Angiogenesis and Cancer

Tumours and Blood Supply

• Tumours require a blood supply to grow and survive.

• They typically achieve this by growing new blood vessels.

Normal Blood Vessel Development

• Microvasculature: Small blood vessels, where most interactions with tumours occur.

  • Orderly structure: Composed of endothelial cells surrounded by a basement membrane, with pericytes and smooth muscle cells present.

• Hemangioblasts: Pluripotent mesenchymal cells that differentiate into angioblasts (endothelial progenitor cells).

  • Angioblasts aggregate to form blood vessels during a process called vasculogenesis.

• Differentiation: Endothelial progenitors develop into mature endothelial cells, forming primitive vascular tubes.

Angiogenic Gradient and Hypoxia

• The angiogenic gradient is created by vascular growth factor signaling molecules from embryonic tissues under hypoxia (low oxygen levels).

• This gradient attracts precursor cells to encourage new blood vessel formation in needed areas.

• Pericyte Role: They migrate to new blood vessels; vascular pruning shapes the vessel network into an organized structure (arborization).

Angiogenesis vs. Vasculogenesis

• Angiogenesis: Sprouting of new vessels from existing ones (most common in tumours).

• Vasculogenesis: Formation of blood vessels from scratch (de novo).

• Angiogenic switch: Process that stimulates angiogenesis by increasing VEGF levels.

Key Steps in Normal Angiogenesis

1. VEGF Signaling: Initiates angiogenesis by stimulating endothelial cells, causing them to dissolve the basement membrane.

2. Pericyte Dropout: Localized dissociation of pericytes promotes new vessel formation.

3. Tip and Stalk Cells Formation:

   • VEGF influences differentiation of a leading tip cell.

   • Surrounding stalk cells proliferate under the tip cell's guidance, forming new blood vessels.

4. Vessel Fusion and Maturation: New vessel sprouts fuse, mature, and establish a vascular loop, completing the angiogenic process once blood flow resolves local hypoxia.

Hypoxia and Cellular Response

• HIF1⍺ Protein: A cellular sensor for hypoxia that drives VEGF expression.

  • In hypoxic conditions, it remains active due to lack of hydroxylation and subsequent degradation.

Tumour Angiogenesis and Characteristics

• Tumour cells produce VEGF and other factors leading to uncontrolled angiogenesis.

• Tumour vasculature is often disordered with:

  • Rough endothelial surfaces

  • Endothelial hypertrophy

  • Hyperpermeability

  • Disorderly branching patterns

Mechanisms of Tumour Vascularization

• Angiogenesis: Main method for tumour blood supply.

• Co-option: Tumours grow along existing blood vessels.

• Vascular Mimicry: Tumours form vessel-like structures using ECM and tumour cells, rather than typical blood vessels.

Potential Angiogenesis Inhibition Strategies

• Targeting various pathways involved in angiogenesis can be a therapeutic approach:

  • Disrupting tumour blood vessels.

  • Inhibiting VEGF signaling and HIF1⍺ activity.

  • Utilizing vascular disrupting agents in clinical practice.

Summary

• Importance of understanding normal vascular development and tumour angiogenesis.

• Hypoxia is a key trigger for angiogenesis in tumours, altering the typical controlled process seen in normal development.