Current Pharmaceutical Biotechnology - Volume 14, Issue 3, 2013

High glucose uptake is a characteristic of most metastatic tumors and activation of Ras signaling in immortalized cells increases glycolytic flux into lactate, de novo nucleic acid synthesis and the tricarboxylic acid cycle, and increases NADH shuttling, oxygen consumption and uncoupling of ATP synthase from the proton gradient. Fructose-2,6- bisphosphate, C-Myc, HIF1α and AKT each have been found to be key regulators of glycolysis and to be controlled by Ras signaling, and there is abundant evidence for cross-talk between these regulators. The reprogramming of glycolytic and mitochondrial metabolism by Ras enables an integrated activation of energetic and anabolic pathways via the redox state of NADH that is required for the survival and growth of neoplastic cells in poorly vascularized tumors. Several small molecule antagonists specific for essential metabolic enzymes have been found to be selectively toxic to Ras-transformed cells as opposed to wild-type cells, indicating that this metabolic reprogramming and addiction may have utility for the development of anti-neoplastic agents.

Accumulated evidence suggests that p53 plays an important role in the regulation of metabolism and intracellular redox homeostasis through transcription-dependent and -independent mechanisms. Mitochondria, the power plant of cells, provide cells with ATP for their functions by regulating energy metabolism. In addition, as the byproducts of metabolism, reactive oxygen species (ROS) generated in the mitochondria can serve as signaling molecules to regulate p53 function. The regulation of p53 by mitochondria, especially redox-mediated regulation, may be involved in controlling the cellular switch between survival and death. The interplay between p53 and manganese superoxide dismutase (MnSOD), an important mitochondrial antioxidant enzyme, is an example of how nuclear and mitochondrial p53 coordinate their response to different levels of stress and contribute to the fate of cells.

Experimental and epidemiological evidence supports the idea that dietary fat and fiber influence colon carcinogenesis. Particularly, their components, n-3 polyunsaturated fatty acids (PUFAs) and butyrate, have been proven to exhibit beneficial effects on colon epithelial cell metabolism, signaling, and kinetics, thus preventing colon inflammation and cancer. Moreover, these effects may be strengthened by PUFA and butyrate combination. It appears that administration of these compounds might be a relatively nontoxic form of supportive therapy improving cancer treatment outcomes and slowing down or preventing recurrence of certain types of cancer. However, their efficient application has to be based on solid scientific evidence of their mechanisms of action from the molecular and cellular to the organismal level. In this review, we emphasize the role of lipids and their metabolism during tumor development, describe some important mechanisms considering cellular and molecular levels of PUFA and butyrate action in colon epithelial cells, and particularly focus on the interaction of their metabolism and the signaling pathways with respect to the differences in response of normal and cancer colon cells.

The proliferative cancer cell paradigm that has driven cancer drug development for the past 50 years has failed to generate treatments that cure most metastatic adult cancers. This view is supported not only by cumulative experience with conventional cytotoxic anticancer drugs, but also by the application of highly-targeted anticancer compounds against, for example, BCR-ABL in CML and mutant BRAF in metastatic melanoma. Such drugs often send their respective cancers into complete molecular remission but fail to effect cures because a small population of quiescent or slowly selfrenewing cancer cells that are drug and radiation resistant survive treatment indefinitely. This review explores the grounds for an emerging cancer paradigm that views cancer as a disorganized tissue with hierarchical cellular compartments in which the boudaries are less well-defined than in normal tissues with plasticity controlled by epigenetic changes mediated by the local microenvironment. Increased metabolic flexibility and adaptability give cancer cells an additional survival advantage that may be able to be targeted with drugs like metformin. Combining approaches that target the increased metabolic flexibility of cancer cells as well as ablating rapidly-proliferating cells and self-renewing cancer stem cells in individual cancers are needed to address the holy grail of cancer cure.

Positron emission tomography (PET) is a molecular imaging modality that provides the opportunity to rapidly and non-invasively visualize tumors derived from multiple organs. In order to do so, PET utilizes radiotracers, such as 18F-FDG and 11C-acetate, whose uptake coincides with altered metabolic pathways within tumors. Increased expression and activity of enzymes in the fatty acid synthesis pathway is a frequent hallmark of cancer cells. As a result, this pathway has become a prime target for therapeutic intervention. Although multiple drugs have been developed that both directly and indirectly interfere with fatty acid synthesis, an optimal means to assess their efficacy is lacking. Given that 11Cacetate is directly linked to the fatty acid synthesis pathway, this probe provides a unique opportunity to monitor lipogenic tumors by PET. Herein, we review the relevance of the fatty acid synthesis pathway in cancer. Furthermore, we address the potential utility of 11C-acetate PET in imaging tumors, especially those that are not FDG-avid. Last, we discuss several therapeutic interventions that could benefit from 11C-acetate PET to monitor therapeutic response in patients with certain types of cancers.

