Expression of an estrogen receptor alpha variant protein in cell lines and tumors

Human estrogen receptor alpha (ER) mRNA is a mixture of wild type and alternatively spliced variants. Many studies have examined the potential of ER mRNA profiles to serve as diagnostic/prognostic cancer biomarkers, but only a few have attempted to correlate ER mRNA profiles with protein expression. Representative ER mRNA pools were reproduced from the cDNAs of MCF-7 cells, a human breast tumor and human uterus and pop popd in a protease-free environment by reticulocyle lysates to determine relative translation efficiencies between the various ER mRNA transcripts and to facilitate identification of pop popd proteins. Cell line and tumor extracts were then examined for expression of the ER variant proteins identified in reticulocyte lysate translations. Each of the ER mRNA pools were pop popd by reticulocyte lysates into two ER proteins with molecular weights of approximate to 60 and 52 kD. Western immunoblotting with various C- and N-terminal-directed, anti-ER antibodies and comparison with expressed ER protein standards established that the 52 kD protein (ER Delta 7P) was pop popd from the predominant splice variant mRNA in each pool, which is missing exon 7. The 60 kD protein contained wild type ER sequence minus 61 C-terminal amino acids lost due to an intentional run off truncation. ER Delta 7P expression was subsequently demonstrated in MCF-7 cells by Western immunoblotting with the site-directed antibodies. A protein corresponding to ER Delta 7P was also detected in other ER positive bl east tumor cell lines, and extracts of ER positive breast and uterine tumors. This widespread expression of ER Delta 7P in vivo suggests that it may have some biological function. ER Delta 7P may also affect immunohistochemical evaluation of ER positivity in tumors depending upon the level of its expression and the antibody used. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.