The Warburg hypothesis and the emergence of the mitochondrial metabolic theory of cancer

Thomas N. Seyfried, Derek C. Lee, Tomas Duraj, Nathan L. Ta, Purna Mukherjee, Michael Kiebish, Gabriel Arismendi-Morillo, Christos Chinopoulos
Journal of Bioenergetics and Biomembranes
Biology Department, Boston College, 140 Commonwealth Ave, Chestnut Hill, Boston, MA 02467, USA

Table of Contents

Overall Summary

Study Background and Main Findings

This paper revisits Otto Warburg's century-old hypothesis that cancer originates from impaired mitochondrial energy production, specifically defects in oxidative phosphorylation (OxPhos), the primary oxygen-dependent pathway for generating ATP (cellular energy currency). The objective is to re-evaluate and bolster this idea, now termed the Mitochondrial Metabolic Theory (MMT), by addressing historical controversies and integrating modern biochemical understanding. The authors employ a literature review methodology, systematically analyzing existing evidence and challenging seven common assumptions that have historically favored alternative theories, particularly the Somatic Mutation Theory (SMT), which posits that cancer arises primarily from accumulated gene mutations.

The key findings center on demonstrating the limitations of historical metabolic measurements and introducing crucial updates to the metabolic picture. The paper argues that oxygen consumption rate (OCR) and lactate production, Warburg's original proxies for OxPhos and fermentation respectively, are inaccurate measures of ATP synthesis in cancer cells. It highlights the significant contribution of an alternative ATP-generating pathway within mitochondria, glutamine-driven mitochondrial substrate-level phosphorylation (mSLP), which was unknown to Warburg and complicates interpretations based solely on OCR. Furthermore, the authors compile extensive evidence, including electron microscopy images and literature data (Tables 1 & 2, Figs 2 & 3), showing widespread structural and functional abnormalities in mitochondria across diverse cancer types, contradicting the assumption that they are normal.

The paper also reinterprets phenomena like lipid droplet accumulation in cancer cells not as fuel storage for oxidation, but as a consequence of impaired OxPhos. It directly challenges the SMT by citing evidence like cancers without known driver mutations, mutations found in normal tissues, and nuclear-cytoplasmic transfer experiments suggesting that the cytoplasm and mitochondria, not just the nucleus, dictate cancerous behavior. The authors propose a model (Fig. 4) where various carcinogenic insults converge on damaging mitochondria, leading to chronic OxPhos insufficiency compensated by increased reliance on fermentation pathways (both cytosolic glycolysis and mitochondrial mSLP), which drives dysregulated growth and other cancer hallmarks.

Ultimately, the paper concludes that the MMT provides a more coherent and credible explanation for the origin of cancer than the SMT. It posits that impaired mitochondrial respiration, compensated by fermentation, is the fundamental metabolic lesion common to most cancers. This perspective, the authors argue, opens avenues for developing less toxic and potentially more effective therapeutic strategies focused on targeting these unique metabolic dependencies, such as ketogenic metabolic therapy (KMT) aimed at restricting fermentation fuels (glucose and glutamine) used by cancer cells.

Research Impact and Future Directions

This paper presents a compelling, albeit controversial, re-evaluation of cancer's origins, strongly advocating for the Mitochondrial Metabolic Theory (MMT) over the dominant Somatic Mutation Theory (SMT). Its strength lies in systematically dismantling long-held assumptions about cancer metabolism by integrating historical context with modern biochemical and cellular evidence. The authors effectively argue that traditional markers like oxygen consumption and lactate production are insufficient proxies for cellular energy (ATP) generation, highlighting the overlooked role of mitochondrial substrate-level phosphorylation (mSLP) and widespread mitochondrial abnormalities in cancer cells. This perspective reframes cancer not primarily as a disease of genetic mutations, but as a consequence of chronic mitochondrial dysfunction leading to a fundamental shift in energy metabolism.

The practical implications are significant, suggesting a shift towards metabolic therapies, such as Ketogenic Metabolic Therapy (KMT) combined with targeted inhibition of pathways like glutaminolysis (the 'Press-Pulse' strategy). These approaches aim to exploit the unique metabolic vulnerabilities of cancer cells (reliance on glucose and glutamine for fermentation) while supporting normal cell function, potentially offering less toxic treatment avenues. However, the argument relies heavily on the re-interpretation of existing data and correlative evidence (e.g., mitochondrial structure defects linked to dysfunction). While nuclear-cytoplasmic transfer experiments offer strong support against SMT being the sole explanation, definitively proving chronic metabolic dysfunction as the primary initiating cause across all cancers remains challenging. The complex interplay between metabolic changes and genetic mutations is likely bidirectional, and the relative contribution of each may vary.

Critical questions remain regarding the precise quantification of ATP derived from different pathways (OxPhos, cSLP, mSLP) in vivo across diverse tumor types and stages. Furthermore, while the MMT provides a unifying framework, the heterogeneity of cancers means that the specific metabolic profile and therapeutic vulnerabilities might differ significantly. The proposed metabolic therapies, while promising, require further rigorous clinical validation to establish efficacy, optimal implementation, and long-term effects. In conclusion, the paper offers a valuable theoretical challenge and a potential paradigm shift, emphasizing that understanding cancer as a metabolic disease opens new, potentially less toxic therapeutic strategies, though the primacy of metabolism over genetics as the root cause requires ongoing investigation and validation.

