The Role of the Androgen Receptor Signaling in Breast Malignancies PANAGIOTIS F

The Role of the Androgen Receptor Signaling in Breast Malignancies PANAGIOTIS F

ANTICANCER RESEARCH 37 : 6533-6540 (2017) doi:10.21873/anticanres.12109 Review The Role of the Androgen Receptor Signaling in Breast Malignancies PANAGIOTIS F. CHRISTOPOULOS*, NIKOLAOS I. VLACHOGIANNIS*, CHRISTIANA T. VOGKOU and MICHAEL KOUTSILIERIS Department of Experimental Physiology, School of Medicine, National and Kapodistrian University of Athens, Athens, Greece Abstract. Breast cancer (BrCa) is the most common decades, BrCa still has a poor prognosis with 5-year survival malignancy among women worldwide, and one of the leading rates of metastatic disease reaching to 26% only. BrCa is the causes of cancer-related deaths in females. Despite the second leading cause of death among female cancers with development of novel therapeutic modalities, triple-negative 40,610 estimated deaths in the U.S. expected in 2017 (1). breast cancer (TNBC) remains an incurable disease. Androgen Breast cancer comprises a heterogeneous group of diseases receptor (AR) is widely expressed in BrCa and its role in the with variable course and outcome. Currently, BrCa is sub- disease may differ depending on the molecular subtype and the classified into distinct molecular subtypes named: normal stage. Interestingly, AR has been suggested as a potential target breast like, luminal A/B, HER-2 related, basal-like and claudin- candidate in TNBC, while sex hormone levels may regulate the low (2, 3). Estrogen receptor (ER), progesterone receptor (PR) role of AR in BrCa subtypes. In the presence of estrogen and HER2 have long been established as useful prognostic and receptor α ( ERa ), AR may antagonize the ER α- induced effects, predictive biomarkers. Hormonal therapy in ER and PR whereas in the absence of estrogens, AR may act as an ER α- positive tumors (4), as well as the use of monoclonal antibodies mimic, promoting tumor. Thus, depending on the BrCa micro- in HER2 over-expressing tumors (5) have shown promising environment, both agonists and antagonists of the AR have been results, however overall survival of metastatic disease remains suggested as therapeutic approaches. Herein, we review the role relatively low (1). To date, androgen receptor (AR) has been of AR signaling in BrCa and the molecular cross-talk suggested to play a key role in breast cancer biology in certain mechanisms with other molecules/pathways, as well as its disease subgroups (6, 7). therapeutic implications in the different subtypes of the disease. AR is expressed in all stages of breast cancer (in-situ, primary and metastatic disease) and its contribution to the Breast cancer (BrCa) is the most common solid tumor among progression of disease may differ depending on the stage (8). women with an annual incidence of 123 new cases/100,000 Overall, AR expression among patients with breast cancer has females according to the United States Cancer Statistics and been estimated at approximately 77% and varies significantly 252.710 estimated new cases in the U.S. in 2017. Despite among molecular subtypes of BrCa (9). Human observational significant progress made in therapeutics during the last 2 studies have associated AR with better outcome in ER + tumors, but this positive effect may be lost in ER- tumors (10, 11). Of interest, AR expression correlates with better This Αrticle is freely accessible online. clinicopathological features in the most aggressive form of BrCa, triple-negative breast cancer (TNBC) (12). AR seems *These Authors contributed equally to the study. to have distinct roles in disease development and progression depending on the tumor’s hormonal environment and Correspondence to: Dr. Michael Koutsilieris, Department of specifically upon relative levels of androgens and estrogens. Experimental Physiology, Medical School, National and The historically used androgens together with anti- Kapodistrian University of Athens, 75 Micras Asias, Goudi-Athens, androgens, Selective AR Modulators (SARMs) and Androgen 115 27, Greece. Tel: +30 2107462597, Fax: +30 2107462571, e-mail: [email protected] Receptor antagonists constitute valuable options for the treatment of specific disease subpopulations. In the past Key Words: Breast cancer, androgen receptor (AR), estrogen decade, a wealth body of studies have focused on AR receptor alpha (ER α), triple-negative breast cancer (TNBC), review. targeting along with hampering major signaling pathways 6533 ANTICANCER RESEARCH 37 : 6533-6540 (2017) implicated in BrCa biology in combinational therapeutic agonist. As discussed above, the effect of androgens on breast approaches. Herein we review the experimental and clinical growth is dependent upon relative levels of estrogens. In the evidence investigating the role of AR in BrCa development, vein to shedding more light