WE Presentation #2 Content to powerpoint posted
Aerobic Glycolysis, lactate production and hypoxia.
The conversion of glucose to lactic acid, which can occur in hypoxic normal cells,
persistent cancer cells despite the presence of oxygen that would normally inhibit
glycolysis through a process termed the Pasteur Effect. We now know that
sustained aerobic metabolism through glycolysis diminished Pasteur Effect and
in certain cancer cells it is linked to the activation of oncogenes or loss of tumor
suppressors. The Warburg Effect came after Pasteur’s findings and was with
cancer cell, where Pasteur was looking at yeast and the relationship of anaerobic
and aerobic production of ATP.
As neoplastic cells accumulate in threedimensional multicellular mass, local low nutrient and oxygen levels triggers the
growth of new blood vessels in the neoplasm. The imperfect neo-vasculature in
the tumor bed is poorly formed and inefficient and hence poses nutrient and
hypoxic stress.
Glucose is transported into cells by facilitated transporters and then trapped
intracellular by glucose phosphorylation. The hexose phosphate is further
phosphorylated and split into 2, 3 Carbon molecules (GAP & DHAP) that are
converted to glycerol for lipid synthesis or sequentially transformed to pyruvate.
Pyruvate is transported into the mitochondria and is converted to acetyl-CoA
were it condenses with OAA to form citrate in the TCA cycle.
Formation of citrate from acetyl CoA and OAA permits a new round of TCA
cycling, generating high-energy electrons, CO2, and carbon skeletons that can
be used for biosynthesis. Citrated self could be extruded into the cytosol and
then converted to acetyl-CoA a by ATP-dependent citrate lyase for fatty acid
synthesis to produce phospholipids and generate new biomembranes. Glucose
through the pentose phosphate pathway generates ribose for nucleic acid
synthesis and NADPH for reductive biosynthesis, and to reduce reactive oxygen
species (ROS).
Glutamine, which circulates with the highest concentration amongst amino acids,
serves as a major bioenergetic substrate and nitrogen donor for proliferating
cells. Glucose and glutamine are required for hexose amine biosynthesis.
Glutamine enters the TCA cycle via the deamination to glutamate and then to αKG a key TCA cycle intermediate. Under hypoxia, the hypoxic inducible factor
(HIF – 1) activates pyruvate dehydrogenase kinase that inhibits pyruvate
dehydrogenase and the conversion of pyruvate to acetyl-CoA, thereby shunting
pyruvate to lactate through homolactic fermentation.
Allosteric regulations of glycolysis (Figure 4 of WE video presentation) confer
metabolic plasticity with respect to local pO2. Enzymes are represented in
italicized blue font and their substrates in bold black. Because the glycolytic flux
is nominally faster than OXPHOS, the Pasteur Effect has been evolutionary
selected to couple both metabolic rates. The energy metabolites glucose-6-P,
ATP, and citrate restrain the glycolytic flux through allosteric inhibition of key
1
glycolytic enzymes, as represented by the red arrows. Inhibition is at its climax
when oxygen is not a limiting substrate for OXPHOS, thus allowing the full
oxidation of glucose. When oxygen levels are limited or when the pO2 fluctuates,
full glucose oxidation, and consequently the levels of ATP and citrate produced
oxidatively are decreased. The Pasteur Effect is reset to less pronounced
inhibition, thus allowing accelerated glycolysis to compensate for defective ATP
production. An extreme situation characterized by full inhibition of the Pasteur
Effect is met under severe hypoxia. The energetic crisis is associated with an
increase in the cellular levels of fructose-1,6-BP, ADP, AMP, and inorganic
phosphate (Pi). These molecules exert a series of allosteric stimulations
(represented by the green arrows) that accelerate the glycolytic flux. Glycolysis
thus becomes the main source of cellular ATP production, a rescue situation
allowing short-term cell survival until the pO2 is restored. Other abbreviations:
GAPDH, glyceraldehyde-3-phosphate dehydrogenase; P, phosphate; PPP,
pentose phosphate pathway; TCA, tricarboxylic acid (cycle).
http://www.frontiersin.org/files/Articles/13090/fphar-02-00049HTML/image_m/fphar-02-00049-g001.jpg
The best-characterized regulatory mechanism is that modulating HIF-1α’stability.
Under well-oxygenated normoxic conditions, prolyl hydroxylation (by prolyl
hydroxylases [PHDs]) and subsequent ubiquitination (by von-Hippel Lindau
(VHL)-containing E3 ubiquitin-protein ligase) of the oxygen-dependent
degradation (ODD) domain of HIF-1α leads to rapid degradation of HIF-1α with a
half life of 5-8 min. Consequently, HIF-1 is inactive under normoxic conditions On
the other hand, under oxygen-deprived hypoxic conditions, HIF-1α becomes
stable because oxygen-depletion directly decreases the PHDs’ activity. Then,
HIF-1α interacts with HIF-1β, forms a heterodimer, HIF-1, binds to its cognate
DNA sequence, the hypoxic-responsive element (HRE), and finally induces the
expression of various genes related to angiogenesis, metastasis and glycolysis
.In addition to the regulation of HIF-1α’s stability, another post-translational
modification of HIF-1α is known to function in the regulation of the
transactivational activity of HIF-1. Under normoxic conditions, factor inhibiting
HIF-1 (FIH-1, Hypoxia-inducible factor 1-alpha inhibitor is a protein that in
humans is encoded by the HIF1AN gene), becomes active and hydroxylates an
asparagine residue (N803) of HIF-1α. The asparaginyl hydroxylation blocks the
interaction of HIF-1α with the transcriptional co-factor p300 and CBP, resulting in
the suppression of HIF-1’s trans-activational activity. Because oxygen is a
substrate of FIH-1 as well as PHDs, HIF-1’s trans-activational activity is restored
under oxygen-deprived hypoxic conditions.
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HIF-1 controls metabolic and pH-regulating pathways. Cells respond to hypoxia
by HIF-1-mediated upregulation of glucose transporters (Glut-1 and Glut-3) and
enzymes of glycolysis, hexokinase (HK), phoshofructokinase-1 (PFK-1) and
pyruvate kinase (PK). Conversion of pyruvate to lactic acid is facilitated by the
induction of lactate dehydrogenase (LDH). HIF-1 also induces pyruvate
2
dehydrogenase kinase-1 (PDK-1), which inhibits the conversion of pyruvate into
acetyl-CoA by pyruvate dehydrogenase (PDH), thus preventing entry of pyruvate
into the TCA cycle. Subunit composition of cytochrome coxidase (COX4) is
influenced by HIF-1 in hypoxia: COX4-2 is induced and COX4-1 is reciprocally
reduced by induction of the protease LON that degrades COX4-1. Switching the
COX subunits ensures optimal efficiency of mitochondrial respiration in hypoxia.
Furthermore, pH homeostasis is maintained by induction of carbonic anhydrase
IX (CAIX) and the monocarboxylate transporter MCT 4 and the
Na+|[sol]|H+ exchanger NHE1.
http://www.nature.com/cdd/journal/v15/n4/images/cdd200812f2.jpg
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