Critical Analysis and Recommendations

Comprehensive and Logical Summary (written-content)
The abstract provides a comprehensive and logically structured summary of the paper's argument, covering Warburg's initial ideas, historical challenges, new concepts (mSLP, biomarkers), and therapeutic implications. This ensures readers grasp the core thesis and scope effectively from the outset.
Section: Abstract
Define Mitochondrial Metabolic Theory Explicitly (written-content)
The abstract introduces the 'mitochondrial metabolic theory' but doesn't define it concisely within the text. Explicitly defining this central theory in the abstract would immediately clarify the paper's theoretical stance for all readers, enhancing comprehension of the subsequent arguments.
Section: Abstract
Clear Historical Context and Controversy (written-content)
The introduction clearly presents Otto Warburg's original hypothesis and the subsequent historical controversy initiated by Sidney Weinhouse. This effectively establishes the foundational context and the long-standing debate the paper aims to address.
Section: Introduction
Explicitly Define Mitochondrial Metabolic Theory (MMT) (written-content)
Similar to the abstract, the introduction mentions the 'mitochondrial metabolic theory' (MMT) but lacks an explicit definition. Defining the MMT early in the introduction would solidify the reader's understanding of the paper's core theoretical framework before delving into detailed arguments.
Section: Introduction
Systematic Argument Structure (written-content)
The main argument is built around systematically addressing seven 'questionable assumptions' that challenge the MMT. This structured approach provides a clear, logical framework that makes the complex refutation of counterarguments easy to follow.
Section: Main Argument
Strong Evidentiary Support (written-content)
The paper effectively integrates diverse and extensive evidence (historical context, biochemical data, ultrastructural observations, experimental results like nuclear transfer studies) to support its claims. This multi-faceted evidentiary base strengthens the refutation of each assumption and the overall argument for the MMT.
Section: Main Argument
Direct Challenge to Somatic Mutation Theory (written-content)
The argument directly confronts the dominant Somatic Mutation Theory (SMT) by highlighting inconsistencies (e.g., mutations in normal tissue, cancer without mutations, nuclear transfer results). This direct challenge positions the MMT as a potentially more comprehensive explanation for cancer's origin.
Section: Main Argument
Visual/Compiled Evidence of Mitochondrial Abnormalities and Lipid Droplets (graphical-figure)
Figures 2 & 3, along with Tables 1 & 2, provide visual and compiled evidence for widespread mitochondrial structural abnormalities and lipid droplet accumulation across various cancers. This evidence is crucial for refuting the assumption of normal mitochondria in cancer and supporting the link between structural defects and functional (OxPhos) impairment.
Section: Main Argument
Integrative Model of MMT (Fig. 4) (graphical-figure)
Figure 4 presents a coherent visual model integrating diverse risk factors, mitochondrial damage, metabolic shifts (OxPhos insufficiency, SLP compensation), signaling pathways (RTG), and cancer hallmarks. This schematic effectively communicates the core tenets of the MMT and its proposed mechanism for cancer development.
Section: Main Argument
Reinforce Link Between Assumption Refutation and MMT Support (written-content)
While refuting assumptions against the MMT, the text doesn't consistently and explicitly state how each refutation directly supports the MMT at the end of each subsection. Adding brief concluding sentences linking each assumption's refutation back to the MMT would reinforce the central thesis and improve argumentative coherence.
Section: Main Argument
Concise Synthesis and Clarification of Historical Context (written-content)
The conclusion provides a clear synthesis, reaffirming Warburg's core insight (OxPhos insufficiency) while clarifying historical inaccuracies (reliance on flawed markers, unawareness of mSLP). This effectively summarizes the paper's updated perspective on Warburg's legacy.
Section: Conclusions
Strong Advocacy for Mitochondrial Metabolic Theory (written-content)
The conclusion strongly advocates for the MMT over the SMT as a more credible explanation for cancer, based on the evidence reviewed. This provides a clear and unambiguous takeaway message regarding the paper's theoretical stance.
Section: Conclusions
Briefly Reiterate Nature of Proposed Therapies (written-content)
The conclusion mentions 'more effective and less toxic therapeutic strategies' but doesn't briefly reiterate the nature of these strategies (e.g., metabolic therapies like KMT targeting SLP). Briefly restating the proposed therapeutic direction would strengthen the link between the theoretical argument and its practical implications.
Section: Conclusions

