In 1923, Dr. Warburg had observed that tumors acidified the Ringer solution when 13 mM glucose was added, which was identified as being due to lactate. When glucose is the only source of nutrient, it can serve for both biosynthesis and energy production. However, a series of studies revealed that the cancer cell consumes glucose for biosynthesis through fermentation, not for energy supply, under physiological conditions. Recently, a new observation was made that there is a metabolic symbiosis in which glycolytic and oxidative tumor cells mutually regulate their energy metabolism. Hypoxic cancer cells use glucose for glycolytic metabolism and release lactate which is used by oxygenated cancer cells. This study challenged the Warburg effect, because Warburg claimed that fermentation by irreversible damaging of mitochondria is a fundamental cause of cancer. However, recent studies revealed that mitochondria in cancer cell show active function of oxidative phosphorylation although TCA cycle is stalled. It was also shown that blocking cytosolic NADH production by aldehyde dehydrogenase inhibition, combined with oxidative phosphorylation inhibition, resulted in up to 80% decrease of ATP production, which resulted in a significant regression of tumor growth in the NSCLC model. This suggests a new theory that NADH production in the cytosol plays a key role of ATP production through the mitochondrial electron transport chain in cancer cells, while NADH production is mostly occupied inside mitochondria in normal cells.
Biomolecules cannot be produced without an energy supply. Growth signaling, driver gene activation, and mTOR activation requires ATP for phosphorylation, and translation machineries including DNA/RNA synthesis enzymes also requires ATP. Therefore, cancer cells need to have huge supply of ATP. What is the major ATP source that cancer cells require for biomolecule synthesis? We have at least three theories of cancer energy metabolism. First, the Classical Warburg effect explains that cancer cells ferments glucose as an ATP source via glycolysis (Fig. 1A). Warburg thought that glucose is the source of cancer growth through fermentation, instead of oxidative respiration (Warburg, 1956a). Until now, it was widely accepted that glucose is the main source of cancer ATP. However, this only happens when glucose is the only nutrient supply under hypoxia. The second theory of cancer metabolism posits that cancer cells elaborate survival through symbiosis: one cancer cell produces lactate with ATP production by consuming glucose (Warburg effect), and the neighbor cancer cell consumes the secreted lactate to produce ATP through the TCA cycle and oxidative phosphorylation (Fig. 1B) (Sonveaux
All cancer cells produce lactate from glucose regardless of thousands of mutation combinations. We need to look carefully this finding that has been discovered by Dr. Warburg. The classical Warburg effect implies that there must be common metabolic pathways for all type of cancer cells to survive. It is a good news for us to find cancer-specific vulnerabilities. It is often explained that cancer glycolysis is responsible for ATP production and biosynthesis (Warburg, 1956a, 1956b). However, although glucose may be used for both biosynthesis and ATP production, cancer cell cannot survive only with glycolytic production of ATP. By simple calculation, glycolysis does not add up sufficient energy supply in cancer cells. It generates 2 molar ATPs, 2 molar NADH, and 2 molar pyruvates from 1 molar glucose. However, only 1 molar ATP is left after consumption of 2 molar NADH for 2 molar lactate production and 1 molar ATP for pentose pathway. It is revealed that glucose greatly contributes towards building biomolecules in cancer anabolic metabolism instead of ATP production (DeBerardinis
People often confuse the TCA cycle with oxidative phosphorylation because Dr. Warburg mentioned that mitochondria in the cancer cell is destroyed. However, mitochondrial membrane potential in cancer cells are more active compared to normal cells, although it does produce lactate avoiding TCA cycle. The TCA cycle is a charger of electron carrier NAD+ to NADH by converting glucose to CO2 by a series of enzymes in the normal mitochondria. Oxidative phosphorylation occurs at electron transport chain in the mitochondrial membrane using NADH from the TCA cycle in the normal cell. One key Achilles’ heel dilemma is, when we admit active oxidative phosphorylation in the cancer cell, we need to admit that cancer cells use oxygenated respiration.
