For our project, Matt Perez and I are looking into the effects that cholesterol has on ER positive breast cancer. This article has made this connection, illustrating that increased levels of cholesterol can work to promote tumor growth and hurt the survivability of the patient. However, it is not cholesterol itself that is the culprit. It is a primary cholesterol metabolite (Figure 1) called 27-hydroxycholesterol (27HC) which works to promote estrogen receptor (ER) positive tumor growth. The figure below illustrates the cholesterol metabolic process and the similarities between Estrogen and 27HC, both tumor promoters. The fact that Estrogen and 27HC are both products of the cholesterol metabolic process provides some reasoning as to why 27HC shows ER agonist activity, that is the ability to bind to a estrogen receptor and mimic its activity, causing for the promotion of tumor growth.
Now just because 27HC showed ER agonist activity, the researchers had to test if it truly promoted cell growth. They used MCF7 cells, cells that mimic ER positive breast cancer cells, as they are an estrogen dependent environment. With these cells, they tested how 27HC binding promoted growth compared to 17B-estradiol (E2), which is a estrogen like steroid that binds ER receptors very well. The findings from the test illustrated that E2 promoted the most growth, with 27HC also promoting some growth, all though not as strong. However, it was strong enough to consider 27HC a promoter of cell growth (Figure 2A). They then went on to test 27HC against tamoxifen-resistant cancer cells, which are cells resistant to the anti-estrogen drug tamoxifen, which can, when taken when the patient is tamoxifen-resistant, promote breast cancer growth, showing agonist activity. In this test, 27HC was the best promoter, followed by E2 and then by tamoxifen (Figure 2B). Granted this is only in tamoxifen-resistant patients, but it illustrates how the unregulated 27HC can cause large amounts of growth. By now it is clear that 27HC can promote breast cancer.
The effects of an important enzyme in the catabolism of 27HC, cytochrome p450 monooxygenase CYP7B1, were analyzed to see if they produced any difference in the tumor latency (Figure 2C) or in the tumor growth (Figure 2D). The mice positive for the enzyme illustrated a longer latency for the tumors, but eventually, at around the same age, all the mice were no longer tumor free. So despite the delaying ability, this at this point it seems to help only a little. However, when it comes to tumor growth, we see that it is much better at inhibiting and limiting the number of primary tumors compared to the mice recessive for CYP7B1. This information is very useful in learning how to inhibit the effects of 27HC. In addition to these tests performed on mice, it was also found that in human occurrences of breast cancer, we see much better outcomes when there is elevated expression of CYP7B1.
Another option for inhibition of 27HC is the use of an estrogen receptor antagonist called ICI 182,780, which would limit the binding of 27HC. As shown in Figure 2F, we see that 27HC has a large effect on tumor growth except when it is used with ICI 182, 780. This competes for binding of the ER and works to keep the tumor from growing as rapidly. Another inhibitor is GW3965, which does not directly inhibit the binding of 27HC, but rather works to bind the liver x receptor (LXR). LXR is a receptor very important to cholesterol synthesis, and as a result, very important to 27HC synthesis. Inhibition of this helps to limit the amount of 27HC and cholesterol that are made and therefore gives the tumor less avenues with which to grow. This inhibition work just as well despite it being earlier in the cycle. We see the effectiveness of GW3965 in figure 2, E and F.
The significance of these findings is mainly preventative. Many researchers are working on studies, such as the one referenced above, to determine if decreased cholesterol in post-menopausal women will help to decrease ER positive breast cancer. This is due to the fact that both 27HC and estrogen are made as a result of cholesterol synthesis. If there is less cholesterol in the system, less of these products will be made, and as a result the cancer will have less promotion to continue growth. However, there are other ways to inhibit breast cancer using this knowledge. As illustrated by my analysis above, mouse models have determined that inhibition of 27HC also works very well when it comes to improvement in breast cancer. It generally leads to better outcomes, less deaths, and overall better treatment of the breast cancer. Inhibitors such as ICI 182,780 (explained above) can competitively bind the ER receptor, better than 27HC, limiting the fuel for the tumor and therefore inhibiting growth. GW3965 works as an inhibitor as well and presents another option for treatment of breast cancer. Finally, the enzyme CYP7B1 has also been shown to improve the overall survivability and outcomes of ER positive breast cancer. What is nice about this enzyme is it is already in the body and some women who have shown natural increases in this enzyme while they had breast cancer had better results. The opportunity to use an enzyme that is natural to the body makes it much easier to give this to patients, or even treat them in such a way as to naturally increase their levels of CYP7B1.
So practically, what does this mean. Well, it means that by a simple change in diet, many women can help to improve their prognosis and treatment, maybe even limiting the mass' growth into a tumor. All this can be achieved by simply limiting cholesterol in the patient's diet. This would be the best option as it is the least invasive and "easiest" in terms of treatment. However, other options also arise from these findings. The use of the inhibitors discussed in the previous paragraph could prove very productive in the treatment of breast cancer. Maybe through injections or pills, these inhibitors can be added to the patients system post-diagnosis to slow the growth of the tumor and improve the outcome. This would also be less invasive than a lot of treatments such as chemotherapy, assuming of course that these inhibitors don't have any adverse side effects. Lastly, we can look at the enzyme CYP7B1, also discussed above, and the role it could play and in some cases has already played in ER positive breast cancer treatment. As discussed above, women who had this enzyme in high quantities during their fight with ER positive breast cancer showed significantly better outcomes. This knowledge can be used to create some kind of stimulus to promote the activity of the enzyme to break down cholesterol and 27HC. Or it could just be introduced into the system through injection. The opportunities are endless. However, to conclude I would like to look back on the option of changing dietary habits. Imagine, if this does prove to be a viable option, that a cancer patient would just have to take on a new diet and therefore limit the amount of anguish and pain that goes along with cancer. It could greatly decrease the amount of chemotherapy needed, lessen the number of drugs with side effects that must be taken, and improve quality of life with this disease tremendously. It is because of these researchers and millions of mice (RIP) that our generation may live in a world where cancer is no longer a large ordeal, but just another curable disease, similar to the common cold.
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2. Silvente-Poirot, S., and M. Poirot. "Cholesterol and Cancer, in the Balance."Science 343.6178 (2014): 1445-446. Web.