Section Analysis

Abstract

Key Aspects

Strengths

Suggestions for Improvement

Introduction

Key Aspects

Strengths

Suggestions for Improvement

Main Argument

Key Aspects

Strengths

Suggestions for Improvement

Non-Text Elements

Fig. 1 ATP production and vulnerability of cancer cells to metabolic stress.
Figure/Table Image (Page 5)
Fig. 1 ATP production and vulnerability of cancer cells to metabolic stress.
First Reference in Text
Figure 1 illustrates the three sources of ATP production in cancer cells.
Description
  • Overview of ATP Production Pathways: This figure is a schematic diagram illustrating the primary ways cancer cells are thought to produce energy, specifically a molecule called ATP (Adenosine Triphosphate), which acts like the cell's energy currency. It shows three main energy production routes: 1) Oxidative Phosphorylation (OxPhos), the highly efficient process occurring in mitochondria (the cell's powerhouses) that uses oxygen; 2) Cytosolic Substrate-Level Phosphorylation (cSLP), a less efficient process occurring outside the mitochondria, primarily through glycolysis (the breakdown of glucose, a sugar); and 3) Mitochondrial Substrate-Level Phosphorylation (mSLP), another direct ATP-generating process occurring within mitochondria, notably linked to the breakdown of glutamine (an amino acid) via a pathway called glutaminolysis.
  • Glucose Metabolism and Glycolysis: The diagram shows how glucose enters the cell and is processed through glycolysis. A key enzyme highlighted is Pyruvate Kinase (PKM), specifically mentioning the PKM2 isoform common in cancer, which is less efficient at ATP production compared to the PKM1 isoform found in most normal tissues. Glycolysis can lead to lactate production, which is often elevated in cancer (part of the Warburg effect).
  • Glutamine Metabolism and mSLP: The figure also depicts how glutamine enters the cell and is processed within the mitochondria through glutaminolysis. Key steps shown include conversion to glutamate and then alpha-ketoglutarate, which enters the Tricarboxylic Acid (TCA) cycle, a central hub of metabolism. A crucial reaction highlighted is the conversion of succinyl-CoA to succinate by the enzyme Succinyl-CoA Synthetase (SCS), which directly generates ATP via mSLP.
  • Uncertainty in ATP Contribution: Red question marks are placed next to the ATP symbols originating from each of the three main pathways (OxPhos, mSLP, cSLP). This signifies the authors' assertion that the precise percentage contribution of each pathway to the total ATP supply in any given cancer cell is context-dependent and difficult to accurately measure, representing a key uncertainty in cancer metabolism.
  • Regulatory Elements and Therapeutic Implications: The diagram includes several regulatory aspects relevant to cancer metabolism. It shows how succinate can stabilize HIF-1a (Hypoxia-Inducible Factor 1-alpha), a protein that promotes glycolysis under low oxygen conditions, but which can also be active in cancer even with oxygen present. It also indicates potential therapeutic interventions: Ketogenic Metabolic Therapy (KMT) is suggested to reduce ATP synthesis via mSLP by diverting a key molecule (CoA). Additionally, the impact of mutations in the IDH1 enzyme (Isocitrate Dehydrogenase 1), found in some gliomas, is shown to potentially reduce mSLP by altering metabolite levels.
Scientific Validity
  • Accuracy of Biochemical Pathways: The diagram accurately depicts the core biochemical pathways involved in glucose and glutamine metabolism leading to ATP production via OxPhos, cSLP (glycolysis), and mSLP (TCA cycle/glutaminolysis). The representation of key intermediates and enzymes is generally consistent with established metabolic charts.
  • Emphasis on Mitochondrial SLP (mSLP): Highlighting mSLP via the SCS reaction as a potentially significant and underappreciated source of ATP in cancer, distinct from OxPhos, aligns with emerging research and the central argument of the review paper. This challenges the traditional dichotomy focusing solely on OxPhos versus glycolysis.
  • Representation of PKM2 Function: The representation of PKM2's role in potentially diverting glycolytic intermediates towards biosynthesis rather than maximizing ATP production via cSLP is consistent with current understanding of this enzyme isoform's function in cancer.
  • Inclusion of Regulatory Interactions: The inclusion of regulatory loops, such as succinate stabilizing HIF-1a, reflects known interactions, although the complexity of these networks is necessarily simplified in a schematic.
  • Representation of Therapeutic/Mutation Effects: The depiction of KMT potentially reducing mSLP by diverting CoA is a plausible hypothesis presented by the authors, though the direct quantitative impact and universality require further experimental validation. Similarly, the effect of IDH1 mutation on mSLP is represented based on current hypotheses regarding its impact on alpha-ketoglutarate and succinyl-CoA levels.
  • Acknowledgement of Quantitative Uncertainty: Acknowledging the uncertainty in quantifying the relative ATP contributions from each source with question marks is scientifically appropriate, reflecting the complexity and context-dependency of cancer metabolism.
Communication
  • Integration of Multiple Pathways: The diagram effectively integrates multiple complex metabolic pathways (glycolysis, TCA cycle, glutaminolysis, OxPhos, SLP) into a single visual representation, highlighting their interconnectedness and potential roles in cancer cell energy production.
  • Visual Organization and Clarity: The use of distinct colors for glucose-derived (blue/purple tones) and glutamine-derived (green tones) pathways aids in distinguishing the origins of metabolites. Arrows generally indicate reaction direction clearly.
