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.
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| Figure 1: Cholesterol and breast cancer.
Deregulations along the cholesterol metabolic pathway may favor the accumulation of metabolites with tumorpromoting activity (such as 27HC) but may also be detrimental to the formation of other metabolites that are beneficial to cell integrity and differentiation (such as dendrogenin A).(2)
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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.
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| Figure 2: The oxysterol 27HC increases tumor growth in several animal models of ER-positive breast cancer.
(A) The estrogenic activity of 27HC is sufficient to support the growth of human MCF7 cell xenografts when propagated in ovariectomized mice. MCF7 cells were injected into the axial mammary pad of ovariectomized, immunocompromised mice, and the mice were administered 27HC by daily injection or given an E2 pellet, as indicated. At day 40, the 27HC-treated mice were randomized into three groups: continued 27HC, 27HC + the antiestrogen ICI 182,780 (ICI), or vehicle treatment (27HC withdrawal) [mean ± SEM (error bars), n = 9 to 10 replicates]. (B) 27HC supports the growth of tamoxifen-resistant, MCF7 cell–derived breast tumors. Tamoxifen-resistant MCF7 cells (TamR) were injected into ovariectomized, immunocompromised mice. The mice were treated for 30 days with E2 (pellet), tamoxifen (pellet), 27HC (injection), or without supplementation. Colored asterisks indicate significant differences from 27HC-treated tumors (mean ± SEM, P < 0.05, n = 5 to 9). The latency (C) and growth (D) of tumors in the MMTV-PyMT mouse model of breast cancer was evaluated in mice in which catabolism of 27HC is attenuated (CYP7B1−/− background). Significance between curves is indicated by a connecting black line and an asterisk (P < 0.05, n = 10 to 28). d, days. (E) Tumor growth in MMTV-PyMT mice is increased by 27HC and attenuated by LXR agonists. MMTV-PyMT mice were injected daily with either 27HC, E2, GW3965, or vehicle as indicated. The growth of tumors in the 27HC- and E2-treated mice were significantly different from those grown in placebo- and GW3965-treated animals (P < 0.05, n = 110 total). (F) The growth of the ER-positive E0771 murine cell-derived xenografts was stimulated by 27HC when grown syngenically. Treatments were by injection as indicated. Colored asterisks indicate significant difference from placebo at the selected time point (mean ± SEM, P < 0.05, n = 7).(1)
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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.
Sources:
1. Nelson, E. R., S. E. Wardell, J. S. Jasper, S. Park, S. Suchindran, M. K. Howe, N. J. Carver, R. V. Pillai, P. M. Sullivan, V. Sondhi, M. Umetani, J. Geradts, and D. P. Mcdonnell. "27-Hydroxycholesterol Links Hypercholesterolemia and Breast Cancer Pathophysiology." Science 342.6162 (2013): 1094-098. Web.
2. Silvente-Poirot,
S., and M. Poirot. "Cholesterol and Cancer, in the Balance."Science 343.6178
(2014): 1445-446. Web.
