¹Department of Pharmacology and Toxicology, University of Otago, Dunedin, New Zealand

Correspondence: Dr RJ Rosengren, E-mail: rhonda.rosengren@stonebow.otago.ac.nz

Revised 14 August 2008; Accepted 4 August 2008; Published online 16 September 2008.

Abstract

We have previously shown that tamoxifen+epigallocatechin gallate (EGCG) is synergistically cytotoxic towards oestrogen receptor (ER)-negative breast cancer cells. To determine if this response would correlate with significant tumour suppression in vivo, athymic nude female mice were implanted with MDA-MB-231 cells and treated with tamoxifen, EGCG, EGCG+tamoxifen, or vehicle control for 10 weeks. Tumour volume in EGCG- (25 mg kg^-1)+tamoxifen (75 g kg^-1)-treated mice decreased by 71% as compared with vehicle control (P<0.05), whereas tumour weight was decreased by 80% compared with control (P<0.01). Epigallocatechin gallate treatment did not alter ER protein expression in MDA-MB-231 cells and thus was not a mechanism for the observed tumour suppression. However, western blotting of tumour extracts demonstrated that epidermal growth factor receptor (EGFR; 85% lower than control), pEGFR (78% lower than control), mammalian target of rapamycin (mTOR; 78% lower than control), and CYP1B1 (75% lower than control) were significantly lower after the combination treatment as compared with all other treatments. Nuclear factor-B (NF-B), b-Raf, p-MEK, S6K, 4EBP1, Akt, vascular EGFR-1 (VEGFR-1) and VEGF expressions were decreased in control but not in the individual treatments, whereas MEK, phospholipase D 1/2, TGF, and ERK expressions were not changed after any treatment. The results demonstrate that tamoxifen at realistic doses (75 g kg^-1) can suppress the growth of ER-negative breast cancer when combined with EGCG. In addition, the dominant mechanism for tumour suppression is the concomitant decrease in tumour protein expressions of mTOR and the EGFR.