TME and balancing tumour growth

• Describe the role of endothelial cells in blood vasculature

  Endothelial cells form a single cell layer that lines blood vessels. Intercellular clefts connect adjacent endothelial cell formed by tight junctions, which control the passage of various molecules across the vessel wall.
• Describe how endothelial cells are altered in tumour blood vasculature

  Tumour endothelial cells differ from normal endothelial cells:

  • Express lower levels of adhesion molecules, which leads to dysregulation of barrier function
  • Express higher levels of immune checkpoint molecules, which results in immunosuppression
• Describe the role of Pericytes in blood vasculature

  Pericytes surround blood vessels and are embedded within the basement membrane of vessels and found adjacent to endothelial cells. Their function is to support the permeability and maturation of the vasculature.
• Describe how Pericytes are altered in tumour blood vasculature

  In tumours, impaired interaction of Pericytes and endothelial cells contributes to a leaky and dysfunctional tumour vasculature. Pericytes interact with other stromal cells and cancer cells via Paracrine mechanisms, leads to modulation of the TME
• Describe the function of lymphatic endothelial cells in vasculature

  Lymphatic endothelial cells form the walls of the lymphatic vessels, which drain fluid from lymph ducts between cells into the venous circulation. Lymph ducts also allow antigen presenting cells to access lymph nodes and prime T and B cells.
• Describe the function of lymphatic endothelial cells in TME

  Lymphatic endothelial cells form the lymphatic vessels, which provide an additional dissemination route for cancer cells in addition to blood vessels. Recently, they have been identified as direct regulators of anti-tumour immunity, via the secretion of chemokines, and can present tumour antigens, but also immune checkpoint molecules.
• Describe arrangement of tumour cells around capillaries

  Tumour cells located more than 0.2 mm away from blood vessels were found to be non-growing, while others even further away were seen to be dying. This is because 0.2 mm represents the distance that oxygen can effectively diffuse through living tissues, therefore tumour cells exceeding this distance will suffer from severe hypoxia and low pH due to lactic acid production from anaerobic respiration. Tissues suffering from hypoxia are at risk of becoming necrotic.
• Describe the angiogenic switch

  During early tumour progression, tumour cells acquire the ability to induce neoangiogenesis. This involves heterotypic interactions among three cell types:

  • Release of unknown signals from tumour cells that recruit mast cells and macrophages
  • Release of MMP-9 by inflammatory cells triggers release of tumour cell secreted VEGF from ECM, creating a high local concentration
  • VEGF triggers Proliferative response of nearby endothelial cells
• Describe the production of VEGF

  Vascular endothelial growth factor production is stimulated by intracellular oxygen tension in TME, which is detected by VHL protein. VHL protein triggers HIF-1Alpha and HIF-1beta transcription factor accumulation. HIF-1 drives expression of angiogenesis related genes, such as VEGF, which can be synthesised by tumour cells, macrophages or myofibroblasts, depending on tumour type and stage of progression.
• Describe capillary formation in tumours

  1. Angiogenic factors, such as VEGF, stimulate proliferation of endothelial cells, which deform to form cylindrical walls of capillaries.
  2. Capillaries move towards the highest local concentration of angiotensin factors, penetrating existing tissue if necessary
  3. Tumour capillaries are 3X wider than normal and assembled haphazardly and often truncated
• Describe the impact of too much VEGF

  Too much VEGF signalling can cause plasma membranes of adjacent endothelial cells to separate. This induces capillary permeability, which results in leaky capillaries.
• Describe the action of PDGF-B

  Localised concentrations of PDGF are required for recruitment of pericytes to capillaries and PDGF-B gets sequestered into the ECM. This creates a high local concentration around endothelial cells, which recruits pericytes to the capillaries.
• Describe why there is high intratumoral hydrostatic pressure

  1. Tumour associated capillaries leak fluid into the parenchymal space of the tumour
  2. Expansion of cancer cell proliferation leads to collapse of lymphatic vessels, which disrupts fluid drainage within the core of solid tumours
  3. PDGF released by carcinoma cells induces contraction of myofibroblasts, which squeezes interstitial fluid

  This complicates the administration of anti-cancer drugs, as they usually rely on a pressure gradient to enter interstitial spaces of the tumour from the circulation.
• Describe the action of anti-VEGF therapies

  This is a form of anti-angiogenic strategy, which utilises anti-VEGF monoclonal antibodies. In mice they can return the vasculature to a normal configuration, which reduces interstitial tumour pressure. Leads to normalisation, but not regression of tumour vasculature. However, excess reduction of tumour vasculature can promote hypoxia, which can induce invasiveness of cancer cells so dosing and timing must be optimised.
• What is cellular senescence?

  This is a typically irreversible form of Proliferative arrest, where cells remain metabolically active but lose the ability to enter the cell cycle. This is usually induced by nutrient deprivation, DNA damage, organelle damage or oncogene induced signalling.
• Describe the central regulators of cellular senescence

  1. Irreparable DNA damage stimulates expression of p53 target genes, including p21 and p16.
  2. These are CDK inhibitors, which are responsible for arresting the cell cycle and inducing cellular senescence.
  3. These CDK inhibitors also suppress E2F transcription factors
• Describe SASP

  Senescence triggers the activation of senescence associated secretory phenotype (SASP). SASP involves the release of chemokines, cytokines and proteases, which vary depending on cell type. SASP factors from senescent fibroblasts have been shown to induce cancer cell proliferation and invasion in culture. SASP gene expression can be activated by:

  • Stabilisation of SASP mRNAs
  • Transcriptional activation of SASP genes
  • Epigenetic regulation of SASP genes
• Describe the tumour suppressive actions of SASP

  In normal tissue, SASP can reinforce senescence in an autocrine manner. SASP can also recruit immune cells to clear themselves, which is known as senescence surveillance.

  • SASP induces autocrine senescence by triggering IL-6 release
  • SASP induces Paracrine senescence by triggering VEGF and TGF-beta release
  • SASP also induces IL-1 and IL-2 release, which recruits neutrophils and macrophages
• Describe the tumour progressive actions of SASP

  SASP factors can promoter cancer progression by enhancing angiogenesis, cancer cell proliferation, EMT and metastasis by enhancing the immunosuppressive environment and suppressing anti-tumour immunity.

  • SASP induces VEGF release, which triggers angiogenesis
  • SASP induces IL-6 release, which triggers EMT and invasions
  • SASP induces CKIa knockout, which induces cancer cell proliferation
  • SASP induces TGF-beta release, which induces metastasis
• Describe the use of SASP in cancer therapy

  CDK4/6 inhibitors are used to inhibit the transition from G1 to S phase in many cancer cell types, specifically breast cancer. One example is palbociclib, which is used to treat oestrogen receptor positive and human EGF receptor 2 negative breast cancers. CDK4/6 inhibitors mimic the function of p16, which means it is likely they induce cellular senescence and induce SASP, which increases vascularity and immune response.