Study Notes on Pyruvate Kinase Type M2 and Tumor Metabolism

Title: Pyruvate kinase type M2: A key regulator of the metabolic budget system in tumor cells
Authors: Sybille Mazurek, Institute of Biochemistry & Endocrinology, University of Giessen, Germany
Journal: International Journal of Biochemistry & Cell Biology 43 (2011) 969–980
Available online: 13 February 2010

Cell proliferation requires a metabolic budget of intermediates for energy and synthesis of building blocks.
In tumor cells, pyruvate kinase isoenzyme M2 (PKM2, M2-PK) determines glucose's fate: either convert it to lactate for energy regeneration (active tetrameric form, Warburg effect) or synthesize building blocks (nearly inactive dimeric form).
Review discusses regulation of M2-PK expression by transcription factors, interaction with oncoproteins, phosphorylation events, and implications for tumorigenesis.
M2-PK offers a potential target for therapeutic approaches and can serve in the early detection of tumors through quantification in plasma and stool.
Evidence suggests both pro-proliferative and pro-apoptotic roles of M2-PK in the nucleus.

Tumormetabolism research (metabolome) began in the 1920s.
Otto Heinrich Warburg discovered tumor cells metabolize glucose to lactate even in aerobic conditions—this is termed the Warburg effect (Warburg et al., 1924).
In healthy tissues, glucose is completely degraded to CO2 and water via glycolysis, citric acid cycle, and oxidative phosphorylation.
At high oxygen levels, ATP is high, inhibiting key glycolytic enzymes and downregulating glycolysis, a process called the Pasteur effect.
In low oxygen conditions, glycolysis is activated due to higher AMP and ADP levels.
High concentrations of certain hexoses can inhibit mitochondrial respiration in normal and some cancer cells, termed the Crabtree effect; the mechanism remains unknown.

Tumor cells often exhibit increased glucose conversion to lactate despite available oxygen.
Warburg hypothesized mitochondrial respiration defects lead to increased aerobic glycolysis.
Variability exists among tumor tissues regarding the connection of glycolysis and oxidative phosphorylation (OXPHOS) for ATP production.
High glycolytic and OXPHOS deficient types are classified; some can generate most energy through OXPHOS.
Pyrimidine synthesis connects to the mitochondrial electron transport chain, implying mitochondrial dysfunction affects RNA and DNA synthesis.

Pyruvate Kinase (PK): Converts phosphoenolpyruvate (PEP) to pyruvate; key for ATP production in glycolysis.
Four isoenzymes based on tissue distribution and function:
- M1-PK: High affinity for PEP, found in brain/muscle; not allosterically regulated.
- M2-PK: Key in proliferating cells, regulates metabolic budget; can form dimeric (inactive) or tetrameric (active) forms.
- L-PK: Found in gluconeogenic tissues; regulated by diet.
- R-PK: Similar to L-PK, in erythrocytes.

Isoenzyme	Tissue Distribution	Encoding Gene	Amino Acids	Molecular Weight	Kinetic Characteristics	Regulation
M1-PK	Brain, muscle	PKM	531	58 kDa	Tetramer, high affinity	N/A
M2-PK	Adipocytes, tumor cells	PKM	531	58 kDa	Tetramer, high affinity	Metabolic intermediates, oncogenes
L-PK	Liver, kidney	PKL	543	59 kDa	Tetramer, low affinity	Regulated by diet
R-PK	Erythrocytes	PKL	574	62 kDa	Low affinity	Regulated by diet

Gene: The M-gene encodes both M1-PK and M2-PK via splicing (exon 9 for M1-PK, exon 10 for M2-PK).
Differentiation: Increased M2-PK in tumor cells; tissue-specific isoenzymes replaced by M2-PK during tumorigenesis.
Induction: Insulin activates M2-PK via PI3K and MAPK pathways in adipocytes, requiring glucose and glutamine.
Transcription Factors: SP1 positively regulates M2-PK expression; oxidative stress can decrease its activity. HIF-1 suggested to influence expression, though mechanisms are unclear.

M2-PK mRNA and activity increase during S-phase of the cell cycle; modifications may occur post-translation.
Phosphorylation: Influences M2-PK activity; serine and tyrosine sites identified.
Nuclear Translocation: Involving proteins like ZO-2 correlates with increased expression and cell proliferation.

Transcription factors such as c-Myc influence alternative splicing toward M2 or M1 forms; hnRNP proteins were identified as repressors.
Missense mutations and SNPs affect M2-PK properties, influencing tumorigenic behavior.

M2-PK exists as dimeric (inactive) and tetrameric (active) forms, with tetramer favoring energy generation and lactate production.
The dimeric form accumulates metabolic intermediates for biosynthetic processes.
Regulation mechanisms involve allosteric effects from metabolic intermediates like fructose 1,6-P2 and amino acids such as serine and alanine.

M2-PK is positioned as a therapeutic target due to its regulatory role in tumor metabolic pathways and potential as a biomarker for cancer detection and therapy monitoring.

Interactions with oncoproteins modify M2-PK activity; phosphorylation by various kinases affects its tetramer-dimer balance.

High levels of Tumor M2-PK in plasma/stool correlate with various cancers, signifying it as a valuable biomarker for early detection and prognostic relevance.

M2-PK serves as an essential metabolic regulator in tumor cells, with potential prospects for cancer therapies and diagnostics, emphasizing the need for further research in these areas.

Understanding M2-PK's role in metabolism could lead to novel therapeutic strategies and diagnostic tools in oncology.

The review is dedicated to Erich Eigenbrodt, whose contributions were pivotal in understanding M2-PK in cancer metabolism and therapy.