INTRODUCTION
- human tumors are heterogeneous in perfusion  large volumes of tumors are chronically
hypoxic
- oxygen is necessary for energy production via mitochondrial respiration
- absence of oxygen  tumor cells have to produce energy via anaerobic glycolysis  leads to
acidosis of tumor tissues and both hypoxia and acidosis lead to more aggressive tumor
phenotypes
1. PASTUER AND WARBURG EFFECTS
- blood vasculature delivers glucose and oxygen to tissues; glucose diffuses to cells in tissues
where it is taken up via specific glucose transporters (GLUT1); once inside cells, glucose
metabolize via glycolysis to pyruvate; in this oxidative process H+ is released
- at pyruvate  continued to oxidizing pyruvate to CO2 (hydrated to HCO3- + H+) via citric
acid cycle and mitochondrial respiration or re-reduce it to lactate (transported out of cell)
- H+ ions produced in both cases  result is an acid load on tissue
- tumor cells consume more glucose for energy production via anaerobic glycolysis when
oxygen perfusion is diminished (anaerobic conditions)
- 18X more energy from one glucose molecule is derived aerobically when compared to
anaerobic glycolysis
- Pasteur effect  inhibition of glycolysis by oxygen; this inhibition is mediated quickly (the
cells respond immediately to the presence of oxygen)
- Warburg noted that the extent to which tumors produce acid under aerobic conditions is
remarkable; he showed that tumors consume glucose rapidly and produce large amounts of
lactic acid (even under aerobic conditions)  Warburg effect
2. FdG PET shows high glycolysis in tumors
- positron emission tomography (PET) can accurately pinpoint source of positron-emitting
isotopes
- positrons are positively charged electrons produced when a proton decays to a neutron;
these positrons migrate and reaction with electron in an annihilation reaction  result is two
gamma rays (511 keV in energy) and radiate 180 degrees apart
- these gamma rays pass through tissue and are captured by a ring of detectors, which are set up
to register events that occur simultaneously in two detectors at the same time (coincidence
detection)  line drawn between two detectors; more lines  pinpoint source of positronemitting isotopes
- 18F (isotope) attached to glucose analog, deoxy-glucose  fluorodeoxyglucose (FdG)
transported by GLUT-1 and trapped in cells by phosphorylation
- tissues that are actively consuming glucose show up positive in FdG PET scans
- conclusion  virtually all metastatic cancer have elevated rate of glucose uptake; FdG PET
has high sensitivity and high specificity
Tumor perfusion
- tumor perfusion is heterogeneous and is illustrated by technique called dynamic contrast
enhanced MRI (DCE-MRI)
- patient injected with contrast reagent (CR), contains chelate lanthanide ion, gadolinium;
gadolinium enhances the magnetization such that signal intensity is higher in volumes that
contain gadolinium
- volumes with highest perfusion take up more CR and have highest signal intensity
- time-dependent enhancement affected by: 1.) vascular volume and 2.) vascular
permeability
- first component  CR in vasculature increases rapidly and decreases slower due to
recirculation-dilution and through kidney clearance  magnitude of peak is directly
proportional to the vascular volume in the individual pixel
- second component  leakage of CR from blood  interstitium, which is affected by
permeability and the surface area of the blood vessel endothelium
Perfusion and Angiogenesis
- angiogenesis is sprouting and growth of new blood vessels; requirement for tumor growth
(drug targets)
- angiogenic tissue is poorly perfused though; vascular beds that are normally finely tuned such
that the resistances across capillary banks are balanced such that flow occurs evenly
- angiogenesis in tumors occurs in the absence of a defined morphogenetic gradient such that
it results in long, tortuous vessels (series, high resistance) or in shunts and extra capillaries
(parallel, low resistance)  resistances are no longer balanced  blood flow is unevenly
shunted to beds with lower resistance
- excessive angiogenesis  heterogeneous perfusion
- angiogenesis induced by specific growth factors: fibroblast growth factor (b-FGF), vascular
endothelial growth factor (VEGF) and angiopoeitins, Ang-1 and Ang-2
- VEGF induced by hypoxia  induces growth of new vessels which relieve hypoxia 
generates balanced vasculature
HYPOXIA INDUCIBLE FACTOR (HIF)
- HIF is the major TF responsible for VEGF induction
- HIF also induces expression of erythropoietin (EPO) expression; HIF is a major
transcription factor (TF) regulating expression of proteins involved in energy metabolism,
iron metabolism, and angiogenesis
HIF-1 is a dimeric transcription factor
- HIF TF is a dimer comprised of HIF-1alpha and HIF-1beta subunits
- HIF-1beta subunits is thought to be constitutively and ubiquitously expressed
- HIF-1alpha is also constitutively expressed, yet its levels are reduced in presence of oxygen
- absence of oxygen  HIF-1alpha increases to levels where it can combine with HIF-1beta to
form an active transcription factor complex  binds to DNA containing sequence CCGCT
(hypoxia response element, HRE)  glycolytic enzymes, EPO, VEGF  increased
glycolysis, increased angiogenesis (VEGF), increased RBCs (EPO, iron)
HIF-1alpha is regulated by oxygen dependent proteolysis
- reduction of HIF-1alpha in presence of oxygen mediated by cytosolic prolyl hydroxylase,
PH (family of iron sulfur hydroxylases that contains the ER prolyl hydroxylase used in the
maturation of collagen)
- oxygen present  PH hydroxylates two proline residues on HIF-1alpha  hydroxylated
HIF-1alpha subunits are recognized by a ubiquitin ligase (vH-L protein, its gene is an
oncogene)  transfers ubiquitin polypeptides to HIF-1alpha  polyubiqiunated HIF-1alpha
recognized by 28S proteosome (its gene is an oncogene) that is responsible for
degradation/turnover of cytoplasmic proteins
- genetic defects in vH-L lead to hereditary renal cell carcinoma (von Hippel-Lindau syndrome)
Coordinated regulation of glucose metabolism, iron metabolism, and angiogenesis by HIF1
- three categories of genes know to be upregulated by HIF-1: energy metabolism, iron
metabolism, angiogenesis (vasoactive proteins), and hormones/receptors
- genes of energy metabolism exclusively involved in regulation of glucose metabolism; a
lack of oxygen  compensated by upregulation of glycolysis  provides cells with energy in
absence of oxygen
- iron metabolism upregulated because iron is essential in hemoglobin and myoglobin for
transport of oxygen to tissues
- vasoactive proteins induce angiogenesis (VEGF) or increase vessel tone  promotes
movement of solutes form blood  interstitium
HIF-1alpha is dysregulated in cancers
- genetic defects in vH-L cause constitutive upregulation of HIF (familial renal cell carcinoma)
- HIF is elevated in many cancers (not a direct consequence of hypoxia); HIF remains elevated
under normoxic conditions
- continued expression of HIF-1alpha under normoxic conditions means that tumors continue
to produce VEGF (even if they are adequately oxygenated); this products disrupts normal
morphogenetic gradients for angiogenesis and will lead to imbalanced vasculature (which is
a hallmark of all tumors)
- overproduction of HIF-1alpha also be responsible for the Warburg effect and the elevated
uptake of glucose observed in virtually all metastatic tumors
THE HOSTILE TUMOR MICROENVIRONMENT
- tumors are heterogeneously perfused  gives rise to significant volumes that are
hypoxic/acidic and substrate-limited
- such microenvironmental conditions have significant impact in the genotypic/phenotypic
characteristics of tumors, including altered gene expression, increase metastasis, and therapy
resistance
- only a few of these are due to alteration in HIF-1alpha
- look at table for therapeutic consequences of this microenvironment 26-8