ROS

Discovery of a new cell death mechanism: Ferroptosis (fair-oh-toe-sis)

I have decided that I want a section of this blog to be about science, not just my frustrations with it. I will summarize some awesome papers (awesome to me) one at a time in as simple of a way as I can. I won’t be able to dismiss all the science jargon, but I will do my best. This topic is very dear to my heart because it is what granted me candidacy (where I went from a graduate student to a Ph.D. Candidate …. super cool title). Here, I present to you, a new cell death – but before we go in to that, I explain cell death first. I wrote this research summary for the undergraduates here at University of Nebraska-Lincoln, and I wanted to share it with all of you (assuming you don’t fall asleep). If you ever want a science-y topic to be discussed, hit me up in the comments below or send me an e-mail!

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There are many types of cell death mechanisms. The one you are probably most familiar with is apoptosis (a-poe-toe-sis), or programmed cell death. In trying to understand apoptosis, more cell death mechanisms were discovered because certain things were just not consistent with apoptosis. Take necrosis, another type of cell death, as an example. In apoptosis, the cells shrink because components of the nucleus (DNA and proteins called chromatin) were condensing. Once everything gets so small, it starts packaging itself into smaller bodies, and breaks away from the original cell. Eventually, all of these dead bodies are taken in by other types of cells and completely destroyed. In necrosis, the cells get larger because the membrane is breaking which allows some of the extracellular fluid to get inside of the cell. This starts happening to all of the compartments within the cell, and eventually all of the contents, including enzymes and proteases, spill in to the extracellular environment. The released enzymes and proteases then chew away at the cell until it is destroyed. The differences between these 2 types of cell death can be seen in the figure.

Cellular differences between 2 major cell death pathways

Cellular differences between 2 major cell death pathways

In May 2012, a group recently started characterizing a new form of cell death, which they decided to call Ferroptosis [1] , mainly because it involves iron (Ferro) and they see death (so they just stuck with the ‘ptosis’). They came across this unique cell death pathway when they were trying to use a drug called Erastin to selectively kill certain types of mutants (RAS family of small GTPases) that cause cancer in 20-30% of all cancer patients. First, they took their cancer cells and incubated them with erastin, and in just 6 hours they noticed a lot of their cells were dead. They then decided to incubate the cancer cells with an iron inhibitor (called deferroxamine, or DFO), and they didn’t see any death at all. Next, they wondered what would happen if they put BOTH the erastin and the DFO on the cells. There was no death! This means that the death caused by erastin depended on iron being present! This is the very first time any cell death mechanism has required iron! Using these same scenarios, they measured the toxic products generated via the mitochondrial electron transport chain, called reactive oxygen species (ROS). This group did some measurements with a flow cytometer and found out that erastin also caused a lot of ROS to build up and the ROS were also dependent on the iron.

In science, just because you have a neat discovery doesn’t mean you neglect some controls. So, this group had to prove ferroptosis was in fact different from apoptosis and other types of cell death. So, they used a different type of cells and chemically induced the different types of cell death. After enough time had gone by, they took those cells and looked at them under a power microscope. They noticed the mitochondria were smalled in ferroptosis but any of the other cell deaths. So far, so good! They did a lot of complicated tests with many cell death inhibitors, and cell death inducers, to really hammer home the idea that ferroptosis was indeed unique.

Now, we have ferroptosis, which isn’t like any other type of cell death, and it requires iron and ROS. This group decided to figure out what makes ferroptosis genetically distinct. To do this, they used 8 different cell lines, and lowered the mRNA levels (with something called short hairpin RNA, or shRNA) for 6 genes they recently found in a screen. They found some iron proteins, and proteins important for translation, to be important in ferroptosis!

Because ferroptosis is new, this group had to find a way to restore growth so they could look at this death mechanism inside of a mouse, and they did this by finding an inhibitor of ferroptosis. They are a witty group and decided to call it Ferrostatin-1. When they were trying to figure out how ferrostatin-1 inhibited cell death, they realized that lipid (fat) generated ROS were crucial for this death mechanism.

Overall, this group found a new form of cell death, which they decided to name Ferroptosis. Ferroptosis requires iron-dependent production of ROS, NADPH-dependent oxidases (proteins that can transfer electrons across a membrane to an oxygen thereby generating superoxide, a type of ROS), and lipid peroxidation (oxygen takes away an electron from the lipids, thereby creating more ROS). With the use of a power microscope, it could be seen that the mitochondria were much smaller in cells that died from ferroptosis. Therefore, ferroptosis is a truly unique way for cells to die!

REFERENCES

1.                   Dixon, S.J., et al., Ferroptosis: an iron-dependent form of nonapoptotic cell death. Cell, 2012. 149(5): p. 1060-72.
IMAGE:      http://www2.uah.es/daviddiaz/Apoptosis/caracteristicas.htm