Two thirds of breast
cancers express
estrogen receptors (ER). ER alpha (ERα) mediates
breast cancer cell proliferation, and expression of ERα is the standard choice to indicate adjuvant endocrine
therapy.
ERbeta (ERβ) inhibits growth in vitro; its effects in vivo have been incompletely investigated and its role in
breast cancer and potential as alternative target in endocrine
therapy needs further study. In this work, mammary epithelial (EpH4 and HC11) and
breast cancer (MC4-L2) cells with endogenous ERα and ERβ expression and T47-D human
breast cancer cells with recombinant ERβ (T47-DERβ) were used to explore effects exerted in vitro and in vivo by the ERβ agonists 2,3-bis (4-hydroxy-phenyl)-propionitrile (
DPN) and 7-bromo-2-(4-hydroxyphenyl)-1,3-benzoxazol-5-ol (WAY). In vivo, ERβ agonists induced mammary gland
hyperplasia and MC4-L2 tumour growth to a similar extent as the ERα agonist 4,4',4''-(4-propyl-(1H)-pyrazole-1,3,5-triyl) trisphenol (PPT) or 17β-estradiol (E2) and correlated with higher number of mitotic and lower number of apoptotic features. In vitro, in MC4-L2, EpH4 or HC11 cells incubated under basal conditions, ERβ agonists induced apoptosis measured as upregulation of p53 and apoptosis-inducible factor
protein levels and increased
caspase 3 activity, whereas PPT and E2 stimulated proliferation. However, when
extracellular signal-regulated kinase 1 and 2 (ERK ½) were activated by co-incubation with basement membrane extract or
epidermal growth factor, induction of apoptosis by ERβ agonists was repressed and
DPN induced proliferation in a similar way as E2 or PPT. In a context of active ERK ½, phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K)/RAC-alpha
serine/threonine-protein kinase (AKT) signalling was necessary to allow proliferation stimulated by ER agonists. Inhibition of
MEK ½ with
UO126 completely restored ERβ growth-inhibitory effects, whereas inhibition of PI3K by
LY294002 inhibited ERβ-induced proliferation. These results show that the cellular context modulates ERβ growth-inhibitory effects and should be taken into consideration upon assessment of ERβ as target for endocrine treatment.