The presence of p53 inhibits cell proliferation. Because p53 is present in almost undetectable amounts in most normal cells, they can properly respond to growth factors and divide when appropriate. A profusion of physiological signals can rapidly elevate the amount of p53 in response to some perceived problem, thereby limiting unwarranted cell proliferation. The protein responsible for regulating p53 has the peculiar name of "mouse double minutes 2" or Mdm2. (The name requires a brief explanation. Double minutes (not 60 seconds, "my-newts") are chromosome fragments that are often found in tumors. They consist of circular pieces of DNA that are present in multiple copies. They often carry oncogenes for, as you may recall, an increase in the number of oncogenes may spur cell proliferation and tumor formation). Mdm2 is an oncogene that was found in a mouse tumor that carried these double minutes. And the protein that it specifies, also called Mdm2, is the major controlling element of p53. Sometime later, mutant versions were found in several human cancers.
How does Mdm2 control p53? In two ways. First and foremost, it helps degrade p53 by attaching ubiquitin to it, marking it for destruction. And what's ubiquitin? Ubiquitin is a small protein found in nearly every human cell (it is ubiquitous, hence its name) and serves to indicate to the proteosomes (remember them?) in the cell that the targeted protein should be broken down. Second, it binds to p53 and prevents it from acting as a transcription factor, thereby interfering with its antiproliferative capacity. Notice that that behavior makes Mdm2 an oncogene, but an oncogene that differs from those that I've written about previously. The others were either out-of-control growth receptors or aberrant enzymes directly in the downstream pathways emanating from these receptors. Mdm2, by contrast, acts by limiting the activity of a tumor suppressor. When it acquires increased activity via a mutation, it prevents p53 from doing its job.
If Mdm2 is continually degrading p53, how does it escape? When an event occurs – see the figure – that signals a problem that p53 can address, several protein kinases may be activated. As you may recall, these enzymes attach phosphate groups on to proteins. They do so to p53 and inhibit Mdm2 from marking it for destruction. In addition, these same kinases may add phosphate groups on to Mdm2, lessening its ability to interact with p53. The result is an increase in the amount of p53. Complicated matters even further, there is at least one another protein that binds to Mdm2, preventing it from destroying p53. It too, then is a tumor suppressor because when it is absent via mutation, Mdm2 will have a freer hand, which in turn will allow p53 to accumulate.
At first it was thought that p53 reacted only to DNA damage. For that reason, it was given the title, "guardian of the genome". But later research showed, as indicated by the figure, that other cellular stresses may cause p53 concentrations to swell. The list is long, but it grows even longer as more are being discovered.
How p53 Guards the Genome
If you've been following along, the next question that might occur to you is: How does p53 work to guard the cell from the problems shown in the figure above? That is, how does p53 limit aberrant cell division? The answer is that p53 is a transcription factor that regulates the activity of a great variety of genes (Weinberg lists 27; a recent review asserts that there are approximately 500!). One surprising target of p53 is the gene for Mdm2. p53 acts to increase the expression of the Mdm2 gene resulting in an elevation in Mdm2 protein concentrations. Thus, p53 sows the seeds for its own destruction. This negative feedback loop ensures that under ordinary conditions, the levels of p53 are kept in check. Mice that bear two inactivated Md2 genes, have greatly increased amounts of p53. That really isn't helpful. While they don't get cancer, they die before birth apparently because their normal cells don't divide as they should.
Another target of p53 is a gene that encodes a protein that inhibits several cyclin dependent kinases – proteins that play a role in advancing cells through the G1 phase of the cell cycle. It is in this way that p53 retards cell division when activated by some defect in DNA damage, allowing cells time to repair problems if they can. Moreover, there is evidence that p53 activates the transcription of genes involved in DNA repair. All this means that when p53 is absent, organisms are at the mercy of agents that damage the genome, such as UV irradiation and chemical mutagens. In fact, mice that lack p53 are extremely prone to tumors when exposed to mutagenic agents. And p53-minus cells grown in culture have aberrant chromosome numbers as compared to controls with the normal complement of p53.
There will be occasions, especially when a cell is teetering toward malignancy, where disturbances to the genome cannot be repaired. At this point, p53 assumes another role – as Dr. Kevorkian, an enabler of suicide. Programmed cell death, apoptosis, is a mechanism built into nearly every cell. It's there for several reasons. During embryogenesis, it helps sculpt organs by removing cells that aren't needed. For example, the formation of the digits of the hand and feet of humans is accomplished by removing cells from between the future fingers and toes (see the figure). The removal of the tadpole tail during metamorphosis is another particularly dramatic example. But apoptosis is occurring all the time. Several references assert that something on the order of a million cells in the human body are dying due to apoptosis each second. In the present context, p53 uses apoptosis to remove cells that pose a danger to the organism because of some damage that cannot be readily repaired. It does so in its role as a transcription factor by increasing the expression of several genes in the apoptosis pathway and inhibiting others that act to suppress apoptosis.
Apoptosis is a phenomenon that has been known to biologists for over one hundred years. However, its mechanism has been uncovered only recently. As you might suspect, it's quite complex, and a lot is still being investigated. Unfortunately, I only have a middling acquaintance with the subject, certainly not enough to write much more about it in depth. For those who want to learn more, I've come across a well reviewed book, "Cell Death - Apoptosis and Other Means to an End" by Douglas R. Green that may be a helpful introduction. For now, I'm going to end my discussion of apoptosis right now and devote the next posting to summarize the role of p53.