Anti-Warburg Effect of Melatonin: A Proposed Mechanism to Explain its Inhibition of Multiple Diseases

Abstract

Glucose is an essential nutrient for every cell but its metabolic fate depends on cellular phenotype. Normally, the product of cytosolic glycolysis, pyruvate, is transported into mitochondria and irreversibly converted to acetyl coenzyme A by pyruvate dehydrogenase complex (PDC). In some pathological cells, however, pyruvate transport into the mitochondria is blocked due to the inhibition of PDC by pyruvate dehydrogenase kinase. This altered metabolism is referred to as aerobic glycolysis (Warburg effect) and is common in solid tumors and in other pathological cells. Switching from mitochondrial oxidative phosphorylation to aerobic glycolysis provides diseased cells with advantages because of the rapid production of ATP and the activation of pentose phosphate pathway (PPP) which provides nucleotides required for elevated cellular metabolism. Molecules, called glycolytics, inhibit aerobic glycolysis and convert cells to a healthier phenotype. Glycolytics often function by inhibiting hypoxia-inducible factor-1α leading to PDC disinhibition allowing for intramitochondrial conversion of pyruvate into acetyl coenzyme A. Melatonin is a glycolytic which converts diseased cells to the healthier phenotype. Herein we propose that melatonin's function as a glycolytic explains its actions in inhibiting a variety of diseases. Thus, the common denominator is melatonin's action in switching the metabolic phenotype of cells.

Keywords: aerobic glycolysis; hypoxia-inducible factor 1α; mitochondrial melatonin synthesis; pentose phosphate pathway; pyruvate dehydrogenase complex; pyruvate dehydrogenase kinase.

Figure 1

Coupling native vitamin E and coenzyme Q10 to the triphenyl phosphonium cation (at position X) creates molecules referred to as Mito E and Mito Q. These synthetically produced antioxidants are more lipid-soluble than their native precursors and, as a result, they concentrate in the mitochondria, the site of maximal reactive oxygen species generation, up to 500-fold. When these modified antioxidants were compared with melatonin at equimolar concentrations, in terms of their anti-inflammatory and antioxidant actions in mice treated with the bacterial toxins lipopolysaccharide and peptidoglycan G, melatonin was equally effective on most parameters measured and in some cases, melatonin was superior. This attests to the outstanding efficacy of melatonin as to its function as an anti-inflammatory agent and use as an antioxidant.

Figure 2

The association of intramitochondrial pyruvate metabolism and its relation to locally-produced melatonin is illustrated here. Only when pyruvate is enzymatically converted to acetyl coenzyme A (acetyl-CoA) by pyruvate dehydrogenase complex (PDC) is melatonin synthesized in these organelles. In healthy cells the glucose metabolite, pyruvate, is transported across mitochondrial membranes by membrane pyruvate carrier (MPC) into the matrix. Here, pyruvate is acted upon by PDC to generate acetyl-CoA, a critically important factor for feeding the citric acid cycle (tricarboxylic acid cycle; Krebs cycle) and aiding the respiratory chain complexes in the production of ATP while also reducing reactive oxygen species production. In addition, acetyl-CoA serves another important task in the mitochondrial matrix by serving as a co-factor/co-substrate for the rate limiting enzyme in melatonin production arylalkyl-N-acetyltransferase (AANAT), which converts serotonin (5-HT) to N-acetylserotonin (NAS). Once formed, N-acetylserotonin forms melatonin (N-acetyl-5-methoxy-tryptamine) under the influence of acetylserotonin methyltransferase (ASMT). Locally-formed melatonin serves a variety of functions in the mitochondrial matrix by stimulating SIRT3, which upregulates a variety of essential actions. In diseased cells, pyruvate is excluded from the mitochondria since PDC is strongly downregulated by pyruvate dehydrogenase kinase (PDK) which is stimulated by hypoxia inducible factor 1-α Without the synthesis of acetyl-CoA in the matrix of diseased cells, efficient ATP production is compromised and melatonin synthesis is precluded. Thus, pyruvate is retained in the cytosol where it is converted by the action of lactate dehydrogenase to lactate. Circulating pineal-derived melatonin can, however, enter cells (both healthy and diseased cells) to inhibit HIF-1α leading to the down regulation of PDK, the disinhibition of PDC and the production of acetyl-CoA which allows melatonin to be locally produced. Endogenous blood melatonin concentrations are only elevated during the night in young and middle-aged individuals. Thus, glucose metabolism is different during the day and night in diseased cells.

Figure 3

The Warburg effect which has been most thoroughly studied in solid tumor cells, is not unique to cancers as indicated in the current review. The major changes include and upregulated glucose metabolism pathway which is associated with a large number of other alterations as summarized in this figure. Another major alteration that occurs is that the cells reduce the activity of the tricarboxylic cycle (TAC) and oxidation phosphorylation (OXPHOS) for ATP production by precluding the synthesis of acetyl CoA due to the down regulation of the associated enzyme pyruvate dehydrogenase complex (PDC). This shunts pyruvate into the cytosolic fermentation pathway which results in the production of lactate. The pentose phosphate pathway (see Figure 4) is also activated leading to the synthesis of a variety of molecules that ensure nucleotide synthesis and underpin cellular proliferation. HIF-1α = hypoxia inducible factor-1α; PDK = pyruvate dehydrogenase kinase; SIRT3 = sirtuin 3.

Figure 4

The pentose phosphate pathway becomes highly activated during aerobic glycolysis (the Warburg effect) (Figure 3) when pyruvate dehydrogenase kinase inhibits pyruvate dehydrogenase complex. This pathway, the initial molecule of which is glucose-6-phosphate, generates the necessary molecular building blocks for the synthesis of nucleotides, which supports cell proliferation, invasiveness and metastasis of cancer cells. Herein, the authors propose that inhibiting the Warburg effect, which interferes with processes that accelerate cell proliferation, etc., may be a common mechanism for melatonin to modulate disease progression not only in cancer but also in other diseased cells that utilize aerobic glycolysis. G1/S = growth 1 to synthesis; ROS = reactive oxygen species; VEGF—vascular endothelial growth factor.

Figure 5

Hypoxia inducible factor-1 alpha (HIF-1α) is a central transcription agent in mediating Warburg-type metabolism within diseased cells. HIF-1α is part of an oxygen sensing system which becomes activated when the partial pressure (pO2) of intracellular oxygen becomes depressed, that is, when cells become hypoxic. A primary means by which HIF-1α promotes Warburg-type metabolism is by upregulating the mitochondrial enzyme pyruvate dehydrogenase kinase which, in turn, inhibits pyruvate dehydrogenase complex thereby reducing the conversion of pyruvate to acetyl coenzyme A in the mitochondria. The reduction of acetyl coenzyme A in the mitochondrial severely restricts the intramitochondrial synthesis of melatonin (see Figure 2). Further, HIF-1α promotes the pentose phosphate pathway (see Figure 4) and upregulates glycolysis by stimulating the glucose transporter (GLUT1), which accelerates the influx of glucose (the “sweet tooth” phenomenon), and promoting the efflux of lactate by stimulating the monocarboxylate transporter 4 (MCT4) which also stimulates neovasculogenesis by upregulating vascular endothelial growth factor (VEGF). Melatonin, by directly or indirectly inhibiting HIF-1α, reverses the Warburg effect and prevents the associated metabolic activities. Reactive oxygen species, which are produced more abundantly under hypoxic conditions, aid in the stabilization of HIF-1α.