Pyruvate Oxidation: Unraveling Cancer’s Metabolic Strategy — Part 3 of 5
By Eva Fata
This is 3rd Part in a 5 Part Series I’ve written comparing cellular respiration in normal cells versus cancer cells. In this part, I will be exploring pyruvate oxidation in cancer cells. For all parts, please see the links at the bottom of this article.
Pyruvate Oxidation in Cancer Cells
Pyruvate oxidation also known as the pyruvate dehydrogenase (PDH) complex reaction is a fundamental process in cellular metabolism where pyruvate is converted into acetyl-CoA in the mitochondria. Acetyl-CoA then enters the citric acid cycle to produce energy in the form of ATP through oxidative phosphorylation.
In cancer cells, pyruvate oxidation can be dysregulated because of various alterations in cellular metabolism.
Cancer cells may exhibit alteration which promotes glycolysis over oxidative metabolism. This may occur through various mechanisms like mutations in regulatory enzymes such as pyruvate dehydrogenase kinase (PDK) or pyruvate dehydrogenase phosphatase (PDP) which control pyruvate oxidation. Increased PDK activity can then lead to the reduction of pyruvate oxidation which then reduces the conversion of pyruvate to acetyl-CoA and promotes glycolysis.
Overall the dysregulation of pyruvate oxidation in cancer contributes to the metabolic reprogramming observed in cancer metabolism which helps support the increased energy demands and biosynthetic requirements which cancer cells need for proliferation and survival.
Regulation of Pyruvate Levels in the Cytosol
In cancer cells, pyruvate metabolism is often dysregulated, leading to impaired pyruvate oxidation in the mitochondria despite the availability of oxygen, a phenomenon known as aerobic glycolysis or the Warburg effect.
Several mechanisms contribute to this scenario such as negative regulation of mitochondrial pyruvate transporters and alterations in enzymes which affect pyruvate availability in the cytosol.
Pyruvate is a critical molecule in cellular metabolism and serves as a branch point between glycolysis and oxidative phosphorylation. It can be formed through the fermentation of glucose by glycolysis where phosphoenolpyruvate (PEP) is converted into pyruvate by pyruvate kinase (PK).
The regulation of cytosolic pyruvate levels can be influenced by lactate dehydrogenase (LDH), an enzyme that catalyzes the reversible conversion of pyruvate to L-lactate while oxidizing NADH to NAD+. This is crucial for cells which are reliant on glycolysis for ATP as it helps maintain NAD+ levels which are required for glycolytic flux ensuring cell survival.
In aggressive cancer, the LDH-5 isoform is often upregulated. This upregulation causes a rapid conversion of pyruvate to L-lactate, which leads to a reduction in cytosolic pyruvate available for mitochondrial oxidation.
Metabolism of Pyruvate in Mitochondria
Under aerobic conditions, pyruvate is mostly transported into the mitochondrial matrix for metabolism by enzymes in the citric acid cycle which then leads to ATP production by the electron transport chain. However, in hypoxic tumour microenvironments, pyruvate may remain in the cytosol and be further diverted into two main pathways in the cytosol
- Conversion to lactate is facilitated by lactate dehydrogenase (LDH), which regenerates NAD+ required for glycolysis to continue.
- Use in other biosynthetic reactions, contributes to the synthesis of macromolecules needed for cell growth and proliferation.
One key aspect of pyruvate metabolism is the transfer from the cytosol into the mitochondrial matrix. Mitochondria possess distinct outer (OM) and inner (IM) membranes that regulate the exchange of metabolites between the cytosol and mitochondrial compartments. The outer membrane is permeable to small molecules through voltage-dependent anion channels (VDAC), also known as porins. VDAC allows the passage of molecules depending on their charge. When closed they restrict the passage of metabolites like pyruvate. VDAC is important for pyruvate transport and mitochondrial functions. Patients who lack VDACs have impaired pyruvate oxidation and ATP production rates. Additionally, in eukaryotic cells, VDACs play a key role in regulating mitochondrial metabolism and intracellular energy flow. The knockdown VDAC1/2/3 has been shown to reduce mitochondrial membrane potential and decrease levels of NAD(P)H, ATP, ADP, and total adenine nucleotides in cancer cells.
