Abstract
BackgroundTamoxifen resistance is a major problem in the treatment of estrogen receptor (ER) α -positive breast cancer patients. Although the mechanisms behind tamoxifen resistance are still not completely understood, clinical data suggests that increased expression of receptor tyrosine kinases is involved. Here, we studied the estrogen and anti-estrogen sensitivity of human breast cancer MCF7 cells that have a moderate, retroviral-mediated, ectopic expression of epidermal growth factor receptor (MCF7-EGFR).MethodsProliferation of MCF7-EGFR and parental cells was induced by 17β-estradiol (E2), epidermal growth factor (EGF) or a combination of these. Inhibition of proliferation under these conditions was investigated with 4-hydroxy-tamoxifen (TAM) or fulvestrant at 10-12 to 10-6 M. Cells were lysed at different time points to determine the phosphorylation status of EGFR, MAPK1/3, AKT and the expression of ERα. Knockdown of target genes was established using smartpool siRNAs. Transcriptomics analysis was done 6 hr after stimulation with growth factors using Affymetrix HG-U133 PM array plates.ResultsWhile proliferation of parental MCF7 cells could only be induced by E2, proliferation of MCF7-EGFR cells could be induced by either E2 or EGF. Treatment with TAM or fulvestrant did significantly inhibit proliferation of MCF7-EGFR cells stimulated with E2 alone. EGF treatment of E2/TAM treated cells led to a marked cell proliferation thereby overruling the anti-estrogen-mediated inhibition of cell proliferation. Under these conditions, TAM however did still inhibit ERα- mediated transcription. While siRNA-mediated knock-down of EGFR inhibited the EGF- driven proliferation under TAM/E2/EGF condition, knock down of ERα did not. The TAM resistant cell proliferation mediated by the conditional EGFR-signaling may be dependent on the PI3K/Akt pathway but not the MEK/MAPK pathway, since a MEK inhibitor (U0126), did not block the proliferation. Transcriptomic analysis under the various E2/TAM/EGF conditions revealed that E2 and EGF dependent transcription have little overlap and rather operate in a parallel fashion.ConclusionsOur data indicate that enhanced EGFR-driven signalling is sufficient to overrule the TAM- mediated inhibition of E2-driven cell proliferation. This may have profound implications for the anti-estrogen treatment of ER-positive breast cancers that have increased levels of EGFR.
Highlights
Tamoxifen resistance is a major problem in the treatment of estrogen receptor (ER) α -positive breast cancer patients
EGFR over expression in MCF7 cells enhances downstream mitogen activated protein kinase (MAPK) and v-akt murine thymoma viral oncogene homolog (Akt) signalling To investigate the role of EGFR on anti-estrogen resistance, we established ectopic human EGFR expression in human MCF7 breast cancer cells
Because the results so far had indicated that EGFR-driven proliferation may be dependent on the Phosphoinositide 3-kinase (PI3K)/Akt pathway and to a lesser extent on the Mitogen-activated kinase kinase (MEK)/MAPK pathway, we investigated PI3K/Akt regulated gene expression
Summary
Tamoxifen resistance is a major problem in the treatment of estrogen receptor (ER) α -positive breast cancer patients. The mechanisms behind tamoxifen resistance are still not completely understood, clinical data suggests that increased expression of receptor tyrosine kinases is involved. Despite the improvement in treatment, therapy resistance remains a major problem in the clinic. Endocrine therapy has become the most important treatment option for women with estrogen receptor (ER) α -positive breast cancer, which is approximately 70% of all breast tumours. The ERα − antagonist tamoxifen is commonly used with these ERα-positive breast cancers. Around 40% of all ERα-positive patients do not respond to tamoxifen treatment (de novo resistance) [1]. Most patients that initially respond to tamoxifen treatment eventually develop resistance (acquired resistance) [2,3]
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