The Golgi apparatus modifies, sorts, and packages proteins to send them to their final destination, either inside or outside the cell.
It is a central function, but little studied in the context of cancer immunology, especially in comparison with other organelles such as mitochondria or the endoplasmic reticulum.
“So we were interested in looking a little bit more at the Golgi apparatus. It’s obviously an important organelle. How is it modified or what is its role in T cells in terms of fighting cancer?” said Nathaniel Oberholtzer, MD/Ph.D. student who worked in the laboratory of Shikhar Mehrotra, Ph.D., co-director of the Cancer Biology and Immunology Research Program at MUSC Hollings Cancer Center and scientific director of the MUSC School of Medicine’s Center for Cellular Therapy.
It turns out that healthy functioning of the Golgi apparatus has a lot to do with how well T cells work to kill cancer cells. Understanding how a signaling axis mitigates Golgi stress, allowing it to function properly, points to a potential new therapeutic target that researchers should pursue to strengthen T cells. Not only that, but Oberholtzer’s research shows how the Golgi could be used as a biomarker to select the strongest T cells for immunotherapy.
Oberholtzer, as first author, and Mehrotra, as senior author, along with a team of Hollings scientists, published the research this month in Science Advances.
T cells, part of the immune system, can kill cancer cells. CAR-T cells are T cells that have been modified in the laboratory to localize proteins to the surface of an individual’s cancer cells. CAR-T cells are custom made for each patient.
Both T cells and CAR-T cells can become “exhausted” in the hostile microenvironment of the tumor. Mehrotra’s lab is looking for ways to stimulate these cells so they can fight cancer for longer.
“The entire tumor microenvironment is conducive to the tumor itself, but not to the other cells that are trying to get in there,” Mehrotra said.
Like people, cells are constantly under stress: stress from biochemical reactions that have become unbalanced and mechanical stress from movement. Transient stress can be good. Stressing muscles through exercise strengthens them, and transient stress on cells can trigger a response that ultimately strengthens them.
“But if this stress remains there, which happens in the tumor microenvironment, the cells are simply in continuous stress, and that will lead to a very different phenotype and death,” Mehrotra said.
However, the researchers found that treating the Golgi apparatus with hydrogen sulfide created T cells that could withstand more stress.
“Hydrogen sulfide is a gaseous signaling molecule present in almost all types of mammalian cells. It is usually a byproduct of different cellular processes, but it has actually been shown to also have very important signaling functions,” Oberholtzer said.
“It can modify proteins through a process called sulfhydration, where it modifies cysteine residues and can change their activity.”
In this project, Oberholtzer discovered that this sulfhydration process, by modifying a protein called Prdx4 within the Golgi apparatus, confers protection in an oxidative environment.
“When you have the stressors that the tumor microenvironment exerts on the T cells, there is an alteration or fragmentation of the Golgi apparatus, where it essentially cannot do its job. Hydrogen sulfide protects against that alteration,” Oberholtzer said. .
Studying this protective effect led researchers to take a closer look at the Golgi apparatus itself.
“Basically, if you use the Golgi apparatus as a simple marker, if T cells have a lot of Golgi rather than less, those with more Golgi are much more robust at killing tumor cells and controlling tumors,” Oberholtzer explained. .
Using cell sorting technology in the Hollings Flow Cytometry and Cell Sorting Shared Resource, the researchers sorted the T cells based on the amount of Golgi they contained. The top 30% was labeled Golgi-hi and the bottom 30% was labeled Golgi-lo.
“Basically, all cells that express a high Golgi apparatus have a very different phenotype. They are less exhausted and much more potent at controlling tumors,” Mehrotra said.
This preclinical work suggests that sorting T cells into Golgi-hi and Golgi-lo and reinfusing only the Golgi-hi cells into a patient would create better chances of controlling the tumor.
“Right now, we’re working on doing some validation studies at the Center for Cellular Therapy so we can start a clinical trial and see if that has translational capacity as well,” Oberholtzer said.
More work is also needed to understand the role of Golgi stress when all organelles in a cell are under stress due to the tumor microenvironment.