Profile of William Kaelin, Peter Ratcliffe, and Greg Semenza, 2016 Albert Lasker Basic Medical Research Awardees

Toward the end of the 18th century, the Swedish apothecary, Carl Scheele, determined that approximately one-fourth of the volume of air was “feuer luft,” or “fire air”: that is, the component of the atmosphere that allows substances to burn. At almost the same time the English theologian and philosopher, Joseph Priestley, found a method to purify this gas; and in both cases, determination and an ingenious use of tools and techniques enabled them to determine that a specific aspect of the air around us is absolutely required for combustion.

After a fortuitous meeting with Priestley, the French aristocrat, politician, and scientist, Antoine Lavoisier, was able to replicate Priestley’s method of purification, and gave the resulting substance the name of oxygen. Both Priestley and Lavoisier later saw their laboratories destroyed by mobs; Lavoisier was executed soon after and Priestley fled to America. Scientists were then, as now, not immune to conflagrations of ignorance.

However, although their laboratories burned, the resulting work and the gas remain central to our understanding of life. In September, the Lasker Basic Medical Research Award was given to three successors to the legacies of Scheele, Priestley, and Lavoisier: two Americans, Gregg Semenza and William Kaelin, Jr., and Sir Peter Ratcliffe of the United Kingdom (Fig. 1). These researchers were recognized for their discovery of how animal cells adapt to variations in oxygen tension.

Although oxygen is a necessary factor in cellular respiration, it has been known since Bechamp and Pasteur that this process is part of a more complex balance of oxygen use: that is, animal cells can use multiple pathways to accomplish energy production, including a switch to anaerobic fermentation when necessary. This capacity was a familiar part of the firmament of biochemical knowledge by the middle of the last century. However, until the discoveries of Semenza, Ratcliffe, and Kaelin, it was not at all clear how this adaptation was regulated at the fundamental level of gene expression.

It was known that a number of physiological and metabolic responses to oxygen levels are transcriptional. The mRNAs encoding many glycolytic enzymes (1, 2), the angiogenic factor VEGF (3), and the hormone erythropoietin (4) had all been shown to increase when oxygen levels in cells drop. Definition of the regulatory regions of genes responsible for this level of control were initially best characterized in the erythropoietin gene. Once this sequence was isolated it was termed an HRE, or hypoxia-responsive element (5). Semenza and Ratcliffe and their coworkers were very involved in the determination and characterization of these regulatory regions. In the race to find the factor binding them, Semenza’s group was the first to clone the transcription factor, which he called HIF, for hypoxia-inducible factor (6).

HREs have since been found in the control regions of many oxygen-regulated genes, and HIF has subsequently been shown to directly regulate the expression of hundreds of mRNAs. The changes wrought by alterations in gene expression through the action of this factor influence virtually every aspect of cell physiology and metabolism, and have been shown to be vital for proper animal cell adaptation to the constantly changing levels of oxygen that occur both within tissues and in the environment.

Early work demonstrated that many aspects of development and homeostasis depended on HIF, and that cancer, inflammation, tissue injury and healing, and a wide range of other challenges to the organism, involved HIF response. This work has continued at a rapid pace, showing repeatedly that HIF-mediated responses to oxygenation are important across biology, from immune response and cardiovascular function to neurological control. Given this finding, it is not surprising that HIF function has been found to influence a large number of pathophysiological responses as well, and has an intimate relationship with many aspects of disease progression.

Semenza showed that HIF consists of two basic helix–loop–helix family factors in a DNA-binding dimer. One member of the dimer is HIF-1α. The second moiety in the dimer was first termed HIF-1β; it was subsequently found to be the previously cloned and characterized aryl hydrocarbon receptor nuclear translocator. This second component is not sensitive to oxygen levels. Thus, a key fact was immediately deduced upon cloning of these two binding partners: that the regulation of the oxygen-sensitive HIF-1α could be an essential facet of the cellular response to hypoxia.

The question then arose: what regulates the regulator? This differed from the usual search for an initiating signal transduction event, in that at its root there had to be a direct response to a biophysical constraint: that is, the level of oxygen itself. And at this point, Kaelin and his collaborators made an important connection: that mutation of the tumor suppressor VHL (associated with von Hippel–Lindau familial cancers) caused increased expression of HIF-regulated genes (7). Because the VHL protein is involved in ubiquitination and protein turnover, a potential regulatory step was revealed; the Ratcliffe group then showed that cells deficient in VHL showed no evidence of oxygen-sensitive turnover of HIF-1α (8).

The critical event that followed was the discovery by both the Ratcliffe and Kaelin groups that specific proline residues in the HIF-1α protein were hydroxylated, and that this hydroxylation step was essential for the binding of the VHL complex (9, 10). Subsequently, Ratcliffe’s group and, independently, Bruick and McKnight found candidate prolyl hydroxylases that catalyzed this modification (11, 12). And because proline hydroxylation necessarily involves molecular oxygen, this set up a model for a conceptually elegant means for cells to regulate HIF levels posttranslationally (Fig. 2). As oxygen levels decrease, hydroxylation of HIF is reduced; this in turn leads to diminished association with the VHL complex, lowered amounts of ubiquitination, and decreased proteasomal degradation of HIF-1α. Ultimately, then, HIF-1α [and its isoform, the subsequently cloned HIF-2α (13–15)] accumulate in the absence of oxygen and do the work of inducing the gene expression needed to adjust cellular lifestyles to the lower levels of available oxygen. A further subtlety was added by Semenza’s discovery of FIH, an inhibitor of HIF (16). This, too, is a hydroxylase, and thus is also susceptible to oxygen-mediated changes in its activity, although it acts in a different region of the HIF-α proteins and alters the transcriptional activation of the pathway instead of modulating HIF protein turnover (17).

Work by many groups have shown the robustness of this pathway and its central role in modulating oxygen-influenced gene expression. And although it took a village to figure out HIF (as someone could have said), these three investigators have remained central figures in this work ever since their pathfinding discoveries. They have been involved in the continuing elucidation of the molecular biology of HIF regulation, and have as well increased our understanding of the physiological roles played by hypoxic response in health and disease.

The discovery of the proline hydroxylases that regulate HIF-1α stability enabled a search for inhibitors; and this has now opened up new pathways for pharmacologic discovery. In fact, a number of potential drugs that influence HIF function are already far along in clinical trials. Clinical development is at the moment chiefly focused on the treatment of anemia, bringing the work back to one of its original goals, manipulating erythropoietin expression. However, given the range of diseases affected by HIF, many more clinical applications will surely follow.

Thus, this honor, the awarding of the Lasker Basic Medical Research Award, is richly deserved: it marks an important turning point in biology, where one of the most fundamental aspects of animal life has had its character and regulation revealed.

It should also be noted that all three of these awardees are physician scientists; and it could be argued that they represent the best of what can be achieved when a medical understanding, and truly superb science, are arrayed against a critical problem in biology.