  • Representation of Uncertainty: The inclusion of question marks next to ATP symbols derived from OxPhos, mSLP, and cSLP successfully conveys the authors' point about the uncertainty in quantifying the relative contribution of each pathway to the total ATP pool in cancer cells, which is a central theme of the review.
  • Density of Abbreviations: While standard abbreviations are used (ATP, NAD+, etc.), the diagram relies heavily on numerous specific abbreviations (PPP, KMT, GLS1/2, GDH, SCS, OXCT1, HIF-1a, UCPs, CL). Although likely defined in the main text or detailed legend, the density of abbreviations within the figure itself can initially hinder rapid comprehension for readers unfamiliar with all terms.
  • Clarity of Regulatory Connections: The diagram attempts to depict regulatory influences (e.g., HIF-1a stabilization by succinate, KMT effects, IDH1 mutation impact) but these connections, represented by dotted lines or text labels, could be visually clearer or more explicitly linked to their points of action within the pathways.
Fig. 2 Abnormal mitochondria and lipid droplets in glioblastoma.
Figure/Table Image (Page 6)
Fig. 2 Abnormal mitochondria and lipid droplets in glioblastoma.
First Reference in Text
Based on the foundational principles of evolutionary biology and in recognition that mitochondrial structure determines function (Darwin 1859; Lehninger 1964; Bhargava and Schnellmann 2017; J. R. Friedman 2022; Brand et al. 1991; Brand and Nicholls 2011; Jezek et al. 2023; Miyazono et al. 2018; Seyfried 2012f; Pedersen 1978), the information in Fig. 2 and Table 1 presents substantial evidence for abnormalities in the number, structure, and function of mitochondria in all major cancers (Seyfried et al. 2020).
Description
  • Imaging Technique and Subject: This figure displays images obtained using Transmission Electron Microscopy (TEM), a technique that uses electrons to create highly magnified images of the internal structures of cells. The images show a section of a human glioblastoma, which is a type of aggressive brain tumor.
  • Mitochondrial Abnormalities (Cristolysis and Dysmorphism): The main focus is on mitochondria, which are often called the 'powerhouses' of the cell because they generate most of the cell's energy supply. The caption and annotations (circles and ellipses) highlight that these mitochondria appear abnormal. Specifically, they show 'total-subtotal cristolysis,' meaning the inner folded membranes (cristae), where energy production mainly occurs, are partially or completely lost. They also exhibit 'dysmorphic cristae,' indicating these inner folds have an irregular or abnormal shape.
  • Presence of Lipid Droplets: The images also clearly show the presence of numerous lipid droplets, indicated by white asterisks. These are small sacs within the cell cytoplasm used for storing fats (lipids). Their abundance is presented as a notable feature of these glioblastoma cells.
  • Magnification and Cellular Context: The figure includes a larger overview image taken at 4000 times magnification (4000x) and smaller inset images providing a closer look at 8000 times magnification (8000x). The letter 'N' labels the nucleus, the central organelle containing the cell's genetic material.
Scientific Validity
  • Direct Evidence of Mitochondrial Structural Defects: The TEM images provide direct visual evidence supporting the claim of structural abnormalities in mitochondria (cristolysis, dysmorphic cristae) within the context of a human glioblastoma sample.
  • Evidence of Lipid Droplet Accumulation: The figure visually documents the presence and apparent abundance of lipid droplets in the cytoplasm of these cancer cells, consistent with observations reported in various cancer types and often linked to metabolic alterations or stress.
  • Source Citation: The image is explicitly sourced from a prior publication (J Electron Microsc (Tokyo). 2008; 57:33-39), allowing readers to consult the original study for methodological details and further context, which adds credibility.
  • Scope of Evidence Provided: While the image shows structural abnormalities, it does not directly measure mitochondrial number or function, which are also mentioned in the reference text as being abnormal in cancer. The figure primarily supports the 'structure' aspect of the claim.
  • Representativeness and Generalizability: The presented image is a representative example from one tumor type (glioblastoma). While illustrative, generalizing the findings to 'all major cancers' as stated in the reference text relies on the collective evidence presented elsewhere (like Table 1) rather than solely on this single figure.
Communication
  • Appropriate Imaging Technique: The use of Transmission Electron Microscopy (TEM) provides high-resolution visualization necessary to observe subcellular structures like mitochondria and lipid droplets.
  • Use of Annotations: Annotations (circles, ellipses, asterisks, 'N' for nucleus) are used to guide the viewer's attention to specific features mentioned in the caption (abnormal mitochondria, lipid droplets, nucleus).
  • Use of Insets for Detail: The main image provides context (cellular layout), while the inset images offer higher magnification (8000x vs 4000x) views, presumably focusing on the specific abnormalities highlighted.
  • Specificity of Annotations: While annotations point to general areas, more specific labels or pointers indicating the exact nature of the abnormality (e.g., specific dysmorphic cristae, areas of cristolysis) could enhance clarity, especially for non-experts in mitochondrial pathology.
