In order to understand the mechanisms underlying the development of resistance to a pure antiestrogen we established three human breast carcinoma cell lines resistant to ZM 182780 (ZM) (Faslodex). Long-term cultivation of the ERalpha-positive, 17beta-estradiol (E(2))-responsive cell lines T47D, ZR-75-1, and MCF-7 with the pure antiestrogen ZM 182780 resulted in the T47D-r, ZR-75-1-r, and MCF-7-r cell lines, which proliferate continuously in the presence of 10(-6)M ZM 182780. The resulting antiestrogen-resistant cells grow equally well in medium with or without E(2) and in medium with or without ZM 182780 indicating that they are no longer estrogen-responsive. ERalpha expression was lost at the protein level in all three resistant cell lines. At the mRNA level, the ERalpha was only faintly detectable in T47D-r, whereas a weak signal was seen in ZR-75-1-r and MCF-7-r. By reverse transcription-polymerase chain reaction (RT-PCR) the ERbeta was detectable in the antiestrogen-sensitive and -resistant breast cancer cell lines, however, ZR75-1-r contained the smallest signal for ERbeta. In all three antiestrogen-resistant cells the PR was undetectable, whereas binding of epidermal growth factor (EGF) and protein expression of epidermal growth factor receptor (EGFR) were increased. To analyse alterations in the gene expression pattern in more detail Atlas arrays were hybridised with RNA isolated from T47D-r and T47D and the two Ca2+-binding proteins calgranulin A and B were found to be up-regulated in T47D-r compared to T47D. Calgranulin A and B were also both up-regulated in ZR-75-1-r and MCF-7-r compared to their antiestrogen-sensitive counterparts. Loss of ERalpha expression may be linked to the acquisition of antiestrogen resistance and enhanced expression of the EGFR and of proteins of the S100 family of Ca2+-binding proteins which may contribute to the outgrowth of resistant cells.