The 2019 winners of the Nobel Prize in medicine, William Kaelin, Gregg Semenza and Peter Ratcliffe discovered the molecular machinery that allows cells to sense and respond to changes in oxygen levels.
This is exactly the kind of thing which got me interested in science to begin with. I remember growing up curious to find out who won the Nobel Prize in various categories each year and in my mind assessing if they were worthy. (Another reason why I needed to learn the names of the Nobel laureates is because we used to have a general knowledge question in science in school and invariably this would be one of the questions in the test and knowing the right answer gave me a one point edge over my peers! ).
Now after being in research for oh so many years I understand what it all really means. Three decades of research by three brilliant and ingenious scientists to understand the fundamental question in physiology- how cells in our body sense oxygen? This research, in a way defines what scientists do. It might seem simple enough that since oxygen is the elixir of life we must be adapted to sense oxygen. We can leave it at that. But what science does is, it takes this question and dissects it to not only the how but also the why and the so what. Now let us look into how these three Nobel Laureates found the answers to these questions.
The ability to sense and respond to changes in oxygen is fundamental for the survival of all prokaryotic and eukaryotic organisms. Oxygen-sensing mechanisms have been developed to maintain cell and tissue homeostasis, as well as to adapt to the chronic low-oxygen conditions found in diseases such as cancer, anemia, stroke and heart attack.
Our body adapts to changes in oxygen levels and tries to maintain the oxygen levels appropriately high in the blood and tissues. For example, at high altitudes where the air is thinner, the body responds to not having enough oxygen, a condition known as hypoxia, by turning on production of erythropoietin. That protein, often called EPO, is a hormone made by the kidneys and signals the bone marrow to make red blood cells. Since red blood cells contain hemoglobin, which carries oxygen throughout the body, making more red blood cells boosts the amount of oxygen in cells and tissues.
Gregg L. Semenza ‘s research
Gregg L. Semenza came up with the initial clues in early 1990s into how cells respond to low oxygen, by examining the hypoxic response element (HRE) of the erythropoietin gene. This led to the identification of a transcriptional activator called hypoxia inducible factor (HIF). HIF is a heterodimer composed of HIF-1α whose expression and transcriptional activity are tightly regulated by the ambient oxygen concentration and HIF-1β (or aryl hydrocarbon receptor nuclear translocator, ARNT), which is constitutively expressed. Once formed, this protein complex migrates to the cell nucleus and, together with the co-activator CBP/p300, binds to the HREs present on the promoter region of genes involved in regulating metabolic supply and demand.
There are a number of genes regulated by HIF including those involved in regulating vascular tone ( iNOS and adrenomedullin), angiogenesis (vascular endothelial growth factor, VEGF), cell metabolism (lactate dehydrogenase A, the glucose transporter GLUT-1), and haemoglobin biosynthesis (erythropoietin). These studies, together with the ubiquitous nature of HIF and genetic studies indicating that animals dificient in HIF have major defects in many core physiological responses to oxygen, proved that HIF is one of the master regulators of the cellular response to hypoxia.
Peter J. Ratcliffe ‘s research
Peter J. Ratcliffe then sort to define the mechanism responsible for the induction of HIF-1α . Through structural and genetic approaches he was able to show that HIF-1α activity is regulated by enzymatic hydroxylation at specific prolyl and asparaginyl residues by a novel 2-oxoglutarate dependent class of dioxygenases. These oxygen sensitive enzymes inhibit HIF activity in a complementary manner since the prolyl hydroxylase domain containing enzymes (PHDs) result in an interaction between HIF-1α and the von Hippel-Lindau protein which targets HIF-1α for proteosomal destruction, and factor inhibiting HIF (FIH) causes asparaginyl hydroxylation and blocks HIF association with CBP/p300. Hence, under normoxic (normal oxygen level) conditions, HIF-1α levels remain low and this prevents the transcription of genes containing HRE promoters.
