| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Trendsetters
1 Physiologische Institut, Medizinische Universität zu Lübeck, D-23538 Lübeck, Germany; and
2 Physiologisches Institut der Universitat Zurich CH-8057 Zurich, Switzerland
Have you ever experienced hypoxia? You have, whenever jogging or biking for example, when your heart or your skeletal or respiratory muscles did not receive enough oxygen to cope with the work. In such circumstances, vasodilating substances such as nitric oxide and angiogenic factors such as vascular endothelial growth factor (VEGF) are released and cause increased blood flow and sprouting of new capillaries. In other, more pathophysiological situations such as anemia or lung disease, hypoxia results in increased production of the hormone erythropoietin, which in turn stimulates production of red blood cells that carry oxygen. In all of these circumstances, lack of oxygen elicits adaptive mechanisms that alleviate the potentially damaging effects of hypoxia by increasing vascular diameter, capillary density, blood oxygen capacity, and cellular glycolysis (6).
Burning questions then arise: is there a common biochemical entity responsible for all of these different processes? If so, is this entity universally represented throughout the animal kingdom? For example, consider the oxygen-dependent manufacture of erythropoietin. The cloning, sequencing, and molecular analysis of the gene encoding this hormone eventually led to the discovery of a family of transcription factors termed hypoxia-inducible factors (HIFs). HIFs are dimers of novel 
-subunits, HIF-, and a previously known ß-subunit commonly known as ARNT (arylhydrocarbon receptor nuclear translocator). ARNT is involved in removing xenobiotics from the organism and is expressed independently of the prevailing partial pressure of oxygen. Both of these HIF subunits are expressed in vertebrates, worms, and flies and therefore are considered to be widely expressed transcription factors, at least in the animal kingdom. What then is so interesting about HIF-? First, it binds, with the help of ARNT, to a DNA consensus sequence that is common to all hypoxia-regulated gene sequences known so far. Second, its cytosolic half-life depends on the partial pressure of oxygen: the lower the partial pressure of oxygen, the longer the half life of HIF- and the stronger its action on the DNA target sequence of relevant hypoxia-dependent genes. Third, it is surprising that the effect of HIF- on the target genes is imitated by iron chelators and cobalt chloride.
Although the widespread DNA consensus sequence to which HIF- binds has been characterized in great detail, much less was known until recently about the cellular events that very rapidly stabilize HIF- under hypoxic conditions (4). A significant step forward in this puzzle has been taken by P. H. Maxwell, C. W. Pugh, P. J. Ratcliffe, and colleagues, who discovered that the von Hippel-Lindau tumor suppressor protein (pVHL) interacts physically with the so-called oxygen-dependent degradation (ODD) domain of HIF- (5). Interaction between HIF- and pVHL is not so "innocent" as it may appear at first glance: in the presence of oxygen it induces the rapid destruction of HIF- by the ubiquitin-proteasome pathway involving an ubiquitin E3 ligase complex. In cells lacking pVHL, HIF- is stable even at normal oxygen concentrations, leading to high levels of VEGF and high rates of glycolysis that eventually contribute to the formation of specific tumor types.
We have now seen that HIF- is being directed by pVHL toward ubiquitination-dependent proteolysis and furthermore have noted that this reaction occurs at a much higher speed under normoxic conditions than it does in hypoxia. Obviously there is still a missing link that connects the ODD domain of HIF- to pVHL as a function of oxygen availability (i.e., the long-sought "oxygen sensor"). Goldberg et al. (1) ingeniously suggested that the biochemical equivalent of such an intracellular "oxygen electrode" is a heme protein. Their hypothesis was based on the observation that the effects of hypoxia could be inhibited by carbon monoxide and mimicked by transition metals such as cobalt and by reagents that chelate iron. Despite this convincing evidence, the exact molecular nature of the "oxygen sensor" remained undefined until recently, when two groups of investigators around P. J. Ratcliffe and W. G. Kaelin Jr. independently reported that an enzyme with prolyl hydroxylase activity could be the missing link in oxygen sensing (2, 3).
Prolyl hydroxylases are dioxygenases that require ferrous iron and 2-oxoglutarate as cofactors. They are well known to researchers in the field of collagen biochemistry as enzymes that attach an oxygen atom, such as a hydroxyl group, to C-4 of proline. A particular feature of this hydroxylation reaction is the need for a reducing agent, such as ascorbate, to keep the iron atom in the ferrous state. With regard to the targeting of HIF-, such a reaction is not simply a "metabolic joke" because both groups of investigators have shown that the highly conserved proline 564 in the ODD domain of HIF- is hydroxylated only under normoxic conditions and in the presence of sufficient amounts of reduced iron. Binding of HIF- by pVHL was demonstrated to be strictly dependent on this metabolic labeling of HIF-. Conversely, decreasing the oxygen availability or chasing the iron with cobalt greatly reduces the interaction of HIF- with pVHL.
Can this enzyme, named HIF- prolyl hydroxylase (HIF-PH), now be regarded as the winner in the race for the discovery of the oxygen sensor? Not quite, but the likelihood is fairly good because this distinct biochemical entity fulfills an impressive number of criteria that were set out for qualification as an oxygen sensor (6). Now we need to identify and clone this enzyme to learn more about its physiological oxygen profile, its cellular regulation in different tissues, and its potential interaction with other redox-sensitive enzymes. However, we should not forget that, apart from protein stabilization, other oxygen-dependent modifications of HIF- have been suggested to be necessary for its full activity. With this in mind, we look forward to more exciting insights in the field of oxygen homeostasis.
| Occasionally, the Editor invites contributions from the authors of published scientific articles that have been identified as being of special interest. All précis to Trendsetters are by invitation only.
|
References
targeted for VHL-mediated destruction by proline hydroxylation: implications for O2 sensing. Science 292: 464–46 2001. to the von Hippel-Lindau ubiquitylation complex by O2-regulated prolyl hydroxylation. Science 292: 468–472, 2001. in response to hypoxia is instantaneous. FASEB J 15: 1312–1314, 2001.
| Occasionally, the Editor of the Trendsetters section invites contributions from the authors of published scientific articles that have been identified as being of special interest. All précis to Trendsetters are by invitation only. |
This article has been cited by other articles:
|
|
 |
|
  H. Hoppeler and M. Fluck Normal mammalian skeletal muscle and its phenotypic plasticity J. Exp. Biol., August 1, 2002; 205(15): 2143 - 2152. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| Visit Other APS Journals Online |
