The healthy breast is a tissue composed of centrally located milk producing glands connected to the nipple by ducts, surrounded by fat tissue and connective tissue. The growth of the breast is primarily mediated by the estrogens, while the androgens mediate tissue homeostasis and protect against growth signals. In breast cancer, the cells of the glands or ducts undergo malignant transformation, and start proliferating in an uncontrollable fashion. Breast cancer is the most common malignancy in women, and it is estimated that 10% of all women will be diagnosed with breast cancer during their life-time. The primary classification of breast cancer is based mainly on the expression of the estrogen receptor, and 70-80% of breast cancers are estrogen receptor positive, and are classified as luminal. The remaining breast cancers are classified into HER2 positive or triple negative breast cancer. Out of all breast cancers, ~80% are androgen receptor positive. This varies in different subtypes, however, with the highest expression in luminal and lowest expression in triple negative breast cancers. The role of androgen receptor varies depending on subtype. It is considered tissue-protective in luminal breast cancer, while it’s role in HER2 positive and triple negative breast cancers is less defined, but is generally considered to be associated with worse outcome. The primary treatment for breast cancer is surgery, followed by chemotherapy and/or radiotherapy to reduce the risk of recurrence. Treatment is also subtype specific, and luminal breast cancers in premenopausalwomen are treated using the estrogen receptor blocker (antagonist) tamoxifen, which blocks estrogen signaling. In postmenopausal women, luminal breast cancers are treated using tamoxifen or aromatase inhibitors, which prevent the formation of estrogen. The knowledge of which patient will respond and who will develop treatment resistance is of great importance, and the development of markers which can be analyzed prior to treatment in order to reduce the risk of unwanted side effects or complications is the focus of a large body of research. One of the primary goals of this thesis was to establish biomarkers for prognosis and tamoxifen treatment in breast cancer, and paper I, paper II and paper III address this aim.
Steroid hormones, including estrogens and androgens, are normally synthesized from cholesterol in the adrenal gland, as well as in gender specific tissues such as ovaries in women or the testis or prostate in men. This synthesis takes place as a number of enzymatic conversions, mediated by several different enzymes, and the expression of these enzymes determines the final product of this conversion. In the adrenal gland, testis and prostate, androgens are the end-product, while the ovaries synthesize estrogens. These hormones are transported through the circulation, and upon reaching their target tissues, they mediate their effect. The impact of the steroids on their destination tissue is dependent on their relative concentration and exposure time, which in turn is dependent on the amount in the circulation, but also on the presence of local steroid converting enzymes, which are present in most tissues. The enzymes of the hydroxysteroid 17β dehydrogenase family are present in most tissues, primarily the oxidative member hydroxysteroid 17β dehydrogenase type 2, which facilitate the conversion of estrogens and androgens to the less active forms, thus protecting the tissues from their effect. In breast cancer, the reductive form, hydroxysteroid 17β dehydrogenase type 1 is often up-regulated, and mediates increased activation of estrogens, resulting in increased estrogen signaling, which results in increased proliferation and growth. The second goal of this thesis was to further study the role of hydroxysteroid 17β dehydrogenase enzymes in breast cancer, and paper I and paper IV address different aspects of their role in breast cancer.
Following reduction of the expression of hydroxysteroid 17β dehydrogenase type 14, an oxidative member of the family, in breast cancer, the expression of C-X-C ligand 10 was found to be altered. In paper I, in order to determine the role of C-X-C ligand 10 and C-X-C receptor 3 in breast cancer, their expression was quantified using immunohistochemistry in breast cancer patients randomized to tamoxifen or no endocrine treatment irrespectively of estrogen receptor status. The expression of C-XC ligand 10 and C-X-C receptor 3 was found to be associated with increased tamoxifen treatment benefit in the estrogen receptor positive group of patients, indicating that they could be useful markers for determining which patient would respond well to this treatment. Further, C-X-C receptor 3 expression was associated with worse outcome in patients who did not receive tamoxifen, and could be a potential target for inhibitors in order to improve patient outcome. The role of the androgen receptor in breast cancer was evaluated. In paper II the expression was quantified using immunohistochemistry in the same cohort as in paper I. We show that in patients with estrogen receptor negative tumors, the androgen receptor is associated with worse outcome. In patients with high tumoral androgen receptor expression, tamoxifen signaling results in significant improvement in outcome, despite lack of the estrogen receptor. The opposite was observed in patients without tumoral androgen receptor expression, and tamoxifen treatment was associated with adverse outcome. Similar findings were made in the triple negative cases. In the luminal cases, the androgen receptor does not provide further information pertaining to outcome. In paper III we evaluated the role of mutations in the androgen receptor in the cohort of estrogen receptor-negative and androgen receptorpositive cases from paper II. The role of mutations in the androgen receptor appear to have a modest role in regard to patient outcome, but rs17302090 appear associated with tamoxifen treatment benefit. The modulation of the members of the hydroxysteroid 17β dehydrogenase in breast cancer is associated with changes in the local steroid balance, and has been associated with worse outcome and changes in the response to tamoxifen. Further, the inhibition of hydroxysteroid 17β dehydrogenase type 1 has been proposed as an alternate treatment for breast cancer, but no inhibitors are currently used in the clinic. In paper IV, we evaluated several different mechanisms by which the expression of hydroxysteroid 17β dehydrogenase type 1 and type 2 are modulated in breast cancer. We show that the most potent estrogen estradiol, in an estrogen receptor dependent fashion, can result in decreased hydroxysteroid 17β dehydrogenase type 1 expression, and a short term reduction in type 2 expression or long term increased type 2 expression. We also show that the most potent androgen, dihydrotestosterone, can increase hydroxysteroid 17β dehydrogenase type 2 expression, but has limited impact on hydroxysteroid 17β dehydrogenase type 1. Further, we show that a number of genes involved in breast cancer, and microRNA are involved in modulating the expression of the hydroxysteroid 17β dehydrogenase type 1 and type 2 in breast cancer. These findings could potentially be used as an alternative to inhibitors, and help modulate the steroidal balance in target tissue.