Once pyruvate enters the mitochondrial matrix, it undergoes different metabolic processes like participation in the citric acid cycle to support the generation of ATP in oxidative phosphorylation or the conversion into other important metabolites like fatty acids or amino acids. This is determined by the activities of enzymes like pyruvate dehydrogenase complex (PDC) and pyruvate carboxylase (PC).
The pyruvate dehydrogenase complex (PDC) is a key enzyme complex which is located in the mitochondrial matrix and catalyzes the conversion of pyruvate to acetyl-CoA, a critical step for its entry into the citric acid cycle.
The activity of PDC is modulated by pyruvate dehydrogenase kinases (PDKs), which phosphorylate specific serine residues in the E1 alpha subunit, leading to PDC inactivation. PDKs are upregulated in many metabolic diseases and cancers where they contribute to the reduction of PDC which then conserves different molecules for cellular growth and promotes glycolytic metabolism.
Pyruvate carboxylase (PC) is a mitochondrial matrix enzyme which catalyzes the conversion of pyruvate to oxaloacetate producing a process known as anaplerosis. The activity is regulated by two PC genes. In certain cancer types, like glioblastoma disruptions in glutamine metabolism can prompt PC expression which makes it sufficient for glutamine-mediated anaplerosis in these tumours.
Structure of MPCs (mitochondrial pyruvate carriers)
Studies in yeast and fruit flies lacking the MPC1 gene showed defects in mitochondrial pyruvate uptake which lead to reduced concentration of acetyl-CoA and citric acid cycle intermediates. The silencing of MPC1 in mammalian cells also impaired pyruvate oxidation. MPC belongs to the SLC54 family of mitochondrial transports. The regulation of pyruvate import into the mitochondria plays a critical role in metabolic adaptation and cell fate determination.
Human diseases associated with pyruvate transport defects are linked to point mutations in MPC1 which indicates its key role in MPC function.
Physiological Functions of MPCs
The transport of pyruvate into the mitochondria plays a key role in cellular homeostasis as it determines whether oxidation phosphorylation or lactic fermentation occurs.
The regulation of MPC activity impacts carbohydrate oxidation differently across different organs which shows the organ-dependent effects of MPC malfunctions of glucose homeostasis. High MPC expression slows down gluconeogenesis, while low MPC expression leads to hypoglycemia. Targeted treatments which are aimed at modulating MPC activity have shown promise in improving insulin resistance in conditions like type II diabetes.
Pyruvate and mitochondrial pyruvate carriers (MPCs) play crucial roles in the central nervous system (CNS) due to the reliance of CNS metabolism on glucose. In neurons, pyruvate is mostly generated through glycolysis and the conversion of lactate produced by astrocytes, a process known as the astrocyte-neuron lactate shuttle. Alterations in MPC activity have been connected to diseases like Alzheimer’s disease and Parkinson’s disease.
MPC Activity in Cancer Cells
Cancer cell metabolism often has high glucose uptake and lactate production even in the presence of oxygen. The control of pyruvate metabolism is key in promoting the transformed phenotype in cancer cells. Alteration in the expression of pyruvate metabolizing enzymes like the upregulation of lactate dehydrogenase (LDH) and glucose transporters can make it possible for increased glucose uptake and lactate production. The inactivation of pyruvate dehydrogenase complex (PDC) which initiates mitochondrial pyruvate oxidation also contributes to the shift towards glycolytic metabolism. The joining of pyruvate kinase M2 slows down pyruvate formation in tumour cells which further impairs pyruvate synthesis. The loss of mitochondrial pyruvate carrier (MPC) activity worsens the Warburg effect favoring glycolysis over oxidative phosphorylation.
