Crosstalk between the androgen and estrogen receptors in breast cancer

Ndlovu, Easter (2016-03)

Thesis (MSc)--Stellenbosch University, 2016.

Thesis

ENGLISH ABSTRACT: Hormone replacement therapy (HRT) is used by post-menopausal women to alleviate symptoms associated with decreased endogenous estrogen levels, such as hot flashes and vaginal dryness. Although HRT is considered an effective treatment method, results from clinical trials such as those conducted by the Women’s Health Initiative (WHI) and Million Women Study (MWS) revealed that HRT usage is associated with increased risk of invasive breast cancer. The mechanism whereby the hormones used in HRT contribute to breast cancer risk is not fully understood. These hormones elicit their biological effects by binding to intracellular steroid receptors such as the estrogen (ER) and progesterone receptor (PR). For many years it was thought that the ER is the main steroid receptor implicated in breast cancer biology, however, data in the literature suggests that crosstalk between steroid receptors play an important role in the development and progression of this disease. Recent evidence suggests that the androgen receptor (AR) can function as a tumour suppressor and thus has the potential of being a prognostic marker and therapeutic target in breast cancer. Given that the AR has been shown to inhibit the transactivation function of ERα, this raised the question of whether the AR would also inhibit the transactivation function of the ERβ subtype. Moreover, considering that ERs have both transactivation and transrepression functions, studies investigating the influence of the AR on the transrepression function of both ERα and ERβ are lacking. This study therefore investigated whether the AR, in the absence or presence of known AR agonists (the natural AR ligand 5α-dihydrotestosterone (DHT)), the androgenic progestins, medroxyprogesterone acetate (MPA) and norethisterone-acetate (NET-A)), could modulate the transcriptional activity of the ERβ. The ER- and AR-negative MDA-MB-231 breast cancer cell line, transiently transfected with expression vectors for either ERα or ERβ and the appropriate promoter-reporter construct, was used as model system. The results showed that the unliganded or liganded AR has no effect on the transactivation function of ERβ on a synthetic estrogen response element (ERE)-containing promoter but downregulates the ERβ-driven mRNA expression of the endogenous PR gene. This study also shows estradiol (E2)-induced transactivation on a synthetic ARE- and an endogenous androgen response element (ARE)-containing promoter via ERα. For transactivation via ERβ, similar results were only seen on the endogenous ARE-containing promoter. Moreover, we show for the first time that both the unliganded- and ligand-bound AR inhibits the transrepression function of ERα, while interestingly only the ligand-bound AR was able to inhibit the transrepression function of ERβ. No E2-induced cell proliferation was observed in the MDA-MB-231 cell line overexpressing either ERα or ERβ under the experimental conditions used. In conclusion, the results of this study are in agreement with previous evidence suggesting that the AR inhibits the activity of ERα. The precise physiological implications of these results remain to be determined. Finally, although the results from this study are preliminary and have certain limitations, it nonetheless highlights the fact that the role of the AR in breast cancer is a complex one. Thus, our findings may aid our understanding of crosstalk between the ER and AR signalling pathways, and how it contributes to the growth and survival of breast cancer cells.

AFRIKAANSE OPSOMMING: Hormoon vervangingsterapie (HVT) word deur na-menopousale vrouens gebruik om simptome geassosieerd met verlaagde endogeen estrogeenvlakke, soos warmgloede en vaginale droogheid, te verlig. Alhoewel HVT beskou word as ‘n effektiewe behandelingsmetode, toon resultate van kliniese toetse soos byvoorbeel die Vroulike Gesondheids Inisiatief (VGI) en Miljoen Vroue Studie (MVS) dat die gebruik van HVT ‘n toename in die risiko van indrigende borskanker veroorsaak. Die meganisme waardeur hierdie hormone, wat gebruik word in HVT, bydra tot die risiko van borskanker word nog nie ten volle verstaan nie. Hierdie hormone voer hul biologiese effekte uit deur te bind aan intrasellulêre steroïd reseptore soos die estrogeen (ER) en progesteroon (PR) reseptore. Vir baie jare was dit geglo dat die ER die hoof steroïd reseptor betrokke is in borskanker, maar data in die literatuur stel egter voor dat ‘n wisselwerking tussen steroïd reseptore ‘n belangrike rol in die ontwikkeling en bevordering van hierdie siekte speel. Onlangse bewyse dui daarop dat die androgeen reseptor (AR) as ‘n onderdrukker van gewasse kan funksioneer, en dus die potensiaal het om as ‘n prognostiese merker en terapeutiese teiken in borskanker te dien. Die feit dat die AR al getoon het om die transaktiverings funksie van die ERα te onderdruk, het gelei tot die opwekking van die vraag of die AR ook die transaktiverings funksie van die ERβ subtipe kan onderdruk. Verder, siende dat die ER’s beide transaktiverings en transonderdrukkings funksies het, is dit belangrik om in ag te neem dat daar ‘n te kort is aan navorsing wat die invloed van die AR op die transonderdrukkings funksie van beide ERα en ERβ ondersoek. Die huidige studie het dus ‘n ondersoek ingestel om te bepaal of die AR, in die afwesigheid of teenwoordigheid van bekende AR agoniste (die natuurlike AR ligand 5α-dihidrotestosteroon (DHT), die androgeniese progestiene medroksieprogesteroon asetaat (MPA) en noretisteroon asetaat (NET-A)), die transkripsionele aktiwiteit van die ERβ kan verander. Die ER- en AR-negatiewe MDA-MB-231 borskankersellyn, tydelik getransfekteer met die uitdrukkingsvektore vir óf die ERα óf die ERβ en die toepaslike promotor-rapporteerder konstruk, was gebruik as die model sisteem. Die resultate het getoon dat die AR geen effek op die transaktiverings funksie van die ERβ op ‘n sintetiese estrogeen respons element (ERE)-bevattende promotor het nie, maar dat dit wel die ERβ-gedrewe mRNS uitdrukking van die endogene PR geen onderdruk. Estradiool (E2)-geïnduseerde transaktivering op ‘n sintetiese androgeen response element (ARE) en ‘n endogene ARE-bevattende promotor deur ERα word ook in hierdie studie aangedui. Soortgelyke resultate is ook waargeneem vir die transaktivering deur ERβ op ‘n endogene ARE-bevattende promotor. Verder, wys ons vir die eerste keer dat beide die ligandlose en ligand-gebonde AR die transonderdrukkings funksie van ERα inhibeer, terwyl interessant genoeg slegs die ligand-gebonde AR die vermoë het om die transonderdrukkings funksie van ERβ te inhibeer. Onder hierdie gebruikte eksperimentele kondisies, was geen E2-geïnduseerde selproliferasie in die MDA-MB-231 sellyn, wat óf ERα óf ERβ ooruitdruk, waargeneem nie. In samevatting, die resultate van hierdie studie stem ooreen met vorige bewyse wat voorstel dat die AR die aktiwiteit van ERα onderdruk. Die presiese fisiologiese implikasies van hierdie resultate moet nog bepaal word. Ten slotte, alhoewel die resultate van hierdie studie slegs voorlopig is en sekere beperkinge toon, beklemtoon dit nietemin die feit dat die rol van die AR in borskanker kompleks is. Dus, ons bevindings mag bydra tot die huigelike begrip van die wisselwerking wat tussen die ER en die AR seintransduksiepaaie voorkom, en mag ook help om te verstaan hoe dit moontlik kan bydra tot die groei en oorlewing van borskankerselle.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/98345
This item appears in the following collections: