy stores are abundant and F2,6BP can override this inhibition and enhance glucose uptake and glycolytic flux. It is therefore not surprising that F2,6BP synthesis is stimulated by several oncogenic alterations which simultaneously cause both enhanced consumption of glucose and growth. Importantly, these studies suggest that selective depletion of intracellular F2,6BP in cancer cells may suppress glycolytic flux and decrease their survival, growth and invasiveness. This review will summarize the requirement of the 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatases for the regulation of glycolysis in tumor cells and their potential utility as targets for the development of antineoplastic agents.

Targeting aspartate aminotransferase in breast cancer

IntroductionGlycolysis is increased in breast adenocarcinoma cells relative to adjacent normal cells in order to produce the ATP and anabolic precursors required for survival, growth and invasion. Glycolysis also serves as a key source of the reduced form of cytoplasmic nicotinamide adenine dinucleotide (NADH) necessary for the shuttling of electrons into mitochondria for electron transport. Lactate dehydrogenase (LDH) regulates glycolytic flux by converting pyruvate to lactate and has been found to be highly expressed in breast tumours. Aspartate aminotransferase (AAT) functions in tandem with malate dehydrogenase to transfer electrons from NADH across the inner mitochondrial membrane. Oxamate is an inhibitor of both LDH and AAT, and we hypothesised that oxamate may disrupt the metabolism and growth of breast adenocarcinoma cells.MethodsWe examined the effects of oxamate and the AAT inhibitor amino oxyacetate (AOA) on 13C-glucose utilisation, oxygen consumption, NADH and ATP in MDA-MB-231 cells. We then determined the effects of oxamate and AOA on normal human mammary epithelial cells and MDA-MB-231 breast adenocarcinoma cell proliferation, and on the growth of MDA-MB-231 cells as tumours in athymic BALB/c female mice. We ectopically expressed AAT in MDA-MB-231 cells and examined the consequences on the cytostatic effects of oxamate. Finally, we examined the effect of AAT-specific siRNA transfection on MDA-MB-231 cell proliferation.ResultsWe found that oxamate did not attenuate cellular lactate production as predicted by its LDH inhibitory activity, but did have an anti-metabolic effect that was similar to AAT inhibition with AOA. Specifically, we found that oxamate and AOA decreased the flux of 13C-glucose-derived carbons into glutamate and uridine, both products of the mitochondrial tricarboxylic acid cycle, as well as oxygen consumption, a measure of electron transport chain activity. Oxamate and AOA also selectively suppressed the proliferation of MDA-MB-231 cells relative to normal human mammary epithelial cells and decreased the growth of MDA-MB-231 breast tumours in athymic mice. Importantly, we found that ectopic expression of AAT in MDA-MB-231 cells conferred resistance to the anti-proliferative effects of oxamate and that siRNA silencing of AAT decreased MDA-MB-231 cell proliferation.ConclusionsWe conclude that AAT may be a valid molecular target for the development of anti-neoplastic agents.

Nuclear Targeting of 6-Phosphofructo-2-kinase (PFKFB3) Increases Proliferation via Cyclin-dependent Kinases

The regulation of metabolism and growth must be tightly coupled to guarantee the efficient use of energy and anabolic substrates throughout the cell cycle. Fructose 2,6-bisphosphate (Fru-2,6-BP) is an allosteric activator of 6-phosphofructo-1-kinase (PFK-1), a rate-limiting enzyme and essential control point in glycolysis. The concentration of Fru-2,6-BP in mammalian cells is set by four 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatases (PFKFB1–4), which interconvert fructose 6-phosphate and Fru-2,6-BP. The relative functions of the PFKFB3 and PFKFB4 enzymes are of particular interest because they are activated in human cancers and increased by mitogens and low oxygen. We examined the cellular localization of PFKFB3 and PFKFB4 and unexpectedly found that whereas PFKFB4 localized to the cytoplasm (i.e. the site of glycolysis), PFKFB3 localized to the nucleus. We then overexpressed PFKFB3 and observed no change in glucose metabolism but rather a marked increase in cell proliferation. These effects on proliferation were completely abrogated by mutating either the active site or nuclear localization residues of PFKFB3, demonstrating a requirement for nuclear delivery of Fru-2,6-BP. Using protein array analyses, we then found that ectopic expression of PFKFB3 increased the expression of several key cell cycle proteins, including cyclin-dependent kinase (Cdk)-1, Cdc25C, and cyclin D3 and decreased the expression of the cell cycle inhibitor p27, a universal inhibitor of Cdk-1 and the cell cycle. We also observed that the addition of Fru-2,6-BP to HeLa cell lysates increased the phosphorylation of the Cdk-specific Thr-187 site of p27. Taken together, these observations demonstrate an unexpected role for PFKFB3 in nuclear signaling and indicate that Fru-2,6-BP may couple the activation of glucose metabolism with cell proliferation.

