The notion of cancer and the images it conjures up—of a rogue cell hijacking the body to form a growing tumor—are eerily reminiscent of science fiction. But cancer is real, far from the fantasy of a sci-fi page-turner. There is no simple switch that can be pressed to make it go away. It is by all accounts a puzzling and elusive disease. Studying the mechanisms by which cancer cells grow and proliferate, a researcher may follow the disease through a variety of pathways. Where they begin their examination can be completely different from where they end up.
Such is the case with Arash Latifkar, a sixth-year graduate student in Chemistry and Chemical Biology. He is a joint student in Richard A. Cerione’s lab, Molecular Medicine/Chemistry and Chemical Biology, and Hening Lin’s lab, Chemistry and Chemical Biology. He works alongside geniuses, as he describes them, in both labs in order to study his project.
Life-Extending SIRT1 and Malignant Tumor Cells: Looking Inside and Outside the Cell
Exactly what his project is, however, has changed in scope more than a few times over the course of his time as a graduate student. He began by looking at the link between a class of extracellular vesicles (small packages filled with substances such as nutrients or proteins that are located outside the cell) known as exosomes. This is a major area of interest in Cerione’s lab. Along with exosomes, he examined a sub-class of enzymes known as sirtuins. Sirtuins are family members of enzymes known as lysine deacylases, which are studied extensively in Lin’s lab. Researchers had previously shown that increasing the levels of a particular sirtuin known as Sirtuin 1 (SIRT1) was linked to longer lifespans. Latifkar, however, was specifically interested in how SIRT1—and the pathways it participates in—is altered in malignant tumor cells.
So Latifkar began outside the cell. “One of our earliest observations was that changes in the levels of Sirtuin 1 impacted the biogenesis of exosomes. We found that decreased expression of SIRT1 increased the number of exosomes produced by the cell,” he says. Latifkar quickly found, however, that the answer to why SIRT1 affected the production of exosomes was not to be found outside the cell but inside.
The life cycle of the exosome actually begins inside the cell with the formation of a multivesicular body, which is a large vesicular structure containing many smaller vesicles. The multivesicular body is transported to the lysosome, the organelle responsible for degrading materials in the cell, where it then fuses with the lysosome membrane and dumps its contents (the smaller vesicles) inside in order to be broken down.
But there is another possibility. Instead of fusing with the lysosome to degrade its contents, it can instead fuse with the plasma membrane of the cell itself and release the vesicles to the outside (at which point the small subtype of these vesicles are known as exosomes). Latifkar noted that if the lysosome wasn’t working normally, then there was an increase in the number of exosomes that were unable to be broken down. If a decrease in SIRT1 led to an increased number of exosomes, he predicted that somehow SIRT1 levels supported the function of the lysosome. Next, he had to figure out how.
Puzzling over Peculiar Cell Functioning: SIRT1 and Lysosome
The environment of the lysosome is usually acidic because it contains digestive enzymes that function best at a low pH (similar to the stomach). What Latifkar observed was that if SIRT1 levels were decreased, the pH of the lysosome increased. A less acidic environment leads to less effective digestive enzymes, which in turn leads to less vesicles being properly degraded and for more to be released outside of the cell in the alternate pathway. Somehow, SIRT1 was helping to maintain the acidity of the lysosome.
“We started by investigating the relationship between SIRT1 and exosome formation and suddenly found ourselves studying lysosomes, which we didn’t set out to do.”
The answer wasn’t so simple. Latifkar explains why, “We and others have observed that Sirtuin 1 is predominantly found in the nucleus of the cell. But the lysosome is found in the cytosol.” The nucleus contains the genetic information for the cell, the blueprints that code for the expression of almost all the structures and proteins the cell needs. Since it holds such valuable information, it is separated from the cell by a membrane of its own. Yet the lysosome is in the cytoplasm (which contains everything else inside the cell besides the nucleus). This evidence suggests that SIRT1 is not acting directly on the lysosome itself to maintain its acidity. Instead, SIRT1 acts on the genetic material that encodes the protein machinery that, once expressed, ensures the lysosome is functional.
Cancer Cells: When SIRT1 Is Not Expressed and Exosomes Predominate
After following exosomes from the outside of the cell to the lysosome and finally to the nucleus, Latifkar was able to get a clearer picture of how SIRT1 was impacting their formation. He found that SIRT1 specifically regulates the protein ATP6V1A, a part of the machinery that maintains the acidity of the lysosome. Mechanistically, SIRT1 stabilizes the RNA sequence that codes for ATP6V1A so that the protein is expressed properly and is able to work on the lysosome.
In certain cancer cells, this pathway is blocked. These cancer cells effectively prevent SIRT1 from being expressed. In turn, the transcript coding for ATP6V1A is no longer stabilized, so the protein machinery itself is not properly assembled. Without this functional machinery, the pH of the lysosome becomes less acidic. The digestive enzymes within the lysosome no longer function as effectively to degrade cellular materials, so fewer vesicles are broken down. Instead, they are shipped out of the cell as exosomes.
Exosomes from a cancer cell can be particularly nasty. Latifkar explains, “These exosomes can transport nutrients and other cargo that support tumor development between tumor cells and other cells in the tumor microenvironment. This allows them to grow faster. It also makes them more resistant to certain treatments.”
Researchers have also shown that exosomes from cancer cells can interact with immune cells and inactivate them. By disabling the normal immune response, the tumor can quickly become much more aggressive.
Studying the interaction between SIRT1 and tumor development, Latifkar’s work has opened up many new questions. Is there a way that researchers can prevent cancer cells from interfering with SIRT1 expression? Can researchers block cancer cells’ access to the cargo in these exosomes? With every new discovery, Latifkar has grown closer to discovering ways to combat tumor development.
“We’ve always tried to follow the biology no matter where it takes us,” he says. “We started by investigating the relationship between SIRT1 and exosome formation and suddenly found ourselves studying lysosomes, which we didn’t set out to do. And now we’re in RNA biology. Both of my faculty mentors have been immensely supportive of my research going in new directions. It’s been a great privilege to watch how they think and to learn how best to go about a scientific problem.”
Compelling Research Options
Latifkar’s research has opened up entirely new avenues for him—so many that it can sometimes even feel like a burden. “It’s been a really wonderful journey because I’ve learned so much about things I’d never thought I would,” he says.
His research has been supported by the Breast Cancer Coalition of Rochester (a $25,000 award) and a predoctoral to postdoctoral transition fellowship from the National Institutes of Health, National Cancer Institute ($84,000 award).
“It’s hard to choose which one area I want to continue with,” Latifkar says. “Now I’m equally captivated by lysosome biology and RNA biology. And one thing that’s on my mind a lot these days is this: Where do these two realms of biology intersect? Not too long ago, it would seem strange to think they overlapped. But recently, there’s been growing interest in lysosomes and how they interact with RNA, and that’s been an exciting area of research for me to watch.”