A mitochondria-K+ channel axis is suppressed in cancer and its normalization promotes apoptosis and inhibits cancer growth

Hypoxia-inducible factor 1 and dysregulated c-Myc cooperatively induce vascular endothelial growth factor and metabolic switches hexokinase 2 and pyruvate dehydrogenase kinase 1

Hypoxia is a pervasive microenvironmental factor that affects normal development as well as tumor progression. In most normal cells, hypoxia stabilizes hypoxia-inducible transcription factors (HIFs), particularly HIF-1, which activates genes involved in anaerobic metabolism and angiogenesis. As hypoxia signals a cellular deprivation state, HIF-1 has also been reported to counter the activity of MYC, which encodes a transcription factor that drives cell growth and proliferation. Since many human cancers express dysregulated MYC, we sought to determine whether HIF-1 would in fact collaborate with dysregulated MYC rather countering its function. Here, using the P493-6 Burkitt's lymphoma model with an inducible MYC, we demonstrate that HIF-1 cooperates with dysregulated c-Myc to promote glycolysis by induction of hexokinase 2, which catalyzes the first step of glycolysis, and pyruvate dehydrogenase kinase 1, which inactivates pyruvate dehydrogenase and diminishes mitochondrial respiration. We also found the collaborative induction of vascular endothelial growth factor (VEGF) by HIF-1 and dysregulated c-Myc. This study reports the previously unsuspected collaboration between HIF-1 and dysregulated MYC and thereby provides additional insights into the regulation of VEGF and the Warburg effect, which describes the propensity for cancer cells to convert glucose to lactate.

The cancer cell's "power plants" as promising therapeutic targets: an overview

This introductory article to the review series entitled "The Cancer Cell's Power Plants as Promising Therapeutic Targets" is written while more than 20 million people suffer from cancer. It summarizes strategies to destroy or prevent cancers by targeting their energy production factories, i.e., "power plants." All nucleated animal/human cells have two types of power plants, i.e., systems that make the "high energy" compound ATP from ADP and P( i ). One type is "glycolysis," the other the "mitochondria." In contrast to most normal cells where the mitochondria are the major ATP producers (>90%) in fueling growth, human cancers detected via Positron Emission Tomography (PET) rely on both types of power plants. In such cancers, glycolysis may contribute nearly half the ATP even in the presence of oxygen ("Warburg effect"). Based solely on cell energetics, this presents a challenge to identify curative agents that destroy only cancer cells as they must destroy both of their power plants causing "necrotic cell death" and leave normal cells alone. One such agent, 3-bromopyruvate (3-BrPA), a lactic acid analog, has been shown to inhibit both glycolytic and mitochondrial ATP production in rapidly growing cancers (Ko et al., Cancer Letts., 173, 83-91, 2001), leave normal cells alone, and eradicate advanced cancers (19 of 19) in a rodent model (Ko et al., Biochem. Biophys. Res. Commun., 324, 269-275, 2004). A second approach is to induce only cancer cells to undergo "apoptotic cell death." Here, mitochondria release cell death inducing factors (e.g., cytochrome c). In a third approach, cancer cells are induced to die by both apoptotic and necrotic events. In summary, much effort is being focused on identifying agents that induce "necrotic," "apoptotic" or apoptotic plus necrotic cell death only in cancer cells. Regardless how death is inflicted, every cancer cell must die, be it fast or slow.

Module-based outcome prediction using breast cancer compendia

Background

The availability of large collections of microarray datasets (compendia), or knowledge about grouping of genes into pathways (gene sets), is typically not exploited when training predictors of disease outcome. These can be useful since a compendium increases the number of samples, while gene sets reduce the size of the feature space. This should be favorable from a machine learning perspective and result in more robust predictors.

Methodology

We extracted modules of regulated genes from gene sets, and compendia. Through supervised analysis, we constructed predictors which employ modules predictive of breast cancer outcome. To validate these predictors we applied them to independent data, from the same institution (intra-dataset), and other institutions (inter-dataset).