  • Clarity of Caption: The caption clearly states the subject (glioblastoma), the key features shown (abnormal mitochondria, lipid droplets), and the imaging technique (TEM), effectively summarizing the figure's content.
Table 1 Mitochondrial abnormalities observed in common cancer
Figure/Table Image (Page 7)
Table 1 Mitochondrial abnormalities observed in common cancer
First Reference in Text
Based on the foundational principles of evolutionary biology and in recognition that mitochondrial structure determines function (Darwin 1859; Lehninger 1964; Bhargava and Schnellmann 2017; J. R. Friedman 2022; Brand et al. 1991; Brand and Nicholls 2011; Jezek et al. 2023; Miyazono et al. 2018; Seyfried 2012f; Pedersen 1978), the information in Fig. 2 and Table 1 presents substantial evidence for abnormalities in the number, structure, and function of mitochondria in all major cancers (Seyfried et al. 2020).
Description
  • List of Cancer Types and Citations: This table serves as a directory, listing various common types of cancer, such as Bladder cancer, Breast/Mammary cancers, Gliomas (brain tumors), Lung cancer, and others. For each cancer type listed, it provides a series of citations, which are references to other scientific publications.
  • Purpose: Referencing Evidence for Mitochondrial Abnormalities: The purpose of the table, as indicated by the caption and reference text, is to point readers towards studies that have reportedly found abnormalities in mitochondria – the energy-producing components within cells – in these different kinds of cancers. The table covers a broad range of malignancies, including solid tumors (like breast, lung, colon) and blood cancers (leukemias/lymphomas).
  • Nature of Content: Literature References: The table does not contain specific data points, numbers, or statistics itself. Instead, it acts as a compilation of literature references intended to support the authors' claim that mitochondrial abnormalities (in number, structure, or function, as mentioned in the reference text) are a common feature across many types of cancer.
Scientific Validity
  • Compilation of Supporting Literature Across Cancer Types: The table effectively compiles a list of references supporting the existence of mitochondrial abnormalities across a diverse range of common human cancers. This breadth strengthens the authors' argument that such abnormalities are widespread.
  • Reliance on Cited Studies: The validity of the claim rests heavily on the quality and findings of the cited studies. The table itself does not provide primary data or summarize the specific abnormalities found in each study (e.g., specific structural defects, functional assays, changes in mitochondrial number).
  • Use of Secondary Source Data: The reference text indicates the table draws evidence from a previous review by Seyfried et al. (2020). While efficient, this relies on the accuracy and potential biases of that secondary source's compilation.
  • Scope of Claim vs. Presented Evidence: The table supports the general claim of mitochondrial abnormalities in common cancers. However, asserting abnormalities in 'all' major cancers (as per reference text) based solely on this list might be an overstatement unless the cited review (Seyfried et al., 2020) provides exhaustive coverage.
Communication
  • Clear Format: The table uses a clear and simple format, listing cancer types in the first column and corresponding citations in the second. This makes it easy for readers to identify relevant literature for specific cancers.
  • Direct Citation: Providing citations directly allows interested readers to verify the claims and delve deeper into the specific mitochondrial abnormalities reported for each cancer type.
  • Lack of Specific Detail within Table: The table itself lacks detail regarding the specific nature (number, structure, function) or prevalence of the mitochondrial abnormalities reported in the cited studies. Readers must consult the references or rely on the authors' interpretation.
  • Caption Clarity: The caption clearly states the table's content: observations of mitochondrial abnormalities in common cancers.
Table 2 Lipid droplets accumulation observed in common cancers
Figure/Table Image (Page 8)
Table 2 Lipid droplets accumulation observed in common cancers
First Reference in Text
Electron microscopy images of lipid droplets from several different cancer types are presented in Figs. 2 & 3.
Description
  • List of Cancer Types: This table lists numerous common types of cancer, including Bladder, Breast/Mammary, Colorectal/Gastric, Gliomas, Kidney/Renal, Leukemias/lymphomas, Liver/Hepatic, Lung, Melanoma, Neuroblastoma, Osteosarcoma, Ovarian, Pancreatic, Prostate, Retinoblastoma, Rhabdomyosarcomas, Salivary Gland/Oral, and Uterine/Endometrial cancers.
  • Purpose: Referencing Evidence for Lipid Droplet Accumulation: For each listed cancer type, the table provides one or more citations, referencing published scientific studies. These citations point to research where the accumulation of lipid droplets (small fat storage sacs within the cell's cytoplasm) has reportedly been observed in those specific cancers.
  • Nature of Content: Literature References: The table itself does not present any numerical data, images, or statistics. It functions as a literature index, directing readers to primary research articles that support the claim that lipid droplet accumulation is a feature observed across a wide variety of common cancers.
  • Relationship to Table 1: The paper mentions that the arrangement of cancers in this table is aligned with Table 1, which lists mitochondrial abnormalities, suggesting a correlation between the two phenomena for the listed cancers.
