Breast Cancer A Wider Angle (Part 2)
Epigenetics and Aging (19)
In considering epigenetic aspects of breast cancer, I will again adopt Bissell’s wide-angle viewpoint, though Bissell herself has never incorporated epigenetics into her research. I will initially focus on those breast cancers that are estrogen receptor positive (about 80% of all breast cancers). Much of what follows, though, applies to other forms of breast cancer as well, as well as other cancers generally.
From the wide-angle point of view, we need to consider, not just the epigenetic landscapes in the cancer cells themselves, but also the epigenetic landscapes in non-cancerous tumor cells, as well as the epigenetic landscapes of cells that constitute the tumor microenvironment. In general, the epigenetic landscapes of cells in the tumor are destabilized, those in the tumor microenvironment initially less so, but in later stages of cancer development they too become increasingly destabilized. Often the destabilization manifests as a reversion to a less differentiated, more stem-like developmental state, called dedifferentiation.
There is “crosstalk” between all these cellular elements in and around the tumor, through the extracellular matrix or direct cell-cell contact. Breast cancer cells influence noncancerous cells and vice versa, in a reciprocal dynamic. Hence alterations in the epigenetic landscape of any cell in the tumor environment can have influences that extend to the epigenetic landscapes of others.
Let’s first consider the epigenetic alterations in the breast cancer cells themselves, the traditional way to view cancer development. As in all cancers the most readily recognizable epigenetic signatures of cancerous cells in breast tissue are alterations in methylation. There is the cancer-typical global demethylation (hypomethylation) that unleashes gene activity in genes that shouldn’t be active. And there is the cancer-typical hypermethylation, therefore suppression, of a few keys genes that would normally inhibit tumor progression. Of the genes prone to epigenetic silencing through hypermethylation are the BRCAs; the epigenetic silencing mimics the effects of inherited mutations in this “tumor suppressor”.
Other key genes that become increasingly epigenetically silenced in response to therapeutic interventions code for estrogen receptors. Estrogen receptor positive (ER+) breast cancers are treated with hormone therapy, primarily tamoxifen. The goal is to down regulate estrogen responsiveness. Tamoxifen does so by binding to the ER, thereby blocking estrogen from doing so. Over time, tamoxifen treatment results in epigenetic changes that cause the virtual elimination of ER, a shutdown of ER genes. The ER gene becomes hypermethylated and therefore suppressed. This effect is reinforced by histone alterations. You might think that that elimination of estrogen receptors would be beneficial for this form of breast cancer but that’s not the case. Estrogen receptors must be present for the hormone therapy to work. The goal is to lower estrogen responsiveness, not eliminate it altogether. Dial it down, not turn it off.
It is thought that the high relapse rate during or after hormone therapy is related to the lack of estrogen receptors. Some epigenetic therapies are designed to undo the epigenetic alterations that eliminated the estrogen receptors and restore the cell’s responsiveness to tamoxifen. A good state-of-the-art review as of 2020 can be found here (33741974). The emphasis is on developing “EPI Drugs” for therapeutic purposes and to overcome the resistance that inevitably develops in hormone therapies.
Beyond the Breast Cancer Cells
So far, we have stayed within the breast cancer cells; now it’s time to zoom out to changes in the epigenetic landscape of cells in the breast cancer microenvironment. In epigenetically engineered therapies, targets in the microenvironment could provide fertile ground. The microenvironment-based epigenetic therapies though, will probably be directed at more basic—or fundamental—metabolic processes than the one involving estrogen, estrogen receptors and their downstream effects. Metabolic processes common to all cells, and hence all cancers.
We discussed one such process in the last post: glucose metabolism. I will pick up that narrative here. Most of the time, in most cells, glucose is converted to energy (ATP) through a multistep process that occurs in the mitochondria, the organelles that function as the cells’ power plant. This is called cellular respiration (oxidative phosphorylation). An alternative pathway to generate energy from glucose is called glycolysis. This process occurs outside of the mitochondria. It is far less efficient than respiration in generating energy from glucose. As such, it was once, erroneously, thought to be primarily a backup for when oxygen levels are low, when the mitochondrial engines are shutdown.
Glycolysis is, though, often associated with low oxygen environments. Low oxygen (hypoxia) is an inevitable byproduct of compromised blood circulation, which occurs in most tumors. Both cancer cells and noncancer cells shift to glycolysis under these conditions. This shift was first noted by Otto Walburg in the 1920s. The signal of this shift was an increase in lactic acid, which is part and parcel of glycolisis. He considered this lactic acid buildup and the consequent drop in pH a hallmark of cancer.