Targeting 6-Phosphofructo-2-Kinase (PFKFB3) as a Therapeutic Strategy against Cancer

In human cancers, loss of PTEN, stabilization of hypoxia inducible factor-1α, and activation of Ras and AKT converge to increase the activity of a key regulator of glycolysis, 6-phosphofructo-2-kinase (PFKFB3). This enzyme synthesizes fructose 2,6-bisphosphate (F26BP), which is an activator of 6-phosphofructo-1-kinase, a key step of glycolysis. Previously, a weak competitive inhibitor of PFKFB3, 3-(3-pyridinyl)-1-(4-pyridinyl)-2-propen-1-one (3PO), was found to reduce the glucose metabolism and proliferation of cancer cells. We have synthesized 73 derivatives of 3PO and screened each compound for activity against recombinant PFKFB3. One small molecule, 1-(4-pyridinyl)-3-(2-quinolinyl)-2-propen-1-one (PFK15), was selected for further preclinical evaluation of its pharmacokinetic, antimetabolic, and antineoplastic properties in vitro and in vivo. We found that PFK15 causes a rapid induction of apoptosis in transformed cells, has adequate pharmacokinetic properties, suppresses the glucose uptake and growth of Lewis lung carcinomas in syngeneic mice, and yields antitumor effects in three human xenograft models of cancer in athymic mice that are comparable to U.S. Food and Drug Administration–approved chemotherapeutic agents. As a result of this study, a synthetic derivative and formulation of PFK15 has undergone investigational new drug (IND)-enabling toxicology and safety studies. A phase I clinical trial of its efficacy in advanced cancer patients will initiate in 2013 and we anticipate that this new class of antimetabolic agents will yield acceptable therapeutic indices and prove to be synergistic with agents that disrupt neoplastic signaling. Mol Cancer Ther; 12(8); 1461–70. ©2013 AACR.

Control of Glutamine Metabolism By the Tumor Suppressor Rb

Retinoblastoma (Rb) protein is a tumor suppressor that is dysregulated in a majority of human cancers. Rb functions to inhibit cell cycle progression in part by directly disabling the E2F family of cell cycle-promoting transcription factors. Because the de novo synthesis of multiple glutamine-derived anabolic precursors is required for cell cycle progression, we hypothesized that Rb also may directly regulate proteins involved in glutamine metabolism. We examined glutamine metabolism in mouse embryonic fibroblasts (MEFs) isolated from mice that have triple knock-outs (TKO) of all three Rb family members (Rb-1, Rbl1 and Rbl2) and found that loss of global Rb function caused a marked increase in 13C-glutamine uptake and incorporation into glutamate and tricarboxylic acid cycle (TCA) intermediates in part via upregulated expression of the glutamine transporter ASCT2 and the activity of glutaminase 1 (GLS1). The Rb-controlled transcription factor E2F-3 altered glutamine uptake by direct regulation of ASCT2 mRNA and protein expression, and E2F-3 was observed to associate with the ASCT2 promoter. We next examined the functional consequences of the observed increase in glutamine uptake and utilization and found that glutamine exposure potently increased oxygen consumption, whereas glutamine deprivation selectively decreased ATP concentration in the Rb TKO MEFs but not the wild-type (WT) MEFs. In addition, TKO MEFs exhibited elevated production of glutathione from exogenous glutamine and had increased expression of gamma-glutamylcysteine ligase relative to WT MEFs. Importantly, this metabolic shift towards glutamine utilization was required for the proliferation of Rb TKO MEFs but not for the proliferation of the WT MEFs. Last, addition of the TCA cycle intermediate α-ketoglutarate to the Rb TKO MEFs reversed the inhibitory effects of glutamine deprivation on ATP, GSH levels and viability. Taken together, these studies demonstrate that the Rb/E2F cascade directly regulates a major energetic and anabolic pathway that is required for neoplastic growth.

