Models of estrogen receptor regulation by estrogens and antiestrogens in breast cancer cell lines.
The expression and stability of the estrogen receptor (ER) is the result of a complex process that is modulated by estrogens and antiestrogens. Regulation of the steady-state ER mRNA and protein levels in breast cancer cells appears to be the result of either of two distinct regulatory mechanisms. Estrogen exposure causes a rapid down-regulation of the steady-state level of ER mRNA and protein in model I regulation, as exemplified by the MCF-7:WS8 cell line. Conversely, in model II regulation, as observed in the T47D:A18 cell line, estrogen exposure causes an increase in the steady-state ER mRNA level and a maintenance of the ER protein level. In both these cell lines, the nonsteroidal antiestrogen 4-hydroxytamoxifen has little effect on the mRNA level but causes a net accumulation of the ER protein over time. In contrast, the pure antiestrogen ICI 182,780 causes a dramatic reduction of the ER protein in both the MCF-7:WS8 and T47D:A18 cell lines. This loss has little effect upon the ER mRNA level in the MCF-7:WS8 cells but leads to a decline in the ER mRNA in the T47D:Al8 cells. The estrogen-independent MCF-7:2A cell line, which has adapted to growth in estrogen free media, expresses two forms of the ER, a wild-type Mr66,000 ER and a mutant Mr77,000 ER (ER77). ER77 is the product of a genomic rearrangement resulting in a tandem duplication of exons 6 and 7 (J. J. Pink et al, Nucleic Acids Res., 24:962-969,1996). This exon duplication has abolished ligand binding by this protein. Here we demonstrate that the loss of ligand binding has eliminated the effects of 4-OHT and ICI 182,780 on the steady-state ER77 protein level. However, in the MCF-7:2A cells, antiestrogens affect the wild-type ER protein in the same manner as observed in the MCF-7:WS8 and T47D:A18 cells. Estrogen regulates the ER mRNA and wild-type ER and ER77 proteins in the MCF-7:2A cells in the same manner as observed in the MCF-7:WS8 cells. Interestingly, treatment of the MCF-7:2A cells with ICI 182,780 causes a slight increase in ER mRNA, which is reflected in a net increase in the ER77 protein but a dramatic decrease in the wild-type ER. The models presented here describe the response of two human breast cancer cell lines in short-term studies. These distinct regulation pathways are predictive of the response of these cell lines to long-term estrogen deprivation. This study illustrates two alternative regulation pathways that are present in ER-positive, estrogen-dependent breast cancer cells. This variable response highlights the diversity of responses potentially present in the heterogeneous cell populations of clinically observed breast cancer.
Pink
J J
JJ
Department of Human Oncology, University of Wisconsin Comprehensive Cancer Center, Madison, 53792, USA.
Jordan
V C
VC
D000230:Adenocarcinoma / Q000473:pathology; D001943:Breast Neoplasms / Q000473:pathology*; D018270:Carcinoma, Ductal, Breast / Q000473:pathology; D004273:DNA, Neoplasm / Q000235:genetics / Q000378:metabolism; D015536:Down-Regulation / Q000187:drug effects; D004958:Estradiol / Q000031:analogs & derivatives / Q000494:pharmacology; D004965:Estrogen Antagonists / Q000494:pharmacology*; D004967:Estrogens / Q000494:pharmacology*; D005260:Female; D000077267:Fulvestrant; D015972:Gene Expression Regulation, Neoplastic / Q000187:drug effects*; D006801:Humans; D006915:Hydroxytestosterones / Q000494:pharmacology; D009363:Neoplasm Proteins / Q000235:genetics / Q000378:metabolism*; D009376:Neoplasms, Hormone-Dependent / Q000473:pathology*; D011485:Protein Binding / Q000187:drug effects; D012333:RNA, Messenger / Q000096:biosynthesis / Q000235:genetics; D012334:RNA, Neoplasm / Q000096:biosynthesis / Q000235:genetics; D011960:Receptors, Estrogen / Q000235:genetics / Q000378:metabolism*; D014158:Transcription, Genetic / Q000187:drug effects; D014162:Transfection; D014407:Tumor Cells, Cultured / Q000187:drug effects
D004273:DNA, Neoplasm; D004965:Estrogen Antagonists; D004967:Estrogens; D006915:Hydroxytestosterones; D009363:Neoplasm Proteins; D012333:RNA, Messenger; D012334:RNA, Neoplasm; D011960:Receptors, Estrogen; C032278:4,17 beta-dihydroxy-4-androstene-3-one; D000077267:Fulvestrant; D004958:Estradiol