Natural Killer cells are part of the immune system, and unlike their T-lymphocyte cousins, they seem to have an innate ability to kill virally infected cells and cancer cells. This is not something they learn from experience, as do other T cells. They act as if by instinct.
Strangely, even though we have this natural defense system in our bloodstream, we still develop cancers that spread, via the bloodstream, to other parts of the body. And it is this spread of a cancer, metastasis, which makes cancer such a lethal disease.
This has been a troubling puzzle for some time now: if we have Natural Killer cells, why don’t they kill cancer cells? Or at least, why not those that leave the primary tumor and enter the blood stream, where NK cells are found?
So it is big news when a study comes along that identifies a way that Natural Killer (NK) cells can be influenced to do a better job killing cancer cells. Such a study has recently been published (1).
It had been known that the deletion of a particular gene from mice provided them with greater resistance to the spread of cancer, and an increased lifetime of mice with cancer, compared to their normal counterparts. That gene encodes a protein with a tongue-twister name called “E3 cbl-b ubiquitin ligase”. Let’s just call it “E3” here.
The question that was addressed is what is the function of that protein, and what cells are affected by the elimination of that protein (via its gene deletion or inhibition of gene expression)?
Along the way they made an unexpected discovery, that low doses of Coumadin (also known as warfarin), the inexpensive and widely used drug that retards the coagulation of blood, is effective at unleashing the cancer-killing ability of NK cells. It reduced colonization of several difference cancer types, and increases the lifespan of mice with cancer. Its impact was just about as good as a specially designed drug that “awakens” NK cells. Coumadin was effective even at concentration so low that blood-clotting is not at all affected, which means that negative side effects are virtually non-existent. It should be emphasized that Coumadin had no effect on the growth of the primary tumor, and it did not totally prevent metastasis, but it slowed the process of metastasis significantly, and reduced the spread of the cancer.
To repeat, the study was limited to mice, to two different strains of mice, so it may be possible that there will be no positive benefits of low dose Coumadin in human beings. But, since Coumadin is so inexpensive, and has been used for so long with so many people, and at the low concentrations used has no negative side effects, and it ie possible, perhaps likely, that it will work in humans as well as in mice, why not prescribe it as soon as a person is diagnosed with any type of solid tumor. It just may slow the spread of cancer enough to buy time, and to allow drugs that prevent the growth of tumors to have their maximal effect.
If I had a solid tumor I would ask this of my doctor. It is not a cure, but together with a drug that limits growth of a tumor, it could offer increased hope.
If you are a doctor reading this (to the end), and who has read the paper referenced below, can you think of a reason for NOT prescribing low dose Coumadin?
Now let’s get back to the study, for those who would like to follow the investigative process.
The researchers started their study by reproducing the published observation that deletion of the E3 gene in mice slows metastasis and extends life. They also observed increased infiltration of NK cells into the tumor mass, indicating that NK cells were somehow influenced by the E3 deletion. That influence could have been indirect, for example, tumor cells with missing E3 might secrete something that attracts NK cells. Or E3 deletion could have directly altered the properties of the NK cells themselves.
They next isolated NK cells from E3–deleted mice and tested them in normal mice that still had the E3 gene. Once again metastasis was slowed. It follows that E3 deletion affects the NK cells directly. Please note that this does not rule out the possibility that other cells are also affected in some way by the elimination of E3.
What does the E3 enzyme do? It adds a chemical group called ubiquitin to existing proteins thereby changing their functional properties. This is referred to a post-translational modification, because it alters a protein after it has already been synthesized (translated from the DNA code).
The researchers then asked, which proteins in the NK cells are modified? They tested 9000 different proteins for ubiquitination by E3, and found that one protein stood out. That protein was one of a three-member of family of proteins (each of which could be ubiquitinated) collectively known as TAM.
There are things known about TAM. It appears in cells other than NK cells, it is part of a trans-membrane protein found at the surface of cells. Its exterior part serves as a signal receptor (known signaling molecules, or ligands, include Gas6 and Protein S) and its interior, cytoplasmic, side has tyrosine kinase activity. When a TAM molecule binds to its ligand, it is ubiquitinated by E3, and this induces endocytosis, or the formation of a phagocytic vesicle.
