Differential expression of estrogen receptor mRNA splice variants in the tamoxifen resistant human breast cancer cell line, MCF-7/TAM R-1 compared to the parental MCF-7 cell line

Abstract

Breast cancer patients with an estrogen receptor (ER) positive tumor can be treated with the anti-estrogen tamoxifen, but development of anti-estrogen resistance is a serious problem. We have analyzed a tamoxifen resistant human breast cancer cell line MCF-7/TAM R-1 for alterations in ER which might explain the tamoxifen resistance. The MCF-7/TAM R-1 cells expressed both wildtype ER mRNA and protein, and by RT-PCR we were able to clone ER cDNAs corresponding to the following mRNA splice variants: ERΔE2, ERΔE4, ERΔE5, ERΔE7 and a new double splice variant lacking both exon 4 and 7 (ERΔE4,7). The existence of the ERΔE4,7 variant was confirmed by RNase protection assay. Semi-quantitative RT-PCR revealed that ERΔE2 mRNA was expressed at a higher level in MCF-7/TAM R-1 cells, whereas the ERΔE5 mRNA was expressed at a significantly lower level in MCF-7/TAM R-1 cells compared with MCF-7 cells. The differential expression of the two ER mRNA splice variants indicates that they may be involved in anti-estrogen resistance, although the present knowledge of their biological function does not provide us with an explanation.