EXPRESSION OF ESTROGEN-RECEPTORS IN A NORMAL HUMAN BREAST EPITHELIAL-CELL TYPE WITH LUMINAL AND STEM-CELL CHARACTERISTICS AND ITS NEOPLASTICALLY TRANSFORMED-CELL LINES

Although approximately two-thirds of breast cancers are estrogen recep tor (ER)-positive, only a small proportion of epithelial cells in the mammary gland express the ER. The origin of the ER-positive breast can cers is unknown, Recently, we have developed a culture method to grow two morphologically and antigenically distinguishable types of normal human breast epithelial cells (HBEC) derived from reduction mammoplast y, In this report, we studied the expression of ER in these two types of cells and their transformed cell lines, The results indicate that T ype I HBEC with luminal and stem cell characteristics expressed a vari ant ER (similar to 48 kd) by Western blot analysis, This variant ER co ntains a deletion in the DNA binding domain (exon 2) as revealed by RT -PCR analysis, The lack of the DNA-binding domain of the variant ER wa s also confirmed by the ER-estrogen responsive element binding assay, as well as by the immunofluorescence staining of the ER using anti-ER antibodies which recognize either the C-terminal or N-terminal region, In contrast, Type II HBEC with basal epithelial phenotype are ER-nega tive. Simian virus 40 (SV40) transformed Type I and Type II HBEC lines also expressed the variant ER, Tumors formed in athymic nude mice by in vitro transformed tumorigenic Type I cell lines, however, expressed a high level of wild type ER which was undetectable in these cells gr own in vitro before and after tumor formation, Thus, there appears to be a differential ER mRNA splicing between the in vitro and in vivo mi leu.