Scientific Validity
  • Compilation of Supporting Literature Across Cancer Types: The table compiles references suggesting lipid droplet accumulation is observed across a broad spectrum of common cancers, supporting the authors' assertion that this is a widespread phenomenon in malignancy.
  • Reliance on Cited Studies: The scientific strength of the table's claim depends entirely on the findings and rigor of the individual studies cited. The table itself does not provide primary evidence.
  • Use of Secondary Source Data: The text indicates this table is based on a previous compilation (Seyfried et al. 2024). Relying on a secondary source introduces the potential for propagation of errors or selection bias from the original review.
  • Interpretation vs. Observation: The link between lipid droplet accumulation and the proposed underlying cause (OxPhos inefficiency) is an interpretation presented in the main text, not directly demonstrated by the table's list of observations.
Communication
  • Clear Format: Similar to Table 1, the table uses a clear, two-column format listing cancer types and corresponding citations, facilitating easy lookup.
  • Direct Citation: The direct citation allows readers to investigate the primary literature regarding lipid droplet accumulation in specific cancer types.
  • Lack of Specific Detail within Table: The table lacks specific details about the nature, extent, or method of observation of lipid droplet accumulation reported in the cited studies. Readers must consult the references for this information.
  • Caption Clarity: The caption accurately reflects the content, clearly stating it pertains to observations of lipid droplet accumulation in common cancers.
  • Alignment with Table 1: The alignment of cancer types listed in this table with those in Table 1 (as mentioned in the text) helps readers correlate findings on mitochondrial abnormalities and lipid droplet accumulation for the same cancers.
Fig. 3 Lipid droplet accumulation in various malignant cancers.
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Fig. 3 Lipid droplet accumulation in various malignant cancers.
First Reference in Text
Electron microscopy images of lipid droplets from several different cancer types are presented in Figs. 2 & 3.
Description
  • Subject and Key Structure (Lipid Droplet): This panel (A) is an electron micrograph showing a portion of a glioblastoma cell (GBM), a type of brain cancer. It highlights a large, roughly spherical lipid droplet (labeled LD), which is a cellular structure for storing fats.
  • Association with Abnormal Mitochondria: The lipid droplet is shown in close proximity to a mitochondrion (labeled M), the cell's energy producer. The caption notes that the mitochondrion appears abnormal, exhibiting features like 'cristae disarrangement and cristolysis' (loss or damage to the internal folds essential for energy production).
  • Lipid Droplet-Mitochondria Interaction: Arrows point to a 'contact site,' indicating a direct physical interaction or very close association between the surface of the lipid droplet and the outer membrane of the mitochondrion. This is described as 'lipid droplet-associated mitochondria.'
  • Magnification/Scale: A scale bar indicates that the length shown by the bar represents 2.34 micrometers ( and extmu{}m), providing a sense of scale for the cellular structures.
Scientific Validity
  • Evidence of Lipid Droplet in GBM: The micrograph provides direct visual evidence of a large lipid droplet within a human GBM cell.
  • Evidence of LD-Mitochondria Association: The image visually supports the concept of close physical association between lipid droplets and mitochondria in this cancer type, a phenomenon increasingly studied in cancer metabolism.
  • Source Citation: The image is explicitly cited from a previous publication (G. Arismendi-Morillo 2011), lending credibility and allowing verification.
  • Focus on Association over Mitochondrial Detail: While morphology suggests mitochondrial abnormality (cristolysis mentioned in caption), the image itself is primarily illustrative of the LD and its association, rather than a detailed depiction of mitochondrial defects compared to normal.
Communication
  • Clarity and Annotation: The image clearly shows the intended structures. Annotations (M for mitochondria, LD for lipid droplet, arrows for contact site) guide the viewer effectively.
  • Scale Bar: A scale bar (2.34 and extmu{}m) is provided, allowing for assessment of the size of the structures shown.
  • Caption-Image Congruence: The caption specifically mentions the interaction between the lipid droplet and mitochondria, which is visually supported by the image and annotation (arrows).
Fig. 4 The origin of cancer as a mitochondrial metabolic disease.
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Fig. 4 The origin of cancer as a mitochondrial metabolic disease.
First Reference in Text
Not explicitly referenced in main text
Description
  • Model Overview and Initiating Factors: This figure presents a schematic model illustrating the authors' theory that cancer originates as a mitochondrial metabolic disease. It proposes that various 'unspecific risk factors' – such as viruses, rare inherited mutations (germline mutations), aging, inflammation, environmental toxins (carcinogens, microplastics, forever chemicals), radiation, and low oxygen levels (hypoxia) – can initiate the process.
  • Mitochondrial Damage and ROS Production: These diverse factors are shown converging on the mitochondria (the cell's powerhouses), causing damage. This damage leads to increased production of Reactive Oxygen Species (ROS), which are chemically reactive molecules containing oxygen that can damage cellular components like DNA, proteins, and lipids.
