New research shows these protein-assembly machines are instrumental to helping cells quickly determine when they have sustained too much damage to continue living.
When life’s routine insults damage genetic material beyond repair, cells destroy themselves. A response initiated by damaged DNA is known to force compromised cells to reckon with their fate. However, new research emphasizes the importance of ribosomes — cells’ protein assembly factories — in cellular suicide. In a new study, biochemist Rachel Green, PhD, a Professor of Molecular Biology and Genetics at the Johns Hopkins University School of Medicine, and her colleagues studied the response that ribosomes initiated in human skin cells exposed to ultraviolet (UV) radiation. Within minutes, the ribosome-mediated pathway became more widely active than its DNA-linked counterpart. Meanwhile, interfering with it prevented the cells from killing themselves. “We think ribosome collisions are a critical sensor for early decisions about whether to live or die,” says Green, a Howard Hughes Medical Institute (HHMI) Investigator at Johns Hopkins. She notes, however, that there’s no question that the DNA-driven response still influences cells’ fate, but maybe not during this earliest time regime.
Making a Fateful Decision
Ultraviolet light, reactive chemicals, or other agents can disrupt DNA’s genetic code. These injuries kick off a response, linked with the DNA damage response, that determines how to cope with the damage or — if that’s not possible — sets off the cell’s own death. Carried out correctly, this process may prevent cancer, because abnormal cells that escape this programmed suicide can proliferate to become tumors.
Like its source molecule, mRNA — a transcription of DNA’s instructions — sustains damage from UV. Ribosomes translate the mRNA code into proteins by moving along this linear molecule. When it encounters an error in the mRNA, a ribosome stalls out and is then rear-ended by the one behind it.
Earlier work from the Green lab found that ribosome collisions activate a stress response linked to decisions about a damaged cell’s fate. Experiments suggested that these collisions switch on a pathway that could lead to cell death via a protein called ZAK.
In the newest study, Green’s team, which was led by Niladri Sinha, a postdoc in her lab, and included Sergi Regot, also from the Johns Hopkins University School of Medicine, and Alban Ordureaue, from Memorial Sloan Kettering Cancer Center, took a closer look at the temporal dynamics of the ribosome-initiated response and how the cell uses it to make its fateful decision.
After exposing the skin cells to UV, a notorious source of cancer-causing mutations, the researchers monitored the responses, initiated by damage to DNA versus damage to mRNA. The group examined early time points after UV exposure using various proteomics with Alban Ordureau, in house biochemistry and imaging techniques with Sergi Regot. Over that time, they saw the mRNA and ribosome collision pathway become activated early in almost every cell. By comparison, the DNA damage response switched on only in those cells in a certain phase of their life cycle, which in this case amounted to roughly a quarter of them.
The team then blocked components necessary to both responses. Interfering with ZAK, from the ribosome response, prevented almost all the UV-damaged cells from dying for 24 hours afterward. But, when they inhibited the DNA pathway by deleting p53, cell death declined only modestly.
“What we see, in this case, is that p53 is making only a minor contribution to early cell death,” Sinha says. “Instead, this early response is totally dependent on ZAK and ribosome collisions.”
Striking a Balance
This concept — that collisions among ribosomes are the primary trigger for inducing cell death — is a new idea, according to Susan Shao, a biochemist at Harvard Medical School who was not involved in this study.
“Historically, it’s thought that the priority of a cell would be to protect the DNA, so the cell death pathway would focus on the genome,” Shao says. “And what this story showed is that this response to damage occurs elsewhere, when RNA is being translated to protein. That to me, makes a lot of sense because at that stage you’re able to induce this really quick response.”
To prevent unnecessary death, cells must balance out this rapid response. Experiments showed they accomplish this through another protein, called GCN2, that limits ribosome collisions. A second mechanism helps cells tolerate a moderate level of collisions by degrading ZAK.
Excessive death isn’t the only detrimental outcome: Too little cell death can lead to the proliferation of abnormal cells — otherwise known as cancer. UV exposure causes skin cancer, so the mechanisms explored in this study are directly relevant to understanding how cells avoid and occasionally succumb to this malignancy, according to Green.
These studies have much broader relevance too, because the ribosome-based response likely occurs throughout the body’s cells after other kinds of assaults, Green says. “Anything that leads the cell to accumulate collisions can use these pathways.”
The work was published in Cell on June 5, 2024. The open-access article is titled “The Ribotoxic Stress Response Drives UV-Mediated Cell Death.”
[HHMI news release] [Johns Hopkins news release] [Cell article]