IEB: Hypoxic Response Pathway (Exam II)

Question 1

What is the role of H1F-1, VHL E3 ubiquitin ligase, and HIF PHD?

Answer 1

The alpha subunits of Hypoxia-inducible factor-1 (HIF-1) are hydroxylated at conserved proline residues by HIF prolyl-hydroxylase domain proteins (PHD), allowing their recognition and ubiquitination by the VHL E3 ubiquitin ligase, which labels them for rapid degradation by the proteasome

This occurs only in normoxic conditions. In hypoxic conditions, HIF prolyl-hydroxylase is inhibited, since it utilizes oxygen as a cosubstrate

The PHDs require molecular oxygen (O2), α-ketoglutarate, Vitamin C, and iron(II) for catalytic activity

Question 2

What is FIH? How does it interact with hypoxia?

Answer 2

Factor inhibiting hypoxia-inducible factor (FIH) is an asparaginyl hydroxylase, and hydroxylation of Asn of HIF (hypoxia-inducible factor) by FIH disrupts the interaction of HIF with the transcriptional coactivators CBP/p300. FIH activity is inhibited during hypoxia.

Question 3

Under hypoxic conditions, what occurs?

Answer 3

Under hypoxic conditions, HIF-1 α subunits translocate to the nucleus, where they heterodimerize with HIF-1ß subunits. The resultant product is an active HIF-1 protein that binds to specific hypoxic response elements present in target genes, ultimately activating transcription of these genes, which encode for erythropoietin, VEGF, various glycolytic enzymes, transferrin, and a variety of other proteins essential for systemic, local, and intracellular homeostasis.

Both HIF-1 subunits are basic-helix-loop-helix proteins containing a PAS domain (O2 sensor protein) whose expression is regulated by extracellular O2 tension.

Question 4

What does hypoxia have to do with glucose metabolism?

Answer 4

Under hypoxic conditions, GLUT1 levels have been shown to increase to increase flux of glucose into cells to allow for higher rate of glycolysis. HIF1 downregulates oxidative phosphorylation, however, to produce lactate for energy.