Breast cancer tissue has been shown to contain alternatively spliced estrog
en receptor alpha (ER-alpha) mRNA variants, which have altered biological a
ctivities compared to the full-length ER-alpha. The development of endometr
ial cancer, as well as drug resistance in breast cancer patients undergoing
tamoxifen therapy, may represent altered ER-alpha function secondary to sp
ecific exon deletions. While the literature is replete with ER mRNA variant
data, little information is available regarding the presence and function
of endometrial ER variant proteins. We evaluated the presence of human ER-a
lpha mRNA and protein variants in six premenopausal, six postmenopausal, an
d six endometrial carcinoma samples. Reverse transcription-polymerase chain
reaction, DNA hybridization, and sequencing techniques identified exon 4,
exon 5, and exon 7 mRNA splice variants in all patients as well as MCF-7 an
d Ishikawa cell lines. Presence of translated proteins for full-length ER-a
lpha, as well as splice variants, was investigated by Western blot analysis
using antibodies directed against the N-terminus, hinge region, and C-term
inus portions of the ER. These experiments confirmed the presence of immuno
positive protein bands of approximately 64-66 kDa in all patients correspon
ding to wild-type ER-alpha. A protein band migrating at 41 kDa, consistent
with an exon 5 splice variant, was only seen in endometrial adenocarcinoma
samples. Premenopausal and postmenopausal endometrial samples did not conta
in detectable amounts of ER splice variant protein. Human ER-alpha mRNA var
iants are present in all human endometrial samples, but detectable levels o
f variant proteins are only observed in patients with endometrial adenocarc
inoma. (C) 1999 Academic Press.