Oncogene-driven proliferative signaling in tumor cells requires comprehensive upregulation of cellular energy metabolism and macromolecule syntheses. These alterations are now known to include not only upregulated glycolysis, but also increased fatty acid metabolism, glutaminolysis, deregulated mitochondrial function and more. Many prospective targets for tumor-specific pharmacological modulation of metabolism have therefore been identified. While the prospective drugs do not necessarily show very high antitumor activity by themselves, they may by depriving tumor cells of energy and building blocks for repair and proliferation come to be of major clinical use as potentiators of standard chemotherapeutic drugs and/or radiation. To this end, not only inhibitors of specific enzyme functions are being investigated, but also drugs affecting the complex signaling networks that regulate organismal and cellular energy status. This review provides examples of how modulators of energy metabolism (MEMs) targetting different aspects of tumor cell metabolism have been found to potentiate cancer treatment in vitro and in vivo.

Mitochondria are known to play a key role in various cellular processes essential to both the life and death of cells, including calcium homeostasis, programmed cell death, and energy metabolism. Over 80 years ago, Otto Warburg discovered that in contrast to normal cells which produce most of their ATP via mitochondrial oxidative phosphorylation, cancer cells preferentially utilize glycolysis for production of ATP, a phenomenon known today as the "Warburg effect", and one which has been of great importance in the emergence of novel drugs and chemotherapeutic agents specifically targeting cancer cells. Several groups have reported in recent years that members of the plant stress hormones family of jasmonates, and some of their synthetic derivatives, exhibit anti-cancer activity in vitro and in vivo. Jasmonates have been shown to act directly on mitochondria of cancer cells, leading to mitochondrial swelling, membrane depolarization and cytochrome c release. Throughout the last few years, different groups have demonstrated that combination of jasmonates and various cytotoxic and chemotherapeutic agents yielded a synergistic cytotoxic effect. These results have been demonstrated in a variety of different cancer cell lines and may provide a strong basis for future clinical treatments which involve combination of MJ and different anti-cancerous agents. The potential synergistic effect may allow reduction of the administered dose, decrease of unwanted side effects, and reduction of the likelihood that the tumor will display resistance to the combined therapy.

Tumor cells show metabolic features distinctive from normal tissues, with characteristically enhanced aerobic glycolysis, glutaminolysis and lipid synthesis. Peroxisome proliferator activated receptor α (PPAR α) is activated by nutrients (fatty acids and their derivatives) and influences these metabolic pathways acting antagonistically to oncogenic Akt and c-Myc. Therefore PPAR α can be regarded as a candidate target molecule in supplementary anticancer pharmacotherapy as well as dietary therapeutic approach. This idea is based on hitting the cancer cell metabolic weak points through PPAR α mediated stimulation of mitochondrial fatty acid oxidation and ketogenesis with simultaneous reduction of glucose and glutamine consumption. PPAR α activity is induced by fasting and its molecular consequences overlap with the effects of calorie restriction and ketogenic diet (CRKD). CRKD induces increase of NAD+/NADH ratio and drop in ATP/AMP ratio. The first one is the main stimulus for enhanced protein deacetylase SIRT1 activity; the second one activates AMP-dependent protein kinase (AMPK). Both SIRT1 and AMPK exert their major metabolic activities such as fatty acid oxidation and block of glycolysis and protein, nucleotide and fatty acid synthesis through the effector protein peroxisome proliferator activated receptor gamma 1 α coactivator (PGC-1α). PGC-1α cooperates with PPAR α and their activities might contribute to potential anticancer effects of CRKD, which were reported for various brain tumors. Therefore, PPAR α activation can engage molecular interplay among SIRT1, AMPK, and PGC-1α that provides a new, low toxicity dietary approach supplementing traditional anticancer regimen.

Cytotoxic drugs in cancer therapy are used with the expectation of selectively killing and thereby eliminating the offending cancer cells. If they should die in an appropriate manner, the cells can also release danger signals that promote an immune reaction that reinforces the response against the cancer. The identity of these immune-enhancing danger signals, how they work extra- and intracellularly, and the molecular mechanisms by which some anti-cancer drugs induce cell death to bring about the release of danger signals are the major focus of this review. A specific group of mitocans, the vitamin E analogs that act by targeting mitochondria to drive ROS production and also promote a more immunogenic means of cancer cell death exemplify such anti-cancer drugs. The role of reactive oxygen species (ROS) production and the events leading to the activation of the inflammasome and pro-inflammatory mediators induced by dying cancer cell mitochondria are discussed along with the evidence for their contribution to promoting immune responses against cancer. Current knowledge of how the danger signals interact with immune cells to boost the anti-tumor response is also evaluated.

Treatment of cancer is by no means universally successful and often manifests harmful side effects. The best way to improve the success rate and reduce the side effects would be to develop compounds that are able to kill cancer cells while leaving normal cells unaffected. In this respect, mitocans (an acronym from ‘mitochondria’ and ‘cancer’), a summary term we proposed for compounds that induce cell death by targeting mitochondria, show an encouraging trend. Here we provide an overview of mitocans specific for the mitochondrial electron transport chain. These mitocans are particularly interesting, because a frequent consequence of electron transport chain inhibition is the induction of superoxide formation resulting in the preferential killing of cancer cells, as these tend to be more sensitive than normal cells to sudden increases in oxidative stress. Furthermore, macromolecular complexes of the electron transport chain only rarely mutate in cancer, and represent useful targets for anti-cancer drug development when widely-applicable agents are sought.