Cancer Biology: Angiogenesis, VEGF, and Anti-Angiogenic Therapies

Blood vessels are lined with endothelial cells, which are simple squamous epithelial cells. Their flat structure facilitates rapid diffusion of substances across the vessel wall. Oxygen diffuses from red blood cells through endothelial cells into tissue cells, while carbon dioxide diffuses in the opposite direction. This single-cell barrier enables efficient exchange between blood and tissues.

The circulatory system delivers oxygen and nutrients to tissues while removing wastes through capillaries—smallest blood vessels with single-layer endothelial walls. Substances cross capillary walls via diffusion (lipid-soluble molecules), facilitated diffusion (glucose via transporters), osmosis (water), and transcytosis (vesicles). Bulk flow occurs through intercellular clefts and fenestrated pores. Filtration happens at the arterial end where higher capillary blood pressure pushes plasma outward, while reabsorption occurs at the venous end. Hydrostatic pressure (higher inside capillaries) drives outward flow, opposed by oncotic pressure from blood proteins (mainly albumin). Since arterial ends are closer to the heart, pressure decreases along the capillary bed. Net filtration exceeds reabsorption, leaving ~15% fluid in tissues, which the lymphatic system returns to circulation.

The cardiovascular system (CVS), also known as the circulatory system, is a major organ system consisting of the heart, blood, and blood vessels that delivers nutrients, oxygen, hormones, and immune cells to all body cells while removing metabolic wastes like carbon dioxide; the heart acts as a natural pump that generates blood pressure to propel blood through arteries (which carry blood away from the heart to organs), veins (which return blood back to the heart), and capillaries (which surround body cells to deliver and absorb oxygen and nutrients).

Capillaries are the finest blood vessels, thinner than a human hair, responsible for delivering oxygen and removing metabolic waste from tissues. At the tips of these blood vessels are endothelial cells, which line the interior walls of blood vessels like tiles in a kitchen. These endothelial cells secrete protein factors called survival factors that keep adjacent tissues alive and healthy.

Endothelial cells are the cells that form the inner lining of capillaries, which are the smallest blood vessels in the body. These capillaries are responsible for transporting oxygen and other nutrients to the brain and other organs throughout the body.

Cell signaling is the communication process between cells, essential for organism development and function. Cells must receive and respond to signals from their surroundings to perform modifications and division. The three main stages are: (1) Receiving the signal when a signaling molecule binds to a receptor, (2) Signal transduction transmitting the signal from exterior to interior, and (3) Response where the cell reacts. Signaling molecules (ligands) are classified by polarity: hydrophilic molecules (peptides, amino acids, neurotransmitters like dopamine, histamine, serotonin, cytokines) cannot cross the plasma membrane and require cell surface receptors, while hydrophobic molecules (steroid hormones, retinoids, nitric oxide, vitamin D) can cross the membrane and bind intracellular receptors. There are four main types of cell signaling: (1) Autocrine signaling - cells respond to signals they themselves secrete, (2) Paracrine signaling - cells respond to signals from nearby cells, (3) Juxtacrine signaling - cells communicate through direct physical contact via transmembrane proteins, (4) Endocrine signaling - signaling molecules travel through the bloodstream to distant target organs. Receptors are classified into cell surface receptors (integral membrane proteins on the plasma membrane) and intracellular receptors (in cytoplasm or nucleus). Cell surface receptors bind hydrophilic signaling molecules and have three domains: extracellular (ligand binding), transmembrane (hydrophobic), and intracellular (signal transduction). Cell surface receptors are further classified into: (1) Ion channel-linked receptors - directly open ion channels when activated, (2) G protein-coupled receptors (GPCRs) - activate trimeric G proteins (alpha, beta, gamma subunits) to transmit signals, (3) Enzyme-linked receptors - have enzymatic activity (like tyrosine kinase) activated by ligand binding.

Cell signaling is the process by which cells communicate through chemical signals. A signaling cell creates molecules (ligands) that carry signals to target cells. The target cell has receptors that catch these signals and generate a response. Receptors are proteins located in the plasma membrane with three domains: extracellular domain (outside the cell, hydrophilic), transmembrane domain (embedded in the membrane), and cytoplasmic domain (inside the cell, hydrophilic). A ligand is a signal molecule that binds to receptors on target cells, also called primary messengers. Primary messengers carry signals from outside the cell to the cell membrane but do not enter the cell. When a primary messenger binds to a receptor, it triggers the activation of secondary messengers inside the cell. Extracellular signaling includes three main types: (1) Endocrine signaling - hormones released into bloodstream for long-distance communication; (2) Paracrine signaling - signals transmitted between nearby cells, such as neurotransmitters between neurons; (3) Autocrine signaling - cells signal to themselves, seen in cancer cells and T-cells. Juxtacrine signaling involves direct physical contact between cells through plasmodesmata in plants or cell junctions in animals. Cell receptors are classified into two main types: (1) Extracellular receptors - located on the cell surface and interact with signals outside the cell, including GPCRs, RTKs, and TGF receptors; (2) Intracellular receptors - located inside the cell (cytoplasm or nucleus) and interact with signals that can cross the cell membrane.

Cellular signaling begins when extracellular ligands bind to cell surface receptors, causing conformational changes that trigger intracellular events. Receptors are classified as extracellular (embedded in plasma membrane, converting extracellular signals to intracellular signals) or intracellular (regulating gene expression). Nuclear receptors directly bind DNA to control gene transcription. Examples include G protein-coupled receptors (seven transmembrane domains, associated with heterotrimeric G proteins), receptor tyrosine kinases (with kinase domains), integrins (connecting cells to extracellular matrix), and ligand-gated ion channels (opening upon ligand binding).

