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"Warburg effect" (/ˈvɑːrbʊərɡ/) describes two unrelated observations in biochemistry, one in plant physiology and the other in oncology, both due to Nobel laureate Otto Heinrich Warburg.

Contents

Plant physiologyEdit

In plant physiology, the Warburg's effect is the decrease in the rate of photosynthesis by high oxygen concentrations.[1][2] Oxygen is a competitive inhibitor of the carbon dioxide fixation by RuBisCO which initiates photosynthesis. Furthermore, oxygen stimulates photorespiration which reduces photosynthetic output. These two mechanisms working together are responsible for the Warburg effect.[3]

OncologyEdit

BasisEdit

Normal cells primarily produce energy through mitochondrial oxidative phosphorylation. However, most cancer cells predominantly produce their energy through a high-rate of glycolysis followed by lactic acid fermentation even in the presence of abundant oxygen, this is called aerobic glycolysis, also termed the Warburg effect.[4] Aerobic glycolysis is an insufficient way of producing ATP.[4]

The Warburg effect has been much studied, but its precise nature remains unclear, which hampers the beginning of any work that would explore its therapeutic potential.[5]

Diagnostically the Warburg effect is the basis for the PET scan in which an injected radioactive glucose analog is detected at higher concentrations in malignant cancers than in other tissues.[6]

Otto Warburg postulated this change in metabolism is the fundamental cause of cancer,[7] a claim now known as the Warburg hypothesis. Today, mutations in oncogenes and tumor suppressor genes are thought to be responsible for malignant transformation, and the Warburg effect is considered to be a result of these mutations rather than a cause.[8][9]

Possible explanationsEdit

The Warburg effect may simply be a consequence of damage to the mitochondria in cancer, or an adaptation to low-oxygen environments within tumors, or a result of cancer genes shutting down the mitochondria, which are involved in the cell's apoptosis program that kills cancer cells. It may also be an effect associated with cell proliferation. Since glycolysis provides most of the building blocks required for cell proliferation, cancer cells (and normal proliferating cells) have been proposed to need to activate glycolysis, despite the presence of oxygen, to proliferate.[10] Evidence attributes some of the high aerobic glycolytic rates to an overexpressed form of mitochondrially-bound hexokinase[11] responsible for driving the high glycolytic activity. In kidney cancer, this effect could be due to the presence of mutations in the von Hippel–Lindau tumor suppressor gene upregulating glycolytic enzymes, including the M2 splice isoform of pyruvate kinase.[12] TP53 mutation hits energy metabolism and increases glycolysis in breast cancer.[13]

In March 2008, Lewis C. Cantley and colleagues announced that the tumor M2-PK, a form of the pyruvate kinase enzyme, gives rise to the Warburg effect. Tumor M2-PK is produced in all rapidly dividing cells, and is responsible for enabling cancer cells to consume glucose at an accelerated rate; on forcing the cells to switch to pyruvate kinase's alternative form by inhibiting the production of tumor M2-PK, their growth was curbed. The researchers acknowledged the fact that the exact chemistry of glucose metabolism was likely to vary across different forms of cancer; but PKM2 was identified in all of the cancer cells they had tested. This enzyme form is not usually found in healthy tissue, though it is apparently necessary when cells need to multiply quickly, e.g. in healing wounds or hematopoiesis.[14][15]

Glycolytic inhibitorsEdit

Many substances have been developed which inhibit glycolysis, and such inhibitors are currently the subject of intense research as anticancer agents,[16] including SB-204990, 2-deoxy-D-glucose (2DG), 3-bromopyruvate (3-BrPA, bromopyruvic acid, or bromopyruvate), 3-bromo-2-oxopropionate-1-propyl ester (3-BrOP), 5-thioglucose and dichloroacetic acid (DCA). Clinical trial for 2-DG [2008] showed slow accrual and was terminated.[17] There is no evidence yet [2012] to support the use of DCA for cancer treatment.[18]

Alpha-cyano-4-hydroxycinnamic acid (ACCA;CHC), a small-molecule inhibitor of monocarboxylate transporters (MCTs; which prevent lactic acid build up in tumors) has been successfully used as a metabolic target in brain tumor pre-clinical research.[19][20][21][22] Higher affinity MCT inhibitors have been developed and are currently undergoing clinical trials by Astra-Zeneca.[23]

Dichloroacetic acid (DCA), a small-molecule inhibitor of mitochondrial pyruvate dehydrogenase kinase, "downregulates" glycolysis in vitro and in vivo. Researchers at the University of Alberta theorized in 2007 that DCA might have therapeutic benefits against many types of cancers.[24][25]

Blood glucose levelsEdit

High glucose levels have been shown to accelerate cancer cell proliferation in vitro, while glucose deprivation has led to apoptosis. These findings have initiated further study of the effects of carbohydrate restriction on tumor growth. Clinical evidence shows that lower blood glucose levels in late-stage cancer patients have been correlated with better outcomes.[26]

Alternative modelsEdit

A model called the "reverse Warburg effect" describes cells producing energy by glycolysis, but were not tumor cells, but stromal fibroblasts.[27] Although the Warburg effect would exist in certain cancer types potentially, it highlighted the need for a closer look at tumor metabolism.[28][29]

Metabolic reprogramming is also observed in neurodegenerative diseases, Alzheimer's and Parkinson's. This metabolic alteration is described by the up-regulation of oxidative phosphorylation - called the inverse Warburg effect.[30] This mode of metabolic reprogramming is characterized by an increase in oxphos activity in the intact mitochondria of impaired neurons. This up-regulation compensates for the diminished efficiency of defective mitochondria.[31][32] This is in contrast to astrocytes, where synaptic activity promotes the up-regulation in glycolysis, an astrocytic Warburg Effect, that is thought to underlie neuronal growth and structural plasticity,[33][34] and – by reducing the rate of oxidative phosphorylation and the generation of reactive oxygen species – may contribute to synaptic activity-dependent neuroprotection.[33][35][36]

Cancer metabolism and epigeneticsEdit

Nutrient utilization is dramatically altered when cells receive signals to proliferate. Characteristic metabolic changes enable cells to meet the large biosynthetic demands associated with cell growth and division. Changes in rate-limiting glycolytic enzymes redirect metabolism to support growth and proliferation. Metabolic reprogramming in cancer is largely due to oncogenic activation of signal transduction pathways and transcription factors. Although less well understood, epigenetic mechanisms also contribute to the regulation of metabolic gene expression in cancer. Reciprocally, accumulating evidence suggest that metabolic alterations may affect the epigenome. Understanding the relation between metabolism and epigenetics in cancer cells may open new avenues for anti-cancer strategies.[37]

ReferencesEdit

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