Conclusions

We show that modules derived from single breast cancer datasets achieve better performance on the validation data compared to gene-based predictors. We also show that there is a trend in compendium specificity and predictive performance: modules derived from a single breast cancer dataset, and a breast cancer specific compendium perform better compared to those derived from a human cancer compendium. Additionally, the module-based predictor provides a much richer insight into the underlying biology. Frequently selected gene sets are associated with processes such as cell cycle, E2F regulation, DNA damage response, proteasome and glycolysis. We analyzed two modules related to cell cycle, and the OCT1 transcription factor, respectively. On an individual basis, these modules provide a significant separation in survival subgroups on the training and independent validation data.

Statin therapy, alone or with rapamycin, does not reverse monocrotaline pulmonary arterial hypertension: the rapamcyin-atorvastatin-simvastatin study

Pulmonary arterial hypertension (PAH) is characterized by excessive pulmonary artery smooth muscle cell proliferation and impaired apoptosis leading to obstruction of resistance pulmonary arteries. We hypothesized that antiproliferative (rapamycin) and proapoptotic (statins) agents, already used clinically for other indications, would decrease experimental PAH, facilitating translation to human therapies. Prior studies in the rat monocrotaline-PAH model have indicated that simvastatin regresses and rapamycin prevents, but cannot reverse, PAH. Two PAH regression strategies (rapamycin monotherapy vs. rapamycin + atorvastatin) and one prevention strategy (simvastatin) were tested in a rat monocrotaline-PAH model. Adult male Sprague-Dawley rats were randomized to saline ( n = 6) or monocrotaline (60 mg/kg ip, n = 36) treatment groups. Monocrotaline rats were randomized to gavage with vehicle, rapamycin (2.5 mg·kg−1·day−1), or rapamycin + atorvastatin (10 mg·kg−1·day−1) treatment groups, beginning 12 days post-monocrotaline. Echocardiographic and hemodynamic end points were assessed 2 wk later. Additional monocrotaline-PAH rats ( n = 20) were randomized to vehicle or simvastatin (2 mg·kg−1·day−1) treatment groups and followed echocardiographically for 4 wk. Monocrotaline-PAH increased lung p70 S6 kinase phosphorylation, and this was reversed by rapamycin, confirming the biological activity of rapamycin. Despite the use of high doses, neither rapamcyin nor rapamycin + atorvastatin improved survival nor reduced PAH, vascular remodeling, and right ventricular hypertrophy. Although prophylactic simvastatin slowed PAH progression, by 4 wk PAH severity and mortality were not different from placebo. Apart from the new finding of p70 S6 kinase phosphorylation in monocrotaline-PAH, this is a negative therapeutic trial (none of these promising therapies improved monocrotaline-PAH). These negative results should be considered as human trials with these agents are underway (simvastatin) or proposed (rapamycin).

The voltage dependent anion channel affects mitochondrial cholesterol distribution and function

We have observed abnormally high membrane cholesterol levels and a subsequent deficiency of oxidative energy production in mitochondria from cultured Morris hepatoma cells (MH7777). Using cholesterol affinity chromatography and MALDI-TOF Mass Spectrometry, we have identified the voltage dependent anion channel (VDAC) as a necessary component of a protein complex involved in mitochondrial membrane cholesterol distribution. VDAC is known to associate strongly with hexokinase, particularly in glycolytic cancers. By constructing an E72Q mutant form of VDAC that inhibits its binding of hexokinase, we report an increase in oxidative phosphorylation activity of MH7777 cells, as well as reduced membrane cholesterol ratios to levels near that of normal liver mitochondria. This paper demonstrates that the ability of VDAC to influence mitochondrial membrane cholesterol distribution may have implications on mitochondrial characteristics such as oxidative phosphorylation and induction of apoptosis, as well as the propensity of cancer cells to exhibit a glycolytic phenotype.