  • Metabolic Shift: OxPhos Insufficiency and SLP Compensation: Mitochondrial damage and ROS are proposed to cause 'OxPhos insufficiency' – a reduced capacity for efficient energy (ATP) production using oxygen. Simultaneously, this triggers a compensatory increase in energy production through less efficient pathways collectively termed Substrate-Level Phosphorylation (SLP), occurring both in the cytoplasm and within the mitochondria.
  • Retrograde Signaling, Genomic Instability, and Oncogene Activation: The mitochondrial stress is shown to activate Retrograde Signaling (RTG), a communication pathway from the mitochondria to the cell nucleus. This signaling is proposed to lead to 'Genome Instability' (increased tendency for DNA mutations and rearrangements) and the activation of oncogenes (cancer-promoting genes) like HIF-1a and c-Myc, which further promote the shift towards SLP-dependent metabolism.
  • Graphical Representation of Metabolic Shift and ATP Energy: A simple graph embedded in the figure depicts this metabolic transition over 'Progression (Time)'. It shows the percentage of ATP production from OxPhos starting high (labeled 88%) in normal mitochondria and decreasing over time, while ATP production from SLP starts low (labeled 12%) and increases as the cell becomes malignant. Despite the shift in source, the energy released by breaking down ATP (ATP hydrolysis) is shown remaining constant at approximately -56 kilojoules per mole (-56 kJ), indicated for both normal and cancer states.
  • Link to Cancer Hallmarks: The model links this fundamental metabolic reprogramming (insufficient OxPhos compensated by increased SLP) to the acquisition of established 'Cancer Hallmarks' – key characteristics of cancer cells, such as self-sufficient growth signals, evasion of apoptosis (programmed cell death), sustained angiogenesis (blood vessel formation), and metastasis (spread to other tissues).
  • Resolution of Oncogenic Paradox: The diagram aims to resolve the 'Oncogenic Paradox' (posed by Szent-Gyorgyi), suggesting how numerous different initial insults can all lead to the same endpoint (cancer) via a common mechanism of chronic mitochondrial damage and the resulting metabolic shift.
Scientific Validity
  • Synthesis of Existing Concepts: The model synthesizes various lines of evidence and hypotheses from the field, proposing a unifying framework where diverse carcinogenic stimuli converge on mitochondrial function. The core concept of mitochondrial dysfunction in cancer is well-documented, although its role as the primary origin versus a consequence is debated.
  • Plausibility of Proposed Cascade: The proposed sequence – initial insult -> mitochondrial damage/ROS -> OxPhos insufficiency -> compensatory SLP -> RTG signaling -> oncogene activation/genomic instability -> cancer hallmarks – presents a plausible, albeit simplified, chain of events based on known biological processes.
  • Emphasis on Metabolic Reprogramming and Energetics: The emphasis on chronic OxPhos insufficiency coupled with compensatory SLP as the necessary metabolic state for cancer aligns with the central thesis of the review and challenges alternative views (e.g., SMT). The constancy of ATP hydrolysis energy (-56 kJ) despite the metabolic shift is correctly represented based on thermodynamic principles.
  • Simplification of Complex Biology: The model simplifies extremely complex interactions. The precise mechanisms by which each listed risk factor induces chronic mitochondrial damage, the intricacies of RTG signaling pathways, and the exact interplay between metabolic changes and specific cancer hallmarks are highly complex and context-dependent.
  • Mechanism of Genomic Instability: While ROS can cause DNA damage leading to genomic instability, the direct causal link specifically through RTG signaling as depicted might be an oversimplification or represent one specific pathway among others.
  • Conceptual Framework Strength: The model provides a strong conceptual framework consistent with the authors' mitochondrial metabolic theory of cancer. Its strength lies in offering a potential explanation for the heterogeneity of cancer causes converging on a common metabolic phenotype.
Communication
  • Integration of Complex Concepts: The figure effectively integrates numerous concepts (risk factors, mitochondrial damage, ROS, metabolic shifts, signaling, hallmarks) into a coherent visual model of the proposed theory.
  • Visual Elements and Flow: The use of icons (virus, radiation symbol, etc.) and distinct visual elements (normal vs. cancer mitochondrion) aids comprehension. Arrows clearly indicate proposed causal relationships and flows.
  • Incorporation of Graph: The inclusion of a simplified graph illustrating the proposed shift from OxPhos dominance (high ATP %) to SLP dominance (lower total ATP % but SLP increases) over time/progression effectively visualizes the core metabolic change.
  • Linkage to Cancer Hallmarks: The connection to established Cancer Hallmarks provides context and links the metabolic theory to broader cancer biology concepts.
  • Information Density and Abbreviations: While visually summarizing the theory, the diagram is dense with information and abbreviations (ROS, RTG, SLP, OxPhos, HIF-1a, c-Myc). Understanding requires familiarity with these terms or reference to the main text/legend.
  • Clarity of Central Hypothesis: The representation of the 'oncogenic paradox' and its proposed resolution through this model is clearly depicted.
Fig. 5 High-throughput synergy between the glycolysis and the glutaminolysis...
Full Caption

