Carcinogenesis of Breast Cancer

Introduction

The oldest documented case of human cancer dates back to 3000 BCE in ancient Egypt. The details were recorded on papyrus, and described a case involving a tumor of the breast. At the time, these tumors were believed to be incurable and “the curse of the Gods,” and this ideology persisted for thousands of years. Approximately 460 to 370 BCE, a Greek physician named Hippocrates proposed the earliest scientific theory on the origin of cancer. Hippocrates theorized that the body consisted of four humors: black bile, yellow bile, phlegm, and blood. When balanced, these humors resulted in health, but when this balance was lost, cancers and other diseases of the human body resulted. In the case of breast cancer, Hippocrates believed that the root cause was excess black bile in the breast.

Although modern research and medicine has moved far beyond Hippocrates’ humoral theories, there still remains much to be discovered about the causes underlying breast carcinogenesis. Carcinogenesis is defined as a multistep process involving the transformation of normal cells to cancerous cells, which can ultimately lead to the formation of a tumor. In the case of a complex, heterogeneous disease such as breast cancer, carcinogenesis can be attributed to various mechanisms including hormone exposure, gene mutations and/or modifications, as well as epigenetic alterations, all of which can dysregulate normal cellular functions and lead to the development of cancer. Although this list is not comprehensive, this chapter focuses on the aforementioned mechanisms leading to breast cancer development.

Hormones in Breast Carcinogenesis

Roughly 80% of all breast cancers are characterized as estrogen receptor (ER)-positive, and of these 65% are also progesterone receptor (PR)-positive. Both estrogen and progesterone are steroid hormones necessary for sexual and reproductive development in women and play an essential role in both menstruation and pregnancy. Estrogen, specifically 17β-estradiol (E2), and progesterone (P4) bind to the ER α/β and PR A/B, respectively, and drive the transcription of unique genes necessary for cell proliferation, survival, and overall growth. Excessive exposure to these steroid hormones, however, can lead to uncontrolled growth of abnormal cells and has been shown to be associated with an increased risk for developing breast cancer.

Physiologic functions of estrogen are mainly mediated by two ERs: alpha (ERα) and beta (ERβ). Both ERα and ERβ are nuclear transcription factors that, upon 17β-estradiol binding, regulate transcription of multiple target genes by binding to hormone response DNA elements. These proteins are expressed in different cells and tissues throughout the body, including the mammary glands. However, ERα is the predominate receptor for 17β-estradiol function in the breast. Animal studies have shown that administration of 17β-estradiol results in breast cancer, while cotreatment with antiestrogens prevents breast cancer development. Furthermore, women who undergo bilateral oophorectomy before age 35 years have a 75% reduction in lifetime incidence of breast cancer. Although the precise molecular mechanisms by which estrogen promotes breast cancer development are not fully understood, one of the current accepted theories is that 17β-estradiol activates ERα-driven transcription of genes to stimulate cell proliferation. Increased proliferation due to cell cycle progression increases the risk of gene mutations due to rapid DNA replication. Constant exposure to 17β-estradiol promotes the expansion of these mutation-harboring cells that ultimately may give rise to a cancer cell.

In addition to direct proliferative effects of 17β-estradiol through ERα, oxidative metabolism of estrogen to reactive metabolites has also been hypothesized to contribute to breast carcinogenesis. A study by Yue and colleagues demonstrated that ovariectomy of ERα knockout (ERKO)/Wnt transgenic mice significantly increased the time to mammary tumor formation, suggesting a role for ovarian hormones in the absence of the ERα. Furthermore, treatment of the ovariectomized mice with 17β-estradiol significantly reduced the time to tumor development to that of the nonovariectomized control mice. These results provided strong support for a genotoxic, ERα-independent mechanism by which estrogens promoted the development of breast tumors. Using an ERα-expressing mammary tumor-derived epithelial cell line (MCF-7) alongside two ERα-negative human mammary epithelial cell lines (MCF-10A and MCF-10F), researchers were able to investigate the mechanism of ERα-independent breast tumor development associated with 17β-estradiol treatment. Results from these experiments demonstrated that estrogens are converted to catechol and reactive quinone metabolites, both of which can cause DNA damage. Quinone metabolites directly bind to DNA and form adducts, while catechol metabolites undergo redox-cycling to generate oxygen free radicals, which damage DNA-bound guanine to form 8-OXO-guanine. Due to their instability, the quinone adducts and the 8-OXO-guanine bases are removed from the affected DNA regions through a process called depurination. In an attempt to repair these affected regions, error-prone DNA repair mechanisms introduce new mutations and it is the accumulation of these mutations which could ultimately contribute to breast cancer carcinogenesis ( Fig. 8.1 ). Thus increased lifetime exposure to estrogens due to early menarche, late menopause, nulliparity, use of combined oral contraceptive pills, and/or hormone replacement therapy have been associated with increased incidence of developing breast cancer.

While the mitogenic activities of 17β-estradiol are well accepted, the role of progesterone in breast cancer carcinogenesis continues to be debated. Synthetic progestogens have been linked to increased risk of breast cancer; however, the role of endogenous progesterone in breast carcinogenesis still remains unclear. In benign breast lesions and healthy adult breast tissue, there are equal amounts of the two predominant PR isoforms, PR-A and PR-B. In breast cancers, the PR-A/PR-B ratio is higher and is associated with a more aggressive tumor phenotype and resistance to endocrine therapies. PR-A is essential for appropriate progesterone response, and current data suggest that progesterone increases the risk of breast cancer by stimulating the normal breast epithelium through paracrine signaling. This, in turn, promotes preneoplastic lesions through stimulation and proliferation of mammary stem cell–like tumor initiating cells. Despite multiple studies implicating progesterone in breast carcinogenesis, there are several studies contradicting a role for progesterone in cell mitotic activity and proliferation of breast epithelial cells. Thus it remains unclear the mechanism by which progesterone contributes to breast carcinogenesis.

In addition to estrogen and progesterone, elevated levels of circulating androgens have also been associated with increased risk of breast cancer. Although their mechanism of action remains unclear, androgens may increase breast cancer risk indirectly through their conversion to estrogens via a multistep aromatization reaction mediated by the aromatase enzyme. Furthermore, high levels of obligate estrogen precursors such as postmenopausal androgens (testosterone and androstenedione) and androgen precursors (dehydroepiandrosterone) correlate with increased breast cancer risk, but whether these effects are dependent or independent of conversion to estrogen remains unclear.

High levels of prolactin, a hormone that influences ductal and luminal epithelial cell proliferation during pregnancy, have also been associated with increased breast cancer risk, possibly through its interaction with progesterone. However, more research is needed to understand the shared signaling pathways between prolactin and progesterone. Overall, a better understanding of the mechanistic roles of different hormones in breast carcinogenesis may uncover novel targets and therapeutic strategies to prevent and/or treat breast cancers.