Is cancer a disease of abnormal cellular metabolism? New angles on an old idea

Glycolytic pyruvate regulates P-Glycoprotein expression in multicellular tumor spheroids via modulation of the intracellular redox state

ABC transporters like P-glycoprotein (P-gp/ABCB1) are membrane proteins responsible for the transport of toxic compounds out of non-malignant cells and tumor tissue.

AIM

To investigate the effect of glycolysis and the tissue redox state on P-gp expression in multicellular tumor spheroids derived from prostate adenocarcinoma cells (DU-145), glioma cells (Gli36), and the human cervix carcinoma cell line KB-3-1 transfected with a P-gp-EGFP fusion gene that allows monitoring of P-gp expression in living cells. During cell culture of DU-145, Gli36, and KB-3-1 tumor spheroids P-gp expression was observed as well as increased lactate and decreased pyruvate levels and expression of glycolytic enzymes. Inhibition of glycolysis for 24 h by either iodoacetate (IA) or 2-deoxy-D-glucose (2-DDG) downregulated P-gp expression which was reversed upon coincubation with the radical scavenger ebselen as shown by semi-quantitative immunohistochemisty in DU-145 and Gli36 tumor spheroids, and by EGFP fluorescence in KB-3-1 tumor spheroids. Consequently endogenous ROS generation in DU-145 tumor spheroids was increased in the presence of either IA or 2-DDG, which was abolished upon coincubation with ebselen. Exogenous addition of pyruvate significantly reduced ROS generation, increased P-gp expression as well as efflux of the P-gp substrate doxorubicin. Doxorubicin transport was significantly blunted by 2-DDG and IA, indicating that inhibition of glycolysis reversed the multidrug resistance phenotype. In summary our data demonstrate that P-gp expression in tumor spheroids is closely related to the glycolytic metabolism of tumor cells and can be downregulated by glycolysis inhibitors via mechanisms that involve changes in the cellular redox state.

Mitochondrial dysfunction in neurodegenerative diseases and cancer

Mitochondria are important integrators of cellular function and therefore affect the homeostatic balance of the cell. Besides their important role in producing adenosine triphosphate through oxidative phosphorylation, mitochondria are involved in the control of cytosolic calcium concentration, metabolism of key cellular intermediates, and Fe/S cluster biogenesis and contributed to programmed cell death. Mitochondria are also one of the major cellular producers of reactive oxygen species (ROS). Several human pathologies, including neurodegenerative diseases and cancer, are associated with mitochondrial dysfunction and increased ROS damage. This article reviews how dysfunctional mitochondria contribute to Alzheimer's disease, Parkinson's disease, Huntington's disease, and several human cancers.

O-GlcNAc signaling: a metabolic link between diabetes and cancer?

O-linked β-N-acetylglucosamine (O-GlcNAc) is a sugar attachment to serine or threonine hydroxyl moieties on nuclear and cytoplasmic proteins. In many ways, O-GlcNAcylation is similar to phosphorylation because both post-translational modifications cycle rapidly in response to internal or environmental cues. O-GlcNAcylated proteins are involved in transcription, translation, cytoskeletal assembly, signal transduction, and many other cellular functions. O-GlcNAc signaling is intertwined with cellular metabolism; indeed, the donor sugar for O-GlcNAcylation (UDP-GlcNAc) is synthesized from glucose, glutamine, and UTP via the hexosamine biosynthetic pathway. Emerging research indicates that O-GlcNAc signaling and its crosstalk with phosphorylation are altered in metabolic diseases, such as diabetes and cancer.

Cancer as a metabolic disease

Emerging evidence indicates that impaired cellular energy metabolism is the defining characteristic of nearly all cancers regardless of cellular or tissue origin. In contrast to normal cells, which derive most of their usable energy from oxidative phosphorylation, most cancer cells become heavily dependent on substrate level phosphorylation to meet energy demands. Evidence is reviewed supporting a general hypothesis that genomic instability and essentially all hallmarks of cancer, including aerobic glycolysis (Warburg effect), can be linked to impaired mitochondrial function and energy metabolism. A view of cancer as primarily a metabolic disease will impact approaches to cancer management and prevention.