Misunderstanding the mitochondrial function in the cancer cell has led to the wrong interpretation of cancer energy metabolism. First, the cancer cell environment provides hypoxic condition due to fast growth of tumors and failure of proper blood supply. Hypoxia stops the glucose driven TCA cycle by HIF (Hypoxia Inducible Factor) induction. HIF triggers induction of pyruvate PDK inactivating pyruvate dehydrogenase to rewire flow of pyruvate to LDH (Kim
Recently, we have reported two independent studies about NSCLC metabolism, which describe the increase of cytosolic NADH production as an electron source of the cancer cell. The cytosolic NADH is transported into mitochondria through the MAS, which results in ATP production through the ETC. The first study was based upon omics data analysis using NSCLC related metabolic enzyme profiling, which suggests close association of ALDH in NSCLC (Kang
The second study was based on drug screening by random combination with conventional drugs and metabolic enzyme inhibitors. We found that GLS1 inhibition is very effective to reduce growth of NSCLC (Lee
This is a very provocative challenge in cancer metabolism, because Dr. Warburg proposed that cancer cells cannot use oxygen as much as normal cell due to mitochondrial malfunction, which leads to an increase of lactate production instead of CO2 production. His idea was established without the concepts of the TCA cycle and oxidative phosphorylation, because those were discovered later following his discovery. O2 consumes in oxidative phosphorylation via electron transfer chain and CO2 is produced from the TCA cycle by carbohydrate dehydration. Cancer cells do not use as much oxygen as normal cells to produce lactate when glucose is the only nutrient supply. However, under glucose-limited conditions, cancer cells may use fatty acids as an energy source through fatty acid oxidation (Carracedo
In our proposed model (Fig. 3), cytosolic NADH as an electron donor is the key player in ATP production in cancer cells. One of major cytosolic NADH production enzymes is ALDH that catalyzes aldehyde to carboxylic acid and NADH. Major substrates of ALDH is fatty aldehyde, acetaldehyde, retinaldehyde, and 10-formyl-tetrahydrofolate in one-carbon pathway. In the cancer condition, ROS radicals are abundant that are usually involved in the peroxidation process. Fatty acid turns into fatty aldehyde under the peroxidation condition, which can be further catalyzed to fatty acid with NADH production by ALDH. Although it remains to be elucidated that peroxidation of fatty acid is increased in cancer cells, knockdown of ALDH expression or treatment with ALDH inhibitor significantly decreased cytosolic NADH and mitochondrial ATP production in cancer cells (Kang
Fast growth of tumor needs efficient ATP supply as well as abundant biomolecules. All recent targeted drugs ultimately aim at cancer translation (anabolism) suppression through inhibiting signaling molecules (Fig. 4A). But blocking signal pathways always induce signal rewiring to promote translation. Although mTOR was a conceptually perfect target for blocking cancer translation, cancer cells evade blocking a biomolecule supply through recycling cell compartments via autophagy (Wu
Continuous growth signaling cannot reach the next level when ATP is not properly supplied in cancer cells, because all kinases requires ATP as a substrate. Therefore, cancer growth should be turned down when the major ATP supplier is blocked. As an alternate approach, targeting ATP supply was attempted by blocking glycolysis and/or inhibiting mitochondrial complex I. Blocking glycolysis using hexokinase inhibitor 2-deoxyglucose, combined with blocking oxidative phosphorylation using mitochondrial complex I inhibitor metformin (Cheong
Recently we have demonstrated that ALDH inhibition using gossypol combined with mitochondrial complex I inhibition using phenformin resulted in up to 80% ATP depletion in NSCLC, which induced significant tumor regression in the cancer xenograft model (Kang
The three major cancer metabolic theories, the “Classic Warburg Effect”, the more recent “Cancer Cell Symbiosis” and “Glutaminolysis,” are based on biosynthesis as promotion of cancer cell proliferation. I proposes a new theory that NADH production in the cytosol using carbohydrate, fatty acid, glutamine and NADH transportation to mitochondria via MAS plays a key role of ATP production through the mitochondrial electron transport complex in the cancer cell (Fig. 3). Further studies of these hot-spots may lead us to answer the question of how we can cure cancer.
This work was supported by the National Research Foundation of Korea(NRF) grant funded by the Korean government (MSIP) (No. 2017R1A2B2003428). I thank Kee-Hwan Kim for English editing.