Re-expression or overexpression of MPC enhances pyruvate oxidation therefore reducing glycolysis and promoting oxidative phosphorylation which can slow down tumour progression. Therefore the regulation of MPC expression plays a key role in various oncogenic processes related to tumour growth and metastasis.
Downregulation of MPC is associated with poor prognosis in many types of cancer which highlights its significance in cancer metabolism and its potential as a therapeutic target.
Overall, understanding and targeting pyruvate metabolism in cancer cells could be possible strategies for cancer treatment.
Regulation of MPC Expression and its Association with Tumour Progression
The loss of pyruvate entering the mitochondria in tumour cells has been linked to malignancy with deletion of MPC and MPC2 being common in cancer. Impaired MPC activity corresponds to increased glycolysis and tumorigenicity. In pancreatic and colorectal cancer suppressed MPC expression enhances epithelial to mesenchymal transition (EMT) which promotes migratory and invasive properties. Negative correlations between MPC1 expression and overall survival are seen in glioblastoma, esophageal squamous carcinoma, cholangiocarcinoma, lung adenocarcinoma, and colorectal cancer.
Induction of Irregular Angiogenesis
Angiogenesis, the formation of new blood vessels, is a feature of tumour progression driven by the irregular expression of pro-angiogenic factors like vascular endothelial growth factor (VEGF). However, despite the formation of new blood vessels, tumours often remain hypoxic due to the dysfunctional nature of the vascular network they develop.
Low oxygen levels in the tumour microenvironment can trigger adaptive cellular responses which are caused by hypoxia-inducible transcription factors (HIFs), particularly HIF-1 and HIF-2. Hypoxia stabilizes HIF-α subunits which can lead to their accumulation and subsequent activation of genes which are involved in angiogenesis like VEGF. This sets up a cycle where hypoxia can drive abnormal angiogenesis which can further make worse the hypoxic conditions in the tumour microenvironment.
In addition, the overproduction of lactate in glycolytic tumours also contributes to abnormal tumour vasculature. Lactade leads to the stabilization of HIF-1α and increased VEGF levels in tumour endothelial cells (TECs) and other cells in the tumour microenvironment. Lactate drives tumour angiogenesis and also promotes vasculogenesis within the tumour which further supports tumour growth and progression.
Lactate and Immunosuppression
Lactic acidosis in the tumour microenvironment deploys immunosuppressive effects which impairs the immune surveillance of immunogenic tumours. When effector T cells are activated they undergo a metabolic switch from oxidative phosphorylation to glycolysis which leads to lactate production. However, tumour-infiltrating lymphocytes with high levels of lactate in the tumour microenvironment disrupt their metabolism proliferation and more processes.
Regulatory T cells, which mostly rely on oxidative phosphorylation for metabolism are less affected by the reduction in glycolysis which is caused by high lactate levels in the tumour microenvironment. Instead, they are reinforced in their oxidative phosphorylation metabolic program which promotes immune tolerance benefiting tumour progression.
In addition, lactic acid and acidosis negatively impact the activity of anti-tumour immune cells like T lymphocytes and natural killer cells and natural killer T cells. Lactate concentrations which exceed 20 millimetres prompt apoptosis in these immune cells while low pH suppresses T-cell effector function and natural killer cell activity.
Why should I learn about this?
Dysregulated pyruvate metabolism in cancer cells presents many potential targets for therapeutic intervention. By understanding the molecular mechanisms behind these metabolic alterations, researchers can identify specific enzymes or transporters which are involved in pyruvate metabolism and can be targeted with drugs to inhibit cancer cell growth.
Furthermore, dysregulated pyruvate metabolism in cancer cells could serve as prognostic or diagnostic biomarkers for certain cancer types. By assessing the expression levels or activity of enzymes which are involved in pyruvate oxidation clinicians can be able to predict tumour aggressiveness, treatment response or patient outcome which can be valuable for personalized cancer management.