on this balance, studies have progression and metastasis, as well as exploring its investigated breast growth under extreme states of androgen therapeutic efficacies in the different subgroups of the signaling. Congenital adrenal hyperplasia, which is disease. Special emphasis has been given on the AR-induced characterized by extremely high levels of androgens, associates signaling in BrCa and the molecular cross talk mechanisms with inhibition of normal breast growth (15), whereas AR with other molecules/pathways that may hold promising knock-out in vivo shows incomplete mammary gland therapeutic implications. development (18). Moreover, during the menstrual cycle, breast cell proliferation is maximal during the luteal phase, when The Androgen –AR Axis in Normal testosterone levels are reduced and estrogen levels are highest, and Cancerous Breast Tissue whereas during the follicular phase, where testosterone levels are stable and estrogens are significantly reduced, breast cell While the estrogen-ER α axis constitutes the predominant apoptosis is increased (8, 23). In general, evidence shows that regulator of female breast development, androgens also play although androgens have an inhibitory effect on breast tissue a significant role through their balanced antagonism with development in the presence of estrogens, AR-induced estrogens (13). The main circulating active androgen in signaling is compulsory for normal breast tissue growth. females is testosterone, which is synthesized both in the ovaries and the adrenal glands, as well as by peripheral Polymorphisms and Post-translational conversion of the inactive androgen precursors such as Modifications of the AR in Breast Cancer dehydroepiandrosterone (DHEA) (14). In the breast tissue, testosterone is converted either to dihydrotestostone (DHT) Nowadays cancer is considered as a genetic disease by 5 α- reductase or to 17 β- estradiol (E2) by aromatase and characterized by both inter-tumoral and intra-tumoral (spatial can function as an AR or ER α agonist respectively, thus and temporal) genetic heterogeneity. As such, it is conceivable having a dichotomous effect on breast development (8, 15). that in the vein of personalized therapeutics, focus has been Studies have shown that testosterone is converted to E2 under turned into studying the genetic background of the distinct estrogen deprivation, but is preferentially metabolized into breast cancer subtypes in selective population subgroups (24). DHT when both hormones are present at physiological levels, The gene encoding for the AR is located on the X thus hampering the E2-induced effects (16, 17). Maintenance chromosome, having a length of more than 90 kb and is of this balance ensures the physiological response of the subjected to alternative splicing modifications giving rise to mammary gland depending on the hormonal needs and the various isoforms. The role of the most common splice variant, menopausal status. V7, has recently been explored in breast cancer tissues. AR Although data remain conflicting, AR is indispensable for V7 was associated with androgen deprivation resistance and normal breast growth. AR-knock-out mice showed reluctant was expressed in more than 50% of BrCa cases. Moreover, ductal extension and branching, as well as markedly reduced treatment with enzalutamide, a non-steroidal anti-androgen, epithelial cell proliferation in the breast tissue (18), whereas led to AR V7 up-regulation suggesting a potential mechanism transcriptional inactivation of AR in another in vivo model of acquired resistance to anti-androgen therapy (25). led to accelerated mammary growth and increased number of Variable length of a CAG repeat in exon 1 of the AR gene terminal end buds, an effect that was reversible with DHT constitutes a functional polymorphism with potential treatment (19). Abnormal cell proliferation could be partially prognostic significance. However, the association between attributed to reduced insulin-like growth factor-1 receptor number of triplet-repeats and risk of BrCa development is (IGF-1R) signaling via cross-talk mechanisms (impaired rather unclear (26). The initial study by Giguere et al. showed MAPK and cyclin D1 activation), underlining the importance that smaller numbers of CAG repeats were protective against of a functional AR for the physiological regulation of breast BrCa development (27). In contrast, the majority of studies development (18, 20) . that followed showed either positive or no correlation between The addressed effects of exogenous androgen treatment on the CAG repeat length and BrCa risk (28), whereas, a meta- breast tissue proliferation are also controversial (21). analysis incorporating 6,788 cases of breast cancer patients Androgens have been shown to inhibit growth of breast tissue and 7,648 controls showed that

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