Q's next: the diverse functions of glutamine in metabolism, cell biology and cancer

Several decades of research have sought to characterize tumor cell metabolism in the hope that tumor-specific activities can be exploited to treat cancer. Having originated from Warburg's seminal observation of aerobic glycolysis in tumor cells, most of this attention has focused on glucose metabolism. However, since the 1950s cancer biologists have also recognized the importance of glutamine (Q) as a tumor nutrient. Glutamine contributes to essentially every core metabolic task of proliferating tumor cells: it participates in bioenergetics, supports cell defenses against oxidative stress and complements glucose metabolism in the production of macromolecules. The interest in glutamine metabolism has been heightened further by the recent findings that c-myc controls glutamine uptake and degradation, and that glutamine itself exerts influence over a number of signaling pathways that contribute to tumor growth. These observations are stimulating a renewed effort to understand the regulation of glutamine metabolism in tumors and to develop strategies to target glutamine metabolism in cancer. In this study we review the protean roles of glutamine in cancer, both in the direct support of tumor growth and in mediating some of the complex effects on whole-body metabolism that are characteristic of tumor progression.

Warburg effect revisited: an epigenetic link between glycolysis and gastric carcinogenesis

In cancer cells, glucose is often converted into lactic acid, which is known as the 'Warburg effect'. The reason that cancer cells have a higher rate of aerobic glycolysis, but not oxidative phosphorylation, remains largely unclear. Herein, we proposed an epigenetic mechanism of the Warburg effect. Fructose-1,6-bisphosphatase-1 (FBP1), which functions to antagonize glycolysis was downregulated through NF-kappaB pathway in Ras-transformed NIH3T3 cells. Restoration of FBP1 expression suppressed anchorage-independent growth, indicating the relevance of FBP1 downregulation in carcinogenesis. Indeed, FBP1 was downregulated in gastric carcinomas (P<0.01, n=22) and gastric cancer cell lines (57%, 4/7). Restoration of FBP1 expression reduced growth and glycolysis in gastric cancer cells. Moreover, FBP1 downregulation was reversed by pharmacological demethylation. Its promoter was hypermethylated in gastric cancer cell lines (57%, 4/7) and gastric carcinomas (33%, 33/101). Inhibition of NF-kappaB restored FBP1 expression, partially through demethylation of FBP1 promoter. Notably, Cox regression analysis revealed FBP1 promoter methylation as an independent prognosis predicator for gastric cancer (hazard ratio: 3.60, P=0.010). In summary, we found that NF-kappaB functions downstream of Ras to promote epigenetic downregulation of FBP1. Promoter methylation of FBP1 can be used as a new biomarker for prognosis prediction of gastric cancer. Such an important epigenetic link between glycolysis and carcinogenesis partly explains the Warburg effect.

BRAFV600E mutation in papillary thyroid carcinoma: a potential target for therapy?

This article reviews the therapeutic significance of the close genotype-phenotype association in papillary thyroid carcinoma, namely regarding the association between genetic alterations in RET, BRAF or RAS genes and the histopathological variants of papillary thyroid carcinoma. Based upon the aforementioned review on morphology and molecular pathology, the most recent prognostic and therapeutic data are reviewed and the role of targeted therapies, namely those interfering with BRAF-activated pathways are discussed, which may play a role in the treatment of patients with papillary thyroid carcinoma unresponsive to radioactive iodine.

MicroRNAs: novel regulators in the hallmarks of human cancer

MicroRNAs (miRNAs) are small non-coding RNAs of 18-25 nucleotides in length that function as negative regulators. miRNAs post-transcriptionally regulate gene expression by either inhibiting mRNA translation or inducing mRNA degradation, and participate in a wide variety of physiological and pathological cellular processes. Recent reports have revealed that the deregulation of miRNAs correlates with various human cancers and is involved in the initiation and progression of human cancers. miRNAs can act as oncogenes or tumor suppressors to inhibit the expression of cancer-related target genes and to promote or suppress tumorigenesis in various tissues. Therefore, abnormal miRNA expression can be regarded as a common feature of human cancers, and the identification of miRNAs and their respective targets may provide potential diagnostic and prognostic tumor biomarkers and new therapeutic strategies to treat cancers. In the present review, we discuss the emerging roles of miRNAs in the hallmarks of human cancers.