Fig. 5 High-throughput synergy between the glycolysis and the glutaminolysis pathways drive the dysregulated growth of glioma cells.

Figure/Table Image (Page 13)
Fig. 5 High-throughput synergy between the glycolysis and the glutaminolysis pathways drive the dysregulated growth of glioma cells.
First Reference in Text
Hence, the reduction of circulating glucose and elevation of ketone bodies will deprive tumor cells of energy, and the glucose carbons needed for the synthesis of growth metabolites (Boros et al. 1998; Mazat 2021).
Description
  • Overall Purpose: Glucose-Glutamine Synergy: This figure is a schematic diagram illustrating how glioma cells (a type of brain cancer) utilize two primary nutrients, glucose (a sugar) and glutamine (an amino acid), in a coordinated way to support their rapid growth and survival.
  • Glucose Metabolism: Energy and Biosynthesis: Glucose (shown in blue) enters the cell and undergoes glycolysis in the cytoplasm (the main cell fluid). The diagram shows that carbons from glucose are used not only for energy (producing some ATP, labeled in the cytoplasm) but are also diverted into important biosynthetic pathways: the Pentose Phosphate Pathway (PPP) for making nucleotides (building blocks of DNA/RNA) and the Hexosamine Pathway. Glucose breakdown also contributes to Serine/Glycine production, important for building proteins and producing glutathione (an antioxidant). Excess pyruvate from glycolysis is often converted to Lactic Acid and exported, contributing to extracellular acidification.
  • Glutamine Metabolism: Nitrogen Source and Energy: Glutamine (shown in green) enters the cell and is primarily metabolized within the mitochondria. It provides nitrogen for nucleotide synthesis and the hexosamine pathway. Glutamine is converted to glutamate, which can be used to make glutathione or further processed in the mitochondria. Mitochondrial glutamine metabolism (glutaminolysis) contributes to ATP production (likely via mSLP, as discussed in Fig 1) and produces succinate and glutamate, which are exported from the cell, also contributing to extracellular acidification.
  • Contribution to Fatty Acid Synthesis: The diagram highlights the contribution of both pathways to Fatty Acid (F.A.) Synthesis, essential components of cell membranes. It notes that under normal oxygen conditions (21% O2), glucose contributes significantly, while under low oxygen (hypoxia, 0.1% O2), glutamine metabolism becomes a key source for fatty acid building blocks (likely via reductive carboxylation, though not explicitly drawn).
  • Regulatory Feedback (Succinate-HIF1a): A key regulatory link is shown where succinate, an end-product of glutaminolysis, stabilizes HIF1a (Hypoxia-Inducible Factor 1-alpha). HIF1a is a protein that promotes glycolysis. This suggests a feedback loop where glutamine metabolism can enhance glucose metabolism.
  • Extracellular Acidification: The export of acidic products like lactic acid, succinic acid, and glutamic acid is highlighted as causing 'Extracellular Acidification,' contributing to the acidic tumor microenvironment.
Scientific Validity
  • Accuracy of Core Metabolic Pathways: The diagram accurately represents the major metabolic fates of glucose and glutamine in cancer cells, including glycolysis, PPP, glutaminolysis, TCA cycle activity, and their contributions to biomass precursors (nucleotides, amino acids, lipids) and waste products (lactate, succinate).
  • Representation of Biosynthetic Contributions: The depiction of glucose carbons feeding into PPP and serine/glycine synthesis, and glutamine providing nitrogen for nucleotides and carbons/nitrogen for hexosamine pathway and glutathione, is consistent with established roles of these nutrients in cancer cell biosynthesis.
  • Oxygen-Dependent Fatty Acid Synthesis Source: Highlighting the differential contribution to fatty acid synthesis based on oxygen levels (glucose in normoxia, glutamine potentially more in hypoxia via reductive carboxylation) reflects current understanding, although the diagram simplifies this complex regulation.
  • Accuracy of Succinate-HIF1a Link: The stabilization of HIF1a by succinate is a known mechanism linking mitochondrial metabolism (specifically TCA cycle dysfunction or high glutaminolysis flux) to the glycolytic phenotype, supporting the depicted regulatory connection.
  • Contributors to Acidification: The figure correctly identifies lactate, succinate, and glutamate as potential contributors to extracellular acidification in cancer.
  • Concept of Metabolic Synergy: The overall concept of metabolic synergy between glycolysis and glutaminolysis being crucial for sustaining the high proliferation rates and biosynthetic demands of cancer cells, particularly gliomas, is well-supported in the literature.
Communication
  • Color-Coding: The diagram effectively uses color-coding (blue tones for glucose pathways, green tones for glutamine pathways) to distinguish the contributions of the two major fuels.
  • Compartmentalization: The layout clearly separates extracellular, intracellular (cytosol), and mitochondrial compartments, aiding understanding of where different metabolic processes occur.
  • Clarity of Flow and Outputs: Arrows generally indicate the flow of metabolites effectively. Key outputs like Biomass, ATP, Protein Synthesis, and Extracellular Acidification are clearly labeled.
  • Illustration of Synergy: The diagram successfully illustrates the concept of synergy by showing how both glucose and glutamine contribute carbons and nitrogen to essential biosynthetic pathways (nucleotides, hexosamine pathway, fatty acids, glutathione, proteins) needed for cell growth.
  • Simplification vs. Detail: While depicting major pathways, the diagram is necessarily simplified. Some connections (e.g., the exact link between mitochondrial metabolism and Fatty Acid Synthesis under different oxygen conditions) could be more explicitly detailed, although this might compromise overall clarity.
  • Highlighting Regulatory Links: The indication of succinate stabilizing HIF1a provides a key regulatory link between glutaminolysis and the upregulation of glycolysis.

Conclusions

Key Aspects

Strengths

Suggestions for Improvement

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