Hypoxia and adaptive landscapes in the evolution of carcinogenesis

Conceptual models of epithelial carcinogenesis typically depict a sequence of heritable changes that give rise to a population of cells possessing the hallmarks of invasive cancer. We propose the evolutionary dynamics that give rise to the phenotypic properties of malignant cells must be understood within the context of specific selection forces generated by the microenvironment. This can be accomplished by using an "inverse problem" approach in which we use observed typical phenotypic traits of primary and metastatic cancers to infer the evolutionary dynamics. This has led to the hypothesis that heritable changes in genes controlling cellular proliferation, apoptosis, and senescence, while necessary, are not usually sufficient to produce an invasive cancer. In addition to these evolutionary steps, we propose that the common observation of aerobic glycolysis in human cancers indicates, via the inverse problem analysis, that adaptation to hypoxia and acidosis must be a major component of the carcinogenic sequence. The details of the hypothesis are based on recognition that premalignant populations evolve within ducts and remain separated from their blood supply by a basement membrane. As tumor cells proliferate into the lumen, diffusion-reaction kinetics enforced by this separation result in hypoxia and acidosis in regions of the tumor the most distant from the basement membrane. This produces new evolutionary selection forces that promote constitutive upregulation of glycolysis and resistance to acid-induced toxicity. We hypothesize that these phenotypic adaptations are critical late steps in carcinogenesis conferring proliferative advantages even in normoxic conditions by allowing the population to produce an acidic environment (through aerobic glycolysis) which is toxic to other local cell populations and promotes extracellular matrix degradation, increasing invasiveness.

The use of glutathione transferase-knockout mice as pharmacological and toxicological models

ADME/Tox studies are of increasing importance because of the necessity to eliminate poor drug candidates early in the development pipeline. The glutathione S-transferases (GSTs) are a family of phase II enzymes that have been shown to play a significant role in the disposition of a wide range of drugs and other xenobiotics. Several GST-knockout mice strains have been developed that can potentially be used in ADME/Tox studies. So far, mice have been generated with deficiencies of mGSTP1/2, mGSTA4-4, mGSTZ1-1, mGSTM1-1, mGSTO1-1 and mGSTS1-1, but studies of drug metabolism in these strains have been limited. As there are 21 recognised GST genes in mice there is potential for many more strains to be made. However, a review of the available data suggests that because of differences in the evolution of the GST gene family between rodents and humans, only some knockout strains can provide insights relevant to human drug metabolism. It is concluded that, of the strains generated so far, only those deficient in mGSTP1-1, mGSTA4-4, mGSTO1-1 and mGSTZ1-1 have direct human orthologues and can be considered as human models. In contrast, there may not be appropriate orthologues of some enzymes, such as hGSTM1-1, that are known to be of relevance in drug metabolism.

Distinct structural mechanisms for inhibition of pyruvate dehydrogenase kinase isoforms by AZD7545, dichloroacetate, and radicicol

Pyruvate dehydrogenase kinase (PDK) isoforms are molecular switches that downregulate the pyruvate dehydrogenase complex (PDC) by reversible phosphorylation in mitochondria. We have determined structures of human PDK1 or PDK3 bound to the inhibitors AZD7545, dichloroacetate (DCA), and radicicol. We show that the trifluoromethylpropanamide end of AZD7545 projects into the lipoyl-binding pocket of PDK1. This interaction results in inhibition of PDK1 and PDK3 activities by aborting kinase binding to the PDC scaffold. Paradoxically, AZD7545 at saturating concentrations robustly increases scaffold-free PDK3 activity, similar to the inner lipoyl domain. Good DCA density is present in the helix bundle in the N-terminal domain of PDK1. Bound DCA promotes local conformational changes that are communicated to both nucleotide-binding and lipoyl-binding pockets of PDK1, leading to the inactivation of kinase activity. Finally, radicicol inhibits kinase activity by binding directly to the ATP-binding pocket of PDK3, similar to Hsp90 and Topo VI from the same ATPase/kinase superfamily.

The nuclear factor of activated T cells in pulmonary arterial hypertension can be therapeutically targeted

