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M= Mouse; R=Rat; H=Human; Rb=Rabbit; G=goat; B=Bovine, MO=Monkey; P=pig; CT= near C-terminus; NT=near N-terminus; Internal=Middle of protein. EC=extracellular; CP=cytoplasmic domains *


HIF-prolyl hydroxylases (PHD1, PHD2, PHD3) 

Oxygen is absolutely critical for the survival of mammalian cells. An abnormal supply (low or high) may perturb the cellular milieu and affect many physiological activities. Therefore, mammals have developed very elaborate and complex cardiovascular, respiratory, and hematopoietic systems to capture oxygen and supply it to every cell. Too much oxygen may generate hyperactive and damaging free radicals, whereas as low oxygen (hypoxia, 1% O2) results into activations of certain genes that increase uptake of glucose and shift from aerobic to glycolytic metabolism. Therefore, there must be very sophisticated mechanisms to sense oxygen levels and execute an appropriate response. Hypoxia induced factor (HIF) is a transcriptional complex that plays a central role in mammalian oxygen homeostasis. HIF is a heterodimer of constitutively expressed Beta subunit (HIF-beta also known as aryl hydrocarbon receptors trabslocator) and oxygen regulated alpha subunits. Although there are 3-types of alpha subunits, HIF1-alpha, HIF2-alpha (also known as the endothelial per/aryl hydrocarbon receptor translocator/Sim omai), and HIF3-alpha, are the product of difference genes. However, the three HIF-alpha subunits are regulated by oxygen in a similar fashion, i.e. by regulated stabilization of the alpha-subunits. Under normal conditions, HIF-alpha Prolines are hydroxylated at Pro-402 and Pro-564. This allows binding of von Hippel-Lindau (VHL), the substrate recognition component of the E3 ubiquinated ligase complex, subsequent ubiquitination and degradation of HIF-alpha by the proteasome. Under hypoxic conditions, hydroxylation of HIF-alpha is inhibited and this prevents HIF-alpha degradation. A conserved HIF-VHL-prolyl hydroxylases (Egl9) pathway have been identified in C. elegan. In mammalian cells, the HIF-prolyl hydroxylases (PHD) are represented by 3 proteins with a conserved 2-histidine-1-carboxylate iron coordination motif at the catalytic site. The genes encoding these proteins were cloned and termed PHD1, PHD2, and PHD3 (also known as HPH1, HPH2, and HPH3). The three PHDs have been identified to hydroxylate the motif, LXXLAP* with *P being the hydroxyproline.

PHD1 (human 407-aa, chromosome 19; also known as Egl nine homolog 2, EGLN2 (Hypoxia-inducible factor prolyl hydroxylase 1) (HIF-prolyl hydroxylase 1) (HIF-PH1) (HPH-3) (Prolyl hydroxylase domain-containing protein 1) (PHD1) (Estrogen-induced tag 6) is a hydroxylates HIF-1 alpha at Pro-402 and Pro-564, and HIF-2 alpha. PHD1 is expressed abundantly in all tissues with highest expression in testis. It is expressed in hormone responsive tissues, including normal and cancerous mammary, ovarian and prostate epithelium.

PHD2 (human 426 aa, chromosome 1q42-q43, also known as Egl nine homolog 1, EGLN1, (Hypoxia-inducible factor prolyl hydroxylase 2) (HIF-prolyl hydroxylase 2) (HIF-PH2) (HPH-2) (Prolyl hydroxylase domain-containing protein 2) PHD2 (SM-20) (PNAS-118 / PNAS-137) is widely expressed. Alternatively spliced isoforms 2 is missing 337-358 aa. It is activated by hypoxia in some cells and tissues.

PHD3 (human 333-aa, chromosome 14; also known as EGL9, C. ELEGANS, HOMOLOG OF, 3, EGLN3, PROLYL HYDROXYLASE DOMAIN-CONTAINING PROTEIN 3;PHD3, HIF-PROLYL HYDROXYLASE 3; HIFPH3) Widely expressed at low levels. Expressed at higher levels in heart (cardiac myocytes, aortic endothelial cells and coronary artery smooth muscle) and placenta. It is activated under hypoxia.

A novel putative proline hydroxylase, PH-4, with an N-terminal EF-hand motif and a C-terminal catalytic domain with 1 transmembrane domain has been identified in endoplasmic reticulum. PH-4 showed approx. 10-15% homology with the collagen prolyl 4-hydroxylases, PH-alpha I, and PH-alpha II. Human PH-4 (502-aa, chromosome 3pp21.3) is highly expressed in most tissues. Like PHD1-3, PH-4 suppressed the HIF transactivation activity, dependent on the consensus oxygen-dependent degradation domain (ODDD) praline residues. Ph-4 levels correlated with the cellular HIF concentration. Therefore, PH-4 may also be involved in cellular oxygen sensing mechanism.

 

© 2005 GENTAUR bvba
 

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