You might expect that the shift from respiration to glycolysis, in and of itself, would constrain tumor growth. But if the glucose supply is sufficient in the tumor microenvironment as a whole, cancer cells do just fine with glycolysis. So fine, in fact, that cancer cells and other cells in the tumor microenvironment continue to use glycolysis even when oxygen levels rise as the tumor develops. This so-called Warburg Effect is why many think that a key to suppressing cancer development is to starve the tumor--and the tumor microenvironment--of glucose. Conversely, unrestrained access to glucose promotes cancer development. Bissell and her coworkers demonstrated the latter for breast cancer (10.1172/JCI63146).
But the energy derived by cancer cells alone, from the glycolysis of glucose, is not sufficient to maintain them long term, and certainly cannot sustain their proliferation. Additional energy must be supplied to them from without. The source of this additional energy are cells in the microenvironment, of which those called fibroblasts are primary candidates. The energy comes in the form of lactate, one byproduct of glycolysis. Lactate is the primary energy fuel in cancer cells.
Fibroblasts
Fibroblasts are quasi stem cells. They have many of the properties of adult stem cells but are more versatile and more effective in tissue regeneration, in response to a wound. They are also much more abundant than stem cells. Fibroblasts are best known as the source of the two main constituents of stroma: extracellular fluid and collagen. (Stroma can also contain blood vessels, lymphatic tissue, immune cells and nerves.) Stroma serves as both structural support and physiological support for tissues throughout the body, including all organs. All solid tumors have stromal support, in which fibroblasts can promote a permissive or repressive environment.
Fibroblasts in the tumor microenvironment can be transformed into a more mobile form, called cancer associated fibroblasts (CAFs). In addition to mobility, cancer associated fibroblasts lose the polarity of typical fibroblasts and hence have an altered shape. The term cancer associated fibroblast is misleading. A better term is activated fibroblasts; cancer associated fibroblasts are those activated fibroblasts that occur in the cancer microenvironment. But activated fibroblasts also function in any wound healing. This is one reason some view cancer as a wound that won’t heal. I will adopt that view here. It is the best way to make sense of cancers—including breast cancers—as an age-related suite of diseases, which, as we have seen, they overwhelmingly are. Inefficient wound healing is a feature of aging.
In all wounds, cancerous or otherwise, fibroblasts influence other cells in the area in various ways. One is metabolic support for other cells at the site (PMC7064590). Some of that metabolic support comes in the form of glycolysis derived lactate. In both cancerous and noncancerous wound healing the shift to glycolysis from mitochondrial respiration (oxidative phosphorylation) is initially induced by low oxygen levels (hypoxia).
Epigenetics of Fibroblast Alterations in Breast Cancer and the Lactate Shuttle
Epigenetic alterations figure prominently in fibroblast metabolic support for breast cancer cells. First, there is the shift from the quiescent state of stromal fibroblasts to the activated state of cancer associated fibroblasts (CAFs). This transition has been attributed to an “epigenetic switch” (26667266), which strikes me as simplistic*. How the CAF state is maintained is unknown (but see 28123515, for some possibilities).
Once transformed, the CAFs undergo further alteration in response to low oxygen (hypoxia), what has been characterized as metabolic remodeling. (Another dubious term but better than metabolic reprogramming, which is also popular; both are puffed up ways of describing a metabolic alteration.) The “remodel” is the shift from mitochondrial respiration (oxidative phosphorylation) to glycolysis, the source of lactate, which fuels both the fibroblasts themselves and the cancerous cells—among others in the
microenvironment—as well. In the meantime, though, the cancer cells themselves have become glycolytic, either in response to hypoxia directly or by way of induction from Fibroblast born biochemicals. Some think it’s the other way around. In any case, something like a mutual induction of glycolysis and exchange of lactate eventually develops. This reciprocity is often described as a “lactate shuttle” between cancer cells and fibroblasts (10.1016/j.celrep.2020.107701), analogous to the 42nd Street Shuttle between Times Square and Grand Central Station.
Glycolysis in Normal Tissue: Implications
Consider this. The use of lactate as an energy source is not confined to the cancer environment or other wound environments. Glycolysis is important in healthy tissues as well. Skeletal muscles, for example, couldn’t function without it. Also noteworthy is that skeletal muscles don’t form tumors, either primary, or secondary by way of metastasis. Cancer can’t get a foothold there. Why is that? To answer this question, we will need to take an even wider view of cancer, using breast cancer as our guide.
* It involves the activation of two DNA methylation enzymes, which combine to inhibit the inhibitor of an enzyme called Janus kinase1 (JAK1), which, in combination with another enzyme, SMK, incites the quiescent fibroblast to shift toward the activated state of cancer associated fibroblasts.