In pulmonary arterial hypertension (PAH), antiapoptotic, proliferative, and inflammatory diatheses converge to create an obstructive vasculopathy. A selective down-regulation of the Kv channel Kv1.5 has been described in human and animal PAH. The resultant increase in intracellular free Ca(2+) ([Ca(2+)](i)) and K(+) ([K(+)](i)) concentrations explains the pulmonary artery smooth muscle cell (PASMC) contraction, proliferation and resistance to apoptosis. The recently described PASMC hyperpolarized mitochondria and increased bcl-2 levels also contribute to apoptosis resistance in PAH. The cause of the Kv1.5, mitochondrial, and inflammatory abnormalities remains unknown. We hypothesized that these abnormalities can be explained in part by an activation of NFAT (nuclear factor of activated T cells), a Ca(2+)/calcineurin-sensitive transcription factor. We studied PASMC and lungs from six patients with and four without PAH and blood from 23 PAH patients and 10 healthy volunteers. Compared with normal, PAH PASMC had decreased Kv current and Kv1.5 expression and increased [Ca(2+)](i), [K(+)](i), mitochondrial potential (Delta Psi m), and bcl-2 levels. PAH but not normal PASMC and lungs showed activation of NFATc2. Inhibition of NFATc2 by VIVIT or cyclosporine restored Kv1.5 expression and current, decreased [Ca(2+)](i), [K(+)](i), bcl-2, and Delta Psi m, leading to decreased proliferation and increased apoptosis in vitro. In vivo, cyclosporine decreased established rat monocrotaline-PAH. NFATc2 levels were increased in circulating leukocytes in PAH versus healthy volunteers. CD3-positive lymphocytes with activated NFATc2 were seen in the arterial wall in PAH but not normal lungs. The generalized activation of NFAT in human and experimental PAH might regulate the ionic, mitochondrial, and inflammatory remodeling and be a therapeutic target and biomarker.

Amino acid residues responsible for the recognition of dichloroacetate by pyruvate dehydrogenase kinase 2

Dichloroacetate (DCA) is a promising anticancer and antidiabetic compound targeting the mitochondrial pyruvate dehydrogenase kinase (PDHK). This study was undertaken in order to map the DCA-binding site of PDHK2. Here, we present evidence that R114, S83, I157 and, to some extent, H115 are essential for DCA binding. We also show that Y80 and D117 are required for the communication between the DCA-binding site and active site of PDHK2. These observations provide important insights into the mechanism of DCA action that may be useful for the design of new, more potent therapeutic compounds.

p53: new roles in metabolism

Virtually all cancers show metabolic changes that result in upregulation of glycolysis and glucose consumption. Although discovered in the 1920s, how this glycolytic switch happens, and whether it is a cause or a consequence of the malignant process, has remained a matter of debate. The p53 tumor suppressor gene, discovered some 30 years ago, has a key role in preventing cancer development. Recent discoveries revealing new functions for p53 in the regulation of glucose metabolism and oxidative stress have brought together these two venerable fields of cancer biology. These activities of p53 appear to be key in tumor suppression, and shed some light on the pathways that underlie the metabolic changes in cancer cells.

Metabolic targeting as an anticancer strategy: dawn of a new era?

As a result of a spectrum of mitochondrial defects, tumor cells often preferentially use glycolysis to generate adenosine triphosphate (ATP), even in the presence of oxygen, a phenomenon known as aerobic glycolysis, or the "Warburg effect." Dichloroacetate (DCA) is an inhibitor of mitochondrial pyruvate dehydrogenase kinase (PDK), which inhibits pyruvate dehydrogenase (PDH), a gatekeeping enzyme for the entry of pyruvate into the mitochondrial tricarboxylic acid (TCA) cycle. In mice, DCA treatment appears to reactivate mitochondrial respiration in tumor cells, induces their selective killing, and suppresses cancer growth. These observations provide intriguing insights into the plasticity of tumor metabolism that may offer new opportunities for therapeutic intervention.

Iron granules in plasma cells

Resistance of cancer cells to chemotherapy is the first cause of cancer-associated death. Thus, new strategies to deal with the evasion of drug response and to improve clinical outcomes are needed. Genetic and epigenetic mechanisms associated with uncontrolled cell growth result in metabolism reprogramming. Cancer cells enhance anabolic pathways and acquire the ability to use different carbon sources besides glucose. An oxygen and nutrient-poor tumor microenvironment determines metabolic interactions among normal cells, cancer cells and the immune system giving rise to metabolically heterogeneous tumors which will partially respond to metabolic therapy. Here we go into the best-known cancer metabolic profiles and discuss several studies that reported tumors sensitization to chemotherapy by modulating metabolic pathways. Uncovering metabolic dependencies across different chemotherapy treatments could help to rationalize the use of metabolic modulators to overcome therapy resistance.