A Ludwig Cancer Research study led by Peter Ratcliffe and colleagues has discovered a previously overlooked oxygen sensor in animal cells that functions very much like one used by plants.
The ability of cells to sense hypoxia, or low oxygen, is vital to most organisms and the identification of a new cellular oxygen sensing system could lead to the development of drugs for many disorders, including heart disease and cancer. For example, the hypoxia at the core of many advanced tumors is associated with therapy resistance and poor prognoses.
This discovery reveals a new mechanism by which human cells, including cancer cells, respond to low oxygen to change their biological circuitry."
Peter Ratcliffe, a member of the Oxford Branch of the Ludwig Institute for Cancer Research
Ratcliffe and his colleagues report in the current issue of the journal Science that a known human enzyme, cysteamine (2-aminoethanethiol) dioxygenase, or ADO, also serves as a sensor of oxygen levels in cells. ADO splits molecular oxygen (O2) and attaches each atom of the pair to an amino acid, cysteine, on its protein targets. This alteration allows the cysteines to be recognized by another enzyme that further modifies them, tagging the proteins for destruction.
"This process is dependent on the oxygen level inside the cell," said Ratcliffe. "As the oxygen level drops, the oxidation of cysteine residues by ADO occurs more slowly. If oxygen is absent, it doesn't occur at all."
Nearly 20 years ago, Ratcliffe's group deciphered an oxygen-sensing system centered on the oxygen-dependent degradation of hypoxia-inducible factors, or HIFs, which govern programs of gene expression that help cells adapt to oxygen starvation. His and other labs showed that HIFs are widely activated in a variety of cancers. Ratcliffe and two other scientists, William Kaelin and Gregg Semenza, won the prestigious Lasker Award in 2016 for their elucidation of the oxygen-sensing system.
At the time, the system's unprecedented mechanism of signaling (by prolyl hydroxylation, a form of protein oxidation, coupled to protein degradation) appeared to be restricted to animal cells. However, it subsequently became clear that other kingdoms of life use different types of protein oxidation coupled to degradation to signal oxygen levels. It was this that set Ratcliffe thinking about whether there might be other oxygen-sensing systems in human cells.
The current study arose from a meeting in Rome between Ratcliffe and Francesco Licausi, a plant physiologist at the University of Pisa and a coauthor on the new paper. The pair wondered what would happen if plant oxygen sensors, known as plant cysteine oxidases (PCOs), were inserted into a human cell.
As a first step, the researchers linked a fluorescent reporter protein to parts of an oxygen-sensitive plant protein that is protected from degradation when oxygen supplies are low. Cancer cells expressing the fusion protein glowed noticeably more in hypoxic conditions than their oxygenated counterparts.
"That told us that something in the human cell was working on the artificial plant protein," Ratcliffe said. "We were surprised and excited and then, when the excitement died down we began to wonder, what the heck is it?"
Scanning databases of human genes, Ratcliffe's group discovered that ADO is one of two enzymes that resemble PCOs in ways that indicate a functional similarity. The similarity also suggests that this oxygen-sensing system first appeared in a common ancestor hundreds of millions of years ago. Ratcliffe and his colleagues identified three of ADO's protein targets inside cells and showed that while both the ADO system and the HIF system sense oxygen in similar ways, they work on different timescales.
"Animals respond to changes in oxygen on different timescales," Ratcliffe said. "For example, the constriction of blood vessels in response to hypoxia has to occur very rapidly, whereas acclimating the body to reduced oxygen at higher altitudes can occur more slowly."
Ratcliffe suspects there are other oxygen-sensing pathways lurking in animal cells that have yet to be discovered. "We haven't found all of the systems," he said. "For instance, we haven't puzzled out the extremely rapid responses that occur on the order of seconds."