**3. Mechanisms of endocrine resistance**

454 Breast Cancer – Focusing Tumor Microenvironment, Stem Cells and Metastasis

independent domain located at the N-terminus of the receptor with its function regulated by phosphorylation. AF-2 is the site where ligand-binding actually occurs and is therefore hormone dependant. Almost all gene promoters are activated through both AF-1 and AF-2, though some are activated independently by AF-1 or AF-2 (Gronemeyer 1991; Osborne et al., 2001). Subsequent to formation of the ER-ligand complex, binding of co-regulatory molecules such as nuclear-receptor co-activator 1 (NCOA1 or SRC1), NCOA2 (TIF2) and NCOA3 (AIB1, TRAM1, RAC3 or ACTR) (Leo & Chen 2000; McKenna et al., 1999) enhance the transcriptional activity of ER accompanied by increased activity of histoneacetyltransferase (HAT) at the promoter site. Other co-regulatory molecules can also partly suppress the transcriptional activity of ER by recruitment of histone-deacetylase complexes such as nuclear-receptor co-repressor 1 (NCOR1) and NCOR2 that influence ER-induced transcription (Chen & Evans, 1995; Horlein et al., 1995). Several of these groups of molecules have been reported to have prominent roles in cancer. AIB1 (SRC-3) is over-expressed in almost two thirds of all breast cancers and associated with a shorter disease-free survival in patients receiving tamoxifen as adjuvant treatment (Osborne et al., 2003). In untreated patients, high levels of AIB1 were associated with improved outcome, consistent with studies that suggest the possibility of an association between an enhanced agonistic effect of tamoxifen and the high levels of co-activators. However, ER can also co-operate with FOS/JUN and bind with other transcription factors such as AP-1 (activator protein-1) and SP-1 (specificity protein-1) at their specific sites on DNA (Kushner et al., 2000; Ray et al.,

1997; Safe 2001) commonly designated as serum response elements (SRE).

tyrosine kinase receptors such as EGFR and ERBB2 (Normanno et al., 2005).

Ligand independent activation of ER can occur via the downstream signaling cascades transmitted through membrane receptor tyrosine kinases such as EGFR, ERBB2, and IGF1R In particular. MAPK/ERK, PI3K/AKT, p90RSK and p38 MAPK pathways can specifically activate ER at key positions (serine 118 and 167 and threonine 311) in the AF-1 domain and in other domains (Bunone et al., 1996; Campbell et al., 2001; Joel et al., 1998; Kato et al., 1995). Expression of ligands and receptors such as transforming growth factor-α (TGFα), IGF1 and IGF1R can be increased by estrogen and those can then initiate signalling while expression of other receptors such as EGFR and ERBB2 is decreased by estrogen signaling (Kushner et al., 2000; Massarweh et al., 2008; Umayahara et al., 1994; Vyhlidal et al., 2000; Yarden et al., 2001). In addition, activation of the PI3K/AKT and the p42/44 MAPK

In addition to its classical mode of action through a nuclear-located receptor, estrogen has also been reported to interact with membrane associated receptors, leading to a more rapid reaction than would be expected from a transcriptionally mediated response, such as initiation of cAMP production (Rosner et al., 1999; Zivadinovic et al., 2005) and activation of intrinsic kinases present in other plasma membrane receptors such as insulin-like growth factor-1 receptor (IGF-1R), epidermal growth factor receptor (EGFR) and ERBB2 (Bunone et al., 1996; Campbell et al., 2001; Font de Mora & Brown 2000) as well as receptors for fibroblast growth factor (FGF), platelet-derived growth factor (PDGF), vascular endothelial growth factor (VEGF) and hepatocyte growth factor (HGF). It has been suggested that interaction of SERMs including tamoxifen with such membrane associated receptors may be responsible for their agonist behaviour. There is however much controversy over this issue, with other studies discounting the involvement of such postulated receptors as G proteincoupled receptor as targets of estrogen action (Otto et al., 2008). However this may be, any non-genomic interactions of estrogen would depend on the levels of the above-mentioned kinases, and they will likely be modest in ER+ breast cancer cells that express low levels of It should be noted that most tumours are heterogeneously composed and a biphasic response to treatment could reflect the survival and eventual clonal outgrowth of an intrinsically resistant minor sub-population.