William Kaelin Jr’s research
William Kaelin first demonstrated that VHL encodes a functional tumor suppressor (pVHL). He researched hereditary forms of cancers such as Von Hippel–Lindau disease (VHL) a familial cancer syndrome that predisposes affected individuals to hemangioblastomas in the CNS and retina, renal cell carcinoma, and pheochromocytoma, a benign tumor of the adrenal gland. VHL tumors, caused by gene mutation, were known to be angiogenic, creating blood vessels that secrete erythropoietin, the hormone known to be part of the body’s mechanism to respond to low oxygen levels in the blood. He therefore hypothesized that there may be a connection between the formation of VHL tumors and the deficiency of the body to detect oxygen. Kaelin’s research found that in VHL subjects, there were genes expressed for the formation of a protein critical in the EPO process, but which the mutation suppressed. Kaelin’s work found that the VHL protein would help regulate the HIF, and in subjects where the VHL proteins were not present, the HIF would overproduce EPO and lead to cancer. Semenza’s group found that HIF-1α was overexpressed in multiple human cancers under both normoxic and hypoxic conditions, while Ratcliffe and his collaborators found that HIF-1α and HIF-2α were expressed in a number of human cancers and tumor-associated macrophages. Together, these studies suggested a broader role for HIFs in oncogenesis, as well as a potential connection to pVHL.
Clinical relevance of the research
The discovery of the HIF pathway for the first time, not only unveiled a new signaling mechanism mediated by oxygen, but also demonstrated that every cell in the body is capable of sensing and responding to oxygen levels. Kaelin, Ratcliffe, and Semenza have all worked to identify new roles for the pathway and to delineate the mechanisms by which HIF signaling is regulated in a given context.
At the cellular level, HIF signaling has a profound effect on metabolism, allowing cells to switch from the oxygen-consuming TCA cycle to glycolysis . HIF signaling also contributes to cell fate decisions, including differentiation, senescence, and apoptosis. At the tissue level, HIF signaling is involved in the development and maintenance of numerous organs and tissues, including those in the cardiovascular, skeletal, and immune systems.
HIF-signaling pathways have been implicated in a variety of diseases states, with HIF playing a beneficial or a detrimental role, depending on the context. HIF signaling has been shown to mediate protective responses in diseases characterized by impaired tissue oxygenation and inflammation, such as coronary artery disease (CAD) , peripheral arterial disease (PAD) , wound healing and organ transplant rejection. Whereas HIF signaling might be detrimental in other diseases like pulmonary arterial hypertension , chronic ischemic cardiomyopathy and obstructive sleep apnea.
HIF signaling in cancer
HIF signaling plays a complex role in cancer. Hypoxia and expression of HIF in tumors are associated with poor prognosis and have been shown to promote tumor angiogenesis, epithelial-to-mesenchymal transition, stem cell maintenance, invasion and metastasis, therapy resistance, and induction of metabolic alterations.
The role of HIF in cancer has best been illustrated in the context of pVHL-defective kidney cancers. In this setting, HIF-2, rather than its better-studied cousin HIF-1, appears to the be the main culprit . The identification of HIF-2 as a driving force in kidney cancer helped to motivate and accelerate the successful development of drugs that inhibit the HIF-responsive growth factor VEGF for the treatment of this disease. Delineation of the pathways and factors that interact with HIF in a specific disease context has and will continue to help identify therapeutic strategies centered on HIF signaling.
HIF as a therapeutic target
Recently, several drugs that act as selective HIF prolyl-hydroxylase inhibitors have been developed for clinical treatment of anemia. By inhibiting prolyl-hydroxylase enzyme, the stability of HIF-2α in the kidney is increased, which results in an increase in endogenous production of erythropoietin.
HIF pathway activators such as ML-228 may have neuroprotective effects and are of interest as potential treatments for stroke and spinal cord injury.
The three Nobel laureates, Kaelin, Ratcliffe, and Semenza addressed a fundamental and what might seem like a simple question in biology ie., how cells in our body sense and respond to oxygen. They not only found the answer to this question with ground breaking research over more than 3 decades but also delineated the mechanism underlying oxygen sensing which lead to several clinical applications with therapeutic outcomes.