Mouse Fibroblasts Lacking RB1 Function Form Spheres and Undergo Reprogramming to a Cancer Stem Cell Phenotype

Activation of the RB1 pathway triggers the cell-cycle arrest that mediates cell-cell contact inhibition. Accordingly, mutation of all three RB1 family members leads to loss of contact inhibition and outgrowth of fibroblasts into spheres where cell-cell contacts predominate. We present evidence that such outgrowth triggers reprogramming to generate cells with properties of cancer stem cells. Fibroblasts with only a single RB1 mutation remain contact inhibited; however, if this contact inhibition is bypassed by forcing the RB1(-/-) cells to form spheres in suspension, cells with properties of cancer stem cells are also generated. These cells not only form tumors in nude mice but also generate differentiated cells. We propose that contact inhibition imposed by the RB1 pathway performs an unexpected tumor suppressor function by preventing cell outgrowth into structures where cells with properties of cancer stem cells can be generated from differentiated somatic cells in advancing cancers.

Selective inhibition of choline kinase simultaneously attenuates MAPK and PI3K/AKT signaling.

Choline is an essential anabolic substrate for the synthesis of phospholipids. Choline kinase phosphorylates choline to phosphocholine that serves as a precursor for the production of phosphatidylcholine, the major phospholipid constituent of membranes and substrate for the synthesis of lipid signaling molecules. Nuclear magnetic resonance (NMR)-based metabolomic studies of human tumors have identified a marked increase in the intracellular concentration of phosphocholine relative to normal tissues. We postulated that the observed intracellular pooling of phosphocholine may be required to sustain the production of the pleiotropic lipid second messenger, phosphatidic acid. Phosphatidic acid is generated from the cleavage of phosphatidylcholine by phospholipase D2 and is a key activator of the mitogen-activated protein kinase (MAPK) and phosphatidylinositol 3-kinase (PI3K)/AKT survival signaling pathways. In this study we show that the steady-state concentration of phosphocholine is increased by the ectopic expression of oncogenic H-RasV12 in immortalized human bronchial epithelial cells. We then find that small interfering RNA (siRNA) silencing of choline kinase expression in transformed HeLa cells completely abrogates the high concentration of phosphocholine, which in turn decreases phosphatidylcholine, phosphatidic acid and signaling through the MAPK and PI3K/AKT pathways. This simultaneous reduction in survival signaling markedly decreases the anchorage-independent survival of HeLa cells in soft agar and in athymic mice. Last, we confirm the relative importance of phosphatidic acid for this pro-survival effect as phosphatidic acid supplementation fully restores MAPK signaling and partially rescues HeLa cells from choline kinase inhibition. Taken together, these data indicate that the pooling of phosphocholine in cancer cells may be required to provide a ready supply of phosphatidic acid necessary for the feed-forward amplification of cancer survival signaling pathways.

6-Phosphofructo-2-kinase (PFKFB3) promotes cell cycle progression and suppresses apoptosis via Cdk1-mediated phosphorylation of p27

The control of glucose metabolism and the cell cycle must be coordinated in order to guarantee sufficient ATP and anabolic substrates at distinct phases of the cell cycle. The family of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatases (PFKFB1-4) are well established regulators of glucose metabolism via their synthesis of fructose-2,6-bisphosphate (F2,6BP), a potent allosteric activator of 6-phosphofructo-1-kinase (Pfk-1). PFKFB3 is overexpressed in human cancers, regulated by HIF-1α, Akt and PTEN, and required for the survival and growth of multiple cancer types. Although most functional studies of the role of PFKFB3 in cancer progression have invoked its well-recognized function in the regulation of glycolysis, recent observations have established that PFKFB3 also traffics to the nucleus and that its product, F2,6BP, activates cyclin-dependent kinases (Cdks). In particular, F2,6BP stimulates the Cdk-mediated phosphorylation of the Cip/Kip protein p27 (threonine 187), which in turn results in p27’s ubiquitination and proteasomal degradation. As p27 is a potent suppressor of the G1/S transition and activator of apoptosis, we hypothesized that the known requirement of PFKFB3 for cell cycle progression and prevention of apoptosis may be partly due to the ability of F2,6BP to activate Cdks. In this study, we demonstrate that siRNA silencing of endogenous PFKFB3 inhibits Cdk1 activity, which in turn stabilizes p27 protein levels causing cell cycle arrest at G1/S and increased apoptosis in HeLa cells. Importantly, we demonstrate that the increase in apoptosis and suppression of the G1/S transition caused by siRNA silencing of PFKFB3 expression is reversed by co-siRNA silencing of p27. Taken together with prior publications, these observations support a model whereby PFKFB3 and F2,6BP function not only as regulators of Pfk-1 but also of Cdk1 activity, and therefore serve to couple glucose metabolism with cell proliferation and survival in transformed cells.

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 mitogen-activated protein kinase and phosphatidylinositol 3-kinase/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 extracellular signal-regulated kinase 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.