J Physiol. 2020 Dec 21.
Contrary to Warburg's original thesis, accelerated aerobic glycolysis is not a primary, permanent and universal consequence of dysfunctional/impaired mitochondria compensating for poor ATP-yield per mole of glucose. Instead, in most tumours the Warburg effect is an essential part of a "selfish" metabolic reprogramming, which results from the interplay between (normoxic/hypoxic) HIF-1-overexpression, oncogene activation (cMyc, Ras), loss of function of tumour suppressors (mutant-p53, mutant-PTEN, microRNAs and sirtuins with suppressor functions), activated (PI3K/Akt/mTORC1, Ras/Raf/Mek/Erk/cMyc, Jak/Stat3) or deactivated (LKB1/AMPK) signalling pathways, components of the tumour microenvironment, and HIF-1-cooperations with epigenetic mechanisms. Molecular and functional processes of the Warburg effect include: (a) considerably accelerated glycolytic fluxes, (b) adequate ATP generation per unit time to maintain energy homeostasis and electrochemical gradients, (c) backup and diversion of glycolytic intermediates facilitating the biosynthesis of nucleotides, non-essential amino acids, lipids and hexosamines, (d) inhibition of pyruvate entry into mitochondria, (e) excessive formation and accumulation of lactate which stimulates tumour growth and suppression of anti-tumour immunity; in addition, lactate can serve as an energy source for normoxic cancer cells and drives malignant progression and resistances to conventional therapies, (f) cytosolic lactate is mainly exported through upregulated lactate-proton symporters (MCT4), working together with other H+ -transporters, and carbonic anhydrases (CAII, CAIX) which hydrate CO2 from oxidative metabolism to form H+ and bicarbonate, (g) in concert with poor vascular drainage these proton export mechanisms are responsible for extracellular acidification, driving malignant progression and resistances to conventional therapies, (h) maintenance of the cellular redox homeostasis and low ROS formation, and (i) HIF-1 overexpression, mutant-p53 and mutant-PTEN which inhibit mitochondrial biogenesis and functions, negatively impacting cellular respiration rate. The glycolytic switch is an early event in oncogenesis and primarily supports cell survival. All in all, the Warburg effect, i.e., aerobic glycolysis in the presence of oxygen and -in principle- functioning mitochondria, constitutes a major driver of the cancer progression machinery, resistance to conventional therapies, and poor patient outcome. However, as evidenced during the last two decades, in a minority of tumours primary mitochondrial defects can play a key role promoting the Warburg effect and tumour progression due to mutations in some Krebs cycle enzymes and mitochondrial ROS overproduction. Abstract figure legend Driving processes causing the Warburg effect during carcinogenesis (upper part), and mechanisms/consequences of metabolic reprogramming in Warburg phenotypes (lower part) leading to survival advantages, malignant progression and, ultimately, poor patient outcome. This article is protected by copyright. All rights reserved.
Keywords: Warburg effect; aerobic glycolysis; glycolytic phenotype; lactate accumulation; metabolic reprogramming; tumour acidosis; tumour glucose metabolism; tumour mitochondria