Bidirectional cross talk between ERα and EGFR signalling pathways regulates tamoxifen-resistant growth

Abstract

We have previously demonstrated that oestrogen receptor alpha (ERalpha) modulates epidermal growth factor receptor (EGFR)/mitogen-activated protein kinase (MAPK) signalling efficiency in a tamoxifen-resistant MCF-7 breast cancer cell line (Tam-R). In the present study we have investigated whether this cross-talk between EGFR/MAPK and ERalpha signalling pathways is bidirectional by examining the effects of EGFR/MAPK activity on ER functionality in the same cell line. Elevated expression levels of phosphorylated serine 118 (S118) ERalpha were observed in the Tam-R compared to the parental wild type MCF-7 cell line (WT-MCF-7) under basal growth conditions. Phosphorylation of ERalpha at S118 was regulated by the EGFR/MAPK pathway in Tam-R cells being increased in response to amphiregulin (AR) and inhibited by the selective EGFR tyrosine kinase inhibitor, gefitinib and the MEK1/2 inhibitor, PD184352. Recruitment of the co-activators p68 RNA helicase and SRC1 to ERalpha, oestrogen response element (ERE) activity and Tam-R cell growth were similarly EGFR/MAPK-regulated. Chromatin immunoprecipitation (ChIP) studies revealed that in Tam-R cells the ERalpha assembled on the AR gene promoter and this was associated with elevated basal expression of AR mRNA. Furthermore, AR mRNA expression was under the regulation of the EGFR/MAPK and ERalpha signalling pathways. Neutralising antibodies to AR inhibited EGFR/ERK1/2 activity, reduced S118 ERalpha phosphorylation and reduced AR mRNA expression in TAM-R cells. These findings suggest that ERalpha function in Tam-R cells is maintained as a consequence of EGFR/MAPK-mediated phosphorylation at serine residue 118 resulting in the generation of a self-propogating autocrine growth-regulatory loop through the ERalpha-mediated production of AR.

Short SULF1/SULF2 splice variants predominate in mammary tumours with a potential to facilitate receptor tyrosine kinase-mediated cell signalling

“…The constitutive activation of the kinase pathways, however, may bypass inhibition of EGFR/HER2 tyrosine kinases, and develop insensitivity to drugs targeting these receptors. This is compatible with reported observations (Britton et al 2006) that estrogen receptor modulates EGFR/kinase signalling in some tamoxifen-resistant breast cancer cells.…”

“…Differences in EGFR cell signalling in triple positive versus triple negative samples could relate to lack of ER signalling in these tumours. EGFR signalling despite short Sulf1 and Sulf2 abundance regulating HSPG sulfation in triple negative tumours thus could not be promoted due to critical dependence of EGFR activity on ER signalling (Britton et al 2006). Triple negative tumours nevertheless showed some pERK and pAKT downstream signalling due presumably to some activation of cMet RTK activity although even BMP signalling has been reported to increase activities of such enzymes in some cases (Ye L et al 2009).…”

Growth of hormone-dependent MCF-7 breast cancer cells is promoted by constitutive caveolin-1 whose expression is lost in an EGF-R-mediated manner during development of tamoxifen resistance

“…Other molecular changes undoubtedly are required for the manifestation of a full tamoxifen resistant phenotype and a number of ERa co-activators are identified as up-regulated in tamoxifen resistant breast cancers compared to tamoxifen sensitive tumours [30]. However, perhaps most significant with respect to our studies is the recognised upregulation of EGF-R levels evident in TAM-R cells [50], consolidating a model of coordinated gain of EGF-R coupled with the loss of caveolin-1 expression as a mechanism central to conferring hormone-resistance.…”

The HOXB7 protein renders breast cancer cells resistant to tamoxifen through activation of the EGFR pathway

“…MCF-7-TMR cells are known to use an autocrine signaling pathway with EGFR ligands through activated EGFR and MAPK for growth (18)(19)(20). Elevated activation of EGFR as a result of HOXB7 overexpression prompted us to examine the possible overproduction of known EGFR ligands (i.e., TGF-α, ARG, and HB-EGF).…”