Ligands are signaling molecules secreted by signaling cells that bind specifically to receptors on target cells, inducing conformational changes to transmit signals; ligands are categorized into membrane-bound and secretory types, with secretory ligands further divided by distance traveled (endocrine, paracrine, autocrine) and by chemical nature (lipophilic molecules crossing the plasma membrane to bind intracellular receptors, versus hydrophilic molecules binding cell surface receptors).

Cellular signaling (cellular communication) is the capacity of cells to exchange information through signals that generate specific responses in other cells. It involves an emitter (signal-sending cell) and a receptor (signal-receiving cell). This process affects all aspects of cellular structure and function, including gene activation/deactivation for protein production, tissue formation, and physiological processes like muscle movements. The process is highly specific—only cells with matching receptors can process signals. Signals are primarily transmitted through exocytosis, though some ligands can diffuse through membranes. Communication only occurs when ligands bind to specific receptors, triggering intracellular cascades. Biological responses include gene modification, cell division initiation, and apoptosis. Cellular signaling is categorized into three types based on distance: paracrine (short-distance, cells coordinate with neighbors), autocrine (cells signal themselves), and endocrine (long-distance through circulatory system). Juxtacrine signaling (contact-dependent) involves ligands remaining bound to the signaling cell and binding to receptors on adjacent cells. Ligands are categorized as intracellular (small, hydrophobic molecules like steroid hormones that cross membranes to bind cytoplasmic or nuclear receptors) or extracellular (water-soluble molecules like neurotransmitters that bind membrane-bound receptors). Membrane-bound receptors include ligand-gated ion channel receptors, enzyme-linked receptors (like receptor tyrosine kinases), and G-protein coupled receptors (GPCRs) with seven transmembrane domains that transmit signals through G-proteins.

Hypoxia-inducible factor (HIF) acts as a master transcriptional regulator that coordinates the cellular response to low oxygen conditions. When activated by hypoxia, HIF regulates the expression of hundreds of genes involved in oxygen sensing, energy metabolism, and vascular development. This central role makes HIF a critical target for understanding and treating diseases characterized by abnormal oxygen metabolism. The HIF pathway represents a fundamental mechanism that cells use to adapt to their oxygen environment, and its dysregulation contributes to various pathological conditions including cancer and chronic kidney disease.

Hypoxia is defined as inadequate oxygen supply to tissues. The body responds by releasing hypoxia-inducible transcription factors (HIF), which act as transcription factors binding to gene promoters to increase glycolysis enzyme production. During hypoxia, the electron transport chain cannot function without oxygen, so cells switch to anaerobic respiration via substrate-level phosphorylation. HIF1 and HIF2 show different expression patterns: HIF1 rises quickly then declines, while HIF2 remains elevated, reflecting their distinct roles in early versus sustained hypoxic responses.

Hypoxia-inducible factor (HIF) is a transcription factor that senses oxygen levels in cells. HIF consists of alpha and beta subunits. Under normal oxygen conditions, HIF-alpha is continuously degraded by prolyl hydroxylases, which require oxygen, iron, and alpha-ketoglutarate. When oxygen is scarce, HIF-alpha is stabilized, translocates to the nucleus, and forms a dimer with HIF-beta to activate genes for hypoxic adaptation. HIF-1 primarily regulates metabolic adaptations, while HIF-2 stimulates erythropoietin and VEGF production. HIF signaling is slower than AMPK because it requires gene transcription. Mutations in the VHL protein cause von Hippel-Lindau syndrome due to inability to degrade HIF-alpha. In hypoxia, cells upregulate glycolysis, lactate dehydrogenase, and glucose transporters while downregulating the electron transport chain, Krebs cycle, and mitochondrial biogenesis.

When oxygen levels are reduced, cells activate hypoxia-inducible factor 1 (HIF-1), a transcription factor that regulates gene expression. HIF-1 increases glucose transporter expression (GLUT1, GLUT3) to enhance glucose uptake, and induces changes in Complex IV of the electron transport chain by promoting subunit replacement through proteasome-mediated degradation. This allows cells to adapt to low oxygen conditions by modifying their respiratory machinery and maximizing energy production from available glucose.

HIF (hypoxia-inducible factor) is a transcription factor that senses oxygen levels in cells. Under normal oxygen conditions, HIF-alpha is rapidly hydroxylated by prolyl hydroxylases and destroyed by the proteasome. Under hypoxic conditions, prolyl hydroxylases cannot function, so HIF-alpha accumulates in the cytoplasm. The accumulated HIF-alpha then enters the nucleus, forms a dimer with HIF-beta, and activates transcription of hypoxia-responsive genes. HIF activates anaerobic glycolysis through hexokinases and lactate dehydrogenase, and increases glucose transporter expression (GLUT1 and GLUT3). HIF also activates erythropoietin (stimulating red blood cell production) and VEGF (promoting angiogenesis). HIF downregulates the electron transport chain and Krebs cycle because these processes require oxygen. In hypoxia, multiple regulatory pathways are activated simultaneously: HIF-1 alpha is activated by low oxygen, AMPK is activated by reduced ATP production, and mTOR activity is decreased. These signals can sometimes conflict, but the cell integrates all signals to determine the appropriate response. The outcome depends on the context and other cellular signals.