2014 The pentose phosphate pathway and cancer

The Pentose Phosphate Pathway (PPP) and Its Role in Cancer

Introduction to the Pentose Phosphate Pathway (PPP)

  • Definition: The PPP branches from glycolysis at the glucose-6-phosphate (G6P) step.

  • Functionality: Essential for ribonucleotide synthesis and a primary source of NADPH.

  • Importance: Supplies cancer cells with anabolic building blocks and aids in oxidative stress management due to high rates of metabolism.

  • Historical Context: The pathway gained attention in the 20th century for its link to hemolytic anemia caused by oxidants affecting glutathione levels.

Structure of the PPP

  • Branches:

    • Oxidative Branch: Generates NADPH and ribonucleotides through three irreversible reactions:

      1. G6PDH Activity: Converts G6P into 6-phosphogluconolactone, producing NADPH.

      2. Hydrolysis: 6-phosphogluconolactone is hydrolyzed to form 6-phosphogluconate.

      3. Decarboxylation: 6-phosphogluconate is converted into Ru5P while generating a second NADPH.

    • Non-Oxidative Branch: Composed of reversible reactions that convert glycolytic intermediates (F6P, G3P) to and from pentose phosphates.

  • Enzymatic Regulation: Enzymes are allosterically regulated, allowing flexible adaptation to cellular needs, especially in rapidly dividing cells.

The Role of PPP in Cancer Metabolism

  • Nucleotide Synthesis: Higher demand for nucleotides in fast-dividing cancer cells drives the PPP activity.

  • NADPH Generation: Essential for various biosynthetic processes, including lipid synthesis and antioxidant defense.

  • Adaptation under Stress: Cancer cells modulate PPP activity to cope with oxidative stress and the tumor microenvironment.

Regulatory Enzymes in the PPP

G6PDH (Glucose-6-Phosphate Dehydrogenase)

  • Main Function: First committed step in the oxidative branch producing NADPH.

  • Regulation: Activity influenced by the NADP+/NADPH ratio; high NADPH levels inhibit G6PDH.

  • Expression: Overexpressed in many tumor types and regulated by growth factors via post-translational modifications.

6-Phosphogluconolactonase (6PGL)

  • Function: Catalyzes the hydrolysis of 6-phosphogluconolactone.

  • Implications in Disease: Mutation linked to hemolytic anemia.

6-Phosphogluconate Dehydrogenase (6PGDH)

  • Role in Cancer: Critical for lung cancer cell proliferation; silencing this enzyme leads to increased ROS and cellular senescence.

Ribulose-5-Phosphate Isomerase (RPI) and Epimerase (RPE)

  • RPI Function: Converts Ru5P to ribose-5-phosphate (R5P), essential for nucleotide synthesis.

  • RPE Role: Converts Ru5P to xylulose-5-phosphate (Xu5P), which affects glycolysis.

Transketolase (TKT) and Transaldolase (TALDO)

  • Functionality in Non-Oxidative Branch: TKT and TALDO facilitate nucleotide synthesis by redirecting glycolytic intermediates.

  • Role in Cancer: Elevated expression in various cancers promotes ribonucleotide synthesis crucial for cell proliferation.

Oncogenic Regulation of the PPP

Tumor Suppressor p53

  • Role: Transcription factor that regulates metabolic genes including those in the PPP.

  • Effects:

    • Inhibits glucose transporters (GLUT1, GLUT4) increasing glucose availability for the PPP.

    • Suppresses glycolytic enzymes, enhancing the oxidative PPP.

Oncogenes (e.g., K-Ras)

  • Activation Effects: Upregulates non-oxidative PPP and increases glucose flux through enhanced HK2 expression, crucial for nucleotide synthesis.

mTORC1 and Nrf2 Pathways

  • mTORC1: Enhances transcription of G6PDH and upregulates oxidative PPP activity for fatty acid synthesis.

  • Nrf2: Activated in response to oxidative stress; increases transcription of PPP enzymes aiding NADPH and nucleotide production.

Implications for Cancer Therapy

  • Therapeutic Target: Targeting PPP may disrupt cancer cell metabolism and reduce oxidative stress resistance.

  • Challenges: Drug resistance linked to upregulated PPP; targeting the pathway must consider the complex interplay between cancer metabolism and therapeutic response.

  • Future Directions: Understanding regulatory mechanisms of the PPP could yield new therapeutic strategies in oncology.