Incidentally, TAM also function in the phagocytosis of apoptotic cells, and mutations in TAM are associated with some types of autoimmune diseases.
It seems that TAM negatively regulates the NK cells; Gas6, which activates TAM and induces endocytosis, prevents killing activity, while E3 deletion which blocks endocytosis awakens NK ability to kill cancer cells.
The investigators reasoned that by inhibiting the tyrosine kinase activity of the receptor they could suppress the negative regulation of NK cells. So they set out to design a small molecule inhibitor of TAM based on the known structure of the active site of the tyrosine kinase portion of the molecule. In this they succeeded, and they called their inhibitor LDC1267.
By the way, this approach stands in contrast to the development of most other drugs, which are discovered by screening thousands of known compounds, many extracted from living plants, fungi and other organisms.
LDC1267 worked. It worked just as effectively as if the E3 gene had been deleted. So it, or a similar drug that may work even better in humans, will likely soon be tested in clinical trials.
Indeed, Foretinib, a drug presently being tested in clinical trials, that inhibits some tyrosine kinase receptors, was recently tested with glioblastoma, a highly invasive and high morbidity tumor (2). The authors write: “Foretinib treatment in vivo abolished MerTK [the M in TAM] phosphorylation and reduced tumor growth 3-4 fold in a subcutaneous mouse model”. When M was completely inhibited, it completely stopped the growth of the tumor. (In this case, interestingly, Foretinib had a direct affect on the tumor cells themselves, in vitro, slowing growth and reducing migration. So TAM activity may regulate both cancer cell behavior as well as NK cell behavior).
It turns out that Gas6 and Protein S, the proteins that activate TAM, are Vitamin K dependent proteins. They need Vitamin K in order to be able to bind to TAM. Vitamin K is a molecule of known importance in bone construction, and in blood coagulation.
Blood coagulation? That brings us back to Coumadin, because Coumadin is an antagonist to Vitamin K. That is why Coumadin is useful as a so-called “blood thinner”. (It doesn’t really thin the blood, but by antagonizing Vitamin K it does decrease the probability of blood clot formation).
Now we have a molecular explanation of just why Coumadin (warfarin) is such a good activator of NK cells. Because Coumadin antagonizes Vitamin K it reduces the functionality of Gas6 and Protein S. In turn that means that their ability to bind to TAM is reduced. Since Gas6 activates TAM and negatively regulates NK activity, Coumadin prevents activation of TAM and positively regulates NK cells In effect, TAM is inhibited, similar to its inhibition with LDC1267.
In their Figure 4, a section of which I copied for this blog, one can readily see the splotches of melanoma cancer that had metastasized to the lung tissue in control (vehicle) mice. By contrast the number of metastatic colonies is significantly reduced when mice are treated with their inhibitor LDC1267. It is striking that common Warfarin (Coumadin) is just about as effective.
It seems like before too long we will see some powerful anti-metastasis drugs made available to cancer patients.
But meanwhile, I hope that doctors will start prescribing very low dose Coumadin to their cancer patients.
1. Paolino M, Choidas A, Wallner S, Pranjic B, Uribesalgo I, Loeser S, Jamieson AM, Langdon WY, Ikeda F, Fededa JP, Cronin SJ, Nitsch R, Schultz-Fademrecht C, Eickhoff J, Menninger S, Unger A, Torka R, Gruber T, Hinterleitner R, Baier G, Wolf D, Ullrich A, Klebl BM, Penninger JM. 2014. The E3 ligase Cbl-b and TAM receptors regulate cancer metastasis via natural killer cells. Nature 507:508-512.
2. Knubel KH, Pernu BM, Sufit A, Nelson S, Pierce AM, Keating AK. 2014. MerTK inhibition is a novel therapeutic approach for glioblastoma multiforme. Oncotarget 5:1338-1351.